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Data will be made available on request.Despite the foreseen transformation that will take place in the next few years, the reduction in oil consumption will not reach 60 % until 2050, with an 80 % reduction when the energy model is zero emissions to the atmosphere [1]. Therefore, it is necessary to continue processing different highly complex crude oils for years, generating high volumes of complex wastewater that must be treated before discharge [2]. The presence of certain organic and inorganic compounds derived from the production processes can cause inhibition or even toxic effects on conventional biological treatment of centralised refinery wastewater treatment plants (RWWTPs) [3,4].Among the different residual aqueous streams produced in refineries, the spent caustic streams are considered a hazardous waste due to their complex features [5]. Spent caustic solutions are derived from multiple sources due to the scrubbing of cracked gas in ethylene crackers or Merox units processing of liquefied petroleum gas, gasoline, kerosene or natural gas [6]. Spent caustic streams are classified into three types depending on their composition and the process in which they are generated: sulfidic, cresylic, and naphthenic caustic streams. Usually, refineries do not separate each type of spent caustic, and they are mixed for a standard treatment rather than implementing specific solutions for each one [7].The spent caustic streams contain a high COD, sulfides that produce very strong odors, aromatic compounds such as phenols, amines, calcium carbonate, sulphates and a very high alkaline pH, which makes their handling and treatment extremely difficult due to corrosion and precipitation problems in industrial facilities [8–11]. The amount of sulphates in spent caustic wastewater is not a problem for aerobic biological treatment in RWWTPs [12]. However, in the case of sulfides, there is an internal limit established in some wastewater treatment plants (WWTPs) of 25 ppm to avoid corrosion of concrete and steel, safety issues for operators and odours, among other problems [12]. The presence of amines in spent caustic streams is also very important. One of the amines most widely used in crude oil refining processes is methyldiethanolamine (MDEA) [13,14]. MDEA is a tertiary alkanolamine used as absorbent in natural gas sweetening process for removal of hydrogen sulfide (H2S) and carbon dioxide (CO2) [14]. Although MDEA is more selective towards H2S, this amine is also used currently in CO2 capture processes. This leads to an increase in the arrival of MDEA to WWTPs of the industries in which it is used [15,16]. Regarding toxicity, contradictory reports can be found in the literature. Even though MDEA does not seem to be toxic to aerobic treatment systems [17], certain problems have been identified due to its inhibitory effect on biological processes such as nitrification [18].Different technologies have been studied for the treatment of spent caustic wastewaters, such as thermally activated persulphate [19], biological treatment based on biomass adaptation to simulated amine concentration [17], ozonation with microelectrolysis [20] and cyclic thermal oxidation processes [21]. In these systems, it must be noted that the highest concentration of MDEA studied was 3,760 ppm from a simulated wastewater working at an acidic pH between 1 and 5 [20]. This concentration is considered in the range of accidentally maximum values found in the inlet streams of biological treatments in RWWTPs.Wet air oxidation (WAO) has proven to be an effective process for the treatment of wastewater with high organic matter content, allowing the total or partial degradation of compounds that are toxic or refractory to biological processes [22–25]. The elimination of organic compounds identified in the wastewater can be practically total, with COD reductions of more than 70 % in most cases. The removal of MDEA in a real wastewater with higher concentration of the contaminant (in the range of g/L) has been studied in two previous works by a WAO process [26] and a catalytic process (known as catalytic wet air oxidation, CWAO) using a commercial activated carbon as catalyst [27]. In the case of CWAO, most works have been performed using synthetic wastewaters with phenols and derivatives as dominant model pollutants [28]. The operation conditions, type of catalyst and removal efficiencies of all these studies are summarized in Table 1_SM. The catalysts used in this process are normally classified as: noble metal catalysts, non-noble metal catalysts and metal-free carbon materials [29]. Synthesised metal-free carbon materials can promote catalytic wet air oxidation processes for wastewater treatment as an alternative to noble metals and rare earth oxide catalysts. The absence of metals in the catalyst based on carbonaceous materials avoids possible leaching and subsequent treatment needs compared to processes using metal-containing catalysts [28,30].Petroleum coke (petcoke) is a black-colored solid composed primarily of carbon, generated as a product of the coking process oil refineries or in other heavy hydrocarbon cracking processes at high temperatures and rotational speeds [31]. It contains limited amounts of sulfur, metals, and non-volatile inorganic compounds. The most extensive use of petcoke is as a source of energy or carbon in different industrial applications [32]. Due to the majority use of coke as fuel in industries traditionally considered as a source of pollutant gas emissions, global demand of petcoke is expected to decline and, therefore, finding novel applications is key [33]. To address this danger, potential applications are being studied in which this co-product can be used due to its special characteristics. In this study, the preparation of activated carbon materials in catalytic wet air oxidation (CWAO) for the treatment of wastewater of the own refinery fits into the circularity guidelines that must govern technological processes in the near future. The use of materials prepared from the own refinery petcoke as possible catalysts for the treatment of wastewater has not been already reported despite the large number of studies described in literature (Table 1_SM).In this work, it is proposed a different approach for the onsite treatment of the spent caustic streams containing MDEA in high concentration coming from the natural gas sweetening process, in which accidentally discharges of MDEA could reach concentrations up to 2.6 g/L. Currently, this highly MDEA-containing stream is mixed with wastewater streams coming from other operation units, diluting the pollutant concentrations upstream of RWWTP. However, mixing is often not sufficient to reduce the concentration to values that can be assumed by a conventional biological process in RWWTPs. The aim of the study is to increase the biodegradability of this wastewater using WAO and CWAO processes. The onsite treatment of highly MDEA-containing wastewater streams rather than at the end-of-pipe after dilution would be beneficial from the point of view of reducing the capital expenditures, as lower wastewater flows would require smaller equipment sizes. Additionally, the reuse of a low value-added material such as refinery petcoke as a catalyst in the CWAO process, which is likely to become a waste product owing to increased legislation on emissions, could reduce the operational costs due to the feasibility of using milder operating conditions of pressure and temperature in the oxidation process.Linde Gas España, S.A.U supplied air pressurised bottles for WAO and CWAO experiments. Potassium hydroxide and hydrochloric acid (37 % v/v) used for the petcoke activation, were purchased from Labkem and Sigma Aldrich, respectively.The spent caustic wastewater used in this study comes from an amine unit located in a petroleum refinery in Spain. Samples were collected and immediately stored at 4 °C to avoid variations in composition. The so-called fuel grade green petcoke comes from a coker unit of the same petroleum refinery. The petcoke sample was stored in a closed container before use.WAO and CWAO experiments were performed in a 500 mL capacity T316 stainless steel autoclave reactor, resistant to high pressures and temperatures: model 4575A manufactured by the Parr Instrument Company, USA. The reactor was equipped with an electrically heated jacket, a turbine stirrer, and a variable speed magnetic drive. The temperature using a thermocouple immersed in the liquid phase and the stirring rate were controlled using a Parr 4842 controller and the pressure by the gas inlet and a gas release valve located on the top of the reaction vessel. The liquid samples were taken through a dip tube immersed in the reaction mixture. A schematic diagram of the experimental setup is shown in Fig. 1 .Typically, 250 mL of the spent caustic refinery wastewater is placed in the reactor. The pH of the refinery wastewater (9.6 ± 0.1) was not modified and adjusted prior or during the treatment. Initially, nitrogen gas flow was passed through the head space of reaction vessel to ensure inert conditions, and continuous stirring was fixed at 400 rpm. Then, the air was supplied up to 15 bar to maintain the wastewater in a liquid phase. Finally, the reactor was heated to the operating temperature and then the air pressure was increased to the selected value. The temperature and air pressure of WAO and CWAO experiments varied between 150 to 250 °C and 10 to 90 bar, respectively [5]. The same procedure is used for CWAO runs but adding 1 g/L of a carbon-based catalyst prepared from petroleum coke in the initial loading of the wastewater into the reaction vessel. The reproducibility and the accuracy of the performed WAO and CWAO runs were evaluated periodically between tests. Methyldiethanolamine (MDEA), Total Organic Carbon (TOC), Chemical Oxygen Demand (COD), sulfides (S2−) and pH were periodically monitored for samples withdrawn along the reaction time for 60 min. Prior to analyses, the samples were filtered through a 0.7 μm glass fiber filter.TOC (Total Organic Carbon) was determined in a combustion/non-dispersive infrared gas analyser model TOC-V Shimadzu. The pH was monitored using a GLP-22 digital pH meter (HACH LANGE SPAIN, S.L.U). Chemical Oxygen Demand (COD), Total Solids (TS), and Volatile Total Solids (VTS) were measured following APHA-AWWA Standard Methods 5220.D, 2540.B and 2540.E respectively. Total Kjeldahl Nitrogen (TKN) was measured using a Vapodest 450 (Gerhardt, Analytical Systems) for the digestion of the samples, following APHA-AWWA Standard Method 4500-Norg C. Sulfides (S2−), nitrates (NO3 −), and ammonium (NH4 +) concentration were determined using a Smartchem 140 (AMS Alliance), following APHA-AWWA Standard Methods [34]. Methyldiethanolamine (MDEA) concentration was monitored by gas chromatography (GC) in a Varian 450-GC equipped with a column HP-PONA (High-resolution Performance column, 50 m × 0.20 mm, 0.50 μm, for the detection of Paraffins, Olefins, Naphthenes and Aromatics) and a Flame Ionization Detector (FID). The injection and detector temperatures were set at 300 and 250 °C, respectively. The oven temperature was maintained at 90 °C for 7 min, raised to 250 °C at a 50 °C/min rate, and finally held for 5 min. The degradation by-products from MDEA oxidation were analysed by direct aqueous-injection gas-chromatography coupled to a mass detector (320 GC–MS) using a Bruker column Stalbiwax-MS (30 m × 0.25 mm, 0.25 μm). This specific column for aqueous samples was initially maintained at 50 °C for 3 min, then heated to 180 °C at 12 °C/min, and finally maintained for 5 min at 250 °C (7 °C/min). The injector was held at 320 °C, and He (1 mL/min) was used as the carrier gas. Biodegradability tests were performed to evaluate the effect of refinery wastewater on acclimated and controlled biomass cultures according to the literature experimental procedure included in the supplementary information. For this purpose, measurements of oxygen consumption (OC) and oxygen uptake rates (OUR) in different pulses of treated wastewater and sodium acetate were made after months of acclimatisation to a sodium acetate-rich feed used as a readily biodegradable substrate of the biomass taken from a local wastewater treatment plant [35]. Table 1 shows the physicochemical characterisation of the spent caustic wastewater. The concentration of MDEA was about 2.5 g/L. The TOC and COD of the corresponding MDEA concentration are almost the 85 % of the total TOC and COD of the wastewater. In the case of nitrogen MDEA represents approximately 97 % of total in the water, mostly in the form of organic nitrogen (MDEA provides more than 98 % of the organic nitrogen). Thus, MDEA is the most abundant compound, with the presence of sulfides (750 ppm) also detected in significant concentrations. The high MDEA concentration is due to an unusual and extreme situation of accidental discharge from ethylene crackers or Merox units to the centralized refinery's WWTP. Additionally, the wastewater showed a low concentration of metals, a very alkaline pH due to the amine content and 36 g/L of TSS. These characteristics allow classify this process stream as a hazardous waste and make it difficult to handle and treat [8,11,36–39].The fuel-grade green petcoke (PC) collected from a coker unit had the typical characteristics (included in the supplementary information) of a refinery coke [32], with a low specific surface area (ca. 10 m2/g), a CHNS distribution of 81.7 %, 3.7 %, 1.4 %, and 5.3 %, respectively, and heavy metals content of approximately 1,700 ppm of vanadium and 400 ppm of nickel (Table 2_SM). Thermogravimetric analyses evidenced, a weight loss lower than 14 % at 1000 °C in an inert nitrogen atmosphere and almost negligible up to 500 °C under air atmosphere (Fig. 1_SM). Table 2 displays physicochemical characterization of activated carbon materials prepared using different KOH:Petcoke ratios [30]. The chemical activation produced an increase in the specific surface area of the petcoke, due to an increase in the porosity of the material [40]. The increase of KOH:Petcoke ratio from 2 to 4 enhanced the BET specific surface area from 1,043 to 3,459 m2/g. These areas are similar or even higher than those reported for other petroleum cokes chemically activated under different conditions [30,41,42]. Nitrogen adsorption–desorption isotherms (Fig. 2_SM) depict typical profiles of microporous materials according to the IUPAC. According to the adsorption isotherms, these materials fill the micropores in a continuous process at low relative pressures (P/P0 < 0.015) [43]. According to the Howarth-Kawzoe model, the pore size distribution was centred in the microporous range with values of less than 2 nm (Fig. 3_SM). The total pore volume of the activated petcoke materials goes from 0.48 cm3/g to 1.63 cm3/g, pore volumes that are up to twice as large compared to other carbonaceous materials obtained by petroleum coke activation [30]. The CHNS elemental analysis showed a significant reduction of N and S contents after chemical activation. Likewise, the content of V and Ni also decreased due to acid washing (HCl) for the removal of KOH. The increase of KOH:PC ratio, calcination temperature, heating ramp and nitrogen flow rate showed a positive effect on the development of more microporous materials with higher total pore volume. A KOH:PC ratio of 4, using a calcination temperature of 790 °C (heating temperature of 20 °C/min and a nitrogen flow rate of 100 mL/min) provide the activated carbon material with maximum specific surface area and total pore volume (3,459 m2/g and 1.63 cm3/g, respectively) and pore size distribution centred at 1.8 nm. X-ray diffraction (Fig. 4_SM ) shows the difference between the structures of the initial petcoke and synthesised carbonaceous materials consisting of graphite microcrystallites characteristic of activated carbon-type materials with a high specific surface area [42]. SEM-EDX analyses of the raw and activated petcoke materials displayed particles of a non-homogeneous morphology and size with a much more porous surface and higher O/C ratios as the KOH:PC increased by the effect of the activation agent (Fig. 5_SM).Wet air oxidation (WAO) has proven to be an effective process for the treatment of wastewater with high organic matter content, allowing the total or partial degradation of compounds that are toxic or refractory to biological processes [44]. The WAO process at different reaction temperatures and air pressure was studied for the concentrated MDEA wastewater stream. Additionally, solid catalysts synthesized from green fuel-grade petcoke in the form of microporous activated carbonaceous materials were also tested. Figs. 2 and 3 show the removal of MDEA, COD, TOC, and sulfides of highly MDEA concentrated spent caustic wastewater for WAO and CWAO experiments, respectively. Preliminary blank experiment at 250 °C and 90 bar under inert nitrogen atmosphere showed no modification of the initial characteristics of the wastewater. This indicates a negligible thermal degradation and the need of dissolved oxygen in the wastewater for the oxidation of pollutants.Initially, the highly concentrated MDEA stream was treated at 150 °C under different air pressures ranging from 10 to 90 bar, which can dissolve oxygen amounts from 0.05 to 0.4 mL O2 per gram of water [45,46]. As it can be seen in Fig. 2, removal of sulfide reached 90 % in all WAO experiments. This fact is attributed to the high reactivity of sulfides as compared to the organic matter contained in the water [8]. This is especially noteworthy for the WAO experiments at the lowest temperature (150 °C), where sulfide removal was above 90 % even with low air operation pressure (10 bar). The increase of air pressure from 10 to 90 bar increased the MDEA elimination but had a slight effect on TOC and COD removals.Sulfides are dissolved in the wastewater due to the basicity of the waste stream (pH = 9.6). If the pH approaches neutrality (pH = 7), the sulfides can abandon solution as acid gas, which must be treated before emission into the atmosphere [8]. Sulfides may exist in three different forms, H2S, HS− and S2− depending on the pH of the medium. At a pH of 7, sulfides are present in the form of H2S gas and HS−. If the pH decreases further, most of the sulfides would be in the form of H2S gas. On the contrary, when the pH increases the majority of the sulfides are in the form of dissolved hydrosulfide (HS−) and sulfide (S2−) [47]. The treatment of these pollutants has been studied by additions of iron salts for their precipitation [48], biological oxidation [49], neutralization combined with conventional or advanced oxidations such as the Fenton process [47], or by physicochemical separation systems such as electro-coagulation [50]. These processes generate secondary sludge effluents that require a further waste management, increasing treatment costs, consumption of chemicals, etc. Besides, in some cases, the efficiency of sulfide removal is not high enough to make the water amenable for conventional biological treatments. The WAO process overcomes these disadvantages, enabling sulfide removal under mild operating conditions without generating secondary waste effluents [51]. It must be pointed out that for WAO experiments at 150 °C and different air pressures, the pH did not decrease from the initial value of 9.6 to<8.5, a value at which most of the sulfides are dissolved in water in the form of S2− [47]. Thus, the elimination of these sulphur compounds by stripping in the form of acid gas is discarded. The elimination is through the oxidation to sulphates (SO4 2−) by the oxygen dissolved in the reaction medium with a stoichiometric consumption of 2 g O2/g S2− [50].Likewise, at 150 °C, the organic matter removal in terms of the total organic carbon (TOC) was 6 %, 9 % and 14 % for operating air pressures of 10, 50 and 90 bar, respectively. The reduction in terms of COD was significantly higher (18 %, 27 % and 28 %) due to the simultaneous removal of sulfides previously mentioned, wich have a contribution to COD. Despite the moderate removal of TOC and COD, the most abundant pollutant identified in the water, MDEA, was eliminated by 10 %, 35 % and 62 %. Therefore, at the temperature of 150 °C MDEA cannot be removed completely resulting in concentrations in the treated effluent that exceed 1,000 ppm even for 90 bar of air operation pressure, which is still too high for the downstream biological treatment, despite the dilution of this effluent by mixing with the rest of the wastewater streams coming from different refinery processes.Therefore, in order to enhance the performance and taking into account the effect of the increase of pressure and temperature on the overall economy of the WAO process due to the rise in energy consumption, the reaction temperature was raised to 200 °C. The increase of 50 °C produced an enhanced in the amount of oxygen dissolved in the water (ca. 1.5 times for the same working air pressures [45,46]). The increase of temperature hardly enhanced the TOC removal, maintaining similar efficiencies to those obtained at 150 °C, whereas the COD removal increased, with reductions of 24 %, 32 % and 38 % for the air pressures of 10, 50 and 90 bar, respectively. In the case of MDEA removal, more than 80 % and 90 % were achieved at air pressures of 50 and 90 bar, leading to final concentrations of 497 and 177 ppm, respectively. In these cases, the dilution of this stream with the rest of the wastewater streams of the refinery would allow reaching MDEA concentration at which very limited effects on the biological process of the wastewater treatment plant are expected. The WAO process is therefore effective for the treatment of wastewater with high MDEA concentration at typical operation conditions of temperature (200 °C) and air operation pressure (up to 90 bar). The main pollutants of the wastewater, MDEA and sulfides, are removed without the need to achieve a high removal of organic matter.At this point, the increase of temperature to 250 °C was also assessed for further reduction of organic matter. The effect of temperature on the amount of dissolved oxygen in the reaction medium is increased by 1.7 times compared to the same working pressures and 200 °C [45,46]. At 250 °C and 50 or 90 bar air pressure, the elimination of MDEA was almost complete (99 %) and over 95 % for sulfides. In this case, the mineralization of organic carbon increases up to 40 %, with COD reductions of more than 65 %. A decrease in final pH of the wastewater occurs from the natural initial value of 9.6 to 4.8 because of the oxidation process. This acidic character can cause corrosion problems in the facilities needed for treatment [52] even having been established as a worldwide problem in sewers due to corrosion caused by H2S in wastewater [53]. This decrease in pH is associated to the organic matter oxidation and conversion of sulfides to sulphates and sulfuric acid by the high pressure WAO process [54]. This acid partially consumes the alkalinity of the spent caustic from the wastewater. In addition, the organic matter contained in the water is oxidized to low molecular weight carboxylic acids that contribute to the acidity of the water. Among the distinct low molecular weight carboxylic acids, WAO process is characterized by promoting the formation of acetic acid as main oxidation by-product [44]. Acetic acid is quite difficult to further oxidize to carbon dioxide and water by WAO process, being necessary to use very extreme operating conditions or to work with catalysts that facilitate the process [55]. In this work, acetic acid concentrations higher than 400 ppm were detected in the wastewater after WAO treatment. This formation of carboxylic acids together with the oxidation of sulfides to sulphates, results in the consumption of alkalinity and consequent decrease in the pH of the water.Initially, operation conditions of 250 °C and 50 bar of air pressure were used for testing the catalytic performance of activated carbonaceous materials. Air pressure at 50 bar was used as the increase of air pressure above this value hardly improved the COD reduction and TOC mineralization for WAO experiments. In these conditions, the catalytic activity of commercial activated carbon (CWAOAC) with a specific surface area of 1,288 m2/g and activated carbon materials from petcoke (CWAOPC) with different specific surface areas (1,013 m2/g, 2,196 m2/g and 3,221 m2/g) was evaluated to compare its effect on oxidation yields (Fig. 3a and b). In general, non-significant differences were observed for the four carbonaceous materials and the non-catalytic WAO process at these operating conditions and 60 min of reaction for all the experiments. The commercial activated carbon achieves near-complete removal of MDEA and sulfides. A slight increase in the mineralization degree of the organic matter and reduction of COD was observed for the CWAOAC compared to the WAO. The carbonaceous material synthesized from petcoke (CWAOPC_2,196 m2/g) shows similar results to the commercial activated carbon, with amine and sulfide degradations of 99 % and 95 %, and TOC and COD reductions of 56 % and 76 %, respectively. These performances are maintained for the carbonaceous material with a lower specific surface area (CWAOPC_1,013 m2/g) and the TOC and COD removals were slightly improved using the carbonaceous material of the highest specific surface area (CWAOPC_3,221 m2/g) and higher O/C ratio according to the SEM-EDX analyses (Fig. 5_SM).In order to determine a higher influence of the catalyst compared to the non-catalytic WAO process, the air pressure and temperature were decreased to milder operating conditions. This study was carried out with the CWAOPC_2,196 m2/g carbonaceous material prepared from petcoke as the most available catalyst of the three CWAOPC materials and similar catalytic performance to commercial activated carbon (CWAOAC).The influence of temperature at constant air pressure of 50 bar and air pressure at constant temperature of 150 °C for the catalyst CWAOPC of 2,196 m2/g is shown in Fig. 3c and 3d, respectively. The performance of CWAO at 50 bar when the temperature decreased from 250 °C to 150 °C evidenced a more remarkable difference between the results of CWAO and WAO experiments. The elimination of MDEA exceeds 90 % for the two upper temperatures of 200 °C and 250 °C, reaching a MDEA removal of 77 % at 150 °C (only 35 % for the WAO treatment). At lowest temperature, the catalyst proves a significant effect, increasing the removal of MDEA, the main pollutant in the actual refinery wastewater stream, by more than twofold. Concerning the removal of sulfides and the reduction in TOC and COD, a slight improvement was also observed in comparison to the results of the WAO process.The effect of the air pressure for the CWAO in the range of 10 and 90 bar was also seen at the lowest temperature of 150 °C. At 10 bar, 61 % of the initial high concentration of MDEA in the wastewater (2,521 ppm) is removed, with the total oxidation of sulfides to sulphates. This is a 50 % improvement in amine degradation referred to the oxidation without catalyst. Increasing the pressure to the upper studied limit of 90 bar yields an MDEA removal of more than 80 % compared to 62 % obtained with the WAO process. Thus, the carbonaceous catalyst CWAOPC_2,196 m2/g improves the performance of the oxidation process. The functional groups of the catalyst promote the reduction of the oxygen in the medium, generating radicals of greater oxidizing power that can oxidize the compounds present in the wastewater [28]. The oxidation under these mild operation conditions with the catalyst results in a higher oxidation of the MDEA and organic compounds with a slight enhancement of COD and TOC removals.Therefore, the catalytic material with high porosity and specific surface area prepared from petcoke proves to be an effective catalyst for the WAO process, improving the yields achieved under mild operating conditions. The CWAO process, which achieves high removals of MDEA and sulfides from the water under mild operating conditions, will probably improve the biodegradability of the wastewater. This is also an important factor in terms of the performance of the centralized biological treatment system of the refinery's wastewater treatment plant. Despite this, the actual biodegradability of the oxidation products generated in the WAO and CWAO processes needs to be studied. This would allow establishing the real effect of the effluent generated in the process on the biological treatment system, as well as the optimal operating conditions at which the pretreatment of the stream with high MDEA should be carried out.Most of the studies of WAO for wastewater treatment are mainly focused on quantifying the degradation of the organic matter in terms of COD and TOC reductions. However, it is essential to identify oxidation by-products or intermediates from nitrogen-containing organic compounds to evaluate their potential toxicity and/or refractory behavior for subsequent biological treatment. The WAO of nitrogen-containing compounds can produce different products including ammonium, nitrate, nitrite, nitrous oxide and nitrogen gas depending on the pollutant and reaction conditions [21–23]. When the main nitrogen input is an amine-containing compound, ammonium is mainly produced as stable end-product at harsh oxidation conditions [51]. Fig. 4 shows the concentration of nitrogen-containing compounds in the wastewater such as the MDEA itself, other nitrogen organic products and inorganic nitrogen as ammonium and nitrates/nitrites, for the WAO and CWAO treated waters after 60 min under different operation conditions. The initial MDEA concentration (2,521 ppm) is about 300 ppm in terms of nitrogen content and contributes to ca. 95 % of the total nitrogen of the wastewater. WAO experiments at 150 °C evidenced a low formation of inorganic nitrogen compounds. MDEA and nitrogen-containing organic by-products were the most abundant compounds. The contribution of MDEA was up to ca. 50 % for the highest air pressure. The increase of temperature at 200 °C resulted in the elimination of MDEA of 27 %, 83 % and 95 % for the 10, 50 and 90 bar, respectively. The increase of air pressure led to 10 % of inorganic nitrogen such as NO3 −/NO2 −. At the highest temperature (250 °C), the increase of air pressure enhanced the inorganic nitrogen products up to 40 %, out of which 30 % corresponding to ammonium.The use of the carbonaceous material synthesized from petcoke as a catalyst (CWAOPC_2,196 m2/g) shows a higher contribution of non-identified organic nitrogen compounds at low temperature (150 °C) and different air pressures in comparison to the analogous WAO experiments, and the oxidation to NO3 −/NO2 − and ammonium was hardly detected. As the temperature was increased up to 250 °C at 50 bar, the oxidation of the nitrogen organic compounds led to ammonium contents higher than those observed in WAO experiments at the analogous operation conditions, and low presence of NO3 −/NO2 −. Thus, the increase in temperature and air pressure using the carbonaceous activated carbon as catalyst for CWAO, decreased the MDEA contribution to the total nitrogen content promoting the generation of ammonium as main inorganic nitrogen by-product. Ammonium obtained as a product of MDEA oxidation has a certain refractory character to the WAO and CWAO processes, resulting in low elimination in the form of nitrogen gas [23] as also attested from the negligible decrease of total nitrogen. Nevertheless, ammonium can be consumed as a nutrient by aerobic biological treatment systems [56].As shown in Fig. 4 , the contribution of non-identified organic nitrogen compounds stemming from the partial oxidation of MDEA (termed as “other organic” in the graph) to total nitrogen varies depending on the operation conditions of WAO and CWAO. The oxidation of MDEA follows a very complex mechanism with a large number of by-products whose formation has not been explained in many cases [13,20,22,23,57–59]. As other amines, MDEA is oxidized to organic acids and glycine as main by-products, but unspecified formyl-amides have also been reported as secondary by-products [57]. Formation of diethylamine (DEA), N-methylamine (MMA), or ethanolamine (MEA) may occur concurrently with a methyl group transfer from MDEA or by direct oxidation. These compounds may react with other compounds formed in the oxidation process of MDEA, leading to a chain of reactions that are difficult to define [57]. In the CWAO, the degradation of MDEA using metal-free carbon catalysts has been demonstrated to be strongly dependent on the chemical surface groups of the materials, which play a key role in the production of active oxidizing species. Thus, it is well recognized that the adsorption of O2 over the carboxyl groups of activated carbon surface produces its dissociation to form O2 − species [60]. Then, hydroxyl radicals (OH) are generated by electron transfer of O2 −or attractingH+ of the carboxyl groups. In addition, the basic groups of the activated carbon surface have also an important role attracting small molecules of carboxylic acids, which can react with OH to generate CO2 and H2O. In the case of MDEA, the hydroxyl radicals can also attack the CN bond of the MDEA to generate other intermediate products which will be oxidized to carboxylic acids, ammonium, CO2 and H2O [27]. In this study, the analysis of the oxidation products of the treated wastewater under the different operating conditions has resulted in the detection of acetic acid, ammonium, and other by-products of the oxidation of MDEA such as dimethylamine, tetrazole-1,5-diamine, 2-amino-1-propanol, and alanine among others.Acetic acid appeared in all the oxidation reactions (WAO and CWAO), being its contribution over the total organic carbon more important as temperature and air pressure is increased or when catalyst was used (Fig. 6_SM of SI). The rest of the identified compounds varied without a clear trend. Fig. 5 shows the potential oxidation reactions of MDEA based on the products identified for the different operating conditions of WAO and CWAO. In these reactions, water and carbon dioxide are also produced because of the mineralization of organic matter as deduced from the reduction of the TOC and COD of the wastewater. (1) 2C5H13NO2 + 8O2 → 5CO2 + 5H2O + 2NH3 + 2,5C2H4O2 (2) C5H13NO2 + 1,5O2 → CO2 + H2O + C2H4O2 + C2H7N (3) 2C5H13NO2 + 7O2 → 5CO2 + 5H2O + NH3 + C2H4O2 + C3H9NO (4) 7C5H13NO2 + 46O2 → 32CO2 + 40H2O + NH3 + C2H4O2 + CH4N6 (5) 2C5H13NO2 + 8O2 → 5CO2 + 6H2O + NH3 + C2H4O2 + C3H7NO2 The complex nature of the petrochemical wastewater and the variety of generated by-products makes necessary to analyze the actual biodegradability of the effluents after the WAO or CWAO treatment. The generation of acetic acid and ammonium as main products of the oxidation process suggests a potentially increased biodegradability of the effluent, which should be easily treated in the conventional biological treatment system of the refinery's water treatment plant [3,56].To evaluate the rapid biodegradability of the treated effluents obtained in different oxidation conditions of WAO and CWAO runs, respirometric tests were performed with an activated sludge culture acclimatised for months to sodium acetate as biodegradable substrate. The toxicity and inhibition effects of the treated effluents on the readily biodegradable substrate were also assessed using the classical respirometric bioassays [35], but no conclusive results could be obtained. The spent caustic wastewater, containing 2,521 ppm of MDEA, high COD, TOC and sulfides, evidenced a low biodegradability of ca. 4 % compared to the sodium acetate solution used as a readily biodegradable substrate. Fig. 6 a depicts the compositional percentage of MDEA and acetic acid (both relative to TOC), and ammonium relative to Total Nitrogen (TN) in relation to the biodegradability of the treated effluents. Fig. 6b shows the results of biodegradability of treated wastewaters at 150, 200 and 250 °C using 50 and 90 bar air pressure for WAO and the same temperatures using 50 bar for CWAO (activated carbon material prepared from petcoke with specific surface area of 2,196 m2/g).The samples of WAO treatment showed a remarkable increase of biodegradability at 250 °C (ca. 50 %) as compared to the results at 200 °C (ca. 20 %), regardless the applied air pressure (50 or 90 bar). This fact is attributed to the absence of MDEA and higher contribution of acetic acid in the remaining TOC of the sample for 250 °C. Moreover, the percentage of ammonium respect to the TN was also much higher at 250 °C, which is decreasing the amount of other nitrogenated organic by-products, presumably with lower biodegradability. At 150 °C, the presence of MDEA in significant amounts and presence of less biodegradable nitrogenated organic compounds, leads to a dramatical decrease of biodegradability (<5 %). In contrast, the CWAO significantly increases the biodegradability at the three studied temperatures using 50 bar of air pressure. At 250 °C, the high content of acetic acid and ammonium are probably boosting the increase of biodegradability up to almost 70 %. At lower temperatures, 200 and 150 °C, the biodegradability decreased up to ca. 30 % and 25 %, respectively, due to remaining amounts of MDEA and lower contents of ammonium, but these values are much higher than those obtained in WAO at the same operation conditions.The fact that the biodegradability cannot reach values close to 100 % is attributed to the presence of nitrogenated organic compounds produced by the oxidation of MDEA, which may require longer degradation times than those established in the respirometric tests. It should be noted that these tests show the biodegradability of the effluents generated in short periods of time, for an activated sludge acclimatized to a model substrate [35]. The performance of an activated sludge process in refinery's wastewater treatment plant should be better, leading to a higher biodegradability of the effluents of the WAO and CWAO processes. These treatment plants have activated sludge systems acclimatized to less biodegradable compounds, with a higher concentration of active biomass than that used in these respirometric tests and operating at higher hydraulic residence times [3,4]. In addition to the improvement expected from operation at the refinery's wastewater treatment plant, biodegradability achieved by WAO and CWAO treatment of the spent caustic stream at the outlet of the unit, where the pollutant load is the highest, makes it viable for the effluent to reach the biological treatment system after dilution with other wastewater streams generated in the refinery. This would further reduce the pollutant load reaching the treatment system making treatment even easier.The WAO process is an effective treatment of the refinery spent caustic stream with high MDEA contamination from amine absorption units in the purification of refinery gas streams. A deep study of the operating conditions has been performed as MDEA is a tertiary amine that can cause significant problems in the centralized biological system of the refinery's wastewater treatment plant. The experiments were performed at the original pH of the wastewater despite its high alkalinity, which assure that sulfides are removed by oxidation to sulphates and using air instead of pure oxygen as source of oxidising agent. The increase of temperature has a more important effect than the air pressure on the performance of the treatment in terms of the reduction of MDEA, sulfides, COD and TOC. The WAO at 250 °C or even 200 °C at 50 bar of air pressure can be considered an effective technology for the on-site treatment of highly MDEA concentrated wastewater. Under these conditions, more biodegradable streams enriched in acetic acid and ammonium compounds were achieved, although their biodegradability could not overpass ca. the 50 %. On the other hand, microporous carbonaceous materials prepared from a refinery petcoke by chemical activation under different conditions were tested as catalysts in CWAO. These materials showed high specific surfaces areas ranging between 1,013 and 3,459 m2/g. A carbonaceous material prepared in this work with a specific surface area of 2,196 m2/g proved a better performance at milder operation conditions than WAO experiments using the same operation conditions. The MDEA removal was improved by more than twofold at 150 °C and 50 bar of air pressure. At 250 °C, the CWAO was able to achieve an increase of biodegradability up to 70 %. The biodegradability decreased up to ca. 30 and 25 % at 200 °C and 150 °C, respectively. But these values were still significantly higher than those obtained for WAO experiments in analogous operation conditions (20 % and < 5 %). It must be noted that these results of biodegradability were estimated according to respirometric tests, whereas biological treatment of the refinery's wastewater treatment plant would operate with a more acclimatized activated sludge and long hydraulic residence times, which can overcome potential problems caused by the presence of remaining MDEA and nitrogenated by-products. In this sense, a compromise between the operating conditions and the resultant biodegradability of the effluent for subsequent mixture with other refinery wastewaters is needed to reduce the operational expenditure of WAO and/or CWAO and determine the techno-economic feasibility of the industrial implementation of this technology as a pre-treatment step for the highly MDEA concentrated wastewater streams.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors thank the financial support of the Community of Madrid through the projects IND2018/AMB-9611 and S2018/EMT-4341 REMTAVARES-CM. Moreover, the authors are grateful to Repsol for providing wastewater samples.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2023.141692.The following are the Supplementary data to this article: Supplementary Data 1

Different operating conditions of wet air oxidation and catalytic wet air oxidation have been studied for the treatment of highly concentrated methyldiethanolamine wastewater streams from amine units of acid gas recovery in petrol refineries. These units occasionally generate streams of high methyldiethanolamine content that require special actions to avoid undesirable impacts on the downstream biological process of the petrochemical wastewater treatment plant due to its inhibition effect. The wet air oxidation treatment achieved remarkable removals of methyldiethanolamine, sulfides, chemical oxygen demand and total organic carbon (99%, 95%, 65% and 38%, respectively). Likewise, activated petroleum coke materials from the own refinery plant were tested as catalysts in the process. These materials were prepared under different conditions (chemical activating agent and thermal carbonization process). The catalytic wet air oxidation treatment using an activated petroleum coke was able to remove the methyldiethanolamine at milder operation conditions keeping a similar performance in terms of wastewater treatment removals as compared to the non-catalytic experiments. This technology significantly increased the biodegradability of the treated effluents ranging from 25 to 70 % due to the formation of more biodegradable substrates (acetic acid and ammonium) for further biological treatment.

Fossil crude is the major raw material used in the petrochemicals industry and the production of transportation fuels. However, the depletion of fossil reserves and the increase in the world's energy demand makes it difficult for the petrochemicals and transportation fuels industry to meet global demand. The more these crude reserves are depleted, the more exploration and refining quality become difficult, which leads to an increase in the price of transportation fuels and petrochemicals (Hosukoglu et al., 2012; Petrequin, 2012; Liu et al., 2011; Ong and Bhatia, 2010; Galvis and de Jong, 2013; Leckel, 2009; Larson et al., 2010). The Fischer-Tropsch (FT) process is a technology that can produce synthetic transportation fuels and chemicals from biomass, coal and natural gas-derived synthesis gas or syngas (H2 + CO). This technology converts the syngas into hydrocarbons and oxygenates free of sulphur (S) and nitrogen (N) over a wide range of iron (Fe)-, cobalt (Co)-, nickel (Ni)- and ruthenium (Ru)-based catalysts. It has been suggested that the following reactions occur during Fischer-Tropsch synthesis (FTS) (Schulz et al., 1994; Idem et al., 2000; Rodríguez Vallejo and de Klerk, 2013; Davis, 2001; van Steen and Schulz, 1999; Ponec, 1978): (1) Alkanes nCO + (2n+1)H2 → C n H2 n +2 + nH2O (2) Alkenes nCO + 2nH2 → C n H2 n  + nH2O (3) Water-gas shift (WGS) CO + H2O ⇄ CO2 + H2 Some other side reactions that occur during FTS are (Nijs and Jacobs, 1980; Lebouvier et al., 2013; Kummer and Emmett, 1953; Lee et al., 2014): (4) Alcohols nCO + 2nH2 → H(-CH2-) n OH + (n-1)H2O (5) Boudouard reaction 2CO → C + CO2 The use of Fe-based catalysts for FTS has been facilitated with chemical promoters such as the group IA elements and structural promoters such as ZnO, Al2O3, MgO and SiO2 (Jager and Espinoza, 1995; Raje et al., 1998; Bukur et al., 1990; Yang et al., 2005). This is due to their ability to undergo WGS reaction to make up the deficient H2 in syngas derived from coal. Fe-based catalysts have a low cost, flexible reaction conditions and high FTS activity, although somewhat less active than Co-based catalysts (Li et al., 2016; Lohitharn and Goodwin, 2008; Griboval-Constant et al., 2014).The promotion of Fe-based catalysts with group IA metals such as Na, K and Cs can lead to a shift in the product distribution and the production of higher molecular weight hydrocarbons in FTS (Uner, 1998; Miller and Moskovits, 1988; Xiong et al., 2015). A typical Fe-based catalyst with a group IA metal as a chemical promoter can influence the dispersion and reduction behaviour of Fe oxides (Xiong et al., 2015). This could also lead to an increase in the CO consumption and dissociation rates, the CO2 production rate and the olefin to paraffin (O/P) ratio, and a drop in the overall FT reaction rate. Na and K have been observed to decrease the CH4 selectivity and exhibit a much higher WGS reaction (An et al., 2007; Dry and Oosthuizen, 1968). Na and K promotional effects are related to the Fe metal local electron density modification and blockage of the catalytic active sites (An et al., 2007; Dry et al., 1969). These can be explained in terms of the Na and K electropositivity, which causes transfer of the charge to the Fe metal leading to a decrease in the adsorption of H2 (Li et al., 2016; Li et al., 2015). Li et al. (Li et al., 2016) reported the promotional effect of Na on Fe-based catalyst in FTS to restrain the reduction property of Fe oxides in the catalyst. And, the Na was also reported to facilitate carbonisation of the Fe-based catalyst, decrease the FTS activity, methane selectivity and paraffins, and increase the production of heavier hydrocarbons (C5+) and olefin. Xiong et al. (Xiong et al., 2015) also prepared a carbon nanotube (CNTs)-supported Fe catalyst with different IA group alkali promoters, and found that Na increased the crystallite size of Fe oxides, while the surface area of the Fe/CNT catalyst decreased in the presence of Na. In addition, the CO conversion and long chain hydrocarbons (C5+) were also found to increase, and the presence of Na in the Fe-based catalyst slightly inhibited the reducibility of Fe oxides.The dependency of Na promotion on hydrocarbon production and CO conversion for Fe-based catalysts in FTS is still not well established in principle. Further research is needed to understand Na effects in Fe-based catalysts. The present study was carried out to investigate the effects of Na in Fe-based catalysts at different reaction temperatures (250 – 310 °C) during FTS. For this reason, an Fe-based catalyst (Fe/Al2O3) and its Na-promoted form (FeNa/Al2O3) were prepared and characterized with various techniques to examine their FT activity at different reaction temperatures. The emphasis was on the Na effects in Fe-based catalysts in terms of the methane formation rate, conversions of H2 and CO, WGS reaction, and olefins and paraffins of the lower, middle and higher range hydrocarbons.The Fe/Al2O3 FTS catalyst used in this study was prepared using the solution of Fe(NO3)3·9H2O (≥ 98%, Sigma-Aldrich) as Fe precursor. This precursor was used for impregnation of the active Fe (15 wt.%) metal on 3 mm alumina (Al2O3) support from Sigma-Aldrich. The incipient-wetness impregnation (IWI) preparation procedure was used to prepare the Fe/Al2O3 catalyst sample. Briefly, 33.1 g of Fe(NO3)3·9H2O (≥ 98%, Sigma-Aldrich) was weighed into 50 mL deionised water (H2O), and stirred at 60 rpm and room temperature for 30 min to obtain the Fe precursor solution. The 3 mm alumina (Al2O3) support (30 g, used as received, without drying) from Sigma-Aldrich was then poured into the solution and continue stirring for 24 h (20–100 °C heating value) to obtain a homogeneity sample. After the impregnation procedure, the catalyst sample was dried at 120 °C for 6 h, calcined in air at 500 °C for 3 h and ground and sieved to 15–60 µm particle size, in order to obtain the Fe/Al2O3 FTS catalyst.Simultaneously, and using the solutions of Fe(NO3)3·9H2O (≥ 98%, Sigma-Aldrich) and Na2O (80%, Sigma-Aldrich) as Fe and Na precursors, respectively, the same IWI preparation technique described above was used to prepare the Na-promoted Fe/Al2O3 FTS catalyst. These precursors were used for impregnation of the active Fe (15 wt.%) and Na (2 wt.%) metals on 3 mm alumina (Al2O3) support from Sigma-Aldrich. Briefly, 22.1 g of Fe(NO3)3·9H2O (≥ 98%, Sigma-Aldrich) and 0.7 g of Na2O (80%, Sigma-Aldrich) were weighed into 50 mL deionised water (H2O), and stirred at 60 rpm and room temperature for 30 min to obtain the Fe and Na precursor solutions. The 3 mm alumina (Al2O3) support (20 g, used as received, without drying) from Sigma-Aldrich was then poured into the solution and continue stirring for 24 h (20–100 °C heating value) to obtain a homogeneity sample. However, the catalyst sample was dried at 120 °C for 12 h after the impregnation technique, calcined in air at 500 °C for 3 h and ground and sieved to 15–60 µm particle size, in order to obtain the FeNa/Al2O3 FTS catalyst.The FTS catalysts were characterized using different techniques. The physical adsorption technique of N2 physisorption was done for the catalysts Brunauer-Emmett-Teller (BET) surface area, pore volume and average pore size distribution using a Micromeritics ASAP 2460 instrument. Prior to this technique, each catalyst sample was degassed at a temperature of 120 °C for 3 h and the results of the analysis were summarized accordingly.The scanning electron microscopy (SEM) technique was performed on the FTS catalysts to determine their morphologies, using a Jeol JSM-7800F Field Emission Scanning Electron Microscope instrument with high resolution in the scanning electron mode. The accelerating voltage and working distance used during this technique were 5 – 10 kV and 15 mm, respectively. Elemental composition analysis of the catalysts was carried out during SEM analysis using the Energy-dispersive X-ray (EDX) spectroscopy technique. Prior to the SEM and EDX analyses, a small amount of each catalyst sample was dropped on the SEM grid and the sample was coated with a thin carbon layer for the catalyst topographic evaluation. The images of the catalyst samples were viewed using a digital charge coupled device (CCD) camera equipped with the SEM instrument.The image characteristics of the prepared FTS catalysts were investigated using a Jeol JEM-2100F Field Emission Transmission Electron Microscope. The transmission electron microscopy (TEM) technique was carried out at an accelerating voltage of 200 kV and the microscope was equipped with a LaB6 source and CCD camera for electron emission and imaging, respectively. Before the TEM technique, each sample of the catalyst was suspended in ethanol and the suspension was then dropped on a copper grid coated with a thin carbon film for the assessment of the catalyst image characteristics. The measurements were performed at a beam spot range of 50 – 100 nm in the transmission electron mode.The catalysts crystallinity was determined by the powder X-ray diffraction (XRD) technique using a PANayltical X'Pert Pro-powder diffractometer instrument. This instrument was fitted with an ID X'Celerator detector of PHD lower and upper level of 6.67 keV and 12.78 keV, respectively. The instrument was also fitted with a programmable divergence slit with a radiation length of 10 mm. The XRD technique was conducted using the 2θ range of 10 to 90° with Cu Kα radiation of λ = 0.15405 nm at 40 mA and 40 kV operation conditions. The diffractometer was configured with a PW3064 sample spinner of 1 second rotation time, and the scan step size and time were 0.0170° 2θ and 87 s, respectively.The hydrogen-temperature programmed reduction (H2-TPR) technique was conducted using a Micromeritics AutoChem II 2920 instrument, where each catalyst sample was put in a pre-heated tubular reactor (quartz) with a thermocouple for continuous measurement of the reduction temperature. Before the H2-TPR technique, each sample of the catalyst was degassed with a high purity argon (99.999%, Ar) at temperature of 150 °C for 1 hour, and after which, the reactor temperature was dropped down to 50 °C. The reduction was carried out with 10% H2/Ar gas mixture of a flow rate of 50 cm3(STP)/min and the reactor temperature was raised to 950 °C at a ramping rate of 10 °C/min against the thermal conductivity detector (TCD) signal.The prepared FTS catalysts were evaluated in a tubular fixed-bed reactor with an internal diameter (ID) of 10.2 mm and a tube length (TL) of 412.8 mm. Prior to the evaluation of the catalysts, 1.0 g of each prepared FTS catalyst was loaded into the isothermal area of the fixed-bed reactor. The upper and lower levels of the reactor were filled with glass beads, thereby placing the catalyst bed in the middle of the reactor. Each catalyst was reduced in-situ for 12 h using a syngas ratio of 0.9, at a temperature of 300 °C and pressure of 1 bar. The FT reaction was carried out using the same reduction syngas (H2/CO) ratio of 0.9, weight hourly space velocity (WHSV) of 3.7 SLph/gcat, four different reaction temperatures (T) of 250 °C, 270 °C, 290 °C and 310 °C, and a pressure (P) of 10 bar(g). The tail-gas was analysed using an INFICON Micro Gas Chromatography (GC) Fusion 4-Module System with different TCDs and columns. Module A, which contained TCD and a Rt-Molsieve 5A column was used to analyse H2, N2, CO and CH4. Module B, which contained TCD and a Rt-U-Bond column was used to analyse CO2, C2H4 and C2H6. Modules C and D contained Rt-Alumina and Rxi-1 ms columns, respectively, and their TCDs were used to analyse the other hydrocarbons in the tail-gas. The liquid, wax and H2O were collected at the cold and hot traps set in between the reactor and back pressure regulator for analysis using a flame ionization detector (FID), DB-5MS, ZB-1HT and packed columns, respectively. The catalyst activity was evaluated by CO conversion, H2 conversion, formation rate and hydrocarbon selectivity on carbon basis (CO2-free), as defined in the equations below: (6) CO conversion : X c o ( % ) = 100 x M C O , i n − M C O , o u t M C O , i n where MCO,in and MCO,out are the molar flowrate (mol/h) of CO at inlet and CO at outlet, respectively. (7) H 2 conversion : X H 2 ( % ) = 100 x M H 2 , i n − M H 2 , o u t M H 2 , i n M H 2 , i n and M H 2 , o u t are the molar flowrate (mol/h) of H2 at inlet and outlet, respectively. (8) Formation rate : r C n ( mol / g cat . h ) = M C n , o u t W c a t (9) CO 2 selectivity : S C O 2 ( % ) = 100 x M C O 2 , o u t M C O , i n − M C O , o u t M C O 2 , o u t is the molar flowrate (mol/h) of CO2 at outlet. (10) Hydrocarbon selectivity : S C n ( % ) = 100 x M C n , o u t x n M C O , i n − M C O , o u t − M C O 2 , o u t M C n , o u t is the molar flowrate (mol/h) of hydrocarbon with carbon number, n at outlet. Wcat (g) is the weight of each catalyst during the FT reaction.The selectivity to C13+ hydrocarbons (including oxygenates) is defined as: (11) S C 13 + ( % ) = 100 − ∑ n = 1 12 S C n where n = 1 to 12 The prepared surface area, pore volume and average pore size of the FTS catalysts (Fe/Al2O3 and FeNa/Al2O3) are summarized in Table 1 . The result of the physical adsorption technique of N2 physisorption show that the Na-promoted Fe/Al2O3 catalyst has a lower BET surface area compared to the unpromoted Fe/Al2O3 catalyst. Na promotion decreased the BET surface area and influenced the textural properties of the Fe/Al2O3 catalyst (Li et al., 2016; Xiong et al., 2015; X. An et al., 2007). The pore volume and the average pore size of the catalysts show no significant differences in the N2 physisorption results. This indicates that no blockage of the catalyst pores was experienced during the loading of the Fe and Na metals on the Al2O3 support.The SEM images showing the morphologies of the Fe/Al2O3 FTS catalyst are shown in Fig. 1 (a) and (c), while that of the Na-promoted FTS catalyst (FeNa/Al2O3) are shown in Fig. 1(b) and (d). The images show that both prepared catalysts formed sphere-like catalyst structures that can clog together to form bigger sphere-like catalyst particles. This is also clearly seen in the TEM images in Fig. 2 (a) and (c) for the Fe/Al2O3 catalyst sample and Fig. 2(b) and (d) for the FeNa/Al2O3 catalyst sample. These images show the characteristic features of the prepared catalyst samples. The particle size distribution ranging from 1 to 5 nm was observed with an evenly and unevenly distribution of the Fe and Na active metals in the TEM images. There was also a slight increase in the particle size when the Fe/Al2O3 catalyst was promoted with Na. This corresponds to what was observed in the N2 physisorption results, as the Na-promoted catalyst showed a smaller BET surface area than the unpromoted catalyst. The TEM images also show that Na promotion inhibited the crystallinity of the Fe/Al2O3 catalyst (Fig. 2(b)). This is also noticeable in the H2-TPR profile of the FeNa/Al2O3 catalyst sample, and it will be discussed later in this section. The particle size distribution can range from 30 to 120 nm when both the prepared catalysts clog together, as seen in the SEM and TEM images.The elemental analyses of EDX performed during the SEM techniques confirmed the presence of Fe and Na active metals in the prepared Fe/Al2O3 and FeNa/Al2O3 FTS catalysts, as summarized in Table 1. The nominal amount of Na prepared relative to the 15 wt% of Fe was 2 wt% in FeNa/Al2O3 FTS catalyst. This nominal amount of Na was used to have a clear indication of Na in the Na-promoted catalyst. There were 15.4 and 11.1 wt% Fe contents detected in the Fe/Al2O3 and FeNa/Al2O3 catalyst samples, respectively. Na was not detected in the Fe/Al2O3 catalyst sample, while 1.6 wt% Na was detected in the FeNa/Al2O3 catalyst sample. This indicates successful loading of both Fe and Na FTS active metals onto the Al2O3 support. Fig. 3 shows the powder XRD patterns of the prepared Fe/Al2O3 and FeNa/Al2O3 FTS catalysts. These patterns contain peaks of the crystallites present in the catalyst samples. It can be noted that the XRD patterns contain haematite (Fe2O3) crystalline peaks at 2θ values of 24.3, 33.3, 49.6 and 64.5° for both the Fe/Al2O3 and FeNa/Al2O3 catalyst samples. The magnetite (Fe3O4) peaks can be found at 2θ values of 35.8, 54.4 and 63.0°, and these are the characteristics of the iron oxides phases, including the haematite peaks. The Fe2O3 and Fe3O4 peak characteristics agree with their reduction steps, which will be discussed later in this section. The XRD patterns also contain alumina (Al2O3) peaks at 2θ values of 41.1, 58.0, 67.2, 72.5 and 75.8°. Na2O and Na2O2 crystalline peaks can be seen at 2θ values of 27.9, 37.4 and 44.6° for the FeNa/Al2O3 catalyst sample. Therefore, the presence of these crystalline peaks suggests the preparation and characteristics of the Fe/Al2O3 and FeNa/Al2O3 FTS catalysts.H2-TPR experiments were performed on the prepared Fe/Al2O3 and FeNa/Al2O3 FTS catalysts to investigate their reduction behaviour, as represented in Fig. 4 . The powder XRD patterns of the catalysts showed the characteristic crystalline peaks of Fe2O3 and Fe3O4. These Fe oxides are known to form active metallic Fe during appropriate reduction conditions. Fe2O3 proceeds to form Fe metal (Fe0) in two or three stages, and it could form iron oxide (FeO) for supported Fe based catalysts (Al-Dossary and Fierro, 2015; Jin and Datye, 2000; Qing et al., 2012). The H2-TPR profile of the Na-promoted catalyst (FeNa/Al2O3) shows that reduction occurred at a temperature of 295 °C (stage b). This could be attributed to the effects of Na in the catalyst, as the 295 °C temperature reduction step is not present in the H2-TPR profile of the Fe/Al2O3 catalyst. The Fe/Al2O3 and FeNa/Al2O3 FTS catalysts H2-TPR profile started at a reduction temperature of 215 and 230 °C (stage a), respectively. Stage c is a reduction step for FeNa/Al2O3 catalyst, at a temperature of 38 5 °C, which is higher than the reduction temperature of 360 °C for the Fe/Al2O3 catalyst at stage c. This indicates that Na promotion inhibited the reduction behaviour of the Fe/Al2O3 catalyst slightly, and the reduction step is thus assigned to reduction of Fe2O3 to Fe3O4 (Xiong et al., 2015; Qing et al., 2012; Li et al., 2013). Stage d occurs at a reduction temperature of 445 °C and is ascribed to reduction of Fe3O4 to FeO in the Fe/Al2O3 catalyst, due to strong interaction between the active Fe metal and Al2O3 support. This stage is not present in the H2-TPR profile of the FeNa/Al2O3 catalyst, as the Na helped to generate some amorphous phases, as seen in the TEM images. Therefore, Na reduced the interaction between the active Fe metal and the Al2O3 support and provided a better reduction step for FeO to Fe0, as seen at stage e (reduction temperature of 550 °C). Stage f (reduction temperature of 765 °C) represents the reduction step of Fe-aluminates present in the both catalyst systems.The catalytic performance of the prepared Fe/Al2O3 and FeNa/Al2O3 catalysts for FTS were evaluated in a tubular fixed-bed reactor. This was carried out at four different reaction temperatures – 250 °C, 270 °C, 290 °C and 310 °C to investigate how the Na-promoted Fe/Al2O3 catalyst behaves at different temperatures compared to the unpromoted Fe/Al2O3 catalyst. Although, some researchers have reported the effects of Na on Fe-based catalysts, limited studies are available on its behaviour at different temperatures (Xiong et al., 2015; Li et al., 2014; H.M.T. Galvis et al., 2013; H.M.T. Galvis et al., 2013). The present study was conducted for each catalyst under the following conditions: a total reaction time on stream (TOS) of circa 360 h; a pressure (P) of 10 bar(g); H2/CO of 0.9; a weight hourly space velocity (WHSV) of 3.7 SLph/gcat. A summary of the FTS experiment results is provided in Table 2 and the TOS performance profiles at reaction temperature of 310 °C are represented in Fig. 5 . Fig. 5 shows a rapid increase in the CO conversion, which stabilised within few hours of TOS. The CO conversion for Fe/Al2O3 catalyst reached a steady state after 20 h TOS, while the CO conversion for FeNa/Al2O3 catalyst reached a steady state after 80 h TOS. The later steady state behaviour of the FeNa/Al2O3 catalyst can be explained with respect to the Na inhibition of the reduction behaviour of Fe/Al2O3 catalyst as discussed earlier for the H2-TPR profiles.The CO and H2 conversions at the steady state were used to evaluate the FTS activity of the catalysts, as shown in Fig. 6 . The behaviour of the FeNa/Al2O3 catalyst did not show the same trend as the Fe/Al2O3 catalyst at different temperatures. The CO and H2 conversions of the Fe/Al2O3 catalyst increased when the reaction temperature was increased, while the CO and H2 conversions of the FeNa/Al2O3 catalyst gave a similar trend at different temperatures. Na as an alkali metal promoter in Fe based catalysts is known to increase CO conversion and decrease H2 conversion during FTS due to increase in the catalyst dissociative adsorption rate of CO and surface basicity, respectively (Li et al., 2016; Li et al., 2015; H.M.T. Galvis et al., 2013; Ribeiro et al., 2010).Na improves the conversion of CO in the low to average conversion range and is ineffective or hinders CO conversion in the high conversion range (Li et al., 2016; Ribeiro et al., 2010). However, comparing the CO conversion of both catalysts in Fig. 6 indicates that the presence of Na increased CO conversion at a reaction temperature of 290 °C and 310 °C. The Fe/Al2O3 catalyst and the Na-promoted Fe/Al2O3 catalyst (FeNa/Al2O3) gave a similar CO conversion rate at a reaction temperature of 270 °C, and Na inhibited CO conversion at a reaction temperature of 250 °C. The CO and H2 conversions of the FeNa/Al2O3 catalyst obtained at reaction temperature of 250 °C are more similar and the difference becomes more obvious as the reaction temperature increases. The reason for these phenomena could be competitive adsorption of dissociative CO and H2 on the Na-promoted catalyst at a reaction temperature of 250 °C and 270 °C (Li et al., 2014). This suggests that Na could be effective at improving CO conversion at certain higher reaction temperatures and hinders CO conversion at lower reaction temperatures. The H2 conversion results for the FeNa/Al2O3 catalyst were much lower than that of the H2 conversion of the Fe/Al2O3 catalyst at a reaction temperature of 250 °C and 270 °C, and closer at a reaction temperature of 290 °C and 310 °C. This also suggests that Na effects in Fe-based catalysts to increase CO conversion and decrease H2 conversion are dependent on the reaction temperature during FTS. Fig. 7 shows a linear relationship between the CO2 selectivity and reaction temperature of the Fe/Al2O3 and FeNa/Al2O3 catalysts. The Na-promoted Fe/Al2O3 catalyst exhibited a much higher CO2 selectivity at a reaction temperature of 250 °C, 270 °C, 290 °C and 310 °C than the unpromoted Fe/Al2O3 catalyst did. This explains the FeNa/Al2O3 catalyst behaviour in terms of the hydrocarbons production rate, where the FT rate is lower than the FT rate of Fe/Al2O3 catalyst at these reaction temperatures, as also seen in Table 2. The higher CO2 selectivity suggests an improved Boudouard (2CO = C + CO2) or WGS (CO + H2O = CO2 + H2) reaction rate at different reaction temperatures, when the Fe/Al2O3 catalyst was promoted with Na. The reason for the increased WGS reaction at all reaction temperatures could not be ascertained, because there was a decrease in the CO adsorption rate at a reaction temperature of 250 °C, and an increase in the CO adsorption rate at a reaction temperature of 290 °C and 310 °C. However, Na promotion decreased the H2 conversion in the Fe/Al2O3 catalyst at all reaction temperatures (Fig. 6). This indicates that H2O or oxygen adsorption could be increased by the presence of Na in the Fe/Al2O3 catalyst, which would lead to an increase in the WGS reaction at all reaction temperatures.The linear relationship between the formation rate of methane (C1) and the reaction temperature of the Fe/Al2O3 and FeNa/Al2O3 catalysts is shown in Fig. 7. The C1 formation rate of the Na-promoted Fe/Al2O3 catalyst that is based on CO and H2 is seen following different trend from CO2 selectivity that is based on CO and H2O. Table 2 also shows the C1 selectivity of the Fe/Al2O3 and FeNa/Al2O3 catalysts with the same trend of C1 formation rate of the Fe/Al2O3 and FeNa/Al2O3 catalysts at different reaction temperatures. The results seen in Fig. 7 indicate that the C1 formation rate of the FeNa/Al2O3 catalyst at a reaction temperature of 290 °C and 310 °C was significantly higher than the C1 formation rate of the Fe/Al2O3 catalyst at the same reaction temperature. This was in the reverse order at a reaction temperature of 250 °C and 270 °C, as the C1 formation rate of the FeNa/Al2O3 catalyst was significantly lower than the C1 formation rate of the Fe/Al2O3 catalyst.Many studies have reported that Na is an effective group IA promoter that decreases the C1 formation rate (Dry and Oosthuizen, 1968; Galvis et al., 2013; Ribeiro et al., 2010). However, the FTS evaluation experiments for the Fe/Al2O3 and FeNa/Al2O3 catalysts show that the C1 formation rate of the Na-promoted Fe/Al2O3 catalyst decreased at certain lower reaction temperatures, but increased at certain higher reaction temperatures. The conclusion for these phenomena could be ascertained to competitive adsorption of dissociative CO and H2 on the Na-promoted catalyst at reaction temperatures of 250 °C and 270 °C (Li et al., 2014; Ribeiro et al., 2010). This indicates that Na effects in Fe-based catalysts to decrease C1 formation rate are dependent on the reaction temperature during FTS.The linear relationship between the selectivity of the lower hydrocarbons (C2°-C4° and C2 p-C4 p) and the reaction temperature for the prepared Fe/Al2O3 and FeNa/Al2O3 catalysts can be seen in Fig. 8 . This can also be seen in Table 2, as the overall C2-C4 selectivity increased with the increasing reaction temperature when the Fe/Al2O3 catalyst contained Na. This indicates improved C2-C4 selectivity for the Na-containing Fe/Al2O3 catalyst at all reaction temperatures during FTS.Many researchers have reported that Na is an effective alkali metal promoter that increases the selectivity of lower olefin (C2°-C4°); and decreases the selectivity of lower paraffin (C2 p-C4 p) (An et al., 2007; Dry and Oosthuizen, 1968; Ribeiro et al., 2010; Abbot et al., 1986; Lama et al., 2018). A slightly higher C2 p-C4 p selectivity was observed with the Na-promoted Fe/Al2O3 catalyst at a reaction temperature of 290 °C. The same trends can be seen in Fig. 8, which shows that C2°-C4° selectivity increased and C2 p-C4 p selectivity decreased at different reaction temperatures when the Fe/Al2O3 catalyst was promoted with Na. The reason for this behaviour is the improved surface basicity of the Na-promoted Fe/Al2O3 catalyst, as its inhibited H2 conversions were also observed at all reaction temperatures. This suggests that the effects of Na in Fe-based catalysts, in terms of increasing C2°-C4° selectivity and decreasing C2 p-C4 p selectivity, are independent of the reaction temperature during FTS. Table 2 shows that overall C5-C12 selectivity increased at all reaction temperatures (250 °C, 270 °C, 290 °C and 310 °C) when the Fe/Al2O3 catalyst was promoted with Na. This suggests that Na-containing Fe-based catalysts can be used to improve selectivity towards middle range hydrocarbons (C5-C12) at different reaction temperatures in FTS. The behaviour of middle olefins (C5°-C12°) and paraffins (C5 p-C12 p) at different reaction temperatures with the Fe/Al2O3 and FeNa/Al2O3 catalysts can be explained using Fig. 9 . It can be seen that the C5°-C12° and C5 p-C12 p selectivity was higher with the FeNa/Al2O3 catalyst at all reaction temperatures compared to that of the Fe/Al2O3 catalyst. Na has been reported to facilitate the carbonisation of an Fe-based catalyst and in context, this can improve the selectivity towards C5+ hydrocarbons (Li et al., 2016; Xiong et al., 2015; Ribeiro et al., 2010). Na was also observed to hinder the reduction behaviour of the Fe/Al2O3 catalyst as earlier discussed. This is due to the increase in the strength of Fe-O bonds present in the Fe2O3 of the FeNa/Al2O3 catalyst, which then leads to an increase in the selectivity towards C5°-C12° and C5 p-C12 p (Li et al., 2016). This indicates that the effects of Na in Fe-based catalysts, in terms of improving selectivity towards C5°-C12° and C5 p-C12 p, are independent of the reaction temperature during FTS. The trend seen with C5°-C12° and C5 p-C12 p selectivity of the FeNa/Al2O3 catalyst is not linear at all the reaction temperatures, as C5°-C12° selectivity tends to be: constant at a reaction temperature of 270 °C, 290 °C and 310 °C; lower at a reaction temperature of 250 °C. The FeNa/Al2O3 catalyst C5 p-C12 p selectivity tends to be: constant at a reaction temperature of 290 °C and 310 °C; linear at a reaction temperature of 250 °C and 270 °C. The trend seen with a Fe/Al2O3 catalyst C5°-C12° and C5 p-C12 p selectivity is linear at all the reaction temperatures. It can also be seen that the increment in the Na-promoted Fe/Al2O3 catalyst (FeNa/Al2O3) selectivity towards C5°-C12° became more obvious as the reaction temperature was raised. This phenomenon could be related to different competitive adsorption behaviours of dissociative CO and H2 at different reaction temperatures in the Na-promoted Fe/Al2O3 catalyst during FTS. Therefore, the Na effects in Fe-based catalysts, in terms of raising the selectivity of C5°-C12° are dependent on the reaction temperature during FTS. Fig. 10 shows the selectivity of the higher hydrocarbons (C13+), including oxygenates (Oxy), as a function of the reaction temperature of the Fe/Al2O3 and FeNa/Al2O3 catalysts. There is a drop in product selectivity to C13+, including oxygenates for the Na-promoted Fe/Al2O3 catalyst (FeNa/Al2O3) at different reaction temperatures, when compared with the unpromoted catalyst (Fe/Al2O3). Although, the drop in C13+ (including oxygenates) product selectivity for the FeNa/Al2O3 catalyst becomes more obvious as the reaction temperature rises, it can still be seen to produce less C13+ (including oxygenates) at all reaction temperatures. The phenomenon indicates that Na hindered the product selectivity of C13+ (including oxygenates) during the FTS experiment. This could relate to an increase in product selectivity of lower and middle range hydrocarbons (C2-C4 and C5-C12) for the Na-containing catalyst (FeNa/Al2O3), at all reaction temperatures studied, when compared with the unpromoted catalyst (Fe/Al2O3). Therefore, Na can be used to reduce product selectivity towards C13+ (including oxygenates) in Fe-based catalysts. Na effect in Fe-based catalysts, in terms of reducing product selectivity towards C13+ (including oxygenates) is independent of the reaction temperature during FTS.The alumina-supported Fe catalyst (Fe/Al2O3) and its Na-promoted catalyst (FeNa/Al2O3) were prepared and characterized with various techniques to investigate their behaviour at different reaction temperatures (250 – 310 °C) in FTS. It was found that there was a slight increase in particle size when the Fe/Al2O3 catalyst was promoted with Na. This corresponds with what was observed in the N2 physisorption results, as the Na-promoted catalyst (FeNa/Al2O3) gave a smaller BET surface area than that of the unpromoted catalyst (Fe/Al2O3). The TEM images showed that Na promotion inhibited the crystallinity of the Fe/Al2O3 catalyst. This was also noticed in the H2-TPR profile of the FeNa/Al2O3 catalyst sample. Na reduced the interaction between the active Fe metal and Al2O3 support to give a better reduction step from FeO to Fe0. The emphasis was on the Na effects in Fe-based catalysts, in terms of: C1 formation rate; H2 and CO conversion; WGS reaction; olefins and paraffins of the lower, middle and higher range hydrocarbons. The results show that the Na effects in Fe-based catalysts of increasing CO conversion and decreasing H2 conversion are dependent on the reaction temperature during FTS. The C1 formation rate of the Na-promoted Fe/Al2O3 catalyst was lower at certain lower reaction temperatures and higher at certain higher reaction temperatures. The conclusion drawn was that they were due to competitive adsorption of dissociative CO and H2 on the Na-promoted catalyst at reaction temperatures of 250 °C and 270 °C. The higher CO2 selectivity suggests an improved WGS reaction rate at different reaction temperatures, when the Fe/Al2O3 catalyst is promoted with Na. It was also found that the Na effects in Fe-based catalysts to raise C2°-C4°, C5°-C12° and C5 p-C12 p selectivity and reduce C2 p-C4 p and C13+ (including oxygenates) selectivity are independent on the reaction temperature in FTS. Therefore, Na-containing Fe-based catalysts could be used to improve selectivity towards light olefins (C2°-C4°) and middle range hydrocarbons (C5-C12) at different reaction temperatures in FTS. The promotion of the alumina-supported Fe catalyst (Fe/Al2O3) and its Na-promoted catalyst (FeNa/Al2O3) with another group IA metal such as K and Cs is recommended for FTS at different reaction temperatures.The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Aliu A. Adeleke: Conceptualization, Methodology, Formal analysis, Investigation, Writing – original draft, Visualization. Muthu Kumaran Gnanamani: Investigation, Writing – review & editing, Supervision. Michela Martinelli: Formal analysis, Investigation. Burtron H. Davis: Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.We would like to acknowledge the financial support provided by the University of South Africa (UNISA) and National Research Foundation (NRF): UID 111348 and 113648.

Na in Fe-based catalysts can be used to increase CO conversion and C2-C4 olefins and decrease the conversion of H2 and C1 selectivity, but its behaviour at different reaction temperatures is of importance in Fischer-Tropsch synthesis (FTS). The dependency of the C1 formation rate, the conversions of H2 and CO, the water-gas shift reaction, the olefins and paraffins of the C2-C4 and C5-C12 hydrocarbons, and C13+ hydrocarbons on the reaction temperature for prepared Fe/Al2O3 and FeNa/Al2O3 catalysts was evaluated in a tubular fixed-bed reactor. This was done to investigate the effects of Na in Fe-based catalyst at different reaction temperatures (250 – 310 °C). The results show that the effects of Na in Fe-based catalysts to increase CO conversion and decrease H2 conversion are dependent on the reaction temperature in FTS. The Na-promoted Fe-based catalyst (FeNa/Al2O3) gave a lower C1 formation rate at certain lower reaction temperatures (250 °C and 270 °C) compared to the unpromoted Fe-based catalyst (Fe/Al2O3). The presence of Na in the Fe-based catalyst improved the C1 formation rate at certain higher reaction temperatures (290 °C and 310 °C). Na was found to hinder the selectivity towards C2-C4 paraffins and C13+ hydrocarbons, including the oxygenates, and improve the formation of C2-C4 olefins and C5-C12 hydrocarbons at different reaction temperatures.

Data will be made available on request.The rational design of crystalline organic-inorganic hybrids such as metal-organic frameworks (MOFs) is an important challenge in materials science [1–3]. In the crystal engineering of extended coordination networks, N,N′-ditopic ligands such as pyrazine (pyz) and 4,4′-bipyridine (4,4′-bipy) are often employed to link inorganic or metal-organic chains or layers into 2D and 3D structures [4–16]. Regarding pyz-pillared 3D materials, the most common types are those derived from metal-carboxylate [8–15] or cyanoheterometallic sheets (Hofmann-type MOFs with the general formula M1[M2(CN)4]) [16–26]. These pillared-layer MOFs have been investigated for applications involving, amongst others, supercapacitors [8], magnetism [9,10], gas storage and separation [11–14,17–20], switchable spin-crossover materials [20–24], iodine capture [25], and rechargeable alkaline batteries [26]. Other pyz-bridged 2D/3D solids include the copper(I) rhenate hybrid CuReO4(pyz) [27], transition metal nitroprusside derivatives of the type Fe1−xTx(pyz)[Fe(CN)5NO] (T = Co, Ni, Cu) [28], metal carbonyl MOFs with the formula fac-M(CO)3(pyz)3/2 (M = Cr, Mo, W) [29], and the molybdenum oxide hybrid compound [Mo2O6(pyz)] [30].The two pyz-bridged Mo-containing materials mentioned above are interesting since they represent two extremes in terms of the metal oxidation state, i.e., from 0 in fac-Mo(CO)3(pyz)3/2 to VI in [Mo2O6(pyz)]. MOFs with low-valent metal nodes are rare and the former tricarbonyl derivative constituted the first crystallographically characterized example of a Mo0-based material [29]. The structure consists of fac-M(CO)3(pyz)3/2 coordination layers that stack along the a-axis, leading to the formation of hexagonal pore channels of 5.5–7.5 Å diameter ( Fig. 1a). The void space inside the pores is occupied by a non-coordinated pyz molecule, resulting in the final formula fac-Mo(CO)3(pyz)3/2·1/2pyz (1). This compound crystallized in the triclinic space group P-1. Syntheses of 1 contained a small amount of a second crystalline phase with the formula fac-Mo(CO)3(pyz)3/2 (2), identified as a dense cubic phase consisting of two interpenetrating coordination networks (Fig. 1b). The structure of [Mo2O6(pyz)] (3) consists of layers of corner-sharing MoO5 square pyramids connected through pyz groups into a 3D covalently bonded metal oxide-organic ligand framework (Fig. 1c), in a manner analogous to that found in [MoO3(4,4′-bipy)0.5] [30,31].The molybdenum-pyrazine coordination compounds described above are potentially interesting as (pre)catalysts for the following reasons: (i) during the last two decades, a broad variety of molybdenum carbonyl complexes, including tricarbonyl complexes such as [Mo(CO)3(L) n ] [L = 1,2,4-triazole (trz, n = 3) or tris(1-pyrazolyl)methane (tpm, n = 1)], have revealed good performance as catalyst precursors for oxidation reactions, especially the epoxidation of olefins [32–36]; (ii) in several cases, the direct use of the aforementioned heteroleptic complexes as pre-catalysts leads to the formation of catalytically active crystalline metal oxide-organic ligand hybrids [37]; (iii) interesting catalytic behaviors have been found for a wide variety of molybdenum oxide-organic hybrids, e.g., 1,2,4-triazole-based hybrids displayed reaction-induced self-separation behavior when applied as catalysts for oxidation reactions [38–40]. With these considerations in mind, the present investigation was undertaken, in which the molybdenum-pyrazine compounds 1–3 were examined as (pre)catalysts for the oxidation of sulfides and the epoxidation of olefins, including biorenewable terpene and unsaturated fatty acid methyl ester (FAME) substrates.The following chemicals, reagents and solvents were obtained from Sigma-Aldrich (unless otherwise indicated) and used as received: (for synthesis) molybdenum hexacarbonyl (Fluka), molybdenum trioxide (Fluka), pyrazine (>99 %), toluene (99.9 %), acetonitrile (99.9 %, Riedel-de Haën), diethyl ether (99.8 %, Riedel-de Haën); (for catalytic tests) cis-cyclooctene (95 %, Alfa Aesar), methyl oleate (99 %), methyl linoleate (95 %, Alfa Aesar), dl-limonene (>95 %, Merck), cyclododecene (mixture of isomers, 96 %), methyl phenyl sulfide (99 %), diphenyl sulfide (98 %), 5.5 M tert-butyl hydroperoxide in decane (<4 % water), α,α,α-trifluorotoluene (≥99 %), acetone (99.5 %, Riedel-de Haën), and the internal standards methyl decanoate (99 %), undecane (>99 %) and mesitylene (98 %).Mo(CO)6 (0.16 g, 0.60 mmol) and pyz (52 equiv.) were added to a PTFE-lined stainless-steel autoclave (40 mL capacity) in a glove box under an argon atmosphere. The autoclave was sealed and heated in an oven to 150 °C at a ramp rate of 0.5 °C min−1. After heating at this temperature for 40 h, the autoclave was cooled to room temperature over a period of 16–24 h, and the resultant dark shiny solid product was transferred to a Schlenk tube and washed with acetonitrile (3 × 20 mL) to remove excess pyz and residual Mo(CO)6. Finally, the solid was vacuum-dried at room temperature for 2 h. Yield: 115 mg, 56 % (based on Mo). Anal. Calcd for C9H6MoN3O3·C2H2N (340.15): C, 38.84; H, 2.37; N, 16.47 %. Found: C, 38.60; H, 2.38; N, 16.81 %.A mixture of Mo(CO)6 (0.48 g, 1.80 mmol) and pyz (10 equiv.) was placed under vacuum (ca. 0.1 bar) for 10 min in a Schlenk tube. Toluene (30 mL) was then added and the mixture was refluxed under a nitrogen atmosphere for 3 h. The resultant dark precipitate was isolated by centrifugation, washed with acetonitrile (3 × 20 mL), and vacuum-dried at room temperature for 2 h. Yield: 0.43 g, 80 % (based on Mo). Anal. Calcd for C9H6MoN3O3 (300.11): C, 36.02; H, 2.01; N, 14.00; Mo, 31.97 %. Found: C, 35.72; H, 2.23; N, 13.89; Mo, 30.5 %.A mixture of MoO3 (0.11 g, 0.76 mmol), pyrazine (60 mg, 0.76 mmol) and Milli-Q water (15 mL) was heated under autogenous pressure and dynamic conditions (20 rpm) for 3 days at 160 °C in a 23 mL Teflon-lined stainless-steel autoclave. The resultant yellow-orange solid was collected by filtration, washed with Milli-Q water (2 × 10 mL), acetone (2 × 10 mL) and diethyl ether (2 × 10 mL), and finally vacuum-dried at room temperature for 2 h. Yield: 0.10 g, 71 % (based on Mo). Anal. Calcd for C4H4Mo2N2O6 (367.98): C, 13.06; H, 1.10; N, 7.61 %. Found C, 13.40; H, 1.15; N, 7.80 %.Elemental analysis for C, H and N was performed using a Truspec 630–200–200 instrument. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analyses (for Mo) were performed at the Central Testing Laboratory, University of Aveiro, using a Horiba JobinYvon Activa M spectrometer (detection limit of ca. 0.02 mg dm−3; an experimental range of error of 5 %). Powder X-ray diffraction (PXRD) patterns were collected on an Empyrean PANalytical diffractometer (Cu-Kα X-radiation, λ = 1.54060 Å) in a Bragg-Brentano para-focusing optics configuration (45 kV, 40 mA) at ambient temperature, using a spinning flat plate sample holder. Samples were step-scanned in the range from 5° to 70° (2θ) with steps of 0.026°. A PIXEL linear detector with an active area of 1.7462° was used with a counting time of 99 s per step. Scanning electron microscopy (SEM) images and elemental mappings (Mo) were obtained on a Hitachi SU-70 SEM microscope equipped with a Bruker Quantax 400 detector operating at 20 kV. Scanning transmission electron microscopy (STEM) images were collected using a Hitachi HD2700 microscope equipped with a Bruker EDS detector. The samples were prepared by depositing a drop of a suspension of the solid sample in ethanol onto holey amorphous carbon-film-coated 400 mesh copper grids (Agar Scientific). Thermogravimetric analysis (TGA) was performed under air using a HITACHI STA 300 system with a heating rate of 5 °C min−1. The textural properties were determined from N2 sorption isotherms at −196 °C, which were measured using a Quantachrome instrument (automated gas sorption data using Autosorb IQ2). The samples were pre-treated at 60 °C for 8 h, under vacuum (< 4 × 10−3 bar). The specific surface area (S BET) was calculated using the Brunauer, Emmett and Teller equation, and the total pore volume (V p) was based on the Gurvitch rule (for a relative pressure (p/p 0) of at least 0.99). The external/mesoporous surface area (S ext) and micropore volume (V micro) were calculated using the t-plot method. Attenuated total reflectance (ATR) FT-IR spectra were measured on a Bruker Tensor 27 spectrometer equipped with a Specac Golden Gate Mk II ATR accessory having a diamond top plate and KRS-5 focusing lenses (resolution 4 cm−1, 256 scans). Diffuse reflectance (DR) UV-Vis spectra were recorded at room temperature in the range 190–900 nm using a JASCO V-780 spectrophotometer equipped with a JASCO ISV-469 integrating sphere, with Spectralon as reference material. The spectra were collected in the reflectance mode, with a bandwidth of 2 nm, a scan speed of 200 nm min−1, and a data pitch of ≈ 0.5.The catalytic reactions were carried out using 10 mL borosilicate batch reactors equipped with a Teflon valve for sampling and a Teflon-lined magnetic stirrer. Initially, catalyst (in an amount equivalent to 18 μmol Mo), α,α,α-trifluorotoluene (TFT) (1 mL) and substrate (1.8 mmol) were added to the reactor, which was then immersed in a temperature-controlled oil bath at 35 or 70 °C under stirring (1000 rpm). After 10 min, the preheated oxidant tert-butyl hydroperoxide (TBHP) (2.75 mmol for cis-cyclooctene (Cy8) and cyclododecene (Cy12), and 4.00 mmol for bio-based olefins and sulfides) was added to the reactor, and this moment was taken as the initial instant of the catalytic reaction. The use of H2O2 as oxidant led to the complete degradation of the solids, forming solutions (no solid-state catalyst), and thus TBHP was preferred. Concerning the choice of solvent, TFT is appealing since it is readily available, relatively inexpensive, noncoordinating, and environmentally benign, and possesses a high boiling point and strong capacity to dissolve a large range of organic compounds [41]. It has been explored as a solvent in various olefin epoxidation catalytic systems, frequently yielding positive results [42,43].The evolution of the reactions was monitored by analyzing freshly prepared samples by gas chromatography (GC), using a Varian 450 GC instrument equipped with a BR-5 capillary column (30 m × 0.25 mm × 0.25 µm) and a FID detector. The quantification of reactants/products was based on calibrations; the internal standards used were undecane for the substrates Cy8, Cy12 and dl-limonene (Lim), methyl decanoate for methyl oleate (MeOle) and methyl linoleate (MeLin), and mesitylene for methyl phenyl sulfide (MPS) and diphenyl sulfide (DPS). The values of initial catalytic activity (mmol gcat −1 h−1) and initial turnover frequency (mol molMo −1 h−1) were calculated based on substrate conversion at 12 min reaction.The catalyst stability was evaluated by reusing the recovered solids in consecutive batch runs, keeping constant the initial mass ratio of catalyst:Cy8:TBHP between runs (the solids are denoted i-runn, where i is the catalyst identification number and n is the number of the batch run from which the solid was isolated). After each run, the solids were separated from the reaction mixture by centrifugation (3500 rpm), thoroughly washed with acetone, dried overnight under atmospheric conditions, and finally vacuum-dried (ca. 0.1 bar) at 60 °C for 1 h. The obtained solids were characterized by ATR FT-IR and DR UV-Vis spectroscopies, PXRD and/or SEM/STEM. Hot filtration (or leaching) tests were performed to check if soluble active species were present in the liquid phase for the system 3/TBHP/TFT/Cy8, at 70 °C, under similar conditions to those used for a typical batch run. Specifically, at 1 h reaction (in the presence of solid), the hot solid-liquid biphasic mixture (catalyst/TBHP/TFT/Cy8, at 70 °C) was filtered through a 0.2 µm PTFE membrane filter, and the hot filtrate was transferred to a separate preheated (70 °C) reactor, after which stirring was continued at 70 °C and GC was used to monitor any further reaction. In addition to the hot filtration tests, ambient filtration tests were performed as follows: at 30 min (for used 1 and 2, referred to as 1-run1 and 2-run1) or 1 h (for 3) reaction in the presence of the solid catalyst, the biphasic solid-liquid mixture (catalyst/TBHP/TFT/Cy8, at 70 °C) was subjected to centrifugation (5 min at 6000 rpm and then 5 min at 9000 rpm) and afterwards filtration (using a 0.2 µm PTFE membrane filter) at ambient temperature; the filtrate was transferred to a separate preheated (70 °C) reactor, after which stirring was continued at 70 °C and GC was used to monitor any further reaction.The influence of the reaction conditions on catalytic performance was evaluated for 1 and 3 using different amounts of catalyst (initial Mo:Cy8 molar ratio of 0.01, 0.005 and 0.0025, maintaining TBHP:Cy8 = 1.5), oxidant (initial TBHP:Cy8 molar ratio of 1.5 and 2.2, maintaining Mo:Cy8 = 0.01) and different temperatures (55 or 70 °C, with initial TBHP:Cy8 molar ratio = 1.5 and Mo:Cy8 = 0.01).The tricarbonyl-pyrazine-molybdenum(0) MOF fac-Mo(CO)3(pyz)3/2·1/2pyz (1) was obtained as a dark shiny crystalline solid in 56 % yield after heating a mixture of Mo(CO)6 and an excess of pyz in a Teflon-lined stainless-steel autoclave at 150 °C for 40 h. The PXRD pattern of 1 ( Fig. 2b) is in agreement with that reported by Voigt et al. for the same phase obtained by heating the reagents in a sealed ampoule rather than an autoclave [29]. When Mo(CO)6 was reacted with a 10-fold excess of pyz in refluxing toluene, an 80 % yield of the cubic phase, fac-Mo(CO)3(pyz)3/2 (2), was obtained, which had previously only been isolated by mechanical separation of single crystals formed as a minor secondary phase in the synthesis of 1 by the ampoule method [29]. The PXRD pattern of 2 (Fig. 2f) matches well with the simulated pattern calculated for the crystal structure (Fig. 2e), which confirms the purity of the as-synthesized MOF. The pyrazine-pillared molybdenum(VI) oxide hybrid [Mo2O6(pyz)] (3) was synthesized in 69 % yield by the hydrothermal treatment of a mixture of MoO3, pyz and H2O in the mole ratio 1:1:1100 at 160 °C for 3 days. Previously, Liang et al. obtained 3 through a similar procedure except that pyrazine-2-carboxylic acid was employed as the pyz source rather than pyz itself, with the former undergoing thermal decarboxylation under the hydrothermal synthesis conditions [30]. The phase purity of 3 was confirmed by PXRD (Figs. 2i and 2j).Crystal morphologies were assessed by SEM ( Fig. 3). MOF 1 shows an irregular rod-like morphology with particle sizes up to 30 µm in length, with some showing a hexagonal cross-section. The particles of 2 generally present an irregular and pseudo-spherical aspect with sizes up to 5 µm. For the hybrid 3, the high crystallinity revealed by PXRD is confirmed by the SEM images, which show rectangular slab- or block-like crystallites with sizes in the range 5–20 µm.The thermal stability of the materials was evaluated by TGA ( Fig. 4). For comparison, the TGA curve of neat pyz is shown. Pyz sublimes at room temperature and ambient pressure, and hence the TGA curve shows an abrupt and complete mass loss between 25 and 70 °C (differential thermogravimetric maximum (DTGmax) at 66 °C). Although the MOF 1 contains free pyz molecules in the hexagonal pore channels, the TGA curve under air does not show a low-temperature (<70 °C) weight loss step corresponding to the removal of these weakly bound moieties. Instead, 1 starts to decompose above 70 °C, showing two main overlapping weight loss steps with DTGmax values of 102 °C and 120 °C. The first of these is probably due to partial decarbonylation combined with removal of the free pyz molecules, while the second step is attributed to the decomposition of the remaining pyz molecules in the framework. Cubic 2 displays a similar behavior with DTGmax values of 97 °C and 118 °C. The molybdenum(VI) oxide hybrid 3 displays high thermal stability, with no weight loss being registered up to 335 °C. Decomposition of the pyrazine pillars then takes place abruptly between this temperature and 380 °C (DTGmax 367 °C). Considering that complete decomposition of 1-3 under air leads to MoO3, the weights of the residues at 400 °C for 1 (41.7 %), 2 (49.6 %) and 3 (78.5 %) are in agreement with the calculated values of 42.3 %, 48.0 % and 78.2 %, respectively.Materials 1-3 possessed relatively low specific surface areas; S BET in the range 9–58 m2 g−1 ( Table 1). MOF 1 possessed higher S BET than 2 (58 and 9 m2 g−1, respectively) and some microporosity (V micro = 0.012 cm3 g−1); 2 did not possess microporosity. These results are somewhat consistent with the dense (cubic phase) framework of 2. Hybrid 3 did not possess microporosity, but the total pore volume was relatively high (V p = 0.44 cm3 g−1 compared to < 0.09 cm3 g−1 for the MOFs), which may correspond to inter/intraparticle mesopores. The three materials did not exhibit well-defined pore size distribution curves, suggesting that they do not possess ordered open pore systems.The pyz-bridged materials 1-3 were explored as epoxidation (pre)catalysts, firstly in the model reaction of cis-cyclooctene (Cy8) using tert-butyl hydroperoxide (TBHP) as oxidant. The three materials led to biphasic solid-liquid mixtures. The influence of the reaction conditions on the performance of 2 and 3 was studied (Fig. S1 in the Supplementary data). For 2 with a reaction temperature of 70 °C, an increasing amount of MOF (initial TBHP/Cy8 molar ratio kept constant at 1.5) led to increasing initial Cy8 conversion, although the initial catalytic activity (mmol gcat −1 h−1) and initial turnover frequency (TOF, mol molMo −1 h−1) decreased, following the order (1991 mmol gcat −1 h−1 and 684 mol molMo −1 h−1 for Mo:Cy8 = 0.0025) > (1292 mmol gcat −1 h−1 and 444 mol molMo −1 h−1 for Mo:Cy8 = 0.005) > (962 mmol gcat −1 h−1 and 330 mol molMo −1 h−1 for Mo:Cy8 = 0.01). For the organometallic MOF 2, the in situ oxidative decarbonylation (discussed below) of the Mo(0) sites into oxidized molybdenum sites is required prior to the catalytic reaction. An increasing amount of 2 represents a decreasing initial molar ratio of TBHP:Mo, which may negatively impact on the oxidative decarbonylation rate of 2 (with TBHP) and consequently on the overall rate of olefin epoxidation. Nevertheless, in the studied range of Mo:Cy8 ratios, 2 led to 92–95 % epoxide (Cy8Ep) yield at 2 h and 97–100 % yield at 4 h (the epoxide was always the only reaction product, i.e., Cy8Ep selectivity was 100 %). For 3, the initial activity and initial TOF were greater than zero solely for the highest Mo:Cy8 ratio (74 mmol gcat −1 h−1 and 14 mol molMo −1 h−1, respectively), and Cy8Ep yield at 24 h increased in the order 69 %, 88 % and 100 % for Mo:Cy8 = 0.0025, 0.005 and 0.01, respectively. As opposed to 2, the oxidative decarbonylation requirement does not apply for 3 (which is already in the oxidized form); a higher amount of 2 corresponds to a higher amount of oxidized metal sites from the initial instant of the catalytic reaction, enhancing the epoxidation reaction rate.Control experiments performed without catalyst or without oxidant showed negligible conversion, indicating that the catalytic reaction required the simultaneous presence of the molybdenum catalyst and the oxidant. This is consistent with mechanistic studies reported in the literature for molybdenum(VI)-catalyzed epoxidations, being generally accepted that acid-base reactions between the metal center (acting as a Lewis acid) and the hydroperoxide oxidant (acting as a base) occur to give active oxidizing species responsible for transferring an oxygen atom to the olefin molecule; this involves the nucleophilic attack of the olefin on an electrophilic oxygen atom of the oxidizing species (heterolytic mechanism), and the concomitant formation of the epoxide and tert-butyl alcohol (co-product of TBHP conversion) [44–46].For both 2 and 3 an increase in the amount of oxidant from TBHP/Cy8 = 1.5 to TBHP/Cy8 = 2.2 increased the epoxidation reaction rate significantly (e.g., conversion at 1 h increased from 87 % to 94 % for 2, and from 24 % to 31 % for 3) (Fig. S1 in the Supplementary data). For a Mo:Cy8:TBHP molar ratio of 1:100:152, a decrease in the reaction temperature from 70 °C to 55 °C led to a significantly slower epoxidation reaction with 2 (initial activity = 367 mmol gcat −1 h−1, TOF = 126 mol molMo −1 h−1).Further catalytic experiments were performed with a reaction temperature of 70 °C and a Mo:Cy8:TBHP molar ratio of 1:100:152. Under these established conditions, 1 led to faster epoxidation than 2 (e.g., leading to a quantitative yield of the epoxide at 2 h, Fig. 5). The slightly higher reaction rate for 1 than for 2 may stem from the different crystal structures, such as the dense framework of 2 vs. the more open framework of 1, which may result in different amounts of accessible molybdenum sites. Control tests performed with (i) the free organic ligand, (ii) the synthesis metal precursor Mo(CO)6, and (iii) the physical mixture of the free organic ligand and Mo(CO)6 (in molar amounts of metal and ligand equivalent to those added together with 1 and 2) led to homogeneous mixtures, and Cy8 conversion at 1 h was 3 %, 41 % and 28 %, respectively, compared to 96 % for 1 and 87 % for 2. Hence, the MOFs give very active species when compared with their individual organic and/or inorganic components.There are few examples of monometallic tricarbonylmolybdenum(0)-based compounds tested for catalytic epoxidation of olefins. To the best of our knowledge, for the model reaction of Cy8/TBHP, only five mononuclear complexes (and no MOFs or hybrids) were previously reported. Table 2 compares the catalytic results of the previously studied complexes to those for MOFs 1 and 2. Under similar reaction conditions, 1 led to superior catalytic results to [Mo(CO)3(1,2,4-trz)3] (trz = triazole) and comparable results to [Mo(CO)3(1,2,3-trz)3] [35]. The mononuclear complexes [Mo(CO)3(tpm*)] (tpm* = tris(3,5-dimethyl-1-pyrazolyl)methane) and [Mo(CO)3(tpm)] (tpm = tris(1-pyrazolyl)methane) led to 96 % and 99 % conversion at 6 h and 2 h, respectively, at 55 °C. Depending on the tricarbonyl metal complex, different species were formed under the epoxidation reaction conditions. Polymeric species with the empirical formula [MoO3(L)] were formed from [Mo(CO)3(L)3] with L = 1,2,3-trz and 1,2,4-trz [35], and the hexamolybdate salt [{MoO2(tpm*)}2(µ2-O)][Mo6O19] was formed from [Mo(CO)3(tpm*)] [48]. The oxidizing species formed were stable in consecutive catalytic batch runs, somewhat in parallel to that verified for 1 and 2 (discussed below).As found for 2, the reaction mixture with 1 as (pre)catalyst was biphasic solid-liquid. The solids were recovered and reused in two consecutive batch runs, which led to similar kinetic curves for each type of material (runs 2 and 3 in Figs. 5A and 5B). There was a slight fall off in initial activity between runs 1 and 2 for both catalysts: from 1142 mmol gcat −1 h−1 to 983 mmol gcat −1 h−1 for 1, and from 962 mmol gcat −1 h−1 to 820 mmol gcat −1 h−1 for 2. For 2, the kinetic curves for runs 2 and 3 merged with that for the first run for reaction times longer than 30 min, while for 1 complete conversion in runs 2 and 3 took slightly longer (4 h) than in run 1 (2 h).The used catalysts (from 1 and 2) were characterized by PXRD, SEM/STEM, ATR FT-IR and UV-Vis spectroscopies. PXRD showed that the recovered solids were X-ray amorphous (Fig. 2). Morphological changes occurred during the first catalytic run with 1 and 2, leading to solids with irregular particle sizes, which included aggregates of nanoparticles (Fig. S2 in the Supplementary Data). The ATR FT-IR spectra of the used catalysts suggest that chemically similar species were formed from 1 and 2 (1-run1 and 2-run1, Fig. 6). The loss of the strong ν(CO) bands near 1765 and 1890 cm−1, coupled with the appearance of a band at about 940 cm−1 (assigned to ν(MoO)) and very broad bands in the spectral range 400–800 cm−1 (assigned to ν(Mo–O–Mo)), shows that the precursor materials underwent in situ oxidative decarbonylation. Changes in the spectral range 1000–1500 cm−1 (associated mainly with internal pyz modes) indicate alterations in the metal-organic ligand coordination modes. The similar spectra found for 1-run1 and 2-run1, and 1-run3 and 2-run3, correlate with the roughly coincident kinetic curves of the used solids (Fig. 5). Accordingly, 1-run1 and 2-run1 gave similar UV-Vis diffuse reflectance spectra consisting of a relatively narrow and intense peak at 250 nm, with a shoulder on the high energy side at about 200 nm, and overlapping shoulders on the low energy side with maxima at about 300 and 335 nm (Fig. S4 in the Supplementary data). The absence of absorption peaks between 400 and 800 nm indicates that practically all molybdenum centers are oxidized to MoVI and no Mo0 or other reduced forms are present (cf. the parent materials 1 and 2 (Fig. S4), which display a strong, very broad absorption band across the whole visible region, consistent with the black color of the solids). Hence, the absorption bands between 250 and 350 nm are assigned to ligand to metal charge transfer transitions (O2– → Mo6+) involving oxygens in bridging (Mo–O–Mo) and terminal (MoO) positions [49,50]. The bands between 300 and 350 nm are especially indicative of a molybdenum oxide substructure containing connected molybdenum(VI)-oxo species in which the Mo6+ centers are octahedrally coordinated. The absorption bands exhibited by 1-run1 and 2-run1 may also have contributions from the pyrazine ligand which, in its free form, displays a band at about 255 nm, assigned to the π-π* electronic transition of the aromatic ring, and a broad band beyond 275 nm (λ max 320–340 nm), assigned to the forbidden electronic transition n-π* [51].Hot filtration (leaching) tests with the recovered solids 1-run1 and 2-run1 led to roughly comparable results to the respective normal batch run in the presence of solid catalyst (Fig. S3 in the Supplementary data). These results may be partly due to the difficult separation of the nanoparticles, which may pass through the membrane filter with a 200 nm pore size. As an alternative, ambient separation tests involving centrifugation and filtration operations were carried out for 1-run1 and 2-run1 (Fig. S3). The recovered solids were used instead of the original MOFs 1 and 2 because during run 1 the latter are converted to the respective oxidized compounds 1-run1 and 2-run1, and thus one cannot exclude the possibility of the reaction mixture of batch run 1 containing side products of the conversion of the original catalyst. The filtrates were left to react further at the catalytic reaction temperature, which led to smaller increments in Cy8 conversion between 0.2 h and 4 h reaction compared with those without solid catalyst separation (i.e., normal batch run 2): for 1-run1, 16 % increment in conversion compared to 33 % without solid catalyst separation; for 2-run1, 17 % increment in conversion compared to 44 % without solid catalyst separation. These results suggest that the solid catalysts contributed to the catalytic reactions. ICP-AES analyses of the respective filtrates indicated that the amount of molybdenum was 209 mg dm−3 for 1 and 119 mg dm−3 for 2, which corresponds to ca. 2 and 1 mol%, respectively, of the initial amount of molybdenum added to the reactor.Catalyst reuse was also explored for the hybrid 3 (Fig. 5C). The kinetic curves are practically identical for the three consecutive batch runs, and epoxide selectivity remained 100 %. The initial catalytic activity was 74, 109 and 103 mmol gcat −1 h−1 for runs 1, 2 and 3, respectively. Characterization of the catalyst recovered after each run by PXRD and ATR FT-IR spectroscopy indicated that the structural and chemical features of 3 were preserved (Fig. 2 and Fig. 6). Morphologically, the particle sizes seemed to decrease, which may have contributed to the slight increase in activity observed in consecutive batch runs (Fig. S2 in the Supplementary Data). A hot filtration test at 1 h reaction for 3 led to a slower reaction without the solid, but conversion was nevertheless significant (50 % at 6 h without solid, compared with 84 % in the presence of the solid catalyst, Fig. S3), suggesting that the system may have a homogeneous catalytic contribution and/or very small solid particles that could not be separated by the membrane filter. The ambient leaching test, which involved centrifugation and filtration operations, was carried out. The filtrate was left to react further at the catalytic reaction temperature, leading to 9 % increment in Cy8 conversion between 1 h and 6 h reaction, compared to 60 % without solid catalyst separation (Fig. S3), suggesting that the solid catalyst 3 contributed to the catalytic reaction. ICP-AES analyses of the respective filtrate indicated that the amount of molybdenum was 3 mg dm−3, which corresponds to only ca. 0.03 mol% of the initial amount of molybdenum added to the reactor.The principal structural motif of 3, comprising perovskite-like layers of corner-sharing {MoO5N} octahedra, is found in a few other organically modified molybdenum(VI) oxide hybrids, namely those containing pyridine [52], 4,4′-bipy [31], 1,2,3-triazole (1,2,3-trz) [53], 1,2,4-triazole (1,2,4-trz) [31] and 4-(1,2,4-triazol-4-yl)benzoic acid (trPhCO 2 H) [54] building blocks. The last four of these have been tested as catalysts for the epoxidation of Cy8 with TBHP. Under the same reaction conditions (70 °C, TFT cosolvent, 1 mol% Mo, 1.5 equiv. TBHP), the hybrids 3 and [MoO3(1,2,3-trz)0.5] [35] led to identical results (83–84 %/100 % Cy8Ep yield at 6/24 h), which were superior to those obtained with [MoO3(1,2,4-trz)0.5] (54 %/91 % Cy8Ep yield at 6/24 h) (Table S1 in the Supplementary data). The hybrid [Mo4O12(trPhCO 2 H)2]·0.5 H2O was only tested at 55 °C, giving 44 %/82 % epoxide yield at 6/24 h [54], while [Mo2O6(4,4′-bipy)] led to 99 % epoxide yield (99 % selectivity) after 2 h in refluxing CHCl3 (using a high catalyst amount of 2.7 mol% Mo) [55]. Table S1 compares the performance of 3 for the model reaction of Cy8 with other polymeric hybrids of the type [Mo2O6(L) x ] in which the Mo(VI) oxide subtopologies consist of 1D chains or ribbons. Under roughly similar reaction conditions, and based on Cy8 conversion at 6 h, 3 performed better than [Mo2O6(Htrgly)]·H2O (Htrgly = 2-(4H-1,2,4-triazol-4-yl)acetic acid; 59 % conversion) [40], [Mo2O6(trpzH)(H2O)2] (trpzH = 4-(3,5-dimethyl-1H-pyrazol-4-yl)-1,2,4-triazole; 74 % conversion), [Mo2O6(m-trtzH)(H2O)2] (m-trtzH = 5-[3-(1,2,4-triazol-4-ylphenyl)]-1H-tetrazole; 21 % conversion), [Mo2O6(p-tr2Ph)]·H2O (p-tr2Ph = 1,4-phenylene-4,4′-bis(1,2,4-triazole); 17 % conversion), and [Mo2O6(tr2ad)]·H2O (tr2ad = bis(1,2,4-triazol-4-yl)adamantane; 18 % conversion) [56], and worse than [Mo2O6(trethbz)2]·H2O (trethbz = (S)-4-(1-phenylpropyl)-1,2,4-triazole; 98 % conversion) [54]. Under different reaction conditions, higher conversion was reported for [Mo2O6(pent-pp)] (pent-pp = 2-(1-pentyl-3-pyrazolyl)pyridine; 98 % conversion at 6 h, 55 °C, Mo:Cy8:TBHP molar ratio = 1:113:172), but this material suffered loss of activity after run 1 and was chemically and structurally unstable [57]. Generally, the literature for the [Mo2O6(L) x ] family of hybrids lacks detailed catalyst stability studies, making it difficult to establish fair comparisons with other catalysts.Catalysts 1-3 were further examined for the epoxidation of other olefins, including the bio-olefins dl-limonene, methyl oleate (MeOle) and methyl linoleate (MeLin), and the reaction scope was expanded to include sulfoxidation (Table S2 in the Supplementary data). dl-Limonene is the racemic mixture of d-limonene and l-limonene, which are the main compounds in citrus and pine needle essential oils, respectively. Upgrading of limonene by catalytic oxidation is a critically important pathway towards its valorization as a renewable platform chemical [58]. The epoxidation of dl-limonene gives mono- and diepoxides, namely 1,2-epoxy-p-menth-8-ene (LimEp) and 1,2:8,9-diepoxy-p-methane (LimDiEp) ( Scheme 1), which can undergo ring-opening reactions to give a wide range of oxygenated derivatives, from simple limonene-1,2-diols (LimDiol), which are useful precursors of bioactive compounds [59,60], to cyclic carbonates [61,62] and a broad variety of bio-based polymers [63,64], such as limonene-based non-isocyanate polyurethanes [61,62]. The reaction of dl-limonene was fast in the presence of 1 and 2, giving 100 % conversion at 2 h and 4 h, respectively ( Fig. 7). Mono and diepoxide products were mainly formed in a total yield of 67 % at 2 h for 1 (LimEp/LimDiEp molar ratio of 3.2) and 66 % at 4 h for 2 (LimEp/LimDiEp molar ratio of 5.7), and LimDiol was formed via LimEp epoxide ring-opening, in 27 % and 28 % yield, respectively. Without catalyst, conversion was only 6 % at 24 h. To the best of our knowledge, the only tricarbonylmolybdenum(0)-based compound that has previously been reported as a (pre)catalyst for the epoxidation of limonene is [Mo(CO)3(1,1,1-tris(methylaminomethyl)ethane)], which led to 18 % conversion of (R)-(+)-limonene at 24 h/55 °C [47]. Although the reaction of dl-limonene was slower in the presence of 3, the combined epoxide selectivity was much higher, with LimEp and LimDiEp being obtained in 64 % and 36 % yield, respectively, at 24 h (LimEp/LimDiEp molar ratio = 1.8) ( Fig. 8).Methyl oleate and methyl linoleate are the methyl esters of oleic acid and linoleic acid, which are the major components of most vegetable oils. The epoxides of fatty acid methyl esters (FAMEs) have widespread use as solvents, lubricants, PVC stabilizers and plasticizers, and as intermediates for the synthesis of biobased polyols for polyurethane production [65,66]. With MeOle as substrate, 1-3 led to 94 %/99 %, 88 %/100 % and 32 %/94 % conversion, respectively, at 4 h/24 h, and methyl 9,10-epoxyoctadecanoate (MeOleEp) was formed with 100 % selectivity (Fig. 7 and Fig. 8, Scheme 1). The reaction of the diene MeLin in the presence of 3 gave mainly the monoepoxide isomers and regioselectivity towards the epoxidation of the 9,10 and 12,13 CC double bonds was similar. Specifically, methyl 9,10-epoxy-12-octadecenoate and methyl 12,13-epoxy-9-octadecenoate (MeLinEp) were formed in equimolar amounts with a total yield of 58 % at 69 % conversion, 24 h ( Scheme 2). The total monoepoxide selectivity decreased slightly between 2 h and 24 h reaction (from 100 % at 12 % conversion to 85 % at 69 % conversion) owing to the formation of the diepoxide methyl 9,10–12,13-diepoxyoctadecanoate (10 % MeLinDiEp yield at 24 h). For 1 and 2, the MeLinEp yields at conversions of 71–74 % (4 h) and 85–91 % (24 h) were 50 %/36 % for 1 and 55 %/49 % for 2 (Fig. 7). The decreasing yields of the monoepoxides (MeLinEp) were accompanied by increasing diepoxide yields (16 %/25 % for 1 and 16 %/31 % for 2 at 4 h/24 h) and increasing total yields of cyclization products (8 %/30 % for 1 and 0 %/5 % for 2 at 4 h/24 h). The cyclization products were methyl 10,13-epoxy-9,12-dihydroxyoctadecanoate and methyl 9,12-epoxy-10,13-dihydroxyoctadecanoate, which may be formed via intramolecular cyclization of the epoxydiol intermediates (Scheme 2). The latter may be relatively unstable due to the proximity of the diol and epoxy ring in the molecule (separated by a methylene group) and thus may not be present in measurable amounts in the reaction medium [67–72].For the two FAMEs, no reaction occurred without catalyst, at 24 h. It is noteworthy that for 1 and 2 the slightly higher catalytic activity observed for 1 with Cy8 as substrate is maintained for the bio-derived olefins. In compliance with the results for Cy8, 1 and 2 display much higher catalytic activity than 3 in the reactions of the bio-derived olefins.To the best of our knowledge, only three hybrid materials of the type [Mo2O6(L) x ] have been reported as catalysts for biomass-derived olefins, namely, [Mo2O6(2,2′-bipy)] [71], [Mo2O6(trethbz)2]·H2O [54] and [Mo2O6(pent-pp)] [57] (Table S1 in the Supplementary data). Based on total epoxide selectivity at high conversions (>90 %) of MeOle and dl-limonene, 3 seems to exhibit the most promising catalytic performance. The MeLin reaction was somewhat slower for 3 than for the 1D hybrid [Mo2O6(trethbz)2]·H2O, which may be due to an interplay of several factors such as differences in the catalysts′ structural dimensionality and solubility in the medium containing the olefin.Catalyst 3 was also studied for the epoxidation of cyclododecene ( Scheme 3). The reaction gave 100 % selectivity to 1,2-cyclododecane epoxide at 86 % conversion, 24 h. Without catalyst, the reaction was sluggish (7 % conversion at 24 h). Comparing the results for cis-cyclooctene and cyclododecene, the reaction is slower for the olefin with the larger ring size, suggesting that steric hindrance effects may be important.To assess the capacity of 1-3 to promote sulfoxidation, the catalytic oxidation of methyl phenyl sulfide (MPS) and diphenyl sulfide (DPS) was studied under mild conditions, at 35 °C and atmospheric pressure. Without catalyst, MPS and DPS conversion was 66 % and 69 %, respectively, at 24 h, and only the corresponding sulfoxide was formed. Catalysts 1-3 promoted the one-pot conversion of sulfide-sulfoxide-sulfone. Thus, 1 and 2 led to quantitative yields of the sulfones after 24 h reaction, while the sulfone yields for 3 were 84 % from MPS and 95 % from DPS ( Schemes 4 and 5). The molybdenum-catalyzed sulfoxidation of sulfides (or sulfoxides) may involve a proton transfer of the hydroperoxide oxidant to an oxido ligand (MoO) forming a Mo-OH group and an additional η 1-hydroperoxido ligand (Mo-OOH). Interactions between the resultant oxidizing species and the sulfur-containing nucleophile (via an oxygen atom of Mo-OOH) leads to O-O bond rupture and formation of a SO bond, giving the sulfoxide (or sulfone) product, and tert-butanol [72].In this work, molybdenum-pyrazine coordination compounds were investigated for the first time in the catalytic epoxidation of olefins and sulfoxidation of sulfides. The organometallic network solids with the framework composition [Mo(CO)3(pyz)3/2] (1, 2) and the polymeric oxomolybdenum hybrid [Mo2O6(pyz)] (3) promoted the epoxidation of cis-cyclooctene, cyclododecene and bio-based olefins (fatty acid methyl esters and terpene substrates), and sulfoxidation reactions, giving products with interesting applications. In general, the reactions were highly selective to the epoxides (in the case of olefins) and sulfones (in the case of sulfides). Compounds 1-3 led to biphasic solid-liquid reaction mixtures. While the solid phase with 3 showed retention of the crystalline structure and could be recovered and reused without loss of activity, the reaction seemed to be partly homogeneously catalyzed by dissolved metal species. On the other hand, the tricarbonyl-pyrazine-molybdenum(0) compounds 1 and 2 were converted in situ to a polymeric oxomolybdenum catalyst which was more active than 3 and performed steadily in consecutive batch runs. The amorphous nature of the oxomolybdenum catalyst (derived from the MOFs) hampers its structural elucidation. The compound warrants, nevertheless, further study due to the convenience of 1 and 2 as catalyst precursors, especially the cubic phase 2 which can be readily prepared on a large scale. Diana M. Gomes: Methodology, Validation, Investigation, Writing - Original Draft. Andreia F. Silva: Methodology, Validation, Investigation, Writing - Original Draft. Ana C. Gomes: Validation, Investigation, Writing - Original Draft. Patrícia Neves: Project administration, Validation, Investigation, Writing - Original Draft. Anabela A. Valente: Resources, Conceptualization, Supervision, Writing - Review & Editing. Isabel S. Gonçalves: Resources, Conceptualization, Supervision, Writing - Review & Editing. Martyn Pillinger: Resources, Funding acquisition, Project administration, Visualization, Writing - Review & Editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020, financed by national funds through the FCT (Fundação para a Ciência e a Tecnologia)/MCTES (Ministério da Ciência, Tecnologia e Ensino Superior) (PIDDAC). We acknowledge support and funding provided within the CENTRO 2020 Regional Operational Program (project references CENTRO-01–0145-FEDER-028031 and PTDC/QUIQOR/28031/2017) and the COMPETE 2020 Operational Thematic Program for Competitiveness and Internationalization (POCI-01–0145-FEDER-030075), co-financed by national funds through the FCT/MEC (Ministério da Educação e Ciência) and the European Union through the European Regional Development Fund under the Portugal 2020 Partnership Agreement. D.M.G. (grant ref. 2021.04756.BD) acknowledges the FCT for a PhD grant (State Budget, European Social Fund (ESF) within the framework of PORTUGAL2020, namely through the Centro 2020 Regional Operational Program). A.C.G. thanks the FCT/MCTES for funding through the Individual Call to Scientific Employment Stimulus (CEECIND/02128/2017).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.cattod.2023.114050. Supplementary material .

The horizons of epoxidation and sulfoxidation processes may be expanded by developing new, efficient, and versatile catalysts. In the present work, three pyrazine-bridged molybdenum(0/VI)-based coordination network solids have been investigated for the epoxidation of olefins and the oxidation of sulfides. The materials studied were the Mo0-based metal-organic framework (MOF) fac-Mo(CO)3(pyz)3/2·1/2pyz (1) with a structure consisting of stacked fac-Mo(CO)3(pyz)3/2 coordination layers, the cubic phase fac-Mo(CO)3(pyz)3/2 (2) with a dense framework consisting of two interpenetrating coordination networks, and the molybdenum oxide-pyrazine hybrid material [Mo2O6(pyz)] (3) with a structure consisting of perovskite-like MoO3 layers pillared by pyz molecules. In the model reaction of cis-cyclooctene with tert-butyl hydroperoxide (TBHP) at 70 °C, quantitative yields of the epoxide were obtained within 2 h for 1, 4 h for 2, and 24 h for 3. Catalysts 1-3 were further examined for the epoxidation of other olefins, including the bio-olefins dl-limonene, methyl oleate and methyl linoleate, and the reaction scope was expanded to include the oxidation of sulfides. In the reactions of the bio-olefins, 3 was highly selective, giving only diepoxide and/or monoepoxide products. While the tricarbonyl-pyrazine-molybdenum(0) compounds displayed higher activity, by-products were obtained in the reactions of dl-limonene and methyl linoleate, namely limonene-1,2-diol and hydroxytetrahydrofuran cyclization products, respectively. Catalysts 1-3 displayed high activity for the selective oxidation of sulfides (methyl phenyl sulfide and diphenyl sulfide) to sulfones under mild conditions (35 °C).

The electrochemical oxygen evolution reaction (OER) is a key component of promising routes for clean energy, such as hydrogen production via water electrolysis, regenerative fuel cells, and electrochemical CO2 reduction (CO2RR) [1–6]. The OER, which involves four coupled electrons and protons, exhibits slow reaction kinetics and high overpotentials that limit the process efficiencies [7–9]. Thus, intensive efforts have been directed toward developing electrocatalysts that enhance the kinetics of the OER. Precious metals such as Ir and Ru are regarded as the best electrocatalysts for the OER in acidic media [10–13].Particularly, numerous studies have focused on Ru-based materials owing to Ru having a higher activity and lower price than those of Ir [14–17]. Electrochemically produced hydrous RuO x shows a remarkable OER catalytic activity because the abundant unsaturated Ru leads to the participation of large amounts of lattice oxygen in the reaction [18–20]. The oxygen participating through the lattice oxygen oxidation mechanism (LOM) significantly reduces the theoretical overpotential of the OER, leading to an enhanced catalytic activity [21,22]. However, hydrous RuO x dissolves to a significant extent in the acidic medium during the OER, resulting in a low stability [23–25]. To suppress the dissolution and increase the stability of Ru-based electrocatalysts, crystalline rutile RuO2 prepared by thermal treatment, which possesses stable lattice oxygen, has been proposed as an OER catalyst. Crystalline RuO2 shows a conventional adsorbate evolution mechanism (AEM) for the OER, which decreases the dissolution to provide a good stability, but leads to a lower catalytic activity than that of hydrous RuO x .A recently proposed strategy to improve the activity and stability simultaneously is to introduce another element that can significantly modify the electronic structure of the Ru oxide. Alloying with Ir is one of the typical approaches for this purpose [23,26–32]. Qiao’s group reported a nanocrystalline Ru@IrO x catalyst with an increased valence state of the Ir in the shell and a decreased valence state of the Ru in the core [27]. This charge redistribution in this structure enhanced the catalytic activity and stability in acidic conditions. Recent studies have reported that other elements, such as Pt, Y, Cr, Te, and Ni, can modify the electronic structure of Ru oxide catalysts adequately [18,33–36]. Lin et al. synthesized a rutile Cr0.6Ru0.4O2 catalyst with a higher Ru oxidation state than that of RuO2 and a superior OER activity and stability [37]. Density functional theory (DFT) calculations demonstrated that this electronic structure reduces the energy barriers for the formation of *OOH. Huang’s research group reported Ru-Ni oxide nanosheets with a downshifted d-band center electronic structure [38]. According to DFT calculations, a modified d-band electronic structure and transformed t2g and eg orbitals reduce the energy barriers for O2 formation. However, few studies have focused on the real behavior and role of the introduced elements in the reaction [39].Here, we investigated the mechanisms through which Ni introduced in Ru oxide enhances the catalytic activity for the OER. A nanosized Ru and Ni oxide (denoted as RuNiO x ) electrode was manufactured via a simple dip coating process modified with a capping agent followed by a thermal treatment. The RuNiO x electrode exhibited a better OER performance than that of a RuO x electrode and was stable for 100 h. The electronic structure of the RuNiO x electrode is close to that of a hydrous RuO x electrode, which possesses oxygen vacancies. The results of in-situ/operando X-ray absorption near-edge structure spectroscopy (XANES), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and on-line inductively coupled plasma mass spectrometry (ICP-MS) analyses demonstrated that the Ni distorts the Ru oxide structure at anodic potential, increasing the amount of oxygen vacancies. This phenomenon increases the participation of lattice oxygen, leading to an enhanced OER catalytic activity of the RuNiO x electrode.All chemicals were of analytical grade and were used without further purification. A commercial Ti foam was purchased from Alantum Corporation. KOH, Ni(NO3)2·6H2O, and polyvinylpyrrolidone (PVP) were purchased from Sigma-Aldrich. RuCl3 was obtained from Alfa Aesar. H2SO4 and ethanol were purchased from Samchun Chemical Co., ltd. (Korea). Deionized water was obtained using an Arium® Mini Ultrapure Water System (Sartorius AG).The RuNiO x electrode was prepared on a Ti foam via a dip coating procedure, which can be easily applied in industrial processes (Fig. S1). To increase the active surface area of the electrode, PVP was used as a surfactant. A piece of Ti foam (TF, 1 cm × 2 cm) was washed with ethanol and dried in nitrogen. Ni(NO3)2·6H2O (5 mmol), RuCl3 (5 mmol), and 50 mg of PVP were dissolved in 9 mL of ethanol and 1 mL of water. The TF was submerged in the solution and shaken for 20 s. The TF was then dried in a convection oven at 70 °C. The coated TF was calcined in air at 450 °C for 1 h; the sample was denoted as RuNiO x . A NiO x electrode was prepared using an identical procedure to that of the RuNiO x electrode, except that the RuCl3 was replaced with Ni(NO3)2·6H2O. A RuO x electrode was prepared with the same procedure, except that the Ni(NO3)2·6H2O was replaced with RuCl3. Finally, a RuO x -L electrode was prepared via the same procedure, but without the PVP.The electrochemical tests were performed in a standard three-electrode system with a potentiostat (VSP, BioLogic) using an Ag/AgCl (3.5 M KCl) electrode and a graphite electrode as the reference and counter electrodes, respectively. All potentials were calibrated to the reversible hydrogen electrode (RHE) scale. Electrochemical impedance spectroscopy (EIS) measurements were conducted at 1.5 V vs. RHE in the frequency range from 1 Hz to 100 kHz.The morphologies of the prepared electrodes were studied via high-resolution scanning electron microscopy (HR-SEM, Regulus 8230, Hitachi). X-ray diffraction (XRD) measurements were conducted using an Empyrean diffractometer (Malvern Panalytical) equipped with a Cu K-alpha radiation source. X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, ULVAC-PHI, Inc.) was used to investigate the chemical states in the electrodes. To analyze the electronic structures, synchrotron-based NEXAFS measurements were conducted at the 10D beamline of the Pohang Accelerator Laboratory (PAL, Pohang, South Korea). X-ray absorption spectroscopy (XAS) analyses were performed at the 1D beamline of the PAL.In situ/operando XAS analyses were conducted using a homemade electrochemical flow cell. The catalyst-coated TF electrodes were placed on a thin carbon-coated Kapton film surface and contacted with a 0.05 M H2SO4 electrolyte. The graphite and Ag/AgCl electrodes were installed on the back side of the flow cell. The set-up details were reported in a previous paper [40].On-line ICP-MS analyses were performed using the electrochemical flow cell. The dissolution was quantified by mixing 1 ppm Re in 1 M HNO3 as an internal standard. The electrode area was 0.1 cm2 for the on-line ICP-MS measurement. The set-up details were reported in a previous paper [40]. The working electrodes were the prepared Ru and Ni electrodes, and a 0.05 M H2SO4 solution was used as an electrolyte. The Ag/AgCl electrode and a Pt wire were used as the reference and counter electrodes, respectively.A membrane electrode assembly (MEA; active area: 10 cm2) was used as the electrochemical cell for the CO2RR tests. An Ag black (APS 20–40 nm, 99.9% in metal basis, Alfa Aesar) pasted gas diffusion layer electrode was used as the cathode, and the prepared RuNiO x and RuO x electrodes and a commercial IrO2 electrode were used as anodes. The membrane (Sustainion® X37-50 Grade RT, Dioxide Materials) was pretreated in a 1 M KOH solution for 48 h and washed with deionized water several times. The MEA was then placed in a homemade CO2 electrolyzer single cell with pin-type and serpentine flow channels on the anode and cathode sides, respectively. Humidified CO2 heated to 90 °C with a heating mantle was used as the reactant gas; the flow rate was 200 cm3 and the gas was fed to the cathode side. A 0.1 M KHCO3 solution was flowed through the anode side using a pump operating at 10 r min−1. The electrochemical single-cell tests were performed using a potentiostat (BioLogic, VSP, VMP3B-10), which can measure up to 10 A at room temperature. The outlet gas was analyzed via gas chromatography (GC, Agilent 7890A). The GC inlet was connected to the cathode through a water trap. Argon (99.999%) was used as the carrier gas. Hydrogen was detected using a thermal conductivity detector, and carbon compounds, such as CO and CH4, were detected using a flame ionization detector (FID). To enhance the detection of CO, a methanizer was installed before the FID. The partial current densities of the products were determined using the total measured current and the volume of each product calculated from the GC peaks.RuNiO x electrode was prepared on Ti foam through dip coating process, which is easily used in industrial process (Fig. S1). The Ti foam substrate was immersed and withdrawn in the Ru and Ni salt contained solution and then calcined in air at 450 °C for 1 h to make stable lattice oxygen. In order to increase catalytic active surface area of electrode, PVP as a surfactant was added in the solution for dip coating. Fig. 1 (a) shows SEM images of the RuNiO x electrode. The particles on the electrode surface have diameters of approximately 10–20 nm and are uniformly distributed on the Ti foam substrate. Similar morphologies were observed for the RuO x and NiO x electrodes (Figs. S2–S4). Without PVP in solution, the achieved Ru electrode (denoted as RuO x -L) had a larger grain size than those of the other electrodes, indicating that the added PVP successively decreases the grain size in the dip coating process (Fig. 1b and Fig. S5). Because of the thin RuNiO x layer, the XRD pattern of the RuNiO x electrode only exhibits a peak corresponding to the Ti foam substrate. Thus, we manufactured carbon-supported RuNiO x , RuO x , and NiO x (RuNiO x /C, NiO x /C, and NiO x /C, respectively) powders via similar synthesis processes to those of the electrodes, but using carbon black (Ketjenblack 300 J, Nouryon) instead of the Ti foam. As shown in Fig. S6, the XRD pattern of NiO x /C is consistent with that of NiO, and RuO x /C shows peaks corresponding to RuO2 and RuClO2. These results indicate that the crystalline oxides formed well during the thermal treatment. RuNiO x /C exhibited a multiphase crystalline structure, which matched with the NiO, RuO2, and RuClO2 peaks. The peaks of RuO x /C were shifted toward higher 2θ values with respect to those of RuO2 and RuClO2, suggesting that Ni was incorporated into the Ru oxide lattice.The electronic structure of manufactured electrodes surface was analyzed by X-ray photoelectron spectroscopy (XPS) and NEXAFS. The Ru 3d XPS peaks of the RuO x and RuO x -L electrodes located at 280.6 eV (Fig. 1c) were attributed to rutile RuO2, which is in good agreement with the XRD results [34]. The Ru 3d peak of the RuNiO x electrode was shifted toward higher binding energies by 0.2 eV with respect to those of the RuO x and RuO x -L electrodes; this is a similar phenomenon to that of hydrous RuO x and in agreement with previous research. This suggests that the incorporated Ni successfully modified the electronic structure of the Ru surface. The Ni 2p XPS spectrum in Fig. 1(d) shows a peak for the RuNiO x electrode at 855.3 eV, which corresponds to the Ni2+ oxidation state of NiO and is identical to that of the NiO x electrode. The NEXAFS spectrum of Ni also shows similar electronic structures for the NiO x and RuNiO x electrodes (Fig. S7). In the O 1s XPS spectrum (Fig. 1e), the strong peaks of the RuO x and NiO x electrodes at 529.3 eV were attributed to lattice oxygen. For the RuNiO x electrode, a peak at 530.8 eV attributed to oxygen vacancies was also observed [41]. This suggests that the RuNiO x electrode possesses a larger number of oxygen vacancies than those of the RuO x and NiO x electrodes [42]. To obtain detailed information about the t2g and eg orbitals, an O 1s NEXAFS study was conducted (Fig. S8). The RuNiO x electrode has a similar O 1s NEXAFS spectrum to those of the RuO x and NiO x electrodes, which indicates that the introduction of Ni had no significant effects on the t2g and eg orbitals. The electronic structure of the synthesized electrodes was further characterized via Ru and Ni K-edge XANES, which is a bulk-sensitive technique and is based on the electron transitions from the 1s orbital to unoccupied 4p orbitals. The Ru K-edge spectra (Fig. 1f) of the RuO x and RuNiO x electrodes show similar peak shapes, in contrast with the XPS results. This indicates that the RuO x and RuNiO x electrodes have similar Ru electronic structures to that of rutile RuO2, but the surface Ru in the RuNiO x electrode has an electronic structure similar to that of hydrous RuO x . The Ni K-edge spectra of the NiO x and RuNiO x electrodes also show identical peak shapes, which is in agreement with the XPS results (Fig. S9). This suggests that the bimetallic oxide of RuNiO x hardly affects the electronic structure of the entire electrode.Energy-dispersive X-ray spectroscopy (EDS) elemental mapping coupled to transmission electron microscopy (TEM) and SEM was used to determine the elemental distribution and composition of the RuNiO x electrode (Fig. 1g and Fig. S10). For convenience, the RuNiO x /C material used in the XRD analysis was used in the TEM analysis as well. Ru and almost Ni are homogenously distributed on the carbon support. According to the EDS elemental analysis of the RuNiO x electrode, the ratio of O to Cl is 96.7:3.3, indicating that most of the RuNiO x electrode has a RuO x structure.The electrocatalytic OER performances of the synthesized electrodes were evaluated in a standard three-electrode system. Fig. 2 (a) shows the linear sweep voltammetry (LSV) results, which were obtained with a 0.05 M H2SO4 electrolyte saturated with O2. The RuNiO x electrode required an overpotential of 217 mV to achieve 10 mA cm−2, a much lower value than those of the RuO x (248 mV) and RuO x -L electrodes (286 mV). Also, it is higher than that of RuNiO x -L (262.3 mV) as shown in Fig. S11(a). These results are in agreement with previous research and reveal that the introduction of Ni enhances the OER catalytic activity of Ru [38]. To determine an optimal Ru:Ni ratio, LSV curves were obtained for RuNiO x electrodes with different Ni ratios (Fig. S12). The optimal Ru:Ni ratio was determined to be Ru1Ni1. The electrocatalytic activity of the RuO x electrode is better than that of the RuO x -L electrode owing to the small particle size, which indicates that the addition of PVP during the dip coating process successfully enhanced the OER performance by increasing the electrochemical surface area. The NiO x electrode showed a deficient OER performance, confirming the poor intrinsic activity of the Ni oxide. The excellent OER performance of the RuNiO x electrode was further confirmed via the analysis of the Tafel plots, as shown in Fig. 2(b) and Fig. S11(b). The Tafel slope of the RuNiO x electrode (56 mV dec−1) is lower than those of the RuO x (70 mV dec−1), RuO x -L (79 mV dec−1), RuNiO x -L (75 mV dec−1) and NiO x (307 mV dec−1) electrodes. This indicates that the catalytic reaction kinetics were enhanced by the introduction of Ni. A list of the OER catalysts is compared in Table S1 to address the large improvement obtained in this work. Also, we carried out CV with multiple scan rates in the potential range of 0.8 to 0.9 VRHE to obtain the ECSA of the as-prepared catalysts (Fig. S13). The calculated double layer capacitance (C dl) can be used to represent the ECSA because the ECSA is directly proportional to C dl. It is noteworthy that the C dl value of RuNiO x (268.24 mF cm−2) is higher than that of RuO x (232.58 mF cm−2), RuNiO x -L (208.12 mF cm−2) and RuO x -L (134.81 mF cm−2). These results demonstrate that RuNiO x could have large active surface area results in higher OER activities. To evaluate the stability of the RuNiO x electrode, a chronopotentiometric experiment was conducted at 10 mA cm−2. The RuNiO x electrode maintained the same performance for 100 h, confirming its good stability in acidic media (Fig. 2c).To investigate the effects of Ni introduction, comparison of XPS before and after OER was performed. In Ru 3d XPS spectrum, the positive shift of binding energy for Ru electrode after OER (A.O) (280.7 eV) can be ascribed to altered, close to hydrous RuO x (280.8 eV) (Fig. 2d). Meanwhile, RuNiO x maintains their electronic structure (280.8 eV). The XPS Ni 2p spectra (Fig. S14) show that the RuNiO x and NiO x electrodes maintain their electronic structures after the OER tests. The changes in the oxygen vacancies after the OER can be studied from the XPS O 1s spectra (Fig. 2e). For Ru-based OER catalysts, the dissolution of Ru has the effect on long-term durability. As the OER potential increases, the dissolution rate increase. Since the RuNiO x catalyst has greatly improved OER activity, the applied overpotential to the anode during the OER stability test was very small. Therefore, since the dissolution rate was very small, it is possible to maintain the performance even for a long-time operation. The surface of all manufactured electrode reveals increased peaks for oxygen vacancy and hydroxide, which is the representative property of hydrous RuO x and lattice oxygen participation. It is clear that relative peak intensity assigned oxygen vacancy and Ru XPS peak position of RuNiO x electrode after OER is higher than that of RuO x electrode. This would be simple reason of enhanced performance by Ni introduction. However, one thing to note is that the chemical state of RuO x electrode surface is also converted to that of hydrous-RuO x after OER. Thus, it would be a bit insufficient to explain the effects of Ni introduction.To confirm the applicability under neutral conditions, the performance of the RuNiO x electrode as an anode for the electrochemical CO2RR was evaluated. In neutral media, the trends are similar to those in acidic conditions, as show in Fig. 2(f) and Fig. S15. The RuNiO x electrode could reach 10 mA cm−2 of current density at an overpotential of only 386 mV, which is a lower value than those of the RuO x (440 mV) and RuO x -L (480 mV) electrodes. The electrochemical CO2RR performance was evaluated using a homemade zero-gap CO2 electrolyzer and gaseous CO2 to accelerate the reduction reaction while minimizing the mass transfer resistance (Fig. 2g). An Ag electrode and a RuNiO x electrode were used as the cathode and anode, respectively, and 0.1 M KHCO3 was used as the electrolyte. For comparison, a commercial IrO2 electrode and the synthesized RuO x -L electrode were evaluated under same conditions. The cell voltage of RuNiO x electrode is lower than other electrodes, demonstrating high OER catalytic activity under CO2RR condition (Figs. S16 and S17). Furthermore, the RuNiO x electrode exhibited a CO selectivity higher than 90% for all values of the applied current density, which represents a similar activity with commercial IrO2 electrode. By contrast, for the RuO x -L electrode, as the applied current density increased, the CO selectivity decreased and the H2 selectivity increased. The high cell voltages of RuO x -L electrode accelerate the dissolution of Ru, leading Ru metal ion crossover to cathode. This can be attributed to the low CO partial current density and poor stability of the RuO x -L electrode under neutral conditions. In the case of RuNiO x -L, the CO selectivity maintains lower than that of RuNiO x as shown in Fig. S18.To gain further insights into the role of Ni in the enhancement of the OER activity, in-situ/operando XANES and on-line ICP-MS analyses were conducted in acidic conditions. The Ni K-edge XANES spectra of the NiO x and RuNiO x electrodes obtained at different applied potentials are shown in Fig. 3 (a–c). The XANES peaks of the NiO x electrode at potentials in the range of the open circuit voltage (OCV) to 1.63 V are similar, indicating that the Ni2+ oxidation state was maintained during the OER (Fig. 3a). However, the pre-edge shape, which represents the coordination geometry, changes with the applied potential [43]. The pre-edge peak of the NiO x electrode appeared at a potential of 1.43 V and shifted toward lower photon energies as the potential increased to 1.63 V. This suggests that the Ni oxide structure was distorted for applied potentials above 1.43 V. For the RuNiO x electrode, the oxidation state of Ni was also maintained when the applied potential changed (Fig. 3b). However, the pre-edge of the RuNiO x electrode shows significant differences with respect to that of the NiO x electrode. A pre-edge peak appeared at the OCV and shifted toward lower photon energies as the applied potential increased. The detailed comparison of the NiO x and RuNiO x electrodes at 1.23 and 1.63 V is shown in Fig. 3(c). Both electrodes retained an oxidation state of Ni which is consistent with Ni2+, but the pre-edge peak of the RuNiO x electrode appeared at a lower potential and was located at lower photon energies than that of the NiO x electrode. These results indicate that the RuNiO x electrode presented distortion at lower potentials and to a greater extent than the NiO x electrode.The changes in the Ru electronic structure during the OER were investigated via in-situ/operando Ru-K edge XANES at potentials in the range of the OCV to 1.63 V. The spectrum for the RuO x electrode maintains its features (which resemble those of RuO2) but shows a slight increase in the white-line intensity at potentials above 1.43 V (Fig. 3d). It has been reported that an increased white-line intensity indicates the presence of hydrous RuO x with a disordered structure and high amounts of structural water [44]. Thus, the results suggest that a transition of the RuO x electrode from rutile RuO2 to hydrous RuO x occurred at a potential of approximately 1.43 V. The Ru XANES spectrum of the RuNiO x electrode shows an increased white line intensity at the OCV and maintains its features as the potential increases up to 1.63 V (Fig. 3e). These tendencies are also confirmed at EXAFS spectra, as shown in the Fig. S19. The hydrous RuO x peak with 2.1–2.25 nm appeared at 1.43 and 1.63 V for RuO x and RuNiO x , respectively. Especially, the Ru–O bonding length of RuNiO x decreased during structure distortion. These results demonstrate that the RuNiO x electrode has the electronic structure of hydrous RuO x at lower potentials than the RuO x electrode does (Fig. 3f). In summary, the distorted Ni and modified RuO x formed at the active sites of RuNiO x changed the initial electronic distribution and coordination environment of the RuNiO x , and hence could enhance its OER performance [45–47].In order to probe the structure transition, on-line ICP-MS study was performed with changing applied potential in acid condition. On-line ICP-MS is a powerful technique to measure real-time electrochemical dissolution which is caused by direct dissolution, structure transition and oxygen lattice participation for OER [48,49]. For the RuO x electrode, the dissolution began at about 1.3 V and increased in extent as the potential was increased (Fig. 3g and h). This indicates the formation of hydrous RuO x at potentials above 1.3 V and the occurrence of oxide-mediated dissolution, which is an indication of lattice oxygen participation in the OER. The dissolution peaks associated with lattice oxygen participation for the RuNiO x electrode at the highest potential are significantly larger than those for the RuO x electrode, indicating a large extent of lattice oxygen participation. Small dissolution peaks were observed for the RuNiO x electrode near 1.0 V, which correspond to the structure transition. Ni dissolution exhibited similar trends to those of Ru dissolution. The NiO x electrode has dissolution peaks only at potentials above 1.3 V, indicating that the transition and direct dissolution began above 1.3 V. For the RuNiO x electrode, after the first cycle, the dissolution peak at the highest potential was reduced, and other dissolution peaks appeared near 1.0 V. This reveals that the direct dissolution was inhibited, and the structural transition occurred at around 1.0 V. These results are consistent with the in-situ/operando XANES results.Based on the ex-situ characterization and in-situ/operando studies, we propose the role of Ni in the enhancement of the OER activity of Ru oxides as shown in Fig. 4 . Before explaining the role of Ni, the behaviors of the RuO x and NiO x electrodes with different applied potentials should be clarified. The NiO x electrode retained its structure until the potential reached 1.23 V. Above 1.23 V, the structure was distorted, leading to asymmetry. The RuO x electrode also maintained its electronic structure until the potential reached 1.23 V, and above said potential, the surface of the rutile RuO2 was converted to hydrous RuO x , which enabled the participation of lattice oxygen in the OER. When Ni was introduced in the RuO2 structure, the Ru oxide lattice distance was slightly reduced due to the short Ni–O distance and the mismatch of the RuO2 and NiO structures, which led to abundant oxygen vacancies. The structure of the RuNiO x electrode was already distorted at the OCV, and it was distorted further as the applied potential increased, generating abundant oxygen vacancies. It is proposed that oxygen vacancies lead to large structural distortion at low potentials. This phenomenon also enables the penetration of water and ions into the RuNiO x structure, modifying the electronic structure further and causing the transition to hydrous RuO x , which enhances the participation of lattice oxygen in the OER. According to theoretical studies, the distorted RuNiO x structure with an eg-dz 2 misalignment can minimize the energy barrier for the OER [38], and the LOM provides a unique local configuration by modifying the metal–oxygen hybridization [21,22]. These changes in the electronic structure and the LOM pathway significantly reduce the energy barriers for each intermediate in the OER. Thus, the introduced Ni not only modified the initial electronic structure of the Ru oxide (which was observed via XPS), but also created a large number of oxygen vacancies at low overpotentials by distorting the lattice oxygen structure. This phenomenon of the RuNiO x electrode can accelerate the conversion of the OER mechanism from AEM to LOM, enhancing the catalytic activity. On the basis of results, we summarized OER mechanisms of RuO x based on previously proposed mechanism in Fig. S20 [22]. Fig. S20(a) showed the AEM of RuO x . The reaction can happen either on Ru site via an OOH intermediate. However, LOM differs by participation of activated lattice oxygen in the reaction as shown in Fig. S20(b). Water attacked the activated oxygen and removed as O2 from the surface leaving behind an oxygen vacancy.In conclusion, we unveiled the role of introduced Ni in Ru oxide which enhanced catalytic activity of OER via in-situ/operando technique. A nanosized RuNiO x electrode was simply fabricated via a modified dip coating process. The electrode demonstrated a remarkable OER performance (an overpotential of 210 mV for 10 mA cm−2) and good stability in acid media (100 h). The pristine RuNiO x electrode possesses an electronic structure close to that of hydrous RuO x , with a larger number of oxygen vacancies than that of the prepared RuO x electrode. According to the in-situ/operando XANES and ICP analyses, the electronic structure of the RuO x electrode became similar to that of hydrous RuO x at potentials above 1.43 V. The introduction of Ni distorted the structure of the oxygen lattice, creating abundant oxygen vacancies and changing the electronic structure to that of hydrous RuO x at a low potential. This increased the lattice oxygen participation and changed the OER mechanism of the RuNiO x electrode from AEM to LOM at a lower potential than that of the RuO x electrode. These findings provide information on the behavior of bimetallic oxide materials during the reaction and constitute fundamental insights into the development of efficient electrocatalysts.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by institutional program grants from the Korea Institute of Science and Technology and Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20224C10300020) and “Carbon to X Project” (2020M3H7A1098229) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT, Republic of Korea. This research was also supported by the National Research Council of Science & Technology (NST) grant by the Korean government (MSIT) (No. CAP21011-100) and National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (NRF-2021R1A2C2093467). We also acknowledge Advanced Analysis Center at KIST for the TEM and 1D XRS KIST-PAL beamline for measuring the hard X-ray absorption spectroscopy (XAS).Supplementary data to this article can be found online at https://doi.org/10.1016/j.jechem.2022.09.032.The following are the Supplementary data to this article: Supplementary data 1

Introducing Ni in Ru oxide is a promising approach to enhance the catalytic activity for the oxygen evolution reaction (OER). However, the role of Ni (which has a poor intrinsic activity) is not fully understood. Here, a RuNiO x electrode fabricated via a modified dip coating method exhibited excellent OER performance in acidic media, and neutral media for CO2 reduction reaction. We combined in-situ/operando X-ray absorption near-edge structure and on-line inductively coupled plasma mass spectrometry studies to unveil the role of the Ni introduced in the Ru oxide. We propose that the Ni not only transforms the electronic structure of the Ru oxide, but also produces a large number of oxygen vacancies by distorting the oxygen lattice structure at low overpotentials, increasing the participation of lattice oxygen for OER. This work demonstrates the real behavior of bimetallic oxide materials under applied potentials and provides new insights into the development of efficient electrocatalysts.

Carbon nanostructures show promise in fuel cell applications as a catalyst support. Much research is still being conducted in order to enhance the catalytic activity and selectivity and reduce the cost of catalyst preparation [1]. There are currently three main synthesis methods used to produce graphitic nanofibres GNFs and/or carbon nanotubes CNTs. The methods are electric arc discharge (EAD) [2], laser ablation technique (LA) and chemical vapour deposition technique CVD [3,4]. Among the techniques stated, CVD is a common method used to produce GNFs and CNTs providing a good carbon yields and low amounts of unwanted soot.The porous nature of amorphous silica provides nucleation sites for catalyst precursor precipitation and hinders subsequent grain growth and sintering. This will lead to the formation of small catalyst particles for carbon growth. The small and uniform size of catalyst particles will lead to the formation of uniform size synthesised CNS [8]. The catalyst particle size is an important factor in carbon nanostructures growth where carbon filament growth rate increases with decreasing catalyst particle size [1,5]. It is reported that by using pores silica [6,7] and pores alumina [8] as a support for a metal catalyst, high quality of CNTs can be grown. It has also been reported that synthesising CNTs using a silica support is very attractive due to uniform pores size of silica, good and homogenous CNTs between 1 and 6 nm diameter were successfully synthesised at 1000 °C [9].Even though unsupported NiO catalyst precursor can give high deposition yields of 57 g g-1 catalyst h−1 of deposited carbon, but the type of graphitic nanostructures formed is a mixture of platelet and herringbone with the diameter of the fibres ranging from 20 to 500 nm, the selectivity for the formation of graphitic structures and the fibres diameter is hard to control [4]. However, the unsupported Fe2O3 catalyst precursor, has much better specific selectivity for the graphitic structure formed, but the fibre diameters are range from 50 to 200 nm but the yield of deposited carbon is also very low, 2.25 g g-1 catalyst h−1 [3]. The hypothesis behind the current study is that a Ni, Fe bimetallic catalyst will have a high yield, high selectivity and if supported on a porous silica, a well-defined size distribution for the deposited carbon.The aim for this study is to investigate the effect of the CVD variables of reaction temperature and gas composition on the nanostructured carbon produced by using a supported NiFe2O4 spinel as a catalyst precursor. The controlled factors for the synthesis are temperature and different ratios of reaction gases, C2H4 and H2 which reported play a vital role in nanostructures formed [8,10].A high purity grade porous silica, 200–500 µm particle size with a pore size of 60 Å, (Sigma Aldrich) was impregnated with a nickel nitrate hexahydrate Ni(NO3)2·6H2O (puriss grade, Sigma Aldrich), iron (III) nitrate nonahydrate Fe (NO3)3·9H2O (ACS reagent, Sigma Aldrich) solution by incipient wetness technique [7,11]. These solutions were then added into weighed porous silica powder to produce a final 5 wt% Ni0.36Fe0.64 reduced metal catalyst in the silica support. The heating process carried out at 130 °C evaporated off the water and left behind iron-nickel nitrate impregnated in the pore sites of the silica. (1) 2 F e ( N O 3 ) 3 . 9 H 2 O l + N i ( N O 3 ) 2 . 6 H 2 O l → Δ 130 ° C N i F e 2 ( N O 3 ) 8 s + 15 H 2 O g The calcination at 400 °C for 4 h caused the iron-nickel nitrate to thermally decompose to iron-nickel oxide (also known as spinel structures catalyst precursor). (2) NiF e 2 ( N O 3 ) 8 s → Δ 400 ° C N i F e 2 O 4 s + 8 N O 2 ( g ) + 2 O 2 g Prior to the reaction, the iron-nickel oxide is reduced to Ni0.36 Fe0.64 metal under the hydrogen flow. (3) NiF e 2 O 4 s + 4 H 2 g → Δ 400 ° C N i F e 2 s + 4 H 2 O ( g ) The catalyst produced analysed using XRD (Bruker D8 Advance) at every stage to ensure formation of NiFe2O4 spinel and Ni0.36 Fe0.64 metal.The growth of carbon nanostructures was carried out at atmospheric pressure in a fixed bed reactor (quartz-tube 5.0 cm diameter and 150 cm length) located in a vertical oven with C2H4 (Research Grade N3.2, BOC) as a carbon source. The feedstock gases H2 and C2H4 were controlled using mass flow controllers (MKS GE250A) providing a mixture of H2/C2H4 of 20/80, 50/50 and 80/20 maintaining a total flow of 100 sccm (standard cubic centimeters per minute). During the initial and the intermediate stages of CVD, the chamber is placed in a vacuum stated by pumping out the chamber using commercial vacuum pump. This is important to ensure the chamber is free from any residual gas that may affect the reaction. Prior to the reaction, the catalyst precursor was first reduced in 10% of H2 for 4 h. The reactions time were kept constant at 2 h. The temperature and gas composition were controlled as outlined in Fig. 1 .All reactions were repeated three times to present reproduced data and minimise the calculation error. For the yield calculation, the percentage of the carbon deposition is defined as follows: (4) yield ( % ) = 100 × weight o f c a r b o n d e p o s i t e d o n t h e c a t a l y s t weight o f c a t a l y s t The output gases and the catalyst activity were monitored using mass spectrometer (MS) and analysed by MASsoft 7 software (Hiden analytical). Scanning electron microscopy (SEM) FEG − ESEM Philips XL − 30, and transmission electron microscopy (TEM) JEOL 2000fx, equipped with selected area electron diffraction (SAED) were used to characterise the GNFs and CNS produced.The samples produced from the CVD were then underwent acid treatments to remove the silica support and the catalyst precursor. First, the samples were treated in 20% concentration of hydrofluoric acid (HF) in three 30 min periods to dissolve the silica. It was then filtered and washed using excess distilled water before further purification in 12 M nitric acid, stirred for 18 h at room temperature before filtered and washed using excess distilled water. Finally, the samples were left to dry in a drying cupboard at 80 °C overnight.Impregnated silica with iron, nickel nitrates calcined at 400 °C for 4 h, showed a weak diffraction pattern due to NiFe2O4 at 2θ values of 37° (311) and 63° (440) as in Fig. 2 . However due to the high relative intensity of silica support, these NiFe2O4 peaks were not clearly observed. It is also believed, for all the catalyst precursor preparations, that due to the slow drying process, these metal precursors formed clusters close to the entrance of the pores in the silica as reported by Bond et al. [12]. Then a heat treatment applied at 400 °C formed a cap of catalyst particle over a pores site of silica preventing the diffraction from the catalyst precursors in the pores.Prior to the CVD reaction, the catalyst precursor underwent a reduction in 10% H2 (in 90 sccm Ar) to form iron-nickel alloy. In the ‘after H2 reduction’ pattern, it was clearly shown that the NiFe2O4 phase had disappeared and a new pattern for iron-nickel alloy was observed at 2θ values of 45° (111) and 52° (200). Fig. 3 shows the XRD profiles for the samples at the different stages of the experiment. The ‘after CVD’ profile did not show the present of any carbon peak in the sample due to the small yield of CNS diffraction overshadowed by the silica diffraction. However, the XRD profile for ‘after acid treatments’ samples showed the high relative intensity of carbon peaks at 2θ values of 26° (002) and 42° (111). The very low intensity of the iron-nickel which is still observed due to the metal catalyst particles trapped inside the carbon nanostructures thus preventing dissolution by the acids. The catalyst particles trapped inside the CNS structures is a common scenario especially when growing MWCNTs similar to previous work proposed [13–16]. Fig. 4 shows that for the silica supported NiFe2O4 catalyst precursor, the carbon deposition increased with the increase of the hydrogen content from 20% to 50%. The maximum yield obtained was around 762%–898% mc/mcat observed at a H2 composition of 50%. However, further increase in the hydrogen content to 80% seemed to reduce the carbon deposition, due to the decreasing carbon source for deposition dropping the yield to 399%–564% mc/mcat. The reduction in yield was found to be the case for all the reaction temperatures studied. Furthermore, the amount of carbon deposition at the temperature of 400 °C, 500 °C, 600 °C and 700 °C was within a similar range irrespective of the gas mixture.The MS plots for the silica supported NiFe2O4 during reduction under hydrogen (10% H2/90% Ar) are given in Fig. 5 . The temperature was increased to 400 °C (taking ca. 40 min), upon reaching this temperature the hydrogen was introduced at a flow of 10 sccm and the Ar was reduced to 90 sccm making 10% H2/90% Ar. No hydrogen partial pressure detected after being introduced and an increase in H2O partial pressure suggested the onset of reduction of NiFe2O4. This can be seen from the production of water (H2O with m/z 18) an increased partial pressure of H2O after the introduction of hydrogen. The partial pressure of hydrogen was at a minimum level during the first 45 min of introduction before it started to increase. The decreased formation of H2O after two hours of the introduction of hydrogen suggested the entire oxide precursor was reduced to iron-oxide metal. Fig. 6 (a–d) shows the MS plot for the samples synthesised using hydrogen reduced silica supported Ni0.36Fe0.64 catalyst during the CVD reaction under different reactant gas compositions. These reactions with different reaction temperature and gas composition were selected to show the catalytic activity of NiFe2O4 throughout carbon deposition analysis. Fig. 6 (a) shows minimal activity of catalyst and low partial pressure of CH4 and C2H6 which later lead to low carbon deposited during synthesis. Fig. 6 (b) shows the MS plot for the reaction at 700 °C with 20% H2/80% C2H4. With the increase of the reaction time, an increase of CH4 and C2H6 were detected. It was found that the dehydrogenations of C2H4 to CH4 and hydrogenation to form C2H6 were maintained throughout the synthesis period. However, by increasing the hydrogen to 50%, the catalytic activity was observed to be increased, as shown in Fig. 6 (c). This can be seen from the steady production of the CH4 and C2H6 throughout the 2 h reaction and until the reaction was stopped. The increase in the hydrogen content assist the hydrogenation process, forming C2H6 and keeping the surface of the catalyst clean for further reactions to occur.The MS plot for the further increase of hydrogen content to 80% is shown in Fig. 6 (d). The dynamic activity of the catalyst can be seen from the maintained CH4 partial pressure and consumption of C2H4 throughout the reaction period, but there was a lower partial pressure of C2H6 and CH4 which lead to a low yield of CNS formation.From the mass spectrometry, the dehydrogenation of C2H4 was observed by the decrease of C2H4 partial pressure and the formation of H2 was found to occur immediately after H2 and C2H4 were introduced into the CVD furnace. This can be observed by the decrease in the partial pressure of C2H4 and the increase of partial pressure of H2 occurring immediately after the reactants gases were introduced.The result of C2H4 decomposition is the production of carbon atoms at the surface of the catalyst particles. The H2 molecules released during this dehydrogenation may reduce the oxide metal catalyst precursor or may interact with the free C2H4 to form higher density hydrocarbons as in agreement with previous experimental works [10,17]. The carbon atoms produced during the C2H4 dehydrogenation will then diffuse onto the metal catalyst surface. The C2H4 dehydrogenation is believed to produce a carbon atom at the surface of the metal catalyst.The diffusion of C atoms into the catalyst particles follows on from the dehydrogenation of C2H4. With the solubility of C atoms into the catalyst surface, it is expected that catalyst particles attain a highly mobile or quasi-liquid state at the reaction temperature for CNS growth. The formation of the metal solid (NixFey-C) then resulted in lowering the melting point of the catalyst particle, which also softened the catalyst particle [18]. The increase in the reaction temperature will continue softening the catalyst particles and will encourage the carbon solubility as compared to the initial solid state of catalyst particles. This can be seen from the higher CNS yield produced as the reaction temperature increased from 400 °C to 700 °C as shown in Fig. 4.After the formation of the quasi-liquid phase, the C atoms become free to diffuse through into the catalyst particles at the area with high carbon concentration. The carbon precipitation stage follows on after the C atom was found to be adsorbed and diffused into the catalyst particle. The precipitation and propagation of the carbon were found to be coinciding with the formation of hydrocarbon gases, e.g C2H6, C3H6 and C3H8 [5,19]. The continuous precipitation and propagation of graphite is determined by the dehydrogenation and hydrogenation on the catalyst surface and the rate of carbon diffusion into the catalyst particles. The precipitation and propagation were also influenced by the reaction conditions: reaction temperature and reactant gas compositions. There was no further analysis such as in situ TEM to determine the exact precipitation and propagation rate or carbon formation on the catalyst. However, from the final carbon yield production during CVD process in Fig. 4, it is possible to study the pattern of optimum reaction conditions for carbon growth. It was found that for silica supported NiFe2O4, the reaction temperatures investigated 400 °C–700 °C and the composition of 50% H2/50% C2H4 as evidenced by the high partial pressure of CH4 and C2H6 produced during the reaction which later gave the maximum carbon produced, hence the maximum rates of carbon precipitation occurred. the reaction of 50/50 ethene/hydrogen was observed to give the highest yield, which suggests that there is a balance between the diffusion of the carbon atom through the catalyst particle leading to the precipitation and propagation of MWCNTs as well as the flux of carbon onto the surface hydrogenation of the carbon onto the catalyst surface to keep the catalyst face clean, allowing sites to remain free for a prolonged reaction.From the MS data, the catalyst deactivation can be detected by an increase of the C2H4 and increased of H2 partial pressure in Fig. 6. The fast deactivation of the catalyst was observed for the reaction using silica supported NiFe2O4 catalyst precursor using 20% H2/80% C2H4, shown a lowest CNS yield production. Further investigation of the MS plot found that the dehydrogenation of C2H4 happened at a very low intensity of hydrogen applied in the reaction. This rapid catalyst deactivation resulted in formation of a graphite layer surrounding the catalyst particles which stopped any further reaction from occurring. The catalyst activity was high at the beginning of the introduction of reactant gases, but later on the reactivity dropped. The trend of the deactivation process found in this study agreed with many other studies that have reported this phenomenon of a reduction in conversion of C2H4 (dehydrogenation) to carbon and H2 or a slow propagation of CNS after a prolonged period [5,10,20,21]. Fig. 7 (a) shows the micrograph of un-impregnated silica support. The CNS was successfully synthesised using silica supported NiFe2O4, Fig. 7(b) showing the growing CNS. More details in morphology of the nanostructures were represented in Fig. 7 (c, d). Given that the limitations of magnification in SEM analysis, details studied on the structures and morphology of the CNS were analysed using TEM. The structures present at the different reaction conditions are represented in the Fig. 8 .The TEM micrographs in Fig. 8 show nanostructures of MWCNT carbon observed when synthesised at 400 °C to 700 °C. At 700 °C reaction temperature, MWCNTs with encapsulation, Fig. 8(a) were obtained at 20% hydrogen composition. By increasing the hydrogen content to 50% and 80%, only the MWCNTs were observed to be found, Fig. 8(b, c). At the lower reaction temperature, 600 °C, disordered CNS with encapsulation, Fig. 8(d) were observed, whereas disordered, Fig. 8(e) and disordered CNS with hollow herringbone GNFs, Fig. 8(f) were observed when the hydrogen content was increased to 50% and 80% respectively. For the CVD reaction at 500 °C, the encapsulated nanostructures with disordered CNS were obtained at 20% and 50% hydrogen content Fig. 8(g, h) and disordered nanostructures with encapsulated CNS found at 80% hydrogen content Fig. 8(i). However, at 400 °C, the encapsulated CNS was observed at all reactant gas compositions Fig. 8(j, k, l).The formations of MWCNTs were observed at the reaction temperature of 700 °C, regardless of the ratio of hydrogen/ethene composition. For better understanding and clarification, detailed analysis of the morphology and structures was undertaken for the reactions at 700 °C conditions. The range of diameters of the CNS observed at700 °C under different gas composition was shown in Fig. 9 .The analysis of 100 fibres shows an average diameter of Davg = 31 ± 4 nm. The tubes diameters were distributed in a range of 25–45 nm where the majority of the tubes were distributed in the range of 30–35 nm as in Fig. 9(a). For the synthesis in a condition of 50%H2/50%C2H4 the average diameter of MWCNTs present in a sample was Davg = 30 ± 4 nm. The fibres were distributed over a diameter range of 25–45 nm where the maximum distribution was at 30–35 nm, Fig. 9(b). However, for the reaction in a reactant gas of 80%H2/20%C2H4. The average diameter of MWCNTs was calculated to be Davg = 32 ± 4 nm. The diameters were distributed over a range of 25–50 nm with the maximum at 35 nm, Fig. 9 (c).This uniformity in CNS diameters was due to the formation of uniform catalyst precursor particles confined within the silica pores. Most of the previous work done also illustrated the formation of a uniform diameter of CNS depending on the pores size of the support [6,11,22–24], while the other studies based on CVD synthesis using the unsupported catalyst showed the nanostructure diameters were in the range of 20–250 nm [4,19,25,26]. In the work presented here the uniformity of the diameter of CNS is controlled by the size of the porous template in the silica particle itself Fig. 10 (a) shows the MWCNT structures present in a sample synthesised at 700 °C with 20%H2/80% C2H4. The presence of some encapsulated carbon is also shown in the figure. The insert in Fig. 10 (b) shows the SAED images with two arcs perpendicular with the structures, confirming this structure is MWCNT. Fig. 10 (c, d) shows the TEM images for the MWCNTS synthesised at a reaction temperature of 700 °C with reactant gas composition of 50% H2/50% C2H4 at a higher magnification with the insert of SAED pattern for the selected fibre. The TEM analysis of the structures of samples synthesised at 700 °C with reactant gas composition of 80% H2/20% C2H4 are shown in Fig. 10 (e, f). The structures present were confirmed to be MWCNTs, shown by images the higher magnification and the SAED pattern inserted in Fig. 10 (f).The predominant nanostructures and compositions present in the samples synthesised at 700 °C with a different H2/C2H4 composition observed on the selected area electron diffraction (SAED) pattern over 50 fibres randomly selected nanostructures are summarised in table 1 . The dominant structures present were observed as MWCNTs, which were present at 20% H2 and 50% H2 (over 80% and 50% ethene respectively). While increasing the hydrogen percentage to 80%/20% ethene, the nanostructures present were still predominantly MWCNTs with a small presence of hollow herringbone GNFs.It was found that the addition of nickel into the iron matrix forming binary iron-nickel alloy found to favour the development of MWCNTs, which can be seen from the table 1.For all the reaction at synthesis temperature of 400 °C–700 °C, the CNS production using silica supported NiFe2O4 catalyst produced a uniform diameter of CNS between 20 and 50 nm and the highest yield produced at the gas composition of 50%H2/50% C2H4. It was also shown the activity of the catalyst during the formation of CNS can be monitored using in situ mass spectrometry by monitoring the exhaust gas from the on-going synthesis. During the CNS precipitation and propagation, it was found that the highest activity of catalyst occurred at the gas composition of 50%H2/50% C2H4.It also shown using silica supported NiFe2O4 as a catalyst precursor, the synthesis at 700 °C will produce MWCNTs at all H2/C2H4 reactant gas compositions. While, the reaction at lower temperature produced disordered CNS and encapsulated carbon.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to acknowledge the financial support of the Ministry of Higher Education Malaysia (MOHE) under the Fundamental Research Grant Scheme FRGS/1/2015/TK05/UPM/02/3/030115-1704FR.

The carbon nanostructures (CNS) were successfully grown during the chemical vapour deposition of ethene (C2H4) and hydrogen (H2) over a supported Ni0.362Fe0.64 catalyst. The temperature of the reaction was varied between 400 °C and 700 °C with different ratios of hydrogen and ethene (20/80, 50/50 and 80/20). The increase of the H2 in the reaction gas gives higher deposition yield of carbon where the maximum yield is observed at a mixture of 50/50 of H2 and C2H4 respectively. The results showed that the structures of the carbon formed by the decomposition of ethene were dependent on the reaction temperature and the gas ratio employed. Graphitic nanofibers (GNFs) and multiwall carbon nanotubes (MWCNTs) were produced when the temperature reached 700 °C, while at the lower temperature 600 °C, disordered CNS with encapsulation and some amorphous nanostructures tended to form.

The continuously growing demand for clean transportation fuels with the depletion of conventional petroleum reserves promotes an increase of poor and heavy oil upgrading. Therefore, it is significant to efficiently convert the residual oil into light fractions oil in the refining industry, in which the composition of residual oil is complex, containing paraffinic, naphthenic and aromatic hydrocarbons with high contents of sulfur (S), nitrogen (N), vanadium (V), nickel (Ni) and so on, but it is difficult and challenging to achieve the efficient conversion of residual oil. Among several residual oil conversion processes, slurry-phase hydrocracking of heavy oil technology is an alternative technology, which has been attracting great attention because of its ability to process various heavy oils and achieve a higher feedstocks conversion (Go et al., 2018; Lim et al., 2018). It is known that the catalyst plays a significant role on the performance in the slurry-phase hydrocracking process.Slurry-phase hydrocracking catalysts mainly include oil-soluble dispersed catalysts and solid powder dispersed catalysts (Watanabe et al., 2002; Al-Attas et al., 2019). Oil-soluble dispersed catalysts were obtained by introducing transitional metals (such as Mo, Ni, Co and Fe) into oil soluble precursors to form organometallic compounds (Kang et al., 2020; Yang et al., 2019; Li et al., 2019). The active metal of catalyst can rapidly saturate the free radicals produced via β-scission reaction of C–C by incorporating hydrogen into the cracked active hydrocarbons, which favor to limit the aromatic condensation reaction and over cracking reaction (Bellussi et al., 2013; Nguyen et al., 2016; Kim et al., 2017). It has been found that oil-soluble Mo catalyst exhibited higher performance compared with other metals catalysts in the slurry-phase hydrocracking process (Kim et al., 2018; Liu et al., 2019). Moreover, the synergy effect of bimetals catalyst on the heavy oil conversion and the yield of light distillates oil was observed (Kim et al., 2019; Nguyen et al., 2015). Oil-solution catalysts possess great catalytic performance in the slurry-phase hydrocracking, but the high-cost during organometallic compounds preparation restrict their wide application in the petroleum refining industry.Solid powder dispersed catalysts, generally inorganic minerals, were widely employed in the early stage of the development of slurry-phase hydrocracking technology, in which inorganic minerals were considered as the hydrogenation active phase and coke carries in the slurry-phase hydrocracking process (Sanaie et al., 2001; Matsumura et al., 2005). Matsumura et al., (2005) used the natural limonite as catalyst to upgrade Brazilian Marlim vacuum residue (VR), VR conversion was less than 80 wt%, C5-340 oC fraction yield was about 34 wt%, and the coke yield was higher than 5 wt%. Red mud containing a mixture of Fe, Al and Ti oxide was employed for the slurry-phase hydrocracking of VR, the result showed that VR conversion was about 65 wt%, and the yield of naphtha and diesel was lower than 34 wt%, as well as the active phase for the hydrocracking reaction was pyrrhotite (Fe(x-1)S x ) derived from iron oxide (Nguyen-Huy et al. 2012, 2013). Quitian et al. (Quitian et al., 2016) found that ore catalyst of molybdenite and hematite were able to inhibit the gas and coke formation caused by decomposition and condensation reactions, and promote the hydrogenation of the free radicals formed primarily via the thermal cracking of C–C bond. Although the inorganic minerals as catalyst have an advantage over the low cost in the slurry-phase hydrocracking process, their inferior catalytic activity can’t meet the demand for highly efficient conversion of heavy oils.In order to overcome the drawbacks of oil-solution dispersed catalysts and fine inorganic minerals catalysts, the transitional metals such as Mo, Co and Ni were supported on carbon, alumina, silica-alumina, even inorganic minerals to prepare the hydrocracking catalysts, which not only can provide more hydrogenation active sites, but also play a role of coke carries (Looi et al., 2012; Park et al., 2019; Viet et al., 2012). MoS2-amorphous-silica-alumina (ASA) catalyst employed in the slurry-phase hydrocracking promoted the cracking reaction and changed favorably the product distribution (Sanchez et al., 2018). Puron et al., (2013) found that NiMo/Al2O3 catalyst exhibited higher asphaltenes conversion with lower coke deposition and a reduced gas yield at achieving similar VR conversion compared with NiMo/ASA catalyst in the slurry-phase hydrocracking of Maya VR, due to its larger pore lessening diffusion limitation of asphaltenes molecules. Sahu et al., (2016) reported that Ni–Mo supported on goethite catalyst showed VR conversion of 80 wt% with the low boiling point liquid products of about 70 wt%, and found the products distribution depending on the physical and chemical properties of the catalysts. In comparison with the chemical synthesis materials of ASA and Al2O3, the natural minerals not only have an advantage in very low cost, but also contains some metals such as Ti and Zn, especially Fe, which can transform to sulfided iron (Fe1-x S) acted as the hydrocracking active sites (Du et al., 2018). Cortes et al., (2019) employed the modified vermiculite as support to prepare the hydrocracking catalyst for Athabasca Bitumen, and observed that Fe in vermiculite favored to improve the catalyst activity for the Bitumen conversion. Our group has studied the catalyst supported on natural bauxite modified by acid-treatment and hydrothermal method for hydrocracking of coal tar, the results shows that the acid-treatment and hydrothermal modifications can enhance the catalyst performance in the hydrocracking process (Yue et al. 2016, 2018). However, the catalyst presented lower conversion even though using relatively light oil of coal tar with boiling point higher than 510 oC fraction less than 40 wt% as feedstock, while the processes of acid-treatment and hydrothermal modification led to wastewater discharge and increasing energy consumption.In this study, natural rectorite after calcination was used as support to prepare Mo catalysts for the slurry-phase hydrocracking of VR, in which the effect of calcination modification on the natural rectorite properties and catalyst performance was investigated. The supports of calcined rectorite and catalysts were characterized by XRD, FTIR, Py-FTIR, H2-TPR and XPS, as well as the catalyst performance was evaluated in an autoclave reactor with VR as feedstock to principally examine VR conversion and the yield of naphtha and middle distillates in the hydrocracking process. This work is significant for the development of high-efficiency and low-cost catalysts for the slurry-phase hydrocracking.Vacuum residue (VR) used as feedstock for the slurry-phase hydrocracking was supplied by China Petrochemical Corporation, its properties are shown in Table 1 .Natural rectorite was supplied by Zhongxiang Rectorite Co., Ltd. (Hubei Province, P. R. China), its chemical analysis compositions are shown in Table 2 . The calcination modification of natural rectorite was conducted at 450, 500 and 600 oC, which are denoted as Rec-450, Rec-500 and Rec-600. Mo supported on calcined rectorite catalyst was prepared by using the incipient wetness impregnation method with aqueous solution of ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O, Adamas, 98%). The wet catalyst after set at 25 oC for 12 h was dried at 120 °C for 10 h, and then calcined at 500 °C for 4 h. The content of MoO3 in catalyst is 5 wt%. The catalysts are designated as Rec-Mo, Rec-450-Mo, Rec-500-Mo and Rec-600-Mo.X-ray diffraction (XRD) analysis of the sample was performed on an Ultima IV diffractometer with Co Kα radiation at 40 kV and 40 mA, and the pattern was recorded in the 2θ range from 5 to 90° with a step of 0.02°. Fourier transform infrared (FTIR) was employed to examine the framework structure of natural rectorite, which was carried out on a Nicolet iS 10 spectrometer with the sample diluted by KBr at the ratio of 1:100, and the FTIR spectra were recorded in the wavenumber range from 4000 to 400 cm−1 with scans of 32. Pyridine adsorbed FTIR (Py-FTIR) of sample was conducted after heated at 350 °C for 5 h under a vacuum of 1.3 × 10−3 Pa, and the adsorption of pure pyridine vapor at 30 °C for 20 min was followed. Adsorbed pyridine was removed by evacuating at 200 and 350 °C. H2 temperature programmed reduction (H2-TPR) of natural rectorite and catalyst was carried out on an ASAP-2920 instrument with a thermal conductivity detector (TCD). The sample was pretreated at 300 °C for 30 min under Ar atmosphere, and then heated from 50 to 950 °C with a rate of 10 °C/min in the 10 vol% H2/Ar stream. X-ray photoelectron spectroscopy (XPS) analysis of catalyst was performed on a Thermo Scientific ESCALAB 250Xi instrument with a monochromatic Al Kα source, and C1s peak with a binding energy of 284.6 eV and Al 2p peak with a binding energy of 74.6 eV were used as the references to calibrate the binding energy scale of Mo species. XPS spectra were recorded by using a XPSPEAK41 software after a background subtraction, and Gaussian-Lorentzian function was used for the spectrum deconvolution. The relative concentrations of the species of MoS2, MoSxOy and Mo6+ oxide for each sulfided catalyst were determined through their corresponding peak area. For example, the relative MoS2 concentration was calculated as following: [MoS2](%) = AMoS2/(AMoS2+AMoSxOy + AMo 6+) where AX represents the peak area of species X.The slurry-phase hydrocracking performances of catalysts were evaluated in a 300 mL stainless-steel autoclave reactor using VR as a feedstock. VR, catalyst and appropriate sulfur powder were loaded into the reactor with the catalyst content of 5 wt%. Prior to the hydrocracking reaction, the catalyst was presulfurized with sulfur powder at 250 oC for 35 min and 350 oC for 35 min. The hydrocracking of VR was carried out at 420 oC under an initial H2 pressure of 13 MPa with the H2 to oil ratio (v/v) of 1000 for 90 min with vigorous agitation. After the hydrocracking reaction, the autoclave reactor was rapidly cooled to ambient temperature. The mixture of product and catalyst was collected, and then centrifuged to separate the liquid product and solid residue. The solid residue was washed with toluene to obtain the toluene insoluble, which included catalyst and coke. The liquid product was divided into four fractions on the basis of the boiling point (BP) in a decompression distillation plant, which are naphtha (BP < 180 oC), middle distillates (BP 180–350 oC), vacuum gas oil (VGO, BP 350–520 oC) and VR (BP > 520 oC). VR conversion and the yields of gas, naphtha, middle distillates, VGO and coke were calculated as following: VR conversion (%) = (mass of >520 oC fraction in feed > mass of >520 oC fraction in product) / (mass of >520 oC fraction in feed) × 100% Gas yield (wt%) = (mass of gas in product) / (mass of the feed) × 100% Naphtha yield (wt%) = (mass of <180 oC fraction in product / mass of the feed) × 100% Middle distillates yield (wt%) = (mass of 180–350 oC fraction in product –mass of 180–350 oC fraction in feed) / (mass of the feed) × 100% VGO yield (wt%) = (mass of 350–520 oC fraction in product – mass of 350–520 oC fraction in feed) / (mass of the feed) × 100% Coke yield (wt%) = (mass of coke) / (mass of the feed) × 100% Fig. 1 shows the XRD patterns of raw rectorite and rectorites calcined at different temperatures. The peaks at about 2θ = 7.0o and 19.8o were assigned to the characteristic diffraction peaks of rectorite crystalline structure (Zhang et al., 2010; Bao et al., 2019), while there were some impurities such as rutile TiO2 corresponding to 2θ = 27.4o and hematite Fe2O3 corresponding to 2θ = 33o and 35.5o in the raw nature rectorite (Nguyen-Huy et al., 2012; Liu et al., 2019), because the rectorite contained Fe2O3 of 8.2 wt% and TiO2 of 4.0 wt% on the basis of the chemical compositions analysis in Table 2. The intensity of diffraction peaks of rectorite crystalline structure decreased after calcination, especially the peak at 2θ = 7.0o, indicating that the crystalline structure of rectorite was destroyed during the calcination process, possibly due to the loss of interlay water under the high temperature calcining. No obvious change was observed for the diffraction peaks of nature rectorites calcined at different temperatures, indicating the calcination temperature higher than 450 oC had no remarkable effect on the rectorite crystalline structure.FT-IR spectra of nature rectorites calcined at different temperature are shown in Fig. 2 . The peaks at 3640 and 3430 cm−1 observed in IR spectra of rectorites were assigned to the bending vibration of hydrogen band of the hydroxyl stretching in SiOH and interlaminar water, respectively. The peak at 1040 cm−1 was associated with the in-plane Si–O–Si stretching vibration, and the peak at 705 cm−1 was ascribed to the bending vibration of Si–O–Al (Zheng et al., 2013). The peak at 3640 cm−1 corresponding to the hydrogen band vibration of SiOH hydroxyl stretching disappeared after rectorite calcination because of the –OH dropping from SiOH structure. Moreover, the intensity of peak at 3430 cm−1 dramatically decreased for the rectorite calcined at 600 oC, nearly disappeared, indicating that the interlaminar water fell off from the rectorite structure when heated at 600 oC. In addition, all IR peaks positions of rectories had no shift during the calcination process.The acid properties of the calcined rectorites were investigated by Py-FTIR. The Py-FTIR spectra of the calcined rectorites measured at 200 and 350 oC are shown in Fig. 3 . The band at 1450 and 1540 cm−1 are assigned to the Lewis (L) and Brønsted (B) acid sites, respectively (Tan et al., 2008; Wei et al., 2019). No obvious peak was observed for raw rectorite at both of 200 and 350 oC in the Py-FTIR spectra. While there was slightly weak peak at 1450 cm−1 for the calcined rectorite measured at 200 oC. The detail amounts of acid sites at 200 and 350 °C for the calcined rectorites are summarized in Table 3 . It is found that the amount of acid sites of calcined rectorites had no distinct increase compared with that of raw rectorite, which were less than 10 μmol/g for both L and B acid sites, suggesting that the calcined modification did nearly not increase acid sites on the rectorite. Fig. 4 displays H2-TPR profiles of nature rectorites calcined at various temperatures. The reduction peaks of H2-TPR profiles can reflect the hydrogen consumption. Raw rectorite exhibited two reduction peaks, which were at center of 495 and 770 oC, respectively. Nature rectorite contained Fe2O3 with the content of 8.2 wt% more than other metallic oxides, except for Al2O3 and SiO2, according to the chemical compositions analysis of rectorite. Thus, the peaks of rectorite were considered as the reduction peaks of iron oxides species. The reduction steps of iron oxides species in hydrogen usually follows as: from Fe2O3 to Fe3O4, and then from Fe3O4 to FeO, finally from FeO to Fe, as reported in the literatures (Mogorosi et al., 2012; Cheng et al., 2015). Hence, the peak at center of 495 oC was ascribed to the reduction of hematite (Fe2O3) to magnetite (Fe3O4), and the peak at center of 770 oC was attributed to formation of Fe though the reduction of FeO. The rectorite calcined at 450 oC also showed two reduction peaks, and the reduction peak positions were similar with that of raw rectorite, suggesting no new iron oxides species formation, but the area of peak at 495 oC was remarkably larger than that of raw rectorite, it may be that calcination made inert iron oxides into active iron phase. A peak at center of 615 oC appeared for the rectorite calcined at 500 oC, associating with the reduction of magnetite to FeO, and it became stronger accompanied with the peak at 490 oC becoming weaker for the rectorite calcined at 600 oC, indicating that some amount of hematite converted into magnetite in the rectorite when calcined at the temperature higher than 500 oC.H2-TPR profiles of Mo catalysts supported on rectorites calcined at different temperatures are shown in Fig. 5 . It is clear that there were two reduction peaks centered at the temperature of 560 and 780 oC for all Mo catalysts supported on rectorites, in which the peak at 560 oC was ascribed to the reduction of octahedral Mo6+ species to tetrahedral Mo4+ species, Fe2O3 species to Fe3O4 species and Fe3O4 species to FeO species, as well as the peak at 780 oC was attributed to the tetrahedral Mo4+ species to Mo and FeO species to metallic iron. In comparison with the catalyst supported on raw rectorite, the two reduction peaks positions had no obvious shift for the catalyst supported on calcined rectorites, however, remarkable change of peak area was observed for the catalyst supported on calcined rectorites, especially the low-temperature reduction peak. The low-temperature reduction peak area of catalysts supported on calcined rectorites decreased compared with that of catalyst supported on raw rectorite, and it changed in the order of Rec-Mo > Rec-500-Mo > Rec-450-Mo ≈ Rec-600-Mo, indicating the amount of octahedral Mo6+, Fe2O3 and Fe3O4 species on catalyst of rectorite calcined at 500 oC was slightly less than that on catalyst of raw rectorite, but more than those on catalyst of rectorite calcined at 450 and 600 oC, it may be that the change of hydroxyl on rectorite during the calcination had an effect on the Mo species phase formation on the catalysts surface.The chemical surface compositions of sulfided catalysts supported on rectorites were investigated by XPS. Fig. 6 displays Mo 3d XPS spectra and deconvolution results of sulfided catalysts supported on rectorites calcined at various temperatures. The Mo 3d XPS spectra include three doublets, the doublets with bending energy at 229.3 and 232.5 eV are related to Mo 3d5/2 and Mo 3d3/2 levels for the Mo4+ in MoS2 phase species, the doublets with bending energy at 230.5 and 233.8 eV are ascribed to Mo 3d5/2 and Mo 3d3/2 levels for the Mo5+ in MoSxOy oxysulfide species, and the doublets with bending energy at 232.7 and 235.9 eV are assigned to Mo 3d5/2 and Mo 3d3/2 levels for the Mo6+ in MoO3 oxide species (Nikulshin et al., 2014; Pimerzin et al., 2017; Zhou et al., 2018). The deconvolution results of different catalysts based on XPS spectra are summarized in Table 4 . It is found that the sulfidation degree of Mo species (Mo4+ proportion) on the calcined rectorites catalysts was higher than that on raw rectorite catalyst, especially the catalyst on rectorite calcined at 500 oC, it may be ascribed that calcination modulated the surface hydroxyl on rectorite, further influenced the interaction of Mo species and rectorite. In general, the catalyst with high sulfidation degree presents high hydrogenation activity in the hydrocracking and/or hydrotreating process (Nikulshin et al., 2014; Pimerzin et al., 2017; Cui et al., 2013; Liu et al., 2020).The performance of catalyst supported on calcined rectorite was evaluated in the slurry-phase hydrocracking of VR at 420 °C under an initial H2 pressure of 13.0 MPa. Fig. 7 shows VR conversions of the different catalysts. The VR conversion of Rec-Mo catalyst was about 77.0 wt%, and no notable change of the conversion was observed for the catalysts supported on calcined rectorites, exceptionally Rec-450-Mo catalyst with VR conversion of 81.0 wt%. It has been reported that the slurry-phase hydrocracking over oil dispersed catalyst or catalyst with few acid sites occurred above 420 oC was considered as a thermal cracking reaction path accompanied with hydrocracking reaction path (Kim et al., 2017; Matsumura et al., 2005; Nguyen et al., 2013). The thermal cracking reaction followed free radical mechanism, mainly depending on the reaction temperature, while hydrocracking reaction followed carbenium ion mechanism, which was principally affected by the acid sites of catalysts at the same reaction condition. There were few acid sites in rectorites examined by Py-FTIR, shown in Table 3. Hence, it is concluded that the almost same VR conversions for all catalysts were attributed to the slurry-phase hydrocracking controlled by the thermal cracking reaction following free radical mechanism.The products distribution is significant for the conversion of heavy and poor feedstocks into light oil fractions in the refining industry, in which more amount of the valuable fractions of naphtha and middle distillates corresponding to the boiling point range of gasoil and diesel was expected to produce, but gas and coke as worthless product, especially coke causing negative effect on the catalyst and reactor, were as infamous fraction. The yields of naphtha and middle distillates are shown in Fig. 7, and the products distribution of the catalysts supported on rectorites calcined at various temperatures are shown in Fig. 8 . The yields of naphtha and middle distillates for the various catalysts increased following as Rec-Mo (40.4 wt%) < Rec-600-Mo (52.7 wt%) < Rec-450-Mo (53.5 wt%) < Rec-500-Mo (61.7 wt%), indicating that the calcination of rectorite was beneficial for the enhancement of the yields of naphtha and middle distillates in the slurry-phase hydrocracking process.The detail yields of gas, naphtha, middle distillates, VGO, residues and coke of the various catalysts were distinctly different, as shown in Fig. 8. Rec-Mo catalyst presented the naphtha yield of 14.9 wt% and middle distillates yield of 25.5 wt%, both of which obviously increased for the catalysts supported on calcined rectorites, especially Rec-500-Mo catalyst with the naphtha yield of 25.4 wt% and middle distillates yield of 36.3 wt%. Moreover, the gas yield of Rec-Mo catalyst was up to 28.2 wt%, distinctly higher than that of the catalysts supported on calcined rectorites, and the gas yield of Rec-500-Mo catalyst reduced to 7.9 wt%. In addition, there was no remarkable change on the coke yield for the various catalysts. It is concluded that the catalysts supported on calcined rectorite had better performance compared with Rec-Mo catalyst on the basis of product distribution, especially Rec-500-Mo catalyst, it is attributed that the higher hydrogenation activity of the catalyst restrained over cracking reaction of intermediate product to produce gas.In this study, the catalysts supported on natural rectorite were prepared, and the effect of calcination modification on the catalysts properties was examined. The catalyst performance was conducted in an autoclave reactor at 420 oC and an initial H2 pressure of 13 MPa. The reaction results show that the catalysts supported on calcined rectotire exhibited similar VR conversions with the catalyst supported on raw rectorite, it is ascribed that the thermal cracking reaction following free radical mechanism controlled the reaction process, because there were few acid sites on the catalyst surface. However, the yields of naphtha and middle distillates for the catalysts supported on calcined rectorite were obviously higher compared with that on raw rectorite, especially the yields of naphtha and middle distillates over Rec-500-Mo catalyst up to 61.7 wt%, indicating that the calcination of rectorite was beneficial for improving the yields of naphtha and middle distillates in the slurry-phase hydrocracking process, it is attributed that the higher sulfidation degree of molybdenum oxide species on catalyst promoted the hydrogenation reaction, thus inhibited the over-cracking reaction of intermediate product to produce gas. This study is significant for the development of high-efficient and low-cost catalyst for the slurry-phase hydrocracking of heavy and poor oil.The authors acknowledge National Key Research and Development program (2018YFA0209403) and National Natural Science Foundation of China (Youth) program (21908027) for financing this research.

In order to develop high-efficiency and low-cost catalyst for the slurry-phase hydrocracking of vacuum residue (VR), the catalyst supported on natural rectorite was prepared, and the effect of calcination modification of rectorite on the catalyst properties and performance was investigated. The support of rectorite and catalyst were characterized by XRD, FTIR, Py-FTIR, H2-TPR and XPS to examine their structures and properties. The comparative reaction results show that VR conversions for the catalysts supported on calcined rectorite were similar with that on raw rectorite, possibly due to the VR cracking reaction controlled by the thermal cracking following free radical mechanism because of few acid sites observed on the catalysts surface. However, the yields of naphtha and middle distillates for the various catalysts were obviously different, and increased following as Rec-Mo (40.4 wt%) < Rec-600-Mo (52.7 wt%) < Rec-450-Mo (53.5 wt%) < Rec-500-Mo (61.7 wt%), indicating that the calcination of rectorite favored the enhancement of the yields of naphtha and middle distillates for the catalyst in the slurry-phase hydrocracking process, it is attributed that the higher sulfidation degree of molybdenum oxide species on the catalyst surface promoted hydrogenation reaction, thus restrained over-cracking reaction of intermediate product to produce gas.

Due to rising environmental concerns and challenges in satisfying future demand, the demand for renewable raw materials to replace petroleum oil-based products is significantly increasing [1]. It is one of the green synthesis choices that can help with long-term sustainability [2]. Vegetable oils (VOs), which currently account for the majority of renewable feedstocks used to make bio-based products, might be a viable alternative for the production of bio-based products [3]. Hence, the use of VOs as a potential feedstock for a variety of functional materials and their applications in a variety of sectors has recently gained greater attention. Long-chain fatty acid triglyceride esters are utilized as a trustworthy starting material for the production of a wide range of bio-based fuels and chemical products [4]. However, the degree of unsaturation present in the oil causes rancidity, stability, lubricity, compatibility, and chemical degradation problems which limit their use as petroleum oil alternatives [5–7]. As a result, the scientific community is paying more attention to the functionalization of VOs via epoxidation. Non-edible vegetable oils, such as castor oil [8], jatropha oil [9], Cynara cardunculus seed oil [1], cottonseed oil [10], Mahua oil [11], etc., have emerged as possible alternatives to address low-cost material demands without competing with food crops. Argemone mexicana oil is an ideal choice for making epoxidized oil, which is a renewable alternative for a variety of applications. This is because of their inherent biodegradability, availability, sustainability, non-toxicity, and ease of chemical modification of VO, as well as environmental concerns and limited supply of petroleum [12].Argemone mexicana oil (AMO) is a non-edible seed oil derived from Mexican prickly poppy seeds, an annual growing weed plant in the Papaveraceae family [13]. It is primarily composed of triglycerides of unsaturated long-chain fatty acids with high linoleic acid (36.6–61.4%) and oleic acid (18.5–40%), which are playing an important role in the epoxidation process [14]. Epoxidation is the most common VOs modification method for functionalizing ethylenic double bonds in VOs and converting them into the highly reactive epoxy group. They are adaptable building blocks for making bio-based products like plasticizers, lubricants, PVC stabilizers, and surface coating formulations [8,15,16]. This can also result in a variety of stable products with a highly reactive oxirane ring that aids in the investigation of a variety of chemical reactions [17]. Generally, the epoxidation of vegetable oils is performed with peroxy acid formed in situ since H2O2 has very low solubility in vegetable oils [11,17–19]. Acetic acid is more selective than formic acid, because the side reaction of peracetic acid decomposition is lower than performic acid, and the side reaction of ring opening is lower than with formic acid [20]. Consequently, epoxidized oil has increased viscosity, lubricity, oxidative stability, compatibility, and thus making the materials more susceptible to microbial degradation, all of which are crucial characteristics of epoxy products [3,12,21,22]. Various studies have been reported on the epoxidation of VOs with peroxy acids formed in situ utilizing homogenous catalysts, acidic ion exchange resins (AIER), or biocatalysts for epoxy formation with a wide range of applications, particularly for PVC plasticizers, and lubricants [10,15,21,23]. During epoxidation employing peroxy carboxylic acid as an oxidizer catalyzed by a homogenous catalyst, mainly sulphuric acid, produces commercially viable epoxidized oils [19]. [24] used peracetic acid to make epoxy canola oil employing sulphuric acid catalyst to convert ethylenic unsaturation to oxirane (81%) at 7 h. However, they can cause a variety of undesirable side reactions as well as corrosion problems [25,26]. Moreover, enzymes are also used as effective biocatalysts for the epoxidation of VOs in recent years [27]. used immobilized lipase as a biocatalyst for epoxidation of genetically modified high oleic acid soybean oil with or without free fatty acid and toluene, resulting in the epoxide conversion of 95% at 35 °C. The chemo-enzyme epoxidation of different VOs such as soybean oil [28], Sapindus Mukorossi seed oil [29], and Karanja oil [30] with other biocatalysts such as Novozym 435, have been reported using hydrogen peroxide as an oxidant. But, the chemo-enzyme epoxidation of VOs is strongly influenced by hydrogen peroxide concentration as well as high temperatures, resulting in enzyme deactivation [26]. The employment of heterogeneous catalysts, including AIER for epoxidation of VOs, would be more advantageous in terms of separation ease, reusability, eco-friendly, and cost effectiveness [31]. [32] have reported on Karanja oil (iodine value, 89 g/100g) epoxidation with peracetic acid catalyzed by Amberlite IR-120 catalyst. The researchers have studied the effects of stirring speed, molar ratio of hydrogen peroxide to ethylenic double bond in the oil, molar ratio of acetic acid to ethylenic double bond in the oil, temperature, and catalyst loading for the epoxidation process [24]. also described on epoxidation of canola oil with in situ generated peroxy acetic acid using Amberlist IR120H resin as a catalyst and obtained oxirane oxygen content of 90% at 7 h.Metal oxide solid acid catalysts are used in a variety of organic synthesis processes, including aromatic nitration, esterification, transesterification, and epoxidation [33]. Thus, heterogeneous metal oxide catalysts are preferred to circumvent the limits of the aforementioned catalysts [3]. There have been a few reports on the epoxidation of soybean oil, canola oil, podocarpus falcatus seed oil, methyl oleate, and sunflower oil using heterogonous metal oxides as a catalyst such as sulfonated –ion exchange resins [32], sulfonated- SnO2 [3], solid sulfonated silica acid [34], Ti–SiO2 [18], Alumina [35], tungsten [36], respectively [34]. used sulfonated silica solid acid catalyst for epoxidation of podocarpus falcatus seed oil with hydrogen peroxide as an oxidizer. The maximal ethylenic conversion to oxirane was reported to be 84.75% under optimal conditions ethylenic double bond to H2O2 molar ratio of (2.5:1), catalyst loading (5%), temperature (70 °C), and time (4 h). Sulfated tin (IV) oxide is also classified as a super solid acid because of its high surface acidity and is employed in the majority of acid-catalyzed processes like esterification and transesterification [19]. [37] reported that a sulfate-doped metal oxide surface can function as a solid acid and an oxidative catalyst [3]. also used sulfated tin (IV) oxide to epoxide unsaturation in canola oil using peroxyacetic acid produced in situ from H2O2 and acetic acid. They reported a maximum epoxide conversion of 100% at optimum epoxidation conditions [33]. studied solid acid sulfated – Zirconia for effective epoxidation of castor oil. Because certain sulfate-doped metal oxides such as SnO2, ZrO2, TiO2, and Al2O3 have both Lewis and Brønsted acid sites derived from metal oxides and sulfates doped on the surface of metal oxides, they have been widely used as a solid acid in organic chemical modifications. Furthermore, as compared to metal oxides without sulfate, they produce super acid materials with high surface acidity and significantly larger surface areas [38]. As a result of their exceptional catalytic activity, they have attracted a lot of attention and are frequently utilized as a solid acid catalyst for a range of organic modifications. As far as the authors knowledge, sulfated tin (IV) oxide solid acid-catalyzed epoxidation of Argemone mexicana oil with peroxy acetic acid formed in situ has not been reported elsewhere.Various researchers have studied kinetic modelling strategies to estimate kinetic constants for the epoxidation of cottonseed oil by Prileschajew method [39]. have used a semi-batch reactor to study kinetic modelling for the epoxidation of cottonseed oil with performic acid by Prileschajew method. The results of their study showed that the reaction enthalpy of epoxidation and ring opening was −230 kJ/mol and −90 kJ/mol, respectively with initial reaction conditions of 50–70 °C, an organic phase 30–40%, formic acid 0.02–0.05 mol/min and time 25–50 min [40]. used a kinetic model under adiabatic conditions to investigate the variables affecting the risk of thermal runaway for the epoxidation of cottonseed oil. It has been noted that adiabatic temperature rise and time to maximum rate were sensitive to the content of acetic acid and hydrogen peroxide [41]. estimated the kinetic constants for the epoxidation of cottonseed oil by peroxyacetic acid using a batch reactor. The authors developed a kinetic modelling technique to predict kinetic constants for the ring opening reaction involving water, acetic acid, and peracetic acid. They reported that ring opening by acetic and peracetic acids more quickly than water and hydrogen peroxide.Therefore, this study aimed to synthesize and characterize sulfated–tin (IV) oxide solid acid as a heterogeneous catalyst for AMO epoxidation with peroxy acetic acid formed in situ. The influences of various AMO epoxidation parameters (viz. molar ratio of the ethylenic double bond in the AMO to H2O2, molar ratio of the ethylenic double bond in the AMO to acetic acid, catalyst concentration, and reaction temperature) were investigated. The physicochemical characteristics of AMO and its epoxidized oil (EAMO) were also examined. Moreover, a kinetic model for AMO epoxidation was analyzed to proceed to the acceptable degree of double bond conversion.Hydrogen peroxide (30%), glacial acetic acid (99.5%), ammonia solution (30%), iodine crystals, HBr solution (48%), sodium thiosulphate, anhydrous sodium sulfate, sulphuric acid (98%), ethyl acetate, Stannous chloride dihydrate (SnCl2.2H2O, 97%) and chloroform (99%) were purchased from Sigma-Aldrich (Germany). The other chemicals and reagents utilized in this experiment were analytical grade.Oil was extracted from Argemone mexicana seed (AMS), collected from Addis Ababa, Ethiopia, using the soxhlet method with chloroform as the solvent. The maximum oil yield was achieved at a temperature near the boiling point of the corresponding solvent [42]. After the complete extraction process, the extracted oil was separated from the solvent using a rotary evaporator and vacuum pump at 70 °C. Prior to the epoxidation process, the obtained oil was refined and stored at −4 °C for further use in the epoxidation process.In this study, sulfate group-doped tin (IV) oxide solid acid catalyst for the AMO epoxidation was prepared by applying the chemical co-precipitation method [3,43–45]. 75 g of SnCl2.2H2O (97%) and 1.5 L deionized water were mixed with a continuous stir followed by a dropwise addition of 30% of aqueous ammonia solution to maintain the desired pH of ∼9.0. At room temperature, a white tin hydroxide (Sn (OH)4) powder gel was precipitated after 6 h. The resultant gel was filtered using Whatman's filter paper and rinsed with distilled water until a neutral solution. The gel was then oven-dried at 100 °C for overnight. Then, 20 g of the obtained gel powder was impregnated with 300 mL of 1 M sulphuric acid solution for 1 h. Further acid-treated tin hydroxide powder gel was oven-dried at 100 °C for 12 h. Both oven-dried tin hydroxide and its related acid-treated gel powder were calcined at 500 °C for 4 h. Finally, several surface characterizations were performed on the resulting pure tin (IV) oxide and sulfate group doped-tin (IV) oxide catalysts using FTIR, XRD, BET/BJH, DSC, TGA, SEM-EDX methods, thereby understanding catalyst activity.FTIR (Thermo fisher FTIR spectrometer-Nicolet iS50), at 4 cm−1 resolution with KBr as a background matrix in the range of 4000–400 cm−1, were used to determine the formation of pure tin (IV) oxide and its sulfated solid acid catalyst. In FTIR analysis, 5 mg of both resultant catalysts and 95 mg of KBr crystal were thoroughly mixed, grounded into a fine powder, and then pelletized using a hydraulic press at 10 tons. Thus, the functional groups present on the catalyst surfaces were identified using this technique.XRD was also conducted on a diffractometer with Ni-filtered CuKα radiation at λ = 0.154 nm in the 2θ range of 10–80° and thus the crystal structure and sizes of both catalysts were determined. Accordingly, the mean crystal sizes of the catalysts were calculated using the Debye Scherer equation as indicated in eq. (1). (1) D = 0.9 λ β cos θ where D –denotes the average diameter of crystalline size (nm), λ –denotes the wavelength of CuKα radiation at 0.154 nm, β –denotes full width at half maximum intensity (FWHM) in radian, and θ –denotes for Bragg angle (օ).Brunauer - Emmett-Teller (BET) devices based on adsorption and desorption of N2 gas isotherms via Quantachrome Nova 2200e Surface Area analyzer (USA) was used to characterize surface areas of both resultant catalysts (pure tin (IV) oxide and it’s sulfate doped - tin (IV) oxide solid acid catalyst). The specific surface areas of both catalysts were determined from a nitrogen adsorption study conducted at a low temperature (−196.15 °C) using the high vacuum conventional volumetric glass system and were evacuated at 250 °C for 2 h before exposure to nitrogen gas at −196.15 °C under reduced pressure (10−5tor). Besides, the micrometric Pore Size analyzer Barret –Joyner Halenda (BJH) technique was used to evaluate the pore volume and average pore diameter of the obtained catalysts.Thermal gravimetric analysis (TGA) was also employed to study the thermal stability of the resultant catalysts under the temperature range of 25–750 °C using TA instruments with SDT Q600 under nitrogen flow in which weight losses were evaluated. Then, further confirmation analysis was conducted using differential scanning calorimetry (DSC).All epoxidation processes were conducted in 500 mL three-necked round bottom flasks with a magnetic stirrer and placed in the hot plate's temperature-controlled water bath. One side of the flask was inserted with a thermometer and used to measure reaction temperature, while the other middle neck of the flask was fitted to a water-cooled reflux condenser. Primarily, the required amount of AMO was mixed with acetic acid and sulfated tin (IV) oxide solid acid catalyst at 30 °C at a stirring speed of 1000 rpm. Then, 30% of hydrogen peroxide solution was added dropwise to the reaction mixture in the first 30 min. The reaction time recording was begun once this oxidizer was completely added to the reaction mixture. The influence of reaction conditions, such as molar ratio of the ethylenic double bond in the oil to H2O2 and acetic acid, catalyst concentration, and the reaction temperature were studied. Upon completion of the epoxidation process, catalysts were removed by filtration. Before analysis, the reaction products were collected and ethyl acetate (50 mL) was used to separate the aqueous and organic oil layers periodically, then washed with both NaHCO3 (5%) solution and distilled water until pH ∼7, and the trace amount of water and other impurities was absorbed with anhydrous Na2SO4. Finally, the resultant epoxy products were separated from ethyl acetate using a rotatory evaporator. The epoxy oxygen content was analyzed using AOCS Cd 9–57 methods in which 0.1 N hydrobromic solution in acetic acid (glacial) was used as a titrate (as per eq. (2).), and the ethylenic double bond conversion into an epoxy group was determined in terms of iodine value (IV). The amount of double bonds in the oil is closely related to the iodine value (IV), which is an indicator of the overall unsaturation in the AMO. As a result, IV was determined to find out the quantity of double bonds in the oil. Hence, the ethylenic double bond conversion was investigated in terms of the iodine (IV) value measurements, using the AOCS Cd 1–25 method according to eq. (3). Further epoxidized oil formation confirmation analyses were conducted using FTIR, 1H NMR, and 13C NMR methods. (2) O O C = 0.1 M x 1.6 x ( B − V ) w e i g h t o f s a m p l e ( g ) where, OOC stands for oxirane oxygen content (%), B stands for volume of hydrobromic acid solution used to titrate a blank, and V stands for volume of hydrobromic acid solution used to titrate the test sample. (3) I V = M x 12.69 x ( B − S ) w e i g h t o f s a m p l e ( g ) where IV stands for iodine value, M stands for molarity of sodium thiosulphate, B stands for mL of sodium thiosulphate used to titrate a blank, and S stands for the mL of sodium thiosulphate used to titrate the test sample. In addition, the ethylenic DB conversion in the AMO to the epoxy group was determined using eq. (4) [46]. (4) E t h y l e n i c D B c o n v e r s i o n ( % ) = I V 0 − I V I V x 100 where, IV denotes the iodine value of the AMO before epoxidation in g I2/100g of oil, and IV is the iodine value of the EAMO in g I2/100g.To better understand the influences of epoxidation reaction conditions on epoxide conversion, the different experimental trials were conducted following the one-variable-at-a-time method. All epoxidation experiments were done with a constant 50 mL of AMO and 1000 rpm mixing speed for 6 h. In this study, the effect of sulfated – tin (IV) oxide solid acid catalyst loading varied from 5 to 15% with corresponding to the weight of AMO on the ethylenic double bond conversion and epoxy oxygen ring content, while other reaction parameters such as the molar ratio of an ethylenic double bond in the AMO to 30% of H2O2 1:3, the ethylenic double bond in the AMO to acetic acid ratio 1:2, and reaction temperature 70 °C were taken from the literature [3]. The effects of the ratio of the ethylenic double bond in the AMO to hydrogen peroxide (30%) on the ethylenic double bond (DB) conversion and epoxy oxygen content were investigated by varying the range from 1:1 to 1:4 at fixed optimal catalyst loading value, the molar ratio of the ethylenic double bond the AMO to acetic acid 1:2, and reaction temperature (70 °C). The effect of the ratio of the ethylenic double bond in the AMO to acetic acid was varied from 0.5 to 2.5 at fixed other epoxidation parameters. The reaction temperature was also altered from 50 to 80 °C to investigate its impact on an ethylenic double bond conversion and thus epoxy oxygen content. The overall AMO epoxidation process is shown in Scheme 1 .The physicochemical characteristics of the epoxidized product (EAMO) such as density, kinematic viscosity at 40 °C (mm2/s), kinematic viscosity at 100 °C (mm2/s), viscosity index, flash point (oC), epoxy oxygen ring content (%), and iodine values (g I2/100g of the AMO) were examined using established methods. Using a Rheometer (MCR 102, USA) instrument, the dynamic viscosity of AMO and its epoxidized oil (EAMO) were measured as a function of temperature ranging from 20 to 100 °C at a constant shear rate of 50 per second. Furthermore, the kinematic viscosity and viscosity index of both AMO and its epoxidized oil was determined using the determined dynamic viscosity value based on the ASTMD2270 standard table. The viscosity index (VI) of EAMO was calculated using eq. (5). The flash point of AMO and its epoxidized oil were also examined. According to a standard procedure, the iodine values (IV) of both samples were tested using eq. (3). (5) V I x = γ A − γ x γ A − γ B x 100 a t 40 o c x 100 Where V I x denotes viscosity index of the AMO/EAMO, γ x denotes kinematic viscosity of Epoxidized oil (EAMO) at 40 °C, γ A and γ B denotes kinematic viscosity of oil A and B at 40 °C are used as reference oil taken from ASTM-D2270-10 table for γ x at 100 °C.FTIR spectra of obtained pure and sulfated tin (IV) oxide catalysts are presented in Fig. 1 a and b. The characteristic peaks at 600 cm−1 show the existence of O–Sn–O stretching. This revealed the complete conversion of tin (IV) hydroxide gel into SnO2 when calcined at 500 °C for 4 h. Similarly, bands at 1286, 1145, and 1018 cm−1 were observed in the spectra after sulfation of tin (IV) hydroxide gel with 1 M H2SO4, indicating symmetric and asymmetric stretching frequencies of the sulfated group (SO) and confirming a bidentate chelation (linkage) mode between sulfate group and tin (IV) oxide. This further indicates the formation of sulfate doped – tin (IV) oxide as a solid acid catalyst after being calcined at 500 °C for 4 h. Because of the existence of sulfate doped on the surface of tin (IV) oxide catalyst, robust acidic properties can be recognized [38]. Thus, improving the properties of sulfate doped tin (IV) oxide catalyst is important for promoting the epoxidation of AMO with peroxyacetic acid generated in situ. Similar research works were reported by Refs. [43–45].The XRD spectra of attained pure tin (IV) oxide and sulfated tin (IV) oxide solid acid catalyst are illustrated in Fig. 2 a. The major absorption peaks at 26.63°, 33.90°, 37.99°, 51.81°, 54.80°, 61.91°, 64.78°, 65.99°, 71.75° are related to diffraction from planes (110), (101), (200), (211), (220) of tin (IV) oxide particles. This shows the complete conversion of tin (IV) hydroxide gel into pure tin (IV) oxide by calcination at 500 °C for 4 h, and thus confirms the tetragonal crystal phase. Similarly, after sulfation of this gel with 1 M H2SO4 treatment, XRD peaks at 26.63°, 33.93°, 37.98°, 51.84°, 54.76°, 61.91°, 64.82°, 65.98°, and 78.73° are related to the above-mentioned diffraction planes. This indicates that the sulfate group doped on the catalyst's surface did not cause any crystalline changes. This is supported via the DCS plot in Fig. 2b, confirming that the prepared catalysts have a single phase. However, sulfation of tin (IV) oxide reduces the crystalline size of the obtained tin (IV) oxide thereby increasing the surface area of the catalyst which in turn enhances its catalytic activity. The mean crystalline sizes of pure tin (IV) oxide and tin (IV) oxide doped with the sulfate group were determined using the Debye Scherrer eq. (1) based on XRD peak width measurements. Accordingly, the calculated mean crystalline size of tin (IV) oxide and sulfated tin (IV) oxide solid acid catalyst was determined to be 35.62 nm and 16.64 nm, respectively. This smaller crystal size of the sulfate group-linked tin (IV) oxide catalyst is related to the addition of sulfate ions. This could be due to sulfate chelation on the catalyst surface, which prevents the tin (IV) oxide particles from coagulating during the calcination process at 500 °C for 4 h. As a result, the surface area of the sulfated tin (IV) oxide solid acid catalyst increased and thus improved its catalytic performance for the AMO epoxidation process. This demonstrates that sulphuric acid treatment has a significant impact on reducing crystalline size, increasing surface area, and so improves the catalytic activity of sulfated tin (IV) oxide for epoxidation. Comparable results were reported in the literature [3,45].Catalyst surface area is one of the critical parameters that have a significant influence on catalytic activity, and thus epoxide conversion. Brunauer – Emmett – Teller (BET) was employed to analyze the surface area of each prepared catalyst while Barret –Joyner Halenda (BJH) method was used to characterize pore volume and average diameters. As indicated in Table 1 the BET surface areas of Tin (IV) oxide and sulfate group linked tin (IV) oxide were 14.84 m2/g and 60.61 m2/g, respectively. The development of sulfate linkage with tin (IV) oxide gives its increased surface area for the sulfate group linked tin (IV) oxide solid acid catalyst. The deposition of sulfates on the surface of tin (IV) oxide increased its pore volume from 0.06 to 0.13 cm3/g and caused an increased in the average pore diameter from 10.97 to 11.20 nm, according to the BJH pore size distribution result. Sulfation of tin (IV) oxide enhanced pore volume and is anticipated to boost epoxide output [12,38].The thermal analyses of pure tin (IV) oxide and sulfated tin (IV) oxide after calcination at 500 °C are displayed in Fig. 3 a and b. The weight loss of both catalysts was determined in the temperature ranges of 25–800 °C. Tin (IV) oxide was determined to be quite stable up to 800 °C, and the weight loss percentage change was found to be insignificant. Up to 600 °C, sulfated tin (IV) oxide was similar stability with very little weight loss, but after 600 °C, the weight loss increased due to the evolution of the sulfate group from the catalyst surface. Sulfated tin (IV) oxide loses weight when heated to 800 °C compared to pure tin (IV) oxide [3,38].The surface morphology of both tin (IV) oxide and sulfated tin (IV) oxide catalysts was characterized by SEM analysis. As indicated in Fig. 4 a and b, the surface morphology of both catalysts has no significant change after the impregnation of sulfate ions. Sulfation of the catalyst improved the catalytic oxidative activity of tin (IV) oxide surface as compared to non-sulfated metal oxide. Thus, sulfation is the key to boosting the conversion of Argemone Mexicana oil to its epoxidized oil [12,33,43].The effect of catalyst concentration on AMO epoxidation is shown in Fig. 5 a. In this study, the influence of sulfated tin (IV) oxide solid acid catalyst on the course of AMO epoxidation was evaluated. It was investigated by varying the amounts of the catalyst 5, 7.5, 10, and 15% of the corresponding weight of oil keeping the ratio of the ethylenic double bond in the oil to acetic acid and 30% H2O2 as 1:3:2 3. All epoxidation reactions were examined at a constant agitation speed of 1000 rpm at 70 °C. As illustrated in Fig. 5a, the ethylenic double bond (DB) conversion in the AMO to its epoxidized oil gradually increased with an increase in catalyst concentration up to 12.5% due to an increment in the active sites of the catalyst. The maximum double bond conversion of 95.05% and related epoxy oxygen content of 6.25 was achieved after 6 h. However, more upsurge in sulfated tin (IV) oxide catalyst concentration resulted in considerably the same conversion or less. This might be an increased rate of oxygen ring cleavage beyond the maximum value of the catalyst external surface active sites content during epoxidation [8]. In the current investigation, a sulfated tin (IV) oxide solid acid catalyst concentration of 12.5% was shown to be the best value for AMO epoxidation. The results of the study showed that pure tin (IV) oxide calcined at 500 °C has no substantial double bond conversion of AMO into an epoxy group under these experimental conditions [3]. Fig. 5b shows the effect of hydrogen peroxide (30%) on the course of AMO epoxidation at a catalyst concentration of 12.5%, temperature of 70 °C, and the molar ratio of the ethylenic double bond in the AMO to acetic acid 1:2. Hydrogen peroxide has a significant influence on in situ epoxidation [47]. Thus, the ratio of an ethylenic double bond in oil to H2O2 (30%) was varied at 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, and 1:4 to study its influence on the in situ epoxidation of ethylenic double bond conversion in the AMO. As shown in Fig. 5b the rate of ethylenic double bond conversion increased with increasing the molar ratio of the ethylenic double bond in the AMO to H2O2 (30%). The molar ratio of the ethylenic double bond in AMO to hydrogen peroxide of 1:2.5 resulted in the maximum ethylenic double bond conversion of 95.5% with the highest epoxy oxygen content of 6.25. As the ethylenic double bond in the AMO to hydrogen peroxide (30%) beyond 1:2.5, the ethylenic double bond conversion declined. This is due to an excess supply of 30% of H2O2 can cause an upsurge in the degradation rate of oxirane oxygen content [25].The effect of the ratio of the ethylenic double bond in the AMO to acetic acid (varied at 1:0.5, 1:1, 1:1.5, 1:2, 1:2.5, 1:3) on the in situ epoxidation is shown in Fig. 5c. Though carboxylic acid such as acetic acid acts as a good oxygen carrier during the AMO epoxidation, it is the main contributor to the degradation of the oxirane oxygen ring formed [1,21]. It was revealed that the epoxide conversion rate improved with an increase in acetic acid concentration, but further addition resulted in oxygen ring degradation. Thus, the molar ratio of the ethylenic double bond in the AMO to acetic acid was found to be 1:1.5. Under this condition, the maximum ethylenic double bond conversion of 95% and the epoxy oxygen content of 6.25 were obtained after 6 h. Beyond this acetic acid value, the epoxy oxygen content was decreased due to epoxy oxygen ring cleavage owing to the higher acetic acid content. Fig. 5d shows the effect of epoxidation temperature (varied at 313, 328, 343, and 358 k) on the development of in situ epoxidation, while other epoxidation parameters (viz. molar ratio of the ethylenic double bond in the AMO to 30% H2O2 of 1:2.5, molar ratio of the ethylenic double bond in the AMO to acetic acid of 1:1.5, and catalyst concentration 12.5%) were kept constant. As shown in Fig. 5d, an increase in temperature up to 358 k increased the rate of the ethylenic double bond conversion. However, after 6 h, the rate of epoxy formation was found to be slightly constant for 343 K and 358 K. Fig. 6 reveals the reusability of sulfate group-doped tin (IV) oxide solid acid on AMO epoxidation was evaluated for four consecutive epoxidation processes under optimized experimental conditions. After the epoxidation reaction, the catalyst was separated and carefully washed, and then refluxed with ethyl acetate to remove reaction products that had formed on the catalyst's surface. Then it was dried overnight in the oven at 100 °C. The in situ epoxidation was carried out at optimal reaction parameters of molar ratio of the ethylenic double bond in oil: H2O2: acetic acid 1:2.5:1.5, employing regenerated sulfate chelated tin (IV) oxide solid acid catalyst 12.5% at 343 K for 6 h. The results of the study showed that repeated washing with ethyl acetate leads to the separation difficulty of oil remnants from the pores of the catalyst which poisons catalyst active sites. This causes a gradual loss of catalytic activity after three repeated cycles, and thus lowers ethylenic double bond conversion. It is revealed that oil interaction with catalyst active sites was limited as the number of repeated cycles increased thereby resulting in the lower double bond conversion of AMO.To explore the reaction mechanism of the AMO epoxidation process catalyzed by sulfated tin (IV) oxide solid acid, experimental runs were conducted at 313, 328, 343, and 358 K to analyze the kinetics and thermodynamics of in situ epoxidation of AMO. The process of epoxidation of AMO is essentially a heterogeneous reaction. To explain the heterogeneous catalytic epoxidation process, the Langmuir – Hinshelwood – Hougen – Watson (LHHW) kinetic expression was suggested. Thus, the reaction that takes place on the catalyst's active sites is principally regulated by three reaction steps: (1) the adsorption of reactants, (2) the surface reaction between adsorbed reactants on the active sites of the catalyst, and (3) desorption of products. However, the in situ epoxidation principally depends on two key reaction steps (i.e. peroxyacetic acid formation and epoxidation steps) since desorption of the product on the catalyst surface is assumed to be weak. In the presence of sulfate group doped tin (IV) oxide solid acid catalyst, epoxidation of AMO with peroxy acetic acid produced in situ from H2O2 and CH3COOH is as in eq. (6): (6) I . H 2 O 2 ( l ) + C H 3 C O O H ( l ) ⇌ K 1 C H 3 C O O O H ( l ) + H 2 O ( l ) Assuming that the adsorption of AMO and peracetic acid on catalyst active sites is a mild reaction. Then, the surface reaction between AMO and PAA indicates the formation of epoxidized Argemone mexicana oil (EAMO) and is written as in eq. (7): (7) I I . A M O + P A A + C ⇌ K 2 E A M O − C The desorption of EAMO is given by eq. (8): (8) I I I . E A M O − C ⇌ K 3 E A M O + C where AMO is Argemone mexicana oil, EAMO is epoxidized argemone mexicana oil, C is sulfated tin (IV) oxide solid acid catalyst, and PAA is peracetic acid.The kinetic analysis for the epoxidation of AMO was conducted on the basis of the subsequent assumptions. (I) The rate-controlling step (the slowest step) is considered to be the epoxidation AMO (eq. (7)) whereas peracetic acid formation (eq. (6)) is a rapid and simultaneous step which does not considered a rate-controlling step. Thus, the overall rate equation of epoxidation of AMO considering the epoxidation surface reaction on active sites of catalyst is a rate-determining step is shown in eq. (9): (9) − r A M O = − d C A M O d t = k ′ C A M O C P A A (II) Since an excess peracetic acid was used for epoxidation, t can be assumed that peracetic acid content is constant during the epoxidation reaction (i. e. CPAA,o = CPAA). Moreover, the epoxidation of AMO is assumed to be a pseudo-first-order reaction. Thus, the rate equation is written as in eq. (10): (10) − d C A M O d t = k C A M O where, k = k’ CPAA The rate-controlling step (the slowest step) is considered to be the epoxidation AMO (eq. (7)) whereas peracetic acid formation (eq. (6)) is a rapid and simultaneous step which does not considered a rate-controlling step. Thus, the overall rate equation of epoxidation of AMO considering the epoxidation surface reaction on active sites of catalyst is a rate-determining step is shown in eq. (9):Since an excess peracetic acid was used for epoxidation, t can be assumed that peracetic acid content is constant during the epoxidation reaction (i. e. CPAA,o = CPAA). Moreover, the epoxidation of AMO is assumed to be a pseudo-first-order reaction. Thus, the rate equation is written as in eq. (10):Thus, eq. (10) in terms of fractional conversion (XAMO) of oil can be rewritten as in eq. (11): (11) − d C A M O , o ( 1 − X A M O ) d t = k C A M O , o ( 1 − X A M O ) Since, C A M O = C A M O ( 1 − X A M O ) Where, CAMO, o denotes the initial concentration of ethylenic double bond in the AMO. (12) − d X A M O d t = k ( 1 − X A M O ) The rate equation can be stated as follows after integrating eq. (12) at XAMO = 0, t = 0 and X = XAMO at t = t. (13) ∫ o X A M O − d X A M O 1 − X A M O = k ∫ 0 t d t Then, after integration of eq. (13), the final rate equation can be written as in eq. (14): (14) − ln ( 1 − X A M O ) = k t As a result, the experimental data were fitted with linear regression, and the epoxidation rate constants (K) at various temperatures were determined using the slope of – ln ( 1 – X A M O ) vs. time plot and tabulated in Table 2 . As shown in Table 2, the rate constant (K) values increased with the corresponding reaction temperature increment, revealing that the reaction was pseudo-first-order with respect to AMO.Arrhenius equation ( k = A E a / R T ) was utilized to compute the activation energy of AMO epoxidation using the slope of –lnk vs. (1/T, k−1) plot, and is presented in Fig. 7 . Therefore, according to the Arrhenius equation plot, the resultant activation energy obtained was 47.03 kJ/mol. This value confirmed that the chemical reaction utilizing sulfate -doped tin (IV) oxide solid acid catalyst was kinetically controlled.The main thermodynamic parameters analyzed under the present study (viz. Gibb’s free energy (ΔG), enthalpy (ΔH), and entropy (ΔS)) can be determined using eqs. (15)–(15)–(17)(15)–(17): (15) Δ H = E a − R T (16) K = R T N h e Δ S R e (17) Δ G = Δ H − T Δ S Using eqs. (15)–(15)–(17)(15)–(17), thermodynamic parameters of epoxidation of AMO using sulfate -doped tin (IV) oxide solid acid catalyst were found to be ΔH = 44.18 kJ/mol, ΔS = −137.91 Jmol−1k−1, and ΔG = 91.12 kJ/mol and tabulated in Table 3 . The ΔH value determined was the enthalpy of activation for the epoxidation process. The positive values of ΔH indicate that the energy input (heat) from an external source is required to raise the energy level and transform the reactants to their transition states. Thus, the positive value of enthalpy of activation reveals that the epoxidation process is endothermic in nature. Similar results were reported in various literature [48–52]. The negative value of entropy revealed that the epoxide product is more stable as compared to the AMO. The positive value of Gibb’s free energy also showed that the epoxidation of AMO is a non-spontaneous process which is also confirmed by the positive value of enthalpy. The present study is in reasonable agreement with the prior reports, which revealed comparable observations of activation energy of 44.85 kJ/mol for epoxidation of soybean oil and 44.65 kJ/mol for palm oleic acid [17,19]. Fig. 8 a and b shows the FTIR results of AMO and its epoxidized oil (EAMO). It was revealed that the removal of ethylenic double bonds in the AMO and the formation of its epoxidized product (EAMO) by their absorption peaks. The bending and stretching vibration of the ethylenic double bond (=C–H of unsaturated fatty acids) in the AMO is visible in bands at 3008 cm−1 and 721 cm−1. However, removing these bands from AMO revealed that the oil has been completely converted to its epoxidized form (EAMO). This was further corroborated by the presence of a new band at 825 cm−1, which was not observed in the AMO, showing that an epoxy oxygen ring (C–O – C) has been formed in the epoxidized oil (EAMO). This matches the appearance of an epoxy oxygen ring in the 785–880 cm−1 absorption peak range [17]. The absence of a broad proton signal of the –OH group in epoxidized oil indicates that no major side reactions occurred during in situ epoxidations of AMO with sulfated tin (IV) oxide soil acid catalyst.Nuclear magnetic resonance spectroscopy (NMR) was used to better understand the synthesis of epoxidized oil (EAMO). This also signifies the ablation of the ethylenic double bond in the AMO and the appearance of an epoxy oxygen ring in the final epoxy product during in situ technique. In Fig. 9 a 1H NMR spectra shows the presence of the ethylenic double bond (-C = C-) in the AMO at a chemical shift of 5.3 ppm. However, these ethylenic double bonds in this oil have been vanished in the epoxidized product (EAMO) as shown in Fig. 9b. Furthermore, the existence of new oxirane oxygen ring bands in the epoxy product at 2.7–3.3 ppm and 1.5–1.87 ppm, confirming the conversion of ethylenic double in the AMO to EAMO by in situ epoxidation using sulfate group doped tin (IV) oxide solid acid catalyst.Moreso, the removal of the ethylenic double bond in the AMO at 130 ppm (Fig. 10 a) and the appearance of a new peak at 53–58 ppm (Fig. 10b) of 13C NMR spectra of EAMO also indicated that the conversion of the ethylenic double bond in the AMO into EAMO.Argemone mexicana oil (AMO) is remarkable a renewable resource for the epoxidation with peroxy acetic acid formed in situ using a heterogeneous solid acid catalyst. Thus, the physicochemical characteristics of AMO were examined as shown in Table 4 . Moreover, the fatty acids content of AMO were 24.92% oleic acid (C18:1), 59.43% of Linolenic acid (C18:2), and 15.65% saturated fatty acids and contained an iodine value (IV) of 118.21 g I2 per 100g of oil. AMO epoxide (EAMO) could be used to synthesize valuable goods such as plasticizers, lubricants, polymers, stabilizers, and others [3]. Thus, the physicochemical characteristics of epoxidized AMO were also examined and tabulated in Table 4. The amount of epoxide generated is dependent on the number of double bonds present in the oil, which is defined by the iodine value. The unsaturation of the raw material increased as the iodine value rises. When compared to AMO, the iodine value decreased from 118.21 to 5.62 g I2/100g of AMO. This shows the conversion of unsaturation present in the AMO into its epoxidized form. The obtained flash point of EAMO was 280 °C. Thus, the epoxidized version of AMO can be utilized as a plasticizer in polymeric materials and as a high-temperature diesel fuel additive as a lubricant because it contains ether and ester functionality which enhances its compatibility [3]. The oxidative stability of AMO and EAMO was determined according to A Metrohm AG Rancimat model 892 (Herisau/Switzerland). It was used to assess the oxidative induction time (OIT) in accordance with AOCS Official Method Cd 12b-92, AOCS 1992. Thus, the oxidative induction time of the extracted AMO and its epoxidized oil (EAMO) was found to be 2.13 h and 68.41 h, respectively (Table 4 and Supplementary Fig. S1).The viscosity of epoxidized products such as lubricants and plasticizers is critical to their lubricity. The viscosity of fluids decreases as temperature rises, and a measure called the viscosity index was employed to enumerate this trend. The greater the viscosity index value, the less the viscosity of the substance changes with temperature [3]. This work investigated the kinematic viscosities of AMO and its epoxidized oil (EAMO) at a temperature ranging from 20 to 100 °C (Fig. 11 ). It was revealed that the viscosity of both AMO and its epoxidized oil (EAMO) decreased with rises in the temperature. The kinematic viscosity of AMO was 31.05 mm2/s at 40 °C and 5.56 mm2/s at 100 °C, and its epoxidized form (EAMO) had a value increased to 131.5 mm2/s at 40 °C and 7.26 at 100 °C (Table 4). The reason behind the increment of kinematic viscosity of EAMO was due to the ethylenic double bond in the AMO was removed through epoxidation. The kinematic viscosities of the present study were within the range of vegetable oil-based epoxy products such as lubricants (5–225 mm2/s at 40 °C and 2–20 mm2/s at 100 °C) [53]. The viscosity index of AMO decreased from 163.3 to 136.8 due to the disappearance of the double bond in the AMO. As a result, the viscosity and viscosity index of the EAMO falls within the given ranges, meeting the ISO VG 100 grade viscosity for industrial applications [53]. Fig. 11 depicts the trend of dynamic viscosity as a function of temperature for AMO and epoxidized AMO (EAMO). It was revealed from Fig. 11 that as the temperature increased the dynamic viscosity of both AMO and EAMO decreased. It was also shown in Fig. 11 that the dynamic viscosity of EAMO is much greater than AMO from 20 to 60 °C. The reason was due to the conversion of the double bond in the AMO to epoxide product, EAMO during in situ epoxidation. Table 5 [7,18,24,54–57], depicts the comparison of literature with other heterogeneous metal oxide catalytic system for epoxidation of vegetable oils. The ethylenic double bond conversion of 89.7%, 75% and 96% were obtained from epoxidation of soybean oil using Ti–SiO2 catalyst at reaction condition of 90 °C and 54 h [18], Alumina catalyst at reaction condition of 80 °C and 10 h [54] and HY zeolite catalyst at reaction condition of 70 °C and 3 h [56], respectively. Similarly, The ethylenic double bond conversion of 90% was obtained from Cardanol oil and Jatropha oil using Amberlite IR 120 at the catalyst loading of 20–22 wt%, and reaction condition of 65 °C, and 7 h. These results were in comparable to the present study as shown in Table 5.Sulfated tin (IV) oxide solid acid catalyst was successfully synthesized and characterized in this study. Sulfated tin (IV) oxide solid acid was an effective catalyst for the epoxidation of AMO with peroxyacetic acid formed in situ. The maximum ethylenic double bond conversion of 95.5% with an epoxy oxygen content of 6.25 was obtained at the molar ratio of the ethylenic double bond in the AMO: H2O2, acetic acid was 1:2.5, 1:1.5, catalyst concentration 12.5% and reaction temperature at 343 k for 6 h. Epoxy group formation was confirmed using FT-IR, 1H, and 13C NMR spectroscopy. The physicochemical characteristics of EAMO indicate improved viscosity and oxidative stability, which leads to high lubricity when compared to its precursor, AMO. The catalyst and the AMO epoxide product were potential sources for PVC bioplasticizers synthesis.1) Fekadu Ashine, Subramanian Balakrishnan, Zebene Kiflie, and Belachew Zegale Tizazu conceived and designed the experiments.2) Fekadu Ashine performed the experiments.3) Fekadu Ashine, Subramanian Balakrishnan, Zebene Kiflie, and Belachew Zegale Tizazu analyzed and interpreted the data.4) Fekadu Ashine, Subramanian Balakrishnan, Zebene Kiflie, and Belachew Zegale Tizazu contributed reagents, materials, analysis tools or data.5) Fekadu Ashine, Subramanian Balakrishnan, Zebene Kiflie, and Belachew Zegale Tizazu wrote the paper.No funding was received to assist with the preparation of this manuscript.We do not have any conflict of interest.All authors mutually agreed that the manuscript to be submitted to the Heliyon Journal and the work has not been published/ submitted or is being submitted to another journal.The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.We request you to kindly consider our manuscript for possible publication in your esteemed journal.The authors would like to thank Addis Ababa Science and Technology University and Chemical and Construction Inputs Industry Development Institute to allow experimental set-up work and analytical instruments for characterization.The following is the supplementary data related to this article: Multimedia component 1 Multimedia component 1 Supplementary data related to this article can be found at https://doi.org/10.1016/j.heliyon.2023.e12817.

In this study, sulfated tin (IV) oxide solid acid catalyst was prepared for the epoxidation of Argemone mexicana oil (AMO) with peroxyacetic acid formed in-situ. The catalyst was synthesized using the chemical co-precipitation method and characterized. The effects of various epoxidation parameters on ethylenic double bond conversion (%) and oxygen ring content were analyzed. The maximum ethylenic double bond conversion of 95.5% and epoxy oxygen content of 6.25 was found at the molar ratio of AMO to 30% of H2O2 = 1:2.5, molar ratio of AMO to acetic acid = 1:1.5, catalyst concentration = 12.5%, and reaction temperature = 70 °C at reaction time = 6 h. The kinetic and thermodynamic features of the epoxidation of AMO were also analyzed with appropriate models. The results of the kinetic study of the epoxidation reaction followed pseudo first order with the activation energy = 0.47.03 kJ/mol. Moreover, the thermodynamic constants of epoxidation of AMO were found as ΔH = 44.18 kJ/mol, ΔS = −137.91 Jmol−1k−1) and ΔG = 91.12 kJ/mol. The epoxidized product of AMO was further analyzed using FTIR, 1H NMR, and 13C NMR. The results of these analyses confirmed the successful conversion of the ethylenic double bond in the AMO to EAMO.

With the rapid development of the global economy, industrialization and urbanization have caused the demand for fossil fuels to expand rapidly. Environmental warming and its associated environmental problems have caused widespread concern around the world; greenhouse gases in the atmosphere are the main cause of global warming, and carbon dioxide is the greenhouses gas with the highest emissions [1]. Carbon dioxide is mainly derived from the burning of fossil fuel, automobile exhaust, animal and plant respiration, and corpse decay. Carbon dioxide emissions are high, but its utilization rate is low. At present, it seems to be meaningful to convert carbon dioxide into different chemicals [2]. Therefore, new catalysts need to be developed, and in-depth research on methods to functionalize carbon dioxide must be performed. To solve this problem, an increasing number of scientists have become committed to developing various technologies to capture and fix carbon dioxide [3].Metal-organic frameworks (MOFs) are a class of porous crystal materials formed by the self-assembly of metal ions or metal clusters and organic ligands and are promising materials with unique properties. MOFs have a large specific surface area, porous structure, and multichemical composition and are easy to be functionalized. MOFs have been applied to gas adsorption, storage and separation; sensors; electrochemistry; and catalysis.Combined with the environmental problems caused by carbon dioxide, at present, many works have reported that metal organic frameworks act as catalysts to fix carbon dioxide [4–7]. Wang W et al., synthesized a metal-organic framework containing zinc metal and the H4tmpe ligand with a stable structure to fix carbon dioxide [4]. Beyzavi MH et al., reported a new Hf-based metal–organic framework (HfNU-1000) incorporating Hf6 clusters [5], which demonstrated a high catalytic efficiency for the activation of epoxides and facilitated the quantitative chemical fixation of CO2 into five-membered cyclic carbonates under ambient conditions, rendering this material an excellent catalyst. Wang S et al., reported a cobalt-containing zeolitic imidazolate framework (Co-ZIF-9) that served as a robust MOF cocatalyst to reduce CO2 by cooperating with a ruthenium-based photosensitizer [6]. Li P et al., successfully constructed a highly porous MOFs via solvothermal assembly of a clicked oct carboxylate ligand and Cu (II) ions, incorporating both exposed metal sites and nitrogen-rich triazole groups [7]. The high-efficiency and size-dependent selectivity toward small epoxides for catalytic CO2 cycloaddition make this MOF a promising heterogeneous catalyst for carbon fixation.The materials discovery process involves several stages, including synthesis, testing and characterization, etc [8]. These steps are usually carried out sequentially, and therefore, only a few materials can be synthesized, tested or measured at a time. The process of discovering and developing new materials currently entails considerable effort, time and expense. For MOFs, the possible combinations of the numerous building blocks under different topological symmetries are almost infinite [9]. Therefore, the construction of metal-organic frameworks is diverse, and people can only construct these frameworks via experience and guesswork. To explore the high-performance MOF materials, a theoretical prediction method with high efficiency and accuracy should be developed and applied.Machine learning technology has achieved great success in finance, medical, biology, and informatics [10–12]. Today, with the development of artificial intelligence, machine learning (ML) is an emerging research paradigm that will revolutionize material discovery [13]. It has been indicated that machine learning can be applied to chemical discovery [14–16]. Machine learning, as the name implies, uses the most primitive learning method of humans (regular learning) to give a machine the ability to process data, train the algorithm using training data, and test the accuracy of the algorithm using test data. ML makes effective decisions through the experience generated by historical data [17]. This material discovery method deviates from the traditional method and can screen large-scale materials with excellent performance at one time. ML is more efficient and faster than traditional methods.In our work, based on the reported experimental results, the characteristics and performance of MOFs for fixing carbon dioxide into cyclic carbonate were extracted to establish a data set, which was further applied to train and test five ML algorithms, including SVM, KNN, DT, SGD, and NN, to setup classifiers. The tested ML algorithms were extended to classify 1311 hypothetical MOFs via screening for high catalytic performance; the characteristics of these MOFs were finally extracted, as shown in Scheme 1 [18–20]. This work applies machine learning methods to small data sets to achieve large-scale screening of catalysts in the hope that these methods can play a guiding role in future experimental work, thereby accelerating the discovery of new materials and saving manpower, material resources, and financial resources.The metal ions/clusters and organic ligands in a MOF determine its structure and properties. Here, the topological structure of a MOF is completely abandoned, and the types of metal atoms and organic ligands are selected as the features of the MOF. The MOF’s characteristics and carbon dioxide conversion rates were collected from approximately one hundred published paper. Data set information is available in the Supplementary Information B. The machine learning algorithm learns a classifier to predict the catalytic properties of MOFs based on their structural features. TOF value is used as an indicator for catalyst performance evaluation, and the calculation method of TOF value is described in Supplementary Information A. The reported reaction temperatures in experiments range from room temperature to 140 °C, which could be directly used in ML. Here, the TOF values have been transformed to the same temperature according to Arrhenius equation, and the specific implementation method is presented in Supplementary Information A.S1. The median of revised TOF value is taken as the classification limit. The target value (revised TOF value) is divided into two categories, a value above the median which classified a good category, marked as 1, and a value below the median is classified a bad category, marked as 0. After processing, the target value distribution of the machine learning data set samples is average.To predict the carbon dioxide conversion rate of MOFs based on their structural properties, supervised learning was applied. In supervised learning, the computer learns from labelled historical examples for which the outcomes are known to make predictions on future data for which the outcomes are unknown. Based on python and the scikit-learn package[21] ], we used five machine learning methods, including SVM, KNN, DT, SGD method and NN, and more information is available in the Supporting Information A.S2. The specific workflow is shown in Fig. 1 .The performance of each ML algorithm was evaluated by calculating the precision, recall and F1 score. As shown in Scheme 2 , the precision was calculated with T P ( T P + F P ) , where TP is the number of true positives and FP is the number of false positives. Precision measures how many of the samples that were predicted to be positive are true positive examples.Recall is T P ( T P + F N ) , where TP is the number of true positives and FN is the number of false negatives. Recall is the ability of the classifier to find all positive samples. Recall measures how many of the positive samples are predicted to be positive.The F1 score, also known as the balanced F-score or F-measure, can be interpreted as a weighted average of precision and recall, and it can be calculated with the following equation: (1) F 1 s c o r e = 2 ∗ ( precision ∗ recall ) ( precision  +  recall ) the F1 score reaches its best value at 1 and worst value at 0, and the relative contributions of precision and recall to the F1 score are equal.The data set was derived from the experimental literature in which MOFs are used as catalysts to immobilize carbon dioxide as a cyclic carbonate [1,5,7,22–81]. Useful information was extracted from the literature, such as the metals, organic linkers, MOFs, reactants in the CO2 fixation reaction, and TOF values. There are five reactants reported in the experimental results, including propylene oxide (PO), epichlorohydrin (ECH), butylene oxide (BO), styrene oxide (SO) and epibromohydrin (EBP), which have similar structures, as shown in Fig. 2 . The database in this study contains 106 data entries (the total MOF number). For each MOF, the number of metal species, organic ligands and reactant types are 2, 2 and 1, respectively, and every MOF was described with 85 structural characteristics composed of 23 metals, 57 organic ligands, and 5 reactants. Each structural property can be encoded as a binary parameter (also called a variable or feature), 0 or 1, indicating its presence or absence, respectively, in a specific MOF [9]. TOF values is taken as the target. The data set information is available in the Supplementary Information B.The machine learning algorithm learns a classifier (also known as a “model”) to predict the catalytic properties of MOFs based on their structural features. The prediction result of the classifier is 0 or 1, which represents materials with a poor prediction performance or excellent performance, respectively. It is hoped that the classifier model can be generalized to predict the approximate performance of a novel species and the values of its structural parameters. If the predicted performance of the new material does not reach the desired level, then there is no point in synthesizing the material, and vice versa. Therefore, an accurate prediction model can guide the synthesis and experimentation of new materials.Before training the models, the data set was randomly divided into 80% for training and 20% for testing. Five machine learning methods, including SVM, KNN, DT, SGD, and NN, were trained by adjusting the hyperparameters. The final models selected were built with the following configuration. The SVM classification with the libsvm implementation method from scikit-learn was used (svm.SVC). The learning of the hyperplane in the SVM algorithm used radial basis function (RBF) kernel functions for the decision function. Our implementation of SVC finds the best parameters; penalty parameter C of the error term is 100, and the gamma value of the kernel coefficient of RBF is 0.01. The K Neighbors Classifier class, with the number of neighbors being 23 and the algorithm parameter being auto, attempts to decide the most appropriate algorithm based on the values passed to the fit method. The Decision Tree Classifier class with a maximum depth of tree of 8 and balanced classes weights was used to build the model. The SGD classifier algorithm possesses hyper-parameter values for loss of hinge and penalty (aka, the regularization term) of l2. The NN model implements a multilayer perceptron (MLP) algorithm that trains using backpropagation. The numbers of hidden nodes in the two hidden layers were set equal to 5 and 5. After training the machine learning models, the next step is to evaluate the models.Five machine learning methods were used to train the model and then test the test set. The model performance for predicting catalysis was evaluated through calculation of the accuracy, precision, recall, and F1 score. The accuracy scores of the five models on the training set and the test set are shown in Fig. 3 (a). SVC, NN and SGD have a strong learning ability for the training data set, and KNN and DT perform are worse than the former three methods. Regarding testing capability, SGD has a strong testing ability with an accuracy of 86.4%, that of SVC and NN are both 81.8%, and KNN and DT is lower than 80%. For SVC, SGD and NN, the prediction results are consistent with the experimental results at least 80% of the tested materials.In some cases, accuracy is not the most comprehensive tool for evaluating models. Indicators such as precision and recall are better for measuring machine learning model performance than accuracy under certain circumstances. Next, to further evaluate the model, precision, recall and the F1 score were used to evaluate the classification performance, and the results are shown in Fig. 3(b). By comparison, we find that SVC, SGD, NN, models have the highest precision of more than 92%, which means that among the catalysts with an excellent predicted performance, over 92% of the MOFs are truly excellent. SVC, SGD, NN models all have the recall of 82%, and among them, SGD has the highest recall rate of 0.864, which means that 86.4% of the excellent MOFs verified by experiments are also predicted with SGD.The F1 score is the harmonic average of precision and recall, and commonly it’s ideal when some model’s F1 score is higher than 80%. Through the analysis of the prediction and testing results, the F1 score of SVC, SGD, NN methods can reach more than 80%, and KNN and DT are lower than that value. Through the reliability analysis for the five classifiers, SVM, SGD, and NN are all get the high cores in accuracy, precession, recall and F1 evaluation. Here, to improve the reliability of the classification, three trained classifiers from SVM, SGD, and NN were combined to improve the reliability, which could be considered as an ensemble learning method.We combined 1311 hypothetical MOFs using 23 metals and 57 organic ligands, which is the component of the reported MOF for fixing CO2, and further applied the trained classifiers to screen out MOFs with high catalytic activity. The five organic reactants in the previous experimental work were used to predict the performance of metal organic framework materials. Finally, 6555 (1311∗5) samples were available for classification.The classified results for the CO2 fixation performance are presented, which contains five layers, and each layer corresponds to one organic reactant and contains 1311 points, which correspond to 1311 MOFs. If a MOF’s performance in CO2 fixation is predicted as “good” by one of the three models at the same time, the MOF is regarded as a highly active material and is specified as a red point. For the other prediction results (none “good” from the three methods), the MOF is regarded as a bad material and is marked as a blue point. Prediction results can be found in the Supplementary Information A.S3.All of the five organic reactants have an epoxyethane group, and their structures are similar to each other. Here, the versatility of a MOF’s catalytic performance among the organic reactants is investigated using the voting method and is presented in Fig. 4 . If a MOF displays “good” activity on more than three reactants, the MOF’s performance is specified as “excellent” and is marked as a red point. If a MOF displays “bad” activity on all five of the reactants, it is specified as “poor” and is marked as a blue point. If a MOF’s activity is only “good” for 1–3 reactants, it is specified as “moderate” and is marked as a green point. In Fig. 4, the abbreviated form of the ligand is used. The full name of the ligand is in the Supplementary Information A.S3.The predicted “excellent” MOFs are what we ultimately need. As shown in Fig. 5 , certain metals could be combined with most organic ligands to form high-performance MOFs, and some organic ligands also combine with most metals to form excellent MOFs. Here, the excellent ratios are evaluated for the metal and ligand, which are calculated with the following formula: (2) Excellent Ratio = Excellent ligands  ( metals  ) + Moderate ligands ( metals ) ∗ 50 % Total ligands  ( metals ) The excellent ratios for metals are presented in Fig. 5. 11 metals, Y, Zr, Ni, Cu, Li, Na, K, Rb, W, V, and Mn have shown slightly better performance than other metals. In the above metals, Li, Na, K, Rb and W, are all derived from the multi-metal MOFs predicted through the machine learning, and the present results could not ensure their single-metal MOFs have good catalytic performances. Here Mn, V, Cu, Ni, Zr, and Y are the most likely candidates.As shown in Fig. 6 , MA, BDC-NHx(Me)3-x(I-), NH2-BPY, NH2-BDC, bpH2, tactmb, tdcbpp, TBPP, TCPP, DABCO, TATAB, AIP, BTC, H3L, TCPE, NDC,BTB, compared with other ligands, display outstanding performance. In the ligands, MA, BDC-NHx(Me)3-x(I-), NH2-BPY, NH2-BDC, bpH2, DABCO, TATAB, AIP, BTC, BTB, NDC, are in the dual-ligand MOFs, and the present results also could not ensure their performance when they applied as ligand singly. TBPP and TCPE in the training data set are only one data, this may lead to the overestimated activity of them through the ML. Then for single ligands, tactmb, tdcbpp, TCPP, H3L are recommended as the most ideal candidates, and their structures are shown in Fig. 7 .The predicted six metals and four organic ligands could be combined to form 24 high-performance MOFs, as shown in Fig. 8 . Six MOFs (specified as stars) have been synthesized and reported [33,34,82–85], and their crystal structures are shown in Fig. 8.In the six excellent metals, V, Mn, Cu and Ni, are 3d metal, and Y and Zr are the first two 4d metals. All of them have multiple valence, similar atomic radius and electronegativity. Due to that the metal ion is always the center of the catalytic activity, the 3d and the beginning of the 4d metal could be considered.For the MOFs reported as catalysts that can fix carbon dioxide into cyclic carbonate in previous experimental works, their structural characteristics and catalytic activities are collected into a data set, which is applied to train classifiers with five ML algorithms, and three classifiers are combined to predicted 1311 novel MOF structures via ensemble learning. The results show that the ML model could predict a MOF’s catalytic performance according to its structural feature. The best metals, Mn, V, Cu, Ni, Zr, and Y, and best ligands, tactmb, tdcbpp, TCPP, H3L, are discovered. The six metals and four ligands could be combined into 24 MOFs that possess strong potential for being catalysts for carbon dioxide fixation.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This study was supported by the National Natural Science Foundation of China (21676004). Authors declare that there are no conflicts of interests.The following is the supplementary data related to this article: Multimedia component 1 Multimedia component 1 Supplementary data related to this article can be found at https://doi.org/10.1016/j.jmat.2021.02.005.

The process of discovering and developing new materials currently requires considerable effort, time, and expense. Machine learning (ML) algorithms can potentially provide quick and accurate methods for screening new materials. In the present work, the features of the metal organic frameworks (MOFs) as a catalyst for fixing carbon dioxide into cyclic carbonate were extracted to build a data set, which were collected from the experimental results of approximately 100 published papers. Classifiers were trained with the data set with various ML algorithms, including support vector machine (SVM), K-nearest neighbor classification (KNN), decision trees (DT), stochastic gradient descent (SGD), and neural networks (NN), to predict the catalytic performance. The ML models were trained on 80% of the data set and then tested on the remaining 20% to predict the carbon dioxide fixation ability. The trained ML model was extended to explore 1311 hypothetical MOFs, and some structures displayed a strong catalytic ability. Finally, the six best metal ions (Mn, V, Cu, Ni, Zr and Y) and four best ligands (tactmb, tdcbpp, TCPP, H3L) were determined. These six metals and four ligands could be combined into 24 MOFs, which are strongly potential catalysts for carbon dioxide fixation. Using machine learning methods can speed up the screening of materials, and this methodology is promising for application not only to MOFs as catalysts but also in many other materials science projects.

No data was used for the research described in the article. No data was used for the research described in the article.As a result of increasing public attention to the major environmental risks posed by the widespread use of fossil fuel sources, a deal was reached to transition to an energy system that relies on cleanliness, safety, and recycled materials [1–2]. Hydrogen energy is an attractive future energy carrier since it can be created easily and without producing any greenhouse effect [3–6]. Despite being the most prevalent element in the universe's elements, in nature, hydrogen isn't found in its purest form. In this context, the production of low-cost, environmentally friendly hydrogen from renewable energy sources is becoming increasingly important. Water is a valuable source of hydrogen, and water electrolysis is the most common method of obtaining hydrogen from it [7–8].The HER has already been examined in both acidic [9–11] and alkaline [12–13] solutions, as well as its dependence on pH and temperature effects. Several electrode materials have been used to investigate HER, including Hg, Rh, Pt, Au, Sn, Cu and Ag [14–17]. It is widely available that increasing any material's surface area can be calculated in general to improve its cathodic efficiency through a synergistic combination of multiple parts engaged in it [18–19]. Moreover, the electrocatalytic productivity of any substance is mostly determined by the metal surface's binding energy with adsorbed hydrogen [20–21]. Based on the preceding, catalysts based on noble metals have already been used as effective HER electrocatalysts [22–23].The first electrodeposition of rhodium plating was demonstrated by Marino in 1912 [24]. According to Cinamon, sulphate and phosphate baths are used for rhodium coating [25]. Rh is very corrosion resistant due to its association with the platinum group of metals. In our research work, we achieved coating using PC and DC techniques. The main reason for selecting PC over traditional DC plating is to avoid the continuous entry of current into the bath, which leads to burning deposits or uneven coating. This led to the PC becoming popular in recent times. Pulse coating also leads to fine grain size deposition by varying the current TON and TOFF formulas. For more than a decade, PC coatings have been used in industries like aerospace, shipyards, and auto manufacturing [26–27]. Catalytic metals, especially Cu, Zn, and Al mixed oxides, are frequently used in chemical engineering and pollution control [28–29]. The presence of catalytically active transition metal species (e.g., Cu, Co, Fe, Ni, V, Rh) allows for easy separation of the end products [30–31].In this paper, Rh has been coated on SS304 by electrodeposition. Pt is always superior. We tried with Rh, to know its ability and get an idea of Rh's comparison to Pt [22]. The prepared Rh specimens were characterised by SEM, EDX, AFM and XRD. HER performance was accessed by LSV, Tafel and Chronoamperometry techniques.The rhodium bath was optimised to coat the rhodium on an SS304 substrate. Even though rhodium is cost-effective, the availability of Rh on the steel surface is very low, and hence the cost of Rh in the study can be controlled. The amount of Rh deposited on the substrate is 0.5 µgcm−2. The bath's composition and working requirements are presented in Table 1 . The rhodium sulphate solution (Rh2(SO4)3) was supplied by Arora Matthey Limited, Kolkata. Using a pH metre (HI2020 edge pH meter, HANNA, USA), the bath pH was kept at 1.3 by adding H2SO4 as needed. All depositions were performed at a temperature of 45°C. Electrodeposition was performed on a specified surface (1.76 cm2) of the polished (abrasive sandpaper of many grades ranging from 80 to 1800 scale) SS304 substrate (15 mm in diameter and 1 mm in thickness). The anode was made of insoluble platinised titanium (Ti anode fabricators private limited, Chennai, India) [32]. During plating, the electrodes, anode and cathode, were placed at a distance of 3 cm apart. The experiment was carried out in a 100 mL capacity beaker (rhodium solution and H2SO4) designed electrochemical cell and a power source (Agilent N6705A DC Power Analyser, USA). All depositions were recorded and completed in 20 minutes under continuous conditions. The coatings were rinsed and dried with distilled water. Various instrumental tools were used to examine the deposited coatings for surface morphology and compositional information.The surface morphology of the rhodium coating was analysed using SEM ((model: FESEM Carl ZEISS), interfaced with EDS (model: Oxford Nanoanalysis 250). The surface morphologies of coated materials have been described using atomic force microscopy investigations (model: Nanosurf® EasyScan 2 AFM & STM) to confirm the evidence of other research methods. X-ray diffraction (Riakgu Mini Flexell Desktop Diffractometer with Cu-Ka (l = 1.5406 Å).) at 40 kV and 40 mA, scanning from 10o to 100o of 2θ was used to identify the phases and crystal structure of the coated samples.In the cell, electrodeposited rhodium electrodes were exposed to cathodic and anodic polarisation to determine the amount of hydrogen produced during the study. The rhodium coatings were electrodeposited on the working electrode and a platinum electrode as a counter electrode. As a reference electrode, Ag/AgCl was used. Sinsil International Private Limited in Bengaluru, India, supplied all of the electrodes. Using the CompactStat.h10800 workstation, Ivium Technologies, The Netherlands, the electrochemical behaviour of the coatings was characterised using CompactStat.h10800. The glass setup is equipped with marked micro-burettes for measuring the quantity of H2 released throughout electrolysis. The distance between the electrodes in our setup is 5 cm, near the cathode and anode exit holes are provided. It is easier for hydrogen and oxygen to go out from the chamber rather than mixing and was achieved by utilising a modified glass tubular cell, as illustrated in Fig. 1 .As illustrated in Fig.2 , SEM is used to study the surface morphology of both PC (with different duty cycles) and DC source coatings, as illustrated in Fig. 2. The optimised current density in our work was 4.1 A/dm2. One of the key factors that determines the rate of HER is current density. In comparison to all the PCs and DC sources (Fig. 2 a), PC 75% samples reveal more homogeneous and smaller granules, which produce a smoother coating surface. The Rh was irregular in scale on the surface in Fig. 2 b, 2c, and 2d because there are variations in its spread, the AFM confirmed this impression (Fig.4). The degree of uniformity decreased from PC 75% to DC. The duty cycle percentage has a considerable impact on the morphology of the surface of Rh coatings. Strong adherence and brightness were obtained for all Rh alloy coating sources, although a 75% duty cycle rendered one appropriate for HER activity [33]. Fig. 2 depicts the EDX spectrum of Rh metal ions incorporated into the Rh bath solution coating. Furthermore, the weight % of Rh is presented in Table 2 and demonstrates that the Rh concentration in PC coatings is less than in DC coatings, despite the same deposition conditions. The experimental data from Table 2 indicates that the Rh content of the coating declines in the bath from DC to PC coating, and the data matches concerning SEM images. (Fig.2). Type SS304 is a grade of austenitic steel with the following chemical composition by weight percentage; C 0.08, Mn 2.00, P 0.042, S 0.032, Si 0.72, Cr 18-20, Ni 8-12, Ni 0.10 and Fe 67-71.This change in the weight percentage of the coating causes a change in the surface roughness, as a result of this, it enhances the electrocatalytic activity [34].The AFM is a strong tool for characterising coating roughness in terms of average smoothness, which is answerable for improved electrocatalytic action. As a result, as shown in Figs. 4(a) and 3 (b), a 3D AFM image of DC duty cycle and PC 75% coatings are captured. In comparison to the PC 75% source, there were considerable alterations of the surface roughness in the DC coating. The excess surface area of the active rhodium on SS304 is caused by spines and corrugations on the DC electrode. The average roughness of the PC technique was 15.9 nm, which is lower than the DC coating of 42.0 nm, indicating that the Rh is deposited relatively consistently and even. This level of uniformity in the PC method increased the electronic charge density during LSV, which contributed to the high HER activity. Fig. 5 depicts XRD patterns for Rh deposited on SS304 at different coating sources in our study. Peaks may be seen at 43.2o, 50.9o and 75.3o respectively. The Rh (111), (200) and (220) planes of the cubic Rh crystal can be indexed by three diffraction peaks Crystallographic search match software and powder diffraction files were used to analyse the peaks of the XRD pattern (PDF no. 1-1213). The grain sizes can be determined using Debye–Scherrer Equation [30] as given below D = K λ β cos θ Where λ is the X-ray wavelength, θ is the Bragg angle, and β is the FWHM of the diffraction peak. The average grain size of the coatings is 7nm, 10nm, 12nm, and 14nm for DC, PCs, 25%, 50%, and 75% duty cycles respectively.As illustrated in Fig. 6 , the Rh electrocatalytic activity was initially investigated for H2 evolution in H2SO4 media. All of the samples exhibit a favourable hydrogen evolution process. At duty cycle 75%, Rh coated by the PC technique has a relatively low overpotential for hydrogen evolution. For HER, the overpotential of a Rh catalyst deposited by a 75% duty cycle sample is comparable to that of pure Pt. These findings support the sample's superior performance (75% duty cycle). During the hydrogen evolution reaction, the 25% and 50% samples show more overpotential and less current. It was predicted that Rh would be particularly active for HER. Indeed, the catalyst Rh demonstrated a catalytic start at virtually zero overpotential, and catalytic current rapidly increased in the sample attained at 75% duty cycle. Further cathodic sweeping revealed H2 bubble development and discharge from the surface are both very active.Tafel plots, as illustrated in Fig. 7 , were used to assess the Rh electrocatalytic activities. Tafel slopes reveal the nature of the HER process. The Volmer Heyrovsky or Volmer-Tafel mechanistic pathways are used to express the kinetics factor of electrocatalytic HER. Table 3 shows the Tafel slopes of 40.7, 61.3, and 74.1 mV/dec for coated Rh deposition with PC, respectively utilising 75 %, 50 % and 25 % duty cycles. The 75 % duty cycle coated Rh catalysts had a much lower Tafel slope, indicating increased electrocatalytic activity, which is indeed more than the prepared DC samples (69.9 mV/dec). The moderate Volmer-Tafel reaction mechanism has a Tafel slope of 40.7 mV/dec is hydrogen atom desorption and that hydrogen atom desorption is the rate-determining step [35–36].The chronopotentiometry (CP) of both PC (75%, 50% and 25% respectively) and DC coatings was investigated, as well as their electrocatalytic stability. A persistent current is administered in-between the two electrodes in this method by monitoring the voltage of one of the electrodes as a function of time regarding the substance of the reference electrode. The CP experiment was carried out at a steady current of -0.35 mAcm−2 for 3 hrs. The electrocatalytic behaviour of the coatings was evaluated using this technique by monitoring the sum of hydrogen freed for an early 180 seconds. The amount of hydrogen liberated is recorded and registered in Fig. 9. When compared to DC coatings, PC coatings emit a greater amount of H2 gas. This demonstrated that the preferred electrode material for HER is PC coating. Fig. 8 depicts the chronopotentiograms of PC 75%, PC 50%, PC 25%, and DC coatings. At first, the graph shows a substantial drop in potential as a function of time for both coatings. This is because, at the start of the electrolysis, the reduction of hydrogen ions and the evolution of hydrogen gas occur at a faster pace due to the rapid supply of current [37–38]. After a few minutes, there was little fluctuation in the potential with time, indicating the development of equilibrium. This shows that the release of hydrogen occurs efficiently on the electrode's surface and measurement of hydrogen release is confirmed by the fitted graduated burette in the three electrode arrangements glass tube as shown in the Fig. 1.The coating of rhodium on SS304 by pulse and direct technique was successfully done by the PC and DC method. The coating grain size was reduced and it was achieved by the PC technique at PC 75% is inferred by SEM. EDX confirmed the presence of Rh in the base metal. The roughness of the surface is highlighted by AFM analysis, which is supported by surface morphology outcomes. The SEM results were XRD verified. LSV demonstrates that the created coating has less overpotential and provides greater current. The lesser Tafel slopes demonstrate the efficacy of the catalysts and validate the HER mechanism explained by the Volmer-Tafel. Values of chronopotentiometry approve the complete consequences by providing a higher hydrogen collecting volume through electrolysis. The present research has the calibre to deliver its importance and to be commercialised for industrial use (Fig. 3).The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Dr. Praveen B.M reports financial support was provided by Board of Research in Nuclear Sciences. Dr Praveen B.M reports a relationship with BRNS that includes: employment and funding grants. Dr. Praveen BM has patent pending to Licensee. Dr. Praveen B.M employee of Srinivas University, College of Engineering and Technology, MangaluruWith the approval of Project No. 37(2)/14/18/2018-BRNS, dated 11/07/2018, instrumentation and financial funding from the Department of Atomic Energy (DAE), Board of Research in Nuclear Sciences (BRNS), Mumbai, Government of India and the authors also wish to acknowledge the management of M.S. Ramaiah College of Arts, Science and Commerce, Bengaluru for the constant support and encouragement through MSRCASC seed money funding granted in the year 2022. Srinivas University, Institute of Engineering & Technology, Srinivas Nagar, Mangaluru, Karnataka, has provided laboratory support

The theory and kinetics of the hydrogen evolution reaction (HER) on electrodeposited rhodium in acidic media (0.5 M H2SO4 solution) were looked into. An electrodeposition approach using direct current (DC) and pulse current (PC) was used to deposit rhodium on a stainless steel 304 (SS304) substrate. Several parameters, including rhodium concentrations, current densities, temperature, pH, and coating duration, were used to optimise the rhodium bath. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) analyses were used to assess the change in surface shape and chemical composition. The best coating was demonstrated at PC 75% duty cycle with an optimised current density of 4.0 A/dm2, which was better than the remaining PC cycles and DC source coating, indicating the most productive activity for hydrogen production. The activity of Rh catalyst coatings resembled that of pure platinum metal. Cyclic voltammetry (CV), chronopotentiometry (CP), and potentiodynamic polarisation techniques were studied to determine the HER. The results obtained from the PC technique with a 75% duty cycle give more HER performance.

Amine compounds are indispensable intermediates in fine and bulk chemical industries for the synthesis of polymers, surfactants, pharmaceuticals, and agrochemicals [1–3]. Currently, a large quantity of industrial relevant amines, such as aliphatic amines, aromatic amines, and aminoalcohols, are manufactured from non-renewable fossil resources via several functionalization steps, which suffer from low energy efficiency and environmental pollution [2]. Catalytic reductive amination (RA) of carbonyl compounds is a well-known class of reaction that is widely used for the clean synthesis of various amines with water as the main by-product [4,5]. Due to the increasing concerns on depletion of fossil recourses and rising CO2 concentration in the atmosphere, the synthesis of valuable amines from biomass-derived compounds has been gaining increasing attention [1,2,6–8].The RA of biomass-derived carbonyl substances including aldehydes and ketones proposes a potential alternative for the high-efficiency synthesis of renewable amines in mild conditions of temperatures ≤ 120 °C and pressure ≤ 4 MPa [2,9]. For instance, Zhang et al. [7] reported a novel bifunctional Ru/ZrO2 catalyst with co-existence of Ru0 and acidic RuO2 species, realized the efficient conversion of lignocellulose-derived glycolaldehyde into useful ethanolamine at 75 °C and 2 MPa H2. Hara and coworkers [10] reported that Nb2O5 supported Ru catalysts could effectively catalyze RA of various carbonyl compounds at 90 °C. Using activated carbon supported Pd nanoparticles, Iborra et al. [11] attained N-substituted-5-(hydroxymethyl)-2-furfuryl amines (yield up to 100%) in the RA of 5-hydroxymethylfurfural at 100 °C and 0.3 MPa H2. Besides noble metals, base metals, such as nickel (Ni/Al2O3 [5], Ni@SiO2 [12], Ni/MC [13]), cobalt (MOF-Co [14], Co/N–C [15]) and copper (Cu/ZrO2 [16]), also presented high performance for the RA of different carbonyl compounds. Despite these achievements, it is still of significant importance to obtain efficient and stable base metal catalysts for the green production of valuable amines from biomass and its derived carbonyl substances via RA reaction under mild conditions.5-Amino-1-pentanol (5-AP) is a valuable amino alcohol that is extensively required as a pharmaceutical intermediate for the manufacture of therapy for cancer and inflammation, especially the alkaloid manzamine with high medicinal value [17]. Recently, our group reported a novel and environmentally friendly process to sustainably synthesize 5-AP by the RA of bio-furfural derived 2-hydroxytetrahydropyran (2-HTHP) with ammonia [18–20] (Scheme 1 ). The in situ ring cleavage tautomerization of 2-HTHP to reactive 5-hydroxypentanal (5-HP) contributes to the high efficiency of the clean synthesis of 5-AP. Note that based on the relatively higher hydrogenation activity (∼80 times) of the 2-HTHP intermediate as compared with the direct hydrogenolysis of tetrahydrofurfuryl alcohol. Huber and coworkers [21] developed a multi-step process for the synthesis of useful 1,5-pentanediol (1,5-PD) with high tech-economy from bio-fufural derived tetrahydrofurfuryl alcohol. Traditional metal oxides especially for ZrO2 supported Ni catalysts [18] and the hydrotalcite-derived Ni–Mg3AlO x catalysts [19] exhibited high activity with 90.8%–93% 5-AP yield in the RA of 2-HTHP at 80 °C and 2 MPa H2. Nevertheless, the stability of these monometallic Ni catalysts was still unsatisfactory due to the sintering and surface oxidation of activity metal particles, even though the stability of Ni–Mg3AlO x catalyst has been improved a lot (losing 18% activity after 120 h time running) by virtue of the unique layered double hydroxides (LDHs) structure [22,23]. Additionally, despite Ru-based catalysts being much cheaper than Rh- or Ir-based catalysts for the synthesis of 1,5-PD, their stability in the direct hydrogenation of 2-HTHP to 1,5-PD was still unsatisfactory, especially for oxide supported Ru catalysts, losing more than 50% of activity in less than 24 h [21]. Clearly, the stability of the metal catalysts played a critical role in the further application of the bio-fufural derived useful 5-AP and 1,5-PD.Taken that bimetallic catalysts generally present improved catalytic activity, selectivity, and stability in comparison to their monometallic counterparts [24–26] and cobalt based catalysts have also been applied to synthesize amines through RA reaction into consideration [14,15,27], NiCo/Al2O3 bimetallic catalysts with LDHs precursor structure were constructed in this work and investigated not only for the RA of 2-HTHP to synthesize valuable 5-AP, but also the direct hydrogenation of 2-HTHP into 1,5-PD. To our knowledge, there were few studies about NiCo bimetal catalysts in RA reactions [28]. The NiCo/Al2O3 bimetallic catalysts were fabricated by a simple co-precipitation method and much attention has been paid to the structure-activity relationship of the NiCo/Al2O3 nanocatalysts for the RA of 2-HTHP to 5-AP owing to the complex reaction network [18,19] and the long-term stability of the catalysts. The incorporation of Co into Ni/Al2O3 catalysts was discovered to greatly enhance the catalytic stability, not only in the RA of 2-HTHP to 5-AP, but also in the direct hydrogenation to 1,5-PD, probably associating with the formation of Ni–Co nanoalloy which inhibited the sintering and surface oxidation of active metal particles. To further elucidate the reaction mechanism and the difference in activity between active metals, DFT (density functional theory) calculations were also performed on the RA of 2-HTHP, in particular on activation of 2-HTHP in the presence of H2, NH3, and Co or Ni.Ru/Al2O3 (5.0 wt%), Pt/Al2O3 (10.0 wt%) and Pd/Al2O3 (10.0 wt%), 1,5-pentanediol (98%), and dihydropyran (99%) were purchased on Alfa Aesar. 1,2-Pentanediol (98%) and 5-amino-1-pentanol (95%) were obtained from Aladdin Chemical Reagent Co. LTD. Aqueous solution of 2-HTHP (∼21.8 wt%) was prepared through the method of an autocatalytic hydration of dihydropyran presented by Huber et al. [29]. Dihydropyran and deionized water with a mass ratio 1:4 were added into a 2 L autoclave and performed under 2.0 MPa N2, 100 °C for 1 h. The concentration of 2-HTHP was determined by gas chromatograph using 1,2-pentanediol as the internal standard.The Ni x Co y Al-LDH hydrotalcite precursors were synthesized via co-precipitation method under pH∼10 with various chemical component [(Ni2+ ​+ ​Co2+)/Al3+ = 2/1, Ni2+:Co2+ = 1:0, 5:1, 2:1, 1:1, 1:2, 1:5, 0:1]. Briefly, a mixed aqueous solution of Ni(NO3)2·6H2O, Co(NO3)2·6H2O, and Al(NO3)3·9H2O with 0.5 M total metal concentration, and a mixture of alkali with Na2CO3 (1 M) and NaOH (5 M) were poured simultaneously into a three-necked flask while being vigorously stirred. Next, the forming suspension was aged under 80 °C maintain 24 h. And the obtained sediments through filtration and abstersion with deionized water to pH blow 7. After drying at 110 °C 12 h to obtain Ni x Co y Al-LDH and then calcined under 700 °C for 3 h to get Ni x Co y Al-LDO (LDO: layered double oxide). The Ni x Co y Al-LDO samples were activated under 650 °C keeping 3 h with flowing H2 to gain Ni x Co y /Al2O3 catalysts. Finally, the samples were transferred into a glovebox filled with Ar gas under sealed quartz tube after cooling to room temperature.X-ray diffraction (XRD) were performed via Rigaku D/MAX-2400 diffractometer in reflection mode with copper Kα radiation source (λ = 0.15406 nm). The mean diameter of nanoparticles was calculated by Scherrer equation [30]. The in situ XRD characterization was got during programmed temperature to 750 °C with H2 (99.999%) at a flow rate of 80 mL/min with a rate of 5 °C/min and the patterns were recorded from 300 to 750 °C interval 50 °C. X-ray photoelectron spectra (XPS) measurements were tested with ESCALAB250xi spectrometer using an Al Kα source (hν = 1486.6 eV). The C 1s at 284.6 eV served as the reference for validating all binding energies. The catalysts by prereduction were passivated under 1% O2/N2 stream at 30 °C maintained 2 h before taking the XRD and XPS tests.The textural characteristics of the samples containing the BET surface area and average pore diameter were determined through nitrogen adsorption isotherms using a Micromeritics Tristar 3020 under the temperature of liquid nitrogen. The samples were disposed of with N2 flow at 300 °C for 4 h before the test. The scanning electron microscopic (SEM) experiments were accomplished using SU8020 electron microscope at 1 kV. Transmission electron microscopic (TEM) patterns were characterized by a JEM2010 electron microscope. The transmission electron microscope of FEI Talos 200x (USA) was employed to carry out high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping investigations.The H2 temperature-programmed reduction (H2-TPR) was characterized by a Quantachrome Automated Chemisorption Analyzer. Before test, the obtained calcined samples (contain ∼10 mg Ni) were pretreated in helium gas stream at 200 °C for 30 min. The samples were cooled to 35 °C, reduced under a 10% H2/Ar stream rate of 30 mL/min, then heated to 900 °C at a heating ramp of 10 °C/min. The equally quantity of sample after calcined was initially pre-reduced at 650 °C for 2 h with a 10% H2–N2 flow (30 mL/min) for the sake of determination the degree of reduction of the reduced catalysts for reaction. The acquired reduced sample underwent a second H2-TPR following a temperature increase to 900 °C after cooling to below 50 °C, and the TCD captured the signal changes. The reduction degree was then calculated with the formula below [31]: R e d u c t i o n d e g r e e ( % ) = T P R p e a k a r e a o f c a l c i n e d s a m p l e − T P R p e a k a r e a o f r e d u c e d s a m p l e T P R p e a k a r e a o f c a l c i n e d s a m p l e A Huasi DAS-7200 automatic chemical adsorption equipment was used to conduct measurements of NH3 temperature programmed desorption (NH3-TPD). Firstly, the 200 mg of calcined samples underwent a 3-h pretreatment at 650 °C under 5% H2–Ar (30 mL/min). After being cooled to 100 °C, the reduced samples were subjected to a 10% NH3–N2 stream for 1 h (30 mL/min). The sample was subsequently heated up to 600 °C with a 10 °C/min rate under He flow, and TCD was used to record the signal of NH3 desorption.NMR spectra of 5-AP were captured at 25 °C on a Bruker AVⅢ400. Chemical shift values for 1H and 13C are given as δ values (ppm) with respect to the deuterated solvent and coupling constants (J) in Hz.A stainless steel autoclave reactor (100 mL) was used to perform the 2-HTHP RA reaction, with the stirring speed set to 800 r/min. The samples from calcination were activated for 3 h at 650 °C under pure H2 (80 mL/min). A typical experiment involved adding 15 g 2-HTHP (21.8 wt%) aqueous solution to the reactor first, followed by adding the reduced catalyst (0.1 g) and 15 g of aqueous ammonia (25 wt%) to the autoclave operated in the glove box fulling of Ar. After replacing with H2 3 times and pressuring to 2.0 MPa, then preheated the reactor to the needed temperature, which remained constant throughout the reaction process.On a tubular fixed-bed reactor, the stability of the screened catalyst was tested at 80 °C and 2 MPa H2. Loading with the calcined Ni x Co y O-LDO sample (2.0 g) in the center section of the reactor, which had mesh sizes ranging from 40 to 60. Both sides were then covered in quartz powder (20–40 meshes). Before the run, the calcined sample was activated with a reduction program to 650 °C kept for 3 h in pure H2 with an 80 mL/min flow rate. When the reactor cooled to the target reaction temperature, and pressurized to 2 MPa with H2, at a weight ratio of 1:1, the mixed solution was infused into the reactor with a rate of 5 g/h containing 2-HTHP aqueous solution and ammonia. The reaction solution was gathered every 5–10 h in the gas-liquid separator.The reactant products were evaluated via a GC-MS system (Agilent 7890A/5975C). The identified products includes 5-amino-1-pentanol (5-AP), 5-imino-1-pentanol (5-IP), 1,5-pentanediol (1,5-PD), THP-oxypentanimine (THPOPI), 5-[(5-hydroxypentyl)imino]-1-pentanol (5-HPIP), and di-1-pentanolamine (DPA). For quantitative analysis, 1,2-pentanediol (1,2-PD) was used as the internal standard. The 2-HTHP conversion and product selectivity were demonstrated as reported previously [19,20].Geometries of molecules and intermediates involved in reactions depicted in Chart 1 were adequately optimized on basis of DFT, adopting the hybrid functional M06-2X [32], with the addition of triple-ζ 6–311++G∗∗ basis for light elements (H, C, N, and O) and the LANL2DZ basis ​+ ​ECP for metals. When required, more isomers were considered. The effects of solvent were included utilizing the implicit solvation model SMD [33]. M06-2X functional and SMD solvation models are acknowledged as accurate for reaction energy prediction. To determine whether the stationary points were actually minima (no negative frequencies), harmonic approximation vibrational frequency calculations were performed on optimized geometries or 1st order saddle points (a negative frequency) for stable molecules or transition states, respectively. Thermochemical values were computed under T = 298.15 K and p = 1.00 atm as well as the harmonic frequencies and IR values.Relaxed scans of the potential energy hypersurface about the ring-opening dihedral angle δ (C(α), C(β), C(α’), O) of 2-HTHP were performed (±5-degree 36 steps). All computations including all atomic species used the integration grid for the electronic density's 974 angular points and 250 radial shells. Double electron integrals and their derivatives were calibrated with an accuracy of 10−12 a.u. The Self-Consistent Field (SCF) methodology employed was the Bacskay-designed quadratically convergent approach, which is considered slower but more accurate than conventional SCF with DIIS deduction [34]. Set the root-mean-square (RMS) variation of the density matrix convergence criterion as 10−10 and the maximum variation of the convergence criterion of the density matrix is set as 10−8. Convergence requirements for geometry majorizations were established as follows: 2 × 10−6 a.u. for maximum force, 1 × 10−6 a.u. for RMS force, 6 × 10−6 a.u. for maximum displacement, and 4 × 10−6 a.u. for RMS displacement. The GAUSSIAN G16.C01 package was used to execute all of the calculations. Table 1 displays the textural properties of Ni x Co y /Al2O3 samples with diverse Ni/Co ratios. The concentrations of Ni and Co in Ni x Co y /Al2O3 catalysts measured by XRF were close to their nominal loadings. The BET surface area decreased generally from 124.3 m2/g for Ni/Al2O3 without Co to 41.6 m2/g for monometallic Co/Al2O3 sample with the decrease of Ni/Co mole ratios in Ni x Co y /Al2O3 samples. In addition, the average pore diameter of the bimetallic Ni x Co y /Al2O3 catalysts was located in the mesopore range of 18.4–27.6 nm (Table 1, Fig. S1). Fig. 1 displays the XRD profiles of Ni x Co y Al-LDH, Ni x Co y Al-LDO, and Ni x Co y /Al2O3 samples with different Ni/Co molar ratios. The diffraction peaks at 2θ = 23.5°, 35.2°, 39.6°, 47.2°, 61.3°, and 62.6° corresponding to (006), (009), (015), (018), (110), and (113) planes for typical LDHs structure were revealed in all Ni x Co y Al-LDH samples (Fig. 1a), respectively [35–37]. No Ni or Co phase was detected as a separate crystalline phase, indicating the successful incorporation of the metals into the LDHs structure. In addition, the characteristic reflections gradually broadened as the Ni content increased, which may associate with the intercalation of more Ni chelates in the hydrotalcite layers [36,38]. Fig. 1b shows the XRD patterns of the hydrotalcite-derived Ni x Co y Al-LDO samples after calcination at 700 °C. The reflection peaks of the LDHs structure disappeared completely and were replaced with the presence of the characteristic diffraction peaks of NiO or Co3O4 or their mixture. For monometallic CoAl-LDO (Fig. 1b, A), the main diffraction peaks at 2θ = 31.2°, 36.8°, 44.8°, 55.6°, 59.3°, and 65.2° were observed, assigning to the lattice planes of Co3O4 spinel with (220), (311), (400), (422), (511), and (440), respectively (JCPDS 78–1970). With the decrease of cobalt amount and the increase of Ni loadings in the Ni x Co y Al-LDO samples, the spiculate of the diffraction peaks assigning to Co3O4 spinel weakened gradually, meanwhile diffraction peaks at 2θ = 37.3°, 43.3°, and 62.9° assigning to the lattice planes of NiO (111), (200), and (220) (JCPDS 71–1179) appeared and intensified (Fig. 1b, B−E). No diffraction peaks belonging to Al2O3 support were observed in all samples, demonstrating that Al2O3 existed in an amorphous form.After activating under 650 °C with H2, the metal oxides diffraction peaks disappeared, meanwhile three distinctive diffraction peaks at around 2θ = 44.4°, 51.7°, and 76.0° were found, assigned to the crystal planes of face-centered cubic (fcc) nickel or cobalt (Fig. 1c) [39]. Detailed characterization presented that the characteristic diffraction peak of the (111) plane for fcc Ni and Co of monometallic Ni/Al2O3 and Co/Al2O3 catalysts centered at 44.5° and 44.3° (Fig. 1d), respectively. Worth mentioning, in bimetallic catalysts, the (111) plane in this region shifted toward the centre, probably attributing to the incorporation of the Co atoms into the Ni lattice in bimetallic catalysts, that is the generation of Ni–Co alloy phase during high temperature treatment in H2 [40,41]. The mean metal crystallite sizes of the Ni x Co y /Al2O3 catalysts as obtained from the Scherrer equation slightly decreased from around 7.9 nm for monometallic Ni/Al2O3 to 7.0 nm for Ni1Co1/Al2O3 and then gradually increased to 11.8 nm for monometallic Co/Al2O3 with increasing Co content (Table 1). The slightly lower crystallite size for the bimetallic Ni1Co1/Al2O3 probably originated from the peak broadening caused by the heterogeneity of the bimetallic composition. Fig. 2 displays the detailed structural variation of the representative Ni x Co y /Al2O3 catalysts during the in situ XRD reduction. For monometallic NiAl-LDO (Fig. 2a), the characteristic peaks of nickel oxide gradually reduced with increasing reduction temperature, simultaneously the characteristic peaks of metal Ni0 vanished at 2θ = 37.3°, 43.3°, and 62.9° after the reduction temperature elevated up to 650 °C, demonstrating the reduction of NiO to Ni0. As for monometallic CoAl-LDO (Fig. 2c), the diffraction peaks of Co3O4 presented to 31.2°, 36.8°, 44.8°, 55.6°, 59.3°, and 65.2° became weaker and eventually disappeared with increasing temperature, meanwhile, the diffraction peak of CoO appeared at 42.3° between 450 °C and 550 °C and subsequently transformed to metal Co0 at a temperature above 600 °C. These results indicate the consequent reduction of Co3O4 to CoO and further to Co0, which is consistent with previous research on supported Co catalysts [42,43]. For the bimetallic Ni2Co1Al-LDO sample, the diffraction peaks of NiO or Co3O4 generally disappeared with increasing temperature to 550 °C and above, accompanied by the appearance of diffraction peaks at 43.8°, 50.9°, and 75.5°, which may relate with the formation of Ni–Co alloy phase as mentioned above (Fig. 2b). The temperature for the obvious presence of metallic phase in the Ni2Co1Al-LDO sample is at least 50 °C lower than that of monometallic NiAl-LDO and CoAl-LDO, showing that the reducibility of the bimetallic oxide sample is greatly improved. Fig. 3 displays the SEM morphology of Ni x Co y Al-LDO and reduced Ni/Al2O3 catalyst. A three-dimensional flower-like layer structure was observed in all calcined samples, indicating that the layered microstructure of hydrotalcite was preserved even after calcination at 700 °C. Note that the stratified structure of the NiAl-LDO was mainly preserved during activation under 650 °C with H2 (Fig. 3d), which is consistent with previous findings [19]. Such unique structure features would provide a high exposure of active sites for the catalytic reactions.TEM images of reduced samples were taken to reflect the microstructure of the monometallic and bimetallic Ni x Co y /Al2O3 (Fig. 4 a–c). High dispersion of uniform metal Ni was clearly described on Ni/Al2O3 catalyst, while the distributions of metal particles became poorer with introducing Co. The average sizes of the metal nanoparticles gradually increased from 7.1 nm for Ni/Al2O3 to 11.4 nm for Co/Al2O3 with increasing Co loading, agreeing well with the XRD results (Table 1). For the Ni2Co1/Al2O3 catalyst, the HRTEM images showed lattice spacings of approximately 0.202 nm for Ni–Co particles (Figs. 4d and S2). Comparing with the standard lattice distance of Ni(111) (0.204 nm) and Co(111) (0.205 nm), the slightly smaller NiCo(111) lattice spacing may associate with the formation of lattice-strained configuration and strong interaction between bimetallic atoms [44]. Furthermore, the high-angle annular dark-field (HAADF)-STEM was also taken for Ni2Co1/Al2O3 (Fig. 4e), which displayed the high dispersion of bimetallic NiCo nanoparticles. The elemental mapping demonstrated homogeneous distributions of Ni, Co, Al, and O elements (Fig. 4f–i). Meanwhile, the line-scan spectra of elements Ni and Co (Fig. 4j) illuminated similar Gaussian distributions along with a single particle, confirming the formation of Ni–Co alloy nanostructure [45]. There have been claimed that the generation of bimetallic alloy particles is helpful for inhibiting the sintering of active metal nanoparticles [24]. Thus, the generation of Ni–Co alloy nanoparticles in our case may contribute to the greatly improved reaction stability of the bimetallic catalysts.H2-TPR experiments were performed to explore the reductive behavior and the interaction of metal oxides with the support of Ni x Co y Al-LDO catalyst precursors (Fig. 5 ). Monometallic NiAl-LDO showed a couple of H2 consumption peaks with a minor one at around 460 °C and a major one at about 737 °C. The minor H2 consumption peak might be associated with the reduction of greatly distributed Ni oxides on the surface, while the major peak might be ascribed to the reduction of Ni2+ species with a strong metal-support association [19]. For the CoAl-LDO sample, two distinct reduction peaks with one at 475 °C and another broad one centered at about 725 °C were observed, which might be associated with the transition of Co3O4 to CoO and then transferring to Co [42,46]. Such a reduction process is in consistent with the in situ XRD study (Fig. 2c). There are two reduction areas for calcined bimetallic Ni x Co y Al-LDO within the scope of 400–500 °C and 600–700 °C (Fig. 5B-F), which are likely attributable to the reduction both of Co3O4 and NiO. Both reduction peaks shift generally towards lower temperatures compared with monometallic samples, indicating that the incorporation of Co species into bimetallic Ni x Co y Al-LDO samples enhanced the reduction of bimetallic samples, which is also reflexed by the calculated reduction degree shown in Table 1. The above results illustrate the possible existence of Ni–Co strong interaction in bimetallic Ni x Co y Al-LDO samples, which facilitates the reduction of metal oxide as a consequence [44].According to the reports that the surface acidic site of the catalyst could facilitate the adsorption and activation of CO group and promote the generation of imine by the condensation of CO with NH3 [7,47]. Meanwhile, the production of the iminium ion may facilitate imine intermediate hydrogenation on account of the abundance of acid sites [48,49]. The surface acidity of the Ni x Co y /Al2O3 catalysts was characterized by NH3-TPD. As shown in Fig. 6 , three distinct NH3 desorption regions were observed at 150–250 °C, 250–400 °C, and 400–600 °C, which might be associated with the acidic sites of weak (WA), medium-strength (MSA), and strong (SA), respectively [7,50]. The concentration of acidic sites especially for medium-strength and strong of Ni x Co y /Al2O3 catalysts gradually decreased with the increase of cobalt content (Fig. 6 and Table S1), which may be result of the increasing amount of CoO x species with a rather low acidity in the catalysts. A previous study by Li and coworkers [44] also found that supported Co catalysts presented lower acidity than those Ni catalysts. Table 2 presents the catalytic performances of a series of LDHs-derived monometallic and bimetallic NiCo/Al2O3 catalysts in the conversion of 2-HTHP by RA to 5-AP at the condition of 60 °C and 2.0 MPa H2. A high conversion of 2-HTHP (91%) was detected even without a catalyst, on account of the high reactivity of 2-HTHP in the presence of ammonia [19]. The main by-products, 5-IP and THPOPI, were found with selectivity of 43% and 29%, respectively (entry 1). The condensation of 5-IP intermediate with 2-HTHP is most likely to generate THPOPI. Similar results were achieved with Al2O3 support as a catalyst (entry 2). The Ni x Co y /Al2O3 catalysts derived from hydrotalcite with different chemical compositions exhibited different catalytic activities for the RA of 2-HTHP (Table 2, entries 3–10). For monometallic Ni/Al2O3 catalyst with 72% Ni loading, a high selectivity of 90% to the target product 5-AP was achieved at 100% conversion of 2-HTHP, and then the selectivity of 5-AP further increased to 93% under an optimum temperature of 80 °C (Table 2, entry 4 and Fig. S3). With the increase of cobalt from a Ni/Co ratio of 5:1 to 0:1, not only the conversion of 2-HTHP dropped to 96%, but also the selectivity of 5-AP decreased monotonously from 87% on Ni5Co1/Al2O3 to 9% over Co/Al2O3, and the imine product 5-IP increased gradually from 0 to 38%. The selectivity to the secondary imine 5-HPIP, generated from the condensation of the target product 5-AP with the 5-HP intermediate, exhibited a trend of first increasing and then decreasing. Meanwhile, a higher amount of DPA, a secondary amine by-product, was detected with a selectivity of 9% over cobalt-riched Ni1Co5/Al2O3 and Co/Al2O3 with 67% and 78.6% Co loadings (Table 1), respectively. These results indicate that cobalt-riched catalysts not only presented lower activity for the conversion of 2-HTHP, but also much inferior in the hydrogenation of imine intermediates to the target amine product as compared with nickel-riched catalysts. The catalytic results of conventionally impregnated monometallic Ni and Co catalysts further supported the above findings (entries 10, 11). Ni/Al2O3-IM showed medium 5-AP selectivity (33%) at full 2-HTHP conversion while Co/Al2O3-IM presented not only lower 2-HTHP conversion (94%) but also rather low 5-AP selectivity (∼0%). Moreover, commercial Al2O3 supported noble metals of Pd, Ru, and Pt (all with 10% metal loading), as well as Raney Ni, were also investigated for the catalytic RA of 2-HTHP (entries 13–16). These catalysts demonstrated rather low catalytic performances for the synthesis of 5-AP, with only the Raney Ni catalyst exhibiting slightly higher 5-AP selectivity of 54% at 94% 2-HTHP conversion.Clearly, the above results indicate that the LDHs-derived Ni-based nanocatalysts, particularly for those with high Ni/Co ratios (i.e. ≥ 5:1), exhibited outstanding catalytic activity for the RA of 2-HTHP. Since the influence of the crystallite sizes and reduction degree of the Ni x Co y /Al2O3 catalysts, especially the catalysts with Ni/Co ratio greater than 1:5 showed lower catalytic activity (Table 1), the monotonous decrease of activity for the generation of 5-AP of the Ni x Co y /Al2O3 catalysts (Table 2, entries 3, 5–10) would be associated with the nature of Ni and Co in the hydrogenation of imine intermediates, and the surface acidity of the catalysts. It has been claimed that the rate-controlling step in RA of carbonyl compounds is the hydrogenation of imines [15], and the hydrogenation of 5-IP to 5-AP is deduced to be the rate-controlling step for this reaction [19]. Therefore, the advantageous catalytic activity of the Ni-riched Ni x Co y /Al2O3 catalysts might primarily account for their higher activity in imine intermediates hydrogenation as compared with the Co-riched catalysts. Larger metal particle size and lower degree of reduction on the Co-riched catalysts (i.e. Ni:Co ratio ≤1:5, Table 1), which mean lower amounts of active surface sites, would also partially result in their inferior hydrogenation activity for the imine intermediates. The experiments on the direct hydrogenation of 2-HTHP to synthesize 1,5-PD at similar conditions as the RA of 2-HTHP except for adding deionized water instead of ammonia solution showed the same trend as the RA of 2-HTHP, that is both the conversion of 2-HTHP and 1,5-PD selectivity decreased with increasing Co content (Table S2). Such finding further supported the inferior hydrogenation activity of the Co component at the low reaction temperature of 60 °C, although several reports showed Co-based catalysts presented high RA activity at temperatures above 100 °C [14,15,26]. Despite 2-HTHP could facilely react with NH3 through reactive 5-HP intermediate to form imine intermediates of 5-IP and THPOPI even without the presence of a catalyst (Table 2, entries 1, 2), the presence of larger amounts of surface acidic sites of the Ni-rich catalysts (Fig. 6) may promote the formation of imine intermediates and their further hydrogenation as reported previously [7,47–49], which would to some extent contributes to the higher RA activity of these catalysts.It is well known that stability is an important fact for a heterogeneous catalyst. Nonetheless, metal catalysts, especially monometallic catalysts, often suffer from sintering, surface oxidation of active sites, and/or loss of active components in the RA reaction in the presence of ammonia [5,8,47]. Although the stability of the Ni–Mg3AlO x catalysts with hydrotalcite precursor structure prepared by co-precipitation improved a lot as compared with the Ni/ZrO2 catalyst synthesized by a conventional method of impregnation in the RA of 2-HTHP, the stability of the former is still unsatisfactory, deactivation obvious after 90 h running [19]. Herein, the stability of the Ni2Co1/Al2O3 bimetallic catalyst derived from hydrotalcite precursor was first evaluated in the RA of 2-HTHP to see whether the incorporation of Co could enhance the stability of the catalyst or not. To our delight, the catalyst maintained high stability during 180 h running (Fig. 7 a). The conversion of 2-HTHP retained under 100% and the selectivity of target product 5-AP just slightly declined from around 90% to close to 82% during the 180 h running. The deactivation rate based on the decrease of 5-AP yield was calculated to be 9% after 180 h time on-stream, which is obviously lower than that of Ni–Mg3AlO x catalysts (18% after 120 h reaction) and Ni/ZrO2 (∼19% after 90 h reaction) reported previously [18,19]. Then, we compared the stability of the NiCo/Al2O3 bimetallic catalysts with the monometallic Ni/Al2O3 at a five times higher feeding rate of WHSV = 2.5 h−1 (Fig. 7b). The yield of 5-AP for Ni/Al2O3 noticeably decreased from 72% to 39% after 60 h reaction. In contrast, the yield of 5-AP for the bimetallic Ni5Co1/Al2O3 and Ni2Co1/Al2O3 just slightly decreased from 70% to 54% and from 62% to 52%, respectively. The deactivation rate for the monometallic Ni catalyst reached 46%, which is ∼2.2 and 2.9 times higher than that of the latter bimetallic catalysts, revealing the outstanding stability of the NiCo bimetallic catalysts. Note that the Ni2Co1/Al2O3 bimetallic catalyst also presented good stability in the reaction of 2-HTHP direct hydrogenation to synthesize useful 1,5-PD. No appreciable decrease in the yield of 1,5-PD was displayed after 180 h of time on-stream (Fig. 7c). Previous studies by Huber and co-workers [21] showed that the oxide supported Ru catalyst, i.e. Ru/TiO2, suffered from rapid deactivation in the hydrogenation of 2-HTHP to synthesize 1,5-PD, losing more than 50% of activity in less than 24 h.Clearly, the above findings reveal the high stability of the NiCo bimetallic catalyst with LDH precursor structure, and the incorporation of Co into the Ni/Al2O3 catalysts could remarkably increase the stability of the bimetallic catalyst, although the incorporation of Co to some extent lowered the selectivity to the product of 5-AP (Table 2), which is probably due to the low hydrogenation activity of Co at the low reaction temperature of 60 °C. As discussed above, the strong interaction between the metal nickel and cobalt species assisted the formation of a highly dispersed alloy structure, which would eventually contribute to the increased stability of the bimetallic catalyst as compared with the monometallic Ni/Al2O3 catalyst [44,51]. It should be remarked here that the Ni–Al2O3 nanocatalysts with similar Ni loadings (∼50 wt%) prepared by a similar co-precipitation method reported previously by our group presented substantially good stability in the RA of not only 2-HTHP (biomass-derived aldehyde) [20], but also 5-diethylamino-2-pentanone (biomass-derived ketone) [52] without appreciable deactivation during 150 and 200 h running, respectively. The somewhat better stability of these Ni–Al2O3 nanocatalysts as compared with the Ni x Co y /Al2O3 catalysts presented in this work is probably due to that the former catalysts prepared with lower metal ions concentration (0.1 M vs. 0.5 M in this work) and thus exhibited higher metal dispersion (smaller Ni particle sizes, i.e. ∼5.4 nm vs. 7–8 nm in this work) [52]. Taken together, despite the stability of the NiCo bimetallic catalysts with LDH precursor structure needs further improvement, the incorporation of Co exactly promoted the stability of the bimetallic catalysts, and may shed light on designing novel bimetallic catalysts for not only RA reaction but also several other reactions, such as hydrogenation and oxidation.To further investigate the improvement in stability with the incorporation of Co, the used Ni2Co1/Al2O3 and Ni/Al2O3 catalysts after 60 h at a higher feeding rate were characterized by XRD, TEM, and XPS. As shown in Fig. 8 a and b, apparent diffraction peaks of Ni0 and Ni–Co alloy were observed at around 2θ = 44.4°, 51.7°, and 76.0°, and no additional peaks appeared after the RA reaction. The Ni–Co alloy characteristic diffraction peaks were well maintained and no separate phases of Ni0 and Co0 could be seen (Fig. 8b). The Ni crystallite size of Ni/Al2O3 catalysts significantly grew from 7.9 nm to 14.6 nm after the reaction, while the Ni–Co alloy particles just slightly grew from 7.8 nm to 10.1 nm under similar reaction conditions, showing that the incorporation of Co could retard the sintering of the active metals. TEM characterizations supported the findings by XRD that a more obvious growth of metal particles on the used Ni/Al2O3 than on the used Ni2Co1/Al2O3 (Fig. 8c and d). XRD and TEM characterization of the Ni2Co1/Al2O3 catalysts after 180 h time-on-stream RA of 2-HTHP or direct hydrogenation of 2-HTHP at WHSV of 0.5 h−1 also confirmed the good stability of the bimetallic catalyst (Figs. S5 and S6). XPS characterization was also presented to reveal the variation of the valence of the surface Ni species before and after the catalysts were used. As displayed in Fig. S4, a large amount of Ni0 species on monometallic Ni/Al2O3 catalyst was surface oxidized to Ni2+ after utilization, as the Ni0/(Ni0 + Ni2+) ratio decreased obviously from 42.6% to 10.9%, while the ratio for Ni2Co1/Al2O3 slightly dropped from 31.3% to 24.0%. It has been reported that zero-valent metal is the active site for the hydrogenation of imine intermediate to form amine [15,52], the decline in the Ni0/(Ni0 ​+ ​Ni2+) ratio means the decrease in hydrogenation sites. Obviously, the above findings by XPS indicate that the incorporation of Co could retard the surface oxidation of the more active Ni0 species, which may associate with its higher reducibility (Figs. 2 and 5) and eventually contributes to the high stability of the NiCo bimetallic catalyst during the reaction. The inevitably dissolved oxygen in the reactant caused the variation of the valence on the surfaced active metal during the long-term time-on-steam reaction. Taking the generation of Ni–Co alloy by incorporating Co into consideration, the Ni–Co alloy species in the bimetallic catalyst could on one hand inhibit the sintering of the metal particles, on the other hand, retard the surface oxidation of active metals during the reaction, and thus remarkably improved of stability of the bimetallic catalysts.Previously, in our proposed reaction pathway for the RA of 2-HTHP, 2-HTHP is first equilibrated with its ring-cleavaged tautomer of 5-HP, which is quickly condensed with NH3 to obtain the 5-IP imine intermediate, followed by the hydrogenation to the 5-AP in the presence of adequate H2 [18,19]. DFT calculations were performed in this work to provide a computational model for potential reaction routes. Reactant 2-HTHP has two possible conformers (namely, A and B), resting with the position of the hydroxyl group (axial or equatorial, respectively). Conformer A is less stable than conformer B (ΔH = 0.8 kJ/mol and ΔG = 0.9 kJ/mol); these two conformers may convert one to the other by means of tautomerization and rotation about σ bond (vide postea). Either conformer A or B admits further rotamers, deciding by the orientation of the OH group about the C–O bond; the energy differences between the stable rotamers are negligible, as the rotational energy barriers. This is found also for other species involved in the reaction paths; in general, the spacial orientation of OH and NH groups origins rotamers, with small energy differences among them. The reactions involving 2-HTHP have been computed for conformers A and B separately; the other reactions are common for the two conformers (and hence the computed values are the same). Reactant 2-HTHP may undergo three primary reactions: (i) a monomolecular reaction of ring opening to give 5-HP (reaction 1 in Chart 1); (ii) hydrogenation to give 1,5-PD (reaction 3); and (iii) addition of ammonia with elimination of water to give 5-IP (reaction 7). It may also react with products of reactions 3 and 7 to give by-products by addition and condensation (reactions 5 and 9, respectively); for these reactions activation energies were not computed. Reaction 1 may hypothetically proceed via two different ways, viz. radical ring opening (1r) or tautomerization (1t). Reaction 1r is very energetically unfavored (Table 3 ), whereas reaction 1t has a lower value of ΔH and ΔG, even without metal presence. Moreover, reaction 1r implies two subsequent steps, viz. the radical breakage of the O–C bond and the subsequent intramolecular hydrogen shift (whose values of ΔH ‡ and ΔG ‡ are depicted in brackets). The values of ΔH ‡ and ΔG ‡ of reaction 1t are much lower than those of reaction 1r; the presence of metal further reduces these values. The computed values of ΔG for reaction 1t are compatible with the experimental values of equilibrium composition at the room temperature reported in the literature [28]. The Boltzmann populations calculated at T = 298.15 K and p = 1.00 atm using the calculated thermochemical quantities are x(1-HTHP): x(5-hydroxypentanal) = 95.4%:4.6%. We can therefore conclude that reaction 1 proceeds via tautomerization and not via ring opening. Once the tautomerization of 2-HTHP has occurred, 5-hydroxypentanal may react either with H2 (reaction 2) or NH3 (reaction 6); reactions 2 and 6 are competitive. However, reaction 6 has a higher rate than reaction 2 ​at a low reaction temperature. Note that the calculations for reactions 1 and 7 were also repeated by modeling the metal surface with a (2 frozen ​+ ​2 relaxed)-layer slab composed of 6 ​× ​6 metal atoms; a few possible anchorings (top, bridge, hollow) and molecule's orientations were tested but only those giving the lowest energy differences were not rejected. As expected, the numerical values are quantitatively different from those obtained with the above (more simplified) model; however, the overall trend and general conclusions that can be dragged from these results remain the same.For reaction 3, ΔH ‡ and ΔG ‡ are not available, since no transition state was identifiable. The target product 5-AP may be obtained either via the addition of ammonia to 1,5-PD and the release of water (reaction 4) or hydrogenation of 5-IP (reaction 8). The activation energy of reaction 4 is rather high, which means the dehydrogenation amination of 1,5-PD is very hindered at low temperatures. Reaction 8 has a larger activation energy than reaction 6; this supports the experimental evidence that reaction 8 is the rate-controlling step for the synthesis of 5-AP [19]. The reason why without a catalyst the reaction of the addition of ammonia is more favored than hydrogenation of 2-HTHP may be the reaction medium include a significant amount of unreacted ammonia, whereas H2 not only needs to get dissolved in the solution, but also needs to be activated over metal sites. The by-product 5-HPIP may not form via two subsequent reactions of 10 and 11, but more likely by means of the reaction between 5-AP and the reaction intermediate 5-HP [19]. Finally, the presence of metal always reduces the activation energies, confirming its role as a catalyst; in particular, Ni is more active than Co in each reaction, supporting the rather low activity of Co catalyst in the RA of 2-HTHP (Table 2).A series of Al2O3-supported monometallic and bimetallic Ni–Co nanocatalysts with diverse Ni/Co ratios derived from hydrotalcite precursors were synthesized through co-precipitation approach and used for the catalytic RA and direct hydrogenation of biofurfural-derived 2-HTHP to synthesis useful 5-AP and 1,5-PD, respectively. Both yields towards the target 5-AP and 1,5-PD products decreased with increasing incorporation of Co. However, the introduction of Co improved the reducibility of the NiCo/Al2O3 bimetallic catalysts and promoted the reaction stability of the bimetallic catalysts in both reactions with over 180 h time-on stream. Detailed characterization of the catalysts before and after the reaction indicated that the incorporation of Co resulted in the formation of NiCo alloy, which helps to inhibit the agglomeration of the metal particles and hinder the surface oxidation of the more reactive Ni0 species as compared with Co0. DFT-based modeling of the reaction mechanisms confirmed the reaction pathway proposed that 5-AP is formed via the hydrogenation of 5-IP intermediate, and also supported the higher reactivity of Ni in the RA of 2-HTHP to synthesize 5-AP as compared with Co. The important findings in this work would shed light on the development of more reactive and stable nanometal catalysts in the RA or hydrogenation of carbonyl compounds from not only biomass but also fossil resources to produce useful chemicals.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by the National Natural Science Foundation of China (21872155, 22102198, and 22272187), the Strategic Pilot Science and Technology Project of the Chinese Academy of Sciences (XDA21010700), and the CAS "Light of West China" Program. The authors also acknowledge the helpful discussion for Prof. George W. Huber at the University of Wisconsin-Madison.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.gce.2023.01.003.

Al2O3-supported monometallic Ni, Co, and bimetallic Ni–Co nanocatalysts originated from layered double hydroxide precursors were synthesized by co-precipitation method, and used for the synthesis of useful 5-amino-1-pentanol (5-AP) and 1,5-pentanediol (1,5-PD) by reductive amination (RA) or direct hydrogenation of biofurfural-derived 2-hydroxytetrahydropyran (2-HTHP), respectively. In both reactions, the yield of the target products decreased monotonously with the increasing amounts of Co in the NiCo/Al2O3 catalysts, owing probably to the replacement of highly reactive Ni by Co component with inferior hydrogenation activity at the low reaction temperature of 60 °C. However, the incorporation of Co could improve the reducibility of the NiCo/Al2O3 bimetallic catalysts and promote the reaction stability of the catalysts, especially for Ni2Co1/Al2O3, in both reactions with over 180 h time-on-stream. Characterization of the catalysts before and after the reaction showed that the incorporating Co could inhibit the sintering of metal particles and hinder the surface oxidation of the more reactive Ni0 species, thanks to the formation of Ni–Co alloy in the bimetallic catalysts. DFT-based modeling of the reaction mechanisms is also performed, supporting the reaction pathway proposed previously and also the much higher activity of Ni in the RA of 2-HTHP as compared with Co.

Alkynyl carboxylic acids are an important class of compounds owing to their existence in a variety of biologically active natural products [1]. The direct carboxylation of terminal alkynes with CO2 is a particularly useful synthetic method since CO2 is inexpensive, easily-available, non-toxic and thus can be considered as an ideal C1 subunit in chemical synthesis [2,3]. To date, a number of catalyst systems for terminal alkyne carboxylations have been reported [4–9], and these reactions are conducted most frequently with noble metal Ag catalysts. Though several homogenous copper catalytic systems were found in the carboxylation reaction [10–13], further application was seriously impeded by the recyclability and reusability of the catalysts as well as the metal contamination of the products. To solve these problems, a handful of Cu-based heterogeneous catalysts have been developed for the alkyne carboxylation with CO2 in recent years. For instance, He and co-workers showed CuBr supported on activated carbon as recyclable catalyst for the carboxylation reaction [14]. Kim's group recently incorporated CuCl2 within polyvinyl imidazolium tri-cationic ionic liquid and found that such a specie exhibited an enhanced activity in catalyzing the coupling of alkyne and CO2 [15].Coordination polymers (CPs) and their unique subclass of metal-organic frameworks (MOFs) are an emerging class of functional organic-inorganic hybrid materials, which can be directly adopted as heterogeneous catalysts or as catalyst supports/precursors for many important organic transformations [16–20]. Recently, a few MOFs, such as MIL-101 [6], ZIF-8 [21], and UiO-66 [22], have been used as supports for the preparation MOF-supported Ag nanoparticle catalysts for the coupling of alkynes with CO2. In contrast, reports on the catalytic CO2 fixation with only MOFs as catalysts are much more scarce [23,24]. The copper(II) MOF-catalyzed carboxylation of terminal alkynes, first reported by Zhao [23] and later developed by Verpoort [24], demonstrated the feasibility of the utilization of copper(II) MOFs as efficient heterogeneous catalysts. Very recently, we reported a mixed-ligand [Cu4(μ3-OH)2]-cluster-based CP catalyst for carboxylation of terminal alkynes with CO2 [25]. Further development of new Cu(II)-based MOF catalysts for these types of reactions is, therefore, highly desirable.According to the Irving-Williams stability series [26], late 3d transition metal ions (soft Lewis acids, such as Co(II), Ni(II), Cu(II) and Zn(II)), being coordinated by N-heterocyclic ligands (soft Lewis bases, such as imidazole-, triazole-, and pyridine-based compounds), tend to favor the formation of stable complexes. In our previous work, the flexible fluorinated bis(1,2,4-triazle) ligand 1,4-bis(1,2,4-triazole-1-ylmethyl)-2,3,5,6-tetrafluorobenzene (Fbtx) has been proved to be a good candidate to assemble with Cu(II) and Co(II), affording stable MOF catalysts for several diverse transformations [27–29]. On the other hand, developing a fast and efficient method for the preparation of MOF-based catalysts is of great significance. In this aspect, the use of microwave approach is much more appealing than the conventional solvothermal method owing to the apparent advantages such as shorter reaction time, fast kinetics of crystal nucleation and growth, higher yields, and better reproducibility [30]. Very recently, we have also reported the application of microwave assistance in the rapid production of a Cu(II)-based MOF catalysts [25]. In this contribution, we report the microwave-assisted synthesis, structural characterization, and catalytic properties of a new Cu(II)-based MOF [Cu(Fbtx)2Br2] n (denoted as CZU-7). Single-crystal X-ray diffraction analysis revealed that CZU-7 features a two-dimensional coordination framework with the 44-sql topology. Furthermore, the crystalline CZU-7 was shown to be an efficient heterogeneous catalyst for the direct carboxylation of terminal alkynes with CO2 under mild conditions.A mixture containing Fbtx (31.3 mg, 0.1 mmol), CuBr2 (22.3 mg, 0.1 mmol) and distilled water (6 mL) was placed in a 100-mL Teflon-lined container. The container was sealed and then irradiated at 100 °C in microwave oven with power 300 W for 100 min. After cooling down to room temperature, blue block crystals of CZU-7 were collected, washed with distilled water, and dried at room temperature (Yield: 79.3% based on Fbtx). Anal. calcd for C24H16Br2CuF8N12: C 34.00, H 1.90, N 19.82%; found: C 34.52, H 1.91, N 19.75%. Selected IR peaks (KBr pellet, cm−1): 3168 (w), 3126 (m), 3088 (m), 3033 (w), 2971 (w), 2946 (w), 1526 (s), 1493 (s), 1438 (w), 1398 (m), 1358 (w), 1329 (w), 1289 (s), 1274 (s), 1224 (w), 1196 (m), 1135 (s), 1044 (s), 1032 (s), 999 (m), 987 (m), 916 (w), 892 (m), 862 (m), 793 (m), 752 (s), 670 (s), 637 (w), 570 (w).In a typical experiment, a 50-mL stainless steel autoclave, equipped with a magnetic stirring bar, was charged with CZU-7 (33.9 mg, 1 mol%), Cs2CO3 (1.96 g, 6 mmol), 1-ethynylbenzene (0.41 g, 4.0 mmol) and DMF (20 mL). Once added, CO2 (0.3 MPa) was introduced into the reaction mixture under stirring at 100 °C for 16 h. After the reaction, the mixture was cooled to room temperature and monitored by high-performance liquid chromatography (HPLC, Shimadzu LC-10 CE).For a long time, developing a fast and efficient method for the preparation of CP-based materials is of great significance. In this work, CZU-7 was synthesized by microwave-assisted hydrothermal reaction at 100 °C, which led to a significant yield (79.3%) with 100 min of reaction time. In contrast, reaction of CuBr2 with Fbtx under hydrothermal conditions at 120 °C for 3 days gave rise to blue single crystals of CZU-7 in 58.2% yield. The phase purity of the samples of CZU-7, prepared by microwave-assisted method and hydrothermal method, was confirmed by similarities between the as-synthesized and simulated PXRD patterns (Fig. S2). Scanning electron microscopy (SEM) analysis was performed to determine the morphology and size of CZU-7 prepared using the microwave method. Fig. S3 shows that the CZU-7 material is mainly consisted of microparticles formed from packing of nanosheets with the thickness of about 120 nm. CZU-7 is insoluble in water and in most of common organic solvents, such as ethanol, acetonitrile, dichloromethane, DMF, and DMSO.X-ray structural analysis revealed that CZU-7 crystallizes in the centrosymmetric triclinic space group P 1 ¯ . The asymmetric unit contains half of a Cu(II) ion, two halves of crystallographically unique Fbtx ligands, and one terminal-coordinated bromide ion. As shown in Fig. 1a, the Cu(II) center displays a distorted octahedral geometry, surrounded by four nitrogen atoms (N1, N1#1, N4 and N4#1) from four Fbtx ligands with the CuN bond distances of 2.003(7) and 2.017(8) Å and two bromide atoms (Br1 and Br1#1) from two bromide ions with the CuBr bond distance of 3.110(7) Å. Each Fbtx ligand adopts the anti-conformation to avoid the steric hindrance and connects adjacent Cu(II) ions to form a 2-D window-shaped layer with 44-sql topology (Fig. 1b). The adjacent layers are stacked in a staggered pattern along the b-axis, which are further interconnected via interlayer π···π interactions between the tetrafluorinated benzene rings with a centroid-centroid separation of 3.41 Å (Fig. 1c). The 3D lattice of CZU-7 has no solvent-accessible area, as calculated by the PLATON package [31]. The catalytic activity of CZU-7 for carboxylation of terminal alkynes with CO2 was investigated with 1-ethynylbenzene using Cs2CO3 in DMF at 100 °C over 16 h. The catalytic activity of CZU-7 for carboxylation of terminal alkynes with CO2 was investigated with 1-ethynylbenzene using Cs2CO3 in DMF at 100 °C over 16 h. The conversion increased with the catalyst loading (1 mol% of Cu) and reached 87% (Table 1 , entries 1–4), and the reaction proceeded with 100% selectivity in favor of phenylpropiolic acid. The catalytic efficiency was slightly lower than that of our previous reported Cu-MOF system [25], and was comparable to those reported for Cu(IN)-MOF [24]. The reaction using the CZU-7 catalyst obtained from microwave method resulted in a slightly higher conversion than that obtained from hydrothermal method (entry 5), which may be attributed to the morphology of nanostructures. Lower temperatures (40 to 80 °C) reduced the conversion, while a higher temperature (120 °C) did not increase the conversion (Table 1, entries 6–9). We also found that a temperature of 100 °C and a reaction time of 16 h were required (Fig. S4) The screening of different solvents (Table 1, entries 1 and 10–15) revealed that DMF was found to be the most effective for the carboxylation reaction to give a 87% conversion (Table 1, entry 4), which may be due to the fact that DMF is a better solvent for both CO2 (DMF is a weak base) and Cs2CO3 than the other organic solvents [32].Variation of Cs2CO3 to the other bases like Na2CO3 and K2CO3 under otherwise identical conditions led to the decrease of the conversion into 17–29% (Table 1, entries 16 and 17). Surprisingly, if we added 3 mmol CsCl to the reaction system of K2CO3, the conversion elevated from 29% to 41% (Table 1, entry 18), thus revealing that Cs+ could promote this reaction. It was observed that CsF and CsOAc were inactive to this reaction (Table 1, entries 19 and 20). The results indicated that Cs2CO3 was the suitable base for deprotonation of terminal alkynes [11,33]. The impact of CO2 pressure on the reaction was also investigated (Fig. S5). The carboxylation reaction took place smoothly under a CO2 pressure of 0.3 Mpa. To gain insight into the catalytic mechanism, several control experiments were conducted. In the absence of CZU-7, the reaction of 1-ethynylbenzene with CO2 resulted in negligible conversion (Table 1, entry 5). The introduction of ligand Fbtx did not promote the reaction at all (Table 1, entry 21). Copper salts (CuBr2, Cu(NO3)2 and CuSO4) and copper oxides (Cu2O and CuO) were found to catalyze the carboxylate reaction with moderate conversion (Table 1, entries 22–26), which implied that the introduction of metal Lewis acidic catalysts is favorable.To extend the substrate scope, our further examination focused on the carboxylation of several representative terminal alkyne substrates (Table 2 ). Under the optimized reaction conditions of CZU-7 (1 mol% of Cu), 6 mmol Cs2CO3, 20 mL DMF, 100 °C, and 0.3 Mpa CO2, the corresponding carboxylation products were obtained in moderate to good yields (65–84%) when aromatic alkynes with either an electron-donating group (Table 2, entries 2–5) or electron-withdrawing groups (Table 2, entries 6–11) were used. Even for an alkyne with a heteroaromatic ring group (Table 2, entry 12), a yield of 72% was achieved. The aliphatic alkyne 1-hexyne successfully underwent the carboxylation reaction to give the corresponding product (Table 2, entry 13).Thermogravimetric analysis (TGA) measurement indicated that CZU-7 can maintain their structural integrity until the collapse of the coordination frameworks beginning at ca. 245 °C (Fig. S6). The temperature-dependent PXRD patterns (Fig. S7) showed that its structure was retained after the crystalline samples were heated at 220 °C in air for 6 h. To check the stability of CZU-7, we carried out leaching and recycling experiments. When the filtrate of the reaction mixture after catalysis was used instead of CZU-7 under identical conditions, the conversion was negligible (Fig. S8). Inductively coupled plasma-mass spectrometry (ICP-MS) analysis showed that no copper species leached into the supernate. These results demonstrate the heterogeneous nature of the catalyst. To determine the catalyst recyclability, the carboxylation of 1-ethynylbenzene with CO2 catalyzed by CZU-7 was performed under the same conditions as described above except for the use of the recovered catalyst. As shown in Fig. 2 , the recovered catalyst could maintain its good activity after five consecutive cycles. PXRD patterns (Fig. S9) and FT-IR spectra (Fig. S10) of the recovered samples are almost identical to those of the as-synthesized one, preserving the structural integrity of the material during catalysis. To determine the oxidation state of copper before and after reactions, XPS experiments of CZU-7 have been carried out (Fig. S11). The binding energy of Cu2p3/2 at 933.41 and Cu2p1/2 at 953.24 eV for the sample is assigned to the characteristics of the Cu δ+ (δ < 2) species [27]. The observation of the Cu 2p peaks demonstrated no change in the oxidization state of copper, further implying the CZU-7 catalyst is stable during the reaction processes. Moreover, the XPSPEAK fitting results of copper species of CZU-7 revealed that simulated peaks for various Cu forms were fit to the measured peak areas. It was observed that the ground complex catalyst with a smaller particle size (100–300 mesh) could accelerate the rate of the carboxylation. Because the surface area of the catalyst is very small, this reaction should be catalyzed by the active sites on the surface of the catalyst.On the basis of the above results and the previously suggested reaction mechanisms involving the Cu(II)-based complexes in alkyne carboxylation reactions [15,24,34], we propose a plausible mechanism shown in Scheme 1 . First, terminal alkyne coordinates to the copper(II) center of the Cu(II) CP, enhancing the acidity of the alkyne CH bond. Then, deprotonation reaction of terminal alkyne by Cs2CO3 affords the Cu(II) acetylide intermediate A. Meanwhile, the insertion of CO2 into sp-hybridized carbon–copper bond leads to the formation of the Cu(II) propiolate intermediate B, which subsequently reacts with another terminal alkyne and Cs2CO3 releasing cesium propiolate, simultaneously regenerating Cu(II) acetylide intermediate A. At the end of the reaction, the acidification of cesium propiolate affords the propiolic acid product.In summary, a new stable Cu(II)-based MOF CZU-7 with the 44-sql topological structure was synthesized via a facile and fast microwave heating method. The as-synthesized CZU-7 catalyst could serve as an efficient and recyclable catalyst for the direct carboxylation of terminal alkyne with CO2. This work highlights the feasibility of using Cu(II)-based MOF materials as heterogeneous catalysts for CO2 conversion. Zhong-Hua Sun: Investigation, Data curation, Writing – original draft. Xin-Yan Wang: Methodology, Validation. Kun-Lin Huang: Software, Visualization. Ming-Yang He: Visualization, Resources. Sheng-Chun Chen: Investigation, Writing – review & editing, Conceptualization, Funding acquisition.We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Heterogeneous catalytic carboxylation of terminal alkynes with CO2 over a copper(II)-based metal-organic framework catalyst”.We gratefully acknowledge financial support by the National Natural Science Foundation of China (21676030), and Jiangsu Province Prospective Industry-University-Research Cooperative Research Program of China (NO. BY2016029-08). Supplementary material 1 Image 2 Supplementary material 2 Image 3 Supplementary material 3 Image 4 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106472.

Developing efficient, inexpensive and robust heterogeneous catalysts for CO2 conversion is greatly important. In this study, a new stable copper(II)-based metal-organic framework [Cu(Fbtx)2Br2] n (denoted as CZU-7, CCDC-2165700, Fbtx = 1,4-bis(1,2,4-triazole-1-ylmethyl)-2,3,5,6-tetrafluorobenzene) was synthesized via a facile and fast microwave heating method. CZU-7 exhibits a two-dimensional layer structure with the 44-sql topology. This material was demonstrated to be an efficient heterogeneous catalyst for the direct carboxylation of 1-ethynylbenzene with CO2, and various propiolic acids were synthesized in moderate to good yields under optimized reaction conditions. Moreover, the catalyst could be easily recovered and reused five times without significant loss of catalytic activity.

Data will be made available on request.Rising environmental issues and diminishing resources caused by increase in the usage of fossil fuels encourage researchers to find alternative sources of energy which is renewable, sustainable, and eco-friendly [1–4]. Biomass has significant potential as an alternative energy source because of its high energy density and minimal environmental impact; however, raw bio-oils derived from biomass contain a mixture of organic compounds with fatty acids as one of the main components, resulting in large amount of oxygen (30–40 wt%). This results in high viscosity, low vapor pressure, high corrosiveness, and low stability of these bio-oils which limit their direct application as transportation fuels [5–8]. Thus, improvement of the quality of bio-oils are indispensable to satisfy the standard requirements of transportation fuels.The fatty acids in the bio-oils can be converted into liquid hydrocarbons by several methods, such as decarboxylation (DCO) [9–11], hydrodecarbonylation (HDC) [12,13], and hydrodeoxygenation (HDO) [14,15]. The ultimate aim of these methods is to remove the oxygen content in the bio-oils. In DCO, the fatty acid is converted by removing the carboxyl group resulting in CO2 as the side product, while in HDC, the fatty acid is initially converted into aldehyde before CC bond cleavage occurs to produce CO as the side product. The difference between these methods lies in the reaction environment. Under inert conditions, decarboxylation mainly takes place over decarbonylation. On the other hand, decarbonylation dominantly occurs over decarboxylation in H2 environment [16,17]. Both reactions generally requires high temperature because of the stability of the CC bond. Due to the removal of CO2 or CO in these methods, the main product is a hydrocarbon with a less carbon atom, which is undesirable in terms of atom economy. Meanwhile, HDO converts fatty acids by using H2 and remove O by forming water in the process, involving series of hydrogenation and dehydration reactions. Hence, compared to DCO and HDC, HDO generally requires higher H2 pressure to occur, depending on the type of feed [10,18]. Nevertheless, HDO is more favorable than the other methods because it maintains the carbon efficiency and produces more environmentally friendly byproduct in the form of water [19–21].The HDO of fatty acids is challenging due to low reactivity of the carboxylic group [22]; therefore, efforts have been made using various catalysts to improve the reactivity. Noble metal catalysts (Pt, Pd, Ru, Rh) are generally used because of their high deoxygenation activity even at low temperatures and hydrogen pressures [23]. Meanwhile, non-noble metal catalysts such as Ni and Co have also been explored [24–27]. Although their deoxygenation activity is not as high as the noble metal catalysts at similar loading level, they are more affordable [28]. Nevertheless, both catalysts are more likely to remove oxygen through DCO or HDC mechanism because of their great affinity towards η1 (C)-acyl configuration [29–32]. For example, Ni/ZSM-5 was used in the conversion of palmitic acid, and favored the formation of pentadecane over hexadecane, highlighting the preference of DCO or HDC mechanisms over HDO [33]. Therefore, modifications of the catalysts have been conducted to alter the deoxygenation pathway. For example, Phan et al. [34] modified Pt/UiO-66 using amine groups for deoxygenation of stearic acid to n-alkanes. The defective-UiO-66 improved the HDO activity significantly due to the increased number of accessible Pt sites, acidity, and basicity of the catalysts. Moreover, defects induced by the amine groups allowed H-transfer for HDO because of enhanced H2 adsorption on the support. Kon et al. [35] loaded MoOx on Pt/TiO2 to modify interaction between Pt and the Lewis acid sites formed by MoOx/TiO2, which led to a synergistic effect on lauric acid HDO to dodecane with full conversion and 86 % dodecane yield at 180 °C. For non-noble metals, Mo has been widely combined with Ni or Co in the traditional HDO catalysts (NiMo/Al2O3 or CoMo/Al2O3) in reduced or sulfided form to modify the carboxylic acid-catalyst interaction [36,37]. Other promoters, such as WOx [38], ReOx [39,40], and Nb2O5 [41,42] have also been used to increase Lewis acid sites for activation of the CO group of fatty acids, thereby preventing HDC/DCO reaction.HDO of fatty acids over modified noble metal catalysts generally results in the formation of alcohols [40,43]. To obtain hydrocarbons as the final product, alcohols must be dehydrated over acidic supports, such as Al2O3 [44,45] and zeolites [46–48]. The dehydration of alcohols leads to formation of alkenes, which can be hydrogenated rapidly over the noble metal sites. This means that the HDO of fatty acids to hydrocarbons over noble-metal based catalysts requires a promoter to control the activity of noble metals and an acidic support to convert alcohols into the final product. These reactions can be seen as a cascade reaction.Ruthenium, one of the noble metals, has an excellent hydrogenation activity similar to Pt and Pd [22,49]; thus, the modification of Ru with promoters is important to regulate the activity of Ru. Addition of Sn [2,50], B [51], or Mo [52] could improve the HDO activity due to the changed electron density of Ru resulting from its interaction with the promoter. For Sn, the interaction of Ru and Sn can result in two different active sites, Ru3Sn7 and Ru-SnOx, which depend on the support used. Both active sites can catalyze the reduction of fatty acids to alcohols, although the mechanism is slightly different. The Ru3Sn7 alloy catalyzes reduction of fatty acids to alcohols through the changed electron density of both Ru and Sn, resulting in higher hydrogenation activity of Ru and higher CO adsorption capability of Sn [50]. On the other hand, the Ru-SnOx reduces fatty acids to alcohols through CO activation on Lewis acid sites formed by SnOx and hydrogenation on Ru [53,54]. Comparing both active sites, the reduction activity is higher over Ru3Sn7 alloy than Ru-SnOx because of lesser side reactions. On Ru-SnOx, the presence of isolated Ru may cause side reactions of HDC or DCO of fatty acids, while the presence of isolated SnOx may catalyze esterification reaction [53]. Therefore, Ru3Sn7 is more preferred active site for the HDO of fatty acids.As aforementioned, in order to achieve full HDO of fatty acids to hydrocarbons, specific sites for dehydration must also exist on the catalyst. Using Al2O3 as support for RuSn catalysts may result in high dehydration activity but formation of the Ru3Sn7 alloy phase may be hindered because of the strong metal-support interaction between Sn and Al2O3 as previously reported [55–57]. Thus, designing supports with low metal-support interaction to enable the formation of Ru3Sn7 alloy without suppressing their dehydration activity is highly desirable, and is the main focus of this study.Herein, we report for the first time the HDO of octanoic acid to C8 hydrocarbons with high yields over RuSn catalysts with Ru3Sn7 as the main active site on SiO2-doped Al2O3 (SiAl) support. The deposition of SiO2 enables the formation of Ru3Sn7 on Al2O3 support, minimizing side reactions. The effect of temperature, H2 partial pressure, contact time, and SiO2 loading were investigated to understand the important factors and derive plausible mechanism for the reaction.The commercial supports used in this study were SiO2 (hydrophilic fumed silica, CAB-O-SIL M−5) obtained from Cabot Co. Ltd. (USA) and γ-Al2O3 from Alfa Aesar Co. Ltd. (USA). The chemicals used in the catalyst synthesis were RuCl3·xH2O (99.9 %), SnCl4·5H2O, (98.0 %), ethanol (>99.5 %), ammonium nitrate (NH4NO3, 99.999 %), polyvinylpyrrolidone (PVP, K-30), sodium borohydride (NaBH4, 99 %), all purchased from Sigma Aldrich Co. Ltd. (USA), tetraethylorthosilicate (TEOS, 99 %), obtained from Alfa Aesar Co. Ltd. (USA), and hydrochloric acid (HCl, 37 %), obtained from Samchun Chemicals Co. Ltd. (South Korea). The chemical used in the reactivity studies was octanoic acid (>98 %), obtained from Sigma Aldrich Co. Ltd. (USA). The standards used for quantitative analysis were octanal (99 %), dioctyl ether (99 %), 1-octene (99 %), trans-2-octene (97 %), all obtained from Sigma Aldrich Co. Ltd. (USA), trans-3-octene (97 %), from Alfa Aesar Co. Ltd. (USA), and octanol (99 %), from Acros Organics Co. Ltd. (USA). The solvent used for gas chromatography (GC) was acetone, purchased from Daejung Chemicals Co. Ltd. (South Korea).The SiAl supports were synthesized via a liquid-phase deposition method as reported elsewhere [58]. Briefly, a solution containing TEOS and ethanol (0.693 g of TEOS corresponds to 200 mg of SiO2) was mixed with 1 g of Al2O3. Hydrolysis of TEOS was conducted by adding 0.13 g NH4NO3 to the solution with continuous stirring at 40 °C for 168 h. The sample was then collected by filtration and dried at 100 °C for 24 h. The sample was calcined at 500 °C for 4 h with a ramping rate of 1 °C/min. The synthesized supports were denoted SiAl(x:y), where x and y are the nominal loadings in wt% of SiO2 and Al2O3, respectively. For example, SiAl(1:9) indicates that the nominal loading of SiO2 in the support is 10 wt%, while that of Al2O3 is 90 wt%.The RuSn catalysts were synthesized by a sol-immobilization method following a reported procedure [59]. For 10 g of support, a certain amount of RuCl3·xH2O and SnCl4·5H2O were mixed in 200 mL of DI water to obtain a Ru loading of 1.4 wt% and Sn/Ru molar ratio of 2 calculated based on previous study [60]. Then, 3.6 g of PVP was added to the solution, which was then cooled to near 0 °C in an ice bath under continuous stirring. An aqueous solution of 0.1 M NaBH4 (NaBH4/metal mol ratio = 5) was added rapidly to the mixture. After 30 min, 10 g of finely ground support (SiO2, Al2O3 and SiAl(x:y)) was then added to the solution and 0.1 M HCl was added to decrease the pH of the solution to 1–3. The solution was stirred overnight at room temperature. The solid suspension in the solution was separated by centrifugation at 3000 rpm for 10 min, and the recovered solid was washed with distilled water five times to remove unreacted precursors, and dried then for 12 h at 100 °C. The catalysts were then pelletized and activated in the reactor at 460 °C for 4 h in H2 atmosphere with a ramping rate of 5 °C/min prior to reaction.The surface area of the catalysts and supports were acquired using an ASAP 2420 apparatus (Micromeritics, USA). The samples were first degassed for 30 min at 90 °C, followed by heating at a constant temperature of 300 °C for 4 h. The bulk phase of the catalysts was analyzed by powder X-ray diffraction (PXRD) using a Rigaku Ultima IV X-ray diffractometer (Japan) with a Cu Kα radiation beam (λ = 0.15418 nm) operated at 45 kV and 200 mA. The measurements were performed within a scanning range of 10 to 80° at a scan rate of 2°/min and step size of 0.02°.X-ray absorption spectroscopy (XAS) was carried out to analyze the local atomic structure of the catalysts at Ru K-edge (22.117 keV). The analysis was performed at the 7D beamline located at the Pohang Accelerator Laboratory (PLS-II, 3 GeV, South Korea). The spectra of the samples were recorded at room temperature in transmission mode. Ru foil and commercial RuO were used as standards for the calibration. The obtained data were analyzed using the Athena and Artemis programs. For the extended X-ray absorption fine structure (EXAFS) analysis, the Fourier-transforms of the background-subtracted data were obtained using a Hanning window in the k range of 3–12 A with a dk of 1 A−1.Temperature-programmed desorption (TPD) measurements were conducted to analyze the acidity of the catalysts and the supports using a Micromeritics AutoChem II 2920 instrument (USA) equipped with a thermal conductivity detector (TCD). The catalysts were reduced at 460 °C for 4 h in H2/Ar. After reduction, the temperature was reduced to 100 °C under pure He, and then 5 %NH3/He was introduced to the sample for 1 h. The NH3 desorption was then carried out at elevated temperatures from 100 to 800 °C with a ramping rate of 5°/min under He flow.The acid sites of the supports were also characterized by pyridine adsorbed infrared spectroscopy (IR) measurements using a JASCO FT/IR-4200 spectrometer equipped with a triglycerine sulfate (TGS) detector. The samples were pressed into self-supporting wafers with a diameter of 2 cm (ca. 30 mg) which was placed at the center of an infrared flow cell equipped with KBr windows maintained at 25 °C. The samples were pretreated at 460 °C in H2 for 1 h and cooled to room temperature under vacuum. Spectra were recorded in absorbance mode (128 scans, 4 cm−1 resolution) in the region of 4000–1000 cm−1. Subsequently, the samples were exposed to 0.5 kPa of flowing pyridine vapor for 15 min and evacuated at 150 °C for 15 min. The spectra were taken at room temperature after evacuation. The amounts of Brønsted acid sites (BAS) and Lewis acid sites (LAS) were determined from the area of the peaks at ca. 1540 and 1445 cm−1, respectively. The molar adsorption coefficient for the band at 1540 cm−1 was 1.67 cm μmol−1 and that for band at 1445 cm−1 was 2.2 cm μmol−1 [61].The Si, Al, and O surface chemical states of the synthesized SiAl supports were obtained by X-ray photoelectron spectroscopy (XPS) analysis with an Axis Supra equipment (Kratos Analytical Ltd., UK) equipped with a monochromatic Al Kα source (1486.7 eV). The Si and Al core-level spectra were recorded in the 2p region, while the O core-level spectra were recorded in the 1 s region. Before analysis, the catalysts were reduced at 460 °C for 4 h in H2/Ar and sealed in a glass vial.Particle size distributions in the catalysts were obtained by Field emission transmission electron microscopy (FE-TEM) analysis using a Tecnai F30 S-Twin (Thermo Fisher Scientific, USA) operated at 300 kV. The catalysts were ground into fine powder and reduced under H2/Ar at 460 °C for 4 h before analysis.The synthesized catalysts were tested for gas phase HDO of octanoic acid. The reaction was conducted in a continuous down flow fixed-bed reactor (SUS 316) with a reactor diameter of 0.75 cm and length of 40 cm. The reactor was equipped with heating jacket, in which the temperature can be controlled to 700 °C. The reactor was also equipped with a pre-heater and a line-heater that were set at 250 °C to ensure vaporization of the octanoic acid before entering the reactor. The H2 gas was mixed with the feed in the pre-heater zone. The flowrate of the gas was controlled using a mass flow controller (MFC, Brooks Instrument 5850E Series (USA)). The reactor was also equipped with a chiller operated at −5 °C to condense reaction products and separate them from the H2 gas.In a typical reaction test, 1 g of catalyst was placed inside the reactor on top of quartz wool. The catalyst volume was set at 3 mL by adding inert glass beads. The catalyst was then activated inside the reactor at 460 °C for 4 h in H2 with a ramping rate of 5 °C/min. The reactor was then cooled to the desired reaction temperature and pressurized with H2 to 20 atm using a back-pressure regulator located at the end of the reactor system. The H2 and vaporized octanoic acid feed were introduced to the reactor at a H2/octanoic acid molar ratio of 70.8. The weight hourly space velocity (WHSV) was set at 1 h−1, calculated using the following equation: (1) W H S V h - 1 = octanoic acid mass f lowrate ( g h ) Amount of catalyst ( g ) The effect of H2 partial pressure and contact time were investigated at 350 °C and 20 atm. For the study of the effect of H2 partial pressure, the total flowrate of gas was fixed at 180 mL/min while the gas composition was varied using inert N2 as balance. To check the effect of contact time, the catalyst amount was varied while the H2/octanoic acid feed molar ratio was kept at 70.8. The WHSV was varied from 1 to 40 h−1 which corresponds to contact time from 0.025 to 1 h, calculated according to the following equation: (2) C o n t a c t t i m e h = Amount of catalyst ( g ) octanoic acid mass f lowrate ( g h ) The reaction products were collected every hour and analyzed using an off-line gas chromatograph (GC, Younglin Chromass 6500 GC, South Korea) equipped with a flame ionization detector (FID), an autosampler (Younglin YL3000A, South Korea), and a DB-624 column (30 m × 0.32 mm × 1.8 μm, Agilent Technologies, USA). The measurements were taken after steady state conditions were achieved, and all the presented data points were obtained by averaging three measurements after stabilization. In all cases, the mass balance reached 98±2 %. The reactants and products were quantified by their column retention time in comparison with standards. Details of the analysis procedure are given in the Supplementary Information (SI).Catalytic activity was expressed by conversion of octanoic acid (Eq. (3)) and selectivity to the products (Eq. (4)), defined as follows: (3) C o n v e r s i o n % = 1 - n OA , 1 n OA , 0 × 100 (4) P r o d u c t s e l e c t i v i t y % = v i n i ∑ v i n i × 100 In these equations, υ is the carbon number of species i (8 for 1-octanol, octanal, 1-octene, and i-octene; 16 for dioctyl ether and octyl octanoate), ηi,1 is the number of moles of the product and ηi,0 is the initial number of moles of the compounds.The physicochemical properties of the catalysts and the supports are summarized in Table 1 . The BET surface area of the supports ranged from 185 to 251 m2/g and did not change significantly after deposition of the metals, indicating no substantial modification of textural properties of the supports.The bulk and crystal structures of fresh and spent catalysts were identified by PXRD shown in Fig. 1 a and Fig. S1, respectively. For RuSn/SiO2, a broad feature can be observed around 2θ of 20°, corresponding to the amorphous SiO2 support. Meanwhile, for the other catalysts, peaks observed at 2θ of 39.7°, 46.0°, and 66.7° (depicted as diamond symbol) correspond to γ-Al2O3. In addition to the support peaks, discernible features can be observed at 2θ of 30.1°, 32.9°, 35.8°, 40.9°, and 43.1°, which match well with the (310), (222), (321), (411), and (420) planes of Ru3Sn7 alloy with cubic crystal structure (PDF No. 26–504). This indicates that the crystalline phase of Ru species in the catalysts was mainly RuSn alloy phase. However, this phase was not observed in RuSn/Al2O3, possibly due to strong interaction of Sn with Al2O3, which stabilized Sn in an oxidation state close to +2 and hampered the reduction of Sn and interaction with Ru [55]. The crystallite sizes of the Ru3Sn7 alloy were calculated from the peak broadening of the (411) plane by the Scherrer equation and the results are reported in Table 1. The crystallite size of the fresh catalysts increased in the order, RuSn/SiAl (2:8) (3.5 nm) < RuSn/SiAl (3:7) (4.2 nm) < RuSn/SiAl (1:9) (5.5 nm) < RuSn/SiO2 (9.4 nm). The crystallite sizes of the spent catalysts increased (Table 1 and Fig. S1) which indicate possible agglomeration of metal sites after reaction.The local atomic structure of the catalysts was examined using X-ray absorption spectroscopy (XAS) at the Ru K-edge as shown in Fig. 1b and c. Fig. 1b shows the normalized Ru K-edge XANES spectra of the catalysts and references with absorption edge energy, E0, shown as vertical lines. Depending on the charge distribution of the observed atom, the E0 might shift to higher or lower energy because of changing chemical environment. The XANES spectra of RuSn/Al2O3 show a similar E0 energy to that of the Ru foil at 22.116 eV, indicating that the charge distribution in RuSn/Al2O3 is similar to the Ru foil. This suggests that there is almost no interaction between Ru and Sn in RuSn/Al2O3. In contrast, the XANES spectra of RuSn/SiO2 and RuSn/SiAl (3:7) show that the E0 shifted to lower energies (E0 = 22.110 keV). This means that the presence of Sn in these catalysts modified the local chemical environment of Ru by changing the electron density of Ru through electron transfer from Sn to Ru. The direction of electron transfer is consistent with Pauling electronegativity differences, where Ru has larger Pauling electronegativity (2.2) than Sn (1.96). Unlike RuSn/Al2O3, the interaction of Ru-Sn in these catalysts gives clear evidence for alloy formation.The Fourier-transform EXAFS spectra (Fig. 1c) show the interaction of Ru atom with other atoms based on radial distances. Peaks corresponding to Ru-O and Ru-O-Ru which are located at radial distances of 1.59 and 2.73 Å were not observed in all the catalysts, suggesting that all catalysts have Ru in the reduced form. For RuSn/Al2O3, an intense peak was observed at a radial distance of 2.34 Å, corresponding to the Ru-Ru bond length. On the other hand, for RuSn/SiO2 a peak located at a higher radial distance of 2.53 Å was observed, attributed to the Ru-Sn bond length [62,63]. Interestingly, the RuSn/SiAl (3:7) shows a similar peak location as RuSn/SiO2, indicating that Ru and Sn interacts strongly in these catalysts, further confirming the result obtained by XRD. Thus, the results provide clear evidence that Ru-Sn interaction exists in both RuSn/SiO2 and RuSn/SiAl (3:7), while only Ru0 phase is present in RuSn/Al2O3.The strength of acid sites on the supports and catalysts were determined by NH3-TPD. Desorption profiles are shown in Fig. 1d-1 for supports and Fig. 1d-2 for the corresponding supported RuSn catalysts. Generally, peaks below 200 °C, between 200 and 400 °C, and above 400 °C are considered weak, medium, and strong acid sites, respectively. The Al2O3 support showed three desorption peaks located at 166, 295, and 406 °C (Fig. 1d-1), which indicates that Al2O3 has three different acid sites that correspond to weak, medium and strong sites [64,65]. The strength of Lewis acidity depends on the coordination of the Al and O atoms, in which the lower coordination number brings stronger acidity [66,67]. With the addition of 30 % SiO2, the peaks that was initially located at 295 and 406 °C shifted to higher temperatures of 313 and 697 °C, meaning that strong acid sites was introduced with deposition of SiO2. The increased acid strength could be attributed to the formation of Brønsted acid sites through the interaction of SiO2 and Al2O3 in the form of isolated silanols (SiOH) anchored on the surface of Al2O3 or the bulk transformation of Al2O3 to SiO2-Al2O3 as reported in several literatures [68–70].When Ru and Sn were added to the supports, different NH3 desorption profiles can be observed (Fig. 1d-2). On Al2O3, the medium and strong acid strengths increased significantly as indicated by the shifting of the peaks at 166 to 170, and 406 to 538 °C. This indicates that the addition of metals could increase the acidity as well. This could be achieved by forming new acid sites such as SnOx due to its strong interaction with Al2O3 [71]. For RuSn/SiAl (3:7) catalyst, the addition of metals slightly shifted the medium acid sites from 313 to 297 °C while two strong acid peaks appeared at 604 and 684 °C. The shifting of medium acid sites could be attributed to the deposition of metal on the support, while the strong acid sites became enhanced caused by the formation of SnOx similar to the RuSn/Al2O3.Further verification of the acid sites on the supports was carried out by pyridine FTIR as shown in Fig. S2 for pyridine adsorption and Fig. 1e for the respective OH region. The pyridine adsorption spectra of SiO2 (Fig. S2) showed no bending vibrations, suggesting that SiO2 has negligible acidity. For Al2O3, bending vibrations were observed at 1450 and 1493 cm−1, corresponding to pyridine adsorbed on Lewis acid sites and on both of Lewis and Brønsted acid sites, respectively [72,73]. The bending vibration at 1540–1550 cm−1 that is characteristic for the Brønsted acid sites was not observed, indicating that acidity of Al2O3 is dominated by Lewis acid sites. With the addition of SiO2, the adsorption band intensity at 1450 cm−1 decreased significantly. This indicates that SiO2 blocked the Lewis acid sites on Al2O3. Although there was no adsorption band of pyridine related to Brønsted acid sites (1540 cm−1), the adsorption band at 1493 cm−1 could be an indicator of the presence of Brønsted acid sites by comparison of the absorbance intensity at that wavelength between the supports. With the addition of 10 % SiO2, the absorbance intensity at 1493 cm−1 was similar to that for the pure Al2O3; however, the intensity at 1450 cm−1 for the former was slightly lower, suggesting that the comparable intensity at 1493 cm−1 was caused by the presence of Brønsted acid sites. A further increase in the SiO2 loading decreased the absorbance intensity at 1493 cm−1, which is due to decrease in Lewis acid sites. Comparing the absorbance intensity between SiAl (2:8) and (3:7), the absorbance was higher for SiAl (3:7) at 1493 cm−1 while the opposite trend was observed at 1450 cm−1, suggesting that the decreasing Lewis acid sites is counterbalanced by the newly formed Brønsted acid sites. In addition, the vibration band at around 1550 cm−1 that belongs to Brønsted acid sites started to appear on the SiAl (3:7) support. Correlating with the NH3-TPD results (Fig. 1d) reveals that the appearance of stronger acid sites for SiAl support was caused by the appearance of the Brønsted acid sites.As aforementioned, the presence of Brønsted acid sites on SiAl supports was due to the formation of silanol species when SiO2 was deposited on Al2O3; however, there are two possible forms of silanol species on Al2O3. The first one is by the bulk transformation of Al2O3, in which Si atoms replaced Al atoms in the lattice to form Si-OH-Al, and the other one is by the formation of silanol species anchored on the top of Al2O3. To determine which of the two silanol species exists in the SiAl supports, the FTIR spectra of the evacuated supports in the OH stretching region were recorded and shown in Fig. 1e. The SiO2 showed one vibration band at 3745 cm−1 attributed to Si-OH species. For Al2O3, the bands at 3768, 3728, 3680, and 3590 cm−1 were assigned to the vibration of terminal OH over one tetrahedrally-coordinated Al ion, terminal OH over an octahedrally-coordinated Al ion, bridging OH, and triply-bridging OH groups, respectively [74,75]. As SiO2 loading increased, a new vibration band at 3738 cm−1 appeared. This vibration band is assigned to the isolated silanols over Al2O3 support [70]. The lower frequency of this band compared to the one in parent SiO2 (3745 cm−1) indicates that the silanols have greater OH bond ionicity, resulting in stronger Brønsted acid sites [69]. This is consistent with the earlier interpretation that the increased acid strength observed by NH3-TPD is related to the presence of isolated silanols over Al2O3. Additionally, the peak at 3610 cm−1 assigned to OH groups of Si-OH-Al was not observed in all the SiO2-containing supports. This suggests that the formation of bulk Si-OH-Al species did not occur, but rather the deposited SiO2 was formed on top of the Al2O3 support, clarifying that only surface modification of Al2O3 occurred with SiO2 deposition.The surface states of the SiAl supports at Si 2p and Al 2p regions were determined by XPS and reported in Fig. 1f and Fig. S3. The adventitious C 1s carbon peak was used as the reference peak at 284.8 eV. The Si 2p spectra of SiO2 showed a very distinct peak at 103.7 eV, a typical peak of the Si4+ of SiO2 [76,77]. The addition of SiO2 on Al2O3 resulted in a shifted spectra to lower binding energy as compared to the pure SiO2 and the shifts became even larger as SiO2 loading increased, indicating a decrease in the Si oxidation state from +4 to between 0< x <4. The reduction of Si oxidation state in SiAl supports means that Si is interacting with other species in the form of Si-O-M, which is typically observed in aluminosilicates, where M is not Si but most likely Al [69]. The increase in the SiO2 loading led to increase in the number of SiO2 interacting with Al2O3 species, resulting in larger shifts in the binding energy. Meanwhile, the Al 2p spectra shows a similar peak location for all samples at 74.2 eV (Fig. S3). However, peak deconvolution reveals two distinct contributions at 75.2 eV that belongs to AlOH species and at 74.2 eV that corresponds to Al-O species [77–79]. The peaks at 75.2 eV did not change significantly with increasing loading of SiO2, while that at 74.2 eV shifted to lower binding energy with increasing SiO2 loading. This indicates that the deposition of SiO2 modified Al-O interaction, and gives additional evidence for interaction between Si, O and Al species. The interaction of Si with Al possibly induced the formation of terminal silanols which are anchored on Al2O3 and are likely the main reason for the increased acidity in SiAl supports, as confirmed previously by NH3-TPD and pyridine FTIR results.The structural images of the catalysts were captured by FE-TEM and are shown in Fig. 2 and Fig. S4. It can be observed that metal particles were distributed evenly in all catalysts. In all the catalysts, except RuSn/Al2O3, the lattice spacing of the metal particles (identified by yellow lines) was calculated to be 0.22 nm, which corresponds well with the lattice spacing of the Ru3Sn7 alloy (411) plane. On the other hand, the lattice spacing for RuSn/Al2O3 was calculated to be 0.21 nm, matching well with the lattice spacing of the Ru0 (111) plane. These results provide further evidence that RuSn alloys were not formed on RuSn/Al2O3 in accordance with the XRD result. Fig. 2 also shows the size distributions of the catalysts and the average particle size determined from 100 particles for each sample observed in the FE-TEM images. It can be seen that the average metal particle size decreased with the presence of SiO2; furthermore, the metal particles became more narrowly distributed. This may be related to strong interactions between Sn and Ru in the alloys which minimized agglomeration of the Ru species [56,80]. Thus, it can be inferred that the presence of SiO2 on Al2O3 modified the interaction between the metals and the supports which was beneficial to the formation of the RuSn alloy phase in the SiAl support compared to the pure Al2O3 support.The reactivity of the catalysts in the HDO of octanoic acid was tested at a total pressure of 20 atm, H2/feed molar ratio of 70.6, temperature of 300–400 °C, and WHSV of 1 h−1. Fig. 3 and Table S1 show the conversion and product selectivity as a function of temperature over all the catalysts. The conversion of octanoic acid exceeded 95 % even at the lowest temperature (300 °C) on all the catalysts except on RuSn/Al2O3, in which the conversion reached 81 % at highest temperature (400 °C). This indicates that all the catalysts except RuSn/Al2O3 possessed a very high activity in the applied reaction conditions. This is likely due to the existence of the Ru3Sn7 alloy phase that can selectively adsorb CO over the Sn sites [2,50]. Contrastingly, the lower activity of RuSn/Al2O3 is attributed to the presence of a different active phase of Ru-SnOx as indicated by the characterization results. Hence, it is clearly proven that the Ru3Sn7 alloy is a key active phase for hydrogenation of CO bond. Fig. 3 confirms that using different supports resulted in significant changes in the product distribution. For RuSn/SiO2, the main product was octanol at all temperatures, followed by octanal and octyl octanoate. At 300 °C, the selectivity to octanol was 74.6 %, before gradually decreasing to 38.7 % with the increase in temperature due to the enhanced formation of lighter C7 products (n-heptane and i-heptenes). For RuSn/SiAl (1:9), C8 hydrocarbons consisting of paraffin (n-octane) and olefins (1-octene, 2-octene, 3-octene, and other i-octenes) were the main products at all temperature regions. When the SiO2 loading increased to 30 %, the selectivity to n-octane also rose significantly and became the main product because of the different acidity possessed by the supports as will be explained further. Meanwhile, the RuSn/Al2O3 produced octyl octanoate and octanal as main C8 products while n-heptane and i-heptenes were also produced significantly as C7 products. Increase in temperature improved the selectivity to the C7 products to 46.6 %. It should be noted that the reaction over RuSn/Al2O3 was conducted from 350 °C because of severe clogging of the reactor at lower reaction temperature possibly due to the extensive formation of octyl octanoate.The products shown in Fig. 3 can be distinguished into hydrocarbons (n-octane, i-octenes, n-heptane, and i-heptenes) and oxygenates (octanal, octanol, dioctyl ether, and octyl octanoate). Fig. 4 shows the distribution of these products and the ratios between them at 350 °C to assess the activity differences of the catalysts. As can be seen in Fig. 3 and Fig. 4(a), the selectivity to C7 products was the highest on RuSn/Al2O3 with a total selectivity of 29 %, indicating that RuSn/Al2O3 showed a decent HDC/DCO activity due to the aforementioned active phase of Ru-SnOx. This implies that isolated Ru species exist and is responsible for the HDC/DCO activity due to their oxophilic property to adsorb acid on the surface.[81–83] In contrast, the other catalysts showed minimum production of C7 compounds, meaning that the Ru3Sn7 alloy phase minimizes HDC/DCO activity and preserves the carbon number of the products. The selectivity to C8 hydrocarbons (Fig. 4a) can be divided into two categories: favoring n-octane or i-octenes. The selectivity to both products differed significantly depending on the supports used. The RuSn/SiAl (1:9) catalyst favored the production of i-octenes with selectivity of 64 %, while the RuSn/SiAl (3:7) catalyst favored the formation of n-octane with selectivity of 82.7 %. In contrast, the proportion of C8 hydrocarbons on RuSn/SiO2 and RuSn/Al2O3 was very low.In terms of the oxygenates (Fig. 4b), RuSn/SiO2 favored the production of octanol, octanal, and octyl octanoate with selectivities of 66.0, 13.7, and 13.8 %, respectively. With the addition of SiO2 on Al2O3 support, the production of these oxygenates decreased, meaning that octanol was immediately converted into i-octenes or n-octane. On RuSn/Al2O3, formation of octanal and octyl octanoate could be clearly observed, suggesting a lower activity and propensity for side reactions such as esterification. It has been discussed previously that the acidity provided by SnOx and Al2O3 could increase esterification activity [2,84]. Fig. 4(c) shows the product selectivity ratio between n-octane and i-octenes, as well as the hydrocarbons/oxygenates ratio at 350 °C. The n-octane/i-octenes ratio increased significantly with increasing SiO2 loading from 0.27 on RuSn/SiO2 to 5.27 on RuSn/SiAl (3:7). The n-octane/i-octenes ratio on RuSn/Al2O3 was zero, indicating no formation of n-octane. Meanwhile, the hydrocarbons/oxygenates ratio also increased significantly from 0.07 on RuSn/SiO2 to 299.83 on RuSn/SiAl (3:7), before decreasing to 0.55 on RuSn/Al2O3. The significant increase in n-octane/i-octenes ratio with increasing SiO2 loading suggests that hydrogenation of i-octenes occurred extensively, while the sharp increase in hydrocarbons/oxygenates ratio on RuSn/SiAl (3:7) indicates high octanol dehydration activity. These results suggest that the different acidity of the supports played a very important role in determining the final product distribution, while the presence of RuSn active phase ensures the high initial activity of octanoic acid conversion.To understand more clearly about the effect of temperature on product distribution over all the catalysts, C7 and C8 hydrocarbons yield as well as their ratios are summarized in Fig. 5 . It can be observed that the overall C7 hydrocarbons yield increased with increasing temperature on all catalysts as a result of high energy input to break CC bonds in the reactant (Fig. 5a). The C7 hydrocarbons yield was much higher on the RuSn/Al2O3 and RuSn/SiO2 catalysts. For the C8 hydrocarbons (Fig. 5b), the increase in temperature from 300 to 400 °C also increased the yield on all the catalysts because of endothermic nature of dehydration. The highest yield was obtained on RuSn/SiAl (2:8) and (3:7) with comparable results, followed by RuSn/SiAl (1:9), RuSn/Al2O3 and RuSn/SiO2. It could be seen that the C8 hydrocarbons yield followed a curved trend with maximum around 350 °C for the RuSn/SiAl catalysts, and decreased slightly at higher temperature because of favorable formation of C7 hydrocarbons. This is more clearly visualized in Fig. 5c which compares the C7/C8 hydrocarbons ratio at different temperatures. At all temperatures, the C7/C8 hydrocarbons ratio was the highest on RuSn/Al2O3, followed by on RuSn/SiO2, decreasing with increasing temperature. The C7/C8 hydrocarbons ratio on the RuSn/SiAl catalysts was similar and increased with temperature.The distinctive behavior observed between the RuSn catalysts supported on pure Al2O3 and SiO2 and those dispersed on the SiAl supports hints at likely participation of the support. The results show that RuSn/Al2O3 prefers to produce C7 than C8 hydrocarbons, although it should be noted that C8 hydrocarbons were still formed mainly as 1-octene at 350 °C and a mixture of i-octenes at 400 °C (Fig. 3). The formation of i-octenes indicates that octanol was dehydrated over Al2O3 during high temperature reactions because of its endothermicity and high dehydration activity possessed by Al2O3 [17,85]. The dehydration seemingly occurred by unimolecular dehydration mechanism that follows an E2 mechanism, in which an octanol is initially adsorbed on Al2O3 surface, resulting in deprotonation of Cβ-H bond and cleavage of Cα-O bond [44,86]. However, the proportion of octenes was minimal in this case due to severe formation of side products, i.e., C7 hydrocarbons and octyl octanoate, caused by the low hydrogenation activity possessed by the active sites. For RuSn/SiO2, the results show preferential formation of C8 oxygenates. In spite of this tendency, C8 hydrocarbons were still observed at high temperature as indicated by the decrease in the C7/C8 ratio. Further inspection of the C8 products shows that the C8 hydrocarbons consist of n-octane and i-octenes mixture (Fig. 4c) caused by the direct deoxygenation of octanol over the metal sites.When SiO2 is deposited on Al2O3, the production of C8 hydrocarbons was favored instead of oxygenates, highlighting the high dehydration activity over the supports which increased with SiO2 loading. As observed in NH3-TPD (Fig. 1d) and pyridine FTIR (Fig. S2) results, the deposition of SiO2 resulted in the formation of Brønsted acid sites in the form of terminal silanols that are highly active for the dehydration of alcohols according to the literature [87,88]. Thus, although the Lewis acid sites decreased with increasing SiO2 loading, the increasing amount of Brønsted acid sites still resulted in high octanol dehydration activity because of the different dehydration activity possessed by Lewis and Brønsted acid sites [88]. The dehydration over Brønsted acid sites follows alcohol adsorption on the support and proton transfer from the silanol groups to the OH groups of alcohol, eliminating water in the process and forming alkoxy group as an intermediate. The alkoxy group then stabilizes by forming double bond, resulting in the formation of α-olefin [89].According to Fig. 3, the C8 hydrocarbons consisted of n-octane, 1-octene, 2-octene, 3-octene, and other i-octenes. Since the dehydration of octanol results in the formation of 1-octene, the formation of 2-octene, 3-octene, and other i-octenes suggests that isomerization occurred during the reaction. In general, isomerization can occur on either Lewis acid sites or Brønsted acid sites. On Lewis acid sites, the isomerization occurs through hydration-dehydration mechanism, in which the formed 1-octene rehydrates to form 2-octanol and dehydrates again to form 2-octene [90]. On Brønsted acid sites, the isomerization occurs through the migration of double bonds achieved by olefin protonation over the Brønsted acid sites [91,92]. Regardless of the isomerization method, the final major product obtained on SiAl supports was n-octane, which comes from the over-hydrogenation of octenes. The increasing amount of n-octane with higher SiO2 loading suggests that Brønsted acid sites highly affects the over-hydrogenation activity by strongly adsorbing octene molecules, resulting in higher retention time for over-hydrogenation to occur on the metal sites.The HDO performance of RuSn/SiAl catalyst was then compared to other reported catalysts as shown in Table S2. The comparison was conducted by calculating the hydrocarbons productivity, which is the production rate of hydrocarbons obtained through HDO mechanism without any carbon loss. In the case of octanoic acid, the HDO products would be the n-octane and i-octenes. As can be observed in Table S2, the RuSn/SiAl (3:7) showed better HDO performance compared to the other catalysts in the cited works, although the different reaction conditions should also be considered.The stability of the catalyst was tested over the course of 60 h using RuSn/SiAl (3:7) as the catalyst. The stability test was carried out at 350 °C, WHSV of 1 h−1, and pressure of 20 bar, and H2/feed of 70.8. It should be noted that the catalyst amount used in this test was 1/4 of the one used in Fig. 2, which explains the different product distribution. The long-term test result is shown in Fig. S5. Minimal change in the catalyst activity with no significant drop in the performance was observed during the reaction, suggesting the high stability of the catalyst although the metal sites marginally agglomerated during the reaction, as evidenced by the XRD of spent catalysts (Fig. S1), surface area result (Table S4), and TEM of the spent catalyst (Fig. S6). Generally, the agglomeration of metal sites results in the deterioration of catalyst activity, but this is not the case in this study. This could possibly mean that formation of alloy was enhanced during the reaction, resulting in the growing of the alloy metal sites since high H2 pressure and temperature with long reaction time were applied. A similar result was obtained by Karim et al. [93], where the formation of PdZn alloy was observed after reaction even without any pretreatment at 250 °C. It was postulated that the reduction of ZnO in the presence of Pd and H2 facilitated the alloying process during reaction. In addition, time-resolved XRD for the reduction of CuO observed by Rodriguez et al. [94] showed that the crystallinity of Cu0 depended on the reduction time of CuO, in which longer reduction time resulted in increased crystallinity. Oezaslan et al. [95] also observed similar result where Pt-Cu alloy crystallinity was enhanced after reducing at high temperature and prolonged time caused by metal insertion and particle growth. According to Luo et al. [50], the formation of Ru3Sn7 alloy started at temperature as low as 350 °C, so enhanced formation of Ru3Sn7 alloy and alloy particle growth during the reaction could be the reason for the high catalytic stability.In order to understand the effect of H2 on the hydrogenation activity of octanoic acid over RuSn/SiAl catalysts, the H2 partial pressure was varied between 3.9 and 19.7 atm with N2 as balance at 20 atm. Fig. 6 shows the octanoic acid conversion and product selectivity as a function of H2 partial pressure. The octanoic acid conversion was insensitive to changing H2 partial pressure, hovering around 70 %. At low H2 partial pressure, the major product was octyl octanoate, followed by octanal, octanol, and i-octenes, with some minor products such as dioctyl ether and n-octane. As the H2 partial pressure increased, the selectivity to octyl octanoate increased, followed by decreasing octanol selectivity. The selectivity to i-octenes remained similar at all H2 partial pressure, meaning that the decreasing octanol selectivity at higher H2 partial pressure was due to the esterification of octanoic acid and octanol to form octyl octanoate, which most likely occurred on the acidic sites of the support. This also means that octanoic acid seemed to preferably adsorb on the support, while H2 molecules adsorbed on the Ru3Sn7 surface. At this condition, saturation of the Ru3Sn7 sites by H2 molecules occurred so further conversion of octyl octanoate either by hydrogenolysis or hydrogenation was suppressed, resulting in higher amount of octyl octanoate. Nevertheless, the conversion remained similar because of the bi-functionality of the catalyst that could convert octanoic acid by both hydrogenation and esterification.To understand the reaction pathway for the conversion of octanoic acid over RuSn/SiAl, a contact time study was carried out over RuSn/SiAl (1:9) at 350 °C, H2/Feed ratio of 70.8, and pressure of 20 atm. Fig. 7 , Table S3, and Fig. S7 show the contact time analysis results in which the products are categorized into hydrocarbons (Fig. 7a) and oxygenates (Fig. 7b).The increase in the contact time raised the conversion value from 70 to 100 % due to prolonged contact of reactants with catalyst. The major hydrocarbon products shown in Fig. 8 a were 1-octene, n-octane, and i-octenes; the minor products were n-heptane and i-heptenes with selectivities less than 1 %. For 1-octene, the selectivity followed a positive curvilinear trend in which it initially increased from 1.1 to 20.2 % at contact time of 0.025–0.25 h and then decreased afterwards to 6.4 %, suggesting that 1-octene is an intermediate product. For i-octenes, a similar trend to that of 1-octene was observed; however, the inflection point was located at higher contact times, suggesting that i-octenes was an intermediate product formed afterwards. It should be noted that significant increase in the selectivity to i-octenes were observed at a contact time of 0.25–0.5 h, which coincided with decrease in the selectivity to 1-octene, suggesting that i-octenes was produced from 1-octene through isomerization. The selectivity to n-octane increased linearly with contact time (0 to 30.6 %), indicating that n-octane is a final product from hydrogenation of i-octenes and 1-octene.For oxygenates, the selectivity to octanal initially increased until a contact time of 0.04 h and decreased subsequently. This trend is similar to the trend of secondary or intermediate products, as shown previously for i-octenes or 1-octene; however, octanal, which should be a primary product in this reaction via the hydrogenation of octanoic acid should exhibit a selectivity trend which follows a continuous downward trajectory. This unexpected trend suggests that the increase in selectivity at short contact time is most likely due to the dehydrogenation of octanol from the equilibrium state of octanal and octanol. However, at longer contact time, the equilibrium shifts to the formation of octanol due to longer interaction and increasing number of reactants on the catalyst surface. In case of octanol and octyl octanoate, both shows similar trend in which the selectivity decreased continuously with increasing contact time. This suggests that octanol is converted into either octanal, octyl octanoate, or dioctyl ether at short contact time and possibly to 1-octene at longer contact time. For dioctyl ether, the selectivity profile followed a curvilinear trend like octanal, meaning that it is an intermediate product that is formed from octanol at short contact time and is converted by subsequent hydrogenation to octanol and 1-octene at longer contact time.Based on the observations above, the following reaction network can be proposed as shown in Fig. 8. First, octanoic acid can be transformed by two different reaction pathways: either by forming octanal (black arrow) through hydrogenation or by forming i-heptenes through DCO (dotted red arrow). With the presence of the Ru3Sn7 alloy phase in the RuSn/SiAl catalysts, DCO activity is suppressed significantly leading to preferential conversion of octanoic acid to octanal. The formed octanal can also undergo two different reaction routes to form either octanol (black arrow) by further hydrogenation or i-heptenes by HDC (dotted red arrow). Again, on Ru3Sn7 alloy such i-heptenes formation is very limited, so octanal is favorably converted into octanol. When octanol is formed, it can be converted by four different reactions: full dehydration to form 1-octene (black arrow), partial dehydration to form dioctyl ether (dotted orange arrow), esterification with octanoic acid to form octyl octanoate (dotted blue arrow), and reverse reaction to form octanal by dehydrogenation (black arrow). The tendency for these reactions depends on the reaction temperature, contact time, and acidity of the support. As can be observed in Fig. 3, the high temperature favors full dehydration over partial dehydration. Under isothermal conditions, short contact time favors reverse reaction to form octanal and octyl octanoate as major products with dioctyl ether as minor product. On the other hand, long contact time favors full dehydration of octanol to 1-octene. Based on the support acidity, the absence of acidity such as in SiO2 inhibits the dehydration process; hence, the formation of octyl octanoate dominates on RuSn/SiO2. On the other hand, the presence of Lewis acid sites in Al2O3 promotes the dehydration of octanol to 1-octene.Both reactions of partial dehydration and esterification are reversible reactions. If dioctyl ether is formed, it can be converted into octanol and 1-octene by decomposition (dotted orange arrow). If octyl octanoate is formed, it can be further converted back into octanol by hydrogenation (dotted blue arrow) or into octanoic acid and octanol by hydrolysis (dotted black arrow). Meanwhile, 1-octene formed from dehydration of octanol can undergo either isomerization to form 2-octene, 3-octene, and other isomers (dotted yellow arrow), or direct hydrogenation to form n-octane (black arrow). Similar to 1-octene, i-octenes can also be converted into n-octane through hydrogenation (black arrow). The presence of medium and strong acid sites enables the isomerization of 1-octene and hydrogenation of octenes by providing H-transfer to the metal site. Overall, the HDO of octanoic acid to n-octane over the RuSn/SiAl catalysts involves sequential reaction steps that occur on two different main sites. The metallic sites of Ru3Sn7 enable the conversion of octanoic acid to octanol, in which with close proximity to the support catalyzes the dehydration of octanol to form octenes. Further hydrogenation by the metallic sites results in the formation of n-octane as the final product.The conversion of octanoic acid was carried out over RuSn/SiAl catalysts with different SiO2 loadings. Characterization of the catalysts suggests that the addition of SiO2 on Al2O3 modified the surface of Al2O3 which was beneficial to the formation of Ru3Sn7 alloy phase. This alloy phase was crucial to the high conversion of octanoic acid to octanol, and the diminished formation of C7 hydrocarbons through HDC/DCO mechanism. The addition of SiO2 on Al2O3 was also beneficial for modulating the acidity of Al2O3, in which the modified acidity enhanced the dehydration of octanol and hydrogenation of octenes. The stronger acidity observed in the RuSn/SiAl catalysts was attributed to the formation of terminal silanols, which induced strong Brønsted acid property on the support. It was revealed that n-octane was formed in significant amounts as SiO2 loading increased due to a longer interaction between octenes and support, facilitating the over-hydrogenation. The synergetic relationship between RuSn metal alloy and SiAl support resulted in high activity for octanoic acid transformation to C8 hydrocarbons with preserved carbon number.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This study was supported by the National Research Foundation of Korea (NRF) (2020M1A2A2080851) and the Institutional Research Program of KRICT (KK2311-10).Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2023.141912.The following are the Supplementary data to this article: Supplementary data 1

The ever-increasing demand for substitute energy sources encourages the usage of biofuels as transportation fuel; however, the unstable properties and low calorific value of biofuels caused by high oxygen content limits their direct application. In this study, we report the upgrading of a model biofuel in octanoic acid by hydrodeoxygenation (HDO) using RuSn supported on SiO2-doped Al2O3 (SiAl). The presence of SiO2 on Al2O3 was crucial for the formation of Ru3Sn7 alloy phase because SiO2 modified the metal-support interaction between RuSn and Al2O3. The alloy phase conferred significant hydrogenation activity to convert octanoic acid to octanol and minimized carbon loss by reducing decarbonylation activity. The deposition of SiO2 also resulted in improved acidity, which increased with increasing SiO2 loading from 10 to 30 %, due to the formation of isolated terminal silanols (Si-OH). The presence of the silanol groups was important for dehydration of octanol to octane, since the silanols retained octenes longer on the support, facilitating over-hydrogenation. The cooperative metal-support activity on RuSn/SiAl enables high and selective hydrogenation and dehydration activity for upgrading fatty acids in biofuel to hydrocarbons.

The root causes of a series of frontier hot issues, such as global climate anomalies, energy supply shortages, and ecological deterioration are the cliff-like decline of energy reserves and insufficient sustainable development [1]. As a clean energy source without supply constraints, The development of clean energy source has become a new trend towards global climate anomalies, energy supply shortages, and ecological deterioration. Solar energy converted by photoelectric devices can meet the urgent needs of a large amount of electronic equipments for human activities. Inspired by the mechanism of “photosynthesis”, biomimetic constructed dye-sensitized solar cells (DSSCs) relied on their cost-effective and environmental-friendly characteristics have posted great potential on photoelectric conversion application. Generally, the DSSCs was assembled with a unique sandwich structure, matching with four components: photoanode coated on conductive glass (TiO2, ZnO, WO3, SnO2, Nb2O5), dye sensitizer, redox electrolyte, and counter electrodes (CEs) [2,3]. Noteworthily, the CEs component concerning on the electrical conductivity, electrocatalysis, and ion diffusion levels plays a non-negligible role in determining photovoltaic performance [4]. Platinum (Pt) as one of the most typical CEs owing to its high price, low abundance, and detrimental effects on the environment, limits large-scale industrial production [5,6]. Hence, it has become an urgent task to explore low-cost, harmless, and stable non-platinum CEs for sustainable photoelectric conversion.Currently, carbon materials featuring high porosity and good chemical inertness to corrosive electrolytes provide a good alternative to Pt [7–9]. [10,11]. Nevertheless, the disadvantage of the carbon materials is that it limites by electrical conductivity, which has a effect on the efficient transfer of electrons between and within the carbon particles, thus leading to higher diffusion resistance of I3 − ions. Moreover, the complete framework obtained from such conventional carbon materials tend to show carbon wholeness at the macro scale of millimeter to centimeter scale, resulting in low bonding strength, poor dispersion, and poor contact between the large particles and the FTO interface.Moreover, considering the higher photovoltaic level, the improvement of the electrocatalytic performance of pure carbon materials still needs to be explored and optimized. Current research focuses on the construction of hetero-structured systems by arranging different materials (selenides, metal alloys, sulfides) [12–16], or by chemically doping heteroatoms (nitrogen, sulfur, phosphorus, boron) [17–19] with carbon defect sites. There are still key issues that need to be addressed, such as the non-renewability and the technical difficulty of the former, the reproducibility and precise control of the position of the doping defects and network pore structure of the latter, and the poor contact between the conductive media. A general design-principle goes beyond these existing methods of structuring carbon materials to achieve precise control of defect doping in structured-carbon. It can create more active sites for the absorption and transfer of I3 − and H + ions, thus achieving the improvement of comprehensive performance of electrical conductivity, catalysis, and electron transport. The precise control of the structural properties of carbon materials, such as particle size, morphology, and porosity, is an exploratory solution to the existing bottleneck.Herein, based on the principle of anisotropic integrated design, this work is expected to generate new physical properties and electronic activities by constructing unique structural arrangements and optimizing the composition ratio, and to generate more active sites by reducing agglomeration and synergistic effects. Specifically, pitaya peel-derived carbon structures were used as the matrix of in situ self-generated N-doped CNTs-coated Ni nanoparticles embedded in N-doped 3D network (Ni@NCNTs/PC-X), realizing the construction of self-generated Ni-N-C hybrid sites in 3D network structures as CEs material. More interestingly, in addition to providing anchor points to stabilize Ni atoms and adjust the electronic structure of surrounding atoms, melamine can also participate in the formation of N-doped porous carbon carriers and N-doped self-grown CNTs, improving the mass transfer catalysis process of Ni@NCNTs/PC-X. The three dimensional network characteristics of Ni@NCNTs/PC-X active sites were revealed by structural characterization, morphology and chemical composition analysis. Based on its good electrical conductivity and electrocatalytic electrochemical oxidation–reduction performance, its photovoltaic performance as a CEs material was initially explored, and its potential application in DSSC was verified. It is expected to promote the application of biomass-based carbon materials in photovoltaic electrodes.Melamine (C3H6N6), nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O), acetic acid (C2H4O2) were provided by Aladdin Chemical Reagent Co., Ltd (Shanghai, China). Polymer spacers (Surlyn, 60 μm thickness), FTO glass slides with a size of 2.5 × 2 cm, resistivity of ∼7 ohm/sq were bought from Yingkou Opivite New Energy Technology Co., Ltd (Liaoning, China).First, the rinsed red pitaya peels were stripped of their folds and roasted at 70 °C for 24 h. The obtained 1 g of pitaya peels were calcined at 800 °C under N2 for 2 h. The calcined product was stirred in 2 g/20 ml KOH solution for 6 h, dried and then annealed at 700 °C under N2 for 2 h. The product (PC) was acid-washed with 1 M HCl for 2 h, washed repeatedly with distilled water and ethanol until neutral, and then dried at 80℃. 1 g of PC, 1 g of Ni(CH3COO)2·4H2O, and 1 g of melamine (2 g, 4 g, and 8 g) were added to 30 ml of 37% CH3COOH solution and stirred until gel-like. Afterward, the gel was annealed at 700 °C at N2 for 2 h to obtain self-grown N-doped CNTs-coated Ni nanoparticles on N-doped porous carbon, named Ni@NCNTs/PC-X (X = 1, 2, 4 and 8), where X indicates the mass ratio of added melamine to PC.Preparation of counter electrodes (CEs): 0.2 g of the prepared CEs material, 10 μL of polyethylene glycol, and 20 μL of Triton X-100 were added into 1 ml of ultra-pure water, and mixed well by sonication. The doctor blade technique was adopted for the preparation of CEs with clean FTO as the substrate. The homogeneous membrane formed by adding 20 mM H2PtCl6•6H2O to the surface of FTO was sintered at 450 °C for 30 min to obtain Pt as CEs.Preparation of photoanodes: FTO conductive glass was washed twice with conductive glass cleaning solution, distilled water, and ethanol in sequence, and dried in an oven. The P25/Ni-2 sol-gel (detailed in Supplementary Information: Fig.S2, Fig.S3, Fig.S4) was scraped to a certain thickness on clean FTO through a filmmaker and sintered at 450 °C for 30 min. In order to obtain dense calcined electrode layers, it is necessary to immerse the above electrodes in 40 mM TiCl4 solution at 70 °C for 30 min and to undergo secondary sintering at 450 °C for 30 min. Finally, dye-sensitized photoanode was obtained by immersing the prepared electrode into 0.5 mM N719 and avoiding light at 25 °C for 18 h.The assembly of DSSCs: Photoanode (P25/Ni-2, P25) and CEs (Ni@NCNTs/PC-X, Pt) were assembled together with a 60 μm Surlyn film to obtain DSSCs with an active area of 0.25 cm2. Then, an electrolyte consisting of 0.025 M I2/0.1 M guanidine thiocyanate/0.5 M 4‑tert‑butyl pyridine/0.6 M 1-methyl-3-propylimidazolium iodide in a mixture of acetonitrile/valeronitrile (85:15 v%) was injected into the CEs cell through the reserved hole. The reserved hole was sealed to prevent the electrolyte from volatilizing.Samples were characterized and measured by X-ray diffractometer (Rigaku D/MAX 2500 V, Japan), Four-point probe meter detector (RTS-8, 4Probes Tech Ltd. China), Field emission scanning electron microscopy (FE-SEM, Zeiss Sigma 300, Germany), X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-Alpha Plus), Fourier transform infrared spectroscopy (FTIR, Thermo Fisher Scientific, USA), Raman spectroscopy (inVia Reflex, Renishaw, UK), A Brunauer-Emmett-Teller (BET, JW-BK400, Jingwei Gaobo Co., Ltd. China), an electrochemical workstation (CS350, Wuhan Corrtest Instruments Corp., Ltd, China), and a solar simulator with a digital source meter (SOFN INSTRUMENTS CO., Ltd, China) (see supplementary information for details).Based on anisotropic integrated design principles, we wish to propose a system of self-grown N-doped CNTs-supported Ni nanoparticles on N-doped porous carbon. The design with abundant active sites provides suitable energy for binding triiodide ions, where N-doped CNTs sites can shorten the diffusion distance of triiodide ions, and N-doped porous carbon matrix provides fast electron transport pathways and acts as physical barriers in process catalysis reactions. Previous literatures have demonstrated that composite metal-based catalysts generally exhibit enhanced catalysis action than single metal-based catalysts because of the synergistic action of multiple elements. Here, we are particularly concerned with the construction of unique Ni-intercalated composite co-doped N-doped CNTs by Ni metal particles on active N-doped PC (Ni@NCNTs/PC-X). Using PC, Ni(CH3COO)2·4H2O, and melamine as precursors, a simple high-temperature in-situ pyrolysis, doping, self-growth, and self-assembly method were used. The nitrogenous hybridization of melamine promoted the formation of N-doped ordered porous carbon and provided the required spatial arrangement. Melamine provided both C and N sources. The carbothermal reduction of Ni(CH3COO)2·4H2O precursor enhanced the graphitization effect of Ni-doped porous carbon. It was partially inserted into the carbon wall of N-doped porous carbon as a Ni-doped catalyst for N-doped self-growing CNTs. The catalytic graphitization and nitridation eventually lead to Ni intercalation and N-doped CNTs (Ni@NCNTs). The synergistic effect between the components is expected to enhance the electrocatalytic performance of this hybrid by the Ni metal component and N-doped CNTs.To achieve this goal, Ni@NCNTs/PC-X composites were designed and constructed in a simple and efficient manner, as shown in Fig. 1 . To further study the effect of experimental parameters on the structure, a series of samples (Ni@NCNTs/PC-X composites) with different melamine contents (1 g, 2 g, 4 g, and 8 g) were designed and prepared based on PC, Ni(CH3COO)2 4H2O and melamine. Ni@NCNTs was in situ immobilized on N-doped PC by Ni-N-C hybridization and uniformly distributed in the non-agglomerated N-doped PC particles to form an interconnected conductive framework. Therefore, a well-networked Ni@NCNTs/PC composite was creatively constructed by in-situ growth of Ni@NCNTs among N-doped PC particles at 700 °C. All of these self-grown N-doped CNTs embedded nickel nanoparticles supported by N-doped porous carbons are discussed in the morphology and composition analyses. To investigate the effect of N and Ni co-doping on PC-modified CEs, electrochemical results confirmed that co-doped Ni@NCNTs/PC-X exhibited better performance than unmodified PC. This detail is discussed in the chapters on photovoltaics, electrocatalysis, and electrochemical properties.The crystallization properties of in-situ self-grown N-doped CNTs-coated Ni nanoparticle composites (Ni@NCNTs/PC-X) of pitaya peel-porous carbon were investigated by XRD patterns and Raman spectroscopy (Fig. 2 a and b). As far as XRD patterns, in addition to the characteristic diffraction peaks of amorphous carbon, Ni@NCNTs/PC-X has broad and weak characteristic diffraction peaks at 2θ=25.8° and 41.5° They represent the typical diffraction peaks in graphitic carbon (002) and (100) planes, indicating the coexistence of amorphous graphitic carbon. And more, there are distinct peaks at 2θ=44.2° (111), 51.5° (200), and 76.2° (220), originating from Ni doping. PC has a broad diffraction peak at 25.8° After the introduction of melamine, the peaks of the Ni@NCNTs/PC-1 to Ni@NCNTs/PC-8 composites gradually narrowed and enhanced at 25.8°, indicating that the graphitization effect was enhanced by Ni catalysis and melamine nitridation. Therefore, Ni@NCNTs/PC-X shows typical characteristics of Ni-containing graphitic carbon compared to the typical amorphous carbon of PC. Furthermore, in Fig. 2b, both PC and Ni@NCNTs/PC-X have two distinct peaks at ∼1348 cm−1 and ∼1588 cm−1, which belong to the D and G peaks of graphite [20]. It cannot be ignored that the ID/IG values of the Ni@NCNTs/PC-X are all lower than that of PC (1.08). This indicates that N-doped CNTs grow on the surface of porous carbon, and the carbon in the composite material has a higher structural order. In addition, the melamine content has an important effect on the ID/IG value of Ni@NCNTs/PC-X, that is, the ID/IG value of Ni@NCNTs/PC-X composites decreased from 1.07 (Ni@NCNTs/PC-1) to 0.99 (Ni@NCNTs/PC-8). The lower ID/IG value of Ni@NCNTs/PC-X composites indicated the increased graphitization, which agreed with the XRD analysis.Ni@NCNTs/PC-X was successfully constructed by this unique Ni-embedded graphical-carbon of self-grown CNTs on the N-doped-carbon surface combined with the abundant defective sites, which has good electrical conductivity, excellent electrolyte diffusivity, and electrocatalytic properties. Ni@NCNTs/PC-X (Ni@NCNTs/PC-1 of 129.1 S cm−1, Ni@NCNTs/PC-2 of 165.3 S cm−1, Ni@NCNTs/PC-4 of 215.7 S cm−1, Ni@NCNTs/PC-8 of 226.8 S cm−1) has a higher degree of graphitization, a more regular and ordered structure, and a higher electrical conductivity than pure amorphous PC with defects (18.3 S cm−1). The increased electrical conductivity benefits from the unique ordered graphite-carbon micro-nano structure through the hybrid by the Ni metal component and N-doped CNTs supported by N-doped PC. These key performances were confirmed by the above-mentioned XRD and Raman analyses, as well as subsequent in-depth analyses of quantitative indicators of elemental composition and chemical state, microstructure and pore structure.In particular, the elemental types-chemical configurations of PC and Ni@NCNTs/PC-4 were further explored by high-resolution XPS spectroscopy. The full XPS spectrum of Ni@NCNTs/PC-4 (Fig. 2c) shows the existence of N and Ni peaks in Ni@NCNTs/PC-4, which indicates that Ni ions and N are doped into PC. In Fig. 2d, the four peaks of high-resolution C 1 s spectrum fitted at 284.5 eV, 284.9 eV, 286.2 eV, and 288.9 eV are CC/C = C, CN, C = N/CO, and C = O, respectively [21,22]. Observations of CN and C = N bonds provide strong support for the arguments for N-doped porous carbon and N-doped CNTs. The high-resolution spectrum of N 1 s at 398.0 eV, 400.1 eV and 403.5 eV represent pyridine N, pyrrole N and graphite N with relative contents of 24.07%, 47.28% and 28.65%, respectively. For Ni@NCNTs/PC-X, the higher ratios of pyridine N and pyrrole N formed in the structure of Ni@NCNTs/PC-4 (24.07%, 47.28%) have better electrocatalytic effects compared with Ni@NCNTs/PC-1 (18.31%, 41.45%), Ni@NCNTs/PC-2 (20.36%, 43.57%), Ni@NCNTs/PC-8 (22.13%, 45.06%) [23]. The predominant pyridine-N with lone pair electrons may be beneficial for electrocatalysis because the electron-donating effect is beneficial for reducing the electronic work function of carbon [23]. Besides, the characteristic metallic Ni in the Ni 2p spectrum was found at 855.3 eV, confirming the thermal reduction of nickel acetate (Ni2+) to Ni as catalysts in carbon under a nitrogen atmosphere [24]. Also, the presence of trace amounts of Ni2+ may be caused by the oxidation of air left over from the heating process.To further confirm the existence and quantitative analysis of the microscopic pore structures of PC and Ni@NCNTs/PC-X composites, N2 adsorption-desorption experiments were measured and analyzed. As shown in Fig. 3 a-b, all samples exhibited typical IV isotherms and obvious hysteresis loop characteristics when the relative pressure (P/P0) was 0.4–1, confirming the existence of mesoporous features. As shown in the inset of Fig. 3b, the pore sizes of the composites are concentrated between 2 and 3 nm with the average pore size of 2.5 nm, which belongs to the mesoporous size. The specific surface area (SBET) of pitaya peels-based PC is 1101.9 m2/g, and the large SBET is beneficial to the charge transport and the growth of CNTs on its surface. The SBET of the Ni@NCNTs/PC-X composites is mainly controlled by the porous PC and CNTs grown on it. With the growth of CNTs, the SBET decreased from 1041.3 m2/g (Ni@NCNTs/PC-1) to 884.1 m2/g (Ni@NCNTs/PC-2), 602.9 m2/g (Ni@NCNTs/PC-4) and 513.7 m2/g (Ni@NCNTs/PC-8). With the increase of melamine content, the growth of CNTs on the surface of porous PC increased and the SBET decreased, leading to the reduction of electrolyte catalyzed by the composite, which may be unfavorable. In Ni@NCNTs/PC-X composites, more electrocatalytic sites can be provided for I3 − to promote electron transport and reduce charge transport resistance by adjusting the amount of melamine, appropriately using porous carbons with high SBET and high mesoporosity, and maintaining a high catalytic activity area.The morphology of the Ni@NCNTs/PC-X corresponds to its SBET, and the increase of the number of carbon nanobundles may have a negative impact on its SBET contribution (Fig.S1), which is not beneficial to provide more electrocatalytic active sites. The morphological characterization of Ni@NCNTs/PC-X composites (Fig.S1) shows that the morphology of the composites can be adjusted to obtain the best electrocatalytic performance by reasonably controlling the mass ratio of melamine to porous carbon. After activation by KOH etching, pores of different sizes appeared on the surface of the porous carbon material, which can accelerate the diffusion of electrolytes in the carbon-based electrode, as shown in Fig. 3c-h. However, small-scale dispersion and non-uniform morphology lead to poor electrical contact performance between materials with high interfacial resistance and high potential barrier, which can adversely affect the charge conduction between carbon matrices. Therefore, it is important to construct an intermediate bridge that can shorten the charge transfer. Interestingly, when a certain mass of melamine was added to the porous carbon material, the high-temperature carbonized precursor and Ni-catalyzed graphitization pyrolysis induced the successful self-growth of carbon nanotube bundles of different lengths on the porous carbon surface. The use of steric hindrance during the preparation of Ni@NCNTs can avoid the increase in the size of PC due to aggregation, ensure a smaller size effect, and also play a positive role in shortening mesoporous channels. Therefore, the protruding carbon nanotube bundles effectively shorten the transport distance of charge in the carbon material, thereby reducing the resistance of charge transport. Furthermore, a three-dimensional conductive interpenetrating network of Ni@NCNTs/PC-X is creatively designed by embedding Ni@NCNTs uniformly into the internal structure and particle gaps of PC. In addition, the EDS mapping shows that C, N, and Ni elements are well homogeneous in Ni@NCNTs/PC-4 (Fig. 3i).The fabrication of DSSCs assembled with P25/Ni-2 as photoanode and Ni@NCNTs/PC-X as CEs is represented in Fig. 4 a. To investigate the electrocatalytic performance of pitaya peel-derived carbon-based composites as CEs, we assembled DSSCs with P25/Ni-2 as photoanode and Ni@NCNTs/PC-X as CEs (Fig. 4b and Table 1 ). The PCE of DSSCs based on the activated pitaya peel-derived PC as CEs was 2.47%, which may be attributed to the high SBET of PC for good catalytic reduction performance for I3 −. It was also found that the PCE values of the DSSCs based on Ni@NCNTs/PC-X as CEs were correlated with the ratio of melamine to PC. When the amount of melamine was 0.5 g, 1 g, and 2 g (X = 1, 2, and 4), the PCE value gradually increased. With the addition of melamine increased to 4 g (X = 8), the PCE value decreased. Using Ni@NCNTs/PC-4 as CE, the optimal PCE is 5.13%, Voc is 0.69 V, Jsc is 13.27 mA/cm2, and FF is 0.56. Compared with P25/Ni-2-Pt, the Voc of P25/Ni-2-Ni@NCNTs/PC-4 decreased slightly, which may be related to the uneven surface of CE. The different PCE values are related to the specific surface area, and the unique micro-nano structure of Ni@NCNTs/PC-X electrode materials. Furthermore, the self-grown CNTs on the surface of the PC effectively shortened the electron transport path, resulting in the formation of a three-dimensional network structure of the dispersed PC. It shows that N doping level and microscopic pore structure had a synergistic effect on photovoltaic performance. The photovoltaic performance of DSSCs assembled with P25/Ni-2 as photoanode and Ni@NCNTs/PC-X as CEs exhibited a small statistical standard deviation, indicating that Ni@NCNTs/PC-X as CEs has good photovoltaic reproducibility.The CV curves of Ni@NCNTs/PC-X (Fig. 4c and d) were used to evaluate the electrocatalytic properties and the kinetics of the I3 −/I −redox reaction of CEs. For CV curves, the position and size of the two pairs of redox peaks are often used to evaluate the electrocatalytic performance of the electrode. The first redox peak is usually defined as Eq. (1) and the other as Eq. (2) [12]: (1) I 3 − + 2 e − = 3 I − (2) 3 I 2 + 2 e − = 2 I 3 − In general, the smaller the potential difference (Ep) between the first redox peaks, the faster the kinetics of the redox reaction, which means a better reduction of I3 − [25]. It can be seen from Table 1, Ep values in descending order are PC (1110 mV) > Ni@NCNTs/PC-2 (668 mV) > Ni@NCNTs/PC-1 (636 mV) > Ni@NCNTs/PC-8 (620 mV) > Pt (588 mV) > Ni@NCNTs/PC-4 (474 mV). Apparently, Ni@NCNTs/PC-4 CE has the best catalytic activity compared with other CEs, which may be due to the best synergistic effect of PC with its self-grown CNTs leading to its good morphology and structure. The CV curve of Ni@NCNTs/PC-4 as CE (Fig. 4e) shows that the diffusion rate of the I3 −/I −ion pairs on the electrode surface increases with the increase of scanning speed, and the Ox-1/Red-1 peak moves in the direction of positive and negative potentials, respectively. Also, the linear relationship between the peak current density and the square root of the scanning rate (shown in the inset) is consistent with Langmuir isotherm theory, implying that diffusion is the controlling mechanism of the redox reaction of I3 −/I −, and there is no interaction between the surface interface of Ni@NCNTs/PC-4 and the electrolyte [26].The Rs values of different CEs based on pitaya peel-derived carbon ranged from 19 to 23 Ω, which was close to 17.18 Ω for CE based on Pt, shown in Nyquist plots (Fig. 4f). The size of Rs has little effect on PCE value, so the key is the impact of Rct on PCE. Due to the lack of sufficient active sites, the activated pitaya peel-derived carbon (PC) has a larger Rct value of 12.53 Ω. The Rct values of Ni@NCNTs/PC-X as CEs are all lower than that of PC, indicating that the growth of CNTs could effectively reduce the Rct value [27]. The Rct value of Ni@NCNTs/PC-X CEs decreases from 10.44 Ω (Ni@NCNTs/PC-1) to 5.30 Ω (Ni@NCNTs/PC-2) and 5.21 Ω (Ni@NCNTs/PC-4), which is close to Pt's 4.91 Ω. Furthermore, the Rct value of Ni@NCNTs/PC-8 increased to 6.26 Ω when the melamine level increased to 4 g (X = 8). The variation of Rct value of Ni@NCNTs/PC-X CEs is consistent with the J-V curve. The reason for the increased Rct value of Ni@NCNTs/PC-8 may be that the number of CNTs growing on PC surface increased, SBET and Rct decreased due to the increase of melamine content. The lower Rct values of Ni@NCNTs/PC-4 imply a higher electrocatalytic performance for I3 −, which is more favorable for electron transport.Meanwhile, the exchange current density (J0) of Ni@NCNTs/PC-4 (0.0013 mA/cm2) is higher than those of PC (0.0007 mA/cm2), Ni@NCNTs/PC-1 (0.0008 mA/cm2), Ni@NCNTs/PC-2 (0.0011 mA/cm2) and Ni@NCNTs/PC-8 (0.001 mA/cm2) (Fig. 4g). Ni@NCNTs/PC-4 has the lowest Rct and the highest J0, indicating that it has a good electrocatalytic effect on the reduction of I3 − [28]. Ni@NCNTs/PC-4 composites exhibit higher Jlim (limiting diffusion current density) values than other CEs materials. According to Eq. (3), it can be confirmed that the surface between the I3 − ions in the electrolyte and the Ni@NCNTs/PC-4 has a large diffusion coefficient (D). (3) J l i m = 2 n e D C N A / l The strong diffusion rate of the as-prepared composite CEs is mainly attributed to its large specific surface area and suitable carbon nanotube growth on its surface, which is of great significance to shorten the diffusion distance of I3 − ions, reduce the charge transfer resistance, and ultimately improve the electrocatalytic performance.The present results were also compared with those of composites prepared by similar methods using carbon from other biomass sources or commercial carbon materials in Table 2 . The PCE value (5.13%) of the Ni@NCNTs/PC-4-based CEs assembly was higher than previously reported biomass carbon-based CEs (PCE=1.09–4.98%). This also confirms the contribution of self-generated Ni-N-C hybrid sites in 3D network structures as CEs for improving the photoelectric conversion efficiency.The CV curves overlapped well after continuous scanning without significant changes, as shown in Fig. 5 (a, c, e, g). Furthermore, no exfoliation of the electrode material from the surface of the FTO substrate was exhibited (insets of Fig. 5), which corroborates the excellent electrochemical stability of Ni@NCNTs/PC-X as CEs in electrolytes. Fig. 5(b, d, f, h) shows the corresponding peak current densities of oxidation and reduction under different scanning times. The increase in the number of scans did not lead to a large change in the curve shape and peak current density of CEs, which also suggested that the CEs prepared by Ni@NCNTs/PC-X composites had good stability in the redox reaction of I3 −/I −. Notably, all CEs exhibited remarkable repeatability in 50 consecutive scan curves, and the as-prepared Ni@NCNTs/PC-X CEs exhibited high electrochemical stability. Fig. 6 takes the Ni@NCNTs/PC-4 composite with the best electrocatalytic performance as an example to explore its electrochemical properties and cycling stability. All CV curves (5–150 mV/s, Fig. 6a) show good shape retention and good capacitance values. At a high scanning rate of 150 mV/s, the shape of the curve can also be well maintained, showing good electrochemical stability and reversibility. The GCD measurements of the composites were simultaneously performed from 1 A/g to 10 A/g. From Fig. 6b, it can be seen that the potential changes linearly with time, and the shape is a typical symmetrical triangle, which also indicates that it is contributed by the Ni@NCNTs/PC-4 electric double layer capacitance. There is almost no voltage drop in the GCD curve, suggesting that the electrode has the advantages of low internal resistance, fast charging and discharging, and good rate performance. The specific capacitances (Fig. 6c) of Ni@NCNTs/PC-4 are 91.0, 90.9, 87.1, 85.1, 83.9, 82.0, 79.7, and 77.5 F/g, respectively. To test the cycling stability of the Ni@NCNTs/PC-4 composite, as shown in Fig. 6d, the capacitance was characterized by 10,000 consecutive charge-discharge cycles at 6 A/g between −1 and 0 V. The results show that Ni@NCNTs/PC-4 electrode has a high capacitance retention rate of 98.91% after 10,000 cycles, which can be clearly confirmed by the last 10 charge-discharge cycles (inset of Fig. 6d). The electrochemical test and analysis of Ni@NCNTs/PC-4 as an electrode material further confirmed its good electrocatalytic performance and charge-discharge stability, and proved its practicability as a photovoltaic electrode from multiple perspectives.The purpose of this study is to solve the problems of conductivity, catalytic performance, dispersion, contact resistance and ion diffusion of traditional biomass carbonaceous materials as electrodes in DSSCs. To this end, a Ni-N-C hybrid 3D ionized sites-network CE was construed in the N-doped 3D network by in-situ self-grown N-doped CNTs and Ni nanoparticles embedded using the carbon structure derived from the pitaya peel as the matrix. The mass ratio of melamine to 3D structured-carbon increased from X = 1 to X = 8, and the number of in-situ self-grown CNTs on the surface of 3D structured-carbon increased gradually, while the surface area (SBET) decreased gradually. The optimal SBET of Ni@NCNTs/PC-4 exhibits excellent electron transfer ability and good stability. In addition to providing anchor points to stabilize Ni atoms, melamine can also participate in the formation of N-doped porous carbon supports and N-doped self-grown CNTs, improving the mass transfer of Ni@NCNTs/PC-4 during catalysis. Structural characterization, micromorphological and chemical composition analyses revealed that Ni@NCNTs/PC-4 had abundant active sites. The DSSCs assembled with Ni@NCNTs/PC-4 CEs exhibited good photovoltaic performance with a PCE value of 5.13%±0.05, which was higher than that of the DSSCs with PC (2.47%±0.05) and close to that with Pt (5.60%±0.10). This novel Ni@NCNTs/PC-x expands the choices of electrodes for designing DSSCs, especially for the purpose of using renewable biomass carbon sources for massive production.This work was supported by the National Natural Science Foundation of China (31960293). Genhui Teng: Writing – review & editing. Baorui Liu: Conceptualization. Zhe Kang: Investigation. Yanhui Xie: Methodology. Dongying Hu: Supervision, Writing – review & editing. Dawei Zhao: Writing – review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.recm.2022.11.001. Image, application 1

The technical bottleneck of carbon materials as counter electrodes (CEs) lies in their limited electrical conductivity, extended ion diffusion paths, poor dispersion, and high contact resistance. Problem-oriented in-situ self-grown N-doped CNTs-coated Ni nanoparticles based on N-doped carbonaceous structures derived from pitaya peel (PC) are adopted to construct Ni-N-C hybrid 3D ionized network sites (Ni@NCNTs/PC-4) as CEs. Structural characterization, micromorphological and chemical composition analyses revealed the 3D network structure of Ni@NCNTs/PC-4 with abundant active sites. They effectively shorten the diffusion distance of I3 − ions with a smaller charge transfer resistance (5.21 Ω) than that of PC (12.53 Ω). DSSCs based on Ni@NCNTs/PC-4 display good optoelectronic properties, in which the short-circuit current density (Jsc) is 13.27 mA/cm2, higher than those of Pt (11.66 mA/cm2) and PC (6.99 mA/cm2). The PCE value (5.13%) of DSSCs based on Ni@NCNTs/PC-4 is also higher than that of DSSCs based on PC (2.47%). Overall, this work provides a preliminary research and new ideas for further in-depth study of biomass-derived 3D structured-carbons that contribute to key electrodes in DSSCs.

Data will be made available on request.The excessive consumption of single-use plastics in the age of consumerism has generated a staggering amount of global plastic production. The short service life of these highly durable plastics creates enormous pressure on municipal waste management systems. Experts estimate an increase in global plastic waste production from 240 Mt/y in 2016–430 Mt/y in 2040 in a business-as-usual scenario [1]. This data translates to the outflow of ∼1.71 and ∼0.75 billion metric tonnes of plastic waste into the aquatic and terrestrial environment by 2020–2040 [1]. Sufficient evidence has demonstrated the possible trophic transfer of the fragmented plastic waste through the aquatic food web, causing an increased risk of toxicity to humans as one of the top predators [2]. Scientists have also pointed out that plastics may hinder the carbon sequestration ability of phytoplankton and zooplankton, hence impeding the role of oceans as the most significant carbon sink on Earth [3]. Therefore, immediate actions are needed to handle the increasing amount of plastic waste, especially the unrecyclable ones. While the gradual bans of single-use plastics could serve as a temporary solution to the crisis, a paradigm shift from a linear to a circular economy of plastics is regarded as a sustainable solution in the long term without compromising the societal benefits of plastics [4,5].At present, there are nearly 28 technology providers around the world that have developed or are currently developing thermal-chemical recycling technologies to promote the circular economy of plastics [6]. Pyrolysis is one of the key technologies used by these providers, where the plastic waste is degraded in an inert environment at high temperatures (400–600 °C) to produce value-added products, including liquid fuels, chemical feedstock, and carbon nanomaterials [7,8]. Given its strategic importance, plastic waste pyrolysis is an essential component in the SuSChem Strategic Innovation and Research Agenda which requires alignment of all actors in the innovation ecosystems [9]. At present, the technological readiness level of plastic pyrolysis stands at 6–7 [10]. Despite the significant progress in plastic pyrolysis development, several important issues limit the potential of large-scale plastic waste pyrolysis. As plastic pyrolysis is an endothermic reaction, the significant energy cost in the scaled-up process reduces the cost-competitiveness of the pyrolysis oil against the fossil-based oil. Most research teams utilized resistive heating in plastic pyrolysis, which is associated with low heating and cooling rates, and therefore low energy efficiency. This limitation is responsible for the high energy consumption and thus aforementioned low economic viability of large-scale plastic waste recycling. A potential solution to this challenge lies in replacing resistive heating with induction heating, which is a non-contact technique involving the induction of eddy current on the surface of a ferromagnetic metal placed in an alternating magnetic field. The eddy current generates the Joule heating effect, which leads to rapid heat generation of the metal. As most plastic pyrolysis reactors are made of stainless steel (which is a good susceptor in an alternating magnetic field), the application of induction heating in plastic pyrolysis could be a critical and innovative solution to achieve higher energy efficiency and, therefore, higher economic feasibility for large-scale application.Several research teams have investigated induction heating in biomass waste pyrolysis. Tsai et al. [11] reported the pyrolysis of biomass wastes to bio-oils in a stainless-steel reactor via induction heating. Muley et al. [12] also developed a two-stage reactor for the pyrolysis of pinewood sawdust with induction heating applied to biomass pyrolysis and catalytic upgrading stages. Compared to resistive heating, the adoption of induction heating in the catalysis stage lowered the degree of coke deposition in catalyst particles. This observation was related to a more efficient heat transfer from the stainless-steel reactor surface (susceptor) to the catalyst bed during the induction heating process. The pyrolysis of electronic waste through induction heating (with graphite crucible as susceptor) also led to a more significant weight reduction (by 7 %) than pyrolysis through resistive heating [13]. Compared with biomass pyrolysis, scientific investigations on the roles of induction heating in plastic pyrolysis are limited in the literature. Nakanoh et al. [14] were the first team who mentioned the feasibility of the process. However, no information was provided on the plastic pyrolysis product yields and compositions. Zeaiter [15] reported on the rapid decomposition of high-density polyethylene waste via induction heating during pyrolysis, resulting in insufficient contact between the pyrolysis intermediates and catalyst particles. This phenomenon ultimately led to high wax/liquid and solid yields but low gas yields. To date, there is an insufficient understanding of the feasibility of plastic pyrolysis via induction heating. To fill this knowledge gap, an exploratory study was performed to investigate the thermal and catalytic pyrolysis of neat plastics in a self-fabricated stainless-steel (SS316) fixed bed reactor using induction heating. Specifically, the effects of catalyst properties on the product yields and compositions from the pyrolysis of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) pellets in a nitrogen environment were investigated. This is the first study that reports the behaviors of thermal and catalytic pyrolysis of different polymers via induction heating of the reactor wall, which will provide valuable insights into novel strategies for plastic waste valorization.The LDPE, HDPE, and PP pellets (particle size: 3–5 mm) were supplied by a Spanish energy and petrochemical company and were used as received. For catalytic pyrolysis, ZSM-5 catalyst (CBV 2314, Zeolyst International, SiO2/Al2O3 = 23) and spent fluid catalytic cracking (FCC) catalyst obtained from a Spanish-based international energy company were used. Before the experiments, the catalysts were calcined at 550 °C in the air for 3 h to remove all the adhered impurities (including moisture) [16]. The calcination step also converts the ZSM-5 zeolite from ammonium form to hydrogen form [17] and removed the coke formed on the FCC catalyst.The thermal degradation behavior of the plastics was analysed using a thermogravimetric analyser (TGA, STAR system, Mettler Toledo). The samples were purged with pure nitrogen gas (20 mL/min) and then heated at 10 °C/min in the temperature range of 25–800 °C [18]. The raw TGA data were differentiated to obtain the derivative (DTG) curves for the samples. The molecular weight distribution (MWD), molecular weight averages (Mw), and polydispersity indexes (PI) of the plastic pellets and waxes from thermal pyrolysis were determined using gel-permeation chromatography (GPC) coupled with infrared detector (GPC-IR6) (Polymer Char, Spain) [19]. The samples were prepared by dissolving approximately 10 mg of plastics in 1 mL of 1,2-dichlorobenzene at 150 °C, followed by in-line filtration. The Mw and PI values referred to the monodisperse PS standards [19].The density and strength of the Brønsted acidic sites on the catalysts were characterized based on the thermogravimetric measurement of temperature-programmed decomposition of n-propylamine (NPA) in a DSC-TGA thermal analyser (Model: SDT Q600, TA Instruments) [20]. The sample was heated at 600 °C for 30 min in nitrogen flow (30 mL/min) to remove all absorbed impurities and then saturated with NPA at 150 °C. After that, the sample was heated in nitrogen flow (30 mL/min) at 150–700 °C with a heating rate of 10 °C/min. The textural properties of the catalysts were characterized based on the nitrogen adsorption-desorption isotherm produced by the Micromeritics Gemini 2360 instrument at 196 °C [21]. Before the analysis, the samples were outgassed at 350 °C for 4 h. The surface area of the samples was computed using the Brunauer-Emmett-Teller's (BET) equation from the nitrogen adsorption curve in the region 0.05 ≤ P/Po ≤ 0.3. The pore size distributions of HZSM-5 and FCC catalysts were computed using the Barrett-Joyner-Halenda (BJH) method from the desorption isotherms. The XRD diffractograms were recorded on a Panalytical X′Pert Pro (The Netherlands) using Cu Kα radiation generated at 45 kV and 40 mA. A scanning range from 5° to 100° was used at a speed of 0.03°/s.A fixed bed reactor system ( Fig. 1) was used for plastic pyrolysis through induction heating. The reactor was an AISI 316 stainless steel tube (length, L: 10.0 cm, outer diameter, OD: 2.2 cm). The plastic pellets and the catalysts were placed in the middle section of the reactor between the layers of quartz wool. The reactor also acted as a susceptor in an alternating magnetic field produced by a 3-turn copper coil (L: 5.0 cm, inner diameter, ID.: 4.5 cm) connected to an induction heater (Easyheat system, Ambrell, UK) with an output power of 1.2 kW. The current frequency was fixed at 315 kHz. A Superwool Plus fiber (13 mm) layer was placed in between the reactor and copper coil to reduce heat losses from the reactor wall to the surrounding. The reactor wall temperature was measured using a pyrometer (CTM-3CF75H1-C3, Micro-Epsilon) via a small opening in the insulation layer.The top part of the reactor was welded to an inlet AISI 316 tube (L: 5.0 cm, OD: 2.5 cm), while the bottom part was welded to an outlet AISI 316 tube (L: 5.0 cm, OD: 1.8 cm). A stainless-steel cold trap connected to the bottom reactor outlet was maintained at 2 °C to collect the condensable reaction products. The non-condensable gases were collected in a water column placed after the cold trap. The total volume of the gas product evolved during plastic pyrolysis was determined based on the volume of water displaced from the water column into a measuring cylinder.In this exploratory study, 1.00 g of plastic pellets (LDPE, HDPE, or PP), together with 0.20 g of catalyst (in the case of catalytic pyrolysis), were placed on 0.20 g of a quartz wool layer in the reactor. As a basis to estimate the effect of the catalyst, a series of blank experiments were carried out, hereafter termed "thermal pyrolysis" in this paper. Before the pyrolysis process, the reactor system was purged with nitrogen gas (120 mL/min) for 10 min to prevent the oxidation/combustion of the plastics during pyrolysis. Next, the nitrogen gas flow was turned off. The induction heater was turned on for 30 min to allow reactor heating. After 30 min, the induction heater was turned off, and the reactor was allowed to cool to room temperature. If the wax product was formed in the cold trap, the wax was collected and weighed. If a liquid product was formed, it was extracted with 3 mL of dichloromethane before being weighed and analysed. All the pyrolysis experiments were performed in duplicates. The average values and per cent errors of product yields are presented in Section 3. In general, the experimental errors fall within 7%.According to the temperature measurement results (Section S1, Supplementary materials), induction heating successfully raised the reactor temperatures to 500–700 ℃ within 10 min. The heating rate of the system is estimated to be 50–85 ℃/min depending on the induction heater power. By definition, the system/process could be classified as fast pyrolysis, which has a typical heating rate of 10–200 ℃/min [22]. Based on the temperature measurement results, the plastic pyrolysis experiments occurred at 650 °C to allow full plastic conversion.As discussed in Section 2.4 and Section S1 (Supplementary materials), the heating power of the induction heater was selected to ensure the complete conversion of the plastic pellets in all experiments. Consequently, no plastic remained in the reactor after pyrolysis reactions. The liquid yield was calculated based on the following equation: (1) Liquid yield , x l % = m l m p × 100 % where mL (g) is the liquid product mass, and mp (g) is the mass of the plastic pellet used. Similarly, the wax yield was calculated based on the mass of the wax product collected. The amount of coke formed on the catalyst was characterized using the temperature-programmed oxidation (TPO) technique described in the literature [23]. The spent catalyst (10 mg) was heated to 800 °C using a thermo-gravimetric analyser (TGA/DSC STAR system, Mettler) at a heating rate of 15 °C/min, followed by a hold for 10 min at 800 °C, with an airflow rate of 100 mL/min. The analysis result was used to calculate the total mass of coke formed on the catalyst. The coke yield was then calculated using the following equation: (2) Coke yield , x c ( % ) = m c m p × 100 % where mc (g) is the total mass of coke formed.The light hydrocarbon compounds in the gas products were analysed using the Shimadzu 2010 GC equipped with an FID column (Equity-1 column, Supelco, L: 50 m, ID: 0.53 mm, stationary phase: poly(dimethylsiloxane), film thickness: 3 µm). Helium gas (31 mL/min) was used as carrier gas. The heating program used was: 50 °C for 6 min, then increased to 238 °C at 12.5 °C/min, and then held for 5 min. The split ratio was 2.0. Both the injector temperature and detector temperature were maintained at 250 °C. The abundance of hydrocarbon compounds in the gas products was reported according to the carbon numbers [24,25]. The chemical composition of the liquid products was analysed using the Thermo Scientific™ Q Exactive™ GC Orbitrap™ GC-MS/MS System with a method adapted from the literature [26]. The GC-MS/MS system was equipped with the Thermo Scientific TG-5SILMS capillary column (internal diameter: 0.25 mm, length: 30 m, film thickness: 0.25 µm, PN 26096–1420). Helium (1.2 mL/min) was used as a carrier gas. The injector was operated at 280 °C, with a split ratio of 25:1. The oven was initially maintained at 40 °C for 2 min, then heated up to 320 °C at a ramp rate of 30 °C/min, and finally held at 320 °C for 15 min. The MS was equipped with an electron ionization source (ionization voltage = 70 eV, m/z = 50–600). The ion source and transfer line were operated at 280 °C and 150 °C, respectively. The peaks in the total ion chromatogram were identified using Xcalibur Qual Browser software (Xcalibur version 4.2.47) by mass spectra searching the National Institute of Standards and Technology (NIST) Mass Spectral Search Program for the NIST/EPA/NIH EI and NIST Tandem Mass Spectral Library Version 2.3. The C7-C40 alkanes standard (1000 µg/mL) purchased from Sigma Aldrich was used to facilitate hydrocarbon compound identification in the liquid products.Information on the thermal behavior of the plastics is essential to determine suitable pyrolysis process conditions. Fig. 2a shows the TGA curves of LDPE, HDPE, and PP. In general, all the samples exhibited a one-step mass-loss process, which is a typical degradation behavior observed in other clean plastic samples [27]. The samples contain negligible moisture evidenced by the absence of a peak at ∼100 °C in the TGA plots. Complete degradation of all plastics occurred within the range of 435–500 °C. The decomposition peak temperatures of the plastics are observed in the following order: PP (458 °C) < LDPE (471 °C) < HDPE (482 °C) (Fig. 2b). Although PP, LDPE and HDPE consist of long hydrocarbon chains, the presence of methyl groups in the PP polymer chain lowers its thermal stability when compared to PE samples [28]. The lower decomposition temperature of LDPE compared to HDPE is related to the higher degree of branching in the former polymer [29].GPC is another critical analysis that provides information on the MWD of the polymer samples. The MWD of the plastic samples is shown in Fig. 2c, while the numerical values of mass average (Mw), number average (Mn), and Z average (Mz) molecular masses of the samples are provided in Table 1. All the samples display single modal peaks, which is similar to the observations made by Cáceres et al. [30] and Zhang et al. [31]. LDPE and HDPE exhibit broader peaks, indicating wider MWD [32]. This observation is accompanied by high PI values (13.4 and 16.8 respectively, Table 1). In comparison, PP produced smaller and higher peaks, with PI values of 6.62. The Mn of the samples is arranged in the following order: LDPE (12600) <HDPE (13300) < PP (32443). These values are very different from those measured in other works [33], as neat plastics properties are often adjusted by the manufacturers for different industrial applications.The Brønsted acidic sites in zeolitic catalysts play an important role as proton donors during the catalytic cracking of hydrocarbons [34], including plastic decomposition products [35], into smaller hydrocarbons. The Brønsted acidic sites in zeolites originate from the protons that balance the negative charges due to the substitution of Si by Al atoms in the zeolitic framework. The abundance of Brønsted acidic sites typically increases with aluminum content, which is represented by the Si/Al ratio of the zeolitic catalysts. When a catalyst saturated with NPA is heated, the Brønsted acidic sites catalyze NPA decomposition into propene and ammonia at a stoichiometric ratio via the Hoffmann elimination reaction [36], enabling estimation of the Brønsted acidic sites density on the catalyst. The strengths of the acidic sites can be estimated based on the product evolution peak temperature(s) [36]. Xian et al. [34] demonstrated different accessibility of the Brønsted acidic sites in a zeolite sample, related to their locations in the crystal grains.As shown in Fig. 3a, HZSM-5 exhibits three peaks at different temperatures. The peaks observed at 400 °C and 460 °C are attributed to the evolution of products from NPA decomposition on Brønsted acidic sites. The peak temperatures observed in this study closely match the HZSM-5 samples used by Xian et al. [34] and Xian et al. [36] (410–420 °C and 470–490 °C respectively), as well as the HZSM-5 sample prepared by Caeiro et al. [37] (415 °C and 485 °C respectively). The peak at 400 °C denotes the presence of stronger Brønsted acidic sites that can catalyze NPA decomposition at a lower temperature. In contrast, the peak at 460 °C represents weaker Brønsted acidic sites that can catalyze NPA decomposition at higher temperatures. A broad peak at 280 °C exists due to the desorption of physisorbed NPA from weak acidic sites, which was also observed by Xian et al. [36]. Previous works by other teams indicate that desorption peaks below 300 °C represent acidic sites that are too weak to decompose NPA, and therefore unlikely to catalyze hydrocarbon cracking [38]. The integration of peak areas (excluding the peak at 280 °C) revealed a Brønsted acidic sites density of 1082.3 µmol/gcat for HZSM-5 (in agreement with data reported by Losch et al. [39]), with 72.4 % contribution from the strong Brønsted acidic sites ( Table 2). When compared to HZSM-5, the FCC catalyst exhibits only two peaks with very low intensity. Similar to HZSM-5, the peak displayed at 212 °C is attributed to the desorption of physisorbed NPA, while the peak at 416 °C is related to the desorption of NPA decomposition products. No additional peak is observed. The quantification of the area under peak (416 °C) indicates a Brønsted acidic sites density of 100.5 µmol/gcat, contributed solely by weak Brønsted acidic sites. In addition to the Y zeolite, the fresh FCC catalyst also consists of binder and filler materials that improve the mechanical performance of the catalyst in the FCC process, while reducing the density of Brønsted acidic sites per unit catalyst mass. Furthermore, the spent FCC used in this study could be poisoned by transition metal contaminants, especially vanadium and nickel, during FCC operations [40,41], rendering a great number of active sites ineffective for cracking plastic pyrolysis intermediates in this study.The textural property of a catalyst is another significant factor that influences product selectivity during the catalytic cracking of plastics [35]. The nitrogen adsorption-desorption isotherms and pore size distribution curves of HZSM-5 and FCC catalysts are shown in Fig. 3b,c. A combination of type II and type IV(a) isotherms is observed for both samples according to the updated classification standards of adsorption-desorption isotherms by IUPAC [42]. At low relative pressure (0 ≤ P/Po ≤ 0.02), the number of nitrogen molecules adsorbed increased quickly due to micropore filling and single-layer adsorption in the mesopores, leading to the convex shape of the curves. Further increase of relative pressure leads to multilayer adsorption. At P/Po ≈ 0.44, condensation of the adsorbate molecules occurs in the mesopores (capillary condensation). Due to the presence of macropores in the samples, adsorption saturation is not observed. The desorption curves of both samples do not coincide with their respective adsorption curves at P/Po ≥ 0.44, and different hysteresis loops are observed. The hysteresis loop for HZSM-5 is the H4 type, indicating a mixture of microporosity and mesoporosity dominated by narrow crack pores [43]. In contrast, an H3-type hysteresis loop is observed for the FCC catalyst indicating plate slit, crack, and wedge structures [44]. HZSM-5 possess a higher micropore area and mesopore areas (Table 2), thus a higher BET surface area (410.6 m2/g) than those of FCC catalyst (147.5 m2/g). Fig. 3c shows that HZSM-5 has a peak distribution below ∼70 Å, with a single peak of 38 Å (3.8 nm), leading to a large micropore volume (0.124 cm3/g) and small mesopore volume (0.082 cm3/g). The observed pore distribution characteristic is typical for HZSM-5 zeolite [45]. FCC catalyst also produces a single peak with similar intensity to HZSM-5. Fig. 3b indicates a significant proportion of HZSM-5 zeolite in such a mixture. Nevertheless, the FCC catalyst has a wider pore distribution, up to 150 Å (15.0 nm), leading to a larger mesopore volume (0.131 cm3/g). This is accompanied by a smaller micropore volume (0.042 cm3/g). The average pore diameter based on the BJH desorption curve is 4.80 nm for HZSM-5, and 11.90 nm for the FCC catalyst.The crystallinity and average crystallite size of the H-ZSM5 and the FCC catalyst is examined using the XRD technique. The H-ZSM5 sample ( Fig. 4) depicts sharp peaks at 2θ = 8.03°, 8.97°, 13.24°, 13.99°, 16.06°, 23.11°, 24.01°, and 30.24°, which are related to (101), (111), (102), (112), (022), (051), (313) and (062) planes in the X-ray diffraction pattern of the MFI topology, indicated by the Powder Diffraction File (PDF) 01–079–1638 [46]. Similar diffraction patterns are also observed in the hierarchical ZSM-5 zeolite synthesized by Jesudoss et al. [47] and meta-promoted ZSM-5 zeolite characterized by [48]. When compared with HZSM-5 zeolite, FCC catalyst displays peaks with lower intensities. This is possibly attributed to the partial destruction of the FCC catalyst crystallinity after repeated usage in the FCC process. The diffraction lines exhibited by the FCC catalyst match the Y-type zeolite (PDF number: 01–077–1549), while the small peaks observed at 45.8° and 67.0° are attributed to the characteristic peaks of Al2O3 [49]. The average crystallite sizes of the HZSM-5 zeolite and the FCC catalyst were estimated to be 35.85 nm and 18.75 nm using the Debye Scherrer equation: (3) L = 0.89 λ β cos θ where L is the average crystallite size (nm), λ is the X-ray wavelength (0.154 Å), θ is the Bragg diffraction angle (in radian) and β is the full width at half maximum (FWHM) (in radian).The product yields for the thermal and catalytic pyrolysis of HDPE, LDPE, and PP are provided in Fig. 5. Thermal pyrolysis of plastics produced high wax yield (72.4–73.9 wt%). Two types of waxes were formed in each experimental run (regardless of the plastic type): white wax in the cold trap and yellow/green wax in the tubing that connected the reactor to the cold trap (Fig. S2, Supplementary materials). This observation signifies the following: when thermal pyrolysis occurs, the plastic vapor fills up the reactor space rapidly. Most of the vapor exited the reactor (due to the volume expansion) and condensed in the cold trap as white wax. As no nitrogen gas flows through the reactor in the experiments, the residence time of the plastic vapor in the reactor was affected by the rate of plastic conversion. The plastic vapor that remained in the reactor was further cracked to form lighter molecules, which then condensed in the tubing as yellow/green wax. This observation is also supported by the production of non-condensable gases up to a reaction time of 30 min, even after the complete plastic conversion. The formation of wax products from plastics indicates the occurrence of random C-C bond scission along the polymer chain, resulting in fragments with high molecular weights. This is a typical observation for plastic pyrolysis in a continuous system with short residence time, for example, a fluidized bed reactor [50]. A longer residence time promotes extensive cracking of the intermediates, producing more gas products and fewer waxes [51]. All the wax samples displayed a single peak with narrower MWD ( Fig. 6) compared to the plastic pellets samples in Fig. 2c. This is accompanied by low PI values (1.32–1.46). The Mw values of the wax samples are arranged in the following order: LDPE (400) < PP (600) < HDPE (900) ( Table 3). Assuming these waxes consisted of -CH2- groups [33], then these waxes contain chains with average carbon numbers of 28 (LDPE), 42 (PP) and 64 (HDPE). These data show that thermal pyrolysis within a short period is sufficient to reduce the molecular weight of the polymers to 1.5–4.5 % of their original molecular weights. When compared to the plastic pyrolysis waxes produced by Arabiourrutia et al. [33], the waxes obtained in this study have smaller Mw and PI values. The observed difference could be ascribed to the higher heating rates used in this study.The presence of catalysts in plastic pyrolysis reduced the wax yield while increasing the gas and liquid yields significantly. When HZSM-5 zeolite was used, all the plastics were converted to liquid products (24.0–27.2 wt%) and gas products (345.0–448.0 cm3/g). A small number of cokes were also formed on the used zeolites, which is discussed further in Section 3.4. Despite the inferior textural properties (compared to HZSM-5) and absence of strong Brønsted acidic sites, the FCC catalyst demonstrated catalytic activity in plastic pyrolysis within a short reaction time. Nevertheless, the catalyst was unable to convert all the heavy hydrocarbons into smaller molecules, and the wax product was mixed with the liquid product in the cold trap. These observations show that catalysts can effectively reduce the activation energy of polymer cracking, which was also observed by Zhou et al. [51] in microwave-assisted pyrolysis of plastic waste. Interestingly, plastic-type seems to influence the distribution of products during catalytic pyrolysis over FCC catalyst. LDPE produced soft wax and gas product without liquid products, while HDPE resulted in a mixture of soft wax and liquid products, together with the gas product. On the other hand, all the wax products from PP pyrolysis were converted into gas and liquid products. A different trend was observed in catalytic pyrolysis over HZSM-5, where the LDPE and HDPE showed closely similar product distribution compared to PP. Such differences could be related to the relatively weaker catalytic performance of the FCC catalyst (compared to HZSM-5), which influenced the product formation from HDPE and LDPE in different ways. More detailed investigations are necessary to fully understand the role of Brønsted acidic sites in the catalytic pyrolysis of plastics assisted by induction heating.The application of induction heating in plastic pyrolysis using different reactor setups has been attempted by several research teams. Whajah et al. [52] reported LDPE pyrolysis over Fe3O4 mixed with Ni- or Pt-based catalysts (1:1 mass ratio for plastic to the Fe3O4/catalyst mixture) in a glass batch reactor. The Fe3O4 particles were heated to 350 ºC, and the pyrolysis reactions lasted for 120 min with 100 mL/min N2 flow. Due to the small amount of heat generated by the metallic susceptor in the alternating magnetic field, a longer reaction time was needed to achieve 94 % plastic conversion, with gas yields of 15–80 wt%, liquid yields of 2–43 wt%, and coke yields of 1.3–2.4 wt%. In contrast, Hassani [53] performed LDPE pyrolysis over HZSM-5 zeolite via the induction heating of a semi-batch reactor wall to 400–500 ºC, with a catalyst loading of 9–25 wt%. The author reported gas yields of 60–75 wt%, liquid yields of 20–55 wt%, and coke yields of 2–4 wt%. Similar to the aforementioned works, this study demonstrated that catalytic pyrolysis of plastics produced high gas yields and comparatively low liquid yields. Nevertheless, induction heating of the reactor wall in this study generated a large amount of heat in a short time and ensured a total plastic conversion. The short pyrolysis time could also reduce the secondary reactions, leading to lower coke yields in this study compared to Whajah et al. [52] and Hassani [53].To obtain further insights on the effects of induction heating on plastic pyrolysis behavior, the evolution rates of gas products during thermal and catalytic pyrolysis of HDPE, LDPE, and PP were recorded ( Fig. 7). As plastic pyrolysis typically leads to the generation of light hydrocarbon gas, Fig. 7 indicates the plastic decomposition rate during pyrolysis powered by induction heating. In all experiments, rapid gas evolution was observed between 3 and 5 min (corresponding to the temperature of 450–550 °C). This observation is in line with the TGA data where plastic decomposition was observed at 450–510 °C (see Fig. 2a). In all experiments, the generation of gas products slowed down after the temperature reached 600 °C after 7 min. After 10 min, complete plastic conversion into volatile products occurred. The experiments were performed without inert gas flow to enable precise observation of the product gas evolution rates in different experimental runs. During the reactions, some of the gas products that remained in the reactor underwent further cracking to produce lighter hydrocarbons leading to a further increase in the product gas volume. The secondary cracking effects could be reduced when a sweeping gas is provided to remove the pyrolysis intermediates from the reactor during plastic pyrolysis.In thermal pyrolysis (Fig. 7a), only a small amount of gas products (95–132 mL/g plastic) were produced, accompanied by wax formation, regardless of the plastic type (Fig. 6). Similar observations were made by Zeaiter [15], indicating the limited degree of polymer chain cracking during thermal pyrolysis. Several research teams reported that plastic pyrolysis with high heating rates (fast pyrolysis) produced more gas products, while pyrolysis with low heating rates (slow pyrolysis) produced more liquid products [54,55]. Fig. 7a also reveals higher product gas evolution (representing more extensive cracking) of PP compared to HDPE and LDPE, as PP (with a branched-chain structure and tertiary carbon) is more susceptible to C-C bond cleavage. Thus, PP can be cracked more easily than PE (linear structure with limited branching) to produce permanent gases, which is also supported by the TGA result (Fig. 2). Similar observations were also made for PP pyrolysis by resistive heating [56] and microwave heating [51].The presence of catalysts significantly increased the product gas evolutions to different degrees (Fig. 7b, c). In the presence of HZSM-5, the gas production volumes increased more than trifolds (345–448 mL/g plastic) when compared to thermal pyrolysis. This is related to the high catalytic property of HZSM-5, which is extensively reported in plastic pyrolysis research. The microporous structure and high acidic properties of HZSM-5, as mentioned in Section 3.2, promoted a high degree of plastic cracking, leading to the formation of liquid products and more C1-C4 products. In comparison, catalytic pyrolysis over FCC catalyst produced a lower amount of gas products (164 −192 mL/g plastics). This is attributed to the less severe plastic cracking due to the lower acidity of the catalyst (Section 3.2). Fig. 7b, c also reveals the lower degree of gas product evolution during catalytic cracking of PP compared to HDPE and LDPE, which is in contrast with the observation in Fig. 7a. Such observation is related to the limited contact between the PP molecules (with branched-chain structure) with the catalyst surface, leading to lower cracking and thus gas product volume. The steric hindrance effect in catalytic pyrolysis of PP was also observed by Li et al. [57] and Palza et al. [58]. Fig. 8 summarizes the composition of gas products from thermal and catalytic pyrolysis (in terms of carbon number), with the details of the gas components provided in Table S1 (Supplementary materials). The gas products from thermal pyrolysis are rich in C2 (12.63–18.27 %) and C3 (67.39–72.19 %) compounds. The presence of catalysts increased the abundance of C3 and C4 compounds to higher extents, at the expense of C1 and C2 compounds. The possible explanation for this observation is the reduced residence time of the plastic vapor in the reactor during catalytic pyrolysis. Due to the abundance of C3 compounds (propane and propene) and C4 compounds (butane and butene), the average molecular weight of the gas products was 41.87–46.67 ( Table 4). The presence of the catalysts increased the rates of plastic decomposition and vapor formation. As the vapors exited the reactor rapidly, there was insufficient time for extensive cracking of hydrocarbon molecules into smaller hydrocarbon compounds.The GC/MS chromatograms (Figs. S2-S7) reveal a significant number of hydrocarbons in the liquid products. A tall peak that corresponds to the C6 compound in all chromatograms could be attributed to the hexane used for cleaning of autosampler of the GC instrument. Therefore, the percentage of C6 compounds in the liquid products was not analysed to prevent data misinterpretations. The presence of doublets and triplet peaks in the chromatograms signifies the presence of alkadienes, alkenes, and alkanes with similar carbon numbers in the liquid samples, which is a typical characteristic of plastic pyrolysis products [18,51]. Detailed information on the liquid product compositions is provided in Table S2 (Supplementary materials). To simplify the data analysis, the abundance of alkanes, alkenes, and aromatic compounds in the liquid products is illustrated in Fig. 9, whereas the details of the compounds in the liquid products are provided in Table S1. A careful examination of the chromatograms shows that catalytic pyrolysis of HDPE and LDPE possessed similar product distributions, while the liquid products from PP pyrolysis showed different product characteristics. This is clear evidence of the influence of plastic structures on the product formation route during pyrolysis.The type of catalyst used in plastic pyrolysis also has an apparent influence on the liquid product compositions. Liquid products from catalytic pyrolysis of HDPE and LDPE over HZSM-5 contained hydrocarbons with carbon numbers in C7-C15, represented by the tall peaks in the chromatograms (Figs. S3–4). HZSM-5 produced a high amount of aromatic compounds in the liquid products from the three plastics pyrolyzed. This observation is related to the high abundance of benzene, toluene, ethylbenzene, xylene and naphthalenes. HDPE pyrolysis produces an exceptionally high percentage of aromatic compounds (93.0 %) at the expense of alkenes (5.5 %) and alkanes (1.49 %). A similar trend is observed for LDPE pyrolysis, which produced 74.8 % aromatic compounds, 18.2 % alkenes, and 7.0 % alkanes. The considerably high proportion of naphthalene in the liquid products is related to the extensive cyclization of pyrolysis intermediates catalyzed by HZSM-5 [59]. A slightly different trend is observed for PP pyrolysis, which produced considerably more alkenes (38.3 %) and alkanes (16.9 %) and fewer aromatic compounds (44.9 %) (Fig. S5). Past investigations show that the highly acidic and microporous structure of HZSM-5 zeolite promotes β-scissions of C-C bonds along the polymer chain to produce carbonium ions and olefinic compounds. The carbonium ions can undergo further β-scissions to produce smaller oligomers, followed by isomerization, oligomerization, cyclization and aromatization of the oligomers, forming a wide range of hydrocarbon compounds [60]. The bulky structures of PP molecules hinder effective contact of the molecules with the catalyst particles, leading the decreased cyclization and aromatization. Due to the high contents of aromatics (mainly C7-C8 compounds), the plastic pyrolysis liquid products over HZSM-5 possess low average molecular weight (AMM) (9.73–11.14).The chromatograms representing catalytic pyrolysis over FCC catalyst (Figs. S6-S8) show different trends to those over HZSM-5. All the liquid products contain C5-C35 hydrocarbons, represented by peaks with lower heights. Such observations indicate a wide product distribution. As FCC catalysts possess lower surface area and weaker acidity than those of HZSM-5, the catalyst was incapable of producing small hydrocarbons with narrow product distribution. Catalytic pyrolysis of plastics over FCC catalyst produced liquid products that contained more alkenes (38.3–67.6 %) and alkanes (16.9–25.1 %), with a small number of aromatic compounds (6.7–9.5 %). Catalytic pyrolysis of HDPE and LDPE produced a slightly higher amount of C19-C40 alkanes (13.4–15.6 % compared to PP, 7.9 %), while catalytic pyrolysis of PP produced a slightly higher amount of C9-C19 compounds (11.0 % compared to HDPE and LDPE, 8.5–9.7 %). Therefore, the AMM of the liquid products over the FCC catalyst are higher (21.74–22.49) than those over HZSM-5. No apparent difference was observed in terms of the abundance of aromatic compounds according to carbon number. These observations show that the strength and density of Brønsted acidic sites play a more critical role than the BET surface area in the aromatization of plastic pyrolysis intermediates. The effects of plastic-type exerted minor influences on the liquid product distribution in this case.The temperature-programmed oxidation of the catalysts used in plastic pyrolysis provides qualitative and quantitative information on the amount and types of cokes formed on the catalysts. Fig. 10 indicate different coking behaviors on the spent catalysts. For the HZSM-5 zeolite, 2–3 peaks were observed (in addition to the peak at ∼100 °C which signifies moisture presence). This observation indicates the formation of different types of cokes that (i) can be decomposed at lower temperatures (150–330 °C), and (ii) can be decomposed at higher temperatures (330–760 °C). In general, cokes are formed via the aromatization of hydrocarbon oligomers on Brønsted acidic sites. Investigations on plastic pyrolysis [61] and alkene aromatization [62] demonstrated that cokes could exist in the spaces between the catalyst particles or on the catalyst surface (termed soft coke), or inside the catalyst micropores, where the Brønsted acidic sites are located (termed hard coke). Soft coke consists mostly of oligomers and can be removed via heating in an inert environment. In contrast, hard coke is a polyaromatic hydrocarbon formed via hydrogen transfer, isomerization, cyclization, and aromatization of plastic cracking products on the Brønsted acidic sites [63]. Due to the more complex structure of the hard coke, higher temperatures are required for its complete oxidation.For the HZSM-5 zeolites, the coke yields are 2.36 %, 2.14 %, and 2.18 % for HDPE, LDPE, and PP, respectively, concerning the mass of plastic samples pyrolyzed. For the FCC catalyst, coke decomposition produced peaks at similar positions to those observed on ZSM zeolites. Strong peaks at ∼640 °C were observed in all the FCC catalyst samples, irrespective of the plastic type. In contrast, small peaks were observed at 220–240 °C on the FCC catalyst used for HDPE and LDPE pyrolysis. The peak is almost non-existent for the FCC catalyst used for PP pyrolysis. The strong peak observed at ∼640 °C is attributed to the coke formed on the weak acidic sites of the FCC catalyst. The coke yields are 1.32 %, 1.38 % and 1.70 % for HDPE, LDPE, and PP, respectively. The characterization results in Section 3.2 indicate the almost non-existence of the strong acidic sites in the regenerated FCC catalyst (Table 2). The formation of coke, despite in small amount, shows that the sites played a non-negligible role during plastic pyrolysis. The difference in the coke yields observed in HZSM-5 zeolite and FCC catalysts are attributed to the higher density and strengths of Brønsted acidic sites in the former catalyst. The high acidity of HZSM-5 is known to aromatize plastic volatiles, which leads to coke formation. The rapid deactivation of HZSM-5 in plastic pyrolysis remains an issue that requires innovative solutions [51]. A more detailed study is required to unravel the possible influence of plastic-type on the catalyst coking behavior.To illustrate the advantage of induction heating in plastic pyrolysis, the electrical energy consumed by the induction heater during plastic pyrolysis was measured using a C.A 8436 Qualistar+ Power Quality analyser (Chauvin Arnoux). Measurements under different conditions show that the electrical energy consumption was independent of the masses of plastics and catalysts during pyrolysis (as heat energy was supplied in excess to the reaction system) and was only related to the induction heater power and reactor dimensions. The electrical energy consumed during 10 min of plastic pyrolysis was 228 kJ (≈63.4 Wh) (Fig. S9a, Supplementary materials). This value is significantly lower than those recorded by Wu et al. [64] for biomass pyrolysis, namely 665.2 kJ at 550 °C (with the presence of metal hollow balls) and 819.9 kJ (without the presence of metal hollow balls) at 600 °C. The observed difference could be attributed to the smaller size of the reactor heated in this study (reactor diameter: 2.2 cm, length of the reactor to be inductively heated: 5.0 cm) when compared to the reactor used by Wu et al. [64] (reactor diameter: 5.6 cm, length of the reactor to be inductively heated: not specified).As plastic/biomass pyrolysis using induction heating is rarely reported in scientific literature, data on the electrical energy consumption of the process is scarce. To provide a direct comparison, electrical energy consumption during plastic pyrolysis in a fixed bed reactor using an electric furnace (based on resistive heating) was also measured using the power quality analyser. In contrast to the reactor heating profile depicted in Fig. S1, a long heating time (25 min) is required to raise the reactor temperature to ∼650 °C, followed by plastic pyrolysis for 10 min. These steps consumed 1225.8 kJ (340.5 Wh) and 270 kJ (75.1 Wh) respectively, resulting in total energy consumption of 1535 kJ (426.4 Wh) (Fig. S9b, Supplementary materials). The measurement values indicate an 85 % saving in electrical energy consumption when plastic pyrolysis is performed via induction heating over resistive heating. These values indicate the potential energy saving for plastic pyrolysis via induction heating. A similar observation was also made by Okan et al. [65], where sewage sludge pyrolysis driven by induction heating (4.62 MJ/kg) consumed less energy than resistive heating (6.65 MJ/kg) under similar reaction conditions. A more accurate comparison of energy consumption in plastic pyrolysis using induction heating and resistive heating will be necessary using the same reactor which can be inductively and resistively heated.In this study, the reactor was heated to a high temperature at a reaction time sufficient to ensure complete plastic conversion. Therefore, the heat energy supplied to the system was in excess compared to the theoretical energy needed to depolymerize the plastic pellets. This statement implies that the electrical energy consumption per unit mass of reactant would decrease at a higher reactant loading, as suggested by Okan et al. [65]. The energy efficiency of the plastic pyrolysis via induction heating can be improved when the potential effects of reaction parameters (frequency of the alternating magnetic field generated by the induction heater, current used for induction heater, the magnetic permeability of the reactor material etc.) which affects reactor temperature during induction heating are fully understood. Statistical process optimization is another strategy to increase the energy efficiency of the plastic pyrolysis process.In literature, different reactor designs have been proposed for plastic pyrolysis to maximize conversion and liquid/gas yields, while minimizing coke formation on catalysts [66,67]. Plastic pyrolysis in batch and semi-batch reactors exhibit satisfactory process performance at the laboratory scale and provide scientists with ample information on the process performance. Nevertheless, the development of reactor designs that allow continuous operation mode is essential to enable process scale-up. In addition, significant energy consumption and long heating time are recognized as a few of the technical challenges in plastic pyrolysis. Microwave-assisted pyrolysis is proven to be an excellent strategy to overcome the aforementioned challenges, as the heat energy can be rapidly produced from metallic susceptors inside the reactor vessel [68], which minimizes the heating/cooling time and energy losses to the surroundings. In addition, the heat transfer limitation can be minimized, as the susceptors are mixed with the plastic feed. Due to the incompatibility with the stainless-steel reactor, microwave-assisted pyrolysis is typically performed in a specialized reaction system, which contains a reactor vessel made of microwave-transparent materials (quartz/ceramic). The limited mechanical strengths of these materials restrain the prospects of microwave-assisted pyrolysis in the plastic recycling sector, especially when considering the possible pressure fluctuations due to the volatile gases released during the process. This study presents the possibility to conduct plastic pyrolysis in a stainless-steel fixed-bed reactor in an alternating magnetic field. The reactor was heated up to 650 °C within 10 min, which was sufficient for complete plastic conversion. Due to the short residence time of plastic pyrolysis intermediates in the reactor heating zone, thermal pyrolysis produced mainly wax products with a small amount of hydrocarbon gas. The addition of zeolitic catalysts significantly improved the yields of gas and liquid products at the expense of the wax product. The product yields and compositions, as well as the catalyst deactivation behaviors, can be explained with references to past investigations on plastic pyrolysis using resistive heating and microwave heating. This exploratory study demonstrates the feasibility of plastic pyrolysis via induction heating of a reaction vessel, which is comparable to microwave heating in terms of heating/cooling time. The use of a stainless-steel reactor allows possible pressure variation during the process, hence increasing the safety margin when considering the process upscaling potential. The avoidance of metallic susceptors in the plastic feed could lead to a more convenient treatment of solid residue in the reactor after plastic pyrolysis. These findings also demonstrate the high flexibility of the inductively heated plastic pyrolysis system in plastic waste valorization into various products, depending on the desired end-uses and market value of these products. It is believed that the application of induction heating can be extended to various existing reactor designs (auger reactor [69], conical spouted bed reactor [70], vertical falling film reactor [71], among others) and process designs (pyrolysis, gasification [72], hydrothermal processing [73] among others), which is an interesting option to improve energy efficiency while retaining the unique advantages related to these reactor/process designs. Nevertheless, more fundamental studies are needed for a comprehensive understanding and control of inductively heated plastic pyrolysis. The lack of data on the yields of H2, as well as C5 and C6 compounds in plastic pyrolysis products in this study, is a significant shortcoming that will be tackled in future studies.The study examined the feasibility of induction heating in the thermal and catalytic pyrolysis of HDPE, LDPE, and PP plastics using two HZSM-5 (MFI topology) and FCC (FAU topology) catalysts, which has not been explored in the literature. In all the experimental runs, the plastic conversion started within 3–5 min of reaction time, and complete plastic conversion was observed within 10 min. This observation corresponded to the rapid reactor temperature increase upon induction heating. Thermal pyrolysis produced a significant amount of wax (72.4–73.9 wt%) and a small amount of non-condensable gas products (rich in C3 followed by C2 compounds). The presence of the catalysts significantly enhanced the gas (70.6–73.9 wt% for HZSM-5 and 62.4–75.2 wt% for FCC catalyst) and liquid yield (24.0–27.2 wt% for HZSM-5 and 0–35.9 wt% for FCC catalyst), at the expense of wax yield (0 wt% for HZSM-5 and 0 −25.4 wt% for FCC catalyst). The gas products for the catalytic pyrolysis were rich in C3 followed by C4 compounds, regardless of the plastic and catalyst properties. On the other hand, a significant variation in terms of liquid product distribution is observed for catalytic pyrolysis. HZSM-5 zeolite produced liquid products with high aromatics content, especially C7-C10 aromatics (representing toluene, ethylbenzene, xylene, naphthalene and several alkylbenzene isomers). The FCC catalyst produced a high number of liquid products, which were richer in alkenes and alkanes in the C9-C40 range. The outlined differences are attributed to the different textural properties and acidity profiles of the catalysts, which led to different reaction pathways during plastic pyrolysis. These observations validate the ability of the catalysts in product distribution modification despite the short residence time. Polymer types also influenced product yields and distribution, in agreement with past investigations in plastic pyrolysis. Coke analysis revealed the formation of cokes (2.14–2.38 wt% for HZSM-5 and 1.32 – 1.70 wt% for FCC catalyst) of different strengths, corresponding to the different acidic sites on the catalysts. Based on the similar decomposition peaks of these cokes in HZSM-5 and FCC catalysts, it is hypothesized that the cokes were formed on the strong and weak acidic sites of these zeolitic catalysts.To conclude, this exploratory study demonstrates the feasibility of new pyrolysis powered by induction heating in the production of different value-added products from plastics. The adoption of induction heating could be an interesting strategy that reduces the time needed for plastic waste pyrolysis, especially at a larger scale. In addition, the prospects of applying induction heating on stainless-steel reactors also provide an interesting scaling-up strategy that allows possible pressure variation. As induction heating is also known to have higher energy conversion efficiency than resistive heating, future studies will be dedicated to the quantification of energy consumption in plastic pyrolysis assisted with induction heating. Syie Luing Wong: Investigation, Methodology, Writing – original draft. Sabino Armenise: Conceptualization, Validation. Bemgba Bevan Nyakuma: Writing – reviewing and editing. Anna Bogush: Formal analysis, Data curation. Sam Towers: Formal analysis, Data curation. Chia Hau Lee: Software, Resources. Keng Yinn Wong: Software, Resources. Ting Hun Lee: Software. Evgeny Rebrov: methodology, Supervision. Marta Muñoz: Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Syie Luing Wong and Sabino Armenise have received support from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant Agreement No. 754382, GOT ENERGY TALENT. The content of this publication does not reflect the official opinion of the European Union. Responsibility for the information and views expressed in this paper lies entirely with the authors. Marta Muñoz also gratefully acknowledges the financial support from the Comunidad de Madrid (S2018/NMT-4411) for a project titled “Additive Manufacturing: from Material to Application.” Syie Luing Wong wishes to thank John Pillier, Li SiRui, Joe Gregory, and Deema Kunda for all the guidance and assistance provided during his research associated with the School of Engineering, University of Warwick, UK. Syie Luing Wong and Sabino Armenise also acknowledge the contributions of Carlos Prieto and the FCC team from CEPSA to this study in terms of materials, technical analysis, and intellectual discussions.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jaap.2022.105793. Supplementary material .

Plastic pollution compromises the environment and human well-being, and a global transition to a circular economy of plastics is vital to address this challenge. Pyrolysis is a key technology for the end-of-life recycling of plastics, although high energy consumption limits the economic feasibility of the process. Various research has shown that the application of induction heating in biomass pyrolysis reduces energy consumption when compared to conventional heating. Nevertheless, the potential of induction heating in plastic pyrolysis is rarely explored. This paper presents an exploratory study on the thermal and catalytic pyrolysis of high-density polyethylene, low-density polyethylene, and polypropylene in a fixed bed reactor through induction heating. An MFI-type HZSM-5 zeolite (SiO2/Al2O3 = 23) and an FAU-type spent fluid catalytic cracking (FCC) catalyst with distinctive Brønsted acidity and textural properties were used. A complete conversion of the plastic feedstocks was achieved within 10 min, even without a catalyst. Thermal pyrolysis produced wax (72.4–73.9 wt%) and gas products, indicating a limited degree of polymer cracking. Catalytic pyrolysis over HZSM-5 and FCC catalyst significantly improved polymer cracking, leading to higher gas (up to 75.2 wt%) and liquid product (up to 35.9 wt%) yields at the expense of wax yield (up to 25.4 wt%). In general, the gas products were rich in C3 and C4 compounds. The liquid product composition was highly dependent on the catalyst properties, for example, the HZSM-5 produced high aromatics, while the FCC catalyst produced high alkenes in the liquid products. The catalyst acidity and textural properties played an essential role in plastic pyrolysis within the short reaction time. This study demonstrated the feasibility of a fast, energy-efficient, and versatile plastic valorization technology based on the application of induction heating, where the plastic feed can be converted into wax, gas, and liquid products depending on the end-use applications.

Hydrogen peroxide (H2O2), as a green and environment-friendly oxidant, is widely used in chemical, biological, environmental, and other industries. Currently, H2O2 production is mainly based on the anthraquinone autoxidation (AO) process. However, the AO process has complicated processes, high production costs, and an unfriendly environment, and the safety risks of high-concentration hydrogen peroxide during transportation, storage, and use cannot be ignored (Fukuzumi et al., 2021). The direct synthesis of H2O2 from H2 and O2 has broad application prospects because of its environmental-friendly, economic efficiency, ready-to-use, and only water as a by-product (Huynh et al., 2021; Lewis and Hutchings, 2018).Although H2O2 directly synthesizing has excellent potential, the formation of H2O2 and H2O from H2 and O2, the hydrogenation and decomposition of H2O2 are both thermodynamically favorable (Han et al., 2021), which makes the preparation of high-performance hydrogen peroxide catalysts more difficult. Many experimental and theoretical studies (Han et al., 2021; Agarwal et al., 2021; Tian et al., 2017; Hang and Chung, 2020; Lari et al., 2017; Lyu et al., 2019; Priyadarshini et al., 2021) were conducted to obtain high-performance hydrogen peroxide catalysts. Palladium is the practical active catalyst for H2O2 directly synthesizing. The Langmuir-Hinshelwood reaction mechanism and proton-electron transfer reaction mechanism are the two main reaction mechanisms for directly synthesizing hydrogen peroxide (Han et al., 2021). These two mechanisms agree for oxygen adsorption but diverge for hydrogen adsorption and dissociation. According to the Langmuir-Hinshelwood reaction mechanism, hydrogen is dissociated on the surface of Pd to form H* (* indicates adsorption on the catalyst surface), and then H* and O2* to form –OOH* intermediate. In comparison, the proton-electron transfer reaction mechanism involves hydrogen dissociated on the surface of Pd to form protic hydrogen (H+) and electrons. Electrons are transferred to O2* by the catalyst, and then the proton H+ reacts with O2* to form –OOH* intermediate. The DFT study concluded that both reaction mechanisms are inverse thermodynamically feasible (Agarwal et al., 2021). Although the mechanism of hydrogen peroxide generation is still somewhat controversial, the role of palladium as an active catalytic center is recognized; therefore, the modulation of palladium can directly affect the synthesis of hydrogen peroxide and ultimately obtain a high-performance catalyst. Changing the catalyst's synthesis process and heat treatment conditions could change palladium's geometry and electronic structure (such as particle size, crystal plane, and Pd0/Pd2+ ratio), which are conducive to improving H2O2 selectivity and production rates (Tian et al., 2017; Hang and Chung, 2020; Lari et al., 2017; Lyu et al., 2019; Priyadarshini et al., 2021; Han et al., 2017). Alloying Pd with various metals (e.g., Au, Ag, Zn, Sn, Co, Ni) (Li et al., 2018; Ricciardulli et al., 2021; Doronkin et al., 2020; Gu et al., 2016; Wilson et al., 2018; Wilson et al., 2018; Kanungo et al., 2019; Nazeri et al., 2021; Tian et al., 2017; Kazici et al., 2017; Zhang et al., 2021; Lee and Chung, 2020; Maity and Eswaramoorthy, 2016; Zhang et al., 2018) enhance H2O2 selectivity by changing the geometric and electronic structure of palladium.Support modification is another important strategy to improve the catalytic performance of Pd catalysts. The surface properties of the support can adjust H2O2 productivity and H2O2 selectivity by affecting the morphology, dispersion, electronic state and alloying degree of the Pd active metal. Support regulation of oxide (SiO2, Al2O3, TiO2, etc.) and carbon materials (Lewis and Hutchings, 2018; Vu et al., 2021; Liang et al., 2020; Edwards et al., 2014; Edwards et al., 2009; Piccinini et al., 2012; Villa et al., 2016; Hu et al., 2014; Gudarzi et al., 2015; Gudarzi et al., 2015; Thuy Vu et al., 607 (2020).; García et al., 2015) (activated carbon, ordered mesoporous carbon, CNT, etc.) are common in H2O2 directly synthesizing. Hutchings (Edwards et al., 2009; Ntainjua et al., 2008) and co-workers found that porous carbon supports were the best supports for Pd and Au-Pd catalysts because of the superior catalytic activity and lowest rate of H2O2 hydrogenation and decomposition side reactions. In the follow-up research works, the team further found that nitric acid pretreatment of carbon supports for Au-Pd catalysts could switch off the sequential H2O2 hydrogenation and decomposition side reactions compared with TiO2, ZrO2, CeO2 and other oxide supports, making porous carbon become a significant support for the direct synthesis of H2O2 from H2 and O2. N-doped is an effective modification method for porous carbon, which could change the surface properties and pore structure and provide plentiful chemically active sites (Wei et al., 2018). Perathoner (Abate et al., 2010) and co-workers found that the Pd-based catalysts supported on N-doped CNTs showed higher catalytic activity and turnover, and the improvement of this activity may be related to the electronic effect of N sites in the support. Rosa Arrigo (Arrigo et al., 2014; Arrigo et al., 2016) and co-workers believed that the N functional groups on the surface of carbon support could form a robust Pd-N bond with Pd, which could not only inhibit the agglomeration of Pd nanoparticles but also provide electrons to Pd, thus improving the stability and activity of Pd catalyst. Ji (Ji et al., 2021) et al. found that an appropriate concentration of N functional groups (2.72 at.%) could maintain the electron-deficient state of Pd, which was conducive to improving H2O2 productivity. N-doped porous carbon was concerned with potential support for the direct synthesis of H2O2 from H2 and O2. However, the current experimental studies cannot well explain the effect of N functional groups on the synthesis of H2O2. To further explore the influence of N functional groups on H2O2 directly synthesizing, the method of theoretical calculation relies on the following: (i) constructing Pd/C heterojunctions with different nitrogen contents; (ii) investigating the effect of N-doping on the formation energy and electron transfer of Pd/C heterojunctions; (iii) investigating the effect of N-doping on O2 and H2O2 adsorption, and OO bond dissociation. Theoretical calculations showed that N-doping could reduce the agglomeration degree of Pd nanoparticles and facilitate electron transfer from carbon support to Pd nanoparticles, which is conducive to the generation of H2O2. However, N-doping reduced the adsorption energy of O2 and H2O2, the active dissociation energy barrier of O2, which is detrimental to the desorption of H2O2 and the inactive dissociation of O2, reducing the selectivity of H2O2. In order to reduce the adverse effects of N-doping, N-doped porous carbons with different pore structures were constructed to accelerate the mass transfer and diffusion performance of Pd/C catalyst. The experimental results achieved the expected goal. Compared to other catalysts, the Pd/NPCs-PSS with a highly developed multistage pore structure showed excellent catalytic performance for the direct synthesis of H2O2 from H2 and O2, the H2O2 productivity and selectivity reached 328.4 molH2O2·kgcat -1·h−1 and 71.9 %, respectively.The periodic density functional theory (DFT) calculations of the present work were performed by the CP2K package (Kuhne et al., 2020) with the spin-polarized gradient corrected functional of Perdew Burke and Ernzerhof (PBE) (Perdew et al., 1996). The Goedecker–Teter–Hutter (GTH) pseudopotentials, DZVP-MOLOPT-GTH basis sets were utilized to describe the core electrons and valence electrons (VandeVondele and Hutter, 2007; Goedecker et al., 1996) (4s24p64d10 for Pd, 2s22p4 for O, 2s22p2 for C, 2s22p3 for N), respectively. 400Ry was set to plane-wave energy cutoff (Lippert and Parrinello, 2010). The electron and force convergence criteria were 1 × 10−6 a.u. and 6 × 10−4 Hartree Bohr, respectively. The transition state of reaction paths was performed using the climbing imagen nudged-elastic-band method (CI-NEB) (Henkelman et al., 2000), including 6 replicas, and the maximum force was 1 × 10-3 atomic units. The transition states had been confirmed by vibrational analysis. The 3 × 3 × 1 Monkhorst–Pack mesh k-points were used for work function calculation. The heterojunction of Pd/graphite consisted of 4 layers of graphite and 2 layers of Pd(111) crystal plane. We use VASPKIT (Wang et al., 2021) software 804 functions to automatically generate heterojunction with optimized p(1 × 1) graphite and Pd(111)-p(1 × 1). The mismatch tolerance of heterojunction is less than 1 %. The interlayer space is set to 2.5 Å. The vacuum space was set to 17 Å along the Z-axis. The final periodic box of Pd/graphite is 9.9 × 9.9 × 39.5 Å. The generated heterojunction model needs to be further optimized before being used. The detailed position information of Pd/graphite heteroatoms is shown in the Supporting Information (Table S5). We replaced carbon atoms with nitrogen atoms in Pd/graphite to build the nitrogen-doped graphite. The adsorption energy (Eads) of O2 over the Pd/graphite (Pd/N-graphite, Pd/2N-graphite) was calculated as below: E a d s = E O 2 / s l a b - E s l a b - E O 2 Where EO2/slab is the total energy of the Pd/graphite (Pd/N-graphite, Pd/2N-graphite) slab with the adsorbed atom(O2); Eslab is the total energy of Pd/graphite (Pd/N-graphite, Pd/2N-graphite) slab, and EO2 is the energy of the O2 in a box of 9.9 Å × 9.9 Å × 39.5 Å.The interface formation energy (EF) of the Pd/graphite (Pd/N-graphite, Pd/2N-graphite) was calculated as below: E F = E t o t a l - E s u p p o r t - E P d ( 111 ) Where Etotal is the total energy of the heterostructure (Pd/graphite, Pd/N-graphite or Pd/2N-graphite). Esupport is the energy of graphite, N-graphite or 2 N-graphite, EPd(111) is the energy of Pd(111).The work functions (Φ) of the graphite, N-graphite, 2 N-graphite and Pd(111) surfaces according to the following equation: Φ = E v a c - E F Where EF is the fermi energy, and Evac is the electrostatic potential of the vacuum level.The activation barriers and reaction energy were calculated as below: Ea = E T S - E I S Δ E = E F S - E I S EIS, EFS and ETS were the energy of the initial state, final state and transition state.We used the VASPKIT code (Wang et al., 2021), Multiwfn, (Lu and Chen, 2012) VESTA (Momma and Izumi, 2011) and VMD (Humphrey et al., 1996) for post-processing of the CP2K calculated data.Palladium chloride (PdCl2, 59–60 %), were purchased from Shanghai Aladdin Biochemical Technology Co., ltd; Sodium polyacrylate (MW = 450–700 W, PAANa), Melamine (C3H6N6, 99 %), potassium bicarbonate (KHCO3, 99.5 %), Sodium polystyrene sulfonate (MW≈70000, PSS), and Diallyldimethylammonium chloride (PDDA, 60 %) were purchased from Shanghai Macklin Biochemical Co., ltd; glucose (C6H12O6, 98 %), sodium borohydride (NaBH4, 98 %), ammonium hydroxide (NH3·H2O, 25–28 %) and hydrochloric acid (HCl, 36.0–38.0 %) were purchased from Sinopharm Chemical Reagent Co., ltd; ethanol (C2H5OH, 99.7 %) and methanol (CH3OH, 99.5 %) were purchased from Tianjin Fuyu Fine Chemical Co., ltd. All reagents were used as received without further purification. Deionized water was used throughout the experiment process.The porous carbon (PCs, NPCs, NPCs-PAANa, and NPCs-PSS; the experimental details for porous carbon are given in the Supporting Information) supported Pd catalysts were synthesized by an absorber-reduction method (Fig. 1 ). In a typical procedure, the porous carbon (100 mg, NPCs-PSS) was added to deionized water (50 ml) on a round flask; after ultrasonication for 30 min, a solution of PDDA (0.5 ml) was added to the mixture and stirred at 35℃ for 24 h. The modified porous carbon was obtained by centrifugation (10000 rpm/min). The modified porous carbon was added to deionized water (50 ml) in a round flask; after ultrasonication for 10 min, a solution of H2PdCl4 (0.5 ml, 0.075 M) was added to the mixture and stirred at 35℃ for 24 h. After impregnation, a fresh sodium borohydride solution (1 ml, 10 % wt) was added to the mixture, and the mixture was stirred at 35℃ for 5 h to reduce Pd entirely. The solid was obtained by centrifugation (10000 rpm/min) and dried in air at 80℃ overnight. The dried solid was placed in a tube furnace and treated in the air at 250℃ for 2 h, followed by H2 reduction at 250℃ for 2.5 h, The sold powder obtained after reduction was named Pd/NPCs-PSS. The experimental details for other catalysts are given in the Supporting Information. Elemental Analyzers, Fourier Transform Infrared Spectrometer (FTIR), Specific surface area and porosity analyzer (ASAP 2460), X-ray diffraction (XRD), Inductively coupled plasma-optical emission spectroscopy (ICP-OES), Raman Spectrometer, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were used to investigate the structure and surface properties of porous carbon and catalyst. The details of the instruments are provided in the Supporting Information.The first-principles calculations based on density functional theory (DFT) were used to investigate the reaction energetics and atomic interactions within the PdNC metal − carbon heterointerface. The calculations were based on the Pd(111) crystal plane. Compared with other crystal planes, the Pd(111) crystal plane is considered the most favorable for the generation of H2O2 (Tian et al., 2013). The carbon supports were simplified as graphite, and the doped N atoms exist in graphitic nitrogen. The Pd/C heterojunctions comprised 4 layers of graphite and 2 layers of Pd(111) crystal plane; furthermore, C atoms on the surface of graphite adjacent to the Pd(111) crystal plane were replaced with N atoms to simulate the PdNC heterojunction. The specific model structures are shown in Fig. S6. The thermodynamic stability of heterojunctions can be described by the interface formation energy (EF). The interface formation energy (EF) of Pd/graphite, Pd/N-graphite, and Pd/2N-graphite were − 0.19 eV, −0.81 eV and − 0.97 eV (Table S3), respectively. The negative formation energy indicated that the Pd/graphite, Pd/N-graphite, and Pd/2N-graphite could form a stable interface; the Pd/N-graphite and Pd/2N-graphite with lower formation energy were conducive to the formation of small Pd nanoparticles, the small Pd nanoparticles were benefited hydrogen peroxide generation (Tian et al., 2017).The work function of the surface is a crucial parameter to studying charge transfer at the heterojunction interface. The work functions of the graphite, N-graphite, 2 N-graphite and Pd(111) surfaces were 4.53 eV, 3.72 eV, 3.68 eV and 5.07 eV (Fig. S7), respectively. The work function of graphite and Pd(111) surfaces agree with the experimental values of 4.6 eV and 5.3 eV (Oshima and Nagashima, 1997), respectively. As shown in Fig. S6, the fermi energy of Pd(111) surface was lower than graphite, N-graphite and 2 N-graphite; when those supports form a heterojunction structure with Pd(111) surface, electrons will flow from supports to Pd(111) surface until there have achieved equilibrium in the fermi energies. The interactions and electronic structure between Pd(111) surface and supports were investigated by calculating differential charge density. Compared with Pd/graphite, the electron density of C atoms on the surface of Pd/N-graphite and Pd/2N-graphite disappears, and the electrons are concentrated around Pd and N atoms (Fig. 2 ). It can be inferred that the graphitic nitrogen in the carbon support promotes the release of electrons from the support to Pd(111) layer, thereby exhibiting an electron transfer effect.Bader charge analysis was performed on the Pd/graphite, Pd/N-graphite and Pd/2N-graphite heterojunction models to quantify charge transfer at the interfaces. Bader change analysis results show that the amount of charge transfer from graphite, N-graphite and 2 N-graphite to the Pd(111) surface were 0.02 e, 0.49 e and 0.48 e, respectively. The plane-average electron difference and the electron displacement curve (Fig. S8) give the same conclusion. The transfer of electrons from N-graphite (2 N-graphite) to Pd(111) was conducive to maintaining a high concentration of Pd0 sites on the surface of Pd(111). Pd0 active site could promote the dissociation of H2 and the activation of O2, which was beneficial to hydrogen peroxide generation. However, excess Pd0 active sites will accelerate the dissociation of the OO bond in O2 and H2O2, resulting in decreased H2O2 selectivity.In the reaction mechanism of the direct synthesis of H2O2 from H2 and O2, the adsorption and activation of molecular oxygen on the surface of Pd were significant steps (Nugraha et al., 2017). The adsorption energy of O2 on the surface of Pd/2N-graphite, Pd/N-graphite and Pd/graphite were − 0.86 eV, −0.83 eV and − 0.82 eV (Table S3), respectively; indicating that the Pd supported on the N-doped carbon surface is beneficial to the adsorption of O2. The OO bond length of O2 after adsorption ranged from 1.38 to 1.39 Å (Table S3) and was higher than in the gas phase. According to the differential charge density figure (Fig. S9), we can indicate that the electrons of the adsorbed O2 molecules are transferred from Pd atoms and OO bond to the oxygen atoms, and weakens the OO bond. The d-Projected density of states (PDOS) of Pd (Fig. S10) were calculated to investigate the influence of N-doped on the d-band center of Pd. It is found that with the increase of nitrogen content, the d-band center of Pd was closer to the Fermi level (Fig. S10); according to the d-band center theory, the closer the d-band center of the metal catalyst to the Fermi level, the higher the metal surface activity and the stronger the adsorption of reactants. As the d-band center of Pd approaches the Fermi level, the adsorption energy of O2 and H2O2 decreases from − 0.82 eV to − 0.86 eV and − 0.38 eV to − 0.40 eV (Fig. 3 a-b), respectively, the lower adsorption energy was unfavorable for the desorption of O2 and H2O2. As shown in Fig. 4 , the activation barriers (Ea) of O2 dissociation on Pd(111) surface decreased from 0.53 eV (Pd/graphite) to 0.49 eV (Pd/N-graphite) and 0.48 eV (Pd/2N-graphite). As mentioned above, N doping could reduce oxygen adsorption and dissociation energy (Ea) on Pd(111) surface, making oxygen more prone to active dissociation. In the reaction mechanism of hydrogen–oxygen synthesis of hydrogen peroxide, the active dissociation of O2 is not conducive to the generation of hydrogen peroxide. The reactive dissociation of O* reacts with H* to produce the by-product H2O, resulting in the loss of intermediates, which should be avoided as much as possible in the direct hydrogen–oxygen synthesis of hydrogen peroxide.The theoretical calculations showed that N doping could reduce the formation energy of Pd/graphite heterojunction; promote the transfer of electrons from carbon supports to Pd particles, which are beneficial to the generation of Pd nanoparticles with a high Pd0/Pd2+ ratio and small particle size. Smaller Pd nanoparticles and a higher proportion of Pd0 have a favorable effect on H2O2 production. However, N doping reduces the adsorption energy of O2 and H2O2, and the dissociation activation barriers of O2*, which is not conducive to improving H2O2 selectivity.In order to suppress the adverse effect of N doping on H2O2 synthesis, a series of Pd/C catalysts were fabricated by adjusting the carbon support pore structure. Firstly, N-doped porous carbon with high specific surface area and hierarchical pore structure were prepared by one-step multiple activation synthesis techniques. The different precursors produced by the hydrothermal treatment of glucose have undergone a one-step multiple activation process, in which potassium bicarbonate was the pore-forming agent, and melamine was the pore-forming agent and source of nitrogen, and simultaneously realized the pore-forming activation and nitrogen doping. Then, the Pd nanoparticles were loaded on porous carbon supports through an absorber-reduction strategy. Finally, the stable Pd/C catalyst was obtained by the further oxidation–reduction heat treatment process.The morphology and size distribution of porous carbon are shown in Fig. 5 . By adjusting the hydrothermal treatment process, the activated hydrothermal carbon has different morphologies and structures. The PCs and NPCs were composed of agglomerated carbon microspheres with an average particle size of 310 nm (Fig. 5a-b, Fig. S1a-b). The NPCs-PAANa prepared by adding trace PAANa dispersants were composed of mono-dispersed porous carbon spheres with an average particle size of 437 nm (Fig. 5c, Fig. S1c) (Gong et al., 2014). The NPCs-PSS prepared by adding trace PSS dispersant were formed by carbon spheres with particle sizes less than 100 nm and a rough surface(Fig. 5d, Fig. S1d) (Gong et al., 2014). All of the activated hydrothermal carbon surfaces have obvious irregular porous structures.The pore structure of porous carbons were analyzed by nitrogen adsorption–desorption isotherm at 77 K (Fig. 6 a-b). The detailed data are displayed in Table 1 . According to the IUPAC classification, the isothermal adsorption–desorption curve of PCs showed type Ⅰ, indicating that it was dominated by microporous (Guo et al., 2020). The isothermal adsorption–desorption curves of NPCs and NPCs-PAANA were type Ⅳ, and the nitrogen adsorption capacity increased sharply at low relative pressure P/P0 less than 0.02 of the N2 sorption isotherms (Fig. 6a), indicating the presence of microporous, the hysteresis loop at relative pressure P/P0 of 0.4–0.8 can be classified as H2 hysteresis, indicating the presence of mesoporous (Rio et al., 2020). Similar to NPCs-PAANa and NPCs, NPCs-PSS also has H2 type hysteresis loop, indicating that NPCs-PSS also has a mesoporous structure. However, when the relative pressure P/P0 of 0.8–1.0, NPCs-PSS still has a surge in adsorption capacity, indicating stacking channels in NPCs-PSS (Gong et al., 2014), which was consistent with the SEM image (Fig. 5d). The specific surface area of porous carbons were calculated by Brunaure-Emmett-Teller (BET) model (Table 1). The specific surface area of PCs, NPCs NPCs-PAANa and NPCs-PSS were 1770 m2/g, 3421 m2/g, 3165 m2/g and 3173 m2/g, respectively. The difference in specific surface area between PCs and NPCs indicates that the volatile gas generated by high-temperature decomposition of melamine will etch hydrothermal carbon aromatic carbon skeleton, forming more pore structures and larger specific surface area (Li et al., 2019; Xu et al., 2020). The pore volumes of porous carbon were calculated by Barrett-Joyner-Halenda (BJH) model. The total pore volumes and the Vmic/Vtotal ratio of PCs, NPCs, NPCs-PAANa and NPCs-PSS were 0.99 cm3/g, 2.16 cm3/g, 2.44 cm3/g and 3.43 cm3/g, 0.79, 0.36, 0.31 and 0.36, respectively. The Vmic/Vtotal ratio of porous carbon indicates that the NPCs, NPCs-PAANA and NPCS-PSS have microporous and mesoporous structures. Fig. 6b shows that all of the porous carbons have micropores and mesoporous structures; micropores dominate the pore structure of PCs. In contrast, NPCs and NPCs-PAANa are dominated by mesoporous with a pore size of 2–5 nm. Besides the similar microporous and mesoporous structures as NPCs and NPCs-PAANa, NPC-PSS has stacked pores with pore diameters of 10–100 nm. The rich mesoporous structure of porous carbon facilitates the mass transfer between catalysts and reactants (Yook et al., 2016).BET analysis showed that the adjusted glucose hydrothermal process combined with one-step multi-activation strategy successfully prepared the high specific surface area and high pore capacity carbon supports with micropore-mesoporous and micropore-mesoporous- macroporous multistage pore structures.The compositions of porous carbons were analyzed by elemental analyzer and XPS. XPS was used to detect the surface composition of the sample. The elemental analyzer was used to determine the contents of C, H, and N in the bulk phase, and the O content was calculated by the difference method. As shown in Table 2 , the nitrogen contents of PCs, NPCs, NPCs-PAANa and NPCs-PSS obtained by elemental analyzer were 0.00 %, 0.85 %, 0.74 % and 0.84 % wt%, respectively, consistent with XPS determination, indicating that nitrogen atoms were successfully doped in the surface of porous carbon (Li et al., 2019). The N content of porous carbon obtained by XPS was consistent with the N-graphite in the theoretical calculation model.The samples were subjected to X-ray diffraction (XRD) to analyze the characteristics of carbon crystal structure. As shown in Fig. S12, The diffraction peak of PCs at 2θ values of 44° and 24° can be assigned as the (100) reflection of graphite and (002) reflection of amorphous graphite, respectively (Cai et al., 2020). The intensity of (002) peaks of the NPCs, NPCs-PAANa and NPCs-PSS were lower than the PCs, indicating that the N-doped damage the crystal structure of graphite and reduces the graphitization degree. The same conclusion can be obtained from the Raman spectra. As shown in Fig. 7 , there are two prominent peaks at ∼ 1343 cm−1 and ∼ 1590 cm−1 for these four kinds of porous carbons, which belong to the D and G bands. The D band comes from the crystallization defect in the carbon material, which corresponds to the sp3 carbon atoms (Wei et al., 2018; Deng et al., 2015); the G band originates from the in-plane vibrations of the SP2 carbon atoms (Wei et al., 2018; Deng et al., 2015). The intensity ratio of the D band to the G band (ID/IG) was used to compare the degree of defects and graphitization in carbon materials (Wei et al., 2018; Deng et al., 2015). The ID/IG of NPCs, NPCs-PAANa and NPCs-PSS were higher than PCs, suggesting a more defective structure with lower graphitization. N-doped seems to promote the formation of defects in the carbon lattice and reduces the degree of graphitization. The carbon lattice defect sites induced by nitrogen-doped were active due to a reduction in symmetry at or near the crystalline edges (Huang et al., 2021; Zhu et al., 2013).XPS was used to investigate porous carbon's chemical states of C, N and O elements. According to the C 1 s spectrum of the porous carbons (Fig. S2), three different functional groups such as CC/CC (284.8 eV), CO (286.3 eV) and O-CO (288.7 eV) were observed. A satellite peak corresponding to the π-π* transition was detected near 290.9 eV, indicating the existence of the graphene-like microstructure on the porous carbon surface (Huang et al., 2020; Shi et al., 2017; Nguyen et al., 2020). The O 1 s spectrum of the porous carbons (Fig. S3) had three different functional groups such as CO (531.7 eV), OCO (533.1 eV) and O-CO (534.4 eV) (Wu et al., 2021; Cai et al., 2020). The N 1 s spectrum can be divided into four functional groups with different binding energies (Fig. 8 ); the peak near the binding energy of 398.0 eV, 400.1 eV, 401.4 eV and 403.6 eV are classified into Pyridinic N, Pyrrolic N, Graphitic N and Oxidized N (Arrigo et al., 2016; Wu et al., 2021; Mao et al., 2020; Mao et al., 2019; Jia et al., 2020; Zhang et al., 2020), respectively. According to Table S2, the Graphitic N was the dominant N functional group of NPCs, NPCs-PAANA and NPCS-PSS, and the contents were 40.20 %, 52.04 % and 54.43 %, respectively, which was consistent with the theoretical calculation model.The Fourier transform infrared (FTIR) spectroscopy show that PCs, NPCs, NPCs-PAANa and NPCs-PSS have similar FT-IR spectra (Fig. S13). The broad peaks around 3000 ∼ 3600 cm−1 could be assigned to the intermolecular hydrogen bonds of carbon and aromatic OH asymmetry stretching vibration. A small peak around 2926 cm−1 could be assigned to the stretching vibrations of –CH3 and –CH2 groups in aliphatic or cycloalkanes (Sheng et al., 2019). A weaker peak at 1387 cm−1 belongs to the stretching vibration of the –CH3 group (Cai et al., 2020; Cuong et al., 2020). The characteristic peaks at 1580 cm−1 and 1178 cm−1 belonged to vibrational stretching of CC and CO in aromatic rings (Chang et al., 2020). No prominent nitrogen-containing groups were found in the FT-IR spectra, which may be related to the low nitrogen content (Gong et al., 2014).As shown in Fig. 6c-d, Pd/C catalysts' pore structure, size distribution, specific surface area, and pore volume are consistent with their carbon supports (Table 2), indicating that the Pd/C catalyst prepared by the absorber-reduction method could inherit the structural properties of the porous carbon support. The specific surface area and pore volume of Pd/PCs, Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS catalysts were 1517 m2g−1, 2915 m2g−1, 2742 m2g−1 and 2839 m2g−1, and 0.85 cm3g−1, 1.81 cm3g−1and 3.06cm3g−1, respectively. The higher specific surface area and pore volume of Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS catalysts are beneficial for exposing a high density of active sites and ensuring fast mass transfer. Fig. 9 displays the XRD patterns of Pd/PCs, Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS in the range of 10-90° (2θ). The Pd/PCs have five different peaks; the peaks at 39.92°, 46.43°, 67.76°, 81.64° and 86.12° can be indexed to the (111), (200), (220), (311) and (222) planes of face-centered cubic (fcc) Pd (JCPDS# 87–0643) (Li et al., 2018), respectively. Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS have only observed the diffraction peak of the Pd (111) plane and are consistent with the theoretical calculation model. Compared with the Pd (111) peak in Pd/PCs, the intense peak around 40° in Pd/NPCs downshifts by 0.3°, and the full-width half-maximum (FWHM) increases by 0.9° (Fig. S14). The same situation also exists in Pd/NPCs-PAANa and Pd/NPCs-PSS, indicating Pd/NPCs, Pd/ NPCS-PAANa and Pd/ NPCS-PSS may have slight lattice stretching. The average size of Pd nanoparticles of Pd/C catalysts were calculated by the Scherrer formula. The calculated Pd nanoparticles size of Pd/PCs, Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS were 14.2 nm, 5.1 nm, 5.5 nm and 3.3 nm, respectively. Fig. 10 a-d presents TEM images of Pd/PCs, Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS, and it can be observed that Pd nanoparticles are evenly dispersed on the surface of porous carbon. The average particle sizes of Pd nanoparticles on Pd/PCs, Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS were 14.9 nm, 4.6 nm, 5.2 nm and 4.2 nm, respectively, agree well with the XRD results. It is worth noting that the average particle size of Pd nanoparticles was strongly correlated with the concentration of N functional groups of the porous carbon (Fig. S5). The average particle size of Pd nanoparticles in nitrogen-containing porous carbons (NPCs, NPCs-PAANa and NPCs-PSS) was smaller than N-free porous carbons (PCs) and negatively correlated with the nitrogen concentration, which is consistent with the theoretical calculation results. Theoretical calculation shows that the formation energy of Pd/C heterojunction was significantly reduced with nitrogen doping and further reduced with increased nitrogen content. The lower formation energy is conducive to anchoring Pd nanoparticles on the surface of the porous carbon support, inhibiting the agglomeration of Pd nanoparticles during redox heat treatment, and facilitating the formation of small particle-size Pd nanoparticles. The smaller Pd particle size could increase hydrogen peroxide production (Tian et al., 2017). The lattice spacing of Pd(111) in Pd/NPCs (0.231 nm), Pd/NPCs-PAANa (0.233 nm) and Pd/NPCs-PSS (0.231 nm) were slightly larger than that of Pd/PCs (0.230 nm) (Fig. 10e-h), agree well with the XRD results. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping Fig. 10i-k showed that the C, N and O atoms were uniformly distributed on the NPCs-PSS support. It is beneficial for the dispersion of the Pd nanoparticles (Fig. 10 l).The electronic state of Pd and N atoms was further researched by Pd 3d and N 1 s X-ray photoelectron spectroscopy (XPS). Fig. 11 shows that the complex Pd 3d was deconvoluted into four different peaks with the binding energies of Pd0 (∼341 eV, ∼335 eV) and Pd2+ (∼343 eV, ∼337 eV) (Liang et al., 2020; Thuy Vu et al., 607 (2020).). These results indicated that the Pd2+ exited in all catalysts, even though the catalysts were heat treatment in the hydrogen atmosphere. It was worth noting that the binding energies of Pd0 (Table S2) decreased in the order of Pd/PCs (335.83 eV) > Pd/NPCs (335.80 eV) = Pd/NPCs-PAANa (335.80 eV) > Pd/NPCs-PSS(335.72 eV), suggesting that N doping can facilitate electron flow from the porous carbon support to the supported Pd nanoparticles (Li et al., 2019), theoretical calculations can infer the same result. Benefiting from electron transfer, the Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS have a higher Pd0/Pd2+ ratio than the Pd/PCs (Table S2). The nitrogen species of catalysts were the same as their supports, but the content was reduced (Table S1), possibly because Pd nanoparticles cover part of the N sites.The above evidence confirms that nitrogen doping can effectively inhibit the agglomeration of Pd nanoparticles and promote the Pd nanoparticles to obtain electrons from the N-dope porous carbon support, which is consistent with the theoretical calculation results. Fig. 12 shows the change of catalytic performance with the time of different Pd/C catalysts for the direct synthesis of hydrogen peroxide. During the reaction period (3.0 h), the concentration of hydrogen peroxide in the solution increased (Fig. 12a), indicating that all the catalysts maintained high activity throughout the reaction. Compared with the Pd/PCs, the Pd/NPCs supported by N-doped porous carbon have higher hydrogen peroxide productivity (Fig. 12b) and hydrogen conversion rate (Fig. 12c) but lower hydrogen peroxide selectivity (Fig. 12d); the experimental result agrees well with the conclusion of theoretical calculations. According to Fig. 12, there are two points worth noting: (i) at the beginning of the reaction (0.5 h), the hydrogen conversion rate was positively correlated with the Pd0/Pd2+ ratio (Table S2), and the Pd/NPCs-PAANa had the highest hydrogen conversion rate. An hour later, the hydrogen conversion rate of Pd/NPCs-PSS exceeded the Pd/NPCs-PAANa (Fig. 12c); (ii) the catalysts (Pd/NPCs, Pd/NPCs-PAANa, and Pd/NPCs-PSS) showed observed differences in H2O2 selectivity. The Pd0/Pd2+ ratio of Pd/NPCs-PSS fall in between Pd/NPCs and Pd/NPCs-PAANa; however, the Pd/NPCs-PSS had the highest hydrogen peroxide selectivity and productivity (Fig. 12d);The size of Pd nanoparticles is one of the factors affecting the direct synthesis of hydrogen peroxide by hydrogen and oxygen. Han's (Tian et al., 2017) research showed that H2O2 selectivity is strongly correlated with particle size when the particle size of Pd nanoparticles is 1.4–2.5 nm, and the effect of particle size on H2O2 selectivity is slight when the particle size is 2.5–30 nm. According to the Fig. 10, the average particle sizes of Pd nanoparticles on Pd/PCs, Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS were 14.9 nm. 4.6 nm, 5.2 nm and 4.2 nm, respectively, especially for Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS with average particle size between 4.2 nm and 5.2 nm. Therefore, the effect of particle size on the catalytic properties of Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS can be ignored in this study. The Pd0/Pd2+ ratio of Pd nanoparticles is another important factor affecting the direct synthesis of hydrogen peroxide (Edwards et al., 2012). Pd0 is the main active site of H2 dissociation, Pd2+ could inhibit the dissociation of H2O2 and improve the H2O2 selectivity. The Pd0/Pd2+ ratios of Pd/PCs, Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS were 1.53, 2.07, 3.33 and 2.35 (Table S2), respectively. A higher Pd0/Pd2+ ratio indicated a higher H2 conversion and a lower H2O2 selectivity. At the initial stage (0.5 h), Pd/NPCs-PAANa showed the highest H2 conversion, while H2O2 selectively decreased in the order of Pd/PCs > Pd/NPCs > Pd/NPCs - PAANa. Although the difference of Pd0/Pd2+ ratio could partly explain point (i) and point (ii), it does not explain the abnormality at point (i) and point (ii). We believe that there are other factors affecting the catalytic performance of hydrogen peroxide. Earlier research suggested that the mesoporous structure of porous carbon could alter the H2O2 selectivity by affecting the mass transfer within the catalyst (Yook et al., 2016; Park et al., 2014; Fellinger et al., 2012). Compared with Pd/PCs, the Pd0 content of Pd/NPCs was increased 7 %, and the H2 conversion was increased 17 %. Compared with Pd/NPCs, the Pd0 content of Pd/NPCs-PAANa was increased 9.5 %, and the H2 conversion only increased 3.1 %. These results imply that compared with the microporous structure, the mesoporous structure is more conducive to the diffusion of H2 in the catalyst and improved H2 conversion, which is consistent with Choi (Yook et al., 2016). After an hour of reaction, the H2 conversion of Pd/NPCs, Pd/NPCs-PAANa and Pd/NPCs-PSS were 63.0 %, 66.1 % and 66.4 %, respectively, and with a decrease of 7.9 %, 7.9 % and 5.8 %, respectively. The slow decline rate of Pd/NPCs-PSS may be related to the particular pore structure. Compared with Pd/NPCs-PAANa, the larger average pore size and the higher mesoporous ratio of Pd/NPCs-PSS are more conducive to H2 mass transfer in the catalyst and improve H2 conversion.According to the mechanism of direct synthesis of hydrogen peroxide (Han et al., 2021), OO bond dissociation is the key that affects the selectivity of H2O2. A lower Pd0 content is beneficial for improving the selectivity of hydrogen peroxide. At the initial stage (0.5 h), H2O2 selectivity was negatively correlated with Pd0 content, except Pd/NPCs-PSS. The H2O2 selectivity of Pd/NPCs-PSS was up to 78.8 %, indicating that other factors may affect the H2O2 selectivity. The solution does not contain hydrogen peroxide at the beginning of the reaction, the dissociation of O2 has an important effect on the hydrogen peroxide selectivity. Xu and co-worker studied (Tian et al., 2020) that the selectivity of hydrogen peroxide is facilitated when O co-adsorbed on the surface of Pd. The Pd/PCs, Pd/NPCs, and Pd/NPCs-PAANa have similar pore structures and average pore diameters, resulting in no significant difference in the diffusion of O2 within these catalysts. The Pd/NPCs-PSS has a special macropore-mesoporous structure and average pore diameters up to 4.32 nm, which could promote the mass transfer of O2 in the catalyst. The good mass transfer of O2 leads to the faster co-adsorption equilibrium of O on the Pd surface, which improves hydrogen peroxide selectivity. As the reaction progress, the concentration of hydrogen peroxide in the solution gradually increases (Fig. 12a), and the dissociation of hydrogen peroxide becomes an essential factor affecting the catalytic performance. Kakimoto's research (Park et al., 2014) showed that the porous carbon's mesoporous structure could improve the catalyst layer's mass transfer and reduce the contact time of H2O2 inside the catalyst, thereby reducing the decomposition of H2O2. In addition to a large number of 2–5 nm mesopores, the Pd/NPCs-PSS also had a large number of 10–100 nm mesopores-macropores. The mesopores-macropores structure enhanced the mass transfer of H2O2, reduced the residence time of H2O2 in the catalyst, and improved the selectivity of H2O2.In addition to the above, two other points are worth noting here, (i). the H2O2 selectivity of Pd/NPCs-PAANa exceeded the Pd/NPCs after reaction an hour, and the difference tended to expand (Fig. 12d); (ii) the difference in H2O2 selectivity between Pd/NPCs-PAANa and Pd/NPCs-PSS gradually decreased as the reaction progressed (Fig. 12d). After fully investigating the effect of Pd nanoparticle size, the surface electronic state of Pd and the pore structure of the catalyst support, these two points seem to imply that the dispersion of the catalyst support also has the potential to affect the selectivity of hydrogen peroxide, which worth to research in the future. Table S4 compares the catalytic performance of Pd/NPCs-PSS catalysts with other carbon-supported Pd-based catalysts. At ambient pressure, the hydrogen peroxide productivity and selectivity of Pd/NPCs-PSS were up to 328.4 molH2O2 kgcat -1h−1 and 71.9 %., respectively. The excellent catalytic performance of Pd/NPCs-PSS indicates that N-doped porous carbon with a macropore-mesoporous-microporous structure is an excellent material for directly synthesizing hydrogen peroxide from hydrogen and oxygen.A series of experiments were performed to study the influence of Pd/C catalysts on the hydrogenation and decomposition of hydrogen peroxide. According to Fig. 13 , the hydrogenation rates of Pd/C catalysts are higher than the decomposition rates, which indicates that hydrogenation was the primary side reaction (Thuy Vu et al., 607 (2020).; Tian et al., 2020). It is worth noting that Pd/NPCs-PSS had the lowest dissociation and hydrogenation rates than Pd/NPCs and Pd/NPCs-PAANa; although Pd/NPCs-PSS has the smallest Pd particle size (Fig. 10d), the difference in metal particle size distribution too small to fully explain the substantial difference in catalytic results. Therefore, we believe that the pore structure of the catalyst support (Fig. 6 b) also influences hydrogen peroxide's hydrogenation and dissociation performance. The unique macropore-mesoporous-micropore structure of the Pd/NPCs-PSS catalyst reduced hydrogen peroxide's dissociation and hydrogenation rates by increasing the diffusion rates of hydrogen and hydrogen peroxide in the catalyst.Reusability is an important property of the catalyst. In-cycle tests were performed on Pd/NPCs-PSS to examine the stability and reusability of the catalyst. After being reused two times, the H2 conversion, H2O2 selectivity and productivity decreased (Fig. S15). ICP, XRD and XPS were used to analyze the Pd invasion, geometry and electronic morphology of Pd/NPCs-PSS after rescue. The ICP analysis of Pd/NPCs-PSS before and after circulation implied that the leaching of Pd during the catalyst recycling could be negligible (Table S2). the XPS analysis of Pd/NPCs-PSS after circulation implied that the catalyst recycling could reduce the Pd0 content (Fig. S16). The XRD analysis of Pd/NPCs-PSS shows that the catalyst recycling changed Pd's morphology and particle size. The crystal plane, unsuitable for H2O2 production, can be observed in the XRD pattern (Fig. S17), such as the (100) crystal plane. The average particle size of Pd calculated by the Scherrer formula was 16.2 nm implying that Pd agglomerated in catalyst cycling.In summary, guided by theoretical calculations, we prepared a series of Pd/C catalysts comprising highly dispersed Pd nanoparticles deposited onto hierarchically porous nitrogen-doped carbon material to efficiently synthesize hydrogen peroxide from hydrogen and oxygen. DFT results indicated that the N doping reduces the formation energy of Pd/C heterojunction, promoting the transfer of electrons from carbon support to Pd nanoparticles, forming Pd nanoparticles with small particle size and high Pd0/Pd2+ ratio, and beneficial to improving the hydrogen peroxide productivity. However, N doping shifts the d-band center of Pd toward the Fermi level, and lowers the active dissociation energy barrier of O2, which reduces hydrogen peroxide selectivity. The experimental results showed that adjusting the pore structure of the N-doped porous carbon supports could reduce the negative effect of N doping for H2O2 selectivity. Compared with other catalysts, the special pore structure of Pd/NPCs-PSS catalyst improved the mass transfer rate of H2, O2 and H2O2 in the catalyst, which was the key to inhibiting the negative effects of N doping. At ambient pressure, the hydrogen peroxide productivity and selectivity of Pd/NPCs-PSS were up to 328.4 molH2O2·kgcat -1·h−1 and 71.9 %, respectively. This study provides a possible solution to design high-performance Pd/C catalysts to directly synthesize hydrogen peroxide from hydrogen and oxygen at atmospheric pressure.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Thanks for the computing support of the State Key Laboratory of Public Big Data, Guizhou University. This work was financially supported by National Natural Science Foundation of China, China (Grant No.22068009), Natural Science Basic Research Program of Guizhou Province, China (Grant No.ZK[2022]088), Cultivation Project of Guizhou University, China (Grant No. [2020]38). the Open Project of Guizhou University Laboratory and Equipment Departments, China (No.SYSKF2022-045) and College Students innovation and entrepreneurship training program of Guizhou Institute of Technology, China (No.S202014440090)Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2022.104452.The following are the Supplementary data to this article: Supplementary data 1

Nitrogen-doped porous carbon is potential support for directly synthesizing H2O2 from H2 and O2. Here, density functional theory (DFT) was used to study the effect of N-doped porous carbon on H2O2 directly synthesized. The theoretical calculation results showed that N-doped improved H2O2 productivity and H2 conversion by increasing the dispersion of Pd nanoparticles and the Pd0/Pd2+ ratio. However, N-doped decreased H2O2 selectivity by reducing oxygen's dissociation energies. The experimental results showed that adjusting the pore structure of N-doped porous carbon could improve the adverse effects of N-doping for H2O2 selectivity. The H2O2 productivity and selectivity of Pd/C catalyst with a macropore-mesoporous-microporous hierarchical porous structure were up to 328.4 molH2O2·kgcat -1·h−1 and 71.9 %, respectively, at ambient pressure. The macropore structure enhances the transfer and diffusion performance of the catalyst and effectively inhibits the effect of N-doping on OO bond dissociation, which improves H2O2 productivity and selectivity. This research provides a possible solution for designing a high-performance Pd/C catalyst to directly synthesize H2O2 from H2 and O2 at ambient pressure.

A major challenge of our century is the substitution of fossil energy carriers by renewable resources. For chemical industry, the independence from crude oil and coal as carbon source is highly desirable. An alternative, ubiquitous, nontoxic, and sustainable carbon source is CO2, but the stable nature of the molecule makes catalytic activation essential [1]. Processes which enable activation and chemical conversion of CO2 require high energy input, desirably provided by renewable sources (e.g. wind, solar, geothermal, etc.). In conjunction with renewable energy, CO2 has the potential to generate a closed carbon cycle, mitigating CO2 emissions and the related issue of global warming [2]. Large-scale industrial processes with the currently greatest economic potential for CO2 utilization are the production of hydrocarbon fuels, methanol, or – for fine chemical production – urea and salicylic acid [3].Reverse water gas shift reaction (rWGS) is among the most promising technologies to convert CO2 into synthetic fuels or CO as their precursor (other examples include direct hydrogenation of CO2 and methane dry reforming – MDR) [4]. For MDR, the high (400 °C–600 °C) operating temperatures and the associated problems of catalyst sintering and coke formation are major drawbacks [5]. Additionally, catalysts are reported to be very sensitive to sulphur impurities, which cause catalyst deactivation [6]. While direct hydrogenation of CO2 is seen as very promising for methanol synthesis on an industrial scale, as it is thermodynamically more favourable than rWGS [7], the indirect route via rWGS and CO is reported to give a 20 % higher methanol yield compared to direct hydrogenation [4]. Furthermore, it has been suggested that rWGS plays a major role in selective methanation of CO2, and it occurs in Fischer-Tropsch reactors operated with high CO2 feeds [8]. In summary, rWGS is a very promising reaction for activation and utilization of CO2, which is the motivation for the present study.The rWGS reaction is an equilibrium-limited reaction (Eq. (1)) and due to its endothermic nature CO formation is favoured at high temperatures. At lower temperatures, the equilibrium favours the water-gas shift reaction (i.e. reverse of Eq. (1)). Moreover, at lower temperatures methanation is a well-known side reaction [4]. (1) H 2 + C O 2   ⇌   C O + H 2 O   Δ r H ° 298 = 42.1   k J   m o l - 1 In recent years, a lot of work was dedicated to the improvement of rWGS catalysts or the design of novel materials. The most widely studied materials are supported catalysts based on copper, platinum, or rhodium [9,10]. Copper has the advantage of lower operating temperatures and suppression of methanation. A comparison of supported platinum catalysts and supported iron and copper reveals improved CO2 conversion rates; however, the selectivity with respect to CO is diminished. [4]. Furthermore, nickel and cobalt based as well as bimetallic catalysts [11,12] were studied. Notably, there is the need for cheap, abundant catalyst materials, as the high cost of noble metals constitutes the major constraint for their large scale application [13]. For instance, Wang et al. showed that the combination of oxygen vacancies and finely dispersed Ni was the reason for the high catalytic activity of their Ni/CeO2 catalyst [11]. Pastor-Pérez et al. demonstrated high CO2 conversion levels exclusively to CO under various reaction parameters for doped FeCu catalysts [14]. Also, perovskite based rWGS catalysts were tested for their performance as demonstrated by Kim et al. for barium zirconate-based materials [15] or by Daza et al. for cobalt-based perovskites [16]. Additionally, various promotion elements were investigated, e.g. the enhancement of the catalytic activity with potassium, as studied by Chwen et al. [17]Among the wide variety of catalysts reviewed in literature, iron-based catalysts have shown the greatest potential, due to their thermal stability and high oxygen mobility [18], while remaining a feasible option in terms of production costs. Ko et al. performed DFT calculations on CO2 dissociation, finding preferred CO2 to CO dissociation on Fe-containing bimetallic particles [19], thus, highlighting the potential of iron-based catalysts even further. A further advantage of application of reducible oxides in rWGS is that their enhanced oxygen mobility prevents coking [20].Perovskite type oxides with the general formula ABO3 consist of two cations of different sizes (A is larger than B). The wide range of possible structures (many A and B cations can be combined) in conjunction with the option to introduce catalytically active or promoting elements via doping facilitate systematic catalyst development [21]. Moreover, many different types of perovskites have been the subject of extensive research and their properties have been studied thoroughly, particularly because of their wide-spread application in many areas such as solid-state electrochemistry, fuel cell technology, and catalysis [22–24]. Among the desired properties of perovskites are their excellent thermal stability (especially important in solid oxide fuel cells due to the high operation temperatures between 600 °C and 900 °C) and their resilience towards catalyst poisons at higher temperatures, including the possibility of catalyst regeneration via redox cycling.A further outstanding ability of some perovskites is nanoparticle exsolution [25]. Upon reductive treatment (e.g. in H2) or under reaction conditions (in sufficiently reducing reaction environments) the perovskite is partially reduced. Consequently, reducible lattice cations move to the surface, exsolve there, and form metallic nanoparticles. Catalytically highly active and easily reducible dopants are exsolved preferentially [21]. This process enables in-situ growth of active catalysts [23] and, in comparison to traditional deposition techniques, more finely and more highly dispersed catalyst nanoparticles are formed [25,26]. Moreover, this process is more economical (both with respect to time and cost), as no expensive precursors or complicated ‘deposition’ procedures are needed [27]. In addition, different studies have found that nanoparticles emerged via exsolution show enhanced sintering stability due to “anchoring”, even at high reaction temperatures [25,28,29]. Hence, perovskites have the potential to resolve a major problem of many rWGS catalysts, i.e. rapid deactivation caused by severe aggregation of metal particles at high temperatures [13]. In fact, perosvkite catalysts are promising candidates to reduce similar problems in many CO2 utilization reactions [30,31]. Moreover, Tsounis et al. could show that tailoring perovskites enables selectivity tuning and can also suppress competing side reactions [32].Intensive research on nanoparticle exsolution and its mechanisms has been done already by the solid-state electrochemistry community [25,29,33]. Whether monometallic or bimetallic nanoparticles are formed during dopant and lattice cation reduction mainly depends on two factors: temperature and reductive power of the gas environment [34,35]. These properties open the possibility of catalyst engineering and tailoring materials for respective reactions.The advantages outlined above were the motivation to synthesize not-yet intensively studied perovskite-based rWGS catalyst materials and to study their catalytic performance in the temperature range of 300 °C–700 °C and with different doping compositions including in situ XRD. The perovskite catalysts studied were La0.9Ca0.1FeO3- δ , La0.6Ca0.4FeO3- δ , Nd0.9Ca0.1FeO3- δ , Nd0.6Ca0.4FeO3- δ , Nd0.6Ca0.4Fe0.9Ni0.1O3- δ , and Nd0.6Ca0.4Fe0.9Co0.1O3- δ . The listed materials were selected due to current research on highly active rWGS materials and to demonstrate a possible design approach for finely tuned catalysts.As described in previous work [21], the Pechini synthesis was used to prepare the samples. The following starting materials were mixed in the appropriate stoichiometric ratios: La(CH3COO)3·1.5H2O (99.9 %, Alfa Aesar), Nd2O3 (99.9 %, Strategic Elements), CaCO3 (99.95 %, Sigma-Aldrich), Fe (99.5 %, Sigma-Aldrich), Co(NO3)3·6H2O (99.999 %, Sigma-Aldrich), and Ni(NO3)3·6H2O (98 %, Alfa Aesar). Solutions (either in H2O or in HNO3 (65 %, Merck) – both doubly distilled) of the needed amounts were produced and mixed. A 20 % excess of citric acid (99.9998 % trace metal pure, Fluka) was added to the resulting mixtures to trigger complex formation. The solvents were evaporated off, self-ignition of the remaining gel was induced by heating, and the formed powders were calcined at 800 °C for 3 h. The Ni doped perovskite Nd0.6Ca0.4Fe0.9Ni0.1O3- δ was additionally calcined a second time at 1000 °C, in order to try to achieve phase purity. The calcined products were homogenised via grinding and the powder samples were used for characterization [using Brunauer-Emmet-Teller (BET) analysis, scanning electron microscopy (SEM), and in-situ X-ray diffraction (XRD)] as well as catalytic experiments. Purchased La0.6Sr0.4FeO3- δ (LSF, Sigma Aldrich) was catalytically tested with the same setup as a benchmark material. A Fe2O3/Cr2O3 WGS catalyst (HiFUEL™ W210, ThermoFischer Scientific) served as further reference. This commercially available catalyst can also serve as a tentative indicator of economic profitability: using the above-mentioned synthesis route and starting materials, production of the presented materials would cost about three times as much as the industrial catalyst. However, this is a very crude comparison, as no optimization or customization for industrial applications were done for the presented materials.A PANalytical X'Pert Pro diffractometer in Bragg–Brentano geometry (with separated Cu Kα1,2 radiation) and an X’Celerator linear detector was used to perform XRD measurements. The in-situ experiments were carried out in an XRK 900 chamber (Anton Paar), providing a gas flow environment at ambient pressure. After a 30 min oxidative pre-treatment at 600 °C, the samples were cooled to room temperature and the reaction atmosphere of H2 and CO2 in a 1:1 ratio, using Ar as balancing gas, was switched on. Flows of 20 mL min−1 for each of the reaction gases and 50 mL min−1 of Ar were used for the experiments with Nd0.6Ca0.4FeO3- δ , while the respective flows were 7.5 mL min−1 and 15 mL min−1 for the experiments with Nd0.6Ca0.4Fe0.9Co0.1O3- δ and LSF. The reaction was conducted at various temperatures and at each temperature step an in-situ XRD measurement was taken. To ensure equilibrium, each temperature was held for a period of 10 min prior to recording of the XRD pattern (∼30 min per measurement), resulting in holding every temperature for about 40 min. Data analysis and reflex assignment were performed with the HighScore Plus software (PANalytical) and the PDF-4 + 2019 database (ICDD - International Centre for Diffraction Data) [36].SEM images were recorded with secondary electrons on a Quanta 250 FEGSEM (FEI Company) microscope with an Octane Elite X-ray detector (EDAX Inc). An acceleration voltage of 5 kV was used for satisfactory surface-sensitivity.To assess the performance of the investigated samples, trial rWGS reactions were conducted in a tubular flow reactor at ambient pressure (the setup was already described in [37]). Continuous sampling of the gas atmosphere was carried out online (with a measurement every 2–3 min) using a Micro–Gas Chromatograph (Micro-GC, Fusion 3000A, Inficon). A carrier gas flow of 6 mL min−1 (Ar) was used, while for both reactive gases (CO2 and H2) a flow of 3 mL min-1 each was set, leading to an overall flow of 12 mL min−1 (the gases were purchased from Messer Group GmbH). To assess the effect of the reactor on the catalytic activity, a blank test without catalyst was conducted. The found value – which was ∼0.5 Mol% for CO at 600 °C – was subtracted from all respective measurements for baseline correction of the measured conversion. The respective amounts of catalyst powder (20−75 mg) were chosen such, that the CO generation remained below the thermodynamic limit – which was tested by repeat measurements with reduced amounts. The chosen flow and catalyst masses resulted in Weight Hourly Space Velocities (WHSV) around 30 L g-1 h-1 (the exact values are given in Table 1 in Section 3.2). To make sure all test reactions start from the same state (fully oxidized perovskite), the samples were oxidized at 600 °C for 30 min in an oxygen atmosphere of 1 bar and a flow of 10 mL min-1 O2.Comparisons of catalytic performance reported in literature tend to be not straightforward: aside from a meaningful indicator regarding the performance, reaction conditions (temperature, pressure, composition of the reaction environment…) need to be given as well. Ref. [13], for instance, offers a nice overview of different catalysts used for rWGS for their given operating conditions. A common measure used by the catalytic community when comparing performances of catalysts are turn over frequencies (TOFs). However, in-depth knowledge about active sites (both nature and number) is necessary to feasibly obtain TOF values. This approach is hindered by the fact that perovskites are highly flexible (i.e. reaction parameter dependent) materials: For instance, the concentration of oxygen vacancies varies strongly depending on temperature. Moreover, the type of surface in contact with the reaction environment might vary (depending on dopants and termination). Also, possible (metal) nanoparticle exsolution influences the number of active sites, and consequently the activity, as well. Therefore, giving a TOF value is not straightforwardly possible. Instead, specific activities (activity per surface area) in mol m−2 s-1 were determined.In order to calculate this specific activity, the specific surface areas a S (in m2 g−1) of the materials (see Table 1 in Section 3.2.) were measured according to the BET method. Relevant isotherms of the degassed samples (4 h at 300 °C under vacuum) were obtained at −196 °C for fitting with a Micrometrics ASAP 2020 system. A specific activity r C O in mol m-2 s-1 [measuring how many moles of product (CO) were formed per m² surface per s] was then derived according to Eq. (2), where the CO formation (mole fraction x C O in the product stream) and the total gas flow n ˙ in mol s-1 were normalized to the catalyst surface area a C A T . The total area a C A T was obtained from the above-mentioned specific activity a s and the used mass of catalyst m C A T . (2) r C O = n ˙ ⋅ x C O a C A T = n ˙ ⋅ x C O a S ⋅ m C A T Six different perovskite powders were manufactured for the current study. The investigated materials were chosen based on the following considerations: (i) Ferrite type perovskites have been selected as starting point, since Fe has been proven to be catalytically active in rWGS reactions [18,38], and, therefore, provides an already active host lattice (ii) Two of the investigated materials were B-site doped with 10 % Co or Ni, respectively. This means that in addition to an already catalytically active host lattice (see (i) above), further enhancements by reducible and catalytically active B-site dopants can be expected [4,39]. The reason for this expected enhancing effect of doping lies in the capability of the dopants of choice to – under the proper conditions (reductive atmosphere, high reaction temperatures) – diffuse to the surface and, by exsolution, form nanoparticles (as could be shown in previous studies [21,40]). Ni and Co are often used in combination with CeO2 and/or Al2O3 as support materials for rWGS catalysts as e.g. shown by work of Wang et al. [39]. (iii) Nd and La were selected as A-cations, as both (as well as most rare earth elements) reportedly positively impact CO2 utilization reactions in general [41]. Usually, the latter is a very wide-spread A-site element, however, additional materials with Nd instead of La were studied as well. This was done to evade potential problems arising from coinciding peaks of lattice elements and dopants (especially Ni) in future X-ray photoelectron spectroscopy (XPS) investigations. (iv) Ca was used as A-site dopant in order to enhance electron and oxide ion conductivities (acceptor doping increases defect concentrations of electron holes and oxygen vacancies). A-site doping allows additional fine-tuning of the catalyst properties: both stability of the crystal as well as exsolution features can be affected [42]. For the doped perovskites here, 10 % and 40 % of Ca doping were chosen. Previous results [43] revealed effects of A-site dopant concentration on the electronic structure (more Ca leads to more partially oxidized Fe4+) and an increase of perovskite stability, thus leading to higher exsolution onset temperatures. For the B-site doped materials enhanced lattice stability was found as well, leading to the assumption that mainly dopants exsolve (given proper reducing conditions), while Fe ions remain in the lattice [21].In short, the perovskite materials La0.9Ca0.1FeO3- δ , La0.6Ca0.4FeO3- δ , Nd0.9Ca0.1FeO3- δ , Nd0.6Ca0.4FeO3- δ , Nd0.6Ca0.4Fe0.9Ni0.1O3- δ , and Nd0.6Ca0.4Fe0.9Co0.1O3- δ , were selected for their content of catalytically active constituents, stability (both thermal and chemical), and exsolution capabilities. For additional comparison, commercial La0.6Sr0.4FeO3- δ (LSF, Sigma Aldrich) was used as reference. Characterization of the produced powders (structure and exsolution capabilities) was performed with XRD and SEM (details have been published in [43]). The XRD measurements revealed that all materials could be prepared successfully and that for all perovskites but the Ni-doped only the perovskite phase was present. They have similar distorted perovskite structures with an orthorhombic lattice, consistent with reports for La(1−x)CaxFeO3 perovskites [44]. In case of Nd0.6Ca0.4Fe0.9Ni0.1O3- δ , minimal amounts of NiO were found (cf. Fig. S2), meaning that Ni was not fully integrated. No additional phases were found for the Co-doped material, indicating complete incorporation. The commercial LSF is, due to the larger Sr cations compared to Ca, differently distorted with a rhombohedral lattice.To obtain the surface area of all synthesized catalyst materials, BET analyses carried out on the freshly prepared powders yielded surface areas ranging from 1.13 m² g−1 to 5.07 m² g−1 (see Table 1 in Section 3.2).To ensure the same starting conditions in all experiments and comparability of the results, all samples were pre-treated oxidatively (30 min in pure O2 at 600 °C). In a second preparation step, the temperature of samples was reduced to 300 °C, and only then, the gas atmosphere was changed to the reaction mixture. For all experiments the CO2 and H2 ratio was set to 1:1. Care was taken with respect to the used mass of catalyst, so that the reaction proceeds away from thermodynamic equilibrium (i.e. at 600 °C and 700 °C the limit is 40 % and 45 %, respectively). This has to be taken into account, since conversion rates around the equilibrium lead to pronounced back reaction contributions (WGS instead of rWGS) [45]. After switching to the reaction mixture, 60-minutes measurements were conducted between 300 °C and 700 °C. The temperature was increased in 100 °C steps. Fig. 1 exemplarily displays the results for rWGS on the B-site undoped perovskite La0.6Ca0.4FeO3- δ . Switching on the reaction environment at low temperatures (300 °C) did not lead to detectable activity. Only after the reaction temperature was raised to 400 °C, the onset of CO formation was observed. Similar onset temperatures where noted by Daza et al. on a related perovskite (LaFeO3) with a CO formation onset temperature of 450 °C and similarly high CO selectivity [16]. When raising the reaction temperature to 500 °C and 600 °C, the CO formation increased significantly (∼15 % conversion at 600 °C). It is worth mentioning that parallel consumption of the educts CO2 and H2 is also nicely visible in Fig. 1. This is especially important to note, since the amount of formed water could unfortunately not be quantified with desired accuracy by the used Micro-GC, despite water being visible in the chromatogram. All comparisons presented in this study use the reactivity values found at 600 °C, since educt conversion at 700 °C is too close to the thermodynamic equilibrium.The same procedure was used to assess the rWGS performance of all synthesized perovskites as well as of commercial LSF. In Fig. 2 a comparative summary of all results is shown. Area specific activities (in mol m−2 s-1) were used for direct comparisons of the CO formation rate. To get the necessary values for such a comparison, the catalytic activity was related to the active surface areas, see Section 2.3 for details. In Table 1, all average specific activities at 600 °C are given.The lowest activities were found for the La-based B-site undoped samples (La0.9Ca0.1FeO3- δ and La0.6Ca0.4FeO3- δ , purple curves in Fig. 2), which are comparable to the results of LSF (Fig. 2, black curve). Increasing the A-site dopant concentration had only a minor effect on the activity. The specific activity at 600 °C was 5.7 ×   10 - 6   m o l   m - 2   s - 1 and 5.9 × 10 - 6   m o l   m - 2   s - 1 , respectively (for LSF it was 4.8 × 10 - 6   m o l   m - 2   s - 1 ). A change of A-site dopant (going from La to Nd) increased the activity (Nd0.9Ca0.1FeO3- δ and Nd0.6Ca0.4FeO3- δ , orange curves in Fig. 2). Both materials showed a CO formation onset when raising the temperature to 400 °C. With every temperature step a further increase of activity was observable. The material with the lower Ca-doping (10 %) exhibited higher CO formation rates than the perovskite with higher Ca content at 600 °C and 700 °C. The specific activities at 600 °C were 11.3 × 10 - 6   m o l   m - 2   s - 1 and 6.6 × 10 - 6   m o l   m - 2   s - 1 for Nd0.9Ca0.1FeO3- δ and Nd0.6Ca0.4FeO3- δ , respectively.A comparison of activities found for the Ni-doped sample Nd0.6Ca0.4Fe0.9Ni0.1O3- δ in Fig. 2 (blue curve) and the undoped Nd0.6Ca0.4FeO3- δ indicates that doping positively affects CO formation at elevated temperatures. Furthermore, it could be observed that at 500 °C CO formation increased initially (unlike for the undoped materials, where the activity during each step was either constant or showed an initial drop). At 600 °C, the specific activity for the Ni doped perovskite was 18.0 × 10 - 6   m o l   m - 2   s - 1 . The material doped with Co exhibited similar activation phenomena (Fig. 2, green curve). Already at 400 °C, a slight increase of the CO formation could be observed in the isothermal regime. At 500 °C, this effect was even stronger. When comparing all activities at all used temperatures, the largest value ( 27.2 × 10 - 6   m o l   m - 2   s - 1 at 600 °C) was found for the Co-doped perovskite.Besides comparing the novel perovskites to LSF, benchmarks of the catalytic performance against an industrial catalyst were conducted as well. Hence, the rWGS reaction was performed with the same parameters on the commercial HiFuel™ high temperature WGS catalyst. It is composed primary of iron oxide with 7 % chromium oxide to enhance sinter stability, which makes the catalyst ideal for comparison to the iron-based perovskites of this study. At 600 °C, the obtained specific activity was ∼ 3 × 10 - 7   m o l   m - 2   s - 1 , which is one order of magnitude lower than for the investigated undoped perovskites. But this result has to be evaluated very critically, as very strong sintering and consequently reduction of active surface area of the commercial catalyst was observed at high reaction temperatures. Consequently, the comparability of the specific activity of this reference material is questionable.To summarize, we found that exchanging La with Nd increases the catalytic activity, and doping the perovskite B-site with Ni or Co enhanced it even further. The Co-doped catalyst exhibited the best performance, highlighting that Co-doping is highly beneficial to rWGS activity. The activity enhancing effects of the addition of Co were reported for other materials, e.g. metal-carbides, as well [46].There are two conceivable reasons for the high activities observed in the Ni- and Co-doped perovskites: (i) exsolution of nanoparticles (which is a well-documented phenomenon in perovskites [25]) and (ii) reducibility of the materials.In literature and in previous work done by the authors, it was shown that nanoparticle exsolution can greatly enhance the catalytic activity of perovskites [34,47]. Moreover, the ongoing process of nanoparticle formation could be an explanation for the increasing activity over time at constant temperature observed for the Ni (500 °C) and Co (400 °C and 500 °C) doped catalysts. Exsolution and the observed structural and morphological changes of our catalysts will be treated in depth in Section 3.3.With respect to reducibility, it should be noted that compared to the undoped perovskites, Nd0.6Ca0.4Fe0.9Ni0.1O3- δ and Nd0.6Ca0.4Fe0.9Co0.1O3- δ are more easily reduced in the reaction environment, as was demonstrated in a chemical looping experiment for Co-doping (see supporting info, Fig. S1). Furthermore, it has been shown by various groups that the rWGS reaction of catalysts containing an easily reducible oxide (e.g. as support) follows a surface redox mechanism (see for example the Pt/ceria or iron oxide based systems in Refs. [48,49]).Both aspects will be discussed more thoroughly in Section 3.5., giving a more detailed mechanistic insight.In addition, the occurrence of any side reactions was checked for all tested materials. Methane, for example, is a very well know side product, as the Sabatier reactions are the main side reactions of rWGS [4]. Here, however, no side products could be detected in the gas chromatograms indicating a high CO-selectivity: A key factor for a high selectivity towards CO – and an important property of perovskites – is the reducibility of the oxide support material, the oxygen ion mobility, and its capability for vacancy formation. For instance, for rWGS on the perovskites BaZr0.8Y0.16Zn0.04O3 and La0.75Sr0.25FeO3 nearly 100 % CO selectivity have been reported [4,15]. Also, O vacancies have been reported to be crucial for the catalytic activity of the Pd/CeO2/Al2O3 system, as they can be filled with O from CO2 [50]. One possible further reason for the high selectivity of the tested systems could be the size and distribution of the exsolved nanoparticles as discussed below. Similarly, Lu et al. observed that low Ni loadings (< 3 %) with well dispersed nanoparticles are highly beneficial to the selectivity towards CO [51]. They reported 100 % CO selectivity in the temperature range from 400 °C to 750 °C.To directly follow the structural changes of the novel perovskites during catalytic reactions and to get insights into the active phase of the different perovskites, in-situ XRD measurements were performed in the reaction environment (i.e. at 1 bar in a flow cell, 1:1 ratio of CO2 and H2) on selected perovskites. Resulting XRD diffractograms for the undoped Nd0.6Ca0.4FeO3-δ are shown in Fig. 3 .At low rWGS reaction temperatures (below 400 °C), only the reflexes corresponding to the perovskite host lattice were visible. Importantly, the perovskite was stable and no decomposition of the material was observed at all reaction temperatures. This is crucial for possible industrial applications, where catalyst regeneration by oxidation/reduction cycles can be realized on stable materials [52]. Although rWGS reaction conditions are reducing, no formation of any metallic Fe-phase could be observed by in-situ XRD over the whole temperature range of the experiment. The chemical potential of the gas phase was not sufficient for the formation of metallic nanoparticles on the surface. However, between 500 °C and 600 °C weak signals (2θ of 30.0°, 35.3° and 62.0°) were evolving, which could be assigned to the occurrence of Fe3O4. This phase transformed to FeO at around 600 °C, indicated by the disappearance of the Fe3O4 signals and emerging signals at 2θ of 36.0°, 41.8°, and 60.5°. This transition agrees with the Fe-O phase diagram [53], which shows a transition around 570 °C. It was previously reported that iron-oxide is an active phase for catalysing rWGS [4], and in the current experiments this phase is forming under reaction conditions.Additionally, at a temperature of 400 °C a small signal evolved at 2θ of 29.4°, resulting from formation of CaCO3 on the perovskite surface. Similarly, above 500 °C trace amounts of a graphite phase could be observed. Formation of carbonates under reaction conditions is a well-known phenomenon for A-site doped perovskites, as reported in literature [54]. The amount of CaCO3 increased at higher reaction temperatures, simultaneously with the formation of the iron oxide species. This might be due to the additional driving force of establishing stoichiometric balance in the perovskite structure after the exsolution of B-site cations. Interestingly, both iron-oxide and CaCO3 signals diminish at the highest temperature of 700 °C.Figs. S3–S6 in the supporting info show diffractograms of all the B-site undoped samples, La0.9Ca0.1FeO3-δ, La0.6Ca0.4FeO3-δ, Nd0.9Ca0.1FeO3-δ, and Nd0.6Ca0.4FeO3-δ, obtained by ex-situ XRD after rWGS reactions (samples from the activity measurements with the last temperature at 700 °C). In agreement with the in-situ XRD results, for Nd0.6Ca0.4FeO3-δ both CaCO3 and an iron-oxide phase were found. The latter is Fe3O4, probably because FeO present at the highest temperatures (see in-situ XRD experiment) underwent a phase transition back to Fe3O4 when cooling down.For Nd0.9Ca0.1FeO3-δ, Fe3O4 was also visible, as well as trace amounts of CaCO3. In contrast, for the samples with La no iron-oxide species could be observed. Furthermore, for La0.9Ca0.1FeO3-δ no new phases were found at all after rWGS, while for La0.6Ca0.4FeO3-δ formation of CaCO3 was observed. Two trends follow from these observations. Firstly, the exchange of La with Nd enables the formation of the iron oxide phases under rWGS conditions. Secondly, more Ca-doping on the perovskite A-site leads to stronger CaCO3 formation during reaction, as would be expected.Another observation arising when comparing the diffractograms before and after rWGS is the slight shift of all reflex positions towards smaller diffraction angles. This effect is strongest for La0.6Ca0.4FeO3-δ and is due to expansion of the unit cell in reducing conditions as the result of oxygen vacancies being formed and the partial reduction of the Fe (from Fe4+ to Fe3+ and from Fe3+ to Fe2+) [55].Formation of CaCO3 was also visible in SEM images recorded after rWGS reactions on La0.6Ca0.4FeO3-δ and Nd0.6Ca0.4FeO3-δ, Figs. S8/S10. After reaction, larger crystals with regular shape (often triangular) and sizes between 200 nm and 400 nm could be observed. These crystals were assigned to CaCO3 (this was supported by EDX measurements performed on the B-site doped materials, see below). In case of lower Ca-doping, La0.9Ca0.1FeO3-δ and Nd0.9Ca0.1FeO3-δ, no CaCO3 can be seen in the SEM images (Figs. S9/S11). For B-site undoped materials, no formation of nanoparticles was observed on the perovskite surface by SEM, although formation of FeO could be observed for Nd0.6Ca0.4FeO3-δ by in-situ XRD and Fe3O4 could be found for both perovskites with Nd in the ex-situ XRD patterns. Conceivably, iron oxide occurred either as exsolved nanoparticles that were below the detection limit of SEM, or decomposition occurred without apparent changes in morphology. Generally, the perovskites preserved their overall surface structure (except for CaCO3 formation), highlighting their excellent thermal stability.The absence of metallic surface iron species on the B-site undoped Nd0.6Ca0.4FeO3-δ could also be confirmed by in situ NAP-XPS data (cf. Fig. S14A).In-situ XRD measurements on the reference material LSF (Fig. S7) showed a very similar behaviour as for Nd0.6Ca0.4FeO3-δ. At low reaction temperatures, only the perovskite phase was observable. At 650 °C, the formation of FeO could be observed, as indicated by the weak signal at 2θ of 41.7°. Moreover, trace amounts of SrCO3 started to appear at 2θ of 25.0°, analogous to the formation of CaCO3 in the Ca doped materials. Unlike for the Ca doped samples, SEM could reveal the formation of small nanoparticles (20−30 nm) on the perovskite surface for LSF, Fig. S12.To investigate the influence of exsolution on the rWGS activity, the experiment on Nd0.6Ca0.4FeO3-δ was repeated with an additional pre-treatment step in H2/H2O (32:1) at 700 °C (60 min). The bottom XRD pattern in Fig. 4 indicates successful Fe nanoparticle formation by reductive treatment (2θ of 44.6° and 62.9°, corresponding to a metallic Fe-phase), which has already been shown in earlier work [21].Upon stepwise increases of the rWGS reaction temperature, the metallic Fe signal started to decrease between 450 °C and 500 °C. At the same time, new diffraction lines appeared at 2θ of 30.0°, 35.3°, 56.6°, and 62.2°, which could be assigned to Fe3O4. Under rWGS reaction conditions, the initially metallic Fe nanoparticles where oxidized to Fe3O4 in the temperature range of 450 °C–550 °C. Between 600 °C–650 °C, a further change of the observed phases occurred. The Fe3O4 nanoparticles where reduced to FeO (2θ of 35.9°, 41.7°, and 60.4°). Additionally, starting from 500 °C, CaCO3 and graphite were formed. At the highest reaction temperature (700 °C), the carbonate phase diminished and only FeO and graphite were left on the catalyst surface.In combination with the activity data of the catalytic measurements (cf. Section 3.2), it can be concluded that the formed Fe3O4 and FeO is correlated to the high rWGS activity. While the specific activities of all the B-site undoped materials were similar at 500 °C, starting from 600 °C, the Nd-based perovskites exhibited higher catalytic activities compared to the respective perovskites with La. The temperature region between 500 °C and 600 °C is exactly where the iron-oxide phases started to evolve in the in-situ XRD experiment with Nd0.6Ca0.4FeO3-δ. Furthermore, an iron-oxide phase only occurred in the ex-situ XRD experiments with the Nd-based perovskites. This phase is formed under reaction conditions, both from the perovskite without reductive pre-treatment and from already exsolved Fe particles. The unwanted formation of CaCO3 observed for the materials with a higher amount of Ca-doping could explain why the activity of Nd0.6Ca0.4FeO3-δ was lower compared to Nd0.9Ca0.1FeO3-δ. These results demonstrate how rich the surface chemistry of perovskites can be, and that the surface is responding dynamically to changes of the chemical potential of the reaction environment, as well as the strong effect these changes have on the rWGS activity.In-situ XRD results for the Co-doped perovskite Nd0.6Ca0.4Fe0.9Co0.1O3-δ are displayed in Fig. 5 . Here, the catalyst was exposed to the rWGS reaction environment without any reductive pre-treatment (only initial oxidation was performed for a defined surface state). Above 525 °C, the formation of a FeO and/or CoO phase could be observed at 2θ of 36.2°, 40.0°, and 60.7°. Unfortunately, it was not possible to find a clear assignment to either FeO or CoO (or a mixed Fe-Co-oxide – FexCo1-xO) due to the overlap of the two signals in XRD and the limited resolution of the diffractometer. However, from previous work with EDX mapping, it is known that due to easy reducibility Co is preferentially exsolved at lower temperatures [21]. At 550 °C, additional formation of a metallic bcc phase could be observed at 2θ of 44.8° and 65.0°. Again, no clear assignment was possible with XRD due to the signal overlap of Fe and Co, however the fact that no metallic phase occurred in the experiment without Co-doping (cf. Fig. 3), the higher diffraction angle compared to the metallic phase in the experiment with B-site undoped Nd0.6Ca0.4FeO3-δ and pre-treatment (cf. Fig. 4), and the preferential exsolution of Co suggest a predominance of Co in this phase. Both the B-site metal oxide and metallic phases are more pronounced than in the case of no Co doping. These results show that doping with Co enhances the exsolution process and also enables the reduction of the exsolved elements to a metallic state within the reaction atmosphere, which is preferred for catalysis. These findings where supported by in situ NAP-XPS data as well (cf. Fig. S14B), with the evolvement of metallic Co during rWGS reaction. Already at 500 °C, a small amount of CaCO3 could be observed at 2θ of 29.3°, which got larger simultaneously with the formation of the B-site cation containing phases. Importantly, the perovskite host lattice was stable over the whole temperature range. Figs. 6 and 7 summarize the results from SEM and EDX measurements for Nd0.6Ca0.4Fe0.9Co0.1O3-δ. After the rWGS reaction, larger, smooth crystals (sizes around 250 nm) on the surface could be observed. They could be identified as formed CaCO3 on the surface, which was supported by the enrichment of Ca and C seen in the EDX spectrum at the position of one of these crystallites (Fig. 7, spectrum C). Furthermore, the formation of finely dispersed nanoparticles with sizes between 20 nm and 50 nm was visible. The EDX spectrum at the position of one particle (Fig. 7, spectrum B) reveals a larger Co L signal (as a shoulder of the Fe L peak) compared to a position in between particles (Fig. 7, spectrum A). This supports the theory that Co has been preferentially exsolved before Fe, and that the nanoparticles can be supposed to be mainly composed of Co. It should be pointed out that the spatial and depth resolution of the EDX analysis is limited, meaning that the obtained spectrum includes signals from the surrounding of the particle. Therefore, an unequivocal determination of the particle composition would require additional TEM studies.The in-situ XRD results suggest that during reaction the nanoparticles were either primarily metallic or already oxidized. Due to the exposure to air (and hence oxidation), when transferring the samples to the SEM, it is no longer possible to distinguish between the two cases. Therefore, it is not clear which of the B-site metal oxide or metallic phases observed with XRD, or even both, correspond to the nanoparticles seen in the SEM images. Astonishingly, these nanoparticles were stable without sintering effects even at high reaction temperatures up to 700 °C. The reason is that nanoparticles formed by exsolution are anchored to the perovskite surface, as has been shown by Neagu et al. [33] in an in-situ TEM study. The stable nature and the prevention of sintering of these formed nanoparticles make perovskite catalysts extremely valuable for industrial applications.As shown in the catalytic data (cf. Section 3.2), the Co-doped perovskite exhibited the highest rWGS activity. A possible reason could be the co-existence of nanoparticles containing metallic Co and the FeO/CoO oxide phase on the perovskite surface (in vicinity to oxygen vacancies of the host lattice), both enhancing hydrogen dissociation and redox activity (further details see Sec. 3.5 below). This is supported by the fact that the activity measurements of the Co-doped catalyst showed the largest increase between 500 °C and 600 °C, the temperature region where these phases started to appear in the in-situ XRD experiment. For Sr-doped lanthanum cobaltite perovskites (La1-xSrxCoO3-δ), Daza et al. reported the importance of metallic cobalt for the conversion of CO2 to CO as well [56].For the Ni doped catalyst Nd0.6Ca0.4Fe0.9Ni0.1O3-δ, a diffractogram was obtained ex-situ after rWGS reaction (Fig. S2 in the supporting information) using the sample from the activity measurement (last temperature 700 °C). Here, the NiO impurity observed in the pristine sample could not be found anymore. Instead, a metallic Ni-phase (fcc) was present, indicated by the reflexes at 2θ of 43.9° and 51.2°. This suggests a reduction of the NiO phase to metallic Ni during rWGS, probably around 500 °C, which is the temperature where increasing activity at constant parameters was observed in the catalytic experiments. The formation of additional metallic Ni (by exsolution) cannot be confirmed conclusively. Also, alloying of Ni with Fe within the fcc phase might be possible by exsolution. Further experiments would be necessary to determine the exact behaviour. Besides the Ni containing phases, formation of CaCO3, as well as trace amounts of Fe3O4 could be observed. This agrees with the results observed for the B-site undoped Nd0.6Ca0.4FeO3-δ. Compared to the latter, there was more CaCO3 and less Fe3O4. This can be explained by the changed driving forces for segregation, evoked by the formation of the Ni phase. As this leads to a B-site sub-stoichiometry in the remaining perovskite, the A-site Ca segregation is enhanced, while B-site Fe segregation is reduced.The larger CaCO3 crystals found on the surface of La0.6Ca0.4FeO3-δ, Nd0.6Ca0.4FeO3-δ, and Nd0.6Ca0.4Fe0.9Co0.1O3-δ were also observed in the SEM images of Nd0.6Ca0.4Fe0.9Ni0.1O3-δ (Fig. S13). Here, they were even bigger with sizes between 400 nm and 700 nm. This larger size matches the result of an enhanced Ca segregation obtained from the XRD measurements. Furthermore, the larger size of the crystals allowed for an EDX mapping to confirm their chemical nature as CaCO3. At high magnification, also very small nanoparticles (< 15 nm) were visible on the surface. These probably consist of Ni, in agreement with the metallic Ni-phase observed with XRD. The formation of Ni nanoparticles can explain the higher catalytic activity of the Ni-doped catalyst compared to the undoped ones, which was observed in the catalytic measurements (cf. Section 3.2).To further investigate the promoting effect of the formed nanoparticles, and to answer if the exsolved nanoparticles are really enhancing the catalytic activity, additional experiments were conducted. These additional activity tests for Nd0.9Ca0.1FeO3-δ and Nd0.6Ca0.4Fe0.9Co0.1O3-δ were performed with linear temperature ramps.For the undoped Nd0.9Ca0.1FeO3-δ, it was found that exsolution of metallic Fe particles does not enhance the catalytic activity (for more details see the supporting info Section “Exsolution Enhanced Catalytic Reaction” and Fig. S15).For the B-doped Nd0.6Ca0.4Fe0.9Co0.1O3-δ, the reaction temperature was increased from 300 °C to 570 °C (heating rate =1 °C min−1). After reaching 570 °C, the reaction temperature was reduced with the same rate (to 300 °C) (see also Fig. S16). For this special experiment 570 °C was chosen as highest temperature, because at this temperature exsolution is already possible, but CaCO3 formation is still minor (see in-situ XRD results, Fig. 5). From the in-situ XRD results, in this temperature range the exsolved particles are expected to be mainly cobalt-oxide.Since the cobalt-oxide-nanoparticles only form upon the first heating in reaction atmosphere (they are formed in-situ at high reaction temperatures where cation mobility is sufficient), heat-up and cool-down behaviour are expected to be different in case the particles affect the activity of the catalyst. If the formed nanoparticles would have no influence on catalytic reactivity, the CO formation rate should be equal for both up and down ramping. Indeed, a hysteresis-like behaviour was found as can be seen in Fig. 8 , with a maximum increase in formed CO by 0.9 mol% at 490 °C. This is a clear indication that for the cooling ramp a higher catalytic reactivity was observed than for the heating ramp, which can be interpreted as an evidence for a catalytic effect of the exsolved nanoparticles.To show that this method is genuinely suitable of determining differences in catalytic activity, a second identical experiment was conducted directly after the first heat-up/cool-down cycle (i.e. without changing the catalyst, and without any intermediate treatment). The catalytic activity during both the second heating and cooling phases followed the cooling ramp of the first cycle exactly (see Fig. 8, dashed curves). This confirms that nanoparticles, evolving upon the very first heat-up, were improving the rWGS reactivity over the whole temperature range of all following heat-up or cool-down ramps, indicating reversible and stable catalytic behaviour of the nanoparticle decorated perovskite catalyst. Consequently, with this experiment strong structural deactivation phenomena in the investigated temperature range (up to 570 °C, prior to the increased formation of CaCO3) could be ruled out as well.In heterogeneous catalysis on reducible oxides, commonly lattice oxygen is found in the reaction product. This effect was explained by P. Mars and D. W. van Krevelen by suggesting a regenerative redox mechanism that consists of two steps [57]: In the first step, the catalyst is reduced by one of the educts, which is oxidized by taking up an oxygen atom from the catalyst and thus creates a vacancy in the catalyst’s oxygen sub-lattice. In the following second step, the catalyst is regenerated by reaction with the second educt, which donates an oxygen atom to the catalyst and hence becomes reduced.The rWGS reaction (Eq. (1)) on oxide catalysts usually also proceeds via a Mars-van Krevelen (MvK)-type redox mechanism. There, in the first step, hydrogen reacts with lattice oxygen of the catalyst forming the first product water, which desorbs leaving an oxygen vacancy behind. The two electrons formed in this reaction step are also transferred to the catalyst oxide ensuring charge neutrality. In Eq. (3) the half reaction of H2 oxidation proceeding on a reducible oxide is written in Kröger-Vink notation with O O × , v O ∙∙ , and e ' denoting regular lattice oxygen (relatively neutral), oxygen vacancy (relatively two-fold positive), and electron (relatively negative), respectively. (3) H 2 + O O × → H 2 O + v O ∙∙ + 2 e ' In the second step, these electrons are consumed by reduction of carbon dioxide, which annihilates an oxygen vacancy by donating an oxygen atom to the catalyst, thus forming the second product carbon monoxide. (4) C O 2 + v O ∙∙ + 2 e ' → O O × + C O Both half reactions are coupled by the electron transfer via the solid catalyst substrate and thus for the case of steady state conditions, the following relationship for the reaction rate densities r holds: (5) r H 2 - o x = r C O 2 - r e d = r r W G S Therein, the subscripts “H2-ox” and “CO2-red” refer to Eqs. (3) and (4), respectively. Both are equal to the net rate of the rWGS reaction rrWGS, which is compared in a surface area-normalised form (thus called specific activity) in Fig. 2.As one can see in this figure, upon changing the composition of the investigated perovskite-type catalysts, the reaction rate is improved. In the following, we thus propose a relation between the observed catalyst activities in Fig. 2 and the corresponding catalyst composition based on the characteristics of an assumed MvK-type redox mechanism on mixed conducting perovskites. To do so, it is helpful to first summarize previous results on similar materials:(i) CO2 reduction proceeds on the perovskite surface and is only little affected by exsolution of metallic particles [40]. The availability of a sufficiently high concentration of electrons in the electro-catalyst increases efficiency drastically. Thus, the CO2 reduction rate is higher on more easily reducible oxides [40].(ii) Exsolved metallic particles can enhance the H2 oxidation rate significantly [34,47]. The improvement of H2 oxidation rate occurs by spillover of adsorbed hydrogen species from the metal to the oxide – i.e. the bare oxide surface suffers from a depletion of reactive species, hence here H2 activation is limiting the net reaction rate, while exsolved metallic particles help sustaining a significant coverage with an active hydrogen species [58].(iii) Surface enrichment of Sr causes performance degradation of perovskite-type electro-catalysts, but enrichment of La shows only minor effects [59,60].With this in mind, let us now look at the catalyst performance data in Fig. 2. Doping LaFeO3 with calcium instead of strontium does not visibly change the catalytic activity of the investigated perovskite. This is in agreement with previous results, since both Ca and Sr do not show any redox activity and their effect on defect chemistry (e.g. electron concentration) is only due to their charge.Changing La against Nd caused an improvement in the specific activity by up to a factor of two (compare the purple and orange curves in Fig. 3). This result is difficult to be unambiguously explained in quantitative terms using the data available so far. However, two potential qualitative explanations shall be briefly discussed here. First, the different redox activity of La and Nd may be the reason for the observed behaviour. While La in the perovskite bulk has only a minor (if any) redox activity, there are indications that surface La actively participates in redox reactions [61]. Assuming a slightly easier reducibility of surface Nd compared to surface La may explain the increased rWGS activity of the Nd-based catalysts via improvement of the CO2 reduction reaction (Eq. (4)). Second, the iron oxide phases observed on Nd0.6Ca0.4FeO3-δ and Nd0.9Ca0.1FeO3-δ may contribute to an increased CO2 reduction rate. Formation of these phases might as well be simply a consequence of the high redox activity of the Nd-containing surface, without a significant enhancing effect of the iron oxide phases on the CO2 reduction. The exact relations are still to be investigated further. However, any role of FeO and Fe3O4 for an improved H2 oxidation activity can be definitely ruled out as demonstrated in Ref. [58].A further noteworthy increase of the catalyst performance was achieved by introduction of the reducible elements Co and Ni, which under reaction conditions both caused decoration of the oxide catalysts by metallic precipitates (see Figs. 2 and 6). Hence – by considering the above-mentioned previous results of H2 oxidation on exsolution-decorated electro-catalysts – we suggest the associated specific activity improvement to be mainly caused by an enhancement of the H2 oxidation rate (Eq. (3)). Some additional activity enhancement may originate from the easier reducibility and the corresponding higher oxygen vacancy concentration especially of the cobalt doped material.A summary of both interpretations is sketched in Fig. 9 , which shows the job sharing of oxide surface and metal exsolution, catalysing CO2 reduction and H2 oxidation, respectively. The connection of both half-reactions is achieved by electrons and oxygen vacancies flowing via the mixed conducting perovskite-type catalyst.To summarize, the high reactivity of Nd0.6Ca0.4Fe0.9Co0.1O3-δ (and to some extent Nd0.6Ca0.4Fe0.9Ni0.1O3-δ) can be explained by the synergistic effect of the perovskite host lattice with its capability for high oxygen vacancy concentration, responsible for effective CO2 activation, the formed CoO (and/or FeO) on the surface, which is contributing to the oxygen chemistry, and the exsolved metal nanoparticles, which are enhancing the H2 adsorption and dissociation ability of the catalyst surface.To assess the deactivation behaviour of the investigated perovskites, rWGS reactions at constant high temperatures (600 °C) were conducted. Here, results for Nd0.6Ca0.4FeO3-δ and Co-doped Nd0.6Ca0.4Fe0.9Co0.1O3-δ are highlighted in Fig. 10 . Results for the other materials are displayed in the supporting info (Figs. S18–S20). To ensure the same well-defined starting point for all experiments, all samples were subjected to an oxidizing pre-treatment at 600 °C for 30 min. Following a cooling down period (to 300 °C in O2), the reaction gas atmosphere was switched on. A sharp increase of reaction temperature to 600 °C in order to find possible pronounced activation or deactivation effects during reaction onset did not reveal any such effects.For Nd0.6Ca0.4FeO3-δ (Fig. 10), a slow deactivation over time was observed which could be seen in the slightly decreasing CO signal. This slow deactivation is very likely attributed to formation of CaCO3 crystallites on the surface, as observed in SEM images and XRD after reaction (Figs. S10 and S4). The CaCO3 is formed in two stages: Ca-segregation to the surface and its reaction with CO2 from the reaction atmosphere. This phenomenon of carbonate formation is a well-known issue for perovskites with alkaline earth metals as A-site cations, as shown in literature e.g. for BaCo0.4Fe0.4Zr0.2O3- δ [54]. The formed CaCO3 crystallites are covering part of the perovskite surface, blocking active sites. The slow deactivation is a result of the proceeding growth of CaCO3. Possible strategies for reducing this deactivation process are either a reduction of the Ca-content, the use of an A-site sub-stoichiometric material or a change of the A-site composition to elements that are less prone to segregation or carbonate formation [62]. For the tested perovskites with lower A-site Ca doping, an already reduced tendency for segregation and CaCO3 formation was observed, as shown by XRD and SEM data (Figs. S3/S9 and S5/S11).Nd0.9Ca0.1FeO3-δ, La0.9Ca0.1FeO3-δ, and La0.6Ca0.4FeO3-δ showed a similar behaviour with respect to catalytic activity with slow deactivation over time (Figs. S18–S20). For the Co-doped Nd0.6Ca0.4Fe0.9Co0.1O3-δ, the same behaviour was observed as well, see Fig. 10 (inset). In in-situ XRD data (Fig. 5), the formation of CaCO3 was observed above a reaction temperature of 500 °C. With increasing reaction temperature, the corresponding reflex grew more intense, indicating the growth of the CaCO3 crystallites.Six rare earth based perovskite-type oxides were investigated with respect to their rWGS performance, as well as their corresponding surface structure and morphology (using XRD and SEM). The highest catalytic activity was achieved when formation of metallic nanoparticles by exsolution occurred during reaction, as could be shown for the Ni- and Co-doped materials Nd0.6Ca0.4Fe0.9Ni0.1O3-δ and Nd0.6Ca0.4Fe0.9Co0.1O3-δ. We propose that this is due to a Mars-van-Krevelen-type mechanism of the rWGS reaction on these materials with a highly beneficial job-sharing ability of the metal-particle-decorated catalyst surfaces. On the one hand, dissociative H2 adsorption takes place on the metal particles. By spillover, the active hydrogen species can subsequently increase the reduction rate (and thus the oxygen vacancy generation rate) in the perovskite backbone compared to the undecorated perovskite. This backbone, on the other hand, is responsible for CO2 activation. It shows a pronounced defect chemistry, including oxygen vacancies, and can reduce CO2, accepting one of its O atoms to re-fill a vacancy.The choice of both A-site and B-site compositions of the perovskite material play an important role for the resulting behaviour and performance. For the design of an optimal catalyst material, several aspects must be considered: (i) The used rare earth metal on the A-site can – via its aptitude to redox behaviour – influence the reducibility of the perovskite backbone and its ability for oxygen vacancy formation. This, in turn, has an effect on the CO2 reduction rate, but also on the exsolution process (this was also seen in previous work regarding exsolution [43]). In this work, we have shown that exchanging La with the presumably more redox-active Nd increased the catalytic activity for rWGS. (ii) Doping of the A-site with an alkaline earth metal is important for the perovskite defect chemistry, affecting electronic and ionic conductivity, oxygen vacancy concentration, and thus CO2 reduction rate and exsolution behaviour. However, alkaline earth metal doping comes with the disadvantage of segregation effects and carbonate formation during rWGS. For our Ca-doped materials, we observed CaCO3 covering the surface, thus blocking active sites and resulting in catalyst deactivation over time. The carbonate formation increased with a higher Ca content and more pronounced B-site metal exsolution. (iii) Using an easily reducible element (such as Co or Ni) as B-site dopant in combination with a less reducible main component (in our case Fe) not only facilitates formation of metal particles on the surface, but also enables a preferential exsolution of the dopant element (found in this study both for Ni- and Co-doping). At the same time, the less reducible main component ensures that a stable perovskite backbone is retained, because it is not completely reduced under the same conditions. The metal particles strongly increase the catalytic activity for rWGS, as was observed for both the Ni- and the Co-doped material. This stable backbone can provide a good anchoring of exsolved nanoparticles, thus preventing sintering and ensuring a high amount of gas/metal/oxide three-phase boundaries. For our Co-doped material, no sintering of the metal nanoparticles could be observed, even up to 700 °C. (iv) The choice of the B-site dopant is crucial. Cobalt is known to be highly active for rWGS, and consequently the Co-doped perovskite showed the best rWGS performance. In contrast, Fe particles exsolved by reductive pre-treatment, did not enhance the catalytic activity. The used rare earth metal on the A-site can – via its aptitude to redox behaviour – influence the reducibility of the perovskite backbone and its ability for oxygen vacancy formation. This, in turn, has an effect on the CO2 reduction rate, but also on the exsolution process (this was also seen in previous work regarding exsolution [43]). In this work, we have shown that exchanging La with the presumably more redox-active Nd increased the catalytic activity for rWGS.Doping of the A-site with an alkaline earth metal is important for the perovskite defect chemistry, affecting electronic and ionic conductivity, oxygen vacancy concentration, and thus CO2 reduction rate and exsolution behaviour. However, alkaline earth metal doping comes with the disadvantage of segregation effects and carbonate formation during rWGS. For our Ca-doped materials, we observed CaCO3 covering the surface, thus blocking active sites and resulting in catalyst deactivation over time. The carbonate formation increased with a higher Ca content and more pronounced B-site metal exsolution.Using an easily reducible element (such as Co or Ni) as B-site dopant in combination with a less reducible main component (in our case Fe) not only facilitates formation of metal particles on the surface, but also enables a preferential exsolution of the dopant element (found in this study both for Ni- and Co-doping). At the same time, the less reducible main component ensures that a stable perovskite backbone is retained, because it is not completely reduced under the same conditions. The metal particles strongly increase the catalytic activity for rWGS, as was observed for both the Ni- and the Co-doped material. This stable backbone can provide a good anchoring of exsolved nanoparticles, thus preventing sintering and ensuring a high amount of gas/metal/oxide three-phase boundaries. For our Co-doped material, no sintering of the metal nanoparticles could be observed, even up to 700 °C.The choice of the B-site dopant is crucial. Cobalt is known to be highly active for rWGS, and consequently the Co-doped perovskite showed the best rWGS performance. In contrast, Fe particles exsolved by reductive pre-treatment, did not enhance the catalytic activity.These considerations can be used to tune the perovskite catalyst composition, achieving a material exhibiting excellent catalyst properties – exsolution of catalytically highly active metal nanoparticles with good sintering resistance and optimal size, distribution, and composition from a stable and reducible perovskite backbone, while showing only minimal carbonate formation. Not only changing the used elements and their ratio, but also introducing a sub-stoichiometry to either of the perovskite sites is conceivable, giving an even wider range of possibilities for optimization. Thus, an optimal rWGS performance tuned to the actual process parameters – with a high catalytic activity and stable operation – can be realized.Our results show that this material class is ideal to meet current challenges for industrial scale rWGS. Moreover, rWGS belongs to the most promising reactions for future CO2 conversion and utilization systems and belongs to the closest to implementation.This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement n° 755744 / ERC - Starting Grant TUCAS). L. Lindenthal: Conceptualization, Investigation, Formal analysis, Validation, Writing - original draft, Writing - review & editing. J. Popovic: Investigation, Formal analysis. R. Rameshan: Data curation, Investigation, Formal analysis. J. Huber: Investigation, Formal analysis. F. Schrenk: Investigation, Formal analysis. T. Ruh: Data curation, Validation, Writing - review & editing. A. Nenning: Data curation, Validation. S. Löffler: Investigation, Formal analysis. A.K. Opitz: Supervision, Validation, Writing - review & editing. C. Rameshan: Conceptualization, Funding acquisition, Project administration, Supervision, Writing - original draft, Writing - review & editing.The authors report no declarations of interest.The X-ray measurements were carried out within the X-Ray Center of TU Wien. The authors acknowledge TU Wien Bibliothek for financial support through its Open Access Funding Programme.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2021.120183.The following is Supplementary data to this article:

Reverse Water-Gas Shift (rWGS) is among the reactions with the highest readiness level for technological implementation of CO2 utilization as an abundant and renewable carbon source, and its transformation for instance into synthetic fuels. Hence, great efforts are made in terms of further development and comprehension of novel catalyst materials. To achieve excellent catalytic performance, catalytically active (nano)particles that are evenly distributed on (and ideally embedded in) an active support are crucial. An extremely versatile material class that exhibits the desired properties are perovskite-type oxides due to the fact that they can easily be doped with highly active elements. Upon controlled reduction or during reaction, these dopants leave the perovskite lattice and diffuse through the material to form nanoparticles at the surface (by exsolution) where they can greatly enhance the activity. Here, six perovskites were studied and their exsolution capabilities as well as rWGS performance were explored. Nanoparticle exsolution significantly enhanced the rWGS activity, with the catalytic activity being in the order Nd0.6Ca0.4Fe0.9Co0.1O3- δ > Nd0.6Ca0.4Fe0.9Ni0.1O3- δ > Nd0.9Ca0.1FeO3- δ > Nd0.6Ca0.4FeO3- δ > La0.6Ca0.4FeO3- δ > La0.9Ca0.1FeO3- δ > La0.6Sr0.4FeO3- δ (benchmark). Moreover, it could be shown that nanoparticles formed due to exsolution are stable at high reaction temperatures. In this paper, the flexibility of the investigated perovskite materials is demonstrated, on the one hand facilitating a material design approach enabling control over size and composition of exsolved nanoparticles. On the other hand, the studied perovskites offer a tuneable host lattice providing oxygen vacancies for efficient CO2 adsorption, activation, and resulting interface boundaries with the ability to enhance the catalytic activity.

Influenced by the continuously growing ecological effects stemming from anthropogenic climate change, carbon dioxide (CO2) conversion and utilization have been receiving increasing interest from the scientific community. 1 , 2 Among all the different CO2 utilization processes, electrocatalytic CO2 reduction reaction (CO2RR) is regarded as an attractive strategy that can not only help to reduce the atmospheric CO2 level and subsequent global warming effect but also relieve mankind’s dependence on fossil fuels for sustainability. 3 , 4 In the past decade, substantial efforts have been dedicated to increase the multi-carbon (C2+) selectivity for CO2RR, in particular, using high-alkalinity electrolytes (e.g., 0.1–10 M KOH) to lower overpotentials and promote the yields of target products. 5 However, the direct synthesis of C2+ products from CO2RR in alkaline electrolyte suffers from the inevitable side reaction of carbonate formation at the catalytic interface, which results not only in the depletion of both CO2 and OH− but also carbonate precipitation on the electrode, which degrades the catalytic performance. 6 , 7 The production of hydrocarbons and oxygenates in CO2RR has been generally proposed as the initial conversion of the CO2 reactant into ∗CO intermediates, followed by further reduction of those adsorbed ∗CO intermediates. 8 Thus, a two-step tandem approach in which CO2 is initially reduced to carbon monoxide (CO) and subsequently into C2+ products has been emerging as an alternative strategy. 6 , 9 Owing to the advance of solid-oxide electrolysis cell (SOEC) technology, 6 high-rate CO2RR to CO has been developed for commercial applications, making this two-step cascade design feasible. On one hand, CO electroreduction reaction (CORR) is a carbonate-formation-free platform that can be operated in high-alkalinity conditions and waive the problem of carbonate formation. 9 On the other hand, as CO is widely accepted as the key intermediate for C−C coupling, 10 the direct use of CO feed for electrolysis can significantly increase the local CO concentration at the catalyst-electrolyte interface, which inherently favors the generation of C2+ products. 7 , 11 Furthermore, as CO∗ is widely accepted as a key intermediate in CO2RR, CO2RR is believed to share common reaction pathways with CORR. Hence, understanding the CORR mechanism can also benefit the mechanistic exploration of CO2RR. The catalyst design strategies of CORR may also be applicable for CO2RR.To date, Cu is generally known as the only monometallic element that can convert CO2/CO into C2+ hydrocarbons and oxygenates with appreciable activities and selectivities. 10 , 12 , 13 Substantial research efforts have been devoted on Cu-containing catalyst surfaces to gain fundamental understandings of CORR. A variety of strategies, such as alloying, 14 facet tailoring, 15 morphology control, 16 , 17 and oxygen engineering, 18 have been pursued on Cu-based catalysts for CORR, targeting steering product selectivity and boosting catalytic activity. Some specific features in CORR, especially compared with its kindred CO2RR, have been discovered. In this review, we will provide an overview of current CORR advances in the mechanistic understanding, representative design strategies for the Cu-based catalysts (Table 1 ), and potential influential factors such as electrolyte and CO coverage. Furthermore, the rational design of CORR electrolysis reactors is also critical for the promotion of CORR performances. Finally, we will summarize the existing challenges that should be overcome to push this technology to the next level, and propose several perspectives for future opportunities.Although Cu shows appreciable selectivity and activity for catalyzing CORR toward C2+ products, the possible products have been reported with a wide distribution. Typically, C2+ products, including ethylene (C2H4), ethanol (EtOH), acetate, and n-propanol, can be obtained simultaneously, 28 , 29 but none of these products can overwhelmingly dominate the product spectrum. From a mechanistic perspective, the broad C2+ product distribution is likely to be caused by the existence of multiple bifurcations in the reaction pathways. 30 , 31 Thus, understanding the mechanism of CORR is of great significance, which can allow the provision of a roadmap for the catalyst design with optimal reaction activities and selectivities. As CO electroreduction is a highly complicated process composed of diversified reaction pathways along with multiple electron-and-proton transfer steps, computational investigations have been employed as major toolkits in providing theoretical insights for the formation of C2+ products. 30 , 31 In addition, spectroscopic studies to identify reaction intermediates have also been widely used to aid the mechanism exploration.In 2013, Koper and co-workers proposed a C−C coupling mechanism through proton and electron transfer as follows: (1) CO adsorption; (2) coupling of ∗CO with CO(g) via electron transfer to form ∗C2O2 −; (3) protonation of ∗C2O2 − to form ∗OCCOH (Figure 1A). 31 The characteristic of this mechanism shows that it can be performed at a relatively low overpotential, and its initial potential was calculated to be −0.4 V (versus reversible hydrogen electrode [RHE]). Using in situ Fourier transform infrared (FT-IR) spectroscopy, the authors further provided experimental evidence for the formation of a hydrogenated CO dimer (∗OCCOH) at low overpotentials during CO reduction on Cu(100) electrodes in LiOH solution (Figure 1B). 32 However, the ∗OCCOH intermediate could not be observed on Cu(111) under identical conditions, suggesting that CO dimerization is a structure-sensitive process, in agreement with previous experimental and computational observations. 35 , 36 Similarly, Nørskov and co-workers predicted that OCCHO∗ should be the most thermodynamically favorable state derived from OCCO∗. 33 Their work proposed that the energy barrier for ∗CO dimerization and surface hydrogenation of OCCO∗ to form OCCHO∗ on Cu(100) are dramatically lower than those on Cu(111) (Figure 1C), 33 indicating that Cu(100) is more active than Cu(111).Garza et al. presented the free energy change with voltage for the reaction ∗CO + CO → ∗COCO on Cu(100) and Cu(111). 34 For Cu(100), only at high overpotentials, ∗CHO was predicted to be more stable than ∗COCO, while on the Cu(111) surface, ∗COCO was highly unstable compared with its components at all the calculated potentials. Thus, on Cu(100) at low overpotentials, ∗CO dimerizes and reduces to ∗COCHO, leading to C2+ products. On Cu(100) at high overpotentials and on Cu(111) at all potentials, ∗CO reduces to ∗CHO, which can lead to both CH4 and C2+ products (Figure 1D). 34 The results were consistent with experiments that C2H4 and CH4 share a common intermediate on Cu(111) and at high potentials on Cu(100), while the C2H4 formation proceeds on a distinctive path at low overpotentials on Cu(100). 35 , 37 , 38 The calculations by Goodpaster et al. showed that the kinetic barrier for CO dimerization on Cu(100) increases with an increasing negative voltage and vice versa for the ∗CHO formation, 39 supporting the mechanism proposed by Garza et al. 34 and Ou et al. 40 There is also a possibility of reducing ∗CO to ∗COH, rather than ∗CHO. Different studies have investigated this question, with different computational techniques employed. 37 , 39 , 41–43 In gas-phase computational hydrogen electrode (CHE) calculations, Nie et al. found that ∗COH was favored, 43 while the opposite trend was observed in explicit solvent simulations. 41 Akhade et al. found that the presence of adsorbed K+ ions increases the selectivity for reducing ∗CO to ∗CHO by stabilizing these species and destabilizing the transition state to form ∗COH. 42 Garza et al. proposed that the stability of ∗COH depends on the adsorption site: on a hole site of a Cu(100) surface, ∗COH is close in energy to ∗CHO. 34 In contrast, on a Cu(100) site (the preferred site of CO top adsorption), 44 the energy of ∗COH is higher than ∗CHO. 34 Thus, the authors concluded that ∗COCHO is much more stable than ∗COCOH, especially at high potentials.However, a recent study by Cheng et al. suggested that the C−C coupling is not the rate-determining step in CORR. 45 The authors investigated the CO adsorption isotherms on Cu in a broad pH range. Combining with the electrokinetic data, the authors demonstrated that the reaction orders of adsorbed CO at p CO <0.4 and >0.6 atm are first and zeroth, respectively, for C2+ products on three Cu catalysts. Thus, the authors proposed that the hydrogenation of CO with adsorbed water is the rate-determining step in CORR, and the site competition between CO and water leads to the observed transition of the CO reaction order.The pathways toward C2H4 and EtOH seem to proceed via several common intermediates, as C2H4 and EtOH are known to have similar onset potentials, suggesting that they share a common potential-limiting step. 46–48 In addition, experimental observations have shown that the Faradaic efficiencies (FEs) of these two products often shift similarly when different alkaline cations are used in the electrolyte. 49 Calle-Vallejo et al. presented that the bifurcation step between C2H4 and EtOH is the conversion of ∗CH2CHO intermediate. 31 Deoxidation of ∗CH2CHO leads to the C2H4 pathway, while hydrogenation of the intermediate to ∗CH3CHO is the penultimate step of the EtOH formation. 31 Garza et al. proposed that the bifurcation step between C2H4 and EtOH on Cu(100) surface is the conversion of ∗COCHO intermediate (Figure 2B). 34 The formation of ∗COCHOH intermediate leads to the C2H4 pathway, of which the activation energy is 0.49 eV at 1.0 V versus RHE. By comparison, the formation of glyoxal (OHC−CHO) leads to the EtOH pathway, with the activation energy of 0.58 eV at the same electrode. 34 These findings may also explain the origin of a more favorable C2H4 production on Cu surfaces.The role of water has also been explored extensively in the formation of C2H4 and EtOH. Xiao et al. used grand canonical quantum mechanics to predict the detailed mechanisms on Cu(111) and suggested that the surface water may directly donate a proton to the OH group of the carbon-containing intermediate and favor the dehydration reaction (the activation energy is 0.86 eV), thus benefiting the formation of C2H4. 30 In contrast, the dehydration reaction is kinetically blocked by surface water (the activation energy is 1.13 eV), thus hindering the formation of EtOH (Figure 2C). 30 To test the possibility that oxygen in the product might arise from water rather than from CO, Lum et al. conducted electroreduction of C16O in H2 18O electrolyte on oriented Cu surfaces and found that 60%–70% of EtOH contained 18O, which was originated from the solvent. 51 The authors further extended the previous all-solvent density functional theory (DFT) meta-dynamics calculations to consider the possibility of incorporating water, 50 , 52 and found a new mechanism involving water molecules in a concerted reaction with the ∗C−CH intermediate to form ∗CH−CH(18OH), subsequently leading to (18O)EtOH (Figure 2D). 50 By comparison, the conversion of ∗C−CH intermediate into ∗C−CH2 leads to the formation of C2H4, which competes with the formation of EtOH.Acetate is not a major CO2RR product on Cu-based electrodes, and the production of acetate with considerable selectivity at high current densities has only recently been demonstrated in CORR under highly alkaline conditions. 14 , 15 Thus, the mechanistic understanding of the acetate formation from CORR/CO2RR is relatively limited compared with other C2+ products. Birdja et al. proposed that acetate and EtOH seem to be formed from the Cannizzaro disproportionation of acetaldehyde. 53 However, in experiments, the molar production of acetate observed often greatly exceeds that of EtOH in CORR, suggesting that the Cannizarro disproportionation is not a dominant pathway. 15 , 54 To gain more insights about the formation of acetate, the isotopic labeling experiments using C18O were conducted to verify the reaction pathways toward acetate. For instance, Lum et al. performed C16O reduction in H2 18O and found that one O atom in acetate originates from CO and the other O is from electrolyte (Figure 3A). 50 Coincidently, Jouny et al. performed isotopic labeled C18O reduction on Cu in flow cells and gained a similar result that only one O of acetate was labeled, supporting that the O from electrolyte was also involved in the formation of acetate. 50 , 54 Based on the results of isotopic labeling experiments, the authors proposed that the observed acetate consisted of one O originating from CO and one O originating from an OH− anion reacting with intermediate species.To further understand the acetate formation pathway on Cu surfaces, Luc et al. performed DFT calculations and proposed a pathway toward acetate (Figure 3B). 15 This pathway involves water incorporation into ethenone (CH2–CO), a ketene species, to form acetic acid (CH3–COOH). In addition, Li et al. suggested a same mechanism on oxide-derived Cu that the formation of acetate probably arises from attacking of OH− on a surface-bound ketene or other carbonyl-containing intermediate after C–C bond formation. 18 This contention is supported by the observation of substantially increased acetate formation when increasing the KOH concentration from 0.1 to 1 M.In the follow-up work, Jouny et al. demonstrated that C–N bonds can be formed through co-electrolysis of CO and NH3 with considerable acetamide selectivity (Figure 3C). 55 The results verified the mechanism for the acetate pathway since ketene or ketene-like intermediate is a key driving force behind acylation of nucleophilic co-reactants. This ketene-involved mechanism may also explain why more CH3COO− is produced by CORR than CO2RR. The basic principle is that the higher local pH under CORR condition provides more OH− for the formation of CH3COO− than that under CO2RR condition.The elementary steps through which the C3 products are formed have not been well studied yet. Ren et al. presented that the formation of n-propanol is initialed from the intermolecular C–C coupling between CO and C2 intermediates, 56 followed by proton/electron transfer to form propionaldehyde (CH3CH2CHO). 49 , 57 Propionaldehyde is then reduced on the Cu sites to n-propanol. Similarly, Xiao et al. 30 and Zheng et al. 10 also reported that C3 formation can proceed via C–C coupling between C2 and C1 intermediates. Hence, most of the related reports use this mechanism to interpret the formation of n-propanol. 19 , 22 , 26 , 27 , 58 , 59 The coupling mechanism between active C1 and C2 species can be classified into two different modes as CO∗–CH2CHO∗ 51 , 56 , 57 and CO∗–OCCO∗, 19 , 26 , 27 respectively. Among them, the coupling of CO∗ and OCCO∗ is mostly used in CORR, as CO species are abundant on the surface (Figure 3D). 26 Another mechanism of the n-propanol formation was also reported by using CO reaction with acetaldehyde. 60 On Cu surfaces, the inadequate stabilization of ∗C2 intermediates leads to desorption rather than further intermolecular reduction with ∗CO for C3 generation. 58 To ensure the production of C3 at high production rates, C2 intermediates must be formed and stabilized on the catalyst surface and thus be available to be coupled with adsorbed CO. 27 Since the hydrogenation processes are critical reaction steps for the conversion of CO to hydrocarbons and oxygenates, the electrolyte pH may also be essential for CORR. Experimentally, highly alkaline electrolytes have been shown to improve the formation rates of C2+ products in CORR on Cu-based surfaces, 61 while the CH4 formation is favored by a low-pH environment. Xiao et al. predicted the atomic mechanisms underlying electrochemical reduction of CO, 52 finding that (1) at acidic pH, the C2+ pathways are kinetically blocked and the CH4 pathway proceeds through ∗CO → ∗COH → ∗CHOH→∗CH2 → ∗CH3 → CH4; (2) at neutral pH, the C1 and C2+ pathways share the common ∗COH intermediate, where the branch to C−C coupling is realized by a CO−COH pathway; and (3) at high pH, the C−C coupling through adsorbed CO dimerization dominates, suppressing the C1 pathway by kinetics, thereby boosting selectivity for multi-carbon products. Liu et al. found a ∼0.36 V shift in overpotential for C2+ products at pH 7–13, which translates to over three orders of magnitude enhancement in C2+ activity (Figures 4A and 4B). 62 This finding was ascribed to the decreased activation barrier of ∗OCCO protonation with increasing pHs (Figure 4C). 62 However, some reports have indicated that the formation mechanisms of C2H4 and EtOH are not affected by pH. Hori et al. presented that the partial current densities of C2H4 and EtOH as a function of potential exhibited good correlations regardless of the pH value (Figure 4D), 57 indicating that the formation of C2H4 and EtOH are likely independent of the electrolyte pH. 57 In this case, pH can affect the formation of C−C bond by changing the electroreduction activity toward CH4. 30 , 57 , 62 For the same CH4 partial current density, an increase in the local electrode pH shifts the potential to a more negative position on both the RHE and the standard hydrogen electrode (SHE) scales. The suppression of CH4 pathway can subsequently lead to enhanced selectivities toward C2H4 and EtOH. 49 , 57 Li et al. varied the concentrations of Na+ and OH− at the same absolute electrode potential, and demonstrated that higher concentrations of cations (Na+), rather than OH−, exert the main promotional effect on the production of C2+ products. 63 The promotional effect of OH− determined at the same potential on the RHE scale is likely caused by larger overpotentials at higher electrolyte pH. The authors also suggested that highly alkaline electrolytes can be beneficial in improving the CORR performance at the device level due to engineering considerations, as the ion-exchange membrane and oxygen evolution reaction (OER) catalysts favor alkaline over neutral conditions. In addition, higher pH environment may reduce the required voltage in full cell operations, because the equilibrium potential of OER can shift to less positive values but the C−C coupling chemistry remains unaffected. 63 For the acetate pathway, as it is proposed to be formed through attacking of OH− on a ketene-like intermediate, high alkaline electrolyte may favor the acetate production. 18 , 54 Correspondingly, the enhanced acetate selectivity was observed at higher KOH concentrations (Figure 4E). 15 As an important part of electrolyte, the nature of cations in the electrolyte may influence the activity and selectivity of Cu for CORR. Hori et al. reported that alkaline cations affect the CORR selectivity on polycrystalline Cu, 49 suggesting that larger cations favor the formation of C2+ species such as C2H4, EtOH, and n-propanol. Cation effects were explained by Hori et al. in terms of the potential variation in the outer Helmholtz plane, which originates from a difference in the hydration number of different cations. 49 Larger cations are less hydrated and expected to adsorb more easily on the cathode surface, shifting the potential to more positive values, thereby steering the selectivity toward C2H4 instead of CH4. Such experimental observations were confirmed by Kyriacou et al. 64 Later, Pérez-Gallent et al. proposed potential-dependent and structure-sensitive cation effects for CORR on Cu electrodes, 65 and found the presence of larger cations may stabilize the hydrogenated dimer intermediate (OCCOH) at low overpotentials (Figure 5A), 65 thereby promoting the formation of C2H4, especially on Cu(100).By systematically varying the concentration of Na+ and OH− at the same absolute electrode potential, Li et al. found that the chelation of Na+ leads to a drastic decrease in the formation rates of C2+ products (Figures 5B–5D). 63 One possibility was proposed by Bell and co-workers that the existence of alkali cations at the outer Helmholtz plane can offer field-assisted stabilization for intermediates with dipole moments, such as ∗CO, ∗OCCO, and ∗OCCHO. 66 , 67 The modified local electric field at higher cation concentrations can lead to higher densities of “hot spots” that favor the formation of C2+ products. Another possibility is that interfacial water structure is modified at higher concentrations of cations, 68 , 69 which can facilitate the C−C coupling pathway by better solvating the transition state complex.In addition to cations, the concentration or species of anions may influence the CO adsorption, thus influencing the CORR process. For example, Sebastian-Pascual et al. investigated the interfacial properties of Cu(111) and Cu(100) in phosphate buffer solutions in the presence of CO. 70 Combining ab initio molecular simulations with voltammetry experiments, they found that CO adsorbs on the surface in the potential region close to the desorption of phosphate species anions from the electrolyte. The predominant adsorbed species is HPO4∗ on Cu(100), while it is PO4∗ on Cu(111). Due to the lower binding energy of PO4∗ on Cu(111), the adsorption of CO on Cu(111) takes place at less negative potentials than on Cu(100). Ovalle et al. systematically explored the adsorption and desorption of CO on polycrystalline Cu electrodes in the presence of specifically and nonspecifically adsorbing anions at different concentrations. 71 They found that, at an electrolyte concentration of 10 mM, the adsorption and desorption of COatop are virtually independent of the identity of the anions. In contrast, at an electrolyte concentration of 1 M, the COatop coverage is significantly affected by the electrolyte anions, and the saturation coverages of COatop are lower compared with those in 10 mM electrolytes. The magnitude and mechanism of the modulation depend on the identity of anions. Weakly or nonspecifically adsorbing anions (SO4 2−, ClO4 −) limit the COatop saturation coverage by blocking a fraction of CO adsorption sites. Chloride ions, which specifically adsorb on Cu electrodes, can lower the CO coverage by modulating the CO adsorption energy.As the adsorption of intermediates is affected by the surface coverage of CO due to adsorbate-adsorbate interactions, 72 tuning the coverage of adsorbed ∗CO may influence the binding of surface intermediate and subsequently the product selectivities. Sargent and co-workers investigated the influence of CO coverage for the formation of ethylene and oxygenates, and found that lower CO coverages stabilize the ethylene-relevant intermediates, whereas higher CO coverages favor the oxygenate formation (Figure 6A). 61 Noskov and co-workers presented a DFT study on the effect of CO coverage for the CO−CO coupling energy on Cu, and suggested that the CO dimerization barrier becomes lower as the ∗CO coverage increases for all facets (Figure 6B). 73 Zheng and co-workers investigated the influence of CO coverage for the formation of acetate, and found that higher CO coverage on the surface favors the C−C coupling as well as the stabilization of ethenone intermediate, which is a key intermediate for acetate formation, thus promoting the formation of acetate. 14 Generally, alloying is an effective strategy to tune the electronic structure and thus modulate the intrinsic adsorption property of intermediates. 74 For example, Wang et al. demonstrated the improvement of CORR selectivity to n-propanol by an Ag-doped Cu catalyst. 19 As a result of strain and ligand effects, the Ag doping in Cu leads to two classes of neighboring Cu atoms with distinct electronic structures. A pair of adjacent Cu atoms with different electronic structures can act as an C−C coupling active site to promote both C1−C1 coupling and C1−C2 coupling, thus resulting in the formation of C3 products (Figures 7A and 7B). 19 These findings are analogous to the enhanced coupling effect of Cu0 and Cu+, proposed by Xiao et al. 75 Li et al. demonstrated a Pd-doped Cu catalyst to tune the adsorption of hydrogen at the Cu surface and thereby promote the formation of alcohols. 20 The introduction of Pd provides optimal H-binding for alcohol production at neighboring Cu sites, hydrogenating post-C−C coupling of reaction intermediates along the alcohol pathway. The synthesized Pd-doped Cu catalyst achieved a Faradaic efficiency of 40% toward alcohols and a partial current density of 277 mA cm−2 from CORR, which was a 2-fold increase in the alcohol-to-ethylene ratio compared with the bare-Cu catalyst (Figure 7C). 20 Wang et al. demonstrated that planar CuAg electrodes can reduce CO to acetaldehyde with over 50% FE and over 90% selectivity at a modest electrode potential. 21 The FE to acetaldehyde was further enhanced to 70% by increasing the roughness factor of the CuAg electrode. The authors indicated that Ag ad-atoms on Cu weaken the binding energy of the reduced acetaldehyde intermediate and inhibit its further reduction to EtOH, suggesting that the improved selectivity to acetaldehyde is due to the electronic effect from the Ag incorporation. 21 Introducing dopants with stronger CO∗ adsorption than Cu may excessively strengthen the surface CO∗ and promote H∗ adsorption concurrently, 74 , 76 while it is also critical to suppress H2 evolution. To enrich the surface ∗CO coverage and inhibit the competing hydrogen evolution reaction (HER) simultaneously, Zheng and co-workers developed an atomically-ordered CuPd intermetallic compound catalyst composed of a high density of Cu-Pd pairs that feature as catalytic sites, enabling FEacetate of ∼70% and an acetate partial current density of 425 mA cm−2. 14 The combination of Cu and Pd can enrich surface ∗CO coverage, stabilize ethenone as a key acetate-path intermediate, and inhibit HER due to the absence of Pd clusters, thus substantially promoting the acetate formation. In contrast to the ordered CuPd catalyst, aggregated Pd atoms hinder ∗CO reduction as they bind those carbonic species too strongly, leading to the formation of H2 (Figures 7D–7G). 14 In addition to binary alloys, ternary alloys were also used to modulate the intrinsic adsorption property of intermediates during CORR. For example, Wang et al. presented a silver-ruthenium co-doped copper (Ag–Ru–Cu) catalyst, which shows high selectivity, production rate, and stability for n-propanol electrosynthesis. 22 The co-doping of Ag and Ru in Cu induces CO adsorption near the C1–C1 and C1–C2 coupling sites, and thus results in higher ∗CO coverage on the surface than Ag–Cu or Cu, which can promote multiple C–C coupling. Additionally, the adsorption energy of key C2 intermediate for C1–C2 coupling on Ag-Ru–Cu is higher than that on Ag–Cu or Cu; this may reduce the desorption of C2 intermediates, thus increasing the residence of C2 intermediates necessary for C3 generation (Figure 7H). 22 In addition to regulating the adsorption of intermediates through electronic effects, alloying can also affect the catalytic performance by changing the catalyst geometry. For instance, Guan et al. reported a series of Cu-Au alloys with different compositions as CORR catalysts, which enabled substantially different product distributions of CH4 and C2H4 (Figures 7I and 7J). 23 Compared with pure Cu catalyst with a good C2H4 selectivity, the introduction of Au causes steric hindrance and increases the distance between two adsorbed CO intermediates at neighboring Cu sites, thus enhancing the C1 pathway by generating ∗CHO intermediate and leading to the formation of CH4.Single-atomic catalysts have been becoming a research hot spot due to their unique electronic structures of metal atoms coordinated with nonmetal atoms, and a variety of metal single-atomic catalysts have been demonstrated with excellent CORR performances. 24 , 25 For example, Bao et al. developed a Cu single-atomic catalyst anchored to Ti3C2Tx nanosheets for CORR. 24 The atomically dispersed Cu–O3 sites favor C–C coupling to generate the key ∗CO–CHO species, and then induce the decreased free energy barrier of the potential-determining step, leading to a high selectivity for C2+ products (Figures 8A–8D). 24 In addition to the single-atomic catalysts, dual-atomic catalysts are also effective to produce C2+ products, in which metal atom pairs may contribute synergistically to favor the coupling of two CO molecules. For instance, Li et al. developed a dual metal atomic catalyst with uniform distributions of two adjacent Cu-Cu or Cu-Ni atoms anchored on nitrogen-doped carbon frameworks, featuring distinctive catalytic sites for CORR. 25 The dual Cu atomic sites facilitate electroreduction of two CO molecules and subsequent carbon coupling toward ethylene and acetate. The replacement of one of the dual Cu atoms with Ni results in too strong CO adsorption, and thus only one single Cu atom functions as the catalytic site for the C1 reduction pathway (Figures 8E–8G). 25 Hence, the rational design of new single-atomic or dual-atomic catalysts is a potential means to tune the product selectivities of CORR.In 1995, Hori et al. firstly reported a facet-dependent selectivity of CO electroreduction. 77 C2H4 was favorably produced on Cu(100), and CH4 was predominantly yielded on Cu(111). The (110) electrode shows an intermediate product selectivity between (100) and (111). Thus, the synthesis of Cu-based catalysts with high Cu(100) exposure serves as a guideline for designing C2H4-selective catalysts. Using online electrochemical mass spectrometry (OLEMS), Schouten et al. monitored the reduction of CO on Cu(111) and Cu(100) and suggested that C2H4 can be formed via two different pathways. 37 On Cu(100) (Figure 9A), CO is reduced to only C2H4 and not CH4 at relatively low overpotentials, presumably through the formation of a surface-adsorbed CO dimer. On both Cu(100) and Cu(111) (Figure 9B), at higher overpotentials, CO is reduced to C2H4 and CH4 simultaneously, suggesting a shared intermediate. 37 Later, those authors further compared the formation of C2H4 on Cu(100) and Cu(111) at different pH values to explore the two pathways for C2H4. 79 It was found that, although the formation of C2H4 on Cu(111) is clearly pH dependent, it is pH independent on Cu(100). The observed pH independence for the formation of C2H4 on Cu(100) supports the formation of the CO dimer on this crystal facet.Furthermore, the selectivity of acetate can be enhanced by decreasing the surface exposed Cu(100), as the ethenone-involved acetate pathway has no facet preference. 14 For instance, Kang and co-workers reported a Cu nanosheet catalyst with two-dimensional triangular-shaped morphology, which selectively exposed (111) facets. 15 The catalyst showed an enhanced acetate selectivity of ∼48% by suppressing the formation of C2H4 and EtOH (Figures 9C and 9D). 15 As a result of the high exposure of Cu(111), an increase in the CH4 selectivity was also observed on these Cu nanosheets at potentials more negative than −0.7 V versus RHE.In addition to the facet types, the grain boundaries can also influence the CORR catalytic performances. Kanan and co-workers showed that the CO reduction activity is directly correlated with the density of grain boundaries in Cu nanoparticles. 78 The authors prepared Cu nanoparticles with different average grain boundary densities, quantified by transmission electron microscopy, and found that the specific activity for CO reduction to EtOH and acetate was linearly proportional to the fraction of Cu nanoparticle surfaces composed of grain boundary surface terminations, suggesting that grain boundaries alter surface properties of the catalyst to lower the reaction barrier (Figures 9E and 9F). 78 They also suggested that grain boundaries may create surfaces with strong CO binding sites and these surfaces are responsible for the catalytic activity. The properties of grain boundaries can also be used to guide the catalyst design to promote the CORR performance. As the Cu(111) facet is C1-selective, whereas Cu(100) is C2-selective, 46 , 80 , 81 to promote the formation of n-propanol, Pang et al. designed a highly fragmented Cu catalyst composed of a mixture of Cu(111) and Cu(100) facets, thereby creating additional opportunities to couple C1 and C2 intermediates. 26 The highly fragmented Cu catalyst presented an n-propanol selectivity of 20%, and a reaction rate that corresponds to a partial current density of 8.5 mA cm−2 for n-propanol.Among numerous catalyst design strategies, regulation of the catalyst morphology is also a way to affect the CORR performance. For instance, Zhuang et al. developed a method of synthesizing open Cu nanocavity structures with a tunable geometry via the electroreduction of Cu2O cavities formed during acidic etching. 27 The cavity morphology can promote C2−C1 coupling inside a reactive nanocavity via the nanoconfinement of C2 intermediates, thus enhancing C3 formation (Figures 10A and 10B). 27 Wang et al. developed a hierarchical Cu nanoflower electrodes to increase the roughness factor of the catalyst, which showed nearly 100% CORR selectivity for liquid oxygenated products at ‒ 0.23 V versus RHE, although the corresponding partial current densities remained relatively low (Figures 10C–10E). 17 The authors demonstrated that the increased roughness factor of the electrode improves the selectivity for C2+ oxygenates by not only enabling operation at low overpotentials but also by suppressing competing HER. 17 Oxide-derived Cu (OD-Cu) has often been reported to present higher CORR selectivities toward C2+ products than polycrystalline Cu. 18 By using in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), Lee et al. showed that CO binds more strongly on the surface of copper oxides than pure metallic Cu. 82 As a result, CO can be densely populated on the surface and subsequently favor C−C coupling toward C2+ products. 82 Kanan and co-workers presented that nanocrystalline Cu prepared from Cu2O (i.e., OD-Cu) produces EtOH, acetate and n-propanol with a total FE up to 57% at modest overpotentials. 18 By comparison, Cu nanoparticles with an average crystallite size similar to that of OD-Cu produce nearly exclusive H2 under identical conditions. The authors demonstrated that the enhanced CORR activity on OD-Cu electrodes relative to Cu nanoparticle electrodes is consistent with the presence of highly active sites on their grain boundary surfaces, 18 suggesting that engineering the grain boundaries by altering the oxide reduction method of nanocrystalline materials can improve activity and selectivity toward long-chain products.Because of the low solubility of CO (∼1 mM) in H2O, the geometric current density achieved in solution-phase CORR is on the order of 1 mA cm−2. 13 Gas diffusion electrodes (GDEs) have been designed to overcome the solution-phase mass-transport limits by creating gas-solid-liquid triple-phase boundaries, comprising CO, Cu-based catalysts, and electrolytes. The use of GDEs can enhance the CORR current densities by at least one order of magnitude. For example, using GDEs, Jouny et al. presented a high CORR performance with a well-controlled electrode-electrolyte interface, with a total current density of up to 1 A cm−2, 54 suggesting attractive potentials of developing CO electrocatalysis in those systems. From a techno-economic perspective, commercially relevant productivity requires a current density of >200 mA cm−2. 83 , 84 Hence, the utilization of GDE-based systems is necessary for the CORR research.To increase the single-pass conversion rates and enlarge product concentrations, various design strategies have been explored for GDE-involved reactors. Kanan and co-workers investigated CO electrolysis with GDEs supplied by inter-digitated flow fields in electrochemical cells with different ion transport properties. 13 Using a cell with the GDE directly contacting with a Nafion membrane, the authors demonstrated >100 mA cm−2 partial current density for CO reduction to C2+ products, and direct production of 1.1 M acetate at a cell potential of 2.4 V for over 24 h (Figures 11A–11C). 13 Wang and co-workers reported continuous generation of high-purity (up to 96%) acetic acid solutions via CORR on Cu nanocube catalysts in a porous solid electrolyte reactor. 85 Different from conventional liquid electrolyte, the porous solid electrolyte layer can efficiently conduct ions and does not introduce impurity ions into the generated liquid products. The porous solid electrolyte reactor can enable the continuous generation of electrolyte-free acetic acid solutions, with a current density of 1 A·cm−2 and a high acetic acid purity up to ∼96% (Figures 11D–11G). 85 Renewable-energy-powered electrocatalysis using Cu-based catalysts has been demonstrated as an attractive approach for converting CO2 or CO into multi-electron-transferred chemicals. Compared with direct conversion of CO2, a two-step cascade approach, in which CO2 is initially reduced to CO and subsequently into C2+ products, has been emerging as a carbonate-formation-free platform that can be operated under high-alkalinity conditions. In the past few years, significant progress has been achieved in CORR, including mechanistic studies, catalyst design, and system development. With the exploration of the reaction and further understanding of mechanism, it is promising to design Cu-based catalysts to selectively produce different C2+ products. The utilizations of GDE-based electrolyzers have exhibited a practical application prospect for CORR in chemical industry. Despite the achievements thus far, several key challenges that preclude the large-scale deployment remain to be solved. Below, we suggest some perspectives to push this technology into the next level.First, it is still difficult to know the comprehensive dynamic process and reaction mechanism of CORR. To understand the reaction mechanisms and further optimize catalyst performances, it is crucial to conduct operando measurements of reaction intermediates and catalytic products under experimental conditions, especially at high current densities. Thus, the development of techniques that can identify these reaction intermediates under in situ and operando conditions are highly desired. Second, computational investigation is a powerful tool to accelerate mechanism understanding. However, it is challenging for the present computational models to precisely describe comprehensive electrolyte-catalyst interfaces. Hence, development of new computational models that can accurately simulate solvated cations, protons, and hydroxide ions at the reaction interface is crucial to the CORR mechanism development. Third, to meet the needs of commercially relevant performances, the stability of CORR should be further promoted by rational designs applied to both catalysts and reaction systems. Finally, to reduce subsequent separation cost, it is critical to optimize the catalyst selectivity for increasing the purity of single C2+ products, which also requires substantial development in the design of reactors.The authors thank the following funding agencies for supporting this work: the National Key Research and Development Program of China (2018YFA0209401, 2017YFA0206901), the National Natural Science Foundation of China (22025502, 21975051), the Science and Technology Commission of Shanghai Municipality (21DZ1206800), and the Shanghai Municipal Education Commission (2019-01-07-00-07-E00045).G.Z. conceived the article’s structure and supervised this work. G.Z., Y.J., and A.G. collected the literature, wrote the original manuscript, and participated in discussions and revisions of the manuscript.The authors declare no competing interests.We support inclusive, diverse, and equitable conduct of research.

Electrochemical reduction of carbon monoxide has recently emerged as a potential approach for obtaining high-value products. Recent studies have shown that carbon monoxide can be electrochemically reduced to C2+ at high reaction rates, high selectivity, and inherently improved stability compared with carbon dioxide, highlighting the attractive potential of the electrocatalytic carbon monoxide reduction reaction (CORR). This review introduces recent progress in CORR mechanistic understanding and representative strategies for the design of copper-based electrocatalysts, such as alloying and doping, single-atom catalysts, crystal facet and morphology design, and oxide-derived copper. Several critical factors that influence the CORR activity and selectivity are also discussed, including pH, electrolytes, and the design of electrolysis reactors. Finally, the challenges and perspectives for further development in this field are summarized.

Atmospheric methane (CH4) from various sources, including biogases and natural gas leaks, is a significant greenhouse gas (GHG) with greenhouse effect much severer than carbon dioxide (CO2) [1–3]. The conversion of biomethane to hydrogen (H2) and useful solid carbon materials can substantially mitigate GHG emission by the carbon-negative effect and meanwhile supply renewable H2 to economic development. However, methane as the most stable hydrocarbon requires extremely high temperature (~1200 °C) for noncatalytic thermal decomposition, which produces H2 and low-value carbon black by (1) CH 4 g ↔ C s + 2 H 2 g , ∆ H r o ≈ 75 kJ / mol In the past years, continued research efforts have been made on catalytic decomposition of methane (CDM) aiming to reduce reaction temperature, enhance conversion rates, and control the morphology of carbon products. In the literature, metallic nanoparticle catalysts are commonly used for CDM at drastically lowered temperatures to form multiwalled carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs). However, CDM is an endothermic and volume-increasing reaction that is still often performed at high temperatures (>700 °C) and far below atmospheric pressure to kinetically and thermodynamically favor the CH4 conversion (χ CH4) [4–12].In general, carbon deposition and CNT growth from hydrocarbons on metallic catalysts involve multiple steps, including (i) metal surface-catalyzed dehydrogenation and carbonization requiring an activation energy (E a, s ), (ii) activated carbon (C) dissolution and diffusion through the metal phase, and (iii) dissociation of metal‑carbon bonds to release graphitic carbons such as graphene layers, CNT, and CNF [6,11,13]. The kinetic and thermodynamic behaviors of hydrocarbon decomposition and carbon formation are thus determined by the E a, s , the C-metal binding energy (∆H f, MC o ), diffusion activation energy (E a, d ), and carbon solubility (S CM ) in the metals. Metals with very large negative ∆H f, MC o values cause transport limitation by the excessive C-metal binding strength while metals with positive ∆H f, MC o lead to reaction limitations due to hindered dissolution and removal of the carbon products from catalyst surface. Thus, metals exhibiting moderate ∆H f, MC o and ∆E a, s and adequate S CM , such as Fe, Ni, and Co, are well-suited for chemical vapor deposition (CVD) of hydrocarbons [14–17]. Also, the ∆E a, s of hydrocarbon dissociation is greater for terrace sites than for step edge sites that results in preferential formation and dispense of graphene layers at the step edges leading to CNT growth on nanoparticles [13,18–21].Non-metallic catalysts, especially metal oxides, could also significantly lower the temperature for CDM but often produce carbon nanomaterials in more disordered structures [4,22]. The mechanisms of CNT formation on oxides are still not fully understood but the insolubility of carbon in solid oxides could cause transport limitation to hinder the formation of long CNTs and CNFs. The CNT formation on various oxides, e.g., SiO2, ZrO2, TiO2, Fe2O3, and Al2O3, have been investigated by in situ spectroscopic characterizations [20,23–25]. It is generally believed that carbonaceous radicals first form by chemisorption and dehydrogenation on the oxide surface and then migrate via surface diffusion to the catalyst tip where supersaturation is achieved to form and dispense CNTs [25–27].In recent studies, mixtures containing carbon monoxide (CO) and CH4 have been demonstrated for CVD conversion to MWCNTs at dramatically lowered temperatures (~ 500 °C) on in situ generated Fe metallic nanoparticles and partially reduced ferrite-type nanoparticle catalysts [22,28]. Such a mixed feed is of practical interest because of the potential to couple with the syngas production from CO2 dry reforming of CH4, i.e., CO 2 + CH 4 ↔ 2H 2 + 2CO [29]. The CVD conversion of the CO/CH4 mixtures apparently involve simultaneous endothermic CDM and the following highly exothermic CO-disproportionation, i.e., the Boudouard reaction, (2) 2 CO g ↔ C s + CO 2 g , ∆ H r o ≈ − 172 kJ / mol Pure CO is a precursor for growing high-purity CNTs on iron-based nanoparticles, which also involve CO surface dissociation, formation of metal carbide, and subsurface carbon transport [30,31]. The metal or oxide catalyzed CVD of pure gases were found to exhibit overall first order for CH4 and second order kinetics for CO at low conversions [32,33]. However, the overall rate of carbon formation by reaction of CH4/CO mixtures on the Fe metal nanoparticles was found to exhibit a linear dependency on the square root of the product of reactant concentrations (i.e., [CH4]0.5·[CO]0.5) [4]. These indicate that synergistic effects between CH4 and CO have caused the mixture reaction kinetics to drastically deviate from those of the pure gas reactions on the catalysts.Here, a Cr-doped ferrite (FeCr) nanocrystalline catalyst is demonstrate for CDM to H2 and MWCNTs with cofed CO at a low temperature of 500 °C. The nanocrystalline FeCr catalyst is a well-known H2S-resistant catalyst for water-gas shift (WGS) reaction with intrinsic stability in H2O and CO2 atmospheres [34,35]. These basic properties of the FeCr catalyst are highly desirable for conversions of biomethane, which commonly contain H2S impurity. In our recent studies, the FeCr nanoparticles were found to induce MWCNT formation while suppressing methanation during WGS reaction at 500 °C under high pressure and H2-lean conditions [22]. The FeCr-catalyzed MWCNT formation in WGS conditions appeared to be associated with the byproduct CH4 and methanation intermediates. The involvements of both CH4 and CO were further evidenced by the rapid conversion of a CH4(20%)/CO(80%) dry mixture to H2 and MWCNTs at conditions corresponding to the WGS reaction. The present work focuses on experimental investigation on the synergistic effects of CH4 and CO on the FeCr-catalyzed CVD conversion of CH4/CO mixtures at a low temperature of 500 °C and atmospheric reaction pressure.The nanocrystalline Cr-doped iron oxide (FeCr) catalyst with an atomic ratio of Fe:Cr = 10:1 was synthesized by the coprecipitation method from a mixed salt solution as explained in our earlier work [36]. The starting solution for coprecipitation contained iron nitrate and chromium nitrate precursors with Fe:Cr atomic ratio of 10:1. An aqueous ammonia solution was added to the mixed salt solution under stirring to induce solid precipitation until completion at pH around 9. The precipitated solids were recovered by filtration and subsequently dried at 80 °C for 12 h. The dried particles were then calcined at 500 °C in air for 3 h using heating and cooling rates of 5 °C/min. The calcined solid was ground intensively and mesh-sieved to remove the large agglomerates. The microscopic images and elemental analysis results, which are presented in later sections, showed that the actual primary grain size and Fe:Cr atomic ratio of the as-synthesized FeCr oxide were 15–25 nm and ~ 10, respectively.The CVD reaction was carried out in a packed-bed reactor (PBR) made of a fused silica-coated alumina tube with outer and inner diameters of 0.57 cm and 0.39 cm, respectively. The tube had a total length of 10 cm with each end mounted with a ~ 1 cm-thick multilayered glass cloth (Fig. 1 ). In most cases, 0.1 g of catalyst particles was distributed on loosely packed quartz wool along the tube reactor. After the PBR was installed in a programmable furnace with a 1.5-m long preheating coil. The catalyst was activated by partially reducing the hematite (Fe2O3) to magnetite (Fe3O4) in a processing gas flow containing H2, CO, CO2, and H2O, which had a reductant to oxidant ratio of R/O = 1.4, where R/O = (p CO  + p H2)/(p CO2 + p H2O ) [34,36]. The activation process was performed at 400 °C for 4 h using heating and cooling rates of 5 °C/min to achieve partial reduction of Fe3+ to Fe2+ without formation of metallic Fe and crystallite growth [22,35]. The catalyst bed was then purged with pure N2 gas at a flowrate of 20 cm3 (STP)/min for at least 3 h while the temperature was increased to 500 °C and stabilized before switching to a reactant gas flow.The entire reaction system was similar to that described in our previous reports [22,37]. It included mass flow controllers for regulating the feed flowrate and composition and an online GC–MS (GC, Agilent GC 6890 N and MS Agilent 5975B) for analyzing product gas compositions. The GC was equipped with a thermal conductivity detector (TCD). The exiting gas flowrate was also monitored by a film flowmeter at a 10-min interval for verification. A digital pressure gauge was used at the reactor entrance while the PBR exit remained at atmospheric pressure. The actual temperature fluctuation of the furnace hosting the PBR was ±3 °C over the reaction durations. The efficient heat exchange through the small-diameter ceramic tube was able to stabilize the gas temperature at the PBR exit, which varied between 495 and 510 °C depending on the feed gas composition.The gas reactions were performed in the PBR immediately after the catalyst activation process. The reaction temperature and exiting pressure were 500 °C and ambient pressure (1.013 bar), respectively. The feed pressure at the PBR entrance was typically around 1.10 bar, which is slightly higher than the ambient exiting pressure to drive the gas flow. The reaction experiments were conducted for feed gases with CH4 mole fraction varying from 100% (i.e., pure CH4, 99.99%) to 0% (i.e., pure CO, 99.9%). The total feed flowrates were kept at around 16 cm3 (STP)/min for mixtures. In most cases, the reaction was terminated when the feed entrance pressure exceeded 1.5 bar under the fixed gas flowrate because of the increased flow resistance from blocking effect of the growing carbon deposits. After terminating the reaction, the reactor was cooled down to room temperature in a continued N2 flow. The solid products, i.e., the carbon-deposited catalysts, were immediately retrieved and sealed in glass vials to minimize possible oxidation by air before various ex situ characterizations.Pure CO and CH4 are common precursors for CNT production on metallic iron-based catalysts usually at temperatures much higher than 500 °C [38]. The low-temperature FeCr-catalyzed reactions were tested for pure CH4 and CO, respectively, with and without the pretreatment with the other. These experiments were performed to study the effects of pretreatment in one pure gas on the catalyst's activity in subsequent reaction with the other.Experiments were first carried out for CH4 and CO single gas reaction without pretreatment in the other. The feed flowrates were kept at ~5 m3 (STP)/min for pure gas reactions. The volume increasing CDM reaction is thermodynamically favored at low pressures. Thus, the reaction was tested for CH4 partial pressure varying from 0.05 bar to 1.013 bar in balancing N2, i.e., molar fraction of CH4 in the CH4/N2 mixture increasing incrementally from 5% to 100% at atmospheric pressure. The endothermic CDM is expected to be kinetically and thermodynamically limited at the low temperature of 500 °C. Hence, pure CH4 reaction was tested for 20 h and 126 h, respectively, to ensure observation of the final carbon morphology. The CO-disproportionation reaction is exothermic and volume-decreasing and therefore, is thermodynamically favored at low-temperature and high pressure. The reaction of the pure CO was conducted at atmospheric pressure (1.013 bar) and 500 °C for 20 h.To investigate the effects of catalyst pretreatment in one gas on the subsequent reaction with the other, experiments were conducted by switching feeds of pure CH4 and CO, respectively. All reactions were performed at 500 °C and 1.013 bar. For studying the effects of CO-pretreatment on the subsequent reaction with CH4, the pure CO flow was first fed a flowrate of 10 cm3 (STP)/min for 30 min; then the catalyst bed was purged by pure N2 gas at 10 cm3 (STP)/min for 1 h; and finally, the pure CH4 gas was fed at a flowrate of 10 cm3 (STP)/min for 21 h. For studying the effects of CH4-pretreatment on the subsequent reaction with pure CO, the pure CH4 was first fed at a flowrate of 10 cm3 (STP)/min for 30 min; the catalyst bed was then purged by pure N2 gas at 10 cm3 (STP)/min for 1 h; and finally, the pure CO gas was fed at a flowrate of 10 cm3 (STP)/min for 21 h. The flowrates and compositions of the exiting gas were monitored during the entire processes. The final solid products were retrieved and characterized by microscopic examinations and chemical analyses.The gas products of the gas reactions were monitored by the online GC, and the solid products including carbon materials and reacted catalyst particles were retrieved and examined by various ex situ characterizations. The crystalline phases of the catalysts were identified by X-ray diffraction (XRD) using a PANalytical X'Pert Pro diffractometer with Cu Kα radiation (λ = 1.5406 Å) and, in some cases, confirmed by transmission electron microscopy (TEM) and electron diffraction (ED). The morphology and dimensions of the solid materials were observed by SEM and TEM and their elemental compositions were determined by the energy dispersive X-ray spectroscopy (EDS) technique. The SEM-EDS analyses were performed by a FEI Scios DualBeam microscope equipped with Ametek Octane Super EDAX. The catalysts at different processing stages were analyzed by X-ray photoelectron spectroscopy (XPS) to study the metal oxidation state variations with reaction conditions. The XPS experiments were performed on a Thermo VG Scientific spectrometer using Al-Kα (1486.7 eV) radiation as the excitation source at room temperature. The pressure of the catalyst chamber was maintained below 10−8 Pa to avoid a large amount of noise in the spectra from contaminants. The obtained binding energies were adjusted by referencing the spectra to the carbon (C 1 s) peak at 284.6 eV. The carbon products were also characterized by Raman shift spectroscopy for assessment of the graphitic phase and crystallinity.The gas and solid products of the FeCr-catalyzed conversion of CH4/CO mixtures had consistent types of gas components and carbon morphology for varied CH4 mole fraction in the feeds (y f, CH4). The gas products contained CH4, CO, CO2, and H2, which were expected from the two primary reactions (1) and (2). Although H2O was not discernable by the TCD-equipped GC, the following secondary reactions likely coexisted [39]: (3) CO 2 g + H 2 g ↔ CO g + H 2 O g , ∆ G r o ≈ + 13 kJ / mol (4) CO 2 g + CH 4 g ↔ 2 CO g + 2 H 2 g , ∆ G r o ≈ + 40 kJ / mol (5) CO g + H 2 g ↔ C s + H 2 O g , ∆ G r o ≈ − 18 kJ / mol where the ∆G r o values are for standard pressure of 1 bar at 500 °C. These secondary reactions cause interdependences between the CH4 and CO reactions because of the participation of H2 and CO2 from their primary reactions (1) and (2). However, the CO2 reduction by H2 (eq. 3) and CO2-reforming of CH4 (eq. 4) are expected to be insignificant because they are thermodynamically unfavorable under such low temperature with low pressures of H2 and CO2. The involvement of CH4 steam reforming is not considered in this case because of the minimal amount of H2O from the very minor reactions (3) and (5) under low χ CH4.The FeCr catalyst was first examined by SEM (Fig. S1), TEM, and EDS, which confirmed its primary particles size range of 10–25 nm and an overall Fe:Cr atomic ratio of ~10.6 (±5%) (Fig. 2a). The XPS examination results (Fig. 2b) verified the partial reduction of Fe3+ to Fe2+ and Cr6+ to Cr3+ was achieved by activation in the processing gas (R/O = 1.4) without forming Fe0 or Cr0 [22,35]. In Fig. 2(c), the XRD pattern of the activated catalyst represents a combination of the Fe2O3 (hematite) and Fe3O4 (magnetite) spectra. There were no appreciable peaks to suggest any segregated phases of chromium trioxide, iron chromate, or chromium ferrite in the FeCr catalyst [40].The activated FeCr nanoparticle catalyst was able to actively catalyzed CH4 conversion to H2 and carbon nanomaterials in the entire range of y f, CH4. However, the morphology of the carbon products and conversions of CH4 (χ CH4) and CO (χ CO ), as defined by Eq. (6), were found to depend on the y f, CH4 and differ markedly between the pure gases and mixtures. (6) χ i = 1 − moles of i in product moles of i in feed i = CH 4 CO The results of SEM and Raman tests for the catalysts after reactions with pure CH4 and pure CO are presented in Fig. 2 (d-f). The carbon products from CH4 (Fig. 2e) and CO (Fig. 2f) were both of graphitic phases and primarily in particulate and rod-shape structures. However, for the same 20-h reaction duration, the amount of carbon from the pure CH4 reaction (Fig. 2d) was far less than that from the CO reaction (Fig. 2f). The carbon product from CH4 reaction developed into large particles with some small fiber-like structures formed after an extended reaction time of 126 h (Fig. 2e) that led to significant increase of the G-band Raman peak intensity. The fiber-like structures were not seen in the products from CO reaction under the same conditions.The mixed feed of CH4 and CO dramatically changed the morphology of the carbon product as compared to the CH4 and CO single gas reactions. As shown by the SEM images in Fig. 3 , the reactions of CH4/CO mixtures resulted in formation of MWCNTs, and the content of MWCNT in the products appeared to depend on the y f, CH4, or CO content (y f, CO =1 − y f, CH4) in the feed. The Raman spectra in Fig. 3 revealed that the crystallinity of the graphitic carbon deposits was also affected by the feed composition. The addition of 5 mol% CO in the feed (i.e., y f, CH4 = 95%) resulted in the formation of MWCNTs as the main carbon product with a small amount of graphitic carbon particles (GCPs). Increasing the y f, CO (i.e., decreasing y f, CH4) in the feed generally led to higher crystallinity for the carbon products as evidenced by the decreasing ratio of D/G peak intensities (I D /I G ). The carbon products were predominantly of MWCNT structure for y f, CH4 varying from 10% to 90% but had significant amounts of GCP for y f, CH4 of 95% and 5%.The TEM images in Fig. 4 unveiled distinct microstructures of the GCPs and MWCNTs in the products of the CH4/CO mixture reactions. In Fig. 4(a), the TEM image on the left shows the coexistence of long MWCNTs and GCP clusters. Compared to the activated FeCr (Fig. 2b), the XPS spectrum exhibited a new peak of metallic iron (Fe0) at around 708 eV in the Fe 2p3/2 envelope (Fig. 4a, right) [22]. Fig. 4(b) shows a HR-TEM image of the catalyst particle at a MWCNT tip, which suggested the formation of metallic Fe in the FeCr catalyst when reacting with the mixtures. The selected area ED pattern (Fig. S2a) was also characteristic of superimposed patterns of nanoscale ferrite, metallic Fe, and graphene [42–44]. In contrast, the TEM (Fig. 4c) and ED pattern of the catalyst/GCP cluster (Fig. S2b) reflected randomly gathered nanoscale ferrite structures [45]. The EDS results showed slightly higher Fe:Cr atomic ratio (~13.1) in the sample of MWCNT and GCP product (Fig. S2c) than the ratio of ~10.6 before reacting with the CH4/CO mixture. This might be caused by Cr6+/Cr3+ segregation from the ferrite solid solution during the formation of Fe0 cluster. The segregated chromium oxides could remain in the exposed oxides to be washed away by the acidic cleaning solution.The microstructures of the MWCNT in Fig. 4(b) and GCP in Fig. 4(c) remained unchanged in the entire range of y f, CH4 but the relative amounts of MWCNTs and GCPs apparently varied with y f, CH4 according to the SEM observations in Fig. 3. The MWCNTs were the principal component in products from feeds with y f, CH4 between 10% and 90% while GCPs were the main component in products from feeds with y f, CH4 of 95% and 5%. The TEM images in Fig. 4(b) from a representative area show that the diameters of the MWCNTs were in a range of 15–35 nm with unfirm wall thicknesses of 5.0 ± 1.0 nm. The MWCNT diameters were dependent on the sizes of the FeCr nanoparticles which ranged between 10 and 25 nm. The individual graphene layers were longer and more ordered in the MWCNTs (Fig. 4b) than in the GCP structures (Fig. 4c).The average χ CH4 and χ CO were determined for varied y f, CH4 over the first hour and second hour of reaction, respectively, and the results are presented in Fig. 5 . The control and stabilization of the vary low CO and CH4 flowrates needed for y CH4 above 95% and below 5%, respectively, were challenging. Thus, mixture reactions were performed for y f, CH4 ranging from 5% to 95%. The experimentally measured conversions had large deviations, typically ±20%, mainly due to gas flowrate fluctuations caused by compaction, partial blockage, and movement of the catalyst bed under fast accumulation of carbons. Nevertheless, as shown in Fig. 5, the general trend of χ CH4 changing with y f, CH4 could be observed with reasonable confidence while the dependence of χ CO on y f, CH4 was less obvious.For CH4 and CO single gas reactions, the average χ CH4 (~2.1%) was much smaller than the χ CO (~25.0%) over the 2-h reaction time (Fig. 5a and b). This large difference of conversion can be explained by the fact that CH4 decomposition is endothermic with an equilibrium conversion (χ e, CH4) of 16.8% while the Boudouard reaction of CO is highly exothermic with equilibrium conversion (χ e, CO ) of 92.1% at 500 °C and atmospheric pressure (Fig. S3). The reaction rate of the CH4 (r CH4) was also much lower than that of the CO (r CO ) with the average r CH4 and r CO over the first hour of reactions estimated to be 0.83 and 15.4 mol/g-cat·h (Fig. 5c), respectively.For reactions of CH4/CO mixtures, besides the morphological change of carbon products from GCPs to MWCNTs, the χ CH4 and r CH4 tended to increase substantially with y f, CH4 up to y f, CH4=95% (Fig. 5a). The addition of small amounts of CO also resulted in drastically greater χ CH4 as compared to that of the CH4 in the inert N2 with the same y f, CH4. For example, the feed with y f, CH4 = 95% (i.e., with 5% CO) had an average χ CH4 ~12%, which was more than five times the χ CH4 for pure CH4 (χ CH4 ~2.1%,). The χ CH4 (<2.0%) was independent of the y f, CH4 for CH4 in N2 but was strongly dependent on y f, CH4 for CH4 in CO (Fig. 5a). The CO-promoted enhancement of χ CH4 weakened as y f, CH4 decreased and eventually diminished at y f, CH4 < 10% (i.e., y f, CO  > 90%). The dependencies of χ CH4 and χ CO on the y f, CH4 suggest that r CH4 and r CO were significantly affected by the mixture feed composition. As can be seen from Fig. 5(c) that the average r CH4 and r CO over the first hour reactions were substantially enhanced by adding small amounts of CO and CH4, respectively. Such a kinetic enhancement for each component declined with increasing the content of the other. Also, for both CH4 and CO, the conversions in the first hour were generally higher than in the second hour of reaction that were expected by the decrease of catalyst surface accessibility due to the growing carbon deposits on the catalyst surface.The above observed effects of CH4/CO feed composition on the gas conversions (χ i ), reaction rates, and produced carbon morphology indicate strong synergistic effects between the CH4 and CO reactions on the FeCr catalyst. These synergistic effects may include thermal and chemical states and interactions of produced gases at the catalyst surface. Firstly, the highly exothermic reaction of CO could increase the catalyst surface temperature to enhance the endothermic CDM both thermodynamically and kinetically. Meanwhile, the CO-facilitated CH4-decomposition could thermodynamically benefit CO conversion by timely consuming the heat generated at the catalytic surface. These mutual benefits of reaction heat effects likely played significant roles in enhancing the χ CH4 and r CH4. Secondly, the coexistence of CH4 and CO with the resultant H2 may help to further reduce the ferrite surface (Fig. 4b) that consequently altered the chemisorbed intermediates and the morphology of the carbon deposits.To study the mutual influences between CH4 and CO reactions on the FrCr catalyst surface, the activated FeCr catalyst was pretreated for 30 min in pure CH4 or CO before reacting with the other. The CH4-pretreated catalyst produced predominantly MWCNTs with a few GCPs when subsequently reacting with the pure CO (Fig. 6a and b). However, the CO-pretreated catalyst generated primarily GCPs with no appreciable MWCNTs in the subsequent reaction with pure CH4 (Fig. 6c and d).The χ CH4 on the CO-pretreated catalyst was <0.5% (Fig. 6d), which was drastically lower than that on the fresh catalysts shown in Figs. 6(b) and 5(a). The χ CO on the CH4-pretreated catalyst presented in Fig. 6(b) was also significantly lower than those on the fresh catalyst for pure CO (Figs. 6d and 5b) or mixtures of low y f, CH4 but comparable to χ CO for mixtures of high y f, CH4 (i.e., low y f, CO ) (Fig. 5b). These decreases of χ CH4 and χ CO can be attributed to the reduced catalytic site accessibility by carbon deposits from the CH4 and CO pretreatments and the lack of beneficial heat effects existing in mixture reactions. The CO-pretreated catalyst suffered more severe activity loss for the subsequent CH4 reaction apparently because of the thicker carbon deposits from the fast CO reaction.The chemical and morphological properties of the pretreated catalyst surface play key roles in determining the structures of the carbon products during the subsequent reactions with different gases. The SEM and EDS examinations revealed similar textures for the catalyst particles after the 30-min pretreatment in CH4 (Fig. 7a) and CO (Fig. 7b), respectively. However, The EDS results in Fig. 7(a) and (b) show that the CO-pretreated catalyst had a substantially greater amount of carbon deposits (C/Fe ~ 1.8) than the CH4-pretreated catalyst (C/Fe ~ 0.8). This difference was expected because the endothermic CH4 reaction (eq. 1) was severely limited both kinetically and thermodynamically at 500 °C (Fig. 5). The thicker carbon deposits could cause greater blockage of the catalytic sites that led to more drastic reduction of χ CH4 on the CO-pretreated catalyst as compared to the decrease of χ CO on the CH4-pretreated catalyst.The XPS spectra for catalysts after the pretreatments by CO and CH4 are presented in Fig. 7(c-f). The Fe 2p spectra of the catalysts in Fig. 7(c) show two distinct envelopes at 710.2–711.0 eV and 723.8–724.3 eV, which are assigned to the Fe 2p3/2 and Fe 2p1/2 sub-levels, respectively. The deconvoluted Fe 2p XPS spectra exhibited two main peaks and a satellite peak for each sub-level, which are related to the contributions from both Fe2+ and Fe3+ ions. The peaks at 709.9–710.6 eV and 723.3–723.9 eV are assigned to Fe2+ states, while the peaks at 712.5–712.8 eV and 725.5–726.8 eV are assigned to Fe3+ ions. Furthermore, the satellite peaks at 718.2–719.2 eV and 732.0–732.7 eV are attributed to both Fe2+ and Fe3+ ions. The spectrum suggests that Fe2+ and Fe3+ ions coexisted on the surface of the Fe/Cr catalyst after the pretreatments in CO and CH4. The CO-pretreated sample showed a blue shift in the binding energy of Fe 2p as compared to the CH4-pretreated sample. Such a shift may be attributed to the higher population of Fe3+ ion on the surface of CO-pretreated sample, which is confirmed by the higher Fe3+/Fe2+ ratio found on the CO-pretreated catalyst as compared to the CH4-pretreated sample (Table 1 ). This confirms that CH4 is a stronger reductant than CO under the current reaction conditions. The XPS spectrum-based estimate of chemical composition was achieved by the area integration protocol described in our previous work [46]. The Fe 2p spectra are accompanied by the broadness of satellite peaks that may have caused the relatively large uncertainty in chemical quantification [47].In the FeCr solid oxide solutions of low Cr contents, the Fe3+ cations are substituted by the doped Cr ions in the ferrite lattice, which is evidenced by the XRD results in Fig. 2(c). The Cr dopant is known to promote the redox cycles of Fe 3+ ↔ Fe 2+ that in turn enhances the catalytic activity of ferrite [46,48]. The Cr 2p XPS spectra of the samples in Fig. 7(d) show the Cr 2p3/2 and Cr 2p1/2 sub-bands. Each sub-level can be divided into two peaks in which the peaks at ~576 eV and ~ 586 eV are ascribed to the Cr3+ states and the peaks at ~578 eV and ~ 587 eV correspond to the Cr6+ states. The Fe 2p binding energy of CO-pretreated sample was higher than that of the CH4-pretreated sample, which could be caused by the presence of more Cr6+ ion on the surface of CO-pretreated sample. This observation is consistent with the fact that surface Cr6+/Cr3+ ratio of the CO-pretreated catalyst was greater than that of the CH4-pretreated catalyst (Table 1). Like the larger Fe3+/Fe2+ ratio in the CO-pretreated sample, the greater Cr6+/Cr3+ ratio of the CO-pretreated catalyst is also attributed to the relatively weaker reducing power of CO than CH4 under the specific conditions.The single-phase hematite structure of the preactivated fresh FeCr catalyst (Fig. 2c) suggests that the following isomorphous substitution for Fe3+ by Cr6+ occurs to create Fe3+ vacancies (V Fe(III)‴), (7) Cr O 3 → Fe 2 O 3 3 O O X + Cr VI Fe III • • • + V Fe III ‴ , ∆ G T o , ∗ The XPS findings of the Fe and Cr oxidation states after activation in the processing gas confirm the transition of hematite to magnetite by partial Fe 3+  → Fe 2+ reduction, e.g., 1 2 Fe 2 O 3 + 1 2 H 2 → FeO + 1 2 H 2 O , ∆ G T o , ∗ , that may be facilitated by the extinction of V Fe(III)‴ via reduction of Cr6+ (Cr(VI)) to Cr3+ (Cr(III)), (8) 3 2 O O X + Cr VI Fe III • • • + V Fe III ‴ + 3 2 H 2 → Cr III Fe III X + 3 2 H 2 O , ∆ G T o , + Although the CO and CH4 can also reduce the above oxides at sufficiently high temperatures, H2 is expected to be the most effective reductant at the low temperature. Thus, the amount of H2 and the content of Cr dopant are critical to the extent of FeCr catalyst reduction and the following equilibrium concentrations of FeO ([Fe2+]) and Cr(III) Fe(III) X ([Cr3+]) are defined by thermodynamics principles, (9) Fe 2 + = Fe 3 + ∙ p H 2 p H 2 O 1 / 2 ∙ exp − ∆ G T o , ∗ RT (10) Cr 3 + = Cr VI Fe III • • • 2 ∙ O O X 3 / 2 ∙ p H 2 3 / 2 p H 2 O 3 / 2 ∙ exp − ∆ G T o , + RT The presence of water vapor on surface may limit the reductions of metal ions and prevent the formation of metallic Fe0, which was confirmed in our previous work on MWCNT formation in H2-lean water gas shift reaction environments containing CH4 and CO [22].For the current FeCr catalyst, metallic Fe was not observed after being pretreated with the pure CH4 and CO but was found after reacting with the CH4/CO mixture (Fig. 4a and b). This may be caused by the significant H2 generated from the CO-promoted CDM, which could induce further reduction of Fe2+ to Fe0 in reaction of the CH4/CO mixtures. The formation of the metallic Fe could also facilitate the growth of MWCNTs with higher crystallinity and uniformity.The gas environment-dependent oxidation state variations of Fe and Cr on the FeCr catalysts dictate the types of surface intermediates, such as chemisorbed‑oxygen and carbonaceous species, that consequently affect the χ CH4 and χ CO and carbon morphology. The deconvoluted C 1 s spectra in Fig. 7(e) show three peaks at ~284 eV, ~286 eV, and ~ 288 eV, which are related to the C–C/C–H, C–O/C=O, and carboxylic groups, respectively. The relative quantity of C–O/C=O and carboxylic species was lower on the surface of CH4-pretreated catalyst than on the CO-pretreated catalyst (Table 1).The O 1 s spectra of the samples in Fig. 7(f) can be fitted into two peaks in which the lower and higher binding energy bands are assigned to the lattice oxygen species (OI) and chemisorbed oxygen species (OII), respectively. Moreover, the CH4-pretreated sample exhibited higher OI/(OI + OII) ratio as compared to the CO-pretreated sample (Table 1). These indicate a decrease of chemisorbed oxygen species on the CH4-pretreated catalyst surface. This is consistent with the fact that the CH4-pretreated catalyst possessed less C–O/C=O and carboxylic species but had greater amounts of CC and CH species.The variations of metal oxidation states and the types of chemisorbed oxygen and carbonaceous intermediates on the catalyst surface determine the gas conversions and carbon morphology in reactions with CH4 and CO. Although carbon coated metallic catalysts, such as Fe and Ni nanoparticles, were found to actively catalyze the MWCNT formation because metal-carbide can form at relatively low temperatures, carbon deposits from CH4 and CO were found to behave differently [49,50]. Literature studies have found that, during CO reaction on the Ni/SiO2 catalyst, some chemisorbed oxygen atoms (OII) could penetrate through the surface to restructure the metal surface that enables subsurface migration of carbon [49]. It was also found that CO carbonization was not determined by the dissociation of CO but by the reaction of adsorbed oxygen with CO via a Langmuir-Hinshelwood mechanism. However, the carbon deposits from CDM were found to remain on the surface due to the inadequate surface OII that hindered the carbon transport and growth.The pretreatment by CH4, a stronger reductant than CO, could cause Fe3+ reduction to greater extents, even possible generation of surface Fe0 atomic clusters [51]. Thus, although the CH4-pretreated catalyst lacked surface OII to sustain fast CH4 decomposition, it could catalyze the subsequent reaction with CO and promote the formation of MWCNT (Fig. 6a) because CO could create a larger OII population (Table 1). On the contrary, the CO-pretreatment was unable to induce metallic Fe at the low pressure and temperature. Consequently, the CO-pretreated catalyst was unable to support the subsurface carbon transport for dispense of MWCNT when continuing to reaction with CO and inefficient in promoting the reaction with pure CH4 due to severe surface carbon blockage (Fig. 6d).In the mixture reaction, CO reaction increases the OII and catalyst surface temperature that enhance the χ CH4 to generate more H2 (Fig. 5a). The increased H2 presence on the catalyst surface may further reduce the surface Fe2+, i.e., FeO + H 2 → Fe 0⋯H 2 O, to form metal Fe clusters on surface (Fig. 4a and b). The O 1 s spectra for the sample reacted in CH4/CO mixtures (Fig. 4a) exhibit a peak at 533.6–533.8 eV, which was absent in the spectra for samples from reactions with pure CH4 and CO. This peak can be ascribed to oxygen in surface groups (OIII) such as chemisorbed carboxyl and/or hydroxyl species.The surface hydroxyl groups (OH −) S may be resulted from reaction with H2, which spontaneously dissociates on Fe clusters to readily react with O and C [31], e.g., 2O O X (O I ) + 2H ↔ V O •• + 2(OH −) S when reaction occur at the metal/oxide boundaries. The resultant V O •• could facilitate the OI/OII transitions that promotes the formation of surface intermediates such as “M⋯O s  − R” where M = Fe 2+/0 and R = CO m (0≤m≤1), CH x (0≤x≤3), or H, depending on the gas composition and temperature. The ab initio and density function theory computational studies have shown lowered reaction barriers for both CO and CH4 dissociations on Fe nanostructures with exothermic path to surface bound elemental carbon (Cs). Such carbon atom formation on nanostructured Fe is key to the initial CNT formation that involves the well-known cap lift-off process in early stage [6,11,31]. The evidence of cap formation was also observed on some premature MWCNT/catalyst particles from mixture reaction but not in the samples of pure CO reaction, where only thick graphitic layers and rods were found (Fig. S4).The magnetite phase nanocrystalline FeCr (10:1) solid oxide solution could effectively catalyze CDM at 500 °C, which is much lower than the temperatures needed for CDM on the conventional metal nanoparticle catalysts. The χ CH4 increase dramatically and the morphology of carbon products altered when a small amount of CO was cofed. For example, a feed with 5% CO (i.e., y f, CH4 = 95%) achieved an average χ CH4 that was over five times the conversion of pure CH4 and obtained predominantly MWCNT product instead of GCPs formed from pure CH4 or CO. The causes of the CO-promoted catalytic activity enhancement for CDM have been found to lie in the synergistic effects of CH4 and CO reactions on the chemical states of the catalyst surface and mutually beneficial reaction heat effects. The strongly exothermic Boudouard reaction increases the local temperature at the catalytic sites to thermodynamically and kinetically benefit the endothermic CH4 decomposition. The enhanced CDM generates more hydrogen at the catalyst surface that induces further reduction of Fe2+ to form Fe0 metal clusters in the catalyst surface. These metallic Fe clusters along with the CO-originated surface oxygens (OII) facilitate the transfer of C and H out of the active surface sites to sustain fast conversions of both CH4 and CO. It can be also inferred by the microscopic observations and XPS results that the surface metallic Fe clusters are imperative to MWCNT formation by enabling subsurface C transfer for continued MWCNT growth. The doped Cr participate in the gas-induced solid state defect reactions that facilitate the redox cycles of Fe 3+/Fe 2+ and transfer of OII, which are vital to maintaining the surface catalytic activities. With its well-known sulfur resistance and low-cost production, nanocrystalline FeCr catalysts is potentially useful for conversion of biogas to produce clean H2 and MWCNTs under desirably mild conditions. CRediT authorship contribution statement: Xinhui Sun: Investigation, Data Curation, Writing (draft). Devaiah Damma: Characterizations, Data Curation. Zishu Cao: Characterizations. NOE T. Alvarez: Validation, Supervision. Vesselin Shanov: Resources (Lab), Supervision. Antonios Arvanitis: Resources (Reactor), validation. Panagiotis G. Smirniotis: Resources (Catalysis Lab), Supervision. Junhang Dong: Conceptualization, Funding acquisition, Resources, Writing – drafting, reviewing, and editing. Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.None.This research was financially supported by the Development Service Agency of Ohio through the Ohio Coal Research and Development program (Grant no. OOECDO-D-17-13) and the U.S. Department of Energy/Office of Science (Grant no. DE-SC0018853). The alumina tubes were provided by Media and Processes Technology Inc., Pittsburgh, USA. Supplemmentary material Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106475.

Catalytic decomposition of methane (CDM) to H2 and multiwalled carbon nanotubes (MWCNTs) was achieved by a nanocrystalline Cr-doped ferrite (FeCr) catalyst at 500 °C and atmospheric pressure with minor cofed CO. The exothermic Boudouard reaction increased the temperature and H2 from CDM at catalyst surface that induced Fe2+ reduction to Fe0. The Fe0 clusters along with the CO-originated surface oxygens enabled transfer of C and H to sustain the surface CDM and CO reactions. The metallic Fe-enabled C transfer led to the formation of MWCNTs. The Cr6+/3+ dopants facilitated the Fe redox cycles and maintained surface oxygens for high catalytic activity.

Electrocatalytic CO2 reduction reaction (CO2RR) is a promising way to convert CO2 into valuable carbon-containing fuels or chemicals at room temperature and atmospheric pressure [1–6]. The development of high-efficiency catalysts for CO2 reduction reaction has become a topical issue. According to the previous studies, metal-based catalysts (Pb, Pt, Au, Ag, Cu, etc.) exhibited good catalytic activity for CO2RR [7–11]. Among these metals, Cu is the only catalyst that has the ability to realize C–C coupling reaction to generate C2+ products, due to its unique adsorption characteristics of these intermediates [12]. Therefore, Cu-based catalysts become the star materials in electrocatalytic CO2 reduction field.CO2 is an extremely stable linear molecule with two CO bonds length of 116.3 pm and bond energy of 750 kJ mol−1, which determine that CO2 activation is a key step limiting the catalytic activity of Cu-based catalyst [13,14]. In addition, Cu has the moderate adsorption properties of intermediates, resulting in a complex variety of hydrocarbon products. So the low CO2RR activity and C2+ products selectivity of Cu-based catalysts is still worth investigating [15–17].In order to improve the activity and selectivity of Cu-based catalysts [18], various strategies were proposed such as regulating morphology structure [19,20], defect [21], and surface/interface engineering [22]. As we know, the CO2RR require multiple steps and involve various intermediates. Generally, the binding strengths of involved intermediates are linearly correlated. For example, both ∗COOH and ∗CO rely on C–metal binding, thus the adsorption strengths of ∗COOH and ∗CO on the metal catalyst surface are essentially correlated. This relationship determines that the binding energy of individual intermediate at Cu site cannot be regulated independently. If we want to improve the activation process of CO2, we need to adjust the stronger adsorption capacity for ∗COOH, which will incidentally bring stronger ∗CO adsorption capacity, thus affecting products selectivity. Cu-based bimetallic catalysts offer the possibility to break this scaling relationship [23,24]. Two kinds of metal component can provide more possible adsorption sites for intermediates, and can more fully realize the regulation of the binding energy of intermediates at different sites, thus improving the activity of CO2RR and optimize the selectivity of products.Since Watanabe et al. reported the Cu-based bimetallic catalysts in 1991, a variety of bimetallic Cu-based catalysts have been synthesized and used for CO2 reduction, such as Cu–Au [25], Cu–Ag [26], Cu–Pb [27], and Cu–Pt [28]. It has been fully demonstrated that Cu-based metallic catalyst can significantly promote CO2 activation. For example, due to the stronger CO2 adsorption ability of Pd than Cu, the reaction kinetics of ∗COOH generation on the Pd surface is faster than on the Cu surface. Therefore, combining Pd with Cu is an effective approach to promote CO2 reduction. In addition, density functional theory (DFT) calculations show that the stepped CuPd(111) surface has a stronger CO2 adsorption and activation capacity than the Cu(111) surface [29]. Experimental results also demonstrated the Cu–Pd alloy can effectively promote the activation of CO2 and inhibit the formation of hydrocarbons on the catalyst surface, resulting in much higher selectivity of CO product [30]. In addition, Cu-based bimetallic catalysts can also promote the C–C coupling reactions. For example, Hoang et al. synthesized a nanoporous Cu–Ag alloy by additive-controlled electrodeposition [31]. The catalyst exhibited a record CO2RR performance with the Faradic efficiency (FE) for C2H4 and C2H5OH reaching 60% and 25%, respectively. The nanoporous surface of Cu–Ag caused low-coordination metal atoms steps and edges, which are favored to facilitate the C–C coupling process. Therefore, Cu-based bimetallic catalysts show an attractive perspective in CO2RR.Although the study about Cu-based bimetallic catalysts is likely to be a future priority, what is the key factor that affects CO2 reduction performance on Cu-based bimetallic catalysts? What is the role of the different metal elements? How can we design Cu-based bimetallic catalysts more rationally? These problems still greatly puzzled researchers. In this review, we begin with a brief background on the CO2RR process and the basic principles that determine the selectivity of Cu-based bimetallic catalysts. Then, we summarize the research progress of Cu-based bimetallic catalysts in CO2 reduction reaction (Scheme 1 ). In short, we first discuss the advantages of the morphology of Cu-based bimetallic catalysts, including dendritic, nanowires, polyhedron, and core–shell structure. Subsequently, we focus on the local electric field effect induced by the Cu-based bimetallic nanoneedle structure. Then, the interface engineering between bimetallic and the series of special phenomena that occurred on the bimetallic interface (including interfacial effects, interfacial atomic arrangements, interfaces strain) are discussed. Next, some commonly studied effects in bimetallic systems, such as the electronic effect and tandem effect are also analyzed. For a deeper understanding, some critical examples that combine experimental and computational studies in the CO2 reduction reaction are given. Finally, a perspective on the research and design of Cu-based bimetallic catalysts is proposed, wishing to better understand the CO2 reduction process on Cu-based bimetallic catalysts and to provide some insights for future studies.With the continuous development of instrument science and the electrocatalytic CO2 reduction reaction, great progress has been made in the CO2 reduction mechanism [32–36]. In general, CO2 reduction reaction involve the multi proton/electron transfer process. The first step is the adsorption/activation of CO2 molecule. Due to the extremely stable property of CO2, the adsorption/activation ability of active sites on CO2 has a great influence on CO2 reduction activity [13]. In most cases, the high energy barrier of CO2 adsorption/activation makes it a rate-determining step (RDS) during CO2 reduction process [37].After CO2 adsorption and activation, various intermediates (such as ∗CO2, ∗COOH, ∗CO, and ∗CH2) will form with the transfer of protons (H+) and electrons. Those intermediates play a critical role in the process to form hydrocarbon products. For instance, ∗CO2 can obtain an H+ to form two possible intermediates: ∗OCHO or ∗COOH, which is determined by the adsorption configuration of CO2. The ∗COOH, as the main intermediate for the generation of CO product, but it is also an important intermediate to form HCOOH (depending on the binding energy of C and O at the active sites) (Fig. 1 ). As a result, there is often a competitive relationship between different products. One critical intermediate to form C1 products is ∗CO. For example, the ∗CHO and ∗COH can form by hydrogenation process of ∗CO, then the ∗CHO can undergo 3 pairs of proton/electron transfer process to form CH3OH, while the ∗COH can form CH4 by 5 pairs of proton/electron transfer process (Fig. 1).Besides, ∗CO is also an important intermediate to generate C2 products. Distinguished from the C1 pathway, the formation of C2 products needs the C–C coupling process. At present, there are three main pathways of C–C coupling: (1) Two ∗CO direct coupling to form ∗COCO; (2) ∗CO coupling with ∗COH to form ∗COCOH; (3) ∗CO coupling with ∗CHO to form ∗COCHO. Moreover, there is also the possibility of coupling between two ∗COH intermediates or two ∗CHO intermediates to form ∗COHCOH or ∗CHOCHO, respectively. And another possible pathway was reported by Janik and his colleagues [38]. The ∗CO intermediate hydrogenation to form ∗COH or ∗CHO, subsequently the ∗CHO or ∗COH undergo hydrogenation and dehydration process to form ∗CH2 intermediate, then two ∗CH2 direct coupling can generate C2H4 product. All in all, the C–C coupling process is a critical and complex step of CO2 reduction. The coverage of ∗CO has a great impact on the C–C coupling process. Huang et al. investigated the influence of ∗CO coverage on the Cu(100) facet and found that with the increase of ∗CO coverage, the energy barrier of ∗CO–∗CO coupling decreasing, which can be attributed to the weakening of the Cu–∗CO bonds [39]. In summary, how to design catalysts for promoting CO2 adsorption/activation and C–C coupling, and how to tune the adsorption/desorption properties of critical intermediates are research hotspots.For the rational design of Cu-based bimetallic catalysts, the role of the second metal should be considered. One important point is that the second metal acts as the direct reaction active site. The Cu-based bimetallic catalysts require the rational utilization of the inherent properties of both metal components, and the incorporation of these properties into the bimetallic systems. For example, in the pathway of HCOOH generation, the O atom in the ∗OCHO intermediate rather than the C atom will adsorb on the surface catalyst active sites. For higher selectivity of HCOOH product, the stronger O affinity of metal sites is required. Previous works have demonstrated that metal electrocatalysts, which has high O affinity and low H (such as Sn, Bi, In, and Pb) affinity exhibit excellent selectivity for HCOOH.Due to the difference in intrinsic catalytic properties of catalytic sites, the main product on the different metal sites will be various. Therefore, the products distribution of the bimetallic catalysts can be adjusted. In bimetallic systems, combining Cu with another metal that produces different intermediates to activate the tandem effect is an effective strategy to improve the C2+ products catalytic performance. Most commonly, combining Cu with Au, Ag, Zn, and other ∗CO-producing metals results in an enrichment of ∗CO on the catalyst surface, the ∗CO will be spillover to the adjacent Cu sites and be further coupled to form C2+ products.Moreover, the introduction of secondary metals may not act as active sites, it can assist in changing the electronic structure of Cu, such as the coordination environment and electronic orbitals of Cu. In this case, the d-band electrons of Cu can pair with the s-band or p-band electrons of the adsorbate, thus facilitating the adsorption of reactants and the formation of intermediates. The introduction of secondary metals may also only change the geometry structure of the bimetallic catalysts, which is manifested by the change of morphology, the creation of interfaces, and the adjustment of atomic arrangements. For example, in porous bimetallic structures, ∗CO intermediate desorption is suppressed due to the domain-limiting effect, which leads to a locally high concentration environment of ∗CO on the catalyst, thus enhancing the selectivity and activity of the C2 products.It is generally believed that the high coverage of adsorbed C1 intermediates on the Cu surface is the key to triggering the interaction of adjacent intermediates for further C–C coupling, the method of assembling secondary metals with different C1 intermediate production capacities in Cu is the key to the goal. This requires full consideration of how the CO2 reduction process fits and cooperates with the two components to maximize the reduction efficiency of CO2 on the catalyst surface. In addition, the interaction between the second metal and the host metal (Cu) should also be considered. We consider and summarize these principles in detail, and present them in the content that follows.Research on Cu-based bimetallic alloys dates back to 1972, at that time, two different papers but discussed the same topic—the change of properties caused by alloying of Ni and Cu were published in the same journal. The one discussed this change from the point of ensemble size, however, the other proposed a view of d-character [40–42]. Although the two papers have different views, they all made convincing explanations and ended up with the same conclusion—alloying the two metals increases the activity of the catalytic reaction [40]. Compared with the single metal catalyst, the performance improvement of bimetallic catalysts is mainly attributed to the multiple factors among structure, interface, and various synergistic effects resulting from the coupling of two metals. For the rational design of Cu-based bimetallic catalysts and understand the promotion mechanism between Cu-based bimetallic catalysts on CO2 reduction. We have sorted out the experimental progress and related derivative studies on those factors and discussed the influence factors in detail in the following parts.Morphology can change the local microstructure of nanocatalysts and determine their catalytic performance [43]. The synthesis of Cu-based bimetallic nanocrystals with different morphology, such as nanodendrite, polyhedral, porous hollow, and core–shell structure, has been widely studied [44]. Each of these structures has its unique advantages (Fig. 2 ). In this chapter, we have briefly classified and discussed the morphology effect of Cu-based bimetallic catalysts for CO2 reduction reaction.Dendrite structures exhibit the advantages of high roughness and high surface area, which is beneficial for CO2 adsorption and electronic transfer. Keerthiga et al. proposed that the dendritic structure can alter the crystal orientation and crystallinity of the metal deposits, thus facilitating the catalytic reaction [45]. Koh et al. found the dendrite structure has high index facets and low-coordination sites, which are beneficial to the stabilization of reaction intermediates [46]. Based on these studies, the high-efficiency CO2 reduction reaction with the multi-electron/proton transfer process can readily occur on the dendritic structure.Roth et al. investigated the effect of Cu–Ag dendrite structure on the distribution and selectivity of CO2 reduction products [47]. Compared with Cu foams, the presence of Ag during electrodeposition significantly changed the size and shape of dendrites in the pore walls of Cu–Ag foams, resulting in increased surface area and roughness. The high surface area enables Cu–Ag dendrite catalysts to have higher CO2 reduction current density, which represents higher catalytic activity (Fig. 3 ). In addition, the study indicated that the production of hydrocarbons depends on the asymmetry of the structure, and the rough surface is conducive to the production of higher hydrocarbons. Thus, dendrite structure was considered to be more favor the production of hydrocarbon. Ingole et al. demonstrated the dendritic structures are more favor for ethylene than other structures. This enhancement is mainly attribute to the nano-hot spots, diffusion control of ionic species and pore size which more favorable for CO adsorption [48]. Similarly, Klingan et al. demonstrated that needle-like structures at the dendrites edges can lead to an increase in local pH, promoting C2H4 production while suppressing CH4 yield [49]. Reller et al. further demonstrated this enhancement is indeed related to the needle-like structure, because the selectivity to ethylene decreases significantly with the coarsening of the edge needle-like structure [50].In addition, due to the higher roughness, and higher density of stepped sites, the dendritic structure can promote CO2 reduction performance by suppressing the hydrogen evolution reaction (HER) [51]. For example, Hoffman et al. prepared Cu–In bimetallic catalysts with a dendrite structure [52]. Due to the exposed stepped sites and variety of surface facets, the dendrite structure exhibited an optimized performance of formate (FE = 62%) at −1 V by inhibition of HER.One-dimensional materials such as bimetallic nanowires [53], nanorods [54], and nanotubes are considered to be one of the most common materials for electrocatalysis as its multiple advantages [55]. Firstly, one-dimensional structures have better conductivity because the charges can efficiently transfer along with the axial orientation. Secondly, the uniform structure can promote the adsorption of reactants, and improve the stability of the catalyst and directional mass transfer. Jang et al. prepared a one-dimensional CuIn nanowire electrocatalyst by anodization and electrodeposition (Fig. 4 a and b) [53]. The CuIn catalyst exhibited performance with over 68% selectivity of CO (Fig. 4c). They proposed that the CuIn nanowires provide a larger surface area and further promote the charge transfer. And alloy formation changes the properties of local active sites. The In atoms replace the Cu atoms and suppress the H2 generation. In addition, due to the oxygen-deficient indium and oxygen of indium hydroxide have slightly positive and negative charges. By delivering electrons and protons, CO2 would be adsorbed and stabilized on the polarized surface as depicted in Fig. 4d.In recent years, one-dimensional nanowire arrays have received extensive attention. The abundant space between adjacent nanowire structures, not only facilitates the mass transport of the reactants at the solution/catalyst boundary, but also promotes the timely release of the bubbles generated by the reaction in the solution, avoiding the inactivation of active sites. For example, He et al. prepared a Cu x Au y nanowire arrays (NWAs) for efficient catalysis CO2 reduction. The authors proposed that the nanowire array structure significantly limits the diffusion of OH− and HCO3 −, which results in high local pH thus facilitating the coupling of CO intermediates. In addition, the nanowire arrays structure can also decrease the diffusion rate of CO generated from the nanowire surface to the bulk solution. Results in a higher CO concentration in the nanowire structure, and increase in the ∗CO/∗H ratio for further reduction on the nanowire, it is beneficial to generate EtOH at low overpotentials [56].Polyhedron of different shapes have also been widely explored because their expose different crystal planes and more active sites. Yang et al. prepared bimetallic Cu–Pd catalysts with different morphologies and compositions, which exhibited different CO selectivity (Fig. 5 a–c) [57]. In particular, spherical Cu–Pd nanoalloys (Cu–Pd–S) have the highest CO conversion faradaic efficiency (93%), while the H2 evolution reaction is dominant on the concave cubic (Cu–Pd–C) and dendritic Cu–Pd (Cu–Pd-D) nanoalloys (Fig. 5d and 5e). This morphology-dependent selectivity is attributed to the equilibrium of rate-determining steps in the CO2 reduction reaction. The severe HER rates of the concave cubic and dendritic Cu–Pd nanoalloys can be mainly attributed to their unique structures providing more active edge/corner sites for water dissociation.Due to the suitable distance of the bimetallic domains in the polyhedral structure, the transformation of intermediate can be effectively facilitated and the reduction performance of CO2 can greatly improve. Recently, Ma et al. realized the confined growth of Cu with (100) facets on Ag nanocubes to form Ag–Cu Janus nanostructure [58]. Compared with Cu nanocubes, the bimetallic Janus structure exhibits very good C2 products selectivity. The exposed Cu(100) facets can lower the energy barrier for C–C coupling and improve the selectivity for C2H4. In addition, the Cu–Ag polyhedral structure also showed higher FE for C2H4 and C2 products than the Ag + Cu mixture. Due to the long distance between the Ag and Cu domains in the Ag + Cu mixture, it is difficult for the CO spillover from Ag to Cu. In the Ag–Cu polyhedral structure, this suitable distance between Cu and Ag is beneficial to the diffusion of the intermediate product ∗CO and reduces the energy barrier of C–C coupling, resulting in the high selectivity of C2 products in Ag–Cu polyhedral.The core–shell structure has the advantages of a larger specific surface area, short diffusion paths, prominent hierarchical structure, and fast charge transfer rate. Therefore, the core–shell structure is attractive as electrocatalysts, as the core is the main active component with specific functions, while the shell acts as a protective layer to enhance the performance of the core material or generate new properties. Huang et al. prepared a Cu@Ag core–shell nanoparticles with tunable shell thickness. The Cu@Ag-2 catalyst with the optimal Ag shell thickness exhibits the highest C2 FE (67.6%). The analysis results show that the Cu@Ag-2 catalyst with appropriate Ag shell thickness can effectively generate and enhance the local CO concentration on the Cu core, and the unique core-shell structure provides more active sites and faster charge transfer. Thus promoting the conversion of CO2 to C2 [59].The core-shell structure can regulate the concentration of intermediates through a confinement effect and thus facilitating C–C coupling. Zhong et al. prepared Ag@Cu core-shell catalysts, achieved the tuning of the pore size in the porous Cu shell (Fig. 6 a–c) [60]. In situ experiments and finite element theoretical simulations demonstrate that the Ag@Cu core-shell catalysts with an average pore size of 4.9 nm induce the highest local ∗CO concentration due to the confinement effect and therefore promote C–C coupling. Thus, exhibited high Faraday efficiency of the C2+ products reached 73.7% under constant current reaction conditions with a total constant current density of 300 mA cm−2 (Fig. 6d and e).Moreover, a heterogeneous nanostructure of Cu-rich cores embedded in an In(OH)3 shell-like matrix was reported [61]. The derivative catalysts show high electrocatalytic performance at moderate overpotential. By comparing the properties before and after the In(OH)3 shell dissolved, they demonstrated that the presence of In(OH)3 is critical to the high selectivity of CO, and thus confirm the heterogeneous nanostructure is essential for improved activity and stability of CO2RR [62].By adjusting the shape and structure of bimetallic catalysts, the activity and selectivity of CO2 reduction can be effectively regulated. According to the previous work, we can conclude that controllable shape and structure can be achieved by various methods such as co–reduction reaction [63], galvanic replacement reaction [64], solvothermal method [65], and seed-mediated method [66]. However, many challenges still need to be solved for nano bimetallic catalysts with controllable structures. For example, since Cu can be combined with a variety of metals to form bimetallic catalysts, strategies of generic and shape controllable synthesis methods suitable for different bimetallic systems are necessary. In addition, during the CO2 reduction process, the sharp dendrite structure is easy to be etched, the nanoparticles are easy to agglomerate, and the nanowires will fracture, thus resulting in performance degradation. Therefore, how to maintain the complete structure of the catalyst during the reaction is very critical. At present, many morphology protection methods have been developed such as: organic coating, construction of second metal protection layer and formation of stable oxide surface. However, the dynamic process of dissolution and reconstruction of electrode morphology should also be the research focus in the future. For example, Larrazábal et al. found the equilibration process may result in the creation of more active sites or generate metal pathways by bypass the less conductive In(OH)3 shell [61]. Weng et al. proposed the Pd atoms pre-deposited on the Cu surface cause sustained morphological and compositional reorganization, which sustained refreshing the catalyst surface and thereby keeping the catalytic performance of CO2 reduction to hydrocarbons [67].As a special morphology, a sharp needle structure with a high curvature is always accompanied by a strong electric field in its vicinity [68]. Dong et al. demonstrated that the strength of the electric field effect is related to the sharpness of the needle by comparing a series of well-defined morphological Cu–Sn bimetallic foils, rods, and cones [69]. The COMSOL finite element analysis shows that the local electric field strength gradually increases as the top diameter (D top) of the Cu cone decreases, while the electric field strength tends to decrease as the bottom (D bottom) radius increases (Fig. 7 ). It is concluded that a high curvature can induce strong electric field effects and promote CO2 reduction. This is consistent with the experimental result that the Cu–Sn electrode with the highest curvature has the best CO faradaic efficiency (FE) of 82.7%.Generally, the low solubility of CO2 in the electrolyte makes it difficult for CO2 molecules to adsorb on the electrode surface. Therefore, it is proposed that an electric field can promote the adsorption/activation of CO2. For example, Nørskov et al. used theoretical calculations to prove the local electric field can significantly alter the free energy of the CO2 reduction to CO on Ag metals [70]. In addition, some studies have demonstrated that alkali metal cations can overcome the limitation of low CO2 solubility through non-covalent interactions with adsorbed CO2. Liu et al. proposed a strategy to improve CO2 adsorption capacity by aggregating alkali metal cations concentration induced by nanoneedle electric field (Fig. 8 a and b). Specifically, the local electric field can increase the K+ concentration on the electrode surface by a factor of 20, and as the K+ concentration increases, CO2 rapidly stabilizes on the sharp electrode [71].Similarly, due to the transfer difficulty of electrons at grain boundaries (GBs), the electrons tend to accumulate at grain boundaries. Zhong et al. designed an Au–Cu bimetallic nanochain aerogel (NC–Au–Cu) containing rich grain boundaries. They found the role of GBs is electron channels to promote the accumulation of charges. The charge accumulation induces a high local electric field for K+ concentration [72]. High K+ concentrations favor the concentration of CO2 near the electrode surface and reduce the formation energy of the rate-determining step of ∗CO2 to ∗COOH intermediate. Lee et al. found the strong electric field is concentrated at the tip of the dendrite [73], the enrichment of K+ inhibits hydrogen generation by preventing H+ from approaching the surface while promoting CO2 adsorption/activation.Furthermore, electric field effects can promote CO2RR by facilitating the C–C coupling process (Fig. 8c and d) [74]. Nørskov et al. reported the theoretical evidence of a CO dimerization mechanism on Cu(111) and Cu(100). The combination of field and solvation effects considerably reduces the thermodynamic and kinetic energy requirements for the formation of CO dimers [75]. This prediction has been proved by experimental results. Zhou et al. reported that the nano-tip arrays with vertical alignment induce stronger electric fields, and that strong electric fields lead to more K+ enrichment, resulting in stronger ∗CO adsorption and a lower energy barrier for C–C coupling (Fig. 8e) [76].The adsorbent interacts with the metal surface can obtain an adsorbate-surface dipole. The interaction of these dipoles with the electric field can have a profound effect on the catalytic reaction [77]. Resasco et al. reported that surface intermediates with larger dipole moments (e.g., ∗CO2, ∗CO and ∗OCCO) can be stabilized by electrostatic fields in the Outer Helmholtz Plane (OHP) [78]. This work provides a theoretical perspective on how the electric field affects the adsorption behavior of intermediates.Although the concentrated electric field at the tip can enhance CO2 adsorption/activation and promote C–C coupling, the low density of the needle tip limits the effective area. It is worth further exploring how to achieve a more pointed tip and a larger effective active region. Safaei et al. reported a hierarchical multi-layered structure to increase the density of the active tip [79]. The performance of CO2RR can be improved by multi nano-tips on porous carbon fibers. In detail, they prepared the new layered structure by partially covering the nanoneedles with thiol and then surface secondary gold electrodeposition. The new catalyst exhibited a stronger local electric field and three times higher current density than a single needle tip. Periodic array structures can also further enhance electric field effects and improve catalytic performance. The ordered nano-tip array structure had a higher local electric field, achieving almost double the selectivity of the C2 products compared to the disordered nano-tip array [76].Although electric field effects do kinetically facilitate the reduction of CO2. And it is undeniable that the electric field effect is indeed a promising strategy for modulating catalytic performance, changing the adsorption energy on the catalyst surface, without changing the catalyst material or structure. However, other side-reactions (e.g., HER) can be equally enhanced by electric field effects, and the competition between side-reactions and CO2 reduction reaction dictates that the tip size is not necessarily the smaller the better [80]. Furthermore, it remains to be investigated whether electric field effects are still present in industrial high current applications and how useful they can be. Beside, although it has been proved that the electric field effect is stronger with higher curvature structure, there are some problems need to be investigated. For example, whether this correlation can be quantified? Whether there has the best value of curvature? Therefore, the construction and utilization of the electric field effect must establish the reasonable model of curvature and electric field strength.Conventional nanostructure and morphology control techniques do not always work in complicated CO2 reduction pathways. To improve the selectivity and activity of Cu-based catalysts, rationally modifying the interface of the bimetallic catalysts may be possible to break the conventional linear scaling relationship and modulate the binding strength of the target intermediate [81–84]. Interface engineering can be designed to contain multiple sites and optimize the chemical environment around the active sites in Cu-based bimetallic catalysts [85]. Experimental and theoretical studies have shown that various synergistic effects of interface engineering maybe present between the different metal elements, resulting in superior catalytic performance. In this chapter, we classify and discuss interface engineering into three parts: (1) interface effect, which highlights the role of intrinsic properties of bimetallic interfaces (such as interface type and size) in CO2 reduction reaction, (2) interface atomic arrangement, which focuses on the influence of changing the mixing patterns of Cu and M atom, and (3) interface strain, which focuses on the changes caused by mixing of two metals with different lattice parameters.The coupling of two metals generally results in the creation of a bimetallic heterogeneous interface, which plays an important role in the CO2 reduction reaction. The bimetallic heterogeneous interface can promote CO2RR through various roles, including the adjustment of intermediate adsorption, electronic/reactant transfer, generation of more active sites, and avoiding catalyst poisoning [85–87]. For example, Zhang et al. found the interaction of the bimetallic interface plays an important role in adjusting the selectivity and activity of CO2RR [88]. Remarkably, the reaction rate was greatly influenced by the interface activity. The Cu–Au interface can effectively promote the dissociation of H2O to H∗, while preferentially enhancing CO2RR and suppressing HER. In addition, they found the adsorb energy of CO is larger on the Cu–Au interface compared with the pure Cu surface, which indicated that the CO is easier to be reduced at the Cu–Au interface. The ∗CHO and ∗CO has stronger adsorption on the Cu–Au interface, which is beneficial to promoting C–C coupling and formation of C2 products.CO2 reduction is always dissatisfied on Cu-based catalysts due to the weak CO2 activation ability of Cu [89]. The adsorption and activation of CO2 can be effectively promoted by regulating the bimetallic interface. Ye et al. conducted a comprehensive understanding of how CO2 and H2O interact with the Cu–Ag interface to promote CO2 adsorption and activation. The synergy effect of Ag and Cu was the key to tune the CO2 (H2O)–Cu–Ag interactions and change the activation process of CO2 [90]. Compared with pure Ag and Cu, the adsorbates (H2O or CO2) on the AgCu interface exhibit different geometric and electronic structures. The observed interface restriction process of Ag and Cu significantly promotes the CO2 adsorption/activation process.Bimetallic heterointerfaces can also effectively promote C–C coupling to generate C2 products. Huang et al. fabricated Ag–Cu heterointerfaces with different Cu domain sizes (Ag1–Cu0.4, Ag1–Cu1.1, and Ag1–Cu3.2) (Fig. 9 a–f) [26], the Ag1–Cu1.1 catalyst with optimal content obtained FE of C2H4 was 3.4-fold compared with pure Cu (Fig. 9g). The adjustable interface of Ag-Cu is related to the FE of C2H4. The size of the Cu can adjust the interface and control the partial Cu oxidation state, then achieve the best ∗CO–∗CO for C2H4 generation (Fig. 9h). Similarly, Wang et al. constructed a bimetallic Ag–Cu catalyst with a sharp interface to promote the reduction of CO2 to C2H4. A high 42% FE of C2H4 can be obtained, which is about two-fold than pure Cu catalyst [91]. In addition to increased C2H4 production, ethanol performance also can be improved by bimetallic heterointerfaces. Yeo et al. obtained a high yield of C2H5OH by adding Ag to Cu2O nanowires and concluded that the improvement of C2H5OH is attributed to the amount of CO released from Ag sites on the Cu-Ag interface.The interface of bimetallic catalysts can also optimize the intermediate adsorption performance to regulate products selectivity. Peng et al. prepared a Cu–Bi bimetallic electrode by using a simple one-step co-deposition method [92]. The Cu–Bi bimetallic catalysts achieved an amazing FE of formate (94.37%) and high current density (27.85 mA cm−2) at −0.91 V vs. RHE. Due to the introduction of Cu atoms into the Bi electrode, the intrinsic and electronic structures of Bi electrodes will be tuned, and thus promote the CO2RR process transfer on Cu–Bi surface to stabilizing intermediate ∗OCHO, which is the important intermediate to generate formate. The surface of pure Bi favors the formation of COOH∗ intermediates, while generating unwanted CO product.By coupling Cu with secondary metals, interface effect have been proven to effectively enhance CO2 adsorption/activation, facilitate C–C coupling, and optimize the adsorption property of intermediates in the CO2 reduction process. Many previous works give us a lot of inspiration on how to understand and use interface effects. Nevertheless, many problems should be studied in the future. For instance, current research on the effect of the interface may pay more attention to the selectivity or activity of the CO2 reduction reaction. Follow-up works need to focus on how to achieve a larger interface between the two metal domains and keep their large interface stable during the electroreduction process. The catalyst design should consider how to engineer advanced nanoheterostructures and generate abundant interfaces to maximize interface effects.In addition to the regulation of the bimetallic interface, the rational design of the interface atomic arrangement of the bimetallic catalysts is also an effective strategy to improve the catalytic CO2 reduction performance. Nishimura et al. investigated the effect of metal atomic modifications of Cu on electrocatalytic CO2 reduction [93]. The results showed that atomic-level bimetallic effects on Cu-based electrocatalysts are typically mediated through the inhibition of undesired side reactions (e.g., the HER). The precise regulation of the relative position and arrangement of the introduced M metal atoms is important [93].In addition, the different interface atomic mixing modes for the two metals have a great influence on the CO2 reduction performance. For example, Ma et al. prepared a series of Cu–Pd catalysts (Cuat: Ptat = 1: 1) with three mixed atomic arrangements of ordered, disordered and phase separated modes [94]. Interestingly, the ordered sample had the best C1 FE (>80%), while the phase-separated structure exhibited the best C2 products selectivity. This result is attributed to the different arrangement of atoms and active sites on the surface of these catalysts. The ordered Cu–Pd catalysts have more alternating Cu–Pd sites than disordered Cu–Pd catalysts. With the CO adsorbed on the Cu atoms facilitating the formation of CHO intermediates and the O atoms adsorbed on the Pd atoms stabilizing the CHO adsorption and thus further facilitating the production of C1. In contrast, the phase-separated structure has more adjacent Cu atoms, which exhibit favorable molecular distances and less steric hindrance. It is favorable for C–C coupling.The intermetallic ordered structures are generally more thermodynamically stable than their disordered counterparts. Moreover, the intermetallic ordered structures provide a unique electronic structure and coordination environment [95]. For example, Kim et al. found that the conversion of Au–Cu (Auat: Cuat = 1:1) nanoparticles from a disordered arrangement to an ordered arrangement enables catalytic selectivity in CO2 reduction reaction (Fig. 10 a) [96]. As shown in Fig. 10b, the most ordered o-AuCu mainly produces CO, while the disordered d-AuCu mainly produces H2. The ordered conversion can form a three-atom-thick Au capping layer, which is responsible for the enhanced CO2 reduction catalytic performance. The DFT calculation compared the thermodynamic limit potentials UL (CO2) (limiting potential for CO2 reduction) and UL (H2) (limiting potential for H2 release) of the three model systems. The UL (CO2) values of the three models indicate that o-AuCu has the best activity to reduce CO2 to CO (Fig. 10c–e). The smaller value of UL (CO2) – UL (H2) means a higher selectivity for CO2 reduction. As shown in Fig. 10e, o-AuCu has better selectivity than d-AuCu.The phase separated structure with moderate atomic spacing and low steric hindrance, can provide a large number of active sites for optimizing the binding of intermediates [97]. For example, Wang et al. prepared a phase-separated Cu3Sn/Cu6Sn5 catalysts by electrochemical deposition of Sn on Cu foams. The catalyst exhibited 82% Faraday efficiency of HCOOH at −1.0 V (vs. RHE) and excellent CO2 reduction stability (about 42 h) [98]. While the main products of both Cu3Sn and Cu6Sn5 are H2. DFT calculations showed that the adsorption energy of ∗HCOO is more negative than ∗COOH on Cu3Sn/Cu6Sn5, indicating that the formate is more likely to be formed on phase-separated Cu3Sn/Cu6Sn5. In addition, the Gibbs free energy of HER on Cu3Sn/Cu6Sn5 is more positive than on Cu3Sn and Cu6Sn5, suggesting that the phase separation Cu3Sn/Cu6Sn5 catalyst can exhibit an inhibition HER effect compared to Cu3Sn and Cu6Sn5.Introducing the metal vacancies on the catalyst surface can regulate surface atomic arrangement for the better electronic structure of adjacent atoms and reaction energy barrier of intermediates. Zhuang et al. reported a Cu2S catalyst with surface Cu vacancies. The core S atoms and Cu vacancies on the shell can efficiently convert C2 alkene products to alcohol products [99]. The ratio of ethanol to ethylene greatly increased. DFT calculations show that the catalyst with Cu2S core and Cu vacancies increases the energy barrier for CO2 reduction to the ethylene pathway, while it does not affect the ethanol pathway (Fig. 11 ).Zhu et al. also reported an Au–Cu alloy catalyst with surface enriched vacancies [100]. In particular, the de-alloyed Au3Cu alloy catalyst (De-Au3Cu) with enriched Cu vacancies exhibited the highest CO Faraday efficiency of 94.3% at −0.43 V vs. RHE (Fig. 12 a). The DFT calculations (Fig. 12b–d) indicated the free energy for adsorption/activation of CO2 (∗CO2 to ∗COOH) on the De-Au3Cu catalyst is lower than Au3Cu and Au(100). CO2 is more likely to be adsorbed on vacancies rather than on metal sites.During the CO2 reduction reaction, atomic arrangement engineering is a promising tool for catalyst design. By adjusting the surface atomic arrangement, changing the atomic spacing and coordination environment between adjacent Cu atoms and secondary metal M atoms, we can effectively change the adsorption mode and configuration of adsorbates on the catalyst surface. The adjusted adsorption strength of intermediates and optimized reaction pathway result in better catalytic activity and higher selectivity of CO2 reduction reaction. However, the implementation of precise controls bimetallic structure (such as the proportion and atomic arrangement pattern of bimetallic atoms) still is a great challenge. Atomic-scale material synthesis methods such as atomic layer deposition techniques should be used and studied.Different lattice parameters of two different metal surfaces can lead to strain effects [101,102]. It is proven that the strain effects can effectively influence the performance of CO2 reduction [103]. The strain effect is usually induced by lattice mismatch and lattice dislocations. In addition, the structure transformations can also result in the strain effect. For example, Zhang et al. reported a FePt/Pt NPs catalyst for high CO2 reduction performance. The high catalytic activity is attributed to the Pt strain. This strain can be changed by structure transition from the fcc (face centered cubic) structure to the fct (face centered tetragonal) structure [104]. Generally, there are two types of strain effects: tensile strain and compressive strain, we focus on the different effects of these two types of strain.The strain effect can enhance the performance of CO2 reduction. For example, due to the smaller lattice parameter of Cu, reducing the number of copper layers in Au@Cu core@shell can increase the tensile strain of the shell, and result in higher selectivity of C2H4 product [105]. Similarly, Reske et al. investigated the effect of Cu cover layers of different atomic thicknesses on Pt [106]. As the thickness of the Cu cover increases, the lattice mismatch between Cu and Pt raises the interatomic distance of Cu and produces a tensile strain effect, thus leading to the suppressive H2 formation. Those studies provided new insight into strain-tuned CO2 reduction performance. This means it is a good approach that controlling the strain effect by varying the thickness of the Cu cover layer on the interface.Strain effects can also enhance the selectivity of CO2RR by inhibiting competing HER. Clark et al. reported Cu–Ag bimetallic catalysts with excellent performance to produce multi-carbon oxygenate by electrochemical reduction of CO2 [107]. This excellent performance was attributed to the selective suppression of HER induced by the compressive strain of a Cu–Ag surface alloy. Specifically, compressive strain causes a valence band modifier, which leads the valence band structure of Cu to move to deeper levels. The weaker adsorption energy of H results in inhibited HER activity in the Cu phase. Similarly, Du et al. obtained different elastic strain states of 32 nm and 5 nm Cu films and investigated their effect on CO2RR performance (Fig. 13 a) [108]. In their study, tensile strain in 32 nm overlay facilitates CH4 production and increases the CH4 Faraday efficiency of CO2RR from 65.02% to 76.48%. In contrast, compressive strain significantly reduces its CH4 selectivity. The compressive strain in the 5 nm overlay reduces the H adsorption energy and thus suppresses the HER (Fig. 13b and c).In bimetallic systems, the presence of strain effects has provided new regulatory strategies for designing catalysts with excellent performance. However, the lack of specific modulation methods for strain effects, the lack of characterization tools, and the absence of a uniform representation of the degree of strain have limited the application of this strategy. Therefore, in future research, in addition to investigating the effect of different strain types on the performance of CO2RR, it is important to further investigate what level of strain can have a significant effect on the performance of CO2RR.Reasonable utilize interface engineering is an important means to improve CO2 reduction performance. Thus, it is important to develop advanced interface research methods, such as in-situ ultrafast temporal resolution and spatial resolution spectroscopic techniques, which can directly observe the transient structure and interface transform during CO2 reduction, provides the possibility to reveal the dynamic evolution of the real active site. Obtain the most comprehensive information on the interfacial reaction process and reaction mechanism, and then summarize the regularity of interface engineering.Regulation of structural properties of the catalysts is the key strategy to alter the catalytic performance, as discussed in the previous section. Other approaches, such as the electronic effect and tandem effect, have been employed to improve the performance of Cu-based bimetallic catalysts for CO2RR. Due to the different electronic configurations, coupling two types of metallic atoms can cause charge redispersion, and leads to the electronic effect. Therefore, the reduction behavior of CO2 on the catalyst is altered and exhibits high selectivity of desired products. Moreover, by rational design the active sites between Cu and the second metal, the bimetallic catalysts can drive the tandem effect to exhibit more deeply reduced multi-carbon products.The electronic effect has a unique ability to facilitate the reaction rate and the activity of CO2RR. From a thermodynamic point of view, due to the different electronic configurations of the host metal and the guest metal, the introduction of the guest metal can change the electronic structure of the host metal, thus changing the binding energy of intermediates [109,110]. In addition, Nørskov et al. reported that the electronic effects can result in the up and down shift of the d-band center, thus determining the reaction activity or binding energies of the intermediates and reactants [109,111]. Similarly, An et al. demonstrated the performance of AuCu3@Au catalyst is related to the binding strength of ∗COOH intermediate, influenced by the surface electronic structure (d-band center energy). The density of states indicates that the introduction of Cu causes the d-band center of AuCu3@Au to move toward the Fermi level, resulting in stranger adsorption of ∗COOH on the catalyst surface [112].In Cu-based bimetallic catalysts, Liu et al. prepared a homogeneous planar film to investigate the electronic effects on the CO2 reduction performance of Au–Cu bimetallic films (Fig. 14 a–d) [97]. The result indicated that as the Au content increases, the d-band center deviates from the Fermi level. The oxygen binding strength is significantly weakened while the desorption energy of ∗CO decreases, so favorable the release of ∗CO and suppressed the formation of ∗OCHO, thus increasing the CO yield and limiting the formation of HCOOH. Kim et al. demonstrated that the increased number of electrons in the s-band and the upward shift of the d-band center position of Au can influence the bonding to CO and result in the activity of electrochemical reduction of CO2 to CO [113].Due to the different binding abilities of intermediates, electronic effects can lead to a change in reaction pathways. Zu et al. proposed that the electronic effect promotes the formation pathway of HCOOH, while inhibiting the CO and H2 production [114]. As we known, the pathway to produce HCOOH is through the C atom of the ∗COOH intermediate bound to the catalyst surface, but meanwhile will produce unnecessary CO and competition HER, thus the selectivity of HCOOH is always unsatisfactory. However, after the doping of Bi atoms in Cu nanocrystals, the electronic effect can result in a high HCOOH selectivity by another pathway to produce ∗OCHO intermediate through the O atoms bound to the catalyst surface. This pathway is more favorable for HCOOH production because the ∗OCHO intermediate almost does not produce CO.The electronic effects originate from the changes in electronic structure, it also can promote CO2 activation and the C–C coupling process. Zhang et al. investigated the changes in CO2 reduction properties by doping a series of transition metals on Cu(100) by the DFT method [115]. The result indicated the order of catalytic activity is Zn > In > Cd > Ni > Sn > Fe > Pd > Co. Due to the doping of Zn, not only alters the electronic around Cu, but also changes the atomic arrangement of the active sites. The active sites on the catalyst surface become electronegative, which are favorable for CO2 activation and lower the energy barrier of C–C coupling.Overall, the electronic effects not only effectively promote the activation of CO2 [116,117], but also optimize the adsorption energy of intermediates [97,113,118], and even promote C–C coupling reactions [115]. However, the electronic effects often co-exist with other effects (such as strain effects), so it is very important to distinguish those effects. The electronic effect refers to the change of the chemical environment around the A atoms caused by the specific chemical properties of the B atoms. The interface strain effect originates from the lattice distortion due to the different atomic radii of the two atoms. In general, the electron effect is a short-range effect, and the strain effect is a long-range effect. The electronic effects require the surface atoms of the catalyst to be located on the surface or in the first or second subsurface layer. However, it is reported the strain effect has no such limitation and only strain effects can influence the reactions of more than multi atomic layers. For example, Strasse et al. found the strain effects could change the distance of surface Pt atoms, even though the other atoms are buried deeper than 1 nm [119]. Maark et al. demonstrated as the number of Cu overlayers increases (from 1 to 3 layers), electronic effects are filtered out, ultimately there are only the strain effects that exist in the Cu overburden [120]. Although it is difficult to distinguish those two effects by the experimental method, DFT calculations can easily examine them individually. For example, by constructing a Cu-M bimetal with the same lattice constant as Cu, it is possible to exclude strain effects and study electronic effects alone.Electrochemical CO2RR is a multiple electron/proton transfer process and requires multiple steps and intermediates. Tandem catalysts offer the possibility of breaking the linear scaling relationship and improving catalytic performance by coupling multi-step reactions [121]. The Tandem catalytic process refers to the intermediates (such as ∗CO) that are selectively produced on metal A sites, in the subsequent CO2RR process, transferred to the metal B sites, and further reduced to other products on metal B sites [122].In Cu-based bimetallic tandem catalysts, ∗CO is the most common intermediate, which can be generated from some metals such as Au, Ag, and Zn. The transfer of ∗CO to the Cu can enhance the possibility of C2 products generation. Therefore, improving the availability of ∗CO intermediates is the key to the subsequent deep reduction reactions. Zhang et al. investigated bimetallic tandem catalysis of Cu-modified Ag and Au, and gave the experimental evidence of ∗CO spillover by ATR-SEIRAS [123]. The free energy barriers for CO spillover on both Au and Ag surfaces are small, and the ∗CO adsorption on surface Cu sites is more stable than on Ag or Au sites, demonstrating the thermodynamic and kinetic feasibility of CO spillover from Ag or Au to Cu surfaces. It is beneficial to C–C coupling and improves the C2 products selectivity.In addition, high coverage of ∗CO allows for more CO spillover and is thermodynamically favorable for CO2RR to multi-carbon products [124–126]. Gao et al. operated the Raman spectroscopy revealed the formation and transfer of ∗CO on Ag/Cu catalysts, ∗CO spillover efficiency of approximately 95% from Ag to Cu, which significantly contributed to the selectivity of the C2 products [127]. The higher ∗CO coverage enables easier ∗CO adsorption of Cu, as well as greater opportunity for C–C coupling.Ren et al. propose a two-site mechanism and ∗CO insertion mechanism to explain the whole tandem reaction process [128]. In the Cu–Zn catalysts, CO2 is firstly being reduced to ∗CO on Cu or Zn (1 → 2 in Fig. 15 a). Secondly, ∗CO can be further reduced to ∗CHO or ∗CH x (x = 1–3) on the Cu site (2 → 3 in Fig. 15a). Thirdly, ∗CO will be desorbed on Zn, and the desorbed ∗CO will diffuse to the Cu site (2 → 3 in Fig. 15a). Then the spilled-over ∗CO can insert into the bond between the Cu surface and ∗CH2, to form ∗COCH2 (3 → 4 in Fig. 15a). Further reduced to CH3CHO (4 → 5 in Fig. 15a) and finally to form C2H5OH (5 → 6 in Fig. 15a). Similarly, Lee et al. further demonstrated that during the formation of C2H5OH, the sample which enriched Cu–Ag biphasic boundary can facilitate the insertion of ∗CO into the ∗CH2 and Cu, and result in the enhancement of the C2H5OH pathway (Fig. 15b) [129]. The homogeneous mixing Cu–Ag biphasic boundary can provide a sufficient interface for enhancing tandem catalytic performance.The Tandem effect can effectively improve the selectivity of C2 products, such as C2H5OH [128,130,131] and C2H4[112]. However, many factors affect the tandem process. Firstly, it depends on the space management of the ∗CO transfer. Zhang et al. showed in a tandem catalytic process, the appropriate spatial management of ∗CO transport can enhance the yield of C2 products. The migration of ∗CO between the ZnO and Cu sites does not require the two active sites to be adjacent [132]. Lee et al. also found the spacing of Ag and Cu should be abundant and suitably, so the insertion process of CO can become beneficial [129].In addition, Buonsanti et al. reported the facet-dependent selectivity for tandem catalytic. The results show that due to the tandem effect of Cuoh ((111) facets with high CH4 selectivity) and Cucub ((100) facets with high C2H4 selectivity), both Cuoh-Ag and Cucub-Ag catalysts can inhibit CH4 and H2, while promoting the selectivity of C2H5OH. Cuoh-Ag has a better selectivity of C2H5OH than Cucub-Ag. Due to the enriched CO generated by Ag, the key step of the C2H5OH pathway is ∗CH x –∗CO coupling process. However, the active sites which can promote this step are only distributed at the edges and corners of the Cucub-Ag catalyst, this means that the Cucub-Ag cannot efficiently reduce CO2 to C2H5OH [130]. Except for this facet-dependent tandem catalysis performance, they reported a size-dependent tandem catalysis on CO2 reduction selectivity [131]. In the Cucub-Ag catalyst, with the enriched ∗CO intermediate, Cucub with a smaller size has higher selectivity of C2H5OH due to its larger edge-to-faces ratio.The tandem effect shows great potential for improving the selectivity of valuable C2 products. In this process, ∗CO is the main intermediate, so its spillover processes are studied in detail. The efficiency of ∗CO spillover can effectively promote the tandem effect. The high ∗CO coverage provides more available CO, which is beneficial to C–C coupling and generation of C2 products. However, it is worth noting that the key evidence of ∗CO spillover is usually obtained from Operando Raman Spectroscopy. More accurate and convenient characterizations should be developed in the future. Based on advanced in situ characterization techniques, future studies on tandem catalysts should focus on the efficient utilization of ∗CO intermediates and their effects on CO2 reduction reaction. For example, it has been demonstrated that under high ∗CO coverage, ∗CO with top adsorption mode is more favorable for the reduction of CO2 to C2 products. In addition, the confinement effect is an effective strategy to enrich the ∗CO intermediate, which deserves further study.By summarizing the CO2 reduction process, it can be concluded that: (1) CO2 adsorption/activation is often the rate-determining step that determined the activity of CO2RR. (2) The C–C coupling process is the key to the generation of the more valuable C2 products. (3) The different adsorption abilities between two metals and intermediates can lead to different reduction products. Cu-based bimetallic catalysts offer new strategies to break the traditional linear scale relationship and improve the activity and selectivity of CO2RR. We summarize recent advances in the reduction of CO2 over Cu-based bimetallic catalysts. The role that second metal act as a reaction site is considered in detail. Such as the strain effect can change the d-band center of Cu, and thus alter the adsorption behavior of the intermediates. The tandem effect enables the coupling of multiple reaction steps to achieve sequential successive catalysis. In addition, the role of the second metal that only change the morphology and electronic structure is also briefly analyzed (morphology effect and electronic effect). For CO2 reduction reaction, provides a comprehensive comprehension of the design of Cu-base bimetallic catalysts in conjunction with the theoretical understanding. However, the reduction of CO2 to valuable additional products at low cost and high efficiency still is a big challenge. It is required to develop more efficient catalysts and a more instructive understanding of the mechanisms of CO2 reduction. Therefore, we briefly discuss the perspective for future studies in these areas. (1) The atmospheric CO2 concentration is extremely low compared to the ideal experimental conditions (saturated CO2 concentration in the electrolyte). It remains to be seen whether the prepared catalysts are still effective under atmosphere circumstances. Therefore, it is important to achieve efficient conversion of CO2 at low concentrations to meet the carbon reduction target. (2) The electrocatalytic reduce CO2 to multiple carbon products in liquid electrolytes faces two challenges: the low activity of the catalyst in non-alkaline electrolytes, and the formation of carbonates in alkaline electrolytes. Therefore, it is necessary to develop a strategy for designing efficient reduce CO2 in liquid electrolytes. In addition, the electrolyte currently used for CO2 reduction is mainly KHCO3, because it has been suggested beneficial to the dissolution of CO2 and thus promotes the reaction. However, the current research on how the electrolyte properties (e.g., concentration, species, pH) affect the CO2 reduction performance is still very limited. Therefore, more attention needs to be paid to the effect of electrolytes in the future. (3) It is well known that the adsorption configuration often influences catalytic performance. However, there are a few studies about whether the reactant molecules/intermediate species affect the catalyst interface property. To maintain the stability of the catalyst. Research in this area should be considered seriously. (4) At present, the reduction products on Cu-based bimetallic catalysts are mostly C2 products. Multi-carbon products, such as C3 and C4 products are still few and have very low selectivity. In the complex process of CO2 reduction to multi-carbon products, it is particularly important to design catalysts with C3 and C4 products. Using the multi-metal system to complete the reactions of each step and in turn, to form a tandem catalytic process to obtain products with high added-value is a possible strategy. Therefore, in the future should focus on the design of Cu-based ternary (or even more than ternary) catalysts. The atmospheric CO2 concentration is extremely low compared to the ideal experimental conditions (saturated CO2 concentration in the electrolyte). It remains to be seen whether the prepared catalysts are still effective under atmosphere circumstances. Therefore, it is important to achieve efficient conversion of CO2 at low concentrations to meet the carbon reduction target.The electrocatalytic reduce CO2 to multiple carbon products in liquid electrolytes faces two challenges: the low activity of the catalyst in non-alkaline electrolytes, and the formation of carbonates in alkaline electrolytes. Therefore, it is necessary to develop a strategy for designing efficient reduce CO2 in liquid electrolytes. In addition, the electrolyte currently used for CO2 reduction is mainly KHCO3, because it has been suggested beneficial to the dissolution of CO2 and thus promotes the reaction. However, the current research on how the electrolyte properties (e.g., concentration, species, pH) affect the CO2 reduction performance is still very limited. Therefore, more attention needs to be paid to the effect of electrolytes in the future.It is well known that the adsorption configuration often influences catalytic performance. However, there are a few studies about whether the reactant molecules/intermediate species affect the catalyst interface property. To maintain the stability of the catalyst. Research in this area should be considered seriously.At present, the reduction products on Cu-based bimetallic catalysts are mostly C2 products. Multi-carbon products, such as C3 and C4 products are still few and have very low selectivity. In the complex process of CO2 reduction to multi-carbon products, it is particularly important to design catalysts with C3 and C4 products. Using the multi-metal system to complete the reactions of each step and in turn, to form a tandem catalytic process to obtain products with high added-value is a possible strategy. Therefore, in the future should focus on the design of Cu-based ternary (or even more than ternary) catalysts.The authors declare no conflicts of interest.The authors acknowledge the financial support from the International Science and Technology Cooperation Program (Grant No. 2018YFE0203400 and 2017YFE0127800), the National Natural Science Foundation of China (Grant No. 22002189, 21872174, and U1932148), Hunan Provincial Natural Science Foundation (2020JJ2041, 2020JJ5691) and Key R&D Program of Hunan Province (2020WK2002). We are grateful for resources from the High Performance Computing Center of Central South University.

Electrocatalytic CO2 reduction reaction (CO2RR) is one of the effective means to realize CO2 resource utilization. Among the high-efficiency metal-based catalysts, Cu is a star material profiting from its ability for CO2 reduction into valuable hydrocarbon products. However, due to the difficulty in activating CO2 and regulating intermediate adsorption/desorption properties, the CO2RR activity and selectivity of Cu-based catalysts still cannot meet the requirements of industrial applications. The design of Cu-based bimetallic catalysts is a potential strategy because the introduction of the second metal can well promote the activation of CO2 and break the linear scaling relationship in intermediate adsorption/desorption. In this review, the synergistic enhancements of Cu-based bimetals on CO2 activation and intermediate adsorption/desorption are analyzed in detail, including the advantages caused by the morphology of Cu-based bimetallic catalysts, the local electric field effect induced by the special nanoneedle structure, the interface engineering (strain effect, atomic arrangement, interface regulation), and other particular effects (electronic effect and tandem effect). Finally, the challenges and perspectives on the development of Cu-based bimetallic catalysts for CO2 reduction are proposed.

The authors declare that the data supporting the findings of this study are available within the article and the supplemental information. The data and results supporting the present study are available from the lead contact upon request.Single-atom catalysts (SACs), comprised of isolated metal atoms anchored on supports, have attracted great attention in recent years because of their high efficiency and excellent selectivity toward various reactions. 1–6 When the metals on the supports are downsized to the minimum, SACs reach superhigh atom-utilization efficiency (close to 100%). 7–10 Moreover, the isolated metal atoms usually deliver different electronic states than that of the metal nanoparticles, endowing excellent catalytic performance due to the strong metal-support interaction and distinctive coordinated environments. 11–13 The unique, well-defined, and uniform active sites of SACs also provide an ideal platform for studying the electrocatalytic mechanism at the atomic level.In the last decade, rational design and fabrication of SACs have been wildly developed, and some inspiring results (including synthesis methods and electrochemical performance) have been reported. 14–17 There novel synthesis methods include the incipient wet chemical method, 18 , 19 pyrolysis method, 20 , 21 physical deposition method, 22 , 23 and atom trapping method among others. 24 , 25 For instance, the wet chemical method has been demonstrated to be a general and effective strategy for the synthesis of SACs and is achieving some inspirational results. However, owing to the limitations of the mechanism, the synthesis of SACs with large-scale production and high metal loading remains a great challenge for the wet chemical method. 26 , 27 The facile pyrolysis method has been reported to show some advantages in the realization of high metal loading of SACs, but the loading could not be easily controlled owing to the complex and unknown high-temperature chemical reaction pathway. 20 , 28 In a word, significant progress has been achieved on the preparation strategies of SACs, while some issues still need to be addressed. Firstly, the metals tend to leach off in the operating conditions due to the high free energy of single-metal atoms, so strong metal interaction or complicated defect strategies are usually needed to stabilize the single-metal atoms. 29 , 30 Secondly, sophisticated experimental design is needed for many synthesis methods owing to the disparate coordination environment between various metal atoms and defect types. 31–34 Thirdly, the SACs derived from the bulk metal salt or compounds still need to be achieved at high temperatures, and the processes are generally time consuming. 35–37 Moreover, the single-batch production ability is usually at the milligram level, which greatly limits its practical application. 38 , 39 Therefore, developing a general, high-efficient, large-scale, and tunable method without high temperature to synthesize SACs is still urgently needed.Herein, we demonstrate a general and high-efficiency “plasma bombing” strategy to directly transform commercially available metal salts into SACs on a large scale. Notably, the proposed strategy can be readily extended to various transition metals, benefiting the synthesis and practical applications of the SACs. As a typical case, the prepared SAC-Fe/NC catalyst displays a remarkable performance and durability both for rotation disk electrode (RDE) and the assembled zinc-air batteries (ZABs). Furthermore, density functional theory (DFT) calculations provide deep insights into the remarkable oxygen reduction reaction (ORR) mechanism that the adsorbent-induced spin-crossover effect is involved and the superb ORR performance is mainly contributed by the pyridinic-type Fe-N4 moieties.The synthesis process of the “plasma bombing” strategy to prepare SACs is shown in Figure 1A (more detailed synthesis information is shown in the experimental procedures section). Briefly, the bulk metal salts are excited and stripped to form the flow of the single-metal atoms under the nitrogen “plasma bombing” treatment, and simultaneously the single-metal atoms are trapped and anchored by the defective and nitrogenous sites on the carbon supports to obtain the SACs.The SAC-Fe/NC is characterized as a model to verify the successful preparation of the “plasma bombing” strategy to fabricate SACs. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of the SAC-Fe/NC display highly disordered carbon structures, and no obvious metal singles (e.g., metal nanoparticles and/or metal clusters) are found on the carbon supports (Figures 1B and S1; supplemental experimental procedures). In addition, the X-ray diffraction (XRD) patterns of pure NC and SAC-Fe/NC are shown in Figure S2: the location and densities of the peaks of SAC-Fe/NC are almost the same as pure NC, indicating no peaks of Fe crystals are detected. Moreover, the Fe signals can be clearly observed from the XPS survey and high-resolution Fe 2p XPS spectrum (Figures S3A and S3B), confirming the existence of the Fe species on the carbon supports. Furthermore, the highly dispersed and homogeneously distributed Fe sites can be observed in the aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM) image (Figure 1C). The HAADF-STEM EDS mapping images show that the Fe and N elements are uniformly dispersed over the whole carbon supports (Figure 1D). The metal content of the Fe in SAC-Fe/NC is as high as 8.5 wt % (Table S1), determined by the inductively coupled plasma optically emitting spectrometer measurement (ICP-OES).X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) are conducted to determine the fine electronic structure of the Fe species on SAC-Fe/NC. Figure 1E exhibits the Fe K-edge XANES of SAC-Fe/NC and standard materials (e.g., Fe foil and Fe2O3). The Fe K-edge absorption energy of SAC-Fe/NC is between the Fe2O3 and Fe foil, indicating the Fe atoms exhibit a positive valence state below +3. A dominate peak at approximately 1.44 Å is detected in the EXAFS data of SAC-Fe/NC, corresponding to the first Fe-N coordination shell (Figure 1F). A Fe-Fe bond at approximately 2.2 Å for Fe foil is not observed in SAC-Fe/NC, confirming the Fe species exist as signal isolated atoms on the supports (Figure 1F). Figure 1G displays the best-fitting result of the EXAFS data of SAC-Fe/NC compared with the experiment data (Fe species are coordinated with four pyridinic-type N atoms), revealing that the Fe atoms are bonded by four pyridinic-type N atoms (Fe-(pd)-N4 coordination structure). The atomic configuration of SAC-Fe/NC is further investigated by the Fe K-edge wavelet transform (WT)-EXAFS, due to the powerful resolution in both k and R spaces of the WT. Different from the WT signals of Fe foil and Fe2O3, we do not find the Fe-Fe coordination in SAC-Fe/NC, yet a bright and clear Fe-N signal is observed, which further identifies the isolated feature of Fe species in SAC-Fe/NC (Figures 1H and 1J).The specific surface area and pore structure of the carbon supports is characterized by the Brunauer-Emmett-Teller. As shown in Figure S4, the specific surface area and pore volume of SAC-Fe/NC are evaluated to be as high as 624 m2 g−1 and 0.95 cm3 g−1, respectively. In addition, the N2 adsorption shows a typical type I behavior of SAC-Fe/NC, indicating the existing small pore size of the carbon supports. The pore size of the SAC-Fe/NC is further estimated to be 1.2 nm by calculating the adsorption branch via nonlinear DFT (Figure S4B). The carbon structure of the prepared catalyst and pure NC are also characterized by Raman spectroscopy. There are two obvious broads obtained for both SAC-Fe/NC and pure NC, which belong to the D band and G band, respectively (Figure S5). The ID/IG values of the SAC-Fe/NC and pure NC are calculated as 1.15 and 1.09, respectively. The higher the ID/IG values are, the more structural defects. The structural defects of the SAC-Fe/NC are generated by the “plasma bombing” treatment, and more defects are beneficial to capturing the dissociated Fe atoms to form the Fe SACs.As a new strategy for fabricating SACs, one of the most important aspects is robust and general. The proposed “plasma bombing” strategy can be readily extended to the preparation of other SACs with the similar synthesizing procedures of SAC-Fe/NC by only varying the metal precursors (detailed information is shown in the experimental procedures section). The atomically dispersed atoms (Mn, Ni) are confirmed by the AC HAADF-STEM images (Figure S6), and XRD patterns as well as HAADF-STEM EDS mapping images of the prepared samples provide more solid evidence for the successful synthesis of various SACs (Figures S7 and S8; Table S1). To demonstrate the superiority of the proposed “plasma bombing” strategy to fabricate SACs, recently reported SAC preparation methods are summarized in Table S2. The proposed “plasma bombing” strategy outperforms most of the previously reported SAC synthesis approaches, in terms of synthesis time, metal loading, universality, and operational difficulty. In short, the proposed “plasma bombing” strategy is a general, novel, and high-efficiency way to synthesize SACs.The ORR performance of the prepared catalysts is measured in O2-saturated 0.1 M KOH solution, and Pt/C is also evaluated for comparison. The SAC-Fe/NC exhibits a high half-wave potential (E1/2) of 0.920 V and kinetic current density (jk) of 9.890 mA cm−2 (Figures 2A and 2B and Table S3), much higher than those of Pt/C (0.860 V, 1.755 mA cm−2) and pure NC (0.768 V, 0.165 mA cm−2). Compared with the Pt/C (72 mV dec−1) and pure NC (114 mV dec−1), the SAC-Fe/NC exhibits the smallest Tafel slope of 67 mV dec−1, confirming much faster kinetics of the ORR process (Figure 2C). Furthermore, a low peroxide yield of less than 4% over the potential range of 0–1.0 V is obtained for the SAC-Fe/NC, indicating a dominant four-electron ORR process (Figure S9). Besides the high activity, there is almost no activity decay in terms of the E1/2 and jk after accelerated durability test (ADT), indicating superb durability of the prepared SAC-Fe/NC (Figure 2D and Table S4).To probe the potential commercial application of the SAC-Fe/NC, it is applied as the air-cathode electrode for the homemade ZABs. As shown in Figure 2E, the peak power density of SAC-Fe/NC-based ZAB is 263 mW cm−2, which is about 1.7-fold higher than the Pt/C-based ZAB (151 mW cm−2). What’s more, the specific capacity of the SAC-Fe/NC-based ZAB reached 803 mAh g−1, much better compared with Pt/C-based ZAB (701 mAh g−1) (Figure 2F). The rate-discharge curves over various discharge current densities of the SAC-Fe/NC-based ZAB are shown in Figure 2G, and a stable plateau is displayed at each discharge current density. Moreover, the SAC-Fe/NC-based ZAB can stably work for over 104 h without an obvious decrease in the discharge voltage, which also reveals that the SAC-Fe/NC-based ZAB possessed excellent discharge stability (Figure S10). Additionally, the unit output of the SAC-Fe/NC is 1.16 g (Figure S11), and considering its efficient preparation process (about 4 h for a single batch), the proposed “plasma bombing” method exhibits great potential for large-scale production. The performance comparison of SAC-Fe/NC catalyst and the recently published non-noble metal-based SACs in both RDE and ZAB level are summarized in Figures 2H and 2I and Tables S5 and S6. It can be seen that SAC-Fe/NC demonstrates outstanding performance among these catalysts, further verifying the highly effective and durable quality of SAC-Fe/NC catalyst synthesized by the “plasma bombing” strategy. The excellent ORR performance of the prepared SAC-Fe/NC catalysts could owe to the unique and uniform active sites, as well as the strong metal-support interaction and distinctive coordinated environments. Moreover, to further verify the high efficiency and practicality of the proposed “plasma bombing” strategy, the ORR activity (including RDE and ZAB level) of the SACs prepared by different synthetic methods is also summarized in Table S2. The ORR performance of prepared SAC-Fe/NC is found to outperform most of the reported catalysts (Table S2). It provides solid evidence to confirm the practicality of the proposed “plasma bombing” strategy.First principal calculations were carried out to provide theoretical insights into the structure and electronic structure evolution of Fe-N4 sites during the oxygen reduction reaction process. Previous investigations on the spin-crossover (SC) ORR mechanism show that the spin states in Fe cations have a visible impact on the ORR performance of Fe-based SACs. 40 To determine the ground state of pyridinic-type (denoted as Fe-pd-N4) and pyrrolic-type FeN4 (denoted as Fe-po-N4), various spin states, including low-spin (LS), intermediate-spin (MS), and high-spin (HS) states, are considered. As summarized in Tables S7 and S8, the type of nitrogen atoms influences the ground state of Fe SAC. Fe-pd-N4 and Fe-po-N4 prefer MS and HS state as the ground state, respectively. The distribution of spin-charge density indicates that the Fe cations make a major contribution to the magnetic moments; see Figure 3A and Tables S7 and S8. It is worth noting that there may be a limited number of transferring charges from Fe cations in FeN4 moieties to O∗ anions; the O∗ anions with partially filled p orbitals exhibit relatively large local magnetic moments of 0.40 and 0.45 μB in Fe-pd-N4 and Fe-po-N4 systems, respectively. Moreover, their ground states are vulnerable to adsorbed intermediates. The OOH∗, O∗, and OH∗ configurations for Fe-pd-N4 exhibit LS, MS, and HS ground state, respectively, while those for Fe-po-N4 are in HS, MS, and HS in the ground state, respectively, revealing an adsorbent-induced spin-crossover (AISC) ORR process in Fe SACs.Based on the computational hydrogen electrode (CHE) methodology, four ORR pathways, including LS, MS, HS, and AISC pathways, are calculated to evaluate the catalytic performance of Fe-pd-N4 and Fe-po-N4. The energy-favoring AISC pathway plays a major contribution to the ORR performance, while the contributions of LS, MS, and HS pathways on catalytic activity are relatively small. The rate-determining step of the LS pathway in Fe-pd-N4 is the formation of OH∗, while that of the others is the desorption of OH∗. As displayed in Figures 3B–3E, the LS and MS pathways exhibit a smaller overpotential than the HS pathway in both Fe-pd-N4 and Fe-po-N4. Moreover, the overpotential of LS, MS, and SC pathways in Fe-pd-N4 is estimated to be 0.57, 0.48, and 0.73 V, smaller than those of LS (0.69 V), MS (0.70 V), and SC (0.90 V) pathways in Fe-po-N4, respectively, indicating that the ORR activity of pyridinic-type Fe-pd-N4 is better than that of pyrrolic-type catalysts.In conclusion, a general “plasma bombing” strategy to transfer bulk metal salts into SACs is developed and validated, which is confirmed to be facile, effective, tunable, and capable of large-scale production. Driven by the energy of the plasma bombing, the surface metal salts are evaporated to generate single-metal atoms, which are trapped and anchored by the defective and nitrogenous sites of the carbon support, forming the isolated SAC-M/NC catalysts (M = Fe, Mn, Ni, and so on). Impressively, the prepared SAC-Fe/NC exhibits both high activity and durability for ORR, and it also achieves high performance for the assembled ZABs. More importantly, DFT calculations provide theoretical insights into the AISC ORR mechanism in FeN4-based SACs and demonstrate that the Fe-pd-N4 sites of the SAC-Fe/NC play a major role in the high ORR performance. The findings open new paths for the rational design of SACs from non-noble bulk metal salts to isolated single-metal atoms, which possess great opportunities for the practical application of SACs in ZABs and beyond.Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Xinlong Tian ([email protected]).This study did not generate new unique materials.In a normal procedure, the FeCl2 (50 mg) and powder of NC (100 mg) were separately placed on the porcelain boat. The porcelain boat was placed in the plasma-enhanced chemical vapor deposition (PECVD), and then started on the tube furnace of the PECVD with the following parameters: the temperature was 400°C, holding time was 60 min, and under the N2 flowing. When the temperature reached 400°C, we turned on the PECVD with the following parameters: radio frequency power was 500 W, the processing time was 40 min, the tube pressure was 50 Pa, and under the N2 flowing. After the temperature of the instrument dropped to room temperature, the SAC-Fe/NC was obtained.TEM, HRTEM, and HAADF-STEM EDX images were performed by using a Thermo Scientific Talos F200X G2 operated at 200 keV. AC HAADF-STEM image was obtained on an aberration-corrected FEI Titan G2 60–300 field-emission TEM (FEI, USA), operated at 300 keV (αmax = ∼100 mrad). XRD was conducted on HAOYUAN powder diffractometer (DX-2700BH). XPS data were collected using a Thermo Scientific NEXSA X-ray photoelectron spectrometer with a monochromatized Al Ka X-ray source (1,486.6 eV). Raman spectra were collected using a Horiba Scientific with 532-nm laser excitation. The actual loading of the metals was determined by using ICP-OES (Aglient 5110). The X-ray absorption spectra including XANES and EXAFS of the sample at Fe K-edge were measured at the beamline 1W1B of the Beijing Synchrotron Radiation Facility (BSRF, Beijing).All ORR performance tests were conducted on a Gamry 1010E electrocatalytic station with a three-electrode electrochemical cell, in which Hg/HgO was applied as the reference electrode, and a graphite rod was used as the counter electrode. A glass carbon electrode (GCE, diameter 5 mm) was applied as the working electrode. The catalysis ink was prepared via ultrasonically 5-mg catalysts dispersed into 1 mL ethanol including 30 μL Nafion (5 wt %). After that, we dropped 7 μL of the catalysis ink onto the GCE to obtain the working electrode. The LSV curves were recorded at a rotating speed of 1,600 revolutions per minute (rpm) with a scan rate of 5 mV s−1 in O2-saturated 0.1 M KOH. The ADT method was applied to test the durability, with 10,000 cycles of potential cycling from 0.6 to 1.0 V at 100 mV s−1, and then we recorded the LSV curves.For the rotating ring-disk electrode tests, the Pt ring electrode was biased at 1.2 V versus reversible hydrogen electrode (RHE), leading to the electro-oxidation of H2O2, which occurred during the ORR process. The H2O2 yield and n per oxygen molecule were calculated by the following equations: (Equation 1) % H 2 O 2 = 200 I R / N I D + I R / N (Equation 2) n = 4 I D I D + I R / N where ID and IR are the disk and ring currents, respectively. N is the ring current collection efficiency (37%).All potentials in this work are quoted with respect to an RHE.In the homemade ZAB, the polished zinc plate was used as the anode electrode, catalyst loaded on carbon paper (catalysts loading: 1 mg cm−2) was applied as the air-cathode electrode, and 6 M KOH solution was used as electrolyte. The polarization curves of the assembled ZAB were recorded via a Gamry 1010E electrochemical workstation. The discharge polarization curve was carried out on a LANHE (CT2001A) battery testing system.Spin-polarized DFT calculations were performed using the Perdew-Burke-Ernzerhof (PBE) 41 functional and the projector augmented wave (PAW) 42 , 43 potential as implemented in the Vienna Ab Initio Simulation Package (VASP). 44 , 45 The Ueff = U – J = 3 eV 46 was applied to Fe’SD orbitals. An energy cutoff of 500 eV and a convergence criterion of 10−5 eV for self-consistent calculations was adopted. All structures were fully relaxed until the total force on each atom was less than 0.05 eV/Å. The solvent effect was included by using the implicit solvation model as implemented in the VASPsol code. 47 , 48 The van der Waals interaction was corrected based on the DFT-D3 scheme. 49 The thickness of the vacuum layer was larger than 15 Å. A 6 × 6 × 1 graphene supercell was used to model the Fe-pd-N4. The Fe-po-N4 model was derived from the pyrrole-type FeN4 model. 30 A Γ-centered k-point with a resolution less than 0.03 × 2π/Å was used. VASPKIT code 50 and VESTA software 51 were used for calculation pre-processing and post-processing.The CHE model 52 was used in our calculations. The Gibbs free energy of molecules and ORR-related absorbates was calculated by G = EDFT + ZPE – TS, where EDFT, ZPE, and S were the DFT energy, zero-point energy, and entropy, respectively, and temperature T was adopted as 298.15 K. The ORR involves four four-electron pathways on the active sites. The theoretical overpotential at equilibrium potential was determined according to η = 1.23 – |▵Gmax/e−|, where ▵Gmax was the maximum free energy change of adjacent electronic steps.This work was supported by the Hainan Province Science and Technology Special Fund (ZDYF2020037, 2020207), the National Natural Science Foundation of China (21805104, 22109034, 22109035, 52164028, 62105083), the Start-up Research Foundation of Hainan University (KYQD(ZR)- 20008, 20082, 20083, 20084, 21065, 21124, 21125).Conceptualization, P.R. and X.-L.T.; methodology, P.R., D.-X.W., and J.-M.L.; investigation, P.R., D.-X.W., J.L., P.-L.D., Y.-J.S., and X.-L.T.; writing - original draft, P.R. and X.-L.T.; writing - review & editing, P.R., D.-X.W., and X.-L.T.; funding acquisition, D.-X.W., J.L., J.-M.L., P.-L.D., Y.-J.S., and X.-L.T.; supervision, X.-L.T.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.2022.100880. Document S1. Figures S1–S11, Tables S1–S8, and supplemental experimental procedures Document S2. Article plus supplemental information

Single-atom catalysts (SACs) have attracted tremendous attention owing to their unique intrinsic properties, while the facile synthesis, especially the direct transformation of metal salts into single-metal atoms, is still a huge challenge. Here we report a practical approach to access the large-scale synthesis of SACs, in which the metal salts are excited and stripped as mobile single-metal atoms flow under the “plasma bombing” treatment. Simultaneously, they are trapped and anchored by the defective and nitrogenous sites of the supports. More importantly, the synthesis approach is quite general, and it can be extended to the fabrication of various SACs. As an illustration, the prepared SAC-Fe/NC delivers a remarkable oxygen reduction reaction (ORR) performance. In combination with the theoretical analysis, an adsorbent-induced spin-crossover ORR mechanism is proposed, and the Fe species coordination with four pyridinic-type N atoms is demonstrated as the main contributor to the high performance of the prepared SAC-Fe/NC.

Non-edible lignocellulose is the most abundant, cheapest and fastest growing sustainable biomass resource, composed of three primary biopolymers: cellulose (a polymer of glucose), hemicellulose (a polymer mainly of pentoses) and lignin (a highly cross-linked polymer of substituted phenols) [1]. In order to produce value-added bio-products which could displace petroleum feedstocks, lignocellulose must first be transformed into simpler and more easily processed platform chemicals. This approach, similar to that used in conventional petroleum refineries, would allow the simultaneous production of biofuels and biochemicals in an integrated facility, a biorefinery [2].In 2004, the U.S. Department of Energy (DOE) [3] released a report, later revised by Bozell et al. [4], identifying the top value-added platform chemicals in a future biorefinery. HMF was identified as one of the most appealing and promising building block molecules. This furan derivative can be produced from agricultural waste and forest residue such as polysaccharides (i.e. cellulose and hemicellulose) by acid-catalysed hydrolysis to C6 monosaccharides, followed by dehydration [5]. In contrast to most petrochemical products, HMF is an oxygen-rich, functionalized compound. Its conversion to value-added chemicals usually involves several chemical transformations (e.g. hydrogenation, dehydration, hydrogenolysis, oxidation, etc.) which are promoted by multifunctional heterogeneous catalysts [6,7]. The development of related catalytic heterogeneous processes has become highly topical to produce valuable bioproducts such as: tetrahydrofuran 2,5-diyldimethanol (THFDM) [8–10], 2,5-dimethylfuran (DMF) [11], 2,5-furandicarboxylic acid (FDCA) [12,13], C6 linear alcohols [14,15], 3-hydroxymethylcyclopentanone (HCPN) and 3-hydroxymethylcyclopentanol (HCPL) [16,17] (Fig. 1 ).One much less studied reaction is the conversion of HMF into the linear diketone derivatives 1-hydroxyhexane-2,5-dione (HHD) and 2,5-hexanedione (HXD). Although a large-scale synthetic route to HHD is currently not available, the presence of a hydroxymethyl functionality offers opportunities for the synthesis of valuable chemicals, as recently highlighted [18–20]. HXD is employed as a solvent and as an intermediate for the synthesis of polymers, amines and surfactants [21,22]. HHD can be produced from HMF in water under H2 pressure via the metal-catalysed selective hydrogenation of the carbonyl group to furan-2,5-diyldimethanol (FDM), followed by the acid-catalysed ring-opening of the FDM unsaturated ring [23]. Notably, this pathway does not involve the formation of a more stable saturated tetrahydrofuran ring, whose ring-opening requires harsh reaction conditions (i.e. T > 140 °C; P > 60 bar) [24]. HXD can also be produced from FDM via hydrogenolysis to DMF, followed by hydrolytic ring-opening of the latter or by HHD via scission of the hydroxyl group (Fig. 2 ) [22,25].The formation of HHD from HMF was firstly reported in 1991 by Schiavo et al. using Pd/C in an aqueous solution of oxalic acid (pH = 2) at 70 bar H2 and 140 °C [23]. In 2009, Luijkx et al. reported a similar process using HCl [26]. In 2014, Liu et al. developed two binary catalytic systems using Pd/C either in CO2/H2O (forming carbonic acid) [27], or in THF with co-added Amberlyst-15 [28], affording in both cases 77% yield of HHD (see Table SI1). Recently we reported that HHD is formed as a low yield intermediate (<7%) in the conversion of HMF to HCPN and HCPL over M-Al2O3 catalysts in H2O (M = Co, Ni, Cu). HHD was rapidly converted to HCPN via an aldol condensation reaction, catalysed by basic sites, followed by hydrogenation [17].The requirement of hydrogenating metal phases and acidic sites for the production of HHD prompted us to investigate the deposition of various transition metals over zeolite supports to prepare easily tuneable bifunctional catalysts. Among the most investigated zeolites, Beta (with BEA topology) exhibits excellent properties to the aimed transformation due to its high hydrothermal stability, large specific surface area (>600 m2 g−1), 3D large-pore channel system (5.5–7.6 Å) and dual Lewis/Brønsted acidity [29]. Beta zeolite-based catalysts have been used for the conversion of furfural into levulinic acid [30,31] and for the hydrodeoxygenation of furoins into alkanes [32]. However, to the best of our knowledge, there are no studies reporting the formation of linear diketone derivatives using Beta zeolite-supported catalysts.Herein, we present the catalytic production of the diketone derivatives HHD and HXD from HMF by zeolite-supported transition metals in H2O. A series of transition metal-loaded (M) Beta zeolites were prepared (M = Co, Ni, Cu, Ru, Pd), characterised and tested in a batch stirred reactor under H2 pressure with Pd showing the highest catalytic activity. Consequently, the effects of Pd particle size, zeolite Si/Al ratio and reaction conditions were investigated, and catalyst stability and recyclability were evaluated. This work establishes Beta zeolite-supported Pd catalysts as promising candidates for the upgrading of HMF into valuable biomass-derived linear diketone derivatives by demonstrating for the first time the conversion of HMF to HHD and HXD in water by a solid state, bifunctional (no acid co-added) catalyst.Metal-loaded zeolites were prepared by incipient wetness impregnation (IWI) of commercially available Beta (Si/Al = 12.5) and ZSM-5 (Si/Al = 11.5) zeolites purchased from Zeolyst Int. The corresponding aqueous solutions of the metal precursors were prepared using: PdCl2 (Sigma-Aldrich), Pd(NO3)2·2H2O (Aldrich), RuCl3 (Aldrich), NiCl2·6H2O (Aldrich), CuCl2·2H2O (Aldrich) and CoCl2·6H2O (Fluka). Prior to impregnation, the parent NH4-zeolites were calcined at 550 °C (heating rate of 2 °C min−1) in static air for 5 h, producing the respective H-zeolites. The deposition of the metal was carried out by adding dropwise the aqueous solution of the precursor to the zeolite support at room temperature (3 wt% for the Pd and Ru samples; 10 wt% for the Cu, Ni and Co samples). After impregnation, the catalysts were dried in a rotary evaporator at 65 °C under vacuum for 1 h. Subsequently, the dried samples were calcined in air at 500 °C for 5 h (heating rate of 2 °C min−1). The reduction treatment was performed under pure H2 flow (100 cm3 min−1) for 5 h at 200 °C (for the Pd and Ru samples), 300 °C (Cu sample) and 500 °C (Ni and Co samples) with a heating rate of 2 °C min−1. Finally, the catalysts were passivated under a flow of 1% v/v O2/N2 (100 cm3 min−1) for 2 h at room temperature.Pd-loaded Beta zeolite was also prepared by deposition-coprecipitation (DP-CP) using the urea-based method developed by Geus et al. [33]. First, 2 g of the calcined Beta zeolite were placed in a 250 ml round-bottom flask. Then, an aqueous solution (100 ml) containing PdCl2 (0.005 M) and urea (1.2 M, Sigma) was added dropwise with constant stirring (550 rpm) at room temperature. The suspension (pH = 4–4.5) was heated to 95 °C to initiate urea hydrolysis. After 3 h, the pH of the suspension remained stable at pH ≈ 7.5. The solution was cooled to room temperature and the precipitate was collected by filtration, washed with deionized water, dried at 110 °C overnight and subsequently calcined in air at 500 °C for 5 h (heating rate of 2 °C min−1). The calcined sample (catalyst precursor) was then reduced under pure H2 flow (100 cm3 min−1) at 200 °C for 5 h (heating rate of 2 °C min−1). Finally, the reduced catalyst was passivated under a flow of 1% v/v O2/N2 (100 cm3 min−1) for 2 h at room temperature. Hereafter, the sample prepared by the DP-CP urea method will be referred as Pd(u)/Beta.Partial dealumination of the calcined Beta zeolite (Si/Al = 12.5) was carried out by acid treatment using HNO3 aqueous solutions of different concentration (0.1, 0.5, 2 and 5 M) at room temperature for 1 h (20 mL g−1 zeolite). After filtration and washing with deionized water, the materials were dried overnight (110 °C) and calcined in static air at 500 °C for 5 h (heating rate of 2 °C min−1). Afterwards, the obtained dealuminated zeolites were impregnated with Pd following the DP-CP urea method described above. Hereafter, the four dealuminated and impregnated samples will be abbreviated as Pd(u)/Beta-dAlx (x = 1–4), where x = 4 refers to the sample showing the highest degree of dealumination (higher Si/Al ratio).The prepared catalysts were characterised by powder X-Ray diffraction (PXRD) on a Panalytical X’Pert Pro diffractometer with Co Kα1 radiation (λ = 1.7890 Å) in the 2θ angle range 10−80° (scanning speed of 0.023° s−1). Metal content of the catalysts was determined by inductively coupled plasma - optical emission spectroscopy (ICP-OES) using an Agilent 5110 SVDV instrument. The samples were digested in a strong acidic medium (10 ml HCl and 20 ml HNO3) and then diluted with water (1:10 v/v). Textural properties were evaluated through N2 adsorption-desorption isotherms at 77 K, using a Micromeritics TRISTAR II instrument. Prior to the measurement, the samples were outgassed under vacuum at 120 °C for 20 h. The BET equation was used for specific surface area calculation, whereas pore volume was determined by the BJH method.Acidity of the catalysts was determined by temperature programmed desorption of ammonia (NH3-TPD) in a Quantachrome ChemBET 3000 unit. Firstly, the samples were outgassed under a He stream (100 cm3 min−1) heating at 10 °C min−1 up to 350 °C. Afterwards, the samples were cooled to 150 °C and saturated under an ammonia stream (100 cm3 min−1) for 10 min. Subsequently, the physically adsorbed ammonia was removed by flowing helium (100 cm3 min−1) for 30 min at 150 °C. Finally, the chemically adsorbed ammonia was desorbed by heating to 650 °C with a rate of 10 °C min−1 under He flow (100 cm3 min−1). Ammonia concentration was monitored continuously using a thermal conductivity detector (TCD).Thermogravimetric analysis (TGA) was carried out on a Q600 TA Instrument; ca. 5 mg of sample were loaded into an alumina microcrucible and heated to 800 °C at 10 °C min−1 under a flow of air (100 cm3 min−1). Elemental analysis (C and H content) of the used catalysts was carried out on a Thermo EA1112 Flash CHNS Analyser. TEM images were obtained with a JEOL 2100 transmission electron microscope operating at 200 kV. The samples were dispersed in acetone, stirred in an ultrasonic bath and deposited on a carbon-coated Cu grid. SEM imaging and energy-dispersive X-ray (EDX) spectroscopy were run on a Hitachi S-4800 Field-Emission scanning electron microscope.Solid state 27Al NMR experiments were performed on a 9.4 T Bruker DSX 400 MHz spectrometer using a Bruker Triple Resonance 4 mm HXY (in double resonance mode) probe under Magic Angle Spinning (MAS) at a rotational rate of 10 kHz. One-dimensional MAS NMR spectra were recorded using a rotor-synchronized (1 period) Hahn echo sequence with a radio frequency pulse of 50 kHz (π/2 pulse of 1.7 μs duration) and a quantitative recycle delay of 1 s. Whilst the quantitative interpretation of 27Al MAS NMR data has to be performed with caution due to non-uniform excitation of sites with different magnitudes of the quadrupolar coupling constants [34], the similar values observed for tetrahedral and octahedral sites (i.e. 1–2 MHz) allow for an estimation of their ratio [35,36].The performance of the catalysts was studied in high pressure 100 ml batch stirred reactors (Parr Instrument Co.) A glass liner was loaded with 45 ml of an aqueous solution of HMF (0.04 M) and 0.06 g of catalyst and placed into the stainless-steel reactor. After sealing the vessel, the reactor was flushed three times with N2 and heated to the required reaction temperature (80–155 °C). Once the targeted temperature was reached, the vessel was pressurised with H2 to the respective value (5–60 bar of H2) and stirring was set to 600 rpm. After the end of the reaction (typically 6 h), the identity and distribution of the products were determined by the combination of 1H and 13C NMR spectroscopy (Bruker AVANCE III HD spectrometer), GC-MS (Agilent 6890 N GC with a 5973 MSD detector) and GC (Agilent 7890A GC with an FID). GC and GC-MS were equipped with a DB-WAXetr capillary column (60 m, 0.25 mm i.d., 0.25 μm). Standard reference compounds used: HMF (Sigma), FDM (Manchester Organics), THFDM (Ambinter) and HXD (Sigma-Aldrich). Details regarding calculations of conversion, yield and selectivity are provided in the Supporting Information (SI).PXRD patterns (2θ = 10–80°) of the Beta-supported metal catalysts after reduction show the characteristic peaks of the corresponding metallic phase (Fig. SI1). No crystalline phases of the metal oxides precursors were observed, confirming their complete reduction under the H2 treatment. Well-defined reflections associated with the zeolitic structures (Beta or ZSM-5) were identified [37], verifying that crystallinity of the zeolitic support was preserved after impregnation. The composition of the prepared catalysts was determined by ICP-OES (Table 1 ), showing metal contents close to the corresponding nominal values (Pd, Ru = 2.7–2.9 wt%; Ni, Cu, Co = 8.7–9.3 wt%).The comparison of the catalytic performance of several zeolite-supported metal catalysts, prepared by IWI, in the conversion of HMF is presented in Table 1 (110 °C, 20 bar H2). Temperature was set at 110 °C in order to minimize the extent of oligomerisation reactions which are favoured by the presence of acidic sites [24].A preliminary control reaction with non-impregnated Beta zeolite (entry 1) showed negligible HMF conversion (4%) to FDM (1% yield), verifying that a reduced metal phase is essential for the conversion of HMF to the targeted diketone derivatives. An additional control experiment using the more reactive FDM intermediate as the substrate over Beta zeolite (entry 2) resulted in 10% FDM conversion with a concomitant colour change of the reaction mixture from pale to dark yellow. However, no products were detected by GC and carbon mass balance (Cmb) was only 90%. The decrease in Cmb suggests that the highly reactive unsaturated intermediates formed by the hydrolytic ring-opening of FDM, such as 1-hydroxyhex-3-ene-2,5-dione (HHED) [27,28], may lead to heavier ill-defined products, such as humins [38,39]. This undesired oligomerisation reaction always takes place in parallel with productive FDM conversion. It should be noted that decarboxylation of HHED to levulinic and formic acid [40] was not observed due to the highly reducing conditions employed.Among the screened active metal phases, the Pd/Beta catalyst (entry 3) afforded the highest HMF conversion (80%) and selectivity to HHD (56%), whereas HXD was also detected as a minor product (8% selectivity). However, Cmb was only 77%, consistent with the formation of undetectable oligomers. The Ru/Beta catalyst (entry 4) showed lower HMF conversion (41%) and HHD selectivity (44%). The non-noble metal based catalysts (i.e. Ni, Cu and Co, entries 5–7) showed even lower HMF conversion (15–24%) despite having considerably higher metal loadings. Moreover, selectivity to HHD was rather poor (13–25%), whereas HXD was not detected. It should be noted that the higher Cmb observed for the Ru, Ni, Cu and Co supported catalysts (84–98%) is a direct consequence of the lower HMF conversion. The superior catalytic activity of Pd relative to other transition metals has also been demonstrated for the hydrogenation of HMF to THFDM in water, using Pd/C carbon [41] or Pd@MIL-101(Al)-NH2 MOF [42].The effect of the zeolitic support was also explored by using ZSM-5 (Si/Al = 11.5) instead of Beta (Si/Al = 12.5) and preparing Pd/ZSM-5 (entry 8). The latter also showed high HMF conversion (74%) and HHD selectivity (41%), albeit slightly lower than Pd/Beta. Notably, Pd/ZSM-5 afforded the highest selectivity to HXD (14%) which can be attributed to the higher concentration of Brønsted acid sites in ZSM-5 [43] and the different structural frameworks (MFI in ZSM-5 vs. BEA in Beta) which affect the shape selectivity by either mass transfer or transition state effects [44,45]. Overall, both zeolite-supported Pd catalysts gave the highest HMF conversion and HHD selectivity but also showed the lowest Cmb due to the formation of heavier undetectable oligomers [27,28].In order to clarify the role of water in the reaction mechanism, an isotopic labelling experiment was performed using D2O as the solvent under the same reaction conditions (110 °C, 20 bar H2). GC-MS revealed the formation of [D3]-HHD and [D4]-HXD as the main products, as well as traces of [D4]-HCPN. FDM was also detected but it was not deuterated (Fig. SI2). Specifically, higher m/z values were observed in the mass spectra of the products when D2O was employed as the solvent instead of H2O: m/z = 118 ([D4]-HCPN), 133 ([D3]-HHD) and 118 ([D4]-HXD) compared to m/z = 114 (HCPN), 130 (HHD) and 114 (HXD). This in turn suggests that two D2O molecules participate in the catalytic mechanism, specifically in the ring-opening of FDM via consecutive hydration-dehydration steps (Fig. 3 ), as originally proposed by Horvat et al. [40]. Importantly, none of the above compounds is formed in water-free reaction mixtures [7]. Therefore, H2O not only serves as an environmentally benign solvent but is also necessary for FDM ring-opening [46].The effect of the Pd particles size in the Beta zeolite-supported catalysts was also investigated. In addition to the catalyst prepared by IWI and PdCl2 (Pd/Beta), two more catalysts were prepared by either (i) IWI and Pd(NO3)2 as the precursor (Pd(n)/Beta) or (ii) DP-CP with urea and PdCl2 (Pd(u)/Beta). Samples were then calcined and reduced as before. The different preparation methods led to different morphologies of the supported Pd nanoparticles (NPs), as deduced by TEM imaging (Fig. 4 and SI3) and PXRD (Fig. SI4).The Pd/Beta catalyst (Fig. 4a) resulted in intermediate Pd NPs (average diameter of 5.2 ± 3.4 nm). Employment of Pd(NO3)2 as the precursor (Fig. 4b) led to much larger Pd NPs with a significantly less uniform particle size distribution (average diameter of 16.2 ± 10.4 nm). The higher dispersion of Pd catalysts with PdCl2 as the precursor has been ascribed to the formation of complex PdxOyClz species on alumina/aluminosilicate surfaces [47,48]. The DP-CP method (Fig. 4c) resulted in the smallest Pd NPs and the most uniform size distribution (average diameter of 3.5 ± 1.5 nm). The smaller and more uniform particle size observed for Pd(u)/Beta can be associated with the slow and homogenous generation of hydroxide ions through the hydrolysis of urea at 95 °C which hinders the uneven precipitation of PdII species due to a sudden, local increase of pH [49,50]. Table 2 shows the conversion of HMF and the obtained product distribution for the three Beta zeolite-supported Pd catalysts after 6 h under 20 bar of H2 at 110 °C. All the catalysts have similar Pd contents varying between 2.6 and 2.8 wt% (based on ICP-OES). A direct correlation between higher HMF conversion and smaller Pd particle size was identified. Thus, the Pd(n)/Beta catalyst (dM = 16.2 ± 10.4 nm) showed the lowest HMF conversion (56%). Moreover, the lower hydrogenation activity of Pd(n)/Beta resulted in the lowest selectivity to HHD (39%) due to a higher degree of oligomerisation of the unsaturated intermediates formed via FDM ring-opening (Fig. 3). On the other hand, the Pd(u)/Beta catalyst (dM = 3.5 ± 1.5 nm) afforded almost complete HMF conversion (96%) and the highest selectivity to HHD (56%).The N2 adsorption-desorption isotherm of the Pd(u)/Beta catalyst was similar to the parent Beta zeolite (Fig. SI5), presenting features of Type I isotherms. The resulting textural properties showed a slight decrease in specific surface area and pore volume after the incorporation of Pd (621 m2 g−1 and 0.297 cm3 g−1 vs. 574 m2 g−1 and 0.286 cm3 g−1) due to partial blockage of the zeolite pores by Pd NPs, characteristic of a highly dispersed metal phase [51]. Thus, the enhanced hydrogenation activity with the decrease of Pd particle size can be attributed to the corresponding higher metal dispersion (higher metal active surface) [52] and to the higher uniformity in Pd particle size which favours the adsorption and hydrogenation of furanic compounds [41,42]. Additionally, well dispersed and uniform Pd NPs increase catalytic lifetime by hindering leaching and sintering of particles [53]. In this sense, SEM-EDX analysis of the Pd(u)/beta catalyst (Fig. SI6) confirmed the absence of residual chlorine (from the PdCl2 precursor) which is known to increase metal atom mobility and cause sintering.The product distribution obtained over the Pd(u)/Beta catalyst (Table 2) revealed that at 96% HMF conversion no product exceeded 1% selectivity apart from the targeted diketone derivatives HHD and HXD. Notably, full conversion was achieved in 24 h and only two well-defined peaks corresponding to HXD and HHD were observed in the respective GC chromatogram of the product mixture (Fig. SI7). However, Cmb was found lower than 80% for all runs. Taken together, these observations suggest that an undetectable by GC fraction of products is produced via an acid-catalysed oligomerisation of unsaturated intermediates, formed via FDM ring-opening [24,27]. The formation of oligomers could be potentially suppressed by co-addition of organic solvents [54]. Alternatively, techniques such as biphasic reactive extraction, adsorbent-based separation or reactive distillation could be applied to separate the oligomer fraction from the targeted compounds [55,56].The time evolution of HMF conversion, product distribution and Cmb over the Pd(u)/Beta catalyst are depicted in Fig. 5 . HMF was swiftly consumed, reaching 87% conversion in 120 min and 96% in 360 min. FDM was detected at early reaction times, with a maximum yield of 4% at 20 min but was fully consumed in 240 min. HHD and HXD yields rapidly increased during the first 120 min (47% and 7%, respectively) but did not significantly change afterwards, achieving final values of 54% (HHD) and 9% (HXD). Notably, exposing a mixture of HHD and HXD to a fresh batch of Pd(u)/Beta with or without H2 (T = 110 °C, 3 h) showed no interconversion between HHD and HXD. A selectivity vs. conversion plot (Fig. SI8) is consistent with HHD and HXD being formed via two separate pathways (Fig. 2). Previous works have shown that hydrogenolysis of FDM to DMF and hydrolysis of the latter can lead to HXD [21,25]. However, we did not detect any trace of DMF, indicating that HXD is formed through a currently unidentified mechanism.The observed time profile supports the proposed reaction mechanism for the conversion of HMF into HHD (Fig. 3) which begins with the hydrogenation of the HMF carbonyl group to form FDM, followed by the hydrolytic furan ring-opening and hydrogenation to form HHD [10,17,20,28]. The intermediate nature of FDM was confirmed by a separate experiment using a lower amount of catalyst (40 mg instead of 60 mg); FDM yield reached a maximum of 23% before gradually decreasing as HHD was being formed (Fig. SI9). The time evolution of the Cmb showed a pronounced decrease within 120 min (from 100% to 73%) but remained practically constant afterwards (71% in 360 min). This is consistent with the formation of undetected heavier oligomers, catalysed by the zeolite acid sites.Since the catalytic properties of the Pd(u)/Beta catalysts are directly related to the acidity of the zeolite framework, it was anticipated that the removal of Al atoms would affect catalyst activity and selectivity [57]. In order to understand the influence of the Si/Al ratio on the two competitive acid-catalysed reaction pathways, i.e. FDM ring-opening and oligomerisation of unsaturated intermediates, four Beta zeolite-supported Pd catalysts with different Si/Al ratio were prepared (Pd(u)/Beta-dAlx, x  = 1–4) via dealumination (acid treatment) and subsequent Pd impregnation (DP-CP with PdCl2). The parent Beta zeolite (Si/Al = 12.5) was partially dealuminated by using gradually more concentrated HNO3 aqueous solutions of 0.1, 0.5, 2 and 5 M which led to increasingly higher Si/Al atomic ratios in the range of 17.8–34.5 (Table 3 ).Crystallinity and microporosity of the zeolitic support were preserved after the dealumination treatment based on the respective PXRD patterns and N2 isotherm profiles (Fig. SI10). Removal of Al resulted in a progressive decrease of acid sites according to NH3-TPD measurements (Fig. SI11 and Table SI2), due to removal of: (i) extra-framework aluminium, associated to Lewis acidity and (ii) tetrahedrally coordinated aluminium, associated to Brønsted acidity [58,59]. Important differences were observed in the catalytic performance of the Pd(u)/Beta-dAlx catalysts (Table 3). A non-negligible increase of Cmb was observed over the dealuminated supports (from 71% for Pd(u)/Beta to 79–82% for Pd(u)/Beta-dAlx). However, a gradual decrease in HMF conversion and selectivity to HHD was also observed as the Si/Al ratio was increased due to the lower extent of the hydrolytic FDM ring-opening, catalysed by Brønsted acid sites.The effect of reaction temperature (80–155 °C) and H2 pressure (5–60 bar) on the production of diketone derivatives from HMF over the Pd(u)/Beta catalyst was also investigated (Table 4 ). Comparison with the original run (entry 1) revealed that lowering the temperature below 110 °C resulted in lower HMF conversion (entries 2–3). Raising the temperature to 125 °C or 140 °C (entries 4–5) restored HMF conversion (≥95%). Notably, HCPN was also detected as a minor product (4–5% selectivity) as the temperature increased due to promotion of FDM ring-rearrangement, resulting in a slight improvement of Cmb (from 71% at 110 °C to 78% at 140 °C). However, the combined selectivity of targeted HHD and HXD was practically not affected (65 ± 1%), although a marginal shift towards HXD was observed (56% HHD and 9% HXD at 110 °C vs. 53% HHD and 13% HXD at 140 °C). Further increasing the temperature to 155 °C (entry 6) led to a significant decrease of HHD selectivity, mainly due to oligomerisation (Cmb = 65%).Having established that T = 110 °C is the optimal temperature for the production of the targeted compounds, the effect of H2 pressure was explored. Running the reaction under 40 bar of H2 afforded again 96% HMF conversion, albeit with a slightly higher Cmb (77%, entry 9). Increasing the H2 pressure to 60 bar resulted in 98% HMF conversion with 66% HHD selectivity in 6 h and 100% conversion with 68% HHD selectivity in 24 h (entries 10–11). Cmb increased to 82%, consistent with a faster hydrogenation rate of the unsaturated intermediates (vs. oligomerisation) to form HHD.The stability of the Pd(u)/Beta catalyst was examined by testing the catalytic activity of the supernatant after physically separating the catalyst from the reaction mixture. HMF conversion did not increase any further (≈50% conversion at 110 °C and 20 bar H2) and product distribution did not change once the catalyst was filtered off after 40 min (Fig. SI12). Likewise, Pd concentration in the supernatant was less than 0.2 ppm (<0.5% of the total Pd content), according to ICP-OES. Both results verify that Pd does not leach into the solution phase and confirm the heterogeneous nature of the catalytic system.The reusability of the Pd(u)/Beta catalyst was investigated by evaluating its catalytic activity upon consecutive runs (110 °C, 20 bar H2). The used catalyst was recovered after each run by filtration at room temperature, washed with deionized water and dried at 25 °C overnight. Fig. 6 a depicts the TGA curves of the fresh and the used Pd(u)/Beta catalysts, showing a noticeable increase in the total weight loss for the used catalyst (28% vs. 7%). Furthermore, the elemental microanalysis of the used catalyst (Fig. 6a inset) revealed a significant carbon content (9.32% weight), consistent with deposition of organic compounds on the catalyst’s surface during turnover. This result compensates to a certain extent (5–6%) for the lower Cmb observed. The PXRD pattern of the used Pd(u)/Beta catalyst (Fig. 6b) showed the expected reflections of the Pd metallic phase, indicating that Pd remains reduced after turnover. However, TEM images of the used catalyst (Fig. 6c and SI13) revealed an increase in Pd particle size (dM = 7.6 ± 2.3 nm, Fig. 6d) compared to the fresh catalyst (dM = 3.5 ± 1.5 nm, Fig. 4c), indicative of aggregation and formation of larger Pd particles [60].Recycling tests were conducted after recovering the Pd(u)/Beta catalyst (Fig. 7 a). HMF conversion gradually decreased from 96% (1st run) to 10% (4th run) due to deposition of organic compounds and aggregation of Pd NPs (Fig. 6 and Fig. SI13). In order to restore the catalytic activity, the used Pd(u)/Beta catalyst (after 4 consecutive runs) was subjected to a regular regeneration treatment: calcination (air/500 °C/5h) and reduction (H2/200 °C/5 h). Catalytic activity was partially restored (5th run), affording 90% HMF conversion to HHD (40% yield) and HXD (4% yield). A non-negligible amount of FDM (7% yield) and HCPN (12% yield) was also detected in the product mixture.In order to investigate the observed change in selectivity, a separate set of experiments was conducted during which the catalyst was regenerated at the end of each run (Fig. 7b). HMF conversion gradually decreased from 96% (1st run) to 56% (3rd run) with a concomitant increase in FDM yield (6% in 3rd run), as also observed for the dealuminated samples (Table 3). Measurement of the 27Al MAS NMR spectra of the fresh and the regenerated Pd(u)/Beta catalyst (Fig. SI14) revealed a decrease in the fraction of tetrahedrally coordinated Al after turnover and regeneration. This in turn suggests a lower number of Brønsted acid sites [61] which promote FDM ring-opening. Moreover, PXRD verified an increase in Pd particle size after regeneration (Fig. SI15), consistent with a lower hydrogenation activity. Therefore, the observed differences in activity and selectivity during recycling can be ascribed to aggregation of Pd NPs and partial loss of Brønsted acidity [61–65].Diketone derivatives such as HHD and HXD were produced from HMF over a bifunctional Beta zeolite-supported Pd catalyst in water under relatively mild reaction conditions. The DP-CP method afforded the most active catalyst, compared to IWI, due to smaller and more uniformly dispersed Pd particles among the zeolitic support. Complete conversion of HMF was achieved at 110 °C and 60 bar of H2 with 68% selectivity to HHD. Leaching of Pd was not observed and catalytic activity could be partially restored after a simple regeneration step. Selectivity to HHD is mainly limited by the formation of heavier ill-defined oligomers. The key distinguishing feature of this study is the synergic effect of the zeolite acid sites and the highly active hydrogenating Pd metallic phase which promotes the hydrolytic ring-opening and subsequent hydrogenation of the FDM intermediate without necessitating co-addition of an acid or use of an organic solvent.This work was supported by the Engineering and Physical Sciences Research Council (EPSRC), UK (EP/K014749).Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.04.038.The following are the Supplementary data to this article: Supplementary Data 1

Conversion of 5-hydroxymethylfurfural (HMF) in water to the linear diketone derivatives 1-hydroxyhexane-2,5-dione (HHD) and 2,5-hexanedione (HXD) was investigated over a series of Beta zeolite-supported transition metal catalysts (Co, Ni, Cu, Ru, Pd). Their catalytic performance was tested in a batch stirred reactor (T = 110 °C, PH2 = 20 bar) with Pd showing the highest activity and selectivity to HHD and HXD. The effects of Pd particle size, zeolite Si/Al ratio and reaction conditions (T = 80–155 °C, PH2 = 5–60 bar) were also investigated. The incorporation of Pd into Beta zeolite by the deposition-coprecipitation method produced the most efficient catalyst, affording complete HMF conversion (T = 110 °C, PH2 = 60 bar) predominantly to HHD (68% selectivity) and HXD (8% selectivity). The combination of a bifunctional acid/redox solid catalyst and water enhances the hydrolytic ring-opening and subsequent hydrogenation of the furan ring. Catalytic activity can be partially restored by a simple regeneration treatment. This work establishes a catalytic route to produce valuable diketone derivatives from renewable furanic platform sources in water.

No data was used for the research described in the article. No data was used for the research described in the article.With the development and progress of industry, air pollution is becoming more and more serious. The various kinds of pollution accumulated in the atmosphere cause harm to plants, river soils, building materials and human health that cannot be ignored [1]. Therefore, a series of measures such as emission reduction, atmospheric governance and resource utilization are being actively launched [2]. An end to pollution control (especially denitrification of flue gas) is urgent, in the terminal treatment of atmospheric nitrogen oxides technology, the most direct and efficient method is to purify pollutants by selective catalytic reduction with ammonia (NH3-SCR), during which, as the core of catalytic reaction, the preparation and configuration of catalysts have become the focus of the field of atmospheric catalysis [3]. At present in denitration management, common catalysts include vanadium, manganese, cerium, titanium and molecular sieve materials. MnOx has rich variable valence states, large specific surface area, high chemical adsorption oxygen content on the surface, such catalyst has rich surface active sites, high crystallinity and rich structural morphology [4,5]. Therefore, scholars gradually began to study MnOx catalysts to improve the NH3-SCR reaction activity at low temperatures. Qiu et al. [6]. prepared ordered mesoporous material MnCo2O4 by nano-casting method with good low temperature SCR activity and N2 selectivity, and showed high anti-poisoning performance with the NOx conversion rate maintained at about 80% after 10 h reaction. The NOx conversion rate was maintained at about 80% after 10 h reaction. Chen et al. [7]. prepared a novel MnCoOx sphere catalyst, which not only exhibited high low-temperature activity for the NH3-SCR of NOx but also significantly enhanced SO2 resistance. Over MnCoOx sphere catalyst the formation of MnSO4 is significantly inhibited in the presence of SO2, so that the reaction over the SO2-poisoned Mn(5)Co(5)Ox catalyst can still proceed by the LH mechanism and maintain its high catalytic performance.However, the conventional single catalyst is easily deactivated by the erosion of H2O and SO2 in the flue gas [8–10]. The catalyst will be inactivated for a long time, so it has been affected in the industrial development [11,12]. Various metals (Cu, Co, Cr, Ni, Fe, Sn, Mg) were doped into MnOx catalyst, and the modification of Co enhanced the performance and tolerance of SCR to SO2 [13,14]. Co for crossing one of the metallic elements, CoOx has unique oxygenation and reduction properties good co-catalyst, which can enhance the catalytic performance in the reaction high activity and high selectivity. Hu et al. [10] synthesized Co3O4 and manganese-doped Co3O4 nanoparticles by co-precipitation method and used them as catalysts for NH3 selective catalytic reduction of NO (NH3-SCR). The NH3-SCR activity of Mn0.05Co0.95Ox catalyst was greatly enhanced by the addition of Mn oxide. The results show that the incorporation of manganese can provide more acidic sites on the catalyst, and the binary nitrate species from NOx adsorption are significantly activated on the surface of Mn0.05Co0.95Ox catalyst. Xu et al. [15] designed and fabricated monolithic porous MnCoxOy nanocubes on a titanium grid, as a denitrification catalyst for NH3-SCR. Characterization results show that the surface of the titanium grid was uniformly coated with cubic arrays, which prevented the migration and aggregation of metal oxides and allowed the components to act synergistically during the catalytic reaction. In addition, due to its robust structure and morphology, the catalyst can maintain a high NOx conversion while exhibiting excellent catalytic cycle stability and good hydrogen resistance. Liu [16]. prepared a series of Ba- and Co-doped MnOx catalysts by citric acid complexation. The effects of Ba and Co doping on the performance of MnOx low-temperature NH3-SCR were investigated. The experimental results showed that the addition of Co then promoted the catalytic performance of manganese oxide. When Ba and Co were co-doped, the performance of the catalyst was significantly improved. 3BaMnCoOx showed the most excellent catalytic performance, with the catalytic activity above 99% when the reaction temperature was higher than 180 °C.But the single structure catalyst is still not resistant to the poisoning caused by SO2 and H2O. Core-shell nanomaterials have cavity and composite properties, the unique shell structure can effectively reduce the contact probability of active substances with H2O or/and SO2, slow down catalyst poisoning, prolong the service life of catalysts [17–20].Up to now, many scholars have prepared core-shell catalysts with different combinations of active components and shell materials [21,22]. The most typical core-shell catalyst is the one with Mn, Ce, Co and other metal oxides as the core and TiO2 as the shell and its SCR reaction at low temperature was studied, the results show that, as opposed to a single active component, core-shell structure catalyst on the basis of no reduction in activity [23,24]. It significantly improved the resistance to H2O and SO2, this is because the shell effectively inhibits the formation of sulfate species on the surface. Therefore, core-shell catalyst has good SO2 resistance [25–28].In this work, a novel Co(3-x)MnxO4@TiO2 core-shell catalyst was prepared by two-step method. The sol-gel method can easily achieve molecular level mixing, and the dynamic coating method can uniformly distribute the material on the surface of the coated material. Therefore, the catalysts prepared by the above two methods have better hybrid type, stability and uniformity at the molecular level. Firstly, MnOx were prepared by sol-gel method, then Co(3-x)MnxO4@TiO2 catalyst was prepared by dynamic coating method, which performed typical core-shell structure. In this paper, surface response method was used to explore the optimum preparation conditions, and on this basis to study the activity and resistance of the catalyst [29,30]. Then, the effect of SO2 in flue gas on SCR activity of core-shell catalyst was studied, and the mechanism and reason of improving SO2 resistance of core-shell catalyst with special morphology were analyzed. Through the analysis of above conclusions, the best proportion of core-shell catalyst was obtained to improve the activity and resistance.Co(3-x)MnxO4 was prepared by sol-gel method. Firstly, CO(NO3)3·6H2O and Mn(AC)2·4H2O were weighed to dissolve in A beaker filled with deionized water, and then C6H12O6 was weighed to dissolve in A beaker filled with deionized water (C6H12O6: Mn+ CO =0.8). Add the solution in A beaker to the solution in B beaker under 60 Hz ultrasound, continue to stir, then aged at 70 °C for 3 h to form wine red transparent sol, black porous powdered forebody was obtained after 12 h drying at 100 °C, finally, the samples were obtained by calcinating in air. The catalysts were denoted as Co(3-x)MnxO4, where x refers to 0.5, 1, 1.5, 2 and 2.5, thus the Mn:Co molar ratio was set as 1:5, 1:2, 1:1, 2:1 and 5:1 [31].After the preparation of the core material, the dynamic coating method was used to coat the shell material. Co(3-x)MnxO4 catalyst prepared in the above experiments was weighed and added with 100 ml anhydrous ethanol and a concentrated ammonia water solution (28wt%, 15 mol/L), by ultrasonication for 30 min. Then, different molar amounts of C16H36O4Ti were added dropwise and the reactions were allowed to proceed for 24 h at 45 °C under continuous mechanical stirring. The resultant products were separated, collected and washed with deionized water and ethanol for several times. Finally, the obtained powders were dried in an electric oven at 100 °C for 12 h and calcined at 500 °C for 2 h. The catalysts were denoted as Co(3-x)MnxO4@TiO2(y), where y refers to the (Mn+Co):Ti molar ratio of 1:5, 1:2, 1:1, 2:1 and 5:1, respectively [31].The Response Surface Methodology (RSM) is also known as Response Surface Design Methodology. The central composite design (CCD) is an experimental design developed based on partial experimental design and 2-level full factorial experimental design approach. The addition of a point to the 2-level experimental design corresponds to an additional level of thus, the nonlinear relationship between response values and factors can be investigated. Suitable for 2∼6 influencing factors. The number of experiments is usually between 14 and 90. In this paper, the 3 factors and 5 levels require 20 groups of experiments. Combining the results of scholars' studies and the literature, the response surface method was used to investigate the effects of core-shell ratio, ammonia addition and calcination temperature on the catalyst performance. The CCD experimental design was carried out using Design expert software and the design parameters are shown in Table 1 . The results of the CCD 3 factor 5 level experimental design using Design expert software and the experimental results are shown in Table 2 .The morphological characteristics of all prepared catalysts were characterized by transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), and element mapping using a FEI TECNAI G2 F20 instrument operated at 200kV. The N2 adsorption and desorption isotherms were obtained by an analyzer (ASAP 2020, Micromeritics, USA) pre-treated at 300 °C for 2h to characterize the pore volume, average pore size, and specific surface area of the nano-composites. X-ray diffraction (XRD) patterns were obtained in a system (D8 forward, Bruker, Germany), with Cu Ka radiation scanning 2θ from 10 to 90°, 6°/min. Raman spectroscopy of the catalyst was performed at room temperature with the Fisher Scientific DXR2 at 532 nm for an exposure time of 50 s.X -ray photoelectron spectroscopy (XPS) spectra were obtained on the Thermo Feather Science Al Kα 250XI spectrometer, calibrated against the C1s peak (binding energy at 284.6 eV) of the surface contamination, and the peak differential simulation was further analyzed using XPS peak software. In-situ DRIFT experiments were performed on the spectrometer (IS50, Thermo, USA), which was equipped with an MCT/ A detector cooled by liquid nitrogen and an in-situ drift reaction unit with a zinc selenide window. Before each experiment, the samples were pre-treated at 400 °C with N2 at 100 mL/min for 1 h. The background spectrum is collected in flowing N2 and automatically subtracted from the sample spectrum. Record 650–4000 cm−1 by collecting 100 times with a resolution of 4 cm−1.The SCR activity of NO by NH3 was performed in a fixed-bed reactor with an inner diameter of 7 mm. The temperature was raised from 75 to 275 °C. The typical reactant gas consisted of 500 ppm NO, 500 ppm NH3, 100 ppm SO2 (if added), 10 vol% H2O (if added), 5 vol% O2 and balance N2 with a gas hourly space velocity (GHSV) of 24000 h–1. Water vapor (10 vol%) was generated by passing N2 through a heated bottle containing deionized water. The NO, O2, NH3 and SO2 were mixed in the mixing tube through the gas flow-meter, and after passing the sample, it enters the Fourier gas infrared analyzer for detection. The NOx (NO and NO2) conversion and N2 selectivity were calculated as follows: (1) NOx conversion ( % ) = C NOx in − C NOx out C NOx in × 100 (2) N 2 selectivity ( % ) = 1 − 2 × N 2 O out NO x in + N H 3 in − NO x out − N H 3 out × 100 Twenty groups of Co(3-x)MnxO4 core materials were prepared under different conditions, and the optimum preparation conditions were investigated by activity detection. The optimum preparation conditions were selected by surface analysis: Mn:Co=2:1, calcination temperature was 450 °C, reaction temperature was 175 °C. On the basis of kernel optimization, the second order surface response was carried out. The coating of TiO2 shell was investigated onto the optimized active component material via the design of three factors (core-shell ratio, ammonia water content and calcination temperature) with five levels, thus, twenty groups of samples were prepared under different combination conditions. The catalytic activity of these catalysts was tested, and then the surface analysis diagram was prepared according to the test results, furthermore, the best coating conditions was selected to make the optimal CoMn2O4@TiO2 core-shell catalyst.The experimental results of 20 tests with 5-level of 3 factors in CCD design (as listed in Table 1) are shown in Table 2. The response surface analysis is not an examination of indicators, but rather multiple indicators, analyzing the interaction between factors and the most important factors affecting the experiment. It is the center point as the core of the surface to build a 3D surface map, so there are repeated experiments that are randomly generated, the purpose is to verify the accuracy and precision of the experimental results. The above results were imported into the software and analyzed to establish an objective function model of NOx conversion (Y), respectively. It should be pointed out that in the regression equation analysis of variance, the corresponding P values of X1X2, X1X3 and X2X3 in Y model was 0.6825, 0.5846 and 0.8682, which was far greater than 0.0001, respectively, so that they were deleted to optimize the regression equation, as presented in Eq. (3), respectively. The quality of a design of experiments is measured by the variability of the estimate of its coefficients and consequently by the variability of the response estimate.It should be noted that the model P value of NOx conversion rate is less than 0.0001, which shows that the NOx conversion rate model is significant. The P value of missing degree is 0.5169, greater than 0.05, it indicates that the degree of absence is not significant. More importantly, the R 2 variance of the regression equation of NOx conversion is 0.9257, close to 1, it shows that the model established by RSM has a good fitting degree. As you can see from Table 2, the central point of the CCD design, a catalyst of 500 °C calcined with 1.5 active component (core-shell ratio 1:1) and 0.5 ml ammonia water (ammonia water to active component ratio 4.17) was prepared, six tests were performed at 225 °C (run 1, 2, 3, 13, 18, 20), and the SCR activity (94.9±3.6%) was higher than 90%. It shows that the experimental method is feasible and the experimental data is repeatable.When the core-shell ratio is 1:5 (Run-19), the NOx conversion rate of Run-19 catalyst is 86.98%, which is lower than that of Run-3 (central point). It indicates that the proportion of active ingredients is too low to play an effective catalytic role. The excess of active component (Run11, core-shell ratio 5:1) also leads to pore clogging and surface accumulation, the effective active site of the catalyst could not be completely dispersed, which cannot show catalytic effect effectively. The SCR activity of Run-4 was compared, high temperature calcination is not good for catalytic reaction (run-5, Run-15, Run-16, and Run-17, also proved this). This may be due to the sintering of active ingredients or the reduction of specific surface area. As one can see from the activity tests on Run-7, less content of ammonia water is not good for catalytic reaction (Run-8, Run-15, Run-16, also proved this). This may be due to that too little amount of ammonia water as a binder may lead to inadequate core-shell bonding, the instability of core-shell structure also results in a decreased activity. Verified by multiple groups of experiments, it can be found that the experimental results designed by CCD have certain repeatability. Therefore, the above model equations can better predict the experimental values, the theoretical basis was laid for RSM optimization of preparation parameters and reaction conditions. (3) Y = 92.98 + 1.33 x 1 + 1.31 x 2 − 7.61 x 3 − 1.64 x 1 x 2 − 2.20 x 1 x 3 − 0.66 x 2 x 3 − 5.63 x 1 2 − 3.89 x 2 2 − 7.08 x 3 2 Fig. 1 shows the 3D response surface diagram of core-shell ratio (a), calcination temperature (b) and ammonia water addition amount (c) to NOx conversion rate, respectively. Fig. 1(a) shows a good 3D hillside shape that the NOx conversion rate increases first and then decreases with the increase of core-shell ratio, this indicates that there is an optimal load value, which can be obtained through model optimization analysis [28]. The reason for this rule may be that too low core-shell ratio results in too few active components, it is difficult to meet the demand of gas molecules, while the excessive core-shell ratio leads to the accumulation and blockage of TiO2 shell materials that prevents gas molecules from contacting the pore [29,30]. According to the analysis in Fig. 1(b), when the calcination temperature is in the range of 400∼480 °C, it has little effect on NOx conversion of catalyst that can reach more than 85%, and the NOx conversion rate is on the rise. In the temperature range of 480∼600 °C, the NOx conversion rate decreases gradually. Many research results show that high temperature may lead to the catalyst sintering, that is, the grain size of active component becomes larger, specific surface area reduction, the catalyst has thermal deactivation [31–33]. As the calcination temperature was gradually increased and the calcination time lengthened, the sintering of catalyst will be aggravated accordingly. With the gradual increase of temperature and the extension of calcination time, the sintering process with crystallization changes will become more serious. Low calcination temperature may lead to incomplete calcination of catalyst [34]. Combined with Fig. 1(a), Fig. 1(c) showed that the core-shell catalyst activity increased first and then decreased with the addition of ammonia water, but to a lesser extent, the activity was above 85%. Therefore, the amount of ammonia water is not the main factor affecting the activity, optimal range of ammonia water content is 0.3∼0.7 mL (ratio of ammonia water to active component: 2.5∼5.8). The reason is that ammonia water acts as an adhesive to facilitate the hydrolysis of C16H36O4Ti, it will not affect the change of active component [35]. However, too little amount will lead to insufficient core-shell bonding, unstable micro-porous structure and easy collapse. Too much amount will affect the distribution of acid sites and reduce the activity of catalyst to a certain extent.The amount of ammonia water is fixed as 0.5, the NOx conversion rate can reach 93.8% within a 5% reduction in maximum activity of 4.7%. The scaling range is selected by the transversal method shown in Fig. 2 , when the amount of active component is 1.1∼2.2, nitrogen oxide conversion can achieve a maximum activity reduction of 5%. To facilitate calculation and preparation, the approximate value of active component is 1∼2 (core-shell ratio is 1:2∼2:1). Taking the boundary value as the limiting condition, two groups of catalysts were prepared, the optimal core-shell ratio was selected by activity and also resistance tests. The purpose of this experiment is to select the core-shell ratio samples with the best nitrogen selectivity and sulfur tolerance under the premise of ensuring a certain high activity.The NOx conversion results of catalysts with different proportions at different temperatures are presented in Fig. 3 (a). TiO2 alone has almost no activity below 225 °C, so when comparing catalysts with different core-shell ratios, the effect of activity brought by TiO2 is not considered. In the range of core-shell ratio of from 1:2 to 2:1, four ratios were selected for the full-temperature window activity detection at 75∼225 °C. It can be analyzed from Fig. 3(a), with the increase of reaction temperature, the SCR activity of catalysts with different proportions also increased, except for the single active component CoMn2O4, activity of other catalysts reached the highest and tended to be stable at 225 °C. The activity of CoMn2O4@TiO2 (1:1) and CoMn2O4@TiO2 (1:2) catalysts was lower than that of CoMn2O4. This is because that, with the same mass of catalyst, a decrease in activity was due to a decrease in the proportion of active components [36]. Compared with CoMn2O4, SCR activity of CoMn2O4@TiO2 (1.5:1) and CoMn2O4@TiO2 (2:1) catalysts was improved to some extent, respectively. It is more obvious below 150 °C, which indicates that the two core-shell catalysts have better activity at low temperature.As shown in Fig. 3(b), the N2 selectivity of the two catalysts decreases with the increase of temperature, which may be due to the oxidation reaction of NO and NH3 in the process of NH3-SCR reaction, which coexists with O2 to produce NO2, N2O and other by-products, leading to the decrease of selectivity [37]. These side effects are more likely to occur at high temperatures, and the storage rate of NH3 increases with the increase of temperature. At the same time, a part of NO is oxidized to NO2, which is conducive to the occurrence of "fast-SCR" reaction. However, excessive NO2 in the system will also lead to preferential reaction with NH3, thus the N2 selectivity is further reduced [38]. As can be seen N2 selectivity of CoMn2O4@TiO2 is always higher than that of CoMn2O4. The results shows that the Ti element coated in shell obviously inhibited the formation of N2O, thus improved the selectivity to N2. Compared to other catalysts of different proportions, CoMn2O4@TiO2 (2:1) always had the highest N2 selectivity to above 80% in full temperature window test, which can reach more than 90% when low temperature range below 175 °C. Therefore, it can be concluded that CoMn2O4@TiO2(2:1) core-shell catalyst has good stability, high N2 selectivity and high SCR activity.Sulfur resistance of catalysts with different core-shell ratios in the presence of 100ppm SO2 is shown in Fig. 4 . Under the condition of 100 ppm SO2 for 20 h at 225 °C, CoMn2O4 catalyst activity decreased significantly to about 40%, SO2 is connected for 20 h, which could not be recovered by removing SO2. It shows that the catalyst is completely inactivated in the presence of SO2 for a long time and the result of this decrease in activity is irreversible. In contrast, the activity of CoMn2O4@TiO2 core-shell catalyst was higher than CoMn2O4 sample after SO2 treatment for 20 h. Importantly, the activity of CoMn2O4@TiO2 could be restored to close to the initial value after SO2 removal. This indicates that CoMn2O4@TiO2 core-shell catalyst has better SO2 resistance and toxicity reversibility than CoMn2O4. This may be due to that the coating of TiO2 shell inhibits the formation of sulfate on the surface of active component, Ti(SO4)O in CoMn2O4@TiO2 protects CoMn2O4 from SO2 passivation [39]. The reason may be reaction between SO2 and TiO2 on the outer surface of the catalyst generates TiSO4O, thus trapping the SO2 in the flue gas outside the catalyst. In addition, SO2 might react with NH3 in the gas to form (NH4)2SO4 There may be the following reactions are TiO2 + SO2 → TiSO4O and 2SO2 + O2 + 2NH3 + 2H2O = 2(NH4)2SO4 (correspond to Eqs. (13) and (14) in 3.4).After SO2 was injected at 225 °C for 4 h, the activity of all CoMn2O4@TiO2 core-shell catalysts decreased to a certain extent. The activity of CoMn2O4@TiO2 (2:1) and CoMn2O4@TiO2 (1.5:1) was still about 80%. Higher than CoMn2O4@TiO2 (1:1) and CoMn2O4@TiO2 (1:2). After SO2 infiltration for 20 h, CoMn2O4@TiO2 (2:1) can still maintain nearly 80% activity, but the catalytic activity of the other three core-shell ratio catalysts further decreases to less than 60%. The activity of CoMn2O4@TiO2 (2:1) catalyst did not decrease significantly that still be maintained at more than 75%, indicating that this catalyst has better SO2 resistance with the core-shell ratio of 2:1 that ensures sufficient active sites on the active components. It also ensures that the uniform coating of TiO2 shell inhibit the formation of shell ammonium sulfate on the core surface, thus it has good resistance to SO2 poisoning [40]. Based on the above research, 2:1 core-shell ratio was selected as the preparation ratio of CoMn2O4@TiO2 core-shell catalyst.It has been reported in the literature that H2O and SO2 in exhaust gas can induce catalyst inactivation [41]. Fig. 5 shows the influence of H2O and SO2 on the NOx conversion of CoMn2O4@TiO2 (2:1) and CoMn2O4 catalysts at 225 °C. In the presence of 10 vol% H2O, the catalytic activity of the two catalysts were both decreased. The catalytic activity of CoMn2O4 catalyst decreases to 45% after 20 h and only recovered to 80% when the water was removed from the flue gas, however, the catalytic activity of CoMn2O4@TiO2 (2:1) remained stable at more than 80% after 20h H2O was introduced, and quickly recovered to the original value after the removal of H2O. The results show that CoMn2O4@TiO2 (2:1) catalyst has good water resistance The stability of catalyst activity may be due to the competitive adsorption of reactants at the active sites on the catalyst surface [42]. When 100ppm SO2 was introduced into the reaction system, the NOx conversion rates for CoMn2O4@TiO2 (2:1) and CoMn2O4 ranged from 100% to 75% and 30%, respectively. After SO2 was stopped, the NOx conversion rate of CoMn2O4@TiO2 (2:1) gradually increases to 100% within 4h and is relatively stable. However, the NOx conversion rate of CoMn2O4 can only be maintained at about 40%. When 10 vol.% H2O and 100ppm SO2 were introduced into the system together, the NOx conversion of the two catalysts showed a decreasing trend. After 24 h, the NOx conversion rate of CoMn2O4@TiO2 (2:1) remains around 78%, while NOx conversion rate of CoMn2O4 can only remain around 58%. After removing the two medium agents, the NOx conversion rate of CoMn2O4@TiO2 (2:1) still recovered to 98% but CoMn2O4 catalyst only recovered to 83%. It shows that CoMn2O4@TiO2 (2:1) has better water and sulfur resistance than CoMn2O4. Considering the consistency of active components, the difference in toxicity resistance of the two catalysts must come from titanium dioxide shell protection in the core-shell structure.The micro-structure and morphology of CoMn2O4 and CoMn2O4@TiO2 (2:1) catalysts were studied by TEM and HRTEM. As shown in Fig. 6 (a) and (b), CoMn2O4 catalyst is in the form of scales, and CoMn2O4@TiO2 catalyst is in the form of cluster centered on a nuclear sphere, with an average diameter of about 200 nm and a shell thickness of about 40 nm. HRTRM images of CoMn2O4@TiO2 catalyst in Fig. 6(d) show high crystallinity with spacing distances between lattice planes of 0.165 and 0.234 nm, corresponding to manganese-cobalt spinel crystal plane and anatase titanium dioxide crystal plane, respectively [43]. The lattice stripes of spinel oxides in the nuclei indicate the presence of crystalline Mn-Co oxides in the catalysts. As can be seen in Fig. 6(e, h), the addition of Ti will cause a certain degree of aggregation of CoMn2O4@TiO2 catalyst. To understand the element distribution of CoMn2O4@TiO2 catalyst, element mapping was used to determine the elements, as shown in Fig. 6(f, g, i, j). The element mapping profiles of manganese and cobalt spinel phases are significantly smaller than those of oxygen and titanium. O and Ti cover the surface of Mn and Co elements, indicating that manganese and cobalt species are covered by titanium dioxide shells. Therefore, CoMn2O4@TiO2 core-shell catalyst coated with titanium dioxide shell is formed.To identify the phase composition of CoMn2O4 and CoMn2O4@TiO2 catalysts, XRD analysis was performed in Fig. 7 (a), diffraction peaks of the two catalysts are similar, and both have obvious CoMn2O4 crystal phase peaks, while CoMn2O4 @TiO2 catalyst also has distinct TiO2 crystal phase peaks. The diffraction peaks of CoMn2O4@TiO2 catalyst are consistent with the typical diffraction peaks of single TiO2 nanocrystals and CoMn2O4 spinel phase [44]. The peak values at 14.2°, 29.4°, 32.96°, 36.5°, 47.2°, 60.8° and 65.3° may be CoMn2O4 phase peaks of different crystal phases [45]. The latter peaks at 25.4°, 38.6°, 48.8° and 75.5° correspond to the crystal signal diffraction peaks of TiO2 in the shell of CoMn2O4@TiO2 at different exposed crystal planes [46]. CoMn2O4@TiO2 (2:1) shows the peak of anatase TiO2 and CoMn2O4 spinel phase, indicating that both anatase TiO2 and CoMn2O4 have good crystallinity. In the Figure, CoMn2O4 peak strength of CoMn2O4@TiO2 (2:1) catalyst is reduced, which may be caused by the good dispersion of anatase TiO2 on the catalyst surface. XRD results showed that TiO2 crystals were likely to be uniformly coated on CoMn2O4 catalyst, which was further confirmed by Raman spectra.More information about the crystal structure was obtained by Raman spectroscopy measurements as shown in Fig. 7(b). Some studies have shown that the vibration characteristics of single manganese-cobalt spinel show low Raman activity, which only appears in the 302, 380, 652 and 853 cm−1 region [47]. The peaks of CoMn2O4@TiO2 are located at 167, 396, 508, and 639 cm−1 respectively, which are typical diffraction peaks of anatase titanium dioxide [48,49]. This result is consistent with the XRD analysis in Fig. 7(a), and also coincides perfectly with the diffraction peak of pure TiO2 crystalline phase in Fig. 7(b). The diffraction peaks of CoMn2O4@TiO2 are almost identical to those of pure TiO2 and pure CoMn2O4 spinel phases, which indicates that CoMn2O4@TiO2 is made from the latter two configurations, and some of the peaks of CoMn2O4@TiO2 are higher than those of TiO2, which may be because the CoMn2O4 diffraction peaks are similar to those of TiO2, so the peaks overlap and show more obvious. It is worth noting that the CoMn2O4 nucleon structure in CoMn2O4@TiO2 shows a low intensity band in the Raman spectrum, and it is difficult to observe any characteristic peaks of metal oxides. These facts proved that no obvious Mn and Co phase was observed due to the cover of TiO2 on the surface of CoMn2O4 cluster centered on a nuclear sphere, and the interaction between metal oxides may be greater. The strong interaction between titanium dioxide and active metal oxides contributes to the activity of NH3-SCR. This result is in good agreement with subsequent XPS results. Therefore, XPS and Raman results support the presence of CoMn2O4 as an active component in CoMn2O4@TiO2 (2:1), which is critical for excellent SCR performance. Fig. 8 shows the FT-IR spectra of the two catalysts and the presence of a certain phase can be determined by detecting some specific functional groups. For CoMn2O4 catalyst, it can be seen from the Figure that there is an obvious peak near the wave number of 500 cm−1. After checking, it is highly consistent with the position of Mn-O bond and Co-O bond in tetrahedral gap, and the peak at 632 cm−1 is consistent with the position of Mn-O bond and Co-O bond in octahedral gap. The formation of the bond is also good, and the CoMn2O4 prepared by the sol-gel method has a spinel structure and good crystal structure. The diffraction peak positions of CoMn2O4@TiO2 catalyst and CoMn2O4 catalyst are similar, but there are obvious vibration bands at 1121 cm−1 and 1405 cm−1, which is due to the relatively strong electronic positioning trend of anatase TiO2 and the vibration peak. This indicates the existence of TiO2 crystal phase, but the peak position is relatively weak, because TiO2 accounts for less in the core-shell catalyst, and the main peak is dominated by CoMn2O4. However, the vibration peaks of Mn-O bond and Co-O bond in CoMn2O4@TiO2 catalyst are relatively weak, which is also because the TiO2 coating on the catalyst reduces the vibration frequency of manganese oxide, resulting in reduced light transmittance.The chemical elements and oxidation states on the surface of the catalyst were determined by X-ray photoelectron spectroscopy (XPS). XPS spectra of Mn, Co, O and Ti are shown in Fig. 9 . Fig. 9(a) and Table 3 shows the Mn2p XPS spectra of CoMn2O4@TiO2 catalyst and CoMn2O4 catalyst, the Mn2p spectra of the two catalysts are typical peaks centered on 641.7eV and 653.6 eV, which are assigned to Mn2p3/2 and Mn2p1/2, respectively. Through peak fitting, Mn2p3/2 spectrum can evolve into Mn2+ peak (640.8 eV) Mn3+ peak (642.0 eV) and Mn4+ peak (644.0 eV), while Mn2p1/2 spectrum evolves into Mn4+ peak (653.44 eV) [50]. As shown in Fig. 9(a), the Mn content of CoMn2O4 is much higher than that of CoMn2O4@TiO2, which is attributed to the core-shell structure of CoMn2O4@TiO2, resulting in a decrease in the proportion of Mn content. It is well known that Mn in different valence states will affect the electron transfer and REDOX ability of the catalyst. Manganese ion is more active in high oxidation state, because Mn4+ ion can promote the oxidation of NO into NO2 [51]. We all know that Mn in different states can influence the electron transfer and REDOX capacity of catalyst. Mn3+ and Mn4+ is considered to be the active site for adsorption and activation, along with redox couples of Mn3+↔ Mn4+. Higher ratios of Mn3+ and Mn4+ are favorable for Mn dissolution species switch between various valence states and promote Catalytic cycle and performance. The (Mn3++Mn4+)/ (Mn2++Mn3++Mn4+) ratio of CoMn2O4@TiO2 catalyst is 77.7%, which is higher than that of CoMn2O4 catalyst 67.4%. Therefore, it is beneficial to the transformation ability of Mn species between different valence states and promotes the catalytic cycle and performance. Therefore, it is more suitable for low-temperature SCR, which may be the reason for the higher reactivity of CoMn2O4@TiO2. The results show that CoMn2O4@TiO2 catalyst can rapidly oxidize the reduced Mn2+ ions to Mn3+ and Mn4+ ions during SCR reaction. This can improve the REDOX capacity of CoMn2O4@TiO2 catalyst, which is conducive to the improvement of catalytic performance.In addition, XPS spectra of Co2p on CoMn2O4 and CoMn2O4@TiO2 catalysts are shown in Fig. 9(b) and Table 3, which are 780.5 eV Co3+ and 782.2 eV Co2+, respectively. The other spin orbital component (Co2p1/2) is at 796.2 eV and 797.8 eV, corresponding to Co3+ and Co2+ configuration, respectively [52]. In addition, peaks centered at 786.6eV and 802eV can be assigned to satellite peaks of Co2p [53]. As can be seen from the Figure, the Co3+/ (Co2++Co3+) of CoMn2O4@TiO2 is 53.5% higher than that of CoMn2O4 (51.8%), indicating that the average valence state of Co ions on CoMn2O4@TiO2 catalyst is higher. Due to the stronger REDOX capacity of Co3+, Co2+ is far less active than Co3+. Therefore, more Co3+ species are conducive to the improvement of REDOX performance, which also determines that CoMn2O4@TiO2 has a better catalytic activity.As shown in Fig. 9(c) and Table 3, the O1s spectra of both catalysts can be divided into two main peaks. A high binding energy peak (531.2eV) corresponds to surface adsorbed oxygen (hydroxyl or oxygen ion, Oα) or defective oxide, while the second low binding energy peak (529.7eV) corresponds to lattice oxygen (O2−, Oβ) [54,55]. The Oα/(Oα+Oβ) ratio of CoMn2O4@TiO2 catalyst decreased from 31.9% to 26.1% compared with CoMn2O4 catalyst. It can be clearly seen that the binding energies of Oα and Oβ at CoMn2O4@TiO2 shift to higher values compared with CoMn2O4 catalyst, and higher binding energies indicate more stable covalent bonds. In addition, the higher concentration of Oα species was conducive to the occurrence of NH3-SCR reaction [56]. The concentration of Oα species in CoMn2O4 is higher than that in CoMn2O4@TiO2, however, a single Oα concentration does not determine a higher activity of CoMn2O4 catalyst, but only leads to a catalyst with the same high activity at the optimum reaction temperature as CoMn2O4@TiO2 above 200 °C, which is consistent with the activity experimental results.For Ti2p XPS spectra in Fig. 9(d) and Table 3, the Ti2p spectra of CoMn2O4@TiO2 catalyst can be divided into Ti2p3/2 and Ti2p1/2 titanium dioxide forms respectively. These bands were observed at 457.9 eV and 463.5 eV, where △BE= gap was 5.6 eV, characteristic of Ti3+ species. The bands at 459.8 eV and 464.5 eV were Ti4+ species [57,58]. For CoMn2O4@TiO2 catalyst, Ti4+ is still the main phase, and there is a certain amount of Ti3+ on the surface of the catalyst. This is because the existence of oxygen vacancies in titanium dioxide can lead to the corresponding charge balance, part of Ti4+ in titanium dioxide receives electrons from oxygen and becomes Ti3+ (Ti4++ E−↔ Ti3+) [59,60].According to XPS analysis, the REDOX capacity of core-shell catalyst CoMn2O4@TiO2 is slightly higher than that of CoMn2O4 catalyst, which explains the increased selectivity of N2 to CoMn2O4@TiO2. In the SCR reaction of CoMn2O4@TiO2 catalyst, there is electron transfer between Mn, Co and Ti, that is Mn4++Co2+↔ Mn3++Co3+, Mn3++Ti4+↔Mn4++Ti3+(corresponding to Eqs. (11) and (12) in 3.4). The strong electron interaction plays an important role in NH3-SCR process under dry and wet conditions.The adsorption of NH3 on the catalyst surface plays a key role in the SCR reaction, so the temperature-programmed desorption of NH3 (NH3-TPD) was used to investigate the number and strength of acid sites on the catalyst surface. The NH3-TPD curves of CoMn2O4 and CoMn2O4@TiO2 catalysts are shown in Fig. 10 . The CoMn2O4 catalyst shows three major desorption peaks centered at 111, 259 and 602 °C, respectively. While the area peaks of CoMn2O4@TiO2 catalyst were at 104, 309 and 736 °C. The NH3-TPD curves for both catalysts can be divided into the following three regions of 50-200 °C, 200-400 °C and 400-800 °C, which can be attributed to NH3 adsorption at weak, medium and strong acid sites, with strong peaks in the high temperature interval probably due to N2 desorption [41]. The corresponding peak areas of the various peaks are denoted as Px (x = 1, 2, 3). The position of the medium-intensity acid peak of CoMn2O4@TiO2 was significantly shifted to a higher temperature range compared with that of CoMn2O4, and this result indicated that the introduction of TiO2 was beneficial to improve the catalytic activity at high temperatures [61]. The strong acid peak sites of CoMn2O4@TiO2 catalyst are much higher than those of CoMn2O4 catalyst, indicating that it has more strong acid sites and stronger acidity. For CoMn2O4, the peak at 259 °C corresponds to NH4 + desorption from the Brönsted acid site, while the peaks at 602 °C correspond to NH3 desorption from the Lewis acid site [62]. In contrast, in the desorption curve of CoMn2O4@TiO2, the broad weak peak at 309 °C corresponds to the desorption of NH3 from the Brönsted acid site, while the sharp peak at 736 °C is from the desorption of the Lewis acid site. It is well known that Lewis acid sites play an important influence in SCR systems, so CoMn2O4@TiO2 has a higher SCR activity compared to CoMn2O4.The percentage of peak area for each acid site was calculated for both catalysts in Fig. 10, and the number of surface acid sites was listed in Table 4 based on the total acid amount. The amount of surface acids follows the order of CoMn2O4@TiO2 > CoMn2O4. It can be clearly seen that the CoMn2O4@TiO2 sample presents stronger acid sites (0.54 μmol/g) compared to the CoMn2O4 sample (0.43 μmol/g). This indicates that the TiO2 shell leads to a catalyst with more acid sites and stronger acidity.The redox performance of catalysts plays an important role in the NH3-SCR reaction, so the reduction of CoMn2O4 and CoMn2O4@TiO2 catalysts was tested using programmed temperature rise reduction (H2-TPR). As shown in Fig. 11 , the H2-TPR spectrum of CoMn2O4 catalyst can be divided into 2 peaks in the range of 50–800 °C. The first peak at around 390 °C could be the combination of MnO2→Mn2O3→Mn3O4 and Co3+→Co2+ [19]. The second peak centered at 552 °C can be explained by the coexistence of Mn3O4→MnO and Co2+→Co0 [19]. Unlike CoMn2O4, another peak exists for the CoMn2O4@TiO2 catalyst, observed at around 208 °C can be attributed to the reduction of surface oxygen species [57]. No obvious electron transfer peak position of Ti was observed in the spectrum, which may be due to several reasons. Firstly, the participation ratio of Ti is too small, much lower than that of (Mn+Co), so the peak position is very low and cannot be visualized. The second reason is that TiO2 is uniformly distributed on the surface with high stability, and it is difficult to observe the valence change of Ti without obvious transition. The third reason may be due to the metal-oxygen bonding between Ti and Mn and Co, so the observed peak positions overlap, which may also be the reason why the peak area of CoMn2O4@TiO2 catalyst is significantly larger than the peak area of CoMn2O4.The presence of lattice defects on CoMn2O4@TiO2 catalysts is related to the appearance of oxygen vacancies caused by crystal structure defects. In contrast, the two reduction peaks of the CoMn2O4@TiO2 catalyst were similar to those of the CoMn2O4 catalyst at 50–800 °C. However, after Ti doping, the position of the reduction peak gradually shifted to a lower temperature, indicating that the catalyst has both better redox performance and higher catalytic activity at low temperatures. Both peaks show an increase in the area of the reduced peaks belonging to Mn3O4 and MnO. This indicates that the oxygen adsorbed on the surface is reduced and Mn4+ is more easily reduced to the lower valence state, which corresponds to the XPS results.The peak areas and total hydrogen consumption of each reduction peak are given in Table 5 . Based on the H2 consumption and reduction temperature of the catalysts, it is concluded that the ease of reduction of the catalysts is in the order of CoMn2O4@TiO2>CoMn2O4. The number of redox sites increases after doping with TiO2 shell layer because H2 consumption increases [63]. Meanwhile, the good interaction of oxides such as Mn, Co and Ti are beneficial to improve the NH3-SCR activity. In-situ DRIFT spectra of adsorbed ammonia species and NO+O2 over time on CoMn2O4 and CoMn2O4@TiO2 catalysts at 225 °C are shown in Fig. 12 . As can be seen from Fig. 12(a), after NH3 pre-treatment for 30min, the surface of CoMn2O4 catalyst is covered by different kinds of adsorbed ammonia species. The bands at 1242, 1340, 1450 and 1540 cm−1 were attributed to the coordination ammonia binding to the Lewis acid site, and the bands at 1607 and 1680 cm−1 were attributed to NH4 + at the Brønsted acid site [64]. After the introduction of NO+O2 mixture gas, the number of ammonia species decreased to a certain extent, and then began to increase with time, indicating that the adsorbed ammonia reacts rapidly with gaseous NO/NO2. After adsorption to the catalyst, NH3 existed in the form of -NH2 and H+. After adsorption saturation of NH3, it further reacted with NO to generate N2 and H2O (corresponding to Eqs. (4) and (5) in 3.4). As shown in Fig. 10(a), bands of ammonia species coincide with several NOx bands, including bridging nitrate (1242 cm−1), nitrite species (1340 cm−1), monotone nitrate (1450 and 1540 cm−1), and nitrogen dioxide (1607 cm−1) [65,66]. In Fig. 12(b), CoMn2O4@TiO2 catalyst also shows Lewis acid sites (1242,1340,1450 and 1540 cm−1) and Brønsted acid sites (1607 and 1680 cm−1) [67,68]. When NO+O2 is added, it can be found that the change trend of CoMn2O4@TiO2 is similar to CoMn2O4. Bridging nitrate bands (1242 cm−1), nitrite species (1340 cm−1), monotone nitrate bands (1450 and 1540 cm−1) and nitrogen dioxide (1607 cm−1) were also presented [69]. The peak intensity of ammonia and NOx species decreased first and then increased with the increase of time. The results showed that ammonia species had strong adsorption and activation capacity for the two catalysts. The results showed that compared with CoMn2O4 catalyst, the reaction adsorption peak of CoMn2O4@TiO2 catalyst was more obvious, and the catalyst had more active intermediates, which was beneficial to SCR reaction. From in situ infrared results, we conclude that both Lewis and Brønsted acid sites are involved in SCR reactions of both catalysts [68]. It is suggested that Eley-Rideal (E-R) mechanism exists, and the adsorbed ammonia reacts with gaseous NO/NO2 to form N2 (or N2O) and H2O [70].On the other hand, the role of adsorbed NOx in CoMn2O4 and CoMn2O4@TiO2 catalysts was studied by in-situ drift spectroscopy under the same reaction conditions. The reaction drift spectra of pre-adsorbed NOx and ammonia are shown in Fig. 12(c) and (d), respectively. The basic reaction principle of this method is the reaction between NH3 and NO in the gas. Firstly, NO oxidizes with oxygen to form NO2, and NH3 gets an H+ to form NH4 +, and then NH4 + reacts with NO2 to form the intermediate NH4NO2, the last, intermediate can further decompose N2 and H2O (correspond to Eqs. (6)–(8) in 3.4). At the same time, the above-mentioned two reaction pathways are accompanied by side reactions. Mn+ reacts with -NH2 produced in the previous process to generate -NH and H+, and then -NH reacts with NO in the flue gas to generate N2O (corresponding to Eqs. (9) and (10) in 3.4). Bridging nitrate bands (1242 cm−1), nitrite species (1340 cm−1), monotone nitrate bands (1450 and 1540 cm−1) and nitrogen dioxide (1607 cm−1) appeared when NO+O2 mixture was introduced for 30 min [71]. The band intensity of different types of NOx increases with time, and then decreases gradually with the addition of NH3. NOx species were quickly covered by the characteristic peak of NH3, and the adsorbed NOx species reacted with NH3. to form N2. When NH3 was added, the peak position remained constant, but the intensity of nitrate species decreased with time. These results indicate that the adsorbed NOx can also participate in the NH3-SCR reactions of the two catalysts. The peak intensities of different nitrates were as follows: nitrogen dioxide > monotone nitrate > bridged nitrate. Combined with reactivity, nitrogen dioxide and monodentate nitrate were the main active species. Nitrogen dioxide has high reactivity at low temperature and is easy to react with adsorbed ammonia to form N2 and water [72]. The diffusion of nitrogen dioxide can be coupled with NO oxidation and “fast SCR” chemistry to promote NH3-SCR reaction. More obvious types of NOx with adsorption peaks can be observed on the surface of CoMn2O4@TiO2. Therefore, the reaction of NH3-SCR to CoMn2O4 and CoMn2O4@TiO2 catalyst mainly follows the typical E-R mechanism, while the L-H mechanism exists but does not play an important role.Reaction between reactants: (4) N H 3 → − N H 2 + H + (5) − N H 2 + N O → N 2 + H 2 O (6) N H 3 ( g ) + H + ↔ N H 4 + ( a d s ) (7) N O ( g ) + 1 / 2 O 2 ( g ) → N O 2 ( g ) (8) e − + N H 4 + + N O 2 → N H 4 N O 2 → N 2 + H 2 O Electron transfer between different elements of the catalyst: (9) M n + + − N H 2 → M ( n − 1 ) + + − N H + H + ( M n + = M n 3 + / 4 + , C o 3 + ) (10) − N H + N O ( a d s ) → N 2 O + H + (11) M n 4 + + C o 2 + ↔ M n 3 + + C o 3 + (12) M n 3 + + T i 4 + ↔ M n 4 + + T i 3 + Blocking effect of the TiO2 shell layer on H2O and SO2: (13) T i O 2 + S O 2 + O 2 → T i S O 4 O (14) 2 S O 2 + O 2 + 2 N H 3 + 2 H 2 O = 2 ( N H 4 ) 2 S O 4 The above reaction mechanism is divided into three main parts, the first part is reaction between reactants, NH3 and NO reaction. After adsorption to the catalyst, NH3 existed in the form of -NH2 and H+. After adsorption saturation of NH3, it further reacted with NO to generate N2 and H2O. In addition to that, NO oxidizes with oxygen to form NO2, and NH3 gets an H+ to form NH4 +, and then NH4 + reacts with NO2 to form the intermediate NH4NO2, the last, intermediate can further decompose N2 and H2O (corresponding to Eqs. (4) to (8) in 3.4). The second reaction mechanism partly involves electron transfer between different elements of the catalyst. Mn+(Mn+ = Mn3+/4+, Co3+) reacts with -NH2 produced in the previous process to generate -NH and H+, and then -NH reacts with NO in the flue gas to generate N2O. In the SCR reaction of CoMn2O4@TiO2 catalyst, there is electron transfer between Mn, Co and Ti, that is Mn4++Co2+↔ Mn3++Co3+, Mn3++Ti4+↔Mn4++Ti3+(corresponding to Eqs. (9) to (12) in 3.4). The third reaction mechanism is partly the blocking effect of the TiO2 shell layer on H2O and SO2. The reason for the high resistance of CoMn2O4@TiO2 may be due to reaction between SO2 and TiO2 on the outer surface of the catalyst generates TiSO4O, thus trapping the SO2 in the flue gas outside the catalyst. In addition, SO2 might react with NH3 in the gas to form (NH4)2SO4 There may be the following reactions are TiO2 + SO2 → TiSO4O and 2SO2 + O2 + 2NH3 + 2H2O = 2(NH4)2SO4 (correspond to Eqs. (13) and (14) in 3.4).Based on the above results, the possible reaction pathway of CoMn2O4@TiO2 was proposed. Although the Langmuir-Hinshelwood (L-H) mechanism exists, it is relatively weak. The Eley-Rideal (E-R) mechanism plays a dominant role in the NH3-SCR reaction, and the electron cycle interacts with CoMn2O4@TiO2 catalyst.It is speculated that the reaction between NH4 + and NO2 will eventually decompose the intermediate into N2. Meanwhile, part of the -NH2 material reacts with gaseous NO to form N2 and water, while the rest is further oxidized to -NH and then reacts with gaseous NO to form N2O. The REDOX cycle is accomplished by electron transfer, transferring reactive oxygen species in SCR reactions. According to Fig. 13 , there is an electron transfer between Mn, Co and Ti. Mn4+ gains electrons to become Mn3+, Co2+ loses electrons to become Co3+, Mn3+ loses electrons to become Mn4+ and Ti4+ gains low electrons to become Ti3+.According to the analysis of catalytic activity and XPS, the catalytic activity below 125 °C at CoMn2O4@TiO2 is low, probably due to the small number of Oα species. With increasing temperature (>125°C), the catalytic activity of CoMn2O4@TiO2 is higher. This is due to stronger Lewis acid sites and higher REDOX capacity. In addition, CoMn2O4@TiO2 showed higher SO2 and enhanced water tolerance. The synthesis of core-shell structure enhanced the surface acidity and REDOX property (Mn4++Co2+↔Mn3++ Co3+, Mn3++Ti4+↔Mn4++ Ti3+) through the coating of TiO2. The reaction of NH3-SCR on CoMn2O4 and CoMn2O4@TiO2 catalysts mainly follows the typical E-R mechanism, while the L-H mechanism is weak. Considering the similar relationship between the two samples, the reason why CoMn2O4@TiO2 has higher SO2 resistance than CoMn2O4 may be its unique core-shell structure. The uniformly distributed titanium dioxide shell of CoMn2O4 core minimizes SO2 poisoning to surface active sites, thus improving the high stability and SO2 tolerance [73–75].In summary, a new type of CoMn2O4@TiO2 core-shell cluster centered on a nuclear sphere catalyst was successfully prepared by sol-gel method and external dynamic coating method. The optimal preparation conditions were selected by the 3D diagram obtained by the two-stage curved surface response experiment (Mn: Co= 2:1), ammonia water and active component addition ratio of 4.17, calcination temperature of 45 °C °C °C, reaction temperature of 225 °C). Then, under the premise of ensuring a certain high activity, the catalyst with a core-shell ratio of 2:1 was selected through SO2 tolerance test, which had high NH3-SCR catalytic activity and SO2 resistance. The catalyst prepared by this method has high activity, excellent N2 selectivity and high stability, achieving more than 95% NO conversion in the 170-225 °C range. It also enhances the resistance to SO2 and H2O in the low-temperature NH3SCR reaction, In the presence of 100 ppm SO2 for 20 h, the catalytic activity can still maintain close to 80%, and after the removal of SO2, the activity can basically recover to the initial level. The reason is uniform distribution of TiO2 shell in CoMn2O4 nucleus reduces the exposure of surface active sites to SO2 and the formation of by-products such as TiSO4O, which provides high stability and improves SO2 tolerance. TEM results showed that CoMn2O4@TiO2 catalyst had an obvious cored shell structure, and TiO2 was uniformly coated on the surface of CoMn2O4, XRD and Raman results show that the catalyst has obvious CoMn2O4 crystal phase and TiO2 crystal phase, and FT-IR results show that CoMn2O4@TiO2 catalyst has higher oxidation reducibility, XPS results show that synthesis of core-shell structure enhanced the surface acidity and REDOX property (Mn4++Co2+↔Mn3++ Co3+, Mn3++Ti4+↔Mn3++ Ti3+) through the coating of TiO2. NH3-TPD shows that the TiO2 shell leads to a catalyst with more acid sites and stronger acidity. H2-TPR shows the ease of reduction of the catalysts is in the order of CoMn2O4@TiO2 > CoMn2O4, the number of redox sites increases after doping with TiO2 shell layer although H2 consumption decreases. In situ DRIFT results show that reaction of NH3-SCR on CoMn2O4 and CoMn2O4@TiO2 catalysts mainly follows the typical E-R mechanism. On the whole, this study provides a new method to solve the SO2 poisoning problem of low temperature SCR catalyst. Zhiyong Qi: Put forward the experimental content and design the experimental scheme. Perform experiments and write papers. Fengyu Gao: Guide the experiment process, help to revise the experiment paper, and propose the modification plan. Songjin Ko: Auxiliary operation experiment, auxiliary modification of the paper. Xiaolong Tang: Guide the experiment process, help to revise the experiment paper, and propose the modification plan. Honghong Yi: Guide the experiment process, help to revise the experiment paper, and propose the modification plan. Hengheng Liu: Auxiliary operation experiment, auxiliary modification of the paper. Ning Luo: Auxiliary operation experiment, auxiliary modification of the paper. Ying Du: Auxiliary operation experiment, auxiliary modification of the paperThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by National Natural Science Foundation of China (U20A20130, 21806009) and Fundamental Research Funds for the Central Universities (06500152).

In this study, the Co(3-x)MnxO4@TiO2 core-shell catalysts prepared by sol-gel and external kinetic coating method were investigated for DeNO x by response surface methodology (RSM) using 3-factor with 5-level experiments. Compared with CoMn2O4 sample, the CoMn2O4@TiO2 catalyst was optimized to obtain almost 100% NO x conversion at 125°C to 225°C that also maintained above 80% SCR activity with 100 ppm SO2 and 10 vol.% H2O in the testing time within 24 h. The XRD and Raman show a clear distinction of that CoMn2O4 spinel phase was well-dispersed on anatase TiO2 over CoMn2O4@TiO2 catalyst, which exhibited a core-shell structure with obvious distribution boundary of CoMn2O4 core coated by TiO2 shell confirmed by TEM images. This intact catalyst presented complete TiO2 coating structure due to the detected bonds Mn-O, Co-O and particular Ti-O via FT-IR analysis. NH3-TPD and H2-TPR profiles indicated that the CoMn2O4@TiO2 catalyst owned more acid sites, stronger acidity and enhanced redox capacities benefiting from its core-shell structure after TiO2 coating. According to XPS analysis, higher content of (Mn3++Mn4+)/(Mn2++Mn3++Mn4+) (77.7%), Co3+/ (Co2+ + Co3+) (53.5%) and surface oxygen (26.1%) were in favour of valence electron interaction (Mn4++Co2+↔Mn3++ Co3+, Mn3++Ti4+↔Mn4++ Ti3+). The NH3-SCR reaction pathways over CoMn2O4 and CoMn2O4@TiO2 catalysts were compared and proposed through DRIFTS experiments, which mainly follows the typical Eley-Rideal (E-R) mechanism, while the Langmuir-Hinshelwood (L-H) mechanism is weak at 225 °C. This study opens up a new avenue for designing efficient and environment-friendly NH3-SCR catalysts and looks promising for practical application.

In 2020, based on U.S energy consumption, the energy consumption summed up to 93 quadrillion Btu. Fossil fuels (petroleum, natural gas, and coal) accounted for 79% of total. Therefore, in order to meet up with the present and the future energy challenges, green diesel is the holy grail of future energy sustainability. Also, as nations round the planet pledge to deal with emission emanated from the use of fossil fuel, here comes an idiom which is becoming new and new from nation leaders: the green hydrogen (biodiesel, propane, solar or hydro, wind, biogas, e.t.c.).Biodiesel also known as green diesel is a renewable and sustainable energy fuel obtained from transesterification of vegetable oils, animal fats, and biomass algae employing ethanol or methanol as organic solvent in the presence of salt of sodium or potassium catalyst [1,2]. With the problems of conventional fuel crisis, toxic effects, environmental problems, high mortality rate recorded against the utilization of conventional diesel, biodiesel as come to stay as the only present and future replacement for conational diesel.However, there are major difficulties to overcome before the green diesel vision can be realized. The major drawbacks against its recognition are the cost of production, low desire output, and high viscous nature. One way to reduce cost of production was the used of wastes materials as feedstocks [3–5]; more vivid way to tackle the problem of high viscous nature and improve the product yield is to increase the methanol-oil molar ratio (Adepoju et al., 2022) [6]. According to stoichiometric ratio, 1:3 is required for reaction to reach completion, but the yield of biodiesel always low with high waste product (glycerol). Twice ratio (1:6) of methanol-oil molar ratio and application of process modelling and optimization software have been identify as a way to tackle the problem [7–10]. Asimina triloba also known as Carica papaya (CP) is a tropical fruit mainly cultivated in tropical climates (Africa, Asian and South America), the fruit contained approximately 15–20% seeds always discarded as a waste after fruit consumption. It has been reported that the dried seed contains 14.1–35% oil content depending on the species of the CP. This oil among all other vegetable oils has been reportedly to be non-edible (acid value = 3.60 mg KOH/g oil) with high degree of unsaturation [11,12]. Hence, its suitability as waste feedstock for green diesel synthesis in the presence of suitable derived base catalyst.Replacing potassium or sodium hydroxide with catalysts derived from agricultural, industrial, and domestic biomass wastes have not only served as feedstock in green diesel synthesis, but also solve the problem of wastes disposal that constituted to the environmental health challenges of plant, animal, and aquatic life [13,14]. Meanwhile, derived catalysts are categories into three categories namely; heterogeneous catalyst, heterogenized-homogeneous catalysis, and biocatalysts. Among these catalysts, heterogeneous catalyst is a solid catalyst of calcium-potassium based compound (Ca–K). It application in synthesis of green diesel (transesterification process) is due to its superior properties such as ease of recoverability and reused, non-toxic, and of low cost. Heterogenized-homogeneous catalysis and Biocatalysts have major shortcomings such as slight reaction conditions, high selectivity and efficiency, in-ability to convert a cellular catalyst into a bioprocess, recoverability problem, in ability to sustainability in harsh environmental conditions, instability in aqueous media, cofactor dependability, reaction allergic, and inhibition inactivation [11,15]. It's not surprising why [16] synthesized CaO from natural waste materials and applied it to conversion of waste vegetable oils blends with hydrotreated kerosene for biodiesel production, while [17] testified the used of cocoa pod husk as heterogeneous catalyst for biodiesel production, while [18] adopted lard oil for the production of biodiesel via heterogeneous catalyst. The work of [19] further emulated the use of heterogeneous catalyst for the conversion of waste cooking oil-Calophyllum inophyllum to biodiesel, but the works reported by Refs. [20,21] utilized the green heterogeneous base catalysts as an effective bio-based for the conversion of oil to biodiesel. These showed that replacement of homogeneous catalyst with derived heterogeneous catalyst has come to stay, and wood ash powder could also serve as biobase material for biodiesel synthesis.Wood ash is the inorganic and organic residual powder remain after the wood combustion or unbleached wood fibre in fireplace, bonfire or in an industrial power plant. Generally, along with oxygen, the major components of wood ash are calcium (Ca), potassium (K), magnesium (Mg), silicon (Si), and phosphorous (P) [22,23]. Due to high Ca–K base present in the residual powder, this can be used for Ca–K base for green diesel synthesis with proper process modeling and optimization of process variables condition [24]. No wonder, the used of wood ash as catalyst for biodiesel production was reported by Ref. [25]. [26] utilisized wood ash for biodiesel production, while wood ash biocatalyst as a green catalyst and its application for synthesis of benzochromene derivate was reported by Ref. [27]. In another study [28], employed calcined wood ash for synthesize of biodiesel, while [29] review the application of wood ash as catalyst in various oil-biodiesel synthesis. These reports showed that catalyst can truly be obtained from the wood ash due to high percentage of calcium and potassium oxide present in its composition.Modeling and optimization of process condition for green diesel production have been reportedly proved to increase the product yield and reduce the production cost making green diesel acceptable as a replacement for fossils fuel [30,31]. Displayed in Table 1 are the past report on the use of different software for process modeling and optimization of synthesized of biodiesel from difference feedstock (oil). It was observed that no report either past or present have modeled or optimized process conditions of biodiesel synthesis from Asimina triloba oil employing I-Optimal design/Integrated Variance. Therefore, this study obtained the optimum biodiesel yields using I-Optimal design in three-level-three-variables process conditions (reaction time, catalyst amount, and ethanol-oil ratio). Catalyst employed was derived from residual wood ash. Economic appraisal of the biodiesel synthesized was also elucidated, and the biodiesel qualities were examined by comparing with biodiesel recommended standard.Matured Asimina triloba seed was obtained from market fruits seller, in Otuoke, Bayelsa, Nigeria. The seed was washed with distil water, sun-dried for two days to semi-dried the seed, oven dried to constant weight at 100 oC for 24 h in a Genlab laboratory oven (fan assisted circulation, high accuracy Pt 100A duplex sensors<0.6 oC, temperature range-ambient +5–100 oC, 8 stages profile control). The dried was separated from the chaff by winnowing, and the cleaned seed was milled into powder using a 4-liter Laboratory blender (single phase, one speed universal motor with maximum ambient temperature rating 40 oC, speed 15, 600 rpm-20,000 rpm, with working capacity 180–1900 mL, dimension 6.5 in), the milled seed powder was then extracted with organic solvent to produce oil. Residual wood ash was obtained from bakery located at Yenegoa, Bayelsa State, Nigeria; the ash was separated from unwanted impurities by using a stainless steel 60 mesh 250 μm. The fine residual ash wood powder (RAWP) was kept in a clean container for further processing.Chemicals used in this work were obtained from Sigma Aldrich Nigeria Limited, and were of standard graded.Extraction of oil from the powder was carried out using a continuous process method known as solvent extraction. 100 g of the powder was loaded in a muslin bag and then put in extraction chambered, the round bottom of Soxhlet extractor (500 ml capacity) was filled with 250 ml of solvent (n-hexane), and the mounted with condenser and set on a four-phase heating mantle at temperature between 68 and 70 oC for until the powder is free of oil. The oil with solvent phase in the flask was transferred to an evaporator reactor (heated at 80 oC) to allow oil made free of solvent by recycled. The recycled solvent was re-used, and the oil was filtered using funnel and filter paper (250 mm size) to remove any particles associated during extraction. The percentage yield of the oil was obtained using Eqn. (1): (1) β ( w / w ) % = m o i l m c p s p X 100 Where β = oil yield, m o i l m c p s p = ratio of mass o oil yield to that of mass of Asimina triloba seed powder.The coated values were taken from the duplicate samples.The properties of oil such as density, moisture content, viscosity, saponification value, iodine value, acid value, and peroxide value via AOAC, 1997 [42] standard procedures. Cetane number, higher heating value and API gravity are computed using Eqns. (2)–(4):Cetane number - [43]. (2) C e t a n e N o = 46.3 + 5458 S V − 0.225 I V Higher Heating Value (HHV) - [43]. (3) H H V ( M J / k g ) = 49.43 − [ 0.041 ( S V ) + 0.015 ( I V ) ] American Petroleum Institute (API) - [44]. (4) A P I = 141.5 S p e c i f i c g r a v i t y @ 15 o c − 131.5 The RAWP characterization was performed using SEM (model 6300F by JEOL solution for innovation, USA) to study the study the surface topography and composition of the catalyst. The XRF (model NEX CG, made by Rugaku, Austin, USA) stimulated over Kά and Cu radiation source, and to confirm the elemental analysis of the samples and the quantitative structure of the samples. FTIR (model 3116465, made in Japan) was used to examine the key functional group and confirm the existence of distinctive absorption bands of the elements in the catalyst powder. The BET isothermal adsorption method (QUANTACHROME, 1KE) and Hammett indicator was used to institute the coefficient of determination along with the pore size, pore diameter, and surface area. The procedures were as follows:Before BET analysis, Initially, the samples were preheated at 150 °C for 45 min underneath helium flow to eliminate some adsorbed loaded entities on the catalyst surfaces, then 5% CO2 was ran over the sample with helium at flow rate 25 ml/min for 40 min. The basic strength of calcined powder and the mixed powder surfaces were assessed by CO2-temperature-programmed desorption (Temperature programmed desorption (TPD) [made in BEL132 Cat, Japan]).The elemental compositions of the samples were checked by (AXS Bruker wavelength) dispersive X-ray Fluorescence (XRF) spectrophotometer with an Rh source and tube of 2.2 kW power. The specific surface area was disclosed by using Brunauer–Emmett– Teller (BET) method via N2 adsorption/desorption isotherm analysis (Surface area & pore size analysis [Belsorb III, Japan]) of the catalyst was carried out on volumetric adsorption analyser at 196°C.FTIR spectrometers (Agilent Technologies Model Cary 122 630 FT-IR spectrometer) with spectral range from 400 to 4000 cm−1 rely on the same basic principle as NDIR analyzers, i.e., the fact that many gases absorb IR radiation at species-specific frequencies. However, FTIR spectroscopy is a disperse method, which means that measurements are performed over a broad spectrum instead of a narrow band of frequencies. This test method is performed by directing an x-ray beam at calcined samples and calcined mixed sample and measuring the scattered intensity as a function of the outgoing direction. The beam is separated, and then scattered when FTIR spectrometer directs beams of IR at the sample and measures how much of the beam and at which frequencies the sample absorbs the infrared light. The spectra were evaluated and identify based on the reference database.Scanning electron microscopy (SEM) is used to examine the morphology of the powder before consolidation. The procedure is straightforward. Two-sided carbon tape is fixed to an SEM sample stub, and the UHMWPE powder is sprinkled onto the surface. A light gold, platinum, was applied (100 Å), and the samples were examined in an SEM chamber. The flakes are 50–100 μm in diameter. The surface morphology was disclosed by field emission scanning electron microscopy (FE-SEM, QUANTA FEG 250).Based on acid value of CP oil, since the Acid value of the oil is greater than the recommended standard (acid value >3.0 mg KOH/g oil) for one step reaction biodiesel production, two steps production processes was adopted as follows:This stage requires the reduction in acid value of the oil. The esterification process involves the use of acid to esterify the oil to acceptable acid value (<3.0 mg KOH/g oil). The procedure reported in our work [45], with little modifications. In this case, chloric acid (HClO3) was used as an acid because of its high acid strength and ability to accelerate reaction condition to earlier completion than HCl and H2SO4. Three necked batch reactor placed on hot magnetic plate fitted with a condenser and magnetic stirrer. 100 ml of oil was measured, and preheated in the reactor on the hot plate with temperature control. 0.15 ml of HClO3 was mixed with 50 ml of methanol in a separate flask and then transferred to the preheated oil in a batch reactor. The reaction time between 30 and 40 min was sufficient to complete the reaction. The resultant product was transfer to a separating funnel for phase separation. The methanol-oil phases was washed with distil water to remove methanol. The washed esterified oil was then dried over anhydrous Na2SO4 to remove washed water, and the Na2SO4 was recovered by filtration. The acid value of esterified oil was determined using official standard method of AOAC, 1990, and the minimum acid value esterified Asimina triloba oil (EASO) was determined via response surface design experiment before transesterification (second stage).In this case, biodiesel was produced using the esterified oil (EASO) with the lowest acid value. The procedures for transesterification of the EASO to biodiesel was as reported in our recent work [46], with little modification. Ethanol was used as a solvent for the synthesis of biodiesel owing to its less chemical toxic and energy carrier ability than methanol.The reaction was carried out in a three-necked batch reactor flask. 80 ml of the cooled EASO was heated at 40 oC for 60 min for oil pretreatment, 2.5 (wt. %) RAWP was dissolved in 16 ml E-OH (ethanol) in a 250 ml flask. The mixture was transferred to preheated oil in the reactor, and the reaction temperature was monitored at 40 oC for further 40 min until reaction completion. The un-dissolved catalyst was separated from the products by decantation, the ethanol-biodiesel-glycerol phases were separated from biodiesel via separating funnel through gravity settling. The catalyst in the biodiesel was recovered by washing with heated ethanolic sodium bicarbonate, and then filtered. The wet biodiesel was dried with an inorganic drying agent (anhydrous Na2SO4). The pure biodiesel was obtained by filtration, and the yield of biodiesel was computed determined. The procedures were carried out in duplicate based on three factors considered for experimental design via response surface design. The produced biodiesel properties were determined using the [42], and the properties were compared with biodiesel recommended standard.Biodiesel production was experimentally designed in two stages with esterification to reduce the oil acid value to minimum level, and transesterification to convert the reduced oil acid value to biodiesel. In the first stage, a total of nine experimental runs were design and carried out without repetition using hybrid design. In the second stage, an I-Optimal design was adopted which generated more experimental runs than box benhken.For esterification experimental design, three factors were considered; HClO3 conc.: K1, reaction time: K2, and Oil/M − OH ratio: K3, along with three levels to established the minimum acid value of the oil. A total of nine (9) experimental runs were designed and carried out based on varied factors design level Table 2 depicts the range of value considered and the variables level.Process transesterification experimental design was carried out using statistical software. Since I-optimal designs provide lower average prediction variance across region of experimentation. It optimality is desirable for response surface methods (RSM) where prediction is important. The algorithm picks points that minimize the integral of the prediction variance across the design space. Hence, the design expert 13.1.4.0 with build time 2632 m.s was adopted for this process by considering three variables in three levels; catalyst amount; X1, reaction time: X2, and E-OH/oil molar ratio: X3, respectively. A total of twenty (20) experimental runs were generated and were experimentally carried out. Table 2 depicts the range of value considered and the variables level.In esterification, the results of experimental values in 9 runs were used as a based to determine the esterified oil with low acid value. Each run was carried out at different acid concentration, reaction time and methanol/oil molar ratio as displayed in Table 2. The minimum acid value was validated in triplicate and the average mean value was used for transesterification stage.Transesterification analysis involved the step by steps analysis with a view to optimize biodiesel production. Regression coefficient, test of significant, hypothesis test, multiple regression, and analysis of variance were used to test the variables significant, coefficient of determination, and interactive effects. The contour and 3-dimensional plots were used to establish the response values and operating conditions variables. The model equation that related the response (biodiesel yield) with the variables interaction is expressed with polynomial regression model of the Kth -order as presented in Eqn. (5). (5) Y = A + B 1 X + B 2 X 2 + B 3 X 3 + … + B K X K Comparative analysis of the hybrid design and Kapla-Meier Estimator (KME) statistical software was evaluated based on the root mean square error (RMSE), coefficient of determination (R-square), adjusted ( R a d j . 2 ) , and predicted ( R p r e d . 2 ) expressed in Eqns. (6)–(8). (6) R M S E = ∑ ( δ i , c a l − δ i , e x p ) N (7) R a d j . 2 = 1 − ∑ i = 1 n ( δ i , c a l − δ i , e x p δ a v g , e x p − δ i , e x p ) 2 (8) R p r e d . 2 = 1 − { ( 1 − R p r e d . 2 ) ( N − 1 ) N − ( N I + 1 ) } With δ i , c a l calculated acid value, δ i , e x p experimental acid value, is the number of experimental runs, δ a v g , e x p average experimental acid value, NI is the number of variables. Table 3 displayed the properties of Asimina triloba oil extracted via solvent extraction as compared with the earlier reported on the same oil. It was observed that the properties were well within the earlier reported by other researchers. The little disparity in the results could be due to how ripe the Asimina triloba was before consumption (unripe Asimina triloba has high acidity which helps remove the bacterial that caused urinary tract infections), the varieties of the fruits, and the growth region. Naturally, Asimina triloba has about seven (7) varieties includes: sunflower, Taylor, Taytwo, Mary Foos Johnson, Mitchel, Davis, and Rebecoas Gold. However, the high acid value of the oil obtained in this study proved that the matured Asimina triloba seed come from unripe fruits, hence, the reason for its potential as feedstock for biodiesel synthesis in a two steps reaction.Since the acid value of oil (3.80 mg KOH/g oil) was found to be higher than the recommended oil (<3.00 mg KOH/g oil) standard for biodiesel production, the oil need to be esterify using a strong acid. To avoid assumption based on trial and error approach, a total of nine (9) runs were design and were carried out based on constraint variables with acid value as the output value. Table 4 present the results obtained for the experimental runs with acid value recorded as the output of the esterified oil.From the table, the experimental runs with lowest acid value was run 6 with acid value of 1.20 (mg KOH/g oil), having an FFA = 0.60. This run with the HClO3 conc. = 0.25 (% vol.); CH3OH/Oil ratio = 5 (vol/vol); and reaction temperature = 60 (oC) was used as the condition for esterify oil transesterification for biodiesel production.The result of BET analysis carried out based on data reduction acquisition employing the DA method with nitrogen as adsorbate for sample weight of 0.12 g, revealed the plot of pore volume against pore diameter in Fig. 1 . It was observed that the highest pore volume was recorded at 0.321 (cc/g) at corresponding value of 2.820 nm pore diameter. It was noticed that higher pore diameter produced low volume which explained while the reaction process was faster to synthesis biodiesel. Further analysis by BJ adsorption method based on liquid density with data reduction parameter evaluation showed a plot of cumulative pore volume, surface area against the pore diameter (Fig. 2 ). The overall maximum values that described the catalyst potentiality for biodiesel production were found at surface area of 441.368 m2/g, pore volume of 0.213 cc/g, and pore diameter of 2.136 nm. The pore size distribution data based on pore with, cumulative pore volume and surface area, dv/d, and ds/d, are as presented in supplementary file (Sup. 1)In order to determine the compositions of the elements present in the catalyst sample, XRS-FP analysis was carried out using Gaussian method and the results was as presented in Table 5 . It was observed that the catalyst consist of various elements, but the major elements are CaO with concentration of 42.516 (wt. %), K2O with concentration of 12.163 (wt. %), and SiO2 with concentration of 23.942 (wt. %). Other elements are present in small quantities, but also aids in biodiesel synthesis. The high CaO indicated that the residual ash has potential to be used as alkaline base for biodiesel production, and the presence of K2O supported the strength of basicity of the catalyst. However, the presence of SiO2 showed that the residual wood ash possesses some acidic strength but SiO2 also help the production of biodiesel due to it weak base nature. The raw results as obtained from the analysis are presented in supplementary file (Sup. 2).The results of SEM analysis carried out on residual wood ash used as catalyst at magnification of 500x and 1000x are presented in Fig. 3 (a–b). Observed from the figures indicated a cohesive jointed-like crack shape with permeable surfaces. The structures showed an ice-like surface mineral particle accountable for the slick flora of the element present. The soapy like look could be attributed to the presence of high concentration of SiO2 found in the catalyst. However, the whiteness, brightness and opacity look could be due to TiO2 present. The fusion adherent effects and the brilliant glossy glaze found in the residual wood ash via XRS-FP Analysis Report could be attributed to the presence of CaO and K2O, which make it as a potential alkaline base catalyst for the synthesis of biodiesel. Fig. 4 represented the results obtained for the FTIR analysis of catalyst sample used for biodiesel synthesis. Various functional elements are found at different wavelengths and angular phases. Normally, the mid-IR spectrum is divided into four regions: the single bond region (2500-4000 cm−1), the triple bond region (2000-2500 cm−1), the double bond region (1500-2000 cm−1), and the fingerprint region (600-1500 cm−1) (Nandiyanto and Ragadhita, 2019). The functional groups present can be identified as follows (Coates, 2000):At fingerprint region, the following functional group can be found (i) aliphatic organohalogen compound such as C–F, C–Cl, C–I, and C–Br. (ii) the Alcohol, OH out-of-plane bend, (iii). Phenol, C–O stretch, (iv) the primary, secondary, and tertiary alcohol, C–O stretch, (v) the primary or secondary, OH in-plane bend, (vi) phenol or tertiary alcohol, OH bend, (vii) the peroxide, C–O–O-stretch, (viii) the Epoxy, oxirane rings, and Aromatic ethers, aryl-O stretch, (viii) the Alkyl-substituted ether, and Cyclic ethers with large rings, C–O stretch, (ix) the primary, secondary, and tertiary, both amine and aromatic CN stretch, (x) the carboxylate salt, the P–O–C, aromatic and aliphatic phosphates, the carbonate ion, sulfate, nitrate, phosphate, silicate e.t.c can be found.At the double bond region, there exists the following functional group: (i) the nitrogen-oxy compounds, the open-chain imino –C = N-, open chain azo –N = N- (ii) the carbonyl compound such as: ketones, carboxylic acid, aldehydes, Ester, Amide, Acid halide, Aryl carbonate (iii) the primary and secondary amine >N–H bend, (iv) Aromatic ring (aryl) such as CC–C aromatic ring stretch (v) the olefinic (alkene) such as Alkenyl CC stretch, aryl substituted CC, conjugated CC.At the triple bond region (2000-2500 cm−1), the following functional groups can be found: (i) Acetylenic (alkyne) such as C ≡ C terminal and medial alkyne, (ii) the transition metal carbonyl, (iii) the Ester carbonyl (iv) the Nitrogen multiple and cumulated double bond compound such as Thiocyanate (-SCN), Isocynate (-NCO asym. stretch), Cyanate (-OCN and C–OCN stratch), aromatic and aliphatic cyanide, (iv) the ether and oxy compound of methoxy, C–H stretch (CH3–O-).At the single bond region (2500-4000 cm−1), there exist the functional groups such as (i) Alkyne C–H stretch, (ii) Olefinic (alkene) such as terminal (vinyl) C–H stretch, the pendant (vinylidene) C–H stretch, medial, cis-or trans-C-H stretch, (iii) Saturated aliphatic (alkene/alkyl) such as methyl C–H asym./asym stretch, methylene C–H asym./sym stretch, methyne C–H stretch, methoxy, methyl ether O–CH3, C–H stretch, methylamino, N–CH3, C–H stretch (iv) The Acetylenic (alkyne) such as alkyne C–H stretch (v) the alcohol and hydroxyl compound such as hydroxyl group, H-bonded OH stretch, normal polymeric OH stretch, Dimeric OH stretch, internally bonded and non -bonded hydroxyl group, OH stretch, primary, secondary, tertiary alcohol, OH stretch, phenols, OH stretch, (vi) Ether and oxy compound of methoxy, C–H stretch (CH3–O-) (vii) Primary and secondary amino of aliphatic and aromatic primary amine NHH stretch, aliphatic and aromatic secondary amine >N–H stretch, heterocyclic amine >N–H stretch, and imino compounds =N–H stretch (vii) The thiols of S–H stretch, (viii) the common inorganic ions such as ammonium ion.The wavelength peak found in this study are within the aforementioned regions, therefore, it can be concluded that the analysis of the calcinated wood ash as catalyst for biodiesel synthesis was viable.Further analysis via quantitative analysis on the residual wood ash used as catalyst based on phase data view showed the plot of intensity against the angular phase diagram. The plot showed a zig-zag plot with graphite as the major compound containing carbon as the major element. Other compound such as Quartz, Adamite, Gahnite, Zincite, and Willemite are present with figure of merit based on weight fraction via the analysis (Fig. 5 .). The presences of quartz identify in the catalyst represent the catalytic current which helped in oscillatory vibration of the phases. The graphite identify in the catalyst responsible for high tension lubrication responsible for low viscous product. The presence of adamite in the catalyst helped in formation of biodiesel colour, usually adamite always yellow in colour which responsible for light yellowish biodiesel produced in this work. Other compounds also aids in biodiesel formation during production. Table 6 presented the I-optimal twenty (20) designs of constraint variable runs, the experimental yield, the residual value (errors), and the leverages. It was observed that the maximum biodiesel yield was obtained at run 3 (98.77% vo./vol.), while the minimum yield was obtained at run 17 with a yield of 84.00% (vo./vol.). This proved that the conversion of Asimina triloba seed oil to biodiesel via two steps reaction processes was successful, and the catalyst derived from residual wood ash is suitable for the synthesis. The experimental values and the predicted value were displayed graphically in Fig. 6 (a), which shows an absolute perfect straight line with no intercept, passing through the origin with all points lie in the line. This also shows that the experimental results via.average value recorded helped in optimal design prediction. The graph of residual plotted against the runs showed a sinusoidal waveform with amplitude of ±2.1, indicated that the experimental runs design were design with both ends equity (Fig. 6(b)).Based on Test of significant, Table 7 represented the full results obtained using I-optimal design, it was noted that all the variables were significant at p-value< 0.005 including the model value. This indicated that the coefficients of determination were found close to 100%. The value of the.Predicted R2 of 99.07% is in reasonable agreement with the Adjusted R2 of 99.78%; i.e. the difference is less than 2%. The model equation based on the coefficient estimate which represents the expected change in response per unit change in factor value when all remaining factors are held constant are presented in Eqns. (9) and (10):In terms of coded value (9) Exp .  Yield  ( % v o l . / v o l . ) = + 89.84 + 5.83 X 1 + 2.00 X 2 − 0.4321 X 3 + 1.36 X 1 X 2 + 0.2902 X 1 X 3 + 0.3966 X 2 X 3 + 1.91 X 1 2 − 2.71 X 2 2 + 0.4578 X 3 2 In terms of actual value (10) Exp .  Yield  ( % v o l . / v o l . ) = + 89.83572 − 2.32016 X 1 + 50.66581 X 2 − 9.75584 X 3 + 0.271285 X 1 X 2 + 0.02901 X 1 X 3 + 0.793211 X 2 X 3 + 0.019149 X 1 2 − 10.83093 X 2 2 + 0.47773 X 3 2 The equation in terms of coded and actual factors can be used to make predictions about the response for given levels of each factor. By default, the high levels of the factors are coded as +1 and the low levels are coded as −1 for coded and in actual, the levels should be specified in the original units for each factor. The coded equation is useful for identifying the relative impact of the factors by comparing the factor coefficients whereas the actual equation should not be used to determine the relative impact of each factor because the coefficients are scaled to accommodate the units of each factor and the intercept is not at the center of the design space.Based on each factor significant contribution towards the response (biodiesel) with p-value highly significant, the significant of each variable on the response are displayed in Fig. 7 (a–c). It was noted that higher reaction time, closely high catalyst amount, and high ethanol to oil molar ratio favoured the response.Further on variable effects on the response, the model quadratic response effects on the response are presented in 3-dimensional contour form displayed in Fig. 8 (a–c). The statistical analysis by confirmation and prediction, predicted a yield of 98.93% (vol/vol) at the following variables conditions; X1 = 59.976 (min), X2 = 3.204 (% wt.); X3 = 6.979 (vol/vol). This value was validated via three experimental runs; an average value of 98.87% (vol/vol) was obtained. These also proved that I-optimal designs provide lower average prediction variance across the region of experimentation in this study, which is desirable for response surface methods (RSM) where prediction and optimum validation are important. The algorithm picks points that minimize the integral of the prediction variance across the design space.To examine the qualities of biodiesel produced from Asimina triloba seed oil using residual wood ash as catalyst, the physicochemical properties of the diesel was examined and the results were compared with biodiesel recommended standard- ASTM D6751 [49] and EN 14214 [50]. Observations from the table (Table 8 ) indicated that the properties of the Asimina triloba biodiesel produced are well within the biodiesel recommended standards. Nearly all the properties of oil have reduced in values as the conversion through two steps processes have proved to be effects ways for synthesis of biodiesel with an active catalyst from residual waste ash (see Table 9).To ascertain the content and composition of esters present in the biodiesel, the characteristics of biodiesel were determined by GC-MS. Table 9 displayed the presence of methyl ester compound present in the biodiesel sample, the retention time, and the molecular weight, while the peaks are attached in supplementary file. Observation from the table indicated that the conversion of Asimina triloba oil to biodiesel in two step approached using wood ash as precursor for the synthesis of CaO–K2O –SiO2 base catalyst was efficacious carried out.The methyl ester compounds found in the biodiesel formed are methyl tridecanoate, tetradecanoic, methyl myristate, methyl palmitoleate, methyl palmitate, methyl ester, methyl elaidate, methyl stearate and methyl elaidate. These results are in agreement with the previous study by Ref. [50], that the GC-MS fatty acid composition of Carica papaya seed oil contained oleic acid, palmitic acid, lauric acid, stearic acid, hexadecenoic acid, linoleic acid, and myristic acid. Table 9 indicated the results of syntheses of FAME using developed catalyst from biomass sources, the percentage yields and the percentage minerals content. Observation from the table showed that the biodiesel conversion yield was high with respect to the concentration of CaO found within the source. The results further showed that most reports derived only CaO from the biomass source which account for low and high yield of biodiesel reported in various work, but the analysis of the wood ash used in this study account for the presence of other compound such as SiO2 and K2O which support the strength of the CaO catalyst during production also found in wood ash. As earlier said SiO2 and K2O are also base and aids in formation of biodiesel. Also, the calcined temperatures reported by researchers were noticed to be around 105–900 oC, these temperature were less than the thermal temperature treatment of 1000 oC reportedly used in this study. Also, the process optimization used for experimental design and validation produced the optimum yield, whereas, most of the biodiesel yield reported by the authors were not validated by any optimization software like I-optimal design adopted in this study; the yields recorded were maximum experimental yields. Therefore, the CaO– SiO2–K2O based catalyst derived from wood ash in this study proved to be suitable Bioresource catalyst for conversion of Asimina triloba oil to biodiesel.To examine the strength of catalyst developed from residual wood ash, the used catalyst after production was recycled, and the impurities in the catalyst was removed by washing primary alcohol, centrifuge for 30 min at 3500 rpm, and then filtered. The residual catalyst was oven dried to constant weight and then cools at room temperature for 40 min. The obtained purified catalyst was reused at the maximum biodiesel yield and variables condition (reaction time of 50 min, catalyst amount of 5.50 (%wt.), and 7:1 (vol/vol) EtOH/OMR).The results obtained at vary reusability test were presented in Fig. 9 using Microsoft excel plot. Catalyst reusability test showed the biodiesel yield approaches ∼100% at first-three cycles (1–3 cycles). This could be attributed to freshly active pore site, pore diameter and large surface area of the catalyst. At 4th to 6th cycles, there was no significant reduction in biodiesel yield <10% reduction. This reduction can be due to the adherent of impurities at the surface of the catalyst during reaction and formation of by-product. At 7th cycle, there exists greater reduction in biodiesel conversion to 90.20% owing to formation of glycerol and ethanol covering the active catalyst sites, hereby rendering the activities of catalyst minima. The 8th-10th cycles the conversion yield reduced greatly (87-80%), this proved that the catalyst interface is completely less active as results of high impurities and the need to introduce new freshly made catalyst for biodiesel conversion cost effectiveness. Hence, catalyst reusability test was altered at 7th cycles. This phenomenon exhibited by wood ash catalyst can be attributed to nonstop intermediate products of mono/diglyceride formed during reaction which blocked the catalyst hovels, and the development of water-oxygen reaction that takes place at the catalyst superficial, hereby reduces the catalyst sensitivity.Meanwhile, the biodiesel conversion yield from wood ash catalyst after 7th cycle may be low, but the yields are still better than most homogeneous catalyst reportedly used for biodiesel production [12,40,57,58], but for the purpose of cost estimation and acceptability of biodiesel as replacement for conventional diesel, the conversion yield must be targeted to optimum value.As earlier reported, the main factor affecting the full application of biodiesel nationwide is its cost of production, hence, the need for its economic appraisal. Generally, the materials used for the production of biodiesel in this study are wastes obtained freely from nearby locations. The key main factors (Table 9) to be considered in cost estimation in developing new solid base catalysts are wastes assemblies; refinement/planning and categorizations [14,32] Basis: 50 L of biodiesel.The cost of production was estimated in Nigeria Naira (N) by Eqn. (5): (11) ∩ ( N ) = α + β + γ + μ + ε + θ Therefore, ∩ ( N ) is N 700.00, which is the cost of producing 50 litre of biodiesel using wastes materials. This is equivalent to $1.68 as at 16th June 2022 (N 1 = $0.0024).Comparing this result with the previously reported works ([59] reported $46.34/kg [32], reported $3.01/kg for 10 L), it can be observed that this is the best result reported so far on cost of biodiesel production ($1.68/kg for 50 L of biodiesel). This proved that replacing conventional diesel with biodiesel/green diesel is highly economical, environmentally friendly and always available [32].In this work, biodiesel was synthesized from Asimina triloba waste seed oil. The oil was obtained from solvent extraction of seeds. The acid value of the oil was high, and was esterified in the first stage by strong acid, HClO3 in ten experimental runs. The esterified oil was converted to biodiesel in second stage using a residual wood ash as catalyst. Catalyst analysis and characterization showed that the produced catalyst has high basic strength with potassium and calcium as major elements. Catalyst reusability test showed that the residual wood ash catalyst can be recycled and used in seven cycles. Statistical optimization via I-Optimal design established the optimum biodiesel yield of 98.87% (vol/vol) at stable constraint variables. Economic appraisal showed that the wastes adopted in this study proved to be the best materials for biodiesel production. The produced biodiesel have some fuel properties when compared with biodiesel standard.No research funds available for this work.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Authors acknowledge the effort of Lillian for her input in this work. The efforts of technical staff of Spectral Laboratory Services of Engineering and Science Analyses, No. 14 Forte Oil Station Polytechnic Road, Tudunwada Kaduna South, Kaduna, Nigeria are highly appreciated.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.cscee.2022.100252.

This study critically examined the visibility of Asimina triloba oil for synthesis of biodiesel in the presence of ethanolic CaO–K2O –SiO2 base catalyst developed from residual wood ash powder. The oil was extracted using continuous extraction method. The physicochemical properties of the oil was determined for its production routes Two steps reaction process were adopted; first to lower the acid value of Asimina triloba oil by considering three factors namely; HClO3 conc., M-OH/Oil molar ratio, and reaction time. The second step convert the esterified oil to biodiesel by considering three variables namely: reaction time: X1, catalyst amount: X2, and E-OH/oil molar ratio: X3 as variables constraint using I-Optimal design by. Catalyst obtained from (RWA) was characterized using SEM, FTIR, XRF-FT, BET isothermal adsorption, and qualitative analysis. Catalyst reusability, and economical appraisal of the biodiesel synthesized were also examined. The results showed that Asimina triloba seed is rich in oil with 40.34%, and the oil is highly unsaturated with 88.90%, having high acid value of 3.80 mg KOH/g oil. Based on first step, the acid value of the oil was reduced to 1.20 mg KOH/g oil which was used as esterified oil in the second stage. A maximum experimental biodiesel yield of 98.73% (vol/vol) was obtained, but I-optimal design predicted 98.93% (vol/vol) yield at the following variables conditions; X1 = 59.976 (min), X2 = 3.204 (% wt.); X3 = 6.979 (vol/vol) which was validated as 98.87% (vol/vol). Catalyst characterization showed that the RWA contained high amount of CaO of 42.516 (% wt.), K2O of 12.168 (% wt.), and SiO2 of 23.942 (% wt.) which serve as base catalyst. Catalyst reusability showed that the catalyst RWA degradation effects start at 8th cycles, and the economic appraisal showed the production of biodiesel using Asimina triloba oil is cost effective fuel properties in line with biofuel standard.

As a clean and efficient energy source, hydrogen energy is pursued after by scientists [1]. The use of solar energy to conduct photoelectrochemical (PEC) catalysis is a highly prospect way to produce renewable energy [2]. Thus, many metal-based compounds are exploited for the realization of potent reaction efficiency [3–5]. Among them, BiVO4 can be regarded as a typical photoanode material for PEC water splitting and has attracted substantial attentions due to its satisfactory light absorption and high theoretical photocurrent density [6–8]. However, the recombination of photogenerated electrons and holes on the electrode surface and the slow reaction kinetics often make the photocurrent density of BiVO4 lower than its theoretical value, thereafter constricting its practical application in photoelectrochemical water splitting [9–11].In order to solve the drawbacks of BiVO4, many studies raised up a variety of effective strategies to promote charge separation and suppression of charge recombination [12,13]. In addition to element doping and morphology control to restrain charge recombination [14–17], proper co-catalyst loaded can not only augment the light absorption of electrode materials, but also enhance the kinetics of water oxidation reaction [18,19]. For instances, Kim et al. successfully prepared a NiOOH/FeOOH/BiVO4 electrode and obtained a photocurrent density of 4.5 ​mA/cm2 with Na2SO3 hole scavenger [20]. Based on this, Zhang et al. prepared an ultra-thin β-FeOOH nano-layer rich in oxygen vacancies. The obtained FeOOH/BiVO4 photoanode showed excellent PEC performance for water oxidation, with 4.3 ​mA/cm2 photocurrent obtained in Na2SO4 solution (At 1.23 ​V vs RHE, AM 1.5 ​G illumination) [21]. Rong et al. took the lead in synthesizing NiFeOOH on carbon nanotubes and confirmed that this structure can expose more active sites. The synergistic effect of carbon nanotubes and NiFeOOH also endowed the composite with excellent stability [22]. Recently, Fang et al. constructed dual co-catalyst systems using NiFeOOH and Co−Pi to form a heterojunction with BiVO4. The core−shell structure of BiVO4/NiFeOOH/Co−Pi electrode displayed excellent photocurrent density (2.03 ​mA/cm2 at 1.23 ​V vs RHE) [23]. And Co−Pi has been commonly regarded as one of the representatives of high-efficiency cobalt-based catalysts, and it can form a stable and effective catalytic system with a variety of materials [24,25]. However, the photocurrent density of BiVO4/NiFeOOH/Co−Pi is still lower than the theoretical value of BiVO4. To this end, finding a more effective and easily synthesized photoanode remains a prerequisite in PEC water splitting. We turned our attention to other cobalt-based salts, hoping to find a more suitable co-catalyst. After trying to load cobalt silicate on NiFeOOH/BiVO4 (Co−Sil/NiFeOOH/BiVO4) [26], we noted Co−Ci co-catalyst can increase the carrier concentration in CO2 reduction and perovskite solar cells [27–29]. Moreover, Yadav and AmalenduChandra studied the dynamic characteristics of carbonate ion in aqueous solution by using the dispersion-corrected density functional theory [30]. The results showed that there was a strong hydrogen bond between carbonate and metal ions, which made the structure more tightened. Therefore, cobalt−carbonate (Co−Ci) co-catalyst is considered one of the candidates for high-efficiency cobalt-based catalysts.Herein, inspired by the above works, we introduced cobalt-based co-catalysts into NiFeOOH/BiVO4, and obtained respectively Co−Ci/NiFeOOH/BiVO4, Co−Pi/NiFeOOH/BiVO4 and Co−Sil/NiFeOOH/BiVO4 electrodes. Three composite electrodes all show higher PEC performance than BiVO4. However, it is obvious that the Co−Ci/NiFeOOH/BiVO4 photoanode has achieved the most excellent performance. The photocurrent density of 4.1 ​mA/cm2 is obtained in 0.5 ​mol/L Na2SO4 solution, and both the injection and separation efficiency have also been enhanced. As a co-catalyst, NiFeOOH plays a role of hole transport layer, making the separated holes more easily captured by Co−Ci. It can also be used as a passivation layer on the surface of BiVO4, because NiFeOOH makes the charge recombination lower at the interface. The promotional effect of Co−Ci is due to rich oxygen vacancies in the thin Co−Ci layer that improve the PEC performance of the photoanode. Compared with Co−Pi and Co−Sil, it also can better match the NiFeOOH/BiVO4 interface. Interestingly, we note that among the three different co-catalysts, carbonate ion has a smaller ionic radius than phosphate and silicate ion, suggesting that the charge around Co−Ci may be more intensive and can accelerate the charge transfer at the interface. The more compact structure of the carbonate allows Co−Ci to better match the NiFeOOH/BiVO4 interface and stimulate the performance of the electrode. As an unpopular co-catalyst in the field of photoelectrochemical water splitting, Co−Ci has brought unexpected performance to BiVO4 photoanodes, and also provides potential application prospects for other photoanodes. In the follow-up work, perfecting the Co−Ci reaction mechanism is an urgent task, and various carbon-based catalysts should be also received continuous attention.Na2SiO3·9H2O and KHCO3 were purchased from Tianjin Kaixin Chemical Co., Ltd. Na2HPO4 was purchased from Shanghai Zhongqin Chemical Reagent Co., Ltd. FeCl2·4H2O, NaOH, Na2SO3, KI, Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Bi(NO3)2·5H2O, Na2SO4 and Na2H2PO4·2H2O were purchased from Sinopharm Chemical Reagent Co., Ltd. p-benzoquinone and vanadium acetylacetonate VO(acac)2 were purchased from Shanghai Aladdin Biochemical Co., Ltd. Fluorine-doped tin oxide (FTO) conductive glass was purchased from Zhuhai Kaiwei Photoelectric Technology Co., Ltd. Hexamethylenetetramine (C6H12N4) was purchased from Yantai Shuangshuang Chemical Co., Ltd. Dimethyl sulfoxide was purchased from Tianjin Damao Chemical Preparation Plant. All chemicals are analytically pure, and deionized (DI) water was used for all experiments.Using the BiVO4 (please refer to the support information for specific steps) prepared by electrodeposition of the classic three-electrode system [20], BiVO4/NiFeOOH was obtained by a simple solution impregnation method. Two aqueous solutions of 2 ​mmol/L Ni(NO3)2·6H2O and FeCl2·4H2O were prepared, and 5 ​mmol/L hexamethylenetetramine was dissolved in the above solution. Then, two aqueous solutions were mixed in a culture flask at a volume ratio of 1:2 and BiVO4 was placed in the culture flask obliquely. Finally, an equal volume of NaOH (20 ​mmol/L) was poured into the bottle and it was stood for 10 ​min [23].For the Co−Ci co-catalyst, a simple and efficient PED method was adopted, using 0.3 ​mmol/L Co(NO3)2·6H2O as a cobalt source and dissolving it in 0.1 ​mol/L potassium bicarbonate [29]. The constant-potential electrodeposition method was adopted, the starting voltage is −0.1 ​V, and the deposition time is 360 ​s (the light source is a xenon lamp that simulates sunlight AM 1.5 ​G 100 ​mW/cm2). Co−Sil or Co−Pi co-catalysts also followed the similar steps, using 0.3 ​mmol/L Co(NO3)2·6H2O as the cobalt source to dissolve in NaSiO3 solution and pH ​7 phosphoric acid buffer, respectively. Co−Sil was prepared by PED for 15 ​s, while Co−Pi was deposited by electrodeposition for 300 ​s.All the instruments used in the characterization can be seen in the supplementary information.In the photoelectrochemical performance test of all photoanodes, we used CHI 660D and CHI 760E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd.) to build a three-electrode system . The prepared photoanode was the working electrode (WE), Pt electrode was the counter electrode [31], and Ag/AgCl (3.5 ​mol/L KCl) electrode was the reference electrode (RE). The electrolyte used in this work was 0.5 ​mol/L Na2SO4 (pH=7.35) aqueous solution. The light source was a xenon lamp produced by Beijing Zhongjiao Jinyuan Technology Co., Ltd. combined with an AM 1.5 ​G filter, and the light power density on the surface of each composite material was equal (the calibration was 100 ​mW/cm2). Gas chromatograph was used to understand the water splitting performance of the composite electrode. In addition, the incident photocurrent conversion efficiency (IPCE) was achieved with a xenon lamp (PLS-SXE300C) equipped with a monochromator (71SWS, Beijing NBeT Technology Co., Ltd.). In all photoelectric tests, the working electrode was backlit.In the first place, BiOI was obtained on FTO conductive glass by using the classical three-electrode system, and BiVO4 was prepared according to the previous reports. We can see clearly in the scanning electron microscope (SEM) that the thin slice BiOI with voids and the interlaced worm-like BiVO4 were obtained by calcining (Fig. 1 a and b). As shown in Fig. 1c, after loading NiFeOOH on BiVO4, the smooth white particles adhered on the tail side of BiVO4 became rough and made the structure more tightened [32,33]. Fig. S1 (a, b, c) show the SEM images of Co−Pi/NiFeOOH/BiVO4, Co−Sil/NiFeOOH/BiVO4, Co−Ci/NiFeOOH/BiVO4, respectively. Furthermore, there is no obvious difference among the above electrodes. Fig. 1d shows a flower-like nanocluster structure with a radius of about 1 ​μm in Co−Ci/NiFeOOH/BiVO4, which can also be seen in NiFeOOH/BiVO4, Co−Pi/NiFeOOH/BiVO4 and Co−Sil/NiFeOOH/BiVO4. From the SEM−EDS element images of Co−Ci/NiFeOOH/BiVO4, it can be clearly seen that Bi, V, O, C, Fe, Ni and Co are uniformly distributed, which proves the successful preparation of this composite electrode (Fig. 2 a). In addition, the SEM−EDS element images of NiFeOOH/BiVO4, Co−Pi/NiFeOOH/BiVO4, and Co−Sil/NiFeOOH/BiVO4 can all be confirmed in Fig. S2, Fig. S3, Fig. S4. The internal structure of Co−Ci/NiFeOOH/BiVO4 was further examined with a high-resolution transmission microscope (HR-TEM). 0.312 ​nm and 0.342 ​nm correspond to the BiVO4 (−121) and NiFeOOH (120) crystal faces, respectively (Fig. 2 b and c) [23]. The amorphous layer at about 4 ​nm was observed from the region contained in the dashed line profile, and combined with X-ray diffraction (XRD) analysis, it can be considered that Co−Ci is amorphous. XRD is usually used to analyze the crystal structure of the material. In Fig. S5a, it is obviously observed that the peaks at 2θ=27° and 33.9° correspond to (110) and (101) planes of SnO2 (JCPDS No. 46–1088) in FTO conductive glass, respectively. The 2θ values of 18.2°, 18.6°, and 28.8° are attributed to (110), (011) and (−121) planes of monoclinic scheelite BiVO4 (JCPDS: 14–0688). Other peaks also basically have crystal planes corresponding to the above two substances. Although the diffraction peaks of α-NiOOH (JCPDS No. 06–0075) and α-FeOOH (JCPDS No. 29–0713) are blocked by SnO2 and BiVO4, but the related signals can still be found in NiFeOOH/BiVO4, Co−Pi/NiFeOOH/BiVO4, Co−Sil/NiFeOOH/BiVO4 and Co−Ci/NiFeOOH/BiVO4. Due to amorphous properties of the cocatalyst such as Co−Ci, no obvious characteristic peaks were found in three different cocatalysts of Co−Ci, Co−Sil and Co−Pi, which was consistent with the results of TEM analysis and previous reports [26,34]. In the XRD spectrum, only the characteristic diffraction peak at 2θ=28.8° of BiVO4 was observed to slightly move to the high diffraction angle after loading the co-catalyst, suggesting that the crystal structure of BiVO4 was not destroyed in the system composed of NiFeOOH and Co−Ci (Co−Sil or Co−Pi) double-layer co-catalysts.X-ray photoelectron spectroscopy (XPS) can be used to prove the composition and chemical valence of elements, and can further prove the successful preparation of Co−Ci/NiFeOOH/BiVO4. Fig. S6 (a-c) shows the XPS whole patterns of BiVO4, NiFeOOH/BiVO4 and Co−Ci/NiFeOOH/BiVO4, and all the elements are marked. The element composition is consistent with the SEM−EDS results. Fig. 3 shows the detailed high-resolution XPS spectra, clearly showing element composition and the changes of chemical bond state. Bi 4f7/2 and Bi 4f5/2 in BiVO4 are located at 158.5 ​eV and 163.9 ​eV, respectively. After co-catalyst was loaded, the two peaks shifted slightly to the low angle direction (Fig. 3a). Similarly, the peak positions of V 2p3/2 and V 2p1/2 also slightly changed (Fig. 3b). As for the O 1s peak, in BiVO4, it can be well fitted for two types of oxygen. One is 529.3 eV O2−, which is caused by the combination of oxygen atoms and metals. The other is 531.9 ​eV adsorbed oxygen (H—O—H) related to hydroxyl groups in adsorbed water molecules (Fig. 3c) [35]. In NiFeOOH/BiVO4 and Co−Ci/NiFeOOH/BiVO4, O 1s can be clearly identified by three peaks, and the newly existing signal is at about 530.7 ​eV. This peak is related to oxygen vacancies, confirming the co-catalyst is helpful to produce many oxygen vacancies, which can better promote water oxidation [21]. Fig. 3d is fitted with two peaks at 855.8 ​eV and 872.9 ​eV, and is accompanied by a satellite peak. These two peaks correspond to Ni 2p3/2 and Ni 2p1/2, respectively, proving that Ni exists in the form of +2. The Fe 2p spectrum shows that Fe exists in the form of +3. The peaks for NiFeOOH/BiVO4 at 711.3 eV and 724.8 eV correspond to Fe 2p3/2 and Fe 2p1/2, and 716.3 eV and 730.8 eV are two satellite peaks (Fig. 3e). By comparing the peak areas of Ni 2p and Fe 2p (≈1:2), the existence of NiFeOOH is proven [23]. Finally, the two characteristic peaks representing Co 2p3/2 and Co 2p1/2 are located at 779.2 ​eV and 796.2 ​eV, respectively (Fig. 3f). Co 2p3/2 can be fitted to two peaks of 779 ​eV and 782.7 ​eV, which are attributed to Co3+ and Co2+, respectively, indicating Co3+ coexists with Co2+ in Co−Ci/NiFeOOH/BiVO4. Combined with C 1S in Fig. S6d, the preparation of Co−Ci is successful [27,36]. In addition, UV–Vis diffuse reflectance is used to compare the absorbance and absorption boundary of different materials to infer the optical properties of the test sample. As shown in Fig. 4 a, the absorbance of NiFeOOH is slightly enhanced compared to BiVO4, and the absorption edge is almost unchanged. After loading three different co-catalysts, Co−Ci, Co−Sil, and Co−Pi, the three dual-co-catalyst-loaded electrodes have better absorbance, indicating that the light absorption capacity of the composite electrode has been enhanced. Compared with BiVO4 and NiFeOOH/BiVO4, the absorption edges of the three composite electrodes have obvious red shift, which proves that the synergistic effect of double layer co-catalyst can broaden the light absorption region and better capture and utilize light. The light harvesting efficiency (LHE) in Fig. S7a reveals that the three photoanodes of Co−Ci/NiFeOOH/BiVO4, Co−Sil/NiFeOOH/BiVO4, and Co−Pi/NiFeOOH/BiVO4 absorb more than 80% of the light. In Fig. 4b, we determine that the band gap of BiVO4 is about 2.41 ​eV by extending the slope of Tauc curve to X axis, which is basically consistent with the known band gap of BiVO4. As the basic characteristics of semiconductors, the band gaps of different catalysts generally do not change. In the subsequent Mott−Schottky figure, the flat band potential (V fb) of the composite electrode is known by linear fitting, which can better explain the excellent catalytic performance of Co−Ci in water oxidation [37].Photoanode oxygen evolution reaction (OER) has the problem of slow reaction kinetics. The reaction kinetics can be accelerated by increasing oxygen vacancies and preparing ultrathin co-catalysts [38,39]. Electrochemical impedance spectroscopy (EIS) was used to clarify the interface charge transfer resistance of Co−Ci/NiFeOOH/BiVO4, Co−Sil/NiFeOOH/BiVO4, Co−Pi/NiFeOOH/BiVO4, NiFeOOH/BiVO4 and BiVO4. It can be observed in Fig. 5 (a and b) that the semicircle diameter of Co−Ci/NiFeOOH/BiVO4 photoanode is small, indicating its optimal charge transfer ability. In the equivalent circuit fitting, the intercept between the curve and the X axis represents the equivalent series resistance [10], and the semicircle reflects the interfacial charge transfer resistance (R ct) between the electrode and electrolyte [40]. Co−Ci/NiFeOOH/BiVO4 obtains 2.4845 ​Ω [10] and 108.9 ​Ω (R ct), which are much smaller than those of other electrodes (Table. S1). It is proven that Co−Ci can effectively improve the transfer rate of photo-induced carrier compared with Co−Pi and Co−Sil, since the radius of Co−Ci is smaller, and thus can match the NiFeOOH/BiVO4 interface more compatibly and provide the shorter transmission channel. Mott−Schottky (M−S) curve is an important mean to analyze the characteristics of semiconductor materials (Fig. 5c and d). Under the applied voltage of 0–0.8 ​V, the slopes of all capacitance−voltage curves are positive, indicating that the five photoanodes are n-type semiconductors in a certain range [41]. By Mot−Schottky formula: 1 C 2 = 2 e 0 ε e N d v − v f − k T e 0 , we can know that the donor carrier concentration (N d) of Co−Ci/NiFeOOH/BiVO4 is 7 times higher than that of BiVO4, and it is also superior to Co−Pi/NiFeOOH/BiVO4 and Co−Sil/NiFeOOH/BiVO4 (Table. S2) [16,42,43]. The increase of the carrier concentration leads to the increase of the conductivity of Co−Ci/NiFeOOH/BiVO4, which in turn increases the photocurrent density, and this is mutually corroborated with the claim that Co−Ci in EIS analysis increases the transfer rate of photo-induced carrier.The PEC performances of BiVO4, NiFeOOH/BiVO4, Co−Pi/NiFeOOH/BiVO4, Co−Sil/NiFeOOH/BiVO4 and Co−Ci/NiFeOOH/BiVO4 were explored. As shown in Fig. 6 a, Co−Ci/NiFeOOH/BiVO4 achieves a photocurrent density of 4.1 ​mA/cm2, which is 530% higher than that of BiVO4, and about 1.9 times higher than that of a single co-catalyst loaded NiFeOOH/BiVO4. At the same time, the initial potential of Co−Ci/NiFeOOH/BiVO4 is reduced to about 0.3 ​V (vs RHE) compared to the 0.41 ​V (vs RHE) of the other four electrodes. In addition, both Co−Pi/NiFeOOH/BiVO4 and Co−Sil/NiFeOOH/BiVO4 showed better water oxidation performance than NiFeOOH/BiVO4, proving that the loaded double-layer cocatalyst was a feasible means to improve the performance of PEC [44]. This is due to the fact that the synergistic effect formed by the double-layer co-catalyst broadens the light absorption range (Fig. 4a), allowing more photo-generated carriers to be generated. Under the condition of dark reaction, the photoanode loaded with co-catalyst also showed lower initial potential, demonstrating that the presence of co-catalyst improved the water oxidation kinetics of the electrode (Fig. S7b). Fig. 6b shows the transient photocurrent curve under chopped light (light-dark interval of 5 ​s). All electrodes show excellent photo-response. The Co−Ci/NiFeOOH/BiVO4 electrode shows the highest photocurrent density in the different voltages, and the photocurrent density is close to 4.1 ​mA/cm2 at 1.23 V (vs RHE), which is consistent with the linear sweep voltammetry (Fig. 6a). 1 mol/L Na2SO3 was used as the hole scavenger in 0.5 ​mol/L Na2SO4 electrolyte solution to explore the charge injection efficiency and charge separation efficiency of the electrode. The charge injection efficiency of BiVO4 is only 15%, which is due to the waste of most of its holes caused by charge recombination. After loading double-layer co-catalysts, the injection efficiencies of Co−Pi/NiFeOOH/BiVO4, Co−Sil/NiFeOOH/BiVO4 and Co−Ci/NiFeOOH/BiVO4 are improved to certain extents. Among them, Co−Ci/NiFeOOH/BiVO4 reached up to 68%, which was much higher than that of other electrodes. In this case, more holes could participate in the reaction (Fig. 6c). It is also confirmed that the good matching between NiFeOOH/BiVO4 and Co−Ci co-catalysts made the electrode present the most favorable reaction interface. The change of charge separation efficiency can be clearly observed in Fig. 6d. Similar to the injection efficiency, the separation efficiency of photo-induced carrier of Co−Ci/NiFeOOH/BiVO4 was also improved, indicating that its charge separation ability was significantly improved. Combined with Co−Ci in EIS, the charge migration rate was greatly improved, and it is concluded that Co−Ci/NiFeOOH/BiVO4 can make the surface reaction kinetics faster.In order to better understand the characteristics of electrode materials, a series of efficiency calculations were carried out. The applied bias photon-to-current efficiency (ABPE) reflects the conversion ability of solar energy to chemical energy of the photoanode (Fig. 7 a). The maximum efficiency of Co−Ci/NiFeOOH/BiVO4 reaches 0.96% at 0.82 ​V (vs RHE), which is much greater than 0.19% of BiVO4 under the same voltage. Thanks to this efficient photoelectric conversion, Co−Ci/NiFeOOH/BiVO4 stands out in the same type of electrodes. The incident photon-to-current efficiency (IPCE) is an important parameter for evaluating electrode materials. In Fig. 7b, it is found that all electrodes are active in the wavelength ranging from 350 ​nm to 520 ​nm, which is highly consistent with UV–Vis diffuse reflectance (Fig. 4a). The IPCE value of all the electrodes almost reaches the peak at 415 ​nm. Co−Ci/NiFeOOH/BiVO4 has 54% conversion efficiency, which means that more than half of the photogenerated electrons are captured. In addition, the light harvesting efficiency LHE and IPCE were used to study the absorbed photon-to-current efficiency (APCE). Similar to IPCE, Co−Ci/NiFeOOH/BiVO4 has the highest conversion efficiency (65%) at 415 ​nm (Fig. 7c). It can be seen from IPCE and APCE that the photon capture and utilization of the electrode modified by the dual-layer co-catalyst are greatly stronger than those of pure bismuth vanadate and BiVO4 modified by a single co-catalyst, so Co−Ci co-catalyst is undoubtedly a very promising one. Meanwhile, electron paramagnetic resonance (EPR) indicates that there are a large number of oxygen vacancies in the composite electrode (Fig. 7d) [17,45]. This is precisely because the rich oxygen vacancies in Co−Ci make the PEC performance of Co−Ci/NiFeOOH/BiVO4 improved, which is mutually confirmed by the O 1s peak in XPS (Fig. 3c).Gas chromatograph was used to understand the water splitting performance of the electrode, and quantitative analysis of oxygen and hydrogen was executed by GC integration every 0.5 ​h for 3 ​h. Generally, H2 and O2 are produced at the same time almost following the ratio of 2:1. In our comparison tests, BiVO4 produces 53 ​μmol of H2 in 3 ​h, while Co−Ci/NiFeOOH/BiVO4 produces 170 ​μmol of H2. The Faraday efficiency is obtained by dividing the actual amount of H2 produced by the theoretical amount of H2 [46]. The Faraday efficiency of Co−Ci/NiFeOOH/BiVO4 reaches 96%, indicating that almost all the photo-generated charges are used to split water (Fig. 8 a). At the same time, in order to analyze the influence of Co−Ci, Co−Sil and Co−Pi on the stability of NiFeOOH/BiVO4 photoelectrocatalytic process, and reasonably evaluate whether the composite electrode has application value, the stability of the electrode was tested under AM 1.5 ​G illumination (100 mW/cm2) and 1.23 ​V vs RHE voltage for 3 ​h (Fig. S8a). The photocurrent density of all electrodes showed a downward trend at the beginning due to the electron−hole recombination, but Co−Ci/NiFeOOH/BiVO4 still maintained the highest photocurrent density within 3 ​h. In addition, Figure S8b shows the SEM image of Co−Ci/NiFeOOH/BiVO4 after a 3 h stability test. It is observed that although the worm-like basic shape of the original porous electrode surface is not destroyed, the gap between BiVO4 becomes relatively close, and the electrode surface becomes rough. Moreover, Fig.S5b provides the XRD pattern of Co−Ci/NiFeOOH/BiVO4 after 3 h stability test, and it is found that the crystal structure of the electrode has not changed, proving that the electrode has a certain degree of durability, which is very important for the practical application of photoelectrochemical water splitting.According to the analyses above, a possible mechanism for the superior performance of Co−Ci/NiFeOOH/BiVO4 to the same type of photoanodes is proposed (Fig. 8b). On the one hand, NiFeOOH is introduced to form the carrier channel to inhibit the rapid recombination of charges. On the other hand, the O2− strength in Co−Ci/NiFeOOH/BiVO4 is enhanced, which may be due to the fact that Co−Ci has stronger metal binding ability and smaller molecular radius, so it can be well matched with NiFeOOH/BiVO4. The oxygen vacancies generated by Co−Ci as a co-catalyst (hole transport layer) can better promote water oxidation, which is conducive to the transmission of photogenerated electrons to the FTO substrate and the generation of hydrogen at the cathode through the external circuit. In Table S3, our work is compared with the recent photocurrent density of BiVO4 photoanode under AM 1.5 ​G (100 ​mW ​cm−2) illumination.In this work, we refer to the Co−Ci co-catalyst used in CO2 reduction and perovskite solar cells, and apply it in the field of photoelectrochemical water splitting through the method of PED. The PEC performance of the related electrodes was compared. All tests showed that this amorphous Co−Ci achieved more outstanding performance than Co−Pi (or Co−Sil) on NiFeOOH/BiVO4. Under AM 1.5 ​G irradiation and pH ​7.35 Na2SO4 solution, the Co−Ci/NiFeOOH/BiVO4 photoanode exhibits a photocurrent density of 4.1 mA cm−2, which is more than 500% that of BiVO4 photoanode and is more than twice that of the traditional Co-Pi/NiFeOOH/BiVO4 electrode. The enhanced PEC activity is attributed to the fact that the Co−Ci co-catalyst not only can bring abundant oxygen vacancies, but also can better match NiFeOOH/BiVO4. Co−Pi/NiFeOOH/BiVO4 broadens the light absorption range, increases the index level carrier concentration and reduces the charge transfer resistance (R ct), resulting in higher photocurrent density, more stable durability and up to 0.97% of the photoelectric conversion efficiency (ABPE). Furthermore, the low cost and simple synthesis methods of Co−Ci provide strong support for its practical application and are expected to be applied to other candidate electrodes.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by the National Natural Science Foundation of China (Nos. 52173277 and 21808189), the Key Science and Technology Foundation of Gansu Province, China (No. 20YF3GA021), and the Natural Science Foundation of Gansu Province, China (No. 20JR5RA523).The following is the Supplementary data to this article: Supplementary data to this article can be found online at https://doi.org/10.1016/j.apmate.2021.11.010.

The cobalt−carbonate (CoCi)/NiFeOOH double-layer co-catalyst was prepared on bismuth vanadate (BiVO4). Compared with the same type of electrode (Co−Pi/NiFeOOH/BiVO4 and Co−Sil/NiFeOOH/BiVO4), the photoelectrochemical (PEC) performance of the composite electrode presents the most excellent performance. The Co−Ci/NiFeOOH/BiVO4 electrode prepared by photoelectric deposition (PED) achieves a photocurrent density of 4.1 ​mA/cm2 at 1.23 ​V vs RHE, and the applied bias photon-current efficiency (ABPE) is up to 0.95%. In addition, with the help of the equivalent circuit fitting in electrochemical impedance spectroscopy (EIS), the charge transfer resistance (R ct) of Co−Ci/NiFeOOH/BiVO4 is only 108.9 ​Ω, which is 16% that of BiVO4. The enhanced PEC performance of Co−Ci/NiFeOOH/BiVO4 in the double-layer cocatalyst system is attributed to the outstanding advantages of Co−Ci cocatalyst in oxygen vacancy defects, superior to other cobalt-based catalysts in promoting charge transfer and improving the kinetics of water oxidation. This makes Co−Ci co-catalyst become one of the favorable competitors in the field of photoelectric catalysis.

Data will be made available on request. Data will be made available on request.There is an increasing urgency to identify alternative energy sources to fossil fuels in order to meet the need to supply sustainable, clean energy as well as reduce greenhouse emissions to mitigate rising global temperatures, extreme and fluctuating weather patterns, and the negative impact on the earth's ecosystem [1–4]. To this end, hydrogen is an attractive energy carrier as a source of clean efficient power in stationary, portable and transport applications [5] as it has a high energy density (142 MJ·kg−1 vs 54 MJ·kg−1 for natural gas) as well as the potential to be generated in high purity from water splitting where the only by-product is oxygen [6–12]. However, hydrogen is a flammable gas which forms potentially explosive environments and, as such, there are significant safety concerns over its storage and transportation; moreover, compression and liquefaction of hydrogen are energy intensive processes. The use of hydrogen storage materials is one of the most promising solutions as they are stable and safe to handle and would allow for the generation of hydrogen on site [6,13-25]. To this end, sodium borohydride has appropriate credentials for use as a storage material as it has a high stability and a high hydrogen content (10.8 wt%) and is nontoxic, inexpensive and water soluble (Eq. 1) [6,13b,c,j,k, 25-32]. (1) NaB H 4 + 4 H 2 O → NaB ( OH ) 4 + 4 H 2 As the thermal decomposition of NaBH4 requires temperatures in excess of 400°C and its hydrolysis in water is slow, considerable effort has been dedicated to developing cost-effective catalysts that can achieve the rapid and controllable release of hydrogen that will be required for this technology to become commercially viable. While homogeneous catalysts have been shown to facilitate the solvolysis of hydrogen-rich boron compounds [33–38], noble metal nanoparticles (NPs) have recently attracted considerable attention as the hydrogen generation rate can be controlled through their size, morphology and environment and the catalyst can be recovered and reused in much the same manner as a conventional heterogeneous catalyst [39–43]. While the high activity obtained with small nanoparticles is due to their high surface area to volume ratio and the large number of active sites, they are unstable with respect to aggregation to less reactive species which limits their practical applications [44–45], for example, integration into hydrogen-based fuel cells for use in vehicles and portable electronic devices [46–48]. One potential solution to overcome aggregation under conditions of catalysis has been to stabilise the nanoparticles by encapsulation into a support such as porous carbon structures [49–60], zeolites [61–65], mesoporous silicas [66–68], porous organic polymers [69–70], metal organic frameworks [71–77] and, most recently, dendrimers [78–80]. Additional benefits of this strategy include control of the growth and morphology due to the confinement [81–87], modification of their properties through surface-support interactions [88–93] and incorporation of functionality to affect synergy, for instance, bimetallic nanoparticles [94–97]. At present, the most efficient supported NP catalyst for the hydrolysis of sodium borohydride is based on RuNPs confined in zeolite-Y; this system gave a turnover frequency of 550 molH2.molRu −1.min−1 [64].Ionic liquids have also been used for the stabilization of nanoparticles [98–101]; however, the weak electrostatic interactions involved do not always provide sufficient stabilisation to prevent aggregation under the conditions of catalysis [102–103]. One possible approach to improve nanoparticle stability has been to introduce a heteroatom donor such as a phosphine, amine, nitrile, ether, or thiol that can supplement this weak stabilization by forming a covalent interaction to the nanoparticle surface [104]. This approach has proven successful with significant improvements in catalyst stability and performance; for example, palladium nanoparticles stabilised by a phosphine-functionalised imidazolium-based ionic liquid are markedly more efficient hydrogenation catalysts than their unmodified counterparts [105–109] while RuNPs stabilised by a phosphine-functionalised ionic liquid exhibited a solvent dependent chemoselectivity for the hydrogenation of aromatic ketones as reactions performed in ionic liquid were highly selective for reduction of the carbonyl group whereas the use of water as the solvent resulted in hydrogenation of both the carbonyl and the arene. Moreover, the phosphine was shown to exert a marked influence on catalyst efficiency as the corresponding phosphine-free RuNP catalyst was markedly less selective in both solvents [110–111]. However, even though this strategy has been shown to improve catalyst performance, functional ionic liquids are prohibitively expensive as a bulk solvent, leaching contaminates the product and recovery, and purification of the ionic liquid can be difficult, which has limited their implementation.These issues have been addressed by grafting ionic liquids onto supports such as mesoporous silica, polymers, and MOFs on the basis that the resulting material would stabilise the nanoparticles in much the same manner as an ionic liquid, while the covalent attachment would prevent leaching of the ionic liquid, facilitate separation and recovery of the catalyst, and reduce the amount of ionic liquid, as the catalyst would be confined within the support [112–117]. Polymers are particularly attractive supports as their modular construction would enable the hydrophilicity, ionic microenvironment, charge density and redox properties to be modified in a rational manner, additional functionality to be introduced and the composition and stoichiometry of the metal precursors to be defined to facilitate access to synergistic bi- and trimetallic nanoparticles. We have recently been exploring this approach and developed heteroatom donor-decorated polymer-immobilised ionic liquids, reasoning that the heteroatom donor could influence the size, size distribution and morphology of the nanoparticles as well as modify their surface electronic structure and, thereby, modulate their efficacy as catalysts. In this regard, there have been an increasing number of reports of the beneficial effect of ligands on the performance of heterogeneous nano-catalysts, which have been attributed to steric, electronic and solubility factors [118]. Our early studies showed that palladium nanoparticles immobilized on a polyethylene glycol-modified phosphine-modified PIIL is a remarkably efficient catalyst for aqueous phase Suzuki-Miyaura cross-couplings [119], the chemoselective hydrogenation of α,β-unsaturated ketones, nitriles and esters, [120] and the hydrogenation of nitroarenes [121]. Moreover, gold nanoparticles stabilized by a phosphine oxide-modified polymer immobilised ionic liquid catalyses the highly selective reduction of nitroarenes to afford N-arylhydroxylamines and azoxyarenes [122] and the corresponding ruthenium nanoparticles catalyse the aqueous phase hydrogenation of aryl and heteroaryl ketones and levulinic acid with remarkable efficacy and selectivity [123].While support-grafted ionic liquids have been used to stabilise catalysts for a wide range of transformations, there appear to be only two reports of their use to support nanoparticle catalysts for the hydrolytic evolution of hydrogen from hydrogen-rich boron derivatives, which is somewhat surprising as polymer immobilised ionic liquids are functional and tuneable supports for molecular and nanoparticle catalysts. An imidazolium-based organic polymer has recently been used to prepare highly dispersed ultrafine AuPd alloy NPs for the hydrolytic release of hydrogen from ammonia borane which outperformed both its monometallic counterparts [124] and we have recently reported that phosphine decorated polymer immobilized ionic liquid stabilized PtNPs are highly efficient catalysts for the hydrolytic generation of hydrogen from NaBH4 [125]. This study has now been extended to investigate the efficacy of phosphine oxide and amine-decorated polymer immobilised ionic liquid stabilised RuNPs as catalysts for the hydrolysis of NaBH4 on the basis that the heteroatom donor could disrupt the key hydrogen-bonded surface-coordinated ensemble between the acidic hydrogen of water and the hydridic hydrogen of borohydride and thereby influence catalyst performance. Herein, we report the results of a comparative study to explore the influence of polymer composition on catalyst performance and reveal that that RuNPs stabilised by an amino-modified polyionic liquid outperform their phosphine oxide-decorated and unmodified counterparts. Kinetic studies in combination with deuterium isotope effects have been used to probe the mechanism and a tandem hydrogenation of 1,1-diphenylethene with hydrogen generated from the catalytic hydrolysis of NaBH4 in D2O gave a mixture of isotopologues resulting from reversible β-hydride elimination/re-insertion at a surface Ru-D competing with reductive elimination.All reagents were purchased from commercial suppliers and used without further purification, RuCl3.3H2O 99.9% (PGM basis) was purchased form Alfa Aesar (47182) and polymers 1a-f were prepared as previously described and their purity confirmed by 1H and 13C{1H} NMR spectroscopy and elemental analysis. Ethanol was distilled over iodine activated magnesium with a magnesium loading of 5.0 g L−1 and diethyl ether from Na/K alloy under an atmosphere of nitrogen.To a round bottom flask charged with 1a (4.0 g, 6.5 mmol) and ethanol (100 mL) was added a solution of RuCl3·3H2O (1.3 g, 6.5 mmol) in ethanol (20 mL). The resulting mixture was stirred vigorously for 5 h at room temperature after which time a solution of NaBH4 (2.0 g, 52.0 mmol) in water (10 mL) was added dropwise and the suspension stirred for an additional 18 h before concentrating to dryness under vacuo. The crude black solid was triturated with cold acetone (2 × 100 mL) then washed with water (100 mL) followed by ethanol (2 × 40 mL) to afford a black solid that was recovered from the washings via centrifugation followed by filtration through a frit. The final product was rinsed with ether until a fine black powder was obtained which was dried under vacuum to afford 2a in 87% yield (4.06 g). ICP-OES data: 5.85 wt% ruthenium and a ruthenium loading of 0.58 mmol∙g−1.Catalyst 2b was prepared from 1b (1.0 g, 0.83 mmol), RuCl3·3H2O (0.17 g, 0.83 mmol) and NaBH4 (0.25 g, 6.64 mmol) in ethanol (25 mL) as described above to afford a fine black powder in 50% yield (0.54 g). ICP-OES data: 7.02 wt% ruthenium and a ruthenium loading of 0.70 mmol∙g−1.Catalyst 2c was prepared from 1c (5.0 g, 6.25 mmol), RuCl3·3H2O (1.30 g, 6.25 mmol) and NaBH4 (1.89 g, 50 mmol) in ethanol (100 mL) as described above to afford a fine black powder in 53% yield (2.82 g). ICP-OES data: 7.24 wt% ruthenium and a ruthenium loading of 0.72 mmol∙g−1.Catalyst 2d was prepared from 1d (4.0 g, 2.68 mmol), RuCl3·3H2O (0.46 g, 2.68 mmol) and NaBH4 (0.81 g, 21.4 mmol) in ethanol (100 mL) as described above to afford a fine black powder in 78% yield (3.32 g). ICP-OES data: 1.83 wt% ruthenium and a ruthenium loading of 0.18 mmol∙g−1.Catalyst 2e was prepared from 1e (5.0 g, 7.75 mmol), RuCl3·3H2O (1.60 g, 7.75 mmol) and NaBH4 (2.34 g, 62 mmol) in ethanol (100 mL) as described above to afford a fine black powder in 67% yield (3.88 g). ICP-OES data: 3.43 wt% ruthenium and a ruthenium loading of 0.34 mmol∙g−1.Catalyst 2f was prepared from 1c (4.0 g, 5.11 mmol), RuCl3·3H2O (1.06 g, 5.11 mmol) and NaBH4 (1.54 g, 40.9 mmol) in ethanol (100 mL) as described above to afford a fine black powder in 79% yield (3.45 g). ICP-OES data: ICP-OES data: 6.97 wt% ruthenium and a ruthenium loading of 0.69 mmol g−1.Comparative catalytic hydrolysis reactions were conducted in water at the appropriate temperature in a thermostated 50 mL round bottom flask. In a typical experiment, a flask charged with a stir bar, catalyst 2a-f (0.2 mol%) and NaBH4 (0.021 g, 0.57 mmol) and fitted with a gas outlet and connected to the top of an inverted water-filled burette designed to monitor the progress of the reaction by measuring the volume of water displaced with time. The flask was stabilised at 303 K and the reaction was initiated by adding water (2 mL) and the system was immediately sealed by replacing the gas outlet; the tap to the water filled burette was then opened, the time zero volume recorded, and the water displacement monitored. The optimum activity for each catalyst was determined by varying the catalyst loadings between 0.08 and 0.32 mol% at 303 K and measuring the hydrogen produced as a function of time. Kinetic studies were also conducted according to the protocol described above using the following catalyst loadings: 0.26 mol% 2a, 0.32 mol% 2b, 0.45 mol% 2c, 0.11 mol% 2d, 0.16 mol% 2e and 0.32 mol% 2f for a range of temperatures (294 K, 298 K, 303 K, 308 K and 313 K) and the corresponding activation energies (Ea) were determined from an Arrhenius plot of the initial rate against 1/T.The reaction order in catalyst was determined by performing the hydrolysis reactions at 298 K with NaBH4 (0.28 M, 0.021 g) in water (2 mL) and varying the catalyst concentration from 0.14 mol% to 0.69 mol% for 2a, 0.16 mol% to 0.63 mol% for 2b, 0.23 mol% to 1.1 mol% for 2c, 0.058 mol% to 0.28 mol% for 2d, 0.12 mol% to 0.27 mol% for 2e and 0.25 mol% to 0.64 mol% for 2f. The reaction order in sodium borohydride concentration was investigated by performing reactions at 298 K in water (200 mL) using 0.026 mmol of catalysts 2a (0.0448 g), 2e (0.0764 g) and 2f (0.0376 g) and varying the amount of sodium borohydride between 6.6 μmole and 185 μmole (i.e. [NaBH4]0 = 0.035, 0.07, 0.13, 0.26, 0.39, 0.53, 0.65, 0.78, 0.9 mM), such that the catalyst:NaBH4 mole ratios ranged from 4:1 and 1:6. The effect of sodium borohydride concentration on the initial rate of hydrolysis at high concentrations of sodium borohydride, i.e. under the conditions of catalysis, was also determined using 2e (0.0026 g, 0.884 μmol) to catalyze the hydrolysis of NaBH4 solutions (2 mL) with varying concentrations of sodium borohydride ranging from 0.55 mmol to 2.2 mmol ([NaBH4]0 = 0.28, 0.56, 0.83, 1.1 M).The effect of the concentration of NaOH on catalyst efficacy was explored by conducting catalytic hydrolysis reactions at 303 K in 2 mL of alkaline 0.28 M NaBH4 (0.021 g) across a range of sodium hydroxide concentrations (i.e. [NaOH] = 0.035, 0.07, 0.14, 0.28, 5.0, 10, 50, 100 mM) catalyzed by 0.26 mol% 2a (0.0025 g) and monitoring the gas evolution.Recycle studies were performed at 303 K as described above using 2 mol% 2a (0.0193 g, 0.0114 mmol) and 2e (0.0335 g, 0.0114 mmol) to catalyze the hydrolysis of sodium borohydride (0.021 g, 0.57 mmol) in water (20 mL). The progress of the reaction was monitored as described above and when the hydrolysis was complete an additional portion of fresh sodium borohydride (0.021 g, 0.57 mmol) was added, and the procedure repeated. After the 5th run samples of the catalysts were isolated and analysed by TEM.A borate-buffered solution was prepared by dissolving Na2B4O7·10H2O (9.53 g, 25 mmol) and NaCl (4.39 g, 75 mmol) in distilled water (900 mL) in a volumetric flask. When the borate was completely dissolved the pH of the solution was adjusted to 7.2 by gradual addition of boric acid (20.99 g, 0.34 mol); the solution was then made up to one liter. Recycle studies were conducted by adding NaBH4 (0.021 g, 0.57 mmol) to a flask containing 1 mol% 2e (0.0165 g, 0.0056 mmol) and 20 mL of the aqueous borate buffer solution. The flask was maintained at 303 K and the progress of the reaction was monitored as described above. When the hydrolysis was complete an additional portion of fresh sodium borohydride (0.021 g, 0.57 mmol) was added, and the procedure repeated for comparison with the recycle study described above in the absence of buffer.Hot filtration studies were conducted at 303 K following the protocol described above using either 0.2 mol% 2a (0.0019 g) or 0.16 mol% 2e (0.0026 g) to catalyze the hydrolysis of sodium borohydride (0.021 g, 0.57 mmol) in water (2 mL). The progress of the reaction was monitored as a function of time and the mixture filtered through a 0.45 μm syringe filter when the conversion reached ca. 50% (10 min for 2a and 7.75 min for 2e), after which the burette assembly was reconnected, and the gas evolution monitored for a further 30 min. In a complementary procedure, a hydrolysis reaction that had reached completion was filtered through a 0.45 μm syringe filter and an additional portion of NaBH4 (0.021 g, 0.57 mmol) added to the filtrate and the gas evolution monitored.A flask was charged with 2 mol% catalyst (2a 0.0186 g; 2e, 0.0335 g), water (20 mL) and sodium metaborate (0.0765 g, 0.57 mmol) and the resulting mixture stirred at 303 K for the predetermined time (t = 0 min, 20 min, 40 min, 60 min) to investigate whether the pre-stirring time influences catalyst efficacy. After pre-stirring for the allocated time, the reaction was initiated by addition of the NaBH4 (0.021 g, 0.57 mmol) and the rate of hydrogen evolution quantified by measuring the volume of water displaced with time.Tandem hydrogenations were performed using two Schlenk flasks connected through tubing. One of the flasks was charged with a stir bar, either NaBH4 (0.042 g, 1.11 mmol) or NaBD4 (0.046 g, 1.11 mol) and 0.26 mol% 2e (0.0025 g) and the hydrolysis started by addition of either D2O (2 mL) or H2O (2 mL). The reaction flask was immediately stoppered, isolated from the second flask by closing the stopcock and stirred for 70 min. The second Schlenk flask was charged with 1,1-diphenylethene (0.180 g, 1.00 mmol), 0.5 mol% Pd/C and either CH3OH (2 mL) or d4-methanol (2 mL). After 70 min the second flask was evacuated briefly before opening the connector to the hydrolysis flask. The reaction was allowed to stir at 303 K for 18 h before the solvent was removed and the residue analyzed by 13C{1H} NMR spectroscopy and GC-MS to establish the composition and quantify the distribution of isotopologues.The polymers required for this study were prepared via radical polymerisation of the corresponding imidazolium-based ionic liquid monomer, either styrene, (4-vinylphenyl)methanamine or diphenyl(4-vinylphenyl)phosphine oxide and the corresponding imidazolium-based ionic liquid cross-linker in the ratio x = 1.84, y = 1.0 and z = 0.16, as previously described [119–123]. Catalysts 2a-f were prepared by the wet impregnation of the polymer support with ruthenium trichloride to afford precursors with a 1:1 ratio of ruthenium to neutral monomer, followed by in-situ reduction of the ruthenium with NaBH4; to afford the product as a fine black powder in high yield; the synthesis and composition of the polymers and the catalysts is shown in Fig. 1 . The composition and purity of polymers 1a-f was determined using a combination of solution and solid state 13C{H} and 31P{H} NMR spectroscopy and elemental analysis while the loaded RuNP catalysts were characterised by solid state 13C{H} and 31P{H} NMR spectroscopy, infra-red (IR) spectroscopy, high resolution transmission electron microscopy (HRTEM), SEM, X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma-optical emission spectroscopy (ICP-OES) (See Fig. 2 and the supporting information for full details). The ruthenium loadings in 2a-f were determined to be 0.18−0.75 mmolg−1 using ICP-OES.The solid state 13C{1H} NMR spectra of 1a-f and 2a-f each contain resonances from δ 121 to 149 ppm, which correspond to the aromatic carbon atoms of polystyrene and the carbon atoms of the imidazolium ring, as well as signals between δ 10 and 51 ppm which belong to the methylene carbon atoms of the polystyrene backbone and the methyl group attached to the imidazolium ring. Additional signals at δ 71 and 59 ppm for 2b, 2d and 2f belong to the carbon atoms of the polyethylene glycol (PEG) chain and the terminal OMe, respectively, and a signal at δ 49 ppm for 2e and 2f is associated with the CH2NH2. The surface of the RuNP catalysts was characterised by X-ray photoelectron spectroscopy by analysing the Ru 3p region as the C 1s and Ru 3d region overlapped. For catalyst 2a, stabilised by unmodified imidazolium-based polymer, a Ru 3p3/2 peak at 463.19 eV was assigned to RuO2, and satellite features were fitted at 465.97 eV (Table S2 and Fig. 2a). The presence of RuO2 species is most likely due to surface oxidation of the pre-formed metallic Ru nanoparticles. The corresponding Ru 3p3/2 peak for catalysts containing the phosphine oxide (2c and 2d) or amine (2e and 2f) was shifted to lower binding energy (462.56 and 461.37 eV for 2c and 2d, respectively and 462.83 and 462.89 eV for 2e and 2f, respectively) compared to the Ru 3p3/2 binding energy of 463.19 eV for catalyst 2a (Table S2 and Fig. 2b-f). A shift to lower binding energy may be indicative of electron transfer from the heteroatom of the phosphine oxide or amine to the RuNPs. Catalyst 2d containing O=PPh2 and PEG heteroatom donors gave the largest shift (-1.72 eV) in binding energy of the Ru 3p3/2 peak (461.37 eV for 2d) relative to 2a. TEM micrographs of 2a-f revealed that the ruthenium nanoparticles were ultrafine and near monodisperse with average diameters between 1.6 and 2.8 nm; representative micrographs and the corresponding distribution histograms based on the sizing of >100 particles for 2a-f are shown in Fig. 2. SEM images revealed that the catalyst materials were far more granular than their polymeric counterparts, which appeared largely smooth.The hydrolysis of sodium borohydride was identified to investigate the efficacy of catalyst 2a-f on the basis that PEG-modified ‘click’-dendrimer stabilised noble and bimetallic metal nanoparticles catalyse this reaction with promising initial TOFs and as such would provide a formative benchmark for comparative evaluation. Preliminary catalyst testing was conducted using recent literature protocols as a lead [75,78]; reactions were initially performed at 303 K using 0.2 mol% of 2a-f to catalyse the hydrolysis of a 0.28 M solution of sodium borohydride (Fig. 3 a, b). The reaction was monitored by quantifying the amount of hydrogen liberated as a function of time using water displacement from an inverted burette assembly and all data were corrected by subtracting the background hydrogen generated over the same time under identical conditions. Hydrogen evolution started immediately with no induction period which is consistent with the metallic state of the ruthenium. Under these conditions, RuNP@NH2-PIILS (2e) gave the highest initial TOF of 135 moleH2.molRu −1.min−1 and reached 92% conversion after 20 min, whereas its PEGylated counterpart RuNP@NH2-PEGPIILS (2f) was less active with a slightly lower TOF of 117 moleH2.molRu −1.min−1. Removal of the amino-group from either of these systems resulted in a reduction in the activity with RuNP@PIILS (2a) and RuNP@PEGPIILS (2b) giving initial TOFs of 121 moleH2.molRu −1.min−1 and 89 moleH2.molcat−1.min−1, respectively. In contrast, under the same conditions, catalysts 2c and 2d, stabilised by phosphine oxide-decorated polymer, were both less active than their respective amino-modified analogues with initial TOFs of 70 moleH2.molRu −1.min−1 and 103 moleH2.molRu −1.min−1, respectively (Fig. 3b). For comparison, 0.16 mol% Ru/C (5 wt%) catalysed this hydrolysis under the same conditions but only reached 57% conversion after 25 min with a TOF of 69 moleH2.molRu −1.min−1. The initial TOF for 2e improved to 177 moleH2.molRu −1.min−1 when the reaction was performed in dilute solution (10 mL) with a reduced catalyst loading of 0.08 mol%. A series of baseline hydrolysis reactions conducted by substituting catalysts 2a-f with their corresponding polymers 1a-f confirmed that the RuNPs were essential for catalysis as the gas evolution did not exceed the background reaction under the same conditions.As there is no clear correlation between the efficacy of catalysts 2a-f and the nanoparticle size, further studies will be conducted to explore the surface electron density of the RuNPs as a function of the support and to investigate whether the amine influences the hydrogen bonded surface ensemble responsible for substrate activation or improves the dispersibility of the catalyst in the reaction mixture and thereby access to the active site. To this end, amine-modified supports have previously been reported to improve the performance of nanoparticle catalysts compared with the corresponding unmodified catalyst. For example, ruthenium nanoparticles stabilised within the pores of amine-modified MIL-53 (MIL-53(Al)-NH2) is a significantly more active catalyst for the dehydrogenation of amine-borane than its unmodified counterpart, MIL(Al)-53; this was attributed to the formation and stabilization of ultra-small RuNPs [76]. There are also numerous additional reports of the beneficial effect on catalyst performance of incorporating an amine onto the surface of a support. For instance, a marked improvement in the activity and selectivity of platinum nanowires for the partial hydrogenation of nitroarenes to N-phenylhydroxylamine [126–127], an enhancement in the activity of RuNPs for the hydrogenation of levulinic acid to γ-valerolactone [128], an improvement in activity for the transfer hydrogenation of nitroarenes catalysed by RuNP confined in an amine-modified porous organic polymer [129], an increase in activity for the PtNP-catalysed hydrogenation of quinoline [130], improvements in activity and selectivity for the Pt/Co and PdNP catalysed semi-hydrogenation of alkynes [131–133], and highly selective reduction of the carbonyl in cinnamaldehyde with MOF-confined Pt nanoclusters [134].Although a comparison of the efficacy of 2a-f with literature reports of other supported ruthenium nanoparticles should be treated with caution because of the vastly disparate experimental conditions and protocols employed to collect data, the initial TOF of 177 moleH2.molRu −1.min−1 is higher than that of 80 moleH2.molcat−1.min−1 obtained with PEGylated click dendrimer-stabilised RuNPs [78], and 105 moleH2.molRu −1.min−1 with ruthenium electrodeposited on nickel foam [135] and a marked improvement on 67 moleH2.molRu −1.min−1 obtained in 5% wt NaOH with RuNPs nanoclusters stabilised by confinement in the framework of Zeolite-Y [64], 25 moleH2.molRu −1.min−1 for RuNP@ZIF-67 [77] and 35 moleH2.molRu −1.min−1 for carbon-supported bimetallic RuCo nanoparticles [136]; but lower than that of 550 moleH2.molRu −1.min−1 obtained with RuNPs stabilised in Zeolite-Y [64] and 505 moleH2.molRu −1.min−1 with nanoporous ruthenium prepared by chemical dealloying RuAl [137]; to the best of our knowledge these latter systems are the most active ruthenium-based catalysts for this hydrolysis.As the highest TOF was obtained with 2e, a thorough study of the reaction kinetics together with deuterium isotope effects, recycle experiments and a tandem reaction using the liberated hydrogen for the tandem hydrogenation of 1,1-diphenylethene with deuterium labelling was undertaken, details of which are discussed herein; for comparison, full details of the corresponding experiments with catalysts 2a-d and 2f are provided in the supporting information and discussed in context where appropriate. There have been numerous reports of an enhancement in activity for the metal nanoparticle catalysed hydrolytic evolution of hydrogen from sodium borohydride and amine borane in the presence of added base. For example, Astruc has reported a marked increase in the initial TOF for the hydrolysis of NaBH4 catalysed by click dendrimer-supported RuNPs from 80 moleH2.molRu −1.min−1 to 186 moleH2.molRu −1.min−1 in the presence of 0.2 M NaOH; an increase in TOF was also observed for a host of other catalysts including Rh, Au, Pd, Co, Ni, Fe and Co nanoparticles with the exception of PtNPs which experienced a strong negative effect [78]. Significant enhancements in TOF were also obtained for the hydrolysis of hydrogen-rich boron compounds with MNP@ZIF-8 (M = Ni, Co), NiPtNP@ZIF-8 and CoPtNP@dendrimer nanocatalysts in the presence of NaOH [72,73,75,80]. This enhancement has been attributed to coordination of the hydroxide to the nanoparticle surface which increases the electron density and facilitates activation of the O-H bond; in contrast, Pt is an electron-rich metal and highly reactive towards oxidative addition and as such the hydroxide ions occupy surface active sites and prevent substrate coordination. Such a large enhancement in activity for a dendrimer-stabilised RuNP-based catalyst prompted us to study the efficiency of 2a for the catalytic hydrolysis of NaBH4 as a function of the concentration of sodium hydroxide; reactions were conducted using 0.26 mol% of 2a to catalyse the hydrolysis of alkaline solutions of 0.28 M NaBH4 with sodium hydroxide concentrations ranging between 0.035 mM to 100 mM (Fig. 4 ). There was no apparent variation in the initial TOF at low concentrations of NaOH (< 0.035 mM) while the TOFs decreased gradually at concentrations above 0.07 mM; this decrease became more dramatic when the sodium hydroxide concentration reached 5 mM and the initial TOF eventually dropped from 136 moleH2.molRu −1.min−1 in the absence of sodium hydroxide to 39 moleH2.molRu −1.min−1 in a 100 mM NaOH solution of NaBH4. To this end, there have been several reports of a decrease in the hydrogen generation activity with increasing NaOH concentration (1-10 wt% NaOH) for the ruthenium-catalysed hydrolysis of NaBH4 [138–142], which were attributed to strong interactions between the hydroxide ions and water decreasing the available free water needed for the hydrolysis of NaBH4 [138]. However, it is interesting to note that high concentrations of NaOH have been shown to enhance the hydrogen generation activity for the non-noble metal catalysed hydrolysis of NaBH4, i.e. these systems tolerate high concentrations of hydroxide and coordination of the OH− to the surface does not appear to prevent substate binding [143–148]. As the decrease in hydrogen generation rate for 2a at a NaOH concentration as low as 0.001 wt% (0.28 mM) is unlikely to be due to a reduction in the activity of water, as described by Amendola, the high rate obtained in the absence of NaOH may reflect the intrinsic activity of ruthenium to facilitate oxidative addition as a late transition metal while the reduction in activity in the presence of even a minor amount of sodium hydroxide (NaOH:catalyst between 0.05:1 and 0.4:1) may be attributed to the hydroxyphilic nature of ruthenium with the hydroxide ions occupying surface active sites and preventing substrate coordination and activation, as described above; even at these concentrations there would be sufficient OH− ions to populate the surface of the nanoparticle and disrupt the strongly hydrogen bonded NaBH4—H2O ensemble involved in the rate limiting O-H bond activation step (vide infra).Kinetic studies were subsequently undertaken to determine the temperature dependence of the rate and obtain activation parameters for the hydrolytic release of hydrogen from NaBH4 for a comparison with related systems reported in the literature. A set of reactions were conducted to monitor the hydrolysis of a 0.28 M solution of NaBH4 as a function of time to determine the initial rates across a range of temperatures from 294 K to 313 K. The apparent activation energies (Ea) for the hydrolysis catalysed by 2a-f, determined from an Arrhenius plot of lnk against 1/T (lnk = lnA - Ea/RT) using the initial rates calculated from the linear slope of the graph, ranged from 38.9 kJ mol−1 to 51.8 kJ mol−1 (Fig. 5 a-b and Fig. S1 in the supporting information). These values lie within the range reported for the hydrolysis of NaBH4 with other RuNP catalysts including 35 kJ mol−1 for RuNPs stabilised in the framework of Zeolite-Y [64], 41 kJ mol−1 for water-dispersible, acetate-stabilized RuNPs [149], 36 kJ mol−1 for RuNPs confined in ZIF-67 [77], 47 kJ mol−1 for RuNPs immobilised by the anion exchange resin IRA-400 [150] and 41.8 kJ mol−1 for ruthenium immobilised on Al2O3 pellets [151], but slightly lower than 61.1 kJ mol−1 for RuNPs supported on amine-modified graphite [139], 56.0 kJ mol−1 for RuNP@IRA-400 [138], 58.2 kJ mol−1 for Ru(acac)3 [152] and 66.9 kJ mol−1 for ruthenium supported on carbon [153]. There does not appear to be a correlation between the activation energies and the initial rates which may be attributed to variations in the number of active sites or their availability as this determines the pre-exponential factor (A) [76, 154].The hydrogen release was next investigated as a function of the concentration of 2e across a range of catalyst loadings from 0.12 mol% to 0.28 mol% in 0.28 M NaBH4 (Fig. 6 a) and the logarithmic plot of the initial hydrogen generation rate versus catalyst concentration gave a straight line with a slope of 1.04 (Fig. 6b), indicating that the hydrolysis of NaBH4 is first order with respect to the catalyst. Similarly, the corresponding slopes for the logarithmic plots obtained with catalyst 2a-d and 2f varied between 0.70 and 1.04, which are all consistent with first order kinetics; full details are presented in Fig. S2 in the supporting information. This data is also consistent with recent reports of noble metal nanoparticle-catalysed hydrogen generation from hydrogen-rich boron derivatives including a slope of 0.73 for RuNPs confined in Zeolite-Y [64], 0.94 for RuNPs stabilized by polyvinylpyrrolidinone [155], 1.06 for Ru(acac)3 [152], 1.17 for porphyrin-stabilised RuNPs [156], 0.85 for PtCoNP@dendrimer [78], and 0.82 for Ni2Pt@ZIF-8 [73]. The variation in the rate of hydrolysis of NaBH4 as a function of the substrate concentration was also investigated using catalyst 2e. As the order of reaction with respect to NaBH4 has been reported to depend on the amount of NaBH4 in solution (i.e. the NaBH4:catalyst ratio), changing from 1 to 0 as the concentration of NaBH4 increases [145], kinetic data was obtained by conducting a series of reactions with 0.026 mmol of catalyst 2e and varying the initial concentration of NaBH4 from 0.066 mM to 0.52 mM as these amounts correspond to catalyst:hydride ratios between 2:1 and 1:4 (Fig. 7 ). Such low catalyst/hydride mole ratios were used to avoid the BH4−induced dynamic saturation of the active sites on the catalyst surface which would give zero order kinetics; under these conditions the surface is not completely covered by NaBH4 and there are active sites. The slope of 1.02 obtained from the logarithm plot of hydrogen generation rate versus concentration of NaBH4 confirms that the hydrolysis is first order in substate, which undergoes rate limiting diffusion on the catalyst surface. Under the same conditions, slopes of 1.02 and 1.01 were also obtained with catalysts 2a and 2f, respectively, which are both consistent with first order kinetics; see Fig. S3 in the electronic supporting information. First order kinetics with respect to NaBH4 have previously been reported for ruthenium on carbon [142], palladium on carbon [157] and Pd and Pt dispersed on functionalised surfaces of carbon nanotubes [158] when reactions were conducted at low concentrations of NaBH4. A similar study conducted with catalyst 2e at much higher catalyst/hydride mole ratios between 1:625 and 1:2500 gave a slope of 0.26 which is indicative of zero order kinetics due to saturation of the active sites on the catalyst surface during the reaction (Fig. S4 in the supporting information), as described by Patel [145]. A slope of 0.17 was also obtained using catalysts 2d which is also consistent with zero order kinetics; similar kinetics have previously been described for ruthenium nanoclusters [159], Ru supported on IRA 400 [150] and ruthenium on carbon [153].The kinetic isotope effect (KIE) is a valuable tool for elucidating information about the rate limiting step (RLS) of a reaction that has been routinely used to probe the catalytic hydrogen generation from borohydride and amine borane (AB) [160,72,79,80]. While the reaction kinetics are complicated and the mechanism still not fully understood [42] it is clear that both NaBH4 and ammonia-borane are hydride donors and provide one of the two hydrogen atoms of the derived hydrogen gas while water provides the other in the form of a proton [41,43] and that the rate determining step involves activation of one of the O-H bonds of water, as measured by the large primary KIE obtained when the hydrolysis is performed in D2O instead of H2O [78,79,80,83,161,162]. Activation of an O-H bond has been proposed to occur via oxidative addition involving a hydrogen-bonded ensemble between a surface-coordinated borohydride and a water proton; the hydrogen could then be liberated either via reductive elimination between a borohydride-derived NP-H and the water-derived NP-H (Fig. 8 , pathway a-c) or a concerted σ-bond metathesis-like process between a surface coordinated [BH4]− and a water-derived NP-H (Fig. 8, pathway d-e), which may be facilitated by hydroxide. Alternatively, the protonic and hydridic hydrogen atoms may be transferred to the nanoparticle surface by oxidative addition of both the O-H and B-H bonds, respectively, to afford a dihydride that would generate hydrogen and BH3-OH via reductive elimination (Fig. 8, pathway f-g), as proposed by Astruc for the CoNP@ZIF-8 catalysed hydrolysis of NaBH4 [75]. While the pathways described in Fig. 8 are all initiated by oxidative addition of the O-H bond of water via a hydrogen-bonded ensemble involving a surface-coordinated borohydride, Jagirdar [163] and Ma [164] have suggested that activation of the O-H bond and generation of H2 could occur via a hydrogen-bonding interaction between a surface adsorbed water and a surface hydride generated via rapid hydride transfer from NaBH4 to the NP surface.The role of H2O in the hydrolysis of NaBH4 catalysed by 2e was explored by conducting the reaction in D2O and monitoring the hydrogen evolution as a function of time to determine the KIE. Reactions were conducted under the conditions of catalysis i.e. 0.16 mol% of 2e was used to catalyse the hydrolysis of 2 mL of a 0.28 M solution of NaBH4 at 30°C. A comparison of the efficacy of 2e as a catalyst for the hydrolysis of NaBH4 in H2O and D2O revealed that the reaction was more rapid in H2O than in D2O with a primary kinetic isotope effect (k H/k D) of 2.31 (Fig. 9 a); similar values of k H/k D were obtained with catalysts 2a (k H/k D = 1.76) and 2d (k H/k D = 1.53) and the corresponding data is presented in Fig. S5a-b in the supporting information. This value is comparable to the solvent isotope effect of 2.25 obtained by Astruc for the gold-ruthenium nanoalloy catalysed visible light-accelerated hydrolytic dehydrogenation of NaBH4 and amine-borane [165] as well as 1.8 determined in a detailed kinetic analysis of the platinum-catalysed hydrolysis of NaBH4 in alkaline media [162], 2.3 for dendrimer-stabilised RhNPs [79], 2.4 for PtCo@dendrimer [80] and 2.49 for NiNP@ZIF-8 [72] and supports a mechanism with rate limiting cleavage of an O-H bond of water in a surface-coordinated hydrogen-bonded ensemble of the type described above and shown in Fig. 8. The same comparison of initial rates between reactions conducted in H2O and D2O under stoichiometric conditions using 26 μmol of 2e for the catalytic hydrolysis of 200 mL of a 0.13 mM solution of NaBH4 at 30°C (catalyst:NaBH4 ratio of 1:1) gave a primary kinetic isotope effect of 1.7 (Fig. S6d in the supporting information), which is also consistent with rate limiting oxidative addition of water. However, this KIE does not distinguish between a rate limiting step in which a surface coordinated NaBH4——HOH ensemble activates an O-H bond towards oxidative addition through a hydrogen-bonding interaction to afford a water-derived metal hydride and a surface-coordinated borohydride, such as that shown in Fig. 8 pathway a, and concerted activation of both the B-H and O-H bonds in a similar hydrogen-bonded ensemble; the latter process would most likely occur via oxidative addition of the O-H bond and rapid hydride transfer from the borohydride (Fig. 8 pathway a-c) rather than oxidative addition of both the O-H and B-H bonds (Fig. 8, pathway f-g) as borohydrides are extremely potent transfer reagents. For the same reason, a subsequent σ-bond metathesis involving the surface-coordinated borohydride and the water-derived RuNP hydride would also be unlikely (Fig. 8, pathway e).Thus, the mechanism was further probed by comparing the rates of hydrolysis of NaBD4 and NaBH4 catalysed by 2e at 30°C. Analysis of the initial rates obtained for the hydrolysis of 200 mL of a 0.13 mM solution of NaBH4 and NaBD4 catalysed by 26 μmol of 2e, i.e., a substrate/catalyst ratio of 1, gave a primary kinetic isotope effect (k H/k D) of 2.72 (Fig. 9b). Reassuringly, comparable values were also obtained with catalysts 2a (k H/k D = 2.25) and 2d (k H/k D = 2.37), full details of which are presented in Fig. S6a-b in the supporting information. These values are comparable to that of 2.2 obtained for the visible light-accelerated H2 evolution from NaBH4 catalysed by a gold-ruthenium nanoalloy; which, together with a KIE of 2.5 obtained for the hydrolysis of NaBH4 in D2O, was taken to indicate that both the O-H and B-H bonds were activated by the ruthenium atoms in the rate limiting step, most likely via concerted oxidative addition-hydride transfer, involving the surface-coordinated hydrogen-bonded [BH3H−]—–H-OH ensemble, rather than oxidative addition of both the O-H and B-H bonds [75,165]. Interestingly though, comparison of the rates obtained under the conditions of catalysis using 2e to catalyse the hydrolysis of 2 mL of 0.28 M solutions of NaBH4 and NaBD4 at 30°C gave a KIE of 0.65 (Fig. 9c); similar values were also obtained with catalysts 2a (k H/k D = 0.87) and 2d (k H/k D = 0.85), full details of which are provided in Fig. S5d-f in the supporting information. These are inverse kinetic isotope effects and would be consistent with a surface-coordinated borohydride activating an O-H bond of water in the hydrogen-bonded ensemble prior to hydride transfer.The hydrogen liberated from the catalytic hydrolysis of NaBH4 was used for the hydrogenation of 1,1-diphenylethene with various labelling experiments to determine the fate of the liberated hydrogen. In the first of these, the tandem reaction was conducted using 0.26 mol% 2a to generate hydrogen from a 0.28 M solution of NaBH4 in D2O at 30°C in a sealed tube; after 70 min the connector was opened to the second flask which contained 1,1-diphenylethene and 0.5 mol% Pd/C in d4-methanol and the resulting mixture was stirred for 18 h. Interestingly, analysis of the crude mixture by 1H, 2H and 13C NMR spectroscopy and mass spectrometry revealed that a mixture of all eight isotopologues of 1,1-diphenylethane had been generated (Scheme 1 ). Analysis of the methine region (δ 44.5 ppm) of the 13C{1H} NMR spectrum was used to identify and assign each of the isotopologues, which appear as a set of four singlets at δ 44.88, 44.81, 44.73, and 44.66 ppm corresponding to I, II, III, and IV, respectively, while V, VI, VII and VIII appear as a set of four 1:1:1 triplets at δ 44.46, 44.39, 44.31 and 44.24 ppm, respectively, resulting from a J CD of 19.5 Hz due to the deuterium atom attached to the methine carbon; the methyl group of these isotopologues has either zero, one, two, or three deuterium atoms. The experimental spectrum of the reaction mixture and the summed simulated spectrum of each isotopologue are shown in Fig. 10 (see Fig. S71 in the supporting information for full details of the simulated spectrum for each isotopologue). The summed simulated spectrum is remarkably similar to the experimental spectrum, which supports the assignment of the isotopologues and their relative proportions and confirms that the coupling constants, chemical shifts and line intensities and widths have been correctly determined. On the basis that the hydrogen generated from the hydrolysis of NaBH4 in D2O should result from a water-derived proton and a borohydride-derived hydride, the deuterium incorporation for all isotopologues II-VIII should be one. To this end, the total deuterium incorporation of 1.3 is slightly higher than expected and could be due to H/D exchange either with the d4-MeOH on the Pd/C during the hydrogenation or from the generation of a mixture of HD and D2 by exchange at the NP surface after O-D bond activation. A complementary experiment using hydrogen liberated from NaBH4/H2O for the hydrogenation of 1,1-diphenylethene in d4-methanol gave a total deuterium incorporation of 0.3, which confirms that H/D exchange occurs on the surface of the Pd/C; moreover, this deuterium incorporation corresponds to the excess of 0.3 above the total deuterium incorporation of one that was expected when the hydrogenation was performed in d4-MeOH with hydrogen generated from NaBH4/D2O. The hydrogenation was also performed in toluene with hydrogen generated from NaBH4 in D2O to investigate exchange at the NP surface. Under these conditions, the total deuterium incorporation of 0.93 was close to one, indicating that H/D exchange at the NP surface is slow; a total deuterium incorporation of 1.76 was also obtained when the hydrogenation was performed in toluene using hydrogen generated from NaBD4 in D2O, which is reassuringly close to the predicted value of two. Finally, the generation of minor amount of isotopologues containing -CHD2 and -CD3 (III, IV, VII and VIII) from each of these deuterium labelling experiments is consistent with H/D scrambling via facile reversible β-hydride elimination from a surface M-CPh2CH2D species, reinsertion of the resulting Ph2C=CHD into a surface M-D followed by reductive elimination from (D)HPd-CPh2CH3-nDn (n = 2, 3); full details of the relative proportions of each isotopologue obtained from these labelling studies are summarised in the supporting information. A higher than stoichiometric incorporation of deuterium recently reported for the hydrogenation of styrene using 'HD' generated from the hydrolysis of tetrahydroxydiboron with D2O using quantum dot stabilised PtNPs was also attributed to facile reversible alkene insertion-extrusion involving metal-hydride/deuteride species [166].Recycle studies were conducted with 2 mol% loading of 2e to investigate its activity profile during reuse and thereby its stability and longevity and potential for use in a scale-up system. The practical issues associated with separating and recovering a small amount of catalyst by filtration without loss of material after each run meant that it was not possible to perform a conventional recycle experiment. As such, a reuse experiment was undertaken by monitoring the hydrolysis until gas evolution was complete, the aqueous reaction mixture was then charged with a further portion of NaBH4 and the gas evolution monitored; this sequence was repeated to map the catalyst efficacy against reaction time and reuse number. While the comparative conversions and TOFs shown in Fig. 11 a, b were obtained during the first 2 min of the hydrolysis to enable a meaningful comparison between runs, complete conversions were obtained for each run within 4 min. The resulting gas evolution-time profile and corresponding conversion-cycle number profile in Fig. 11a, b shows a minor but gradual drop in conversion across five reuses, from 89% after 2 min in the first run to 78% after the same time in the 5th run. The drop in catalyst activity in successive runs, defined as the percentage reduction in the initial TOF, shows that 2e retains 71% of its activity across five reuses (Fig. 11b, red); this is comparable to recycle studies reported for other noble metal nanoparticle catalysts including; RuNPs immobilised in ZIF-67 [77], PtCoNPs supported on carbon nanospheres [167], ruthenium nanoparticles immobilised within the pores of amine-functionalised MIL-53 [76], ruthenium supported on graphite [139], RuCo nanoclusters incorporated in PEDOT/PSS polymer [168], RuNP stabilized by polyvinylpyrrolidone, zeolite-confined RuNPs [64], click dendrimer-stabilized PtCo, Rh and Pt nanoparticles and gold-transition metal nanoalloys [72,73,78,79,80,165] and Ru-RuO2/C [141].Sneddon et al. previously reported that the use of a borate buffered solution for the rhodium-catalysed release of hydrogen from ammonia triborane extended the catalyst lifetime such that Rh/Al2O3 showed little change in the hydrogen release rate over 11 cycles [169]. Following this lead, a preliminary comparative recycle hydrolysis conducted in freshly prepared aqueous borate buffer (pH maintained between 7.2 and 8) containing 0.28 M NaBH4 and 1 mol% 2e resulted in a marked increase in activity as evidenced by the initial TOF of 133 moleH2.molRu −1.min−1 obtained for the first run compared with 95 moleH2.molRu −1.min−1 for the corresponding reaction in water. The initial TOF increased to 146 moleH2.molRu −1.min−1 in the second run but then decreased gradually in subsequent cycles to 109 moleH2.molRu −1.min−1 in the final run (Fig. 11d); even though this represents a 26% reduction in activity over the 5 cycles, it remains higher than the TOFs obtained in water under the same conditions. Interestingly, the data in Fig. 11c, d also shows that the conversion-time profile changes quite dramatically in successive cycles such that the conversion increases from 54% after 5 min in the first run to 80% at the same time interval in the final run; in contrast, for reactions conducted in the absence of buffer, conversions decreased gradually in successive runs (Fig. 11b). A hydrolysis catalysed by 1 mol% 2e was also conducted in 0.34 M boric acid to provide a benchmark as the borate buffer solution was prepared with this concentration of boric acid and, under otherwise identical conditions, the initial TOF of 66 moleH2.molRu −1.min−1 was significantly lower than that obtained in the aqueous borate buffer solution (See Fig. S7 in the supporting information). Further studies are currently underway to identify an optimum buffer for this reaction and to develop an understanding of the changes in the conversion-time profile in consecutive runs as well as the origin of the enhancement in activity obtained when the catalysis is conducted in aqueous buffer.ICP-OES analysis of the aqueous reaction mixture recovered after the fifth run revealed that the ruthenium content was below the detection limit, suggesting that the reduction in activity was unlikely to be due to leaching of the ruthenium to generate a homogeneous species that was less active. Hot filtration studies were also conducted to explore whether soluble ruthenium species might be responsible for the gas evolution. Following a typical protocol, a hydrolysis reaction catalysed by 2 mol% 2e was filtered through a 45-micron syringe filter at ca. 50% conversion. The hydrogen liberated from the filtrate was monitored and corresponded to the background hydrolysis in the absence of catalyst (Fig. 12 , blue line), indicating that the active species had been removed in the filtration i.e. it is heterogeneous, and that leaching does not generate active soluble ruthenium species. In a complementary hot filtration study a catalytic hydrolysis that reached completion was filtered through a syringe filter (0.45 μm) and a fresh portion of NaBH4 added to the filtrate. The hydrogen liberated also corresponded to the uncatalyzed hydrolysis providing further support that the active species is heterogeneous (Fig. 12, orange line). TEM analysis of the catalyst isolated after the fifth run revealed that the ruthenium nanoparticles remained essentially monodisperse with a mean diameter of 1.8 ± 0.5 nm compared with 1.8 ± 0.6 nm for the freshly prepared catalyst (Fig. 12b) which suggests that agglomeration is not responsible for the drop in conversion with increasing use.There have been several reports that the sodium metaborate tetrahydrate by-product generated during the hydrolysis of NaBH4 deactivates the catalyst by adsorption on the surface [67,71,76,80,135,170-172], although Wie has demonstrated that the activity of deactivated Ru on nickel foam catalyst can be partially replenished by washing the catalyst with deionised water and completely replenished by washing with HCl to remove the NaBO2 [135]. As such, a series of poisoning studies were undertaken to examine the influence of the by-product on catalyst performance; this involved pre-stirring an aqueous suspension of 2e with 100 equivalents of sodium metaborate prior to addition of NaBH4 and monitoring the progress of the reaction as a function of the pre-stirring time. A 11B NMR spectrum of a typical reaction solution confirmed that the tetrahydroxyborate anion B(OH)4 was the sole by-product as the spectrum contained a single sharp resonance at δ 2.2 ppm [162,173]; no other species such as partially hydrolysed intermediates were detected. A comparison of the hydrogen evolution in the absence of NaBO2 against the corresponding reaction with added NaBO2 as a function of the pre-stirring time (Fig. 13 a,b) confirms that the addition of metaborate passivates the catalyst. The conversions obtained after a reaction time of 2 min and the corresponding initial TOFs as a function of pre-stirring time reveal that the passivation is instantaneous as the TOF drops from 84 moleH2.molRu −1.min−1 in the absence of NaBO2 to 80 moleH2.molRu −1.min−1 immediately after the addition of the NaBO2 with no pre-stirring (time = 0 min); the TOFs continue to drop gradually to 57 moleH2.molRu −1.min−1 as the pre-stirring time was increased to 60 min.Finally, the formation of NaBO2 can also be monitored by measuring the pH of the reaction solution as a function of time for the catalytic hydrolysis of a 0.028 M solution of NaBH4 using 2 mol% of 2e. Fig. 14 shows that the pH of the reaction solution clearly maps to the conversion with a gradual increase from pH 8.3 at time = 0 min, recorded immediately after addition of the NaBH4, to pH = 11.1 after ca. 2.5 min when the gas evolution had finished; for comparison a 0.028 M solution of NaBO2 in the absence of catalyst or NaBH4 has a pH of 11.30, which correlates with the pH of a hydrolysis reaction at high conversion.Ruthenium nanoparticles stabilized by polymer immobilized ionic liquids catalyze the hydrolytic evolution of hydrogen from sodium borohydride; catalyst stabilized by an amino-modified imidazolium-based polymer was the most active with an initial TOF of 171 moleH2.molRu −1.min−1, this is among the highest to be reported for a RuNP-based system. Kinetic studies revealed that the reaction was first order in catalyst as well as sodium borohydride at low hydride/catalyst mole ratios but zero order with respect to NaBH4 concentration with high hydride/catalyst mole ratios. The apparent activation energies of 38.9 kJ mol−1 to 51.8 kJ mol−1 are in the region commonly reported for the platinum group metal catalyzed hydrolysis of hydrogen rich boron derivatives; the apparent activation energy of 38.9 kJ mol−1 for RuNP@NH2PIILS is lower than each of the other catalysts tested and consistent with its higher initial TOF. A kinetic isotope effect (kH /kD ) of 2.3 obtained for reactions conducted in H2O and D2O and a kH /kD of 2.72 for reactions conducted with NaBH4 and NaBD4 at a low catalyst/hydride mole ratio indicate that both the O-H and B-H bonds are activated by the ruthenium atoms in the rate limiting step, most likely via a concerted oxidative addition-hydride transfer involving the surface-coordinated hydrogen-bonded [BH3H-]—–H-OH ensemble rather than oxidative addition of both the O-H and B-H bonds. Interestingly though, the kH /kD of 0.67 obtained from comparing the initial rates of hydrolysis for NaBH4 and NaBD4 under conditions of catalysis, i.e. at a high catalyst/hydride mole ratio, is an inverse KIE which would be consistent with a surface-coordinated borohydride activating an O-H bond of water in the hydrogen-bonded ensemble prior to rapid hydride transfer. Reuse experiments showed that RuNP@NH2-PIILS retains 79% of its activity over 5 runs and poisoning studies conducted by adding NaBO2 to a catalytic reaction suggest that the reduction in activity is most likely due to passivation of the catalyst by absorption of the metaborate by-product on the nanoparticle surface. A tandem hydrogenation of 1,1-diphenylethene in d4-MeOH with hydrogen generated from the catalytic hydrolysis of NaBH4 in D2O gave a mixture of all eight possible isotopologues with a total deuterium incorporation greater than one while the use of toluene for the hydrogenation using NaBH4/D2O gave a total deuterium incorporation close to one. This is consistent with slow H/D exchange at the NP surface and fast H/D exchange on the surface of the Pd/C coupled with H/D scrambling via facile reversible beta hydride elimination-reinsertion during the hydrogenation. This programme is currently exploring the use of PIIL supported bimetallic nanoparticles with varying proportions of noble and earth abundant metals to establish how the composition of the NP influences catalyst performance with the aim of identifying an optimum synergism that will be suitable for use as a hydrogen generation system for portable applications of proton exchange membrane fuel cells (PEMFC). In addition, PIILs are an ideal support to investigate how polymer properties such as charge density, the number and type of heteroatom donor and functionality, porosity and hydrophilicity influences the size, morphology, and efficacy of the nanoparticles as well as to tailor catalyst-support interactions to enhance efficacy. Ultimately, this catalyst technology will be extended to include the hydrogen evolution reaction to develop stable, durable, highly active cost-effective catalysts for use in AEM based electrolysers and fuel cells.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Reece Patterson, Anthony Griffiths reports financial support was provided by Engineering and Physical Sciences Research Council.R.P. gratefully acknowledges the Engineering and Physical Sciences Centre for Doctoral Training in Renewable Energy Northeast Universities (‘ReNU’) EP/S023836/1 for a studentship and A.A. thanks Taibah University, Saudi Arabia for a Scholarship. We also thank (Dr Tracey Davey) for the SEM images (Faculty of Medical Sciences, Newcastle University) and Zabeada Aslam and the Leeds electron microscopy and spectroscopy centre (LEMAS) at the University of Leeds for TEM analysis. This research was funded through a studentship (Anthony Griffiths) awarded by the Engineering and Physical Sciences Centre for Doctoral Training in Molecules to Product (EP/SO22473/1). The authors greatly acknowledge their support of this work. This article is dedicated to the memory of Professor Stephen A. Westcott (Canada Research Chair holder in the Department of Chemistry & Biochemistry, Mount Allison University, Canada) who recently passed away; a fantastic scientist, a great ambassador for chemistry teaching and research in Canada and across the globe, a generous, genuine and kind human being but most of all the best of friends.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.mcat.2022.112476. Image, application 1

Ruthenium nanoparticles stabilised by polymer immobilized ionic liquids catalyse the hydrolytic release of hydrogen from sodium borohydride. The composition of the polymer influences performance and ruthenium nanoparticles stabilised by an amine-decorated imidazolium-based polymer immobilised ionic liquid (RuNP@NH2-PIILS) was the most efficient with a maximum initial turnover frequency (TOF) of 177 moleH2.molRu −1.min−1, obtained at 30°C with a catalyst loading of 0.08 mol%; markedly higher than that of 69 molH2.molRu −1.min−1 obtained with 5 wt% Ru/C and one of the highest to be reported for a RuNP catalyst. The apparent activation energy (Ea) of 38.9 kJ mol−1 for the hydrolysis of NaBH4 catalysed by RuNP@NH2-PIILS is lower than that for the other polymer immobilized ionic liquid stabilised RuNPs, which is consistent with its efficacy. Comparison of the initial rates of hydrolysis in H2O and D2O catalysed by RuNP@NH2-PIILS gave a primary kinetic isotope effect (k H/k D) of 2.3 which supports a mechanism involving rate limiting oxidative addition of one of the O-H bonds in a strongly hydrogen-bonded surface-coordinated [BH3H−]—-H2O ensemble. The involvement of a surface-coordinated borohydride is further supported by an inverse kinetic isotope effect of 0.65 obtained from a comparison of the initial rates for the hydrolysis of NaBH4 and NaBD4 under the conditions of catalysis i.e., at a high hydride/catalyst mole ratio. Interestingly though, when the comparison of the initial rates of hydrolysis of NaBH4 and NaBD4 was conducted in dilute solution with a hydride/catalyst mole ratio of 1 a kinetic isotope effect (k H/k D) of 2.72 was obtained; this would be more consistent with concerted activation of both an O-H and B-H bond in the rate limiting step, possibly via a concerted oxidative addition-hydride transfer in the surface-coordinated hydrogen-bonded ensemble. Catalyst stability and reuse studies showed that RuNP@NH2-PIILS retained 71% of its activity over five runs; the gradual drop in the initial TOF with run number appears to be due to passivation of the catalyst by the sodium borate by-product as well as an increase in viscosity of the reaction mixture rather than leaching of the catalyst.

Direct photocatalysis using plasmonic metal (gold, silver, copper, or aluminum) nanoparticles (NPs) under visible-light irradiation, also called plasmonic catalysis, has drawn significant interest in the last decade. 1–13 When plasmonic metal NPs are illuminated, they can efficiently absorb visible light due to the excitation of localized surface plasmon resonance (LSPR), where the conduction electrons of the NPs collectively oscillate with the electromagnetic (EM) field of incident light. 14 The plasmon-resonant excitation generates energetic hot electrons that can trigger chemical reactions of a reactant adsorbed on the plasmonic NPs, under mild reaction conditions. 1–7,12,13 However, only a limited number of the transformations can be directly catalyzed by NPs of plasmonic metals, especially when compared to the range of transformations accessible with transition-metal complexes in homogeneous catalysis systems. For specific reactions, photocatalysts of alloy NPs of a plasmonic and a transition metal of inherent catalytic activity for specific reactions have been developed to expand the application of the plasmonic photocatalysis to a wider range of selective organic synthesis reactions. 15 Transition-metal complexes are widely used for the homogeneous catalytic synthesis of many important organic compounds. 16–19 Combining the optical function of plasmonic metal NPs with the inherent bond-forming or bond-breaking ability of transition-metal complexes may enable the transition-metal complexes to catalyze reactions with the light energy harvested by plasmonic metal NPs under mild conditions. With traditional catalysis reactions, intense thermal heating is often required to bridge reaction activation barriers and achieve sufficient catalytic efficiency. When plasmonic metal NPs are introduced as antennas to direct energy to the active reaction site, there may be no need for such intensive heating. This difference in the way catalysis is expected to prove beneficial for selective synthesis. In a combined system, the plasmonic NP surfaces do not act as catalytically active sites themselves; instead, the NPs facilitate chemical transformations via the transfer of energy and light-generated hot electrons to reactive transition-metal complex sites, as schematically illustrated in Figure 1 . The adsorption and activation of reactants at the complex sites of the new catalysts are different from those on metal NP surface.It is well known that the LSPR light absorption of plasmonic NPs can generate EM near-fields, with intensities much higher than that of the incident radiation. 4,6–11 The plasmon field enhancement can significantly change the light-matter interaction in excitonic systems. 14,20 For instance, the field intensity in a narrow junction between Ag NPs has been predicted to be 106 times that of the incident light alone. 4 These narrow junctions between closely spaced NPs are the so-called hot spots. 21,22 Whether the high intensity of EM fields at the hot spots can be utilized for catalysis by surface-bound active sites has not been investigated. We consider that the strong EM fields at hot spots may radically change the interaction between reactant and transition-metal complexes when the complexes are immobilized within the hot spots. A strong interaction may direct the energy of the incident light to the reaction site. Also, since the number density of plasmonic NPs is larger at hot spots than other regions in a sample, the number of hot electrons at hot spots is also predicted to be higher. These properties may facilitate a chemical transformation and thus improve the catalytic efficiency of the metal complexes.Electron transfer may contribute to the catalysis process by changing the oxidation state of the transition metal complexes, a key mechanism that can promote catalysis of many reactions. 23 In a combined system, the transfer of energy and hot electrons is determined by energy alignment: the energy of photoexcited electrons have to be sufficiently high to be injected into the metal complex sites (via a medium) and change the metal oxidation state. The light harvested by plasmonic NP antennas promotes conduction electrons to energy levels above the Fermi level, 4,12,14 and energy distribution of hot electrons depends on the energy of the incident photons and the light absorption mechanism of the plasmonic metal NPs.In addition to the intraband electron excitation in the plasmonic NPs due to the LSPR effect, interband excitation of d → sp transitions by visible light can generate electron-hole (e–-h+) pairs in Au NPs, which can participate in a chemical reaction of adsorbate on the NP surface. 24 The interband excitation in NPs of non-plasmonic metals can also facilitate chemical reactions. 25 It is of great interest to know whether the interband excitation can promote catalytic performance of the metal complexes at a distance. Small Au NPs possess only weak LSPR absorption and the light absorption of the NPs exposed to short wavelengths (<450 nm) induces predominantly interband excitation. 12 This property of small Au NPs can therefore be used to investigate whether interband excitation can promote the catalysis even when the NP surfaces are not catalytically active sites themselves. Hence, the mechanisms of the systems with the plasmon-antenna-promoted catalysts will be different from those of the plasmonic metal NP catalysts and homogeneous metal complex catalysts. These features are of great interest from a fundamental research perspective.To verify the efficacy of the proposed strategy of exploiting the antenna-effect of plasmonic metal NPs and to develop protocols for transition-metal-catalyzed reactions under mild conditions, we designed a structure as illustrated in Figure 1. In this structure, Ni2+ complexes and Ag or Au NPs are immobilized on γ-Al2O3 nanofibers. Immobilizing Ni2+ complexes to the γ-Al2O3 supports maintains the complexes at fixed locations relative to the metal NPs, allowing us to investigate the influence of the high intensity of EM near-fields and transfer of hot electrons from the metal NPs to Ni2+ ions. Such a structure can achieve stable photocatalytic performance and make the catalysts recyclable.Ni2+ complexes have been extensively used as homogeneous catalysts. 26–29 Nickel is a common, inexpensive transition metal and has been used to catalyze reductive cleavage of C–O bonds. 30–35 This reaction was chosen as a model reaction because it is the essential reaction step for the production of high-value aromatic chemicals from biopolymer lignin. 36 Aryl ether C–O bonds are relatively unreactive; for example, the dissociation energy of the aryl ether C–O bonds in the α-O-4 linkage is 218 kJ mol−1. 31 So catalysts usually function at high temperatures and hydrogen pressures (>120oC and added hydrogen pressure), 37 inevitably yielding saturated hydrocarbons. To avoid undesired hydrogenolysis of aromatic rings and to achieve selective cleavage of the aryl ether C–O bonds, the reaction should ideally be conducted under mild reaction conditions (low temperature and pressure). Hence, the catalysts must be highly active and selective to the C–O bond. Cleavage of C–O bonds in aryl ethers with a photocatalyst [Ir(ppy)2(dtbbpy)]PF6 at room temperature was reported recently. 38 Although these types of organometallic homogeneous photocatalysts are efficient, they are generally expensive and their use requires costly recycling processes.In the present study, photocatalysts with immobilized Ni2+ complexes and plasmonic metal NPs were applied to the hydrogenolysis of benzyl phenyl ether that has an α-O-4 linkage. 39 We demonstrate that by varying the metal NP loading on the alumina support, the number of the plasmonic hot spots could be substantially altered. The hot electrons generated by both interband excitation of small Au NPs and intraband excitation (via LSPR effect) of Ag NPs can promote the catalysis. A significant observation is evidence for a light-induced, enhanced chemisorption of the reactant molecules at the catalytic active Ni2+ sites. This is beneficial to the catalytic performance of the Ni2+ sites. We also find that transfer of the hot electrons from the plasmonic metal NPs to the Ni2+ complexes via a “bridge” of the aromatic ring of the reactant is essential to the performance of the photocatalysts.The photocatalysts were prepared following the procedures shown in Figure S1. The prefix number of the sample name indicates the plasmonic metal content in weight percentage (wt %), and ASN (alumina-slilane-NH2) is the support of Al2O3 nanofibers grafted with a silane containing an amino group. The metal content in the catalysts was measured by inductively coupled plasma optical emission spectrometry (ICP-OES), confirming the contents of Ni and plasmonic metal (Table S1). For example, the 2.5Au-ASN-Ni2+ catalyst contains 2.5 wt % of Au NPs, and Ni2+ ions are immobilized on the ASN support.The γ-Al2O3 nanofibers are ∼5 nm thick and 100 nm long, which were sintered to form a highly porous framework of randomly oriented fibers. 40 The cage-like nanofiber configuration (see Figure 1) could confine the NPs formed within the structure and allow reactant molecules to readily diffuse to the Ni2+ complex reaction sites in the vicinity of the metal NPs through inter-fiber voids. Plasmonic NPs (Ag or Au) and Ni2+ were immobilized on the alumina fibers in the procedure schematically illustrated in Figure 1. The transmission electron microscopy (TEM) images indicate that the metal NPs were dispersed throughout the γ-Al2O3 fiber support (Figures 2A and 2D). The supported Au NPs were relatively small, most of them smaller than 5 nm (Figure 2B and inset of Figure 2A), the mean particle size is 2 nm. The particle size distributions of the Ag NPs (Figure 2E and inset of Figure 2D) were broader, most of the Ag NPs were smaller than 15 nm and the mean particle size is about 9 nm. Line scan analysis (energy dispersion X-ray spectroscopy [EDX]) of plasmonic metal and Ni compositional fluctuations in Figures 2C and 2F indicate that the Ni2+ ions were immobilized on the bare support between the metal NPs, instead of accumulating on the surface of the plasmonic metal NPs.The other structural and chemical properties of the catalyst were characterized by nitrogen adsorption, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) and are shown in Table S2 and Figures S2–S6. The results verify that the photocatalysts possess the assembled structure as we designed.Diffuse reflectance ultraviolet-visible (DR UV-vis) spectra of the samples are shown in Figures 2G and 2H. γ-Al2O3 fibers and grafted γ-Al2O3 fibers (ASN) have no obvious absorption in the visible-light region (wavelength > 400 nm), while two broad bands peaked at wavelengths of 378 and 652 nm are clearly observed for Al2O3-silane-NH2-Ni2+ (ASN-Ni2+). These two bands can be assigned to the 3A2g (F) → 3 T 1g (P) and 3A2g (F) → 3 T 1g (F) transitions for distorted octahedral Ni2+ complexes. 41,42 These absorption bands in 2.5Au-ASN-Ni2+ and 4.3Ag-ASN-Ni2+ samples overlap with the strong absorption of the supported metal NPs in the samples. Both 4.3Ag-ASN-Ni2+ and 4.3Ag-ASN samples have absorption bands centered at 405 nm, which are the LSPR absorption of Ag NPs. 8 The 2.5Au-ASN-Ni2+ catalyst shows a broad light absorption and the absorption intensity decreases when the wavelengths are longer than 550 nm. No sharp characteristic LSPR peak of Au NPs at ∼530 nm 43 is observed. This could be attributed to that the small Au NPs (with a mean size of 2 nm). The interaction of Au NPs with the amino group of the grafted silane also changes the electron density of the small Au NPs and their light absorption due to LSPR effect 44,45 more significantly than that of the relatively larger Ag NPs (with mean size of 9 nm). Figure 3 shows the catalytic performance for the C–O bond cleavage of benzyl phenyl ether (one of the α-O-4 model compounds) by the photocatalysts prepared with different amounts of plasmonic metal NPs (parameters given in the caption). Both the 2.5Au-ASN-Ni2+ and 4.3Ag-ASN-Ni2+ catalysts gave high conversion of the ether (98% and 96%, respectively) under visible-light irradiation of 0.96 W cm−2, while no reaction was observed in the dark. The selectivity toward production of phenol was around 50%, indicating that a homolytic C–O bond scission of benzyl phenyl ether occurred. There are no products of undesired hydrogenolysis of aromatic rings; the high selectivity is because the moderate conditions of the photocatalytic process induce over-reduction.The catalytic activities of a sample prepared with Ni2+ complexes but without the metal NPs, a sample with the metal NPs but no Ni2+ ions yielded very low conversion (6% or nil). This highlights a significant synergistic effect occurred when the Ni2+ complexes were immobilized in the presence of plasmonic metal NPs: when the light absorption by the metal NPs combines with the inherent catalytic capability of Ni2+ complexes, this results in superior photocatalytic activity of the composite. It is also evident that conversion of the reactant increased with increased metal NP loading of the catalyst. The synergistic effect becomes significant when the metal NP content is high.Simply the mixture of Ni(NO3)2 and 4.3Ag-ASN exhibited negligible activity. Hence, the possibility that Ni2+ species in liquid phase are active for the catalytic reaction is excluded. The mixture of Ni(NO3)2 and 2.5Au-ASN exhibited a low conversion of about 20%. It appears that the gold NPs can interact with Ni2+ species in liquid while Ag NPs do not. The detailed reason is unknown. In those control experiments, the overall contents of Ni2+ ions and silver in the mixture of Ni(NO3)2 and 4.3Ag-ASN are the same as those in the 4.3Au-ASN-Ni2+. This is also the case for the Au sample. The significant differences in catalytic performance between the mixtures and our designed catalysts suggest that the interaction between metal NP-ASN and immobilize Ni2+ ions has substantial impact on the overall photocatalytic process, the reaction is a complete heterogeneous catalysis process for Ag-ASN-Ni2+ catalysts and proceeds mainly by heterogeneous catalysis process for Au-ASN-Ni2+ catalysts. Plasmonic NPs considerably enhance the catalytic activity of Ni2+ complexes only when they are immobilized in the close proximity to the NPs.It was reported that photothermal effect of plasmonic NPs could elevate the temperature of the NPs, which may play a critical role in the catalysis. 46 To understand if local heating will contribute to the Ni2+ catalysis, we conducted the photo-reaction using the ASN-Ni2+ catalyst at elevated temperatures (80°C –120°C); only 2% of conversion increase of C–O bond cleavage was observed (see Table S3), which means that a 40°C increase in reaction temperature could not enhance the catalytic activity of the Ni2+complexes in our reaction system. Therefore, photothermal effect caused by irradiation could not drive the reaction in the present study. The hot electrons from the illuminated metal NPs may play the critical role in activating the aryl ether C–O bonds as is the case for some other plasmonic catalysts reported in the literature. 1–13 But the Au and Ag NPs are not the active catalytic sites in the present study, as evidenced by the experimental data (see Figure 3). Hence, these catalysts may be expected to operate differently from the well-known mechanism where light-generated hot electrons promote reactions. 2–8 We compared the catalytic performance enhancement of Ni2+ when the number density of metal NPs are different by using catalysts with different metal NP contents and maintaining the metal NP content in the reaction systems and other experimental conditions identical. For instance, the reaction vessel was loaded with 66 mg of 1.3Ag-ASN-Ni2+ and 20 mg of 4.3Ag-ASN-Ni2+. The quantities of the two catalysts were calculated from the Ag content (Table S1). Figure 4 A shows that the percent conversion of reactant in the reaction driven by 4.3Ag-ASN-Ni2+ is significantly higher than by 1.3Ag-ASN-Ni2+ although the content of immobilized Ni2+ ions in the latter is 3.8 times of that in the former (derived from data in Table S1). The turnover number (TON) with respect to the Ni2+ sites in 4.3Ag-ASN-Ni2+ was calculated from the conversion, being >18 times higher than that in 1.3Ag-ASN-Ni2+. Given that the Ag NP content in the two systems is the same, the overall Ag NP content in the catalytic system cannot be the reason for the large difference in the catalytic performance.The major difference between the two photocatalytic systems shown in Figure 4A is that the Ag NPs are more closely packed in 4.3Ag-ASN-Ni2+. The junctions between closely spaced NPs can generate “hot spots” where the strong EM field coupling of the closely spaced NPs generates huge EM field enhancement. 4,6 In a unit volume of catalyst, the Ag NPs in the 4.3Ag-ASN-Ni2+ is more than three times the Ag NPs in the 1.3Ag-ASN-Ni2+ catalyst, and therefore, there are a greater number of nano-junctions between Ag NPs. The results in Figure 4A can be interpreted as a manifestation of the fact that the photocatalytic activity heavily depends on the number of Ag NPs’ nano-junctions in the photocatalysts. An environment of high number density of Ag NPs can significantly promote catalytic performance of the immobilized Ni2+ complexes around the particles.The effect of catalyst mass on photocatalytic activity was examined, and the results are provided in Table S4. The conversion increased as more catalysts were used. But the conversion did not increase linearly with catalyst mass. The deviation from the linearity may be attributed to the screen effect. Figure S7 shows EDS mapping results of the 4.3Ag-ASN-Ni2+ photocatalyst, which reveals the existence of closely spaced NPs, consequently supporting the role played by plasmonic hot spots. 21,22 Analysis of the results (Figure S7) indicates the distribution of plasmonic hot spots regions overlapped with concentrations of surface-bound Ni2+ complexes co-located in the plasmonic hot-spot regions.A similar phenomenon can be found in the xAu-ASN-Ni2+ photocatalytic systems as seen in Figure 4B. However, the effect of high number density of Au NPs on the catalytic performance is weaker than that of Ag NPs. The performance improvement caused by increasing the number density of small Au NPs (from 0.7 wt % Au to 8.4 wt % Au; see TEM images in Figure S8) is less than that caused by increasing the number density of Ag NPs (from 1.3 wt % Ag to 4.3 wt % Ag; see TEM images in Figure S9).We performed simulations of the near-field enhancement of single Au NP (2 nm) and dimer Au NPs (2 nm) with 1 nm gap between them (Figure S10) by using an electrostatic eigenvalue method; the details of the simulation methodology are from Davis and Gómez. 47 Under 400 nm excitation, the intensity of EM fields at the surface of an isolated Au NP is ∼10 times larger than the field intensity of the incoming photon flux, while the enhancement of EM field occurring in the middle of the dimer is ∼12 times. In contrast, the EM field localization upon LSPR excitation of Ag NPs is significantly higher (Figure S11). The intensity of EM fields at the surface of an Ag NP (9 nm) is ∼300 times larger than the field intensity of the incoming light, while the enhancement of EM field occurring at the hot spot of the dimer is ∼8,000 times. The hot spot regions will have high concentrations of energetic electrons (because the high number density of the plasmonic NPs) and interaction of the EM near-field with reactant in these regions is likely to be very strong. Thus, these sites are considered most important for plasmonic photocatalysis.We note that our wet-chemistry synthesis approach results in distributions of sizes of the plasmonic metal particles and a range of inter-particle distances in the catalysts as previously reported. 1,2 Uniform metal particle size and inter-particle distance is not a prerequisite for formation of plasmonic hot spots. 46 The 4.3Ag-ASN-Ni2+ catalyst has many more hot spots, compared to 1.3Ag-ASN-Ni2+ as shown in Figure S9. The intense field enhancement at the hot spots between the metal NPs clearly influences the catalytic performance of Ni2+ complexes under mild reaction conditions, and the results of Figure 3 represent a statistical average which is dominated by the response augmented by plasmonic hot spots.The existence of hot spots has also been experimentally observed. Highly intense EM near-fields affect the properties of molecules within them; 48 for example, the intensity of signals in surface-enhanced Raman scattering (SERS) of the molecules. 48–50 SERS predominantly originates from EM near-field enhancement, and its intensity is approximately proportional to the square of the EM field intensity. Thus, the intensity of SERS active peaks of reactant is expected to be commensurate with intensity of the enhanced EM near-field around Ag NPs experienced by the reactant molecules. The Raman spectra of benzyl phenyl ether adsorbed on the catalysts with different Ag NP contents were compared (seen in Figure 5 A).The results show that when the aryl ether molecules were adsorbed on the catalysts with a high content of Ag NPs (4.3Ag-ASN-Ni2+), peaks at 1,153 cm−1 ascribed to the aryl ether C–O bond vibration become much more intense, even when compared to their concentrated counterpart (pure benzyl phenyl ether). No SERS signals were detected with the catalyst samples prepared with a lower Ag NP content (1.3Ag-ASN-Ni2+ and 1.3Ag-ASN) or with small Au NPs. Given that the photocatalysts have a similar specific surface area (Table S2), the adsorption of the aryl ether molecules on the alumina support has no obvious effect on the Raman signal intensity. The SERS results can be attributed to three possibilities: (1) there is a higher concentration of the aryl ether adsorbed on the samples with a higher Ag NP content; (2) the EM near-field is much more intense (there are more hot spots) in the photocatalysts with increased number density of Ag NPs, and this significantly enhances the Raman signal of adsorbed molecules; or (3) the pronounced Raman signals are due to synergistic effect of the two reasons above. The near-field enhancement of the sample with small Au NPs (the 2.5Au-ASN-Ni2+) is relatively weak as indicated by the simulation results and the fact that there is no LSPR absorption peak at about 520 nm wavelength. This is consistent with the result that no enhanced Raman signal is observed.The reactant adsorption is a key step prior to surface reaction in heterogeneous catalysis, the impact of light irradiation on the adsorption has not been extensively investigated to date. To explore whether the light irradiation can affect reactant adsorption, we conducted adsorption experiments of benzyl phenyl ether on different catalysts in the dark and under light irradiation with different light intensities. The results are summarized in Figure 5B. The adsorption amount is given by the percentage of the initial concentration of the reactant before light irradiation, which was adsorbed by the catalyst under irradiation. The adsorption capacity is calculated based on the Ni amount collected in Table S1. In the experiment, the cleavage reaction did not proceed as KOH was not added. No chemicals other than those added were detected. The adsorption capacity (AC) was calculated as AC ( % )   =  0 . 01  ×  ( C 0 − C C 0 n Ni )  ×  100 , where 0.01 is the total molar amount of benzyl phenyl ether, C0 and C are the concentrations of the ether before and after adsorption experiment, respectively, and nNi is moles of nickel (calculated by the data shown in Table S1). The adsorption properties of ASN-Ni2+, 1.3Ag-ASN-Ni2+, and 4.3Ag-ASN-Ni2+ catalysts are compared in Figure 5B.The initial concentration used for the adsorption experiment (0.01 M) was much lower than that used in the reaction. The amount of Ni2+ (the adsorption site) in these samples was 0.011–0.013 mmol, and as discussed below, only the adsorption on a fraction of the Ni2+ sites, which are in the intense EM fields, was influenced by light irradiation. When a high initial concentration of reactant was added, it was difficult to accurately detect the concentration changes cause by light-induced adsorption.It is also worth noting that only trace amounts of benzyl phenyl ether were adsorbed on the illuminated 4.3Ag-ASN catalyst, the sample without Ni2+ but with high Ag NP content. This implies that the EM near-fields alone are not leading to increased reactant adsorption. One needs to consider the interaction between the ether and Ni2+ complex sites of the catalysts too. We measured FTIR spectra of the catalysts after the ether adsorption experiments (Figure S12). For the ASN-Ni2+ samples that adsorbed the ether in the dark or under light irradiation of intensity of 1.11 W cm−2, the position of the peak located at 1,151 cm−1 (which is assigned to the C–O bond stretch mode of the ether) remains unchanged (Figures S12A and S12D).However, for both catalysts containing Ag NPs and the surface-bound Ni2+ complexes (1.3Ag-ASN-Ni2+ and 4.3Ag-ASN-Ni2+), this band is blue-shifted to a higher wavenumber under light-irradiation conditions. This indicates an enhanced interaction can occur between the ether and Ni2+ sites when hot spots are activated (Figures S12E and S12F). Also, the intensity of the band at 1,350 cm−1 (the vibration mode of NO3 − that coordinates with Ni2+ sites) decreased markedly after the sample was illuminated (Figures S12B and S12C). This can be explained if NO3 − is displaced by the ether during adsorption under visible-light irradiation. The light-induced adsorption of the ether is irreversible since it remained adsorbed after the light was switched off. The specific adsorption sites (Ni2+ sites), the coordination of the adsorbate to the sites, the requirement of activation energy (light harvested by the plasmonic metal NPs) for the adsorption, and irreversible adsorption process suggested chemisorption of the ether at Ni2+ sites.The adsorption of the ether increases significantly with the light intensity increases, on the photocatalyst 4.3Ag-ASN-Ni2+ (Figure 5B). The adsorption of the ether on 1.3Ag-ASN-Ni2+ catalyst (with much less hot spots) is considerably lower, regardless of the light intensities used. These data demonstrated that the ether adsorption depends on the number density of Ag NPs and intensity of the incident light but not on the overall number of Ni2+ complexes in a sample. The higher light intensity and the Ag NPs density determine the stronger EM near-fields generated close to the active sites Ni2+. 1.3Ag-ASN-Ni2+ sample contains slightly more Ni2+ complexes than the 4.3Ag-ASN-Ni2+ sample (Table S1). It follows that the chemisorption only appears at the Ni2+ sites subject to the EM fields of high intensity. In 1.3Ag-ASN-Ni2+, a small number of sites with the sufficient EM field intensity for chemisorption as the sample has small number of hot spots. Increasing the incident light intensity amplifies the intensity of the EM near-fields, and as the number of such sites increases, so does the chemisorption. Such an increase is insignificant for 1.3Ag-ASN-Ni2+ compared with that for 4.3Ag-ASN-Ni2+. To the best of our knowledge, there have been no reports on light-enhanced chemisorption of molecules with irradiation of continuous-wave visible-light of moderate intensity (∼1 W cm−2). Very recently, we reported that a plasmonic alloy system can selectively concentrate the reactant molecules to catalyst surface and thus increase the reaction rate by several orders under light. 51 It is proven that light irradiation generates an optical plasmon force that can add to van der Waals force and selectively attract reactant molecules to the active sites. In the plasmonic antenna system in the present study, similar optical plasmon force contributes to concentrate the ether to catalyst surface and thus enhances the chemisorption of ether on Ni2+ sites. This provides essential knowledge on how visible-light irradiation significantly promotes the catalytic performance of metal cations by plasmonic metal.Following the chemisorption, the dependence of photocatalytic performance on the intensity of the light source is also investigated in Figure 6 . The temperature of the reaction mixture was carefully maintained at 90°C or 80°C ± 2°C to guarantee that thermal effects could be discounted. It is clear that the higher the light intensity, the greater is the contribution of light irradiation to driving the reaction. When the light intensity is 0.96 W cm−2, over 90% of the conversion is due to light irradiation for the C–O bond cleavage of benzyl phenyl ether. Clearly, light irradiation is the principle driving force of the reaction. Higher light intensity generates more hot electrons in metal NPs and also generate stronger EM field to increase the reactant chemisorption; thus, irradiation-induced enhancement of chemisorption and reactant conversion are correlated.Correlating the light intensity dependence of the chemisorption and the reactant conversion with observation that the catalytic performance is not regulated by the overall Ni2+ content in the photocatalyst system (Figure 4A), we infer that the C–O bond cleavage predominantly takes place on the Ni2+ sites within the hot spots, where the ether molecules chemically adsorbed. The adsorbed ether molecules will experience an intense oscillating EM field, which could facilitate the activation of the molecules for the cleavage reaction. The hot spots can capture both light energy and reactant molecules, have higher concentration of hot electrons and thus, are an ideal location for the catalysis.Besides, as can be seen in Figure 6, increasing the light intensity to higher than 0.76 W cm−2 resulted in a dramatic increase in the conversion of the C–O bond cleavage reaction catalyzed by 2.5Au-ASN-Ni2+ (or 4.3Ag-ASN-Ni2+). There exists an energy threshold to activation of benzyl phenyl ether molecule. It is between 0.59 and 0.76 W cm−2 for 2.5Au-ASN-Ni2+ catalyst at 90°C and between 0.76 and 0.96 W cm−2 for 4.3Ag-ASN-Ni2+ at 80°C. The intensity threshold is a common feature in photocatalysis systems mediated by plasmonic NPs and influenced by various factors. 52 The catalytic performance variation with the wavelength reveals the energy alignment of the combined system. The action spectrum (Figures 7A and 7B) shows the dependence of the irradiation wavelength on the conversion efficiency of the photocatalytic reactions. The reaction rates were converted to the apparent quantum yields (AQYs), which were calculated as AQY  ( % ) = Y light − Y dark n × 100 , where the Ylight and Ydark are the amount of reactant being converted under light irradiation and dark conditions, respectively; 15 n is the number of incident photons. The number of reactant being cleaved in the dark, Ydark, was subtracted from that observed when the system was irradiated, Ylight, to clearly illustrate the contribution of light irradiation to the overall conversion. The AQY is a wavelength-dependent quantity that is given by the ratio of the number of molecules produced to the number of incident photons. By definition, it is a quantity that is independent of the intensity of the incident radiation, and its trend provides physical insight on the mechanism, accounting for the conversion of photonic energy into chemical potential energy.For the Ag-Ni2+ photocatalytic system, the highest conversion rate was achieved when illuminating with a 400 nm peak wavelength (Figure 7B). According to the UV-vis spectra of the photocatalyst in Figure 2H, the most intense LSPR absorption of Ag NPs in this photocatalyst occurs at 405 nm, well matched with the LED wavelength with which the highest cleavage reaction rate conversion was observed. Also, the AQY spectrum generally matches the absorption of photocatalyst (Figure 7B). Given the temperature of the reaction mixture and the irradiation energy of each wavelength were carefully maintained identical, the significant AQY variation with irradiation wavelength corroborates that the reaction was mainly driven by light. The results evidence that the enhancement of the catalytic performance is caused by the LSPR absorption of Ag NPs. The LSPR absorption in the range between 400 and 600 nm can generate hot electrons with sufficient energy to reduce Ni2+ in the complexes.For the 2.5Au-ASN-Ni2+ photocatalyst, the AQYs show a mismatch with the catalyst light absorption: the highest conversion rate was achieved when illuminating with a 400 nm peak wavelength (single-color LED source with band width ± 10 nm), while there is no absorption peak at this wavelength (Figure 7A). Hence, light absorption is not the sole factor determining the catalytic reaction over the small Au NPs. The high AQY at 400 nm suggests that there is an energy alignment governing the electron transfer: only hot electrons of sufficient energy can trigger the reaction that takes place at Ni2+ complex sites.According to the diagram shown in Figure 7C, photons with 397 nm wavelength (∼3.12 eV) are able to excite d electrons of Au NPs to the energy level for Ni2+ reduction. Photons with longer wavelengths have insufficient energy to reduce Ni2+ ions. On the other hand, a much lower reaction rate is observed under irradiation with wavelength < 380 nm (Figure 7A). It has been reported that visible-light illumination can make the reduction potential of Ni2+ complex shift negatively. 53 We also found that the UV irradiation shifted Ni2+/Ni0 reduction potential by 0.2 eV increasing the difficulty in reduction of Ni2+ complexes. Another possible reason is that light with wavelength around 380 nm causes the 3A2g(F) → 3T1g(P) transition for the distorted octahedral Ni2+ complex (which exhibits light absorption peaked at 378 nm). A fraction of light can be absorbed for this transition; accordingly, fewer hot electrons are generated. Consequently, the reduction of Ni2+ complex, which is critical for the C–O bond cleavage as discussed later, proceeds at a slow rate. Therefore, there is a narrow energy window at about 3.12 eV, in which photons generate hot electrons by interband electron excitation in the Au NPs to reduce Ni2+ ions driving the reaction. The energy alignment in Figures 7C and 7D may not be strict as the alignment is affected by other factors such as temperature, pH, and solvent. However, it provides a good approximation of the alignment. As shown in Figures 7C and 7D, isopropanol acts as a sacrificial electron donor and is oxidized into acetone on the metal particles, 2,15 accomplishing the electron-hole separation in high efficiency.In contrast, the energy level of d band in silver is much lower than that in gold 54 so that the hot electrons generated by interband transition in Ag NPs do not have sufficient energy to reduce Ni2+ ions directly (Figure 7D).The catalysts with Au NPs or Ag NPs have similar architecture but distinct differences in LSPR absorption characteristics. The weak LSPR absorption of the small Au NPs (seen in Figure 2G) can be expected to generate a much weaker EM near-field. The EM field enhancement at the hot spots of small Au NP dimer is also predicted to be weak (Figure S10). Hence, the LSPR absorption is possibly too weak to be the main driving force for promoting the reaction. Light-excited interband transition (from d band to sp band) in the small Au NPs should be main mechanism for generating hot electrons that induce the catalytic reaction.Interestingly, light-induced adsorption phenomenon occurred in 2.5Au-ASN-Ni2+ photocatalytic system as well (Figure S13). It is known that metal NPs produce large field gradients in a wide wavelength range (not only LSPR wavelength) 55 as the oscillating EM field of incident light always changes the charge distribution of metal NPs. Also, electrical charges tend to accumulate in sharp edges and high curvature surface of metal particles (lightening rod effect). The higher charge density at surface of the smaller particles results in sharper gradient around the particles. The two effects are applicable to small Au NPs, leading to the increasing chemisorption under light irradiation.The above observation implies that the reaction is operated via the transfer of energy and light-excited hot electrons from metal NPs to Ni2+ active sites, modulating the oxidation state of Ni2+ in the process. In many reactions catalyzed by the transition-metal complexes, the change in the oxidation state of the transition metal enables the bond-breaking or bond-forming processes (oxidative addition and reductive elimination). 23 The electron paramagnetic resonance (EPR) spectra of the photocatalysts shown in Figure 8 support our inference. The EPR spectra of the photocatalysts (4.3Ag-ASN-Ni2+ and 2.5Au-ASN-Ni2+) with the reactant benzyl phenyl ether and solvent isopropanol (IPA) after purple light irradiation (400 nm of wavelength) exhibits a six-line signal (traces b and d in Figure 8), which is similar to that observed from the ASN-Ni2+ sample after reduction in H2 atmosphere at 200°C (trace e in Figure 8). The g-factor (∼2.01) is close to the reported Ni NPs with a characteristic of paramagnetic behavior. 56 So a fraction of the nickel in this H2 reduced sample exists in the Ni0 state. The paramagnetic signals are not observed in the spectra of other samples, even the system of the photocatalysts in IPA solvent after light irradiation (without the ether, see Figure S14). The results indicate that in the presence of the ether, a Ni2+ → Ni(0) transformation takes place in the photocatalyst under irradiation of 400 nm. The hot electrons generated on the illuminated metal NPs migrate to the Ni2+ complex and reduce Ni2+ ion to Ni0. However, the results shown in trace d in Figure S14 that direct electron transfer from Ag NPs to the Ni2+ complex is not evidenced by the EPR spectrum of the photocatalyst dispersed in IPA solvent in the absence of α-O-4 reactant under irradiation of 400 nm wavelength. It seems that the ether molecules are involved in the reduction of Ni2+ to Ni0. Surprisingly, we observed the Ni2+ → Ni0 reduction after illumination, when the photocatalyst was dispersed in benzotrifluoride (BTF) or toluene (traces f-i in Figure 8) in the absence of the ether, but this phenomenon was not observed when BTF was replaced with N,N-dimethyl-formamide (DMF) (traces j and k in Figure 8). It has been reported that for the C-C stretching modes of unsaturated hydrocarbons, temporary electron transfer from plasmonic metal NPs into the normally unoccupied anti-bonding π∗ orbitals can happen in a SERS system. 57 Therefore, the aromatic ring in the molecules of the aryl ether, BTF, and toluene could serve as a “molecular bridge” for the transfer of the photo-induced hot electrons from the plasmonic metal NPs to Ni2+ sites (inset in Figure 8). The hot electrons have sufficient energy to migrate to the unoccupied anti-bonding π∗ orbitals of the aromatic ring.For the Au-Ni2+ system, the ether molecules are also necessary for the charge transfer process to produce Ni0 state species (comparing the results of trace d in Figure 8 and trace a in Figure S14). So generation of hot electrons on Au NPs surface is a prerequisite but not sufficient condition. The transfer of the hot electrons plays a critical role in the Ni2+ reduction.No products were detected when toluene or BTF was used as the solvent (Table S5), suggesting that the IPA is necessary in the reaction as the hydrogen source for the reductive cleavage. The reducing agent IPA adsorbed on the surface of Ag NPs or Au NPs releases H atoms at the surface and is itself oxidized as a sacrificial electron donor in the presence of KOH and acetone was detected. 2 Using IPA as reduction agent avoids high pressure H2 that has a safety risk. The aryl ether molecules adsorbed at Ni2+ complex sites, and molecules with aromatic rings could assist transfer of the light-generated hot electrons from the illuminated NPs to Ni2+ sites. The reduced Ni0 state is chemically unstable in the reaction environment because the reductive potential of Ni0/NiII (−1.2 V versus SCE) 28 is more negative than that of benzyl phenyl ether (−0.62 V versus SCE, seen in Figure S15). Therefore, the reduced Ni0 sites could catalyze the cleavage of the C–O bonds in the aryl ether compound. Given the dependence of the chemisorption on the EM near-field intensity and the required molecular bridge for the hot electron transfer, the reaction should take place predominately at the Ni2+ complex sites which are simultaneously close to the metal NPs and subject to intense EM fields, i.e., within plasmonic hot spots.The efficiency of electron transfer by benzyl phenyl ether bridge should be also correlated to the ratio of NP to Ni2+. The mean density of the Ag NPs and Au NPs are estimated and summarized in the Table S6 and correlated to mean densities of APTMS and Ni complexes. The distributions of supported NPs and Ni2+ complexes based on the mean densities are presented as below. The densities are averaged data, summarized from TEM images in Figures 2, S8, and S9. The Ag NPs density in 4.3Ag-ASN-Ni2+ sample is 8 times of Ag NPs density in 1.3Ag-ASN-Ni2+ sample. The density of the Ag and Au NPs determines the ratio of NP:Ni2+. The higher the NP:Ni2+ ratio, the higher the efficiency of electron transfer by benzyl phenyl ether bridge. Thus, the electron transfer efficiency in 4.3Ag-ASN-Ni2+ is much higher than in 1.3Ag-ASN-Ni2+ because there are 36 Ni2+ corresponds to per Ag NP in 4.3Ag-ASN-Ni2+ and 508 Ni2+ corresponds to per Ag NP in 1.3Ag-ASN-Ni2+. Similarly, the Au NP:Ni2+ ratio in 8.4Au-ASN-Ni2+ is much higher (1:10) than the other two catalysts, 1:17 and 1:47. The electron transfer by benzyl phenyl ether bridge in 8.4Au-ASN-Ni2+ and 2.5Au-ASN-Ni2+ catalysts is more efficient than that in 0.7Au-ASN-Ni2+ catalyst.On the basis of these results, we propose a tentative reaction pathway for the cleavage, the main features are depicted in Figure 9 . The aryl ether molecules first diffuse within the framework of the photocatalyst and physically adsorb onto the catalyst surface (the adsorption observed in the dark; Figure 5B). Visible-light irradiation induces chemisorption of the ether at Ni2+ complex sites (i-ii), the ether molecules coordinate to Ni2+ ions replacing the NO3− ions. Then photo-generated hot electrons transfer to Ni2+ sites (via the unoccupied anti-bonding π∗ orbitals of the aromatic ring of the ether) and reduce the ions to Ni0 species (iii). The reduced Ni0 species can catalyze the C–O bond cleavage, as described by literature reports, 37,58 yielding the final products (v). Here, IPA is oxidized on the plasmonic metal NPs (iv), similar to the mechanism reported. 2 The IPA oxidation releases hydrogen and electrons, which are needed in hydrogenolysis. The synergistic effect between the optical antenna function of plasmonic metal NPs and the catalytic ability of Ni2+ sites dominates the transformation of the aryl ether. The synergistic effect is accredited to light-induced chemisorption of reactant molecules and the generation and function of the hot electrons as shown in Figure 9.The stability and recyclability of the catalysts investigated. The silane was grafted onto the Al2O3 support in toluene at 110°C, so that the Al–O–Si bond is stable at the reaction temperatures (80°C and 90°C). We compared the contents of Si in 4.3Ag-ASN-Ni2+ and reused 4.3Ag-ASN-Ni2+ photocatalysts measured by the EDX (Figure S16), they are similar (2.2–2.4 wt %). The recycle stability of the photocatalyst is illustrated in Figure S16. The 4.3Ag-ASN-Ni2+ catalyst was reused for 3 cycles of C–O bond cleavage of benzyl phenyl ether under light irradiation. The product conversion moderately decreased during recycling, and the leaching of Ni2+ ions during the reaction caused the decrease in Ni content of the used catalyst. The soluble Ni2+ ions were not active for the hydrolysis at 80°C. The decrease could also be due to the partial loss of Ag NPs on the external surface of aggregates of randomly oriented fibers and leaching of nickel, thus lowering the chemisorption of reactant amount and formation possibility of Ni0 species, as seen in the EDS mapping result displayed in the figure.In summary, with the designed photocatalyst structure we have demonstrated that Au NPs or Ag NPs can act as optical antennas to absorb visible-light and promote the catalytic performance of Ni2+ complex immobilized in the hot spots. The new plasmonic-antenna-promoted catalysts with 2.5 wt % of small Au NPs (∼ 2 nm) or 4.3 wt % of Ag NPs (∼ 9 nm) exhibited superior catalytic activity for the cleavage of relative stable C–O bonds in benzyl phenyl ether under visible-light irradiation and mild reaction conditions without reduction of the aromatic rings. The results signify a new mode to activate chemical reactions by combining the advantages of plasmonic metal NPs and the chemical bond formation ability of transition-metal complexes, which is complementary to known plasmonic catalysis and transition-metal complex catalysis.We have provided strong evidence suggesting that the catalytic performance is enhanced by the synergy of the following two effects: hot-electron transfer to the catalytically active Ni2+ complex sites and light-enhanced chemisorption of reactant species. Both of these effects are greatly augmented by the strong EM near-field localization attainable with plasmonic hot spots. The energy alignment is commonly a key issue for the hot-electron transfer in the new photocatalysts. We believe these results will stimulate further research into the creation of novel plasmonic-antenna-promoted catalysts that exploit the high chemical reactivity of transition-metal complexes.The chemicals were purchased from commercial suppliers and used as provided: benzyl phenyl ether (Sigma-Aldrich, >98%), isopropanol (Sigma-Aldrich, >99.5%, anhydrous), toluene (Fisher, >99.99%, GC assay), N,N-dimethyl formamide (Sigma-Aldrich, >99.8%, anhydrous), α,α,α-trifluorotoluene (Sigma-Aldrich, >99%, anhydrous), acetic acid (Ajax Finechem, >99.7%), nitric acid (Ajax Finechem, 68%–70%), C12-14H25-29O(CH2CH2O)5H surfactant (Sigma-Aldrich), (3-aminopropyl)trimethoxysilane (Sigma-Aldrich, >97%), potassium hydroxide (Sigma-Aldrich, >99.99%), phenol (Ajax Finechem, AR), silver nitrate (Merck, AR), gold chloride trihydrate (Sigma-Aldrich, >99.9%), nickel(II) nitrate hexahydrate (Scharlau, >98%), sodium borohydride (Sigma-Aldrich, >98%), sodium aluminate (Sigma-Aldrich, anhydrous), H2 (Supagas, >99.999%), and Ar (Supagas, >99.99%)The photocatalysts were prepared following the illustration shown in Figure S1. γ-Al2O3 nanofibers were prepared following a procedure reported previously 59 and used as support of the catalysts. Boehmite (with a chemical formula of AlOOH) nanofibers were prepared from NaAlO2 and then converted to γ-Al2O3 fibers by calcination at 450oC for 5 h. The details of the fiber preparation are provided in the Supplemental Information. Amino groups were then grafted on 3.0 g of γ-Al2O3 fibers by refluxing in 50 mL of toluene solution of 9.7 mmol of (3-aminopropyl)trimethoxysilane (APTMS) for 40 h. The solid sample was collected by washing and filtrating with H2O and ethanol, and finally dried at 60oC (Al2O3-silane-NH2 is abbreviated to ASN).Plasmonic Ag or Au NPs were prepared on ASN support by an impregnation-reduction procedure. For example, in the synthesis of 1.3 wt % of Ag NPs on ASN support, 1.0 g of the ASN support was dispersed into 133 mL of deionized water under vigorous stirring with a magnetic stirrer for 20 min. 27.8 mL of 0.01 M AgNO3 aqueous solution was then added to the suspension and stirred for further 20 min. Next, 74 mL of NaBH4 (0.038 M) aqueous solution was added dropwise to the suspension over 30 min under continuous stirring. The suspension was aged overnight, and then the solid was separated by washing with water and dried at 60°C under vacuum. The obtained sample was labeled as 1.3Ag-ASN.The Ni2+ ions were introduced via complexation with the free amine groups of the silane grafted on the γ-Al2O3 fibers. The procedure for introducing Ni2+ ions to Ag-ASN samples is as follows: 0.5 g of obtained sample with supported Ag NPs (xAg-ASN, where x denotes the weight percentage of silver in the catalysts) was mixed with 30 mL of Ni(NO3)2 aqueous solution (0.017 M) by a shaker for 24 h at room temperature. Then the solid was washed with water 3 times before drying at 60oC under vacuum. The catalysts obtained were labeled as xAg-ASN-Ni2+. This procedure immobilizes Ni2+ complexes on the sample surfaces, including the narrow gaps between metal NPs, where hot spots are likely to be generated. The γ-Al2O3 nanofibers are ∼5 nm thick and 100 nm long, which were sintered to form a highly porous framework of randomly oriented fibers. 40 The fibers possessed a large specific surface area of 260 m2·g−1, as seen in Table S2 and Figure S2, where most of the surface area was available for grafting of the silane possessing an NH2 group to form Ni2+ complexes and immobilizing plasmonic NPs. Moreover, the cage-like structure (see Figure 1) could confine the NPs formed within the structure and allow reactant molecules to readily diffuse to the Ni2+ complex reaction sites in the vicinity of the metal NPs through inter-fiber voids.The photocatalytic reaction was conducted in a light reaction chamber. A 10 mL Pyrex glass tube was used as the reaction container. After adding the reactants and catalyst, the tube was filled with argon and sealed with a rubber septum cap. Then the tube was placed above a magnetic stirrer with stirring, and illuminated under a halogen lamp (Philips Industries: 500W, wavelength in the range of 400–750 nm). An air conditioner was set to the light reaction chamber to control the reaction temperature. Reactions were also conducted in the dark at the same temperature for comparison. The reaction temperature in the dark was maintained the same as the reaction under irradiation. All the reactions in the dark were conducted using an oil bath placed on a magnetic stirrer. The tube was wrapped with aluminum foil to avoid exposure of the reaction to light. After the reaction, the mixture was collected and filtered through a Millipore filter (pore size 0.45 μm) to remove the solid photocatalyst. The products were analyzed by an Agilent 6890 gas chromatography (GC) with HP-5 column. An Agilent HP5973 mass spectrometer was used to identify the product. All the products concentrations were calibrated with an external standard method.XRD patterns of the samples were recorded on a Philips PANalytical X’Pert PRO diffractometer using CuKα radiation (λ=1.5418 Å). The working power was 40 kV and 40 mA. The diffraction data were collected from 10° to 80° with a resolution of 0.01° (2θ). FT-IR measurements were conducted on PerkinElmer Spectrum. 2 The samples were prepared in KBr pellets and stabilized under controlled relative humidity before acquiring the spectrum. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were collected by a Nicolet-5700 spectrometer in the wavenumber range between 4,000 and 620 cm−1 at resolution 4 cm−1. The particle size and morphology of the catalyst samples was characterized with a JEOL2100 transmission electron microscope, equipped with a Gatan Orius SC1000 CCD camera. Nitrogen physisorption isotherms were measured at −196°C on the Tristar II 3020. Prior to each measurement, the sample was degassed at 120°C for 24 h under vacuum. The specific surface areas of the samples were calculated by using the Brauner-Emmet-Teller (BET) method and the nitrogen adsorption data in a relative pressure (P/Po) range between 0.05 and 0.2. Varian Cary 5000 spectrometer was used to collect the data for the diffuse reflectance UV-visible (DR-UV-vis) spectra of the samples. EPR spectra were recorded with a Burker EPR ELEXSYS 500 spectrometer operating at a frequency of 9.5 GHz in the X-band mode. Metal content in catalysts were obtained by inductively coupled plasma optical emission spectrometry (ICP-OES) carried out on a R4 PerkinElmer ICP-OES 8300DV instrument. Prior to analysis, the powder samples were dissolved in HNO3 (70%) and diluted by deionized water. Electrochemical measurements were conducted with Bio-Logic SAS potentiostat model VSP. All Raman spectra were taken with a 532 nm excitation laser, 10 s exposure, one-time accumulation, and a Raman imaging setup based on a Renishaw Invia Raman microscope was used.We acknowledge financial support from the Australian Research Council (DP150102110). The electron microscopy work was performed through a user project supported by the Central Analytical Research Facility (CARF), Queensland University of Technology.P.F.H. performed all the experiments. H.Y.Z. and S.S. supervised the project. E.W. and Q.X. provided valuable discussion and revised the manuscript. D.G. provided simulation of EM field. The manuscript was written through contributions of all authors.The authors declare no competing interests.Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.2019.07.022. Document S1. Figures S1–S16 and Tables S1–S6 Document S2. Article plus Supplemental Information

Plasmonic catalysis has drawn significant interest recently, as the catalysis can be driven by visible light. Here, we show a new tactic to apply low-flux visible-light irradiation on plasmonic metal nanoparticles (NPs) to initiate catalysis with surface-bound transition-metal complexes under mild conditions. Ni2+ complexes (as catalytic reaction sites) and Au or Ag NPs were immobilized on γ-Al2O3 nanofibers to produce plasmonic-antenna-promoted catalysts. The light irradiation on Au or Ag NPs enhanced photocatalytic activity of the Ni2+ complexes for reductive cleavage of C–O bond by 18-fold or 17-fold, respectively. The intense electromagnetic near-fields of the plasmonic metal NPs significantly increased the chemisorption of the reactant to the Ni2+ active sites. The light-excited hot electrons transfer via a molecular bridge of the aromatic ring of the reactants. The light-enhanced chemisorption plays a key role in this photocatalyst’s structure that comprises a plasmonic antenna and catalytically active metal complex sites.

Levulinic acid (LA) is regarded as a representing chemical derived from lignocellulosic biomass[1–3]. GVL has been identified as a platform molecule and a promising option for producing value-added chemicals, which can be efficiently obtained through the hydrogenation of LA[4–9]. Besides, GVL can also be used as solvents and fuel additives due to their excellent thermal stability and energy storage ability[10,11].The catalytic reduction of LA to GVL has been reported with various supported metal and bimetal catalysts[12–18]. Ru metal-based catalysts were especially intrinsically in this process owning to their high activity to activate carbonyl groups[8,12,19,20]. Yan et al[21] reported 96% of the selectivity of GVL with 63% LA conversion at 160 °C, 4.5 MPa H2 for 6 h. Almeida et al [22]developed a TiO2-SiO2 hybrid support loaded Ru catalyst. High LA conversion (96%) can be obtained at 130 °C, 3.0 Mpa H2 for 6 h with 5.0 wt% Ru-TS catalyst. In recent reports, a second metal was used in this reaction to reduce the use of noble metal and improve the reaction activity. A 0.5 wt % Ru-5 wt % Ni/MMT bimetal catalyst was developed and 93% conversion of LA can be achieved at 220 °C for 4 h with the pressure of 2.5 Mpa[23]. For the Ru-Sn/C catalyst, the Sn addition can lead to the formation of bimetal Rn-Sn species and stabilized the catalyst. Complete LA conversion at 180 °C with 3.5 Mpa H2 was achieved[24].Zeolites have been applied as a promising catalyst for the conversion of biomass owing to their high thermal stability, complex porous structure, and rich acid sites[25]. Benefiting from these unique structures, it could be an interesting attempt to fabricate the acid-metal bifunctional catalysts. However, diffusional restriction lowers the catalytic performance in both the solvent and gaseous-based liquid phase reaction for biomass upgrading. In this work, we consider using an MCM-49 zeolite as a carrier to prepare highly efficient bimetallic zeolite catalysts. The active center of MCM-49 zeolite was claimed to be located at the 12 MR cups (0.71×0.71×0.91 nm) on the external surface of zeolites[26]. Thus, the more easily accessed sites make this zeolite potential support for biomass conversion which often involves diffusion problems.Herein, Ru-Mn bimetal supported on the MCM-49 zeolite catalyst was designed, converting LA to GVL. It is shown that the Mn promoted Ru/MCM-49 catalysts are efficient catalysts for the hydrogenation of LA into GVL. A large amount of accessible acid sites in layered MCM-49 zeolites is favorable for catalyzing the cleavage of C-O bonds, a key step in the hydrogenation of LA to GVL. The effect of Mn addition on the yield of GVL was investigated. This work contributes to an in-depth understanding of the complicated structure-performance relationship of bimetallic Ru-based zeolite catalysts for the LA hydrogenation reaction.MCM-49 zeolite was obtained by SINOPEC (RIPP). Levulinic acid, γ-valerolactone, ruthenium trichloride hydrate (RuCl3·xH2O, 99.98%), and Mn(CH3COO)2·4H2O (≥99%) were obtained from Sigma-Aldrich. Ru-Mn/MCM-49 catalysts with different weight percent of Mn were prepared by the two-step incipient wetness impregnation. The MCM-49 zeolite was dried at 120 °C for 6 h before impregnation. In a typical procedure, Mn(CH3COO)2·4H2O was dissolved in 4 mL of water, followed by the addition of a calculated amount of MCM-49 support with stirring. The precursor was dried at 60 °C overnight and then calcined at 550 °C for 3 h to obtain the Mn/MCM-49 samples. After that, Ru (2.0 wt%) was loaded onto the Mn/MCM-49 catalyst by incipient wetness impregnation. After impregnation and aged overnight, samples were dried at 80 °C and were further calcined at 400 °C for 2 h. The obtained samples were denoted as Ru-MCM-49, Ru-Mn(x)/MCM-49, where × represents different weight percent (0.2, 0.5, 0.7 and 1.0 wt%) of Mn.X-ray diffraction patterns were measured on a Rigaku X-ray diffractometer with Cu Kα radiation and a tube current of 35 mA. FTIR spectra of zeolites were recorded on the Perkin Elmer's System 2000 IR spectrometer (4000–400 cm−1). Element contents were obtained by ICP-AES on an ICP 6000 SERIES instrument.The N2 adsorption-desorption isotherms were recorded at 77 K using a Micromeritics ASAP 2004 surface area analyzer. The microporous volume, surface area, and pore size distribution were calculated by the DFT method. SEM analysis was performed on a Hitachi 4800 with an accelerating voltage of 20 kV. TEM analysis was carried out on a JEOL JEM-2100FMII microscope and operated at an accelerating voltage of 200 kV. XPS measurements were carried out on Phi510 X-ray photoelectron spectrometer with Mg Kα radiation operating at 250 W.The thermogravimetric study of the samples was carried out on the German NETZSCH STA 449 F3 thermal analyzer, with a heating rate of 10 °C/min (air flow: 50 mL/min). NH3-TPD analysis was performed on a Micromeritics-2920 instrument. The sample was treated at 300 °C for 2 h in He gas. Then, the sample was purged with NH3 in He (30%) at 60 °C for 1 h. TPD was measured in the range of 100–750 °C. Pyridine-adsorbed FTIR spectrum was acquired on a Nicolet Magna-IR 560. A self-supporting wafer with 30 mg of zeolite was loaded on an in-site cell equipped with a CaF2 window. Before pyridine adsorption, the powder zeolite was evacuated at 400 °C under vacuum (P < 10−3 Pa) for 3 h. After equilibration, the sample was evacuated at 200 °C and 350 °C, and data were recorded accordingly.The H2-TPR analysis was carried out on the Autochem-2920 instrument. For one experiment, the sample of 200 mg was pre-treated with N2 for 2 h. The signals were collected by rising the temperature from 50 to 500 °C with a rate of 5 °C/min using H2/He mixture (5% H2). The CO sorption experiment was conducted on a Bruker Vertex 70 FTIR system equipped with a Harrick diffuse reflectory accessory. The sample was reduced in-situ at 400 °C for 2 h. The background data was acquired at 160 °C after purging with He gas. CO saturation for the catalyst was conducted with CO/He mixture (5/45 mL/min) at 160 °C synchronized with the adsorption time. The CO “cut off” experiment was performed using He as the balanced gas (50 mL/min).The in-situ XAFS data were taken at 8-ID (ISS) in the National Synchrotron Light Source II (NSLS II), Brookhaven National Laboratory (BNL). The in-situ measurements were done using a Clausen cell where around 1 mg sample was loaded in a 1.0 mm (OD) and 0.9 mm (ID) quartz capillary.Aqueous-phase LA hydrogenation was conducted in a 50 mL batch reactor under the conditions of 160 °C, 2.5 MPa H2. The as-prepared catalysts were reduced at 10% H2/Ar flow at 400 °C for 2 h prior to the reaction. Subsequently, 10 mL LA aqueous solution (50 g/L) and 50 mg catalyst were added to the reactor, which was then purged with N2 three times. The reactor was then sealed with H2 and heated up to the desired reaction temperature. Reactions were conducted at 160 °C for 180 min under a constant stirring speed of 800 rpm. The collected liquid products were analyzed by Agilent GC-7820 with FID detector.Powder XRD patterns for the reduced catalysts are shown in Fig. 1 . Typical diffraction peaks of MCM-49 can be observed for all the samples. No obvious Ru2O3 phase was found, indicating the highly dispersed Ru species. The crystalline structure of MCM-49 zeolite was also confirmed by the FTIR spectra (Fig. S1). There was no significant morphology change demonstrated by SEM images for the catalyst with different Mn concentrations which can be seen in Fig. S2. The N2 physisorption profiles for samples are shown in Fig. 2 a. The uptake at the P/P 0 = 0.5–0.9 indicates the presence of mesopores in the catalysts. Pore size distribution (Fig. 2b) calculated by DFT method suggests that all the samples present mesopores at 1.9 nm and 2.7 nm. Table 1 gives the porous parameters of samples. Compared with the MCM-49 zeolite, both the microporous area and the external surface area decreased after metal loading. At high Mn loading (˃0.7 wt%), the external surface area of the catalyst significantly decreased while the microporous area was kept unchanged. It can be speculated that the micropores of zeolite can be easily blocked by metal sites located at the micropores and mesopores. In this situation, the excess amount of Mn tends to partly render the diffusion of reacting molecules in the reaction.The physical chemistry parameters of the typical catalysts are collected in Table 2 . The Mn modified Ru catalysts exhibit nearly the same Ru loading about 2.0 wt%. It was observed that the Ru concentration at the surface of the catalyst is much smaller than that at the bulk, which suggests that Ru particles prefer to locate at the micropores of the MCM-49 zeolite. CO chemisorption was applied to estimate the diameter and dispersion state of Ru particles. It was found that the size of Ru particles decreased with the increase of Mn content. High Ru dispersion was obtained by Mn doping. Furthermore, the EDS-mapping images of Ru-Mn(0.7)/MCM-49 in Fig. 3 further revealed the highly dispersed RuO nanoparticles. Consequently, the results illustrated that highly dispersed Ru species occurred in Ru-Mn(0.7)/MCM-49 through the presence of the Mn element. Fig. 4 gives the activities of the tested catalysts for the LA hydrogenation. In previous reports, some kinds of derivatives, such as 1,4-pentanediol, MTHF and pentanoic acid were observed in the LA hydrogenation reaction[27–30]. In this report, GVL was found to be the main product and targeting intermediate 4-hydroxypentanoic acid (4-HPA) was detected. No other products such as methyl-tetrahydrofuran and pentanoic acid formed by hydrogenation products of GVL were observed. When Ru/MCM-49 was used as the catalyst, the LA conversion was relatively low (64%) with the GVL selectivity of 87%. Mn modified samples exhibited enhanced hydrogenation activity. However, LA conversion and GVL selectivity were substantially boosted over Ru-Mn/MCM-49 catalysts as evidenced by Fig. 4. With increasing the Mn content, the LA conversion continuedly increased. When 0.7 wt% Mn was loaded onto Ru/MCM-49 catalyst, the LA conversion was improved significantly to 98%, and high selectivity (100%) to GVL. The high activity of the Ru-Mn/MCM-49 catalyst in this reaction suggests that the catalytic activity is determined by the Mn species. The comparison results in Table 3 from the literature demonstrate the high performance of the Ru-Mn/MCM-49 catalyst for the LA hydrogenation in this study.As shown in Fig. S5, the yield of GVL decreased from 98% in the first run to 63% in the fourth run at the reusability test. However, this stability was much better than the Ru/C catalyst in the literature[31,32]. Further, the TGA results (Fig. S6) showed that the carbon deposit was the important reason for the loss of activity. The mass loss between 200 °C and 400 °C was the adsorption of small amounts of residual organic species or carbon deposits of surface metal potentials on the catalyst. In the TGA profile, a significant weight loss in the range of 200 °C – 400 °C can be seen, indicating that carbon deposition mainly occurs on the surface of the catalyst. The carbon deposits covered the active metal sites on the surface of the catalyst so that the activity of the reaction was reduced.The acid sites of the catalysts were evaluated by NH3-TPD and Py-FTIR studies. NH3-TPD plots of samples, shown in Fig. S3, exhibit that all the catalysts present similar acid amounts and acid strength. The desorption peaks at the temperature range (220–330 °C) are ascribed to the weak acid sites while NH3 desorption peaks at higher temperatures (380–460 °C) are assigned to strong acid sites. For the impregnated zeolites catalysts, the acid properties of the catalyst were mainly determined by supports[33]. In some reports, a slight change in the acid sites can be observed for the Ru exchanged zeolites[34,35]. In this study, due to the low Ru metal loading, MCM-49 zeolite contributed to the main acid sites, including the weak and strong acid sites, and all the samples showed similar amounts of acid sites. The Py-FTIR spectra of all samples were carried out to estimate the amounts of Brønsted acid sites and Lewis acid sites. As shown in Fig. S4, the band at 1540 cm−1 from the bonding of Brønsted acid sites appeared on the sample Ru/MCM-49. The quantitive results are listed in Table 4 . For the various Ru-Mn/MCM-49 catalysts, the amounts of the Brønsted acid sites of catalysts decreased with the Mn promotion. The band at 1445 cm−1 and 1605 cm−1 can be assigned to Lewis acid sites. The L/B ratio was also calculated on the basis of the two kinds of acid sites. It can be found that the amount of Lewis acid sites is rather stable for all the catalysts, but the L/B ratio increases significantly with Mn modification. To identify the intrinsic activity of the catalyst, the turnover frequency (TOF) was calculated and was listed in Table 5 . The Ru-Mn(0.7)/MCM-49 catalyst exhibited the highest TOF value of 1529 h−1, which was 1.6 times higher than that of the Ru/MCM-49 catalyst.The reaction pathway for the LA hydrogenation was shown in scheme 1 . The hydrogenation of levulinic acid into 4-hydroxypentanoic acid (4-HPA) followed by the dehydration of 4-HPA into GVL are the two main steps of the reaction. The first step of the reaction is the hydrogenation reaction which requires metal sites and for the dehydration reaction (second step) the active component is acid sites[14,36–39]. The relatively high TOF among the test catalysts indicated that a high L/B ratio clearly had a positive effect on the hydrogenation reaction. Because the Lewis sites preferentially interact with the CO bonds of LA, the L/B ratio is closely related to this reaction. However, an increase in Brønsted acid acidity (Ru/MCM-49) led to the significant generation of the intermediate product 4-HPA, with a concomitant decrease in the GVL selectivity as shown in Table 5. As a result, the Lewis acid sites originated from the zeolite, enabling the activation of LA and the followed dehydration reaction, improving the efficiency of this reaction.In Fig. 5 , H2-TPR profiles are present for the catalysts. Main reduction peaks at 110–120 °C were observed for all the catalysts. The reduction peak lower than 150 °C can be ascribed to the reduction of highly dispersed Ru particles located at the external surface of zeolite[40]. Obviously, the Ru existed as RuOx in the MCM-49 zeolite support, which can be reduced at relatively low temperatures. The value of the reduction temperature for the ruthenium species was aligned with the results reported for Ru/zeolite and Ru/SiO2 [41,42]. The Mn loading can efficiently improve the reduction temperatures of the catalysts. The highest reduction temperature was obtained for the sample Ru-Mn(0.7)/MCM-49, indicating the presence of strong metal-support interaction in the sample.To further elucidate the dynamic changes in the local coordination environment, in-situ XAFS characterizations were performed to investigate the chemical state and local structure of Ru in the reduced catalysts. As should in Fig. 6 (a), both reduced samples and Ru foil show very similar XANES structures, indicating the samples have been fully reduced to metallic Ru. Moreover, the Fig. 6(b) EXAFS data give a similar conclusion that both reduced samples have average local structures very similar to metallic Ru. These results demonstrate that the phase of Ru can be fully reduced as observed in the XANES results.XPS characterizations were performed to investigate the chemical state of the catalysts. Fig. 7 shows the major C, O, Mn, Ru, Si and Al peaks, the Ru 3p spectra, and the Ru 3d spectra. Ru 3d (Fig. 7b) was not suitable to determine the chemical state of Ru species which was easily obscured by carbon C 1 s peak[43–45]. As shown in Fig. 7c, the binding energy of 461.8 eV designed at Ru 3p3/2 evidenced the metallic state of Ru(0) for Ru-Mn/MCM-49. After doping Mn, the binding energy of Ru 3p3/2 shifted to a low value, and it becomes 461.3 eV for Ru-Mn(1.0)/MCM-49 catalyst. The change of peak value indicates that electronic interaction excites between Ru and Mn species. With the increase of Mn content, the enhancement of electron density of metallic Ru can be inferred because lower binding energy of Ru 3p was observed. The shift of binding energy for Ru species suggests the catalysts exhibit higher H2 dissociation ability after loading Mn[46]. In-situ CO-Drifts were further employed to reflect the sensitives of environment change and the electronic state of the adsorption site by the CO vibration frequency. The spectra of CO adsorption on the Ru/MCM-49 and Ru-Mn(0.7)/MCM-49 catalysts were obtained in Fig. 8 . Low adsorption band at 2050 cm−1, high adsorption bands around 2115 cm−1, and high adsorption bands around 2171 cm−1 are observed. The band at 2171 cm−1 can be designed as the CO molecular in the gas phase rather than the multicarbonyl species on Run+ as reported in previous report[47], which is also demonstrated by the CO– cut-off experiment as shown in Fig. S5. The band at 2056 cm−1 was assigned to the Ru particles. The peak at higher wavenumber (2115 cm−1) falls in the range of high dispersed Ru metal sites. In this work, two different kinds of dispersed Ru sites were confirmed. It is clear that the addition of Mn changes the state of metallic sites on the Ru-Mn/MCM-49 catalyst. The relative percent of Ru particles in Ru/MCM-49 was higher than that in Ru-Mn/MCM-49, which was in agrees with the H2-TPR results. Then a high GVL selectivity was observed for the sample Ru-Mn/MCM-49. It has been reported by Novodárszki et al[48] that the LA conversion was closely related to the adsorption state of the surface intermediate and the GVL selectivity can be enhanced by the efficient intermediate adsorption and the high surface H coverage. However, too strong H coverage may decrease the GVL selectivity by further hydrogenation. In the LA hydrogenation reaction, hydrogen was adsorbed on the surface of Ru through the formation of hydrogen bonds between hydrogen and Ru. Hence, an increase of the high dispersed Ru species was beneficial to the dissociation and activation of H2 on the catalyst surface; thus, the reaction rates for the hydrogenation of LA can be enhanced. Hence, the results of in-situ CO-DRIFTS confirm that Ru species exists as high dispersed metallic species in the sample Ru-Mn/MCM-49, in agreement with the above H2-TPR and EXAFs observations.Bifunctional Ru/MCM-49 catalyst modified by Mn species was developed for the hydrogenation of LA to GVL. XPS result revealed the presence of Mn-Ru interaction with the Mn modification. The shift of position for metallic Ru 3p to lower energy suggested the catalyst exhibited higher H2 dissociation ability by Mn addition. It was also found that the amount of Brønsted acid sites for the sample Ru-Mn/MCM-49 was significantly decreased, which could be beneficial for the formation and stabilization of GVL. Doping of Mn to Ru-MCM-49 remarkedly improved the activity, and the activity of the catalyst was related closely to the Mn concentration. Ru-Mn(0.7)/MCM-49 catalyst exhibited the highest activity, with the TOF value of 1529 h−1. Under the reaction conditions, 98% LA conversion and 100% GVL selectivity were achieved. The electron density of metallic Ru can be enhanced by the addition of Mn which improves the dissociation ability of Ru sites in the reaction as revealed by XPS and CO-DRIFTS. The stable adsorption and activation of LA and H2 can efficiently proceed on the metallic Ru sites facilitating the hydrogenation reaction. Wenlin Li: Conceptualization, Funding acquisition, Supervision. Feng Li: Investigation. Xin Ning: Investigation. Kaixi Deng: Investigation, Methodology. Junwen Chen: Investigation, Supervision. Jiajun Zheng: Data curation. Ruifeng Li: Resources.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Financial support from the State Key Laboratory of Catalytic Materials and Reaction Engineering (RIPP, SINOPEC, No. 18-ZC0607-0007) is gratefully acknowledged. This research used resources 8-ID (ISS) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.Supplementary data to this article can be found online at https://doi.org/10.1016/j.crcon.2022.05.003.The following are the Supplementary data to this article: Supplementary data 1

Selective hydrogenation of Levulinic acid (LA) to γ-Valerolactone (GVL) is an important reaction to produce high value-added chemicals and fuels but remains a big challenge. Herein we reported a Ru/zeolite catalyst with Mn promotion, which exhibited excellent catalytic performance (yield: 98%) towards LA to GVL. The intrinsic activity (TOF) also increased obviously with the Mn addition. The particle size of Ru gradually decreased with the increase of Mn loading and a strong interaction between Ru and support was observed for the Ru-Mn/MCM-49 catalyst. The addition of Mn not only offered a good dispersion of Ru species on MCM-49, but also increased the L/B ratio of the catalyst, thereby contributing to the high GVL selectivity. High dispersed Ru sites were the intrinsic active sites of the catalyst verified by the in-situ experimental studies. The dissociation of the reactants was significantly enhanced, resulting in higher catalytic activity.

H2 is considered an ideal clean energy carrier to replace traditional fossil fuels. 1–4 Electrocatalytic H2 evolution reaction (HER) from renewable energy by water splitting is a promising technology for efficient H2 production. 5–8 Various Earth-abundant metal-based catalysts with considerable HER activities have been broadly reported to replace precious platinum group metal (PGM) electrocatalysts. 9–14 Unfortunately, a large gap still exists between non-precious metals and PGM catalysts toward HER, owing to the lack of efficient active sites. The occurrence of too weak (e.g., for Cu and Zn) or too strong (e.g., for Co, Ni, and W) H adsorption results in a lower reaction rate through affecting the initial H adsorption and/or the ultimate molecular H2 desorption from the catalyst surface. 15 In particular, metallic Ni and its alloys are widely regarded as the most promising non-precious-metal electrocatalysts alternative to PGM for industrial-scale water electrolysis; yet, the strong H adsorption on Ni has severely affected its durability. 16 The Fe triad elements (i.e., Fe, Co, and Ni) share similar chemical and physical characteristics, are abundant on Earth, and have been popularly used as catalysts for many hydrogenation applications. 17 , 18 For example, metallic Co is the most widely used catalyst in Fischer-Tropsch synthesis (FTS) for converting H2 and carbon monoxide from coal or natural gas into long-chain hydrocarbons. 19 , 20 However, metallic Co and Fe typically show poor activity toward HER, although Co- and Fe-based chalcogenides, 21 nitrides, 22 carbides, 23 and phosphides, 24 and their interaction with heteroatom-doped carbon and macrocyclic Co complexes 25 , 26 have been reported as having considerable activity for HER. Co can exist in two crystallographic structures: the hexagonal close packed (hcp) and the face-centered cubic (fcc) phase. Many reports have revealed that the hcp Co is more active than fcc Co, due to the intrinsic stacking faults that offer a greater number of active sites. 27 , 28 Nevertheless, the large size of nanoparticles (NPs) with limited surface area and strong H binding over Co greatly slow down the desorption of H to evolve H2 and limit the HER kinetics. To achieve enhanced HER catalytic performances on Co, downsizing the catalysts to fine NPs and creating disordered structure to engender more exposed active sites, along with optimized electronic structures for H binding, are critical.Here, we show a hcp metallic Co catalyst modulated by vanadium oxide (VOx) clusters (denoted as Co(VOx)) with extraordinary HER activity as well as an atomic-level understanding of the activity origins. The VOx-modulated metallic Co catalyst ((Co(VOx)-y, y is the V5+/Co2+ precursor molar ratios) is prepared by a facile electrodeposition process. Due to the strong interaction between Co and VOx, the crystallographic, electronic structure, and coordination environment can be rationally regulated at atomic scale by adjusting the doping levels during the electrodeposition process. The VOx doping plays an important role for achieving a refined Co NPs and highly disordered Co structure. The optimal Co(VOx)-3% catalyst exhibits extraordinarily low overpotentials and high activity and stability toward HER, which are distinctly different from the poor HER activity and durability of metallic Co. X-ray absorption spectroscopy (XAS), X-ray crystallography, operando Raman spectroscopy, and density functional theory (DFT) calculations are applied to study the roles of VOx clusters, uncovering the highly disordered structures and partial electron transfer from Co to VOx, which dramatically decreases the H adsorption on V-Co(001) to achieve enhanced HER.The Co(VOx)-y (y = 0%, 1%, 3%, and 6%) catalysts with different doping levels are electrodeposited directly on Ni foam (NF) substrate, and all of the electrodes after electrodeposition show a black color. The X-ray diffraction (XRD) pattern of Co and Co(VOx) samples electrodeposited on carbon fiber paper to remove the effect of diffraction signals from NF were collected by using the Empyrean PANalytical diffractometer in the grazing incidence measurement mode. As shown in Figure 1 D, for pure Co, the characteristic peaks located at 41.5°, 44.5°, and 47.3° are indexed to the (100), (001), and (101) planes of hcp metallic Co (joint committee on powder diffraction standards [JCPDS]: 05-0727), 20 respectively. VOx doping has a remarkable influence on the crystal structure of Co. With the increment of VOx doping, the two characteristic peaks at 41.5° and 47.3° almost disappear, while the (001) peak broadens and decreases sharply, indicating the formation of a poorly crystallized Co structure and refined NPs, as illustrated in Figure 1A.To identify the structural transformation of Co by VOx doping, the electrodeposition behavior of the Co(VOx)-y electrodes was analyzed. The representative Co(VOx)-3% electrode is discussed in the following sections, unless stated otherwise, as it shows the highest HER activity. Figure S1 shows that pure Co was electrodeposited at a more positive potential than that for pure VOx deposition from the NH4VO3 precursor. However, the deposition of VOx can significantly influence the Co crystallization, as evidenced by a negative shift of the electrodeposition potential for Co(VOx)-3%. This negative shift in potential indicates that VOx regulates the nucleation and crystallization process of metallic Co. Because the formed VOx are ultrafine NPs in nature (see scanning electron microscopy [SEM] information below), thus, the crystallization structure and size of Co can be well regulated by VOx.The morphology of the obtained catalysts was first characterized by SEM. Figure S2 shows the SEM image of uniformly and densely distributed Co NPs on NF. At high magnifications, it is revealed that the size of NPs is in the scale of tens of nanometers. For the electrodeposited VOx, the NF substrate is uniformly covered with the composite constituted by a large number of NPs with sizes of merely several nanometers (Figure S3). Figure S4 depicts the different magnified SEM morphology for Co(VOx)-3% with respect to the individually deposited Co or VOx. In the presence of vanadium, the well-ordered structure of Co NPs was transformed into an irregular nanocluster structure composed of fine NPs with a size of few nanometers, indicating the crystalline refinement effect of VOx on Co. Transmission electron microscopy (TEM) was used to learn more about the structure of Co(VOx) composites. As seen in Figures 1B and S5, the TEM morphology of pure Co shows large NPs with sizes ranging ∼30–50 nm. The clear lattice patterns and diffraction rings from the selected area electron diffraction (SAED; inset in Figure 1B) both indicate the well-crystallized Co structure. The TEM images of the Co(VOx)-3% sample reveal the uniformly distributed fine NPs structure with a size of ∼5–10 nm (Figure 1C). The high-resolution TEM (HRTEM) image in Figure 1C shows a lattice fringe of 0.205 nm, which is consistent with the (001) plane of Co and corresponds well with the SAED pattern (Figure 1C, inset). However, the poorly defined rings in SAED demonstrate the poor crystalline phases and a disordered Co structure. The elemental distribution was confirmed by TEM-energy dispersive spectroscopy (EDS) mapping, in which all the elements are uniformly distributed throughout the whole sample (Figures 1E–1H). In addition, the HRTEM morphologies of Co(VOx)-1% and Co(VOx)-6% samples (Figures S6 and S7) depict Co NPs with sizes between ∼15–20 nm and ∼5–10 nm with good and poor lattice fringe for Co (001) plane, respectively, demonstrating that both the morphology and crystal structure of Co NPs can be well tuned through VOx modulation.X-ray photoelectron spectroscopy (XPS) was used to investigate the valence states of the as-prepared Co and Co(VOx) samples. As seen from Figure S8, in the Co2p region, the two main peaks for Co2p3/2 and Co2p1/2 are located at 781.2 eV and 797.3 eV, accompanied by two shake-up satellite peaks (786.6 eV and 803.1 eV), demonstrating an oxidized Co surface. In addition, an enhanced oxidation degree of Co after the increment of the doped VOx causes a slightly negative energy shift in Co2p configuration. 29 The high-resolution XPS of the V2p region shows characteristic peaks at 517.1 eV and 516.4 eV assigned to V5+ and V4+ species, 30 and a slightly negative shift of V2p was observed with increasing VOx doping. The energy shift on Co2p and V2p reveals a direct interaction between Co and VOx and a charge transfer from Co to VOx. 31 The O1s XPS spectra show a main peak at ∼531.1 eV, corresponding to O in VOx and the oxidized Co-O in the composite. It is noted that the metallic Co peaks were not observed in the XPS spectra because of the surface oxidized layers.The HER performance of the as-prepared Co(VOx) catalysts with different VOx concentrations (0%–6%) along with the commercial 20 wt % Pt on carbon black (Pt/C) electrodes are evaluated in 1 M KOH without ohmic potential drop (iR) correction (Figure 2 A). Linear sweep voltammetry (LSV) of all the prepared Co(VOx) electrodes show significantly enhanced HER activity compared to the bare Co electrode. In particular, the Co(VOx)-3% electrode shows the best HER activity, suggesting the electrodeposited VOx with optimal doping plays an important role in the activity enhancement. Specifically, to deliver a current density of −100 mA cm− 2, the Co(VOx)-3% electrode requires an overpotential of merely 178 mV, which is much smaller than 344 mV and 275 mV required for Co and Co(VOx)-1%, respectively, and 203 mV for Co(VOx)-6%. The obtained performance of Co(VOx)-3% is comparable or outperforms recently reported state-of-the-art noble-metal-free HER catalysts, although it still shows a larger overpotential than Pt/C catalyst (93 mV) (Table S1). The corresponding Tafel slopes were derived from the LSV curves to investigate the reaction kinetics. As shown in Figure 2B, the Co(VOx)-3% and Co(VOx)-6% electrodes have similar Tafel slopes of 40 and 36 mV dec−1, respectively, which are much smaller than those of Co(VOx)-1% (60 mV dec−1) and Co (125 mV dec−1), suggesting that HER follows the Volmer-Heyrovsky mechanism at the Co(VOx) electrodes. Also, a Tafel slope of 23 mV dec−1 was obtained for the Pt/C electrode. The significantly decreased Tafel slope of Co(VOx)-3% and Co(VOx)-6% indicate fast intrinsic kinetics of HER due to the formation of the fine and disordered Co structure (see below).The long-term electrocatalytic durability is a pivotal parameter for a HER electrode. Figure 2C shows that the Co(VOx)-3% catalyst retains a stable HER activity for over 50 h. In contrast, the bare Co electrode shows a noticeable activity decay during the continuous HER operation. Additionally, the almost overlapped LSV curves obtained on the Co(VOx)-3% electrode before and after stability demonstrate the robustness of the Co(VOx)-3% electrode (Figure 2D). The stable activity of the catalyst was further studied by the post-HER characterizations. As shown in Figure S9, the almost unchanged morphology of the fine NPs and the retained uniform distribution of the constituting elementals both indicate the rigid structure of the Co(VOx)-3% catalyst, and the HRTEM also reveals the existence of the lattice fringe for metallic Co.To understand the origin of the enhanced HER activity on Co(VOx) catalysts, the electrochemical active surface area (ECSA) was first determined by measuring the double-layer capacitance (C dl) of the electrodes derived from cyclic voltammetry (CV) curves in a non-Faraday region with different sweep rates (Figure S10). 32 The C dl values obtained on Co(VOx) electrodes are all larger than that on pure Co (12 mF), with the Co(VOx)-3% electrode being the largest (30 mF), followed by Co(VOx)-6% (24 mF) and Co(VOx)-1% (17 mF) (Figure 2E). These results demonstrate that the fine NPs and disordered structure provide more active sites for HER. Moreover, the intrinsic catalytic activity of each active site was evaluated by normalizing current against ECSA. Figure S11 shows that the Co(VOx)-3% electrode still exhibits a higher HER activity than Co and Co(VOx)-1%, demonstrating the profound role of VOx in enhancing the intrinsic activity of each active site. It should be noted that the ECSA normalized HER activity of Co(VOx)-3% and Co(VOx)-6% electrodes almost overlapped, suggesting that the more exposed active sites are contributing in the improved HER on Co(VOx)-3%, and this observation is consistent with the projected DFT simulations (see below). In addition, the charge transfer process of the prepared samples was investigated by electrochemical impedance spectroscopy (EIS), and an equivalent resistor-capacitor circuit model (R s, resistor; R ct, charge transfer resistance; C, capacitance) was used to fit the impedance spectra. As shown in Figure 2F, the EIS spectra reveal a significantly smaller R ct for the Co(VOx)-3% electrode (12 Ω) than that of Co(VOx)-6% (22 Ω), Co(VOx)-1% (81 Ω), and Co (107 Ω), verifying the fast electron transfer kinetics of HER on Co(VOx)-3%. Furthermore, the Faradaic efficiency close to unity is obtained on the Co(VOx)-3% electrode by measuring the generated H2 gas by using gas chromatography (Figure S12).To obtain further information on the elemental oxidation states and in particular to determine the influence of VOx on the atomic structure of Co and eventually the HER performance, the Co K-edge X-ray absorption near edge structure (XANES) was studied in detail. As seen from Figure 3 A, the XANES spectra of Co and Co(VOx)-1% contain similar pre-edge characteristic features, referring to an oxidation state close to metallic Co for Co in these catalysts. With increasing the VOx amount, both the positively shifted pre-edge peak and higher white line intensity demonstrate a higher oxidation degree of Co (Figure 3B). In other words, the Co(VOx)-3% and Co(VOx)-6% catalysts are oxidized and disordered more at higher VOx doping levels. Figure 3C shows the resulting Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) for all prepared samples. There are fewer scattering contributions from the Co-O bond for Co and Co(VOx)-1%, but the other three nearest-neighbor (NN; 2nd–4th) shells show strong intensity, which indicates the formation of metallic Co with highly ordered atomic structure. 33 It should be noted that from XPS spectra, we observed that these two samples are oxidized, but the XAS spectra reveal a metallic phase structure. This is because XPS is a surface-sensitive technique, whereas XAS is designed for bulk-averaged atom-atom correlation information. With the increment in VOx doping, the significantly reduced intensity amplitude for Co(VOx)-3% and Co(VOx)-6% demonstrates the existence of an abundant disordered structure. Furthermore, the increased 1st NN shell intensity reveals the higher oxidation degree, whereas the decreased 2nd–4th NN shell intensity indicates a much less efficient atomic packing and a greater structural disorder of metallic Co due to the regulation effect of VOx, which corresponds well to the XRD and XPS results. Ex situ experiments yield valuable information on the chemical nature of Co(VOx); yet, the origin of catalytic activity in these NPs remains largely unknown. To address this issue, operando Raman spectroscopic experiments were performed to detect structural changes during HER conditions. To conduct operando experiments, the catalyst was held at the applying potential for 10 min before acquiring the spectrum on Co(VOx)-3% and Co electrodes. As shown in Figure 3D, several intense peaks are observed at 518 cm−1, 679 cm−1, and 806 cm−1 on Co(VOx)-3% under open circuit potential (OCP). The detected bands at 518 cm−1 and 679 cm−1 are assigned to Co oxide, which correspond to F1 2g and A1g vibrations, respectively. 34 The Raman peak at 806 cm−1 is attributed to O-V-O stretching vibrations. As the potential was increased to more negative values, e.g., −1.1 V, the vibration band peak at 679 cm−1 almost disappeared, whereas the characteristic peak at 518 cm−1 weakened, which is attributed to the reduction of the Co oxide layer. The intensity of the VOx peak at 806 cm−1 was also decreased, which is mainly due to the dissolution of loosely bonded or physically adsorbed VOx in KOH. It should be noted that the Co-O-V features cannot be totally removed, as the peak at 806 cm−1 remains with increasing the overpotential. This is further evidenced by the inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis (V/Co = 2.7% for fresh sample versus V/Co = 1.4% after stability) as well as the V/Co atomic ratio of 1.2% from TEM-elemental spectra (Figure S9F) after long-term stability test. In addition, the operando Raman spectra of the pure Co electrode shows two well-defined Raman peaks at 523 cm−1 and 691 cm−1 for Co oxide under the OCP condition (Figure 3E). However, the peak intensities are much lower than those of the Co(VOx)-3% NPs, and a similar trend for the reduction of the Co oxides can be found when higher negative potentials are applied.From the above results, it can be concluded that under HER conditions, Co oxides are first reduced to expose Co-O-V active sites to electrolyte for H2 evolution. This is further proven by applying a negative potential at different LSV cycles on the Co(VOx)-3% electrode, for which a profound Co oxide reduction peak was observed during the first 3 cycles (Figure 3F), demonstrating the fast reduction of Co oxide for the exposure of Co-O-V active sites toward HER. It is noted that the developed electrodeposition approach can be easily extended to other conductive substrates, such as carbon fiber paper and copper foam. As seen from Figure S13, similar performances were achieved as those on the NF substrate, where the Co(VOx) electrode always shows a significantly enhanced HER activity compared with the electrodeposited bare Co electrode, demonstrating this is a general synthetic approach to fabricate active Co(VOx) electrodes on conductive substrates.Understanding the doping effect of the oxidized V (VO4 cluster) on the HER activity on Co(001) catalysts as the determined structure of the catalysts was followed by a series of DFT calculations to elucidate the HER on Co(001) and V-doped Co(001) (V-Co(001)) catalysts under different VO4 coverage levels (θ = 0.25, 0.50, and 0.75). The structural model of bare Co(001) was constructed as a 4 × 4 periodic supercell (Figure S14). We considered the key reaction steps in alkaline HER, including the water dissociation reaction and the adsorption/combination of reaction H intermediates (H∗). Figure S15 shows the calculated reaction energy diagram of water dissociation on Co(001) and V-Co(001) (θ = 0.25). The energy barriers for water dissociation are similar on these two surfaces (0.90 eV and 1.01 eV on Co(001) and V-Co(001) (θ = 0.25), respectively), which are quite smaller than those for other active catalysts for alkaline HER, such as Ni2P/NiTe2. 7 These results indicate that the water can be efficiently dissociated on both Co(001) and V-Co(001) surfaces with similar energy barriers.Then, we considered all possible adsorption sites for H∗ closed to the VO4 cluster (Figure S16) and all possible stable structures of V-Co(001) with θ ≥ 0.50 (Figures S17 and S19). Figure 4 D shows the calculated free energy diagram for HER on bare Co(001), V-Co(001) (θ = 0.25-0.75), and V-Co(001) (θ = 0.50) with the highest HER activities, which are shown in Figures 4A, 4B, S18, and S20. For the bare Co(001), the Δ G H ∗ is highly negative (−0.32 eV), which indicates a strong interaction between H∗ and Co(001), 35 manifesting poor HER reaction kinetics. Introducing a low coverage of the VO4 cluster on Co(001), namely V-Co(001) (θ = 0.25), significantly increased the value of Δ G H ∗ to −0.25 eV, suggesting an enhanced HER activity compared to bare Co(001). The deformation electronic density calculation (Figures 4C and S21) and Bader analysis show that the VO4 cluster has led to an increase in electronic charge density on VO4 and a loss of electron charge density on the surrounding six Co atoms, and there are 0.23 electrons transferred from each Co to VO4 based on the Bader analysis. Fewer electrons localized on Co sites closest to VO4 results in the weak H adsorption on V-Co(001) and thus the enhanced HER activity. Moreover, we found that a further increase of the coverage of VO4 clusters on the Co(001) surface can further increase the values of Δ G H ∗ . For example, the Δ G H ∗ for V-Co(001) (θ = 0.50) and V-Co(001) (θ = 0.75) are −0.12 eV and 0.14 eV, respectively. These results indicate that the HER activities of V-Co(001) with a high VO4 coverage (θ ≥ 0.50) are much higher than those with low VO4 coverage (θ ≤ 0.25). In addition, considering that the number of active sites for V-Co(001) (θ = 0.75) (Figure S20) are fewer than those of V-Co(001) (θ = 0.50) (Figure S18), we conclude the HER activity follows the order of V-Co(001) (θ = 0.50) > V-Co(001) (θ = 0.75) > V-Co(001) (θ = 0.25) > Co(001), which is consistent with experimental results. It has to be noted that the binding of reaction intermediates on low-coordinated metal atoms is stronger than that on high-coordinated metal sites. 36 The small particle size of Co can significantly increase the proportion of low-coordinated metal atoms, such as step atoms, kink atoms, and vacancies, which can increase the binding strength of H∗. Hence, the absolute value of adsorption energy of H∗ should be decreased as Co particle size increases. In other words, the downsized Co NPs should have a stronger H∗ adsorption and decrease HER activity. Although the downsized Co particle itself has a strong H∗ adsorption energy, for our developed Co(VOx) catalyst, the VOx modification can significantly decrease the H∗ adsorption energy compared with that of bare Co, thus leading to the excellent HER performances.In summary, a Co(VOx) catalyst was developed with a refined NP size, disordered structure, and a significantly enhanced alkaline H2 evolution activity. The best Co(VOx) electrode can only be achieved at appropriate VOx doping levels to offer suitable H binding as well as abundant active sites during HER. This study provides a promising approach for achieving notable HER performance with metallic Co, which is known to be a poor catalyst for HER, and sheds light on the crucial role of atomic structure modification through a facile VOx engineering strategy. The development of highly active metallic Co-based catalysts for HER would find applications in the water electrolysis industry and also potentially provide a multifunctional catalyst for the FTS industry for which metallic Co is currently most widely used for the synthesis of valuable long-chain hydrocarbons from H2 and carbon monoxide. The proposed approach can also be potentially extended to other non-active metallic Fe triad materials for developing efficient low-cost electrocatalysts for H2 production.Further information and requests for resources should be directed to the Lead Contact, Prof. Chuan Zhao ([email protected]).This study did not generate new unique materials.The authors declare that data supporting the findings of this study are available within the article and the Supplemental Experimental Procedures. All other data are available from the lead contact upon reasonable request.The Co(VOx) electrodes were prepared by a direct electrodeposition approach in a three-electrode system with the NF as the working electrode, graphite plate as the counter electrode, and a double junction saturated calomel electrode (SCE) as the reference electrode. The NF was cleaned in a 5 M HCl solution for 10 min and rinsed Milli-Q water before use. The electrodeposition was carried out in an electrolyte consisting of 0.45 M CoCl2·6H2O, 4.5 to 27 mM (V/Co = 0%–6%) NH4VO3, and 0.35 M H3BO3, by dissolving the chemicals in 50 mL Milli-Q water under sonication. The electrodeposition was performed on a CHI 760D electrochemical workstation at –1.9 V (versus SCE) for 600 s. The Co(VOx) mass loading on NF was 2.3 ± 0.4 mg cm−2. The preparation of other control samples can be found in detail in the Supplemental Experimental Procedures.All the electrochemical measurements were carried out on a CHI 760D electrochemical workstation by using the prepared Co(VOx) and Co electrodes on NF as the working electrode and graphite plate and double junction SCE (saturated KCl) as the counter and reference electrode, respectively. All potentials measured were calibrated to the reversible H2 electrode (RHE) through the following equation: E RHE = E SCE + 0.241 V + 0.059 pH. The linear sweep voltammetry (LSV) polarization curves were recorded at a scan rate of 3 mV s−1 in 1 M KOH without iR compensation. Chronoamperometric measurements were obtained under the same experimental setup. EIS spectra were measured at 150 mV overpotential in the frequency range of 0.1–100000 Hz with an amplitude of 10 mV in 1 M KOH electrolyte.All of the spin-polarized DFT calculations were performed using the vienna Ab initio simulation package (VASP) program, 37–39 which uses a plane-wave basis set and a projector augmented wave method (PAW) for the treatment of core electrons. The Perdew, Burke, and Ernzerhof exchange-correlation functional within a generalized gradient approximation (GGA-PBE) 40 was used in our calculations. For the expansion of wave functions over the plane-wave basis set, a converged cutoff was set to 450 eV. Spin-polarization effect and dipole correction were considered in all cases.The structural model of bare Co(001) was constructed as a 4 × 4 periodic supercell (Figure S14A), which contains four atomic layers with the bottom two layers fixed in their respective bulk positions and all the other atoms fully relaxed. Experimental observations show that V-dopant atoms are oxidized and adsorb on the Co(001) surface. In order to simulate V-doped Co(001) catalysts, VO4 clusters are added to the Co(001) surface to model V-doped Co(001) catalysts with different VO4 coverages (V-Co(001) (θ = 0.25, 0.50 and 0.75)). For example, Figure S14B shows the simulation model of V-Co(001) (θ = 0.25). The vacuum space was set to larger than 18 Å in the z direction to avoid interactions between periodic images. In geometry optimizations, all the structures were relaxed up to the residual atomic forces smaller than 0.005 eV/Å, and the total energy was converged to 10−4 eV. The Brillouin zone was sampled using 3 × 3 × 1 Γ-centered mesh. The deformation electronic density of the V-Co(001) was defined as Δρ ( r ) =  ρ ( r ) V − Co ( 001 ) − ρ ( r ) Co ( 001 ) − ρ ( r ) VO 4 , where ρ ( r ) V − Co ( 001 ) represent the charge density of the V-Co(001) system, and ρ ( r ) Co ( 001 ) and ρ ( r ) VO 4 represent the charge density of the bare Co(001) and the VO4 cluster at the same coordinates as those in the V-Co(001) system, respectively.The overall HER mechanism was evaluated with a three-state diagram consisting of an initial H+ state, an intermediate H∗ state, and 1/2 H2 as the final product. The free energy of H∗ ( Δ G H ∗ ) is proven to be a key descriptor to characterize the HER activity of the electrocatalyst. A electrocatalyst with a positive value leads to low kinetics of adsorption of H, whereas a catalyst with a negative value leads to low kinetics of release of H2 molecules. 35 The optimum value of | Δ G H ∗ | should be zero; for instance, this value for the well-known highly efficient Pt catalyst is near zero as | Δ G H ∗ | ≈ 0.09  eV . 5 The Δ G H ∗ is calculated as 35 (Equation 1) Δ G H ∗ = Δ E H ∗ + Δ E ZPE − T Δ S H where Δ E H ∗ is the binding energy of adsorbed H and Δ E ZPE and Δ S H are the difference in zero point energy (ZPE) and entropy between the adsorbed H and H2 in the gas phase, respectively. As the contribution from the vibrational entropy of H in the adsorbed state is negligibly small, the entropy of H adsorption is Δ S H ≈ − 1 / 2 S H 2 , where S H 2 is the entropy of H2 in the gas phase at the standard conditions. Therefore, the Δ G H ∗ value for the Co(001) surface should be Δ E H + 0 . 24  eV . 35 More details of the characterization methods followed, XAS data collection, and operando Raman measurements are provided in Supplemental Experimental Procedures.All physical characterizations were carried out at the Mark Wainwright Analytical Centre (MWAC) and Electron Microscope Unit at the University of New South Wales (UNSW). XAS spectra were recorded on the multiple wiggler XAS beamline 12 ID at the Australian Synchrotron (AS1/XAS/15778). Thanks to Dr. Hangjuan Ren for the schematic drawing. Thanks to Dr. Rosalie Hocking at Swinburne University of Technology and Dr. Bernt Johannessen at Australian Synchrotron for the help of XAS data collection. Thanks to Mr. Kamran Dastafkan for proofreading the paper. This research was undertaken with the assistance of resources provided by the Pawsey and the National Computational Infrastructure (NCI) facilities at the Australian National University, which were allocated through the National Computational Merit Allocation Scheme and supported by the Australian Government and the Australian Research Council grant (LE190100021). C.Z. is grateful for the award of a Future Fellow from Australian Research Council (FT170100224).C.Z. and Y.L. designed the experiments. Y.L. undertook electrochemical experiments; performed the XRD, TEM, and XAS; and interpreted data. X.T. and S.C.S. undertook the DFT calculation. W.Y. collected the Faradic efficiency data, X.B. helped to collect the XAS data, Z.S. performed XPS, and T.Z. collected SEM. C.Z., Y.L., X.T., and S.C.S. co-wrote the paper. All authors discussed the results and commented on the manuscript.The authors declare no competing interests.Supplemental Information can be found online at https://doi.org/10.1016/j.xcrp.2020.100275. Document S1. Figures S1–S21 and Table S1 Document S2. Article plus Supplemental Information

Promoting active and stable H2 evolution reaction (HER) on metallic Fe triad materials is important yet challenging. Here, we report a metallic Co catalyst modulated by vanadium oxide (VOx) clusters (denoted as Co(VOx)) for active alkaline HER activity. Systematic X-ray absorption spectroscopy (XAS) and X-ray crystallography studies verify that VOx clusters endow Co with a highly disordered lattice and downsized particle size. The best Co(VOx) electrode is achieved with an optimal doping level of 3%, which delivers −100 mA cm−2 at an overpotential merely of 178 mV, in contrast to 344 mV and continuous activity decay on pure Co. The lower or higher doping level is unable either to regulate the atomic structure and reduce the H binding or to provide fewer active sites. Density functional theory (DFT) calculations reveal that VOx enables efficient electron transfer from Co to VOx, thus decreasing the H-adsorption on V-Co(001) for enhanced HER.

Data will be made available on request.Worldwide, there is a great desire to develop technologies for the efficient capture and conversion of carbon dioxide to fuels and chemicals, such as methanol [1–3]. Methanol is a carrier of carbon and hydrogen [4,5] and, as an energy carrier, it can be used directly as a fuel in direct methanol fuel cells (DMFCs) and internal combustion engines (ICEs) [6,7]. Additionally, methanol can be used in the production of high value-added chemicals (e.g. formaldehyde, methyl tert-butyl ether and acetic acid) and as a feedstock to produce hydrocarbons (such as alkanes, olefins or aromatics) and inherently fuels [6–8].Traditionally, Cu/ZnO-based catalysts have been employed in the industrial production of methanol from the syngas stream (CO/CO2/H2) generated in the steam reforming of natural gas [9–11]. The mechanism of methanol formation from CO/CO2 has been under debate for decades [12,13]. Since CO is the predominant carbon-containing molecule in syngas [2,12] and Cu is an outstanding metal for CO2 reduction to CO (through reverse water-gas shift, CO2 + H2 ⇌ CO + H2O), CO has been considered the primary carbon source in methanol production for decades (CO + 2 H2 ⇌ CH3OH) [10,12]. It was in the late 1980 s when the use of 14C-labeled isotopes provided evidence to suggest CO2 as the main carbon source in the methanol production (CO2 + 3 H2 ⇌ CH3OH + H2O) from CO2/CO/H2 mixtures [14]. Since then, many studies have been driven in the same direction [10,12,15–17], and currently, CO2 is perceived as the major reactant under industrial conditions. However, as Grabow and Mavrikakis suggested based on density functional theory (DFT) calculations and microkinetic modeling [12], under typical methanol production conditions, both CO and CO2 hydrogenation routes can coexist.In addition to the active discussion about the carbon source, the knowledge gained regarding the nature of the active sites in the CO2-to-methanol (CTM) process has grown exponentially in recent decades [18–20]. CO2 is a stable molecule and greatest difficulties in achieving great methanol selectivity in CO2 hydrogenation are related to kinetic limitations [21]. Therefore, a molecular understanding of the key aspects that govern the activity and selectivity of a catalyst is crucial. Overall, over copper-based catalysts, the CO2 hydrogenation to methanol reaction has been described as a structure-sensitive reaction in which not all of the surface atoms have the same role and activity [15,18,22]. For copper-zinc oxide (Cu-ZnO) binary systems, copper is responsible for the adsorption, dissociation and spillover of atomic hydrogen (H*) [23], while zinc oxide enhances the dispersion of Cu nanoparticles and facilitates the adsorption of CO2 [24]. The Cu-ZnO interface and surroundings have been described as the most active sites responsible for the activity wherein the intermediate species (e.g. carbonates and formates) are further hydrogenated to methanol [11,25–27]. Lately, ZrO2-based catalysts are emerging as active [18,28,29] and cost-effective solutions for the efficient synthesis of methanol [30] and a few experimental studies have explored the synergistic interactions of Cu/ZnO/ZrO2 catalysts and the active interplay toward methanol production [10,18,31,32]. Alone on ZrO2, both Cu [28,33] and ZnO [34,35] can also display some activity in the hydrogenation of intermediate species to methanol. Also, the ZrO2/Cu inverse configuration has shown excellent properties for an efficient methanol synthesis from CO2 [36,37].In recent years, more investigations have described not only the synergistic effect of binary Cu/ZnO catalysts but also the effects of the locations of both the active metal (Cu) and promoter (ZnO) on methanol production from CO2 [19,38,39]. The formation of ZnO particles or reduced Zn on the Cu surface have been found to improve the Cu dispersion and eventually, the accessible Cu surface area [11,19,25,39]. Experimental and computational studies on CO2 hydrogenation to methanol have pointed out that the formation of ZnO aggregates on top of Cu particles promoted methanol production which may be related to the increase in the number of active ZnO-Cu pairs [19,38]. Palomino et al. [38] experimentally demonstrated that ZnO added on top of Cu (100) and (111) surfaces yields a superior methanol production compared to the inverse copper-added-on-top-of-zinc oxide catalyst. Moreover, the highest production of methanol was observed at a relatively low surface coverage (θZn) of 0.15–0.20 monolayer (ML) and similar values of θZn ≈ 0.20 ML were reported by Nakamura and coworkers for ZnO over polycrystalline copper [40], and Kattel and collaborators over Cu(111) substrates [19]. By a combination of experimental, and DFT calculations and modeling based on thermodynamics, Kuld and coworkers [41] found the highest methanol turnover frequency (TOF) at a surface coverage of θZn ≈ 0.47 ML, with the TOF being greater when using larger Cu particles.The above examples highlight the potential of and interest in synthesizing and testing zinc-on-top-of-copper catalysts with ZnO surface coverages of approximately θZn ≈ 0.1–0.2 ML. An atomic-scale synthesis technique, such as atomic layer deposition (ALD), is an efficient technique to reach this range of surface coverage. The ALD technique is based on the sequential use of self-terminating gas–solid reactions and can offer accurate atomic level control of the deposited metal concentrations [42,43]. To modify metal oxide interfaces, single atoms can be uniformly distributed on high surface area supports by ALD [43–45]. The first studies in atomic-scale synthesis (by ALD) toward CTM reaction have already been reported (e.g. ZrO2-ALD on Cu/SiO2 [46] and Ni-ALD on Cu nanoparticles on γ-Al2O3 [47]), bringing out the benefits of this technique to enhance the catalyst activity, selectivity and stability. In a recent publication, Saedy and coworkers [48] applied preferential chemical vapor deposition (PCVD) and incipient wetness impregnation in the synthesis of ZnO/Cu/Al2O3 catalysts for the CTM reaction. Zinc oxide introduced by the PCVD method on a prereduced copper phase resulted in a more active and selective catalyst compared to the impregnated catalyst [48]. By means of various diffraction, spectroscopic and microscopy characterization techniques, the investigators demonstrated a more efficient production of active/selective ZnO/Cu interfaces compared to the more traditional impregnation method [48]. Furthermore, the authors suggested that the inverse ZnO/Cu interface may result in a more active system than the conventional Cu/ZnO interface [48]. In a recent research [49], ZnO was added by ALD (323 K, diethylzinc as a precursor) on copper hydroxide nanowires and the size of ZnO was tuned from isolated species to nanoparticles by increasing the number of ZnO cycles from 1 to 20. The maximum methanol production rate was found after 3 cycles.In this work, we focused on studying the catalytic performance of diverse copper-zinc oxide on zirconia catalysts for the hydrogenation of carbon dioxide to methanol. Atomic layer deposition (ALD) and incipient wetness impregnation were applied for the incorporation of Zn and Cu into the catalyst, respectively. Zn was deposited by ALD in one ALD cycle, which in practice should correspond to atomically dispersed ZnO species covering 10–20% of the surface (0.1–0.2 ML). By alternating the order in which Cu and Zn were attached to the catalyst, we created different metal-oxide configurations. A combination of various characterization techniques, such as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), hydrogen temperature programmed reduction (H2-TPR), carbon dioxide temperature programmed desorption (CO2-TPD), X-ray photoelectron and absorption spectroscopy (XPS and XAS) and scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDS), DFT calculations and catalytic tests, led us to identify the most active configuration. We expect that the findings of this research shall enhance the understanding of the elemental features in the zinc oxide/copper/zirconia system and help to consider the location of zinc oxide (promoter) and copper (metal) as a crucial parameter to produce active sites for the efficient hydrogenation of carbon dioxide to methanol.In total, five samples were synthesized: ZnO/ZrO2, Cu/ZrO2, ZnO/Cu/ZrO2, Cu/ZnO/ZrO2 and ZnO/Cu/ZnO/ZrO2. The self-made samples were prepared following two different methods, depending on which metal was incorporated into the porous structure of the support (monoclinic zirconia, ZrO2). Thus, Cu was added by incipient wetness impregnation (IWI), while Zn was added by atomic layer deposition (ALD). Monoclinic zirconia, provided by Saint-Gobain NorPro as cylindrical pellets (length 5 mm, diameter 3 mm) was used as a support material (surface area of 70 m2 g−1). Prior to its utilization, ZrO2 was crushed and sieved to a particle size of 250–420 µm and calcined in a muffle furnace (Nabertherm P330) in ambient air at 873 K for 5 h (10 K min−1) to remove possible surface impurities. Cu nitrate trihydrate, Cu(NO3)20.3 H2O (CAS: 10031–43–3, Sigma Aldrich, 99–104% purity) and Zn acetylacetonate, Zn(C5H7O2)20.3 H2O (Zn(acac)2, CAS: 14024–63–6, Volatec) were used as copper and zinc precursors, respectively. The targeted areal number density for Cu and Zn was 2 atoms/nm2 (Zn/Cu atomic ratio of one). The Cu and Zn loadings in wt% measured by ICP–OES are shown in Table 1, and a scheme that shows the sequence of samples prepared is shown in Fig. 1.For the impregnation of Cu by IWI, either on ZrO2 or ZnO/ZrO2, the corresponding amount of Cu precursor was dissolved in the exact amount of deionized water needed to fill the pore volume of the support. The water uptake capacity of the support (≈ 0.3 mL g−1) was experimentally estimated by adding deionized water drop by drop to a known amount of dried support (393 K, 24 h). Approximately 3–5 drops of the Cu nitrate solution were added at a time to an Erlenmeyer flask containing the dried support. Next, the partially wet support was first gently mixed, and then the flask was shaken for 2–3 min to ensure an even distribution of the solution. After adding the final drops of the solution, the slightly damp material was aged for 5 h at room temperature and dried overnight at 393 K in an oven under static air. Finally, the dried material was calcined at 673 K for 2 h (5 K min−1) in a tube furnace with a constant flow of 100 mL min−1 of synthetic air (AGA 99.999% purity, 20% O2, 80% N2).The deposition of Zn on ZrO2 by ALD started by treating the calcined support in a flow-type fixed bed F-120 ALD reactor (ASM Microchemistry) at 523 K for 10 h to remove moisture before the actual ALD process. Then, the solid zinc acetylacetonate reactant was vaporized at 393 K in flowing nitrogen and reacted to the pretreated ZrO2 support by one cycle of the ALD process for 3 h at 473 K and a pressure of ca. 3 mbar. Reactant-originated acetylacetonate ligands were removed by oxidative treatment in a tube furnace in synthetic air flow (100 mL min−1) at 773 K for 2 h (5 K min−1). The same procedure was followed when Zn was deposited on ZrO2, Cu/ZrO2 and Cu/Zn/ZrO2. Fig. 2 shows a conceptual scheme with the configurations of the various copper-zinc-zirconia catalysts that were synthesized and tested in this research.The metal content of the catalysts was determined by inductively coupled plasma atomic emission spectroscopy (ICP–OES). Samples (ca 0.100 mg) were weighed in Teflon vessels, and a mixture of nitric acid (HNO3, 65%, 2.5 mL) and hydrochloric acid (HCl, 37%, 7.5 mL) was added. Vessels were closed and placed in a microwave oven (Milestene, Ethos) and heated (1 h, 200 °C). After cooling, the samples were diluted with MQ-grade water, and the Cu- and Zn-contents were determined with an F-AAS instrument (Varian 220 F) using an air-acetylene burner.The surface area and cumulative pore volume of zirconia were obtained by nitrogen physisorption isotherm (liquid nitrogen, 77 K) in a Thermo Scientific Surfer equipment. The support sample was weighted to a quartz glass burette (ca. 200 mg) and degassed at 573 K for 3 h. Specific surface area was calculated from the isotherm according to the Brunauer-Emmett-Teller (BET) method [50]. The cumulative pore volume was calculated based on the Barrett-Joyner-Halenda (BJH) method [51].The phase/crystallinity of the support was studied by X-ray diffraction that was carried out on a ground sample in a PANanalytical X‘́Pert Pro MPD Alpha 1 device equipped with Cu Kα1 radiation (45 kV and 40 mA). The X-ray scanning range was from 10° to 100° (2θ) with a step size of 0.0131° and a time per step of 51 s. The results are shown in the supporting information (Fig. S1). The characteristic monoclinic phase (JCPDS 37–1484) was identified with main reflections at 24.5°, 28.3°, 31.5°, 34.2°, 35.4°, 40.8°, 49.3°, 50.2°, 54.1° and 55.5°.The reducibility of the metal oxides was studied by hydrogen temperature-programmed reduction. The experiments were performed using an Altamira AMI-200 characterization system with a thermal conductivity detector (TCD) connected to an OmniStarTM mass spectrometer (MS) produced by Pfeiffer Vacuum. A total of 150 mg of sample was placed in a U-shaped quartz reactor and treated in constant He flow (AGA 99.999% purity) at 473 K for 60 min and cooled back to 303 K. The sample was then heated from 303 to 873 K or 1173 K in 2% H2/Ar (AGA 99.999% purity) with a heating ramp of 5 K min−1. The TPR measurement of ZrO2 and ZnO/ZrO2 was performed up to 1173 K, while 873 K was used as the maximum temperature for the Cu-containing samples. The TPR results are qualitative (given as arbitrary units) and the areas under the peaks cannot be compared with each other. The total gas flow was set at 50 mL min−1 (STP conditions) during the whole measurement.Similarly, carbon dioxide desorption was studied by CO2 temperature-programmed desorption. The experiments were performed using an Altamira AMI-200 characterization system with a TCD connected to an OmniStarTM MS produced by Pfeiffer Vacuum. A total of 150 mg of sample was placed in a U-shaped quartz reactor, treated in He flow at 473 K for 60 min, and cooled back to 303 K. Then, the sample was activated by heating the solid from 303 to 623 K in 2% H2/Ar with a heating ramp of 10 K min−1 and 60 min hold time. The sample was then cooled to 323 K in He flow and maintained at that temperature for 30 min. Thereafter, the reduced sample was exposed to a constant flow of 10% CO2/He (AGA 99.999% purity) for 30 min at 323 K and flushed for 60 min at the same temperature in He flow to remove the physisorbed CO2. Finally, the sample was heated from 323 to 1073 K in He flow to desorb the chemisorbed CO2. The possible desorbed products were continuously monitored during the experiment by mass spectrometry. The amount of CO2 desorbed for each measurement was quantified by using calcium carbonate (CaCO3) as an internal standard. About 4 mg of CaCO3 was mixed with the sample and decomposed during the desorption step into CO2 (g) and CaO (s) in the range of 800–950 K. The total gas flow was set at 50 mL min−1 (STP conditions) for all measurements.The electronic structure of the samples was studied by high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS). The experiments were performed at the ID20 beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) [52,53]. The beam was monochromated by a combination of a Si(111) premonochromator and a Si(311) channel-cut monochromator. The spectrometer was a von Hamos spectrometer based on three Si(333) crystal analyzers.X-ray photoelectron spectroscopy (XPS) was used to study the surface composition of the reduced samples (623 K for 30 min, 2% H2/Ar, transfer to XPS was done through air). The measurements were performed with a Kratos AXIS Ultra DLD X-ray photoelectron spectrometer using a monochromated AlKα X-ray source (1486.7 eV) run at 100 W. A pass energy of 80 eV and a step size of 1.0 eV were used for the survey spectra, while a pass energy of 20 eV and a step size of 0.1 eV were used for the high-resolution spectra. Photoelectrons were collected at a 90° take-off angle under ultra-high vacuum conditions, with a base pressure typically below 1×10−9 Torr. The diameter of the beam spot from the X-ray was 1 mm, and the area of analysis for these measurements was 300 µm x 700 µm. Both survey and high-resolution spectra were collected from three different spots on each sample surface in order to check for homogeneity and surface charge effects. All spectra were charge-corrected relative to the position of C-C bonding of adventitious carbon at 284.8 eV.Scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) images were acquired for the prereduced samples (623 K, 60 min, 50 mL min−1 of 2% H2/Ar, STP conditions) by a JEOL JEM-2200FS double aberration corrected, high-resolution microscope, operated at 200 kV acceleration voltage. The chemical elemental mapping analysis was conducted with an X-ray energy-dispersive spectroscopy (EDS) detector. The samples were drop-casted using acetone onto a gold grid coated with an ultrathin holey carbon film.The evolution of the surface species during the cyclic CO2-H2 adsorption was measured by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) carried out in a Nicolet Nexus FTIR spectrometer with a high temperature/pressure Spectra-Tech reactor chamber equipped with a dome and ZnSe windows. The gas flow leaving the chamber was monitored with an OmniStar GSD 301 spectrometer by Pfeiffer Vacuum. The scans were collected from 4000 to 600 cm−1 at a scan resolution of 4 cm−1. Prior to the experiment, a background spectrum was acquired under room conditions using an aluminum mirror in constant Ar flow. Approximately 20 mg of crushed sample powder was placed in the sample holder, heated from room temperature to 673 K in a constant flow of 10% O2/N2/Ar (synthetic air: AGA 99.999% purity, 20% O2, 80% N2; Ar: AGA 99.9999% purity) and kept there for 60 min to remove possible surface impurities. The temperature was then decreased to 623 K in the same atmosphere, and the gas was switched to 10% H2/Ar (H2: AGA 99.999% purity; Ar: AGA 99.9999% purity) and kept for 60 min to activate the metal oxides. The catalyst was then cooled to the desired reaction temperature, either 450, 500 or 550 K, and flushed with Ar for 30 min. The cyclic adsorption of CO2-H2 consisted of three consecutive full cycles of the following sequence: i) CO2 adsorption for 12 min under flowing 10% CO2/He (AGA 99.999% purity), ii) switching to Ar flow for 12 min, iii) switching to H2 adsorption for 12 min under flowing 10% H2/Ar and, iv) switching to Ar flow for 12 min. During each step within each cycle, a spectrum (100 scans, approximately 2 min) was recorded at 0, 5 and 10 min to monitor the surface of the catalyst with respect to time on stream. The total gas flow was set at 50 mL min−1 (STP conditions) during the whole experiment. A summary of the experimental conditions (timing, gases, flow rates, and temperatures) used for DRIFTS experiments is depicted in Fig. 3.To assist infrared band identification, density functional theory (DFT) calculations were performed using the BEEF–vdW exchange–correlation functional [54] as implemented in the GPAW [55] software. The monoclinic zirconia was described by a two-layer-thick slab model, built from a 3 × 2 m-ZrO2(111) supercell with periodic boundary conditions used in the lateral directions. The final cell measurements were 20.67 × 14.79 × 24.0 Å with angles of 90°/90°/116.5°. Cu(111) and stepped Cu(110) surfaces were modeled as three-layer periodic slabs, where the bottom layers were kept fixed in their bulk geometry in unit cells of 4 × 4 and 3 × 4, respectively. The core electrons of all elements were described by projector-augmented wave (PAW) [56] setups in the frozen-core approximation. A real–space grid basis was used with a maximum grid spacing of 0.2 Å. Periodic boundary conditions were used in two directions and the reciprocal space was sampled at the Γ point. A Hubbard U correction [57] of 2.0 eV was applied to the d–orbitals of the zirconium atoms. The atomic structures were optimized using the Fast Inertial Relaxation Engine (FIRE) algorithm as implemented in the Atomic Simulation Environment (ASE) [58,59] package until the maximum residual force was below 0.005 eV Å−1. Vibrational frequencies for modes involving adsorbate and binding catalyst surface atoms were determined using the Frederiksen method [60] . The frequencies of the combined modes were obtained by adding up the frequencies of their individual contributions.The catalytic performance was evaluated in a high-pressure continuous-flow fixed-bed equipped with a stainless-steel tube reactor with a mesh placed in the midsection of the reactor. One gram of calcined catalyst, sieved to 0.25–0.42 mm, was loaded in the reactor. Prior to the catalytic reaction, the catalyst was activated by in situ reduction at 623 K for 60 min with a constant flow of 10% H2/N2 (v/v; H2: AGA 99.999% purity, N2: AGA 99.999% purity). Activity tests were conducted at 450, 500 and 550 K with a total pressure of 3.0 MPa and a gas hourly space velocity (GHSV) of 7500 h−1 (STP conditions: 273.15 K and 1 bar). The reaction mixture was composed of H2/CO2/N2 (⁓ 71/23/6, v/v/v; H2: AGA 99.999% purity; CO2, AGA: 99.995%; N2: AGA 99.999% purity). The volumetric flows were 6.3 L h−1 of H2, 2 L h−1 of CO2 and 0.6 L h−1 of N2 (STP conditions) and the volume of catalyst used per experiment was approximately 1.2 10−3 L. For each catalyst, the reaction temperature was increased in steps of 50 K. Initially, the reactor temperature was stabilized at 450 K and kept there for 90 min. Next, the temperature was increased to 500 K (10 K min−1), stabilized, and kept there for another 90 min. Finally, the procedure was repeated for the highest temperature of 550 K. Thus, the data depicted in this manuscript were collected after 90 min under each operating condition. The unreacted gases and reaction products were continuously monitored in an Agilent 490 Micro Gas Chromatograph (microGC) fitted with a thermal conductivity detector (TCD) and equipped with two columns: a) MS-5 molecular sieve for permanent gases H2, N2, CH4 and CO and, b) PoraPLOT U for CO2, CH3OH and H2O. The CO2 conversion (X CO2, Eq. 1) and product selectivity (Si, Eq. 2) were calculated by internal normalization standard with N2. The CO2 conversion, the selectivity of CH3OH, CO and CH4 and the space-time yield of CH3OH (STY CH3OH, mmol h−1 gcat −1, Eq. 3) were calculated according to the following formulas: (1) X CO 2 ( % ) = F CO 2 , in - F CO 2 , out F CO 2 , in * 100 % (2) S i ( % ) = F i , out F CH 3 OH , out + F CO , out + F CH 4 , out * 100 % (3) STY C H 3 OH ( mmol h − 1 g cat − 1 ) = F CO 2 , in * X CO 2 * S C H 3 OH m cat where F CO2 and F i are the molar flow rates of CO2 or products (CH3OH, CO and CH4) and m cat is the mass of catalyst in grams.The list of samples (support and self-made catalysts), the corresponding sample code used throughout the manuscript, the Cu and Zn metal loadings measured by ICP–OES and the Cu and Zn areal number density, the amount of CO2 desorbed in the CO2-TPD experiments and the number of reduction peaks (for Cu-species) and temperature at the maximum height of the peak determined by H2-TPR are listed in Table 1.The STEM-HAADF images and EDS mapping of elements for the support and self-made samples are depicted in Fig. 4. Based on the results, the elements mapped for each sample seem to be evenly distributed throughout the samples and no high concentration spots were detected in any of the samples. The estimation of the Cu and Zn particle size distributions was impossible due to the poor contrast between Cu, Zn and Zr.Also, the percentage of a monolayer of ZnO (% ML) was estimated as a ratio between the areal number density of ZnO in each sample (Zn atoms per nm2, calculated from the Zn metal loadings measured by ICP–OES) and the average areal number density of Zn in one bulk ZnO monolayer obtained through equation reported elsewhere [42] (12.0 nm−2, from ZnO density = 5.61 g cm−3). According to these calculations, one Zn-ALD cycle (Zn/Zr, Cu/Zn/Zr and Zn/Cu/Zr samples) yielded approximately 15% of a bulk ZnO ML equivalent.The electronic structure (oxidation states) of copper and zinc was investigated by high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS). The Cu and Zn absorbance at the Cu and Zn K-edges are depicted in Fig. 5 for the various catalysts and references (Cu foil, Cu2O, CuO, Zn foil, ZnO and Zn(acac)2). Along with the samples prepared and reported in Table 1, two more samples were prepared and analyzed: Zn/Cu/Zr (5 wt% Cu) and Zn/Cu/Zr (15 wt% Cu). These samples were synthesized to observe a possible effect of the Cu loading on the electronic structure of Cu and Zn. All the samples analyzed by HERFD-XAS were studied after the calcination treatment detailed under Section 2.1 (673 K or 773 K when Cu or Zn were added last, respectively), except a Zn/Zr sample that was also studied before the calcination step, i.e., after the Zn-ALD cycle.Analyzing the Cu K-edge HERFD-XAS and first derivative (relative to incident energy) spectra (Fig. 5a-b) of the Zn/Cu/Zr samples with various Cu loadings, the spectra of the samples with 5 wt% and 15 wt% Cu were similar to that of CuO, which indicated a clearer presence of bulk CuO when increasing the Cu content. The characteristic edge transition for CuO can be observed at ca. 8.984 keV [32]. The rest of the samples (Zn/Cu/Zr with 1.1 wt% Cu, Cu/Zn/Zr and Cu/Zr) showed the absence of the first main shoulder that caused an apparent shift of the first peak of the derivative spectrum toward higher energies (Fig. 5b). The shoulder in CuO stems from 1 s to 4p transitions with a charge transfer from the ligand (1s13d10 _L_ final state, with _L_ denoting a ligand hole) [61,62]. This ligand effect decreases from CuO to the catalysts and it is typically absent in tetrahedral Cu(II) complexes [63]. The local coordination in bulk CuO is nearly square planar while the difference in the catalysts indicates a different local coordination in the surface-dominated Cu species. Thus, the pre-edge intensity (at 8.979 eV) decreases from CuO to the catalysts, indicating more likely an octahedral coordination in our samples, where the pre-edge is dipole-forbidden. However, the absence of the characteristic Cu1+ 1 s-4p transition feature at ca. 8.98 keV [63] indicates the absence of Cu2O in any of the samples. Thus, it can be concluded that the Cu valence state seemed to be Cu2+ for all the catalysts.For the Zn K-edge (Fig. 5c-d), the HERFD-XAS spectra and 1st derivative were very similar for all the samples, which suggests that the chemical environment for Zn is not significantly influenced either by the order in which Cu and Zn were added to the catalyst or by the Cu loading. Comparing the HERFD-XAS spectra of the samples with the ZnO (wurtzite) reference, there were significant differences; however, the presence of the Zn2+ oxidation state can be assumed. The peak at 9.679 eV present on ZnO reference, assigned to multiple scattering on atomic neighbors beyond the first shell [64], was not visible in any of the samples (except for a small peak observed on the Cu/Zn/Zr sample) which evidenced the presence of atomically dispersed ZnO species. It was computationally [64] and experimentally [49] demonstrated that ZnO XANES features are developed while increasing the number of ZnO atomic shells and, consequently, the ZnO cluster size. To the best of our understanding, it was expected that Zn was atomically dispersed as lone ZnO units with an oxidation state of Zn2+.To complement the bulk structural-chemical information obtained by HERFD-XAS, the surface composition was analyzed by XPS. The samples were reduced ex situ and momentarily exposed to the atmosphere during the preparation before the analysis.The elemental composition and the relative amount of the diverse zinc components are included in Table 2. The survey spectra of the samples in the range of 0–1200 eV are included in Fig. S2 and the high-resolution XPS spectra of the Zn 2p and Cu 2p regions are depicted in Fig. 6. As expected, all samples exhibited zirconium and oxygen, the relative amount of which decreased when adding other components (Table 2). The surface concentration of copper and zinc content measured by XPS varied between 2–4 and 3–7 at%, respectively, for the samples. The Zn/Cu atomic ratio was close to two for the samples with one ALD cycle of Zn and Cu by impregnation. Compared to the bulk atomic ratio of about one (see Table 1), the observed Zn/Cu ratio is consistent with zinc located on the surface and copper somewhat clustered. Addition of zinc on Cu/Zr decreased the surface concentration of copper, as expected (from 3.0 to 1.9 at% for Zn/Cu/Zr). According to XPS, the Cu/Zn/Zr sample had a higher surface concentration of both copper and zinc than the inverse Zn/Cu/Zr. While the reason for this observation is not fully clear, we speculate that it may have to do with the details of impregnation (zinc oxide is amphoteric [65] and the impregnation solution was acidic; part of the zinc may have dissolved and migrated to the outer surface during drying). Addition of zinc on Cu/Zn/Zr again decreased the surface concentration of copper (from 4.0 to 2.8 at% on Zn/Cu/Zn/Zr).Taking a closer look at the high resolution XPS results, the Zn 2p3/2 region (Fig. 6a) could be deconvoluted to three different species of Zn2+. These correspond to ZnO-like species (1021.5 eV) and Zn mixed state denoted a (1023.1 eV) and Zn mixed state denoted b (1024.4 eV). The presence of metallic zinc can be discarded due to the absence of a peak at a slightly lower binding energy (at ⁓ 1021 eV [66]). The highest fraction of zinc in ZnO-like species was in the Zn/Cu/Zr sample (over 85%). The Cu 2p3/2 region (Fig. 6b) showed the presence of Cu metal (Cu0) and Cu2+ in all Cu-containing samples. Although the samples had been reduced before the XPS measurements, sample transfer through air to XPS had evidently been sufficient to oxidize part of the Cu0 to Cu2+.The reducibility of CuO was studied by H2-TPR (hydrogen temperature-programmed reduction). The results are shown in Fig. 7. The reduction profiles for Zr and Zn/Zr samples are also shown in Fig. 7 as a reference. The Zr sample showed a very small and broad reduction peak with a maximum at 860 K that can be related to the formation of surface oxygen vacancies on the support [67,68]. On the Zn/Zr sample, a single reduction peak was found at 815 K, which can be associated with the partial reduction of the support and/or with the partial reduction of ZnO to metallic Zn [69,70].The Cu-containing samples showed various reduction peaks in the range of 360–510 K, depending on the order in which Cu and Zn were incorporated into the support. Reduction of CuO in the Cu/Zr sample generated up to three reduction peaks: two overlapping peaks with maxima at 405 and 418 K and a third peak with a maximum at 477 K. The double reduction peak at relatively low temperatures (frequently named the α and β peaks [71–74]) has been previously reported in samples with relatively low CuO loadings of approximately 5 wt%. In accordance with these authors, well-dispersed Cu2+ species are first reduced to Cu+ (α-peak) and subsequently to Cu0 (β-peak). The third peak at higher temperature can be assigned to the reduction of more poorly dispersed bulk-like CuO particles or CuO particles with a stronger metal-support interaction [72,73,75].The deposition of Zn on top of Cu/Zr (Zn/Cu/Zr sample) led to a partial improvement in the reducibility of CuO. The previously reported α and β peaks in the Cu/Zr sample merged into a single reduction peak at 418 K, while the reduction at 477 K in Cu/Zr shifted to a lower temperature. This latter observation suggests that the addition of Zn after Cu improved the dispersion of bulk CuO particles [19,76]. Surprisingly, when zinc was added by ALD before copper impregnation (Cu/Zn/Zr sample), higher reduction temperatures than in the Zn/Cu/Zr sample were observed with two reduction peaks at ca. 448 and 500 K, the latter presumably generated by the reduction of larger CuO particles. The positive effect of zinc addition after copper impregnation on the reducibility of bulk CuO is also observable in the Zn/Cu/Zn/Zr sample with a shift of the peak maximum from 500 to 475 K.The CO2-TPD (carbon dioxide temperature-programmed desorption) profiles of pure Zr and the self-made Cu-Zn on Zr samples are displayed in Fig. 8. Up to four different desorption peaks or desorption domains can be identified. The first two peaks (at ca. 380 and 445 K) can be assigned to weakly basic sites with different gas–solid interactions, while peaks at approximately 565 and 700 K can be attributed to moderate and strong basic sites, respectively [77,78]. Accordingly, the desorption profile of pure Zr showed the presence of a significant amount of desorbed CO2 in the range of 340–600 K with a main desorption peak at 380 K (most likely bicarbonate species on surface -OH groups) and a less significant and broader peak at 445 K. The addition of Cu and/or Zn resulted in the appearance of new desorption peaks at higher temperatures while the main peak at approximately 380 K became broader and less intense.The amount of desorbed CO2 (Table 1) followed the order (from higher to lower): Zn/Cu/Zr > Zn/Cu/Zn/Zr > Zn/Zr > Zr > Cu/Zn/Zr > Cu/Zr ranging from 115 to 63 µmol of CO2 per gram of catalyst. In general, ZnO played an important role for the adsorption of CO2, especially when it was introduced after copper. The deposition of ZnO on Cu/Zr sample (Zn/Cu/Zr sample) increased the total amount of adsorbed CO2 from 63 to 115 μmol CO2 gcat −1 (∼80% higher) while the deposition of ZnO on Cu/Zn/Zr (Zn/Cu/Zn/Zr sample) increased the adsorbed CO2 from 85 to 109 μmol CO2 gcat −1 (∼30% higher). This trend shows the benefits of the zinc-after-copper pair in promoting the adsorption capacity of the catalyst and the advantages of adding the Zn atoms after Cu. When the results were expressed as molecules of CO2 per Zn atom (Table 1), Zn/Cu/Zr was still a superior catalyst (0.67 molecules of CO2 per Zn atom); however, the Zn/Cu/Zn/Zr sample performed worse, turning out to be the catalyst with the lowest capacity among the Zn-containing catalysts (0.3 molecules of CO2 per Zn atom). These results showed that Zn has an essential role in the adsorption of CO2 but the amount of CO2 and the Zn loading are not linearly correlated, the Zn/Cu/Zr being the preferred configuration to maximize the amount of adsorbed CO2.To understand the CO2 and H2 adsorption of Cu and Zn on ZrO2, the surface species were monitored during three consecutive adsorption cycles of CO2 and H2 by in situ DRIFTS. To follow the catalyst surface with the experiment, representative spectra of the first and third cycles of adsorption are depicted in Fig. 9 in the fingerprint region (1700–1200 cm−1). The results are shown for the studied catalysts at three temperatures (450, 500 and 550 K). The selected spectra in Fig. 9 show the surface species after either 5 or 10 min of a certain gas flow (CO2, H2 or Ar). In the supplementary material (Figs. S3-S7), monitoring with respect to time on stream (first five minutes of exposure to CO2 and H2) of the surface species during the first cycle of CO2 and H2 adsorption is displayed. An example of the evolution of several MS signals during the DRIFTS experiment carried out at 500 K with the Zn/Cu/Zr sample is displayed in Fig. S8.The cyclic adsorption on the Zr sample (Fig. 9a) was considered to serve as a reference for the CO2 and H2 adsorption capabilities of the support material at various temperatures. After 10 min of CO2 flow, the surface showed the presence of bicarbonate species, HCO3 - at ca. 1625, 1427 and 1220 cm−1 and bidentate carbonates CO3 2- at ca. 1560–1530, and 1330 cm−1, with the band positions exhibiting good agreement with the literature [79–82]. According to the literature, the presence of terminal -OH groups is required for the formation of bicarbonate species, while carbonates (either monodentate, bidentate or polydentate) require the presence of coordinately unsaturated (c.u.s.) Zr4+ and O2- sites [79,80,82]. Experimentally, the highest intensity for bicarbonate species was observed at the lowest temperature (450 K), and the intensity decreased with increasing temperature, especially from 450 to 500 K. In contrast, the intensity of bidentate carbonate species was not significantly affected by the temperature of the experiment. The spectrum after one minute of CO2 flow (Fig. S3) did not differ from the spectrum after 10 min, which indicated a rapid saturation of the surface with carbonate and bicarbonate species. After switching the gas flow from CO2 to Ar, bicarbonate species vanished almost completely during 10 min of Ar purge. At 450 K, vibrational bands at ca. 1564 and 1330 cm−1 corresponding to bidentate carbonate species remained slightly visible, which suggested a stronger adsorption of these species to ZrO2 compared to bicarbonate species. A similar observation was made by Kouva and coworkers [79] when studying the adsorption of CO2 on ZrO2 by DRIFTS in the range of 373–673 K. Both the first and third cycles of adsorption yielded comparable spectra with no apparent accumulation of any species throughout the experiment.The incorporation of Zn atoms onto ZrO2 by ALD (Zn/Zr sample, Fig. 9b) clearly modified the CO2 adsorption capacity of ZrO2. After 10 min of CO2 exposure (either during the first or third cycle of adsorption), a crowded carbonate region (1600–1300 cm−1) was observed. The vibrational band at 1220 cm−1 during the CO2 flow indicated the presence of bicarbonate species, most likely on unsaturated Zr or Zn sites [83,84]. After 10 min of Ar purge after the first cycle of CO2 adsorption, the spectra still displayed a busy carbonate region, indicating that Zn or the Zn-ZrO2 interface can store CO2-related species (especially carbonates at ca. 1535 cm−1) with a greater adsorption strength than ZrO2. The switch from Ar to H2 flow led to the formation of new species such as formates (*HCOO-) located at ca. 2966, 2873 (Fig. S4), 1575, 1379 and 1365 cm−1 (Fig. 9b), while the intensity of carbonate species (at ca. 1535 cm−1) decreased in parallel. The formation of formate species was already visible after 1 min of H2 flow (Fig. S4), which indicated a speedy hydrogenation of carbonates to formates on a Zn/Zr sample. In addition, more formates were clearly detected with increasing experimental temperature and number of adsorption cycles. This indicated: i) the ability of the Zn or Zn-ZrO2 interface to accumulate carbonates during the CO2 flow and to further convert them to formates, and ii) the high stability of formates on the Zn/Zr sample even at high temperature since they did not disappear or further react between cycles.The computational and experimentally observed infrared vibrational frequencies for formate species are displayed in Table 3. The frequencies for formate species were computed on different model systems such as m-ZrO2 (111), ZnO/m-ZrO2 (111), Cu (111), and Cu (110). The model surfaces used in DFT calculations are displayed in Fig. S9. Additionally, the computed IR frequencies for possible intermediates, such as formic acid *HCOOH, carboxyl *COOH and dioxymethylene *H2COO, which are intermediate species in methanol formation [19,85], are given in Figs. S10-S13. According to the DFT calculations, formate species produced four different bands on ZnO/ZrO2 in the 3000–1200 cm−1 region that correspond to different functional groups and types of vibration. The bands were located at 1326, 1358, 1535 and 2961 cm−1, and they correspond to symmetric stretch υs(O-C-O), bending δ(C-H), asymmetric stretch υas(O-C-O) and stretching υ(C-H), respectively. The experimental band at ca. 2970 cm−1 can be attributed to the combination of C-H bending and asymmetric O-C-O stretching modes [86].The first cycle of CO2 adsorption over the Cu/Zr sample (Fig. 9c) led to a less crowded fingerprint region compared to Zn/Zr sample. As for the Zr sample, the main species detected were bicarbonates (at ca. 1625, 1430 and 1220 cm−1) and bidentate carbonates (at ca. 1557 and 1330 cm−1): the bicarbonates were rapidly removed after 10 min of Ar purge. Additionally, according to the band observed at 1535 cm−1, carbonates were present on the catalyst surface. Similar to the Zn/Zr sample (Fig. 9b), formates were formed during exposure to H2; however, the position of the band of the O-C-O asymmetric vibration differed (1575 cm−1 on the Zn/Zr sample and 1569 cm−1 on the Cu/Zr sample). A lower IR frequency for formate species on Cu (111 and 110) compared to that on a Zn/Zr sample was also predicted by DFT calculations (Cu(111) vs. ZnO/m-ZrO2, Table 3). After three consecutive cycles of CO2 and H2 adsorption, formates reached the highest intensity at 500 K, followed by 550 and 450 K. During the 3rd cycle of H2 flow on the Cu/Zr sample, the intensity of formate species slightly decreased from 5 to 10 min on stream, which evidenced the strong adsorption of formate species on the Cu/Zr sample. Herein, by using a combined experimental and computational (DFT calculations) approach, Larmier and coworkers [85] found that on a Cu/ZrO2 catalyst, formate species exhibit a low Gibbs free energy, which dictates their strong adsorption on the catalyst surface.With Zn/Cu/Zr (Fig. 9d) and Cu/Zn/Zr (Fig. 9e) samples, the detected species and the discussed trends with respect to temperature and CO2 and H2 cycles displayed a combination of the results observed based on Zn/Zr and Cu/Zr samples. Thus, mainly due to the presence of Zn in both Zn/Cu/Zr and Cu/Zn/Zr samples, the fingerprint region was highly occupied by CO2-related species after exposing the catalysts to CO2. Formate was the main species detected after completing the cycles of adsorption of CO2 and H2, and no further hydrogenated species were observed (such as formic acid, *HCOOH, dioxymethylene, *H2COO or methoxy, *H3CO).Based on the DRIFTS results discussed above, carbonates were present on all samples, and formate was the prevalent hydrogenated species in the selected experimental conditions. To compare the ability of each catalyst to hydrogenate carbonates into formates, a “formates to carbonates intensity ratio” was calculated and followed throughout the first to third cycles of adsorption for the various temperatures (Fig. 9f). Bands at 1535–1533 and 1575–1569 cm−1 were chosen as representative frequencies for carbonates and formates, respectively. Comparing the evolution of the ratio by temperature, clear trends could be observed at 500 and 550 K, while at 450 K, the ratio remained roughly constant at ca. 1 throughout cycles, which indicated a poor reduction of carbonates to formates at this temperature. At 500 and 550 K, the ratio was approximately 1.5 and 1.3, respectively, which pointed out the positive effect of temperature in the transformation of carbonates to formates. In all cases, the increase of the ratio originated from the concurrent decrease in the intensity of carbonate and the increase in the intensity of formate. At 500 K, the Zn/Zr sample yielded the highest ratios at the end of each cycle which indicated the crucial role of the ZnO-ZrO2 interface to form formate from carbonate. Especially outstanding was the increase of the ratio for the Zn/Cu/Zr sample at 500 and 550 K when hydrogen was introduced into the system (increase highlighted with solid black in lines in Fig. 9f). This behavior revealed the good ability of the Zn/Cu/Zr configuration in the hydrogenation of carbonate to formate. In addition, the Zn/Cu/Zr was the unique sample that exhibited a rising ratio during H2 flow under any condition. This indicated that the Zn/Cu/Zr configuration was able to remain active in the transformation of carbonate to formate during the 10 min of H2 flow, presumably due to a greater ability to keep CO2 adsorbed, as observed in Fig. 8.The catalytic performance of Cu-Zn on zirconia catalysts is illustrated in Fig. 10 for the three selected reaction temperatures: 450, 500 and 550 K. Fig. 10a shows the CO2 conversion, Fig. 10b-c show the production of CH3OH expressed as the space-time yield in mmolCH3OH gcat −1 h−1 and in mmolCH3OH gCu −1 h−1, respectively, and Fig. 10d shows the product selectivity for CH3OH, CO and CH4. Among the three reaction temperatures tested, all the catalysts showed activity at 500 and 550 K, while there was no measurable activity at 450 K. Greater CO2 conversion values were achieved for all the samples with increasing reaction temperature. The CO2 conversion values attained with both Cu- and Zn-containing samples, i.e., Zn/Cu/Zn/Zr, Zn/Cu/Zr and Cu/Zn/Zr, were higher than those achieved with Zn/Zr, Cu/Zr and Zr samples, which evidenced the importance of the Cu-Zn interactions in the CO2 hydrogenation reaction in accordance with the literature [18,19,87]. Interestingly, the addition of Zn on top of Cu (Zn/Cu/Zr and Zn/Cu/Zn/Zr samples) promoted CO2 conversion, and the most remarkable improvement occurred at 550 K, with conversion values of approximately 9% for Zn/Cu/ZrO2 and 6.5% for Zn/Cu/Zn/ZrO2. Under the same operating conditions, the other samples (Cu/Zn/Zr, Cu/Zr and Zn/Zr) yielded CO2 conversion values between 2% and 4%.Regarding the product selectivity (Fig. 10d), temperature had a remarkable effect on the product distribution. At 500 K, the prevailing product was methanol with all catalyst combinations (except on the Zr sample, which only produced CO). Thus, the highest CH3OH selectivity was achieved at 500 K with Zn/Cu/Zn/Zr sample (close to 80%) followed by Zn/Cu/Zr (71%) and Cu/Zn/Zr (68%) samples. The selectivity toward CO increased significantly when increasing the temperature from 500 to 550 K, according to the endothermicity of the reverse-WGS reaction (ΔH°298k = + 41 kJ mol−1) [88]. Advantageously, the Zn/Cu/Zr sample did not produce any methane under any of the conditions, in contrast to the Zn/Cu/Zn/Zr and Cu/Zn/Zr samples that produced methane at both 500 and 550 K with selectivities around 3–6%.In terms of methanol production (Fig. 10b-c), the most efficient catalysts were Zn/Cu/Zn/Zr and Zn/Cu/Zr, with similar production rates at 500 K (⁓ 1.9 mmolCH3OH gcat −1 h−1) and 550 K (⁓ 3.8 mmolCH3OH gcat −1 h−1). For Cu/Zn/Zr sample, the methanol production was lower (1.3 and 2.9 mmolCH3OH gcat −1 h−1 at 500 and 550 K, respectively). Moreover, Zn/Cu/Zr and Zn/Cu/Zn/Zr samples showed a space-time yield of methanol approximately three times higher than that achieved with Cu/Zr and Zn/Zr samples. When the methanol production was referred to in terms of grams of copper (Fig. 10c), the Zn/Cu/Zr sample produced 165 and 340 mmolCH3OH gCu −1 h−1 at 500 and 550 K (similar rates were attained with the Zn/Cu/Zn/Zr sample), while the Cu/Zn/Zr sample produced 107 and 241 mmolCH3OH gCu −1 h−1 under the same operating conditions. Therefore, the activity results highlight the potential interest of adding Zn by atomic layer deposition after Cu to promote methanol production catalysis.The presence of zinc oxide overlayers on top of copper particles in industrial-type Cu/ZnO/Al2O3 catalysts during the hydrogenation of carbon oxides to methanol has been demonstrated in diverse studies during the last decade [11,89,90]. Schott et al. [91] reported the unique properties of ZnO layers on the surface of copper particles due to the partial reduction of ZnO to a less strongly oxidized Znδ+ state under the reducing conditions of methanol synthesis. Similarly, Behrens and collaborators [11] demonstrated that the Cu-ZnO synergy lies in their strong metal support interaction leading to partial coverage of the copper surface with ZnOx under reducing (activation) conditions. Thus, ZnO nanoparticles dispersed on top of copper are special entities that could show particular physico-chemical properties not observed for bulk oxides [38,89,92].As previously described in the introduction there seems to be agreement among scholars in setting the optimal coverage of copper particles by ZnOx for a higher catalytic activity at relatively low values of 15–20% monolayer (ML) [19,38,40]. Nevertheless, higher coverages (θZn = 0.47) have also been reported as optimal values [41]. Although this difference is not fully understood, all of these studies agree that larger coverages of ZnO may negatively affect the catalyst activity in terms of methanol production [19,38,40,41]. Furthermore, the computational studies carried out by Kuld and coworkers [41] predicted a greater TOF of methanol for ZnO particles smaller than 7 nm, sizes that can be easily accomplished by ALD. Based on our results, we observed that zinc added by one ALD cycle yielded ⁓15% ML of ZnO, while we speculate that Cu formed larger structures from nanoparticles to small clusters. Based on this remark, the addition of copper by impregnation after zinc ALD might have disabled some of the ZnO species and prevented their assistance in any further reaction. This speculation is supported by the similar methanol production rates observed for Zn/Cu/Zr and Zn/Cu/Zn/Zr samples.To place these arguments and calculations on a more solid basis, we have compared the results included in this manuscript with the results reported in the literature for earlier studies ( Table 4). In our work, with the Zn/Cu/Zr sample, we report rates of 165 and 340 mmolCH3OH gCu −1h−1 at 500 and 550 K, respectively, while 169 and 321 mmolCH3OH gCu −1h−1 were produced with the Zn/Cu/Zn/Zr sample under the same temperature conditions. Recently, Saedy et al. [48] accomplished a methanol productivity of ⁓ 56 mmolCH3OH gCu −1h−1 at 523 K and 3.0 MPa with a ZnO/Cu/Al2O3 catalyst (Zn added by PCVD) and a Zn/Cu atomic ratio of 0.5. Higher Zn/Cu atomic ratios (up to 1.64) did not significantly affect the production of methanol. It is also important to highlight that we achieved a similar areal number density and % of ML of ZnO for the Zn/Cu/Zr sample to those reported by Saedy et al. (⁓ 1.5 Zn atoms/nm2, ⁓ 13% ML). In a similar study, Gao et al. [27] synthesized ALD ZnO-coated Cu/SiO2 catalysts with various exposure times and ALD cycles and tested them in the hydrogenation of carbon dioxide. Among the prepared catalysts, the most active catalyst (exposure time of 30 s to the Zn precursor and one ALD cycle) was more selective toward carbon monoxide (96 mmolCO gCu −1h−1) than toward methanol (10.6 mmolCH3OH gCu −1h−1) at 523 K and 4.0 MPa. However, for that particular ZnO/Cu/SiO2 catalyst, the areal number density was significantly lower than the value that we report in our manuscript (⁓ 0.6 Zn atoms/nm2, ⁓ 5% ML).In general, the data included in Table 4 highlight the good performance of the catalysts prepared and tested in the present work for carbon dioxide hydrogenation to methanol. It is worth mentioning that this level of activity was achieved with relatively low Cu and Zn metal loadings (⁓1–2 wt%) which led to considerably high methanol production rates when they were expressed per gram of copper. This range of metal contents seemed to be relatively efficient to achieve a good and even distribution of both copper and zinc and to produce a significant amount of active ZnO-Cu sites. As a future challenge, it would be worth investigating whether it is viable to obtain similar methanol production rates per gram of copper with higher metal contents. In this context, atomic layer deposition (ALD) can be an outstanding synthesis method for the scale up of catalysts toward higher metal loadings.Tuning the interaction of zinc and copper must be considered an important parameter to control the catalytic performance toward methanol formation. In this work, by alternating the order in which metal (copper) and promoter (zinc) were added to the catalyst, a series of catalysts with various metal-promoter-support configurations were synthesized. The order in which the zinc promoter was introduced onto the catalyst by atomic layer deposition (ALD) compared to the active copper metal by impregnation affected the catalytic activity. Zinc ALD after copper impregnation (zinc-on-copper) yielded higher CO2 conversion and methanol production rates than copper-on-zinc, although the overall copper and zinc loadings were similar. Advantageously, unlike the other catalysts, the zinc-on-copper zirconia catalyst (Zn/Cu/Zr sample) did not produce any methane with high methanol production rates under the tested operating conditions.Infrared studies of cyclic adsorption of CO2 and H2 revealed that zinc ALD on impregnated copper accumulated carbonates and bicarbonates (CO3 2-, HCO3 -) exceptionally well during the carbon dioxide feed and transformed them into formate species (*HCOO) during the hydrogen feed. Together with the higher CO2 conversion and methanol production rate achieved with the Zn/Cu/Zr sample, this suggests that the catalytic activity to some extent relies on the ability of the catalyst to transform carbonates to formates. The DFT calculations accurately predicted the band position for formate species on different model surfaces (i.e, ZnO/ZrO2, Cu(111) and Cu(110)) compared to the experimental bands observed by DRIFTS. The formate pathway was the favored mechanistic route of carbon dioxide hydrogenation to methanol under the selected experimental conditions. In addition, the CO2 temperature programmed desorption analyses showed the great capacity of the zinc-copper on zirconia catalyst for the adsorption of CO2. When evaluating the molecules of CO2 adsorbed per atom of zinc, the zinc-on-copper configuration adsorbed more CO2 molecules than the copper-on-zinc configuration (0.67 versus 0.45 molecules CO2 per zinc atom). Additionally, according to TPR studies, the zinc deposited after copper impregnation improved the homogeneity of copper oxide species and the reducibility of the bulk CuO.Overall, this work provides insight into the significance of the zinc oxide/copper/zirconia interactions for selective hydrogenation of carbon dioxide to methanol and highlights the potential of atomic layer deposition (ALD) in the synthesis of atomically dispersed metal catalysts for an efficient methanol synthesis. Aitor Arandia: Conceptualization, Investigation, Methodology, Formal analysis, Validation, Visualization, Supervision, Writing – original draft, Writing – review & editing. Jihong Yim: Investigation, Methodology, Formal analysis, Validation, Writing – review & editing. Hassaan Warraich: Investigation, Formal analysis, Methodology. Emilia Leppäkangas: Investigation, Methodology. René Bes: Investigation, Formal analysis, Writing – review & editing. Aku Lempelto: Investigation, Software, Formal analysis, Visualization, Writing – review & editing. Lars Gell: Investigation, Software, Formal analysis, Visualization, Writing – review & editing. Hua Jiang: Investigation. Kristoffer Meinander: Investigation, Formal analysis, Visualization, Writing – review & editing. Tiia Viinikainen: Methodology, Writing – review & editing. Karoliina Honkala: Conceptualization, Writing – review & editing, Funding acquisition. Simo Huotari: Investigation, Formal analysis, Visualization, Writing – review & editing. Riikka L. Puurunen: Conceptualization, Methodology, Resources, Writing – review & editing, Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The work at Aalto University has been financially supported by the Academy of Finland (COOLCAT consortium, decision no. 329977 and 329978; ALDI consortium, decision no. 331082). This work made use of Aalto University Bioeconomy, OtaNano and RawMatters infrastructure. Hannu Revitzer (Aalto University) is thanked for the ICP-OES analysis, Aalto workshop people (especially Seppo Jääskeläinen) for working on the reactor modifications. The DFT calculations were made possible by computational resources provided by the CSC — IT Center for Science, Espoo, Finland (https://www.csc.fi/en/) and computer capacity from the Finnish Grid and Cloud Infrastructure (urn:nbn:fi:research-infras-2016072533). The University of Helsinki acknowledges support from Academy of Finland (project 295696) as well as ESRF for beamtime and Blanka Detlefs and Christoph Sahle for expert support. Preliminary XANES measurements were performed using the Helsinki Center for X-ray Spectroscopy Hel-XAS instrument under the proposal number 2021–0011.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2022.122046. Supplementary material. .

The development of active catalysts for carbon dioxide (CO2) hydrogenation to methanol is intimately related to the creation of effective metal-oxide interfaces. In this work, we investigated how the order of addition of copper and zinc on zirconia influences the catalytic properties, the catalytic activity and selectivity toward methanol. Regarding the carbon dioxide conversion and methanol production, the catalysts on which the promoter (zinc) was atomically deposited after copper impregnation (i.e., ZnO/Cu/ZrO2 and ZnO/Cu/ZnO/ZrO2) were superior catalysts compared to the reverse copper-after-zinc catalyst (Cu/ZnO/ZrO2). Temperature-programmed experiments and in situ diffuse reflectance infrared Fourier transform-spectroscopy (DRIFTS) experiments allowed us to elucidate the benefits of the zinc-after-copper pair to store CO2 as carbonate species and further convert them into formate species, key intermediates in the formation of methanol. This research provides insights into the potential of atomic layer deposition in the development of tailored heterogeneous catalysts for efficient CO2 valorization to methanol.

Metal-organic framework, denoted as MOF, is defined by the International Union of Pure and Applied Chemistry (IUPAC) as “a coordination network with organic ligands containing potential voids” [1]. Since the early 1990s, after the first scientific reports on the development of a new class of porous materials, there has been strong interest in this topic. Almost 30 years of intense research has led to numerous potential applications of MOFs in a wide variety of fields including gas adsorption, separation, catalysis, photocatalysis and bio-sensing. Intensive studies on MOF applications have also included their application in fuel cells and supercapacitors [2–9]. Several synthesis routes of metal-organic networks have been developed over the years. The most utilised are conventional solvothermal and non-solvothermal, microwave-assisted and mechanochemical methods [2,4,10]. Numerous scientific papers report on both solvothermal and non-solvothermal syntheses of MOFs, giving the exact synthesis procedures, and the changes of MOFs’ parameters by the modification of synthesis conditions can be found in the literature. Several MOFs have been synthesised using non-solvothermal methods which require the selection of metal precursors, organic linkers and solvents, as well as the appropriate synthesis temperature. The remarkable success of MOFs in a wide range of applications has pushed scientists to use MOF materials as precursors to obtain catalytic materials with unprecedented properties. However, despite the fact that the recent development in synthesis of metal organic frameworks pushes the limits of the chemical and mechanical resistance of those materials, they are used in a wide range of industrial applications based on catalysis. The next milestone in the application of metal organic frameworks in industry may be not only further improvements in the chemical and mechanical endurance of those materials, but also their structuring into monolith-like, short channel structures membranes or arranged structures which guarantees high heat and mass transport properties. Since the remarkable success in development of structured catalysts in industry-based heterogeneous catalysts including gas exhaust abatement in the automotive sector and stationary source abatement, water gas shift, combustion and NOx abatement [11], the structuring of MOFs into structured catalysts seems to be a natural step forward in their evolution.Several works have recently been published describing the ways of the preparation of structured materials based on metal organic frameworks [12–18]. In the work written by Chen et al. [18], various attempts to produce composite HKUST/Fe3O4 materials in different bodies like pellets, films and foams are described. The authors have developed a method of shaping of composite HKUST/Fe3O4 materials by using carboxymethylcellulose as a binder. By using freeze-drying or gel-induced surface hardening, various foam-like or thin films with high porosity properties have been developed. A complementary method for the preparation of MOF-based foams is described in the work published by Garai et al. [19], where the shaping of metal organic frameworks by transferring them into areogel or xerogel and further solvent removal was proposed. However, despite the versatility of proposed method, the use of foams derived by the aerogel and xerogel method is limited, due to a high fragility of derived structures. In the deposition of metal organic frameworks on the metallic surfaces, much attention has been paid to the preparation of electrodes for lithium-ion batteries [20]. The deposition of metal organic frameworks based on zeolite-imidazole frameworks was performed by annealing treatment. The porous zinc-cobalt oxide porous plates prepared in this way revealed remarkable, high reversible properties as anode materials and considerable lithium storage capacities.Despite the fact that the metal organic framework materials demonstrate great catalytic properties in many catalytic reactions including catalytic oxidation [21–25], selective catalytic reduction [26], alkylation, transesterification [10], water gas shift and conversion of methane to fuels, their heat and mass transfer properties may be successfully tuned up by either their direct shaping into structured catalysts or their deposition on existing carriers. Although several works describing the use of three-dimensional printing of metal organic frameworks to monoliths have recently been published [17], literature reports describing deposition of MOFs on supported carriers are scarce.Structural reactors owe their significant success mainly to their wide use in the automotive and energy industries, where the ceramic or metal monoliths are commonly in use for oxidation and selective catalytic reduction reactions [27]. The catalytic oxidation of hydrocarbons is one of the most important reactions for the conversion of hydrocarbons to obtain valuable products. Over the numerous catalytic reactions, the oxidation of cyclic hydrocarbons such as cyclohexane or cyclohexene results in the formation of value-added products that can be further used in fine chemical synthesis. The exemplary oxidation of cyclohexene with H2O2 may be used as an alternative method for the synthesis of adipic acid, which is further used in production of Nylon-66 [23]. Additionally, the oxidation of cyclohexene may also result in the formation of epoxides and unsaturated ketones and alcohols which are valuable products in organic syntheses and the fragrance industry. Recently, the catalytic oxidation of cyclohexenene to the mixture of oxygen-containing products has been reported for SBA-15 [28], core shell-structures [24] and MIL-101 [21] or modified Ni-MOF-74 catalyst [29]. Although literature reports provide information on the successful use of metal organic frameworks on cyclohexene catalytic oxidation instead of conventional mesoporous catalysts, a common feature of the work is the use of powder catalysts which practically eliminates their wider application. The main reason for that is the necessity of additional mixture/catalyst filtration to receive products instead of simple structured catalyst removal from the batch reactor.In this work, we present an optimised method for the preparation of composite metal organic frameworks for structured catalysts based on metallic plates, woven gauzes and metallic foams as catalysts for aerobic oxidation of cyclohexene. The choice of those types of structures is not accidental, as they are used as catalyst supports: metal monoliths for oxidation and reduction reactions, meshes for oxidation/separation processes and foams for oxidation reactions. The prepared structured catalysts with deposited thin metal organic frameworks have revealed considerable surface areas and remarkable, good adhesion parameters. The catalytic activity tests have proven that the composite metal organic framework catalysts may be successfully used in aerobic oxidation of cyclohexene to produce value-added fine chemicals.All chemicals used in this study were reagent grade and are commercially available. They include nickel acetate tetrahydrate, cobalt acetate tetrahydrate, zinc acetate dihydrate, nickel nitrate hexahydrate, cobalt nitrate hexahydrate, zinc nitrate hexahydrate, 2,5-dihydroxyterephthalic acid (DHTP), all from Sigma-Aldrich, and methylene chloride, n-hexane, N,N-dimethylformamide (DMF), n-propanol, from Chempur Poland.The synthesis protocol used in this study consisted of three steps: support pre-treatment, in situ MOF deposition and material activation. Structured supports used in this study were FeCrAl plate (GoodFellow, 0.3 mm thick Fe 72.8%, Cr 22%, Al 5%, Y 0.1%, Zr 0.1%), steel woven gauzes (17.5 mesh/in., wire diameter 0.1 mm; Fe 73%, Cr 20%, Al 5%) and NiCr foams (Recemat BV; 27–33 ppi, estimated average pore diameter 0.6 mm, Ni 60–80%, Cr 15–40%, Fe 0.5%, Cu 0.1–0.3%).Prior to the deposition of MOF on to the structured carriers, the structures were cut into small pieces – FeCrAl plates 1 cm × 1 cm, FeCrAl gauze 1 cm × 1 cm, NiCr foams 1 cm × 1 cm – and subsequently cleaned in an ultrasound bath using acetone, n-propanol and distilled water to remove impurities. Subsequently, FeCrAl plates and wire gauzes were calcined at 1100 °C in a ventilated oven for 24 h to obtain a thin alumina layer. This procedure of FeCrAl alloy treatment was previously reported as enhancing further adhesion between alloy and deposited material [30].In the second step, the M(M = Zn; Ni; Co)--MOF-74 layers were deposited in situ by modifying the solvothermal method for powder synthesis recently reported in the literature [31,32]. The detailed synthesis conditions are summarised in Table 1 .The first layer deposition of Zn-MOF-74 was performed from Solution I by using zinc acetate as a metal precursor. After dissolution of the appropriate amounts (see Table 1) of metal salt and 2,5-dihydroxyterephtalic acid (DHTP) in N,N-dimethylformamide DMF, the metal salt solution was added to the DHTP solution dropwise to avoid precipitation. The resulting solution was then transferred to Teflon liners with structured carriers previously suspended on scaffolding. The as prepared stainless-steel bombs with Teflon vessels were tightly capped and placed in oven at 100 °C for 20 h. The resulting structured carriers with deposited MOF layers and non-deposited MOF crystals were washed using the sequence proposed elsewhere [33]: methyl chloride three times, and n-hexane three times. The resulting materials were then dried at room temperature and activated in a vacuum drier at 180 °C for 6 h. The double and triple deposition of Zn-MOF-74 was performed by changing synthesis solution I to synthesis solution II with zinc nitrate as a metal precursor.The general procedure for deposition of Co-MOF-74 and Ni-MOF-74 was performed as for deposition of Zn-MOF-74, with the difference that the appropriate metal nitrate (Co or Ni) was used as a metal precursor in all three-layer deposition steps.The crystallinity of prepared materials was determined by XRD analyses using an X'Pert Pro MPD (PANalytical) diffractometer with CuKα radiation at 30 mA and 40 kV. The diffraction patterns were collected in the range of 5–80° 2θ with a 0.033° step for 12 min. The determination of crystallinity M(M = Zn; Ni; Co)-MOF-74 layers deposited on FeCrAl plates was determined by means of Grazing Incidence X-Ray Diffraction analysis (GIXRD). Analyses were performed only for M(M = Zn; Ni; Co)-MOF-74 layers deposited on FeCrAl plates due to the GIXRD method limitations. The GIXRD analyses were performed in 5–75° 2θ range with a 0.033° and constant omega angle 1°.The morphology of prepared structured catalysts was determined by using a Nova Nano SEM 300 FEI Company scanning electron microscope for high-quality magnification imaging. To enhance the visibility of the structure of and the distribution of the Me-MOF-74 layers on structured carriers, the obtained materials were pseudo-coloured using Fiji software. The exact colours of LUT's were determined of an activated MOF samples by using AvaSpec-ULS3648 High-resolution spectrometer equipped with a high-temperature reflection probe (FCR-7UV400-2-ME-HTX, 7 × 400 μm fibres, Avantes BV) and a Mikropack DH-2000-BAL Deuterium-Tungsten Halogen Light Source working in the 200–1000 nm spectral range. The exact colour of the prepared material was determined by AvaSoft 8 software with colour measurements extension (Avantes BV). The determined colours were presented using HEX and RGB values (Table 1).Kr and N2 sorption experiments were performed on ASAP 2020 (Micromeritics) for structured supports, powder samples and MOF layers deposited on FeCr plates and NiCr foams, respectively. Prior to analyses, the samples were outgassed at 250 °C for 12 h. The BET specific surface areas were calculated for p/p0 in the range of 0.06–0.2 and for Kr adsorption and p/p0 = 0.06–0.2 for N2 adsorption experiments.The Me-MOF-74 layers deposited on FeCrAl plates were examined by X-ray Photoelectron Spectroscopy with an ESCA Prevac spectrometer equipped with a hemispheric XPS analyser of charged particles and AES analysers (VG Scienta R3000) and Mg/Al anticathodes. The sample charging effect was corrected using C 1s band at 248.8 eV.The prepared Me-MOF-74 samples were characterised by FTIR spectroscopy using two modes: ATR FTIR for non-deposited MOF crystals that were collected after in situ MOF deposition, and by in situ DRIFT for composite Me-MOF-74 samples deposited on FeCrAl plates. The ATR-FTIR studies were carried out using a Bruker Vertex 70v spectrometer equipped with Bruker Platinum ATR (diamond crystal), by averaging 128 scans in the range of 4000–400 cm−1 with a 4 cm−1 resolution. The in situ DRIFT spectra were collected by using a Thermo Nicolet iS 10 equipped with MCT detector and Praying Mantis High Temperature Reaction Chamber with ZnSe windows (Harrick). The in situ experiments were performed on dehydrated at 110 °C for 1 h in He flow (AirProducts) catalysts samples. To avoid the presence of water and oxygen, the He line was equipped with an Agilent moisture/oxygen trap. The spectra were collected by averaging 128 scans with 4 cm−1 resolution and BaSO4 as a background.The FTIR sorption experiments by using CO (Linde) and CD3CN as probe molecules were performed by using a NICOLET iS 10 spectrometer. The spectra were taken in the 4000-650 cm−1 range with 4 cm−1 resolution by averaging 128 scans. Prior to the spectroscopic measurements, the obtained Me-MOF-74 crystals were pressed into the self-supporting wafers and activated under vacuum at 270 °C with 5 °C/min temperature ramp. The qualitative determination of the nature of the active sites in prepared MOF-74 samples was determined by low temperature (−100 °C) carbon monoxide (Linde) and room temperature CD3CN (Sigma Aldrich) chemisorption. Prior to the chemisorption of probe molecules, the adsorbed gases were distilled by freeze and thaw cycles to remove impurities. The resulting spectra were presented as a substructured spectra after each portion of adsorbed probe molecule and activated sample as a background.To determine the nature and the chemical distribution of deposited metal organic frameworks on structured carriers, the μRaman mapping analyses were performed by using high resolution confocal Raman microscope - Witec Alpha 300 M+ equipped with three ZEISS lenses (x10, x50, x100), two diffraction gratings 600 and 1800, and two 633 nm and 488 nm with power of approximately 50 and 75 mW, respectively. The μRaman spectra were taken for FeCrAl plates due to the optical microscope limitations.The effectiveness and stability of the prepared structured metal organic framework materials was determined in two ways. The effectiveness of MOF-74 in situ deposition was determined by weighing the washed and activated composite materials before and after layering. The mechanical stability test was performed by ultrasound irradiation methods proposed recently in literature for structured catalysts [34–36]. In brief, the washed and activated structured catalysts were immersed in polypropylene jars filled with n-propanol and irradiated in a 40 kHz ultrasound bath (Ultrasonix proclean 0.7 M, 60 W). The weight loss was determined after 15 min of ultrasonic irradiation.Catalytic activity during the aerobic oxidation of cyclohexene was measured under atmospheric and 10 bar O2 pressure for powder samples and MOF deposited on NiCr foams as representative for structured catalysts. The aerobic oxidation of cyclohexene was measured under atmospheric conditions and were performed in glass reactor vessel equipped with a reflux condenser. In a typical experiment, the 50 mg of catalyst (for MOF/NiCr foams 50 mg of catalyst refers to the 50 mg of MOF deposited on NiCr foam) and 10 cm3 of cyclohexene were placed in the reactor and heated to 80 °C for 4 h under oxygen flow. The oxygen flow (Oxygen 5.0, Linde Gas) was controlled by Bronkhorst mass flowmeters and set to 20 ml/min. Prior to the reaction, the glass reactor was purged with molecular oxygen for 15 min with 20 ml/min flow. The experiments under 10 bar O2 pressure were performed in a Buchi Miniclave Stainless Steel reactor. The catalytic experiment procedure was similar to experiments at atmospheric pressure. The O2 pressure was set to 10 bar by using a Buchi manometer at the reactor vessel. Prior to the catalytic experiments, the pressure reactor was purged with molecular oxygen for 15 min.The catalytic reaction products were analysed by the method described in ref. [21], using a gas chromatograph (Thermo Scientific A Trace 1310) coupled with a single quadrupole mass spectrometer (ISQ) equipped with an RXi-5MS capillary column (Restek, USA, 30 m, 0.25 mm ID, 0.25 mm film thickness.). Prior to analysis, the reacting mixtures were thoroughly cooled down in an ice bath to avoid CH evaporation, and approx. 10 mg of PPh3 was added to reduce cyclohexenyl hydroperoxide to 2-cyclohexen-1-ol and avoid further mixture oxidation.The migration of metal (Zn, Ni, Co) from prepared MOF samples to the reaction mixture during the catalytic reaction was determined by atomic absorption spectrometry using a Thermo Scientific ICE3000 series AAS spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). To determine the metal content in post-reaction mixtures, the external standard method was used. The results were processed using Solaar 2.01 software. All standards and reagents were of trace analysis grade.The synthesis of metal organic frameworks may be performed in various conditions by using metal precursors and organic linkers, of which metal nitrates and acetates are commonly used [2]. Since the choice of the starting reagents for synthesis of MOF in powder form may influence the crystal size and the synthesis time, the application of the in situ crystal deposition over the metallic structures should consider crystal-surface interactions [37]. It followed from this that acetates and nitrates were natural choices due to their acidic properties in a liquid solution. The choice of the acetates and nitrates is dictated by their dual role as metal precursors and acidic environment generators. The acidic environment is favourable and commonly used in structured reactor preparation in metallic support pre-treatment [30]. It was previously reported that the use of an acidic environment induces the formation of thin alumina layer on FeCrAlloy material, which increases further adhesion of the deposited layer [38]. Another problem related to the nature of the precursor is that, while acetates can be used for synthesis of various MOF, their use for MOF-74 synthesis is limited for the preparation of Zn-MOF-74 though conventional synthesis and Ni- and Co-MOF-74 through dry-gel synthesis [39]. Based on available literature reports, we used zinc acetate as a starting point mixture in the optimisation of in situ synthesis. To monitor the acidity of the synthesis solutions, we performed measurements of pH before and after in situ solvothermal synthesis (Table 2 ). The acetate solutions’ pH values before the synthesis are very close to neutral point, whereas nitrate-based precursor solutions are strongly acidic (pH ≈ 2.7). Despite the fact that the in situ synthesis of Zn-MOF-74 resulted in well crystallised MOF-74, as already been postulated in the literature [39,40], the amount of MOF-74 deposited on structured carriers was considerably low. Hence, for the double and triple synthesis of MOF layers on metallic supports, we used metal-nitrates as metal precursors. However, it has to be pointed out that the use of the metal nitrate as an MOF metal precursor at the first layer deposition did not result in either deposition of the MOF layer at the metallic carrier or formation of the Zn-MOF-74 crystals on the bottom of the reaction vessel. To confirm the crystallinity and the purity of obtained materials, PXRD for non-deposited powder MOF-74 (Fig. 1 , left column) and GIXRD for MOF-74 deposited on FeCrAl plates (Fig. 1, right column) were performed. In all prepared materials, as well as for the non-deposited crystal phase and thin layer deposited on metallic carriers, the presence of Zn-MOF-74 (JCPDS 00-062-1198), Ni-MOF-74 (JCPDS 00-62-1029) and Co-MOF-74 (JCPDS 00-063-1147) structures without impurities [39,41,42] was confirmed. The use of GIXRD analysis allowed high quality diffraction patterns on MOF layers deposited on FeCrAl plates to be obtained. Despite the fact that the GIXRD measurement was performed at a low angle, we could still observe reflections at 25.6, 35.1, 37.8, 43.5, 52.6 (024) and 57.6°, which are characteristic of α-Al2O3 [43] (JCPD 04-005-4503) from FeCrAl support. The α-Al2O3 is the result of the FeCrAl support calcination at 1100 °C which enhances the adhesion of deposited MOF layers. The detailed phase analysis was previously reported in our previous paper [44] and also in GIXRD profile analysis in supporting information (Figs. S1-S2). It may be seen that the intensity of characteristic α-Al2O3 reflections decreases in the Co-MOF-74 >Ni-MOF-74> Zn-MOF-74 order, which may suggest that the thickness of metal organic framework layers in prepared structured catalysts increases. It is also worth mentioning that, in all considered materials, we observed that the crystallisation of MOF material over the metallic support was strongly influenced by the number of metallic supports placed in the Teflon liners for in situ deposition. Once the total amount of metallic supports exceeded 1 g per synthesis, we did not observe the metal organic framework crystals either in reacting vessels or deposited on the structured carriers.To determine the structure and the purity of the MOF layers deposited on FeCrAl plates, the XPS analysis of triple deposited MOF-74 layers on FeCrAl plates was performed. The results of the XPS analyses are presented in Fig. 2 . The survey spectra of the triple deposited MOF-74 layers deposited on FeCrAl plates (black lines) and calcined FeCrAl plates are presented in Fig. 2 A, D, G. It may be seen that the survey spectra of Zn-MOF-74, Ni-MOF-74 and Co-MOF-74 do not reveal any lines originating from calcined FeCrAl plates (cf. red lines) and only signals from Me(Zn, Ni, Co) 2p, O1s and C1s may be observed. Since the alumina is mainly present at the calcined FeCrAl plate surface due to the migration of alumina at 1100 °C calcination, we used the signal at 75 eV originating from Al 2p [45] as an internal marker to determine the purity deposited MOF-74 layers. The zoomed area for 75 eV region for Me(Zn, Ni, Co)-MOF-74 catalysts are presented in Fig. 2 B, E, H. It may be seen that, for all considered cases, the Al 2p line does not occur at the XPS spectra of Me (Zn, Ni, Co)-MOF-74 catalysts. The XPS spectra for Zn 2p, Ni 2p and Co 2p for Me (Zn, Ni, Co)-MOF-74 are presented in Fig. 2 C, F, I. The Zn-MOF-74/FeCrAl catalyst reveal two main peaks at 1022.2 and 1045.3 eV (Fig. 2 C) that may be attributed to Zn 2p3/2 and Zn 2p1/2 [46]. For the Ni-MOF-74/FeCrAl catalyst two main group bands were detected with the peaks at 855.9 and 873.6 eV and associating satellite peaks at 860.7 and 879.4 eV, which may be attributed to Ni 2p3/2 and Ni 2p1/2 [47], respectively. At the XPS spectrum of Co-MOF-74/FeCrAl, catalyst peaks at 781.9 and 797.8 eV and associating satellite peaks at 785.8 and 802.6 eV are observed. These may be attributed to Co 2p3/2 and Co 2p1/2 [48], respectively.The effectiveness of the in situ MOF deposition over structured supports was determined gravimetrically after each deposition. The results are presented in Fig. 3 A. The effectiveness of the MOF deposition on the structured carriers was presented as a mass increase per geometrical surface area of metallic support. Such deposition results are commonly used for the comparison of coating loading in structured reactors engineering [27,49]. The lowest MOF loading was observed for the layers deposited on FeCrAl plates. For this support, the individual deposition of Zn- and Co-MOF-74 layers never exceeds 0.32 mg/cm2 (maximum value achieved for Zn-MOF-74 after double deposition). The maximum mass increase after triple deposition was achieved for Co-MOF-74, and was equal to 0.669 mg/cm2. The deposition of MOF layers of on FeCrAl wire gauze results in considerable MOF mass increase on metallic support. In general, the MOF loading on wire gauze increases on average by a factor of two, with some minor derogations for Co-MOF-74 at single deposition where this value increases almost four-fold, and for Zn-MOF-74 at triple loading, where the mass increase is almost one order of magnitude higher than for the FeCrAl plate. When considering the total mass increase on the FeCrAl wire gauze in comparison with the FeCrAl plate, the mass loading factor increases in a arrange 2.9-fold for Zn-MOF-74, two-fold for Co-MOF-74 and up to 2.1 times for Ni-MOF-74 (cf. Table S1). The highest metal organic metal loading by in situ deposition was achieved for NiCr foam. Analysis of the obtained MOF loading values (Table S1) reveals that the maximum MOF loading was achieved after triple deposition of Co-MOF-74. Considerable high values were achieved for double deposition of Zn-MOF-74. It must be emphasised that the total mass increase forms the following order Co-MOF-74>Zn-MOF-74>Ni-MOF-74, which is similar to MOF loading on the FeCrAl plate and wire gauze. It must be also pointed out that the Ni-MOF-74 indicated the worst adhesion properties on all considered metallic carriers.The morphology of deposited coatings on structured supports was determined using two methods: digital photography and SEM microscopy. The results of digital photography imaging are presented in supplementary materials in Figs. S3–S5 for Me (Zn, Ni, Co)-MOF-74 layers deposited on FeCrAl plates, FeCrAl wire gauzes and NiCr foams, respectively. In the case of Zn-MOF-74, the single deposition on each structured support is barely seen in digital pictures. Considerable changes in layer deposition on each structured support may be observed after double and triple deposition (Figures S3-S5, B and C). For Ni and Co-MOF-74 layers, the single deposition of MOF material may be observed. To determine in detail the morphology of prepared structured catalysts, SEM analysis was performed. To enhance the visibility of SEM images, pseudo colouring by using defined RGB colours determined by UV–Vis spectroscopy was performed. The SEM images are presented in Fig. 4 for three structured carriers, and in Fig. 5 for triple deposited MOF layer on NiCr foams with 2000x magnification. Since the whole matrix contained 27 images per single SEM magnification, the results for each deposition for M(M = Zn; Ni; Co)-MOF-74 are presented in supplementary materials in Figs. S6–S14. The deposition of Zn-MOF-74 on structured carriers is presented in Figs. S6-S8. It can be seen that, after single deposition, surfaces of all three structured carriers at the lowest magnification (200x) do not show any substantial changes in carrier morphology. This changes upon increasing magnification from 2000x up to 5000x. The surface seems to be coated with a thin layer of MOF with visible small crystals of irregular shape. This phenomenon changes after double in situ coating (Fig. S7). In this case, even a quick look at the catalyst's surface at low-magnification images reveals the complete coverage of the structured carrier. The crystals began to grow in more regular shape, similar to hexagonal rods. The shape of the Zn-MOF-74 structures is more evident for wire gauze and foam structures. The MOF-74 growth on structures is evident, and good adhesion may be observed. The higher magnifications also reveal smaller crystals found on larger ones (Figure S6-S8 E-F). The triple deposition reveals full surface coverage in all three structured carriers. The MOF crystals reveal full developed shapes. Detailed analysis of SEM images allows the thickness of the Zn-MOF-74 layers to be determined, which in that case is equal to 40 μm. The important feature of Zn-MOF-74 layers is depicted in Fig. 4 A1, B1, C1 as well as in Figs. S6–S8 G-I, where, for the foam carrier, the MOF crystals are perpendicularly oriented to the foam surface, in contrast to the FeCrAl plates and wire gauzes, where the stochastic orientation prevails.The SEM images for Ni-MOF-74 are presented in Fig. 4 A2, B2, C2 for triple deposition and in Figs. S9–S11 for single, double and triple deposition. It may be seen that the crystal morphology is far different from that of Zn-MOF-74 crystals. The surfaces of all three structured carriers are covered with spherical crystals with an average diameter of 10 μm. However, it must be emphasised that the crystals form a thin layer which is more visible after double and triple coating of wire gauze and foam carriers. One can observe that surface coverage is uniform after double deposition on structured carriers. After triple deposition, the carriers’ surfaces reveal point-crystal growth (Fig. 4 B2 and C2). The thickness of the Ni-MOF-74 layers was equal to the average MOF particle diameter, i.e. 10 μm. The average thickness after triple coating was approx. 30 μm (cf. Fig. S11 G).The Co-MOF-74 morphology is presented in Fig. 4 A3, B3, C3 and Figs. S12–S14. The crystal morphology exhibits more regular hexagonal shape in comparison with Zn-MOF-74. It can be seen that complete carrier coverage is achieved after single deposition in all considered carriers (Figures S12 A-I). It must be emphasised that, for single deposited Co-MOF-74 on NiCr foam, there is different morphology in comparison with Co-MOF-74 deposited on the FeCrAl plate and wire gauze. The foam surface seems like it was treated by some kind of MOF primer and forms the incubation-like centres for further crystal growth. The morphology of the Co-MOF-74 crystals is similar for all kinds of metal supports after single deposition (Figures S12 A-I). In all considered structured carriers, the hexagonal crystal is perpendicularly oriented to the metallic carriers. The triple deposition of Co-MOF-74, however, causes crystal aggregation, and local crystal spots can be observed especially for the FeCrAl wire gauze and NiCr foam. However, the presence of the local crystal hypertrophies is not evident as in the case of Zn- and Ni-MOF-74 layers. It must also be pointed out that the thickness of the Co-MOF-74 layers is lower than for Ni-MOF-74 and is equal to 20 μm (average single crystal size). Due to the growth of the MOF crystals perpendicular to the support surface, the crystal tends to fill the free space between crystals rather than to overgrow already grown crystals.The results of the krypton and nitrogen adsorption on bare structured carriers, MOF powders and MOF deposited on metallic supports are summarised in Table 3 . The krypton adsorption on structured supports revealed that structured carriers are non-porous solids (Table 3 A). The measured SBET for the FeCrAl plate, wire gauzes and NiCr foams were equal to 0.027, 0.012 and 0.039 m2/g, respectively. The nitrogen adsorption on powder samples (Table 3 B), collected using the in situ solvothermal method, revealed that the specific surface SBET areas of prepared samples were approx. 1000 m2/g for all prepared powder MOF-74 samples, which corresponds well with the results presented in the literature [39,42]. Since for the characterisation of metallic structured catalysts with deposited porous metal organic framework layers there is no proposed methodology for the presentation of the SBET results, the data presentation was two-fold. To compare the specific surface of the M(M = Zn; Ni; Co)-MOF-74 layer over representative FeCrAl support, the SBET was referred to the mass of MOF-74 deposited on the metallic carrier. This value was determined gravimetrically after M(M = Zn; Ni; Co)-MOF-74/FeCrAl plate activation. However, to compare the values of the specific surface between the supported catalysts, the SBET was referred to the total mass of the structured catalyst. When analysis of SBET for the FeCrAl plate referred to the deposited MOF layer (Table 3 C), it may be seen that the values for SBET are lower than the calculated specific surface areas for powder samples, and are equal to 331.6 m2/g for Zn-MOF-74, 823.5 m2/g for Ni-MOF-74 and 716.7 for m2/g for Co-MOF-74. It may be observed that a considerable decrease was observed for Zn-MOF-74, where the value of specific surface area was approx. 700 m2/g lower than for its powder counterpart. The difference between the calculated SBET values may be two-fold. The successful in situ synthesis of Zn-MOF-74 over metallic structures was achieved by the optimised triple synthesis, where the primer layer on Zn-MOF-74 was prepared from the zinc acetate solution, whereas double and triple deposition was synthesised by using a nitrate solution as zinc precursor. For Ni- and Co-MOF-74 catalysts, the observed SBET decrease was lower and equal to approx. 200 m2/g and 300 m2/g. In this case, however, the Ni- and Co-MOF-74 the triple deposition may cause crystal overgrowth which may influence the overall SBET value. Additionally, the multiple layer deposition may also influence the availability of micro and mesopores for adsorbed molecule. Analysis of the SBET values referred to the total mass of the structured catalyst (Table 3 D; mass of the metallic carrier + mass of the deposited layer) leads to the general conclusion that the amount of the deposited metallic organic frameworks on the structured support increases in the following order: FeCrAl wire gauze > FeCrAl plate > NiCr foam, which is different than the gravimetrical measurements from Table S1 and Fig. 3. However, it must be emphasised that the values determined by the gravimetrical method were performed after structure catalyst washing after in situ deposition and are not impacted by the high temperature UHV activation of catalysts samples in the sorption meter. Analysis of the literature data on TGA analysis of the metal organic frameworks leads to the conclusion that, at approx. 300 °C, M (M = Zn; Ni; Co)-MOF-74 is equal to 30 wt %. of the initial mass [39,40]. In this study, the activation of MOF prior to the N2 sorption was performed under 250 °C to ensure effective activation. Since the metal supports used in this study are non-porous solids, we can estimate the mass of the catalyst deposited on the surface of the structured supports by formula previously proposed in the literature [50]: (1) m M O F d e p o s i t e d o n t h e s u p p o r t = S B E T , M O F d e p o s i t e d o n t h e s u p p o r t S B E T , M O F p o w d e r · m M O F p o w d e r · 1000 , m g where: mMOF deposited on the support is the approximated mass of the deposited MOF layer on structured support, SBET, MOF deposited on the support is the specific surface area of the structured reactor (structured support with MOF layer), MOF powder is the mass of the powder used to calculate SBET equal to 1 g. The calculated values of MOF mass deposited on different structured supports lead to the conclusion that the MOF-74 layers are favourably deposited on NiCr foams and FeCrAl plates. However, to fully characterise the effectiveness of the in situ layering, the type of the MOF-74 by metal should be considered. It may be seen that the lowest calculated MOF masses were obtained for Zn-MOF-74. Despite the fact that, in the case of Zn-MOF-74, XRD analyses revealed a characteristic pattern for MOF-74 crystals at the metallic support, deep analysis of the SEM pictures for individual depositions shows that the well-defined crystals are formed after triple deposition (Fig. 5 and Figs. S6-S8). The first two layers should therefore be defined as intermediate MOF-layers or primer MOF-layers. The decrease in calculated MOF referred to the total mass of the structured catalysts using N2 sorption is related to the low contribution of well-crystallised MOF on the overall mass of the deposited layer. The opposite situation can be observed for Ni- and Co-MOF-74 layers deposited on structured carriers. Here, the total mass of deposited MOF calculated from N2 sorption gives two-order of magnitude higher values of deposited MOF when comparing to Zn-MOF-74. The SEM results clearly show the growth of well-defined crystals on structured supports after single deposition (Figs. S9-S11).To determine the molecular nature of the prepared structured MOF catalysts, IR and Raman analyses were performed. The detailed IR analysis of prepared samples using ATR, DRIFT and transmission IR can be found in supporting information (Fig. S15). The characterisation of the active centres in prepared materials was performed by the sorption of two probe molecules: carbon monoxide and CD3CN. Both probe molecules are commonly used to study acidic and basic properties of heterogeneous catalysts. The results of CO and CD3CN adsorption are presented in Fig. 6 and Fig. 7 , respectively. The low temperature of carbon monoxide adsorption on M(M = Zn; Ni; Co)-MOF-74 gives the rise of the main band at 2160-2180 cm−1, which corresponds to Me2+-CO adducts formed in the prepared metal organic framework catalysts. It has been previously reported in the literature [51,52] that the values of the main CO adsorption bands for M(M = Zn; Ni; Co)-MOF-74 decreases in the following order: Ni (2180 cm−1) > Zn (2173 cm−1) > Co (2162 cm−1). The high C–O stretching frequencies are derivative of the smallest size and the highest polarisation of Ni2+ ion for the Ni-MOF-74 sample (Fig. 6 B) [51,52]. It must be emphasised that, upon increase of partial pressure of carbon monoxide, the minor bands at 2150-2100 cm−1 and 2200-2250 cm−1 can be observed and may be attributed to some combination overtones of ν(CO). It was also observed that, at high CO coverages, for Zn-MOF-74 and Ni-MOF-74 an additional band at around 2135 cm−1 is formed, which was previously assigned to liquified CO in the MOF pores [53].The results of CD3CN probe molecule adsorption on M(M = Zn; Ni; Co)-MOF-74 catalysts are presented in Fig. 7. The adsorption of CD3CN probe molecules shows rise of a sharp and intensive band at 2110 cm−1, which is characteristic of deuterated ν(CD3) vibrations, and two intense bands at 2237 and 2290 cm−1, which may be attributed to physiosorbed CD3CN and coordinated CN species to Lewis acid sites, respectively [53,54]. The acidic properties of various MOF materials by using CD3CN as a probe molecule has recently been reported for MIL-140C (Zr), MIL-140D (Zr) [55], MIL-100 (Al, Fe, Cr) [54]. It must be pointed out that the values of ν(CD3) and ν(CN) vibrations are similar to those reported for MIL 140C, D and MIL-100 metal organic frameworks, which may lead to the conclusion that they possess similar acid strength.The complementary experiments of molecular properties of prepared samples were performed by μRaman spectroscopy. The results of μRaman analysis were presented as a μRaman maps (Figs. 8 and 9 ), for two reasons. The μRaman mapping allowed us to show the distribution of the MOF over the metallic carrier. Comparison of the μRaman maps leads to the conclusion that the most uniform distribution was achieved for Ni and Co-MOF-74 samples (Fig. 8 C and D). Indeed, the μRaman maps also exhibit local layer overlapping (brighter spots at μRaman maps), which is in good agreement with SEM images for samples after MOF triple deposition. However, it must also be pointed out that the determination of the surface homogeneity using only μRaman maps must be carried out with a high degree of caution, since μRaman maps for the homogeneous calcined FeCrAl plate also reveal some local increase in Raman intensity. The corresponding Raman spectra (Fig. 9) exhibit the structure of prepared composite samples. The Raman spectrum of the calcined FeCrAl plate (Fig. 9 A) reveals bands at 418, 630 and 750 cm−1, which may be attributed to α-Al2O3 of hexagonal symmetry (band at 418 cm−1) [56], α-Fe2O3 (band 630) and γ-Fe2O3 [57]. The μRaman of the FeCrAl plate may be treated as a marker. Since the depth of the sample penetration is relatively high for Raman scattering, the presence or absence of a marker band may be useful in determining the surface thickness. In our previous work, we reported that the use of various characterisation techniques such as XPS, μRaman and EDX allows the determination of the in-depth distribution of the active phase [58]. Here, we can observe that, for the M(M = Zn; Ni; Co)-MOF-74 composite catalysts deposited on metallic support, there was no signal originating from the metallic support. The Raman spectra of M(M = Zn; Ni; Co)-MOF-74 reveal two main band group regions: to 820 cm−1 and 1200-1700 cm−1. The 1200-1700 cm−1 reveals bands at 1275, 1412, 1501, 1560 and 1619 cm−1, which may be attributed to ν(C–O) from deprotonated hydroxyls, symmetric ν(COO−) and stretching and deformation vibrations of benzene rings [41], respectively. The bands at lower frequencies, at approx. 820 and 560 cm−1, may originate from benzene ring bending and deformation vibrations, respectively [41,51]. The additional bands, at approx. 413 cm−1, can be due to ν(Me–O) vibrations [51]. Comparison of the Raman maps for M(M = Zn; Ni; Co)-MOF-74 structured catalysts and the FeCrAl plate lead to the conclusion that the metallic carrier is uniformly covered with the MOF layer. Similar observations can be observed from the analysis of XPS results (cf. Fig. 2 A).The adherence of the deposited on metallic support M(M = Zn; Ni; Co)-MOF-74 layers was evaluated by using an ultrasound bath mechanical resistance test. This type of examination is frequently used for layer adherence testing in structured catalyst characterisation [37,59,60]. The results of the MOF layer adherence performance for various structured supports are presented in Fig. 3 B. The results are presented as a percentage of mass loss during ultrasonic irradiation treatment. The best adherence properties were observed for NiCr foams. After the ultrasonic irradiation test for Zn-MOF-74, almost 50% of the deposited material remained at the support surface. This value was slightly lower for the Ni and Co-MOF-74 layer, with 40% and 35% of the material deposited over a metallic foam. The metal organic framework layers deposited on FeCrAl wire gauzes indicated lower adherence to the structured support. In the case of Zn-MOF-74, almost all of the deposited material was removed from the structured support, whereas, for Ni and Co-MOF-74, 10% and 20% of the deposited material remained on the support. Comparison of the layer adherence to the support carrier after mechanical resistance testing for FeCrAl plates and NiCr foams leads to the conclusion that the stability of the deposited MOF material is derivative either of the available geometrical area and its shape or of the total volume of the support which is sonochemically treated. During the mechanical stability experiment, the structures were stochastically placed in an ultrasonic bath. Their natural arrangement in the bath left one of the sides less subjected to ultrasounds. What is more, comparison of the support structure morphology for wire gauzes and foams may lead to the conclusion that intensity of ultrasound waves can be gradually screened by the bone-like structure of NiCr foam. It must be emphasised that the literature reports on the deposition of metal organic frameworks on metallic supports is rather scarce, which makes comparison of the obtained results with other literature reports impossible. Since the metal organic frameworks are mainly formed into the desired shapes, such as pellets, foams or monoliths with the addition of a binder [17–19], or as required in the case of their use as the electrodes [13], the influence of the other kinds of forces of the prepared materials has been considered.The catalytic activity of prepared M(M = Zn; Ni; Co)-MOF-74 powders and Me (Zn, Ni, Co)-MOF-74 deposited on NiCr foams was measured in the aerobic oxidation of cyclohexene. The results are summarised in Table 4. It must be emphasised that bare metallic supports revealed no activity in the aerobic activation oxidation of cyclohexene. The result of catalytic activity is expressed as a function of total conversion of cyclohexene and individual selectivity to the main products: cyclohexene oxide, 2-cyclohexen-1-ol, 2-cyclohexen-1-one and trans-cyclohexane-1,2-diol. It may be seen that the activity of all prepared powder catalysts exceeds 50% conversion. The activity of prepared powder samples was: 66.5% for Zn-MOF-74, 59.0% for NiMOF-74 and 52.3% for Co-MOF-74 catalysts. Analysis of the selectivity for prepared samples shows that, for Zn- and Ni-MOF-74 catalysts, the oxidation reaction proceeds mainly to 2-cyclohexen-1-ol and 2-cyclohexen-1-one. In the case of the Zn-MOF-74 catalyst, the selectivity to cyclohexene-1-ol and 2-cyclohexen-1-one was 65.4% and 13.9%, whereas for Ni-MOF-74 it was 74.3% and 13.3% respectively. The selectivity for the cyclohexane oxide was 12.5 and 8.7% for Zn-MOF-74 and Ni-MOF-74, respectively. However, when analysing the oxidation reaction results for Co-MOF-74, it may be seen that the cobalt oxide favours the epoxidation reaction, with cyclohexane oxide as the main product with almost 19% selectivity, whereas the contributions of the 2-cyclohexen-1-ol, 2-cyclohexen-1-one and the other products were lower. Moreover, among the products, trans-cyclohexane-1,2-diol was not detected. Additionally, the contribution of the side products reached 30%. Although in the literature [29,61] we can find some results on cyclohexene catalytic oxidation over Me-MOF-74 catalysts, comparison of the obtained results is impossible due to different synthesis procedures for MOF-based materials and their different physicochemical properties. For example, Ruano et al. [61] synthesised the catalysts from metal acetate solutions (Zn, Co, Ni, Mn and Cu)-MOF-4 with another nanocrystalline structure. Furthermore, the morphology of prepared MOFs in Ref. [61] was far from that of our materials. The SBET values presented in Refs. [61] were 948, 693 and 514 m2/g for Zn-, Co- and Ni-MOF-74, respectively. These SBET results are considerably lower than the SBET values presented in this work. The next difference between our work and [61] lies in the fact that, during the catalytic activity tests, Ruano et al. [61] used H2O2 or tetr-buty hydroperoxide (TBHP) as an oxidising agent together with atmospheric oxygen. Indeed, both oxidising agents can be used to either initialise radical reaction (TBHP) or oxidise cyclohexene, but the oxidising effect is supposed to be higher than in the case of molecular oxygen. Despite this fact, the authors presented cyclohexene conversion reaching 71.5% for Co-MOF-74, 40% for Ni-MOF-74 and 5% for Zn-MOF-74, and analysis of the reaction product was performed by gas chromatography equipped with flame ionisation detector. In relation to the work written by Sun et al. [29], the preparation results were different from the preparation conditions presented in this study.When analysing the oxidation results under 10 bar O2 pressure, a general increase of the activity for Ni- and Co-MOF-74 samples can be observed. The conversion of cyclohexene for Ni-MOF-74 increases up to 81.7%, whereas for Co-MOF-74 the conversion is equal to 67.9%. The individual selectivity for the oxidation products changes for Ni-MOF-74 at 10 bar O2, with considerable increase to 2-cyclohexen-1-one, cyclohexane-1,2-diol and other products. In the case of Co-MOF-74, with the reaction at elevated O2 pressure, the selectivity of oxidation products remains at the same level, with a slight increase of selectivity to cyclohexane-1,2-diol. For Zn-MOF-74 powder catalysts, we could see no considerable changes in either conversion or selectivity. Catalytic activity was also determined for MOF catalysts deposited in situ on NiCr foams. Through analysis of the results of the catalytic activity under 10 bar O2 pressure over structured M(M = Zn; Ni; Co)-MOF-74 deposited on NiCr foams, a general decrease in conversion of cyclohexene can be observed. It can be seen that, in all considered MOFs deposited on NiCr foams, the conversion of cyclohexene decreased by a factor of two. The reason of this phenomenon can be explained by the decrease of the effectiveness factor of the catalyst in cyclohexene oxidation. Despite the fact that in all catalytic experiments the same catalyst amount was used (50.0 mg), it must be pointed out that, in the case of powder catalysts, the availability of the active sites is higher due to the wide distribution of catalysts in the reacting mixture. The comparison of the SEM results in Figs. S6-S14 for both supported catalysts and powder MOF-based materials shows that the size of the individual grains varies from 5 to 10 μm, whereas the thickness of the deposited layer is as high as 40 μm. The considerable thickness of the MOF layer on the support may lead to a considerable decrease in the catalytic activity of prepared materials according to the Thiele modulus. However, the calculation of the Thiele modulus and effectiveness factor calculations exceeds the scope of this article, indicating future directions for the application of structured reactors with deposited MOFs.The characterisation of MOF materials in the catalytic oxidation of cyclohexene should consider also a factor related with the migration of a metal from MOF structure to the reaction solution. The results of the metal content in post-reaction mixtures are presented in Table 4. Analysis of the obtained results leads to the conclusion that, in the case of Zn-MOF-74 and Ni-MOF-74, the metal content in the post-reaction mixture was below the detection limit. Only small amounts of zinc ions were detected in the post-reaction mixture (0.12 mM). Noticeable amounts of metal in the post-reaction mixture were observed for Co-MOF-74. The amount to detected cobalt was approx. 3 mmol for the Co-MOF-74 powder sample for the oxidation reaction under atmospheric and 10 bar O2 pressure. However, for MOF deposited on NiCr, the value of detected Co was one order of magnitude lower, and was equal to 0.39. The decrease of cobalt migration to the reaction mixture may be related with the generally lower activity of the Co-MOF-74/NiCr catalyst and the good adhesion of the MOF to the NiCr foam surface. It must be emphasised that, in the case of MOF catalysts deposited on NiCr foams, the catalysts were placed in the reaction vessel and simply removed after the reaction, whereas cobalt catalysts in powder form required additional filtration to separate the reacting mixture and powder catalyst. The lack of additional filtration of the post-reaction mixture and catalyst in the case of MOF deposited on NiCr may be a fundamental step towards the wider application of MOF materials as heterogeneous catalysts.The aim of this paper was to obtain and characterise thin metal organic framework layers on various metallic structured supports by using spectroscopic and microscopic methods, and to determine their potential application in the catalytic oxidation of cyclohexene. The in situ deposition of metallic organic framework thin layers consists of three steps, including support pre-treatment, in situ solvothermal deposition and MOF-layer activation to remove residual solvents from synthesis protocol. The prepared structured carriers with deposited MOF-74 layers were characterised with various characterisation techniques to determine the surface morphology and their molecular structure. The in situ deposition of metal organic frameworks was the most effective for Zn- and Co-MOF-74 on NiCr foams, giving the approx. 4 mg/cm2 mass increase after triple coating. We have indicated that there is no difference in molecular structure between in situ deposited and non-deposited crystalline phase of metal organic frameworks. The high mechanical resistance of prepared M(M = Zn; Ni; Co)-MOF-74 layers on NiCr foams and FeCrAl plates was confirmed by the ultrasonic irradiation performance.The activity of prepared MOF catalysts both in powder form and MOF deposited on NiCr foams was measured in the catalytic oxidation of cyclohexene. The prepared catalysts revealed high activity in the studied reaction, with the conversion exceeding 50% for powder catalysts under both atmospheric and elevated pressures. The catalysts deposited on NiCr foams revealed twice lower conversion in comparison with their powder counterparts. However, the use of structured catalysts did not require their additional filtration from the reaction mixture, which makes them favourable for further testing as heterogeneous catalysts in the organic reagents oxidation.We believe that the in situ deposition of metal organic frameworks from Me2(dobdc) group, proposed in this study, will lead to the substantial development of MOF materials and their further application in heterogeneous catalysis as structured reactors. P.J. Jodłowski: Formal analysis, Investigation, Data curation, Writing - review & editing. G. Kurowski: Formal analysis, Investigation. K. Dymek: Formal analysis, Investigation. R.J. Jędrzejczyk: Formal analysis, Investigation. P. Jeleń: Formal analysis, Investigation. Ł. Kuterasiński: Formal analysis, Investigation. A. Gancarczyk: Formal analysis, Investigation. A. Węgrzynowicz: Formal analysis, Investigation. T. Sawoszczuk: Formal analysis, Investigation. M. Sitarz: Formal analysis.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to acknowledge dr Jakub Marchewka (Faculty of Materials Science and Ceramics, AGH University of Science and Technology) for digital photography of prepared structured catalysts and also Maciej Bik (Faculty of Materials Science and Ceramics, AGH University of Science and Technology) for GIXRD profile fitting and phase assignment.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.micromeso.2020.110249.

The aim of this study was to obtain and characterise thin metal organic frameworks layers supported on various metallic structured carriers such as FeCrAl plates and woven gauzes and NiCr foams. The thin layers of the metal organic frameworks were fabricated by in situ solvothermal deposition, optimised by the selection of metal precursor and the layering/washing order. The parameters of the resulting metal organic framework coatings were characterised in terms of layer thickness in correlation with the fold overlap, morphology, chemical properties and mechanical resistance to ultrasonic irradiation. Several techniques were used to characterise metal-organic framework layers, including in situ FTIR, μRaman mapping, XRD, low temperature sorption of liquid nitrogen, and SEM. The results of structural analysis of prepared structured catalysts revealed that the surfaces of the structured carriers are uniformly covered with Me-MOF-74 thin layers. The mechanical stability tests showed that the metallic foams possessed high mechanical resistance and may be considered as a structured support for heterogeneous catalysts.

Data will be made available on request.The progress in industrialization and urbanization have created different environmental pollution issues worldwide. Regardless of much developments, industrial units are one of serious threats to the environment. Continuous production waste and their discharging into water resources is extremely terrible due to the mutagenic and carcinogenic nature of the pollutants [1–4]. Behind this water pollution, the textile, paper, leather, plastic, pharmaceutical, food processing and cosmetics units are principal offenders. The discharged water from such industries contains both organic and inorganic particulates that are malignant to human beings and aquatic life [5–8]. So, for healthy environment of all the living organisms, the removal of these wastes from water is necessary. In this regard, several strategies have been employed, i.e., ozonation, electrocoagulation, electrochemical destruction, photo-Fenton degradation, membrane filtration, coagulation, ion exchange and adsorption [3,9–13]. Among these, the photocatalytic process is proved to be highly efficiency and if the catalyst is active under visible light, then this process also viable economically. Various catalysts have been studied that showed promising efficiency for the removal of toxic pollutants, i.e., composites, metallic oxides (Mn3O4, CoO, NiO, CuO, ZnO, CeO2, SnO2) doped materials (Sn doped titania, N-doped Zirconia, NiFe2O4 and ZnO heterostructures and BaFe12O19) have been applied as photocatalysts and their response was promising [3,14–19]. Amongst these materials, ferrites have been investigated extensively owing to their ease of preparation, high stability as well as magnetically recoverable nature. There are six types of hexaferrite materials (M−type, U-type, W-type and X-Z-types). M−type hexaferrite possess hexagonal crystal lattice with P63/mmc space group in which 11 symmetry sites are present with 64 ions per unit cell. Due to high magnetization, chemical stability and large microwave magnetic loss, these hexaferrites are highly explored for different applications. Ferromagnetic oxides include M−type hexaferrites (MFe12O19), where M may be divalent cation, i.e., Ba and Sr. The SrFe12O19 (SFO) belongs to metallic oxide-type hard magnetic compounds and has been used usually in many technological applications [4,20–22]. This compound with magneto plumbite structure exhibits electromagnetic behavior owing to its higher magnetic parameters, low conductivity loss and high permeability. It has been extensively employed in microwave absorbers, permanent magnetic designs, magnetic recording media (MRM), high-frequency electromagnetic (HFEM) devices, sensors and EM (electromagnetic) protecting devices [23]. The SFO being n-type semiconductor has been utilized as magnetic component in the fabrication of metal oxide photocatalysts [3,16,17,24]. Owing to their magnetic nature, these composite photocatalysts can be recollected from reaction mixture via simple and economic method by using a magnet. Xie et al. [25] fabricated magnetic Bi2O3/SrFe12O19 heterojunction and observed the enhanced photocatalytic performance of SrFe12O19 due to the heterojunction of p-n type. These hexagonal ferrites have been studied occasionally in heterogeneous catalysis for photodegradation of dyes. Different methods have been used to synthesize SFOs, i.e., co-precipitation, sol–gel, hydrothermal and microemulsion method etc [4,16,23]. Among these synthesis strategies, the micro-emulsion approach offers various advantages versus other synthesis techniques, i.e., to control the particle shape, size, surface area, morphology and homogeneity. It is also a facile, fast and eco-benign versus others [3].Based on aforementioned facts, microemulsion is facile and low temperature operating strategy which imparts good homogeneity and produces crystallites in nano range. Herein, we have fabricated pure SrFe12O19 (SFO) and Ni2+ doped SrNixFe12-xO19 (x = 0.05–0.25) NPs named as SFNO (1–5, Ni content = 0.0, 0.05, 0.10, 0.15, 0.20, 0.25 are named as SFO, SFNO1, SFNO2, FNO3, SFNO4 and SFNO5, respectively) via facile microemulsion route. Hence, the fabricated materials were assessed for the removal of CV dye under the irradiation of visible light. Furthermore, the effect of Ni substitution on ferroelectric and dielectric behavior of SFO material was also investigated.The iron nitrate nonahydrate (≥99 %), nickel nitrate hexahydrate (≥99 %), strontium nitrate (99.99 %), cetyltrimethyl ammonium bromide (CTAB), ammonia (25 %) and CV dye (C25H30N3Cl) were precured from Sigma Aldrich and were utilized as received. Distilled water was utilized for the washing purposes and solutions preparation.All the metal nitrate solutions were mixed according to required compositions (stoichiometric amounts) followed by magnetic stirring and heated up to 60 °C. To minimize the aggregation of particles and to well control the crystallites size, the aqueous CTAB in equal volume was poured to all the mixture. Drop wise pouring of NH3 solution resulted into precipitation of metallic hydroxides at 10–11 pH. The precipitates were neutralized up to pH 7 by washing with water. To minimize moisture contents, the precipitates was subjected to drying in an electric oven at 90 °C for 8 h, then these were sintered at 850 °C where metallic hydroxides were transformed into desired mixed metal oxide (hexaferrite) lattice. Lastly the powdered material was transformed into pallets for dielectric and ferroelectric characterizations (Fig. 1 ). Total six samples were prepared at using × (Ni content = 0.0, 0.05, 0.10, 0.15, 0.20, 0.25), which were named as SFO, SFNO1, SFNO2, FNO3, SFNO4 and SFNO5, where × concentration is 0.0, 0.05, 0.10, 0.15, 0.20, 0.25, respectively.The crystalline phase of the prepared composites was studied by Philips X pert PRO (3040/60 model) X-ray diffractometer (Cu K-alpha radiation and λ = 1.5406 A°) in 2-theta range of 20-60°. Characteristic functional groups were identified by FTIR analysis (Nicolet FTIR interferometer (model-8400S) in 400–4000 cm−1 range. Raman spectra were traced in 0–800 cm−1 range through spectrometer (model 6400 triple JobinYvon-Atago/Bussan). At room temperature, the ferroelectric loops were studied by P-E (M/s Radiant Instrument USA) tracer. Wayne Kerr 6500B Impedance analyzer with DC bias current from 0 to 100 mA was used to analyze dielectric response in frequency range 20 Hz to120 MHz using silver coating pellets of 6 mm thickness at 300 K. The PL spectra were acquired by fluorescence spectrophotometer (SHIMATZU-RF 5301PC). UV–Visible diffuse reflectance absorption spectra for all composites were measured via double beam spectrophotometer (Carry, Agilent).A 10 mg/L solution of CV dye was prepared in distilled water and 500 mL of dye was taken out and 10 mg of catalyst was added. Prepared mixture was stirred for 10 min to stabilize the reaction mixture. To acquire equilibrium, the mixture (catalyst and dye) was shaken for 20 min in dark followed by illuminating to visible light (150 W Xe lamp) for various time intervals. Analyte was withdrawn for every 10 min, centrifuged and absorbance (588 nm) was recorded. The percentage CV dye removal was assessed using Eq. 1. (1) D e g r a d a t i o n ( % ) = 1 - A t A o × 100 where, At and Ao shows the absorbance of dye solution at specific time interval’t’ and at zero time.The XRD patterns of pure SFO and SNFO (1–5) composites in 2-theta range of 20°-60° are shown in Fig. 2 a. Intense diffraction peaks were evolved at 2 theta, i.e., 20.27°, 23.15°, 30.32°, 32.35°, 35.22°, 40.00°, 49.93°, 54.94° and 56.22°, which corresponds to reflection planes of (103), (006), (110), (007), (108), (205), (209), (217) and (2011), respectively and were well in line with standard (JCPDS # 00–033-1340). Absence of any secondary peak confirmed the formation of single phase hexaferrite lattice. On increasing the dopant amount, the position of peaks shifted towards the lower diffraction angle, where the peak intensity was also declined (Fig. 2b). The observed trend might be endorsed to distortion in crystalline lattice, which may originate due to dissimilarity in cationic radius of host Fe3+ (64 pm) and Ni2+ cations (69 pm) [3]. The mean crystalline size (D) was determined using Eq. 2. (2) D = 0.94 λ β c o s θ where, λ and β represent the wavelength used of X-rays used and full width at half maximum whereas θ is Bragg’s angle. Cell volume (Vcell) was calculated from lattice constants ‘a’ and ‘c’ as shown in Eq. 3. The other structural parameters, i.e., theoretical density (ρX-ray) and bulk density (ρbulk) values were determined using Eqs. 4–5. (3) V cell = a 2 c 0.866 (4) ρ x - r a y = 2 M N A V cell (5) ρ b u l k = m π r 2 h Where, M and NA denotes the molar mass and Avogadro’s number, while m and r are the mass and radius and h is the height of pallet correspondingly.The average crystalline size was found to be in 34–50 nm range. Lattice constant ‘a’ remained constant, while ‘c’ was decreased by increasing the concentration of Ni because the ionic radius of dopant (Ni) is larger than the host (Fe) cations, which leads to expansion in Vcell [26]. The c/a ratio was less than 3.98, which indicated that the hexagonal structure was formed with P63/mmc space group [27]. The X-ray density decreased from 5.08 to 4.93 to g/cm3, whereas bulk density was increased from 2.127 to 3.421 g/cm3 with increase in Ni dopant (Table 1 ). The ρX-ray was found to be greater than ρbulk, which probably is an indication of certain amount of pores in the lattice, highly beneficial for to enhance the photocatalytic and optical properties of the fabricated material [3]. Fig. 3 a depicts the FTIR spectra of bare SFO and SFNO (1–5) in 4000–400 cm−1 range. The two Fe–O stretching vibrational bands at 447 and 593 (cm−1) at tetra and octahedral sites were appeared in 400–800 cm−1 range. The octahedral band appears in 400–500 cm−1 range, while the tetrahedral band originates in 500–700 cm−1 range [28]. Considerable change in band position takes place, which might be ascribed to the variation in Fe+3–O-2 and Ni+2–O-2 bond lengths in doped lattice. Possibly the variation in magnetic dipole moment and ionic replacement, the band intensity decreases with increase in of Ni substitution [29]. The Fe-O bending vibrational band was evolved at 1129 cm−1, which was in agreement with already reported literature [27]. The –OH anti-symmetric stretching band was appeared at 1384 cm−1. The deformational vibration and stretching vibration correspond to hydroxyl group that was detected at 1633 cm−1 [30]. Band at 2337 cm−1 represents the deformation of water molecule. The band appeared near 3400 cm−1 might be associated with hydroxyl group due to presence of moisture contents [16].In order to explore the characteristic vibrational modes of fabricated materials, the Raman analysis was employed in 0–800 cm−1 range (Fig. 3b). There are 64 atoms in the unit cell of SrFe12O19 where only 42 modes consisting of 11A1g + 14E1g + 17E2g were found as Raman active. Raman bands associated to octahedral site vibrations confirmed that the composites had magneto plumbite structure of strontium hexaferrite lattice. According to literature data used by Kreisel et al. [31], the peaks observed in Raman patterns have been ascribed to different vibrational bands. Bands appeared at 173 and 329 (cm−1) correspond to E1g symmetry of whole hexaferrite block and E2g vibrations [32,33]. Mode observed at 410 cm−1 can be associated to A1g vibrations at octahedral (12 k) sites [34]. The strong E2g band detected at 329, 529, 606 (cm−1) might be ascribed to Fe-O bond of FeO6 octahedral vibration [35]. The E2g band evolved at 606 cm−1 can be linked to stretching vibrations of Fe-O at 4f2 octahedral sites of hexaferrite lattice [36]. The band appeared at about 677 cm−1 might be present owing to vibrations of bipyramidal (2b) sites of A1g symmetry. For doped materials, a slight shifting in Raman peaks towards lower wavenumber was observed with increase in Ni doping content that is possibly due to difference in ionic masses of host Fe (55.85 amu) and Ni (58.71 amu) dopant [3].Ferroelectric (P-E) loops of SFO and SFNO (1–5) composites were measured at 300 K under −10–10 mV/cm electric field (Fig. 4 a). On applying the field, the + ve side of field showed increasing polarization, while –ve side revealed a decrease in electric polarization. None of the prepared material attained saturation polarization (Psat), maximum polarization (Pmax). On increasing the Ni content, the Pmax, coercivity (Er) and remnant polarization (Pr) were increased. Maximum polarization and remnant polarization were increased from 4.06 to 10.98 mC/m2 and 4.0 to 10.6 mC/m2, respectively for highly doped material (Fig. 4b). The observed trend in substituted SFNOs might be due to that the one FeO6 octahedron within sub-unit of SrFe12O19 unit cell at 3 crystallographic sites, i.e., 4f2, 2a and 12 k wherever Fe is sited normally at octahedron center. When external field is applied the Fe cations demonstrate off-center shifting within the hexaferrite lattice, which probably prompt the electric polarization. Also, the enhanced polarization in substituted materials might be endorsed to Ni doping at Fe site where the cationic radius of Ni is slightly bigger than Fe, which may cause a variation in Fe-O and Ni-O bond length [3]. Pristine SFO indicates slightly less conductive behavior that seems to be decrease on increasing the Ni doping in crystallite lattice. Remnant polarization is less than Pmax for all the materials that is in well agreement with reported studies [3,20,21]. Such ferroelectric behavior of Ni doped SrFe12O19 indicates their potential utilization in various electronic devices [3].Field dependency of dielectric constant (έ) for all the materials was analyzed in 20 Hz-1.5 GHz at 300 K. Dielectric constant was measured from capacitance values using Eq. 6. (6) ε ̇ = CD ε oA Where, C and D denote the capacitance and pallet thickness while εo and A are permittivity constant of free space and cross-sectional area of the pallet. For all the samples, at lower frequency, the dielectric constant was higher showing dispersion in this regime whereas in high frequency region, it remains almost constant with their lowest value (Fig. 5 a). The observed behavior might be ascertained to electronic polarization (induced due to fluctuation in oxidation states of metallic cations) as well as space charge type polarization. At high frequency, the constant and lowest ε′ value is possibly due to incapability of dipoles of dielectric material to follow the rapid changing AC field that starts to trail behind the field thereby resulting of drop in έ. The dependence of frequency on ε′ might be described via Koop’s polarization theory established on Maxwell and Wagner model of double layer [17]. According to this, a dielectric material contains bilayer structure where first layer having conducting grains of high conductivity is separated from second layer of less conductive grain boundaries. When electric field is applied, the electrons via hopping mechanism get arrived at boundaries where they accumulate due to high resistivity of grain boundaries. This charge accumulation at grain boundaries enhances the interfacial type polarization that intern rise the ε′ in low field. The electronic flow through grain boundaries inhibits at high frequency that results in drop of dielectric constant [3]. Fig. 5b illustrates the dielectric constant versus doping content at specific frequencies for pure and doped SFNOs. The SFNO5 shows highest ε′ than undoped SFO, which is possibly owing to reduction of B-site Fe cations that decreases the hooping rate among Fe+2 and Fe+3 cations thus increasing the έ value [16,37]. Fig. 6 a demonstrates the frequency dependent dielectric loss (ε”) of pure SFO and Ni doped strontium hexaferrite different compositions (1–5). The dielectric loss shows similar trend to the dielectric constant that is it is high at low frequency. As the frequency was increased, it starts to drop and outside specific frequency, it responds rather independently from applied frequency. Such behavior can be associated to Maxwell–Wagner polarization of interfacial type, which occurs in heterogeneous dielectric medium [3]. Fig. 6c demonstrates the variation of tanδ of hexaferrites versus applied field at room temperature. At about 100 kHz, the tanδ dropped down swiftly and later, it remains almost constant. This initial decline in loss tan might be described by Koop’s phenomenological model. The dielectric losses in hexaferrite materials are normally represented in resistivity terms that is the dielectric materials with lower resistivity values display high dielectric loss and vice versa. Tangent loss showed inverse relation with Ni doping in SrFe12O19, which probably is due to low electron hopping rate among Fe2+ and Fe3+ state responsible for high conduction [16]. In ferrites, polaron hopping perhaps the basic reason for dielectric loss when frequency is high and electron hopping was the reason for dielectric loss at low frequency. The low dielectric losses in these materials are beneficial for their utilization in different high frequency applications [17].The impact of frequency on AC resistivity of doped and undoped Sr hexaferrite samples is shown in Fig. 7 a. Like dielectric constant, AC resistivity shows dispersion at low field, however, on increasing the frequency, resistivity of all material was dropped and at highest frequency, it was lowest with constant value. The basic reason for variation in resistivity might be that on octahedral sites of structure, the hopping rate of charge carriers takes place among Fe+2 and Fe+3 states. Hopping of charge carriers increases by the external field when the frequency is high. Electrical conduction is improved by the hopping of charge carriers as it leads to decrease in resistivity [3].In region of low frequency, the AC conductivity of all the compositions did not change and was low values, but gradually enhanced on increasing the applied field. In structure of ferrites, the conductivity is generally associated to octahedral sites where the hopping of electrons takes place among Fe+3 and Fe+2cations. At higher frequency region, it started to increase with frequency and attained its maximum value at highest AC field. Perhaps the degree of crystallinity, crystallite size and temperature of a material may affect the hopping phenomenon between localized states. AC conductivity was enhanced probably due to accessibility of hopping between electrical charges when the frequency was high. In higher frequency region, the free charges are able to move as they have sufficient energy and thus lead to enhance the conductivity process [17].To investigate the charge separation ability and recombination of photo-electron-hole pairs, the PL analysis was performed in 400–800 nm range. At specific excitation wavelength, a strong emission peak was detected at 470 nm (Fig. 8 ). For pristine SFO, the sharp PL peak intensity indicates its fast recombination rate of charge carriers, while for doped compositions, the PL intensity dropped quickly showing their low recombination rate. In case of substituted materials, the low recombining rate increases the life time of the light induced carriers, which intern improves the optical efficiency of photocatalytic material [17]. The better separation ability of doped materials might be endorsed to O2 vacancies and creation of extra energy levels within the conduction band and valence band possibly owing to intrinsic defects on increasing the doping amount in hexaferrite lattice [38].The UV–vis analysis of fabricated materials were studied to analyze the optical behavior and bandgap (Eg), which is highly useful to investigate the photocatalytic application. Fig. 9 a shows the UV–vis diffuse reflectance spectra of pure SFO and SFNO(1–5) composites in 200–800 nm range. Direct optical bandgap for all the composites was estimated by employing the Tauc’s model (Eq. 7) by plotting hυ vs αhυ2. (7) α h υ = k h υ - E g 1 / 2 Where, α, k and υ are absorption coefficient, Boltzmann constant and photon frequency of visible light used. Bandgap was estimated on extrapolation of linear part of plot at α = 0 (Fig. 9b). The bandgap for undoped strontium hexaferrite was 2.31 eV, while for doped composites, a substantial decline in Eg was observed. Bandgap energy was tuned from 2.11 eV (SFNO1) to 1.66 eV (for SFNO5) (Fig. 9b). The observed decline in bandgap for substituted materials might be explained on the basis of specific impurity levels that may appear within the forbidden energy band on increasing the Ni dopant in lattice, which accelerates the intensification of donor level overhead the prior valence band, while acceptor level under prior conductance band [39]. Other factors such as structural strain, variation in average crystallite size or surface area/volume might have definite consequence on optical bandgap [16,17].The catalytic capability of pure SFO and highly doped SFNO5 material was appraised for CV dye removal under visible light illumination. Fig. 10 (a-b) demonstrates the visible light absorption curves of CV dye for SFO and SFNO5 materials recorded at specific time intervals. Pristine SFO showed a degradation of 55 %, whereas highly doped SFNO5 catalyst demonstrated superior efficiency of 91 % removal of CV in 90 min under the exposure of visible light irradiation. This better photocatalytic competence of SFNO5 material for degradation of CV dye might be endorsed to structural strain created on doping by metallic cations with different cationic size and charge, as it is most probable that a definite level of oxygen or cationic vacancies may be generated within the lattice to balance the electrical charge neutrality. Actually, the Ni doping tuned not only the Eg of substituted catalyst (SFNO5), but also minimize the recombination of e--h+ pair via acting as efficient trappers for photoactive charge carriers [3]. In comparison to previous studies (Table 2 ), it is concluded that the SrNixFe12-xO19 showed promising efficiency for the degradation of CV dye, which could have potential applications as a photocatalyst under solar light irradiation.For photo-degradation reaction (PDR) of CV dye, the apparent rate constant (kapp) was calculated from Langmuir–Hinshelwood relation as shown in Eq. 8. (8) - l n A t A 0 = k t The kapp was determined from slope of plot, i.e., -lnAt/Ao versus irradiation time (t). The linear fitting of obtained line with Adj R2 > 0.95 confirmed that the degradation pathway of dye followed pseudo first order kinetics. The kapp values for PDR of CV dye over undoped SFO and SFNO5 catalysts under visible light irradiation were determined as 8.02 × 10-3 min−1 and 2.337 × 10-2/min, respectively (Fig. 11 a-b).In order to recognize the mechanism of charge transferring and to analyze the key radicals involved during the photo-degradation of CV dye over substituted SFNO5 catalyst, scavenging experiments were conducted. The AgNO3 and EDTA and 2-propanol (TP) were taken as trapping agents being scavengers of electron (e-), h+ and hydroxyl (•OH) radicals, respectively. Without using scavenging agent, the percent removal of dye was 91 %. A radical drop in decolorization was noticed with TP, which showed the most active specie, hydroxyl (•OH) radicals involved in the CV dye removal. A significant decrease in degradation was noted also with AgNO3 that indicated that h+ were the active agents that played imperative role in removal of CV dye over SFNO5 catalyst. Though, no substantial change in degradation was observed with EDTA, indicating no direct impact of e- towards the photo-gradation of dye. For above-mentioned scavengers, the percent degradation of CV dye was dropped to 22, 33 and 43 (%) showing the rate constant of 0.0009, 0.0017 and 0.0029 (min−1), separately. The experimental results showed that hydroxyl radicals were key active species which played the major part in decolorization of CV dye. The h+ also influenced the degradation of CV dye significantly, whereas e- showed slight contribution in degradation over SFNO5 (Fig. 12 a). The order of scavenging effect was found as, •OH > h+ > e-.Degradation of dye effluents on photocatalysts are based on creation of electron-hole pair within the semiconducting material when it is irradiated to the light, which operates the oxidation- reduction processes of adsorbed species on the surface of photocatalyst [17]. These photo-induced e- are excited from the valance band (VB) and entered into conduction band leaving behind the h+ in VB. This e-/h+ pair on the surface of SFNO5 probably plays its crucial role for oxidation/reduction reactions involved during the degradation of CV dye (Fig. 13 ). The O2 (dissolved in water) in dye medium captures the photo-excited electrons, where super oxide (O2 •-) radical is produced. On the other hand, H2O interact with h+ and resultantly, generates OH• radical [16]. The OH• degrade the CV dye by oxidative process and convert them into CO2, H2O and inorganic ions. An overall CV dye degradation mechanism is presented in Eqs. 9–19. (9) SrN i x F e 12 - x O 19 + h v → S r N i x F e 12 - x O 19 e CB - + h VB + (10) SrN i x F e 12 - x O 19 h VB + → S r N i x F 12 - x O 19 + H + + O H (11) SrN i x F e 12 - x O 19 h VB + + → S r N i x F e 12 - x O 19 h VB + + O H . (12) SrN i x F e 12 - x O 19 e CB - + → S r N i x F e 12 - x O 19 e CB - + O 2 ∙ - (13) O 2 ∙ - + H + H O 2 ∙ (14) H O 2 ∙ + H O 2 → H 2 O 2 + O 2 (15) SrN i x F e 12 - x O 19 e CB - + H 2 O 2 → S r N i x F e 12 - x O 19 + O H ∙ + O H - (16) H 2 O 2 + O 2 ∙ - → O H ∙ + O H - + O 2 (17) H 2 O 2 + h v → 2 O H ∙ (19) Dye + O H ∙ → I n t e r m e d i a t e s (19) Intermediates → C O 2 + H 2 O + i n o r g a n i c i o n s The Ni doped SrNixFe12-xO19 (for ×  = 0.0–0.25) were fabricated through simple micro-emulsion method and influence of substitution on the dielectric, optical and photo-catalytic behavior was analyzed. Ferroelectric loops got widened on doping, which enhanced the maximum polarization, coercivity and remnant polarization. Dielectric parameters showed dispersion in low frequency region, but remained almost independent at higher frequencies. Bandgap was reduced from 2.31 eV (pure SFO) to 1.66 eV for highly doped SFNO5 composition. The substituted SFNO5 material showed much better photocatalytic efficiency regarding the removal of CV dye versus undoped SFO under visible light irradiation. Effect of various scavenging agent to assess the key active species for photodegradation of dye was studied, where OH• radicals were observed as major species involved in degradation process. The Ni doping affected the optical, conductivity and dielectric properties significantly of doped SrNixFe12-xO19. The photo-catalytic efficiency revealed a potential application for the photodegradation of dye in wastewater under visible light irradiation, which will be highly economical versus UV light based catalytic process.This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project (Grant No. PNURSP2022R124), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. Zunaira Irshad: Investigation, Writing – original draft. Ismat Bibi: Conceptualization, Supervision. Aamir Ghafoor: Methodology. Farzana Majid: Formal analysis. Shagufta Kamal: Project administration. Safa Ezzine: Validation. Zainab M. Elqahtani: Resources, Data curation, Funding acquisition. Norah Alwadai: Software, Writing – review & editing. Noureddine El Messaoudi: . Munawar Iqbal: Visualization, Writing – original draft, Writing – review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors express their gratitude to Princess Nourah bint Abdulrahman University Researchers Supporting Project (Grant No. PNURSP2022R124), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University, Saudi Arabia for funding this work through the Research Groups Program under grant number R.G.P.1: 255/43.

A series of Ni doped SrNixFe12-xO19 (x = 0.0, 0.05, 0.10, 0.15, 0.20, 0.25) hexaferrites were fabricated through facile micro-emulsion approach and doping content effect was investigated based on structural, dielectric, optical and photocatalytic properties. The prepared materials were characterized via X-ray diffraction (XRD), Raman, Fourier transformed infrared (FTIR), photoluminescence (PL) and UV–vis techniques. The Ni doped SrFe12O19 structure was hexagonal having space group P63/mmc with mean crystallite size in 34–50 nm range. Ferroelectric polarization, coercivity and Remnant polarizations were increased with Ni doping. Dielectric analysis showed the higher values of dielectric parameters with dispersion in low frequency region, which were independent of at high frequency. Optical bandgap was reduced on substitution, which was in 2.31–1.66 eV range, which was in good association with decline in PL analysis. The PCA (photocatalytic activity) of pure SrFe12O19 (SFO) and doped SFNO5 (x = 0.25) material was assessed for removal of crystal violet (CV) dye under visible light illumination. The SFNO5 showed much better PCA and 91 % dye was removed in 90 min with a rate constant of 2.337 × 10-2 min−1 versus pristine SFO (only 55 % with rate constant 8.02 × 10-3 min−1). Results revealed that tuned bandgap and enhanced AC conductivity of doped materials make it crucial candidates for optoelectronic and SOFCs (Solid oxide fuel cell) applications. Owing to the excellent PCA and magnetically separable nature, SrNixFe12-xO19 has potential for the dyes removal from the effluents under solar light irradiation, which could be more economically versus UV based photocatalytic process.

Data will be made available on request.In recent years, there has been a concerted effort to decrease the reliance of modern consumer products on the fossil fuel industry and the need for a sharp decline in their use to keep the temperature increase below 1.5 °C (Welsby et al., 2021). One of the main products from the petroleum industry are chemical feedstocks, which represents around 10 % of the global petroleum industry in terms of production volume, but a significantly higher financial value due to the increased price of petrochemicals compared to fuels. In particular, BTX are used to manufacture common plastics like poly ethylene (PE), polystyrene (PS), polyurethanes (PUL), and nylon (NY) among other chemicals (“Biogreen Energy Syngas,” 2020). In consequence, there is growing interest in replacing fossil derived with bio derived feedstocks, although it has been suggested that despite biomass is an “indispensable” resource for the circular economy, combusting it to electricity only is inefficient and the path to chemicals should be pursued (Hamer, 2020). Accordingly, lignin as renewable source of aromatics is an important element in achieving the ‘net-zero emission by mid-century’ target outlined by The Paris Agreement (Griffin et al., 2018). Lignin possesses a phenolic polymer structure comprised of p-hydroxy-phenyl, guaiacol, and syringyl groups, and hence has good potential to provide renewable aromatic compounds for future use as feedstocks to the chemical manufacturing industry. Aromatic production could be an economical way of kick-starting large-scale lignocellulosic pyrolysis as a means to reduce dependency on oil derived chemicals and help promote renewable biomass as a staple industry in the global economy.Fast pyrolysis, which is the rapid heating of biomass in absence of oxygen for a short residence time followed by rapid quenching of the produced vapours can be used to produce either fuels or aromatics, with appropriate variations in process conditions, which includes the use of catalysts (Liu et al., 2021a; Farooq et al., 2022; Hendry et al., 2020). On an economic level, biomass pyrolysis has been assessed for fuel production. A report published in 2015 by the US Department of Energy shows an economic projection of how advanced bio-fuels can be made cost competitive vs fossil fuel derived transportation fuels (Dutta et al., 2015). The conclusion of this report was that biomass pyrolysis can compete with fossil fuel derived fuels based on a total yield of combined gasoline and diesel equivalent fuels of 78 gallons / US ton dry of biomass and a fuel price of around $3.50 /gallon final product. The report outlines that the total product yield should be minimum 25 wt% of dry feedstock for the whole process, including ex-situ HDO upgrading of pyrolysis vapours.Recent studies show that pyrolysis under hydrogen atmosphere operating at a rapid heating rate namely hydropyrolysis (HyPy) is a promising technology for converting biomass into liquid fuels (e.g., bio-oil and C4 + hydrocarbons), since the addition of H2 enhances the H/C ratio of the bio-oil, reduces the O/C ratio, facilitates hydrotreating reactions that involve CC coupling, hydrocracking, alkylation, decarboxylation, decarbonylation, hydrogenation, HDO, and recombination (Oh et al., 2021; Stummann et al., 2021). In particular, HDO of oxygenated aromatics (e.g., phenolic compounds) occurs via hydrogenation of the aromatic ring followed by deoxygenation (Liu et al., 2021b). The second advantage is that the H• radicals ‘stabilise’ the highly reactive intermediates species (e.g. ketone and aldehyde) and prevent them from condensing into coke, which reduces the carbon recovery and lead to higher operational costs (Resende, 2016). Presence of hydrogen also enhances demethylation of methoxy groups in phenolic products and favours dehydration vs C loss pathways (–CO, –CO2). For example, non-methoxy phenolics reached 19.68C% at 30 bar and 500 °C, where monocyclic and polycyclic aromatic hydrocarbons as well as condensable aliphatic hydrocarbons were observed from lignin hydropyrolysis (Wang et al., 2022). Temperature plays an important role in hydropyrolysis, where HDO takes place at increasing temperatures to with efficient elimination of furanic and phenolic oxygen-containing compounds at T > 500 °C (Wang et al., 2013). However, Tian et al (2021) showed that when hydropyrolysis temperature is increased from 700 to 800 °C and higher, the hydropyrolysis oil (from pine sawdust) decreased from 35 wt% to 25 wt% at expenses of gas phase (from 50 to 70 wt%) (Tian et al., 2021).The majority of studies show that hydrogen atmosphere (either at pressure or ambient) with no catalyst can be sufficient to provide the desired bio-oil composition and vaporisation before the HDO step (Zheng et al., 2017). This pathway demands a good understanding of the HyPy products so that a suitable HDO catalyst can be selected. For BTX production, the pyrolysis vapour should consist of simple phenols and the HDO catalyst should selectively produce BTX molecules by removing the oxygen content (Jan et al., 2015). Some studies show HDO catalysts further cracking the bio-oil phenols into small olefins or alkane gas molecules, which have lower value than aromatic BTX compounds (Marker et al., 2013). Additional reduction of BTX molecules can promote char build-up and catalyst deactivation, which is undesirable.There are two main catalyst types used in the hydropyrolysis of lignocellulosic biomass, the pyrolysis catalyst and the HDO catalyst, where the most well-known conversion processes include both of them, such as the integrated hydropyrolysis and hydroconversion (IH2®)) and a two-step biofuel process (H2Bioil) (Venkatesan et al., 2020). The pyrolysis catalyst is primarily used to increase the hydrogenation of the biomass molecules and promote the breaking of the weakest CC bonds in the structure which in turn, breaks down the lignin and cellulose polymer chains into oligomers, dimers and monomers (Sirous-Rezaei and Park, 2020). The most efficient catalysts for this are either powered metal oxide catalysts on a fluid bed reactor or acid site zeolite-based structures. Zeolites are relatively expensive to prepare and have several well-documented shortcomings when used in biomass pyrolysis, namely char build-up, fast deactivation and high regeneration energy, suggesting they may not be suitable for large scale processes, although they promote aromatics production (Stummann et al., 2018; Zhu et al., 2022; Jindal et al., 2022).In the literature the main success in HDO catalysts for BTX production have been in metal oxides on a porous, surface-active supports under high H2 pressure (Resende, 2016; Stummann et al., 2021; Venkatesan et al., 2020; Wang et al., 2013). Metal oxide reducing powders appear to be a better catalyst due to their higher reactivity, which does not depend on molecule mobility rates through zeolite pores, as well as regeneration being easier and faster than that of zeolites (Jan et al., 2015). Palladium is a precious metal in the same chemical group as Nickel and Platinum and is used widely in industry as a hydrogenation catalyst, usually for the reduction of CC or CO bonds in organic chemistry. Jan et al published a study in 2015 that showed very high yields for BTX from lignin samples using in-situ palladium doped HZSM-5 zeolite (Jan et al., 2015). The reported yield was 40 wt% aromatics at 600 °C and 17 bar of H2. However, the catalyst was used with a catalyst to biomass ratio of 10:1 or 20:1, with the 1:1 resulting in only 4 wt% aromatics produced, with C6-C10 aromatic and polyaromatic molecules counting in that total. BTX consisting of C6 and C7 aromatics made up approximately 18 wt% with 20:1 catalyst to biomass ratio. The high catalyst to biomass ratio would cause problems when scaling up these experiments as the assumed reason for such high catalyst ratio is the high tendency for palladium and zeolites to quickly form char that leads to rapid deactivation. Stummann et al. (2018) studied the hydropyrolysis of beech wood at 26 bar and 450 °C in presence of ‘bog-iron’ as a hydropyrolysis catalyst and nickel-molybdenum alloy supported on aluminium oxide as the HDO catalyst. This study showed 24.7 wt% of condensed organic and C4 + molecules, which can be used as gasoline additive (Stummann et al., 2020). This is important as one of the reaction paths to aromatics from hydropyrolysis is through C4 + intermediate species (Norinaga et al., 2014). Sirous-Rezaei and Park (2020) studied the HyPy/HDO of kraft lignin using HY as in-situ catalyst and Fe/HBeta, FeReOx/MCM-41, Fe/ZrO2 and FeReOx/ZrO2 as ex-situ HDO catalyst, under a continuous flow of H2 at 1 atmosphere pressure (Sirous-Rezaei and Park, 2020). While FeReOx/ZrO2 resulted the most efficient HDO catalyst (7.1 wt% aromatic hydrocarbons of which 4.8 wt% BTX), all FeReOx/MCM-41, Fe/ZrO2 and FeReOx/ZrO2 led to significantly lower yield of coke compared to a zeolite-supported catalyst like Fe/HBeta. Similar aromatics (7.6 wt.%/5.4 wt% BTX) were obtained using palladium rhenium oxide (Sirous-Rezaei and Park, 2020). The lower cost ferrous rhenium catalyst material and low reaction pressure is significant because it represents a relevant advantage if this process is scaled up. The products from the iron-based catalyst also gave 11.4 wt% oxygenates of which 8.7 wt% was phenol. These compounds are usually the primary targets for HDO catalysts suggesting phenols deoxygenation can be improved. The same research group showed the efficiency of ferrous rhenium oxide on zirconium oxide support in the upgrading and deoxygenating of mono-phenols into BTX (Starting material – BTX wt%: Guaiacol – 21.6 wt%, m-Cresol – 61.7 wt%, Anisol – 48.3 wt%) (Sirous-Rezaei et al., 2018). However, the starting materials for these studies were pure lignin/phenols without any additional species that could interfere with the catalyst action (e.g. cellulose fraction). An early research show that Etek (Etanol Teknik) lignocellulosic filtration residue from acid straw hydrolysis resulted in 34 % gas, 61 % liquid and 5 % coke (down from 19.6 % in N2) when underwent hydropyrolysis at 800 °C with both lignin and cellulose derived compounds accounted in the liquid product (Windt et al., 2009).Overall, the literature shows that integrated Hy-Py/HDO is the most effective pathway for deoxygenating pyrolysis bio-oil and also that bifunctional zirconia supported metal catalysts such as FeZrO2 can turn lignin into valuable aromatics under mild hydropyrolysis conditions. However, selectivity on BTX is a unresolve challenge. Therefore, this study wants to investigate in-house synthetised ZrO2 supported Ce, Na Fe, PdFe catalysts in the integrated Hy-Py/HDO of Etek lignin waste under ambient H2 pressure at 600 °C using a pyroprobe reactor coupled with a gas chromatograph/mass spectrometer (Py-GC/MS), with the aim to (i) maximise BTX selectivity on oil product, (ii) deoxygenation of the Hy-Py bio-oil and (iii) compare the effect of hydrogen and the selected catalysts on the products distribution with those obtained in presence of nitrogen.Etek (Etanolteknik AB, SE) lignin is an industrial filtration residue remaining after a two stages weak acid hydrolysis of soft wood and consists of 41 % holocellulose and 59 % lignin (Nowakowski et al., 2010). The Etek lignin contained 51 wt% C, 5.7 wt% H, 1.6 wt% N and 37.7 wt% O (by difference), while the proximate analysis gave 76.6 wt% volatiles, 19.4 wt% fixed carbon and 4 wt% ash. Na2CO3 (99.6 % purity), CeO2 and ZrO2 (99.0 % purity), ZrOCl2·8H2O (99.5 % purity) and PdCl2 and FeN3O9x9H2O used for the synthesis of the catalysts were all purchased from Sigma-Aldrich.Na/ZrO2 (1.5_1), CeNa/ZrO2 (1_1_1), CeNa/ZrO2 (2_1_1) were synthetized mixing by pestle & mortar Na2CO3, CeO2 and ZrO2 and then calcining the mixture at 900 °C for 4 h in air. ZrO2 support for Fe/ZrO2 and PdFe/ZrO2 was instead prepared by dropwise addition of ZrOCl2·8H2O to water (100 mL). After addition, the suspension was aged 20 h at 90 °C, dried in an oven at 110 °C for 15 h and subsequently calcined in flowing air (30 mL/min) at 500 °C for 3 h. 5 %Fe and 1 % Pd on ZrO2 were then prepared by incipient wet impregnation method with PdCl2 and FeN3O9x9H2O as precursors. The dried resulting salts were then calcined in air at 500 °C for 3 h to achieve good dispersion (Hendry et al., 2020).A -micro-pyrolysis reactor coupled with a gas chromatography/mass spectrometer (Py-GC/MS) was used for the experimental campaign. The system consisted of a CDS 5250 (Chemical Data Systems, USA) pyroprobe hyphenated to a GC–MS Trace DSQ II (Thermo Scientific, USA). Fig. 1 schematises the reactor setup, consisting in an autosampler quartz tube (1.9 mm diameter) where one mg of the Etek lignin was sandwiched between 2 layers of 5 mg of selected catalyst and the two materials were kept in place and separated by layers of quartz wool.The reaction tests were performed at 600 °C (heating rate of 10 °C/ms) in presence of hydrogen (100 mL/min) with final temperature maintained for 30 s. Although the true temperature in the microprobe is likely 75-100° C lower than the set point (Thangalazhy-Gopakumar et al., 2011). The temperature of 600 °C was selected since literature indicates that this temperature was required to maximise recovery of phenolics from Etek lignin pyrolysis (Hendry et al., 2020). The volatiles generated in the pyroprobe were collected in a tar trap for separating the non-condensable gases and further analyse them by GC–MS. The tar trap consisted in a 1/8″ Tenax kept at 40 °C. The condensed species were then desorbed from the tar trap at 300 °C for 3 min and sent to a GC–MS for analysis by a transfer line heated at 350 °C. The bio-oil was injected in the GC at 280 °C, using 25 mL/min of helium, as a carrier, and the split ratio of 80:1. The column used was an Agilent: HP-5MS, 19091S-433; length, 30 m; internal diameter, 250 μm; film thickness, 0.25 μm The GC oven temperature started at 40 °C for 2 min, heated up to 320 °C at 12 °C/min and kept at temperature for 15 min. The raw area percentages were recalculated by excluding the GC–MS peaks after 20 min (see supplementary material) and normalising to 100 %. This was done because those peaks belonged to contaminants leached out from the GC column at high temperature. The peaks were finally identified by the NIST library. The Total Sum Normalization (TSN) method for normalising GC–MS data, which consists in dividing the area of each peak in each chromatogram by the total sum of all peaks within that chromatogram, was used in this work. Sum normalised data were multiplied by 100 and expressed in terms of their percent contribution (area%) to the total area. To evaluate the catalysts’ ability on steering the HyPy/HDO reactions to produce BTX in the oil product from the starting lignin fraction in Etek lignin, the term selectivity was used. The product was studied in term of functional groups using Excel. Non-aromatic molecules were classified in the following order: cycloalkene, cycloalkane, epoxide ring, esters, ketones + aldehydes and olefins as summarised in Table 1 including a brief description of the main uses. Alcohol groups was a very common feature so were ignored in terms of classifications. Hence there are molecules in the cycloalkene class that are also ketones and have ester groups on branches off the cycloalkene ring. 2-Dodecenal is an example of an olefin chain with an aldehyde group but was classified in the ketone + aldehyde group and not the olefin group as per the order above. The BTX content based on starting Etek lignin weight was estimated considering (i) the bio-oil wt% resulting from Etek lignin hydropyrolysis in absence of catalyst (61.2 wt% based on dry lignin) under similar conditions (ambient pressure H2, 800 °C) (Windt et al., 2009); (ii) a fraction of bio-oil detectable by GC–MS (∼16.4–40 %) (Windt et al., 2009); (iii) the fraction of HDO oil (23 wt%) obtained from lignocellulosic material under similar conditions (450 °C, H2 at ambient pressure and 5 %Pd catalyst + NiMo/Al2O3) (Gholizadeh et al., 2016) and (iv) the fraction of BTX from the GC–MS analysis (63 %) assuming this is proportional to the actual weight content of BTX.Proximate and ultimate analysis of the Etek lignin were obtained by using an Exeter CE-440 Elemental Analyser and a Mettler Toledo TGA2 thermogravimetric analysis, respectively. The catalysts characterisation included XPS spectra that were acquired using a PHI Quantera II Scanning XPS Microprobe -instrument. Samples were sputter-cleaned using argon ions prior to analysis. Calibration of spectra was done using the C 1 s peak (284.8 eV) for adventitious carbon. SEM-EDX analysis was done using a Zeiss Leo 1530 microscope equipped with a FEG and operated at a voltage of 2.5 kV and equipped with an Oxford Instruments X-Max silicon drift detector (SDD) on non-coated samples. TEM analyses were performed using a Titan Themis 200 scanning/transmission electron microscope (S/TEM) equipped with an X-FEG Schottky field emission gun and a Super-X high sensitivity windowless EDX detector complemented by a Gatan Enfinium EELS Detector. The catalyst samples for TEM were ultrasonically dispersed in ethanol and then deposited on carbon-coated copper grids using capillary and dried in air for 30 min. XRD was instead run using a Bruker Nonius X8-Apex2 CCD equipped with an Oxford Cryosystems Cryostream (typically operating at 100 K), and an X-ray source with a Cu anode working at 40 kV and 40 mA, and an energy-dispersive one-dimensional detector. The Fe amount on the Fe/ZrO2 catalyst was determined by atomic absorption spectroscopy (AAS) using a PerkinElmer Analyst 100, while surface analysis was carried out using a Tristal II Plus Micromeritics analyzer. H2-TPR was carried out using a lab-made instrument (Bagnato et al., 2020). The catalyst was first dried at 110 °C for 18 h. and then heated at 10 °C/min from 25 °C to 900 °C in a 5 % H2/He flow (40 mL/min). The effluent gases were analysed by a TCD detector. Surface acidity was analysed by NH3-TPD using an AutoChem II system (Micromeritic, USA). Table 2 shows the HyPy oils’ chemical composition from the GC–MS analysis together to the biomass component each group is originated from. The HyPy oil product using CeNa/ZrO2 (1_1_1) (see supplementary material) contained 107 distinct compounds. This shows the low selectivity of the catalyst, with Ketones + Aldehydes (11.04 %), and Guaiacols (22.25 %) being the most abundant functionalities. The effect of hydrogen vs nitrogen during the Etek lignin pyrolysis was compared (see supplementary material) using the data produced in a previous work (Hendry et al., 2020). The high HyPy temperature used (600 °C) was necessary to maximise lignin break-down into monomers, which would then be converted into hydrocarbon via hydrogenation, decarbonylation, deoxygenation pathway generating fuel grade hydrocarbons (Hendry et al., 2020; Jindal et al., 2022). Clearly, the distribution of the bio-oil compounds shifted to lower molecular weight compounds, such as butanone and cyclopentadiene, although the most abundant compounds in N2 atmosphere (e.g. phenol; phenol-2-ethoxy; 3-creosol; levoglucosan) remained qualitatively unchanged when H2 was used. Very limited benzene was detected suggesting that both Ce and Na are not good in hydrogenating and deoxygenating the substituted aromatic rings. The GC–MS analyser from the hydropyrolysis reactions in presence of the same catalyst but with two moles of Ce instead of one [CeNa/ZrO2 (2_1_1)] identified 46 distinct compounds in the oil product. This shows an increased peak area of the catalyst over CeNa/ZrO2 (1_1_1), with the 20 most abundant molecules making up 87.73 % of the total bio-oil, most of which presented a lower retention time compared to CeNa/ZrO2 (1_1_1) and larger content in anhydrosugars derived from cellulose, suggesting that Ce is more effective in breaking down cellulose into small oligomers/monomers including some cycloalkenes and cycloalkanes. Presence of Ce/ZrO2 (1_1) resulted in 81 distinct compounds, mostly Oxygenated sugars (34.7 %), Guaiacols (23.5 %) and Esters (7.66 %), indicating its propension in converting cellulose-derived components than CeNa/ZrO2 (2_1_1). The GC–MS analysis in presence of Na/ZrO (1.5_1) identified 60 distinct compounds with Ketones + Aldehydes (23 %) and Guaiacols (42.3 %) being the most abundant. Basic sites of the catalyst can favour reduction of acids and deoxygenation via ketonization and aldol condensation reactions (Stefanidis et al., 2016). In comparison, when Na/ZrO3 (1.5_1) was pyrolysed in presence of N2, the GC–MS resulted in 47 distinct compounds mainly Guaiacols (62.4 %) (Hendry et al., 2020). This suggests that the catalyst had good reactivity to both the cellulose and lignin components of the biomass in presence of H2. Olefin intermediates quickly reform into larger molecules, eventually leading to polyaromatic hydrocarbons and char (Norinaga et al., 2014). The presence of H2 gas stabilises the olefin intermediates and stops further reactions that produce coke on the catalysts’ surface. This also explains why the tests in presence of H2 showed much higher cellulose derived compounds.The GC–MS analysis with 1 wt% Pd and 5 wt% Fe on ZrO2 identified 62 distinct compounds, which are much closer to the desired products (i.e. deoxygenated hydrocarbons) than Na/ZrO2 and the other two Na/Ce catalysts. Ketones + Ketone Alcohols, Cycloalkenes, Guaiacols, and BTX were the main functionalities with total area from each being 12.06 %, 18.67 %, 7.88 % and 16.31 %, respectively. The presence of polyaromatic hydrocarbons (5.74 % - highest in any of the catalytic tests) shows that olefins created from cellulose reduction are contributing to the end products and indicate that species are over-reduced and forming char deposits on the catalytic acid sites, limiting further catalytic activity (Stefanidis et al., 2016). The higher area% of cycloalkenes and lower yield of Ketones + Aldehyde shows that PdFe/ZrO2 is more active and more reducing than Na/ZrO2. Likewise, the 16.3 area% of BTX and low relative area% in Alkylphenols and Guaiacols shows the reducing factor is equally applied to the lignin component as well as the cellulose component.The HyPy bio-oil obtained using Fe/ZrO2 showed the best characteristics among all the employed catalysts. The GC–MS analyser identified only 11 distinct compounds in the product flow from the hydropyrolysis- test, which was duplicated for consistency. Ketones (acetone), Cycloalkenes and BTX were the most abundant functionalities with total area from each being respectively: 15.3 %, 13.5 %, and 66.8 % (35.4 % benzene, 25.6 % toluene and 5.8 % xylene). This shows a very high selectivity of the catalyst compared with the other catalysts. FeZrO2 also resulted in a carbon distribution of 85.5 % C5-C10 (10.5 % C5, 41.1 % C6, 27.6 % C7 and 6.3 % C8, 3.3 % C10), which was similar to the 84 % aromatic hydrocarbons (C6-C11) obtained by the Hy-Py/HDO of pine sawdust at 500 °C and 20 bar over a hydrocracking catalyst (20:1 cat:biomass wt ratio) bed maintained at 500 °C (Venkatesan et al., 2020). They also showed that at 1 bar, ∼63 % aromatics (34 % BTX of which 12.8 % B., 13.8 % T. and 5.8 % X.) and significant amount of Poly Aromatic Hydrocarbons (PAH) (∼16 %) were produced at 500 °C, which confirm the higher selectivity of FeZrO2 on BTX under the studied conditions. Similarly, Agarwal et al. (2017) produced alkylphenolics (17 wt% on lignin) and aromatics (4 wt% on lignin) for a total of 34 wt% lignin oil, from the hydrotreating of Kraft lignin at 450 °C, 100 bar H2, 4 hrs using Fe rich limonite catalyst (Agarwal et al., 2017). To compare the BTX yield with those in literature, the weight % of the BTX (based on starting Etek lignin) was estimated (1.5–3.6 wt%) resulting in between those obtained using FeZrO2 (1.3 wt%) and PdReOx/ZrO2 (5.4 wt%) under lower temperature (350 °C), where oxygenated phenolics were the predominant oil components (Sirous-Rezaei and Park, 2020). However, proper BTX quantification would be required.to confirm this. Moreover, the deoxygenation power resulted the best with only about 5.5 wt% O in the HyPy oil compared to 15.5 wt% for FePd/ZrO2, based on GC–MS HyPy oil composition and larger for the other catalysts, which confirms the selective recovery of C ad H in the HyPy oil.This study proposes a reaction pathway leading to BTX considering the already proposed mechanisms for the cracking and reforming of biomass. To evaluate if the main products distribution from the HyPy of Etek lignin was dictated by the thermodynamical favourability, the main reactions for three representative model compounds (phenol and 2-methoxy-phenol for lignin and glucose for cellulose) were simulated using HSC Chemistry 5.11, Outokumpu between 300 and 800 °C, at ambient pressure. For phenol and 2-methoxy-phenol the thermodynamic viability of hydrodeoxygenation to benzene (R1), cyclohexane (R4), toluene (R6) and methyl-cyclohexane (R7), the hydrogenation to cyclohexadiene (R2), cyclohexanol (R3), methyl-cyclohexanol (R8) and the methanation reactions (R5 and R9) were evaluated. For the cellulose fraction of Etek lignin instead, the HDO of glucose to 1,3-cyclopentadiene (R10), acetone (R11) and 3-cyclopentene 1,2-diol (R12) were considered as listed below: (1) R1: C6H6O + H2 = C6H6 + H2O (2) R2: C6H6O + 2H2 = C6H8 + H2O (3) R3: C6H6O + 3H2 = C6H12O (4) R4: C6H6O + 4H2 = C6H12 + H2O (5) R5: C6H6O + 10H2 = 6CH4 + H2O (6) R6: C7H8O + H2 = C7H8 + H2O (7) R7: C7H8O + 4H2 = C7H14 + H2O (8) R8: C7H8O + 3H2 = C7H14O (9) R9: C7H8O + 11H2 = 7CH4 + H2O (10) R10: C6H12O6 + 3.6H2 = 1.2C5H6 + 6H2O (11) R11: C6H12O6 + 4H2 = 2C3H6O + 4H2O (12) R12: C6H12O6 + 4.8H2 = 1.2C5H8O2 + 3.6H2O ΔG, ΔH, ΔS and the equilibrium constant (Kc) were calculated as function of temperature and the resulting log Kc for the above reactions are shown in Fig. 2 , where values of Kc larger than zero indicate spontaneity and larger the number greater the thermodynamic viability. As can be seen for the two phenolic species, in general lower the temperature more thermodynamically favourable are the reactions R1 to R10 (due to exothermicity of all these reactions) and only the methanation and HDO to benzene and toluene are thermodynamically favoured at temperature between 500 and 600 °C as used in this work, with the first being more favourable. This is in agreement with the experimental findings with the exception of cyclohexadiene (R2), where only benzene and toluene were detected. Typically, the phenolic ring is hydrogenated to form cyclohexanol or cyclohexanone at low temperature (as supported by the Kc in Fig. 2) and this explains the absence of these products in this work. The HDO reactions of glucose (R10-12) are instead endothermic and therefore favoured at higher temperature as shown in Fig. 2, where it can be seen that they are thermodynamically favourable under the studied temperature. If acetone (R11) is common to both Fe/ZrO2 and PdFe/ZrO2, the HDO to 1,3-cyclopentadiene occurs only in presence of Fe/ZrO2 suggesting that Fe is more prone to deoxygenating cellulose. Due to the reducing nature of the H2 atmosphere, it is expected that cellulose is reduced to non-condensable gases as shown by V.K. Venkatakrishnan et al (2014), with the remaining contribution from cellulose being reduced to simple chain molecules like Ketones, Aldehydes, Olefins as well Cyclopentenes (Venkatakrishnan et al., 2014). Recently, Li et al (2022) showed that cellulose hydropyrolysis under 25 bar hydrogen and 500 °C mainly produced C5-C7 ketones with carbon yield of 27.2 % (Li, Miao, et al., 2022). In particular, C5-C7 chain ketones and cyclic ketones were generated by furfural through HDO and hydrocracking reactions. The olefin-radical pathway for cellulose to BTX conversion is not prevalent in H2 atmosphere because the presence of H2 will protonate and stabilise the radicals and prohibit further aromatisation reactions from cellulose derivatives. However, it is assumed that cyclopentadiene is formed via this route.Various reaction pathways have been previously proposed to explain the mechanism for lignin hydropyrolysis and bio-oil vapour HDO upgrade into BTX (Jan et al., 2015; Liu et al., 2019; Sirous-Rezaei et al., 2018; Sirous-Rezaei and Park, 2020). It involves the hydrogenation of the lignin structure during pyrolysis into phenolic monomers, including guaiacols, which then are reduced to BTX. At high temperature as used in this work, phenol deoxygenation occurs either via decarbonylation or dehydration of the alcohol group (RC), resulting in mono-aromatic hydrocarbons (MAHs). Over acid sites, these MAHs may undergo alkylation, cyclization, minor rates of hydrogenation to form cycloalkanes and polymerization (RD) to form naphthalenes and poly-aromatic hydrocarbons (Gamliel et al., 2018). Based on previous literature, it is expected that coking reactions took place at 600 °C (Gholizadeh et al., 2016).The GC–MS analysis clearly indicates that FeZrO2 can selectively produce BTX from the HyPy/HDO of Etek lignin under the studied conditions. Therefore, the properties of this catalyst were examined by different techniques to elucidate the reasons beyond it and correlate them to the proposed reactions mechanismss. AAS confirmed that the Fe content in the FeZrO2 catalyst was 4.98 wt%, while the surface analyses resulted in a BET surface of 65 m2/g, a pore volume of 0.26 cm3/g and a BJH average pore size of 20–30 nm (see supplementary material). The relatively large volume of mesopores represent an important factor for the promotion of hydrotreating reactions, since it favours the diffusion of large molecules (e.g. guaiacols) into the pores where they can then react in the active sites. Guaiacol HDO in presence of Pd/meso-ZSM-5 catalyst exhibited superior guaiacol conversion and product distribution when compared with Pd supported on conventional microporous ZSM-5, due to the improved diffusion and accessibility of active sites inside meso-ZSM-5 (Wang et al., 2020). X-ray powder diffraction pattern of Fe-doped ZrO2 catalyst (see supplementary material) appears as a very well crystalline mixture of different zirconium oxide phases. ZrO2 monoclinic patters typical at ambient temperature (2-θ of 28.2°, 31.5°, 38.5°, 50.1° and 59.8°) are visible, although additional XRD peaks (30.4°, 31.4° and 35.4°) suggest the presence of other ZrO2 phases. ZrO2 has been shown to be a good support for hydropyrolysis reactions due to its thermal stability and surface acidity (Li, Su, et al., 2022), XPS spectra of the FeZrO2 catalyst were taken (see supplementary material). The Fe 2p binding energy (BE) bands indicate presence of Fe3O4 (711 and 715 eV) and Fe2O3 (725 eV) (Bagnato et al., 2020), while the Zr3d bands indicate presence of ZrO2 (182.5 eV and 183.9 eV) and ZrOx suboxide (180.1, 182.5 eV). Finally, the O1s spectra show the oxygen corresponding to ZrO2 (531 and 532.8 eV), ZrOx (530.6 eV) and Fe species (528.9 eV). SEM-EDX (see supplementary material), which was used to establish the overall size of the particles and the content in iron, suggests particles in the 10–50 nm size with presence of some larger agglomerates and confirm that the weight content of iron close to 5 wt% as per synthesis. TEM (see supplementary material) confirms that the FeZrO2 particles are of 20–30 nm size, but do not allow for a clear distinction between Zr and Fe due to poor contrast difference, although the darker area of ∼ 5 nm can be assigned to Fe. It is well known that a good dispersion of metals on the support surface is related to good catalytic activity (Rhodes et al., 2005). Therefore, to evaluate the dispersion of Fe on the ZrO2 support, TEM-EDX (see supplementary material) was analysed. The TEM-EDX show a good dispersion of the Fe on the ZrO2 support and confirmed that the Fe nanoparticles are well dispersed in the rage of 5 nm, which can be linked to the catalyst performance. In this context, the low selectivity of PdFe vs Fe could be linked to the poor dispersion of Pd as shown by TEM-EDX (see supplementary material). The acidity, basicity and H2 reducibility of FeZrO2 were also analysed. The H2-TPR (see supplementary material) showed an H2 uptake of 280 μmol/g with reduction to Fe0 occurring between 450 and 600 °C with peak at 550 °C, which confirm that reported in a previous work (Bagnato et al., 2020; Hendry et al., 2020; Spreitzer & Schenk, 2019). NH3-TPD resulted (see supplementary material) in an acidity of about 260 µmol NH3/g, which consisted of weak and mild acid sites denoted by desorption of NH3 from 100 to 400 °C, while FePdZrO2 had considerably lower acidity as shown in previous work (Bagnato et al., 2020). These findings support the fact that under the studied conditions PdFeZrO2 performance was not good as the mono metallic FeZrO2 catalyst and suggest that the selectivity and deoxygenation capability of FeZrO2 is due to the synergic activity of weak and mild acid sites in bonding the carbonyl group and the capacity of Fe of adsorb H2 at high temperature and hydrogenating the aromatic ring. It was recently shown that aromatic rings are repulsed by oxyphilic Fe/ZrO2, which instead adsorb and hydrogenate the carbonyl group followed by deoxygenation in the ZrO2 acid sites (Yung et al., 2019; Li, Miao, et al., 2022)). Moreover, Kumar et al. (2021) studied the influence of the support ZrO2, for Co based catalyst at different reaction temperature, 300–600 °C, at 1 bar for the hydropyrolysis of prot lignin (Kumar et al., 2021). ZrO2 resulted in 29.8 % H-type phenolics at 600 °C due to enhanced demethoxylation. In addition, the Fe performance can be ascribed to a better distribution of active Fe inside the support mesopores and the ability of monoaromatics to enter the mesopores to reach the closely located Fe and ZrO2 active sites. A similar effect was observed in the hydrotreating of guaiacol with Ni-ZrO2 on mesosilica, where mesopores facilitated confinement of Ni and ZrO2 nanoparticles, which proximity was considered necessary for achieving a high selectivity into deoxygenated products HDO (Lopez et al, 2020).Therefore, the remarkable catalytic activity and selectivity of Fe/ZrO2 was linked to the iron oxophilicity, the strong reduction potential of zero-valent iron, the good dispersion of Fe nanoparticles on the support and the presence of mesopores and Lewis acid sites on the zirconium oxide support, which increased the interactions between lignin derived phenolic molecules and the catalyst surface. Fe/ZrO2 showed also good potential to also reduce cellulose-derived molecules to acetone and cyclopentadiene. The above suggest that Fe/ZrO2 can be used for hydrodeoxygenation of biomass-derived oxygenates under the tested conditions, which is crucial to achieve a cost-effective scale-up of the process.The hydropyrolysis and hydrodeoxygenation of Etek lignin was studied at 600 °C in presence of zirconia supported metal catalysts under ambient pressure. Fe/ZrO2 led to considerably enhanced HDO of lignin-derived phenolics to aromatic hydrocarbons in comparison with the other catalysts. The Fe/ZrO2 mild-strong reduction sites provided- good reducing activity, selectively hydro-deoxygenating carbonyl groups in phenolic and guaiacol compounds to benzene (35.4 %), toluene (25.6 %) and xylene (5.8 %), as the main products with a carbon distribution of 85.5 % in C5-C10 hydrocarbons. William Lonchay: Investigation, Formal analysis, Visualization, Validation, Writing – original draft. Giuseppe Bagnato: Investigation, Validation, Writing – review & editing. Aimaro Sanna: Conceptualization, Methodology, Supervision, Validation, Writing – review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by Innovate UK/KTN (KTP project no. 10013135). We kindly acknowledge Joe Perkins-Hall and Karen Sam, CDS-Analytical for Py-GC support and Aaron Naden, Department of Chemistry, University of St. Andrews for TEM-EDX analysis.Supplementary data to this article can be found online at https://doi.org/10.1016/j.biortech.2022.127727.The following are the Supplementary data to this article: Supplementary Data 1

The use of lignin to produce Benzene, Toluene and Xylene (BTX) is a promising pathway to strength the economic case, over the production of advanced bio-fuels alone. In this work, Ce, Na, Pd and Fe supported on zirconium oxide were evaluated for the ex-situ hydropyrolysis (HyPy)/hydrodeoxygenation (HDO) of Etek lignin under mild conditions (600 °C, 1 atmosphere) towards the production of BTX. Fe/ZrO2 was able to selectively produce BTX (67 area%) and cycloalkenes (13.5 area%) and strongly deoxygenate the HyPy oil to about 5 wt% oxygen content, resulting in an oil with a carbon distribution of 85.5 % in C5-C10 hydrocarbons. The high selectivity of Fe/ZrO2 was related to the iron oxophilicity, the strong reduction potential of zero-valent iron, the good dispersion of Fe nanoparticles on the support and the presence of mesopores and acid sites, which enhanced the interactions between the reacting species and the catalyst surface.

No data was used for the research described in the article.The development of catalytic converters for neutralization of exhaust gases has a history of almost 50 years and is constantly accompanied by an increasing stringency of emission standards for hazardous atmospheric pollutants CO, NOx and CHx [1]. To meet these standards at today's level, significant improvements in the performance of three-way catalysts (TWCs) are required. One of the main challenges in this regard is to solve the problem of a cold-start when the catalytic converter and exhaust gases have a temperature below the ignition temperature. The maximum emission of hazardous pollutants is observed during the first 30 s from engine start [2,3]. One approach to solving this problem is to localize the catalyst in the closest position to the engine, i.e. close coupled position. This reduces the time to reach ignition temperature to 10–15 s [1]. However, during engine operation the temperature in this zone varies in a wide range and reaches 1000 °C and more. This poses new challenges for development of catalysts with increased thermal stability while maintaining high activity at low temperatures. [4,5]. The use of natural gas, containing mostly methane, as a clean fuel for natural gas fueled vehicles (NGVs) and for power generation in gas turbines is becoming increasingly common as an alternative to conventional diesel or gasoline fuel [6–8]. Over the past 10 years, the number of NGVs worldwide has more than doubled [6]. The increased use of NGVs is related to lower emissions of pollutants, such as carbon monoxide and volatile organic compounds, compared to conventional engines [9,10]. However, the residual unburned methane in exhaust gases poses a great potential threat to the environment because of its strong greenhouse effect, which is ∼30 times greater than that of CO2. Therefore, an effective aftertreatment system is required to reduce the fraction of residual methane [11]. Complete catalytic oxidation of methane has become the main technology for purifying the exhaust gases of NGVs [12]. This process is complicated due to high strength of the CH bond in the methane molecule, low temperature of exhaust gases, not exceeding 500 °C, and high concentration of water in the exhaust gases, poisoning the catalyst [11,12]. Under these conditions, Pd-based catalysts proved to be the most active [13]. To prevent the decomposition of active PdO into less active metallic Pd and to minimize the high-temperature formation of NOx and SOx, it is necessary to lower the methane oxidation temperature. [7,14]. However, the exhaust temperature may rise to 800–850 °C during fast driving. [7]. To maintain high low-temperature activity under these conditions, it is necessary to increase the thermal stability of both the support and the highly dispersed oxidized forms of palladium, which exhibit the greatest activity in methane oxidation.Ceria is an indispensable component of modern TWC catalysts. First of all, ceria serves as an OSC component to minimize the drop of conversion when the composition of the reaction mixture fluctuates around the stoichiometric air/fuel ratio [1]. An important feature of CeO2 is the accumulation and release of oxygen under redox conditions, which are realized during the operation of TWC catalysts. This is achieved due to the Ce4+-Ce3+ redox couple, which has the ability to change from Ce4+ (CeO2) under oxidizing conditions to Ce3+ (Ce2O3) under reducing conditions and vice versa [15]. However, the thermal stability of cerium oxide is not sufficient, so the doping of CeO2 with transition metal ions is used to increase thermal stability and preserve OSC. The most studied is the modification of CeO2 by Zr4+ ions in a certain concentration (ZrO2 ≤ 33 mol%), which does not change the fluorite structure of Ce1-xZrxO2 solid solution [16], but increases the oxygen capacity and thermal stability [17,18].The use of tin for CeO2 doping has been studied in a number of works and has shown to provide a significant increase of OSC in series of CexSn1-xO2-δ oxides with low amount of tin (x ≥ 0.8) [19–22]. Doping with tin was also found to prevent the growth of mixed oxide crystallites leading to increase of the specific surface area [19,23,24]. The enhancement of OSC as a result of modification by tin is explained by an increase in the concentration of defects in the mixed oxide lattice. This leads to the formation of additional oxygen vacancies associated with the redox couple Sn4+/Sn2+ (along with the Ce4+/Ce3+ pair) [19,25,26] promoting increased oxygen mobility. The authors of [19] also observed a lower reduction temperature of mixed oxides, which was attributed to the lower oxygen binding energy in Ce-O-Sn than in Ce-O-Ce or Sn-O-Sn.The doping of SnO2 with cerium also leads to increase in the thermal stability of the SnO2-based catalysts. The SnCe binary oxides are characterized by a larger specific surface area and a smaller particle size than undoped SnO2 [27], which determines the dispersion of deposited Pd and thus affects the activity [28]. Generally, the activity of Pd/SnO2 catalysts in CO oxidation reaction is significantly lower than the activity of Pd/CeO2 and Pd/Ce-Sn-O catalysts. The onset of the CO oxidation on Pd/SnO2 is observed at temperatures above 100–170 °C [28,29], whereas in the case of Ce-based and Ce-Sn-based catalysts the CO oxidation starts at room temperature and even below. At the same time, Sn-rich catalysts (SnO2 phase) have a larger specific surface area compared to Ce-rich catalysts (CeO2 phase) [30]. A lower reduction temperature was observed for Sn-rich catalysts, which the authors attribute to the higher oxygen mobility in Sn-rich catalysts due to the coexistence of surface and bulk oxygen vacancies. In contrast to Sn-rich catalysts, in Ce-rich catalysts oxygen vacancies exist predominantly on the surface [30]. For Ce-doped tin dioxide nanoparticles an increase in thermal stability and resistance to sintering at high temperature is observed. In this case the thermal stability is explained by the processes of cerium segregation on the surface of tin oxide particles with the formation of a Ce-rich surface layer responsible for particle growth inhibition [31]. It should be noted that Ce-doped SnO2 in Sn-rich catalysts shows higher activity in methane oxidation than Sn-doped CeO2 in Ce-rich catalysts [30]. On the other hand, the introduction of small amounts of Sn into CeO2 lattice increases the activity of the catalysts in CO oxidation [19].Pd and Pt being widely known for their high activity in oxidation of CO and hydrocarbons, are used together with cerium oxide in TWC catalysts [1,32–36]. The deposition of Pd on cerium oxide promoted with transition metal ions can increase the thermal stability and preserve the active component in a highly dispersed state due to the strong metal-support interaction. Increased thermal stability and catalytic performance in CO oxidation for Pd-Ce-Sn-O co-oxides have been reported in [22,29,37–39]. A study of Ce-rich and Sn-rich Pd/Sn1-xCexO2 catalysts showed that the Ce-rich Pd/Sn0.2Ce0.8O2 sample has a higher CO oxidation activity than the Sn-rich Pd/Sn0.8Ce0.2O2 and non-promoted Pd/CeO2 and Pd/SnO2 catalysts [37]. Compared to the non-promoted SnO2 and CeO2 oxides, the binary CeSn oxides have smaller crystallite sizes and larger surfaces. The authors concluded that the active forms of oxygen in Ce-rich and Sn-rich supports are lattice and chemisorbed oxygen, respectively. However, to accurately establish the nature of oxygen and the mechanisms of the catalytic reaction the additional studies are required, including investigation of catalysts with different compositions of mixed oxides.Pd/CeO2-SnO2 catalysts with varying Ce/Sn ratio showed high activity in methane oxidation [40]. The highest activity is achieved by Pd deposition on Sn0.9Ce0.1O2 support calcined at 1100 °C. However, in this work the catalyst was calcined only at 450 °C that doesn't give an opportunity to judge about preservation of activity under operating conditions at high temperature. Therefore, the study of Pd/CeO2-SnO2 catalysts with varying Ce/Sn ratio calcined at high temperatures is promising for advancing the development of thermostable methane oxidation catalysts.In our previous work [29] the Pd/Ce-Sn-O catalysts synthesized by the counter precipitation method with an equimolar Ce/Sn ratio were studied. It was shown that the calcination of samples at temperatures of 800–1000 °C leads to an increase in catalytic activity in CO oxidation reaction at low temperatures below 150 °C. It was shown that upon calcination above 600 °C a nanoheterophase catalyst structure was formed, which is a catalytically active PdxCe1-xO2-δ dispersed phase on the surface of SnO2 nanoparticles. The formation of such structure in Pd/Ce0.5Sn0.5O2 catalyst provided high thermal stability up to 1000 °C and preserved active PdOx clusters stabilized on the surface of PdxCe1-xO2-δ solid solution. Despite the relationship between increase in activity and formation of nanoheterophase structure of the catalysts detected during thermal activation, it remains unclear whether the increase in reaction rate compared to unmodified catalysts is a consequence of the specific spatial structure of the catalyst, or whether modification of the active center of the catalysts occurs.This work presents a detailed study of the local structure of active centers of Pd/Ce(Sn)O2 catalysts using XRD, TEM, XPS, Raman methods and is aimed at establishing the relation between the catalyst structure and catalytic properties. For this purpose we studied the effect of Ce/Sn ratio in Pd/CeO2-SnO2 catalysts calcined at high temperature of 800, 900 and 1000 °C on physico-chemical properties and activity in CO and methane oxidation.The Pd/CeSn samples with varying Ce/Sn ratio were synthesized by counter precipitation method used in our previous work [29]. The initial (NH4)2[Ce(NO3)6] was synthesized from Ce(NO3)3·6H2O (JSC Reaktiv, analytical grade) according to Ushakov et al. [41]. [Pd(H2O)2(NO3)2] was obtained from metallic Pd (Krastsvetmet, 99.9%) in compliance with Khranenko et al. [42]. Na2[Sn(OH)6] was prepared from SnCl4·5H2O (Reachem) by dissolving in a 10% excess of NaOH with subsequent crystallization. Purity of the products was verified by XRD.(NH4)2[Ce(NO3)6] and [Pd(H2O)2(NO3)2] were dissolved in water or in an aqueous solution of HNO3 to obtain an acid solution (30 ml). Na2[Sn(OH)6] was dissolved in water or an aqueous solution of NaOH (50 ml). The amounts of NaOH and HNO3 were calculated for the complete hydrolysis of all components. The amount of [Pd(H2O)2(NO3)2] was calculated so as to equalize the molar ratio Pd/(Ce + Sn) to the molar ratio Pd/Ce in a reference sample 1wt%Pd/CeO2 (0.0163) synthesized by the procedure described in [29]. The amounts of the reagents for the synthesis of 4 g of catalyst and Pd loading (wt%) are given in Table 1 .The resulting solutions were rapidly mixed with each other which led to the neutralization and precipitation of hydroxides of all the metals. The reaction mixture was then cooled to −15 °C and held for a day at this temperature. The precipitate was isolated on a filter at 2 °C and repeatedly washed with cold water and then with acetone. The obtained precipitates were dried in an oven at 80 °C for 4 h. After that, the catalyst samples were placed in the preheated furnace and calcined for 2 h in air at 450, 600, 800, 900, and 1000 °C.The synthesized catalysts are denoted as Pd/CeSnX-T, where T is the calcination temperature, X describes the atomic ratio of the support components and is calculated from atomic fraction of Sn (X Sn) and Ce (X Ce) in the catalyst by the following eq. X = X Sn / (X Sn + X Ce)·100.In all cases, atomic absorption spectrometry (AAS) did not detect palladium in mother liquors and rinsing waters. CeO2 and SnO2 are chemically inert compounds and therefore it is hard to prepare solutions for analysis. To determine the ratios of elements we dissolved the hydroxide precipitate, which was obtained after the mutual neutralization, in 6 M hydrochloric acid. The results of ICP-MS analysis (with Quadrupole ICP-MS iCAP spectrometer) confirmed the ratio of the metals to be close to initial loadings. The atomic ratio Pd:Ce is 0.017:1 for Pd/Ce reference catalyst (1.04 wt% in terms of “Pd + CeO2” stoichiometry) and Pd:(Ce + Sn) is 0.0158:1, 0.0163:1, 0.0163:1 and 0.0163:1 for Pd/CeSn75, Pd/CeSn50, Pd/CeSn25and Pd/CeSn15 series, correspondingly.An X-ray diffraction study (XRD) was carried out on a Shimadzu XRD–7000 diffractometer (Cu K α radiation, Ni filter on the reflected beam). Diffraction patterns were recorded in a stepwise mode with the accumulation time necessary for recording reflections of the phases that are present in small amounts. The refinement of lattice constants and quantitative phase analysis were performed by Rietveld full-profile analysis using PowderCell2.4 software [43]. The crystallite sizes (D) were calculated on the base of Scherrer Eq. [44] after exclusion of the instrumental contribution. For the fluorite phase, the DFluorite value was calculated from reflex 220 (2Θ = 47.5°); for the rutile phase, the DRutile value was taken as the average of that calculated from reflexes 200 (2Θ = 38.0°) and 111 (2Θ = 38.9°). Taking into account the results of work [45] the independence of the integral broadening method on the size and shape distribution of the samples was applied for calculations. The procedure of deconvolution and fitting of the XRD lines was performed using the WinFit 1.2.1 software [46].The catalysts microstructure was studied by transmission electron microscopy (TEM) using a Thermo Fisher Scientific Themis Z double Cs-corrected electron microscope operated at an accelerating voltage of 200 kV. The spectrum imaging data were obtained using a Super-X G2 EDX detector and a HAADF detector for image registration in scanning (STEM) mode. Crystal lattices on the obtained (S)TEM images were analyzed using the Fourier method. The samples were dispersed by ultrasound in ethanol and deposited on standard copper grids covered with a holey carbon film.The X-ray photoelectron spectra were obtained using an Ultra Axis DLD (Kratos Analytical, UK) and ES-300 (Kratos Analytical, UK) photoelectron spectrometers. The analyzers were calibrated relatively Au 4f 7/2 and Cu 2p 3/2 lines with standard binding energies 84.0 eV and 932.7 eV, respectively [47]. Mg K α source with hν = 1253.6 eV was used as a primary radiation. Spectral data acquisition was carried out in the regime of constant pass energy of the analyzer. The decreased X-ray source power 65 W was used to prevent ceria photoreduction during spectra recording. The calibration of experimental spectra was performed using the U'''-component of the Ce 3d line, the binding energy of which was taken equal to 916.7 eV [48]. The spectra processing was carried out using the XPSCalc software developed at the Boreskov Institute of Catalysis SB RAS, approved for films and powder catalytic systems [49–51]. The spectra decomposition into individual components was described by the Gaussian-Lorentzian distribution with subtraction of the background of inelastically scattered electrons by Shirley model. The quantitative composition was calculated from areas of the lines taking into account the atomic sensitivity factors [47].Raman spectra were obtained using an InVia (Renishaw, UK) confocal Raman dispersion spectrometer equipped with a Leica microscope with a 50× objective. Excitation was performed with continuous lasers: a semiconductor laser with 785 nm and 100 mW and a solid-state Nd:YAG laser, second harmonic, 532 nm, 100 mW. To prevent sample heating, only 10% of the maximum laser intensity was employed together with 50% defocusing mode and signal accumulation time up to 100 s. The Raman spectra were measured in the 100–3200 cm−1 range with a spectral resolution of 2 cm−1 and 1 cm−1 upon excitation at 532 and 785 nm, respectively. Data are reported for the range of 100–1000 cm−1, in which important changes were observed in the spectra.An automated setup with a flow reactor [50] was used for investigation of catalytic properties in temperature-programmed reaction with CO (TPR-CO), in temperature-programmed CO oxidation reaction (TPR-CO + O2) and in temperature-programmed CH4 oxidation reaction (TPR-CH4 + O2). The concentrations of CO (m/z = 12, 28), CO2 (m/z = 12, 28, 44), O2 (m/z = 32), H2 (m/z = 2) and CH4 (m/z = 13) were measured with aid of the quadrupole mass spectrometer RGA 200 (SRS). Neon was used as the reference inert standard for precise calculation of the component concentrations in the reaction mixtures. Digital mass-flow controllers for individual gases and mass-spectrometer operate at room temperature.In the TPR-CO experiments the reaction mixture containing 1.0 vol% CO, 0.5 vol% Ne and helium the balance was introduced at a flow rate of 100 cm3/min to the catalyst sample (0.2 g) preliminary cooled in the reactor to −20 (−40) °C. As the steady-state concentrations of CO and CO2 were established, the sample was heated from −20 (−40) to 450 °C at 10 °C/min heating rate. The concentrations of CO, CO2, O2, H2 and H2O were measured during the reaction. Before each TPR-CO experiment, the catalysts were pre-treated by 20% O2/He gas mixture at 450 °C during 2 h with subsequent cooling in this mixture and with subsequent purging with helium.The activity in TPR-CO + O 2 and TPR-CH 4  + O 2 was measured using the following experimental conditions. The catalysts grain size was 0.14–0.25 mm. The catalysts weight and catalysts volume were 0.2–0.3 g and 0.25 cm3 for TPR-CO + O2 accordingly. The reaction mixture velocity and GHSV were 1000 cm3/min and 240,000 h−1, accordingly. A 0.05 g of catalyst was diluted by quartz to 0.25 cm3 for TPR-CH4 + O2. For CH4 oxidation the reaction mixture and GHSV were 100 cm3/min and 120 L g−1 h−1, accordingly. For CO oxidation initial gas composition contains 0.2 vol% CO, 1.0 vol% O2, 0.5 vol% Ne, and helium the balance. For CH4 oxidation initial gas composition contains 0.1 vol% CH4, 1.0 vol% O2, 0.5 vol% Ne, 0 (10.0) vol% H2O and helium the balance. 10 vol% of steam (H2O) were introduced by flowing the helium through a temperature-controlled saturator. The catalysts were cooled to −20 °C if the experiments were carried out in a CO oxidation reaction. If experiments were carried out in CH4 oxidation, the initial temperature was 50 °C.The experiments were carried out using repeated cycles of the catalysts heating/cooling in the reaction mixture in the temperature range from −10 (+50) °C to 450 (600) °C at constant heating rate 10 °C/min for CO (CH4) oxidation. Changes in the concentrations of CO (CH4), O2 and CO2 during the reaction were monitored by measuring at a frequency of 0.34 Hz.Rates of the CO + O2 or CH4 + O2 reactions were calculated using the initial sections of TPR − CO + O2 (TPR-CH4 + O2) curves in the region of conversions not higher than 20% under differential reactor operation, where diffusion limitations and changes in the reaction mixture composition over the catalyst bed can be neglected (Fig. S1). As it is reported in previous work [29] the internal and external diffusion effects and heat transfer effect could be neglected during the kinetic experiments. The specific reaction rate was determined per catalyst surface using the formula W(molecules/cm2 × s) = C 0  × X × V RM / m × S sp , where C 0 is the initial concentration of CO (or CH4) (molecules/cm3), X is the CO conversion, V RM is the space velocity of the reaction mixture (cm3/s), m is the catalyst weight (g), and S sp – specific surface area (cm2/g). The weight reaction rate was determined per 1 g of Pd using the formula W(molecules/g(Pd) × s) = C 0  × X × V RM / m, where C 0 is the initial concentration of CO (or CH4) (molecules/cm3), X is the CO conversion, V RM is the space velocity of the reaction mixture (cm3/s), and m is the weight of Pd (g).The catalytic properties of Pd/CeSn samples with various content of tin were studied in the CO oxidation reaction. The previously discovered tendency towards an increase in the catalytic activity of Sn-modified catalysts with calcination temperature extends to the entire studied range of Ce/Sn ratio. Comparison of the weight rates (per 1 g Pd) of CO oxidation for catalysts calcined in the temperature range of 450–1000 °C (Fig. S2) showed that the activity of the catalysts increases significantly with an increase in the calcination temperature up to 800 °C and above. In this regard, the main emphasis in this work is placed on a detailed study of catalysts calcined at temperatures of 800, 900, and 1000 °C.In our previous work [29] Pd/CeO2-SnO2 catalysts with eqimolar Ce/Sn ratio were investigated. In the present work, the synthesis of a catalyst with a ratio Ce/Sn = 1 was reproduced. Fig. S3 shows the dependences of CO conversion for two different samples of 1%Pd/CeSn50–800. From the presented data, we can see that the dependences of CO conversion are almost identical, which indicates a good degree of reproducibility of the synthesized samples. Figure 1 shows the dependences of T 50 (temperature of 50% CO conversion) and the weight rates (per 1 g Pd) of CO oxidation at 25 °C on the catalyst calcination temperature. The temperature dependences of CO conversion and the Arrhenius dependences of the CO oxidation rate are shown in Fig. S4. The presented data shows a dome-shaped dependence of the T 50 values on the relative tin content. The minimum T 50 value is attained in the case of Pd/CeSn50 and Pd/CeSn25 calcined at 800–900 °C, while the maximum values are observed for the Pd/CeSn15 and Pd/CeSn75 catalysts. The dome-shaped dependences are also observed for the rate of CO oxidation on Pd/CeSnX with rate maxima at X = 50 (T calc = 800–1000 оC) and X = 25 (T calc = 800–900 оC). The activity of the Pd/CeSn25 catalyst is similar to that of the Pd/CeSn50 catalyst after calcination at 800–900 °C. After calcination at 1000 °C the activity of Pd/CeSn25 and Pd/CeSn75 catalysts becomes significantly lower than that of Pd/CeSn50. Thus, the activity of the catalysts strongly depends on the amount of tin oxide introduced and the activation temperature. At the same time, the observed dome-shaped dependence on the composition may be related both to the implementation of different catalyst morphologies and to the formation of new active centers on the surface. It can be assumed that the nature of the active oxygen in these centers differs significantly, determining its reactivity.The temperature-programmed reaction with CO was used to determine the reactivity of oxygen in the catalysts. The shape of the CO consumption curves versus temperature and the temperature profile of the CO2 evolution are related to the presence of oxygen with different bonding energies in the catalyst. Fig. S5 shows a typical TPR-CO experiment showing the time dependence of CO consumption, evolution of CO2, O2, H2, H2O, and temperature as well. A description of the experiment is provided in the Supplementary Material. Based on previous work [29,51–53], the CO2 release at temperatures above 200 °C is associated with the interaction of CO with bulk oxygen of CeO2 and SnO2 particles or particles of their solid solutions based on fluorite and rutile phases. The presence of Pd in these phases leads to a decrease in the temperature of interaction of oxygen with CO due to activation of oxygen in these phases and its diffusion to the palladium active sites (PdOx, PdO) on the catalyst surface [54]. A sharp peak of CO2 evolution in the temperature range of 180–200 °C is associated with the interaction of CO with oxygen of PdO particles [29,51–53]. The evolution of CO2 in low-temperature region of −40–100 °С takes place due to interaction of CO with oxygen of PdOx(s)/Pd-Ce-O(s) clusters.As it can be seen from the TPR-CO data in Fig. 2 , the main release of CO2 occurs in the range of 150–450 °C. According to Table S1 and Fig. S6, the total amount of reactive oxygen varies significantly depending on the tin content in the samples. The data presented in Table S1 and Fig. S6 show that calcination at 800 °C produces the maximum amount of reactive oxygen in all samples studied. However, considering all calcination temperatures, the amount of reactive oxygen passes through a maximum depending on the concentration of tin in the samples. The calcination at 800 °C yields the maximum amount of oxygen for the equimolar Ce/Sn ratio, while for higher calcination temperatures of 900 °C and 1000 °C the maximum amount of oxygen is observed for the Pd/CeSn25 sample.TPR-CO curves of Pd/CeSn25–1000, Pd/CeSn75–900 and Pd/CeSn75–1000 show peaks at about 190 °C, which are related to the reduction of PdO nanoparticles [51,52]. The amount of PdO calculated from the area of these peaks is 14, 37 and 49% of the total Pd content in Pd/CeSn25–1000, Pd/CeSn75–900 and Pd/CeSn75–1000 catalysts, respectively. It should be noted that PdO particles are not formed during the calcination of the samples at 800 °C, although the above data indicate the highest amount of active oxygen in the catalysts calcined at 800 °C. The formation of PdO particles in the Pd/CeSn50 catalysts is also not observed. It can be assumed that in those samples where the PdO phase is not formed, the maximum interaction of palladium with structures of cerium and tin oxides occurs.The most noticeable differences in TPR-CO curves are observed in the low temperature region, as shown in Fig. 2 d-f. The release of CO2 at the lowest temperatures is an extremely important factor, since it largely determines the activity of the catalyst. Fig. 2 d-f shows that the onset of CO2 evolution depends on both the tin content and the calcination temperature. For Pd/CeSn15 and Pd/CeSn25 catalysts the onset of CO2 evolution shifts towards higher temperatures from −20 to 20 °C as the calcination temperature increases from 800 to 1000 °C. For the Pd/CeSn50–800 catalyst, CO2 evolution starts at the temperature of 0 °C which remains practically unchanged with increasing calcination temperature of the catalyst up to 1000 °C, thus demonstrating high thermal stability. For the Pd/CeSn75 catalyst the characteristic feature is the ability to release CO2 at very low temperatures around −40 °C if the catalyst is calcined at 800 and 900 °C. These data allow us to conclude that the Pd/CeSn75 catalyst has the most weakly bound and reactive oxygen among the catalysts calcined at 800–900 °C, but the amount of oxygen of this type is quite low. After calcination at 1000 °C, the CeSn75–1000 catalyst loses weakly bound oxygen, whereas the Pd/CeSn50–1000 catalyst has the highest amount of weakly bound oxygen in this case. The presented data show that the Pd/CeSn50 catalyst has a compromise property of retaining the highest amounts of weakly bound oxygen at high calcination temperatures up to 1000 °C.Fig. S7 shows the temperature dependence of CH4 conversion and the Arrhenius dependence of CH4 oxidation rate. Based on these data, the values of T 10 and weight rate of CH4 oxidation (per 1 g Pd) at 450 °C were calculated as a function of relative tin content X for Pd/CeSnX catalysts (X = 15, 25, 50, 75) calcined at 800, 900 and 1000 °C. These data presented in Fig. 3 show that the highest T 10 values are observed for the Pd/CeSn15 catalyst with lowest tin content. An increase in the tin content leads to a decrease in the T 10 value. The lowest T 10 values and, accordingly, the highest reaction rate are achieved for the Pd/CeSn75 catalyst. Being calcined at 1000 °C the Pd/CeSn75 catalyst is characterized by a significantly higher reaction rate compared to the Pd/CeSn50–1000 and Pd/CeSn25–1000 catalysts (4 and 10 times higher, respectively). Thus, the catalytic testing of the studied catalysts in the reaction of methane oxidation shows that the activity of the Sn-rich catalysts is significantly higher than the activity of the Ce-rich ones.The effect of water vapor on the catalytic activity was studied on the Pd/CeSn75 catalyst, which shows the highest activity in the oxidation of methane in the series of Pd/CeSnX catalysts. As shown in Fig. 3 с, the Т10 value for the Pd/CeSn75–800 catalyst increases from 380 to 505 °C when 10% H2O is added to the reaction mixture. At the same time, the reaction rate decreases by about an order of magnitude (Fig. 3 d). Figure 4 shows XRD patterns obtained for Pd/CeSn catalysts of various compositions calcined at 450, 800, 900, and 1000 °C. Table S2 presents the structural parameters and phase content calculated from the XRD data. Due to the low palladium content in the samples, the diffraction patterns do not comprise any reflections that could be attributed to known palladium-containing phases.Pd/CeSnX catalysts calcined at 450 °C are characterized by a significant broadening of diffraction reflections due to the small crystallite size, the presence of crystalline structure defects in the formed phases and local fluctuations of phase composition. The poor resolution of the reflections does not allow a correct quantitative phase analysis to compare the amounts of coexisting phases with the structure of fluorite and rutile. Moreover, for Ce-rich samples (Pd/CeSn15–450 and Pd/CeSn25–450) it is not possible to confidently state from the XRD data whether a phase with rutile structure is present or not. Diffraction data for catalysts calcined at 450 °C also do not allow the crystal lattice parameters to be determined with sufficient accuracy. Nevertheless, it can be confidently stated that the lattice parameter of fluorite phase in all samples a Fluorite = 5.26–5.40 Å differs significantly from the lattice parameter of pure cerium oxide (a(CeO2) = 5.411 Å, ICDD PDF-2 card #34–394). This indicates the incorporation of tin ions into the cerium oxide lattice with the formation of a Ce1-ySnyO2 (or Pd x Ce 1-x-y Sn y O 2-δ ) solid solution [55]. An increase in the calcination temperature of catalysts to 800 °C and higher leads to an increase in the crystallite size due to coarsening. This is accompanied by narrowing of the reflections in the diffraction patterns. Fig. 4 and Table S2 show that the Pd/CeSn catalysts calcined at 800 °C represent two-phase mixtures consisting of a phase with fluorite structure and a phase with rutile structure. A slight shift of the lattice parameters in the observed phases (Table S2) relative to the lattice parameters of pure cerium oxide (a = 5.411 Å, ICDD PDF-2 card #34–394) and tin oxide (a = b = 4.738 Å, c = 3.187 Å, ICDD PDF-2 card #41–1445) indicates the incorporation of tin and cerium atoms into the lattice of the CeO2 and SnO2 phases to form primary solid solutions based on fluorite (Ce1-xSnxO2-δ) and rutile (Sn1-yCeyO2-δ). Quantitative phase analysis shows that the phase content of the samples correlates well with the nominal composition of the samples. It should be noted that the samples with a higher tin content comprise crystallites of a smaller average size compared to Ce-rich samples (Table S2), and this decrease in particle size is relevant to both the fluorite and rutile phases. Thus, the fluorite phase in the Pd/CeSn25–800 catalyst has a crystallite size of 6.8 nm, while in the Pd/CeSn75–800 sample it is 4.5 nm, and the rutile phase in these two samples has crystallite sizes of 9 nm and 4.3 nm, respectively. The diffraction patterns of the samples calcined at higher temperatures (900 and 1000 °C) are characterized by narrow, well-resolved reflections (Fig. 4, Table S2). At the same time, the tendency for the crystallites in the observed phases of fluorite and rutile to be smaller in Sn-rich samples is retained.HRTEM images of catalysts calcined at 450 °C show crystallites of mixed CexSn1-xO2-δ oxides of various compositions (Fig. 5 ). The crystallite size in all samples varies from 1 to 4 nm; in the Pd/CeSn50–450 sample individual largest crystallites with a size of 4–7 nm are observed.As can be seen from the Fast Fourier Transform (FFT) image shown in Fig. 5 a, all interplanar distances observed in the crystallites of the Pd/CeSn25–450 catalyst correspond to the reflections of the fluorite phase. Also, there are areas in the sample containing poorly crystallized, amorphous species. Apparently, these species are the nuclei of the rutile phase. Thereby the rutile phase in the sample has not yet been formed, which explains the absence of rutile reflections on XRD patterns (Fig. 4). Similar amorphous regions are observed in the Pd/CeSn50–450 sample, however, crystallites of the rutile phase are present – the characteristic reflections appear in FFT patterns, corresponding to distances in rutile phase (Fig. 5 b). The additional ring in the selected area electron diffraction (SAED) pattern between the (220) and (311) rings of CeO2 is clearly visible, the position of this ring corresponds to the distance of 1.76 Å and thus can be attributed to the (211) ring of the SnO2 rutile phase. HRTEM images of the Pd/CeSn75–450 catalyst clearly show crystallites with the interplanar distances corresponding to the fluorite and rutile phases. The intensity of the rutile phase reflections in the FFT patterns (Fig. 5 c) exceeds the intensity of the fluorite ones. Also, HRTEM images show the presence of amorphous, poorly crystallized particles.According to EDX analysis (Fig. 5 d, f), Pd/CeSn25–450 and Pd/CeSn75–450 catalysts show quite homogeneous distribution of Sn and Ce, i.e. particles of CexSn1-xO2-δ oxides with various composition are strongly mixed in the volume of the samples. On the contrary, the Pd/CeSn50–450 sample is notable for a “two-colour” EDX pattern, where tin-rich and cerium-rich regions are clearly distinguished (Fig. 5 e). Quantitative analysis gives atomic ratios Ce:Sn ≈ 1.7 in cerium-rich regions and Ce:Sn ≈ 0.3 in tin-rich regions.Palladium in all samples is in a highly dispersed state and is not detected as a separate phase. A low-intensity signal from palladium can be seen in the EDX spectra, quantified from 0.4 to 1.5 wt% in different sections of the samples, with higher amounts of palladium being detected in the Ce-rich regions.TEM study of Pd/CeSn catalysts showed that calcination at 800 °C leads to formation of polycrystalline agglomerates consisting of two types of oxide nanocrystallites (Fig. 6 ). According to EDX data regions enriched in either tin or cerium can be distinguished. Analysis of interplanar spacings allows us to attribute the Sn-rich crystallites to rutile phase and the Ce-rich ones to fluorite phase, in agreement with the XRD results. The crystallites of rutile and fluorite phases are spatially mixed in the bulk of the sample agglomerates, thus forming a nanoheterophase structure over the studied range of tin content. The crystallite size of both phases has a non-linear dependence on the tin content in the catalyst. The highest dispersion is observed for the Pd/CeSn75–800 catalyst, where the size of the fluorite crystallites varies within 3–6 nm, and the size of the rutile crystallites is about 5–10 nm. The crystallite size of the fluorite phase in the other two samples is slightly higher, 5–10 nm. The size of rutile nanocrystallites varies in the range of 5–15 nm in Pd/CeSn25–800 and reaches up to 20 nm in the Pd/CeSn50–800 catalyst (Fig. S8). Larger Sn-rich crystallites are surrounded by smaller Ce-rich crystallites, thus forming composites with intercrystalline mesoporosity. Only for the PdCeSn75 catalyst no preferential localization of cerium on the surface is observed, most likely due to too high tin content in the sample.All the catalysts calcined at 800 °C comprise palladium in a highly dispersed state, no large palladium-containing structures are detected in TEM images. EDX analysis shows rather uniform distribution of palladium in the studied regions without agglomeration (Fig. S8).The catalysts calcined at high temperatures of 900 °C and 1000 °C were investigated by TEM. The calcination of the Pd/CeSn25 catalyst at 900 °C and higher leads to a noticeable sintering and enlargement of a part of the fluorite phase crystallites. As it can be seen from Fig. 6, Ce-rich particles of 50–100 nm in size appear in the Pd/CeSn25–900. These particles are characterized by developed intracrystalline microporosity. Such pores do not form in smaller fluorite particles.Earlier studies of Pd/CeSn50–900 and Pd/CeSn50–1000 catalysts [29] showed formation of heterogeneous structures, represented by the SnO2-based particles covered with CeO2-based particles. A higher amount of tin in these catalysts was shown to prevent cerium oxide sintering, the crystallite size of the fluorite phase does not exceed 20 nm.The particle size of the fluorite phase in Sn-rich Pd/CeSn75–900 catalyst is even smaller – about 5–8 nm, the particles of the rutile phase are enlarged to 5–20 nm (Fig. 6). Thus, the crystallite size of both phases in Sn-rich catalysts remains the smallest in the whole series of catalysts.All catalysts are characterized by the formation of intercrystalline mesopores after thermal treatment. The most developed porous structure is achieved with a small spread in the size of the support crystallites, which is typical for the samples with the maximum tin content – Pd/CeSn75–900 (Fig. 6) and Pd/CeSn75–1000.TEM data for the Pd/CeSn75–900 catalyst (Fig. 7 ) show that palladium leaves the support lattice to form PdO nanoparticles with a size of about 5–10 nm. These particles are densely surrounded by the CexSn1-xO2-δ nanoparticles, forming structures of the core@shell type. As can be seen from Fig. 7, PdO particles are most closely in contact with Ce-rich nanoparticles, and the Sn-rich particles are located on the surface of these agglomerates. Thus, these particles can be considered as having a core@shell1@shell2 structure, where the core is PdO, the shell1 consists of the fluorite CexSn1-xO2-δ (x > 0.6) nanoparticles, and the shell2 is formed by the rutile SnyCe1-yO2-δ (y > 0.6) ones.The calcination of the Pd/CeSn75 catalyst at 1000 °C leads to further segregation of palladium from the support lattice and enlargement of the Pd-containing particles. The formed particles are clearly visible in the TEM and STEM images, their size is about 100 nm (Fig. 8 ). It should be noted that rather low oxygen signal is detected in the EDX spectra acquired from the central area of particles (Fig. 8 e). At the same time the Pd signal in center is noticeably higher, and an inner border in Pd map is distinguishable. This assumes the core@shell particle structure with metal palladium core covered by PdO shell. These core@shell Pd@PdO particles are covered with nanoparticles of cerium‑tin oxides, which size varies in the range of 5–15 nm. In this case, due to the large size of palladium particles, the Ce-rich and Sn-rich oxide nanoparticles are mixed randomly without particular localization on the surface of certain phases, which was observed for the Pd/CeSn75–900 sample.The nanoscale palladium phase was detected only in Sn-rich catalysts after high-temperature treatment (900–1000 °C). In all other studied samples palladium is in a highly dispersed state.HRTEM investigation (Fig. 9 ) reveals the presence of small species on the support surface, which can be attributed to PdOx clusters [53]. It should be noted that the observed clusters were located on the surface of fluorite particles. The size of the clusters does not exceed 1 nm. Under the influence of the electron beam, the clusters detected on extended facets of ceria exhibit dynamic behavior and move intensively across the surface until they reach the surface defects. We associate this dynamic process with the partial reduction of clusters under the influence of the electron beam, which decreases their stability on extended ceria surfaces. The Pd-containing clusters are stabilized on the defects of the support, such as surface steps and intercrystalline boundaries. Analysis of the interplanar distances in these clusters shows the correspondence to metallic palladium. More often, epitaxy between the lattice of such clusters and the fluorite crystals is observed, that is typical for Pd/CeO2 systems [56–58]. Nevertheless, many of the clusters do not exhibit the periodic contrast due to the small size and resulting structural rearrangement. In [59] similar amorphous Pd particles stabilized on ceria surface were shown to be highly active and stable towards CO oxidation.A high-resolution EDX mapping (Fig. 10 ) for the Pd/CeSn50–800 sample revealed that not only palladium but also tin signals are recorded intensely in the area where the cluster is located. Meanwhile, the cluster is located on the surface of Ce-rich nanoparticle. Apparently, the calcination of the catalyst results in the segregation of both palladium and tin from the lattice of mixed Ce(Pd,Sn)O2 oxides with the formation of mixed palladium‑tin clusters. Most likely, such clusters are initially oxidized under atmospheric conditions, but get easily reduced during TEM investigation under the action of the electron beam [51,60].Despite the formation of a nanosized palladium phase in Sn-rich catalysts, part of palladium remains in a highly dispersed state after calcination at 900–1000 °C. The HRTEM and HAADF-STEM images presented in Fig. 11 show a region of a Pd/CeSn75–900 sample. It can be seen that clusters <1 nm in size are located on the surface of the CexSn1-xO2-δ particles. Using high-resolution EDX mapping, we confirm the localization of the clusters on the surface of Ce-rich crystallites. Also, individual clusters are located at the grain boundaries between Ce-rich and Sn-rich crystallites thus having an interface with the rutile phase too. Again, in the presented EDX-mapping data the Sn signal is clearly visible in the region containing the clusters. Thus, the earlier conclusion about the formation of PdSnxOy clusters in the samples during the segregation of Pd and Sn from the fluorite solid solution is confirmed.High-temperature treatment at 1000 °C is accompanied by further segregation of doping elements from the crystal lattices of the fluorite and rutile phases. As a result, HRTEM images (Fig. 12 ) show numerous clusters and amorphous species on the surface of the support particles. These highly dispersed species undergo constant movement under the influence of the electron beam followed by slight agglomeration near the surface defects. Due to the reducing effect of the electron beam some TEM images of the clusters show interplanar spacings of about 0.23 nm, which corresponds to the (111) spacing in metallic palladium.Thus, all catalysts retain the highly dispersed active state of palladium even after calcination at 1000 °C. Only in Sn-rich catalysts a part of palladium undergoes sintering with the formation of PdO nanoparticles stabilized by the CexSn1-xO2-δ support in the form of core@shell structures. Figure 13 shows the Pd 3d spectra obtained for all catalyst series depending on the composition and calcination temperature. All spectra in Fig. 13 are normalized to the total integral intensity of Ce 3d + Sn 3d lines. The Pd 3d spectra were decomposed using three individual doublets with E b(Pd 3d 5/2) = 337.8 eV, 336.2 eV and 336.6–337.2 eV. The provided spectra show that for the Pd/CeSn25 and Pd/CeSn50 catalyst series palladium is predominantly in one state, characterized by E b = 337.8 eV (Fig. 13 a, b). According to the literature [4,35,52,53,61] this state is interpreted as Pd2+ ions incorporated into the lattice of cerium oxide particles. An additional state with E b(Pd 3d 5/2) = 336.0–336.2 eV is observed in Pd/CeSn25–800 (Fig. 13 a, curve 2), Pd/CeSn50–900 and Pd/CeSn50–1000 (Fig. 13 a, curves 3,4), which can be reliably referred to PdOx surface clusters [53,60]. In the case of Pd/CeSn25 catalyst calcination at higher temperatures of 900 and 1000 °C leads to a shift of this doublet by 0.5 eV towards higher binding energies to the value E b(Pd 3d 5/2) = 336.6 eV, which is associated with the formation of PdO particles [62]. In the series of catalysts with high tin content (Fig. 13 c) the intensity of the palladium line is significantly lower, indicating either a larger size of palladium particles or their encapsulation by the support. Calcination of the Pd/CeSn50 from 800 to 1000 °C leads to a shift of the Pd 3d 5/2 line from 337.5 to 336.7 eV, which implies the transition from isolated Pd2+ ions in the oxide matrix to nanosized PdO particles.Quantitative data on the atomic ratio of Pd/(Ce + Sn) depending on the catalyst calcination temperature are shown in Fig. S9. Thus, the XPS data show that the Pd/(Ce + Sn) ratio decreases with increasing tin content in the catalysts, and this effect is pronounced with an increase in the calcination temperature. After calcination of the Pd/CeSn25 catalyst at 600 °С, the ratio Pd/(Ce + Sn) equals 0.016, which is the typical value for isotropic distribution of palladium over the catalyst, while for the Pd/CeSn50–600 and Pd/CeSn75–600 catalysts the palladium concentration in the surface layers is significantly lower. The calcination procedure affects this Pd/(Ce + Sn) ratio differently. For catalysts with a high cerium content (Pd/CeSn50 and Pd/CeSn25), calcination at 800–1000 °C leads to an increase in the Pd/(Ce + Sn) ratio compared to this value for catalysts calcined at 600 °C. This suggests a partial segregation of palladium ions to the surface and/or dispersion of palladium particles and their participation in the formation of active centers. This effect was previously observed in catalysts with equimolar content of cerium and tin in the Pd/CeSn50 catalyst [29]. It should be noted that the highest Pd/(Ce + Sn) ratio was obtained for the Pd/CeSn25–800 catalyst, which exhibits high specific activity (Fig. 1 b). In the case of the Pd/CeSn75 catalyst with low cerium content no segregation of palladium into the surface layers of the catalyst is observed, only sintering or encapsulation of palladium occurs in accordance with the interpretation of the shifts of the Pd 3d line.Analysis of the Ce 3d line shows that the proportion of Ce3+ in the samples increases with increasing tin content (Fig. S10). In the case of the Pd/CeSn75–800 catalyst an abnormally high Ce3+ content of about 40–45% of the total cerium content is observed. Such high values of Ce3+ concentrations are unattainable within the crystal lattice of CeO2 fluorite and may indicate that part of cerium ions, namely Ce3+, is outside the ordered phase of CeO2 particles. These cerium ions are either located in the amorphous layers on the surface of cerium oxide particles or they dope the lattice of tin oxide. Figure 14 shows the Raman spectra of the catalysts obtained with laser excitation wavelengths of 532 nm and 785 nm. It was established that the spectra of the catalysts calcined at 450 and 600 °C are almost identical. Therefore, Fig. 14 shows the Raman spectra of the catalysts starting from the calcination temperature of 600 °C.For Pd/CeSn samples calcined at 600 °C, a strongly broadened Raman peak is observed in the region of 460 cm−1, corresponding to the well-known F2g vibrational mode of the crystalline cubic fluorite type structure of ceria, originated by the tension vibrations of the oxygen atoms that surround the cerium atoms [63]. As the calcination temperature increases, this peak narrows, with the strongest narrowing observed for the Pd/CeSn50 sample. For the Pd/CeSn75–450 and Pd/CeSn75–600 samples with low cerium content the F2g vibrational mode is almost not manifested, indicating that the ordered structure is formed during high-temperature annealing. Also, for the samples calcined in the temperature region of 600–800 °C, a general rise in the Raman spectrum is observed in the vibration region below 750 cm−1, which indicates significant lattice distortions of the fluorite structure of cerium oxide. One can also observe a weak peak in the region of ∼835 cm−1 (at ex = 532 nm), which can be attributed to stretching vibration of peroxide species formed on CeO2 surface according to literature data and our recent studies [64,65]. This peak does not appear during further annealing.Tin oxide with a tetragonal rutile structure has a complex structure of vibrational states – 18 oscillations in the first Brillouin zone [66]. Of these, the symmetrical A2u and Eu modes are IR active, whereas the remaining A1g, B1g, B2g, and Eg modes are Raman active. It should be noted that the Raman spectrum of SnO2 is well manifested only for well-crystallized and rather large particles (tens of nm) or bulk samples [66]. In our case we see intense SnO2 Raman peaks only for the Pd/CeSn75 sample calcined at 900 and 1000 °C - this is an intense narrow band A1g vibration at ∼635 cm−1, excited by 532 nm radiation. For the other samples with lower tin content we observe only a weak A1g band at 635 cm−1.Of great interest is the broad band in the region of ∼234 cm−1, which is formed in all samples during calcination at T ≥ 800 °C and is observed when using radiation of 785 nm, while the radiation of 532 nm does not excite this type of vibration.It is known that vibration of IR active Eu mode of SnO2 lie in this region, which usually does not show up in Raman spectra. However, these vibrations become active in Raman spectra when the structure of SnO2 is distorted, for example, as a result of a lack of oxygen. Thus, in [67] the bands of vacancy-related Raman modes at 234, 573, and 618 cm−1 related to Eu, B1u, and A1g modes were observed. Therefore, the band in the Raman spectrum at 234 cm−1 can be attributed to the Eu vibration of the distorted rutile SnO2 structure; the less intense peaks in the region from 250 to 650 cm−1 also refer to different SnO2 modes (Eu, Eg, B1u, and A1g) in the distorted structure.The incorporation of Pd into the matrix is confirmed by Raman spectra (a broad shoulder in the region of 500–700 cm−1 with excitation at 532 nm for samples calcined at low temperatures - up to 800 °C) [60].Thus, the Raman spectroscopy data indicate a high degree of lattice distortion of the rutile and fluorite phases, which is consistent with the XPS data and confirms the structural data obtained by XRD and TEM methods.The study of the catalytic properties of Pd/CeSn catalysts for the CO oxidation reaction allowed us to establish two general patterns. The first one describes the catalytic activity as a dome-shaped dependence on the Ce/Sn ratio. The highest activity in CO oxidation is observed for catalysts with cerium content from 75 to 50%. In agreement with [37] this range extends for Ce-rich catalysts with 80% cerium content. However, the activity decreases noticeably already at 85% Ce content in accordance with our data. The second pattern is related to the effect of calcination on the catalytic activity. The effect of thermal activation is observed as the calcination temperature increases from 450 (600) °C to 1000 °C, and the maximum activity is observed at T = 800–900 °C. The effect of thermal activation is manifested to some extent for all the compositions studied, but the greatest increase in activity is observed for the equimolar Ce/Sn ratio. It is noteworthy that the amount of reactive oxygen passes through a maximum depending on the concentration of tin in the samples. The calcination at 800 °C yields the maximum amount of oxygen for the equimolar Ce/Sn ratio, while for higher calcination temperatures of 900 °C and 1000 °C the maximum amount of oxygen is observed for the Ce-rich sample. Such high amount of reactive lattice oxygen is explained by formation of additional oxygen vacancies associated with the redox couple Sn4+/Sn2+ (along with the Ce4+/Ce3+ pair) [19,25,26] promoting increased oxygen mobility.The activity of catalysts in the methane oxidation reaction increases with increasing Sn content, and the ignition temperature of the reaction decreases significantly in accordance with the work [40].The study of the catalytic properties of Pd/CeSn catalysts shows a strong mutual influence and synergy of Ce and Sn components, which are responsible for the formation of the support morphology and structure.The addition of tin to Pd/CeO2 system leads to a significant thermostabilizing effect, which involves the suppression of sintering processes due to the formation of a nanoheterophase structure consisting of crystallites of the tin-doped fluorite phase and the cerium-doped rutile phase. Phase inhomogeneity sterically hinders the sintering of crystallites. Mutual doping of the fluorite and rutile phases with Sn and Ce ions leads to a change in the lattice parameters and a strong distortion of the corresponding crystal lattices. In work [31] the increased thermal stability of the CeO2-SnO2 system was explained by the cerium segregation on the surface of tin oxide particles with the formation of a Ce-rich surface layer. Due to the distortion of the crystal lattice, a large number of defects are formed, which, in turn, serve as centers for stabilization of palladium in a highly dispersed state with formation of various microstructures.Even after calcination at high temperatures up to 1000 °C high dispersity of rutile and fluorite phases is retained due to the spatial mixing of particles forming a nanoheterophase structure. The mutual arrangement of the particles of two phases is determined by their content and the size of the crystallites. If the Ce:Sn atomic ratio is ≥1, there is a tendency for fluorite phase nanoparticles to localize on the surface of larger Sn-rich particles, forming composites with intercrystalline mesoporosity. It is this microstructure that is favorable for catalysts that are highly active in low-temperature CO oxidation.The formation of isolated Pd2+ ions with their preferential localization throughout the volume of CeO2-based fluorite phase particles is characteristic when using catalysts with higher cerium content. This type of interaction between palladium and cerium oxide can be characterized as the strongest from a chemical point of view and is associated with the relatively small size of CeO2 particles and their defectiveness. In this case, palladium is in the most dispersed (atomic) state and can penetrate into the volume of the fluorite particles, which in this case are highly defective due to the introduction of Sn4+ ions into their lattice and the formation of a CexSn1-xO2-δ solid solution. Calcination at temperatures of 800–1000 °C stimulates the transfer of palladium atoms to the upper layers of CexSn1-xO2-δ particles. Thus, the number of active catalytic centers on the surface significantly increases, leading to an overall increase in the activity of catalysts per weight content of palladium. The calcination of Pd/CeSn catalysts leads not only to the mentioned transfer of Pd atoms, a similar process also occurs with the Sn ones doping the fluorite phase, resulting in formation of active PdxSnyOz clusters (oxidized due to the contact with the atmosphere and with the fluorite particles). The Sn atoms are mostly localized closer to the surface of the fluorite particle on which the cluster is located. Most likely, the segregation of Sn occurs as an interface layer of SnOx, on which palladium is localized in the form of PdOx. Some of the Sn atoms are assumed to be incorporated directly into the active palladium particle. In summary, the data obtained indicate the formation of a new type of catalytically active centers in which PdOx clusters are modified by tin during calcination at T = 800–900 °C. This effect of modification of PdOx active centers by tin atoms during catalyst calcination should affect the reactivity of oxygen. Indeed, the onset of the CO2 evolution upon interaction with CO can occur even at temperatures below 0 °C for the catalysts calcined at 800 °C and higher. The catalysts calcined at a low temperature of 450 °C show the onset of interaction with CO only at 50 °C and above (Fig. S11). Note that within the Mars-Van Krevelen mechanism, the most reactive oxygen forms determine the ignition of the reaction and the low-temperature activity of the catalysts. Thus, the thermal activation of catalysts with high cerium content is associated with the reorganization of the morphology and structure of catalysts with the formation of active centers based on PdOx clusters with their modification by tin atoms.The calcination of catalysts with high tin content results in such a reorganization of the morphology and structure of the support that leads to encapsulation of palladium in the form of PdO particles inside a shell of nanoparticles of fluorite and rutile phases. Previously, the formation of catalysts with the PdO@CeO2 structure was studied in a number of articles [68–70]. These works demonstrated high activity of such catalysts in the reaction of low-temperature oxidation of CH4. Catalytic data obtained in the reaction of the CH4 oxidation for the Pd/CeSn system show that the Sn-rich sample has significantly higher activity than the samples with lower tin content. Comparing the structural and catalytic data, we can conclude that the formation of PdO particles during calcination of the Sn-rich sample at 800–1000 °C is responsible for the catalyst activity with respect to methane oxidation. A characteristic property of the Sn-rich sample is the preservation of activity regardless of the calcination temperature in the range of 800–1000 °C, which is apparently a consequence of the formation of core@shell PdO@(CeO2 + SnO2) structures.A study of the effect of water vapor on the activity of catalysts showed inhibition of methane oxidation, which was demonstrated previously in a number of works [10,12,71]. According to [72,73], an increase in methane conversion can be achieved by increasing the Pd content. In turn, that increase in the conversion is expected to lead to a shift of the methane conversion curve to the low-temperature region, which is the goal of creating catalysts for the efficient oxidation of methane at low temperatures [7]. A detailed study of the effect of water vapor on catalysts with increased Pd content is planned for further research.In this work we carried out the detailed study of the structural and catalytic features of the thermostable 1%Pd/CeO2-SnO2 catalysts. It was demonstrated that by varying the Ce/Sn ratio and calcination temperature we can regulate the structure of the Pd-centers active in the reactions of CO and CH4 oxidation.The Pd/CeO2-SnO2 catalysts were synthesized by counter precipitation varying the Ce/Sn ratio from 15/85 to 75/25 and the calcination temperature in a wide range of 450–1000 °C. To establish the structural features of the catalysts and their catalytic activity, the catalysts were studied by a complex of structural (XRD, HRTEM), spectral (XPS, Raman), and catalytic (TPR-CO + O2, TPR-CH4 + O2, TPR-CO) methods. The catalysts, due to the synergism between components, were shown to possess high thermal stability after calcination up to 1000 °C with preservation of low-temperature (T < 100 °C) activity in the CO oxidation reaction at a Ce/Sn ratio from 3/1 to 1/3. In the case of the catalyst with the highest tin content (Ce/Sn = 1/3), high-temperature treatment leads to a decrease in the activity in the CO oxidation, but significantly increases the activity in the CH4 oxidation reaction.It was found that the effect of thermal activation for Pd/CeSn catalysts is associated with a change in the state and structure of active centers involving palladium. The initial samples calcined at 450–600 °C comprise palladium in ionic form (Pd2+) located in the lattice of the support nanoparticles, especially the fluorite ones. Annealing at 800 °C and higher is accompanied by the segregation of doping ions from the support structure with the formation of PdOx clusters on the surface of fluorite phase. The active PdOx clusters were found to be modified by Sn atoms, which also segregate from the fluorite lattice during calcination. The preservation of active cluster forms in the highly dispersed state during high-temperature treatment is due to their stabilization on the defects of the support surface. Due to parallel precipitation of tin and cerium oxides during the synthesis of catalysts, the two support phases are mutually doped with cerium and tin, respectively, which leads to distortion of crystal lattices and defectiveness of nanoparticles. Efficient spatial mixing of defective nanoparticles of the rutile and fluorite phases ensures the resistance of the support to sintering due to the formation of a nanoheterophase structure.In the Pd/CeSn75 catalyst with high tin content, the thermal activation is also associated with the reorganization of the support morphology, but it also includes the formation of a new phase based on palladium oxide. The segregation of Pd2+ ions in this case leads to the formation of both the tin-modified PdOx clusters and the PdO nanoparticles occluded inside the agglomerates of the support particles. The formed core@shell structure PdO@(CeO2 + SnO2) shows high thermal stability and provides catalyst activity in methane oxidation reaction. Olga A. Stonkus: Investigation, Visualization, Writing - original draft. Andrey V. Zadesenets: Investigation. Elena M. Slavinskaya: Investigation, Writing - review & editing. Andrey I. Stadnichenko: Investigation, Visualization. Valery A. Svetlichnyi: Investigation, Visualization. Yury V. Shubin: Investigation, Visualization. Sergey V. Korenev: Methodology, Project administration. Andrei I. Boronin: Conceptualization, Project administration, Writing - review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the Ministry of Science and Higher Education of the Russian Federation within the governmental order for Boreskov Institute of Catalysis (project АААА-А21-121011390053-4) and for Nikolaev Institute of Inorganic Chemistry (project #121031700315-2). The TEM studies were carried out using facilities of the shared research center “National center of investigation of catalysts” at Boreskov Institute of Catalysis. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2022.106554.

1%Pd/CeO2-SnO2 catalysts with varying Ce/Sn ratio were synthesized by counter-precipitation followed by calcination in a wide temperature range. The catalysts with Ce/Sn < 3/1 possess high thermal stability after calcination up to 1000 °C while maintaining low-temperature activity in CO oxidation. The PdOx clusters serving as active centers in CO oxidation are modified by Sn upon calcination. High tin content (Ce/Sn = 1/3) provides the activity of the catalysts in CH4 oxidation due to stabilization of PdO nanoparticles in the form of core@shell PdO@(CeO2 + SnO2) structures. Formation of the nanoheterophase structure upon calcination plays a key role in the stabilization of Pd-active centers of different types.

The catalysed formation of linear or isomerised aliphatic and aromatic hydrocarbons from the polymerisation of syngas (H2/CO mixture) is known as Fischer-Tropsch synthesis (FTS) [1]. FTS typically takes place under two environments: at high temperatures (330–350 °C) on an iron catalyst, and at low temperatures (180–250 °C) on a cobalt catalyst [2]. Whilst cobalt is the more costly metal, it shows higher catalytic activity and selectivity towards the formation of longer chain hydrocarbons than its iron congener [2].Many details of the FTS mechanism are still uncertain due to limitations around studying FTS in situ with an applied heterogenous catalyst. Consequently, there are several aspects of the mechanism that remain intensely debated. For example, activation of CO, a theorised step in FTS initiation, has been explained using different mechanistic pathways, such as associative or dissociative adsorption and with or without hydrogen assistance [3]. Another area of debate in the FTS mechanism is the process of chain growth, as spectroscopic characterisation of absorbates and reaction intermediates face many practical limitations [4–6]. Namely, chain growth intermediates (CxHy) are present in low concentration under steady state conditions alongside significantly higher surface coverages of CO, H, and products. Such compexity makes it difficult to determine which species are active in the chain growth mechansim and further to understand what the active sites are for this aspect of the reaction.A number of groups have used model approaches to study both CO activation and intermediate CxHy adsorption and chain growth on Co(0001) single crystal surfaces [7–9]. Regarding chain growth mechanisms, unsaturated absorbates such as ethylene and propene were shown to either dissociatively chemisorb to Had and the corresponding alkyne, or, in the presence of co-adsorbed CO, be hydrogenated to alkylidyne species [9]. When sufficiently heated, these alkylidyne species are shown to dimerise [8]. Whilst this shows that the presence of co-adsorbed CO impacts the reaction pathway of hydrocarbon polymerisation to form more reactive alkylidyne intermediates, spectroscopic characterisation of these intermediates under more realistic conditions, specifically at pressure, remains a challenge. In addition to this “pressure gap”, there is a significant difference between single crystal and polycrystalline applied catalytic materials, known as the “materials gap” [10].Inelastic neutron scattering (INS) and quasi-elastic neutron scattering (QENS) are vibrational and diffusional spectroscopic techniques that could provide insight into catalytic processes [11,12]. As neutrons have no electrical charge, they do not have the same limitations found in electromagnetic vibrational spectroscopy. All vibrational modes should theoretically be visible with INS/QENS, although the large incoherent scattering cross section of 1H results in these techniques being uniquely sensitive to hydrogen nuclei. QENS can measure the diffusional dynamics and mobility of hydrogen and CXHY species on the a catalyst surface, providing a more in-depth insight into the FTS mechanism. Several INS studies by Lennon and Parker have been performed on conventional supported and unsupported Fe FTS catalysts [13–15]. Recently, they have reported their first paper on Co surfaces using supported and unsupported Co catalysts [16]. These studies have focoused on the nature of hydrocarbon and coke species post FT catalysis, in addition to seminal work on the nature of adsorbed hydrogen on these surfaces. To date, no specific neutron scattering studies have been carried out that follow the FTS chain growth mechanism from model CxHy intermediates.As with all studies of specific adsorbates on supported metal catalysts, differentiation of species bound to metal or support is challenging. Skeletal metals, commonly known by their commercial name of Raney metals or as sponge metals, are nanoporous catalysts with a high surface area and limted or no support structure. The absence of adsorbate-support interactions as well as the resulting high thermal conductivity make skeletal metals potential candidates for studying CXHY adsorption and bridging the “pressure” and “materials gap”. Most neutron spectroscopy used with skeletal metals have been INS studies performed with skeletal nickel [17–21], with only a handful of studies using skeletal cobalt [16,22,23]. This includeds a recent report by Lennon and Parker on H2 adsorption on skeletal nickel and cobalt [16].Here we report preliminary experiments investigating skeletal cobalt as an adsorbate for an ethylene adsorption study using INS and QENS. The adsorbed ethylene is used as a model for CxHy FTS chain growth intermediates, as previously pioneered through the UHV single crystal work of Weststrate and co-workers [8,9]. The experiments intend to investigate the viability of using neutron scattering and skeletal cobalt catalysts, in first emulating surface science findings and then in the future proceding to perform experiments at elevated pressures. These neutron techniques were supported by temperature programmed reduction (TPR) experiments and X-ray photoelection spectroscopy (XPS) studies. Limitations in the use of skeletal cobalt as a bridge to UHV single crystal studies are discussed.500 g of reagent grade, >98% Sigma-Aldrich NaOH pellets were dissolved in 2 L distilled water at 40 °C. 100 g of Goodfellow© 33 wt% Co:66 wt% Al alloy catalyst precursor was slowly added to the base solution and stirred [Diffraction patterns of the Goodfellow© alloy precursor sealed in cellulose tape were recorded using monochromated Cokα1 radiation using a Bruker D8 Discover diffractometer operating in transmission mode over the 2θ range 10–110°, with a 0.00750404° step)] over a period of 13 h. Phase analysis was performed using the ICDD PDF database.The reaction proceeded for 1 h at 40 °C. The resultant catalyst was washed with distilled water until neutral (pH indicator paper was used). This reaction was repeated two more times and washed as before ensuring that the product remained submerged in water throughout. Due to the large mass of catalyst present, drying on a Schlenk line was impractical. As such, ~ 50 g of wet catalyst slurry was decanted into an Inconel 718 can until full, then sealed with a copper gasket. The gas inlet and outlet lines on the can were then connected to hydrogen and helium inlet gas lines and placed into a furnace. The catalyst was then dried under 270 SCCM He at 130 °C for 16 h, after which hydrogen was blended into the gas mixture at 130 SCCM at 280 °C for 3 h. The hydrogen supply was then turned off and the sample was held at 280 °C for 30 min under He flow.Ethylene gas lines were connected to the Inconel 718 canister as it was lowered into the neutron beam pathway. The sample was cooled to <10 K and an INS spectrum was recorded for 12½ hours. A buffer volume of 0.0175 mol (868.4 mbar in 499.76 cm3 at 298 K) of pure ethylene gas (SIP analytical N5.0 grade) was introduced to the catalyst sample at 200 K for 10 min. Sample was then cooled back down to <10 K and another INS spectrum was recorded for 13 h 20 min. The canister was then heated up to room temperature in order for any condensed ethylene gas to evaporate and adsorb onto the catalyst. Sample was held at room temperature for 10 min. Sample was then cooled back down to <10 K and a final INS spectrum was recorded for 16½ hours. Data were recorded on the TOSCA spectrometer and the MAPS spectrometer at the ISIS neutron and Muon Source.Catalyst slurry was prepared as described in Section 2.1 and decanted into an Inconel 718 can and dried, reduced, and washed as previously described. The catalyst was then decanted into a cylindrical aluminium canister inside an Ar glovebox and sealed with indium wire.The aluminium canister was connected to a pure ethylene gas line and placed into the neutron beam. A base temperature measurement was recorded at <10 K before heating to 300 K with data collected at intermediate temperatures. The sample was then dosed with 0.0202 mol (1000.0 mbar in 499.76 cm3 at 298 K) pure ethylene gas (N5.0) and allowed to equilibrate for 10 min before the measurement was repeated. Residual gas was removed by evacuation at room temperature before dosing with a gas mixture of 50 mbar pure (N5.0) CO and 850 mbar pure ethylene gas in 499.76 cm3 at 298 K. After equilibration for 10 min, the sample was cooled for measurement. Data were recorded on the IRIS spectrometer at the ISIS neutron and Muon Source using the pyrolytic graphite analyser.Surface area analysis required pre-passivation of sample to allow exposure in air, no other procedure required sample passivation. 5000 ppm O2 in He gas mixture was flowed into a Schlenk flask containing 100 mg of catalyst before TPR/TPD treatment at 30 mL min−1 after a needle was inserted through the Suba-seal to prevent building up of internal pressure. Sample was left to oxidise for 30 min. Passivated samples were degassed at 150 °C for 24 h. BET surface area and porosity was measured using N2 substrate gas at 77.3 K (liquid N2) with a Quantachrome Quadrasorb and a Micromeritics ASAP 2020 Plus.Temperature programmed reductions were performed on the as synthesised material and after ethylene dosing using an Altamira AMI-300 Lite. Approximately 75 mg (exact measurements recorded for each experiment) of catalyst was loaded in-between quartz wool and reduced under 5% H2/Ar flowing at 30 SCCM with a 5 °C min−1 ramp rate from room temperature to 300 °C. Measurement of hydrogen consumption was determined by a thermal conductivity detector (TCD) and calibrated by pulsing argon through a 574 μL sample loop. Samples were then cooled to room temperature and dosed with an excess of ethylene. Multiple TPR cycles without exposure to air were also performed on dosed and un-dosed samples to probe ethylene deposition.XPS analysis was performed using a Kratos Axis SUPRA XPS fitted with a monochromated Al kα X-ray source (1486.7 eV), a spherical sector analyser and 3 multichannel resistive plate, 128 channel delay line detectors. Samples were transferred from glovebox to the instrument using the vacuum transfer arm in order to avoid exposure to the air. All data were recorded at 150 W and a spot size of 700 × 300 μm. Survey scans were recorded at a pass energy of 160 eV, and high-resolution scans recorded at a pass energy of 20 eV. Electronic charge neutralization was achieved using a magnetic immersion lens. Filament current = 0.27A, charge balance = 3.3V, filament bias = 3.8V. All sample data were recorded at a pressure below 10−8 Torr and a room temperature of 294 K. Data were analysed using CasaXPS v2.3.20PR1.0 and the spectra were calibrated with C1s peak at 284.8 eV.Following the synthetic procedure, skeletal cobalt was prepared by the caustic dealumination of the cobalt-aluminium alloy. XRD analysis (Fig. 1 ) demonstrated that the material comprised of a mixture of small FCC and HCP Co metal crystallites, in keeping with numerous other observations of nanoparticular support Co and skeletal cobalt catalysts [24]. The synthesised material was found to have a surface area of 23 m2g−1, with a pore volume of 0.07 cm3 g−1 and average pore diameter of 3.8 nm, in keeping with previously synthesised materials [25].Based on the recent findings from Davidson et al. that the surface of skeletal cobalt is dominated by hydroxylated and oxidic species, a temperature programmed reduction of the catalyst was performed (Fig. 2 ). The profile of the TPR shows has three features centred at 140 °C, 160 °C and 180 °C and finally a series of residual reduction features up to 300 °C. As anticipated, the volume of hydrogen consumed is significantly lower, at 0.96 mmol g−1, than that seen for bulk reduction of Co3O4 or CoO to Co (16.6 and 13.3 mmol g−1 respectively) and is indicative of a passivated surface oxide/hydroxylated layer on the catalyst. A temperature of 280 °C was therefore used to reduce the catalyst for ethylene dosing experiments. Notably, this reduction procedure resulted in the surface area of the material dropping to 12 m2 g−1. Given that a key criterion of this model material is high Co surface area, to maximise CxHy concentration and make INS/QENS experiments viable, this degree of sintering is concerning. Further studies to remove surface hydroxyl species with milder reduction treatments are required as part of ongoing work.After reduction of surface hydroxyl species, the material was transferred to TOSCA and ethylene dosed at −73 °C (200 K) and then again at 30 °C. The spectra collected from TOSCA after ethylene dosing and background subtraction are shown in Fig. 3a. It is noted that the observed signal in both spectra was relatively low and indicates that sintering of the sample limited the concentration of H containing species. Both samples show peaks at 988 and 1102 cm−1, which in reference to skeletal Ni [26] suggests hydrogen bound in the threefold-site (i.e. Co3H). If the C2H4 was partially dehydrogenated to adsorbed acetylene and H2, then features should be present c. 300–1200 cm−1 dominated by the symmetric and asymmetric CH bending modes. The absence of these bands supports the explanation that complete dehydrogenation to form adsorbed hydrogen and coke-type species might be occurring. The 85 cm−1 band could represent external vibrational modes of associatively adsorbed ethylene that have broadened and shifted due to interaction with the metal surface [27]. The presence of both adsorbed hydrocarbon and completely dehydrogenated species suggests that different sites on the cobalt surface exist with very different reactivity. Alternatively, the 85 cm−1 mode could be interpreted as another hydrogen adsorption mode; vide infra.After the second dose at 30 °C, a richer spectrum was obtained with a strong, sharp peak at 300 cm−1, assigned to a CH3 torsion from a metal-bound methyl species [28]. This is higher in energy than a typical C-CH3 group and makes the presence of an ethylidyne species unlikely [29]. Although, specific surface coverage and adsorption onto a metal surface could shift features, making definitive exclusion of ethylidyne not possible. DFT studies, as part of potential future work, to provide simulated spectra present an opportunity to resolve this. The assignment (of a methyl group) was further confirmed from the higher energy transfer data recorded on MAPS (Fig. 3b) that shows a peak at 2940 cm−1 indicating a CH stretch from a saturated hydrocarbon [30]. It should also be noted that there is no apparent signal around 3400 cm−1 that would indicate the presence of hydroxyl groups. Regarding the other modes seen on TOSCA, the 85 cm−1 mode seen at lower temperature dosing was retained, while the hydrogen on the threefold Co site appears slightly decreased, and the intensity of the broad band between 550 and 900 cm−1 increases. Due to its low effective mass physisorbed ethylene would show molecular recoil in this energy range which results in no well-defined peaks [27] and represents a transfer of momentum from the neutron to the sample without excitation of vibrational modes. CH modes from hydrocarbon fragments would be expected in this region, but other interpretations can also be considered.A recent INS study of hydrogen adsorption on Ni and Co catalysts by Davidson et al. [16] has tentatively suggested the formation of bound molecular hydrogen on cobalt that resemble Kubas complexes [31]. In this model the dihydrogen molecule is chemisorbed to the metal through an η2 σ-bond. This increases the HH bond length, causing the H2 rotational band to shift to 46 cm−1 from the 118 cm−1 of pure hydrogen. If this is also the origin of the 85 cm−1 band on our catalyst, it suggests that Co can support a stable intermediate in the dissociation to the hydride species that may have a strong influence on catalytic reaction mechanisms. Furthermore, it suggests that this interaction strength may easily be followed by the frequency of this mode and can be directly linked to the length of the HH bond, in this case calculated at 0.96 Å. Bound molecular hydrogen would lead to increased structure in the obtained spectra [32] which was not reported in the study by Davidson et al. However, comparison with H2 complexes in solution [26] suggest that the symmetric HMH stretch and the rock, wag and torsion modes would fall in the range of 950–400 cm−1, which could explain at least some of the intensity of the broad feature in the observed spectra. Unfortunately, this cannot be taken in any way as a confirmation of the chemisorbed molecular hydrogen. Similar features where they appear in [16] are assigned to bound M-H species on different adsorption sites, which is an equally valid interpretation for our data. Furthermore, the presence of overlapping hydrocarbon modes in this region further complicates the assignment.The spectra thus show that mixed-mode adsorption is occurring on the catalyst at lower temperature. This does not contradict the possibility of a Kubas-type chemisorbed molecular hydrogen and metal hydride, although the more conventional assignment would be for associatively and dissociatively chemisorbed ethylene. The second dose at higher temperature shows that the adsorbates have reacted, with strong evidence for a methyl species appearing at 300 cm−1 in the spectrum with other features that may be assigned to complexed molecular hydrogen and/or bound saturated hydrocarbon fragments. Interestingly, Weststrate and co-workers on ethylene adsorption on single crystal (0001) Co surfaces under UHV, suggests that at <−75 °C ethylene should associatively absorb [8,9]. While at 30 °C ethylene would dehydrogenate into adsorbed hydrogen and acetylene on the skeletal cobalt. Observations, from INS on the skeletal cobalt, of dissociatively chemisorbed species at low temperature, and the clear presence of a methyl group at higher temperature, demonstrates a notable deviation from the surface chemistry seen by Weststrate.Given the dramatic differences in reaction conditions and catalytic surface, this discrepancy is perhaps unsurprising, with several possible factors being possibly responsible. The higher reactivity of certain sites on the polycrystalline surface could reduce the temperature, from the 125 °C seen by Weststrate, required for the dehydrogenation of acetylene to form surface coke [8]. The role of the diverse surfaces, steps and kinks on the Co mediated Fischer-Tropsch mechanism is unresolved and remains an area of debate. Theoretical studies have shown that the relative stability of C1Hx vs C2Hx is dependent on the surface termination, with close packed surfaces favouring C2Hx and higher index surfaces C1Hx [33]. The results invoking a concept of an ensemble of active sites where CO activation occurs at high index sites and chain growth on close packed surface. The propensity of higher index planes towards CO activation but also CC breaking being reflected in the significant dehydrogenation activity and probable CC cleavage (re. evidence of methyl groups) observed in this study using the polycrystalline skeletal catalyst vs a close packed Co (0001) surface. Madey and co-workers showed that the INS spectrum of ethylene adsorbed on skeletal Ni changed dramatically on heating to room temperature due to dehydrogenation, although the specific species could not be identified [17]. Interestingly, they note that this dehydrogenation was far more significant than observed on analogous EELS studies on a Ni(111) surface and postulated that steps and defects on the skeletal catalyst reduced the activation energy of dehydrogenation. To further clarify the presence of multiple dehydrogenated species, a TPR was performed on a skeletal cobalt that had been dosed with ethylene at 30 °C (Fig. 2). Two significant features were observed between 200 and 250 °C with a small 3rd feature centred at 300 °C. Unfortunately, given that a TCD was used instead of mass spectrometry, it was not possible to identify the desorbed hydrogenated species. However, the results confirm that multiple dehydrogenated carbon species were formed during ethylene adsorption. Further, it is anticipated that ambient pressure hydrogenation of coke species would require higher temperatures than observed.Alternatively, the potential hydroxylation/oxidation of the surface through contamination which was noted to be highly facile by Davidson et al. could have reduced surface coverage and altered the reactivity of adsorbed ethylene species. Water and surface hydroxyl species, present under high conversion in Fischer-Tropsch influence Co structure and oxidation state and have been implicated in a possible CO activation pathway [34]. However, little evidence of hydroxyl groups can be found from INS of the catalysts after reduction; vide supra. Further, TPR analysis post ethylene dosing showed no reduction features in the temperature range associated with original hydroxylated sample. Yet, analysis of the reduced sample by XPS appears to contradict these findings. As indicated from the Co 2p spectra XPS (Fig. 4 ) shows that the surface of the sample was completely oxidised to Co2+, with a multiplet peak at 780.34 eV and satellite structure indicative of Co(OH)2 [35]. The O1s spectra further indicate surface metal oxide and hydroxide with peaks at 529.94 and 532.06 eV respectively. However, a note of caution is required, as Davidson et al. highlighted the potential for rapid surface hydroxylation on sample transfer, even within a glovebox, as done for the present XPS analysis. A reference TPR experiment where the skeletal cobalt was reduced, then cooled to room temperature and stored under a static environment for 60 min, before being exposed to a second TPR step is illuminating. As shown in Fig. 2, this procedure results in a reduction feature at broadly the same temperature as that seen for the as synthesised material, indicating that simple storing under a static inert environment provided sufficient oxygen/water ingress to facilitate surface hydroxylation. Consequently, XPS is shown to be sensitive to the effect of handling the sample, even when carried out within a glovebox. It can therefore be concluded that experiments must be conducted in situ and that the surface of sample is probably free of hydroxyl species within such experiments (INS/QENS/TPR).Lastly, the diffusional properties of adsorbed species were investigated by QENS. Fig. 5 shows the peak intensity integrated between −17.5 and 17.5 μeV and normalised to this value at the lowest temperature, representing the total elastic scattering from the sample as temperature varies. Very little change is seen in the undosed cobalt; however, the dosed material show significant decreases around 75 K. This indicates the onset of motion that may be considered a phase change. Pure ethylene has a melting point of 104 K, so the depression of this melting point demonstrates there is an interaction between the adsorbate and the cobalt catalyst. Full fitting of the Q-resolved data at 300 K to obtain diffusion parameters was attempted, but the diffusion was too rapid for the instrument to measure, which is suggestive of rapid rotational motions of the methyl group confirmed by INS. Interestingly, repeating the experiment with co-dosed CO increased the temperature when elastic intensity decreased and implies that CO is inhibiting the motion of surface bound CxHy species. This result suggests that future INS experiments should be performed with co-dosing of CO to identify possible changes in adsorbate speciation.A skeletal cobalt catalyst was synthesised and used in a preliminary INS/QENS study of adsorbed ethylene. We show that the material requires substantial reduction prior to use in adsorption studies. While samples were stable under a reducing atmosphere, any transfer of the sample, even within a glovebox, was sufficient to result in complete surface hydroxylation. In addition, this prerequisite reduction step can easily result in significant sintering of the skeletal cobalt. Recorded INS spectra had relatively weak and broad signals possibly associated with a small uptake of ethylene, due to sintering during reduction. It should also be noted that these data were the result of a subtraction to remove the strong background signal from both the Co sample and the Inconel sample holder which is unfortunate and could lead to distortion of the spectra. However, without adequate sample containment the sample will oxidise, and this treatment is therefore unavoidable. INS showed that the adsorbed hydrogen and CxHy environment is significantly more complex on a skeletal cobalt catalyst than analogous single crystal UHV studies, with a range of CH and adsorbed hydrogen species being observed on the former. This is hypothesised as being due to the presence of highly reactive defects and high index surfaces on the skeletal cobalt catalyst. Dissociated hydrogen adsorbed in a 3-fold site was observed in addition to a possible Kubas-type chemisorbed molecular hydrogen. CH fragments could also possibly be observed, with dosing at room temperature resulting in definitive methyl species. Distinction between M-CH3 or M-C-CH3 cannot be made from the literature available and requires further investigation. QENS studies confirmed that ethylene reacted with the skeletal cobalt surface, although diffusional parameters were too rapid to be measured. The co-dosing of ethylene with CO inhibited the motion of the adsorbed ethylene or dehydrogenated species.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors wish to thank Prof. Stewart Parker for discussions about neutron scattering and performing the MAPS measurement. The ISIS Neutron and Muon source is thanked for their grants of beam time for the neutron experiments (doi: https://doi.org/10.5286/ISIS.E.RB2010089, doi: https://doi.org/10.5286/ISIS.E.RB2010090 and doi: https://doi.org/10.5286/ISIS.E.RB2190135-1). The X-ray photoelectron (XPS) data collection was performed at the EPSRC National Facility for XPS (“HarwellXPS”), operated by Cardiff University and UCL, under Contract No. PR16195. UK Catalysis Hub is kindly thanked for resources and support provided via our membership of the UK Catalysis Hub Consortium and funded by EPSRC grant: EP/R026939/1, EP/R026815/1, EP/R026645/1, EP/R027129/1 or EP/M013219/1 (biocatalysis). Edward Jones acknowledges funding for this work through his studentship received from EPSRC Sustainable Hydrogen CDT (EP/S023909/1).

To bridge the materials gap of single crystal work on the Fischer-Tropsch chain-growth mechanism, ethylene adsorption on a model skeletal cobalt catalyst was studied. Speciation and mobility of surface species were characterised using inelastic (INS) and quasi-elastic (QENS) neutron scattering. INS spectra demonstrated that highly reactive sites facilitated ethylene dehydrogenation at lower temperature than in single crystal studies. Adsorbed hydrogen was assigned to Co3H and potentially a Kubas species. After adsorption at 30 °C, methyl groups were identified. CO co-adsorption was shown to modify the dynamics of the adsorbed species. Further analysis demonstrated the sensitivity of skeletal cobalt to surface hydroxylation.

Over the past decades, the global environment has been rapidly deteriorated due to the increasing use of fossil fuels [1]. In the current age, it is highly anticipated to develop green energy devices. Rechargeable metal-air batteries are the environmental-friendly energy conversion devices with high performance and practical feasibility [2–4]. Among them, the rechargeable zinc-air battery (RZAB) is particularly attractive [5], with several advantages such as intrinsic safety, high specific capacity, and high theoretical energy (1086 Wh kg−1, including O2) [6,7]. The air–cathode of RZAB needs a bi-functional catalyst having activities for both using O2 (when discharging) and producing O2 (when charging) efficiently, that is, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) [8]. Commercially available catalysts are usually platinum on carbon (Pt/C) for ORR and iridium dioxide or ruthenium dioxide for OER, which all involve noble metals and exhibit mono-functional catalytic performance [4,5]. Such disadvantages limit the wide use of RZAB.As alternatives to noble metallic elements, first-row transition metal oxides and their derivatives are considered as good ORR/OER catalysts [9–11]. However, simple metal oxides often exhibit relatively low electrical conductivity [12] and small specific surface area (<400 m2 g−1) [13]. Hence metal oxides should be integrated with conductive materials for the improved performance. It is known that conductive carbon black (CB) is commercially mature activated carbon having high electrical conductivity and plentiful pores, which provide large surface areas for chemical modifications [12,14,15]. In this sense, the combination of conductive carbon with metal oxides can enhance the ORR/OER electrochemical performance [16].In general terms, nevertheless, the degree of combination of metal oxides and carbon is relatively low, leading to the easily leaching of metal oxides from carbon and the resultant unstable performance. That being so, it is reasonable to look for a binder that helps to anchor metal oxides to the carbon surface, and such binder can be polymers. In this scheme, monomers are first anchored within a porous carbon matrix followed by the polymerization of them, forming the polymer-grafted carbon, and then heteroatom groups in polymer chains are able to act as ligand donors to bind metallic ions in a coordination mode, making possible the subsequent formation of metal-oxide active sites anchored on polymer-grafted carbon.Unfortunately, a substantial number of polymers are prepared using radical initiators that include organic/inorganic peroxide and azo compounds, which are either dangerously explosive or toxic to human health and the environment [17]. On the other hand, acoustic cavitation generated by ultrasonic irradiation can produce numerous micro-bubbles and cause violent collisions of particles, thereby leading to high temperatures locally (>5000 K) and a large number of free radicals [18]. It has been demonstrated that free radicals produced by ultrasound are able to induce the polymerization of monomers [19–21]. Moreover, the ultrasonic environments are favourable for the formation of metal oxides without using extra chemical reagents or apparatus [22,23]. From the green chemistry perspective, ultrasonication is an environmentally friendly and highly energy efficient method for the preparation of polymers and metal oxides.In this work we introduce the convenient route to the effective combination of earth-abundant tri-metallic oxide and conductive CB using an amide-type polymer induced by ultrasonic cavitation. The monomer N-isopropyl acrylamide (NIPAm) undergoes the polymerization using ultrasonication instead of any radical initiator. The polymer chains formed are grafted on the surface of CB, and the amide groups in the polymer can bind three types of metal ions (i.e., Mn2+, Ni2+ and Fe2+) through the efficient coordination, resulting in the production of metal oxide by the second round of ultrasonic irradiation. Thanks to the amide-type polymer as a binder, the Mn-Ni-Fe tri-metallic oxide anchored on polymer-grafted CB has both the appropriate amount of metal-oxide active sites and a sufficient number of hierarchical pores, leading to the enhanced ORR/OER electrocatalytic performance compared to its ultrasonication-free counterpart and commercial catalysts, which proves to be suited as a redox bi-functional catalyst in the RZAB. Accordingly, this work provides appealing insight into the effective combination of the two inherently incompatible parts for the construction of composite materials.N-isopropyl acrylamide (NIPAm, 98%, Adamas-beta), ferrous sulfate heptahydrate (FeSO4·7H2O, Adamas-beta), manganese (II) chloride (MnCl2, 99%, Adamas-beta), nickel (II) chloride hexahydrate (NiCl2·6H2O, 99%, Adamas-beta), isopropyl alcohol (C3H8O, 99.7%, Sinopharm Chemical), Ketjenblack® CB (EC300J conductive carbon black, Japan Lion), Nafion® perfluorinated resin solution (5 wt%, Sigma-Aldrich), α-Al2O3 with diameter of 50 mm (99%, Tianjin Aida), platinum on carbon (Pt/C, 10 wt%, Sigma-Aldrich), ruthenium oxide (RuO2, 99.95%, Adamas-beta), potassium hydroxide (KOH, >90%, General reagent), zinc acetate (Zn(ac)2, 99.5%, Adamas-beta), high-purity nitrogen and oxygen gases (99.999%, Xuzhou Special Gases), and ultrapure water (18.2 MΩ cm, Sartorius arium).Field-emission scanning electron microscopy (FESEM) images were acquired using Quanta FEG 250. Transmission electron microscopy (TEM) as well as high-resolution TEM (HRTEM) images were obtained on FEI Tecnai TF30. Nitrogen adsorption–desorption isotherms were obtained by Quantachrome Autosorb iQ. Pore width distributions were gained based on the quenched solid density functional theory (DFT) model. X-ray diffraction (XRD) patterns were acquired by PANalytical X'pert3 Powder using the Cu Kα radiation (λ = 1.54178 Å). X-ray photoelectron spectroscopy (XPS) measurements were conducted on Thermo Fisher ESCALAB 250Xi using the Al Kα radiation (1486.6 eV) and binding energies were calibrated according to the C 1 s peak (284.8 eV). An ultrasonic exfoliation instrument with six-sided distributed ultrasonic transducers (Scientz-CHF-5B, Ningbo Scienz) was used to intensify the dispersion of CB. An ultrasonic instrument equipped with a microwave generator (Scientz-IIDM, Ningbo Scienz) was employed to synthesize catalysts.From the beginning, 0.5 g of CB was dispersed in 100 mL of ethanol in a 250 mL sealed beaker and ultrasonically treated using the ultrasonic exfoliation instrument for half an hour (20 kHz, 576 W). Then, 1.5 g of NIPAm was dissolved in 100 mL of ultrapure water, which was added to the black suspension of the dispersed CB. After that, the mixture was degassed by high-purity N2 using a capillary glass-made gas inlet for 10 min at a flow rate of 50 sccm. Further, the degassed mixture was transferred to a three-neck flask and placed in the ultrasound/microwave instrument. The ultrasonic horn was inserted 5 cm below the liquid level through the centre neck of the three-neck flask. As for the other two necks, one was linked to a reflux condensation maintained at 0 °C, and the other was sealed using a polytetrafluoroethylene (PTFE) cap. The mixture was then treated under a 500 W ultrasound for half an hour with the assistance of microwave (200 W) at the initial 500 s for rapid warming. Subsequently, 10 mL of 0.05 M NiCl2, 10 mL of 0.05 M FeSO4 and 10 mL of 0.05 M MnCl2 solutions were added to the mixture, followed by the second round of ultrasonic irradiation for another half an hour (20 kHz, 500 W). Finally, the ultrasonically treated mixture was centrifuged and dried at 80 °C overnight. The obtained material was denoted U-MnNiFe@NCB. To investigate the ultrasonic effect, a sample was prepared using stirring (at 80 °C) to replace the ultrasonic treatments mentioned above, leaving the other conditions unchanged. This control sample was denoted [email protected] tests were conducted using an electrochemical workstation (Iviumstat.h, Ivium Technologies) at 25 °C. In a three-electrode configuration, a rotating disc electrode with glassy carbon surface (GC-RDE) connected with a rotator (AFMSRCE, Pine Research) was employed to load a catalyst, acting as a working electrode. Additionally, a Ag/AgCl electrode (3.5 M KCl) connected to the main cell through a Luggin capillary was employed as a reference electrode, and a platinum foil was used as a counter electrode. All the potentials measured in this paper were converted to the reversible hydrogen electrode (RHE) based on the following equation: E(RHE) = E(Ag/AgCl) + 0.2046 + 0.059∙pH. The catalyst ink was made by mixing 3.0 mg of the catalyst, 70 μL of isopropyl alcohol, 170 μL of ultrapure water as well as 10 μL of 5 wt% Nafion® perfluorinated resin solution together under ultrasonication for half an hour to form a homogenized mixture. Meanwhile, the GC-RDE (6 mm in diameter for GC) was polished with α-Al2O3 and washed thoroughly with ultrapure water and ethanol. Afterwards, 8 μL of the catalyst ink was transferred onto the polished surface of GC-RDE using a pipette and dried naturally. The catalyst loading was 0.49 mg cm−2. Regarding the inks of the commercial catalysts, 1 mg of 10 wt% Pt/C or 2 mg of RuO2 was mixed with 10 μL of Nafion® perfluorinated resin solution, 170 μL of isopropyl alcohol and 70 μL of ultrapure water under ultrasonic irradiation for half an hour. The loadings of Pt/C or RuO2 were 0.16 mg cm−2 and 0.32 mg cm−2, respectively.Regarding ORR experiments, the gas (O2 or N2) with a flow rate of 80 sccm was first fed into the electrolyte (0.1 M KOH) for half an hour and then through the headspace above the electrolyte solution during measurements. Cyclic voltammetry (CV) measurements were undertaken to activate catalysts with a scan rate of 50 mV s−1 for 50 cycles prior to electrocatalytic measurements. Linear sweep voltammetry (LSV) experiments at different rotating rates (400–2025 rpm) with a scan rate of 10 mV s−1 were conducted. As for LSV curves, the currents obtained in the N2-saturated solution were subtracted from those in the O2-saturated solution in order to eliminate strong capacitative currents from porous carbon. The Koutecky-Levich (K-L) equation shown below was employed to obtain the electron-transfer number [24]: (1) 1 j = 1 j k + 1 B ω 1 / 2 where ω is rotating rate (rpm), jk is kinetic current density (mA cm−2), j is total current density (mA cm−2), and B is the K-L slope, which can be expressed as [24,25]: (2) B = 0.2 nFC 0 D 0 2 / 3 v - 1 / 6 where F is the Faraday constant (96485C mol−1), n is the electron-transfer number, C 0 is the saturated O2 concentration (1.2 × 10–3 mol L–1), D 0 is the diffusion coefficient of O2 (1.9 × 10–5 cm2 s−1), and v is the kinematic viscosity in 0.1 M KOH (0.01 cm2 s−1). For the OER measurements, the LSV curves were recorded with a scan rate of 10 mV s−1 at a rotating rate of 1,600 rpm after the electrolyte (0.1 M KOH) was saturated with N2 with a flow rate of 80 sccm. The iR-drop compensation was made to minimize the ohmic potential loss of electrodes due to the solution resistance.Custom-made RZABs comprising air-cathodes and zinc-plate anodes were assembled, each with three poly(methyl methacrylate) (PMMA) plates. The air–cathode was made by pressing catalyst-loaded carbon paper, air-breathable water-proof layer (placed in the middle), and nickel foam at 10 MPa. The catalyst ink for the RZAB measurements was prepared by mixing 3 mg of U-MnNiFe@NCB, 70 μL of isopropyl alcohol, 140 μL of ultrapure water and 10 μL of 5 wt% Nafion® perfluorinated resin solution together under ultrasonication for half an hour. As for the Pt/C-RuO2 catalyst, 1 mg of 10 wt% Pt/C and 2 mg of RuO2 were mixed in 70 μL of ultrapure water, 170 μL of isopropyl alcohol and 10 μL of 5 wt% Nafion® perfluorinated resin solution under ultrasonication for half an hour. For either of these catalysts, the catalyst area on the electrode is 1.77 cm2, and the mass loading of catalyst is 1.69 mg cm−2 in each battery. The electrolyte comprising 0.2 M Zn(ac)2 and 6.0 M KOH was circulated by a peristaltic pump. The specific capacity was calculated by the following equation [26]: (3) C s = I Δ t m Zn where I is current (mA), C s is specific capacity (mAh g−1), Δt is time from the beginning until zinc was totally consumed (s), mZn is the consumed zinc mass (g).The pristine CB undergoes the two-round ultrasonic treatments, being converted to the polymer/metal oxide-modified material U-MnNiFe@NCB, as shown in Fig. 1 a. The FESEM images of CB and U-MnNiFe@NCB at different magnifications (Fig. 1b-g and Fig. S1) show that the two samples have the similar morphology of nanosized carbon granules, suggesting that the ultrasonic modification does not change the basic structure of the CB. Free radicals generated by the ultrasonic cavitation are able to induce the polymerization of NIPAm, resulting in the formation of PNIPAm that is grafted onto the carbon granules. The grafted polymer chains contain the structural units terminated by amide groups, which can act as good donor groups bonded to metal ions. To determine whether PNIPAm was successfully grafted and its content, U-MnNiFe@NCB, MnNiFe@NCB and CB were vacuum heated at 250 °C for 6 h with a ramp rate of 10 °C min−1 separately. As shown in Table S1, the grafting proportion of U-MnNiFe@NCB is ca. 23.0%, revealing that its surface is indeed modified with sufficient quantities of PNIPAm by the ultrasonication. By comparison, the grafting proportion of MnNiFe@NCB is only 6.7% (Table S1), showing the low content of volatile organic components in the material without undergoing the ultrasonication. This difference in the grafting proportions can be explained by the fact that the polymer PNIPAm with the network of carbon skeletons formed using the ultrasound favour the adherence to the surface of porous carbon compared to the monomers.The TEM images of U-MnNiFe@NCB at different magnifications (Fig. 2 a and b) also exhibit its typically granular morphology, consistent with the FESEM observations. Further, the HRTEM image of U-MnNiFe@NCB (Fig. 2c) shows its overwhelmingly amorphous feature, together with a few lattice fringes. A magnified view of the lattice fringes by the inverse fast Fourier transform (IFFT) of the circled region is shown in the inset of Fig. 2c. The d-spacing is 0.34 nm, which can be ascribed to the (002) surface of graphitized carbon [27]. Also, another two types of lattice fringes are found in the HRTEM image of U-MnNiFe@NCB (Fig. 2d). As magnified in the insets of Fig. 2d, the d-spacings of these lattice fringes include 0.21 nm and 0.30 nm, which should be ascribed to the metal oxide formed by the ultrasonication. Fig. 2e–k exhibit the high-angle annular dark-field (HAADF)-TEM and energy-dispersive X-ray spectroscopy (EDS) elemental mapping images of U-MnNiFe@NCB. The three metallic elements Fe, Mn and Ni coexist with O, indicating the formation of the tri-metallic oxide on the CB.The XRD patterns of U-MnNiFe@NCB, MnNiFe@NCB and CB are exhibited in Fig. 3 . The widened bands centred at 24.0° are seen from all the three patterns, corresponding to the (002) plane of graphitized carbon [28] and consistent with the HRTEM observation (Fig. 2c). The widened characteristics of diffraction peaks also confirm the three materials possess the amorphous carbon matrices. By comparison with MnNiFe@NCB and CB, U-MnNiFe@NCB has the two additional small yet sharp peaks centred at 30.3° and 43.4° observable from its XRD pattern, which can be ascribed to the (220) and (400) planes of Mn-doped NiFe2O4, respectively [29,30]. The d-spacings of the (220) and (400) planes of Mn-doped NiFe2O4 are 0.30 nm and 0.21 nm, respectively [29,30], which are consistent with the HRTEM data (Fig. 2d). Thus, it can be inferred that the metal ions Ni2+, Fe2+ and Mn2+ attached to the polymer-grafted carbon granules by the amide groups are converted to Mn-doped NiFe2O4 under the ultrasonication.The elemental compositions of Mn-doped NiFe2O4 anchored on the polymer-grafted CB, along with the control samples MnNiFe@NCB and CB, are evaluated using the XPS technique. The wide-scan XPS spectra of the three materials are shown in Fig. S2a and the corresponding elemental information is summarized in Table S2. The pristine CB contains only two elements C and O, whereas U-MnNiFe@NCB possesses the largest quantities of N, O, Mn, Ni and Fe among the three materials, and MnNiFe@NCB prepared without using the ultrasonication has the intermediate amounts of these elements, indicating that the grafted polymer chains produced by the ultrasonic cavitation, acting as adhesives, allow the CB to bind with the metallic elements effectively. Without the ultrasonication or any radical initiator, the polymerization of NIPAm could not be triggered spontaneously. Because the polymer PNIPAm formed in U-MnNiFe@NCB binds more tightly to the carbon surface than its monomer NIPAm as mentioned above, MnNiFe@NCB contains the lower amounts of potential N-/O-donor groups anchored on the surface of CB and thus the reduced number of the coordinated metal ions. For the non-metallic elements, the narrow-scan C 1 s spectrum of U-MnNiFe@NCB (Fig. 4 a) can be split into the four subpeaks C=C (284.7 eV) [31,32], C–C/C–O/C–N (285.4 eV) [31], C=O (286.4 eV) [31] and O=C–O (289.2 eV) [32]. Likewise, the narrow-scan N 1 s spectrum of U-MnNiFe@NCB (Fig. 4b) can be split into the three peaks N–Q (quaternary nitrogen, 401.5 eV) [33], HNC=O (400.2 eV) [34] and metal-N (399.2 eV) [35], and its O 1 s counterpart (Fig. 4c) can also be decomposed into the three subpeaks O–C=O (533.5 eV) [31], N–C=O (532.0 eV) [36] and O2– (530.5 eV) [37]. By contrast, the C 1 s spectrum of CB that lacks nitrogen or metallic elements (Fig. S2b) can be split into the three subpeaks C=C (284.8 eV) [31], C–C/C–O (285.4 eV) [31] and O=C–O (289.2 eV) [32], whereas its O 1 s counterpart (Fig. S2c) can also be split into the two peaks O–C=O (533.5 eV) and C–OH (532.3 eV) [31]. Indeed, the chemical bonding of the non-metallic elements is different between U-MnNiFe@NCB and CB, because of the existence of the grafted polymer and metal oxide in the former. Evidently, the existence of the metal-N bonding on the surface of U-MnNiFe@NCB shows that the metal ions are coordinated to the amide groups of the grafted PNIPAm. Additionally, the existence of the lattice oxygen O2– indicates the formation of transition metal oxide [37,38]. In other words, the metal ions bind to the N and O donor atoms of the amide groups that are part of the grafted polymer in the coordination mode, forming the metal oxide under the ultrasonication. Regarding the metallic elements, on the other hand, the high-resolution Mn 2p, Fe 2p and Ni 2p XPS spectra of U-MnNiFe@NCB (Fig. 4d–f) show the typical characteristics of 2p3/2 Fe3+ (711.7 eV), 2p1/2 Fe3+ (724.9 eV), 2p3/2 Mn2+ (642.4 eV) and 2p3/2 Ni2+ (855.8 eV) [39,40]. The surface contents of Mn, Ni and Fe in U-MnNiFe@NCB are 0.28 at%, 0.23 at% and 0.91 at%, respectively, as summarized in Table S2; that is, the atomic ratio of Mn to Ni to Fe is 1:0.82:3.25. Thus, the chemical formula of Mn-doped NiFe2O4 anchored on the polymer-grafted CB synthesized by the ultrasonication can be written as Mn0.6Ni0.5Fe2O4, the formation of which should involve the following steps under the ultrasonic irradiation [41]: (4) H 2 O → ) ) ) H · + O H · (5) 2 H O · → ) ) ) H 2 O 2 (6) 2 F e 2 + + H 2 O 2 → ) ) ) 2 F e 3 + + 2 OH - (7) Fe 2 + + 2 F e 3 + + 8 OH - → ) ) ) Fe 3 O 4 + 4 H 2 O (8) Ni 2 + + 2 H 2 O 2 + 2 OH - → ) ) ) N i O + 3 H 2 O + O 2 (9) 3 Mn 2 + + H 2 O 2 + 6 OH - → ) ) ) Mn 3 O 4 + 4 H 2 O (10) 4 F e 3 O 4 + 3 N i O + 1.2 Mn 3 O 4 → ) ) ) 6 M n 0.6 Ni 0.5 Fe 2 O 4 It can be seen that ultrasound plays important roles in the generation of the tri-metallic oxide: the ultrasonic cavitation induces the formation of free radicals that can produce peroxide, which reacts with the metal ions in a series of steps until the formation of Mn0.6Ni0.5Fe2O4. Without the ultrasound, metal oxide could hardly be obtained under the given conditions. This can also explain why the diffraction peaks of Mn0.6Ni0.5Fe2O4 or any other metal oxide are absent in the XRD pattern of MnNiFe@NCB obtained without employing the ultrasound (Fig. 3). Accordingly, the ultrasonication helps to induce not only the polymerization of NIPAm but also the formation of the tri-metallic oxide Mn0.6Ni0.5Fe2O4, and the grafted polymer chains act as binders to anchor the active sites (i.e. the metal oxide) to the porous electrode (i.e. the CB).In addition to active sites, porous properties have significant effects on the catalytic performance, and in this case the CB provides the porous domains for mass transport. N2 adsorption–desorption isothermal experiments were conducted to investigate pore structures of the three materials. The isotherms of U-MnNiFe@NCB, MnNiFe@NCB and CB (Fig. 5 a) can be classified as Type I/II [42,43]. Specifically, the largely increased adsorption volumes at p/p0 ≈ 0 indicate the existence of micropores [44,45], the hysteresis loops at p/p0 = 0.45–0.95 are caused by mesopores [46,47], and the large uptakes at p/p0 > 0.95 imply the presence of macropores [48]. Both U-MnNiFe@NCB and MnNiFe@NCB are characteristic of hierarchical pores originating from the CB. The surface areas of U-MnNiFe@NCB, MnNiFe@NCB and CB are 414 m2 g−1, 615 m2 g−1, 725 m2 g−1, respectively. The change of surface areas reflects the degree of pore blockage by the grafted polymer chains, that is, the more the polymer on the surface, the higher the degree of pore blockage and the smaller the surface area, which is consistent with the corresponding grafting proportions of the materials mentioned above (Table S1). Moreover, the pore width distributions of U-MnNiFe@NCB, MnNiFe@NCB and CB based on the DFT method are shown in Fig. 5b. Overall, the three curves look similar to each other, confirming the pore structures of U-MnNiFe@NCB and MnNiFe@NCB stem from the characteristics of the CB and that no major changes are made to the pore width distributions after the surface modifications either with or without the ultrasonication. On the other hand, there is a noticeable difference between U-MnNiFe@NCB and CB in the micropore domains. U-MnNiFe@NCB has the two lower peaks centred at 0.8 nm and 1.4 nm than the CB, whereas their mesopore counterparts are comparatively close to each other, indicating that the ultrasonic modification has the stronger effect on micropores. This can also be quantitatively demonstrated according to the data from Table S3. The CB has the surface area and pore volume of micropores roughly three times greater than U-MnNiFe@NCB, while the differences between CB and U-MnNiFe@NCB in surface areas and pore volumes of mesopores are very little. It can then be inferred that the grafted polymer tends to ‘bury’ the micropore sites. In other words, the ultrasonic modification does not lead to a significant loss of mesopores that are responsible for mass transport, whereas micropores act as effective anchor points for the ultrasonically-induced polymer chains that capture the metal-oxide active sites, therefore achieving the fine balance between active sites and mass transport in this material design.Next, the electrochemical measurements were conducted to evaluate the catalytic activities towards ORR and OER for U-MnNiFe@NCB, MnNiFe@NCB and CB. Regarding the ORR performance, Fig. 6 a shows the LSVs of the three samples alongside commercial 10 wt% Pt/C. U-MnNiFe@NCB achieves an acceptable level of ORR catalytic activity, having the onset potential (Eonset ORR) of 0.92 V, the half-wave potential (E1/2 ORR) of 0.73 V and the limiting current density of 5 mA cm−2 at 1,600 rpm, superior to MnNiFe@NCB, CB and 10 wt% Pt/C. The inferior ORR performance of MnNiFe@NCB and CB can be ascribed to the lack of metal-oxide active sites in these two materials. The ORR electron-transfer numbers of U-MnNiFe@NCB, MnNiFe@NCB, CB and 10 wt% Pt/C are 3.7, 3.2, 2.3 and 3.6, respectively, according to the corresponding K-L plots (Fig. 6b and Fig. S3), showing the ORR by U-MnNiFe@NCB is predominantly the 4-electron process that is energetically favourable for electrochemical devices. Moreover, the OER catalytic performance of U-MnNiFe@NCB is compared with that of MnNiFe@NCB, CB and RuO2. The LSVs of the four materials concerning their OER behaviours are exhibited in Fig. 6c. U-MnNiFe@NCB shows the onset potential (Eonset OER) of 1.64 V at 10 mA cm−2, more negative than MnNiFe@NCB (1.79 V), CB (1.89 V) and RuO2 (1.67 V), which indicates the better OER performance of U-MnNiFe@NCB than the other three samples [49]. This can also be confirmed by its minimal overpotential requirement for OER. The OER overpotential of U-MnNiFe@NCB (at 10 mA cm−2) is 410 mV, the smallest value among the samples (MnNiFe@NCB: 560 mV; CB: 660 mV; RuO2: 440 mV). Furthermore, considering the ORR/OER activities holistically (Fig. 6d), U-MnNiFe@NCB exhibits the superior bi-functional performance to the commercial Pt/C and RuO2. The ΔE gap (Eonset OER–E1/2 ORR) can reflect the ORR/OER performance of electrocatalysts: the wider the ΔE gap, the less efficient the bi-functional catalyst [50]. As shown in Fig. 6d, the ΔE gap of U-MnNiFe@NCB is the narrowest among the catalysts studied, showing its reasonable performance towards ORR/OER. Clearly, the major reason why U-MnNiFe@NCB exhibits the much improved bi-functional ORR/OER performance compared to MnNiFe@NCB is the much larger amount of metal-oxide active sites (produced by the ultrasonication) anchored on the surface of CB by the grafted polymer chains (also produced by the ultrasonication) for U-MnNiFe@NCB compared to its non-ultrasonically treated counterpart.Custom-made RZABs were designed to evaluate the bi-functional ORR/OER electrocatalytic performance of U-MnNiFe@NCB in real electrochemical devices. Fig. 7 a shows the schematic illustration of the two zinc-air batteries connected in series, each made of two electrodes—a zinc-plate anode and an air–cathode. The electrolyte that contains 0.2 M Zn(ac)2 and 6.0 M KOH flows in each battery by an external peristaltic pump. The air–cathode is composed of nickel foam, air-breathable water-proof layer and carbon paper loaded with a catalyst. Fig. 7b presents the open-circuit voltage (OCV) performance of the two RZABs using U-MnNiFe@NCB and the mixture of 10 wt% Pt/C and RuO2 as the cathode catalysts (abbreviated as the U-MnNiFe@NCB battery and the Pt/C-RuO2 battery respectively). It can be seen that the U-MnNiFe@NCB battery exhibits an OCV of 1.47 V, the voltage being slightly larger than the Pt/C-RuO2 battery (1.43 V). Additionally, Fig. 7c shows the charging/discharging polarization curves of the U-MnNiFe@NCB and Pt/C-RuO2 batteries. There is a larger voltage gap between the charging and discharging data of the Pt/C-RuO2 battery compared to its U-MnNiFe@NCB counterpart, indicating the enhanced performance of the latter [3]. As exhibited in Fig. 7d, the power density of the U-MnNiFe@NCB battery achieves the maximum (96 mW cm−2), the value being much larger than for the Pt/C-RuO2 battery (45 mW cm−2). Furthermore, the specific capacity of the U-MnNiFe@NCB battery was measured to be 746.6 mAh gZn –1 (Fig. S4), which is larger compared to the Pt/C-RuO2 battery (630.2 mAh gZn –1). Indeed, U-MnNiFe@NCB proves to be the fine bi-functional electrocatalyst for RZABs. Finally, the stability test of the U-MnNiFe@NCB battery was undertaken at a current density of 5 mA cm−2 with repeatedly 5-min charging and 5-min discharging for 160 cycles (96,000 s in total). The result in Fig. 7e shows that the voltage gap between charging and discharging undergoes almost no obvious change (from 0.73 V at the beginning to 0.76 V in the end), revealing the good stability of U-MnNiFe@NCB. Fig. 7f shows the demonstration of a white LED light powered by the two U-MnNiFe@NCB batteries connected in series. Though still in the infancy, these gadgets are instructive as the non-noble metallic oxides produced ultrasonically have the comparable bi-functional catalytic activities to the noble metal counterparts.To summarize, this work demonstrates the important roles of ultrasonication in the fabrication of metal oxide/porous carbon composite materials in which polymer chains act as binders. Reactive free radicals generated by ultrasonic cavitation not only help to polymerize monomers without using dangerous radical initiators but also favour the formation of the Mn-Ni-Fe tri-metallic oxide that is otherwise impossible to obtain without ultrasonication. Our investigations have also shown that ultrasonically induced polymers are inclined to anchor within micropore domains instead of mesopores, which would not block mesopore pathways. This is particularly important, because the increase in the number of active sites (metal oxide) does not come into conflict with the enhancement in mass transport (relevant to mesopores) in this material design. With numerous active sites and fast mass transport, U-MnNiFe@NCB possesses the superior bi-functional ORR/OER electrocatalytic performance to its ultrasonication-free counterpart and commercial catalysts. Moreover, two in-series connected RAZBs using U-MnNiFe@NCB as air-cathodes are capable of powering small electronics, demonstrating its potential to work well in a real device. As a consequence, this work provides useful frameworks of ideas for understanding multiple effects of ultrasonication on the assembly of composite materials that comprise inherently incompatible components. Bolin Jin: Data curation, Formal analysis, Investigation, Validation, Visualization, Writing – original draft, Writing – review & editing. Peiyao Bai: Formal analysis, Investigation, Validation, Visualization, Writing – original draft. Qiang Ru: Formal analysis, Investigation, Validation. Weiqi Liu: Formal analysis, Validation. Huifen Wang: Formal analysis, Visualization. Lang Xu: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Writing – original draft, Writing – review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Natural Science Foundation of China (51702358), the Natural Science Foundation of Jiangsu Province (BK20170281) and the Fundamental Research Funds for the Central Universities (2019ZDPY02). L.X. holds the Jiangsu Specially-Appointed Professorship.Supplementary data to this article can be found online at https://doi.org/10.1016/j.ultsonch.2021.105846.The following are the Supplementary data to this article: Supplementary data 1

As a promising electrochemical energy device, a rechargeable zinc-air battery (RZAB) requires cost-effective cathode catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Some earth-abundant transition metal oxides have certain levels of bi-functional ORR/OER catalytic activities yet low electronic conductivities. The addition of high-electronic-conductivity material such as carbon black could result in another problem because there is low compatibility between metal oxide and carbon. In this work, polymer chains are ultrasonically prepared to act as binders to anchor metal-oxide active sites to porous domains of carbon black. The monomer N-isopropyl acrylamide is polymerized under ultrasonication instead of using conventional radical initiators which are dangerous and harmful. Reactive free radicals produced by ultrasonic irradiation can also help to form the Mn-Ni-Fe tri-metallic oxide. Thus, aided by the amide-type polymer as an adhesive, the tri-metallic oxide anchored on polymer-grafted carbon black prepared by ultrasonication possess a large number of metal-oxide active sites and hierarchical pores, contributing substantially to the enhanced ORR/OER electrocatalytic performance in the RZABs. Accordingly, this work provides interesting insight into the effective combination of inherently incompatible components for the fabrication of composite materials from an ultrasonic standpoint.

Shape-selective synthesis of ceria nanoparticles is of scientific and technological importance. Unique shape, size and surface defect-dependent ceria properties play a key role for various promising industrial applications including catalysis, electrochemistry, biology and optics [1–3]. Several strategies, such as hydrothermal [4–8], microwave-assisted hydrothermal [9], solvothermal [10] and micro-emulsion methods have been developed to synthesize CeO2 materials with well-defined cube, rod, polyhedron, nano-sheet morphologies [11]. Among all these strategies, hydrothermal synthesis is the most popular and convenient technique for preparing nanostructured CeO2 through varying precursor concentration, ratio of template agents, pH, hydrothermal temperature and aging time [8,12–14].In this context, cubic CeO2 materials often display remarkable structure-catalysis properties due to tunable oxygen vacancy (Vö) on (100) facets. Up to date, many reports so far have also synthesized cubic CeO2 nanoparticles using this strategy [4–7,15,16]. However, high concentration of alkalis (6 mol/L) and prolonged reaction time (24–48 h) make this strategy very environmental and economical unfriendly. In addition, the size of resultant ceria materials often range from 20 to 60 nm, which show limited amounts of surface defects thus hindering further improvement of catalytic properties. Therefore, manufacture of finely dispersed ceria clusters with high surface defective contents are needed for advanced catalytic processes.Catalytic dehydrogenation of bio-derived oxygenates has been widely known as one of the key reactions for sustainable production of fuels and chemicals [17–19]. Catalytic activation of C–H bond is known as the key reaction step for a variety of catalytic applications such as dehydrogenation, hydrogenolysis, dehydration and amination [20,21]. In particular, catalytic C–H bond cleavage of bio-polyols leads to formation of various value-added carboxylic acids, which are essential building blocks for bio-degradable plastics and a variety of multifunctional polymers [22–25]. Typically, TOF values measured for conversion of bio-polyols such as glycerol, xylitol and sorbitol are in the range of 50–1500 h−1 at 160–200 °C over monometallic Pt, Pd, Ru, Co, Ni, Cu and bimetallic NiCo, CuPd, PtCo, PtSn, AuPt catalysts [19,20,22,26–29]. As it is generally accepted that the cleavage of C–H is a metal-activated reaction, considerable research efforts have been primarily focused on metal composition and morphology of metal particles for tunable activity and selectivity [15,22,27,30]. However, the morphological and electronic features (e.g., shape, size and surface defects) of heterogeneous supports, are yet to be detailed investigated for dehydrogenation of bio-polyols in aqueous phase. According to current literatures, CeO2 (100) and (110) facets show superior performances for facile C–H activation of bio-polyols compared with other facets [31]. Large particle size (>20 nm) of ceria with low content of Vö are, however, major bottlenecking issues prevention further catalyst development for biomass conversion. The critical role of small size reflects on exposed surface of ceria materials. As already mentioned, previous studies have been primarily focused on synthesizing ceria size of >20 nm. Controllable fabrication of small ceria still remains a grand challenge in this area. Therefore, developing reliable synthetic approaches for small sized ceria materials is important for rational design of highly active catalysts for dehydrogenation as well as other energy applications.In the field of renewable H2, dehydrogenation of glycerol and polyols to green H2 co-producing valuable carboxylic acids, is known as an emerging technology for future bio-refineries (Scheme 1 ). Herein, we report a facile “lactic acid (LA) assisted hydrothermal method” for synthesizing cubic CeO2 of 6 nm in size (CeO2-LA) with enhanced metal-support catalysis for dehydrogenation of glycerol and other bio-polyols. The structure and surface physicochemical properties analysis based on XRD, TEM, BET, Raman and XPS characterizations demonstrated that, the formation of finely sized cubic CeO2 leads to enhanced surface area and abundant surface Vö defects. The growing mechanism of finely dispersed clusters is ascribed to the synergism that, hydroxyl and carboxylic groups in LA molecule induce morphological confinement. The proposed PtCo/CeO2-LA catalysts with abundant surface Vö defects exhibit remarkable catalytic activity and good selectivity, leading a record high TOF value of 29,241 h−1 at 200 °C in dehydrogenation of bio-derived polyols. The influence of shape-, size-, Vö-dependent catalytic properties on C–H bond cleavage has been studied to reveal the underlying reasons for such performance enhancement. In addition, the investigated nano ceria materials display remarkable durability in hydrothermal conditions. The proposed synthetic strategy to tailor the Vö defect content can be potentially applied in rational design of other metal oxide catalysts for energy and environmental applications.Chloroplatinic acid hexahydrate (H2PtCl6·6H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), cerium nitrate hexahydrate (Ce(NO3)3·6H2O), NaOH, Na2CO3, lactic acid, isopropanol, propanoic acid, glyceric acid, glycolic acid, acetic acid, formic acid, propylene glycol, ethylene glycol, ethyl alcohol, methyl alcohol, glycerol, xylitol, sorbitol, arabitol and mannitol were purchased from Sinopharm Chemical Reagent Co., Ltd. PtCo/CeO 2 -LA were prepared by LA-assisted hydrothermal method. A typical procedure is presented in Fig. S1. Solution A (3.03 g of Ce(NO3)3·6H2O dissolved in 25 mL deionized (DI) water) and solution B (0.8 mol/L NaOH and 0.25 mol/L Na2CO3) were simultaneously added dropwise to 250 mL beaker with 50 mL DI water, under continuous stirring at ambient temperature. The pH value of slurry in beaker was kept at 10.0–10.5 throughout the synthetic process. After stirring for 30 min at ambient temperature, solution C (0.06 g H2PtCl6·6H2O and 0.03 g Co(NO3)2·6H2O dissolved in 10 mL DI water) and solution B were dripped into the beaker. After stirring another 30 min, the slurry was reduced with 0.1 g NaBH4 dissolved in 50 mL DI water (slowly added dropwise). Then the slurry was transferred to a 100 mL autoclave (2 g LA and 0.86 g NaOH were pre-added) and crystallized for scheduled time (1 h, 2 h, 3 h, 6 h, 10 h, 18 h and 24 h) at 200 °C. Furthermore, replacing to LA, isopropanol or propanoic acid were also selected as assistant to study growth kinetic of CeO2-LA. Finally, the slurry was filtered and washed three times with DI water. The solid sample was dried under 70 °C to obtained the final catalysts. The Pt and Co loadings of 1.36 wt% and 0.47 wt%, respectively, were determined by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). PtCo/CeO 2 were synthesized by deposition–precipitation method. Cubic CeO2 materials were first prepared by conventional hydrothermal method [32]. In a typical procedure, 1.74 g of Ce(NO3)3·6H2O and 19.2 g of NaOH were dissolved in 80 mL DI water with stirring at ambient temperature. The resulting slurry was transferred to a 100 mL autoclave and crystallized for 24 h at 180 °C. Finally, the slurry was filtered, and washed three to five times with DI water. The solid sample was dried under 70 °C to obtained the final cubic CeO2. Supported PtCo/CeO2 catalyst was prepared by deposition–precipitation synthesis. In a typical procedure, solution A (0.06 g of H2PtCl6·6H2O and 0.03 g of Co(NO3)2·6H2O dissolved in 50 mL DI water) and solution B (0.25 mol/L Na2CO3 aqueous solution) were simultaneously added dropwise to 500 mL beaker with 100 mL DI water and 1 g powder of cubic CeO2 under continuous stirring at ambient temperature. The pH value of slurry in beaker was kept at > 9 throughout the synthetic process. After stirring for 30 min at ambient temperature, the slurry was reduced with 0.1 g NaBH4 dissolved in 50 mL DI water (slowly added dropwise). After stirring another 12 h under ambient temperature, the slurry was filtered and washed three times with DI water. The solid sample was dried under 70 °C to obtained the final catalysts. The Pt and Co loadings of 1.30 wt% and 0.50 wt%, respectively, were also determined by ICP-OES.The morphologies and crystal structure of the PtCo/CeO2 and PtCo/CeO2-LA catalysts were measured by transmission electron microscope (TEM, JEOL JSM-2100F), high resolution TEM (HR-TEM) and X-ray diffraction (XRD, X'pert PRO MPD diffractometer instrument using Cu-Kα radiation with a scanning angle (2θ) of 10° - 80°, operated at 40 KV and 40 mA). The BET surface areas, pore volume and pore size were calculated according to N2 adsorption isotherms. Composition and valence states of surface metals were collected by X-ray photoelectron spectroscopy (XPS) measurements. Surface oxygen vacancy defect was measured by Raman spectra (LabRAM HR Evolution (HORIBA JobinYvon)) with the 514 nm laser lines. The Pt and Co loadings were determined by ICP-OES (VARIAN 720-ES, America Varian technologies).Evaluation of catalysts performance was carried out in a 30 mL autoclave. In a typical experiment, 0.05 g catalyst, 0.6 g NaOH and 15 mL glycerol aqueous solution (1.0 mol/L) were charged into the Parr reactor, following sealed and purged thrice with pure N2 (>1 MPa). Then, the reactor was heated to 200 °C and held at continue stirring in 1000 rpm under 1 MPa N2 pressure (N2 was used as an inert gas which does not affect experimental results. See Table S7). It is also important to mention that, N2 pressure was charged only for lab safety purpose. Thus 1 MPa was chosen for the benefit of conducting experiments. The activity of used catalysts was measured by conducting experiments with conversion <25%, roughly 1.0 h of batch time using 0.01 g of solid catalysts.After a designed reaction time, the liquid phase products were collected for quantitative analysis using a Shinadze HPLC LC-20AT system equipped with Phenomenex chromatographic column (Rezex ROA-Organic Acid H+ (8%), 300 × 7.7 mm) and refractive index (RID-10A) detectors. A typical operating condition were carried out at 60 °C with 0.005 mol/L H2SO4 aqueous solution as the mobile phase flowing at 0.8 mL/min. The definition of turn over frequency (TOF), conversion and selectivity is similar as previously described [22]. From the concentration values, conversion (X), selectivity (S) and turnover frequency (TOF based on Pt atom and calculated by initial reaction rate) were calculated as defined below. The yield was defined based on X and S (carbon based). Conversion = Carbon m o l e s , r e a c t a n t i n i t i a l − Carbon m o l e s , r e a c t a n t f i n a l Carbon m o l e s , r e a c t a n t i n i t i a l × 100 % Selectivity = Carbon m o l e s , p r o d u c t s f i n a l Carbon m o l e s , r e a c t a n t i n i t i a l − Carbon m o l e s , r e a c t a n t f i n a l × 100 % TOF =  ▵ N r e a c t a n t N m o l e , P t × R e a c t i o n t i m e (mole of loading Pt were determined by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)). Yield = Conversion × Selectivity Fig. 1 (a) and S1 depict the proposed two-step preparation of cubic CeO2-LA materials. Different from conventional one-step synthesis methods without using LA, Ce3+ was first precipitated at ambient conditions, the slurry of which was then further hydrothermally treated in a stainless steel reactor at 200 °C. In this process, LA was selected as soft template agent to control anisotropic growth of CeO2 nuclei. As shown in Fig. 1(b), the TEM image presents that clearly cubic CeO2 crystal was formed via our LA-assisted hydrothermal method. Furthermore, the HRTEM image and fast Fourier transform (FFT) analysis of as-obtained cubic CeO2-LA (Fig. 1(c)) show that two kinds of clear lattice fringe directions ascribed to (200) and (220) were observed, suggesting that the (100) planes are exposed on the surface of CeO2 nanoparticles [16,32]. The XRD pattern (Fig. 1(d)) further reveals the pure cubic phase (fluorite structure, JCPDS 34-0394, space group Fm-3m) of the cubic CeO2-LA materials [16,32]. Notably, the particle size statistics (inset in Fig. 1(b)) show that the average size of cubic CeO2-LA nanomaterial is 5.8 ± 0.9 nm, which is much smaller than cubic CeO2 synthesized by conventional hydrothermal methods (28.5 ± 24.2 nm, Fig. S2). Obviously, in XRD pattern, the broadening of reflections of cubic CeO2-LA sample implied the significantly smaller cluster sizes (detailed calculation presented in Table S1) in comparison with conventional approaches (Table 1 ), which agrees well with the results of the particle size statistics based on TEM image. Incidentally, its surface areas is also increased significantly (see Table 1).The promising results on unique textural properties such as cubic crystal, small particle size and enhanced surface area, motivated us to inspect the plausible growth mechanism of CeO2-LA clusters. In general, kinetically controlled crystal growth in solution follows two major pathways: oriented attachment and Ostwald ripening (dissolution and precipitation) [33–35]. The conventional hydrothermal synthesis strategy of cubic CeO2 nanocrystals is known to follow oriented attachment pathways through initial nucleation of CeO2, subsequent effective collision and phase agglomeration of nuclei in concentrated OH− medium, which eventually results in large cluster size [35]. Interestingly, the collision and fusing of CeO2 nuclei is not significant in the presence of LA. As shown in Fig. 2 , instantaneous dissolution and recrystallization (Ostwald ripening) [36] of irregular nanoclusters to cubic CeO2 particles were observed in aqueous LA medium. Continued dissolution and recrystallization of ceria and cerium carbonates nuclei with extended hydrothermal time from 0 to 10 h, results in formulation of <6 nm sized cubic CeO2 particles. Notably, different from previous works [4–8,15], it is found that prolonged hydrothermal time from 3 h to 10 h only leads to slight enhancement of cluster size from 5.1 nm to 5.8 nm (Fig. 3 (a)), suggesting the collision and fusing of CeO2 nuclei are significantly inhibited in the proposed LA-assisted synthesis. Clearly, the Oswald ripening processes are manipulated by selectively promoting surface reconstruction rather than simple oriented attachment of cerium carbonates and CeO2 species.In particular, as shown in Figs. 2 and 3(b), when synthesis time is less than 3 h, the majority of the observed samples were nano-spheres, and composed of mixed cerium carbonates (PDF#44–0617) and CeO2 species. However, when the hydrothermal treatment time prolonged to 3 h and further to 6 h, the uniform cubic CeO2 nanomaterials were generated, with pure CeO2 diffraction peaks retained in XRD patterns. Li and coworkers [37] reported that the transformation of cerium carbonates phases go through the formation of amorphous cerium hydroxide and subsequent crystallization to form CeO2. Slowly dissolution of cerium carbonates in this work is beneficial for the recrystallization process leading to cubic CeO2 nanomaterials with smaller particle size.The existence of LA in aqueous medium clearly plays a key role in determining the fine dispersion of ceria clusters. To gain more insights for the formation mechanism, we performed further control experiments to understand the contribution of hydroxyl and carboxylic groups in LA molecule in formulating cubic ceria clusters during hydrothermal synthesis (see Fig. 4 ). Therefore, we selected isopropanol (IP) and propionic acid (PA) as soft templates and synthesized CeO2 nanomaterials under similar conditions to establish the possible mechanism for the formulation of (200) and (220) facets.The critical role of hydroxyl and carboxylic groups on the formation of cubic ceria with <6 nm in size has not been well-documented in literature. According to TEM and XRD characterization in Fig. 4, it is clear that, carboxylic groups quickly enable the formation of cerium carbonates phases. Under hydrothermal condition, the existing hydroxyl groups eventually facilitate the formation of cubic clusters with <6 nm in size.One should also be aware of the critical role of concentration of LA, IP and PA. Obviously, we have conducted synthesis under a condition where cubic morphologies are thermodynamically and kinetically favorable. Thus the size of ceria can be well controlled.As can be seen from Fig. 4(a), mixed cubes, polyhedron and nano-spheres of CeO2 nanomaterials are coexisting when IP was used as the template. The average particle size of CeO2-IP (9.2 nm) is significantly larger than cubic CeO2-LA clusters (5.8 nm). Such observation indicates the CeO2-IP crystals in solution grow in an anisotropic fashion. When PA was used (Fig. 4(b)), the obtained CeO2-PA clusters exhibit similar cubic shape and particle size to CeO2-LA clusters (Fig. 4(c)). Taking into account the result of control experiments with time-dependent particle size data of CeO2-LA, it implies that the carboxyl functional groups play a critical role in manipulating morphological rearrangement through Ostwald ripening processes during hydrothermal treatment. Furthermore, the carboxyl functional groups are also found to stabilize the CeO2 nuclei, thus kinetics of self-assembly of cubic CeO2 clusters was significantly inhibited, by the repulsive forces through bonding the carboxyl functional groups in the soft template [38]. Teo and colleagues [35] also reported that, at a low pH, the growth of CeO2 particle favors the Ostwald ripening process through instantaneous dissolution and recrystallization of CeO2 nuclei, inducing the formation of small and individual particles. Hence, the dominant carboxyl mediated Ostwald ripening process results in the formation of cubic CeO2 with smaller size.Another interesting finding is that the pure CeO2 phase was generated within CeO2-IP clusters, while the as-prepared CeO2-PA clusters are mainly composed of both CeO2 and cerium carbonates phases (Fig. 4(d)), suggesting that the hydroxyl groups control the reconstruction of Ce2(CO3)3 nuclei. This phenomenon was also found in time-dependent XRD patterns of CeO2-LA presented in Fig. 3(b). As the hydrothermal treatment time prolong, the cubic CeO2 finally formed with dissolved cerium carbonates phases, indicating that the dissolution rate of cerium carbonates tuned by hydroxyl groups play a greatly role in determining the eventual morphology of cubic CeO2. Furthermore, the hydroxyl groups have reduction ability, which is favorable for the formation of surface Vö defects through reducing Ce4+ to Ce3+ [39,40]. Hence, detailed study of the surface features of CeO2-LA sample were conducted in the following sections.In summary, the experimental results shown above confirm that the formation of cubic shape and fine dispersion of CeO2-LA sample is induced by the strong synergism of hydroxyl and carboxyl functional groups during structural evolution (see Scheme 2 ). The carboxyl groups play a critical role in manipulating morphological rearrangement, thus the cubic shape and particle size can be well controlled. Hydroxyl groups determines the surface reduction rates of cerium carbonates phases. As a result, uniform CeO2-LA clusters with smaller particle size and abundant surface Vö defects can be achieved in the two-step preparation method proposed in this work.N2 adsorption/desorption results of cubic CeO2-LA and control samples are listed in Table 1. It is found that the surface areas and pore volume of CeO2-LA are 101.3 m2/g and 0.2 cm3/g, respectively, which are 5-fold and 10-fold higher compared with the control sample prepared in the absence of LA (20.6 m2/g and 0.02 cm3/g). The significantly enhanced surface area and pore volume often indicates more catalytically sites and defects are accessible to reactant molecules. Especially the Vö defects, created by removal of surface oxygen species simultaneously reduction of Ce4+ to Ce3+ species (Scheme 3 ), are proposed to reactive hot spot on CeO2 surface for many redox reactions catalyzed by metal oxides [3]. Therefore, we prepared CeO2-LA immobilized bimetallic PtCo catalysts and evaluated the performances for dehydrogenation of polyols as the model reaction. Prior to activity tests, metal valences and surface Vö defects of as-obtained bimetallic PtCo/CeO2-LA catalysts were characterized using XPS and Raman to gain insight into the unique surface and structural features.XPS spectra were collected to elucidate the chemical state of surface Ce and O species, which can reveal the content of surface defects [40]. The Ce 3d spectra (Fig. 5 (a)) of CeO2-LA was resolved into eight groups, suggesting the coexisting of Ce3+ and Ce4+ species in cerium oxides [6]. The peak positions and their attribution are summarized in Table S2 and Table S3. The 3d5/2 peak labeled V1 (885.1 eV) and 3d3/2 peak labeled U1 (903.3) are ascribed to Ce3+. In addition, the peaks labeled V0 (882.5 eV), V2 (888.4 eV), V3 (898.4 eV), U0 (900.8 eV), U2 (907.4 eV) and U3 (916.8 eV) are ascribed to Ce4+ species. To our best knowledge, it is generally accepted that the proportion of Ce3+ could reveal the concentration of Vö on CeO2 surface [3]. Following methodologies reported in the previous literature [6,32,40], the surface Ce3+ content was calculated based on the peaks areas of eight groups. The results show that the proportion of Ce3+ (23.7%) in cubic CeO2-LA sample is higher than CeO2 material synthesized in the absence of LA (19.0%), suggesting more abundant Vö defects on the surface of CeO2-LA. Fig. 5(b) presents the O 1s spectra of samples for further determining the chemical state of surface Ce species. The O 1s region is resolved into three groups, including the peak at 531.8 eV attributed to surface hydroxide and adsorbed H2O, the peaks at 530.2 eV and 529.2 eV assigned to Ce2O3 and CeO2, respectively (Table S4 and Table S5) [9,41]. These results further demonstrate the coexistence of Ce3+ and Ce4+ species in ceria samples. The peak area of lattice oxygen assigned to Ce2O3 in CeO2-LA sample is increased compared to the CeO2 material synthesized in the absence of LA, which means higher proportion of Ce3+ in CeO2-LA sample. This observation clearly shows good agreement for the Ce3+ and O 1s analysis, suggesting abundant Vö defects available on the surface of CeO2-LA.To further investigate the Vö defects content on CeO2-LA surface, the Raman spectra were collected and results are shown in Fig. 5(c). The Raman spectra curves further confirmed different O storage capacity for cubic CeO2 synthesized by different method. The sharp characteristic peak at ∼456 cm−1 is assigned to the Raman active optical-phonon F2g vibration mode of CeO2 with a fluorite structure [42]. In addition, the other low intense peak at ∼573 cm−1 is detected on high energy sides of F2g peak, which is responsible for the Vö [43]. In general, the ratio of peak areas for the bands at ∼573 cm−1 and ∼456 cm−1 is defined as the concentration of Vö [44]. Detailed inspection on the two samples reveal that Vö concentration is significantly increased on CeO2-LA (27.2%), compared to the CeO2 material (3.3%) synthesized in the absence of LA. This result is good consistent with the observation from XPS analysis. Moreover, the relation between Vö concentration and hydrothermal reaction time was also investigated in detail (Figs. S3 and S4). It is clearly observed that Vö concentration increases with extended hydrothermal time, possibly due to the removal of more O species in the reductive atmosphere (-OH from LA and H2 released from NaBH4).More detailed catalyst characterization such as TEM-EDX and Aberration corrected STEM were performed to reveal the metal distribution of Pt and Co metals in the case of the PtCo/CeO2-LA and PtCo/CeO2 catalysts. As shown in Fig. 6 , the STEM-EDX mapping clearly shows that the Pt and Co elements have a good dispersion on PtCo/CeO2-LA catalyst compared with PtCo/CeO2 catalyst, which also confirms the presence of Pt and Co content in the catalysts. This information confirms the more introduction of Vö defects in the CeO2 surface leads to a better dispersion of the Pt and Co metals, which is consistent with other reports [45,46]. From aberration corrected STEM of PtCo/CeO2-LA catalysts in Fig. 6(c) and Fig. S6, it is shows that cubic morphology of CeO2 has been maintained very well after immobilizing Pt and Co content. Moreover, from the HR-TEM images in Fig. S5, it is found that the lattice fringes corresponding to Pt (111) and Pt (200) planes are present along with Co (100) and Co (101) planes on PtCo/CeO2 and PtCo/CeO2-LA catalysts, indicating the presence of Pt–Co surface on both catalysts.More importantly, detailed inspection on Fig. 6 (c) further reveals the well dispersion of Pt species on cubic ceria. Taking into account the EDX mapping in Fig. 6 (b), it is clear that, both Pt and Co elements are well dispersed on catalyst surface.Metal–metal interfacial strong interaction are believed to be most essential to modulate cascade C–H, C–O and CO bond cleavage and formation in efficient conversion of biomass resources, due to tunable electronic structures at the interface [47]. Such, XPS of the PtCo/CeO2 and PtCo/CeO2-LA catalysts were further investigated to reveal the metal–metal coupling effect. In Fig. 7 , the electron binding energy of Pt 4f5/2 and Pt 4f7/2 for both catalysts are 74.5 eV and 71.1 eV, respectively. The consistent electron binding energy of Pt 4f for the PtCo/CeO2 and PtCo/CeO2-LA catalysts indicates similar Pt–Co and Pt-support coupling effect on both catalysts, which is further confirmed by the similar electronic structures of Co 2p for both samples. Hence, we can conclude that the more introduction of Vö defects in the CeO2 surface improve more sites for the growing of Pt, leading to a better dispersion, however, it cannot enhance Pt–Co and Pt–Ce electron coupling effect.As already mentioned in the above sections, catalytic activation of C–H bond is known as the key reaction step for a variety of reactions such as dehydrogenation, hydrogenolysis, dehydration and amination [20,21]. In particular, catalytic C–H bond cleavage of bio-polyols leads to formation of various value-added carboxylic acids, which are essential building blocks for bio-degradable plastics as well as other multifunctional polymers [22–25]. The catalytic performances of the PtCo/CeO2-LA and PtCo/CeO2 are investigated to understand possible size- and Vö-dependent properties during conversion of bio-polyols to LA under N2 pressure. Glycerol, a byproduct of biodiesel production and chemical syntheses of perfumes, fragrances and pharmaceuticals from vegetable oil and animal fat, is selected as model compound. As shown in Fig. 8 (a) and Fig. 9 (detailed comparison see Table S6), the PtCo/CeO2-LA catalyst displays a record high activity (TOF: 29,241 h−1) for C–H cleavage of glycerol (rate-determining step, see Scheme 4 ) [22]. In contrast, PtCo/CeO2 catalyst prepared in the absence of LA only shows a TOF value of 8079 h−1 under identical condition. Control experiments with uncalcined PtCo/CeO2 and PtCo/CeO2-LA catalysts were conducted to confirm that, negligible conversion of glycerol was found over those catalysts (X: 0.2–3%). Obviously, calcination is necessary to generate intrinsically active sites for glycerol conversion. In addition, the remaining LA on solid catalysts surface has been removed to exposed active sites. While the electron structure and surface morphology of Pt species show negligible differences for the two samples (Figs. 6 and 7), LA-assisted formation of abundant surface and Vö defects are the key contributing factor for such dramatic enhancement in activity.The plausible mechanism for activity enhancement for C–H cleavage of glycerol (dehydrogenation) is discussed in this section. It is necessary to mention that, Pt-catalyzed dehydrogenation is the key step for glycerol conversion to LA [17,19,20,22]. As already shown in Scheme 4, one mole of glycerol should generate equivalent amount of H2, thus high glycerol conversion indicates significant generation of H2 in gaseous phase. Therefore, it is not surprising that H2 is the main gas phase product during experiments. Clearly, Vö defects are critical for dehydrogenation reactions. Among all critical parameters that have been extensively studied in literature, Vö is believed to be the key for surface redox properties of metal oxides [3,31,48–51]. This is because Vö defects can bind reactants and intermediates more strongly in their activation [3]. In this work, more Vö defects on the surface of PtCo/CeO2-LA catalysts can promote chemical adsorption of OH−, which intrinsically facilitates glycerol dehydrogenation to form glyceraldehyde as key intermediate [22]. Hence, the record high activity was obtained over PtCo/CeO2-LA catalyst. We further investigated the effect of Vö concentration on glycerol conversion under N2 pressure. It is found in Fig. 8(b) that the conversion of glycerol displays optimal values with Vö content, suggesting that the synergism between Vö and PtCo sites is critical for C–H bond cleavage of glycerol. It is generally accepted that metal catalyzed C–H bond cleavage is often restrained by poor activity for nucleophilic attacking activity of metal centers. The presence of Vö intrinsically facilitate electronic transfer from Ce3+ to metallic centers [22]. As a result, the α-bond activation mode is selectively promoted on the surface PtCo clusters. This result further confirms that adjusting chemical adsorption of reactants and OH− through tailoring Vö content is an effective approach for tunable dehydrogenation reactivity of bio-polyols.The catalytic reusability of the PtCo/CeO2-LA catalyst for glycerol were also tested at 1 MPa N2 pressure and 200 °C (Influence of pressure on conversion is shown in Table S7). As shown in Table 2 , the TOF of PtCo/CeO2-LA catalyst show slight decreases under the recycle reaction. Thus, the PtCo/CeO2-LA catalyst is stable under the reaction conditions.It was previously found that, actually lowering the operating pressure favored conversion of glycerol to LA (Table S7). This is possibly because that, this reaction involves generation of one mole H2 and LA from one mole of glycerol molecules. Therefore, it is not surprising that, lowering N2 pressure slightly enhances glycerol conversion.TON values were also measured in Table 3 . It is found that, TON value for PtCo/CeO2 catalyst was 20,197 at 200 °C. In contrast, PtCo/CeO2-LA catalyst shows a much higher TON value of 81,874 under the same condition. In addition, the regenerated PtCo/CeO2-LA catalyst also shows 79,899. It is clear that, the LA assisted PtCo/CeO2 catalyst displays superior activity and durability for glycerol conversion to LA and co-products.The catalytic performance of the PtCo/CeO2-LA catalyst for conversion of other sugar-derived polyols (xylitol, sorbitol, arabitol and mannitol) were also studied at 1 MPa N2 pressure and 220 °C. It is observed that synergistic C–H bond cleavage was also achieved on the surface of the proposed catalyst, resulting in good selectivity of LA as the main product. As shown in Table 4 , the selected long-chain bio-polyols are high efficient converted to C3 and C2 products with a high conversion (>90%), suggesting promising performances of PtCo/CeO2-LA catalyst for selective C–C bond cleavage. And we observed that the C6 bio-polyols have a higher conversion but lower selectivity for LA, compared with C5 bio-polyols.As already mentioned in the above sections, More Vö defects on the surface of PtCo/CeO2-LA catalysts can intrinsically facilitate C–H bond cleavage of bio-polyols. However, notably, the Ce3+, formed due to loss of considerable amounts of oxygen from its lattice, are readily oxidized to Ce4+ during storing at air atmosphere and quenching of partial Vö defects [52]. The loss of a large of number of Vö defects will inevitably reduce the catalytic activity. Hence, the performance of the PtCo/CeO2-LA catalyst with different storage time was evaluated, which can further understand the effect of surface Vö defects on the C–H bond cleavage of bio-polyols. As shown in Fig. 10 , the fresh PtCo/CeO2-LA catalyst obviously displays higher activity with conversion of 39.9%, compared with aged PtCo/CeO2-LA catalyst in air at prolong time with conversion of 32.5% (1d), 30.2% (2d) and 25.4% (4d), while the selectivity of LA displays a negligible reduction. Pt 4f spectra of the fresh and aged PtCo/CeO2-LA catalyst in Fig. 11 (a) shows consistent electron binding energy, indicating similar Pt–Co and Pt-support coupling effect on both simples. However, the weakened characteristic peak of Ce3+ on aged PtCo/CeO2-LA catalyst compared with fresh PtCo/CeO2-LA catalyst (Fig. 11(b)) suggests the oxidation of Ce3+ to Ce4+ and quenching of Vö defects. Moreover, the Raman spectra also reveals the reduction of Vö defects (fresh: 12.1%, aged 1 day: 11.4%) on the surface of PtCo/CeO2-LA catalyst after an aging at air. This information indicates that the quenching of Vö defects decreased the activity of glycerol conversion, further confirming the great role of Vö defects in dehydrogenation of bio-polyols.Quenching of Vö defects on the surface of PtCo/CeO2-LA catalyst reduces its C–H bond activity during conversion of bio-polyols, hence, it is urgent to develop a simple and efficient strategy for the regeneration of Vö defects. Several strategies, such as high temperature hydrogenation, ion doping, high-energy particle bombardment, atmosphere deoxygenation, mechanization and chemical reaction methods have been developed to form Vö defects in the past decades [53–57]. Among all these strategies, thermal reduction method is the most popular and convenient technique for forming Vö defects through varying reduction temperature and reducing atmosphere. However, this method usually requires high energy and seriously destroys the intrinsic structure of materials, hence it's not the best strategy for the regeneration of the PtCo/CeO2-LA catalyst.In this content, we developed a simple method for the regeneration of Vö defects on surface of the PtCo/CeO2-LA catalyst. Hernia radiation is known to create a large quantity of free radicals on the surface of metal oxides [58–60]. The formation of free radicals can decompose the unstable oxidative species on the surface of ceria to regenerate Ce3+. As shown in Fig. 12 (a), the PtCo/CeO2-LA catalyst was irradiated using hernia lamp, and it is found that the Vö defects content increased significantly with the extension of irradiation time (Fig. 12(b)). Then, the regenerated catalysts with different irradiation time was evaluated at same reaction condition to reveal its reaction performance. In Fig. 12(c), it is clearly that the activity of the PtCo/CeO2-LA catalyst was gradually recovered after regeneration of Vö defects with hernia lamp irradiation. When the irradiation time is 10 h (Vö defects content: 16.5%), the conversion of glycerol reached 42.3%, which even exceeds the conversion of the fresh PtCo/CeO2-LA catalyst (39.9%). However, the conversion began to decrease with the prolong irradiation time, because excessive Vö defects content (18.1%) will promote the enrichment of OH− on the surface of catalyst and inhibiting the transformation of glycerol [47].In summary, we have developed an environmental friendly and efficient strategy to synthesize cubic CeO2 supported PtCo bimetallic catalysts via LA-assisted hydrothermal method. The carboxyl and hydroxyl groups in LA synergistic promote phase transformation in hydrothermal process to the formation of cubic CeO2 with smaller particle size and enhanced surface area. Meanwhile, reductive atmosphere plays a key role in tailoring surface Vö content. Smaller particle size, enhanced surface area and abundant Vö defects contribute to improved dehydrogenation activity (TOF: 29,241 ± 1202 h−1) at 200 °C, during aqueous phase conversion of glycerol to LA. The PtCo/CeO2-LA catalyst also displays significant potential in conversion of long-chain bio-polyols. We also found that quenching of Vö defects on the surface of PtCo/CeO2-LA catalyst at air atmosphere reduces its C–H bond activity during conversion of bio-polyols. In this content, a simple strategy to regenerate the quenched Vö defects and activity of the PtCo/CeO2-LA catalyst is also developed by irradiating of the deactivated catalyst hernia lamp. The methodology on shape-selective synthesis of metal oxides could be potentially utilized in other materials synthesis.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors want to thank financial supports National Natural Science Foundation of China (22078365, 21706290), Natural Science Foundation of Shandong Province (ZR2017MB004), Innovative Research Funding from Qingdao City, Shandong Province (17-1-1-80-jch), “Fundamental Research Funds for the Central Universities” and “the Development Fund of State Key Laboratory of Heavy Oil Processing” (17CX02017A, 20CX02204A) and Postgraduate Innovation Project (YCX2021057) from China University of Petroleum.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2022.08.008.

Dehydrogenation is considered as one of the most important industrial applications for renewable energy. Cubic ceria-based catalysts are known to display promising dehydrogenation performances in this area. Large particle size (>20 nm) and less surface defects, however, hinder further application of ceria materials. Herein, an alternative strategy involving lactic acid (LA) assisted hydrothermal method was developed to synthesize active, selective and durable cubic ceria of <6 nm for dehydrogenation reactions. Detailed studies of growth mechanism revealed that, the carboxyl and hydroxyl groups in LA molecule synergistically manipulate the morphological evolution of ceria precursors. Carboxyl groups determine the cubic shape and particle size, while hydroxyl groups promote compositional transformation of ceria precursors into CeO2 phase. Moreover, enhanced oxygen vacancies (Vö) on the surface of CeO2 were obtained owing to continuous removal of O species under reductive atmosphere. Cubic CeO2 catalysts synthesized by the LA-assisted method, immobilized with bimetallic PtCo clusters, exhibit a record high activity (TOF: 29,241 h−1) and Vö-dependent synergism for dehydrogenation of bio-derived polyols at 200 °C. We also found that quenching of Vö defects at air atmosphere causes activity loss of PtCo/CeO2 catalysts. To regenerate Vö defects, a simple strategy was developed by irradiating deactivated catalyst using hernia lamp. The outcome of this work will provide new insights into manufacturing durable catalyst materials for aqueous phase dehydrogenation applications.

Data will be made available on request.As a prototypical model probe reaction, catalytic oxidation of carbon monoxide (CO) has attracted much research attention for many years [1]. With the development of proton exchange membrane fuel cell (PEMFC) technology, removal of CO from hydrogen raw materials by preferential oxidation has gradually become a research focus [2]. With expanding awareness of environmental conservation, the removal of CO from air has become a pertinent topic. CO could not only bind to human's hemoglobin, but also affect climate atmospheric chemistry and the ozone layer [3–5]. The catalytic oxidation of CO is one of the most effective strategies to mitigate the issues above. Typically, noble metal-based catalysts (Pt, Pd, Rh and Au etc.) are utilized for CO oxidation. However, large-scale applications of noble metal-based catalysts are restricted by their limited reserves and high cost, making it practically urgent to develop high-efficiency and low-cost catalysts for CO oxidation based on earth abundant elements [6].Recently, transition metal oxides (such as Co3O4, Fe2O3, Mn3O4, CuO and NiO etc.) have been employed as high active catalysts for CO oxidation [7,8]. Among the multifarious candidates, copper-based catalysts attracted much attention owing to their economic viability, environment-friendliness, and high catalytic activity [9]. For instance, CuO-CeO2 catalyst developed by Liu et al. could deliver a high CO conversion of 96% at 86 °C [10]. Regarding the progress achieved in copper-based catalysts and the features of active metal species, it has been established that only well dispersed copper species exhibited satisfactory catalytic activity, while that of bulk Cu species are negligible [11]. Consequently, it has aroused much attention to exploit highly dispersed copper-based catalysts to further improve its catalytic performance.In general, supports surfaces are utilized to load highly dispersed and active catalytic sites in a homogeneously distributed fashion, thereby maximizing utilization efficiency, catalyst stability, while minimizing aggregation of the active species and thus costs. In particular, metal-support interactions can modify the geometric and electronic structure of the interfacial active sites, which can further enhance catalytic activity and stability [12,13]. Recently, Papadopoulos et al. synthesized atomically dispersed copper-ceria catalysts using surfactant-assisted hydrothermal method and the catalytic activity of the catalyzers were greatly improved due to highly dispersed active centers and strong copper-ceria interaction [14].CuO/ZnO catalysts have been utilized in various thermochemical reactions, e.g., E.L. Reddy et al. reported that the surface area of Cu metal in catalyst significantly affects the catalytic activity of water-gas shift (WGS) reaction, when 30 wt% of CuO and 70 wt% of ZnO were utilized [15]. Yupeng Xie et al. found that CuO could be reduced to Cu (under reducing conditions) in CuO/ZnO catalysts in supercritical water gasification of lignin at 500 and 600 °C. The reduction may release active oxygen species to improve the decomposition and gasification of biomass. Additionally, metal Cu could also serve as the catalyst to accelerate the WGS reaction and carbon conversion [16]. C. Mateos-Pedrero et al. reported that in CuO/ZnO catalysts the copper dispersion and surface area increased with increasing the surface area of the ZnO support, resulting in better catalytic activity. In methanol steam reforming reaction, the selectivity increases with increasing the polarity ratio, which might be due to the presence of more selective Cu–ZnO sites at the Cu–ZnO polar interface of CuO/ZnO catalyst [17].Furthermore, the catalytic CO oxidation performance have been demonstrated to depend greatly on the sort of exposed crystal plane of oxide support [18,19]. Among the various metal oxides, ZnO support is more attractive owing to its low cost, nontoxicity, ease of preparation and its morphology can be well controlled to preferentially expose certain crystal facets. Therefore, ZnO has been utilized as supports in various catalytic reactions [20]. For instance, the selective hydrogenation of CO2 to CH3OH production was examined using Cu loaded on ZnO with various morphologies [19]. It was found that the platelike ZnO support based copper catalyst exhibited the highest selectivity towards methanol synthesis. To the best of our knowledge, though the morphology effect of ZnO carrier towards many reactions have been investigated [18,19], the influence of morphology engineering of CuO loaded ZnO supports on its catalytic performance for CO oxidation has been seldom reported.In this work, ZnO with different morphologies including nanorods (NRs) with varied aspect ratios and nanodisks (NDs) were synthesized, which served as support for CuO nanoparticles loading via deposition-precipitation method. The morphology engineering of CuO/ZnO catalysts for CO oxidation was systemically investigated. The ZnO support with different morphologies and the as-prepared catalysts derived from them were characterized by nitrogen adsorption, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopic (XPS) and inductively coupled plasma mass spectrometry (ICP-MS). The catalytic performance of the CuO/ZnO catalysts for CO oxidation was evaluated via fixed bed reactor. And a mechanism was proposed to highlight the importance of morphology of ZnO in CuO/ZnO catalysts for CO oxidation.ZnO NRs were prepared according to the reported procedure after modification [21,22]. ∼ 10.5 g of Zn(NO3)2·6H2O and 28 g of NaOH were dissolved into 420 mL and 325 mL ethanol, respectively. And then the two solutions were mixed, followed by adding 231 mL ethylenediamine (EDA); and the mixture was kept at 120 °C for 12 h. Afterwards, the products were centrifugated and washed using deionized water and ethyl alcohol several times and calcined at 300 °C for 3 h in air flow. The ZnO NRs with different aspect ratios were also prepared with similar procedure except the reactions were carried out at 120 °C for 24 h. The products were named as ZnO-NR1 and ZnO-NR2, respectively.The ZnO NDs were prepared according to the reports with some modifications [19,23]. ∼ 12 g of Zn(OAc)2·2H2O and 7.68 g of hexamine were continuously dissolved into 50 mL deionized water to obtain solution, respectively. And then the two solutions were mixed and reacted at 100 °C for 2 h. Afterwards, the as-prepared products were centrifugated and washed with deionized water and ethyl alcohol for at least 3 times and calcined at 300 °C for 3 h in air flow. And the products were denoted as ZnO-NDs.In order to load CuO onto the ZnO support via deposition precipitation method [24,25], ∼1 g of ZnO (including commercial Zinc oxide (ZnO-C), ZnO-NR1, ZnO-NR2 and ZnO-NDs) were dispersed into 40 mL deionized water under ultrasonic vibration condition. Then 10 mL deionized water containing 0.16 g of Cu(OAc)2·H2O were put into the solution containing ZnO and the pH values of the mixture were regulated to 8–8.5 using 0.1 mol/L K2CO3. And then the above-mentioned mixture was transferred from beaker to a Teflon-lined stainless-steel autoclave and heated at 80 °C for 12 h. Afterwards, the obtained products were centrifugated and washed with deionized water and ethyl alcohol for at least 3 times, which were transferred to an oven and kept at 80 °C for 12 h. The as-prepared catalysts are denoted as CuO/ZnO-C, CuO/ZnO-NR1, CuO/ZnO-NR2 and CuO/ZnO-NDs, respectively.TEM and SEM images were collected using JEM-2100F field emission microscopy and Nova Nano SEM 450, respectively. XRD patterns were measured using a Lab XRD-7000 s powder X-ray diffractometer with Cu Kα (λ = 0.154 nm) radiation. XPS measurements were carried out on an ESCALAB250Xi instrument. The charging effect was corrected by adjusting the binding energy of C1s to 284.6 eV. For XPS surface elemental compositional analysis/atomic fractions calculations, the relative ratio of atoms was calculated according to Eq. (1): (1) n i n j = I i I j × σ j σ i × E k j 0.5 E k i 0.5 (n: Number of atoms, I: Intensity (Integral peak area), E k: Kinetic energy, σ: Photoionization cross section) [26–28]. N2 adsorption-desorption curves were collected using Autosorb-iQ-C instrument (Quantachrome) at 77 K. To determine the loading amount of Cu species on ZnO supports, ICP-MS were measured using a 7900 (Agilent) instrument.The CO catalytic oxidation performance was evaluated in a fixed-bed reactor using ∼60 mg of screened catalysts (40–60 mesh) in a gas mixture (0.6% CO, 16.8% O2 and 82.6% N2) at a flow rate of 50 mL min−1. Before each measurement, the CuO/ZnO catalysts were pretreated under air at 250 °C for 1 h. The effluent from the reactor was analyzed by an online gas chromatograph (Techcomp-GC7900) equipped with a flame ionization detector with Ni reformer. For kinetic studies, the reaction rates were measured at 398.15 K, while the conversion of CO was kept below or within 20% to assure differential reactor conditions. The conversion of CO was obtained according to the Eq. (2): (2) CO conversion = A CO 2 A CO 2 + A CO where A(CO2) and A(CO) are the concentrations of carbon dioxide and carbon monoxide, respectively.The morphological characteristics of the ZnO supports were examined using SEM. As illustrated in Fig. S1, the ZnO-NR1 and ZnO-NR2 show rod-like morphology, with average length of 160 and 750 nm, respectively. And the rod morphology can be observed more clearly from the TEM images (Fig. S2, Fig. S5(b)). ZnO-NDs exhibit regular hexagonal disk-like morphology with a diameter of 1–2 μm and a thickness of 0.5–1 μm. In comparison, the commercial ZnO support exhibits plate/flake morphology with rough surface and the thickness of the flake is 30–60 nm. Further loading copper species does not alter the morphology of the ZnO supports (Fig. 1 ).The presence of copper species can be inferred from the elemental mapping images of CuO/ZnO-NDs, the copper species are uniformly distributed on ZnO nanodisks (002) polar facet (Fig. 2 ). Moreover, the copper species in the as-prepared catalysts were characterized using ICP-MS measurements, and the copper contents are found within the range of 4.2–5.2 wt% (Table S1), which agrees well with the nominal copper contents during catalysts preparation.Fig. S3 illustrated the XRD patterns of the powder ZnO supports before loading CuO nanoparticles. All the diffraction characteristic peaks could be attributed to the wurtzite ZnO, and the peaks at 31.77°, 34.42°, 36.25°, 47.54°, 56.60°, 62.86°, 66.38°, 67.96° and 69.10°, respectively correspond to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) planes, respectively of ZnO (JCPDS: no. 36–1451). It is obvious that the ZnO supports exhibit different intensity of (002) plane owing to the different morphology. As inferred from XRD patterns, the intensity ratio of (002) to (100) plane (I(002)/I(100)) for ZnO-NR2 is 0.66 due to its highest length to diameter ratio in the samples with nanorod morphology and it possesses the most exposed (100) plane. With shortening of the nanorod length, the intensity of (002) plane increased gradually, and the I(002)/I(100) for ZnO-NR1 is 0.810, which is even larger than that of commercial ZnO carrier. ZnO-NDs exhibit the largest I(002)/I(100) value of 1.186, due to the most exposed (002) planes induced by the nanodisk morphology. After loading copper species onto the powder ZnO supports and catalyst palletization, the XRD pattern of CuO/ZnO still shows the typical peaks of ZnO as well as a rather weak peak for CuO(111) [JCPDS: 45–0937] at 38.73o, showing well dispersed small-sized CuO (Fig. 3 ), which is well in agreement with the TEM results.Fig. S4(a, b) shows the representative TEM image of the as-prepared CuO/ZnO catalysts, showing CuO nanoparticles dispersed onto ZnO commercial support as reference. As can be seen in the particle sizes derived from the TEM image (Fig. S4(c)), the mean size of CuO is ∼3.1 ± 0.9 nm.N2 adsorption-desorption isotherms were collected to investigate the specific surface areas of the ZnO supports and CuO/ZnO catalysts. As demonstrated in Fig. 4(a), all the ZnO samples with different morphologies exhibit similar adsorption isotherms with negligible N2 uptake at low relative pressure and a rapid increase above relative pressure of 0.9, suggesting that the pores are mainly accumulated pores from the nanoparticles of ZnO. The Brunauer-Emmett-Teller specific surface areas are 50, 35, 3.7 and 43 m2g−1 for ZnO-NR1, ZnO-NR2, ZnO-NDs and commercial ZnO, respectively. It is demonstrated that the ZnO-NR1 exhibits the largest specific surface area due to its small size, and ZnO-NDs exhibit the smallest specific surface areas owing to their relatively big size. The specific surface area of the CuO/ZnO catalysts likewise exhibits the same trend as that of ZnO, and the specific surface area decreases a little after CuO loading as shown in Fig. 4(b).To investigate the catalytic performance of the CuO/ZnO catalysts, CO oxidation was measured in the normal mixture gas, and the effect of temperature on the CO conversion is illustrated in Fig. 5(a). All the prepared CuO/ZnO catalysts exhibit higher catalytic activity than CuO/ZnO-C in terms of much lower conversion temperature, and the catalytic activity exhibits a trend of CuO/ZnO-NDs > CuO/ZnO-NR1 > CuO/ZnO-NR2 > CuO/ZnO-C. For comparison, the values for 50% CO conversion (T50) are tabulated in Table S1, which reflect the same trend as that above mentioned. From another viewpoint, at a certain temperature of 150 °C, the CO conversion for CuO/ZnO-NDs is 52%, while the values are 33%, 25% and 25% for CuO/ZnO-NR1, CuO/ZnO-NR2 and CuO/ZnO-C, respectively. As can be analyzed, though the specific surface area of CuO/ZnO-NDs is much lower than that of CuO/ZnO-C derived from commercial ZnO support, its catalytic activity is more than 2 folds higher to that for CuO/ZnO-C at 150 °C. The specific reaction rates for all catalysts are provided in Table S1, which also reflect the same trend. The better catalytic performance of CuO/ZnO-NDs could partially be ascribed to its higher fraction of (002) polar plane (as inferred from XRD patterns and ZnO disk morphology from SEM and TEM images). And the ICP and XPS measurements (discussed below) show that ZnO-NDs attract and retain greater copper contents or CuO phase on the surface. From CuO/ZnO-NR1 to CuO/ZnO-NR2, both the specific surface area and (002) polar plane fractions lowered with increasing the length of ZnO nanorods, so that decreased catalytic performance was observed. In contrast, ZnO-NDs have the highest (002) plane fraction and thus generally show higher catalytic CO oxidation activity.Besides the catalytic activity, the stability of a catalyst is also an important parameter that determines its potential reuse and operational/catalyst replacement costs. As shown in Fig. 5(b), after six consecutive catalytic reaction cycles, the conversion of CO over CuO/ZnO-NDs remained ∼50% at 170 °C without apparent reduction of activity performance, implying its good cyclic stability.XPS measurements were carried out prior to reaction tests and after CO oxidation utilizing spent catalysts to investigate the relative surface composition, oxidation states for constituent elements and interaction between copper species and ZnO supports for the CuO/ZnO catalysts. Fig. 6(a) and Fig. 6(g) show the Cu2p spectra for ZnO NR2 and NDs presenting pre-mortem catalyst surface analysis before CO oxidation reaction, respectively. The Cu 2p3/2 peak is centered at 933.6 and 933.7 eV (with Spin-orbit splitting (S.O.·S) value at 19.9 eV) for ZnO NR2 and NDs, respectively, which is attributed to CuO/Cu2+ species. The related prominent signature CuO satellite peaks can also be seen [28]. Fig. 6(d) and Fig. 6(j) presents the Cu2p spectra for the same ZnO NR2 and NDs revealing post-mortem analysis of the catalyst surface after reaction tests, respectively. The Cu 2p3/2 (CuO) peak is centered at 933.13 and 933.63 eV for ZnO NR2 and NDs, respectively. Fig. 6(b) and Fig. 6(h) show the Zn2p spectra for ZnO NR2 and NDs before CO oxidation reaction, respectively. The corresponding binding energies of Zn 2p3/2 are centered at 1021.4 and 1021.7 eV (with S.O.·S value of 23.0 eV), which is attributed to Zn2+ of stochiometric ZnO. Fig. 6(e) and Fig. 6(k) show the Zn2p spectra for ZnO NR2 and NDs after reaction tests, respectively. The corresponding binding energies of Zn 2p3/2 (of ZnO) are centered at 1021.63 and 1022.1 eV, respectively [29]. Fig. 6(c) and 6(i) show the O1s spectra of the ZnO NR2 and NDs prior to the reaction tests, respectively. The O 1 s spectra can be deconvoluted into three peaks centered at ca. 530.0, 531.5 and 533.3 eV, corresponding to the lattice oxygen of ZnO/CuO, and adsorbed hydroxyl (OH) /surface oxygen for CuO/ZnO NR2 and CuO/ZnO-NDs catalysts, respectively. Fig. 6(f) and 6(l) show the O1s spectra of the ZnO NR2 and NDs after CO oxidation reaction, respectively. The above-mentioned O 1 s spectra corresponding three peaks are centered at ca. 530.25, 530.15, 531.5, 531.45, 532.5 and 533.45 eV for NR2 and NDs, respectively [30].For the XPS spectra, prior to CO oxidation, compared to CuO/ZnO-NR2, the binding energies of CuO/ZnO-NDs for Zn2p and Cu2p shift positively, demonstrating a stronger interaction between CuO and ZnO-NDs. It is suggested that a p-n heterojunction may be formed at the interface of CuO and ZnO, resulting in the shift of binding energy as observed from XPS measurement [31]. Herein, the polar plane of ZnO could induce stronger interaction between ZnO and CuO, and correspondingly the binding energy shifts positively.From XPS compositional analysis, we also calculated associated surface atomic fractions of both the catalysts prior to the reaction and after the CO oxidation tests. And from O1s peak areas analysis post deconvolution, we computed the ratio of the surface oxygen/lattice oxygen (Table S2). From the surface atomic fraction analysis of the corresponding constituent elements, compared with the amount of CuO/Cu2+ species on ZnO NR2, the contents of CuO/Cu2+ species is higher on ZnO-NDs catalysts surface before reaction tests, as can be seen in Cu2p spectra (Fig. 6(a, g)) for CuO-NDs is less noisy and indicated by Cu/O and Cu/Zn ratios (as well as ICP results). However, there is an observable reduction in the fractions of CuO/Cu2+ species after reaction tests for both catalysts, as demonstrated by Cu2p XPS peaks becoming weak in Fig. 6(d) and Fig. 6(j) and evident from variation in Cu/O and Cu/Zn ratios. Similarly for the samples before test, we also observed that the exposed Zn2+ of ZnO is higher on the surfaces of CuO/ZnO-NR2 than CuO/ZnO-NDs, probably due to ZnO surface coverage by higher surface Cu2+ species (as discussed above) and higher adsorbed active oxygen species (Fig. 6(c, i)) in CuO/ZnO-NDs catalysts and indicated by Zn/O ratios. The higher copper contents on ZnO-NDs are also verified by ICP results (Table S1), as well as qualitatively verified by strong signal of Cu2p XPS as discussed (Fig. 6(a, g)). Because the same nominal amount of copper precursor and precipitation protocol was strictly followed, the greater Cu content retention on ZnO disk polar facets (Fig. 2) may well be regarded as a function of the greater fractions of the polar surfaces of ZnO-NDs for which the copper interaction is stronger than non-polar facets of ZnO-NRs [19] which is an interesting new insight in this work. After the CO oxidation tests, XPS compositional analysis reveals that there is a reduction in the amount of Zn2+ (ZnO), as indicated by Zn/O and Zn/Cu ratios. For the samples before reaction, from O1s peak analysis, we see that the percentage of surface/adsorbed Oxygen is much higher on CuO/ZnO-NDs as can be seen by proportion of the deconvoluted peak areas, and which was verified by increase in corresponding peak area ratios of OH/O-(Zn, Cu) (Table S2).The abundance and availability of activated surface oxygen species on ZnO-NDs is an attributing factor for enhanced catalytic activity, in addition to the relatively higher Cu2+ species [23]. From O1s peak area and compositional analysis after the reaction tests, we observe an increase in surface oxygen species for CuO/ZnO-NR2 and CuO/ZnO-NDs catalysts. O1s peak area analysis reveals an enhancement in the percentage of surface/adsorbed oxygen after the tests, which is much higher for CuO/ZnO-NDs verified by the proportion of the deconvoluted peak areas in Fig. 6(f) and Fig. 6(l), and which was verified by increase in corresponding peak area ratios of OH/O-(Zn, Cu) (Table S2) for the two catalysts. Thus, Our XPS study clearly identifies CuO/Cu2+ species from Cu2p peak analysis. The Cu2+ species surface atomic fractions can be compared easily between the samples before the reaction separately and after the reaction separately, in turn, as the catalyst samples conditions for the two sets are same. Additionally, the weakening of Cu2p peak due to active oxygen on ZnO NDs is indicative of the greater active oxygen species available for catalyzing CO oxidation, as can simply be qualitatively inferred from the O1s peak analysis as well. Since, we have pretreated our catalysts under air before CO oxidation at 250 °C for 1 h, there is a single CuO phase to the exclusion of any other oxidation state present on the sample. Thus, Cu contents reflected in ICP can be considered for quantitative CuO analysis. Thus, ICP clearly identifies higher Cu contents on NDs polar facets than on NR2 nonpolar facets, indicating stronger interaction. In summary, the enhancement in adsorbed active surface oxygen species, together with higher CuO/Cu2+ species (and thus higher interfacial sites) for CuO/ZnO-NDs catalysts, results in corresponding increase in catalytic turnover.In addition to the XPS analysis after CO oxidation, spent catalysts were evaluated after CO oxidation reaction utilizing XRD, SEM and TEM to assess support morphology and ZnO, CuO stability. Fig. S5 shows the representative TEM images of the typical CuO hemispherical shaped nanoparticles anchored onto ZnO lattice. Fig. S5(a) highlights the typical situation for CuO/ZnO-ND in spent catalysts, while Fig. S5(b, c) shows the interface in our typical CuO/ZnO-NRs catalyst, before and after reaction tests respectively.Fig. S6 shows the XRD patterns of the spent catalysts after CO oxidation, which reveals that the ZnO and CuO peaks remain essentially unchanged, with CuO still existing as weak as before the tests, illustrating the absence of significant Cu growth. Fig. S7 shows the SEM images of the spent catalysts after CO oxidation tests, which revealed the final morphology of the zinc oxide support. No morphology change was observed on the ZnO-NDs support which exhibited the highest thermal/shape stability, followed by ZnO-NR2, ZnO-NR1 and ZnO-C. Fig. S8 shows the corresponding elemental mapping images of the spent CuO/ZnO-NDs catalysts, which exhibits the ZnO wurtzite crystal hexagonal morphology and presence of Zn, O and Cu elements after CO oxidation tests. The ZnO disk morphology is intact after the CO oxidation tests showing its thermal stability at high temperatures and the uniform distribution of copper is maintained over the entire nanodisk top of the polar (002) facet with only negligible growth. Additionally, Fig. S9 shows the elemental mapping images of CuO/ZnO-C catalysts identifying distribution of Zn, O and Cu elements before and after CO oxidation tests. Even after the reaction tests, the Cu species show uniform distribution analogous to that before the reaction, elucidating the absence of drastic growth.The catalytic activity of copper-based catalysts is always influenced by oxidation states of copper, generated on the surfaces when usually exposed to air or under O2-rich reaction conditions. In nanoparticle catalysts comprising three valence states of copper, Cu2O phase is reported as more active than either metallic Cu or CuO [32–34].Li et al. [20,23] investigated ZnO shape effects in ZnO disks and rods. Based on diffraction intensity ratio of (002) polar plane to (100) nonpolar plane (I(002)/I(100)), ZnO disks were found to have a higher value. From SEM imaging and shape geometrical interpretation, it was concluded that the proportion of polar planes (with preferably more oxygen vacancies) is higher for disks than rods. Such oxygen vacancies on polar planes are found to be more crucial for catalytic activity than surface area, which agree with our findings as well. Mclaren, et al. [35] also reported that hexagonal nanodisks (with higher fractions of the more active polar ZnO surfaces) were found to have more than five times higher activity in the photocatalytic decomposition of methylene blue than nanorods. Very recently, Lyu et al., [36] synthesized Cu x O/ZnO catalyst which agreed well with our observations that the active sites of the catalysts are exposed Cu x O particles and the heterojunction at the interface. It was shown that due to the lower work function of the ZnO, electrons transfer from ZnO to CuO, thus generating higher active Cu species. Such electron rich interface is beneficial for the activation and dissociation of oxygen molecules and hence CO oxidation reaction. As regards mechanism, Luo et al. [37] reported a surface activation process on an atomic scale for copper species during CO oxidation utilizing environmental TEM. Surface roughening and low-coordinated Cu atoms formation upon CO exposure, and quasi-crystalline CuO x phase formation upon O2 exposure were observed. CO molecules formed strong Cu-CO bonds with Cu atoms on the surface, and then intercalated into the lattice (2–3 atomic layers) with the aid of O2, which induced lattice expansion. It was concluded that, on active CuO x surface, the reaction proceeds as follows: first O2 molecules dissociate due to the undercoordinated Cu atoms, then this lattice O atom combines with adsorbed CO and CO2 is produced. Sarkodie et al. reported promotional effects of Cu x O on the activity of Cu/ZnO catalyst towards efficient CO oxidation [38]. The fraction of active CuO x phase directly correlated with the catalytic activity and more Cu+ species resulted in enhanced CO adsorption ability. CO is chemisorbed on Cu+ surface, forming Cu+-CO, and then CO drifted to the interface between copper oxide and zinc oxide, where CO combined with activated O atoms from oxygen vacancies in the support lattice and CO2 is formed.In our case, as stated, since all the catalysts were pretreated in air at 250 °C for 1 h before each test, Cu species should be in oxidation state of CuO. Indeed, XRD, XPS and TEM results clearly confirmed that it was in CuO state, hence CuO worked as active centers that catalyzed the CO oxidation. We observed that the Cu x O/ZnO derived from ZnO-NDs showed greater CuO contents for the same copper-deposition-precipitation route as confirmed by ICP and XPS. This highlights that NDs attracts greater CuO fractions on its polar facets and this correlates well with the enhanced catalytic activity. Additionally, more active adsorbed oxygen species on the catalyst surface was confirmed by XPS spectra, as compared to the other catalysts.Based on our own findings and the literature, a reaction mechanism of CO oxidation was proposed, as described in Fig. 7 . CO2 is generated via two possible pathways. The first pathway involves CO molecules from gas phase bonding to the Cu atoms in CuO nanoparticle surface, forming Cu2+-CO complex. Then the O2 molecules from the gas phase dissociate near the lattice O vacancy sites on CuO. Finally, the adsorbed CO molecules combine with the lattice O from CuO, which is replenished by molecular O2, to form CO2. In the second reaction pathway, the reaction proceeds at the interfacial sites between CuO and ZnO. The CO molecules either adsorbed on top surfaces of CuO drift to the interface between CuO and ZnO or adsorbed on the CuO interfacial perimeter sites, which react with the lattice O atoms on ZnO surfaces, thus forming CO2 molecules. Consumption of lattice O atoms creates O vacancies, which are filled by dissociating O2 molecules from gas phase. Due to the strong interaction between ZnO polar plane and CuO, the adsorption and oxidation of CO may be energetically more favorable in CuO/ZnO catalysts derived from NDs, resulting in better performance compared to other catalysts. The gas phase CO may adsorb on CuO nanoparticles, while the lattice Oxygen may be provided by CuO surface sites or ZnO at the interface between CuO and ZnO.In conclusion, we prepared a series of CuO/ZnO catalysts via altering the morphology of ZnO support and their catalytic activity towards CO oxidation was investigated. The CuO/ZnO-NDs with the most exposed (002) polar plane fraction derived from ZnO nanodisk exhibits more than 2 folds higher catalytic activity at 150 °C compared with Cu/ZnO-C using commercial ZnO as carrier. The resulting rationally tailored CuO/ZnO catalysts with nanodisk morphology were able to achieve complete CO conversion at 218 °C, which was much lower than that of Cu/ZnO-C (246 °C). The catalytic activity results indicated that the polar plane of ZnO played an important role in boosting CO oxidation via greater CuO retention and more adsorbed active oxygen, despite having lower specific surface area. This strategy can be applied to tailor ZnO based catalysts for industrially attractive applications such as CO2 hydrogenation reactions or be extended to other complex oxide-based catalysts.Shengnan Lu: Investigation, Writing-review & Editing.Houhong Song: Investigation, Writing-review & Editing.Yonghou Xiao: Resources, Conceptualization, Supervision, Project administration, Writing -review & Editing.Kamran Qadir: Resources, Conceptualization, Writing-review & Editing.Yanqiang Li: Writing -review & Editing.Yushan Li: Writing-review & Editing, Validation, Analysis.Gaohong He: Writing-review & Editing, Supervision.There are no conflicts of interest to declare.This research was supported by the Project of Central Government for Local Science and Technology Development of China (2022JH6/100100050), National Natural Science Foundation of China (21776028) and Liaoning Key Laboratory of Chemical Additive Synthesis and Separation (ZJKF2001). Supplementary material Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.colcom.2023.100698.

Catalytic oxidation of CO is an effective route to eliminate the pollution of automobile exhaust. Tuning the fraction of exposed polar and non-polar crystal facets in ZnO provides an effective strategy to tailor metal-oxide interfacial active sites. Herein, the morphology of ZnO support was tuned to alter the fraction of exposed (002) polar/(100), (101) non-polar facets. ZnO nanodisks and nanorods were prepared, followed by deposition of ∼3.1 ± 0.9 nm CuO nanoparticles. The impact of the ZnO morphology upon CuO deposition and catalytic activity towards CO oxidation were investigated. The CuO/ZnO derived from ZnO nanodisks (NDs) with the most exposed (002) polar plane exhibited more than 2 folds higher catalytic activity at 150 °C than that over CuO/ZnO with commercial ZnO support (CuO/ZnO-C), highlighting the morphology influence of ZnO. For its enhanced catalytic activity and cyclic stability, the CuO/ZnO-NDs could be a promising catalyst for CO oxidation.

Data will be made available on request.Nowadays, a large number of environmental problems caused by excessive CO2 emissions from anthropogenic sources occur frequently, such as melting glaciers, rising sea levels, drastic climate changes, etc [1]. Therefore, it is essential to develop an effective approach to mitigate carbon emissions. Considering that more than 60 % of the current energy demand comes from fossil fuel-based power plants, capture and sequestration (CCS) as well as capture and utilization (CCU) become indispensable for the mitigation of carbon emissions in the short term [2]. Among the two alternatives, CCU is the most promising solution and can be closely integrated with current industrial processes such as power plants and cement manufacturing [3]. However, the energy consumption in the separation, enrichment and transport of CO2 significantly increases the total cost of the CCU process.Recently, integrated CO2 capture and utilization (ICCU) technology has been proposed, which reduces the cost of the overall process by eliminating CO2 transport and storage [4]. This technology uses dual function materials (DFMs) and is gaining increasing interest by combining CO2 adsorption and utilization in a single reaction unit [5–7]. ICCU is a process of two periods or steps in which the feeding is varied in each one. In the first period, the DFM is saturated with CO2 by feeding an exhaust gas stream with a diluted concentration of CO2 (4–15 %). Then, in the second period, a reducing agent is introduced and the conversion of the adsorbed CO2 and the regeneration of the adsorption sites take place.Depending on the reducing agent, the composition of the DFM and the operating conditions different process can take place: dry reforming of methane (ICCU-DRM) [8,9], reverse water gas shift (ICCU-RWGS) [10,11] or methanation (ICCU-methanation) [12–14]. In addition, ICCU technology can also be an effective solution for storing surplus electrical energy in chemicals. For that, the energy surplus from renewable sources would be used to produce H2 from the hydrolysis of H2O and then reacted with the captured CO2. Therefore, ICCU can address the problem of intrinsic intermittency of renewable sources [15]. Among the different options, the ICCU-methanation technology is the most promising and its operation would approach a carbon neutral cycle. The first work of ICCU-methanation was published in 2015 [16], and since then the publications on CO2 adsorption and CH4 hydrogenation cycles are growing exponentially [17].DFMs are a combination of a compound based on Na [18,19], Ca [16,20], Mg [13,21] or K [13,22], as adsorbent, and on Ru [18,23], Ni [22,24] or Rh [25] as catalytic phase. Both phases are usually supported on a large surface area support. Specifically, γ-Al2O3 is proposed as the best support [26]. In a previous work, we studied the effect of nickel loading in DFMs based on Na or Ca [20]. The highest activity was presented by DFM based on Na as adsorbent with 10 % Ni. Next, the addition of different promoters was studied. We conclude that the yield of CO2 adsorption and hydrogenation to CH4 in DFMs Ni-Na/Al2O3 is favored by the presence of a promoter [24]. Specifically, the sample promoted with Ru presents the best results. The presence of Ru creates synergistic aspects with Ni. It improves the reducibility of nickel and increases the ability of DFM to chemisorb H2.In the development of adsorbent materials with high capacity, reversibility and stability, Al-Mamoori et al. [27] studied Na and K doping of Ca-based adsorbents. The authors concluded that the addition of K and Na improved the performance of CaO since these materials had high CO2 adsorption capacity, fast kinetics and good stability above of 300 °C. On the other hand, Lee et al. [28] performed a comparative study of the adsorption and regeneration kinetics of Na2CO3 doped and conventional CaO adsorbents. In this study, the authors concluded that the addition of sodium carbonate to the calcium adsorbent can improve the cyclic stability of CO2 adsorption with fast kinetics. Recently, we have studied, for the first time in the scientific literature, the joint presence of Na and Ca in the same DFM [29]. DFMs are based on 4 % Ru as catalytic phase and 16 % adsorbent phase with different Na2CO3/CaO ratios. It has been possible to modulate the basicity and improve the Ru dispersion by varying the ratio and improve the CH4 production.In this work, the possibility of improving the activity of Ni-based DFMs is analyzed. For that, DFM 10Ni-16Na/Al2O3 is selected as reference. In the first place, the influence on the physicochemical properties and on the activity in the cyclic process of the joint presence of Na and Ca is analyzed. It is analyzed if it is possible to modulate the basicity and promote the CH4 production as in the DFMs based on Ru. Secondly, the influence of the incorporation of a small amount (1 %) of Ru to the reference DFM is studied. Note that the price per unit of mass of Ru is around 935 times higher compared to Ni (September 2022). The synergies that take place between both metals are studied. Third, the joint presence of Na and Ca and the simultaneous Ru incorporation is analyzed. For this, all the DFMs are extensively characterized and their activity is analyzed in CO2 adsorption and CH4 hydrogenation cycles. Finally, it is studied how hydrothermal aging in the presence of O2 modifies the physicochemical properties and the activity of the DFMs studied.Four DFMs were prepared by wetness impregnation. Firstly, appropriated amounts of Ca(NO3)2·4H2O (Merck) and/or Na2CO3 (Riedel de-Haën) were impregnated over γ-Al2O3 (Saint Gobain). The impregnated powder was dried at 120 °C overnight and then calcined at 550 °C for 4 h (1 °C min−1). Afterwards, Ru(NO)(NO3)2 (Sigma Aldrich) and/or Ni(NO3)2·6H2O (Fluka) were impregnated over the previous calcined powder. After drying at 120 °C, the DFMs were stabilized by calcining again at 550 °C for 4 h (1 °C min−1). Table 1 lists the complete formulation of prepared DFMs and their nomenclature used in this work. The 10 %Ni/Al2O3 sample was also prepared as a reference.The calcined DFMs were placed in their granulated form (0.3–0.5 mm) in a quartz tube reactor and were heating from RT to 500 °C at 10 °C min−1 during 1 h under 10 % H2/Ar (50 cm3 min−1).For studies of hydrothermal aging in the presence of oxygen, the DFMs were placed in their granulated form (0.3–0.5 mm) in a quartz tube reactor placed into a horizontal furnace. The DFMs were aged under 5 % H2O and 5 % O2 in Ar for 3 h, at a total flow rate of 550 cm3 min−1 at 550 °C.The specific surface, pore diameter and pore volume were determined from the N2 adsorption-desorption analysis. The reduced DFMs were pre-purged with nitrogen for 10 h at 300 °C using SmartPrep degas system (Micromeritics). Then the analysis were carried out at the nitrogen boiling temperature (−196 °C) using an automated gas adsorption analyser (TriStar II, Micromeritics).X-ray diffraction spectra were obtained in a Philips PW1710 diffractometer. The samples were finely ground and were subjected to Cu Kα radiation in a continuous scan mode from 5° to 70° 2θ with 0.02° 2θ per second sampling interval.Metallic dispersion was determined using the H2 chemisorption method in a Micromeritics ASAP 2020 equipment. Prior to the experiments, the samples (0.2 g) were reduced with pure H2 for 2 h at 500 °C. After that, the samples were degasified at the same temperature for 90 min. Finally, H2 was dossed for obtaining the adsorption isotherm at 35 °C.The morphology of the samples was analysed by transmission electron microscopy (TEM) in a JEM-1400 Plus instrument using a voltage of 100 kV. The reduced DFMs were dispersed in distilled water ultrasonically, and the solutions were then dropped on copper grids coated with lacey carbon film. In addition, High Angle Annular Dark Field (STEM-HAADF) images were obtained in a CETCOR Cs-probe-corrected Scanning Transmission Electron Microscopy microscope (ThermoFisher Scientific STEM, formerly FEI Titan3) operating at 300 kV and coupled with a HAADF detector (Fischione). The instrument had a normal field emission gun (Shottky emitter) equipped with a SuperTwin lens and a CCD camera. The samples were mixed with ethanol solvent and dropped onto a holey amorphous carbon film supported on a copper grid. In order to obtain spatially resolved elemental chemical analysis of the samples, the TEM apparatus was also equipped with an EDAX detector to carry out X-ray Energy Dispersive Spectroscopy (EDS) experiments. A 2k x 2k Ultrascan CCD camera (Gatan) was positioned before the filter for TEM imaging (energy resolution of 0.7 eV). The acquisition time for the analysis was 50 ms per spectrum and the used energy dispersion was 0.2 eV pixel−1.H2-TPD experiments were performed in a Micromeritics AutoChem II equipment. The samples (0.1 g) were pretreated at 500 °C (10 °C min−1, 1 h) under 5 % H2/Ar (50 cm3 min−1) and then cooled down to 40 °C. After that, a 50 cm3 min−1 stream of pure hydrogen was fed long enough for complete saturation (60 min). Subsequently, DFMs were flushed out with Ar for 60 min in order to remove physisorbed H2. Finally, the desorption was conducted increasing temperature up to 750 °C (10 °C min−1). The hydrogen desorbed was continuously monitored with a TCD detector.CO2-TPD experiments were carried out in a Micromeritics AutoChem II equipment. The samples (0.1 g) were pretreated at 500 °C (10 °C min−1, 1 h) under 5 % H2/Ar (50 cm3 min−1) and then cooled down to 50 °C. Then, the samples were exposed to a gas stream composed of 5 % CO2/He (50 cm3 min−1) for 1 h at 50 °C to saturate the catalyst with CO2. Subsequently, the samples were exposed to He (50 cm3 min−1) for 90 min to remove the physically adsorbed CO2 and, finally, they were heated from RT to 1000 °C at 10 °C min−1. The CO2 released was measured by mass spectrometry (HIDEN ANALYTICAL HPR-20 EGA).TPSR experiments were carried out in a quartz tube reactor placed in a horizontal furnace. The DFMs (0.3 g) were pretreated under 5 % H2/Ar at 500 °C (10 °C min−1, 1 h), and then, the sample was cooled down to 50 °C. Subsequently, the DFMs were exposed to a gas stream composed of 25 % CO2/Ar with a flowrate of 700 cm3 min−1 for 20 min at 50 °C to saturate the catalyst with CO2. Previous to the experiments, the DFMs were purged in Ar (700 cm3 min−1) for 90 min to remove the physically adsorbed CO2. Afterwards, the samples were heated from 50° to 600 °C at 10 °C min−1 in a 5 % H2/Ar mixture with a flowrate of 700 cm3 min−1. The MultiGas 2030 FT-IR analyzer was used to quantify the formation of products during the reduction in the reactor effluent gas.H2-TPR experiments were carried out in a Micromeritics AutoChem II equipment. The samples (0.1 g) was loaded in a quartz tube reactor and pretreated at 350 °C for 15 min under 5 % O2/He (30 cm3 min−1) and then cooled down to 35 °C. The reducing gas flow was 30 cm3 min−1 of 5 % H2/Ar and the temperature was increased from 35° to 1000°C with a heating rate of 10 °C min−1. The water formed during reduction was trapped using a cold trap and the hydrogen consumption was continuously monitored with a TCD detector.The catalytic activity in the cyclic operation of CO2 adsorption and hydrogenation to CH4 of the DFMs was evaluated in a vertical tubular stainless steel reactor. The reactor was loaded with 1 g of DFM whose particle size was between 0.3 and 0.5 mm. Prior to the cycles, the DFM were reduced with a stream composed of 10 % H2/Ar. First, the temperature is increasing from RT to 500 °C and then is maintaining for 60 min. Once the DFMs were reduced, the temperature is stabilized at 280 °C in Ar and cycles of CO2 adsorption and hydrogenation to CH4 were started. The reaction temperature was varied between 280 and 520 °C, with intervals of 40 °C. At each temperature several isothermal cycles have been carried out until obtaining three consecutive quasi-identical cycles (cycle-to-cycle steady state). During the adsorption period, a stream with 10 % CO2 (Ar balance) was fed for 1 min, followed by a purge with Ar for 2 min to remove weakly adsorbed CO2. Next, during the hydrogenation period, a stream consisting of 10 % H2/Ar was fed for 2 min, followed by an Ar purge for 1 min before starting the adsorption period again. The total flow rate in the whole experiment was 1200 cm3 min−1, which corresponds to a space velocity of 45000 h−1. The flue gas composition was continuously measured using the MultiGas 2030 FT-IR analyzer for quantitative analysis of CO2, CH4, CO and H2O.The amount of CO2 stored was calculated from Eq. (1). For that, the amount that leaves the reactor was subtracted from the amount fed. To determine the amount of CO2 fed, the stream from the feed system was led directly to the analyser. This profile corresponds to the actual CO2 input that was fed to the reactor. (1) stored CO 2 ( μ mol g − 1 ) = 1 W ∫ 0 t [ F CO 2 in ( t ) − F CO 2 out ( t ) ] d t The CH4, H2O and CO productions were calculated from the following expressions: (2) Y CH 4 ( μ mol g − 1 ) = 1 W ∫ 0 t F CH 4 out ( t ) d t (3) Y CO ( μ mol g − 1 ) = 1 W ∫ 0 t F CO out ( t ) d t (4) Y H 2 O ( μ mol g − 1 ) = 1 W ∫ 0 t F H 2 O out ( t ) d t CH4 selectivity is determined by relating the CH4 and CO productions since they were the only carbon based products that were detected: (5) S CH4 ( % ) = Y CH 4 Y CH 4 + Y CO × 100 The carbon balance check was carried out from the following expression: (6) s CB ( % ) = ( Y CH 4 + Y CO s t o r e d C O 2 − 1 ) × 100 Table 2 lists the textural properties of the alumina and the four DFMs synthesized after the reduction protocol. The alumina used has a specific surface area of 197 m2 g−1 and the specific surface values of the DFMs are between 101 and 119 m2 g−1. Al2O3 has a mean pore size of 122 Å and a pore volume of 0.62 cm3 g−1. On the other hand, the DFMs present larger mean pore size values (147–158 Å) and smaller pore volume values (039–0.46 cm3 g−1). For that, the decrease in the specific surface area is attributed to a lower proportion of alumina in the formulation and to a partial blocking of the pores caused by the incorporation of the adsorbent and the metal. Comparing the DFMs with each other, it is observed that the joint presence of Na and Ca decreases the specific surface to a greater extent (101–103 vs. 113–119 m2 g−1). Note that the total adsorbent content is the same in all DFMs (16 %). The joint presence of Na and Ca reduces the pore volume to a greater extent (0.39–0.40 vs. 0.43–0.46 cm3 g−1). This fact indicates that the impregnation of two adsorbent phases blocks a greater number of pores. Fig. 1 collects the diffraction spectra of the alumina used as support and of the reduced DFMs. The spectrum of alumina shows broad peaks of low intensity, characteristic of an amorphous solid. This background is also seen in all the DFMs spectra, in which only two additional peaks appear at 44.6 and 51.8° 2θ belonging to metallic nickel. In contrast, no peaks are identified for Na and Ca phases, which indicates that they are highly dispersed and/or they present poor crystallinity. Furthermore, no peaks are detected belonging to Ru-based phases in 1Ru10Ni-Na and 1Ru10Ni-NaCa formulations, indicating a high dispersion. Table 3 collects the metallic dispersion values estimated by H2 chemisorption of the DFMs. Disparate values (2.2–5.9 %) are obtained depending on the composition of the DFMs. The Ru-promoted DFMs show a noticeably higher dispersion (5.3–5.9 %) compared to the bare Ni DFMs (2.2–2.4 %). Taking into account that ruthenium is also reduced and both compounds can chemisorb H2 with a stoichiometric H/Me= 1/1 (Me=Ni or Ru) [30], these results denote that Ru promotes metallic phases dispersion.In order to analyze if the increase in dispersion is only due to the presence of a higher metallic content or if a synergistic effect is also being created between both metals, the chemisorbed H atoms for each DFM in the H2 chemisorption experiments are also determined and collected in Table 3. The bimetallic DFM 1Ru10Ni-Na presents a dispersion of 5.9 %. This dispersion corresponds to an H chemisorption capacity of Ru and Ni of 106.3 μmol g−1. In a previously prepared monometallic Ru-DFM under similar conditions (4Ru-Na, [29]) a dispersion of 19.6 % has been obtained, which corresponded to an H chemisorption capacity of 77.6 μmol g−1. Considering that 4Ru-Na DFM was composed of 4 % Ru and in the 1Ru10Ni-Na DFM the ruthenium content is of 1 %, we estimate that the ruthenium in 1Ru10Ni-Na DFM would be capable of chemisorbing 19.4 μmol g−1. Therefore, the rest of H chemisorbed in the bimetallic sample (86.9 μmol g−1) should correspond to nickel phase. This H chemisorption capacity is significantly higher to that observed for monometallic Ni-based sample (10Ni-Na = 37.5 μmol g−1). Therefore, the presence of Ru seems to promote the H chemisorption capacity of nickel.Similar results are obtained for DFMs with the joint presence of Na and Ca. The 10Ni-NaCa monometallic DFM has an H chemisorption capacity of 40.9 μmol g−1, while the contribution of nickel in the bimetallic DFM is 70.9 μmol g−1. The contribution of ruthenium (24.6 μmol g−1), in 1Ru10Ni-NaCa is estimated from the 4Ru-NaCa DFM [29]. Based on these results, the synergistic effect between nickel and ruthenium is confirmed, which leads to a larger exposed metallic nickel surface with respect to monometallic 10Ni-NaCa and 10Ni-Na samples. At this point, some authors have indicated that due to the higher melting point of ruthenium (2334 °C) compared to nickel (1455 °C), Ru can act as a shell with Ni in the nucleus, remaining protected it from sintering during the calcination stage [31,32]. On the other hand, the presence of Na or Na/Ca together does not seem to have a significant influence on the metallic dispersion. Fig. 2 collects the TEM micrographs of the DFMs. Based on the assignment made in previous works [33] for a similar formulation (Ni/Al2O3 catalysts), where the authors assigned the dark areas in the micrographs to the Ni metal particles, while the gray areas to the γ-Al2O3 support, we assign the darker spherical areas in the Ni-only DFMs (10Ni-Na and 10Ni-NaCa) to Ni particles. In fact, this assignment is in line with their higher atomic number (28) with respect to the adsorbents (Na = 11 or Ca = 20) and the support Al = 13).Regarding to the bimetallic DFMs (1Ru10Ni-Na and 1Ru10Ni-NaCa), the dark spherical particles are assigned to particles of Ni, Ru, or alloys of both in line with the observed by Li et al. [31] for the reduced Ni-Ru nanoparticles supported on SiO2 by TEM images. Similar assignment was made by Zhou et al. [34] in their study of bimetallic Ru–M/TiO2 (M=Fe, Ni, Cu, Co) nanocomposite catalysts, where Ru-Fe, Ru-Ni and Ru-Co nanoparticles were observed in darker contrast. In fact, this assignment was further verified by EDX in their study. Thus, these results suggest the conformation of bimetallic Ni-Ru particles in the bimetallic DFMs (1Ru10Ni-Na and 1Ru10Ni-NaCa).Further evidences of this aspect as well as of the disposition of the metals and the adsorbent phases on the support have been obtained by HAADF images and EDX maps. Fig. 3 shows the EDX maps obtained for each compound (Ru, Ni, Na, Ca and Al) in the DFM with the more complex composition (1Ru10Ni-NaCa). As can be observed, both adsorbent phases (Na and Ca) are homogenously disposed, covering the entire surface of the support (Al). In contrast, Ru and Ni coexist and are preferentially placed in the central area of the image. In fact, smaller Ru nanoparticles seem to be deposited on large Ni particles. Thus, these results confirm the existence of bimetallic Ni-Ru particles, as previously suggested by TEM images (Fig. 2).An average size of metallic or bimetallic particles is determined by counting at least 100 particles (Fig. 2) and the values are shown in Table 2. Very close values are obtained for the four DFMs (9–11 nm), although the lowest values are presented by the DFMs with ruthenium in its formulation. At this point, a greater difference in metallic dispersions (estimated by H2 chemisorption) compared to particle sizes is observed. It is suggested that some of the particles observed in the TEM images of the DFMs are not completely reduced.In order to clarify this discrepancy, the hydrogen adsorption capacity by H2-TPD is also determined. Fig. 4 shows the H2-TPD profiles of alumina and the synthesized DFMs. It can be seen that all the profiles exhibit two bands, before and after 450 °C. The bands below 450 °C are attributed to H2 chemisorbed on metallic particles (type I). On the other hand, the bands at higher temperatures are associated with the H2 desorption from the subsurface layers of the alumina or with the spillover phenomenon (type II) [35,36]. The alumina profile does not show variation below 350 °C. At higher temperatures, an intense band appears, which is related to the dehydroxylation of alumina. On the other hand, the bands of the DFMs at low temperatures can be divided into several peaks. Ewald et al. [37], in their study of Ni/Al2O3 catalysts, assigned the main peak to hydrogen chemisorbed on the Ni surface. On the other hand, the peaks located at lower temperatures were attributed to hydrogen adsorbed on the corners of large Ni particles or on better dispersed particles. In DFMs, the position of the main peak shifts towards lower temperatures and its intensity increases with the addition of Ru. This fact shows a greater number of exposed metal atoms. The amount of H2 desorbed below 450 °C is quantified and shown in Table 3. It can clearly be seen that in the DFMs promoted with Ru the amount desorbed is more than double compared to the DFMs only with nickel. Note that the ability of DFMs to supply dissociated hydrogen increases markedly with the addition of Ru. The increase in the number of exposed metal atoms has also been reported by other authors who incorporated Ru [38], Cr [39] or Fe [40] to Ni/Al2O3 catalysts.Based on H2 chemisorption (Table 3), TEM micrographs (Fig. 2) and H2-TPDs experiments (Fig. 4), it can be concluded that the Ru incorporation in the DFMs notably increases the dispersion and, consequently, the metallic surface area exposed. On the other hand, the presence of Na or the joint presence of Na and Ca does not significantly modify the disposition of the metals (Ni and Ru).The basicity of the samples is evaluated by CO2 temperature programmed desorption. Fig. 5 shows the evolution of the CO2 signal measured by a mass spectrometer (m/e=44) as a function of temperature during the CO2-TPD experiments of alumina, the reference sample only with nickel (10Ni) and the DFMs. Depending on the desorption temperature, weak, medium and strong sites are distinguished. Weak basic sites are unstable and easily decompose below 200 °C. Medium basic sites decompose between 200 and 600 °C while strong basic sites are highly stable and decompose at temperatures above 600 °C.Alumina exhibits a small desorption peak at low temperature. This peak is assigned to the decomposition of bicarbonates resulting from the interaction between CO2 and the surface hydroxyl groups of the alumina [41,42]. The incorporation of 10 % nickel (10Ni) does not modify the profile, obtaining also only a small desorption peak at low temperature. Porta et al. [13] did not observe a modification of the desorption profile with the incorporation of Ru over alumina support.The incorporation of Na, or the joint incorporation of Na and Ca, notably modifies the desorption profiles. With the incorporation of the adsorbent, the amount of CO2 desorbed increases remarkably. Porta et al. [14] studied by infrared spectroscopy the nature of CO2 adsorbed on DFMs based on potassium and barium as adsorbents and on ruthenium as metal. The authors only observed bands belonging to adsorbed carbonates on the adsorbent phases and did not observe surface species typical of CO2 adsorption on alumina. Based on these results, they were able to conclude that the adsorbent phases were covering the surface of the alumina.In order to compare the evolution of the different basic sites concentration depending on sample formulation, Table 4 shows the weak, medium, strong and total basicity for the reference samples and the DFMs. Alumina and the reference sample with nickel (10Ni) only show weak basicity. Also, as mentioned above, the amount desorbed is much lower compared to DFMs. The four synthesized DFMs present the three types of basicity. The addition of 1 % ruthenium does not notably modify the distribution of the basic sites. However, the joint presence of Na and Ca promotes medium basicity and strong basicity, and therefore total basicity (471–446 vs. 372–387 μmol g−1). This aspect is of special interest since in the cyclic process of CO2 adsorption and hydrogenation to CH4, only the CO2 that remains adsorbed during the storage period is the one that can be converted into methane in the hydrogenation period.The reactivity of CO2 adsorbed on DFMs is studied by temperature programmed surface reaction (TPSR) experiments. First, the sample is reduced at a controlled temperature up to 500 °C with an isothermal step of one hour, to simulate the state of the DFMs in reaction. Next, the DFMs are saturated with CO2 and, finally, a stream with H2 is passed while the temperature is increased with a ramp of 10 °C min−1 up to 600 °C. During the experiment, the reactor outlet gases are continuously measured by an FTIR analyzer. Fig. 6 shows the evolution of CH4 production with temperature for the four synthesized DFMs. In general, a main peak with a shoulder that extends up to 600 °C is observed. The main peak is assigned to the hydrogenation of the CO2 adsorbed on the basic sites with weak strength, while the shoulder is assigned to the adsorbed on the sites with medium and strong strength basic sites. The DFMs without ruthenium in their composition (10Ni-Na and 10Ni-NaCa) show the start of CH4 production around 300 °C. On the other hand, the incorporation of Ru leads to the start of production at a lower temperature (250 °C). The amount of CH4 produced in each experiment is obtained by integrating the profiles and the values are shown in Table 5. The presence of ruthenium in the DFM, in addition to shifting production to a lower temperature, increases the amount of CH4 produced. On the other hand, the presence of Na or the joint presence of Na and Ca does not significantly modify the TPSR profiles.If the values of total basicity obtained (Table 4) are compared with the production of CH4 in the TPSR (Table 5), in all cases lower values are obtained for the TPSR experiments. In fact, methane/basicity ratios of 0.30–0.47 are obtained. Note that part of the CO2 is released at low temperatures when DFMs are not active for CO2 methanation. The highest ratios (0.43–0.47) are presented by the DFMs with ruthenium in their composition. Consequently, the DFMs 1Ru10Ni-Na and 1Ru10Ni-NaCa show the start of production at lower temperatures (Fig. 6), a greater amount of CH4 produced (Table 5) and the highest methane/basicity ratios. The best results are attributed to the synergies between Ni and Ru and to the higher intrinsic activity in CO2 methanation of ruthenium compared to nickel [43], which displaces the start of methanation at lower temperatures.The reducibility of the prepared samples is determined by H2 temperature programmed reduction (H2-TPR). Fig. 7 shows the evolution of H2 consumption during the H2-TPR experiments of the DFMs prepared together with the reference sample without adsorbent (10Ni). The H2 consumption profile for the 10Ni reference sample (Fig. 7) can be deconvolved into three main contributions. According to the reported in the literature [44], the reducible Ni2+ species are generally classified as: α-NiO, β-NiO and γ-NiO. The α-NiO species are related to free and easily reducible NiO species. β-NiO species are reduced at intermediate temperatures and are related to Ni2+ species that are not fully integrated into the nickel aluminate spinel structure. Finally, the reduction of the γ-NiO species occurs at higher temperatures, related to the Ni that integrates into the nickel aluminate structure. These species are reduced to metallic nickel according to the following reactions: NiO+H2⇆Ni+H2O and NiAl2O4 +H2⇆Ni+Al2O3 +H2O, with Ni/H2 = 1 stoichiometry. In fact, the experimental H2/Ni ratio obtained is 1.01. This fact indicates that nickel is found as Ni2+, either as NiO, NiAl2O4 or a mixture of both.With the incorporation of the adsorbent, the profiles vary significantly. H2 consumption increases and shifts to lower temperatures. For the DFMs with only nickel, 10Ni-Na and 10Ni-NaCa, the H2/Ni ratio is 2.5 and 2.1, respectively. On the other hand, in the DFMs with ruthenium in their formulation, this is also reduced according to the RuO2 + 2 H2⇆Ru+ 2H2O reaction. Therefore, Ru is reduced with a H2/Ru ratio of 2. In both DFMs there is 1 % Ru, which consumes 99 μmol g−1 of H2 in its reduction. If the H2 consumption relative to ruthenium is subtracted from the total, the H2/Ni ratio results in 2.7 and 2.2, for the DFMs 1Ru10Ni-Na and 1Ru10Ni NaCa, respectively. At this point, an additional phenomenon is evident that contributes to the increase in hydrogen consumption observed for the four DFMs. Fig. S1 shows the evolution of the concentration of NO, NH3 and CH4 measured by the FTIR analyzer during the pretreatment H2-TPR of the TPSR experiment. The formation of NO is related to the decomposition of residual nitrates belonging to the adsorbent and metal precursors that have not been completely decomposed during the calcination step. The start of NH3 formation is detected at higher temperatures. NH3 formation requires the noble metal in its metallic state, so NH3 formation can be used as an indirect way of determining the temperature at which nickel begins to reduce. On the other hand, the formation of CH4 is attributed to the hydrogenation of the CO2 adsorbed on the samples, due to exposure to the environment before the experiment.With this additional information, the hydrogen consumption profiles of the DFMs can be correctly interpreted (Fig. 7). H2 consumption is related to the reduction of metals, residual nitrates and carbonates. On the other hand, the displacement of consumption at lower temperatures is assigned to a lower interaction between nickel and alumina due to the presence of the adsorbent. Keep in mind that in the DFM synthesis process, the adsorbent is incorporated in the first impregnation and the metals in the second.The H2 consumption profiles are different depending on the composition of the DFMs (Fig. 7). The DFMs 10Ni-Na and 10Ni-NaCa show the consumption start at 345 and 375 °C. In this peak, the reduction of the α-NiO species occurs, as well as the hydrogenation of the nitrates and carbonates. The peak centered at 600 °C is assigned to the reduction of β-NiO species and the peak centered at 770 °C to the reduction of γ-NiO species. On the other hand, with the addition of 1 % Ru (1Ru10Ni-Na and 1Ru10Ni-NaCa) the start of consumption shifts to 250 and 280 °C and a peak with a larger area is obtained. In Fig. S1 it can be seen how in the DFMs with Ru in their composition, the production of NH3 and CH4 shifts towards lower temperatures and the amounts produced increase. On the other hand, the reduction shoulder of the β-NiO species disappears. However, the peak centered at 770 °C assigned to the reduction of the γ-NiO species is detected. It is evident that Ru promotes Ni reduction at lower temperatures. This fact is of great importance since Ni and Ru in their metallic state are the active sites for the hydrogenation of CO2 and, therefore, the hydrogenation activity would be expected to increase with the amount of reducible nickel.The presence of γ-NiO species is detected in all four DFMs. The H2 consumption of these species is approximately 250 μmol of H2 g−1, therefore 16 % of nickel is in the form of spinel and is not reduced during pretreatment. On the other hand, part of the β-NiO species in the DFMs without ruthenium are not reduced either. At this point, the percentage of nickel reduced in the DFMs 1Ru10Ni-Na and 1Ru10Ni-NaCa is 84 %, while in the DFMs 10Ni-Na and 10Ni-NaCa this percentage decreases to 70 %. This difference explains the similar particle sizes obtained in the TEM images. It is confirmed that some of the particles observed in the TEM images of the DFMs are not reduced and therefore are not capable of chemisorbing H2. The calcination-reduction temperature selected in this study (550–500 °C), has been optimized in a previous work [24]. Finally, based on H2-TPRs (Fig. 7 and Fig. S1), it can also be concluded that the reduction pretreatment, carried out before the activity tests, completely eliminates the residual nitrates and carbonates.The catalytic activity is evaluated in cycles of CO2 adsorption and hydrogenation to CH4. Each cycle has a total duration of six minutes. First, it starts the adsorption period by introducing a stream of 10 % CO2/Ar for one minute, followed by a two-minute purge. The hydrogenation period is then started by introducing a stream of 10 % H2/Ar for two minutes and an additional one-minute purge takes place before starting the next cycle. Fig. 8 shows the evolution of the concentration of CO2, CH4, H2O and CO in a complete cycle at 400 °C for the four DFMs. The detailed description of the temporal evolution of reactants and products, as well as the mechanism, is detailed in previous works [18,20,29], as well as its modeling and simulation [45,46]. Table 6 collects the reactions suggested in each period. In the adsorption period, CO2 remains adsorbed forming carbonates. CO2 can be adsorbed on oxide (Eq. 7 and Eq. 8) or hydroxide sites (Eq. 9 and Eq. 10). On the other hand, in the hydrogenation period, the carbonates are decomposed by the presence of hydrogen (Eq. 11 and Eq. 12), the desorbed CO2 is hydrogenated to CH4 (Eq. 13) and part of the produced water remains adsorbed forming hydroxides (Eq. 14 and Eq. 15).If the evolutions of the reactants and products are compared (Fig. 8), depending on the DFM formulation, different concentrations are observed. For a more detailed interpretation, the quantities of CH4 and CO produced (Eq. 2 and Eq. 3, respectively), at each operating temperature, are determined and plotted in Fig. 9. In all cases, the carbon balance closes with an error less than ± 5 % (Eq. 6) and the H2O/CH4 ratio is very close to 2, in agreement with the stoichiometric of the Sabatier reaction (Eq. 13).CH4 production (Fig. 9a) shows a maximum around 400 °C. At low operating temperatures, the ability to extract the stored CO2 is limited (Fig. 5 and Table 4), while at high temperatures the storage capacity is limited during the adsorption period due to the destabilization of carbonates. The DFM 10Ni-Na presents the lowest CH4 production in the temperature range studied, even so, it reaches 172 μmol g−1 at 400 °C.The joint presence of Na and Ca (10Ni-NaCa) promotes the CH4 production at medium-high temperatures (360–520 °C). In the CO2-TPD experiments (Fig. 5 and Table 4) it has been observed that the presence of both adsorbents promotes total basicity since it increases the sites with medium and strong strength. At this point, this DFM is capable of retaining a greater amount of CO2 in the storage period, which is reflected in a greater CH4 production in the hydrogenation period. Specifically, the DFM 10Ni-NaCa produces 202 μmol g−1 at 400 °C.The incorporation of 1 % Ru (1Ru10Ni-Na) to the DFM also improves CH4 production. On this occasion, it is promoted in the entire temperature range studied (280–520 °C), although especially at low-intermediate temperatures (280–440 °C). The DFM 1Ru10Ni-Na reaches to produce 250 μmol g−1 at 400 °C. The production enhancement is assigned to a synergistic effect between Ru and Ni. As has been observed by H2-TPR (Fig. 7), Ru promotes the reduction of Ni in a more extent at lower temperature. In addition, the presence of Ru increases markedly the dispersion, thus increasing the number of active sites for CO2 hydrogenation. Liu et al. also observed an improvement in activity after the incorporation of Ru in a nickel-based catalyst for the continuous methanation of CO2 [38]. The authors also highlighted the synergistic effects between Ni and Ru bimetallic catalysts, as well as their enhanced H2 and CO2 chemisorption capabilities.Finally, the DFM 1Ru10Ni-NaCa presents the highest CH4 production in the entire range of temperature studied. The promotion of low-temperature production is mainly attributed to the presence of Ru and the synergistic effect between Ru and Ni as above described. On the other hand, the promotion at high temperature is assigned to the modulation of basicity by the joint presence of Na and Ca. At this point, in the DFM 1Ru10Ni-NaCa, the contact between carbonates and metallic sites is promoted to a greater extent and consequently CO2 adsorption and hydrogenation to CH4 are also improved. Specifically, the DFM 1Ru10Ni-NaCa produces 298 μmol g−1 at 440 °C. Fig. 9b shows the CO production for the four formulations of DFMs. An upward trend is observed with temperature since its increase favors the RWGS (Eq. 16) in contrast to the methanation reaction (Eq. 13). Table 7 shows the selectivity (Eq. 5) of the DFMs operating at 400 °C together with the CH4 production. DFM 10Ni-Na has a selectivity of 87.8 %. The incorporation of Ru increases the selectivity to 96.4 % and the joint presence of Na and Ca to 94.1 %. The improvement due to the presence of Ru is attributed to the fact that the greater number of available metallic sites favors the total hydrogenation of CO2 to CH4. On the other hand, the addition of Ca to the catalysts for CO2 methanation improves the selectivity to CH4 by strengthening CO2 chemisorption, while the addition of Na favors the formation of CO [47,48]. In addition, it has recently been concluded that the joint presence of both adsorbents further limits CO production by favoring contact between carbonates and metal sites [29]. Finally, the DFM 1Ru10Ni-NaCa presents the highest selectivity value (98.4 %). It is suggested that the two positive effects are added up. (16) CO2+H2⇆CO+H2O In order to analyze the resistance of DFMs to the presence of O2 and H2O vapor, the effect of hydrothermal aging in the presence of oxygen is studied. For this, the DFMs were placed in a tubular quartz reactor placed in a horizontal furnace and a stream with 5 % H2O steam and 5 % O2 was fed for 3 h at 550 °C. This strategy is commonly used for NSR or SCR catalysts for NOx removal in diesel vehicle engines [49–51]. In this way, the state of the catalyst at the end of the vehicle's life can be simulated on a laboratory scale. Fig. 10 shows the CH4 production at 400 °C from the DFMs before (1st column) and after (2st column) the hydrothermal aging in the presence of O2. The aging process limits CH4 production from all DFMs. Specifically, production is reduced between 37 % and 47 %. Table S1 shows the textural properties and metal dispersion values of the aged DFMs. The aging process causes a reduction in specific surface area and pore volume and an increase in pore size. Continued exposure of DFMs to temperature in the presence of O2 and H2O causes sintering of the catalytic phase and agglomeration of the adsorbent, causing irreversible blockage of the smallest pores. Burger et al. [52] observed a progressive decrease in the specific surface area of NiAl2Ox and NiFeAl2Ox catalysts as the operation time with continuous CO2 and H2 feeding increased. The authors attributed the decrease to Ni particle growth and sintering of the mixed oxide phase. De-La-Torre et al. [49] also observed a reduction in specific surface area after hydrothermal aging for Pt-Ba/Al2O3 and Pt-Ce-Ba/Al2O3 NSR catalysts. The authors attributed the decrease to the formation of barium aluminate and blockage of the alumina pores by platinum and cerium. On the other hand, the aging process also causes a drastic reduction in the metallic dispersion of the DFMs (Table S1). The dispersion values of the aged DFMs are reduced between 53 % and 62 %. Again, the DFMs with ruthenium in their formulation (1Ru10Ni-Na and 1Ru10Ni-NaCa) show the highest values, 2.1 % and 2.8 %, respectively.If the CH4 productions of the aged DFMs (Fig. 10) are compared, they show a trend similar to that observed in Fig. 9. The lowest production is shown by the DFM 10Ni-Na (108 μmol g−1) with a selectivity of 88 %. Then the DFM 10Ni-NaCa produces 128 μmol g−1 with a selectivity of 96 %, followed by the DFM 1Ru10Ni-Na which produces 131 μmol g−1 with a selectivity of 97 %. It is confirmed that the promotion with Ru or the joint presence of Na and Ca also promote the production of CH4 after aging. Finally, DFM 1Ru10Ni-NaCa shows the highest production (155 μmol g−1) with the highest selectivity (98 %). Consequently, the contact between carbonates with metal sites and consequently the adsorption of CO2 and hydrogenation to CH4 is still promoted to a greater extent.The feasibility of boosting the CH4 production of the DFM 10Ni-16Na/Al2O3 through the joint presence of Na/Ca and the Ru incorporation has been studied. For that, four DFMs have been prepared by sequential wetness impregnation in which their formulation has been varied. The Ru incorporation notably increases the metallic dispersion. Synergistic aspects are created between Ni and Ru that limit nickel sintering and induce its reduction at lower temperatures. The reducibility is increased from 70 % (10Ni-Na and 10Ni-NaCa) to 84 % (1Ru10Ni-Na and 1Ru10Ni-NaCa). Furthermore, in TPSR experiments, CH4 production increases and starts at lower temperatures, confirming the synergies observed by other techniques. In short, the DFMs with Ru in their composition have a greater number of exposed metal atoms, so they have a greater capacity to supply dissociated hydrogen. On the other hand, the joint presence of Na and Ca does not seem to influence the disposition of the metallic phases.The four synthesized DFMs present three types of basicity (weak, medium and strong). On this occasion, the addition of 1 % ruthenium does not significantly modify the distribution of basic sites. However, the joint presence of Na and Ca promotes medium and strong basicity, and therefore total basicity. Note that all the DFMs present the same adsorbent loading (16 %), therefore the joint presence of Na and Ca improves the adsorption capacity, especially at medium-high temperatures.In the operation in cycles of CO2 adsorption and hydrogenation to CH4, once the cycle-to-cycle steady state is reached, the process is cyclic and repetitive. For all DFMs and at all operating temperatures (280–520 °C) the error with which the carbon balance is closed is less than ± 5 % and the H2O/CH4 ratio is very close to 2, according to the stoichiometric of the Sabatier´s reaction. CH4 production shows a maximum around 400 °C. At lower temperatures the extraction capacity of the adsorbed CO2 is limited, while at higher temperatures the storage capacity is limited. The lowest CH4 production is presented by the reference DFM 10Ni-Na, even so, it produces 172 μmol g−1 at 400 °C. The joint presence of Na and Ca (10Ni-NaCa) improves the production at intermediate-high temperatures by boosting the adsorption capacity of the DFM, specifically it produces 202 μmol g−1 at 400 °C. On the other hand, the Ru incorporation boosts the production in the entire range of temperatures studied, although especially at low-intermediate temperatures. The improvement is assigned to the synergistic aspects between Ni and Ru and to the higher number of exposed metal atoms. The DFM 1Ru10Ni-Na produces 250 μmol g−1 at 400 °C. Finally, the joint presence of Na and Ca and the simultaneous Ru incorporation (1Ru10Ni-NaCa) presents the highest CH4 production in the entire range of temperatures studied. It is concluded that both effects are added. Mainly, at low temperatures, the presence of Ru boost the production, while at high temperatures, the greatest adsorption capacity is due to the joint presence of Na and Ca. Specifically, it produces 298 μmol g−1 at 440 °C. In addition, it is the DFM with the highest selectivity to CH4 and the one that also presents the highest activity and selectivity after hydrothermal aging in the presence of O2.Alejandro Bermejo-López: Validation, Methodology, Investigation, Writing – original draft. Beñat Pereda-Ayo: Conceptualization, Methodology, Visualization, Writing – review & editing. Jon A. Onrubia-Calvo: Methodology, Visualization, Writing – review & editing. José A. González- Marcos: Methodology, Data curation, Supervision, Funding acquisition. Juan R. González-Velasco: Conceptualization, Supervision, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Support for this study was provided by Project PID2019–105960RB-C21 by MCIN/AEI /10.13039/501100011033 and the Basque Government (Project IT1509-2022). The authors thank for technical and human support provided by SGIker (UPV/EHU Advanced Research Facilities/ ERDF, EU).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2023.109401. Supplementary material .

ICCU-methanation is a promising technology that would approach a carbon neutral cycle. In this paper, the feasibility of boosting the activity of the dual function material (DFM) 10Ni-16Na/Al2O3 through the joint presence of Na/Ca and the Ru incorporation is studied. Four DFMs are prepared by sequential wetness impregnation and are extensively characterized by N2 adsorption/desorption, XRD, H2 chemisorption, TEM, STEM-HAADF and temperature-programmed techniques (H2-TPD, CO2-TPD, TPSR, and H2-TPR). The catalytic behaviour of DFMs in the cyclic process of CO2 adsorption and hydrogenation to CH4 is evaluated. The joint presence of Na/Ca improves CH4 production at intermediate-high temperatures by boosting the CO2 adsorption capacity. On the other hand, the Ru incorporation promotes CH4 production at low-intermediate temperatures by presenting synergistic aspects with nickel that lead to a greater number of exposed metal atoms. The Ru incorporation increases the metallic dispersion and the Ni reduction. Finally, the joint presence of Na/Ca and the simultaneous Ru incorporation presents the best activity results. It is concluded that both positive effects are added. Specifically, the DFM 1Ru10Ni-NaCa produces 298 μmol g−1 at 440 °C with a CH4 selectivity of 98.4 %. Furthermore, it is also the most active and selective DFM after hydrothermal aging in the presence of O2.

Biomass gasification is considered one of the most efficient routes to convert biomass feedstock into gaseous fuel through a partial oxidation process at high temperatures [1,2]. However, one of the main shortcomings of biomass gasification lies in the presence of tars in the product stream, which leads to fouling/clogging and corrosion of downstream equipment [3–5]. Hence, in order to minimize the amount of tar and improve the syngas composition, its catalytic conversion is one of most promising routes [6,7]. This process involves the oxidation of the tar components using steam to produce a syngas richer in H2 and, furthermore, the presence of the catalyst allows a more effective tar removal at lower temperatures than those in the non-catalytic tar conversion [8].The tar is as a complex mixture of condensable hydrocarbons, ranging from single-ring to five-ring aromatic compounds along with other oxygen-containing hydrocarbons and complex polycyclic aromatic hydrocarbons (PAH) [9]. Tar model compounds have been widely used in order to ascertain the catalyst performance and determine suitable operating conditions. In this work, toluene was selected as a tar model compound because it is a stable aromatic structure, especially at relatively low temperatures, apart from being one of the major tar species in the biomass gasification [10–14].Many parallel and consecutive reactions involve tar conversion, with the product distribution being the result of their competition. The main products obtained are hydrogen, carbon monoxide and carbon dioxide, and the major reactions occurring in the process are as follows:Toluene steam reforming (1) C7H8+7 H2O→7 CO+11 H2 Toluene steam dealkylation (2) C7H8+H2O→C6H6+CO+2 H2 Water gas shift (WGS): (3) CO+H2O↔CO2+H2 Thermal cracking (4) C7H8→x CnHm+z H2 (5) C6H6→x CnHm+z H2 Hydrodealkylation (6) C7H8+H2→C6H6+CH4 Toluene dry reforming (7) C7H8+7 CO2→14 CO+4 H2 Boudouard reaction: (8) C+CO2↔2 CO Methanation (9) CO+3 H2↔CH4+H2O Coke heterogeneous gasification: (10) C+H2O→CO+H2 Amongst these reactions, the most important ones are steam reforming (Eq. (1)) and water gas shift (WGS) (Eq. (3)).The selection of temperature and catalyst determines the extent of these reactions and the selectivity towards the different products [7,15–18].Tar catalytic conversion methods are classified as in-situ (or primary) and post-gasification (secondary) ones [14,19]. In the former, the tar reduction occurs during the gasification stage, with the catalyst being located in the gasifier itself. In the secondary approach, the gas produced in the gasifier is treated downstream in a secondary reactor where the catalyst is placed. Regardless the strategy followed, essential aspects conditioning the gasification process are those involving the reactor configuration, operating conditions and type of catalyst [20]. Fluidized bed reactors are one of the most developed technologies for biomass gasification, which require an appropriate catalyst in terms of activity and stability in order to reduce the tar content to 2  g m−3 and avoid the need of a more expensive secondary catalytic reactor downstream [21,22].A wide range of materials with significant activity for cracking and reforming of heavy aromatic compounds have been investigated as primary catalysts [12,14]. Natural minerals, such as olivine and calcined dolomite, have been widely used in the steam gasification in fluidized beds, as they are active for tar removal, apart from being inexpensive and abundant [23]. Acid catalysts, such as alumina or zeolites, have also been used (prior to and after metal impregnation) as catalysts for tar abatement [24]. Nevertheless, the performance of all these primary catalysts can be greatly improved by metal phase addition [19,25–28]. Thus, support features, such as mechanical (resistance to attrition), physico–chemical (surface area, porosity, acidity, composition and density) and catalytic ones (activity / selectivity and stability) play a relevant role in the metal-support interactions, as well as in the reforming reaction mechanism itself [29].From a catalytic point of view, nickel is known to be the most interesting metal phase for reforming applications [30]. Ni-based catalysts have been widely applied in the steam reforming of biomass tars due to both their high activity for breaking CC and OH bonds and performance in terms of H2 production [7,12,13,31–35]. However, their main drawbacks are related to their rapid deactivation, mechanical fragility and high cost compared to natural minerals or alumina [36]. Currently, use of iron as an active phase is gaining more attention for tar reduction due to its lower cost, abundance and lower toxicity compared to nickel [37]. Iron is known to be an active species for aromatic hydrocarbon destruction (breakage of CC and CH bonds), as well as for the WGS reaction. In fact, it has been proven effective for the aforementioned reactions in different oxidation states [24,38–40]. Therefore, iron impregnation of olivine, dolomite or Al2O3 seems to be interesting for synthetizing in-bed primary catalysts for gas–solid contact reactors, such as fluidized or spouted beds, from both economic and environmental perspectives. Nevertheless, the lower activity of the iron species with respect to the Ni ones requires higher amounts of dopant, generally in the 10–30 wt% range [41].Accordingly, the aim of this work is to analyse the performance of olivine, dolomite and γ –Al2O3 as primary catalysts, as well as the effect the impregnation of each catalyst with 10 wt% Fe has on the elimination of toluene, which has been selected as the model compound of biomass gasification tar. Furthermore, a detailed characterization of the fresh and deactivated Fe-doped catalysts has been carried out in order to determine the main deactivation mechanisms in this process. The results obtained will provide essential information for the selection of optimal primary catalysts for biomass gasification in the bench-scale unit equipped with an improved spouted bed reactor developed by our research group [21,42,43]. Furthermore, the results obtained may also be extrapolated to industrial gasification reactors, which are mainly fluidized beds. This study addresses multiple aspects that have not been jointly approached in the literature, as are catalysts preparation and characterization, influence of temperature, catalyst performance at zero time on stream, stability of Fe-loaded catalysts and the main deactivation causes.Six catalysts have been tested in the toluene steam reforming process. Three of them (olivine, dolomite and γ –alumina) are primary catalysts, whereas the other three are those obtained by impregnating the aforementioned primary catalysts with Fe, i.e., Fe/olivine, Fe/dolomite and Fe/Al2O3. Besides, runs with silica sand were carried out for comparison purposes. Minerals Sibelco supplied the olivine and dolomite, and Alfa Aesar the γ-Al2O3. These three primary catalysts provided satisfactory results in a previous study of biomass gasification in a fountain confined conical spouted bed reactor (CSBR), as they allowed reducing tar formation, as well improving the yield and composition of the syngas [21,42,43]. The catalyst particles were sieved in order to retain the fractions within the ranges of 90–150 μm for olivine, 150–250 μm for dolomite and 250–400 μm for γ - Al2O3, which allow attaining similar fluidization regimes with these materials of different densities. Prior to use, dolomite was calcined at 900 °C for 4 h in a muffle oven in order to complete the decarboxylation of calcium and magnesium carbonates.The Fe loaded catalysts were prepared by wet impregnation of the supports with an aqueous solution of Fe(NO3)3·9H2O (Panreac AppliChem, 98 %). The amount of saline precursor added was that corresponding to the desired final catalyst composition. The concentration of Fe was fixed at 10 wt% in order to compare the catalytic activity and selectivity of the three catalysts for same amount of metal loaded. Subsequent to the impregnation process, the prepared catalysts were dried at 100 °C for 24 h and then calcined at 1000 °C for 4 h. Given that the catalytic activity of iron species generally increases with their reduction state (Fe2O3 < Fe3O4 < FeO < Fe(0)) [26], these iron-impregnated catalysts were used once they had been subjected to an ex situ reduction process at 850 °C for 4 h under 10 vol% H2 stream, which ensured full reduction of ferric oxides into their metallic phase. The particle sizes of the Fe loaded catalysts were the same as those of their primary counterparts.The physical properties of the catalysts were determined by N2 adsorption–desorption in a Micromeritics ASAP 2010 instrument. Based on the information of these isotherms, the catalysts features, such as those involving specific surface area and porous structure (average pore size and pore volume), were calculated by the Brunauer–Emmett–Teller (BET) method. Prior to the analysis, and in order to remove any impurity, the samples were degassed at 150 °C until a pressure below 2·10−3 mmHg was reached. The chemical composition (wt%) of each catalyst was measured by X-ray fluorescence (XRF). More detailed information about the XRF methodology can be found elsewhere [43].The temperature-programmed reduction (TPR) of the catalysts was carried out in an AutoChem II 2920 Micromeritics, which allowed determining the catalyst reduction temperature before using it. This method consists in exposing the solid to a reducing gas flow of 10 vol% H2/Ar, while temperature is increased with a constant heating rate of 5 °C min−1 from ambient one to 900 °C. The reduction temperature of each catalyst was ascertained by monitoring the H2 consumed.The crystalline structure of the fresh and deactivated catalysts was analyzed using X-ray powder diffraction (XRD) patterns. A Bruker D8 Advance diffractometer with Cu Kα1 radiation was used to conduct XRD. The detailed procedure followed is described elsewhere [44]. The metal crystallite size was calculated by using the Scherrer formula. Metal dispersion was calculated from metal crystallite size using the equation D (%) = 97.1/d (nm) and assuming that the size of Fe atom is the same as that of Ni atom, as reported elsewhere [45,46].The values of total acidity of the catalysts have been obtained by monitoring the differential adsorption of NH3 at 150 °C using simultaneously calorimetry and thermogravimetry in a Setaram TG-DSC 111 equipment.The amount of coke deposited on the used catalysts was determined by temperature-programmed oxidation (TPO) in a thermobalance (TGA Q5000TA Thermo Scientific). This TGA is connected on-line to a Blazer Instruments Mass Spectrometer (Thermostar) and the procedure followed to determine the coke deposited on each sample is as follows: (i) signal stabilization with He stream at 100 °C for 30 min, and (ii) a ramp of 5 °C min−1 to 800 °C in a stream of O2 diluted in He, with this temperature remaining constant for 30 min in order to ensure full coke combustion.The experiments of toluene conversion with the different catalysts were performed in an Inconel fluidized bed reactor (300 mm in length and 10 mm in internal diameter), as shown in Fig. 1 . The reactor is located within a radiant oven, which provides the heat for operating up to 900 °C. The temperature was measured and recorded by means of two K-type thermocouples, with one being located inside the reactor, approximately in the middle zone of the bed, and the other one close to the wall of the electric oven.The water for generating the steam and toluene were introduced by means of a high-pressure pump (ASI 521) and a syringe pump (PHD 4400), respectively. Their pumping flowrates were maintained constant in all the runs, with the values being 0.24 mL min−1 for water and 0.06 mL min−1 for toluene, which correspond to a steam/toluene ratio (S/T) of 4 and a molar steam/carbon (S/C) ratio of 3.35. Prior to feeding into the reactor, these two compounds were pumped separately into an evaporation system at 350 °C, which ensures their full vaporization. This plant is also provided with a nitrogen mass flow meter (Brooks SLA5800) that allows feeding up to 1 L min−1. In fact, a nitrogen flow rate of 300 mL min−1 was used as fluidizing agent during the heating process prior to the reaction.The gaseous stream leaving the reactor was passed through a heater, whose temperature was kept at 300 °C in order to prevent the condensation of the products before entering the on-line analysis system. Then, the volatile stream circulated through a condensation device consisting of two coalescence filters, which ensured total condensation and retention of the non-reacted steam and toluene, as well as toluene derived products.This study deals with the effect of operating conditions in a catalytic process for tar elimination process, i.e., reforming temperature (in the 800–900 °C range), catalysts type (olivine, dolomite and alumina, as well as their counterparts with Fe impregnation) and catalyst stability. Olivine was chosen to analyse the effect of temperature, whereas 850 °C was established as the suitable operating temperature to study the influence of catalyst type and time on stream. The effect of reaction time was studied for the Fe loaded catalysts in the 5–115 min range in order to assess the evolution of catalyst activity and stability.Given that the density of the primary catalysts differs significantly (3300 kg m−3 for olivine, 1666 kg m−3 for alumina and 1275 kg m−3 for dolomite), and in order to operate under the same hydrodynamic conditions in the fluidized bed reactor, the same bed volume was used in all experiments. Accordingly, as mentioned above, suitable particle sizes were chosen. Thus, 3.8 cm3 of the corresponding catalyst (or sand in case the experiment was carried out without catalyst) were placed in the bed in all cases, corresponding to a gas hourly space velocity (GHSV) of 820 h−1. Experiments at zero time on stream were repeated at least 3 times to ensure reproducibility of the results and the carbon mass balance closure was above 95 % in all runs.The analysis of the volatile stream leaving the reactor was conducted on-line by means of a GC (Agilent 7890) provided with a flame ionization detector (FID). The sample was injected into the GC prior to condensation by means of a line maintained at 280 °C in order to avoid the condensation of heavy tar compounds. The analysis of the non-condensable gases (after separating the tars from the gaseous stream in the condensation system) was carried out by means of a micro GC (Agilent 4900). The three independent modules with different columns (molecular sieve, porapak and plot alumina) allowed identifying and quantifying the gaseous products previously calibrated. This analysis methodology allowed a detailed quantification of the entire product stream.The conversion and product yields were taken as reaction indices to monitor and assess process performance. The carbon conversion of toluene was defined as the moles of carbon in the gaseous product stream divided by the moles of carbon in the toluene feed (Eq. (11)). Note that the moles of CO, CO2 and C1-C4 hydrocarbons formed (corresponding to the total amount of carbon moles in the gas) have been determined from the micro-GC analysis, whereas the moles of carbon in the feed were calculated based on the total amount of toluene introduced into the reactor (total volume of toluene injected in the run). (11) C conversion % = moles of carbon in the product gas moles of carbon in the feed · 100 The product yields were calculated as the ratio between the grams of each product (H2, CO, CO2 and CH4) in the gaseous stream and the grams of the model compound in the feed: (12) Y i e l d w t % = g of the compound in the product gas g of model compound in the feed · 100 Moreover, H2 potential was also determined as the ratio between the concentration of H2 in the effluent gas and the maximum allowed by stoichiometry: (13) H 2 p o t e n t i a l % = moles of H 2 i n t h e p r o d u c t g a s maximum moles of H 2 a l l o w e d b y s t o i c h i o m e t r y The maximum number of H2 moles allowed by stoichiometry was calculated by considering toluene reforming reaction and that of WGS. Thus, H2 potential is defined based on the maximum number of H2 moles obtained when toluene is fully reformed to CO2 and H2. Table 1 shows the physical properties (specific surface area, pore volume and average pore diameter) and chemical composition of the primary catalysts and those impregnated with Fe. As observed, olivine has the lowest specific surface area (1.92 m2 g−1) and pore volume (0.002 cm3 g−1) due to its non-porous structure. After impregnation with Fe(NO3)3·9H2O solution, the specific surface area and the pore volume of dolomite and Al2O3 decreased mainly due to metal deposition, as it blocks some of the micropores of the catalysts. According to Kumar et al. [47], the presence of iron on alumina accelerates the shrinkage of alumina and transforms the alumina from gamma into other phases, which decreases the surface area because Fe2O3 particles act as heterogeneous nucleation sites for α-Al2O3 particles at high temperature. Nevertheless, the opposite trend was observed in the Fe/olivine, i.e., the specific surface area increased due to the deposition of Fe on the external surface area, and the pore volume and average pore size became larger, which suggests the collapse of the inter-pore structure of olivine. Note that the same trend has been observed for metal impregnation on supports with low porosity surfaces [22,48,49]. Apart from the impregnation process, the high calcination temperature also contributes to reducing the BET surface area and porosity of the Fe/ Al2O3 catalyst, although to a lesser extent. In a previous study [50], the same Al2O3 used in this study was calcined with air at 1000 °C during 5 h and its BET surface area and pore volume reduced to 87 m2 g−1 and 0.38 cm3 g−1, respectively.Dolomite is a calcium magnesium carbonate, i.e., CaMg(CO3)2, and therefore carbonates are decomposed into CaO and MgO in the calcination, which are the main constituents in the calcined dolomite, as shown in Table 1. Moreover, the XRF revealed that there is a high content of Fe in the Fe/olivine. In fact, the content of Fe in the raw olivine was of around 5.3 wt% and after impregnation, the Fe amount in the catalyst increased significantly to 17 wt%, which confirmed that the metal content was close to that corresponding to the impregnation (∼10 wt%) plus that in the original olivine. In the other two catalysts, namely Fe/Al2O3 and Fe/dolomite, the initial Fe content was negligible and after the impregnation increased up to 9.9 and 9.3 wt%, respectively. Thus, the Fe content of the three studied catalysts is consistent with the targeted metal loading of 10 wt%. Table 2 shows the metal dispersion of each catalyst which was estimated based on the metal crystallite size obtained by XRD analysis (by applying Debye-Scherrer equation). As observed, the highest metal dispersion is attained for Fe/Al2O3 (2.6 %), whereas the poorest value is for dolomite (0.5 %). This result confirms that the physical structure of the support plays an essential role in the dispersion of the metal phase; that is, the support with the highest surface area as that of Al2O3 leads to the highest metal dispersion.The XRD patterns of the primary catalysts and Fe reduced catalysts are shown in Fig. 2 a and 2b, respectively. As observed, the three Fe doped catalysts show an intense peak of the metal iron phase at 2θ = 44° and two smaller ones at 2θ = 65° and 82°. Note that iron oxide phases were not detected in these catalysts, which is evidence of their full reduction. In both olivine (Fig. 2a) and Fe/olivine (Fig. 2b), the main crystalline phases observed are those corresponding to olivine (Mg1.81Fe0.19·(SiO4)) and enstatite (MgSiO3). Further diffractogram of unreduced Fe/olivine catalyst can be found elsewhere [22]. Regarding Fe/dolomite (Fig. 2b), apart from the metal iron phase, those of Ca(OH)2, CaO and MgO were also observed, with all of them being derived from the calcination of calcium magnesium carbonate, which is the main mineral species in the dolomite [51]. These last three phases (Ca(OH)2, CaO and MgO) were also observed in the XRD diffractogram of calcined dolomite (Fig. 2a). These alkaline earth oxides (CaO and MgO) containing Lewis basic sites may promote adsorption and migration of H2O and OH groups on the catalyst surface, and therefore promote carbon gasification and reduce carbon deposition [52]. The Ca(OH)2 diffraction peaks are evidence that CaO (a highly hygroscopic compound) absorbed humidity from the ambient and formed Ca(OH)2. In the Fe/Al2O3 catalyst, typical diffraction peaks corresponding to the Al2O3 support were detected, as well as hercynite (FeAl2O4), whose diffraction lines are located at 2θ = 31°, 36°, 51°, 59° and 64°. The high calcination temperature used (1000 °C) allowed the formation of hercynite spinel (FeAl2O4), which occurs at temperatures above 600 °C by the interaction between Fe species (Fe0, FeO and Fe3O4) and Al2O3, following the reaction mechanism reported in the literature [53,54]. Moreover, comparing the XRD patterns of Al2O3 before and after impregnation and calcination stages, there is a phase change from γ-Al2O3 to a more stable one, which is probably the most stable one (α-phase) due to the high temperature of calcination used (1000 °C). The peaks assigned to Al2O3 in the Fe loaded catalyst in Fig. 2b are clear and sharp, which is evidence of its high crystallization degree, whereas the peaks assigned to Al2O3 in Fig. 2a are broad and low, thereby suggesting an amorphous structure with a small crystallization degree. Note that the same diffraction peaks than those observed for Al2O3 crystalline phases in Fig. 2a and b have been reported in the literature and correspond to γ-Al2O3 and α-Al2O3, respectively [55,56]. Therefore, phase transformation is the consequence of the thermal degradation of the support, which affects adversely the physical properties of the catalyst by reducing catalyst surface area, thereby reducing catalyst activity. Several authors have called this process support sintering [35,57].The temperature programmed reduction (TPR) profiles of calcined Fe/olivine, Fe/dolomite and Fe/Al2O3 catalysts are shown in Fig. 3 . Given that metal iron is expected to be the active phase for breaking CC and CH bonds [24,58], the reducibility of the catalysts is of great relevance. According to the literature [26,59] the reduction of Fe2O3 generally proceeds in two steps, as are: the reduction of Fe2O3 to Fe3O4 in the 350–500 °C range and the reduction of Fe3O4 to metal Fe in the 500–900 °C range. However, according to certain studies, the intermediate FeO is formed in the reduction from Fe3O4 to Fe0 [60,61]. These two regions associated with two or three reduction steps from Fe2O3 are observed in the three reduced catalysts, although differences in the interactions between the iron and the supports shifted the location of the peaks. In the TPR profile of the Fe/olivine, a broad reduction zone between 350 and 700 °C is observed with 3 peaks. The first two (at 470 and 530 °C) are associated with the reduction of Fe2O3 and Fe3O4/FeO, respectively, whereas the latter peak above 600 °C is due to the Fe atoms that migrated into the olivine support to form a very stable MgFe2O4 spinel phase [62]. Peaks at 380 °C and 500 °C appear in the Fe/dolomite, which are characteristic of iron species reduction, but there is also a broad peak at 750 °C, which corresponds to the reduction of Fe3+ from the calcium iron oxide (srebrodolskite, Ca2Fe2O5) to Fe, as was suggested by Zamboni et al. [63,64]. These authors observed the formation of this phase when iron nitrate was used in the wet impregnation of dolomite. In this study, no evidences of Ca2Fe2O5 are observed in the XRD diffractogram (Fig. 2), probably due to its low crystallinity. In the Fe/Al2O3 catalyst, apart from the two peaks identified at 380 and 580 °C, which are associated with the reduction of iron species (Fe2O3, Fe3O4 and FeO) , a third broad reduction zone appears between 700 and 900 °C, which is attributed to the reduction of iron aluminates (FeAl2O4), also identified in the XRD spectra [65]. Different authors suggested that the presence of alumina stabilizes Fe2O3 phase and the reduction goes through the formation of FeAl2O4 spinel, whose reduction occurs above 700 °C [66,67].The influence of temperature on toluene abatement on olivine catalysts is displayed in Fig. 4 . Fig. 4a shows the evolution of carbon conversion and H2 potential. As observed, temperature has a great influence on carbon conversion and H2 potential, since their values increase from 3.6 and 2.6 % at 800 °C to 46.0 and 23.6 % at 900 °C, respectively. This increase in both parameters is attributed to the endothermic nature of the toluene reforming reactions, as well as of those involving decomposition and dehydrogenation, as all of them are promoted at high temperatures [68]. The same trend of carbon conversion and H2 potential with temperature on olivine catalysts was observed by other authors in the tar steam reforming [58,69].The yields of the compounds in the product stream is displayed in Fig. 4b. An increase in temperature leads to higher yields in gaseous compounds (including benzene) due to the promotion of both reforming and cracking reactions, with the highest yields being those of CO and CO2 at 900 °C (49.8 and 26.8 wt%, respectively). The yield of CH4 increases with temperature, but it is lower than 2.2 wt% at the three temperatures studied. It should be noted that the yield of C 2-C4 hydrocarbons is hardly noticeable (below 0.01 wt%), and has not therefore been included in Fig. 4b. CH4 is mainly formed from dealkylation of the methyl group in the toluene structure and, to a minor extent, from the methanation of CO [8]. However, steam reforming of CH4 prevails over these reactions, since the content of CH4 in the products is very low [12]. The presence of an undesired compound (benzene) is due to incomplete decomposition of toluene [70], which is confirmed in Fig. 4b, where benzene yield increases from 0.6 wt% at 800 °C to 12.6 wt% at 900 °C at the expense of a decrease in toluene yield. Several reactions, such as steam dealkylation (Eq. 2), thermal cracking (Eqs. 4–5) or hydrodealkylation of toluene (Eq. 6) lead to the formation of benzene (all of them enhanced at high temperatures) [11,15,17,71]. However, the small amount of CH4 in the product stream is evidence that hydrodealkylation reaction (Eq. 6) is not significant [72]. It should be noted that the benzene produced from the aforementioned reactions can undergo reforming reactions to produce further CO and H2, although these reactions are limited due to benzene stability [11,12]. The yield of polycyclic aromatic hydrocarbons (PAHs, referred to the compounds heavier than toluene) also increases with temperature due to the promotion of condensation reactions of lighter tars. However, the low yield of these PAHs (below 1.2 wt% in the whole range of temperatures studied) is evidence that the extent of these reactions is almost negligible, probably due to the presence of steam [21,73]. Swierczynski et al. [3] also observed a yield of around 6 wt% of benzene and 14 wt% of polyaromatics in the product stream of toluene steam reforming at 850 °C when they used olivine as primary catalyst. Fig. 4c displays the gas composition in the 800–900 °C range. It can be observed that the effect of temperature on the gas composition is not very pronounce above 850 °C, i.e., the concentration hardly changes above this temperature. Between 800 and 850 °C, certain trends are observed when temperature is increased, as are: a slight decrease in H2 and CO2 concentrations (from 69.1 to 66.2 vol% and from 8.1 to 6.8 vol%, respectively) and an increase in that of CO (from 21.8 to 25.5 vol%). This result is explained by the promotion of the reverse WGS reaction due to its exothermic nature. The same trend with temperature was observed in other studies of catalytic reforming of tar model compounds, with this effect being attributing to the exothermic nature of the WGS reaction [68,74].In order to study the performance of primary catalysts, toluene conversion on olivine, dolomite and alumina was monitored at 850 °C and the results obtained are displayed in Fig. 5 . The effect of thermal cracking was ascertained by comparing the results of carbon conversion (Fig. 5a), product yields (Fig. 5b) and concentration of gaseous compounds (Fig. 5c) obtained with the catalysts and those obtained with inert sand. As observed, the presence of any catalyst improves the overall efficiency of the process by increasing carbon conversion and the yields of gaseous compounds, especially those of H2, CO and CO2, as well as reducing that of toluene. This improvement over the results obtained with inert sand is associated with the promotion of steam reforming (Eq. 1), cracking (Eqs. 4–5) and WGS reactions (Eq. 3). The presence of primary catalysts also promotes steam dealkylation (Eq. 2) and thermal cracking (Eq. 4) reactions, since the concentration of benzene in the product stream is higher than that obtained with sand.Comparing the efficiency of the primary catalysts (Fig. 5a), Al2O3 leads to the highest conversion (58.4 %) followed by dolomite (39.1 %). However, the H2 potential with both catalysts is similar (28.5 % for Al2O3 and 28.9 % for dolomite). This latter result can be explained by the lower activity of Al2O3 and the higher of dolomite in the WGS. Thus, the higher activity of dolomite in the WGS reaction is related to CaO and MgO basic sites, with activity being higher as the Ca/Mg ratio is increased [75,76]. Furthermore, the presence of CaO and MgO also explains the higher yield of benzene at the expense of lowering that of toluene [77,78]. Moreover, olivine has the smallest influence on the toluene steam reforming, since it provided the lowest carbon conversion and H2 potential values. In this case, although the presence of Fe promotes reforming reactions, the low BET surface area (1.91 m2 g−1) and pore volume (0.002 g cm−3) are the factors leading to the low efficiency of this catalyst in the toluene elimination process. Studies reported in the literature confirm that dolomite and Al2O3 were more active than olivine for reducing the amount of tar derived from biomass gasification, as the extent of the WGS reaction is enhanced with dolomite [43,79]. Fig. 6 compares the parameters involving toluene conversion (carbon conversion and H2 potential (a), product yields in the outlet stream (b) and the concentration of gaseous compounds (c)) for the Fe loaded catalysts. Fig. 6a reveals that Fe incorporation into the primary catalysts leads to higher carbon conversion and H2 potential than those on the primary catalysts in all cases, Fig. 5a, which is evidence of their higher catalytic activity for toluene reforming. Thus, on the one hand, it is well stablished that metal iron is active for CC and CH bond breakdown, which enhances hydrocarbon reforming and cracking reactions [58,80]. On the other, the addition of Fe promotes the WGS reaction because the adsorption of water molecules on the catalyst active sites is favoured, thus leading to higher H2 yields [81]. This improvement is especially remarkable with olivine, whose carbon conversion and H2 potential increases from 18 and 10.5 % to 73 and 31.9 %, respectively.As occurred with primary catalysts, that of Fe/Al2O3 provided the best results in terms of carbon conversion (87.6 %) and H2 potential (38 %) (Fig. 6a). However, the trends were reversed for Fe/olivine and Fe/dolomite after Fe incorporation, attaining higher carbon conversion in the former. This result is closely related to the change in the surface area of the catalysts caused by the impregnation, which definitely affects metal dispersion. As observed in Table 1, the surface area increased in the olivine when Fe was introduced, whereas it significantly decreased in the dolomite (from 17.42 to 3.55 m2 g−1). Furthermore, the results in Table 2 confirm the better dispersion of Fe on the olivine than on the Fe/dolomite, which suggests that the active sites are more accessible for the reactants in the former, as all the iron is located on the catalyst surface. This implies a higher catalytic activity of Fe/olivine, which explains the better results of carbon conversion on this catalyst than on Fe/dolomite, whose metal dispersion is the poorest.Comparing the product yields shown in Fig. 6b, the highest yields of CO and CO2 (mainly derived from the reforming and WGS reactions, respectively) and benzene (a cracking product) are obtained on the Fe/Al2O3 catalyst, whereas that of toluene is the lowest (below half of those obtained on Fe/olivine or Fe/dolomite). Note that Fe acts as the active phase for the reforming and WGS reactions, whereas the alumina support provides the acidity required for cracking reactions, i.e., the combination of both provides Fe/Al2O3 catalyst with the highest activity for these reactions. Moreover, a comparison of Fig. 6b with Fig. 5b shows that the yield of benzene increases greatly when Fe is added to the primary catalysts. It seems that the presence of Fe mainly catalyzed the conversion of toluene to benzene. Some studies suggested that temperatures higher than 800 °C increase the hydrodealkylation activity for the steam reforming of toluene on iron-based materials [72,82], whereas other researches concluded that the activity of iron-based materials leads to the decomposition of large tar compounds into small fragments of carbon species, which subsequently form benzene [83]. Therefore, it can be concluded that the higher benzene content is a combined effect of cracking and hydrodealkylation of toluene molecules on Fe active sites. The higher CH4 yields observed on Fe loaded catalyst than on primary catalysts also confirms this hypothesis. Fig. 6c displays the gas composition obtained with the three Fe-impregnated catalysts. A comparison of these results with those for primary catalysts (Fig. 5c) shows the relevance of metal iron in the WGS reaction (Eq. 3), since the concentration of CO2 greatly increased in all the cases, whereas that of CO reduced. This is consistent with previous studies in the literature, in which a high activity of Fe is reported in the WGS reaction [38,40,84]. Analysing Fe loaded catalysts, Fe/Al2O3 led to the lowest concentration of CH4 and CO2 and the highest of H2 and CO, which is evidence of a high extent of steam and dry reforming of hydrocarbons (Eq. 1 and 7). According to Adnan et al. [85,86], this fact is attributed to the basic sites of Fe/Al2O3 catalysts, which promote endothermic CO2 reforming of hydrocarbons.The differences observed among these Fe-impregnated catalysts are the consequence of various factors. As previously stated, one of the most influential factor is related to the metal dispersion on the catalyst support, which plays a key role in the initial catalyst activity. A suitable metal-support interaction enhances the migration of metal crystallites, thereby obtaining a better dispersion of Fe on the support [26]. Furthermore, the physical structure of the support greatly influences the dispersion of the metal phase, as shown in Table 2, in which the highest Fe dispersion was obtained for Al2O3 (the support with the highest BET surface area and pore volume). The results in Fig. 6 confirm that the better surface properties of the Al2O3 support promote the dispersion of the active phase, and therefore lead to higher catalyst activity. Besides, the higher dispersion of Fe on olivine also explains the higher carbon conversion than on Fe/dolomite.Another factor is related to the activity of the support for cracking and/or reforming reactions, which is directly linked to its acidity [30,57]. Thus, the porous structure of olivine and dolomite barely have micro or mesopores, whereas alumina has a more developed porous structure, as shown in Table 1. Adnan et al. [85] suggested that toluene conversion reactivity is dominated by strong acid sites in the catalyst, which are directly attached to the surface of the catalyst. Thus, a higher surface area of the catalyst increases the number of strong sites available to contact with toluene, thereby leading to a higher acidity of the catalysts, and consequently to a higher conversion of toluene, as is the case of Fe/Al2O3, which has the highest acidity (Table 1) of the three Fe loaded catalysts [87]. Besides, Adnan et al. [88] stated that a higher content of Fe in the catalyst also promotes catalyst acidity, and therefore toluene conversion. Comparing the acidity of primary and Fe doped catalysts (Table 1), the presence of Fe increases the acidity of Fe/olivine and Fe/dolomite catalysts from 2.4 to 8.8 and from 8.7 to 10.5 µmol NH3 gcat −1, respectively, which explains the higher toluene cracking capability of Fe doped ones. Regarding the acidity value of Fe/Al2O3 (11.4 µmol NH3 gcat −1 ), it is much lower than that of the raw γ-Al2O3. Indeed, as previously stated, the reduction in BET surface area caused by the calcination and impregnation stages leads to the blockage of some pores and reduces the number of acid sites available, thus reducing the total acidity of the catalyst. However, comparing Fig. 5b and 6b, benzene yield is higher when Fe/Al2O3 is used than when the primary Al2O3 is used, which suggests that the cracking activity of Fe/Al2O3 is higher. This result is explained by the combination of two issues. On the one hand, as was previously stated, the better performance of Fe for reforming and WGS reactions leads to higher H2 partial pressures in the reaction environment, thus promoting hydrodealkylation reactions (Eq. 6) which lead to higher benzene contents. On the other hand, the real acidity of γ-Al2O3 under reaction conditions is much lower than that given in Table 1, as the high temperatures used in this study (850 °C) and the presence of steam accelerate the collapse of the porous structure and the transformation of γ-alumina into other more stable phases, as stated elsewhere [47]. Thus, the blockage of pores and the transformation of γ-phase into other ones (δ, θ or α) reduces the number of acid sites available, and therefore its cracking activity.Other important issue involving catalytic activity is the reduction state of the iron species, with activity being higher as Fe species are further reduced (metal Fe is the most active phase). Thus, the XRD patterns in the three fresh catalysts reveal the presence of metal Fe, whereas the presence of other species with different reduction states, such as Fe2O3, Fe3O4 or FeO, was not initially observed (Fig. 2b). This is an evidence that the difference in the catalytic activities of Fe impregnated catalysts is mostly attributed to the interactions between the metal iron and the supports, as well as their physical structure. Thus, the better properties of Al2O3 (it acts as a textural promoter preventing the fast sintering of the iron metal, as well as stabilizing active sites on its surface) lead to better dispersion of the Fe oxide phase, and therefore better performance for toluene steam reforming and WGS reaction [38].Claude et al. [26] analysed the effect of Fe doped olivine and alumina catalysts and revealed that Fe/Al2O3 provided also higher toluene conversion than the Fe/olivine catalyst when temperature was 850 °C. Nevertheless, the olivine catalyst was the one of better performance at 750 °C. It should be noted that these authors reduced both catalysts in situ in the reactor with an inlet reactant gas mixture containing 31.5 %vol. H2, which simulates a feed containing a fraction of the reforming outlet stream. Therefore, the differences at this temperature can be explained by the presence of iron species with different reduction states (active for steam reforming) in the olivine, whereas Fe was only present as hercynit in the case of alumina. Fig. 7 displays the evolution of carbon conversion and H2 potential with time on stream in the toluene steam reforming at 850 °C on the three catalyst tested. As observed, Fe/Al2O3 provided the highest stability in terms of carbon conversion (Fig. 7a) and H2 potential (Fig. 7b), since it allowed operating for the longest period with the highest conversion (85.9 % after 35 min on stream). Fe/dolomite and Fe/olivine catalysts provided a rather stable activity for the first 15 min, but the deactivation rate increased greatly subsequent to this time, and therefore toluene conversion and H2 potential decreased rapidly. The decrease in these parameters in the range from 15 to 25 min is more pronounce on the Fe/dolomite catalyst (35.1 % and 18.0 %, respectively, at the end of 25 min on stream). Subsequent to this time, the Fe/olivine catalyst underwent more severe decrease to 31.0 % and 15.8 %, respectively, after 45 min on stream. Overall, the conversion level and H2 yield decreased gradually with reaction time when either catalyst was used, reaching similar steady values of around 30 % and 15 %, respectively, which is evidence of the deactivation underwent by the catalysts.The evolution of component yields in the product stream with reaction time is shown in Fig. 8 for the three Fe doped catalysts. As observed, the yields of toluene and benzene follow opposite trends in the three catalysts. Once the activity for toluene reforming and cracking is low due to catalysts deactivation, the yield of toluene increases, whereas that of benzene decreases. Given the higher activity and stability on the Fe/Al2O3 catalyst for a longer time, benzene yield is higher and remains at around 45 wt% for a longer period than on Fe/olivine and Fe/dolomite. However, Fe/Al2O3 catalyst deactivation is more pronounced, attaining yields of H2 and CO lower than 10 wt%. Thus, Fe/dolomite provided lowest yields of toluene and highest of benzene when it was deactivated (65.2 and 14.6 wt%, respectively, for 65 min on stream), which means it is more active for toluene cracking than the other ones subsequent to this time. As previously stated, dolomite is well-known as an active catalyst for tar conversion when it is in the calcined state, i.e., CaO and MgO state, and therefore these species are still active phases for toluene cracking subsequent to the mentioned time [89].Moreover, as time on stream increased, H2 and CO2 yield decreased for the three catalysts, which is evidence of the lower extension of reforming and WGS reactions when the catalysts undergo deactivation. This reduction is more pronounced in the Fe/Al2O3 catalyst, in which the H2 yield decreased steadily from 17.5 to 7.3 wt% and that of CO2 from 105.0 to 36.0 wt% for 115 min on stream. These results and those corresponding to benzene and toluene yields confirm that, although the Fe/Al2O3 catalyst was able to maintain its reforming/cracking capacity for longer time, the deactivation was more severe.Regarding the CO yield, it was almost constant for Fe/Al2O3, whereas it decreased slightly for Fe/olivine and Fe/dolomite (from 13.7 to 6.9 wt% on both catalysts), although the latter allows operation for longer time on stream until reaching this final yield. This trend is a consequence of a balance between the attenuation of reforming (Eqs. 1 and 7) and WGS (Eq. 3) reactions and CO formation by mainly decarbonylation (cracking) [90,91].Regarding CH4 yields, they decreased slightly as reaction proceeded, attaining a value of around 1.25 wt% in the three catalysts. These results are evidence of the attenuation of the hydrodealkylation reaction (Eq. 6) when deactivation proceeded, although CH4 may still be formed by the cracking of the hydrocarbons in the reaction environment or by methanation (Eq. 9). A similar explanation holds for the slight increase in the yield of heavier hydrocarbons with time on stream. In this case, the attenuation of hydrocarbon reforming reactions by catalyst deactivation enables hydrocarbon rearrangement reactions, such as polymerization and/or cycloaddition, which lead to higher molecular weight species than toluene [21,92].The faster deactivation of olivine and dolomite catalysts shown in Figs. 7 and 8 suggests that the role of metal-support interactions, as well as the structural characteristics of the supports, in the metal dispersion may be relevant in the catalyst deactivation mechanism [37]. In fact, the poorer metal dispersion on dolomite and olivine catalysts, and therefore the lower amount of Fe active sites available for reforming and WGS reactions, enhances catalyst deactivation, either by sintering, coke deposition or iron phase change (reduction in metal active sites by the oxidation of iron species). As reported by other researchers, iron is more active for tar cracking/reforming when it is in the metal state than oxidized, but the oxidizing nature of steam at high temperatures promotes the oxidation of Fe metal sites [37,83]. Furthermore, given the lower dispersion of Fe on Fe/olivine and Fe/dolomite catalysts, most of it will be deposited on the catalyst surface, which leads to faster coke deposition, and therefore faster deactivation [24]. In fact, the deactivation mechanism by coking for toluene steam reforming is well established in the literature on Fe based catalysts [58,88]. The deactivation mechanism for the three catalysts will be further discussed in the next section.Prevention and attenuation of catalyst deactivation is essential for improving the viability of this catalytic process at larger scale. Therefore, a detailed characterization of the deactivated catalysts was carried out in order to understand the main causes of catalysts activity decay. Based on the results obtained in this study and others reported in the literature, the main factors causing the deactivation of Fe impregnated catalysts are coke deposition and active phase oxidation. Nevertheless, sintering or iron loss by attrition may also be relevant [22,25,26,62]. Table 3 shows the textural properties of deactivated Fe/olivine, Fe/dolomite and Fe/Al2O3 catalysts once they were used for 115 min on stream. Comparing these properties with those displayed in Table 1 for fresh catalysts, the BET surface areas remained almost constant for Fe/olivine and Fe/dolomite catalysts, whereas the Fe/Al2O3 underwent a reduction from 12.48 in the fresh one to 7.63 m2 g−1 in the deactivated one. Pore volume and pore diameter of the Fe/olivine catalyst decreased from 0.017 cm3 g−1 and 234 Å to 0.010 cm3 g−1 and 217 Å, respectively, thus revealing a partial blockage of the catalyst pores, but not their complete clogging. Moreover, the pore volume also decreased in the deactivated Fe/Al2O3, but the pore diameter increased from 206 to 285 Å, which suggests that the smallest pores are fully blocked by coke deposition. Similarly, the pore diameter increased in the spent Fe/dolomite, which reveals blockage or partial obstruction of the smallest pores. Table 3 also displays the chemical composition of the deactivated catalysts. A comparison of these values with those of the fresh ones (Table 1) allows concluding that there is not significant iron loss by attrition phenomena. Thus, the iron oxide content decreased slightly in the deactivated catalysts, i.e., 3.15 wt% in the Fe/olivine, 0.49 wt% in the Fe/Al2O3 and 1.48 wt% in the Fe/dolomite. The absence of attrition phenomena was also checked by sieving the deactivated catalysts particles. Thus, their size is approximately the same as the fresh ones, i.e., 90–150 μm for olivine, 150–250 μm for dolomite and 250–400 μm for alumina. The higher loss of Fe in the Fe/olivine and Fe/dolomite catalysts is related to the weaker interaction between the Fe and the support, as the metal species is mainly located on the surface [26]. Overall, these results of XRF analysis reveal that catalyst deactivation is not caused by the loss of metal phase. Fig. 9 shows the XRD patterns of the spent catalyst, which allow assessing the influence of the active phase oxidation in the deactivation process. As observed, the diffraction lines attributed to metal iron (2θ = 45° and 2θ = 65°) are only present in the Fe/dolomite sample, although their intensity is greatly reduced compared to those of the fresh one (Fig. 2). The XRD profiles for neither Fe/olivine nor Fe/Al2O3 contain these lines, which explains the slightly higher activity of Fe/dolomite for toluene reforming and WGS at longer times on stream. Nevertheless, lines for other iron phases have been detected in the three catalysts, with some of them being absent in the diffractograms for the fresh ones. The X-ray pattern of the deactivated Fe/Al2O3 only shows the presence of iron strongly incorporated into alumina (hercynite, FeAl2O4). A new phase of calcium iron oxide (Ca2Fe2O5) appears in the Fe/dolomite, which is formed due to the interaction between the metal iron and calcium oxide in the presence of steam [63]. The presence of Ca2Fe2O5 (reducible at high temperatures) promoted the redox reaction of Fe3+ to Fe0 due to its great oxygen-carrying capacity, which explains the presence of metal iron in the Fe/dolomite after the steam reforming process [78,93]. Given that MgO-iron oxide is detected, it can be assumed that iron only reacts with CaO, which was also concluded by Di Felice et al. [94]. Note that the spent Fe/dolomite catalyst shows certain decrease in the intensity of MgO and CaO peaks with respect to the fresh one (Fig. 2b), as well as the presence of the calcite phase (not observed in fresh catalyst), probably due to a very limited carbonation at 850 °C in the gasifier. These results confirm that CaO and MgO are still the most important active phases for toluene cracking on the spent catalyst. Concerning the Fe/olivine XRD profile, apart from the Mg1.81Fe0.19(SiO4) olivine phase previously detected in the fresh catalyst, new Fe3O4 lines appear at 2θ = 18°, 21°, 30°, 54° and 57°. These new iron phases, together with the absence or sharp reduction in Fe0 lines, are evidence of a loss of active phase by metal iron oxidation under the reaction conditions used on the three catalysts analysed. However, Fe3O4 phase in the Fe/olivine and Ca2Fe2O5 in the Fe/dolomite are still active for the reforming and WGS reactions, which explains their slightly higher carbon conversion than Fe/Al2O3 when they underwent deactivation (Fig. 7a).Another cause of catalyst deactivation with time on stream is the coke deposited on these catalysts. Therefore, spent catalysts were subjected to temperature programme oxidation (TPO) in order to assess the amount and nature of the coke deposited. This coke blocks the access of reactants to the metal sites or encapsulates the Fe particles, thereby deactivating the crystallite. The TPO analyses revealed that the highest amount of coke deposits were formed on the Fe/Al2O3 catalyst, followed by Fe/olivine and Fe/dolomite, with values being 4.10, 2.36 and 1.17 wt%, respectively. It is well stablished in the literature that the rate and extent of coke formation increases by increasing the acid strength of the catalyst [57,95]. Thus, the higher coke deposition on the Fe/Al2O3 catalyst is explained by the higher acidity of alumina than olivine and dolomite. Moreover, the lowest coke formation rate in the Fe/dolomite catalyst is explained by two facts: (i) the presence of Ca2Fe2O5 phase (oxygen carrier) improves oxygen mobility on the catalyst surface, and therefore leads to faster carbon removal by oxidation, and (ii) the presence of CaO and MgO in the dolomite favours steam-carbon reactions, thus hindering polymerization reactions that promote coke development [89]. However, it should be noted that the catalytic activity of CaO for steam reforming decreases dramatically when the carbonate is formed [96]. Zamboni et al. [63] suggested that the oxygen vacancies in the Ca2Fe2O5 structure favour the reduction of water and, furthermore, Ca2Fe2O5 rearranges by releasing oxygen, which oxidizes carbon species to CO2. Note that the CO2 yield (45.5 wt%) is the highest on the Fe/dolomite, even though the H2 yield is similar in the product stream once catalysts have been deactivated (Fig. 8). Fig. 10 displays the TPO profiles of the spent catalysts, which allow determining the nature and the possible location of the coke within the catalyst. Apart from the mentioned differences in the amount of coke deposited, Fig. 10 shows that the nature of the coke varies depending on the type of support. A prevailing peak is observed at 450 °C for Fe/olivine and Fe/Al2O3, which is related to the combustion of the amorphous coke (hydrogenated composition) deposited on metal particles. Two small peaks are also observed at 600 °C for these catalysts, which are related to a more structured coke located on the catalyst support, even though its content is almost negligible due to the low intensity of both peaks. Another oxidation peak was detected at around 270 °C for the Fe/olivine catalyst, which is related to a less structured coke or heavy hydrocarbon deposits [37]. Virginie et al. [24] also detected 3 peaks in the TPO of a 10 wt% Fe/olivine deactivated catalyst. The first one at around 360 °C, which is attributed to surface carbon oxidation, the second one at around 500 °C, which is due to the oxidation of iron carbide, and the third one at around 610 °C assigned to filamentous carbon oxidation. The Fe/dolomite catalyst seemed to be more effective than the other two catalysts for preventing coke formation and, furthermore, its coke burns at lower temperatures (the main peak below 400 °C). This is explained by the presence of Ca2Fe2O5 phase, which increases oxygen mobility on the catalysts surface, thus favouring coke gasification and inhibiting its growth and evolution towards a more structured coke [93].In view of these results, it may be concluded that the active phase oxidation is the main deactivation cause, but the coke deposited on the Fe active sites also causes their blockage, and therefore contributes to the catalysts deactivation. The deactivation of Fe/dolomite and Fe/olivine catalysts is faster because the iron is mainly located on the external surface, and therefore coking reactions encapsulate more easily the metal particles. Indeed, the surface area of olivine and dolomite supports is limited, and therefore Fe dispersion is more restrictive than in the alumina support. The latter undergoes more severe deactivation by coke deposition, but this occurs for longer reaction periods.This study approaches tar elimination by feeding toluene as a representative tar compound and shows that Fe incorporation into olivine, dolomite and alumina increases the activity and selectivity towards hydrocarbon reforming and WGS reactions. The results are evidence that an increase in temperature to 900 °C leads to an increase in carbon conversion and H2 potential due to the enhancement of toluene reforming and cracking reactions. Concerning the efficiency of the primary catalysts, alumina provides the highest carbon conversion followed by dolomite, with their H2 potential being similar. In fact, the higher acidity of alumina promotes catalytic cracking reactions, thus leading to higher carbon conversions, even though its activity for WGS reaction is more limited. Dolomite is the one of highest activity for WGS reactions, which is related to CaO and MgO basic sites obtained after calcination. In addition, the presence of these species also improves tar decomposition, which in turn increases benzene yield at the expense of decreasing that of toluene.Regarding Fe loaded catalysts, Fe/Al2O3 provides the best performance in terms of carbon conversion and H2 potential. In fact, the higher porosity and BET surface area of alumina compared to those of olivine and dolomite improves the dispersion of Fe, which acts as the active phase for reforming and WGS reactions, whereas the alumina support provides the acidity required for cracking reactions. Furthermore, Fe/Al2O3 is the most stable catalyst and allows operating for longer periods with higher conversion values, whereas Fe/olivine, and especially Fe/dolomite, undergo faster deactivation, as evidenced by the sharper decrease in the reaction indices.The analyses of spent catalysts show that the main deactivation cause is the active phase oxidation followed by coke deposition on Fe active sites during the toluene conversion process. The XRD patterns show new iron oxidized phases and the absence of Fe0 lines (except for Fe/dolomite, with their intensity being significantly lower than those of the fresh one), whereas TPO analyses reveal a higher coke deposition for Fe/Al2O3 (4.4 wt%), which fully blocks the smallest pores of the catalyst. The coke deposited in all the spent catalysts has an amorphous nature and blocks the access of reactants to the metal sites, thereby deactivating the catalysts. Maria Cortazar: Investigation, Data curation, Methodology. Jon Alvarez: Writing – original draft, Writing – review & editing, Formal analysis, Visualization. Leire Olazar: Investigation, Data curation. Laura Santamaria: Validation, Data curation. Gartzen Lopez: Conceptualization, Methodology, Supervision. Heidi Isabel Villafán-Vidales: Methodology, Validation. Asier Asueta: Supervision. Martin Olazar: Supervision, Conceptualization, Writing – review & editing, Project administration.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was carried out with the financial support of the grants RTI2018-098283-J-I00 and PID2019−107357RB-I00 funded by MCIN/AEI/ 10.13039/501100011033 and by “ERDF A way of making Europe” and the grants IT1218−19 and KK-2020/00107 funded by the Basque Government. Moreover, this project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 823745.

The performance of olivine, dolomite and γ-alumina primary catalysts was evaluated in the continuous tar elimination process in which toluene was selected as the biomass gasification tar model compound. Iron was incorporated into these catalysts in order to improve their catalytic activity. All the experiments were performed in a continuous flow fluidized bed micro-reactor, with a steam/toluene ratio of 4 and a space velocity (GHSV) of 820 h−1, which corresponds to a catalyst amount of 3.8 cm3. The effect of temperature was studied using olivine in the 800–900 °C range, which allowed concluding that 850 °C was the best temperature for tar removal. The fresh and deactivated catalysts were characterized by N2 adsorption–desorption, X-ray fluorescence (XRF), X-ray diffraction (XRD) and temperature-programmed oxidation (TPO). Tar conversion efficiency was assessed by means of carbon conversion, H2 yield (based on the maximum allowed by stoichiometry), gas composition and product yields, with Fe/Al2O3 leading to the highest conversion (87.6 %) and H2 yield (38 %). Likewise, Fe/Al2O3 also provided the highest stability, as it allowed operating for long periods with high conversion values (85.9 % after 35 min on stream), although it underwent severe deactivation. The analysis of the spent catalysts revealed that deactivation occurred mainly by coke deposition on the catalyst surface and iron phase oxidation, with Fe/olivine and Fe/dolomite leading to the faster deactivation due to their poorer metal dispersion related to their reduced surface area. The TPO profiles showed that the coke deposited on the three catalysts was amorphous with a very small contribution of highly structured carbon.

Fuel cells are electrochemical devices that efficiently convert the chemical energy of fuel and an oxidant into electricity. Specifically, SOFCs have advantages such as high efficiency for power generation and fuel flexibility, without the requirement for precious metal catalysts due to their high operating temperature (e.g. from 600 to 900 °C) [1–6]. Stationary combined heat-and-power fuel cell systems have already been commercialized. For example, the number of residential fuel cell systems (commonly called “ENE-FARM”) installed in Japan has already exceeded 400,000 as of August 2021 [7]. The polymer electrolyte fuel cell class of ENE-FARM was launched in 2009, followed by the SOFC class in 2011, and the percentage of the SOFC class is gradually increasing largely due to its higher efficiency [8]. In addition, a 250 kW class SOFC-micro gas turbine hybrid system has been commercialized by Mitsubishi-Hitachi Power Systems (now Mitsubishi Heavy Industries), with a 1 MW system in the planning stage [9]. Meanwhile, Ceres Power in UK has set a target of producing SOFC systems with a cumulative capacity of several hundred MW, having signed commercial contracts with Robert Bosch in Germany, and Doosan in Korea [10]. Moreover, Bloom Energy in the U.S. sold a cumulative 80.9 MW of SOFC system capacity by 2018, representing a leading effort in the commercialization of SOFC systems [11]. However, research and development into SOFCs that operate, e.g., at relatively lower temperatures are still in progress due to the limited availability of materials which can maintain their performance for long periods of time at elevated temperature [12,13].Nickel cermets are widely used as SOFC anodes, and are comprised of Ni and an electrolyte component such as yttria-stabilized zirconia (YSZ) [1–6,14]. However, the Ni-based electronic conducting network in these anodes can be disrupted by repeated cycling of redox reactions, associated with the coarsening and aggregation of Ni particles [15]. Such redox processes occur when the fuel supply is interrupted upon system startup and shutdown, resulting in the deterioration of the overall efficiency of SOFC systems [16–26]. The development of SOFC anodes with high redox cycle durability has another important practical benefit, eliminating the need for an inert gas supply to prevent Ni oxidation during system shutdown, significantly simplifying system design and control.Therefore, it is desirable to develop SOFC anodes with high durability and resilience against redox cycling for the more widespread use of SOFCs. To solve this issue, the use of more redox stable materials such as strontium titanate (SrTiO3) may be helpful [27–30], whilst it can be doped with a higher valence cation at the A site to improve its electronic conductivity [31–35]. Strontium titanate doped with La3+ at the A site (i.e, Sr0.9La0.1TiO3, commonly known as LST) exhibits high electronic conductivity under SOFC operating conditions [31]. Furthermore, LST has a comparable thermal expansion coefficient to that of zirconia-based electrolyte materials [32]. Futamura et al. demonstrated that, impregnation of porous anodes with fine metallic catalyst nanoparticles such as Ni and Rh, high durability against redox cycling could be achieved when supplied with 3% humidified hydrogen fuel, without sacrificing electrochemical performance [30]. However, when hydrogen-related fuel gas is supplied to SOFC systems, the water vapor concentration is reported to increase near the fuel gas outlet, and thus the Ni anode catalysts tend to oxidize [21], one of the major degradation mechanisms of SOFC anode materials. The electrochemical performance of impregnated anodes at high fuel utilization (i.e. at a higher water vapor concentration in the fuel feed) is slightly lower than that of conventional anodes [29,30]. For successful commercialization, both high performance and durability must be achieved simultaneously. The stability of anodes at high fuel utilization, if improved, enables higher power generation efficiency, because a larger fraction of the fuel can be utilized for power generation.As an alternative approach, novel SOFC anodes are being developed using Ni alloyed with transition metal elements rather than using pure Ni. In general, metallic Ni-alloys can exhibit higher electronic conductivity compared to ceramics such as donor-doped SrTiO3. When such alloy-based cermet anodes are exposed to an oxidizing atmosphere, the formation of a surface oxide layer suppresses further oxidation within the alloy particles, and it is possible that this could improve the durability against redox cycling in SOFCs. However, it is well known that doped elements in alloys generally act as scattering sites detrimental to electronic conductivity [36]. Therefore, it is important that Ni-alloys should exhibit both sufficient electronic conductivity and catalytic activity for efficient SOFC operation, even under a hydrogen-based fuel supply. Araki et al. already reported the synthesis of various Ni alloy anodes using a spray drying technique, and investigated the effect of alloying on SOFC efficiency and H2S poisoning [37]. Meanwhile, Ishibashi et al. prepared NiO-containing complex oxide powders and various Ni alloy anodes by ammonia co-precipitation, and reported electrochemical performance and redox cycling durability, finding that cobalt is a promising element for the formation of surface oxide layers for enhanced durability against redox cycling [38].Although the durability may be improved using alloying, cobalt is classed as a critical raw material, i.e. an element of high economic importance, but limited supply [39]. The price of Co will continue to increase due to the rise in demand for lithium-ion batteries, especially in battery electric vehicles (BEVs) [39–41]. As such, the overall amount of cobalt in SOFCs should be minimized where possible, so that the redox durability can be enhanced without risking supply chains or increasing the cost. The source of cobalt and the related geopolitical situation should also be considered.The objective of this study is to develop highly durable Ni–Co alloy cermet anodes, whilst keeping the Co content as low as possible. Gadolinium-doped ceria (GDC) is selected as the electrolyte component in the anodes, and the effect of systematically adjusting the Ni:Co ratio on SOFC performance and microstructure will be investigated. Long-term tests will also be conducted at high fuel utilization (i.e. using a highly-humidified hydrogen supply).A stability diagram was computationally obtained using the software FactSage (Version 7.4, Thermfact Ltd., Canada) in order to examine the thermochemically stable phases of Ni–Co alloys in reducing and oxidizing atmospheres at 800 °C (a typical SOFC operating temperature). In this study, we used the Phase Diagram module of FactSage to derive the Ni–Co and NiO–CoO phase diagrams and the Ni/NiO and Co/CoO phase boundaries.NiO and CoO composite oxide powders were prepared by ammonia co-precipitation, as described in Fig. 1 . Ni(NO3)2⋅6H2O and Co(NO3)2⋅6H2O (Kishida Chemical Co., Ltd., Japan) were used as the precursors for co-precipitation. Briefly, ammonia solution was added dropwise to an aqueous solution containing Ni and Co ions (in various ratios) to simultaneously precipitate these complex hydroxides [38]. The precipitates were then filtered, dried at 100 °C for 10 h in air, and calcined at 1000 °C for 2 h in air. The resulting composite oxide powders were mixed with GDC (Ce0.9Gd0.1O2, Rhodia, ULSA grade, USA) in a mass ratio of 48.1:51.9, resulting in a volume ratio of 50:50 for pure NiO (slightly changing when CoO was also incorporated). Ni–Co-GDC cermet anodes were prepared with a variety of molar ratios, Ni:Co = (100-x):x, where x = 0, 5, 10, 20, and 30, herein referred to as Ni(100-x)Cox-GDC. Lanthanum strontium manganite (LSM, (La0.8Sr0.2)0.98MnO3, Praxair, USA) and scandium-stabilized zirconia (ScSZ, Daiichi Kigenso Kagaku Kogyo Co., Ltd., Japan) were used in the cathode. At the cathode side, a 1:1 LSM:ScSZ mass ratio was used close to the electrolyte, and 100% LSM powder was used close to the current collector [15,38]. Fig. 2 shows a schematic of the cell structure. In this study, the classical electrolyte-supported structure rather than electrode-supported or metal-supported structures is selected, in order to extract the anode-side voltage losses using a reference electrode [38]. The solid electrolyte is comprised of a plate of ScSZ (10 mol% Sc2O3, 1 mol% CeO2, 89 mol% ZrO2) with a diameter of 20 mm and a thickness of 200 μm. The anode layer was screen-printed onto the ScSZ electrolyte followed by heat-treatment at 1300 °C for 3 h in air. The cathode layer was then screen-printed onto the opposite side of the electrolyte followed by heat-treatment at 1200 °C for 5 h in air. The anode and cathode layers were both approximately 40 μm in thickness after heat-treatment. To separate the anode and cathode overvoltages, a reference electrode with an area of ca. 4 mm2 size was placed 2 mm away from the cathode, using Pt paste. A Pt mesh was attached to the surface of each electrode as a current collector, set on the electrodes after screen printing, and then each electrode was heat-treated. The electrode area was 8 × 8 mm2 (0.64 cm2). Fig. 3 shows the configuration of the electrochemical experimental setup. Cell performance tests were conducted using automated fuel cell evaluation systems (AutoSOFC, TOYO Corporation, Japan). Before measuring the cell polarization curves, the cell was sealed using a Pyrex glass ring and increasing the temperature to 1000 °C at 200 °C h−1. Then, the cell temperature was maintained at 1000 °C, and 3% humidified hydrogen was supplied at 100 mL min−1 for 1 h to reduce the metal oxide in the anode to the metallic state. The selected reduction temperature (1000 °C) is sufficient to reduce NiO to metallic Ni in the thin porous anode layers of the electrolyte-supported cells [15,21,29,30,38], whereas a higher temperature may be required for e.g. anode-supported and metal-supported cells with thicker anode layers. The current-voltage (I–V) characteristics were then measured at 800 °C with 3% humidified hydrogen fuel supplied to the anode. The anode voltage (potential) was measured relative to the reference electrode on the cathode side. As the anode voltage includes both ohmic and non-ohmic overvoltages, these overvoltages (i.e., anode-side ohmic loss and anodic overvoltage) were separated via the current interrupt method [15,38]. Each electrochemical measurement was repeated three times, and the average values and their standard deviation are given to check their uncertainty.Redox cycling tests were conducted at 800 °C to evaluate the durability of the anodes, as described in Fig. 4 [15]. First, the cell was operated for 1 h at a current density of 0.2 A cm−2 with 3% humidified hydrogen fuel supply (i); then, the fuel supply was interrupted for 1.5 h (ii); and finally, the supply of 3% humidified hydrogen was restarted (iii). These three steps are regarded as one cycle, and 50 cycles were applied in total. The anode voltage, non-ohmic anodic overvoltage, and anode-side ohmic loss were measured during these cycling tests.The I–V characteristics and durability were also evaluated during continuous power generation up to 1000 h under 80% humidified hydrogen supply. The I–V characteristics were measured under the same conditions as the cell performance test section described above. The anode voltage, anodic overvoltage, and anode-side ohmic loss were measured at a current density of 0.2 A cm−2 and 80% humidified hydrogen over a course of 1000 h.Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) was performed to evaluate the distribution of elements in the whole of anodes using dual-beam SEM (Versa 3D, FEI, USA). High-resolution observation and elemental analysis on the Ni(100-x)Cox-GDC anodes were carried out by means of scanning transmission electron microscopy (STEM, JEM-ARM200F, JEOL, Ltd., Japan).Phase diagrams of Ni–Co and NiO–CoO systems were calculated to estimate the stable phases in both reducing and oxidizing atmospheres. The resulting phase diagrams are shown in Fig. 5 , where the triangular symbols specify the positions at 800 °C for the following compositions: Ni-GDC (black); Ni95Co5-GDC (red); Ni90Co10-GDC (green); Ni80Co20-GDC (blue); and Ni70Co30-GDC (pink). Fig. 5(a) predicts that all five compositions will form Ni–Co alloys with fcc structure in reducing atmosphere. Fig. 5(b) predicts that all five compositions will form a solid solution with a single-phase NiO crystal structure in oxidizing atmosphere. Therefore, in the reducing atmosphere of the SOFC anode, the material is expected to be a true alloy, with Co dissolved in the Ni lattice. Fig. 6 shows a stability diagram calculated after Ishibashi et al. [38], in which the coexisting equilibrium oxygen partial pressures of the metallic and oxide states of Ni and Co are plotted as a function of temperature. In this figure, the equilibrium boundary between Co and CoO is located at a lower oxygen partial pressure compared to Ni/NiO. This indicates that Co is slightly more easily oxidized compared to Ni. As such, when the fuel supply is interrupted in SOFC anodes, Co will be oxidized first, forming an oxide surface layer on the Ni–Co alloy. Evidence of the formation of a Co-rich layer on surface of the Ni–Co alloy has already been observed via higher-resolution TEM [38]. Therefore, based on the calculated thermochemical stability of Ni–Co alloys in reducing and oxidizing atmospheres, it is expected that the redox cycle durability could be improved by alloying Ni with Co through the formation of a protective surface oxide layer. Further analysis is of scientific and technological interest, such as in-situ microstructural observation of redox processes at Ni–Co alloy surfaces. Fig. 5 also confirms that a solid solution will be formed at elevated temperature in both the Ni–Co and NiO–CoO systems. Whilst the sintering temperature of the anodes was around 1300 °C, which is relatively low compared to typical anode-supported cells, these phase diagrams predict the formation of complete solid solutions even at higher sintering temperatures above 1300 °C. Fig. 7 shows representative plots of (a) the anode voltage (anode potential) relative to the reference electrode, (b) the anodic overvoltage, and (c) the anode-side ohmic losses for each anode material, with error bars indicating their standard deviation, measured at 800 °C. Fig. 7(a) reveals that the pure Ni-GDC anode exhibits the highest I–V performance. As shown in Fig. 7(b) and (c), the non-ohmic anodic overvoltage and the ohmic losses of anodes for which x = 10, 20, and 30 are higher than those of the pure Ni-GDC anode. These results suggest that Ni is more active than Co, and that alloying Ni with Co leads to a decrease in electronic conductivity. The dependence of overvoltage on the Co concentration above 10% will be a matter for future studies.In contrast, the anode for which x = 5 exhibits similar I–V characteristics as the pure Ni-GDC anode, as shown in Fig. 7(a). This result indicates that the decrease in power generation performance due to the addition of Co is negligible when the content of Co is sufficiently low. This result suggests that, by optimizing the ratio of Ni and Co, it is possible to prepare alloy-based cermet anodes with high redox cycle durability while maintaining power generation performance.Following the protocol outlined in Fig. 4, the durability against redox cycling was measured three times for each of the five anode compositions. Typical results for the change in (a) anode voltage; (b) anodic overvoltage; and (c) anode-side ohmic loss during the test, measured at 800 °C, are shown in Fig. 8 . The percentage of anode voltage drop from the first cycle to the 50th cycle is shown in Fig. 9 , with error bars based on the standard deviation calculated from the results of three cells for each composition.In Fig. 8, the observed changes during the early stages of the tests could be caused by e.g. the thermal history of the cells during the sintering processes. Regarding durability, Fig. 8(a) reveals that the anode voltage for pure Ni-GDC significantly drops over 50 redox cycles, finally reaching around 0.7 V. In contrast, all the Ni–Co alloy anodes display a much smaller drop in anode voltage, dropping to ca. 0.87 V after 50 redox cycles. Fig. 9 plots the percentage change in anode voltage after 50 cycles, and a clear trend of decreasing degradation with increased Co content emerges. The largest difference is observed between the pure Ni-GDC anode and the Ni95Co5-GDC anode, indicating that the addition of even a small amount (5%) of Co can considerably improves the durability against redox cycling. Fig. 8(b) and (c) reveal that the addition of Co suppresses increases in both the non-ohmic anodic overvoltage and the ohmic losses during redox cycling. This is attributed to the preferential formation of a Co-based surface oxide layer on the Ni–Co alloy, helping to suppress redox-induced aggregation of Ni particles, and thus preventing a decrease in electrode reaction area and maintaining electron-conducting network.Based on the fact that the durability of SOFC against redox cycling can be significantly improved even with the addition of just 5 mol% Co, herein the Ni95Co5-GDC anode is selected as a suitable anode composition for combining durability with minimal content of critical raw materials. Therefore, in the next section, the performance and durability of SOFCs with pure Ni-GDC and Ni95Co5-GDC anodes with 80% humidified hydrogen are evaluated.The electrochemical performance and long-term durability of the pure Ni-GDC and Ni95Co5-GDC anodes are herein investigated under more severe operating conditions, namely at higher water vapor partial pressure, simulating higher fuel utilization. The I–V characteristics and durability up to 1000 h were measured at 800 °C using 80% humidified hydrogen supply. Fig. 10 (a) compares the initial anode voltage, whilst (b) and (c) compare the initial anodic overvoltage, and the initial anode-side ohmic loss, respectively. Meanwhile, Fig. 11 shows (a) the anode voltage, (b) the anodic overvoltage, and (c) the anode-side ohmic loss throughout the 1000-h durability test. Fig. 10 confirms that the Ni-GDC and Ni95Co5-GDC anodes exhibit almost the same I–V characteristics under 80%-humidified hydrogen supply, with the anodic overvoltage and the anode-side ohmic loss also being very similar. This confirms that the addition of a low concentration of Co has negligible or slightly positive effect on the I–V performance even at high fuel utilization, simulating the conditions at the downstream of the fuel supply in fuel cell systems, where redox-related degradation tends to occur. The reactions responsible for the slight improvement in electrochemical performance via alloying Ni with Co under these highly-humidified conditions are of scientific interest, where e.g. dealloying, segregation of secondary phases, formation of nano-composites, and/or their co-catalyst effects could occur.The anode voltage measured over the 1000-h durability test at 800 °C is shown in Fig. 11(a). As the anode voltage in (a) remained stable during 1000 h, certain noisy fluctuation in the raw data shown in (b) and (c) may not influence the overall durability. Fig. 11(a) confirms that the addition of Co slightly improves the long-term durability under highly humidified hydrogen supply. Fig. 11(b) and (c) confirm that this improvement in the long-term durability is mainly associated with a decrease in non-ohmic anodic overvoltage, which is suppressed by the addition of Co.The non-ohmic overvoltages were averaged, and their standard deviations derived in every 100 h throughout the test, giving 80.2 ± 6.1 mV for the pure Ni-GDC anode, and 44.7 ± 2.0 mV for the Ni95Co5-GDC anode. Matsui et al. previously reported that the Ni surface can be oxidized under highly humidified hydrogen supply, reducing the extent of the triple-phase boundary [42], increasing the non-ohmic overvoltage. As such, it is concluded that the addition of Co does not accelerate this process, and therefore can be used in SOFC anodes even in highly-humidified fuel streams.To elucidate the reasons for changes in the performance and durability, the distribution and chemical state of Ni and Co in the Ni–Co alloy anodes were measured before the durability test. Fig. 12 shows representative elemental distribution maps of the Ni95Co5-GDC and Ni70Co30-GDC anodes, analyzed by SEM-EDS. As shown in Fig. 12, in both samples (a) and (b), the location of Ni and Co within the anode layers is exactly the same. This analysis confirms that the elemental distributions of Ni and Co are almost identical in both anodes. The molar ratios of Ni and Co obtained by point analysis compiled in Table 1 are very close to the nominal ratios targeted in the co-precipitation synthesis step, namely Ni:Co = 95:5, and Ni:Co = 70:30, confirming the validity of the phase diagram shown in Fig. 5(a). Fig. 13 shows STEM-EDS elemental distribution maps of the Ni-GDC anode before and after the redox cycling test, for Ni, O and Ce. Before the test, the elemental O distribution largely overlaps with Ce, but not with Ni, suggesting that pure Ni mainly exists as a metal. In contrast, after the durability test, oxygen is observed to additionally be present at the surface of the Ni particles, confirming that nickel particles are oxidized during the durability tests.Meanwhile, Fig. 14 shows elemental distribution maps for the Ni95Co5-GDC anode before and after the redox cycling test, in this case also including the Co distribution. Before the test, the distributions of Ni and Co closely overlap, and the distributions of O and Ce overlap. However, the O distribution does not overlap with either Ni or Co. This confirms that Ni and Co co-exist as an alloy, as expected from the phase diagram in Fig. 5 (a). In contrast, after the durability test, oxygen can additionally be found uniformly on the surface of the Ni–Co alloy particles. However, in this case, Co also appears to be more concentrated near the surface of the alloy particles.Quantitative evaluation of the different ratios of these elements near the surface of the alloy particles was performed using point analysis. Fig. 15 (a) shows a STEM image of the Ni95Co5-GDC anode after the redox cycling test, and Fig. 15(b) shows a higher-magnification image of the highlighted area in Fig. 15(a). The ratios of Ni, Co, and O in the Ni95Co5-GDC anode were then compared by point analysis at positions 1, 2, and 3, with 1 being closest to the surface, and 3 being furthest away. The particle on the right side of position 1 is GDC. The results of this quantitative analysis are shown in Table 2 , confirming that the ratio of O is higher nearer the surface of the alloy particle. Meanwhile, the Ni:Co ratios were 62.6:37.4 at position 1; 91.4:8.6 at position 2; and 92.8:7.2 at position 3, confirming that the Co ratio also increases near the surface. These results clearly indicate that a Co-rich oxide film is formed at the surface of the alloy particles during the redox cycling durability test.These results confirm that a Ni-based oxide layer is formed on the Ni-GDC anode, while a Co-containing oxide layer is formed on the Ni–Co alloy anode during the redox cycling test. As shown in the stability diagram in Fig. 6, Co is more stable as an oxide than Ni. When NiO is reduced to metallic Ni, aggregation is promoted by the formation of Ni fine particles due to volume shrinkage, as verified by Matsuda et al. [43]. Therefore, when the fuel supply is restarted, the dense oxide layer formed on the Ni–Co alloy anode surface is more stable and more difficult to be reduced compared to the Ni-GDC anode, preventing aggregation during the reduction from NiO to metallic Ni.The EDS images of the Ni-GDC and Ni95Co5-GDC anodes after 1000-h power generation test under 80% humidified hydrogen supply are shown in Fig. 16 . In this case, the Ni-GDC anode images reveal thick oxide layers on the surface of the Ni particles. In contrast, images of the Ni95Co5-GDC anode reveal that both O and Co are present at the surface of Ni–Co alloy particles. These micrographs indicate that a Co-containing dense oxide layer is formed at the surface of the Ni–Co alloy particle even under high water vapor pressure. As mentioned in the previous section, the Ni95Co5-GDC anode suppressed the increase in non-ohmic anodic overvoltage during the 1000-h test under 80% humidified hydrogen supply, compared to the Ni-GDC anode. This can now be directly attributed to the difference in the surface compositions of these two anodes.Because Ni acts as an excellent catalyst in SOFC anodes, a decrease in the Ni ratio on the particle surface is expected to decrease the electrocatalytic activity. However, in this case the electrode activity is actually maintained in Ni–Co alloys even when the Ni ratio on the electrode surface decreases. In addition, the alloying of the surface may suppress the aggregation of Ni–Co particles. More detailed mechanisms to explain how the high electrocatalytic activity is retained in alloys is of scientific interest in future studies, and further investigation is needed. This will be performed via e.g. quantitative particle size analysis, impedance measurements, and distribution of relaxation time (DRT) analysis.When these alternative anode materials are applied to practical SOFCs, it will also be essential to evaluate the effects of using impurity-containing fuels [44–51]. Sulfur is a typical SOFC fuel impurity causing cell voltage drop [44–46]. Chlorine is another typical trace impurity in e.g. tap water [44,45,47], while phosphorous causes serious degradation even at ppb levels [44,45]. Siloxane is a contaminant in e.g. digester gas causing silica formation [44,45]. Trace impurities in biofuels affect the performance and durability of SOFCs [48–51]. As such, it will be important to evaluate impurity poisoning in Ni–Co alloy anodes in future studies, associated with various chemical reactions with Ni and/or Co.Ni–Co alloy cermet anodes for SOFCs were prepared and compared to pure-Ni cermet anodes. The initial I–V performance of the Ni–Co alloy anodes was comparable to that of the Ni-based anode. The redox cycling durability was considerably improved by alloying, even at low concentrations of 5 mol%. A 1000-h durability test was performed using highly humidified hydrogen supply, confirming that alloying can suppress increases in non-ohmic anodic overvoltage without compromising the I–V performance, even at high fuel utilization. Electron microscopy revealed that for pure Ni-based anodes, a nickel oxide layer is formed on the surface of the Ni particles, while in the case of alloy anodes, a Co-containing oxide layer is formed during operation. Since Co is a more stable oxide compared to Ni, this Co-containing oxide layer suppresses aggregation of Ni-based particles under redox cycling, or at high water vapor partial pressure. As such, Ni–Co alloy can be regarded as a robust cermet anode material for SOFCs, realizing high electrochemical performance, redox cycling durability, and long-term durability even under a high water vapor atmosphere, and with a Co content as low as 5 mol%. As such redox-stability is more critical for anode-supported cells than for electrolyte-supported cells, it is of technological interest to apply the redox-stable alloy anode material to anode-supported SOFCs and related electrochemical cells.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.A part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO) (Project No. JPNP20005). Collaborative support by Prof. H. L. Tuller, Prof. B. Yildiz, and Prof. J. L. M. Rupp at Massachusetts Institute of Technology (MIT) is gratefully acknowledged. Abbreviations DRT distribution of relaxation time EDS energy-dispersive X-ray spectroscopy GDC gadolinium-doped ceria LSM lanthanum strontium manganite LST lanthanum-doped strontium titanate ScSZ scandium-stabilized zirconia SEM scanning electron microscopy SOFC solid oxide fuel cell STEM scanning transmission electron microscopy distribution of relaxation timeenergy-dispersive X-ray spectroscopygadolinium-doped cerialanthanum strontium manganitelanthanum-doped strontium titanatescandium-stabilized zirconiascanning electron microscopysolid oxide fuel cellscanning transmission electron microscopy

Ni alloys are examined as redox-resistant alternatives to pure Ni for solid oxide fuel cell (SOFC) anodes. Among the various candidate alloys, Ni–Co alloys are selected due to their thermochemical stability in the SOFC anode environment. Ni–Co alloy cermet anodes are prepared by ammonia co-precipitation, and their electrochemical performance and microstructure are evaluated. Ni–Co alloy anodes exhibit high durability against redox cycling, whilst the current-voltage characteristics are comparable to those of pure Ni cermet anodes. Microstructural observation reveals that cobalt-rich oxide layers on the outer surface of the Ni–Co alloy particles protect against further oxidation within the Ni alloy. In long-term durability tests using highly humidified hydrogen gas, the use of a Ni–Co cermet with Gd-doped CeO2 suppresses degradation of the power generation performance. It is concluded that Ni–Co alloy cermet anodes are highly attractive for the development of robust SOFCs.

Nowadays, the increasing energy demand and environment pollution have stimulated the research for the utilization of clean energy, in which hydrogen is considered as one of the most potential energy carrier [1]. For hydrogen economy, the greatest challenge lies in the hydrogen storage. Compared with gas-state and liquid-state hydrogen storage, solid-state hydrogen storage materials with higher energy density are safer [2]. Mg is not only used as a structural material [3,4] or biomedical material [5], but is also regarded as a particularly promising candidate for hydrogen storage, due to its low density, abundant resource and high theoretical hydrogen storage capacity [6,7]. Nevertheless, its thermodynamic stability, sluggish reaction kinetics and inherent low thermal conductivity impede the process of practical applications [8–11]. In the past decades, many solutions have been developed to overcome these problems, such as nanosizing [12,13], catalyzing [14,15], alloying [16], etc.Mechanical milling is widely used to disperse different catalysts. Particularly, transition metals (Nb, V, Ti, Co, etc.) [17,18], their oxide [19,20] and non-metal element [21,22], are usually dispersed into MgH2/Mg system. Among the transition metals, Ni-based compounds exhibit excellent catalytic effect on the hydrogen absorption/desorption of MgH2 [23–25]. When the size is reduced to nanoscale, the catalytic effect will be further enhanced. Liu et al. [23] reported that Mg-Ni nano-composite, prepared by a wet chemical method, could absorb 85% of its maximum hydrogen capacity within 45 s at 125 °C. Furthermore, Ni nanofibers with a diameter of ∼50 nm and porous structure could enhance the desorption properties of MgH2 [26]. For example, the onset temperature (143 °C) and peak temperature (244 °C) of dehydrogenation are much lower for Mg with 4% Ni nanofibers addition, than that mixed with 4% Ni powders (300 °C for onset temperature and 340 °C for peak temperature). In addition, many carbon materials, including graphite [27], single-walled carbon nanotubes (SWNTs) [28], graphene nanosheets (GNs) [29] and activated carbon [30], have been studied to improve the hydrogen storage properties of MgH2. Moreover, recent theoretical calculation has shown that the combination of nanosizing and carbon materials leads to further enhanced hydrogen storage properties [31]. Huang et al. [32] revealed that carbon played an important role on inhibiting the aggregation of the catalysts. All in all, among the carbon materials, graphene, with unique 2D nanostructure, excellent electronic and thermal conductivity, and high chemical stability, is an ideal supporter to host disperse nanoparticles (NPs), which exhibits great catalytic effect in the hydrogen storage areas [33,34]. The combination of the transition metal and carbon seems to be more efficient on the improvement of hydrogen storage [35,36]. Liu et al. [37] synthesized porous Ni@rGO with GO and NiCl2·6H2O as raw materials. Ni NPs loading on the rGO has better catalytic effect on the desorption kinetics of MgH2 than that of Ni or rGO alone. Zhang et al. [38] reported that Ni decorated graphene nanoplate (Ni/Gn), which was prepared with Gn and Ni(NO3)2·6H2O, enhanced the sorption properties of MgH2. The sample of Mg@Ni8Gn2 absorbed 6.28 wt.% H2 in 100 s at 100 °C. To maximize the effect of graphene on preventing the growth and aggregation, a simple solvothermal method was adopted to prepare ordered structure of MgH2 NPs with good dispersion on graphene by Xia et al. [39]. The hydrogen capacity of the composite did not decay after 100 cycles, which is due to the stable structure.To sum up, carbon supported Ni NPs shows a positive effect on enhancing the hydrogen storage properties of MgH2. However, it is still a challenge to prepare uniformly dispersed Ni NPs on the surface of graphene, and the evolution of Ni during the hydrogenation/dehydrogenation cycles is not clear. In this work, Ni@rGO was successfully synthesized through annealing the Ni(OH)2@rGO with aqueous solution of GO and Ni(NO3)2·6H2O as raw materials, and then it was doped into MgH2 by mechanical milling to improve the hydrogen absorption/desorption properties. The effects of milling time and Ni@rGO with different Ni loading amount have been investigated in detail. The evolution of Ni during the hydrogenation/dehydrogenation cycles has been clarified.Typically, 25 mL aqueous solution of GO (8 mg/mL, Shenzhen Matterene Technology Company) was diluted to 1 mg/mL, and then 248 mg Ni(NO3)2·6H2O (AR) was added to the solution by ultrasonic vibration for 1 h. Freshly prepared aqueous solution of NaBH4 (5 mL, 1 M) was added to prepare composite precursor. After stirring the mixture solution at room temperature for 2 h, the products were collected by centrifuge and washed with deionized water and ethanol several times. Then, it was dried in a blowing dry oven at 100 °C for 12 h. Afterwards, the products was processed with experienced heat treatment at 500 °C for 2 h in a tube under Ar/H2. The mass ratio of Ni and GO was 2:8, 4:6 and 6:4, and the corresponding samples were denoted as Ni2@rGO8, Ni4@rGO6, and [email protected] hydride was prepared by reactive ball milling method according to our previous work [40]. Firstly, magnesium (purity > 99.9%, 400 mesh) was mechanically milled under Hydrogen atmosphere with an initial pressure of ∼ 1 MPa, followed by a long-period hydrogenation at 350 °C.The as-synthesized Ni2@rGO8 catalysts and MgH2 powders mixed in a fixed mass ratio of 10:90. The milling was then performed under the hydrogen pressure of 1 MPa, with the rotational speed of 450 rpm, and the ball-to-powder mass ratio of 40:1. The mixtures were milled for different periods (2 h, 5 h, 10 h, and 20 h). The corresponding samples were denoted as Mg-Ni2@rGO8-2h, Mg-Ni2@rGO8-5h, Mg-Ni2@rGO8-10h, and Mg-Ni2@rGO8-20h. The corresponding samples with different Ni@rGO addition were denoted as MH-Ni2@rGO8, MH-Ni4@rGO6 and [email protected] loading amount of the as-prepared Ni@rGO was quantified by differential scanning calorimetry combined with thermogravimetry (TG-DSC, STA449 F3, Netzsch). The structure and morphology of the synthesized materials were characterized by powder X-ray diffraction (XRD, D8 Advance, Bruker), scanning electron microscopy (SEM, Sigma 500, Zeiss) and transmission electron microscopy (TEM, F200, FEOL), respectively.The decomposition performance of the composite was measured by DSC. Hydrogen storage properties were determined by using a homemade HPSA-auto apparatus [41], in which the modified Benedict-Webb-Rubin (MBWR) EOS was applied to calculate the compression factor of H2 gas. To reduce the error of the system, two high-accuracy pressure transducers (Keller, ±0.05% FS) and three thermocouple of Pt 100 were used to monitor the pressure and temperature. Absorption-desorption measurements were performed at various temperature with an initial pressure of 3 MPa for hydriding and 0.0004 MPa for dehydriding, respectively. Before ab/desorption measurements, the samples performed dehydrogenation and rehydrogenation at 340 °C. All materials handling was performed in a glove-box filled with purified argon (99.999%), in which water vapour and oxygen levels were below 1 ppm by a recycling purification system to prevent samples from hydroxide formation and/or oxidation.The XRD patterns of the as-prepared Ni2@rGO8, Ni4@rGO6, and Ni6@rGO4 are shown in Fig. 1 . The peak at 26° is contributed by rGO (002). The characteristic diffraction peaks at 44.5° (111) and 51.8° (200) are attributed to face-centered crystalline nickel. Firstly, GO was reduced by the addition of NaBH4, and Ni(OH)2 was formed from the Ni2+ in the solution and anchored on the rGO (Fig. S1). After annealing in H2/Ar flow gases at 500 °C, the Ni(OH)2 was further reduced to Ni. Based on Scherrer equation, the average grain size of Ni in Ni2@rGO8, Ni4@rGO6, and Ni6@rGO4 were estimated to be 8.5 nm, 13.1 nm, and 15.4 nm, respectively.As-prepared catalysts of Ni2@rGO8, Ni4@rGO6, and Ni6@rGO4 were also characterized by SEM. As shown in Fig. 2 , it can be seen that the Ni NPs are uniformly anchored on the surface of rGO for all samples. The average particle diameter is around 11.5 nm for Ni2@rGO8, 13.5 nm for Ni4@rGO6, and 16.5 nm for Ni6@rGO4, suggesting that the particle size increases with the increasing of Ni loading amount. It is worthy of noticing that, after an exposure in air for a week, Ni NPs doped on the rGO were not oxidized, suggesting that there is a strong interaction between Ni NPs and rGO.The catalyst of Ni4@rGO6 was further characterized by TEM (Fig. 3 ). TEM images show that Ni NPs are uniformly spread over the rGO without aggregation even if the Ni loading amount is up to ∼66 wt.%. The diameter of Ni NPs is about 15 nm. Such superior confinement should be attributed to rGO, which can serve as a support for in-situ formation of Ni NPs and prevent the aggregation and growth during the following heat treatment [39]. In addition, the high-resolution TEM (HRTEM) shown in Fig. 3b demonstrates lattice fringes with interplannar distance of 0.205 nm, 0.177 nm and 0.221 nm, which corresponds to the lattice planes of Ni (111), Ni (200) and graphitic carbon (100), respectively. Besides Ni and C, NiO (111) with interplannar distance of 0.241 nm was also observed. The present of NiO may be due to the oxidization of Ni during the TEM sample preparation. It is observed that the rGO appears in the surrounding of Ni and leaves the Ni facets with high activity exposed [42], which can enhance the “synergistic effect”.To verify Ni loading amount of Ni@rGO, TG measurement was adopted. The as-prepared samples were heated to 900 °C at a rate of 5 °C min−1 under high pure air atmosphere. As a result, the rGO would be oxidized to carbon dioxide and released. Ni would be oxidized to NiO and remained as a residual with the increasing temperature. The Ni loading amount in Ni2@rGO8 was calculated to be 45 wt.% (Fig. S2), 66 wt.% for Ni4@rGO6, and 77 wt.% for Ni6@rGO4. It is worth noting that the Ni loading amounts are higher than the designed values, which is due to the loss of oxygen-containing functional group, i.e., carboxylic, hydroxyl, and carbonyl during the reduction process.After the preparation of the catalysts of Ni2@rGO8, catalyzed MgH2 was then made by reactive ball milling method, which is to mill the catalysts and MgH2 together under a hydrogen pressure of 1 MPa. In order to research the effect of milling time on the hydrogen storage properties, various milling time of 2 h, 5 h, 10 h, and 20 h were applied, and the XRD patterns of the MH-Ni2@rGO8 with different milling time are shown in Fig. 4 . The peaks of all samples mainly correspond to MgH2. However, the peaks of Ni phase were not found in the XRD patterns of all samples. In addition, with the increase of milling time, the diffraction peaks become wider and weaker, indicating that the crystallite size decreases. The grain size of MgH2 was estimated by Scherrer equation, which shows that the grain size decreases in an order of 16.0 nm, 13.8 nm, 11.9 nm, 9.4 nm with the increase of milling time. Decreased grain size of MgH2 would provide more diffusion channel of hydrogen, leading to an improvement of hydrogen sorption kinetics [29].The morphology of as-milled MH-Ni2@rGO8 for different time was further observed by SEM. As shown in Fig. S3, big particles (> 2 μm) can be clearly observed in the sample of MH-Ni2@rGO8-2h. While the milling time is increased to 5 h and 10 h, the particle size is noticeably decreased. However, when further prolonging the milling time to 20 h, the particle size becomes much larger, and flaky particles appeared. It is noted that the variation trend of particle size observed in SEM images is different from that of grain size calculated by XRD results, which indicates that the grain size could be reduced by increasing the milling time. However, powder particles may be much bigger due to the welding and aggregation during the long milling process.The dehydrogenation properties of MH-Ni2@rGO8 with different milling time were investigated by DSC at a constant heating rate (5 °C min−1). As shown in Fig. 5 , both the onset temperature (Tonset) and peak temperature (Tpeak) of dehydrogenation for all MH-Ni2@rGO8 samples shift to low temperature compared with pure MgH2. For the samples of Ni2@rGO8 catalyzed MgH2, The Tpeak of hydrogen desorption is increased in an order of Mg-Ni2@rGO8-5h (280 °C), Mg-Ni2@rGO8-10h (290 °C), Mg-Ni2@rGO8-2h (295 °C), and Mg-Ni2@rGO8-20h (306 °C). It is obvious that 5 h is the best milling time for hydrogen desorption in our experimental condition. The desorption temperature reduces ∼ 98 °C compared to pure MgH2. The hydrogen desorption result is in accordance with the particle size of ball milled samples. When the milling time is short, the particles are not fully ground and still big; when the milling time is too long, the particles will be welded due to the strong mechanical energy input. The reduced particle size leads to the decreasing of diffusion distances for H atom and enhances the hydrogen absorption/desorption kinetics [39]. As shown in Fig. S3, the particle size of MH-Ni2@rGO8-2h (about 5 μm) is bigger than that of MH-Ni2@rGO8-5h (about 1 μm). While the milling time increases to 20 h, large particles (> 2 μm) can be easily observed. Therefore, with the increase of milling time, the Tpeak of hydrogen desorption decreases firstly and then increases.The hydrogen absorption/desorption kinetics of composite MH-Ni2@rGO8 ball milled for various times was then studied by volumetric method. The initial hydrogen pressure during absorption and desorption is 3 MPa and 0.0004 MPa, respectively. The hydrogenation curves at 200 °C are plotted in Fig. 6 a, and the dehydrogenation curves at 300 °C are plotted in Fig. 6b. The results show that the composite of MH-Ni2@rGO8-5h has the fast hydriding/dehydriding rate. It can absorb 5.6 wt.% H2 within 60 s at 200 °C and 4.8 wt.% H2 within 30 min at 100 °C (inset of Fig. 6a). For the desorption properties, MH-Ni@rGO-5h can desorb 6.2 wt.% H2 in 20 min at 300 °C and 6 wt.% H2 in 60 min even at 280 °C (inset of Fig. 6b). When the temperature reaches to 300 °C, all the Ni@rGO-containing samples reach 95% of their maximum dehydriding capacity in 20 min, indicating that faster hydrogen desorption kinetics are obtained at higher temperature. The hydrogen desorption behavior measured by volumetric method is the same as that by DSC. Fig. 6 shows that the increase of the ball milling time would lead to a negative influence on the hydrogen storage kinetics when the milling time is longer than 5 h. The composite of MH-Ni2@rGO8-20h can only absorb 3.9 wt.% H2 within 30 min at 100 °C.Above results showed that the composite of MH-Ni@rGO with a milling time of 5 h had the best hydrogen storage properties. Thus, the milling time of 5 h was used for further study about the catalytic effect of Ni@rGO with different Ni loading amount on the hydrogen storage performance of MgH2. Fig. 7 presents the DSC profiles of the as-milled composite of MH-Ni2@rGO8, MH-Ni4@rGO6 and MH-Ni6@rGO4. The MH-Ni4@rGO6 displays the lowest desorption peak temperature (Tpeak = 259 °C) in comparison of MH-Ni2@rGO8 and MH-Ni6@rGO4. Furthermore, it is worth to note that the onset temperature of MH-Ni4@rGO6 (Tonset = 190 °C) is much lower than that of MH-Ni2@rGO8 (Tonset = 240 °C) and MH-Ni6@rGO4 (Tonset = 270 °C). In addition, MH-Ni6@rGO4 shows two desorption peaks at higher temperature, one is at 285 °C and the other is at 320 °C, which indicates that the excessive Ni NPs may aggregate on the rGO and weaken the interaction between Mg and rGO. Obviously, too much Ni loading would degrade the catalytic effect. Fig. 8 gives the hydrogen absorption and desorption kinetics curves of MH-Ni2@rGO8, MH-Ni4@rGO6, and MH-Ni6@rGO4. The MH-Ni4@rGO6 can absorb 3.7 wt.% H2 at 100 °C in 10 min, higher than that of MH-Ni6@rGO4 (3.0 wt.%) and MH-Ni2@rGO8 (2.7 wt.%). The dehydrogenation kinetics is further improved in the sample of MH-Ni4@rGO6. At 300 °C, the MH-Ni4@rGO6 can desorb 6.1 wt.% H2in15 min, while the MH-Ni2@rGO8 needs 20 min to reach the same capacity. Moreover, the MH-Ni6@rGO4 can only desorb 5.7 wt.% H2, which is due to the higher Tpeak (320 °C).To facilitate comparison, representative hydrogen absorption/desorption data for MgH2 system (with metal/carbon catalysts, prepared by ball milling method) are summarized in Table 1 [29,32,37,43,44,45]. Obviously, the Ni4@rGO6 shows outstanding catalytic efficiency in enhancing the ab/dehydrogenation kinetics of MgH2. In comparison with other reported systems, the sample of MH-Ni4@rGO6 is competitive in the hydrogen absorption at low temperature (100 °C) and hydrogen desorption at 300 °C.The composite of MH-Ni4@rGO6 exhibits excellent hydrogen absorption and desorption properties. The PCT measurements of hydrogen absorption and desorption for the sample were performed at 300 °C, 320 °C, and 340 °C in a hydrogen pressure range from 0.05 to 2 MPa. Particularly, to ensure equilibrium, each absorption/desorption stage lasts at least 2 h. As shown in Fig. 9 a, the plateau pressure was measured as 0.205, 0.320, and 0.499 MPa for absorption and 0.162, 0.270, 0.420 MPa for desorption at 300, 320, 340 °C, respectively. The corresponding van't Hoff plots (Eq. 1) for both hydrogen absorption and desorption are shown in Fig. 9b. (1) ln P = 1 T ( − Δ H R ) + C Where P is the H2 pressure, T is the temperature, ΔH is the enthalpy, R is the gas constant (8.3145 J mol− 1 K−1), and C is a constant. The C value equals to ΔS/R, in which ΔS refers to entropy.According to the fitting result, the hydride formation and decomposition reaction enthalpy value (ΔH) are calculated to be −64.8 and 69.6 kJ mol−1, respectively. The values are lower than the theoretical values of MgH2 [38]. The results indicate that the addition of Ni@rGO can destabilize the MgH2, which might be a reason for lower onset dehydrogenation temperature shown in the DSC curves. Therefore, it can be concluded that the Ni@rGO can not only improve the absorption/desorption kinetics, but also change the thermodynamics.A number of kinetic models for the gas-solid reaction were adopted to analyze the evolution of kinetics, such as Johnson-Mehl-Avrami-Kolmogorov (JMAK) model [46,47], Chou model [48,49], etc. The classical JMAK model can well describe the hydrogenation and dehydrogenation of nucleation-growth-impingement mode. Thus, the improved kinetics of hydrogenation and dehydrogenation of MH-Ni4@rGO6 was further determined by the JMAK model (see Eq. 2) through fitting the absorption and desorption curves of MH-Ni4@rGO6 and the activation energy (Ea ) can be calculated according to the Arrhenius equation (Eq. 3). (2) ln [ − ln ( 1 − α ) ] = n ln t + n ln k (3) k = A exp ( E a R T ) Where k is the reaction rate constant, n is the Avrami exponent of the reaction order, α is the fraction transformed at time t, and A is the temperature-independent coefficient. The reacted fraction of 0.2 < α < 0.8 was used in this study. Fig. 10 shows the JMAK plots for the absorption of the MH-Ni4@rGO6 at the temperature of 100, 150, 200 °C, and desorption at 280, 300, 320 °C. Generally, the rate-limiting process, growth dimensionality and nucleation behavior of the hydrides can affect the reaction order n. The n values of hydrogenation (0.91, 1.03 and 1.09) are close to 1 (Fig. 10a), indicating that the hydriding reaction of the sample follows a diffusion-controlled mechanism [50,51]. There are numerous growth and nucleation scenarios consistent with a value of n = 1, including nucleation and growth along one-dimensional (1D) dislocation lines and thickening of cylinders, needles and plates. Jeon [52] pointed out that hydrogen atoms rapidly nucleate and accumulate along the defects and form a metal hydride layer in one dimension, followed by subsequent growth and thickening from the metallic core. Similarly, the n values are in close proximity to 1.5 for decomposition of MH-Ni4@rGO6 at 300 and 320 °C (Fig. 10b). Thus, the phase transformation from MgH2 to Mg in this case exhibits a zero nucleation rate, which is consistent with previous report [28]. Besides, the Ea of MH-Ni4@rGO6 is calculated to be 47.6 ± 3.4 kJ mol−1 for hydrogenation (Fig. 10c) and 117.8 ± 3.4 kJ mol−1 for dehydrogenation (Fig. 10d). The value for hydrogenation is lower than that of MgH2-5 wt.% GNs (Ea of hydrogenation: 78.4 kJ mol−1) [53], and the dehydrogenation value is also lower than pure MgH2 (157 kJ mol−1) [37].Above results show that Ni4@rGO6 is an excellent catalyst to improve the hydrogen storage properties of MgH2. Generally, the uniformly distributed ultrafine Ni NPs could be beneficial to the decomposition of H2 and recombination of atomic hydrogen during hydrogen absorption/desorption [54]. Moreover, the graphene can also provide more nucleation sites for the alloy or hydride and hydrogen diffusion channel [39], exhibiting a “synergistic effect” with Ni. To gain a further insight into the catalytic mechanism, XRD measurement was carried on the samples of MH-Ni4@rGO6 at following five states: as-milled MH-Ni4@rGO6, after first dehydrogenation, after first rehydrogenation and after 8th de/rehydrogenation. As shown in Fig. 11 , XRD pattern of as-milled MH-Ni4@rGO6 sample exhibits diffraction peaks corresponding to MgH2 and Ni (Fig. 11a), indicating that Ni has not reacted with Mg during ball milling process. After first dehydrogenation, only two phases can be detected, Mg and Mg2Ni (Fig. 11b), implying that Ni has already reacted with Mg and transformed to Mg2Ni. During the next rehydrogenation and dehydrogenation cycles, except for the main phase transformation of Mg/MgH2, the phase transformation of Mg2Ni and Mg2NiH4 occurred (Fig. 11c–e).It is well acknowledged that the XRD investigation can not give the information of trace amount of sample or amorphous sample. Thus, the evolution of Ni during the dehydrogenation process of MH-Ni4@rGO6 was also detected by TEM. As shown in Fig. 12 a, the size of dehydrogenated MH-Ni4@rGO6 particles is around 200–600 nm, and some small catalyst particles are anchored on the surface of matrix. Further HRTEM analysis (Fig. 12c–d) shows that MH-Ni4@rGO6 sample was fully dehydrogenated to Mg and Mg2Ni with a crystallize size of smaller than 10 nm. Moreover, Ni NPs can't be found in the HRTEM images, implying that the Ni NPs were totally reacted with Mg and yielded Mg2Ni during the hydrogenation of MH-Ni4@rGO6, which agrees with the result of XRD (Fig. 11). Moreover, it is worthy of note that, a metastable alloy of Mg6Ni was also identified by HRTEM, and evidenced by selected area electron diffraction (SAED) pattern (Fig. 12b). It has been reported that Mg6Ni alloy can be formed in a Mg83Ni17 alloy due to the solute accumulation in solidification, and decomposes into Mg and Mg2Ni with a low velocity at the temperature of 300-350 °C [45]. In this study, the as-milled MH-Ni4@rGO6 sample was composed of MgH2 and Ni rather than Mg-Ni alloys (Fig. 11). During the following dehydrogenation process, Mg6Ni and Mg2Ni alloys might be formed due to the entering of Ni atoms into the lattice of Mg. At high temperature, Mg6Ni alloy is not stable and transforms into Mg and Mg2Ni again. In addition, as shown in Fig. 12d, Mg2Ni and rGO grains were clearly dispread in the surrounding of Mg, which indicates that the co-catalyst of Mg2Ni and graphene nanosheet may have a “synergetic effect” on the hydrogen storage properties of Mg.Hydrogen desorption kinetics of as-milled MH-Ni4@rGO6 and rehydrogenated MH-Ni4@rGO6 are presented in Fig. 13 . It can be clearly seen that the hydrogen desorption rate of rehydrogenated samples are faster than that of as-milled sample. According to the above XRD and HRTEM results, the only difference between as-milled and rehydrogenated sample is chemical surrounding of Ni. We believed that the change of dehydrogenation kinetics of MgH2 in the sample of MH-Ni4@rGO6 is due to the change of Ni: elementary Ni for as-milled sample and Mg2NiH4 for rehydrogenated samples. The result indicates that the in-situ formed Mg2Ni/Mg2NiH4 may have better catalytic effect than Ni.Interestingly, we found that Ni NPs in the sample of MH-Ni2@rGO8 seems to be more stable than Ni NPs in the sample of MH-Ni4@rGO6. As shown in Fig. S4, although Ni is not detectable in the as-milled MH-Ni2@rGO8, the diffraction peaks of Ni still present in the rehydrogenated samples even after 8 cycles. And Mg2Ni/ Mg2NiH4 is not appeared in all MH-Ni2@rGO8 samples, implying no reaction between Ni and Mg. For the sample of MH-Ni2@rGO8, the hydrogen desorption kinetics of as-milled sample is almost the same as that of rehydrogenated sample (Fig. S5). The results further proved that the in-situ formed Mg2Ni has more effective catalysis than Ni. It is well acknowledged that, Mg2NiH4 is easier to release hydrogen compared with MgH2. Therefore, the in-situ formed and uniformly dispersed Mg2NiH4 on the surface of rGO can serve as a “hydrogen pump” to enhance the dehydrogenation kinetics [18,41]. The rGO could provide more active “catalytic sites” and H “diffusion channels” to reduce the dehydrogenation temperature and enhance the dehydrogenation kinetics [36], leading to a “synergetic effect” with Mg2NiH4. According to the results, it is assumed that there is a strong interaction between Ni NPs and rGO. When the amount of Ni is low, the binding may be strong enough to prevent the reaction between Ni and Mg/MgH2. However, if the amount of Ni is high, the interaction between Ni NPs and rGO will be weakened, therefore, the Ni NPs can react with Mg/MgH2 more easily. It is worth noting that too much Ni loading will weaken the interaction between Mg and rGO and degrade the catalytic effect. (1) Ni@rGO with different loading amounts was synthesized by wet chemical method, and the average crystallites size of Ni for Ni2@rGO8, Ni4@rGO6, Ni6@rGO4 were calculated to be 8.5 nm, 13.1 nm, and 15.4 nm, respectively. (2) The MH-Ni4@rGO6 composite absorbs 5 wt.% hydrogen in 20 min at 100 °C. And the composite shows enhanced dehydrogenation rate: it can release 6.1 wt.% hydrogen within 15 min at 300 °C. The activation energy for the rehydrogenation of MH-Ni4@rGO6 is 47.6 ± 3.4 kJ mol−1. Hydride formation and decomposition reaction enthalpy (ΔH) are determined to be −64.8 and 69.6 kJ mol−1, respectively, indicating a little thermodynamic change for the composite. (3) We found that the in-situ formed Mg2Ni/ Mg2NiH4 exhibits better catalytic effect than Ni. Ni couldn't react with Mg due to the strong interaction between rGO and Ni NPs when the loading amount of Ni is low. Ni@rGO with different loading amounts was synthesized by wet chemical method, and the average crystallites size of Ni for Ni2@rGO8, Ni4@rGO6, Ni6@rGO4 were calculated to be 8.5 nm, 13.1 nm, and 15.4 nm, respectively.The MH-Ni4@rGO6 composite absorbs 5 wt.% hydrogen in 20 min at 100 °C. And the composite shows enhanced dehydrogenation rate: it can release 6.1 wt.% hydrogen within 15 min at 300 °C. The activation energy for the rehydrogenation of MH-Ni4@rGO6 is 47.6 ± 3.4 kJ mol−1. Hydride formation and decomposition reaction enthalpy (ΔH) are determined to be −64.8 and 69.6 kJ mol−1, respectively, indicating a little thermodynamic change for the composite.We found that the in-situ formed Mg2Ni/ Mg2NiH4 exhibits better catalytic effect than Ni. Ni couldn't react with Mg due to the strong interaction between rGO and Ni NPs when the loading amount of Ni is low.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Natural Science Foundation of China (Grant No. 51671118), the research grant (No. 16520721800 and No. 19ZR1418400) from Science and Technology Commission of Shanghai Municipality. The authors gratefully acknowledge support for materials analysis and research from Instrumental Analysis and Research Center of Shanghai University.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jma.2019.06.006. Image, application 1

Uniform-dispersed Ni nanoparticles (NPs) anchored on reduced graphene oxide (Ni@rGO) catalyzed MgH2 (MH-Ni@rGO) has been fabricated by mechanical milling. The effects of milling time and Ni loading amount on the hydrogen storage properties of MgH2 have been investigated. The initial hydrogen desorption temperature of MgH2 catalyzed by 10 wt.% Ni4@rGO6 for milling 5 h is significantly decreased from 251 °C to 190 °C. The composite can absorb 5.0 wt.% hydrogen in 20 min at 100 °C, while it can desorb 6.1 wt.% within 15 min at 300 °C. Through the investigation of the phase transformation and dehydrogenation kinetics during hydrogen ab/desorption cycles, we found that the in-situ formed Mg2Ni/Mg2NiH4 exhibited better catalytic effect than Ni. When Ni loading amount is 45 wt.%, the rGO in Ni@rGO catalysts can prevent the reaction of Ni and Mg due to the strong interaction between rGO and Ni NPs.

Propylene is one of most important petrochemical products, as it could build huge variety of chemical commodities. In traditional petrochemical technology network, cracking technology undertake most supply of propylene. Nowadays, more than three-fourths propylene is produced as a by-product by naphtha steam cracking and fluid catalytic cracking (FCC) (Bhasin et al., 2001; Hu et al., 2019; James et al., 2016; Nawaz, 2015; Wang et al., 2020; Al-Douri et al., 2017). However, the growth requirement of propylene cannot be satisfied. Facing challenges from supply and demand side, it is urgent to improve novel propylene generation technology. In recent years, a lot of attempts have been adopted to give traditionally cracking technology more flexibility and increase yield of propylene (Alabdullah et al., 2020). Coupling catalytic and thermal cracking process has been got attention, which deepen the cracking degree of reactant and widen feeding to wide-range oils. A novel cracking technology called DCC was developed by Sinopec Research Institute of Petroleum Processing. Base on FCC technology, this technology further takes higher temperature and partial pressure of steam, which deepen cracking degree and thus produce more propylene (Akah and Al-Ghrami, 2015). However, the selectivity of propylene in these technologies is still poor and oil-feeding would cause serious energy-consumption (Blay et al., 2017). Some novel technologies called on-purpose propylene production technology has been attractive alternatives to traditional cracking process, such as oxidative coupling of methane, methanol to propylene, Fischer-Tropsch synthesis and direct dehydrogenation of propane (PDH). Compared with the traditional process, they have highly selectivity and don't need oil-feeding. (Hu et al., 2019). Recently, PDH due to cheap and widely available raw material (propane) receives much attention, this transformation from gas fuel (propane) to chemical product (propylene) also indicated huge economic profits. Thus, PDH is widely considered to the most promising propylene production technology in the future.For improving industrial sufficiency of PDH, developing eco-friendly, cheap and active catalysts play the core role. At present, Pt and Cr-based catalysts are successfully industrialized, and various kinds of (Ga, V, Zn, Zr, Ni, Co, Fe, Al, Sn, Mo and other noble metals) the catalysts have been studied and reported widely (Hu et al., 2019; Nawaz, 2015; Sattler et al., 2014). In order to provide a scientific understanding on these numerous catalysts, researches had made a lot of excellent reviews. Sattler et al., 2014 classified common PDH catalysts by element and make a very detailed review on their reaction mechanism, promoter and support. Hu et al., 2019 further widened types of catalyst and classified them into three categories: metal, metal oxide and carbon catalysts by an intuition of overall structure of materials. Chen et al. (2020) subdivided the existing metal and oxide catalysts based on the active site structure. However, the structure-performance relationship is rarely summarized and explained in basic principles of chemistry. For the oxide catalysts, most reviews are classified by elements (Hu et al., 2019; Nawaz, 2015; Sattler et al., 2014). In addition, the role of adsorbed gas and coke product on activity is also lacked summary. Therefore, the structure-performance relationship in propane dehydrogenation still needs to be outlined systematically.In this review, we summarize theoretical and experimental researches achievement of catalysts in recent years. Reaction mechanism and nature of active sites are discussed in detail assisted by researches on well-defined model catalysts and density functional theory (DFT) calculations. Furthermore, we outline the effect of supports and promoter on structural and electronic properties of catalysts as well as their effect on catalytic performance. Coke formation, as the main culprit causing deactivation, its deposition mechanism and structure-activity relationship is highlighted. In the end, the influence of adsorption of co-feeding gas is introduced to emphasize importance of process intensification. We hope this paper would contribute rationally design and optimization PDH catalyst in the future.For understanding PDH processes intuitively, Fig. 1 displays a simplified process of main and side reactions in PDH reaction. Formation of equimolar proplyene from propane is the main reaction of PDH. Side reactions include cracking, hydrogenolysis and coke deposition. Some other side reaction such as oligomerization, cyclization also happened but occupy a small part. Cracking includes a group reaction of cleavage hydrocarbons to two smaller alkene or alkane. Their pathway in PDH includes two mechanisms: thermal cracking and catalytic cracking. Coke deposition refers to formation various carbon-rich hydrocarbons or macromolecule solid carbon though deep dehydrogenation. Due to its gas-solid two-phase reaction, thus it is not limited by reaction equilibrium in PDH. Both catalytic cracking and coke deposition are directly catalyzed through active sites or Lewis/Bronsted acidic sites. Specially, hydrogenolysis reaction initiate on the surface of metal, in which alkane would be converted into two smaller alkanes assisted by hydrogen (Sattler et al., 2014).In the process of catalytic PDH, hydrogen atoms in propane would play a protecting role and prevent activation of C–C bonds. DFT calculations also proved breaking the C–C bond of alkane when the C–H bond is abundant are difficult, fracture energy of C–C bonds and C–H bonds would be lower with taking away hydrogen atoms(Yang et al. 2010, 2012; Huš et al., 2020). Therefore, propylene is widely thought as the key intermediates of side product of side reaction rather than propane, and it has a series reaction like relationship between side reaction and main reaction. (Yang et al., 2010; Nykänen and Honkala, 2011; Valcárcel et al., 2002; Yang et al., 2010). The barriers of propylene desorption and dehydrogenation is often used as criteria to evaluate selectivity (Sun et al., 2018).The main reaction of PDH is strongly endothermic and increases in total moles of gas, thus lower partial pressure benefits the appropriate conversion. The high temperature is also necessary for breaking the limits of thermal balance. To maximize propylene generation, temperature needed in PDH reaction is about 550–700 °C (Yang et al., 2012). It is noted that some other methods are used to break through the limit of thermodynamics, such as oxidative dehydrogenation (Atanga et al., 2018) (the addition of CO2, O2 or other oxides into reaction gas to increase the conversion by removing H2), adsorbed membrane reactor(James et al., 2016; Kim et al., 2016) (remove H2 by membrane exchange) and partial alkane combustion, however, they all have their own limitations and have not been used in industrial propane dehydrogenation. (Kong et al., 2020).In summary, for PDH, fracture of C–H bonds is the focused question. Catalyzing cleaver of C–H bonds is an important organic reaction. For heterogeneous catalytic cleavage of C–H bond, its mechanism is often considered as oxidation mechanism or electrophilic mechanism and could be catalyzed by oxide or metal catalysts (Lillehaug et al., 2004). For alkane dehydrogenation, after a long screen in the past, PtSn- and Cr-based/Al2O3 catalysts have been applied in industrial application(Nawaz, 2015). Other catalysts, such as Ga (Yu et al., 2020; Im et al., 2016), V (Wu et al., 2017), Zn (Camacho-Bunquin et al., 2017; Schweitzer et al., 2014), Zr (Otroshchenko et al., 2017), Sn (Wang et al., 2016), Fe (Tan et al., 2016) based-catalysts, are also widely studied and may be competitors for applications in the future. However, they all have their own limits in stability, selectivity and conversion. Deep understanding structure-activity relationship of metal and oxide catalysts is very important to improve catalytic performance. During reaction condition, the formation of coke deposition and the adsorption of gaseous species also have important influence on the structure of the catalyst.VIIIB metals have good performance in catalyzing dehydrogenation reaction. Among them, Pt has the best catalytic performance (Sattler et al., 2014). Pt-based catalysts have excellent one-way stability, high selectivity and undoubtedly first-class activity, which provide great convenience to product separation and long-time continuous reaction. The industrialized PtSnK/Al2O3 catalysts have stability up to several hours and high selectivity, which has been used to provide high purity propylene for polypropylene industry (Sattler et al., 2014). The whole process is powered by a semi-continuous interstage heater and regenerated dynamically in moving bed for maintaining a uniform propylene outlet flow rate. Another famous Pt-based catalyst developed by Uhde co-feed steam and thus further improve anti-coke stability (Nawas et al., 2015). However, high-level price still limits widely utilization of Pt-based catalysts in PDH. Pt has almost 100 times higher price than elements used in oxide catalysts. In general, loading of Pt is need to be controlled lower than 1 wt% to maintain its economic benefits. Recently, the latest platinum-based catalyst of UOP even has only a 0.3w%Pt loading (Ji et al., 2020). Cost-efficient has been the key challenges for Pt-based catalysts. At the same time, pure Pt-based catalysts always have low selectivity and suffered by coking and sintering, it is necessary to improve the supports, promoters and synthesis methods (Liu and Corma, 2018) (Liu et al., 2019). Except Pt, other VIIIB metal catalysts due to their lowly intrinsic selectivity or poor activity are less used in PDH. However, some of them have higher C–H cleaver activity and lower price than Pt, this gives them great potential in future (He et al., 2018).For metal catalysts, the whole PDH reaction could be catalyzed by metal cluster from multiple atoms to even single metal atom theoretically (Yang et al., 2011). A whole adsorption diagrams during dehydrogenation and deep dehydrogenation are shown in Fig. 2 a. Reaction mechanism of PDH in metallic catalysts is widely recognized as a reverse Horiuti-Polanyi (HP) mechanism, which could be divided into four steps(Sattler et al., 2014):Propane would firstly adsorb on surface of metal thought a weak physisorption drove by van der Waals force. (Lian et al., 2018; Yang et al., 2010). Then, two adjacent C–H bonds on propane are successively dissociated by two adjacent metallic atoms. Finally, propylene and H2 desorbed from metal surface. Two C–H cleaver steps always have the highest energy barrier in this mechanism, thus PDH is also widely considered as a surface reaction-controlled process follows Langmuir−Hinshelwood kinetics.While main reaction could occur on single metallic atom, undesired reaction step such as deep dehydrogenation and cleaver of C–C bonds in PDH need more adjacent active sites. (Yang et al., 2011). Hydrocarbons adsorbed on metal surface would more like retain their alkane structure(Yang et al., 2010). For example, propylene adsorb on Pt (111) face though di-σ bonds, these bonds would still keep their SP3 hybridization structure. While the hydrogen atoms continuously dehydrogenate from propane, in order to maintain original coordination morphology of propane, more consecutive sites are favored(Lian et al., 2018). Pure metal catalysts due to their multiple sites always would suffer severe side reaction.(Pham et al., 2016). Generally, 3-follow active sites are considered to be the smallest site that could catalyze the side reactions effectively(Zhao et al., 2015) Purdy et al. (2020) synthesized a variety of Pd-based catalysts alloyed with different promoting metals including Zn, Ga, In, Fe and Mn. As showed in Fig. 2b, Pb atoms in PbZn and PbIn alloy nanoparticle catalysts only exist as the isolated Pb sites and thus have the highest selectivity. Pb3Fe and Pb3Mn alloy have relatively lower selectivity because of the presence of three-fold Pb sites. For Pb3Ga alloy, although 3-fold Pb sites exist, the positive triangle structure of threefold Pb ensemble is distorted by alloying, so it has higher selectivity than Pb3Fe and Pb3Mn. This result reveals the importance of site continuity for selectivity.The coordination environment of metal atoms also plays important role. Defects in end of atomic arrangement, such as the edges or corners, have low coordination number and high electron deficiency. These unsaturated coordination metal atoms strongly adsorb reactants and is more preferred to dehydrogenation and deep dehydrogenation. Size or structure of nanoparticles would affect amounts of defect directly. Zhu et al. (2015) synthesized a serious Pt/Mg(Al)O catalysts with different particle size form 1 nm–10 nm for PDH. They found sample with medium particle size shows the highest propylene formation rate. When particle size increases, propylene selectivity increase but conversion rate of propane decreased, which result a volcano curve relationship between particle size and propylene formation rate showed in Fig. 2c. Author think this structural-sensitivity-like phenomenon is attributed to the surface of small platinum particles have more unsaturated step atoms which not only have a higher dehydrogenation activity but also stronger C–C cleavage tendency than terrace atoms (Nykanen et al., 2013).In sum, the continuity and coordination environment of metal active sites play important role in catalytic performance. Nature of active sites in metal catalysts could be regulated by the interaction of other components such as promoters and supports. Addition of second metal could alloy with active metal, resulting metal-metal bonds could adjust catalytic performance. Support provides adsorption sites and surface area available for dispersing metal particles. In some cases, interfacial area between support and metal also plays complex electronic and geometric interaction. Next two parts, we would outline these structure-activity relationships in detail.Pure metal catalysts always have low selectivity and suffered by serious coking and sintering. Alloyed with second promoting metal could effectively modify catalytic performance from ensemble and electronic effect. Sn is the most used promoter in Pt-based catalyst and famous as industrial promoter used in commercial PtSnK/Al2O3 catalysts. It has been widely reported that the addition of Sn could restrain side reaction and improve catalytic conversion (Pham et al., 2016; Wang et al., 2019). In-situ characterization techniques have identified transformation of chemical state of Sn from oxide state to metallic state and Sn atoms move into platinum lattice forming Pt–Sn alloy during the PDH process (Deng et al., 2014; Iglesias-Juez et al., 2010; Kaylor and Davis, 2018). However, microscopic interaction between Sn and Pt in nanoparticle is still illusive due to complexity of real alloying metal particles. Ensemble effect and electronic effect has been used to explain the role of Sn. Electronic effect thinks Sn atoms would play its role as an electronic donor and increase the electronic density of 5d band of platinum atoms, enrichment of this electronic state benefit rapid desorption of proplyene due to repel force between electron-rich π bonds in propylene and metal surface (Deng et al., 2018; Long et al., 2016). Ensemble effect think non-reactive Sn atoms would dilute large platinum ensembles into small clusters by formation of alloy and thus increased the selectivity of the reaction due to structural sensitivity of side reaction in PDH (Zhu et al., 2014; Zhu et al., 2017).For explaining role of Sn, the researchers have made a lot of effort in modeling ideal surfaces and DFT calculation. Yang et al. (2012) performed a DFT research of dehydrogenation of propane on Pt, Pt2Sn1 and Pt3Sn1 (111) face for explaining the ensemble and electronic effects of Sn in various surface of Pt–Sn alloy. Authors found that the Sn atom causes a downshift of d-band center of adjacent platinum atoms, which results in the weaker bond between carbon atoms of hydrocarbon and Pt atoms. However, downshift of d-band center also improves energy barrier of dehydrogenation step, thus lead to decrease in conversion. Results of calculation also indicate Pt3Sn bulk is the most suitable alloying surface. Another explanation think Sn atoms would preferentially cover and deactivate Pt atoms in edge, corner which are electron deficient and are apt to catalyze side reaction (Virnovskaia et al., 2007). Nykänen and Honkala, 2013 compared the PDH performance on step sites of pure Pt (211) and Pt3Sn (211) face in which Sn atoms located on the step edge of platinum based on DFT. In Fig. 3 a, as the Pt (211) step sites with coordination unsaturation would adsorb propylene tightly, after decorated by Sn, Pt3Sn (211) edge would have a obviously weaker bond with propylene. Sn atom located at edge significantly inhibited deep dehydrogenation and weaken propylene adsorption, and improve selectivity. For understanding catalytic process in experiment, well-defined Pt–Sn alloy surface for experimental verification are also ongoing to provide a better understanding. Zhu et al. (2014) synthesize a serious of Sn surface-enriched Pt–Sn nanoparticles though a surface organic chemistry (SOMC) method and support them on MgAl2O4. As shown in Fig. 3b, with the increase of Sn usage, the surface of nanoparticles gradually appears an enrichment of Sn and form spherical shell-like structure, while the particle size remains unchanged. With the process of Sn surface enrichment, the selectivity and conversion gradually increase. It is directly proved that isolation effect of Sn has an important effect on the PtSn catalysts.Although the electronic and ensemble effect have successfully explained many experimental phenomena in PtSn catalysts, these studies are only limited to the surface of PtSn nanoparticles. Under really experimental conditions, the interaction between Sn and Pt is far beyond surface interaction, the difference between the inner and outer layers in alloy could still cause large difference in reaction performance (Wu et al., 2018). Ye et al. (2020) synthesized a series of PtSn@Pt/SiO2 catalysts with a well-defined Pt1Sn1 surface. With the increase of the number of subsurface Pt–Sn coordination bonds, the selectivity almost remains unchanged while the TOR continue increasing. Therefore, author considered that the isolated effect by surface Sn atoms mainly affect selectivity, while the TOR is more related to electronic effect of subsurface Sn. The structure-performance relationship is shown in Fig. 3d. It can be seen that the surface and internal structure of Pt–Sn catalyst have important effects on the catalytic performance.In addition, Sn element has many other effects on Pt-based catalysts. Pham et al. (2016) studied the structural changes of Sn of PtSn/γ-Al2O3 in regeneration-reaction process and found interaction between Sn and Al2O3 would play an important role. After the process of oxidative regeneration, Sn would de-alloy from Pt–Sn nanoparticles and anchor on the Al2O3 support as Sn atoms or clusters. These adsorbed Sn sites provide nucleuses for recreation Pt–Sn nanoparticles. Therefore, Pt–Sn catalysts would recover their own high dispersion after redox process used Al2O3 as support, while the size of pure-Pt catalysts will continue increasing after several regenerations. Although the traditional idea is that Sn can only improve the activity of catalyst in the form of alloy, recent studies have shown Sn in oxide state could also improve the catalytic performance and like a strong metal-support interaction (SMSI). Deng et al. (2018) reported a SMSI effect between Pt and SnOx in Pt–Sn/SiO2 catalysts. Traditionally, Sn is pretreated by H2 to form an alloy structure though ensemble and electronic effect, author found Sn in Pt–SnO2/SiO2 pretreating by O2 and N2 would play similar role, even all Sn species are only existed as SnO2. A more detailed structure modeling and comparison of catalytic performance is shown in Fig. 3c. Compared with the Pt catalyst without Sn, the electronic state of Pt with SnOx was effectively increased after pretreatment of nitrogen, resulting in the improvement of selectivity and conversion. Identify promoting role of Sn still needs to further researches.Other metals, such as Ga (Bauer et al., 2019), Zn (Rochlitz et al., 2020) and In (Xia et al., 2016), Cu (Ren et al., 2018; Han et al., 2014), V (Purdy et al., 2020), Co (Cesar et al., 2019), Mn (Wu et al., 2018; Fan et al., 2020) and Fe(Cai et al., 2018), are also widely used in Pt-M catalysts (M stands for promoting metal) and their role of promoters are always explained like Sn. Nakaya et al. (2020) synthesized a novel PtGa-Pb/SiO2 ternary alloy catalyst with isolated Pt atom structure. In the surface of PtGa nanoparticles, Pd atoms would selectively locate on threefold Pt sites driving by thermodynamic effect and only keep isolated Pt active sites. PtGa-Pb SAAC has higher stability and selectivity than the sample without Pb. X Sun et al. (2018) synthesized a novel kind of PtCu Single-atom alloy catalyst (SAAC). By addition of Cu, Pt ensembles were dispersed to isolated single-atoms. PtCu SAA has a similar TOF but higher selectivity than pure Pt catalyst. DFT calculation shows that the Pt atom dehydrogenation activity on Pt–Cu alloy is almost unchanged, but the propylene desorption ability increases significantly which prevent deep dehydrogenation and other side effects. By precisely controlling solid state transformation of Mn atom into Pt nanoparticles, Wu et al. (2018) synthesize and identified Pt@Pt3Mn core-shell nanoparticle catalysts and pure Pt3Mn nanoparticle catalysts. The selectivity of Pt@Pt3Mn is effectively lower than that of Pt3Mn alloy. DFT calculation indicated Mn atoms in the subsurface would reduce the surface adsorption of propylene, thus inhibit side reaction. Interaction between these various promoters and platinum still needs more extensive study.Except Pt, other VIIIB metals could also be modified by second promoting metals. Ni, Co, Fe metals have higher dehydrogenation activity than Pt, but prone to generate cracking products or coke than producing propylene. (Chen et al., 2020; Saelee et al., 2018). Some other noble metals, such as Pd, Rh and Ru, have also been used in PDH because of similar electronic and geometric structures to platinum (Ma et al., 2020; Purdy et al., 2020; Natarajan et al., 2020). Addition of appropriate promoters has potential to improve these essential shortcomings. He et al. (2018) synthesized a Ni–Ga nanoparticle catalyst supported by Al2O3 (70% delta, 30% gamma phase). This alloying NiGa catalyst had a high initial selectivity of 94% and long-term stability, while pure Ni has a poor selectivity approached to 0 and bad stability due to serious coking deposition. For studying the effect of Ga on Ni deeply, researchers further synthesized a serious of catalysts with different Ni:Ga surface ratio. Higher surface gallium content is main reason of the high selectivity in NiGa/Al2O3 catalyst. Raman et al. (2019) studied performance of RhGa/Al2O3 catalyst in PDH. Addition of Ga would form solid intermetallic phases with Rh and improve catalytic performance. Continuing to increase amount of gallium would result in gradual formation of Rh–Ga liquid metal solutions and a sharp rise in selectivity and activity will be observed while alloy is all liquid state. DFT showed present a synergistic effect between Ga and Rh, activation of propane is happened on single-atom Rh and has formed proplyene would diffuse and desorb from Rh to the Ga, and it may be the reason of high activity of isolated Rh atom.For metal-based catalyst, the role of the support is dispersing metal particles and avoiding the irreversible sintering at the high temperature of PDH. Support with high surface area is obviously benefit to disperse and stabilize uniform and ultra-fine metal particles. In addition, appropriate pore diameter which is matched to particle size would show higher resistance to sintering which is also called confinement effect. Because of the above property, aluminum and silicon-based supports with higher specific surface area and adjustable pore structure are widely used in metal-catalyzed PDH process, and ordered mesoporous materials and microporous molecular sieves have unique advantages in stabilizing nanoparticles though confinement effect. In terms of support-metal interaction, supports used in PDH are usually electrically inert and hardly reduction for avoiding undesired side reaction or structural collapse, coordination unsaturated defect on these nonreducible oxide support would provide strongly local-unsaturated adsorbed sites to anchor these metal particles (Ji et al., 2020; Kwak et al., 2009). Thus, adjusting the surface defected sites are important in improving metal particles dispersion and stability.Among aluminum-based materials, γ-Al2O3 is the most common commercial support, which has low price, high thermal stability and strong resistance to abrasion (Sattler et al., 2014). In the surface of γ-Al2O3, coordinatively unsaturated pentahedral coordination Al3+ could anchor metallic component and prevent metallic nanoparticles aggregation (Gong and Zhao, 2019; Yu et al., 2020; Kwak et al., 2009). The amounts of unsaturated coordinated sites are closely related to the preparation method and morphology. Shi et al. (2015) synthesized a novel PtSn/Al2O3 sheet catalyst for PDH. Rich-defected Al2O3 sheet would attribute to more unsaturated pentahedral Al3+ sites than commercial γ-Al2O3, and thus effectively stabilize ultra-small raft-like Pt–Sn clusters. PtSn/Al2O3 sheet catalyst has an extraordinary selectivity up to 99% and only suffered trace deactivation happened during 24h reaction test. Sheet-like structure also has a positive impact on mass transfer, and benefit better activity and selectivity at high space velocity. Gong and Zhao, 2019 synthesized a novel peony-like alumina nanosheet (Al2O3-MG) with richer pentahedral Al(Ⅲ) by glocuse-assisted hydrothermal method and supported PtSn catalyst on it. The strong interaction between pentahedral Al(Ⅲ) sites and PtSn nanoparticles improved anti-sintering ability, thus, PtSn/Al2O3-MG catalyst has an unexcepted stability and conversion.Although γ-Al2O3 is good for achieving high dispersion of metal particles, however, acid sites on Al2O3 surface would catalyze undesired coke deposition and cracking reaction. Introducing basic promoters to adjust surface acidity is common solution. Addition of K, Na et al. alkali metal would partially cover the strong acidic sites and result in the reduction of side reactions (Sattler et al., 2014a). Other basic oxide such as Mg, Zn, Ca and some rare earth elements (Vu et al., 2016; Im et al. 2016) are also widely used for similar purpose. In addition to adjusting acidity, formation of MgAl2O4, ZnAl2O4 Spinel phase after addition of Mg, Zn would provide additional anchoring effect on metal particles by epitaxial metal-oxide interfacial caused by similar structure with Pt (111) face (Belskaya et al., 2016). Ren et al. (2018) systematically studied the promoting role of IB metals in Pt-M/MgAl2O4 and effect of MgAl-spinel on propane dehydrogenation. Compared with γ-Al2O3, although the original size of particle in both supports are very similar, MgAl2O4 evidently provide a stronger interaction to Pt nanoparticles and showed a better reaction-regeneration performance in multiple regeneration. After the introduction of IB metal promoters (Cu, Ag, Au), selectivity and conversion of all Pt-M catalysts have improvement, among them Cu has the most preferred promoting effect. This may due to worse affinity with Pt of Ag and Au, and thus result in formation of amorphous alloy, while Cu would form stable Pt–Cu intermetallic alloy with high stability and better dispersion. Some low melting-point oxides are also used in propane dehydrogenation to partially cover the acidic sites on the surface of alumina. Aly et al. (2020) found that introducing B species into Pt/Al2O3 catalysts can a reduction of coke formation and side reaction. DFT calculation indicated formation of finely dispersed amorphous B2O3 on alumina, which covered stronger acid sites on Al2O3 and reduce unwanted side reaction. The improvement of alumina-based support still needs further more exploration and research.Except for aluminum-based materials, pure silica materials, especially pure silica zeolites, have been widely studied in the field of PDH because of their low acidity, high specific surface area and adjustable, homogeneous and ordered pore structure. However, inert property of silica material also leads weak adsorption capacity. At preparation process, due to the lack of strong electronic attraction, metal precursors could be often evenly distributed on the surface of silicon oxide (Fan et al., 2020). At the reaction and regeneration process, due to the lack of strong adsorption sites, oxidized metal particles would suffer intensive sintering (Kaylor and Davis, 2018). Therefore, the research on silicon materials mainly focuses on improving the interaction between support and metal in both preparation and reaction-regeneration process.Creating the adsorbed sites on SiO2 materials by metal doping is a very common method to produce adsorption sites for metallic nanoparticle. Fan et al. (2020) used MnOx modified mesoporous silicon nanoparticle as a support to disperse platinum. The modified of MnOx provide strong electron adsorption to Pt precursors thus result better dispersion of Pt. Moreover, metallic Mn produced from reduction also play promoting role though alloyed with Pt. DFT calculation shows that the formation of PtMn alloy not only promote the proplyene desorption, but also can keep a good activity of initial dehydrogenation. For zeolite support, although presence of aluminum atom provides additional anchored effect to metal particle, caused strong acidity would lead to serious coke deposition. Many studies have tried to exchange the framework structure of silica-alumina zeolite with other metals like Zn (Zhang et al., 2015), Sn (Li et al., 2017), Ti (Li et al., 2017), Fe (Waku et al., 2003) to obtain a relatively low acidity but keep stronger interaction. Zhang et al. (2015) synthesized a Zn-ZSM-5 zeolite though use Zn precursor instead of Al precursor. Compared with the traditional Al-ZSM-5, the Zn-ZSM-5 has lower acidity which reduce coke deposition. At the same time, Zn also provide a strong interaction than Al-ZSM-5. Formation of Pt–Zn nanoparticles also improve selectivity and conversion.Except for the introduction of heteroatoms, surface organometallic chemistry method (SOMC) also provides an optional method to directly anchoring stable metal nanoparticles by pure silicon material. Although silicon oxide materials due to its electric neutrality could not provide strong adsorption capacity for metal precursors in water through the electric attraction, hydroxyl groups on the surface of SiO2 can be connected with metallic organic precursors by SOMC and form highly dispersed even isolated metal sites (Xu et al., 2019). Searles et al. (2018) prepared a novel PtGa/SiO2 catalyst via grafting Pt and Ga precursor onto the surface of silica gel and form Ga and Pt single-sites, after reduction, they obtained homogeneous and ultrasmall Pt–Ga alloying nanoparticles. This catalyst had amazing catalytic performance which high conversion (31.9%), selectivity (99%) by only usage of only 0.001g catalyst in a very high space velocity with ultrahigh stability and regeneration ability. Two different kinds of Ga species were observed in PtGa/SiO2: metallic Ga and remained single Ga3+ on SiO2. Metallic Ga formed bimetallic particles with Pt thus the selectivity and stability is improved. In addition, isolated Ga3+ on the surface of SiO2 is related to formation of strong Lewis acid sites, which may enable to the nucleation and stabilization of ultra-small Pt–Ga nanoparticles. Another PtZn/SiO2 catalyst which has similar synthesis method was also prepared by Rochlitz et al. (2020) PtZn/SiO2 catalyst has also a high conversion, selectivity, stability like mentioned-above PtGa/SiO2 catalyst. The formation of Pt–Zn alloy is considered as an important reason to the high selectivity of PtZn/SiO2.Through dealumination of Si–Al sieves, the silanol nest formed after dealumination on sieve has strong adsorption capacity than common silica material, these sites could effectively disperse metal precursor sites at the initial stage of preparation. Xu et al. (2019a,b) further introduced this SOMC method into the synthesis of PtSn catalyst supported by dealuminated beta zeolite. Though directional interaction between organic functional group, isolated Sn atoms would localize in the framework of beta zeolite and prefer to form smaller PtSn clusters with Pt. PDH test showed that the Pt/Sn2.00-Beta catalyst had the highest conversion (50%) and selectivity (99%). It is worth noting that similar stable Pt clusters have been obtained by the same method on Y zeolite, which indicates that this method may be a general sintering inhibition method to Silica alumina zeolites. Ryoo et al. (2020) co-impregnated Pt(NH3)4NO3 and nitrate ions of rare earth in the de-Ga molecular sieve. Silanol nest formed by dealumination would stabilize rare earth ions exist in the form of single atom and thus easy to be reduced. After reduction treatment at 700 °C in H2, rare earth metal-platinum alloy catalyst was obtained. This catalyst has high selectivity, conversion and shocking stability up to several days.Encapsulation of Pt-M clusters into micropores of zeolite by in-situ method or post-synthesis could also synthesize ultrasmall Pt alloyed cluster and effectively inhibit sintering due to confined effect. Wang et al. (2020) synthesize a novel PtZn@S-1 catalyst via hydrothermal method. XPS found some of the Zn species exist as Zn2+ and located into lattice of S-1 zeolite, and others form Pt–Zn alloys with Pt species. Ultrasmall Pt–Zn alloys particles with high catalytic performance are encapsulated inside S-1 zeolite though in-situ synthesis. The confinement effect of S-1 channel effectively prevents the sintering of Pt–Zn particles. Liu et al. (2020) synthesized a novel K–PtSn@MFI catalyst. Through XAS, TEM, author proved sub-nano Pt clusters(0.6 nm) are confined in the sinusoidal 10R channels of MFI. Interaction between Pt and Sn can be effectively controlled by adjusting the reduction method. Increasing the reduction temperature or time could can help Sn to enter into Pt clusters and provide promoting effect, thus increasing selectivity and slowing down deactivation rate. Under a condition similar industrial process, the K–PtSn@MFI catalyst exhibited a selectivity of up to 97% and an initial conversion of 20% with only 3% deactivation in 70h.The traditional supports such as Al2O3 and SiO2 are electric inertia generally interact with metals by local defect sites. Although these defects have some anchoring effects, but they still failed to provide more effective electronic interaction with Pt. Therefore, some supports which provide strong metal-support interaction (SMSI) have received the much attention from researchers. The SMSI effect provides additional electronic interaction and modify interface structure between support and metal, thus it is expected to an interesting way to change catalyst performance. (Liu et al., 2020). Jiang et al. (2014) studied the promoting role of TiO2 on Pt/Al2O3 catalyst. After doping Ti into Al2O3, both selectivity and conversion have obvious improvement, despite Pt/TiO2–Al2O3 and Pt/Al2O3 has similar particle size of about 2 nm. This phenomenon may be due to SMSI between TiO2 and Pt would increase electronic density of Pt atoms. Electron-rich Pt promote desorption of propylene and prevent side reaction. A high electronic density also weak attachment of coke precursor, so promote migration of coke from metallic nanoparticles to support. Liu et al. (2017) synthesized a novel Pt/ND@G (graphene shell in nano diamond) catalyst which has high selectivity (90%) and shows slight deactivation in 100h. The electron transfer from defective graphene to platinum nanoparticles improved anti-sintering ability of Pt/ND@G. In addition, this electron transfer also increases electronic density of Pt surface and inhibit coke deposition and side reaction effectively.In regeneration process of PDH, treating catalysts with high temperature air would oxidize part Pt (0) to volatile Pt2+, Pt4+ species, and finally lead agglomerate (Kaylor and Davis, 2018; Pham et al., 2016; Xu et al., 2018; Uemura et al., 2011). A strong interaction between Pt species and support under air condition is also important to inhibit sintering in regeneration process. CeO2 is well known for its special trapping platinum atom ability in high temperature. Xiong et al. (2017) synthesis a novel PtSn/CeO2 catalyst. The strong interaction between Pt and CeO2 effectively minimized sintering of small Pt–Sn clusters. Interestingly, during the oxide regeneration process, Platinum clusters supported on CeO2 would be dispersed into single-atoms. In reduction period, the dispersed Pt atoms will be self-reassembled to Pt–Sn cluster and restore original dispersion, while in Pt/Al2O3 Pt particle would aggregate violently after regeneration. This self-assembly process is shown in Fig. 4 . Xu et al. (2018) added β-Ga2O3 with different shapes to Pt/Al2O3 and studied their influence on propane dehydrogenation. PDH results suggest Ga2O3 can stabilize the particle size of platinum in air at high temperature. This may be because Pt particles could be captured by Ga2O3, which prevents Pt from being oxidized into PtOx large particle. Recent catalysts are shown in Table 1 .Oxide catalysts in PDH could be divided into supported catalysts and bulk catalysts. Compared with Pt, oxide catalysts have their own advantage such as significantly lower price and convenient regeneration process, while Pt catalysts need using dangerous Cl2 to redisperse large particles (Sattler et al., 2014; Liu et al., 2016; Zangeneh et al., 2015). However, most oxide catalysts are suffered by serious coking deactivation and have much shorter one-way life than Pt catalysts, thus frequent regeneration is necessary (Nawaz et al. 2015). In industry Catofin technology, CrOx-K2O/Al2O3 catalysts are regenerated every 10 min, several fixed bed reactors work in turn to keep a constant propylene outflow rate (Sattler et al., 2014). Thus for oxide catalysts, stability during regeneration process always needs to pay more attention than one-way stability. Solid reaction or leaching in regeneration process are culprits of deactivation in regeneration process. For example, in industrial CrOx/Al2O3 catalysts, migration of Cr atoms into framework of Al2O3 would form inactive spinel phase and thus lead to irreversible deactivation. The inactivation process can be reduced by selecting supports and promoters properly. In addition, the intrinsic activity of the oxide catalyst is often tens of times lower than metal-based catalysts, which makes the oxide catalyst often need a high loading to achieve appropriate conversion.Catalytic mechanism of oxide catalysts in PDH is basis on metal-organic chemistry, unsaturated metal-oxide pairs are considered to be the source of active sites. After reduction activation process, Metal oxides would lose a part of oxygen atoms and produce unsaturated metal-oxygen sites, these unsaturated M-O pairs would tend to coordinate with carbon atoms and hydrogen atoms on propane and thus reduce activation energy of C–H cleavage (Chen et al., 2020; Estes et al., 2016; Schweitzer et al., 2014; Conley et al., 2015). In Cr-based catalysts, coordination unsaturated Cr3+ and Cr2+ reduced from Cr6+ and Cr5+ have been widely confirmed as active sites by a series of in-situ characterization (Nawaz, 2015; Santhoshkumar et al., 2009; Puurunen et al., 2001; Gao et al., 2019). Similar to Cr-based system, coordination unsaturated V3+ and V4+ reduced from V5+ are widely thought as active sites in V-based catalysts (Sokolov et al., 2012; Langeslay et al., 2018; Bai et al., 2016; Zhao et al., 2018; Liu et al., 2016; Xie et al., 2020; Rodemerck et al., 2017). In some cases, the valence of oxides after reduction would not change such as Ga, Zn, but their coordination conditions are changed to unsaturation due to loss of oxygen atom (Sattler et al., 2014). For Ga-based catalysts, tetrahedral (IV) coordination Ga3+ reduced from octahedrally (VI) coordinated Ga3+ are the main active sites. (Sattler et al., 2014; Schreiber et al., 2018 Choi et al., 2017; Kim et al., 2017; Szeto et al., 2018; Zheng et al., 2005). Although in most oxide catalysts, increasing the reduction degree of the active site could greatly improve its activity due to increase of unsaturation, however, deep reduction is not necessarily beneficial to the oxide catalysts (Sun et al. 2014, 2015; Dai et al., 2020). For example, the active site of Co-based catalyst is considered to be unsaturated Co2+O. If CoOx is reduced to metallic Co nanoparticle, this will result in a sharp decrease in selectivity and stability (Dai et al., 2020). In Zn-based catalysts, Zn metal formed by excessive reduction is inactive and easy to be sublimated and lost from the support due to its low melting point, resulting in irreversible deactivation.Reverse H–P mechanism is also widely used to describe mechanism of oxide catalysts, but there is a little different in adsorption sites: while physical adsorption completed, propane would be dissociated to alkyl and hydrogen atom, alkyl adsorb on coordination unsaturated metal atoms, while hydrogen atoms adsorb on oxygen atom near these metal atoms. After that, the alkyl would undergo a β-hydrogen transfer and desorbed hydrogen would be adsorbed on metal ions, while propylene adsorbs on the top of metal ions by π-bond(Huš et al., 2020). Then, propylene and hydrogen are desorbed successively(Liu et al., 2016; Estes et al., 2016). This process has been showed on Fig. 5 b. The whole reaction is also considered as a surface reaction control mechanism and dehydrogenation step is rate-limited step (Xie et al., 2020; Liu et al., 2016). Although traditional model of oxide catalysts only includes metal-oxide pair M-O and follow HP mechanism, under different chemical conditions, active site structure and reaction mechanism may be different. (Olsbye et al., 2005; Zhao et al., 2018; Dixit et al., 2018). DFT calculation is of great significance in predicting and interpreting influencing mechanism and detailed information of active sites in oxide catalysts. Zhao et al. (2018) found that pretreating VOx/Al2O3 by hydrogen could converse VO bonds on VOx to V–OH gradually, while pretreating with C3H8 would directly lead to the fracture of VO. Active sites with V–OH structure have lower activity but slower deactivation (Fig. 5a). With the increase of treating time of H2, both of the initial activity and deactivation rate would decrease. This also shows the complexity of active sites on surface of oxide. Although the active sites of most oxides are considered to be metal-oxygen pairs, Dixit et al. (2018) made a systematic study in sites with different coordination environment and present of hydroxyl of PDH mechanism on (110) surface of Al2O3 by DFT. They also found a volcano relation between binding energy of dissociated H2 and TOF of PDH. Desorption of H2 in highest active sites is more suitable to be the rate-limiting step rather than traditional breaking of C–H. AlIII−OIII sites of hydroxylated Al2O3 (110) which has compromising Lewis acid/base ratio have highest active and follow a concerted mechanism. In addition to alkyl mechanism, mechanism participated with free radicals and carbocation have also been proved to have lower energy barrier in some cases (Mansoor et al., 2018; Lillehaug et al., 2004). Schreiber et al. (2018) found traditional HP mechanism may not be suitable in Ga/H-ZSM-5 catalysts. They indicated while first step of dehydrogenation is still the common heterolytically dissociation process, the second step of dehydrogenation should happen with an intermediate state of carbenium rather than direct cleaver of C–H in alkyl. These mechanisms are listed in Fig. 5b.In sum, the active sites of the oxides are formed by metal-oxygen pairs after reduction, these sites are often coordination unsaturated. Which oxygen is reduced (M-O-M or M-O-S et al., M is the active metal atom and S is the support), how many oxygen atoms are reduced, and whether forming adsorbed species such as hydroxyl decide the final micro-structure of active sites. In order to modify the reducibility of oxide catalysts, commonly used methods include regulating support and promoter. In addition, supports and promoters also affect the dispersion of active sites in oxide catalysts.Support plays a key role in oxide catalysts for PDH reaction. First and most obviously, support provide landing surfaces for dispersing active species and effect structure of active sites (Bai et al., 2016). As the intrinsic activity of most oxide catalysts is much lower than that of Pt, a support with larger specific surface area is needed to increase the loading of oxide catalyst in order to achieve higher conversion. However, as increasing loading, polymerized structure of oxide would change from isolated sites to oligomer and finally crystals (Wu et al., 2017) (Fig. 6 ). The degree of aggregation not only determines amount of exposed active sites, but also plays decisive role in reduction tendency of oxygen atom and active structure during PDH process (Ruiz Puigdollers et al., 2017; Liu et al., 2016; Zhao et al., 2019). A large number of studies in oxide catalysts focus on studying relationship between polymerization state and PDH catalytic performance. Isolated sites are generally considered to have special advantages. They not only have the maximum dispersion theoretically, but also prevent coke precursors contact each other to from graphitized coke (Rodemerck et al., 2017; Liu et al., 2020; Dai et al., 2020; Hu et al., 2015; Hu et al. 2015, 2015). Supports with high specific surface area have obviously advantages in keeping them working as isolated sites. Rodemerck et al. (2017) found that in the surface of VOx/MCM-41, isolated V sites have the highest activity and coke resistance. Although isolated V species have higher acidity than oligomerized counterparts, the coke precursors are hard to contact with each other and form coke, thus they would have both higher activity and stability in PDH. Synthesis method also plays an important role in dispersing oxide to isolated sites. Traditional impregnation method often leads to the uneven distribution of oxide species, some novel synthesis method such as hydrothermal one-pot method (Dai et al., 2020), sol gel method (Hu et al., 2018) and precursor modification has unique advantages. Zhao et al. (2019) using Zn-based metal organic framework as precursor and synthesized highly dispersed ZnO@CN/S-1 catalyst. ZnO nanoparticles are highly dispersed and encapsulated into CN material. CN layers effectively hindered the loss of Zn in the high temperature due to physical wrapping effect. In addition, the electron interaction between CN materials and ZnO also promoted the desorption of propylene. This ZnO catalyst has excellent conversion, selectivity and stability.Evolution surface structure of oxide with the change of loading would also be largely determined by type of support, although some inert supports have large specific surface area, they could not disperse the active components well. Santhoshkumar et al. (2009) found Cr is more likely to exist as isolated Cr (VI) species under low loading on the SBA-15. These isolated sites have highly activity and stability. However, with the increase of the loading amount, α-Cr2O3 gradually formed from isolated Cr species and result in a decrease of activity. On the contrary, although in the low loading Cr exists as oligomers on the Al2O3 with relatively low activity, α-Cr2O3 oxide does not appear even in a high loading. Therefore, CrOx/Al2O3 have higher conversion than CrOx/SBA-15 in high loading. Rossi et al. (1992) studied the catalytic activity and texture properties of CrOx species on SiO2, Al2O3 and ZrO2. The result of PDH show CrOx supported on ZrO2 has a much higher activity due to the highly dispersed chromium species, this may be due ZrO2 has higher ability to disperse chromium catalyst as isolated sites. Supports also could affect coordination environment of active metals though M-O-S bonds (M is the active metal center, S is the support), and result in influence on the reducibility of supported oxide. Xie et al. (2020) found ZrO2 can effectively improve the TOF of VOx than the sample using Al2O3 as support. The VOx/ZrO2 catalysts shows more loss of coordination oxygen atoms than VOx/Al2O3, and activity of lower coordinated V–O pair on ZrO2 shows six times higher than Al2O3. Author also measured reducibility of V–O–V dimer on Al2O3 and ZrO2 though a DFT calculation, result showed V–O–S and V–O–V bond in VOx/ZrO2 are weaker than VOx/Al2O3.In the isolated active sites, metal-oxygen active pairs are closely connected with the support though M-O-S bonds, their catalytic performance are more greatly affected by the support. Organic precursor directed synthesis make M-O-S interaction in well-defined oxide catalysts possible. Szeto et al. (2018) synthesized well-defined isolated Ga catalysts supported respectively on Al2O3 and SiO2 by SOMC method, which Ga species exist as isolated single-atoms on Al2O3 and double-atom on SiO2. Ga1/Al2O3 appears to show more active and selective catalyst than Ga2/SiO2. This may be due to Ga-O-Al sites have higher C–H cleaver activity than Ga-O-Si, thus proved the importance of support in oxide catalysts. This strong interaction between oxide and support decide isolated sites sometimes may not be the best active site, for example, strong support-oxide interaction would cause isolated oxide sites difficult to be reduced into coordination unsaturation state, and thus resulting in low activity. Liu et al. (2016) found isolated VOx has lower TOF than V2O5 crystalline in VOx/Al2O3. The lower reducibility of V–O–Al bond than V–O–V bond may be the reason of their poor activity, which result in most vanadium species are only reduced to V4+ instead of highly active V3+. The M-O-S bonds sometimes lead to the formation of B-acid sites which would catalyzes side reactions (Castro-Fernández et al., 2021).. Therefore, compatibility of isolated oxide catalysts should be carefully considered according to the types of supports.In some bulk catalysts such as ZrO2 (Zhang et al., 2018), TiO2 (Li et al., 2020), Al2O3(Wang et al., 2020), Ga2O3 (Zheng et al., 2005), particle size, crystal phase and degree of surface defects would play a similar role instead of dispersion. For Zr-based catalysts, oxygen vacancies on zirconia surface are widely considered as the source of activity. (Otroshchenko et al., 2015). Zhang et al. (2018) found that ZrO2 monoclinic crystal with smaller particle size shows higher intrinsic activity than bigger one. The effect is more obvious when the particle size is below 10 nm. This is due to more amount of surface defects such as corner, edge exist on the small ZrO2 crystal and results a higher oxygen vacancy concentration. At the same time, coking selectivity would decrease with the decrease of ZrO2 crystal size, this may be due to small nanoparticle have more dispersed active sites. Zhang et al. (2019a,b) also studied the effect of crystal phase of ZrO2 on catalytic performance. Decreasing of particle size would increase activity of ZrO2 in either monoclinic phase or tetragonal phase. However, although they have similar activation energy, monoclinic ZrO2s have a higher conversion than tetragonal ZrO2. This phenomenon can be explained by that lattice oxygen in monoclinic ZrO2 has a better mobility than that on tetragonal ZrO2. Zheng et al. (2005) tested catalytic performance of various Ga oxide with different morphology. Catalytic activity tests found β-Ga2O3 has the highest specific activity of PDH in different polymorphs of Ga2O3. NMR and NH3 adsorption indicated that β-Ga2O3 has higher Lewis acid sites and tetra Ga3+ density than other polymorphs of Ga2O3, this shows that the Lewis acid sites is closely related to the tetra coordinated Ga3+ and these Lewis acid sites are related with activity strongly.For oxide catalysts, adding appropriate promoters could effectively change the geometry and electronic structure of the active sites, thus affecting the performance of the catalysts (Sattler et al., 2014). Interaction between promoters and active oxide could be classed to two interaction: oxide-oxide and metal-oxide interaction. This interaction depends on the existing form of promoter. For oxide-oxide interaction, heteroatom would effectively change redox properties or affects the acidity and basicity of oxide surface, thus effectively effect catalytic performance (Ruiz Puigdollers et al., 2017). However, due to complexity of oxide catalysts, systematic researches on predicting what kind of element would in oxide catalysts is still difficult. From the experience, oxyphilic or basic elements often lead to lower activity but higher stability, while the dopants with acid elements (most of them could be used as independent active components) are conducive to the improvement of dehydrogenation activity (Liu et al., 2020; Li et al., 2016; Zhang et al., 2019). Oxide-oxide interaction could also affect the aggregation morphology of active oxides though M-O-M bonds and thus affecting the dispersion of active sites. For metal-oxide interaction, metals especially noble metals could activate hydrogen molecules into high energy hydrogen atoms, so they could introduce hydrogen spillover or reverse spillover effect to adjust the reducibility of oxide or part in hydrogen evolution step in PDH (Otroshchenko et al., 2015; Sattler et al., 2014). The metal-oxide interface could also affect geometry and electronic structure of the active sites in some case (Liu et al., 2016).Doping metal element could be achieved by simple co-impregnation or sequential impregnation method and effectively effect catalytic performance, thus they have been widely studied. Alkali metals such as Na and K are the most common metal promoters used in PDH. They could poison strong acid sites on surface of catalysts, which are considered to prone catalyzing side reactions such as coking. In addition, the addition of K is also conducive to improve dispersion of the oxide species on the support (Sattler et al., 2014). Other elements, such as Ce (Zhang et al., 2019), Zn (Liu et al., 2020), Ni (Li et al., 2016), have also been used in Cr-based catalysts as promoters. Zhang et al. (2019) studied the role of Ce in CrOx/Al2O3 catalyst. The interaction between Ce and Cr results in more oxygen vacancies and after modification of Ce. Addition of Ce effectively reduced the inactive Cr6+ species and the dispersion of CrOx species, thus stability, selectivity and conversion are effectively improved. Wu et al. (2017) explained in detail the structure-reaction relationship of Mg promoter in VOx/Al2O3. The addition of Mg would disperse V2O5 crystals (which is mainly contributors of coke) into oligomeric or isolated VOx structure (Fig. 7 c). This effect not only effectively decrease quantity of coke, but also results in a better conversion due to higher dispersion. However, excessive addition of Mg oxide would form MgO aggregation and cover active VOx, which lead to decline of activity. Precious metals are used as promoters. Liu et al. (2016) prepared a novel ZnO/Al2O3 catalyst doped by trace amount (0.1 wt%) of platinum for PDH. Comparing to traditional ZnO catalyst, the addition of trace amount platinum greatly improves the reduction resistance of zinc oxide and reduces the formation of unstable metallic Zn. In addition, Pt improves the desorption of H2 and C–H activation on the surface of Zn oxide. Further characterization indicate that Pt clusters are highly dispersed and covered by ZnO. Authors indicate these Pt clusters would increase the Lewis acidity of zinc though electronic interaction and promotes the desorption of adsorbed hydrogen atoms, this effect could reduce overreduction of unstable metallic Zn and increase ZnO activity (Fig. 7b). Sn supported by SiO2 catalyst recently has high selectivity and activity, but at high temperature, the reduced Sn will aggregate and lead to the decrease of activity (Wang et al., 2016; Wang et al., 2020; Liu et al., 2020). Wang et al. (2016) found that after adding a small amount (0.05 wt%) of Pd into Sn/SiO2 would effectively increase stability of catalysts, the life of Sn catalyst was prolonged by more than two times. Authors found introduction of Pd could decrease aggregate degree of Sn species and reduce the loss of tin at high temperature, thus improve activity and stability.Except for metal elements, some non-metallic oxides such as SOx, POx can effectively improve the negative electron properties of oxide. Sun et al. (2014) found that the introduction of SO4 2− into Fe2O3 could effectively improve the yield of propylene and stability. This better catalytic performance may be due to the presence of SO4 2− also lead to electron deficiency of Fe atoms, which makes absorb negatively charged second carbon of propane easier and provide a higher activity. In addition, promoting sulfur could also stabilize Fe chemical state and the inhibiting formation of harmful species, such as FeCx or FeS (Sun et al. 2014, 2015). Tan et al. (2016) synthesized a serious phosphorus containing Fe/Al2O3 catalysts. The addition of phosphorus plays an important role in creating more active sites, while the Fe2O3 without phosphorus will coked severely and have a low selectivity. After adding phosphorus, the catalyst with best performance has a conversion about 14% and a selectivity of 80%. In addition, longer duration experiment indicated that the conversion and selectivity of catalyst can be almost unchanged in 24 h. Gu et al. (2020) modified vanadium oxide surface supported by Al2O3 with phosphorus by gas phase reduction method. After proper modification, the activity of the catalyst decreased slightly, but the stability increased greatly. This may be due to P dispersing VOx polymerization to isolated V sites, which have lower activity but higher stability. The addition of phosphorus also reduces the acidity of the oxide, thus improving the overall stability of the catalyst.While further improving second metal loading, dopant would increase in diffusion probability and react with the original oxide to bimetallic compound structure or solid solution. Formation of stable bimetallic oxides often leads to decreasing number of active sites and poor catalytic performance. But sometimes the formation of new structures is also conducive to the formation of better active sites than supported oxide. Chen et al. (2008) synthesized a kind of Ga2O3–Al2O3 solid solution catalyst with a spinel structure. Compared with the traditional Ga2O3/Al2O3 catalyst, the formation of solid solution can effectively disperse and stabilize the gallium oxide species. Although Ga2O3–Al2O3 solid solution has a slight decreasing in the initial conversion rate compare to Ga2O3/Al2O3, it has more stable catalytic performance, this enhancement effect is shown in Fig. 7d. Otroshchenko et al. (2017) studied the promoting role of Cr in CrZrOx bimetallic bulk oxide. The addition of Cr promotes the lattice oxygen removal by activate H2 molecule to active hydrogen atom and thus improve coordinatively unsaturated Zr4+ sites formation, which has high dehydrogenation activity. Therefore, CrZrOx has higher activity than pure ZrO2 catalysts and even industrial CrOx/Al2O3 catalysts. In addition, lattice CrOx species are also responsible to reduce the strength and amount of strong acid sites on ZrO2, which reduce side reaction caused by Lewis acid sites. Compared with the supported CrOx/LaZrOx catalysts, the bimetallic CrZrOx oxide catalyst has better regeneration stability and activity at the same acidity number.Metals especially noble metals could also play a role of promoter by introducing hydrogen spillover effect, this effect could recombine adsorbed hydrogen atoms or adjust the reducibility of oxide catalysts. Sattler et al. (2014) synthesized a Pt–Ga2O3/Al2O3 with trace platinum content of only 0.001w%. Although both platinum and gallium oxide are considered to have catalytic activity in some previous studies, this catalyst has better conversion and selectivity than one of them exist alone. It is assumed that promoting role of Pt is accelerating recombination of the hydrogen atoms to H2 which is reverse reaction of hydrogen spillover. The stability and size of Pt particles would greatly affect the hydrogen spillover effect. Han et al. (2019) studied the promoting effect of Cr in a series of supported CrZrOx catalysts. They found Cr atoms would activate H2 molecule to active hydrogen atom and promote coordinatively unsaturated Zr4+ sites formation, which has high dehydrogenation activity. This synergy effect could effectively reduce the usage of toxic Cr and expensive Zr but keep the activity unchanged.Promoter has a similar effect to improve catalytic properties of bulk oxide. Otroshchenko et al. (2017) studied the influence of doping Li, Ca, Mg, Sm, La in MZrOx catalysts (M stands for dopant). Doping of ZrO2 with Ca or Li would result a low activity. On the contrary, Addition of La, Sm, Y would effectively enhance propylene formation rate of ZrO2. Creation of more surface defects on ZrO2 after adding heteroatoms may be the cause of these activity changes. Noble metals such as Pt, Rh, Ir and some hydrogen-activated metals such as Cu could provide hydrogen spillover effect, which improve reducibility and amounts of active sites (Otroshchenko et al., 2015). Zhang et al. (2020) carried out a more systematic study interaction between ZrO2 and Rh nanoparticles in Rh/ZrO2 catalyst. Experiment and calculation results indicated the addition of Rh increase the oxygen vacancy number on the surface of ZrO2 which is widely considered to main active sites, thus effectively improve conversion. However, excessive addition of Rh would lead to a decrease in activity, this may be due to too many oxygen vacancies would restrain the desorption of C3H6 and hindered PDH reaction (Fig. 7a). Recent oxide catalysts are shown in Table 2 .Coke is the general term of deep dehydrogenation alkyls or graphitized carbon deposition (Sattler et al., 2014). In PDH, coke deposition path mainly includes four steps: deep dehydrogenation, C–C bonds breaking, formation of aromatic hydrocarbon and graphitization (Huš et al., 2020; Lian et al., 2018; Zhao et al., 2015). But actually, its detail mechanism and key intermediates is still ambiguous until now. Researchers have proposed some possible processes in deep dehydrogenation process basis on DFT. Valcárcel et al. (2006) studied stability of a variety of intermediates of deep dehydrogenation on Pt (111) include 1-propenyl, propylidyne, propenylidene, and propyne. Among these intermediates, propylidyne has the lowest energy and preferentially adsorb on hollow of three platinum atoms. This work pointed out the most stable hydrocarbon intermediate in PDH on Pt (111) surface. In another DFT calculation of Yang et al. (2010), they indicated with the process of dehydrogenation the energy barrier of C–C scissor decreases continuously. Propyne was found to be the most likely starting point for C–C scissor to coke deposits. Many other studies have obtained similar conclusion (Saerens et al., 2017). In addition to coking directly caused by active sites, acidic sites also catalyze the coking process, Lewis or Bronsted acid sites would catalyze coke formation like a catalyzed aromatic hydrocarbon process and follow an acid-catalyzed carbocation mechanism (Sattler et al., 2014).Although the mechanism of coke deposition is still uncertain, C1 and C2 species are generally considered as main precursor of coke deposition rather than C3. These species have more lone electrons and would attract each other and form aromatic rings through surface-mediated mechanism. (Larsson et al., 1996; Lian et al., 2018; Saerens et al., 2017). Jackson et al. (1997) proved polycyclic aromatics formed on Pt/Al2O3 in PDH are more likely derived from C1 species rather than C3 species used a mathematical derivation. After detecting coke by mass spectrometry, they found main component of coke are pyrene and methyl pyrene and they cannot be divided by three. In addition, isotopic labeling experiments also proved C3 species would be divided into C1 in the process of coke deposition. Up to now, most of the DFT experiments use the polymerization of C1 or C2 to aromatic ring as the main carbon deposition process. (Lian et al., 2018; Saerens et al., 2017). After the formation of the first aromatic ring, the aromatic ring precursor expands continuously to polycyclic aromatic hydrocarbons through a Diels−Alder mechanism, finally form highly graphitized coke.For either metal catalysts or oxide catalysts, coke deposition are the main culprit of deactivation. In industrial process, Pt-based catalysts need to be regenerated every 7–8 h. For Cr-based catalyst, this regeneration would be more frequent and one-way reaction time is only about 10 min(Sattler et al., 2014). Although sometimes coke also show some advantages in Cr-based catalysts, such as they could provide additional energy in burning regeneration step. But for platinum-based catalysts, heat from combusting coke would lead to serious sintering even more serious than reaction step (Kaylor and Davis, 2018). In order to understanding effect of coke deposition, researches on coke deposition in PDH could be described from three levels: macro-levels, meso-levels and micro-levels (Ye et al., 2019). In view of macro-level, coke will influent mass and heat transfer process, its influence is closely related to type of reactor and the reaction technology. Limitation of research focus of this paper, we would ignore detailed discussion for this influence. Recently, understanding and inhibiting coke deposition at a meso- and micro-level has attracted a lot of attention.In view of meso-level, coke product would narrow and finally block pore channels, causes diffusion resistance and leads to deactivation (Ye et al., 2019). Proper pore structure could improve carbon capacity and thus reduce block effect. In addition, pore structure could play an important role in diffusion of propylene, and thus shorten the contact time between propylene and active sites to reduce coke deposition. Ye et al. (2019) built a pore network model of PtSn/Al2O3 and use it to simulated its coke deposition process. According to the simulation results, increasing of pore connectivity and volume-averaged pore radius did not obviously influence the coke formation rate, but increased the maximum coke deposition. Decrease of pore size distribution would not affect the rate of coke deposition, but significantly increase maximum coke capacity. In addition to pore structure, the pore radius can't make a great impact on carbon capacity but is proportional to the rate of coke deposition. Ye et al. also simulated the in-situ change of pore structure in deactivation process. Due to obvious diffusion limitation, in the first stage, propylene is difficult to diffuse from the inside to outside and thus coke form mainly in the nearly center part of catalyst, catalysts would suffer a rapid deactivation step. In the second stage, the pores in the center are almost blocked, so coke mainly exists in the outer region of the particles and the rate of coke deposition slows down.How to control the appropriate pore structure to minimize coke deposition has attracted extensive attention of researchers. Straight uniform pore is widely considered to be more favorable for propane dehydrogenation. Accumulation of particles could form some natural pores, but these inter-crystalline pores usually don't possess uniform size and have many structural defects such as curved and closed structures, which are not conducive to mass transfer. In industrial, shaping catalysts with high pressure is needed to homogenize size of channel of supports, this usually requires an ultra-high pressure of more than 100 MPa. Therefore, some monolith materials such as ordered mesoporous oxide, molecular sieve which has uniform and natural pore structure have attracted largely attention. Creation of ordered multistage pores also plays an important role in promoting mass transfer. Li et al., 2017 prepared a series of PtSn/TS-1 catalysts with different particle sizes by hydrothermal synthesis. Although TS-1 is a microporous molecule sieve, with the decrease of particle size, mesopores gradually appear on surface of TS-1 and the hierarchically porous structure is formed. Compared with TS-1 with large particles, TS-1 with small particles has apparent advantages in conversion, selectivity and stability. The diffusion of products and reactants may be the key to explain this phenomenon. Calculation of Weisz–Prater criterion indicated the PtSn/TS-1 of large particles is seriously affected by internal diffusion but small one has a low resistance to inter-crystalline diffusion. The hierarchically pore structure doesn't only significantly accelerate the diffusion of propylene, but also accelerating the diffusion of propane and avoiding the internal diffusion control. Liu et al. (2020) embedded tin into dendritic mesoporous SiO2 nanoparticles for PDH. Dendritic mesoporous SiO2 nanoparticles have mesoporous structure and radial 3D pore with highly connectivity. Changing the ratio of template could regulate the pore structure with different pore size and connectivity, an appropriate pore structure could improve the reaction rate and selectivity.Dynamic radius of propane (4.3 Å) and propylene are both relatively small, which reduces researchers' attention to the influence of pore structure. Because PDH is one-step reaction, the role of diffusion effect in PDH is often ignored. However, due main and side reactions of propane dehydrogenation is series reaction, the pore structure plays an important role in the selectivity of propane dehydrogenation. At present, most researches on the process of coke deposition focus on the microscopic reaction mechanism of coke. Next, we will discuss the micro-process of coke deposition in detail.At a micro scale, coke and coke precursors are rich in lone electrons or electron-rich π bond, active sites would adsorb them strongly and thus result in deactivation. Therefore, those highly unsaturated coordination sites or electrophilic acidic sites are easier to be poisoned by coke. For example, step, edge sites on Pt nanoparticle and over reduced oxide sites are more prone to tightly adsorb C atom and suffered by deactivation (Zhu et al., 2015). Coke would first generate and cover these sites tightly, and thus lead to decrease of conversion. Unsaturated sites are also the main sites which are prone to catalyze cracking and other side reactions, thus coke deposition process is always accompanied by the increase of catalyst selectivity (Gorriz et al., 1992; Larsson et al., 1996). However, even for terrace of Pt, coke deposition has also been proved to improve selectivity. Lian et al. (2018) explained effect like double-edged sword of coke through a DFT-based kinetic Monte Carlo simulation on Pt (111) in PDH. At the beginning, although conversion in Pt-based catalysts is very high, but the main reaction is deep dehydrogenation rather than formation of propylene. As carbon deposits cover the surface, Consumption rate of propane decreases rapidly, and accompanied by a sharp increase in selectivity. Snapshots indicated the formation of coke covered and separated the Pt ensembles like alloying promoters such as Sn, and thus increase selectivity.Reducing coke deposition from both structural and electronic aspects has been widely used in PDH process. Isolated active sites are beneficial to reduce the contact between coke precursors and is helpful to reduce structural-sensitive coke deposition reactions. For metal catalysts, alloying metals as promoter could separate active metal ensembles and deep dehydrogenation species are difficult to form stable transition state on these small ensembles. Isolated catalysts such as SAACs have lower tendency to produce coke (Sun et al., 2018). Electronic effect also plays an important role in inhibiting coke deposition. The promoting metal could transfer the electron to active metal as an electron donor, the electron-rich active sites would exclude the same electron rich π orbitals of propylene and avoid strong adsorption or deep dehydrogenation (Wang et al., 2018). Yang et al. (2012) found Pt alloyed with Sn could effectively broaden d-bandwidth and lead to a downshift of d-center. This downshift of the center of d-band would lead to a higher dehydrogenation barrier, thus Sn slower carbon deposition rate though an electron effect. It should be noticed SMSI can also transfer electron to active sites similar to promoters and thus reduce the deep dehydrogenation reaction (Jiang et al., 2014). In the oxide, the highly isolated active sites are also proved to be beneficial to reduce the formation of carbon deposition, because hydrocarbon precursors adsorbed on these sites are hard to combine with each other and form aromatic rings (Zhao et al., 2019). In addition, the addition of basic promoters could modify affinity of the active site to electron, thus inhibiting coke deposition.In PDH, precursor of coke and formed coke deposition attached to active sites loosely would be constantly moving on active surface and even move to support surface derived by high temperature, which is also called a self-cleaning effect. This effect is confirmed in two different peaks of temperature programmed oxidation (TPO) experiments of spent catalysts, which showed coke may move into surface of support and further are dehydrogenated to more graphitized coke with higher combusted temperature (Jiang et al., 2014; Redekop et al., 2016; Wang et al., 2018). Some researchers also attribute two kinds of combustion peak to some coke would migrate to far distance from platinum, which could lead oxygen overflow effect of platinum need to a higher energy barrier (Larsson et al., 1996; Redekop et al., 2016). Rich electron density would weaken interaction between coke precursor and active sites thus improve this self-cleaning effect (Iglesias-Juez et al., 2010).Although the amount of coke deposited on Pt–Sn catalysts are even more than on pure Pt catalysts in similar condition, Pt–Sn catalyst still has higher stability. At the same time, the movement of coke is greatly affected by size of platinum particles. Peng et al. (2012) observed by a high-resolution TEM, binding graphene layers will slough off from platinum particles to support due to the strain of structure, which is closely dependent on the particle size. Large Pt particles are more likely to form a carbon layer which coated on the surface of platinum particles. Smaller Pt particles would cause a greater tension on the carbon layer and thus coke would be form of carbon nanotubes or sheets, which are easier to desorb from Pt surface. The movement of the carbon layer will also cause the deformation of platinum nanoparticles due to its strong adsorption and result in the change of reaction performance (Wu et al., 2016). Due to the complex structure of carbon materials, how to correctly understand the role of graphite layer coke still needs further study.In sum, strategies of anti-coke deposition could be started from both mesoscopic and micro perspectives. On the meso-scale, coking deactivation are mainly caused by channel block, increasing pore size could effectively reduce this process. In addition, proper pore structure is also important for reducing coke reaction. Straight, uniform and ordered channels are benefit to reduce the residence time of propylene inside catalyst, thus reducing coke deposition. On the micro-scale, deactivation of catalysts mainly comes from the covering of active sites by coke deposition. Isolated active sites are beneficial to inhibit the structure sensitive coke deposition process. From the view of electronic effect, the active sites rich in electrons can weaken the adsorption of coke precursors on the catalyst surface, thus inhibiting the deactivation of coke deposition. These structural and electronic effects could be achieved by adjusting promoters and supports reasonably.Co-feeding gas in PDH get more and more attention. Traditional feeding gas in PDH is pure propane, sometimes, in order to reduce coke deposition, a mixture of propane and hydrogen is also used in some cases (Saerens et al., 2017). In industrial process, hydrogen would promote reverse reaction of deep dehydrogenation and thus inhibit coke formation, but it also inhibits propane dehydrogenation at the same time. Steam is another widely used co-feeding gas, it could eliminate coke though a water-gas conversion process and reduce the partial pressure of reaction gas. Some mild oxides such as N2O, CO2 (Xie et al., 2019) are also used in propane dehydrogenation to improve conversion and reduce coke deposition. But strictly speaking, these are the category of oxidative dehydrogenation after addition these gases. Besides participating in the reaction, these gases also have important influence on the active site structure on the catalyst. How to understand their effect on the reaction is very important to optimizing technology.Hydrogen is the most important co-feed gas of PDH process in serious industrial Technology. In general, the role of H2 is considered to promote the deep dehydrogenation and dehydrogenation reverse reaction, and thus reduce the conversion and improve the stability. It should be noted that although hydrogen could participate in the reverse reaction of deep dehydrogenation, it could not reduce has graphitized carbon deposits, which also proves the irreversibility of graphitized coke deposits (Larsson et al. 1996; Redekop et al., 2016). DFT calculations and experiments also has proved hydrogen adsorbed on the surface would also promote the desorption of propylene and increase the energy barrier of dehydrogenation, thus inhibit further dehydrogenation of propylene to coke deposition. Sattler et al. (2013) found addition of H2 not only reduces the total amount of coke deposition, but also effect the structure of coke deposit. Though the analysis of Raman spectrum and TGA, researcher observed the coke deposit has a smaller size and a more graphitized structure after adding hydrogen. This finding also verifies DFT's conclusion. In fact, sometimes the addition of hydrogen will lead to the improvement of both stability and conversion, this anti-common phenomenon is considered to be related to adsorption effect. Saerens et al. (2017) tested influence mechanisms of hydrogen in Pt-based catalysts on the activity of PDH though a microreactor simulation. In addition to increasing the dehydrogenation barrier, highly partial pressure of hydrogen in feedback gas would not only reduce deep dehydrogenation species but also free active sites. Thus, appropriate addition of H2 would create more free sites on the surface and increase in activity. On the oxide, the addition of hydrogen could also change the properties of active sites, resulting in the change of reaction performance (Zhao et al., 2018). Therefore for different types of catalysts, hydrogen atoms could not only participate in surface reactions, but also change the structure of active components, the role of hydrogen should be carefully considered due to different hydrogen absorption ability.Steam is a good choose of feeding gas in industrial process which can reduce the partial pressure of propane, promote the reaction equilibrium moving forward and at the same time can be used as a good heat conducting material. Though a reaction similar to water gas conversion, steam can reduce the rate of coke deposition. However, in high temperature, co-feeding steam will accumulate in breaking of structure of supporters, such as Al2O3, zeolite, basic metals are commonly used in improving stability of these materials. In some oxide catalysts, such as zirconia, the addition of steam would cover the active sites and completely inactivate the catalyst (Otroshchenko et al., 2017). In some systems, the chemical reaction of water vapor with active component also plays a positive role. Shan et al. (2015) found the addition of steam would change alloying structure of Pt–Sn catalyst. After proper steam pretreatment time, the size of platinum particles will decrease and thus improve activity. Promotion of steam would oxidize the Sn(0) component to SnOx, resulting in the partly dealloying of Pt–Sn catalysts and formation of highly active alloying components Pt3Sn, thus decrease dehydrogenation barrier. As a cheap additive, steam has high research value.Today, uncertain volatility of oil price, decreasing crude oil resource, energy consumption and carbon emission are driving traditional petrochemical industry to reexamine the utilization efficiency of carbon atoms. Utilization of propane is a very challenging but promising part in petrochemistry, while it traditionally is used as gas fuel with a lot of CO2 emission. In order to utilize these resources in green way, PDH technology need pay more attention. From the perspective of atomic economy, PDH is a fuel-chemical conversion process with high carbon utilization, in which propylene is its only product. In PDH, hydrogen as a by-product with almost zero pollution, which is an important low-carbon fuel and reaction gas. Traditional hydrogen comes from fossil fuels and is accompanied by a large amount of CO2 emissions. Undoubtedly, widening propane dehydrogenation process will provide a “1 + 1 > 2” effect in either environment friendliness and economic benefits. However, traditional industry catalysts have their intrinsic shortcoming such as fast coke deposition and high toxicity of Cr-based catalysts and expensive price of Pt-based catalysts. In order to further improve this potential technology, structure-activity relationship of catalysts should be paid a lot of attention.For metal-based catalysts, it is proved that PDH takes place on adjacent metal atoms from one to multiple. The activity and selectivity of propane dehydrogenation are closely related to coordination environment and continuity of metal atoms. Catalytic performance of metal active sites can be effectively adjusted by forming alloy with second metal, electronic effect and ensemble effect often be used to reveal promoting role of alloy. Traditional support in metal-based catalysts is Al2O3 or SiO2. Highly dispersed alloy particles could be achieved by precisely adjusting the defect site structure or surface geometry of them. Recently, some semiconductor material such as TiO2, CeO2 were also used as supports due to their electronic effect. We think future research in metal catalysts should focus on the structure of alloys, defect sites engineering of inert supports and interaction between alloy nanoparticles and “active” support. For alloying nanoparticles, different surface alloying structure such as substitutional solid-solution alloys, interstitial solid-solution alloys, intermetallic alloy and amorphous alloy with different alloying elements should be studied separately to detail the role of alloying promoter. The sub-structure and internal structure of nanoparticle should also be paid enough attention due to their important electronic interaction with surface atoms. In the aspect of support, the commonly used supports such as alumina and silica have high specific surface area and adjustable hole structure but still need to be further improved their interaction between metal and support. Their morphology and defect sites should be precisely modified to improve dispersion of supported nanoparticle. Considering that most of the traditional supports are electrically inert, those “active” supports which interact with metal nanoparticle though electronic effect provide new possibilities for the further development of metal catalysts. How to make better utilization of their interaction and improve the specific surface area is more important.For oxide-based catalysts, the active sites of oxides are often considered to be metal-oxygen pairs with unsaturated coordination. Reduction plays an important role in determining the micro-structure of active sites include coordination of unsaturated metallic cation, hydroxyl et al. Catalytic mechanisms on oxides catalysts are varied, which are closely related to the properties of metals and oxygen. Oxide could exist as different degree of polymerization on the support, such as isolated sites, polymerized sites or crystalline on the support. Due to the different bonding between the metal center and the support, their catalytic performance is also significantly different. Isolated sites are widely considered as the most favorable active sites for oxides because of their high dispersion and low coke deposition tendency, but considering their strong interaction with support, catalytic performance of isolated sites still needs to be carefully deliberated according to the coordination structure. Oxide catalyst could be further modified by adding metal or oxide promoters. Though metal-oxide interaction or oxide-oxide interaction, promoter would change the degree of polymerization or the electronic structure of active sites. Based on the above analysis, we outline that future development direction of oxide catalysts should focus on deeply understanding the mechanism and interface effect of oxide-oxide and metal-oxide. At present, most of the catalytic mechanism of oxide catalysts come from mechanism of homogeneous C–H activation process catalyzed by metal complexes. However, the mechanism of heterogeneous catalysis is still unclear because detecting evolution of active sites in solid-gas two-phase is much more difficult than single-phase solution. Considering that there are many kinds of oxide catalysts, structure of active sites and reaction mechanism in each of them should be carefully analyzed individually. Metal and oxide promoters play an important role in improving the performance of oxide catalysts. Although many studies have found that a variety of element have positive effects in oxide catalysts, due to complex structure of interface, their promoting mechanism is still unclear. Adjusting their interaction through interface engineering is very promising to not only obtain higher activity but also establish structure-activity relationship between promoter and oxide. Especial the noble metal-oxide interaction, some recent reports even found that noble metals as promoters in oxide can achieve higher catalytic performance than noble metals as active components, regulatory interaction between noble metal and oxide though control the interface need more attention for replacing expensive noble metal catalysts with cheap oxides.Except for structure-activity relationship dominated by chemical effect, pore structure and mass transfer should be also emphasized. Reasonable design of highly interconnected, uniform pore with appropriate size plays an important role in further improving catalyst performance, especially anti-coking performance. The influence of pore structure should be paid more attention in the synthesis of supports. Influence of co-feeding gases on structure of active sites is also highly concerned in this review. We think future research in co-feeding gas should focus on some alkane-associated gas or gas easy to be separated in order to further improvement of dehydrogenation process.This work was supported by the National Natural Science Foundation of China (21872163, 21972166), National Engineering Laboratory for Mobile Source Emission Control Technology (NELMS2017A05), Beijing Natural Science Foundation (2202045, 2182060), PetroChina Innovation Foundation (2018D-5007-0505).

Dehydrogenation of propane (PDH) technology is one of the most promising on-purpose technologies to solve supply-demand unbalance of propylene. The industrial catalysts for PDH, such as Pt- and Cr-based catalysts, still have their own limitation in expensive price and security issues. Thus, a deep understanding into the structure-performance relationship of the catalysts during PDH reaction is necessary to achieve innovation in advanced high-efficient catalysts. In this review, we focused on discussion of structure-performance relationship of catalysts in PDH. Based on analysis of reaction mechanism and nature of active sites, we detailed interaction mechanism between structure of active sites and catalytic performance in metal catalysts and oxide catalysts. The relationship between coke deposition, co-feeding gas, catalytic activity and nanostructure of the catalysts are also highlighted. With these discussions on the relationship between structure and performances, we try to provide the insights into microstructure of active sites in PDH and the rational guidance for future design and development of PDH catalysts.

With the acceleration of industrialization, water pollution caused by the uncontrolled discharge of a large number of wastes and hazardous pollution into water has become a critical issue worldwide [1–3]. Various advanced oxidation processes (AOPs), including photocatalysis, electrocatalysis, Fenton-like oxidation, persulfate oxidation, and others have been proved to be sustainable solutions to toxic chemical pollutants [4–6]. Catalysts, especially heterogeneous catalysts, play a central role in AOPs for environmental remediation because of their easy recyclability and separation [4,7]. Heterogeneous catalysts usually contain transition metals or metal oxides and supporting materials [7–9]. However, the immobilization of metal or metal oxides decreases the dispersion of active sites and enhances the diffusion resistance on the surface of a solid catalyst, limiting its catalytic efficiency [9,10]. Hence, developing cost-effective advanced catalysts with high atom utilization efficiency and perfect stability is critical for environmental catalysis.Recently, single-atom catalysts (SACs) have become ideal catalysts with excellent activity and stability in variety of catalytic reactions [11–13]. Due to the almost 100% atom utilization efficiency of metal atoms, unique electronic features and ultralow metal loading, SACs have been used extensively in environmental catalysis [13–15]. In 2017, single-atom Ag was modified on mesoporous graphitic carbon nitride and used to activate peroxymonosulfate (PMS) to degrade bisphenol A (BPA) under light conditions [16]. Subsequently, abundant novel SACs with distinctive catalytic properties have been developed for environmental remediation [17–20].Although various significative review articles over SACs were published in succession [4,5,21,22], comprehensive classification of AOPs over SACs for environment remediation was not well summarized and discussed. In this review, we summarize the technical classification of AOPs over SACs in environment application and discuss the roles of SACs in AOPs for water treatments. As such, this review classified the water treatment process according to catalytic technology, including photocatalytic degradation technology, electrocatalytic degradation technology, Fenton-like reaction, persulfate oxidation reaction and coupling reaction (Fig. 1 ). We believe that SACs are interesting and promising materials for the degradation of water pollutant. Finally, the challenges and research tendency of SACs applied to water remediation are discussed.As advanced catalysts, SACs are a good choice for remediating organic pollution compared to traditional catalysts. The advantages of SACs are as follows (Fig. 2 ): (1) the preparation route is relatively simple such as direct impregnation and pyrolysis; (2) dramatically reduced metal consumption due to the ultra-low metal loading and almost 100% atom utilization; (3) remarkable catalytic activity attributed to abundant atomically dispersed active sites and the unique electronic structures; (4) perfect stability originating from the special covalent bond between metal and non-metallic element; (5) the catalytic mechanisms and active sites were liable to identified. In general, SACs have been proved as excellent candidates and provided a powerful solution to achieve desirable AOPs catalysts with high activity and stability for environmental remediation (Table 1 ).To clarify the advantages of SACs, we compare the reaction pathways on SACs and metal nanoparticles in AOPs. In the process of degrading organic pollutants, metal nanoparticles mainly play the role of trapping and transferring electrons, which can effectively improve the catalytic performance of the catalyst. For SACs, thanks to the formation of abundant chemical bonds with strong bond energies between single metal atom and non-metallic atom, numerous charges accumulate and redistribute on the surface of SACs, which not only significantly increase polarized active sites, but also promote the generation of reactive species (such as SO4 •-, O2 •-, •OH and 1O2).Photocatalysis is a kind of technology that can oxidize organic contaminants absorbed on the surface of catalyst under light irradiation. It is an efficient, safe and environmentally friendly technology to deal with environmental pollution [23,24]. The key to achieve the perfect performance of photocatalyst is to ensure the efficient separation of photo-generated-carrier on the surface of photocatalytic material. To promote photocatalytic activities, it is an ideal strategy to decorate metal nanoparticles on the surface of conventional semiconductor photocatalyst [23,25]. For a specific photocatalytic reaction, SACs can affect the three key steps of photocatalytic reaction, including light absorption, charge separation and transfer, and surface catalytic reactions [26], as shown in Fig. 3 A. The introduction of single metal atoms can not only effectively improve the light capture ability of photocatalytic material, but also enhance the efficiency of photo-generated carrier separation [24,27]. Moreover, atomically dispersed single atoms endow abundant active sites to SACs due to metal-support interaction.In recent years, different types of single atoms, including noble metal (such as Au, Ag, Pt) and non-noble metal (Cu, Co, Ni, Ba, Zn, Bi, Mo and so on) have been loaded on the surface of various semiconductor nanomaterials in the form of atomic dispersion (Fig. 3B), which are widely used in photocatalytic degradation of contaminations in water [23,24,26]. As a classic semiconductor catalyst, TiO2 has been widely used to prepare SACs due to its non-toxicity, low cost, reliable stability and suitable optical properties [28,29]. For example, Xu et al. [30] reported on the nanopores TiO2 film loaded atomic Pt and applied to the photocatalytic degradation of trace ethenzamide. They found that the nanopores of TiO2 film could accelerate the eddy diffusion and the abundant Pt active sites could promote the molecular diffusion of low concentration pollutants at the same time. More important, the single Pt atom dispersed on TiO2 film could act as the electron capture center to reduce the recombination efficiency of photo-generated carriers and prolong the lifetime of photogenic holes, which enhance the degradation efficiency of low concentration of ethenzamide under vacuum ultraviolet (VUV) and ultraviolet (UV) illumination. In addition, ideal photoactive materials can not only absorb sun light, but also provide binding pockets to stabilize individual atom. In this respect, thanks to the abundant triazine rings and N sites, g-C3N4 could anchor the metal atoms to separate and transfer charge carriers, which could enhance the photocatalytic activity [4,31–33]. For instance, Xin et al. [32] successfully synthesized single atom Ag anchored g-C3N4, which significantly enhanced the degradation of rhodamine B (RhB) and tetracycline (TC). They confirmed that the catalytic activity of the single-atom photocatalyst was modulated by the doping amount of monatomic Ag due to the ideal concentration of Ag atoms could cause a higher position of the conduction band, which endued the catalyst with stronger reduction performance. Yang et al. [33] introduced single Co atom into polymeric carbon nitride (pCN) via a facile in situ growth strategy. They found strong covalent bands of Co-O bond and Co-N bond could be formed between Co single atom and pCN, which could efficiently expand the absorption of visible light and accelerate the separation and transfer of photo-generate carrier and promote the photocatalytic degradation efficiency of oxytetracycline.Electrocatalytic oxidation process, which is the most direct means to convert the input electrical energy to contaminant oxidation through direct electrode reactions or radical generation, has led to growing interests in degrading persistent organic pollutants [34]. The construction of efficient electrode materials is one of the major challenges to improve the electrocatalytic degradation of pollutants. Recently, unique SACs have been employed to produce electrode coating materials with high electron transfer rates and stability [35,36]. On the one hand, the abundant monatomic active centers in the cathode can form intermediate products with strong oxidation capacity from O2 and H2O, which is beneficial to promote the oxidative degradation of organic pollutants (Fig. 4 A). On the other hand, the non-metallic active sites in SACs can serve as effective binding sites for immobilizing metal atoms to augment the constitutive redox activity of the metal sites [37–39]. Generally, carbon materials are used as supports for anchoring metal single atom due to its perfect conductivity and numerous versatile covalent linkages (Fig. 4B). For example, Pan et al. [39] prepared a conductive membrane consisting of single Co atom and N atom co-doped graphene (NG-Co) for electrochemical degradation of MB and RhB. The single Co site served as major active site, while the pyrrolic N groups graphene acted as the key binding sites to immobilize the Co-active site on graphene, which improved the electrochemical catalytic performance of NG-Co. Zhao et al. [40] designed an N-doped porous carbon electrode loaded with a bimetallic FeCu single-atom (FeCuSA-NPC) to degrade chlorinated pollutants (CPs). The synergistic effect of single-atom Fe and Cu is benefit to the rapid transfer of electrons and the formation of abundant •OH in FeCuSA-NPC, leading to excellent mass activity of CP pollutants removal.As a “green” degradation technology, Fenton or Fenton-like reactions play an important role in the decomposition of refractory contaminants via producing strong reactive •OH from H2O2 in aqueous media [41,42]. Traditional Fenton reaction mainly relies on Fe2+/Fe3+ catalytic decomposition of H2O2 to generate a large number of active species [43,44]. However, the inherent disadvantage of easy aggregation of iron activity sites still exists. To maximize the dispersion of the active sites and achieve higher atom utilization efficiency, single-atom iron was anchored on the surface of supported catalyst in Fenton-like reaction [45–47] (Fig. 5 A and B). Yin et al. [46] anchored single-atom Fe onto nanopore SBA-15 (SAFe-SBA) by one step of calcination. The extended X-ray absorption fine structure (EXAFS) analysis confirmed the formation of four Fe-O bonds in SAFe-SBA. Moreover, SAFe-SBA catalyst was used to catalyze H2O2 for p-hydroxybenzoic acid and phenol degradation. The results showed that the degradation rate of SAFe-SBA catalyst was significantly enhanced compared with the aggregated iron sites (AGFe-SBA). The superior catalytic activity of SAFe-SBA was attributed to the single atomic dispersion, which benefited to the exposure of the maximum Fe active sites for H2O2 decomposition to induce more •OH.Generally, the narrow working pH range (generally 2.0–3.0) of Fe-based catalysts hinders the large-scale utilization of Fenton-like reaction. Thus, single atoms Cu and Co are used in a wide pH range with H2O2 as the oxidant [48–50]. For instance, Wu et al. [48] first loaded single Cu atoms decorated on N-doped graphene (Cu-SA/NGO) with atomically dispersed CuN4 moieties via an innovative pyrolysis after freeze drying method. The Cu-SA/NGO catalyst displayed excellent activity and stability in the degradation of various organic contaminants at neutral pH. The authors proposed that Cu-SA/NGO could provide more CuN4 sites for H2O2 activation to produce •OH active species with lower energy barriers under acidic and neutral conditions. This provides a foundation for the application of single atom Cu-based catalysts in the degradation of organic pollutants. Xu et al. [49] incorporated single Cu atoms in graphitic carbon nitride (Cu-C3N4) to activate H2O2 to generate •OH at neutral pH. Then the Cu-C3N4 was rationally designed for Fenton filter to oxidize the organic contaminants in the wastewater treatment system (Fig. 5C–E).Recently, thanks to the generated high oxidizing species of free radicals (SO4 •- and •OH), the persulfate oxidation technology (POT) has obvious advantages in effectively degrading organic wastewater. Many works reported that persulfate such as peroxymonosulfate (PMS) and peroxydisulfate (PDS) can degrade pollutants directly [51,52]. In 2018, Li et al. [53] first anchored single-Co-atom on porous N-doped graphene to degrade BPA via the activation of PMS, SACs were began to apply into POT for organic contaminants degradation (Fig. 6 ).PMS (HSO5 -) has an asymmetric structure and long superoxide O-O bond (1.326 ​Å). Thanks to the higher oxidation potential and long half-life of SO4 •-, PMS-based catalytic system combined with SACs has been emerged as a promising method for environmental remediation. There are various catalytic mechanisms of SACs in PMS system, mainly including free radicals (SO4 •-, O2 •-, •OH) and reaction intermediate ROS (1O2 and high-valence metal oxidation species) (Fig. 6A). At the same time, metal species can regulate the electronic properties of the surrounding non-metal elements, forming metal-non-metal active centers, which not only facilitates PMS activation, but also accelerates the formation of intermediate ROS [54]. Recently, various metal-based SACs have been used as PMS activators to degrade organic pollution [54,55]. In particular, Fe-based and Co-based SACs are most widely studied [57–59]. For instance, Duan et al. [56] loaded single Fe atoms onto g-C3N4 (SAFe-CN) for PMS activation to degrade o-phenylphenol (OPP). The SAFe-CN coated water treatment filter remained 100% of OPP removal after 100 ​h of operation. More importantly, the author found that the dominated mechanism was not the non-radical pathway but electron transfer, which improved the utilization efficiency of PMS. Zhao et al. [58] successfully anchored atomically Co in B-doped-CN network (BCN/CoN[2+2]) adopted structural constraint engineering strategy, and this catalyst can effectively activate PMS to degrade TCL with 80% removal in 5 ​min. The experiment test and density functional theory calculations verified that the BCN/CoN[2+2] could drive the complete switching of PMS to 1O2 owing to the existence of the active site Co. Wang et al. [59] anchored singly Fe on graphite carbon nitride (Fe-SA/PHCNS). They reported that the formation of ≡FeN6=O species could activate the core catalytic site of ≡Fe-N6, which endowed 97.2% average PMS utilization rate and less affected by environment factors. Moreover, a novel filter composed of Fe-SA/PHCNS and carbon felt displayed high efficiency filtration and oxidation performance for ACE degradation (Fig. 7 ).In addition, other active transition metals (Cu, Mn) have been investigated in PMS [60–62]. Chen et al. [61] embedded single-atom copper in reduced graphene oxide (SA-Cu/rGO) to activate PMS for the degradation of various antibiotics. They reported that the synergistic interaction between rGO matrix and single-atom Cu active sites could promote the formation of reactive species on the catalyst surface and in-situ decomposition of contaminants, improving the catalytic performance of the catalyst. Yang et al. [62] successfully prepared SA Mn-N4 catalyst, which could highly activate PMS to oxidize BPA.PDS, another parent persulfate of SO4 •-, has a symmetric structure (-O3S-O-O-SO3-) and a short peroxide O-O bond (1.322 ​Å), which makes PDS more stable. Compared with PMS, PDS is cheaper and lower toxicity. Moreover, PDS has no significant effect on pH value of water. Therefore, PDS has received widespread applied for refractory organic pollutants [63–65]. Huang et al. [63] doped single-atom Fe in the plane of 2D MoS2 nanosheets. The catalyst displayed efficient catalytic activity and stability for propranolol degradation via activation of PDS, which attributed to the catalytic sites and the interaction between Fe and Mo. Du et al. [65] anchored single iron atom on nitrogen-doped carbon (SAFe-N-C) for activating PDS to degrade chloramphenicol (CAP), the removal rate of SAFe-N-C outstandingly enhanced in degradation of CAP compared to N-C (from 15.3% to 93.1%). Instead of generating free radicals, Fe-N x and graphitic N active sites were oxidized by PDS to produce 1O2 on the catalyst, which immediately oxidized CAP into small molecules.Considering the problems encountered by using one catalytic technology alone in the environmental remediation process, coupling with different catalytic technologies can realize multifunctional co-catalytic strategies, which are extensively applied to degrade pollutants [3,4]. Commonly used coupling technologies include photo-Fenton, electro-Fenton, photo-PS, ozonation-PMS, piezo-PMS, and others [66–74]. Compared with the single catalytic technique, the coupled catalytic technology has perfect catalytic performance and unique reaction mechanism (Fig. 8 ).For example, in order to overcome the high consumption of H2O2 in Fenton-like reaction, combination photocatalysis and electricity with Fenton-like reaction have been adopted, which can facilitate electron transfer in catalytic process [66,67]. Su et al. [66] designed a defect engineered single Fe atom catalyst (Fe1-Nv/CN). They reported that nitrogen vacancies in Fe1-Nv/CN could act as electron trap sites, which can actuate the photoelectrons to concentrate on single Fe atoms for •OH production under light irradiation, which could restrain the single-atom catalyst per se from quenching •OH and promote the conversion efficiency of H2O2. As a result, the Fe1-Nv/CN catalyst displays a higher ciprofloxacin degradation activity under light irradiation. Liu et al. [69] supported single-atomic-site Cu on carbon nitride (CN) by a pyrolyzing coordinated polymer strategy, and compared the degradation performance of tetracycline (TC) in SAS-Cu1.0/PS/Light and other similar systems. The SAS-Cu1.0/PS/Light system displayed outstanding catalytic performance for degradation of TC compared to other systems, indicating that light, catalyst and PS play a synergistic role in the degradation progress, which could generate abundant active species (O2 •-, SO4 •-, •OH, 1O2). Generally, the reaction between O3 and H2O2 can spontaneously generate •OH, but the reaction rate constant is extremely limited under acidic conditions. In this regard, Guo et al. [74] designed a novel catalyst by anchoring single Mn atoms on g-C3N4, which effectively generated •OH in situ through a new path in acid solution (pH=3) in the peroxone process, thus showing excellent activity and stability for oxidation of organic pollutants. Lan et al. [13] immobilized Fe single atom on the surface of piezoelectric MoS2 nanosheet and then coupled the piezoelectric activation of PMS in Fenton-like reaction to degrade various contaminants. The introduction of Fe atomic sites could not only enhance the piezoelectric polarization and the piezoelectric charge separation of MoS2, but also improve the activation of PMS during the piezoelectric catalysis.Over the past decade, the introduction of SACs in environmental remediation has been considered as an effective strategy to degrade various organic pollutants due to their high intrinsic activity. In this review, the technical classification (including photocatalysis, electrocatalysis, Fenton-like, PS and coupling reactions) of SACs in environment application is summarized. Concurrently, the reactive mechanism of SACs in different catalytic reactions is discussed. Generally, SACs exhibit perfect catalytic properties due to the synergistic effects between metal single atom and the supports materials. Owing to the optimized mono-atom dispersion properties, the unique electronic properties and the formation of covalent bands between metallic and nonmetallic atom, SACs display unique advantages such as maximizing the utilization efficiency of metal atoms, excellent stability, ultra-low metal loading, as well as perfect catalytic activities in environment remediation. What's more, the development of advanced identification methods for SACs has made it possible to study mechanism at atomic level, which endows deep understanding of the basis of catalytic engineering.Undoubtedly, SACs show excellent catalytic performance in different catalytic technology for environment remediation, but the problems of stability and selectivity of SACs still need be overcome and addressed in the future (Fig. 9 ). 1. During the process of water treatment, the reaction process is carried out in a suspension, which makes it difficult to separate and recover SACs from the suspension. Therefore, how to immobilize the SACs in water treatment system is a challenge. Some effective strategies have been applied to improve the recovery and reuse ability of SACs in water treatment applications. For example, SACs can be coated on the surface of carbon materials (carbon felt, carbon cloth, functional cotton fiber, and so on) with a porous structure, and can be used as filters in water treatment processes. Moreover, SACs can be wrapped in magnetic materials to solve the problem of separation and recovery of catalysts in water treatment processes. 2. To avoid aggregation, the loading amount of metal atoms in SACs is generally minimal, which leads to unsatisfactory catalytic efficiency in practical application. Therefore, how to balance the catalytic activity and the limited single metal loading of SACs is of great significance. 3. Although the single metal atoms can be fixed on the surface of supports through strong covalent bands, they are still easy to fall off during catalysis, leading to inactivation of the active site. Hence, it is necessary to solve the problem of the structural stability of SACs. 4. During catalysis process, abundant highly reactive oxygen species will indiscriminately attack both the contaminants and the substrate material, which threatening the stability of SACs. Hence, how to improve the selectivity of SACs is an urgent problem to be solved. During the process of water treatment, the reaction process is carried out in a suspension, which makes it difficult to separate and recover SACs from the suspension. Therefore, how to immobilize the SACs in water treatment system is a challenge. Some effective strategies have been applied to improve the recovery and reuse ability of SACs in water treatment applications. For example, SACs can be coated on the surface of carbon materials (carbon felt, carbon cloth, functional cotton fiber, and so on) with a porous structure, and can be used as filters in water treatment processes. Moreover, SACs can be wrapped in magnetic materials to solve the problem of separation and recovery of catalysts in water treatment processes.To avoid aggregation, the loading amount of metal atoms in SACs is generally minimal, which leads to unsatisfactory catalytic efficiency in practical application. Therefore, how to balance the catalytic activity and the limited single metal loading of SACs is of great significance.Although the single metal atoms can be fixed on the surface of supports through strong covalent bands, they are still easy to fall off during catalysis, leading to inactivation of the active site. Hence, it is necessary to solve the problem of the structural stability of SACs.During catalysis process, abundant highly reactive oxygen species will indiscriminately attack both the contaminants and the substrate material, which threatening the stability of SACs. Hence, how to improve the selectivity of SACs is an urgent problem to be solved.In a word, there are still many challenges and opportunities to explore the feasibility of SACs in water treatment field in the future. This paper reviews the research progress on catalyst design, catalytic mechanism, different catalytic techniques and environmental applications over SACs, and makes efforts for its future industrial applications.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was funded by the Guangdong Basic and Applied Basic Research Foundation (Nos. 2020B1515020038), Science and Technology Program of Guangzhou (No. 202201020545) and Pearl River Talent Recruitment Program of Guangdong Province (2019QN01L148).

Single-atom catalysts (SACs), consisting of metal single atoms and supporting materials, have shown remarkable potential due to their ultrahigh catalytic performances, maximum atomic utilization and environmental friendliness. More recently, SACs have become ideal catalyst materials and have been extensively applied in water treatment. This review summarizes the classification of advanced oxidation processes (AOPs, e.g., photocatalysis, electrocatalysis, Fenton-like reactions, persulfate oxidation and multi-technology coupling reaction) for the degradation of organic pollutants in water on SACs. The corresponding mechanisms for the removal of organic pollutants over SACs in the above technologies are also discussed. Distinguished from traditional nanoparticles and nanoclusters, the unique electronic properties of single metal atoms and the formation of covalent bands between metallic and nonmetallic atom promote the rapid generation of reactive oxygen species (SO4 •-, O2 •-, •OH and 1O2), which endow SACs with excellent removal efficiency of organic pollutants. Finally, the opportunities and challenges of SACs applied in practical water treatment are proposed.

Data will be made available on request.An aqueous-phase bio-cured oil separated from the pyrolysis oil of woody biomass contains up to 60% of the carbon in the original biomass [1]. The aqueous fraction is composed of various oxygenated organic compounds, mainly small carbonyl compounds such as ketones, aldehydes, and organic acids [2,3]. Recently, researchers have begun to valorise carbon contained in aqueous-phase crude oil such as small olefins and aromatics with HZSM-5 via catalytic conversion [1].In line with the growing demand for bio-jet fuel [4], studies to prepare fuel precursors through C–C bonding reactions (aldol condensation) of small oxygenates in the aqueous fraction are also being conducted [5,6]. These medium-chain fuel precursors are transformed to liquid biofuel (n-alkanes) through a hydro-deoxygenation process [7]. The medium-chain alkanes produced in this manner are suitable for the carbon range of jet fuel, and further conversion into branched alkanes through isomerization is required to obtain high quality bio-jet fuel (Fig. A1 ). For reference, since n-alkanes obtained from oil-based biomass are mainly long-chain hydrocarbons (C16 and C18), hydro-upgrading (hydro-cracking and isomerization) is performed in the last step to obtain high-yield and high-quality jet fuel. The catalyst used for hydro-upgrading is a bi-functional catalyst (Pt/HY, Pt–Mg/HY, Pt–Pd/HB, Pt/SAPO-11, etc.) wherein active metal and acid sites coexist and the cracking and isomerization reaction of paraffin occur simultaneously according to the reaction mechanism [8–10].However, unlike the existing hydro-upgrading catalysts suitable for long chain alkanes, more careful catalyst design is required for the isomerization of medium-chain hydrocarbons (C8, C13, etc) obtained through the C–C bonding reactions of small oxygenates. This is because, during the isomerization reaction on the bi-functional catalyst, a cracking reaction of the medium-chain alkane is accompanied, thereby lowering the yield of jet-fuel. Therefore, in order to obtain a high-yield jet fuel while preserving the carbon number in medium-chain alkanes synthesized from small oxygenates, a catalyst in which isomerization is dominant rather than cracking of the medium-chain alkanes is required. In particular, it is important to find a composition with excellent catalytic performance at low temperatures, since hydro-cracking easily occurs at high temperatures due to endothermic reactions [11].Many studies have been conducted to improve the yield of isomers by changing the catalyst properties in the isomerization reaction of medium-chain alkanes [11,12]. As a result of performing the isomerization reaction of n-dodecane with Pt/SAPO-11 prepared under different synthesis condition for SAPO-11 (particle sizes of 65 nm to 4.5 ㎛), it is found that both suitable acidity and suitable particle size of SAPO-11 for shorter diffusion path are closely related to the yield of isomers [12]. In the isomerization reaction of n-octane, Ni–Cu/SAPO-11 was prepared to suppress hydrogenolysis of n-alkane [11]. Hydrogenolysis was inhibited by reducing the active ensemble size by diluting the active metal (Ni) with inactive Cu, resulting in an isomer yield of about 63% at 340 °C. Low-temperature isomerization of n-hexadecane was performed using Pt–Pd/HB (Si/Al = 25) with different Pt and Pd ratios. Bi-metallic catalysts showed an enhanced metal dispersion (92%) relative to mono-metallic catalysts (54%) and the conversion was also increased along with the metal dispersion (65%–77%) [13].A small amount (0.5 wt%) of rare earth metals (Ce, La, and Re) were loaded on Pt/ZSM-22 to suppress the Pt sintering during reduction treatment since the Pt dispersion is important in the low-temperature isomerization of alkane [14]. Ce or La oxides helped to protect the nano-sized Pt metal and induced an electron-deficiency state of Pt. However, the Ce-modified Pt/ZSM-22 showed better isomerization performance than the La-modified catalyst due to the strong interaction of a Pt–O–Ce bond. Rare earth metal (Ce, La, Nd, and Yb) - exchanged Pt/HB catalysts were also prepared [15,16]. The Ce-exchanged HB (0.2–0.8 wt% loading) exhibited higher conversion and selectivity for isomerized products than the parent HB catalyst due to the reducibility of Pt species facilitated by the ion-exchanged Ce [16]. On the other hand, in the case of La-exchanged Pt/HB, only a small amount of loading (0.3 wt%) had a slight positive effect on the isomerization performance due to the formation of new Lewis acid sites [15]. As such, in the bi-functional catalytic system, the catalytic performance for the isomerization reaction of n-paraffin is greatly affected by the pore size and acid site strength and density [12], the residence time of the olefinic intermediate in the pores [17,18], the metal dispersion (distance between metals) [11,13], and the metal/acid balance [17,19], as well as the reaction conditions, and studies are being conducted to increase the degree of isomerization at relatively lower temperatures. Nevertheless, it is still necessary to explore catalyst compositions that can maximize the isomer yield while minimizing the cracking reaction, in order to achieve high-yield and high-quality fuel through isomerization of medium-chain alkanes.In this work, a series of Pt–La/HB (Beta zeolite, Si/Al = 38) catalysts were prepared for low-temperature isomerization of n-dodecane as a model compound of medium-chain alkane. In general, a Pt catalyst impregnated on an acidic support with a low Si/Al ration is more active but less selective for the isomerization reaction. However, beta zeolite with a low Si/Al ratio (B38) was selected to increase the conversion of n-alkane at relatively low temperatures and La was added on Pt/B38. The textural properties, metal dispersion, and acid properties of the La-loaded catalysts were characterized using various techniques and the accessible metal/acid ratio and an average acid step between two metal sites were discussed to highlight the importance of balancing the accessible metal and acid sites related to conversion and isomer yield.CP814C (B38) as a powder type beta zeolite was purchased from Zeolyst, and the zeolite was calcined before use at 500 ֠C for 1 h in air. As Pt and La precursors, chloroplatinic acid solution (Sigma-Aldrich, 5 wt%) and lanthanum nitrate hexahydrate (Samchun Chem., >98%) were used. The catalyst was prepared by the wetness impregnation method using a mixed aqueous solution of Pt and La precursors, followed by calcination at 450 ֠C for 4 h in air. To determine the effect of La addition, the loading amount of Pt was fixed at 1 wt%, and the prepared catalysts were denoted as follows: Pt–La'x’/B38, where x is the tentative La content.To determine the textural properties of the prepared catalyst, a Brunauer-Emmett-Teller (BET) analysis was conducted by using 3Flex (Micromeritics Co., LTD.). The sample was degassed at 200 ֠C overnight, and was cooled to −196 ֠C for N2 adsorption. The structural property was estimated by an X-ray diffraction analysis (XRD, SmartLab 9kW/Rigaku Co. Ltd.). To observe the acidity of the catalyst's surface, NH3-temperature programmed desorption (NH3-TPD) and Fourier transform infrared spectroscopy (FTIR) were utilized by using a BELCAT II catalyst analyzer (BELCAT) and a Nicolet iS50+ (Thermo Scientific), respectively. In the case of NH3-TPD, NH3 was adsorbed on the sample at 50 ֠C, and then desorbed with increasing temperature to 800 ֠C. Meanwhile, pyridine, employed as a probe molecule in the FTIR analysis, was adsorbed at 100 ֠C, and then desorbed. In the desorption profiles of pyridine at below 300 ֠C, two bands overlap around 1445–1444 cm−1, corresponding to pyridine interacting with the hydroxyl group and pyridine bonded to a relatively stronger Lewis site, respectively. Thus, we chose the pyridine desorption profile of 300 ֠C to confirm the strong Lewis site because the pyridine bonded to the hydroxyl group disappears below 300 ֠C. A H2-temperature programmed reduction (H2-TPR) experiment was carried out to evaluate the temperature at which hydrogen consumption for Pt oxides occurred using an AutoChem II (Micromeritics). The contents of Pt in the prepared catalysts were determined by using inductively coupled plasma – optical emission spectroscopy (ICP-OES, OPTIMA7300DV/PerkinElmer), and a CO-pulse chemisorption (BELCAT II catalyst analyzer/BELCAT) was used for measuring the Pt dispersion and size. Visual images of the catalysts were obtained by using a field emission-scanning transmission electron microscope (FE-S/TEM, HF5000/Hitachi Co. Ltd.) equipped with an energy dispersive spectrometer (EDS). Before the TEM analysis, the catalysts were reduced at 350 ֠C for 1 h with H2.Low-temperature hydro-isomerization was conducted with low temperature and atmospheric pressure in a continuous fixed bed reaction system. The prepared catalyst (400 mg) was inserted into the tubular reactor (quartz, I.D. 5 mm), and then the reactor was mounted on a furnace. Hydrogen (Special Gas Co. Ltd., 99.999%) was injected with a flow of 100 ml/min into the reaction system. The reactor was heated to 350 ֠C for the catalyst's reduction, and then the hydro-isomerization was progressed at 160–340 ֠C. The temperature deviation was maintained below 1%. As a model compound, n-dodecane (Sigma-Aldrich, >99%) was employed and pumped into the reaction system with 5.85 h−1 weight hourly space velocity (WHSV) by an HPLC pump (Optos 1HM, Eldex). The liquid sample was collected by using a cold trap, and the trap was cooled by ice to condense the products derived from the reactor.The collected sample was analyzed by a gas chromatograph (GC, Simadzu Co. Ltd./GC-2010 plus) equipped with a frame ionization detector (FID) and mass spectroscopy (MS, Simadzu Co. Ltd./GCMS-QP2010 SE). Both instruments used the same RTX-5 column (30 m × 0.25 mm x 0.25 μm) and oven program (held to 50 ֠C for 1 min, heated to 220 ֠C (10 ֠C/min and held for 1 min), and further heated to 280 ֠C (15 ֠C/min and held for 3 min)). To quantitatively measure unconverted n-dodecane and isomerized products (iso-C12), n-pentadecane (Aldrich, >99%) was used as an internal standard, and the response factor of isomerized products was assumed to be the same with n-dodecane. The terminologies were defined by the following equations: C o n v e r s i o n ( % ) = t h e a m o u n t o f c o n v e r t e d n − d o d e c a n e ( g ) t h e a m o u n t o f i n j e c t e d n − d o d e c a n e ( g ) × 100 i s o − H C 12 y i e l d ( % ) = t h e a m o u n t o f p r o d u c e d i s o − C 12 ( g ) t h e a m o u n t o f i n j e c t e d n − C 12 ( g ) × 100 i s o − H C 12 s e l e c t i v i t y ( % ) = t h e a m o u n t o f p r o d u c e d i s o − C 12 ( g ) t h e a m o u n t o f c o n v e r t e d n − C 12 ( g ) × 100 i s o − H C 12 n − H C 12 r a t i o ( − ) = i s o − H C 12 i n p r o d u c t ( g ) n − H C 12 i n p r o d u c t ( g ) , a n d n P t n B A ( − ) = t h e a m o u n t o f e x p o s e d P t m e t a l ( m m o l g ) t h e a m o u n t o f m e d i u m − s t r e n g t h B r ø n s t e d a c i d s i t e s ( m m o l g ) The textural properties and Pt dispersion of the prepared catalysts are summarized in Table 1 . The specific surface area and total pore volume decreased with the impregnation of Pt and La on the bare B38 support. In particular, an increase in the amount of impregnated La had a greater effect on the reduction of micropore volume than the meso/macropore volume. As a result, the mean pore diameter increased from 2.15 nm to 2.29 nm. This reduction in micropores and increase in mean pore diameter are advantageous in terms of reducing the cracking reaction of olefinic intermediates, because retention of the intermediate in micropores induces cracking reactions at acid sites. The Pt dispersion of the Pt/B38 catalyst was 41.7%, whereas the Pt dispersion increased to more than 60% when impregnated with La on Pt/B38.The XRD patterns of the prepared catalyst are shown in Fig. A2 . Compared with the XRD pattern of B38, the intensity of the characteristic peak of B38 decreased as the loading amount of La on the catalyst increased. Also, no characteristic peaks related to La were found even when 12 wt% of La was impregnated on the B38. This indicates that an amorphous lanthanum oxide (LaOx) was formed, which can be confirmed from the TEM images presented later. FE-STEM/TEM and EDS images of the prepared catalysts (Pt/B38, Pt–La2/B38, and Pt–La10/B38) are shown in Fig. 1 and Fig. A3. In the case of Pt/B38 (Fig. 1(a) and Fig. A3(a)), large Pt particles (approximately, 26–36 nm) are exposed to the external surface of the support, and small size Pt particles are distributed between them. Small Pt appears to be present within the pores rather than on the surface of the catalyst particles. The low Pt dispersion confirmed by the CO pulse method (Table 1) appears to be due to the large Pt particles exposed to the outside. As shown in Fig. 1(b) and Fig. A3(b), when Pt and a small amount of La were impregnated on B38, large and small Pt particles were also mixed. However, it is observed that, unlike Pt/B38, the size of Pt exposed to the outside was drastically reduced (approximately, 6–14 nm). As observed in the EDS mapping of the Pt–La2/B38, Pt is distributed evenly on the support with La. It appears that co-impregnated La improves the dispersion of Pt, as shown in Table 1 (Pt dispersion). When 10 wt% of La was co-impregnated with Pt on B38 (Pt–La10/B38), all large-sized Pt particles disappeared and nano-sized Pt particles (∼approximately, 3.3 nm) were uniformly distributed (Fig. 1(c) and Fig. A3(c)). It thus can be seen that as the amount of co-impregnated La increases, the uniformity and dispersion of nano-sized Pt particles appear to increase. However, unlike the STEM/TEM images, the Pt dispersion (63.7–68.4%) measured by the CO pulse (Table 1) was similar regardless of the amount of La loading for the Pt–La5∼12/B38 catalysts. This appears to be due to the Pt size gradually decreasing when the amount of La loading exceeds a certain amount (about 5 wt%), whereas as the catalyst surface is covered with lanthanum, the number of externally exposed Pt (accessible Pt) decreases little by little. However, they still show higher dispersions (more than about 63.7%) than Pt/B38.As shown in the STEM image (Figs. 1(c-3)), amorphous LaOx was observed, which is consistent with the XRD result where La-related peaks did not appear (Fig. A2). Nevertheless, as shown in both EDS mapping (Figs. 1(c-4)) and line-EDS (Fig. A3(c)), La and Pt are well distributed throughout.H2-TPR was employed to further confirm the reduction in size of Pt particles by La addition to Pt/B38 (Fig. A4). For Pt/B38, the reduction peaks centered at 80 °C and 355 °C were attributed to the reduction of PtOx particles loaded on the external surface and dispersed in the internal pores of B38, respectively [19]. The reduction temperature for PtOx loaded on the external surface of B38 was gradually shifted to higher temperatures (above 200 °C) as the loading amount of La increases. This indicates that the size of PtOx particles loaded on the external surface of the support becomes smaller, which is accordant with the results of the FE-S/TEM analysis. However, excessive loading of La (12 wt%) reduced the amount of Pt species exposed on the surface, and thus the hydrogen consumption was relatively reduced. La thus appears to play a role in preventing agglomeration of co-impregnated Pt and facilitating uniform nano-size dispersion. Table 2 summarizes acid properties of the prepared catalysts, based on the results of NH3-TPD and pyridine-FTIR (Fig. A5). When 2 wt% of La was loaded on Pt/B38, the total acid sites increased. However, as the amount of La was further increased, the amount of acid sites decreased slightly because the amorphous lanthanum oxides covered the acid sites. (Fig. 1 and A3). However, they still have more acid sites than B38 or Pt/B38. In addition, the Brønsted acid sites related to the skeleton isomerization of the reactant also decreased according to the La loading, while Lewis acid sites were generated due to the formation of amorphous LaOx. Considering the ratio of accessible Pt and Brønsted acid sites (nPt/nBA), Pt–La2∼10/B38 had a higher nPt/nBA value (0.117–0.132) than that of Pt/B38 (0.076), but there was not a significant difference among the values. It is noteworthy that although nPt/nBA shows similar values, the size of accessible Pt becomes smaller as the amount of loaded La increases (Fig. 1, Fig. A3, and Fig. A4).The desired isomerization reaction entails dehydrogenation of n-alkane on Pt site, skeleton rearrangement of olefinic intermediate on active acid sites, and hydrogenation of iso-olefin on Pt site occurring continuously and smoothly, while preserving the number of carbons in the reactant. Fig. 2 shows the conversion and the selectivity of iso-dodecane (a), the yield of iso-dodecane (b), and the distribution of product (c and d) for B38, Pt/B38, Pt–La10/B38, and La10/B38. B83 and La10/B38 were inactive for isomerization of n-dodecane at reaction temperature below 280 °C, but n-dodecane started to be converted above 300 °C where a severe cracking reaction was predominant, and thus the liquid product was hardly recovered. In the case of Pt/B38, n-dodecane stared to be converted at 180 °C and showed 100% conversion at 240 °C. However, most cracked hydrocarbons with less than five carbons and gases were generated in the high conversion section (Fig. 2(c)), and as the conversion increased according to the reaction temperature, the selectivity of iso-C12 fell inversely. As a result, the maximum yield (22.9%) of the desired iso-C12 was obtained at 200 °C with a conversion of about 30% (Fig. 2(b)). For Pt–La10/B38, the conversion was delayed by about 20 °C compared to that of Pt/B38, but the selectivity of iso-C12 was maintained high in the range of 200–250 °C. The selectivity of iso-C12 at 260 °C, which showed 100% conversion, decreased sharply, and the maximum iso-C12 yield (59.2–56.2%) was thus obtained in the range of 240–250 °C (Fig. 2(b)). As shown in Fig. 2(c), distributions (mono-branched and multi-branched isomers and cracked hydrocarbons) in the product varied according to the conversion of n-dodecane over the bi-functional catalyst. n-dodecane was mainly transformed into cracked hydrocarbons over B38 acid catalyst. In the case of Pt/B38, isomers were mainly produced at low conversion (less than about 30%), but as the conversion increased, cracked products became the dominant species due to their secondary transformation. This appears to be due to poor hydrogenation resulting from low nPt/nBA (0.076). This trend was similar when La was co-impregnated with Pt on B38, but the conversion at which the secondary transformation began to appear in Pt–La10/B38 was delayed by about two times (at about 60% conversion). For comparison, hydro-isomerization of n-dodecane was performed using a Pt/SAPO-11 which is one of the well-known catalysts for hydro-isomerization (not shown here). The conversion of 64.9% and the isomer yield of 52.1% were obtained at 300 °C. Thus, Pt–La10/B38 shows better catalytic performance even at 250 °C than Pt/SAPO-11 that have been studied recently.The ratio of cracked product and isomer (C/I ratio) and the ratio of multi- and mono-branched isomers (multi/mono ratio) are provided in Fig. 2(d). Both C/I and multi/mono ratios of Pt–La10/B38 were lower than those of Pt/B38. This means that skeleton isomerization and hydrogenation of n-dodecane are balanced on the Pt–La/B38 catalyst. In conclusion, as shown in Table 2, it can be seen that the high nPt/nBA (0.117) of Pt–La10/B38 is suitable for producing iso-C12 under the present reaction conditions, where the carbon number of the feed (n-dodecane) is preserved while minimizing the cracking reaction in the range of low reaction temperature.The average number of active acid sites that one n-dodecane molecule encountered during the catalytic reaction, nas, was estimated based on the initial product distribution to observe the characteristics of the diffusion path of olefinic intermediates between two Pt metal sites and the results are summarized in Table 3 [18]. B38 was converted at high temperature (above 280 °C in Fig. 2(a)) because there was no metal active site for dehydro/hydrogenation, and mostly cracked products were produced, traveling approximately 3.8 acid steps. When Pt was impregnated on B38, isomers were mainly generated in the initial reaction, and the number of active acid sites involved in the transformation was greatly reduced. In the case of Pt–La10/38, the nas value was close to 1.1. This is considered a result of the reduced the distance between nano-sized Pt particles, as can be seen from TEM images and Pt dispersion. This is consistent with the results of previous studies [18]. However, it is difficult to find a significant difference between nas values estimated from the initial reaction of two catalysts (Pt/B38 and Pt–La10/B38). This is because the possibility that inactive acid sites exist under the operating conditions (180 and 200 °C) cannot be excluded. This is confirmed by observing the change of nas values according to the reaction temperature, shown in Fig. A6. As the conversion of n-dodecane increased, the nas values of two bifunctional catalysts increased. This indicates that the increase of the reaction temperature causes more acid sites to become active, and also increases the diffusivity of the olefinic intermediates. However, it is clear that Pt–La10/B38 still shows lower nas values than Pt/B38 at similar conversions, and the gap between the two values increases as the temperature (conversion) increases. This means that in the Pt–La10/B38 catalyst with well dispersed nano-sized Pt particles, even if the reaction temperature rises, the average number of active acid sites that the intermediates encounters while traveling through the surface of the catalysts is small due to the short distance between Pt particles. Fig. 3 shows the results of hydro-isomerization of n-dodecane according to the La content impregnated on the Pt/B38 at 250 °C. Although the conversion was close to 100% on Pt/B38 at 250 °C, the recovered iso-C12 yield was very low due to the predominant cracking reaction (Fig. A7). As the amount of La loading increased, the conversion of dodecane decreased, but the selectivity and yield of the desired iso-C12 tended to increase. The maximum yield of iso-C12 was obtained in the 10% La impregnated catalyst.The reason for the change in catalytic properties (conversion, selectivity and yield) in the Pt–La series catalysts even with similar nPt/nBA values is (1) an increase of uniformity of nano-sized Pt and (2) the smaller distance between two Pt particles caused by La loading, resulting in predominant hydrogenation of the olefinic intermediates. That is, the optimal loading of La reduce the distance between Pt particles and enhances the intimacy between the uniformly distributed nano-sized Pt and the active acid sites. A further increase of the La loading (more than 12 wt%) resulted in lowered catalytic performance, likely due to the coverage of the Brønsted acid and Pt sites by LaOx with Lewis acid sites, as discussed in the results of FE-S/TEM and H2-TPR, consistent with a previous study showing that the conversion is related to the externally exposed Pt over the catalyst [19]. For the Pt–La2∼10/B38 catalysts, the ratio of iso-C12/n-C12 was greater than 2.1. Note that the ratio decreases as the content of La increases because uncovered n-dodecane remains. Fig. 4 shows the time-on-stream stability of Pt–La10/B38 in hydro-isomerization of n-dodecane. Initial conversion and isomer yield were 86% and 57%, respectively, but the isomer yield gradually decreased (46%) until 4 h of time-on-stream. After shutting down the reaction system, the catalyst was in-situ cleaned with acetone without any other regeneration process. In the subsequent reaction, the conversion and isomer yield were maintained within 75–80% and 55–60%, respectively, which means that the catalyst can be regenerated only by washing with acetone.Based on the above results, the hydro-isomerization reaction over the bifunctional catalysts is schematically drawn in Fig. 5. The mechanism of hydro-isomerization on the bi-functional catalyst has been well documented in several studies [13]. In general, the n-alkane is dehydrogenated to the alkene intermediates on the Pt active site and the intermediates are protonated and isomerized on the Brønsted acid sites, and then hydrogenated on the nearby Pt active site to form a desired iso-alkane. Meanwhile, the low nPt/nBA value and the diffusion limitation of the intermediates in the microporous channels of acid supports increases the contact opportunity and the contact time with the acid sites, resulting in further cracking to obtain unwanted cracking products. That is, a proper arrangement of metal and acid sites is very important in bi-functional catalysts, as well as the textural structure of the catalysts. For Pt–La10/B38, the size of Pt was very small and uniform. Such uniformly distributed nano-sized Pt particles (∼approximately, 3.3 nm) give the reactant many opportunities for dehydro/hydrogenation reactions, resulting in the production of many olefinic intermediates at the same time. The intermediates undergo skeletal rearrangement at an adjacent acid site and then hydrogenate at a nearby Pt site to form isomers. In addition, the La loading reduces the volume of micropores, thereby suppressing the cracking reaction that occurs from the diffusion limitation of the intermediates.A series of Pt–La/B38 catalysts were prepared for low-temperature isomerization of n-dodecane and the effect of La addition on the catalytic performance (conversion, selectivity and yield) was investigated. Based on the results of CO pulse chemisorption, FE-S/TEM images, and a H2-TPR analysis, La (10 wt%) co-impregnation in Pt/B38 resulted in an increase of the Pt dispersion (41.7%–68.4%). Moreover, the size of Pt loaded on the external surface of B38 was significantly reduced from approximately 26–36 nm–∼3.3 nm and the uniformity in Pt size was notably enhanced. La thus appears to play a role in preventing agglomeration of co-loaded Pt and facilitating uniform nano-size Pt dispersion. Pt/B38 showed good activity at relatively low reaction temperature (100% conversion at 220 °C), but most cracked hydrocarbons with less than five carbons and gases were generated in the high conversion section, resulting in a maximum yield, 22.9%, for the desired iso-C12. Meanwhile, for Pt–La10/B38, the conversion was delayed by about 20 °C compared to that of Pt/B38, but the selectivity of iso-C12 was maintained high in a range of 200–250 °C, resulting in the maximum iso-C12 yield (59.2–56.2%). Both the C/I and multi/mono ratios of Pt–La10/B38 were lower than those of Pt/B38, indicating that skeleton isomerization and hydrogenation of n-dodecane are balanced on the Pt–La/B38 catalyst. In conclusion, the desirable arrangement of active sites with higher nPt/nBA (0.1777) and lower nas (1.11) caused by La loading in the Pt–La/B38 catalyst enhances the catalytic performance for low-temperature isomerization of n-dodecane.A National Research Foundation of Korea grant funded by the Korea government. Il-Ho Choi: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. Hye-Jin Lee: Methodology, Formal analysis. Kyung-Ran Hwang: Writing – review & editing, Supervision, Project administration, Investigation, Conceptualization.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF2020M1A2A2079802).The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.micromeso.2022.112294.

A series of Pt–La/B38 catalysts were prepared for low-temperature isomerization of n-dodecane and the effects of La addition on the textural properties, metal dispersion, acid properties, and catalytic performance were investigated. La co-impregnated with Pt on B38 significantly reduced the Pt size and notably enhanced the uniformity in Pt size. A higher ratio of accessible Pt and medium-strength Brønsted acid sites and a shorter distance between two Pt particles of Pt–La10/B38 resulted in the maximum iso-C12 yield (59.2–56.2%) in the range of 200–250 °C, due to the reasonable arrangement of active sites caused by La loading.

Data will be made available on request.Wastewater generation in urban cities is increasing steadily, and more than 48 % of global wastewater is discharged without treatment [1]. Many technologies, including aerobic and anaerobic treatment systems, have been reported for wastewater treatment. However, these conventional technologies have high energy requirements and are not self-reliant due to the need for an energy supply from fossil fuel sources [2]. Therefore, developing new water treatment technologies with integrated, efficient energy systems and sustainable practices will provide a positive approach toward a circular economy [3].One technology with the potential to significantly reduce the energy requirements in wastewater treatment plants is the microbial fuel cell (MFC). This bio-electrochemical system has gained widespread attention as a sustainable energy generation and wastewater treatment technology. In this system, the electroactive microorganisms are able to oxidize the organic substrates in wastewater under aerobic conditions, thereby releasing electrons and protons. The electrons are then transferred to the cathode via an external circuit, while the protons diffuse through a proton exchange membrane (PEM) [4]. The MFCs have demonstrated advantages such as low pressure and temperature operation and a minimal negative environmental footprint. However, their practical application has been limited by their low power generation, mainly due to the limited transfer of electrons from the microorganisms to the anode, and the high cost of their component materials [5]. Moreover, domestic wastewater contains many complex molecules that are difficult for the microorganisms to break down and oxidize [6]. Consequently, many studies have used synthetic, lab-produced wastewater. However, these synthetic wastewaters do not represent the actual characteristics of real wastewater and often require the addition of selective electroactive microorganisms cultured elsewhere. Therefore, to overcome the challenges of MFCs and make them compete with the conventional technologies for wastewater treatment and energy generation, new materials and designs must be developed to enhance the MFC performance, and these must be tested under operation using real wastewater for practical applications.The anode material of the MFC is a critical component in determining the power generation and enhancing the MFC performance due to its direct contact with microorganisms and its significant influence on the efficiency of the electron transfer process. Generally, the anode material should have a large surface area, electrical conductivity, good hydrophilicity, chemical stability, and biocompatibility [7]. Although various carbonaceous materials, including carbon cloth, felt, fiber, foam, reticulated vitreous carbon (RVC), and graphite, have been reported as MFC anode materials [8], they are limited by their high hydrophobicity, low surface area, and minimal porosity [9]. Consequently, many researchers have attempted to improve the properties of carbonaceous electrode materials by using various methodologies such as chemical, physical, and thermal treatment or coating with nanostructured materials [10–12]. In particular, the nanostructured coating of the anode is a simple and efficient technique. It provides a large surface area, porosity, stability, and selectivity for better interaction between the electrode and microorganisms, thus resulting in enhanced biofilm adhesion and acceleration of the electron transfer process. Metal and metal oxides such as Ni, Fe, Co, Au, Pd, Co2O3, MnO2, and Fe2O3 have been investigated extensively as anode catalysts for MFCs [13]. These materials possess remarkable properties, such as good electrical conductivity and electrocatalytic activity, for improving the performance of MFC [14,15]. However, their poor corrosion resistance and inadequate bacterial adhesion hinder their large-scale application in MFC [16]. It is noteworthy that the biocompatibility of metal and metal oxides as anode modification materials in MFC does not have a unified conclusion in the literature. Some metal and metal oxides, such as Fe, Fe2O3, TiO2, MnO2, FeO2 are reported to have good biocompatibility [12], while others, such as Cu, Ag, and Au, have limited biocompatibility with microbes [17]. Some authors hybridize metal and metal oxides with graphene-based materials to modify their properties for MFC applications [13].In this regard, tungsten nitride (W2N) and tungsten carbide (WC) have attracted considerable attention as specific transition metal groups due to their electrochemical stability, corrosion resistance, and platinum-like properties [18,19]. For example, previous work by the present authors investigated the use of WC on graphene oxide as an efficient anode catalyst for the MFC [20]. The results showed an enhancement in the generated power and current density due to increased surface wettability, surface area, and electric conductivity. Similarly, titanium nitride (TiN) nanoarray was in situ grown on carbon cloth (CC) for the anode in MFCs. The growth of TiN on the anode facilitated the enrichment of a large amount of Geobacter soli on the anode surface. The efficient growth of a large number of electroactive microorganisms was due to the metallic conductivity of TiN and its strong affinity towards microorganisms [21]. Towards the same aim, other researchers have investigated carbonaceous materials coated with three-dimensional macroporous structures, such as carbon foam, sponge, and carbon nanotubes, as high-performance anodes [22,23]. However, even though 3D microporous structures possess many advantages, their large-scale applications are hindered by the low interaction between the microorganisms and the anode surface, the complex fabrication procedures, and the high cost of these materials [15].MXenes, a class of two-dimensional inorganic materials with layers (a few atoms thick) consisting of transition metal carbides, nitrides, or carbonitrides, was first reported by researchers from Drexel University in 2011 [24]. MXenes have unique properties, such as good stability, high electrical conductivity, and rich surface functionalization, including a layered morphology for accelerated charge transfer and biocompatibility [25]. Consequently, the applications of MXenes in electrocatalysis [26], ion batteries, supercapacitors, water treatment, electronic devices, fuel cells, and bio-electrochemical systems have expanded rapidly [27,28]. Many studies have shown that MXenes are promising materials in electron transfer applications [29–31]. For example, due to their exceptional conductivity, they have been investigated for cathodic and anodic reactions in energy storage devices. This is due to the interlayer structure and oxygen termination groups that promote efficient electron transport [32,33].The first synthesized MXene was titanium carbide (Ti3C2 Tx ), after which the family grew to include zirconium carbide (Zr3C2 Tx ), molybdenum carbide (Mo2CTx ), titanium nitride (Ti2N), niobium carbide (Nb2C), tantalum carbide (Ta4C3) [34]. Nevertheless, Ti3C2 Tx remains the most widely investigated MXene due to its more accessible synthetic pathways. This material is biocompatible, can bind strongly to carbonaceous substrates, and increases both the electron transfer rate and catalytic ability of the substrate [35,36]. However, despite the many outstanding properties of the reported MXenes, and the presence of rich surface functional groups, the fabrication of MXene-based composites via a simple and efficient process remains challenging due to the need for extreme synthesis conditions, such as high temperatures above 300 °C, which sometimes lead to damage of the crystal structure and the loss of its advantageous features [37,38]. In 2018 Ti3C2 Tx MXene was first reported as an anode for improved MFC [29]. This work showed an increase in power generation attributed to lower internal resistance and the promotion of microbial nanowires (pili), which increase the conductivity of the biofilm.Similarly, an MFC with a nickel ferrite/MXene on a carbon felt anode exhibited higher power density [31] due to more efficient electron transport and increased biological activity. MXene was also combined with a metal–organic framework (MOF) named ZIF-67 to form a Ti3C2-ZIF-67 hybrid anode catalyst to improve the performance of MFCs. Due to the large specific surface area of MOFs, large pore size, biocompatibility, and particle size combined with the better hydrophilic surface and interlayer morphology of MXene, the power generation, and bacterial colonization were improved significantly [39]. The anolytes used in these works are synthetic wastewaters or solutions that do not represent the complexity of the wastewaters and lead to a different population of microorganisms, which then require that they are cultured separately.Herein, a W2N-Ti3C2 Tx composite catalyst (where Tx  = O, F, and OH) is synthesized, characterized, and tested as the anode in an MFC to produce electricity during wastewater treatment. The aim is to improve the extracellular electron transfer and microorganism adhesion by depositing the as-synthesized material on a standard carbon cloth anode. The anolyte solution is real domestic wastewater without any additional inoculated microorganisms. The crystallinity, structural morphology, wettability, and biocompatibility of the composite catalyst are investigated via X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), and atomic force microscopy (AFM). The composite catalysts coated on carbon cloth are tested in situ using a single-chamber air–cathode microbial fuel cell. The present work highlights the great potential of such composite catalysts to enhance the performance of electrochemical systems.The Ti3C2 Tx MXene (where Tx represents surface functional groups such as –O and –F, OH) was produced by exfoliating the titanium aluminum carbide MAX phase precursor (Ti3AlC2) via the hydrofluoric acid (HF) etching approach shown schematically in Fig. S1a of the Supplementary Information [25,40]. In brief, the Ti3AlC2 MAX phase (1 g; ≥90 %, ≤100 µm, Sigma-Aldrich) was added gently to a hydrofluoric acid solution (10 ml; 40 wt%, SungYoung Chemical Limited) with magnetic stirring. The solution was mixed for 6 h to complete the etching process. Following the reaction, the residual solids were separated by centrifugation and washed with deionized water until the pH of the supernatant was ∼ 7. The wet sample was then filtered under vacuum and dried at 80 °C overnight to obtain the MXene powder.The W2N was synthesized via the urea glass route, as shown schematically in Fig. S1b [20,41]. First, the metal precursor tungsten chloride (WCl6; 1 g, 99.9 % based on trace metals, Acros Organics) was dissolved in ethanol (2.54 ml). The solution was mixed for 30 min to form a stable brown W-orthoester solution. Then, urea (1 g) was added to the solution and mixed until a viscous solution was obtained. Finally, the solution was dried overnight at room temperature, followed by thermal treatment at 800 °C (with a ramping rate of 5 °C min−1) under a flow of nitrogen (N2) for 4 h to obtain the desired product as a silvery black powder.To synthesize the W2N-MXene composite catalyst, 100 mg each of W2N and MXene were separately dispersed in ethanol (10 ml each) to form solutions A and B, respectively. Each solution was then sonicated for 1 h at room temperature, as shown schematically in Fig. S1c. Solution A was then gently added to solution B with stirring. The stirring was continued for 20 min until the ethanol had evaporated. The wet sample was then dried at 70 °C for 90 min, followed by heat treatment at 300 °C for 4 h under a flow of N2 at 5 °C min−1, to yield a dark powder.The as-synthesized catalyst was coated onto carbon cloth via a simple ink-dropping approach, as shown schematically in Fig. S1c. First, the catalyst ink was prepared by adding the catalyst powder (10 mg) to a mixture containing 5 wt% Nafion binder (15 µl; Sigma-Aldrich), absolute ethanol (500 µl), and deionized water (500 µl). The mixture was then shaken several times, followed by sonication for 30 min to form a well-dispersed catalyst ink. Then, using a micropipette, the ink was repeatedly deposited onto the anolyte-facing side of the carbon cloth (Electro Chem. Inc., USA) until the entire surface was covered with catalyst ink. Finally, the sample was dried at room temperature to obtain the W2N-Ti3C2 Tx modified carbon cloth anode.The crystal structures of the as-synthesized catalysts were investigated under Cu Kα radiation using a Bruker D88 advanced diffractometer operated at 40 kV and 40 mA, in the angle range of 5° to 90° with a step size of 0.05. The morphologies of the as-synthesized composite catalysts were examined via field emission scanning electron microscopy (FE-SEM) with energy-dispersive X-ray spectroscopy (EDX; FEI TENEO VS) at an acceleration voltage of 20 kV and a working distance of 10 mm. High-resolution transmission electron microscopy (HR-TEM) was performed on an FEI-Titan ST electron microscope operated at 300 kV. After collecting the HR-TEM images, the same microscope was used to collect high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and to characterize the elemental distributions on the as-synthesized catalysts. The thicknesses of the as-synthesized MXenes were determined by atomic force microscopy (AFM) using a dimension icon SPM scanner (Bruker) with an RTESPA-150 probe. Water contact angle analysis was performed at room temperature using a CAM 200 goniometer (KSV Instruments, USA). The formation of biofilm on the anode surface was investigated by the bio-SEM images using a Zeiss Merlin Gemini II microscope equipped with an in-lens critical point dryer (CPD300, Leica) and Everhart-Thornley detector (ETD).The ex-situ investigation of the prepared anodes was performed in a three-electrode cell comprised of the bio-anode, a Pt wire, and an Ag/AgCl electrode as working, counter, and reference electrodes, respectively, using a potentiostat cyclic voltammetry (CV) (Gamry Reference 600, United States of America) at a scan rate of 25 mV s−1 from + 0.8 V to −0.8 V. Electrochemical impedance spectroscopy (EIS) was used to analyze the electron transfer behavior; the measurements were carried out using the same aforementioned electrochemical setup. The frequency range used for the analysis was 0.01 Hz to 100 kHz with an amplitude of 5 mV.As shown schematically in Fig. S2, the single chamber air–cathode MFC reactor consisted of highly corrosion-resistant stainless-steel plate current collectors maintained at a constant separation of 2 cm, along with an anode composed of plain carbon cloth (CC), the Ti3C2 Tx /CC, or the W2N-Ti3C2 Tx /CC, and a 2.4 cm × 2.4 cm cathode consisting of platinum deposited onto carbon paper (Pt/CP), separated by a Nafion 115 membrane (Membrane International Inc. NJ, USA). All experiments were performed at room temperature and atmospheric pressure. In each case, the MFC was charged with domestic wastewater obtained from the KAUST wastewater treatment plant in Thuwal, Saudi Arabia. The anode chamber had a volume of 80 ml, which contained 60 ml of wastewater and 20 ml of glucose solution. Before each experiment, the anolyte chamber was purged with N2 to remove any dissolved oxygen and maintain anaerobic conditions. In contrast, free atmospheric oxygen was used as an electron acceptor in the cathodic compartment. The open circuit voltage (OCV) and electrode potentials were measured using a potentiostat and an Ag/AgCl reference electrode (HA-151B), respectively. All conducted measurements were recorded over time using a GL240-Graphtec data logger. After achieving a stable OCV, the circuit was closed using an external resistance, and the polarization curve was obtained via linear sweep voltammetry at a scan rate of 1 mV s−1. The obtained current was used to calculate the power (P = IV). The current and power densities were obtained by normalizing the current and power to the anode geometric surface area (5.76 cm2), as given by Eq. (1) and Eq. (2), respectively. A current stability test was performed for 12 h at a constant cell voltage of 200 mV to evaluate the performances of the MFCs based on the as-synthesized catalysts. (1) C u r r e n t D e n s i t y m A m - 2 = I A an (2) P o w e r D e n s i t y m W m - 2 = IV A an where I is the current generated, Aan is the anode geometric surface area, and V is the voltage of the MFC.The wastewater treatment efficiency was evaluated by measuring the COD before and after each MFC test. As given in Eq. (3), the coulombic efficiency (CE) was assessed based on the ratio of the recovered charge to the theoretical value, assuming that all substrate removal was converted entirely to electricity. Where 8 is the constant used for COD based on the ratio of the molecular weight of oxygen (O2) and the number of electrons exchanged per mole of O2 (b = 4), I is the current, F is Faraday’s constant, Van is the volume of liquid in the anode compartment, tb is the time of MFC operation and Δ C O D is the difference between the COD of the influent and effluent. The COD removal percentage was evaluated using the ratio of the difference between the influent and effluent CODs to the influent COD, as given by Eq. (4). (3) C E % = 8 ∫ t b 0 I d t F V an Δ C O D ∗ 100 % (4) C O D r e m o v a l % = COD influent - COD effluent COD influent ∗ 100 % The Ti3C2 Tx material is structurally defined by the parent Ti3AlC2 MAX phase, which contains ionic-solid/covalent bonds between the Ti and C atoms, along with weaker metallic bonds between Ti and Al atoms. During the reaction, the etchant attacks the weak Ti–Al bonds to selectively etch and remove the Al layer, while failing to break the Ti–C bonds due to the high stability of the metal carbide in the acidic environment [42]. This is confirmed by the XRD pattern of the as-prepared Ti3C2 Tx and the Ti3AlC2 precursor in Fig. 1a, where the Ti3C2 Tx (red line) exhibits two significant peaks at 2θ = 8.9° and 18.2° due, respectively, to the {002} and {004} crystal planes of the MXene [25] (see magnified peaks inseted in Fig. 1a). In contrast, the characteristic peak at 39° due to the Al {104} crystal plane observed in the precursor (green line) is no longer visible after the etching process (red line). The peak at 41.5° in the XRD pattern of the MAX phase precursor is attributed to the presence of TiC as an impurity. Meanwhile, the XRD pattern of the W2N-Ti3C2 Tx (black line, Fig. 1b) exhibits the peaks at 8.9° and 18.2° due to the Ti3C2 Tx , along with additional peaks at 38.35, 43.16, 61.97, 73.3, and 75.87° due, respectively, to the {111}, {200}, {220}, {311}, and {222} crystal planes of W2N [43]. The latter peaks are observed in the XRD pattern of the pristine W2N (blue line, Fig. 1 b) along with peaks attributed to tungsten nitride (WN) and tungsten (W) due to traces of tungsten mononitride along with metallic tungsten obtained during the synthesis process.The structures of the as-prepared Ti3C2 Tx and W2N-Ti3C2 Tx can be further elucidated by using Bragg's diffraction equation to calculate the d-spacing and c-lattice parameters. The d-spacing of an MXene indicates the complete interlayer spacing between single MXene sheets, while the c-lattice parameter encompasses two consecutive MXene sheets along with two complete interlayer spacings [44]. In the present work, the d-spacing and c-lattice parameters of the Ti3C2 Tx are calculated to be 0.985 nm and 1.970 nm, respectively. They are higher than those calculated for the Ti3AlC2 MAX phase (i.e., 0.923 nm and 1.846 nm, respectively). The difference of 0.062 nm between the d-spacings of the Ti3C2 Tx and the Ti3AlC2 MAX phase corresponds to the average atomic radii of oxygen (O) and fluorine (F) atoms. It is due to the intercalation of the Ti3C2 Tx MXene with these atoms during the HF-based preparation. Meanwhile, the calculated d-spacing and c-lattice parameters of the W2N-Ti3C2 Tx are 0.964 nm and 1.928 nm, respectively, which are lower than those of the MXene due to the deposition of W2N onto the surface and between the Ti3C2 Tx layers.The structure, morphology, and compositions of the as-prepared Ti3C2 Tx and W2N-Ti3C2 Tx are revealed by the SEM images in Fig. 2 . The Ti3AlC2 exhibits a compact 3D morphology (Fig. 2a). In contrast, the Ti3C2 Tx exhibits an accordion-like structure with visible interlayer spacing (Fig. 2b). This accordion-like morphology is typical of MXenes that are synthesized using HF with a concentration above 30 %. The appearance of an accordion-like structure is due to the exothermic nature of the HF reaction with Al, which releases H2 gas [40]. The Ti3AlC2 structure is composed of stacked Ti3C2 layers separated by Al atoms. When Ti3AlC2 powder is immersed in an HF solution with high concentration, an exothermic reaction occurs between the Al atoms and HF and releases gas bubbles, presumed to be H2. This reaction removes the Al atoms between the layers, resulting in the exfoliation of Ti3C2 layers due to the loss of metallic bonding holding them together with the Al atoms. Because the experiments were conducted in an aqueous environment rich in fluorine ions, oxygen, hydroxyl, and fluorine are the most probable termination groups on the surface [25,45]. The visible interlayer spacings confirm the removal of the Al layers in agreement with the XRD results mentioned above. The HAADF-STEM images of Ti3C2 Tx and the W2N-Ti3C2 Tx composite in Figs. S3 and S4 further confirm the structure and reveal the presence of –O and –F groups on the Ti3C2 Tx MXene surface.Further, the SEM image of the W2N-Ti3C2 Tx in Fig. 2c demonstrates that the MXene structure is maintained in the composite and reveals that the W2N is deposited in the interlayer spacings and on the surface of the MXene. As well as preventing the 2D MXene layers from restacking and promoting the electron transfer process.The SEM + EDX elemental mappings of the MAX phase precursor and the Ti3C2 Tx MXene are provided in Fig. S5. These reveal that, after the etching process, Al atoms are no longer present, while C and Ti atoms are well distributed. These results confirm the presence of–O and –F functionalities on the MXene surface, in agreement with the calculated c-lattice parameters and the results in Fig. S3. Finally, a comparison of these results with the SEM + EDX mappings of the W2N-Ti3C2 Tx in Fig. 2d indicates that the addition of W2N does not cause any significant rearrangement in the Ti and C distributions in the catalyst. The numerous functional groups on the surfaces of the as-synthesized Ti3C2 Tx and W2N-Ti3C2 Tx are expected to play a significant role in their hydrophilic and adhesion properties. The quantitative elemental distributions of the MAX phase, the MXene, and the W2N-NPs-MXene obtained from the EDX mappings are summarized in Table S1.The 2D structure and interlayer spacing of the as-prepared Ti3C2 Tx are further revealed by the low- and high-resolution TEM images in Fig. 3 . Here, the d-spacing is seen to be 0.98 nm (Fig. 3b), which agrees with that calculated from the XRD data. Further, the AFM results in Fig. 4 a and c indicate that the average thickness of the Ti3C2 Tx multilayer sheet, which consists of three-four stacked Ti3C2 Tx sheets, is 2.9–3.4 nm. Meanwhile, the average thickness of the multilayer sheet in the W2N-Ti3C2 Tx composite is 3.9 nm (Fig. 4b and d). This is attributed to the deposition of W2N between the MXene multilayers and on the surface and the exfoliation of some of these stacked sheets.The hydrophilicity of the plain CC and the carbon cloth coated with Ti3C2 Tx (Ti3C2 Tx /CC) and W2N-Ti3C2 Tx (W2N-Ti3C2 Tx /CC) are demonstrated by the water contact angle (WCA) results in Fig. 5 . The WCA is seen to decrease significantly from 144° for the pristine CC to 70° for the Ti3C2 Tx /CC. The hydrophobic surface of the pristine CC is mainly due to the lack of oxygen-rich groups and can hinder the attachment of microorganisms. By contrast, the surface of the Ti3C2 Tx /CC is hydrophilic due to the presence of –O and –F groups. However, the WCA of the W2N-Ti3C2 Tx /CC is seen to increase slightly relative to that of the Ti3C2 Tx /CC (i.e., from 70° to 77°) due to the removal of some –O and –F groups during the synthesis of the composite [46]. Even though the addition of W2N exhibited a slight increase in WCA from 70° to 77°, the surface characteristics are still in the hydrophilic zone, whereas decreasing WCA from 144° in the case of CC anode to 77° for W2N-Ti3C2 Tx /CC is a significant improvement in the CC anode wettability [47]. Therefore, improving the hydrophilicity of the CC by coating it with the as-fabricated W2N-Ti3C2 Tx composite catalyst is sufficient for promoting microorganism attachment and biofilm growth on the anode surface.The ex-situ electrochemical behavior of CC, Ti3C2 Tx /CC, and W2N-Ti3C2 Tx /CC was investigated in a three-electrode cell. Fig. S6a shows the CV profiles of the plain CC, Ti3C2 Tx /CC, and W2N- Ti3C2 Tx /CC anodes. The results reveal that the W2N-Ti3C2 Tx /CC anode exhibited the highest current density (6.2 A m−2), followed by Ti3C2 Tx /CC (3.3 A m−2) and finally CC (0.3 A m−2). This finding indicates that the electron transfer process was improved by the deposition of Ti3C2 Tx , and W2N- Ti3C2 Tx catalysts layers on the surface of CC. The presence of the catalyst layer enhanced the bio-catalytic activity, surface wettability, and electrical conductivity. Because of these improvements, the electron transfer resistance was reduced, thereby increasing the current density. EIS analysis was used to analyze the resistance to electron mobility of CC, Ti3C2 Tx /CC, and W2N-Ti3C2 Tx /CC in the three-electrode cell using real wastewater and 1 M glucose as the electrolyte solution. The obtained results further explain why the coated CC (Ti3C2 Tx /CC, and W2N-Ti3C2 Tx /CC) achieved higher current density compared with the CC. As shown in the Nyquist plot (Fig. S6b), plain CC exhibited the highest resistance to electron mobility. This is due to the high interfacial resistance (710 Ω) between the wastewater and the hydrophobic surface (WCA of 144°) of CC [48,49]. On the other hand, the Ti3C2 Tx /CC, and W2N- Ti3C2 Tx /CC anodes achieved an interfacial resistance of 350 and 200 Ω, respectively. This is attributed to the significant improvement in the anode wettability after coating the catalyst layer. The interfacial resistance achieved from the EIS analysis exhibits the same order as the internal resistance calculated from the slope of the polarization curve of the MFC (Fig. 8c.).As detailed in the Experimental section, the OCV and anode potentials (Ean) were measured for MFCs equipped with the CC, Ti3C2 Tx /CC, and W2N-Ti3C2 Tx /CC anodes, and the results are presented in Fig. 6 . Here, each cell exhibits a decrease in anode potential (Fig. 6a) and an increase in OCV (Fig. 6b) with time. This is attributed to the accumulation of electrons generated by microorganisms during the accumulation stage. The bacteria degrade the organic substrate (glucose) in the wastewater solution, release Adenosine triphosphate (ATP) and Nicotinamide adenine dinucleotide-hydrogen (NADH), and convert the latter to Nicotinamide adenine dinucleotide (NAD+) to complete the metabolic cycle [21,50]. The released NAD+ accumulates within the growing bacteria until a steady state is reached, and a biofilm layer is formed with the maximum quantity of released electrons on the anode surface. Notably, the highest OCV (789 mV) and lowest anode potential (–0.118 V) are obtained for the W2N-Ti3C2 Tx , followed by Ti3C2 Tx /CC (785 mV and −0.214 V), and the plain CC (710 mV and –0.125 V). These results demonstrate the enhanced attachment of the microorganisms and biofilm formation (Fig. S7 shows an image of the used anode with biofilm attachment.) on the surface of the modified anodes due to the enhanced hydrophilicity and affinity of these anodes towards the microorganisms. In addition, it is worth noting that the W2N-Ti3C2 Tx /CC takes longer to reach the stable state relative to both the Ti3C2 Tx /CC and the plain CC, thereby indicating the continuous growth of a larger community of electroactive microorganisms on the former electrode surface.The process of biofilm formation on the surfaces of the plain CC, the Ti3C2 Tx /CC, and the W2N-Ti3C2 Tx /CC anode is further elucidated by the bio-scanning electron microscope images obtained after testing the MFCs (Fig. 7 ). Here, the plain CC anode (Fig. 7a and b) exhibits the lowest colonization of microorganisms compared to those of the Ti3C2 Tx /CC (Fig. 7c and d) and W2N-Ti3C2 Tx /CC (Fig. 7e) anodes, while the two modified anodes exhibit high colonization of many types of microorganism. This is due, in turn, to the hydrophobic nature of the plain CC, which is unfavorable for bacterial attachment, and the enhanced hydrophilicity and biocompatibility of the modified anodes. Moreover, the synergy between the W2N and Ti3C2 Tx in the composite catalyst significantly enhances the conductivity, increases the available surface area for microorganism attachment, and enhances the growth of electroactive microorganisms. This, in turn, promotes the electron transfer rate between the bacteria and the anode surface. Consequently, the W2N-Ti3C2 Tx /CC exhibits an extensive biofilm with structures that resemble microbial nanowires or pili (Fig. 7e), which are known to play a role in direct electron transfer between the electroactive bacteria and the electrode surface. Notably, the W2N-Ti3C2 Tx exhibits the maximum microorganism attachment among the three anodes, along with a diverse bacterial community, which can significantly influence the time required to achieve a high and stable OCV during the electron accumulation stage of MFC operation [51].The current densities, power densities, polarization curves, internal resistances, COD removal, and CE values of the MFCs containing the plain CC, the Ti3C2 Tx /CC, and the W2N-Ti3C2 Tx /CC anodes are presented in Fig. 8 . The W2N-Ti3C2 Tx is seen to provide the maximum current density of 2.3 A m−2 at 0.2 V, followed by the Ti3C2 Tx /CC, with a current density of 1.2 A m−2, and the plain CC, with a current density of 0.5 A m−2 (Fig. 8a). This corresponds to improvements of 360 % and 91 %, respectively, for the modified anodes relative to the plain CC anode and is attributed to the high electrical conductivity of Ti3C2 Tx (15000 S cm−1), which helps to promote the extracellular transfer of electrons from the microorganisms to the electrode surface, thereby significantly reducing the electron transfer resistance of the MFC. Notably, this result correlates with the observation mentioned above on the SEM images of the anode obtained after MFC operation (Fig. 7). Meanwhile, the power densities of the MFCs can be evaluated from the plots of cell voltage vs current density in Fig. 8b. The results indicate that the MFC operated with the W2N-Ti3C2 Tx /CC anode exhibits the highest power density of 548 mW m−2, which is respectively 52 % and 84 % higher than that obtained with the Ti3C2 Tx /CC (263 mW m−2) and with the plain CC (88 mW m−2). Because the power generated by the MFC is directly correlated with the voltage and current generated during operation, the main factors that improve the current generation can be the same factors that directly promote power generation [8]. Therefore, the enhanced power density of the modified anode is mainly due to the high electrical conductivity and hydrophilicity of the Ti3C2 Tx , which improve the charge transfer and microorganism attachment and decrease the electron transfer resistance. Further, the slopes of the polarization curves in Fig. 8b can be used to estimate the internal resistance, which can provide a comprehensive understanding of the activation losses, ohmic losses, and bacterial metabolic losses in the MFC [52]. The as-calculated internal resistances are presented in Fig. 8c, where the MFC operating with the W2N-Ti3C2 Tx /CC anode exhibits a significantly reduced internal resistance of 498 Ω, compared to 984 Ω for the Ti3C2 Tx /CC, and 1687 Ω for the plain CC anode. Further, the enhancements in power generation and current density obtained in the present study are compared with those reported previously in Table S2, where the as-fabricated catalyst is highly competitive.Finally, the wastewater treatment capability of each MFC is indicated by the COD of its influent and effluent in Fig. 8d. Here, the MFC based on the W2N-Ti3C2 Tx /CC anode exhibits the highest COD removal of 68 %, followed by the Ti3C2 Tx /CC anode (60 %), and the plain CC anode (40 %). Moreover, Fig. 8d also presents the CE of the tested MFCs, which were calculated according to the relationship between the removal of organic substrate and the generated current of the MFCs. Thus, the W2N-Ti3C2 Tx /CC anode exhibits the highest CE of 81 %, followed by the Ti3C2 Tx /CC anode (46 %) and the plain CC anode (31 %). The enhanced COD removal capacity and CE of the W2N-Ti3C2 Tx /CC anode are attributed to the more rapid growth of electroactive microorganisms relative to non-electroactive microorganisms on the surface of this anode, which leads to an increase in the number of electrons generated, an enhancement in the electron transfer process, and an increase in the induced current density. Moreover, the synergy between the W2N and Ti3C2 Tx significantly improves the bio-electrochemical kinetics at the anode by reducing the internal resistance and increasing the generated power and current densities [53].The large-scale utilization of MFCs significantly depends on the anode efficiency to interact with the microorganism for improved biofilm growth and stability to achieve optimum electron transfer for power generation. Challenges such as limited electron transfer, poor microbial colonization, stability, and high internal resistance are directly affected by the property of the material used as an anode in MFC. Therefore, emphasis must be placed on the design of cost-effective hybrid catalysts with the ideal chemical properties, surface area, wettability, biocompatibility, and electrocatalytic activity to drive better electroactive microorganism growth and biofilm stability for achieving high current and power density in MFC. Moreover, it is vital to emphasize that any commercial treatment approach must assure stability and reusability [54].Our work introduces a cost-effective synthesis approach for a W2N-Ti3C2 Tx composite catalyst, which enhances the power generation of an MFC operating with domestic wastewater if used as an anode. We must indicate that we used domestic wastewater without any additional external microorganism inoculation or pretreatment.The W2N-Ti3C2 Tx composite catalyst was synthesized, characterized, and then used as an anode to enhance the power generation of an MFC operating with domestic wastewater. The introduction of the composite catalyst on the surface of CC benefited the attachment of microorganisms. It promoted the significant growth of biofilm with structures resembling microbial nanowires (pili) that can enable direct electron transfer. Further, the high hydrophilicity of the Ti3C2 Tx MXene and the excellent affinity of the W2N towards the electroactive microorganisms combined to promote an enhanced interaction between the biofilm and the anode surface, compared to the Ti3C2 Tx MXene alone.These combined features enabled extracellular electron transfer from the electroactive microorganisms to the electrode surface. As a result, W2N-Ti3C2 Tx /CC anode generated 4.6 and 6.2 times higher current and power densities, respectively, than the plain CC anode. Furthermore, the synergy between the W2N and the Ti3C2 Tx improved the COD removal capacity and CE by 1.8 and 2.6 times, respectively, relative to the plain CC.In brief, the present study has introduced a simple, practical approach to synthesizing highly active, electrically conductive, hydrophilic, and cost-effective composite anode catalysts for use in high performance MFCs.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.We greatly acknowledge the funding provided by the King Abdullah University of Science and Technology (KAUST), BAS/1/1403. We also acknowledge the KAUST Electron Microscope and Surface Science Core Labs for helping with the SEM-EDX, TEM, and AFM analysis.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2023.141821.The following are the Supplementary data to this article: Supplementary data 1

Microbial fuel cells (MFCs) have enormous potential to treat wastewater and reduce the energy demands of wastewater treatment plants while generating electricity using active microorganisms as biocatalysts. However, the practical application of MFCs is limited by the low power density produced, mainly due to poor anode performance. A tungsten nitride (W2N)-MXene composite catalyst is introduced to modify the anode surface for use in microbial fuel cells during domestic wastewater treatment. The aim is to improve the wettability, electrical conductivity, electron transfer efficiency, and microorganism attachment capability of the anode and ultimately increase the overall performance of the microbial fuel cell to produce electricity during wastewater treatment. In detail, a hydrofluoric acid etching approach is used to synthesize the Ti3C2 Tx MXene, the urea glass technique is used to prepare the W2N particles, and an adequate mixing and heat treatment approach is used to produce the W2N-Ti3C2 Tx composite catalyst. The W2N-Ti3C2 Tx composite on carbon cloth anode provides one of the best performances recorded for MXene in this type of fuel cells and using real domestic wastewater: with a 523 % increase in the power density (548 mW m−2), an 83 % decrease in the chemical oxygen demand (COD), and a 161 % increase in the electron transfer efficiency compared to those of the plain carbon cloth. We demonstrate that this outstanding performance is due to the improvements in hydrophilicity and microorganism attachment, particularly nanowires (or pili) which promote electron transfer. The present work offers an exciting avenue toward the process scale-up and optimization of single-chamber microbial fuel cells.

Nitrogen-containing heterocycles are ubiquitous scaffolds of many natural products and are being widely used in the field of pharmaceuticals [1–4]. Among them, quinazolinone derivatives are of utmost interest for a wide cross-section of chemists due to their remarkable bioactivities (Fig. 1 ) [5–8]. They exhibit a range of biological activities including cardiovascular, adipogenesis inhibitor, kinesin spindle protein inhibitor, antiinfective, anticancer, anticonvulsant, etc [9–14]. In addition, the use of quinazolinone derivatives in agriculture is still rare [15–18]. Therefore, there is still a hotspot in developing new means for synthesizing quinazolinone derivatives and application [19–24].Over the years, CH bond functionalization is considered as powerful and shortest approach for construction of carbon–carbon and carbon-heteroatom bonds, which have potential application in the synthesis of biologically relevant molecules [25–27]. The azaarenes are among an important class of organic molecules whose synthesis have been described based on the concept of CH functionalization [28–33]. 2-Alkyl azaarenes such as 2-methyl pyridine, 2-methyl quinolone and 2-methyl quinazolinone have been successfully functionalized via addition to a variety of carbonyl compounds, such as aldehydes, isatins, α-oxoesters, etc [34–40]. Because ketones have an additional electron-donating group, which not only reduces their partial positive charge on the carbonyl carbon but also contributes to steric hindrance, they usually have lower reactivity than aldehydes. These factors make it challenging for ketones to go through nucleophilic addition compared to aldehydes. As a strategy to overcome this problem, an electron-withdrawing CF3 moiety adjacent to the reactive carbonyl carbon site is effective in activating the ketonic carbon. Shaikh reported a Yb(OTf)3-catalyzed benzylic CH bond functionalization of alkyl azaarene with a-trifluoromethylated carbonyl compounds [41]. In 2015, Teo described the InCl3-catalyzed functionalization of the CH bond of α-alkylazaarene with trifluoromethyl ketones [42]. Although the reactions proceed successfully, expensive rare metal catalysts and high reaction temperature were necessary. Based on this precedent, Teo observed that the reaction can also be catalyzed by FeCl3 catalyst (Fig. 2 ) [43]. In 2022, we developed a novel reaction for the synthesis of azaarene-equipped CF3-tertiary alcohols through addition of azaarenes to CF3-ketones under metal-free conditions [44].The literature survey indicated that most reported reactions are limited to using metal catalysts. Therefore, considering the potential of the greener approach in organic synthesis, the development of new strategies via improving the atom economy of chemical processes and simple and efficient coupling under solvent- and catalyst-free approach for the construction of quinazolinone derivatives are a challenging task and are in great demand.Coumarin is an important heterocyclic skeleton frequently found in numerous natural products, pharmaceutical molecules, fluorescent probes, and materials [45–50]. The combination of some privileged structures, a benzopyrone ring, a trifluoromethyl moiety and a quinazolinone ring, for the synthesis of quaternary carbon organic molecules could be of significant importance, especially for new drugs and materials.In continuation of our study on the application of 3-(trifluoroacetyl)coumarins [51–56], our aim was to develop a direct, metal free, environmentally benign route to access the quinazolinone derivatives along with avoidance of the aforementioned drawbacks. Herein, we report an efficient, eco-friendly direct approach to access the quinazolinone derivatives from 2-methyl quinazolinones with 3-(trifluoroacetyl)coumarins under catalyst- and solvent-free conditions (Scheme 1 ).All chemicals, except 2-(trifluoromethyl)-2-hydroxy-2H-chromene 1 and 2-methyl quinazolinone 2, which were synthesized according reported procedure, were purchased from commercial sources and used without further purification. 1H NMR and 13C NMR spectra were obtained using a Bruker DPX-400 spectrometer in CDCl3 or DMSO‑d 6 solution with TMS as an internal standard. HR-MS(APCI) spectra were performed using a Waters Q-Tof MicroTM instrument, and X-rays were measured at 293 K on a Rigaku RAXIAS-IV type diffractometer.To a mixture of 2-(trifluoromethyl)-2-hydroxy-2H-chromene (0.4 mmol) and aryl ketone (0.4 mmol) in acetic acid (4 mL) was added NH4OAc (0.8 mmol) and the resulting mixture was heated under reflux. After completion of the reaction, the mixture was concentrated under vacuum to yield the crude product, which was further purified by column chromatography.White solid, mp: 213.2–214.7 ℃. 1H NMR (400 MHz, CDCl3) δ 10.13 (s, 1H), 8.41 (s,1H), 8.23 (d, J = 7.9 Hz, 1H), 7.71 (m, 2H), 7.57 (m, J = 7.5, 4.2 Hz, 3H), 7.45 (t, J = 7.5 Hz, 1H), 7.32 (m, J = 12.4, 5.4 Hz, 2H), 4.03 (d, J = 15.3 Hz, 1H), 3.58 (d, J = 15.3 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 161.3, 160.7, 153.7, 152.1, 147.4, 146.51, 134.8, 133.2, 128.8, 127.3, 126.7, 125.1, 124.3, 122.8, 121.4, 118.3, 116.6, 76.2, 75.9, 36.9.19F NMR (376 MHz, CDCl3) δ −80.95. HRMS (APCI): m/z calcd for C20H14F3N2O4 [M + H]+ 403.0906, found 403.0923.White solid, mp: 257.2–259.8℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.08 (s, 1H), 8.43 (s, 1H), 7.86 (d, J = 7.6 Hz, 1H), 7.78 (d, J = 8.0 Hz, 1H), 7.66 (t, J = 7.8 Hz, 1H), 7.40 (m, 3H), 7.20 (d, J = 8.1 Hz, 1H), 4.34 (d, J = 15.7 Hz, 1H), 3.37 (s, 1H), 2.51 (s, 3H), 2.20 (s, 4H). 13C NMR (101 MHz, DMSO‑d6 ) δ 162.2, 159.0, 153.8, 151.3, 146.8, 143.2, 132.9, 129.4, 128.4, 126.9, 125.6, 125.2, 124.0, 123.0, 119.2, 116.1, 75.4, 75.2, 74.9, 74.6, 36.4, 20.8, 12.6. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.52. HRMS (APCI): m/z calcd for C22H18F3N2O4 [M + H]+ 431.1219, found 431.1190.White solid, mp: 219.6–221.0 ℃. 1H NMR (400 MHz, CDCl3) δ 10.35 (s, 1H), 8.47 (s, 1H), 8.10 (d, J = 7.8 Hz, 1H), 7.82 (s, 1H), 7.57 (m, 3H), 7.32 (m, J = 10.4, 4.9 Hz, 3H), 4.16 (d, J = 15.4 Hz, 1H), 3.61 (d, J = 15.4 Hz, 1H), 2.50 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 161.9, 160.3, 153.8, 151.3, 146.4, 145.9, 135.5, 134.6, 133.1, 128.7, 126.8, 125.0, 124.5, 122.4, 121.2, 118.4, 116.6, 76.1, 75.8, 36.4, 17.7. 19F NMR (376 MHz, CDCl3) δ −80.62. HRMS (APCI): m/z calcd for C21H16F3N2O4 [M + H]+ 417.1062, found 417.1083.White solid, mp: 207.8–209.9 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.13 (s, 1H), 8.41 (s, 1H), 7.84 (m, 2H), 7.66 (t, J = 7.4 Hz, 1H), 7.46 (d, J = 8.3 Hz, 3H), 7.38 (d, J = 7.5 Hz, 1H), 7.01 (d, J = 8.3 Hz, 1H), 4.21 (d, J = 15.4 Hz, 1H), 3.39 (d, J = 15.5 Hz, 1H), 2.36 (s, 3H). 13C NMR (101 MHz, DMSO‑d6 ) δ 161.7, 158.9, 153.7, 152.2, 146.1, 143.6, 136.5, 136.1, 133.0, 129.5, 126.7, 125.5, 125.2, 123.9, 121.1, 118.8, 116.2, 75.8, 75.5, 75.2, 74.9, 36.1, 21.1. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.12. HRMS (APCI): m/z calcd for C21H16F3N2O4 [M + H]+ 417.1062, found 417.1029.Yellow solid, mp: 244.3–246.6 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.73 (s, 1H), 8.73 (d, J = 2.6 Hz, 1H), 8.40 (m, 3H), 7.86 (d, J = 7.6 Hz, 2H), 7.67 (d, J = 7.4 Hz, 2H), 7.48 (d, J = 8.3 Hz, 1H), 7.39 (s, 2H), 7.25 (d, J = 9.0 Hz, 2H), 4.36 (d, J = 15.3 Hz, 0H), 3.45 (s, 0H). 13C NMR (101 MHz, DMSO‑d6 ) δ 161.1, 159.0, 156.9, 153.7, 152.4, 145.1, 143.7, 133.1, 129.5, 129.0, 128.6, 126.7, 125.2, 125.0, 123.8, 122.3, 121.4, 118.8, 116.3, 75.7, 75.4, 75.2, 74.9, 36.8. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.31. HRMS (APCI): m/z calcd for C20H13F3N3O6 [M + H]+ 448.0756, found 448.0732.White solid, mp: 210.4–212.7 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.57 (s, 1H), 8.41 (s, 1H), 8.24 (s, 1H), 7.90 (d, J = 8.5 Hz, 1H), 7.83 (d, J = 7.8 Hz, 1H), 7.63 (t, J = 7.8 Hz, 1H), 7.38 (m, 3H), 7.25 (d, J = 8.5 Hz, 1H), 4.33 (d, J = 15.4 Hz, 1H), 3.43 (d, J = 15.4 Hz, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 161.1, 159.0, 155.7, 153.7, 150.7, 143.6, 133.0, 130.9, 129.5, 128.4, 126.9, 126.7, 125.5, 125.1, 123.6, 122.8, 121.5, 118.8, 116.2, 75.7, 75.5, 75.2, 74.9, 36.6. 19F NMR (376 MHz, DMSO‑d6 ) δ −61.06, −78.31. HRMS (APCI): m/z calcd for C21H13F6N2O4[M + H]+ 471.0780, found 471.0797.Yellow solid, mp: 204.3–205.9 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.49 (s, 1H), 8.31 (s, 1H), 7.79 (d, J = 6.4 Hz, 1H), 7.59 (d, J = 7.2 Hz, 1H), 7.53 (dt, J = 8.3, 2.9 Hz, 2H), 7.40 (d, J = 8.2 Hz, 1H), 7.33 (d, J = 7.6 Hz, 1H), 7.27 (s, 1H), 4.31 (d, J = 15.4 Hz, 1H), 3.34 (s, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.4, 159.2, 158.1, 158.1, 158.0, 158.0, 155.6, 155.4, 153.9, 152.9, 143.0, 135.0, 134.9, 132.7, 129.3, 126.7, 125.5, 125.0, 123.9, 123.8, 123.7, 119.0, 116.1, 110.4, 110.1, 110.1, 109.9, 106.9, 106.9, 106.7, 106.7, 75.7, 75.4, 75.1, 74.8, 36.3. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.34, −110.70, −121.00. HRMS (APCI): m/z calcd for C20H12F5N2O4 [M + H]+ 439.0717, found 439.0701.White solid, mp: 199.2–200.2 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.25 (s, 1H), 8.40 (s, 1H), 7.85 (d, J = 6.8 Hz, 1H), 7.65 (d, J = 7.3 Hz, 1H), 7.60 (m, J = 8.1, 2.5 Hz, 1H), 7.47 (d, J = 8.3 Hz, 1H), 7.38 (t, J = 7.5 Hz, 2H), 7.14 (m, J = 10.9, 8.2 Hz, 1H), 6.86 (d, J = 8.2 Hz, 1H), 4.24 (d, J = 15.4 Hz, 1H), 3.37 (s, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 162.2, 159.6, 159.0, 154.1, 153.7, 150.4, 143.5, 135.5, 133.0, 129.5, 126.7, 125.2, 123.9, 122.9, 118.8, 116.2, 113.3, 113.1, 110.9, 75.7, 75.4, 75.1, 74.8, 36.1, 19.0. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.26, −111.33. HRMS (APCI): m/z calcd for C20H13F4N2O4 [M + H]+ 421.0811, found 421.0801.White solid, mp: 250.0–251.2 ℃. 1H NMR (400 MHz, DMSO) δ 12.66 (s, 1H), 8.39 (s, 1H), 7.93 (d, J = 2.4 Hz, 1H), 7.88 (d, J = 2.4 Hz, 1H), 7.84 (d, J = 7.7 Hz, 1H), 7.63 (d, J = 7.1 Hz, 1H), 7.43 (d, J = 8.3 Hz, 1H), 7.39–7.30 (m, J = 18.3, 10.2 Hz, 2H), 4.43 (d, J = 15.6 Hz, 1H), 3.38 (s, 1H). 13C NMR (101 MHz, DMSO) δ 160.5, 159.2, 154.3, 154.0, 144.1, 143.2, 134.3, 132.7, 131.8, 130.6, 129.3, 126.8, 125.6, 125.0, 124.4, 123.9, 119.4, 116.0, 75.6, 75.3, 75.0, 74.8. 19F NMR (376 MHz, DMSO) δ −78.52. HRMS (APCI): m/z calcd for C20H12Cl2F3N2O4 [M + H]+ 471.0126, found 471.0106.White solid, mp: 220.7–222.8 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.39 (s, 1H), 8.40 (s, 1H), 7.93 (d, J = 2.5 Hz, 1H), 7.82 (m, J = 7.8, 1.4 Hz, 1H), 7.62 (m, J = 8.0, 5.6 Hz, 2H), 7.43 (d, J = 8.3 Hz, 1H), 7.33 (d, J = 7.4 Hz, 1H), 7.09 (d, J = 8.7 Hz, 1H), 4.37 (s, 1H), 4.27 (d, J = 15.4 Hz, 1H), 3.45 (s, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.8, 158.9, 153.6, 146.8, 143.6, 134.9, 132.9, 131.1, 129.5, 129.0, 126.7, 125.1, 123.8, 122.6, 118.8, 116.2, 75.7, 75.4, 75.1, 74.8, 36.3, 18.9. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.25. HRMS (APCI): m/z calcd for C20H13ClF3N2O4 [M + H]+ 437.0516, found 437.0530.White solid, mp: 222.2–224.2 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.23 (s, 1H), 8.39 (s, 1H), 7.84 (m, J = 7.7, 1.3 Hz, 1H), 7.65 (t, J = 7.1 Hz, 1H), 7.52 (s, 1H), 7.46 (d, J = 8.3 Hz, 1H), 7.37 (m, 3H), 6.97 (d, J = 8.1 Hz, 1H), 4.22 (d, J = 15.3 Hz, 1H), 3.36 (s, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.0, 159.0, 153.8, 150.7, 143.5, 134.6, 132.9, 129.5, 129.1, 126.7, 126.4, 125.2, 123.9, 118.8, 118.4, 116.2, 75.7, 75.4, 75.1, 74.8, 36.1. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.24. HRMS (APCI): m/z calcd for C20H13ClF3N2O4 [M + H]+ 437.0516, found 437.0505.Yellow solid, mp: 225.0–226.0 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.39 (s, 1H), 8.40 (s, 1H), 8.08 (d, J = 2.3 Hz, 1H), 7.82 (d, J = 7.7 Hz, 1H), 7.74 (m, J = 8.7, 2.4 Hz, 1H), 7.63 (m, J = 11.4, 4.2 Hz, 1H), 7.44 (d, J = 8.3 Hz, 1H), 7.35 (m, J = 8.9, 5.3 Hz, 2H), 7.02 (d, J = 8.7 Hz, 1H), 4.26 (d, J = 15.4 Hz, 1H), 3.40 (s, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.7, 159.0, 153.7, 147.2, 143.6, 137.7, 133.0, 129.5, 129.2, 128.3, 126.7, 125.1, 123.9, 123.0, 119.2, 118.8, 116.2, 75.4, 75.3, 75.3, 74.8, 36.4. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.25. HRMS (APCI): m/z calcd for C20H13BrF3N2O4 [M + H]+ 481.0011, found 481.0027.Yellow solid, mp: 205.1–206.4 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.21 (s, 1H), 8.39 (s, 1H), 8.12 (d, J = 2.4 Hz, 1H), 8.02 (d, J = 9.0 Hz, 1H), 7.78 (d, J = 2.4 Hz, 1H), 7.62 (s, 1H), 7.40 (m, 3H), 7.07 (d, J = 8.1 Hz, 1H), 4.23 (d, J = 15.6 Hz, 1H), 3.41 (s, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 161.8, 158.50, 152.8, 148.1, 142.2, 135.3, 134.9, 131.5, 126.8, 126.2, 123.8, 121.4, 120.8, 118.5, 116.8, 75.7, 75.4, 75.1, 74.8, 36.2. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.14. HRMS (APCI): m/z calcd for C20H13BrF3N2O4 [M + H]+ 481.0011, found 480.9995.White solid, mp: 218.4–219.6 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.19 (s, 1H), 8.43 (s, 1H), 8.17 (d, J = 2.3 Hz, 1H), 7.88 (d, J = 7.4 Hz, 1H), 7.81 (m, J = 8.8, 2.4 Hz, 1H), 7.43 (m, J = 14.7, 5.8 Hz, 3H), 7.28 (t, J = 7.6 Hz, 1H), 4.35 (d, J = 15.7 Hz, 1H), 3.38 (s, 1H), 1.85 (s, 3H). 13C NMR (101 MHz, DMSO‑d6 ) δ 162.1, 158.5, 152.8, 151.5, 147.0, 142.0, 135.2, 134.8, 131.4, 127.0, 126.8, 126.2, 123.9, 121.3, 121.0, 118.5, 116.8, 75.56, 75.3, 75.0, 74.7, 55.4, 36.4, 16.9. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.43. HRMS (APCI): m/z calcd for C21H15BrF3N2O4 [M + H]+ 495.0167, found 495.0154.White solid, mp: 200.9–201.6 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.11 (s, 1H), 8.39 (s, 1H), 8.13 (d, J = 2.3 Hz, 1H), 7.79 (m, 2H), 7.50 (s, 1H), 7.44 (t, J = 6.5 Hz, 2H), 6.99 (d, J = 8.3 Hz, 1H), 4.20 (d, J = 15.5 Hz, 1H), 3.38 (d, J = 15.7 Hz, 1H), 2.35 (s, 3H). 13C NMR (101 MHz, DMSO‑d6 ) δ 161.7, 158.5, 152.7, 152.0, 146.1, 142.3, 136.5, 136.1, 135.3, 131.5, 129.5, 126.7, 125.5, 123.8, 121.1, 120.7, 118.5, 116.7, 75.7, 75.4, 75.1, 74.8, 36.1, 21.1. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.07. HRMS (APCI): m/z calcd for C21H15BrF3N2O4 [M + H]+ 495.0167, found 495.0143.White solid, mp: 187.2–189.4 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.41 (s, 1H), 8.40 (s, 1H), 8.13 (m, J = 16.5, 2.2 Hz, 2H), 7.81 (m, J = 8.3, 5.9, 2.3 Hz, 2H), 7.44 (t, J = 9.6 Hz, 2H), 7.02 (d, J = 8.7 Hz, 1H), 4.26 (d, J = 15.6 Hz, 1H), 3.40 (s, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.8, 158.5, 153.7, 152.7, 147.1, 142.4, 137.7, 135.3, 131.5, 129.2, 128.3, 126.6, 123.8 (s, 2H), 123.0, 120.7, 119.1, 118.5, 116.8, 75.7, 75.4, 75.1, 74.8, 56.5, 36.4. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.19. HRMS (APCI): m/z calcd for C20H12Br2F3N2O4 [M + H]+ 558.9116, found 558.9077.Yellow solid, mp: 222.4–224.9 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.22 (s, 1H), 8.41 (s, 1H), 8.19 (s, 2H), 8.04 (d, J = 6.9 Hz, 1H), 7.66 (t, J = 7.7 Hz, 1H), 7.58 (s, 1H), 7.42 (t, J = 7.5 Hz, 1H), 7.07 (d, J = 8.1 Hz, 1H), 4.21 (d, J = 15.6 Hz, 1H), 3.41 (d, J = 15.7 Hz, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 161.7, 157.9, 152.8, 149.7, 148.1, 142.1, 137.3, 134.9, 131.3, 127.51, 126.8, 126.2, 123.7, 121.7, 121.4, 116.9, 110.2, 75.7, 75.4, 75.1, 74.8, 66.8, 36.1. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.11 (s, 1H). HRMS (APCI): m/z calcd for C20H12Br2F3N2O4 [M + H]+ 558.9116, found 558.9084.White solid, mp: 221.4–222.8 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.09 (s, 1H), 8.45 (s, 1H), 8.20 (d, J = 9.4 Hz, 2H), 7.79 (d, J = 8.0 Hz, 1H), 7.49 (s, 1H), 7.21 (d, J = 8.0 Hz, 1H), 4.33 (d, J = 15.7 Hz, 1H), 3.39 (s, 1H), 2.23 (s, 3H), 1.81 (s, 3H). 13C NMR (101 MHz, DMSO‑d6 ) δ 162.1, 157.8, 151.1, 149.7, 146.7, 143.4, 142.0, 137.2, 132.7, 131.1, 128.4, 127.8, 126.7, 123.9, 123.0, 121.9, 119.3, 116.9, 110.2, 75.4, 75.2, 74.9, 74.6, 36.4, 20.8, 12.6. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.45. HRMS (APCI): m/z calcd for C22H16Br2F3N2O4 [M + H]+ 586.9429, found 586.9394.White solid, mp: 161.9–163.9 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.18 (s, 1H), 8.42 (s, 1H), 8.20 (m, J = 6.6, 2.2 Hz, 2H), 7.88 (d, J = 7.2 Hz, 1H), 7.48 (d, J = 6.6 Hz, 2H), 7.28 (t, J = 7.6 Hz, 1H), 4.33 (d, J = 15.6 Hz, 1H), 3.36 (d, J = 15.8 Hz, 1H), 1.84 (s, 3H). 13C NMR (101 MHz, DMSO‑d6 ) δ 162.1, 157.8, 151.4, 149.8, 146.9, 141.9, 137.2, 135.1, 134.7, 131.1, 127.8, 126.7, 126.3, 123.8, 121.9, 121.4, 116.9, 110.2, 75.5, 75.3, 75.0, 74.7, 55.4, 36.3, 16.8. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.39. HRMS (APCI): m/z calcd for C21H14Br2F3N2O4 [M + H]+ 572.9272, found 572.9242.Yellow solid, mp: 215.5–217.1 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.12 (s, 1H), 8.40 (s, 1H), 8.18 (s, 2H), 7.83 (s, 1H), 7.59 (s, 1H), 7.48 (m, J = 8.3, 1.9 Hz, 1H), 6.99 (d, J = 8.3 Hz, 1H), 4.17 (d, J = 15.6 Hz, 1H), 3.40 (d, J = 15.7 Hz, 1H), 2.37 (s, 3H). 13C NMR (101 MHz, DMSO‑d6 ) δ 161.7, 157.8, 151.9, 149.7, 146.0, 142.1, 137.3, 136.6, 136.2, 131.3, 127.5, 126.7, 125.52, 123.7, 121.7, 121.2, 116.9, 110.2, 75.7, 75.4, 75.1, 74.8, 66.8, 36.0, 21.12. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.04. HRMS (APCI): m/z calcd for C21H14Br2F3N2O4 [M + H]+ 572.9272, found 572.9244.Yellow solid, mp: 217.3–218.9 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.76 (s, 1H), 8.74 (d, J = 2.6 Hz, 1H), 8.42 (t, J = 4.5 Hz, 2H), 8.19 (s, 2H), 7.51 (s, 1H), 7.26 (d, J = 9.0 Hz, 1H), 4.33 (d, J = 15.6 Hz, 1H), 3.45 (d, J = 15.6 Hz, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 161.1, 157.9, 156.6, 152.3, 149.7, 145.2, 142.4, 137.4, 131.3, 128.9, 127.2, 126.5, 123.7, 122.3, 121.6, 117.0 (s, 11H), 110.3, 75.3, 75.0, 74.7, 36.6. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.24. HRMS (APCI): m/z calcd for C20H11Br2F3N3O6 [M + H]+ 603.8967, found 603.8925.White solid, mp: 204.5–206.5 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.55 (s, 1H), 8.34 (s, 1H), 8.18 (s, 2H), 7.62 (m, 2H), 7.43 (s, 1H), 4.33 (d, J = 15.5 Hz, 1H), 3.37 (s, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.4, 158.0, 155.5, 152.6, 149.8, 141.4, 137.1, 134.9, 131.1, 128.0, 126.6, 123.7, 121.8, 116.7, 110.5, 110.2, 106.9, 75.5, 75.1, 74.8, 36.3. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.23, −110.56, −121.79. HRMS (APCI): m/z calcd for C20H10Br2F5N2O4 [M + H]+ 594.8927, found 594.8881.White solid, mp: 232.8–234.4 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.66 (s, 1H), 8.36 (s, 1H), 8.18 (s, 2H), 7.94–7.92 (m, J = 6.4, 2.4 Hz, 2H), 7.42 (s, 1H), 4.39 (d, J = 15.5 Hz, 1H), 3.37 (s, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.5, 158.0, 154.0 (s, 4H), 149.9, 144.0, 141.8, 137.0, 134.4, 131.7, 131.1, 130.7, 127.8, 126.5, 124.5, 123.8, 122.2, 121.3, 116.8, 110.2, 75.4, 75.1, 36.5. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.35. HRMS (APCI): m/z calcd for C20H10Br2Cl2F3N2O4 [M + H]+ 626.8336, found 626.8307.White solid, mp: 107.3–108.6 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.43 (s, 1H), 8.42 (s, 1H), 8.18 (m, J = 7.4, 2.2 Hz, 2H), 8.12 (d, J = 2.3 Hz, 1H), 7.81 (m, J = 8.7, 2.3 Hz, 1H), 7.51 (s, 1H), 7.03 (d, J = 8.7 Hz, 1H), 4.24 (d, J = 15.7 Hz, 1H), 3.41 (d, J = 15.7 Hz, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.7, 157.8, 153.5, 149.7, 147.7, 147.1, 144.7, 142.4, 141.1, 139.1, 137.5, 136.2, 135.1, 134.1, 133.0, 132.2, 131.3, 130.4, 129.4, 128.3, 128.1, 75.0, 74.7, 36.23. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.18. HRMS (APCI): m/z calcd for C20H11Br3F3N2O4 [M + H]+ 636.8221, found 636.8177.Yellow solid, mp: 212.5–214.0 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 11.79 (s, 1H), 9.06 (s, 1H), 8.57 (d, J = 7.4 Hz, 1H), 8.23 (s, 2H), 8.18 (m, J = 11.1, 4.3 Hz, 1H), 7.98 (d, J = 8.1 Hz, 1H), 7.91 (t, J = 7.6 Hz, 1H), 7.81 (m, J = 6.1, 3.2 Hz, 1H), 7.74 (m, J = 7.0, 3.7 Hz, 2H), 4.75 (d, J = 15.9 Hz, 1H), 4.38 (s, 3H), 4.13 (d, J = 15.9 Hz, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 161.8, 158.7, 153.1, 148.1, 146.6, 143.9, 143.0, 134.8, 129.6, 126.8, 126.1, 125.3, 125.1, 123.9, 121.4, 121.0, 120.5, 119.4, 115.0, 75.7, 75.4, 75.1, 74.8, 36.1, 19.0. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.28 (s, 1H). HRMS (APCI): m/z calcd for C21H16F3N2O5 [M + H]+ 433.1011, found 433.1028.White solid, mp: 223.6–225.7 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.21 (s, 1H), 8.42 (s, 1H), 7.89 (d, J = 7.9 Hz, 1H), 7.46 (d, J = 7.2 Hz, 1H), 7.41 (d, J = 7.1 Hz, 1H), 7.31 (m, 4H), 4.38 (d, J = 15.6 Hz, 1H), 3.93 (s, 3H), 3.39 (d, J = 9.8 Hz, 1H), 1.87 (s, 3H). 13C NMR (101 MHz, DMSO‑d6 ) δ 162.1, 158.7, 151.7, 146.9, 146.6, 143.5, 143.1, 134.9, 126.9, 126.2, 125.8, 125.1, 123.9, 123.7, 121.3, 120.4, 119.7, 115.0, 75.6, 75.3, 75.0, 74.7, 36.4, 16.9. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.50. HRMS (APCI): m/z calcd for C22H18F3N2O5 [M + H]+ 447.1168, found 447.1182.White solid, mp: 204.2–206.3 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.59 (s, 1H), 8.41 (s, 1H), 8.29 (s, 1H), 7.97 (m, J = 8.6, 1.7 Hz, 1H), 7.35 (m, J = 16.7, 12.3, 3.6 Hz, 6H), 4.33 (d, J = 15.5 Hz, 1H), 3.94 (s, 3H), 3.44 (d, J = 15.5 Hz, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 161.2, 158.7, 155.7, 150.7, 146.6, 144.0, 143.1, 131.0, 128.4, 127.0, 126.7, 125.5, 125.2, 123.8, 123.6, 122.8, 121.5, 120.6, 119.4, 115.1, 75.5, 75.3, 75.1, 74.8, 56.6, 36.6. 19F NMR (376 MHz, DMSO‑d6 ) δ −60.99, −78.38. HRMS (APCI): m/z calcd for C22H15F6N2O5 [M + H]+ 501.0885, found 501.0868.White solid, mp: 89.7–91.5 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.53 (s, 1H), 8.33 (s, 1H), 7.65–7.55 (m, J = 22.7, 9.0 Hz, 2H), 7.33 (dt, J = 12.1, 7.6 Hz, 4H), 4.34 (d, J = 15.5 Hz, 1H), 3.93 (s, 3H), 3.36 (d, J = 15.7 Hz, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.5, 159.2, 154.3, 154.0, 144.1, 143.2, 134.3, 132.7, 131.8, 130.6, 129.3, 126.8, 125.6, 125.0, 124.4, 123.8, 119.4, 116.0, 75.6, 75.3, 75.0, 74.8, 66.8, 36.4. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.39, −110.70, −120.93. HRMS (APCI): m/z calcd for C21H14F5N2O5 [M + H]+ 469.0823, found 469.0790.White solid, mp: 193.2–195.7 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.24 (s, 1H), 8.38 (s, 1H), 7.59 (m, 1H), 7.33 (m, 4H), 7.13 (t, J = 9.5 Hz, 1H), 6.87 (d, J = 8.1 Hz, 1H), 4.23 (d, J = 15.5 Hz, 1H), 3.92 (s, 3H), 3.41 (s, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 162.16, 159.6, 159.2, 158.7, 154.2, 150.3, 146.6, 143.8, 143.0, 135.5, 126.7, 125.2, 123.9, 122.9, 120.6, 119.4, 115.1, 113.3, 113.1, 110.9, 36.1. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.33, −111.33. HRMS (APCI): m/z calcd for C21H15F4N2O5 [M + H]+ 451.0917, found 451.0899.White solid, mp: 186.5–188.6 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.67 (s, 1H), 8.36 (s, 1H), 7.94–7.88 (m, J = 17.6, 2.4 Hz, 2H), 7.39 – 7.36 (m, J = 7.2, 1.9 Hz, 1H), 7.34 – 7.28 (m, 3H), 4.43 (d, J = 15.7 Hz, 1H), 3.92 (s, 3H), 3.37 (d, J = 15.7 Hz, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.5, 159.0, 154.3, 146.6, 144.0, 143.4, 134.3, 131.8, 130.6, 126.7, 125.7, 125.0, 124.4, 123.8, 120.4, 119.9, 114.8, 75.6, 75.3, 75.0, 74.7, 56.6, 36.4. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.56. HRMS (APCI): m/z calcd for C21H14Cl2F3N2O5 [M + H]+ 501.0232, found 501.0211.White solid, mp: 148.3–150.3 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.45 (s, 1H), 8.43 (s, 1H), 7.98 (d, J = 2.5 Hz, 1H), 7.68 (m, J = 8.7, 2.5 Hz, 1H), 7.41 (m, J = 6.9, 2.1 Hz, 2H), 7.33 (t, J = 4.9 Hz, 2H), 7.16 (d, J = 8.7 Hz, 1H), 4.31 (d, J = 15.5 Hz, 1H), 3.95 (s, 3H), 3.44 (d, J = 15.7 Hz, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.8, 158.7, 153.7, 146.9, 146.6, 144.0, 143.0, 135.0, 131.1, 129.1, 126.7, 125.1, 123.9, 122.6, 120.5, 119.4, 115.0, 75.7, 75.4, 75.1, 74.8, 56.5, 36.3. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.34 (s, 1H). HRMS (APCI): m/z calcd for C21H15ClF3N2O5 [M + H]+ 467.0622, found 467.0598.Yellow solid, 232.7–234.9 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.24 (s, 1H), 8.37 (s, 1H), 7.53 (t, J = 8.0 Hz, 1H), 7.38 (m, J = 10.1, 4.7 Hz, 2H), 7.32 (m, J = 13.1, 6.5 Hz, 3H), 6.99 (d, J = 8.1 Hz, 1H), 4.21 (d, J = 15.4 Hz, 1H), 3.92 (s, 3H), 3.37 (s, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 159.9, 158.7, 153.9, 150.7, 146.6, 143.8, 143.1, 134.7, 132.8, 129.2, 126.7, 126.4, 125.2, 123.9, 120.6, 119.4, 118.4, 115.1, 75.6, 75.4, 75.1, 74.8, 56.6, 36.0. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.32. HRMS (APCI): m/z calcd for C21H15ClF3N2O5 [M + H]+ 467.0622, found 467.0605.White solid, mp: 173.6–174.7 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.42 (s, 1H), 8.40 (s, 1H), 8.12 (d, J = 2.3 Hz, 1H), 7.80 (m, J = 8.7, 2.4 Hz, 1H), 7.36 (m, 4H), 7.06 (d, J = 8.7 Hz, 1H), 4.27 (d, J = 15.5 Hz, 1H), 3.94 (s, 3H), 3.40 (d, J = 15.5 Hz, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.7, 158.7, 153.8, 147.2, 146.6, 143.9, 143.0, 137.8, 129.3, 128.3, 126.7, 125.2, 123.9, 123.0, 120.6, 119.4, 119.2, 115.1, 75.5, 75.1, 74.8, 56.6, 55.4, 36.3. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.33. HRMS (APCI): m/z calcd for C21H15BrF3N2O5 [M + H]+ 511.0166, found 511.0092.Yellow solid, mp: 170.7–171.5 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 11.74 (s, 1H), 9.01 (s, 1H), 8.57 (m, J = 7.9, 1.2 Hz, 1H), 8.18 (d, J = 8.7 Hz, 3H), 7.99 (d, J = 8.0 Hz, 1H), 7.91 (t, J = 7.1 Hz, 1H), 7.40 (m, J = 8.7, 2.4 Hz, 1H), 7.32 (d, J = 2.4 Hz, 1H), 4.72 (d, J = 15.8 Hz, 1H), 4.36 (s, 3H), 4.09 (d, J = 15.8 Hz, 1H). 13C NMR (101 MHz, DMSO‑d6 ) δ 164.3, 161.2, 159.7, 156.4, 154.8, 147.8, 146.0, 134.9, 130.6, 126.9, 126.5, 123.9, 122.0, 120.4, 113.4, 112.7, 100.3, 76.1, 75.8, 56.0, 35.8. 19F NMR (376 MHz, DMSO‑d6 ) δ −80.16. HRMS (APCI): m/z calcd for C21H16F3N2O5 [M + H]+ 433.1011, found 433.1022.White solid, mp: 212.7–214.7 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 11.72 (s, 1H), 9.03 (s, 1H), 8.41 (d, J = 7.9 Hz, 1H), 8.22 (d, J = 8.7 Hz, 1H), 8.09 (s, 1H), 8.02 (d, J = 7.2 Hz, 1H), 7.79 (d, J = 7.5 Hz, 1H), 7.42 (d, J = 8.7 Hz, 1H), 7.34 (s, 1H), 4.81 (d, J = 15.9 Hz, 1H), 4.37 (s, 3H), 4.08 (s, 1H), 2.80 (s, 3H). 13C NMR (101 MHz, DMSO‑d6 ) δ 163.3, 162.1, 159.3, 155.7, 151.8, 146.9, 143.4, 135.0, 130.4, 127.0, 126.2, 124.1, 123.8, 121.6, 121.3, 113.1, 112.7, 100.4, 75.6, 75.3, 75.0, 74.7, 36.3, 19.0, 17.0. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.64. HRMS (APCI): m/z calcd for C22H18F3N2O5 [M + H]+ 447.1168, found 447.1176.White solid, mp: 210.2–210.9 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.19 (s, 1H), 8.32 (s, 1H), 7.60 (m, 3H), 7.47 (m, 2H), 7.36 (d, J = 8.4 Hz, 2H), 7.14 (d, J = 2.6 Hz, 2H), 6.88 (d, J = 8.2 Hz, 1H), 4.24 (d, J = 15.4 Hz, 1H), 3.37 (s, 2H), 2.36 (s, 4H). 13C NMR (101 MHz, DMSO‑d6 ) δ 162.2, 159.5, 159.1, 154.1, 151.9, 150.4, 143.4, 135.5, 134.4, 133.8, 129.1, 128.4, 126.7, 125.1, 123.9, 122.9, 119.0, 118.6, 117.3, 115.9, 115.2, 113.8, 75.1, 36.1, 20.7. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.30, −111.35. HRMS (APCI): m/z calcd for C21H15F4N2O4 [M + H]+ 435.0968, found 435.0947.White solid, mp: 217.9–219.5 ℃. 1H NMR (400 MHz, DMSO‑d6 ) δ 12.38 (s, 1H), 8.32 (s, 1H), 7.95 (d, J = 2.4 Hz, 1H), 7.65 (m, J = 8.7, 2.5 Hz, 1H), 7.60 (s, 1H), 7.45 (d, J = 8.5 Hz, 1H), 7.33 (d, J = 8.4 Hz, 2H), 7.09 (d, J = 8.7 Hz, 1H), 4.26 (d, J = 15.5 Hz, 1H), 3.37 (d, J = 15.7 Hz, 2H), 2.34 (s, 4H). 13C NMR (101 MHz, DMSO‑d6 ) δ 160.8, 159.1, 153.6, 151.9, 146.9, 143.5, 135.0, 134.4, 133.8, 131.1, 129.3, 126.7, 125.1, 123.9, 122.6, 118.6, 116.0, 75.7, 75.4, 75.1, 74.8, 36.3, 20.7. 19F NMR (376 MHz, DMSO‑d6 ) δ −78.29. HRMS (APCI): m/z calcd for C21H15ClF3N2O4[M + H]+ 451.0672, found 451.0649.Green metrics calculations for compound 3aa. 1. % Yield = (0.362 × 100)/0.402 = 90 % 2. Atom Economy = (402 × 100)/(242 + 160) = 100 % 3. Carbon Efficiency = [(1 × 20) × 100]/[(1 × 11)+(1 × 9)] = 100 % 4. RME = (0.362 × 100)/(0.242 + 0.160) = 90 % 5. Mass intensity = (0.242 + 0.160)/(0.362) = 1.11 6. E-Factor = (1.11–1) = 0.11 % Yield = (0.362 × 100)/0.402 = 90 %Atom Economy = (402 × 100)/(242 + 160) = 100 %Carbon Efficiency = [(1 × 20) × 100]/[(1 × 11)+(1 × 9)] = 100 %RME = (0.362 × 100)/(0.242 + 0.160) = 90 %Mass intensity = (0.242 + 0.160)/(0.362) = 1.11E-Factor = (1.11–1) = 0.11Antifungal assays were performed against Fusarium oxysporum (F. oxysporum), Fusarium graminearum (F. graminearum), Phytophthora parasitica var ni-cotianae (P. nicotianae), Fusarium moniliforme (F. moniliforme), and Rhizoctonia solani Kuhn (R. solani) in vitro by the plate growth rate method. The synthesized compounds were dissolved in 2 % DMSO to yield a 10 mg/mL stock solution. Then, each solution was added to sterile potato dextrose agar (PDA) to give final concentrations of 0.1 mg/mL. After the mixture was chilled, the mycelium of the fungi was transferred to the test plate and incubated at 26 °C. When the mycelium of the fungi reached the edges of the control plate (without sample), the inhibitory index was calculated as follows: In-hibitory index (%) = (Db-Da)/(Db-Dc) × 100 %, where Da is the colony diameter of the growth zone in the test plate, Db is the colony diameter of the growth zone in the con-trol plate, and Dc is the diameter of the mycelial disc. The median effective concentra-tion (EC50) of each compound with a significant fungicidal activity was further evalu-ated in three independent experiments. The statistical analyses were performed using SPSS software (IBM SPSS Statistic 26).Inevitably, with our ongoing interests in the development of simple and mild methodologies for organic syntheses, we began our studies by employing 3-(trifluoroacetyl)coumarin 1a and 2-methyl quinazolinone 2a as model substrates for the CH functionalization reaction, and the results are summarized in Table 1 . An initial trial under a mild condition consisting of 20 mol% Na2CO3 in 1,4-dioxane at 90℃ gave only 20 % yield of 3aa was isolated after 24 h (entry 1). Subsequently, a comprehensive screening of reaction catalysts (acids, bases as well as amino acids) was carried out (entries 2–6). However, acids or bases used in the reaction did not give promising results and the isolated yields of desired product 3aa were poor (<50 %, entries 1–4). Moving toward greener catalysts, that is amino acid, did not give satisfactory results (entries 5–6). We further continued our screening under catalyst- and solvent-free conditions, surprisingly, we obtained the product 3aa in higher amounts (67 %) and short reaction time (2 h) compared to other previous conditions (entry 7). Therefore, we further performed a set of the reaction under solvent- and catalyst-free conditions at elevated temperatures (entries 8–12). To our delight, on only increasing the reaction temperature, the better yield of desired product 3aa was obtained (90 %, entry 9). Further increasing or decreasing the reaction time diminishes the yield of 3aa (entries 11–12). Therefore, the reaction of 1a with 2a at 120 °C under solvent- and catalyst-free conditions for 2 h was the optimized eco-friendly reaction condition for the synthesis of quinazolinone derivative 3aa (entry 9).Next, we started exploring the scope of the reaction using various substituted 2-methyl quinazolinones and 3-(trifluoroacetyl)coumarins under the optimized reaction conditions (Table 2 ). Generally, the reactions of quinazolinones with electron-withdrawing and -donating groups with 3-(trifluoroacetyl)coumarin 1a all proceeded smoothly to give corresponding products 3aa-3al (entries 1–12). In the case of disubstituted quinazolinones, the reaction led to decreased yields (entries 2, 7 and 9). To further expand the scope of the methodology, we subsequently investigated the reactions with various 3-(trifluoroacetyl)coumarins and quinazolinones (entries 13–37). In all cases, the reactions ran efficiently to give the desired products in moderate to high yields. Meanwhile, the structure of 3di was also determined by analogy on the basis of X-ray (CCDC 2225572).Additionally, various green chemistry parameters were investigated and demonstrated by the green assessment diagram for the synthesis of quinazolinone derivatives (Fig. 3 ). Our developed methodology has remarkable advantages in terms of several green parameters, for example, atom economy of the synthesis of 3aa was reached 100 %; carbon efficiency of the reaction was 100 %; reaction mass efficiency was observed up to 90 %; and most importantly the E-factor was reduced to 0.11.The in vitro antifungal activity of target compounds against F. oxysporum, F. graminearum, P. nicotianae, F. moniliforme and R. solani is summarized in Fig. 4 . Triadimefon was used as positive controls at a concentration of 10 μg/mL. In general, most of the title compounds exhibited a certain degree of fungicidal activity at a concentration of 500 μg/mL. Overall, most of the desired quinazolinone derivatives showed fungicidal activities against the abovementioned five fungi. Gratifyingly, the antifungal activity against R. solani was obviously better compared to other four fungi, with a moderate to good inhibitor rate. Particularly, compounds 3bl, 3ce, 3 cl and 3ea displayed good (>93 %) in vitro fungicidal activity. Among them, compound 3 cl was the most potent and had the EC50 values of 10.6 μg/mL.In summary, we have described an environmentally benign, straightforward access for the synthesis of quinazolinone derivatives in the absence of catalyst and solvent in excellent yields. The advantages of the developed methodology are experimental simplicity, easy work-up, and excellent yields of products. This eco-friendly methodology also eliminates toxic metal catalyst-related environmental hazards and pollutions along with increase in atomic economy. Furthermore, the preliminary in vitro antifungal activity revealed that most of the synthesized compounds displayed promising fungicidal activities. Among them, compound 3 cl exhibited 95 % fungicidal activity against R. solani, with an EC50 value of 10.6 μg/mL. Xiaodan Chang: Conceptualization, Investigation, Data curation. Liangxin Fan: Resources, Formal analysis. Lijun Shi: Data curation. Zhenliang Pan: Formal analysis. Guoyu Yang: Validation, Visualization. Cuilian Xu: Resources, Writing – review & editing. Lulu Wu: Conceptualization, Methodology, Investigation, Writing – original draft. Caixia Wang: Resources.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by Natural Science Foundation of Henan Province (22230420459, 222300420456) and Science and Technology Innovation Fund of Henan Agricultural University (KJCX2020A19).Supplementary data to this article can be found online at https://doi.org/10.1016/j.jscs.2023.101621.The following are the Supplementary data to this article: Supplementary data 1

An environmentally benign highly atom-economic protocol for the construction of the CC bond has been developed under catalyst- and solvent-free conditions. This protocol involves the efficient coupling of 2-methyl quinazolinones with 3-(trifluoroacetyl)coumarins for the access of quinazolinone derivatives in excellent yields (up to 90 %). The crystal structure of compound 3di was investigated by X-ray diffraction analysis. The biological activities, such as in vitro antifungal activity of the quinazolinone derivatives against Fusarium graminearum, Fusarium moniliforme, Fusarium oxysporum, Phytophthora parasitica var nicotianae, and Rhizoctonia solani Kuhn, were investigated. The bioassay results indicated that most of the target products exhibited promising fungicidal activities, and compound 3 cl exhibited 95 % fungicidal activity against R. solani, with an EC50 value of 10.6 μg/mL.

Air pollution has always been a hot issue of public concern; substantial works have been applied to air pollution control for decades [1]. Volatile organic compounds (VOCs) have extensive sources, mainly including industrial and traffic exhaust emissions. The discharged pollutants such as benzene, toluene, formaldehyde, xylene and so on are toxic [2,3]. Secondary reactions and more complex pollutants are caused because of the diversity of VOCs [4]. It is an important factor that intensifies the atmospheric compound pollution, which directly affects human health and the environment [5]. In the last few decades, VOCs control is mainly based on end treatment, including single technologies such as adsorption [6–10], membrane separation [11], plasma [12], biodegradation [13], thermal catalysis [14–18], photocatalysis [19–21] by various environmental functional materials [22–25]. However, the increasing demands of VOCs control are difficult to be satisfied by single traditional technologies. The coupling and collaborative processing between several technologies have shown new vitality, such as adsorption-catalysis [26,27] adsorption-plasma [28] and plasma-catalysis [29], adsorption-plasma catalysis [30].Photothermal catalysis is a new coupling technology in recent years and has been rapidly developed, which can simultaneously solve the high-energy consumption of thermal catalysis [31,32] and the low efficiency of photocatalysis [33,34]. Photothermal catalysis is not a simple superposition of thermal catalysis and photocatalysis. It can make use of solar energy and thermal energy simultaneously and further improve catalytic efficiency through synergistic effect [35]. At present, several research methods of photothermal catalysis mainly include as follows:(1) Light driven thermal effect, without additional heat source; (2) To introduce additional heat source into the photocatalytic system; (3) To introduce light irradiation in the thermal catalytic system. The application of photothermal catalysis in the environmental field is limited, accounting for only about 2.1% (Fig. 1 ). In recent years, photothermal synergy has been widely studied in VOCs oxidation (Fig. 2 ) [36–49], carbon dioxide (CO2) reduction [50,51], hydrogen production [34,52], CO2 methanation [53,54] and methanol reaction systems [55,56]. Some progress has been made in catalyst design and the removal of complex pollutants. The reported photothermal catalysts can effectively degrade VOCs under different light irradiation conditions, and have excellent catalytic stability, such as MnOx [40,57], Co3O4 [58–60], TiO2 [12,61], perovskite [42,62] and noble metal composites [44,63], etc.Recently, the relevant review on photothermal catalysis mainly focuses on the design and application of nano-catalysts [64,65]. The mechanism of light-driven thermal catalysis is also briefly summarized by An et al. [66]. However, the research on the mechanism of photothermal oxidation of VOCs, also the standardized comparison based on the performance over different photothermal catalytic systems are still rare. It is difficult to make a systemic comparison of the catalytic performances of various photothermal catalytic reactors because of the differences in performing parameters. On this basis, this paper focuses on the research progress of photothermal catalytic performance and the VOCs elimination mechanisms in batch and continuous systems, as well as the fabrication of photothermal catalytic materials (Fig. 3 ). Considering the basic research and practical application, this review provides an opportunity to compare the efficiency of each method on a parallel basis. In addition, the application of theoretical calculation in photothermal catalysis and related mechanisms of VOC destruction are summarized and discussed. Finally, the existing problems are pointed out to guide photothermal catalysis technology in the future. This review will be devoted to point out a potential direction in the field of photothermal catalysis and make the related research work in the environmental field more successfully.In general, photothermal catalysts mainly include two types according to the current researches. One is metal oxide with both photo responsive properties and thermal catalytic activity, such a single oxide can be excited by both synchronously by light and heat. Another one is composite catalyst composed by noble metal particles (or transition metal oxides) and semiconductor oxide carrier. Among them, noble metal particles (or transition metal oxides) are dedicated to the thermal catalytic activity, and semiconductor oxide carriers play an excellent light response performance. Each part of the composite is activated by light and heat, respectively, and the synergistic effect is produced through material coupling. The light required for these catalysts comes from external light sources such as Xe lamp and Hg lamp used in the laboratory. The heat comes from the photothermal conversion caused by light irradiation or an external heat source. To achieve the effective performance of photothermal catalysis, the optical properties, photothermal conversion efficiency, also thermal catalytic activity of the catalyst should be considered [19,67,68].The photocatalytic efficiency is limited due to the low utilization rate of solar energy and the fast recombination of current carriers by traditional semiconductors with a wide band gap. To make full use of the solar energy, the catalyst should be designed to achieve broadband absorption between 300 and 2500 nm to maximize energy capture from solar light and activate the reaction. Different strategies have been developed to improve the light absorption capacity of catalysts, including the use of strong light absorption carriers (such as silica nanowire arrays [69,70] and MXene [71]) and the construction of plasma superstructures [72]. In addition, electron–hole pairs are generated when the semiconductor absorbs photons with energy equal to or above the band gap. Eventually, these photo-generated carriers can migrate to the semiconductor surface and be transferred to the adsorbed molecules, thereby reduction or oxidation are initiated [19,73]. Therefore, to enhance solar spectral absorption or reduce the band gap is conducive to obtain a smaller carrier recombination rate, thus improving the photocatalytic efficiency.Photothermal conversion refers to the process of concentrating solar irradiation energy through reflection, absorption, or other ways and converting it into high temperature. When materials surface is exposed to electromagnetic irradiation, part of the photon energy is converted into other energy, such as thermal, electricity, chemical, and biomass energy [74–78]. To realize the efficient thermal utilization of solar energy, photothermal conversion materials are the basic medium required [79]. In general, materials with energy level transitions equivalent to that of photon energy are often used for photothermal conversions, such as metals, metal oxides, metal sulfides, and semiconductors [80–86]. In addition, another effective way to improve the photothermal conversion efficiency is to reduce the heat dissipation [87,88]. The photothermal conversion efficiency is an important index. In general, the ratio of the thermal energy converted by a material to the incident light energy is defined as the photothermal conversion efficiency (η) which can be calculated by Equation (1) [89]. (1) η = Ethermal /Ephototons × 100% E phototons represents the energy of the incident photon, and E thermal represents the thermal energy converted after the catalyst absorbs the simulated light irradiation. Detailed calculation can be carried out according to the following formula [90]: (2) η = Cp, i × mi × (T - Tsurr) / (A × q × t) × 100% The Cp, i represent the specific heat capacity, which can be measured by a Heat Conduction Analysis Meter. The mi is the mass of the catalyst. The T and Tsurr are the catalyst temperature and ambient temperature, respectively. The A, q and t are the surface area of the catalyst, the power density of light source, and the irradiated time, respectively. The η value depends on the absorbance of light and the surface area of the catalyst. Higher platform temperatures can be obtained through strong light absorption and surface area.Thermal oxidation is the preferred method for industrialization because of its economic and environmental advantages. The reaction is affected by many factors, such as low reaction temperature, competitive reaction, small amounts of active sites, brief residence time and catalyst deactivation. The presence of these factors leads to the formation of abundant intermediate products in the reaction system. During the catalytic process, the stubborn carbonaceous intermediate products are deposited on the surface of the catalyst, which hinders the contact between the pollutant molecules and the active sites. According to the photothermal conversion properties of the catalyst and the incident light intensity, the introduction of light in the thermal catalytic system has a positive effect on the temperature rise of catalyst surface. Catalysts with good thermal catalytic performance can directly utilize the heat energy generated by light, thus further reducing energy consumption, and having a positive impact in promoting the reaction. At present, the thermal catalytic activity of materials can be effectively enhanced through loading, doping modification [91,92], and defect engineering.Based on the description in Section 2.1, plenty of catalytic materials with photothermal synergistic effects have been designed and applied. Some reported photothermal catalysts for VOCs degradation and their properties are summarized in Table 1 [36–39,42–49,58–63,93–114].Transition metal oxides, which are usually polycrystalline and polyvalent, exhibit significant activity in the removal of gaseous pollutants [115–119]. At present, the photothermal catalysis of VOCs by metal oxides has received a lot of attention, including Co3O4 [58,59], MnOx [102], TiO2 [57], etc. Co 3 O 4 . As an excellent catalytic material, Co3O4 has excellent light capture and electron-mediated properties [120,121]. As described in Section 2.1, a better photothermal catalysis performance can be improved by enhancing light absorption and heat conversion. The light absorption properties of catalysts could be modulated by shape and structure regulation or metallic element doping. Chen et al. found that structure management and secondary metal doping techniques were effective to improve the solar light utilization efficiency of the catalyst [58]. NiOx/Co3O4 composites derived from Ni-doping ZIF-67 were prepared by the impregnation method. The full spectral absorption of NiOx/Co3O4 composites was significantly enhanced after Ni doping due to metal-to-metal charge transfer (MMCT). The MMCT process occurs when two metal ions with different valence states are bridged together. The difference in charge causes an electron transition and the two metal centers are reduced and oxidized, respectively. The transition oxo-bridged and the all-inorganic heterobinuclear units can extend light absorption from the ultraviolet region to the visible region [122]. In the meanwhile, the hollow structure of NiOx/Co3O4 allowed multiple reflections of light in the internal cavity, thus effective contact between photons and matter was promoted. Wang et al. prepared ultrathin mesoporous Co3O4 nanosheets on stainless steel mesh (SS-Co3O4) with high photothermal properties by electrochemical deposition method [59]. SS-Co3O4 exhibited strong absorption throughout the entire solar spectrum. The temperature of SS-Co3O4 rapidly raised from room temperature to 110 °C and finally stabilized at 175 °C, which revealed its efficient photothermal conversion. The combined properties of SS-Co3O4 ultrathin with two-dimensional shape, foam porous structure and metal substrate resulted in the maximum photothermal conversion, more surface catalytic active sites, lattice oxygen species and mobility.In addition to improving the light absorption and heat conversion of Co3O4, it is also possible to improve the thermal catalytic activity at low temperatures. Reactions are more likely occurred when the atoms on the catalyst surface are in an unsaturated coordination state [123]. When Co3O4 forms a bimetallic catalyst with other elements, the electronic structure of the cations on the surface can be adjusted by weakening the Co-O bond [60]. Jin et al. reported a quenching method that hot Co3O4 nanosheets were poured into copper nitrate solution and Cu2+ modified Co3O4 was obtained. Different from hydrothermal synthesis, Cu2+ only modified the surface layer of Co3O4 after quenching and Cu2+ was easier to replace Co2+than Co3+, thus more active sites were generated [60]. MnO x . MnOx has good thermal catalytic activity at low temperatures [124–126], most of the photothermal modification of MnOx focuses on light absorption and photothermal conversion. Yang et al. synthesized a new hollow pellet composed of tightly packed nanosheets (R-MnO2-HS) [96]. R-MnO2-HS could effectively convert solar energy into thermal energy under full spectrum, visible and infrared light irradiation, the sample temperature raised from room temperature to 226, 220, 211 °C, respectively. Besides, there was almost no photocatalytic performance when the reactor temperature is kept at ambient temperature under full spectral irradiation, so the catalytic performance is mainly due to light-driven thermal catalysis. Therefore, the catalytic performance of R-MnO2-HS is mainly attributed to the light-driven thermal catalysis. The researchers also improved the light absorption and photothermal conversion of MnOx by combining multiple materials. Carbonaceous materials, especially graphene, display high capability of photo-absorption form the ultraviolet to near-infrared region [127–129], and then light energy can be converted into thermal energy through non-radiative decay [130]. On this basis, multivariate manganese oxide catalysts have been reported widely. Dong et al. prepared 2D graphene oxide, MnOx and polymerized carbon nitride nanosheets as building blocks through a filtration method (Fig. 4 a). The superior photothermal effect of graphene caused the temperature of the film to rise to 85 °C in 15 min, which then initiated the thermal catalytic reaction of manganese oxide. Furthermore, the 2D/2D/2D assembly of these nanosheets in the membrane facilitated the transfer of energy and charge carriers between the various nanosheets. The increased temperature would activate the lattice oxygen and adsorbed oxygen molecules of MnOx, and promoted the transfer of photogenerated electrons and holes to the surface of the carbon nanotubes and the subsequent surface reactions [48]. Except for traditional manganese oxides, manganese potassium ore octahedral molecular sieve (OMS-2) has been widely used due to the unique characteristics of porosity, mixed-valence state, and easy release of lattice oxygen. Li et al. [39,93,94] synthesized a series of OMS-2 composites doped with metal elements (Mg, Fe, Ce). The OMS-2 catalyst substituted by nanometer Mg (or Fe, Ce) ion had strong absorption in the whole solar spectrum region, it could effectively convert the absorbed solar energy into thermal energy. The catalyst surface temperature was higher than VOC ignition temperature, which promoted the catalytic reaction. TiO 2 . TiO2 is one of the most promising photocatalysts due to its excellent activity, good stability, non-toxicity, and relatively low price. However, TiO2 has a wide band gap and only responds to ultraviolet light. Moreover, the surface of TiO2 is easily occupied by toxic by-products and deactivated in the process of photocatalysis. The effect of reaction temperature on the photocatalytic oxidation of refractory carcinogenic benzene by anatase TiO2 nanosheets with (001) crystal faces were studied by Li et al. [99]. The TiO2 nanosheets were coated on the surface of the Hg lamp, and the efficient photothermal catalytic oxidation of benzene was realized by using the heating effect of the UV and infrared light, without an extra heater. At the same time, it is an effective way to construct photothermal catalytic materials by combining other metal oxides with TiO2. MnOx/TiO2 [98], Co3O4/TiO2 [61], TiO2/CeO2 [103], CeMnxOy/TiO2 [101] and nano-TiO2-supported amorphous manganese oxide [104] were prepared through hydrothermal redox reaction in the presence of TiO2 (P25). These nanocomposites could effectively convert the absorbed solar energy into thermal energy, the surface temperature would be significantly higher than the light-off temperature of benzene oxidation [101]. In addition, the deposition of carbonaceous intermediates on TiO2 surface was inhibited by photothermal catalysis, thus the catalytic activity and durability were significantly improved [61,98]. Polymetallic oxides. Perovskites can provide a stable framework for elemental doping and electron band structure regulation because of the stable structure, abundant metal ions and anions [62,131]. In the meantime, perovskites show a good prospect in visible light-driven photocatalysis because of their good band-edge potential and changed band structure. Chen et al. [42] synthesized ABO3 type perovskite (A = La, Ce, Sm; B = Cr, Mn, Fe, Co, Ni), which was successfully applied to photothermal catalytic degradation of VOCs under visible light for the first time. All the synthesized ABO3 type perovskites showed excellent UV and visible light absorption, especially for LaMnO3 and LaNiO3. Besides, the band gap could be changed with different metal ions since B ion was located on the 3d orbit to form the conduction band, thereby the photo response, redox activity and other properties were accordingly affected. The band gaps of these perovskites followed the order: LaCrO3 (3.1 eV) > LaCoO3 (2.9 eV) > LaMnO3 (2.5 eV) > LaNiO3 (2.4 eV) > LaFeO3 (2.1 eV). The narrower band gap allowed for more efficient transfer of photogenerated electrons and separation of electron–hole pairs [132]. Even under the premise of making full use of visible light, the utilization rate of the full solar spectrum is still low. The energy in the infrared region is wasted mainly by heat dissipation. Making full use of the energy in the infrared region becomes a hot issue that must be considered. Xu et al. [47] found that CeO2/LaMnO3 composites had the characteristics of wide wavelength absorption (800–1800 nm), which could be used as a highly active photothermal response catalyst for the decomposition of VOCs under infrared irradiation. Under the infrared irradiation intensity of 280 mW/cm2, the maximum photothermal conversion efficiency of CeO2/LaMnO3 was 15.2%.The crystal structure of spinel contains tetrahedron and octahedron coordination sites, the accommodated different metal cations in the structure result in more reactive oxygen species. Chen et al. synthesized ACo2O4 (A = Ni, Cu, Fe, Mn) spinel by co-precipitation method. The MMCT effect between Co ion and A ion was affected by the d–d indirect transition band gap of A ion, which led to the difference in light absorption capacity. The strong light absorption and high solar thermal effect of NiCo2O4 in the whole solar spectrum (200–2500 nm) provided enough heat energy for the catalytic degradation of toluene [106].Compared with the high energy consumption in the catalytic process of metal oxides, noble metals are widely used in the thermal catalysis and photocatalysis of VOC due to their excellent activity [45,46,133–135]. As noble metal particles tend to aggregate at high temperature, semiconductor metal oxides are often used as carriers [136]. The noble metal–semiconductor structure is beneficial to broaden the spectral response region and the separation of photogenerated carriers. Schottky potential barriers between the noble metals and semiconductors can be generated due to the deposition of noble metals, thus promoting the separation of photogenerated electron–hole pairs. Cai et al. [63] reported that Pt nanoparticles were well dispersed on porous γ-Al2O3 (Fig. 4b (i)). Pt nanoparticles were used as a light absorbent and catalytic active site. As shown in Fig. 4b (ii), Pt/γ-Al2O3 showed obvious strong absorption in the wavelength range of 200–2500 nm, while pure γ-Al2O3 has no obvious light absorption. The extraordinary optical absorption further confirmed that the thermal-electronic and photothermal effects of Pt nanoparticles could efficiently drive the catalytic reaction. On the other hand, the temperature of the catalyst's surface was monitored during the irradiation process (Fig. 4b (iii). The temperature of catalyst raised to the platform temperature under irradiation in 15 min without an additional heater due to the excellent photothermal conversion. There was a positive correlation between temperature and Ag loading. The highest surface temperature was obtained over 2.81 Pt/γ-Al2O3 (169 °C) [63]. The combination of noble metals with other transition metals or carbonaceous materials can also effectively improve light absorption and photothermal conversion. Jia et al. reported that the photothermal conversion efficiency of hybrid nanomaterial Pt-rGO-TiO2 reached 14.1% under the infrared irradiation intensity of 116 mW/cm2 due to the effective photothermal conversion, increased light adsorption and well-dispersed Pt nanoparticles [45]. The light absorption, photothermal conversion are promoted mainly due to the localized surface plasmon resonance (LSPR), the strong metal-support interaction (SMSI) [45,114,137,138].As shown in Fig. 5 a, the LSPR effect on the surface of the noble metal is enhanced under light irradiation. The energy of the hot electrons generated by the electrons' collective oscillation is much higher than that of the electrons in a thermodynamic equilibrium state [63,137]. In essence, hot electrons are the products of surface plasmon oscillations in nanostructures that pump electrons from a lower energy level to a higher energy level through Landau relaxation [139,140]. The LSPR effect of noble metal occurs under light, and the generation of hot electrons is conducive to the rapid heating of the system and the activation of semiconductor lattice oxygen. The range of LSPR induction can be broadened by manipulating the plasma nanostructure [41]. Mao et al. [44] reported that Pt nanoparticles were limited to mesoporous micron-sized CeO2 successfully. A new hot electron-induced photoactivation process was proposed. The intense surface plasma absorption of Pt nanoparticles was beneficial to the catalytic activity and regional heating effect. Pt nanoparticles could absorb visible light photons and generate hot electrons, while the visible light photons could not be absorbed by CeO2 due to the large band gap. Besides, the LSPR thermal electron conversion to chemical reaction energy is inefficient due to the rapid relaxation of the carriers. It is valuable to design catalysts that are highly responsive to visible light irradiation and highly efficient in converting photons into chemical energy. Zou et al. found that Pd-Ce catalyst was synthesized by a liquid-phase reduction method assisted with cetyltrimethylammonium bromide (CTAB), and more active interfaces were produced. The transfer of hot electrons from Pd particles to CeO2 because of surface plasmon resonance promoted the dissociation of adsorbed oxygen. The maximum light utilization rate of Ce/Pd catalyst for toluene oxidation and CO oxidation reached 0.42% and 1%, respectively, which was attributed to the effective Ce/Pd interface [41].The importance of SMSI in reducible oxides supported noble metal nanoparticles has been recognized in many thermal catalytic systems. SMSI promotes the activity of the loaded noble metals mainly by affecting the formation of negatively charged noble metals nanoparticles and chemisorbed oxygen. The noble metal nanoparticles were stabilized by geometric modification and electronic modification [108,141]. The strong interaction between the noble metal and the carrier interface can be enhanced by doping and modification of the carrier, different synthesis methods and the formation of composite catalysts, which is of great significance for improving the catalytic performance of photothermal synergistic catalytic materials. Yang et al. reported that the SMSI in strontium titanate (STO) supported Pt nanoparticles (NPs) significantly promoted the photothermal oxidation of toluene under visible light. Chemisorbed oxygen and negatively charged Pt NPs were formed due to the SMSI effect, the oxygen activation and the surface plasma resonance effect of Pt NPs were promoted in the meanwhile. The above phenomenon promoted visible light absorption, photoionizing separation, and generation of reactive oxygen species [108].In recent years, defect chemistry and engineering techniques of metal oxides have attracted extensive attention because of their important role in regulating catalytic performance (Fig. 5b) [142,143]. The existence of defects promotes the adsorption and activation of substrate molecules, thus accelerating reaction thermodynamics and kinetics [144,145]. In photocatalysis, electrons can be trapped by the defects and release thermal energy to the surface, which improves charge utilization and speeds up the surface reaction rate. In order to pursue more excellent catalytic performance, defect engineering has been widely studied in the past decades [146]. Li et al. reported for the first time that porous Co3O4 nanorods (Co3O4-MNR) with a large amount of Co2+ vacancy defects greatly enhanced the photothermal catalytic activity of Co3O4 (Fig. 5c) [95].It is found that the photocatalytic/catalytic performance of oxides significantly depends on the type and concentration of defects [142,143,146–149]. Oxygen vacancy (OV) is the most utilized defect. OVs can effectively separate the photoexcited electrons and holes, thus improving the photocatalytic performance [150]. It is a natural defect of oxides and can be easily incorporated into the lattice of oxides by plasma treatment technology [151], acid treatment [152] and annealing in an anoxic atmosphere [153]. Zhang et al. proposed that a solution plasma processing technique could be applied to process pre-synthesized TiO2 containing a large amount of OVs. Hydrogen dopants were added to the TiO2 lattice to produce defects, and the original visible absorption of colored TiO2 was still retained [12]. Huang et al. demonstrated a simple and effective route to introduce OVs into (001) face of BiOI nanosheets by modification with low concentration nitric acid for the first time. The vacancy increased the maximum valence band of the BiOI nanosheet and ensured that more charge carriers were converted to ·O2− in photothermal catalysis. In addition, the defective BiOI nanosheets could absorb more visible light than the original BiOI nanosheets, which was important for improving photothermal catalytic performance [111]. Carbon deposition in the process of catalytic oxidation of VOCs is the key factor that restricted the stability of catalysts. An et al. introduced OVs into CeO2 through redox and steam treatment (ARCeO2), which showed high coke resistance performance. The abundant OVs in ARCeO2 enhanced light absorption, improved charge separation, and increased the generation of reactive oxygen species, thus improving its photothermal catalytic performance [49]. Although OVs can promote the catalytic reaction, however excessive OVs will act as the complex center and reduce the charge transfer to the catalyst surface [111]. Quantitative analysis and characterization of OVs have always been a difficulty in related studies. Some characterizations have been employed to measure oxygen vacancies, such as electron spin resonance (ESR) [154,155], x-ray photoelectron spectroscopy (XPS) [156,157], situ UV-Raman [158], etc. These studies further reveal that rational surface defects engineering is an extremely effective and advanced route in the promotion of photothermal catalysis.Moreover, the heterojunction was constructed for photothermal catalysis of VOCs because of the unique charge separation and transfer behavior, remarkable light response, as well as strong redox ability [114,159]. Z-scheme Ag3PO4/Ag/SrTiO3 Heterojunction [159], a new type of WO3/Ag/GdCrO3 named as Type B heterojunctions [50] and band gap-broken Ag3PO4/GdCrO3 heterojunction [51] were constructed by Rui et al. Fig. 6 a clearly revealed the charge transfer in WO3/Ag/GdCrO3 heterojunction. The thermal electrons and holes generated under photothermal process broke through the energy barrier of WO3/Ag/GdCrO3, and then combined with the photogenerated electron–hole pairs generated by the semiconductor. Thus, the spatial separation of photoinduced charge was realized, and the strong light absorption and redox ability were retained [50]. In order to further clearly observe the effect of photothermal synergy on charge migration in heterojunction, photoinduced charge migration of Ag3PO4/GdCrO3 and Ag3PO4/Ag/GdCrO3 under different conditions was conducted by Rui et al. (Fig. 6b). The Ag3PO4 and GdCrO3 produced electron–hole pairs in the irradiation of visible light, and hot electron–hole pairs also induced by LSPR of Ag NPs. At the same time, the local electromagnetic fields induced by Ag LSPR effect could accelerate the electron generated by Ag3PO4 transfer to the Ag3PO4-Ag interface, which was beneficial to the production and separation of the light induced charge in Ag3PO4. When the temperature increased, the Ag NP enhanced LSPR effect can promote the formation of hot holes and improve the scattering rate of hot electrons. The photoinduced charge separation of GdCrO3 and Ag3PO4 was enhanced through interfacial charge transfer by plasma Ag NP as a bridge under photothermal conditions [51]. Therefore, the construction of heterojunction has great research potential and application value in the field of photothermal catalysis.Besides studying the oxidation mechanism of VOCs, the catalytic reactor also plays an important role in the combustion. In this review, the VOC photothermal catalytic system is divided into the batch system (Fig. 7 a) and the continuous system (Fig. 7b). On the one hand, this classification is intended to facilitate the comparison of the performance of photothermal catalysts, since the operation reactor has a significant effect on the performance. On the other hand, batch reaction systems can usually simulate the variation of pollutants in indoor or confined spaces, while continuous reaction systems can simulate the variation of pollutants in outdoor and industrial emissions.The equipment and operation of a batch photothermal catalytic reactor are relatively simple, which is commonly applied in laboratory research (Fig. 7a). The system is easy to adapt to different operating conditions and pollutants. Also, it is suitable for the degradation with small air flow and multiple VOC varieties. Basically, batch photothermal catalysis occurs in a closed reactor and an external light source irradiates the inside of the reactor through a quartz window of a certain size. Meanwhile, the reaction temperature is maintained through insulation equipment; Sometimes additional heating equipment is used to provide the corresponding thermal energy. Some of reactants are fed into the system and interact with each other. However, it is difficult to achieve continuous degradation in batch system and the reaction time is long. In this review. The photothermal catalysis, photocatalysis and thermal catalysis performance and experimental parameters of various VOCs in the batch system are summarized in Table 2 [36–40,44,46,93–104,110,113,160]. Obviously, photothermal catalysis has a higher degradation rate and shorter reaction time than thermal catalysis and photocatalysis. Aromatic hydrocarbons with benzene rings are stable and require high temperatures to destroy. Most of the researches on photothermal catalysis VOCs focuses on benzene series. Moreover, the light response range of the catalyst is continuously expanded to obtain better catalytic activity and solar energy utilization.Traditional photocatalysis is carried out under the irradiation of ultraviolet light. Li et al. coated the surface of the UV lamp with TiO2 and Pt/TiO2. The ultraviolet irradiation and nonradiative heat energy emitted from the UV lamp were fully utilized, and the efficient photothermal catalytic oxidation of benzene (∼70 ppm) was realized (Table 2) [36]. In addition to the light driven thermal effect under UV irradiation, the combined effect of UV light and external thermal sources on the catalytic reaction had been further investigated. Wang et al. successfully improved the mineralization of gaseous benzene over TiO2 by adjusting the temperature from room temperature to 280 °C under UV irradiation. The high-temperature desorption could accelerate the mineralization rate of inactive sites. However, at high temperatures, the reduction of ∙OH on the surface weakened the oxidation of benzene, while heat promoted the oxidation rate [110].The solar spectrum only contains about 5% ultraviolet (UV). Expanding the absorption and utilization of visible and infrared light is extremely urgent. Therefore, some works have been conducted to improve infrared and visible responses. Li et al. reported that efficient photothermal catalytic oxidation of benzene (∼250 ppm) was realized by coating TiO2 on the surface of the Hg lamp. The thermal effect of infrared light was further employed without an extra heater. Under the same irradiation conditions, the photothermal catalytic activity of TiO2 was increased by 42.3 times compared with that at room temperature and showed excellent durability [99]. Ji et al. commercialized SrTiO3 by constructing fluorine ions and Ag nanoparticles (Ag/F-STO). Under visible light irradiation at 90 °C, Ag/F-STO could degrade 95% of benzene, toluene, and xylene (800 ppm) through photothermal catalysis after 6 h with a high degradation rate constant [46]. Kong et al. prepared bifunctional 0.1 wt % Pt/SrTiO3-x, achieved 100% mineralization against toluene (500 ppm) under visible light irradiation and mild thermal energy input (≤150 °C) after 1 h. The introduction of photocatalysis reduced the activation energy in the conventional thermal catalysis process, the generation of ∙O2− and ∙OH was also reduced by activating oxygen and consuming water [113].In order to make full use of solar energy in practical application, the research of full spectrum is gradually coming into view. The Pt/CeO2 nanocomposite prepared by Mao et al. showed high photothermal catalytic activity under the full solar spectrum, visible light-infrared, or infrared light irradiation. Pt nanoparticles had strong surface plasma absorption in the whole solar spectrum region. The surface temperature of the catalyst was stabilized to 173 °C, and the toluene (∼4900 ppm) was completely degraded after 25 min of irradiation [44]. The mesoporous Co3O4 nanorods prepared by Lan et al., which contained a large amount of Co2+ vacancy defects, showed high photothermal catalytic activity for benzene oxidation under UV-Vis-IR. Benzene (∼4900 ppm) was completely oxidized to CO2 after 40 min of irradiation [95]. The performance of a single metal oxide is limited, so the composite catalysts have become the research trend of many researchers [61,98]. The MnOx/TiO2 [98], Co3O4/TiO2 [61] nanocomposites were prepared and showed good catalytic activity and durability for benzene oxidation under light irradiation. Xie et al. found that OMS-2/SnO2 nanocomposites showed good photothermal catalytic activity and stability of benzene under UV-Vis-IR irradiation. The surface temperature of the catalyst was stabilized to 260 °C, and the toluene conversion rate reached about 88% after 35 min of irradiation. The initial CO2 yield of OMS-2/SnO2 with photothermal catalytic degradation of benzene was 83.3 μmol min−1g−1, which was 37.2 times higher than that of pure SnO2 [100].In addition to aromatic hydrocarbons, photothermal catalysis of other VOCs in batch systems has also been reported, such as aldehyde and alkanes. The MnOx-CeO2 mixed oxide prepared by Jiang et al. had strong light responsiveness and low-temperature reducibility, a good catalytic synergistic effect was founded in formaldehyde emission reduction. The degradation rate of formaldehyde (250 ppm) reached 90.4% after 3 h of infrared irradiation at 75 °C. In addition, the depleted material after a long period of dark reaction showed encouraging self-repair under in-situ light [160]. Kang et al. catalyzed propane over Pt/TiO2-WO3 catalyst. After the introduction of UV-vis light, the reaction temperature required for 70% conversion of C3H8 was reduced to a record-breaking 90 °C from 324 °C [138].In the batch reaction system, if the operation time is long enough and there is no side reaction or catalyst deactivation, the conversion rate per unit volume of the batch reaction system is higher. From the view of experimental research, the batch system is easy to operate and control, it can provide unique insights for the analysis of catalytic mechanism, kinetic parameters, and overall performance. However, the batch reaction system is not widely used in large-scale industries because of its high operating cost and low versatility. At present, the catalysts studied in batch VOC treatment system mainly focus on transition metal oxide catalysts, but there is still some space for improvement compared with noble metal catalysts, which needs further research and practical application promotion.On an industrial scale, most chemical processes employ continuous systems due to their excellent heat and mass transfer performance. In addition, the continuous working mode uses a higher energy light source to improve performance. For example, at higher energy levels, the light source can handle more intake volume per unit of time than other operation modes. After all, continuous systems are considered easier to scale up to the practical application. A typical continuous photothermal catalytic system is shown in Fig. 7b.There are few studies on the photothermal degradation of VOCs by continuous systems. The continuous system is mainly based on toluene. In order to improve the traditional photocatalytic mode and expand the application of solar energy, the main irradiation light sources are concentrated in infrared, visible and full solar spectrum. Due to the short contact time between gas flow and the catalysts in the continuous system, the VOC concentrations in the current experimental study are mostly concentrated in the range of 50–200 ppm. Some relevant research advances are summarized in Table 3 [43,45,47,49,58,62,63,105–107].In the study with infrared light as the source, Li et al. found the surface temperature of CeO2/LaMnO3 composites raised rapidly to 275 °C due to excellent photothermal conversion. The toluene (200 ppm) conversion rate was 89% and the CO2 production rate was 87% under the infrared irradiation intensity of 280 mW/cm2. Such enhanced photothermal catalytic activity was mainly attributed to the synergistic effect of ultra-broadband, strong light absorption, efficient photothermal conversion, good low-temperature reducibility, and high lattice oxygen mobility, which was caused by the strong interaction between LaMnO3 and CeO2 [47]. Li's group also reported that the significant toluene conversion rate of Pt-rGO-TiO2 was 95% (150 °C), and the CO2 yield was 72% under the infrared irradiation intensity of 116 mW/cm2 [45].In the studies of visible light irradiation, An et al. proposed that CeO2 (ARCeO2) with OVs performed high photothermal catalytic performance of typical VOCs (50 ppm) including styrene (T90 = 226 °C), n-hexane (T90 = 459 °C) and cyclohexane (T90 = 563 °C). In addition, rich OVs and weak acid ARCeO2, together with the synergistic effect of photothermal catalysis, promoted the oxidation of intermediates, thus enhanced the coke resistance of high photothermal catalysis. The photothermal catalytic activity of ARCeO2 did not decrease significantly at 200 °C for 25 h. Its excellent photothermal catalytic stability could be attributed to fewer intermediates and limited coke accumulation on ARCeO2 [49]. Chen et al. studied the removal of toluene (220 ppm) on ACo2O4 (A = Ni, Cu, Fe, Mn) spinel by collecting inexhaustible solar energy to provide thermal energy. The order of photothermal catalytic performance of ACo2O4 was as follows: NiO2O4 > CuCo2O4 > FeCo2O4 > MnCo2O4. NiCo2O4 showed the highest photothermal catalytic activity at 214 °C (toluene conversion rate was 93%, CO2 production rate was 80%) and good toluene oxidation stability (at least 20 h). The excellent photocatalytic performance of NiCo2O4 was mainly due to its strong visible light absorption. It was found that visible light irradiation could enhance the mobility of reactive oxygen species, and thus significantly improved the photothermal catalytic activity of NiCo2O4 [106].Similarly, the full utilization of full-spectrum light is of great significance for the development of continuous photothermal catalytic systems. Chen et al. reported that NiOx/Co3O4 composite exhibited high photothermal catalytic activity (about 95% conversion and 80% mineralization at 295 °C) in toluene oxidation (210 ppm, gas space velocity per hour = 32,000 mL g−1 h−1) under simulated sunlight [58]. Cai et al. evaluated the toluene degradation performance of Pt/γ-Al2O3 under full solar spectrum irradiation. Pt/γ-Al2O3 displayed tunable optical properties and outstanding photothermal conversion due to the plasma photothermal effect of Pt NPs. 1.81 wt% Pt/γ-Al2O3 showed highly efficient catalytic activity with 87% toluene conversion and 84% CO2 yield at 165 °C with solar irradiation intensity of 320 mW/cm2, as well as a decent stable continuous operation for 30 h [63].Up to now, there are few studies on VOCs degradation by continuous photothermal system, and there is a lack of theoretical support from basic experimental data. Furthermore, the evaluation of catalyst performance mainly focused on single contaminant component. The coexistence should be expanded to include mixtures such as multicomponent VOCs, NOx, and water vapor, to better simulate the actual industrial processing conditions. In the continuous system, noble metal catalyst shows more excellent catalytic performance, but the current catalyst test conditions are low concentration, low space speed conditions, and the industrial conditions are quite different. Therefore, the study of noble metal catalyst in continuous VOC treatment system should focused on improving its stability and durability under high space velocity and high concentration conditions.At present, some reports have proposed the routes and mechanisms of photothermal catalytic VOCs with heterogeneous catalysts based on experimental and theoretical researches. Although abundant studies have been conducted on different types of VOCs and catalysts, the comprehensive oxidation mechanism of VOCs involved in the degradation process is rarely summarized in detail. Therefore, the mechanisms of photothermal catalytic VOCs were systematically summarized in this paper.Light-driven thermal catalysis based on photothermal effect is the most common route in photothermal catalysis. The catalyst converts the absorbed light into thermal energy to heat up the surface of the catalyst. It can provide enough energy to reach the combustion temperature of VOCs on the surface of the catalyst without the need for an auxiliary heat source [107]. The photothermal effect of catalysts mainly caused by surface plasmon resonance, non-radiative relaxation in semiconductor and molecules thermal vibration [161]. Light can be converted to heat by one or several photothermal conversion routes depending on the properties of the material. Fig. 8 provide a diagram of light-driven thermal catalysis. The catalyst surface temperature increases due to the photothermal conversion when thermal catalysis is driven by solar light. VOC oxidation reaction occurs when the temperature reaches the light-off temperature. The appropriate catalyst can efficiently absorb light and release it as heat energy, resulting in a temperature rise to activate the oxidation process. Localized heating effects caused by strong surface plasma absorption of noble metal nanoparticles have also been reported [44]. Noble metal nanoparticles rapidly heat up after absorbing visible light, the heat energy generated allows the reactants to cross the barrier needed for the reaction. The light intensity around noble metal nanoparticles and the interface of nanoparticles are stronger after the surface plasma absorbs light. The photons produced by the surface plasmon effect can be converted to heat energy, the area around the noble metal particles will be heated to a higher temperature. Zou et al. [41] reported the photothermal catalysis of toluene by the composite catalyst Pd/CeO2. It was suggested that the enhanced catalytic activity was mainly attributed to the strong surface plasmonic resonance of Pd hot electrons. Under light irradiation, the hot electrons were transferred to CeO2 rapidly through the Pd-CeO2 interface; The service life of hot electrons was prolonged and the system heated up rapidly; In the meanwhile, the surface adsorption of oxygen and the activity of lattice oxygen were improved, which were favorable to the catalytic activity.The solar light-driven thermal catalysis reaction system has the following advantages:(1) No additional heat input is required and high energy efficiency; (2) Due to the photothermal effect, the surface temperature of the catalyst rises instantly, and the heat is mostly concentrated on the surface of the catalyst; (3) In some cases, the photothermal catalysis can effectively inhibit the deactivation of the catalyst and increase the selectivity of the desired product.Based on the photochemical effects, catalysts are excited by light irradiation to produce hot carriers (such as electrons and holes), which can participate in the reactions [162]. The energy of hot carriers generated by photochemical effects is much than that produced by thermal excitation. At the meanwhile, the hot carriers can be transferred to unoccupied molecular orbitals of adsorbate molecules, thus resulting photochemical transformation. In addition to the advantages of promoting electron transfer, light also inhibits the carrier recombination. Jiang et al. found that abundant oxygen vacancies existed in the lattice of Ce1-xBixO2-δ catalyst with the introduction of Bi3+. The introduction of Bi3+ ions would expand the response to visible light and enhance the redox activity at low temperatures. At the same time, the vacancy greatly promoted the migration of oxygen ions, which further promoted the low-temperature integration. The migration of oxygen ions captured by holes effectively inhibited the recombination of charge carriers under UV-Vis and the thermal activation under infrared light. The high temperature caused by infrared would further enhance the coupled ion and electron conductivity [38].It is interesting to note that the photochemical effect has a positive effect on reducing carbon deposition [49], enhancing water resistance [113] and catalyst self-remediation in the reaction process [160]. Jiang et al. proposed a self-healing mechanism based on photo-remediation during photothermal process (Fig. 9 a). The reduction of high valent metal ions and the toxicity of intermediate products would deactivate the catalyst after a long period of thermal catalysis. However, photo-remediation could partially repair depleted catalysts: (1) Photo oxidized species mainly recover reduced metal ions; (2) Harmful reaction intermediates could be removed from the closed active site; (3) The photoinduced holes transformed the inert –OH into strongly oxidizing ∙OH, which further reduced the VOCs and released the active sites [160].In fact, it is difficult to completely distinguish the photothermal and photochemical catalytic pathways, which are intertwined in the photothermal process. The photothermal catalytic system can dissipate the absorbed photon energy into heat energy under incident light irradiation, which can promote the transfer of charge carriers and improve catalytic activity. At the meanwhile, hot carriers generated by light irradiation can participate in catalytic reactions. Thus, the reaction mechanism of heterogeneous photothermal catalysis is usually the photothermal and photochemical effects.Under photothermal conditions, irradiation can promote the activation of lattice oxygen, and thus improve its thermal catalytic performance. Ji et al. proposed that in the photothermal degradation of toluene on Ag/F-SrTiO3, the high-energy electrons on Ag nanoparticles induced by visible light could be transferred to the adsorbed oxygen to assist the activation of O2 (Fig. 9b). The transfer of activated oxygen species (e.g. O2 −) from noble metal nanoparticles to the carrier had a positive effect on the oxidation reaction. On the other hand, the electron transfer could be accelerated by increasing the reaction temperature, thus the photogenerated electron–hole separation rate could be promoted. The heat in the photothermal catalysis process was beneficial to the production, transfer, and oxidation capacity of reactive oxygen species, which was essential for improving the photothermal catalysis rate and the deep oxidation of accumulated surface intermediates. Furthermore, the thermal effect induced by the surface plasmon resonance effect of Ag nanoparticles could further promote the surface reaction of Ag nanoparticles, which was helpful to the photocatalytic activity [46]. Another photothermal synergistic pathway is proposed based on the Mars-van Krevelen (MvK) theory. The MvK mechanism [163] refers to the reaction between the reactant and the lattice oxygen of the catalyst. The reaction diagram is shown in Fig. 9c. In this reaction, the reactants are oxidized and the catalyst is reduced, and oxygen vacancies are produced. The OV is further replaced by adsorbed oxygen, which causes the catalyst to be re-oxidized. Li et al. have pointed out that the photothermal synergistic catalytic degradation mechanism of benzene, toluene and acetone conforms to the MvK mechanism [89].In general, the mechanism of photothermal synergism is mainly reflected in the following aspects: (1) The absorption of solar light by the catalyst accelerates the rate of charge separation and the ability of reactive oxygen generation; (2) Under simulated solar irradiation, the generation of reactive oxygen is beneficial to speed up the MvK redox cycle and reduce the activation energy; (3) Improving the photocatalytic efficiency through thermal catalysis can promote the deep oxidation of coke and alleviate the negative effect of carbon deposition on photocatalysis [105]. The improvement of catalytic activity after irradiation is not a simple effect of photoinduced heating, but a more complex photothermal synergistic effect and the specific mechanism still needs to be further explored.Theoretical calculations based on density functional theory (DFT) are widely applied to study and predict catalytic reactions due to great advances in computational methods and related software [164]. Computational chemistry has become a powerful tool to explore degradation processes and mechanisms. The cooperation between experimental research and theoretical calculation can provide a deeper insight into the mechanism of catalysis [165,166]. At present, DFT calculation is often used to calculate the formation energy of OV (Evo) and the adsorption energy (Eads) of gas molecules on the surface of the catalyst.OV formation energy refers to the energy required for an oxygen atom removed from the lattice oxygen of metal oxide to form a defect. As mentioned above, the catalytic oxidation of metal oxides is generally believed to be carried out through the MvK mechanism. The organic molecules adsorbed on the surface of the catalyst are oxidized by lattice oxygen, and the oxygen vacancies are subsequently supplemented by gas-phase O2. The lattice oxygen activity of the catalyst under sunlight irradiation can be further studied by DFT theory calculation. Hou et al. considered the effect of the substitution of K+ by Ce4+ in the tunnel on the OV through DFT calculation and calculated the Evo in OMS-2 supercell. For pure OMS-2 (Fig. 10 a (i)), the Evo was 2.32 eV. When four K+ ions were replaced by one Ce4+ ion, the Evo increased to 3.52 eV (Fig. 10a (ii)). The theoretical results showed that only the substitution of K+ ions by Ce4+ ions in the OMS-2 channel was not conducive to oxygen vacancy formation, thus reducing the catalytic activity of OMS-2. When four K+ ions were replaced by two Ce4+ ions to form a Mn4+ ion vacancy to maintain the charge balance (Fig. 10a (iii)), the Evo in the lattice near the Mn ion vacancy decreased to 2.23 eV, suggesting the high lattice oxygen activity. The lattice oxygen activity and catalytic activity of OMS-2 could be significantly improved by designing the unique Ce ion substituted with Mn vacancy nanostructure [167]. The results also provided a reference for other catalysts.Effective adsorption is a necessary condition for the rapid decomposition of VOCs. DFT is also used in the photothermal catalysis process to calculate the adsorption energy of various gas molecules in the reaction process on the catalyst surface. Borjigin et al. studied the adsorption of benzene on the surface of CeO2 and Ag3PO4 during the photothermal degradation over Ag/Ag3PO4/CeO2. The optimized structure and adsorption energy obtained are shown in Fig. 10b. For the CeO2 (111) model, a shorter O-C bond length (3.737 Å) and higher adsorption energy (−0.63 eV) were obtained between the benzene ring and CeO2 (111) (Fig. 10b (i)). The electron transfer of C6H6 to the CeO2 surface was also confirmed by the charge density differences transection maps (Fig. 10b (ii, iii)). In addition, the calculation results of the Ag3PO4 (100) model showed that a longer O-C bond (3.853 Å) and smaller adsorption energy (−0.31 eV) was formed between Ag3PO4(100) and benzene (Fig. 10b (iv)), and the electron transfer is not clear (Fig. 10b (v, vi)). According to theoretical adsorption analysis, the adsorption capacity of the CeO2 (111) surface was better than that of the Ag3PO4 (100) surface. Therefore, it could be considered that the CeO2 component in the composite promoted the decomposition of VOCs through strong adsorption, so the pollutants were enriched to the catalyst surface [114].Photothermal catalysis aims to convert harmful compounds into harmless substances. However, there are still many challenges in reaction mechanisms and practical application. (1) Reaction mechanism. In the current photothermal catalysis mechanisms, the solar light-driven thermal catalysis is essentially based on thermal catalysis, and light irradiation just plays the role of providing thermal energy. The reaction paths photocatalysis and thermal catalysis are different, it is important to define their differences in the photothermal synergistic process. In the current research on photothermal synergistic catalysis of VOCs, the performance–temperature curves under light and dark conditions are measured to judge whether light or heat participate in the reaction. At present, the surface temperature of catalyst is measured by thermocouple. The selectivity and yield of catalysis products can be observed and compared at the same temperature to determine whether light plays a heating role in the reaction. However, the accuracy of surface temperature measurements for catalysts has been debated in recent years. As a macroscopic measurement method, thermocouple has limited application in the nanometer scale and cannot accurately reflect the localized temperature of nanoparticles. More accurate temperature measurements such as scanning thermal microscopy (SThM) [168], non-contact measurement contact technology based on Raman spectroscopy/in situ infrared spectroscopy [169,170] need more exploration. Therefore, the relationship between light and heat in photothermal catalysis and the synergistic mechanism needs to be further explored. More experiments and theoretical studies need to be carried out, such as product yield and selectivity under different temperatures and different light intensities, activation energy calculation, photon utilization rate calculation, and other core experimental data analysis. (2) Catalyst deactivation. In catalytic reactions, catalyst deactivation can be affected by the change of catalyst performance, intermediate products, or byproduct poisoning. For example, in the catalytic degradation process of benzene, the coke will deposit on the surface of the catalyst, resulting in the deactivation of the catalyst. The deactivation of the catalyst results in a significant decrease in the overall removal efficiency and the generation of intermediate by-products. In addition, the operating cost of the system will increase significantly when the catalyst is frequently regenerated or replaced. It is a feasible option to improve the performance of the catalyst since regeneration is difficult and uneconomic. (3) Lack of performance data under actual working conditions. Most of the research on photothermal catalysis of VOCs has been conducted in the laboratory. VOCs concentrations are usually in the range of 50–2000 ppm (Tables 2 and 3). Although the highly stable performance of photothermal catalysts has been reported, further evaluation under practical conditions is necessary. At present, the data of photothermal catalytic VOCs are scanty and usually limited to conditions without moisture, so there is a certain disparity between this assessment and the actual working conditions. Moreover, the evaluations of performance mainly focused on individual components, and the coexistence atmosphere should be expanded to include multiple VOCs (especially containing chlorine VOC, sulfur VOC, etc), NOx and other mixtures, so the industrial conditions can be better simulated. (4) Accuracy of theoretical calculations. In the process of theoretical calculation, active catalytic species or intermediates have been difficult to be studied by experimental methods. However, there are still many challenges in studying the mechanism of catalytic transformation, such as the limitations of the accuracy of the computational methods. Furthermore, the large number of intermediate products involved in the catalytic process increases the complexity of theoretical calculation. In addition, the complexity of commonly used organometallic complexes and ligands, as well as the solvent and additive effects in the process, pose additional challenges. Reaction mechanism. In the current photothermal catalysis mechanisms, the solar light-driven thermal catalysis is essentially based on thermal catalysis, and light irradiation just plays the role of providing thermal energy. The reaction paths photocatalysis and thermal catalysis are different, it is important to define their differences in the photothermal synergistic process. In the current research on photothermal synergistic catalysis of VOCs, the performance–temperature curves under light and dark conditions are measured to judge whether light or heat participate in the reaction. At present, the surface temperature of catalyst is measured by thermocouple. The selectivity and yield of catalysis products can be observed and compared at the same temperature to determine whether light plays a heating role in the reaction. However, the accuracy of surface temperature measurements for catalysts has been debated in recent years. As a macroscopic measurement method, thermocouple has limited application in the nanometer scale and cannot accurately reflect the localized temperature of nanoparticles. More accurate temperature measurements such as scanning thermal microscopy (SThM) [168], non-contact measurement contact technology based on Raman spectroscopy/in situ infrared spectroscopy [169,170] need more exploration. Therefore, the relationship between light and heat in photothermal catalysis and the synergistic mechanism needs to be further explored. More experiments and theoretical studies need to be carried out, such as product yield and selectivity under different temperatures and different light intensities, activation energy calculation, photon utilization rate calculation, and other core experimental data analysis. Catalyst deactivation. In catalytic reactions, catalyst deactivation can be affected by the change of catalyst performance, intermediate products, or byproduct poisoning. For example, in the catalytic degradation process of benzene, the coke will deposit on the surface of the catalyst, resulting in the deactivation of the catalyst. The deactivation of the catalyst results in a significant decrease in the overall removal efficiency and the generation of intermediate by-products. In addition, the operating cost of the system will increase significantly when the catalyst is frequently regenerated or replaced. It is a feasible option to improve the performance of the catalyst since regeneration is difficult and uneconomic. Lack of performance data under actual working conditions. Most of the research on photothermal catalysis of VOCs has been conducted in the laboratory. VOCs concentrations are usually in the range of 50–2000 ppm (Tables 2 and 3). Although the highly stable performance of photothermal catalysts has been reported, further evaluation under practical conditions is necessary. At present, the data of photothermal catalytic VOCs are scanty and usually limited to conditions without moisture, so there is a certain disparity between this assessment and the actual working conditions. Moreover, the evaluations of performance mainly focused on individual components, and the coexistence atmosphere should be expanded to include multiple VOCs (especially containing chlorine VOC, sulfur VOC, etc), NOx and other mixtures, so the industrial conditions can be better simulated. Accuracy of theoretical calculations. In the process of theoretical calculation, active catalytic species or intermediates have been difficult to be studied by experimental methods. However, there are still many challenges in studying the mechanism of catalytic transformation, such as the limitations of the accuracy of the computational methods. Furthermore, the large number of intermediate products involved in the catalytic process increases the complexity of theoretical calculation. In addition, the complexity of commonly used organometallic complexes and ligands, as well as the solvent and additive effects in the process, pose additional challenges.Photothermal catalysis is a promising and sustainable technology, which can significantly improve the catalytic activity and regulate the reaction pathway through the synergistic effect of photochemical and thermochemical process. Photothermal catalytic VOCs shows many advantages in solving the problems of high-energy consumption of traditional thermal catalytic oxidation technology and low efficiency of photocatalytic VOCs purification technology. This paper reviews the research progress of photothermal catalytic technology, including catalyst design, the performance over different systems and the main photothermal catalytic mechanisms. Meanwhile, the challenges and development prospect of VOCs photothermal elimination technology is illustrated. There are still some scientific and technical issues needs be solved, which mainly include the effective design of catalysts, the deep exploration of essential mechanisms of photothermal synergistic effect, the synergistic control of multiple pollutants, etc. It is hoped that this review can improve deeply understanding of photothermal catalytic VOC technology and provide some reference for practical application.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work is sponsored financially by the National Natural Science Foundation of China (No.21906104 and No.12175145), the Shanghai Rising-Star Program (21QA1406600).

Photothermal catalysis realizes the synergistic effect of solar energy and thermochemistry, which also has the potential to improve the reaction rate and optimize the selectivity. In this review, the research progress of photothermal catalytic removal of volatile organic compounds (VOCs) by nano-catalysts in recent years is systematically reviewed. First, the fundamentals of photothermal catalysis and the fabrication of catalysts are described, and the design strategy of optimizing photothermal catalysis performance is proposed. Second, the performance for VOC degradation with photothermal catalysis is evaluated and compared for the batch and continuous systems. Particularly, the catalytic mechanism of VOC oxidation is systematically introduced based on experimental and theoretical study. Finally, the future limitations and challenges have been discussed, and potential research directions and priorities are highlighted. A broad view of recent photothermal catalyst fabrication, applications, challenges, and prospects can be systemically provided by this review.

1. Introduction Rice husk is a lignocellulosic biomass consisting of cellulose, hemicellulose, and lignin [1]. Cellulose can be converted into energy sources in the form of platform chemicals such as 5-hydroxymethylfurfural (5-HMF) and levulinic acid (LA) [2]. Cellulose conversion to 5-HMF and LA has been widely carried out using homogeneous catalysts, such as sulfuric acid and hydrochloric acid [3,4], as well as heterogeneous catalysts such as Nafion, SAC-13 [4], Dowex, cellulase-mimetic solid acid catalysts [5], and ZSM-5 zeolites in media based on water [6,7]. Glucose conversion to 5-HMF and LA carried out using MOFs [8,9] have also been reported. However, not many articles discuss the conversion of cellobiose to LA, in example [10] or 5-HMF. From all heterogeneous catalysts, hierarchical ZSM-5 zeolite modified with Mn3O4 species has drawn much attention, due to its unique properties of large surface area, distinctive structure, and reactivity [6,7,11]. Since the hierarchical ZSM-5 catalyst possesses the hierarchical pore system i.e micro- and mesoporosity [12,13], it has high thermal stability and well-spread active sites as well as fast mass transport [14]. As we can see from the previous study on Table 1 , Chen et al. [11], manage to obtained 1.17% HMF, Krisnandi et al. [6], obtained an LA % yield of 15.83%, and Pratama et al. [7], obtained an LA % yield of 39.75% with conventional heating method.However, the typical conversion reaction of biomass-based cellulose to 5-HMF and LA [6,7] usually is time-consuming although it has used a catalyst and medium heat. As mentioned above, the conversion of delignified rice husks into LA using hierarchical Mn3O4/ZSM-5 had an optimal yield of 39.75% at a reaction time of 8 h [7]. In another study, the conversion of cellulose to LA obtained optimal yield (91%) at 6 hours of reaction (using Ni-HMETS-10 catalyst, 180 °C) [15] and optimal yield (55%) at 3 hours of reaction (using C4(Mim)2(2HSO4)(H2SO4)2 catalyst, 100 °C) [16]. Also, most of the time, it is difficult to reuse the catalyst since it is difficult to be separated from the very thick product after 8h reaction and most of the catalysts were decomposed [6].Therefore, it has drawn our interest to use a microwave-assisted method in performing this conversion reaction to shorten the reaction time and to avoid severe damage to the catalyst. Many researchers are starting to carry out catalytic conversion using microwaves instead of conventional methods because the heat transfer process in conventional heating is relatively slow and less effective [11]. The microwave-assisted organic synthesis (MAOS) method is currently widely used because it gives a fast, easy, and clean pathway route [17]. Other advantages of MAOS include low energy, short time, high yield, solvent-free, and recyclability of catalyst. Reactions such as Aldol, Claisen, and Michael may benefit from using microwave assistance because of the high temperatures they require [17]. Muley et al. [18], stated that microwaves give a positive impact on heterogeneous catalysts properties. With microwaves, the polar reactant molecules have a faster rotation rate and enter the transition state more quickly than with conventional heating systems. Rapid microwave heating and higher temperatures can increase product selectivity, as in the case of micro-plasma formation at metal sites, microwaves lead to increased product selectivity and rapid activation of reactants such as methane [18].Ren et al. [19], carried out a microwave-assisted synthesis in SO3H-functionalized ionic liquids (SFILs) as an efficient catalyst for the direct conversion of cellulose to LA using laboratory microwaves. The yield of LA rose up with the reaction time increasing from 5 to 30 min and decreasing afterward. As a result, a 5 min at 160 oC reaction produces 15.7% LA (Table. 1) and the most optimal LA (55%) was obtained by reacting for 30 min with the same temperature. In addition, the microwave-assisted catalytic conversion of cellulose to 5-HMF in ionic liquid also showed a higher 5-HMF yield compared to when using the oil-bath method [20]. From those results, it can be suggested that microwave heating has several advantages including short reaction time, a high percentage of conversion, product yield, and selectivity.Microwave irradiation assistance also applies to green chemistry because the time and energy are lower. This is in line with the UN SDG goal number 7 which is access to affordable, reliable, sustainable, modern, and clean energy for all [14]. However, there are challenges to scaling up the reaction-assisted microwave, such as the reduced intensity (attenuation) of electromagnetic waves at small penetration depths of the most absorbent solvents [21]. To overcome this limitation, the combination of microwave irradiation with ultrasound waves can help produce an increased depth of penetration beyond intensifying mixing and mass transfer [21,22].Biomass conversion using catalysts can also play an important role in supporting the UN's SDGs. Biomass conversion can produce carbon-neutral and even carbon-negative energy [23–25]. This is in line with UN SDG number 12, ensuring sustainable consumption and production patterns [26].The aim of this comparative study is to carry out the biomass conversion to LA and 5-HMF using a household microwave as a heating method and compare it to the reaction using the conventional yet established thermal heating method using an oil bath. The catalyst used was hierarchical ZSM-5 zeolite, impregnated with Mn3O4 which has been the chosen catalyst and used to great extent in our previous work [6,7], and the composition in the reaction mixture also followed a similar reported procedure. To determine the optimal performance of the microwave-assisted catalytic conversion reaction of biomass to LA, microwave power and reaction time were varied. The results then are compared to those obtained using conventional heating methods. Furthermore, the catalyst was reused for three cycles to confirm its reusability.The substrate materials in this conversion are delignified cellulose, from lignocellulosic biomass of rice husks, cellobiose, and glucose. Recently, there has been much interest in using cellobiose as a cellulose model compound [27,28]. Cellobiose is the most thermodynamically stable subunit of crystalline cellulose [29]. Cellobiose is a bridge between monosaccharides and cellulose, which contains two D-glucose units linked by a β-(1,4)-glycosidic bond [29]. Therefore, cellobiose has a conversion pathway similar yet shorter to cellulose, so it is very interesting to use it to develop efficient and effective catalytic systems of lignocellulosic biomass. 2. Materials and methods 1.1. Materials Delignified cellulose (24.07% cellulose, 21.21% lignin, and 33.33% holocellulose) was prepared from local Indonesian rice husks following the previous method [7], and the result is available in SI. 1, Table S1. The chemicals used in this experiment were all of the analytical grades: phosphoric acid (89.0 wt %), hydrogen peroxide (30.0 wt %), sulfuric acid (96.0 wt %), ethanol (95.0 wt %), sodium hydroxide (99.0 wt %), manganese chloride (99.0 wt%), and glucose were obtained from Merck (Darmstadt, Germany). Cellobiose was obtained from TCI (Tokyo, Japan) and used directly without purification. Meanwhile, tetraethyl orthosilicate (TEOS, 98.0%), sodium aluminate (99.0%), tetrapropylammonium hydroxide (TPAOH, 1.0 M), and polyacrylamide-co-diallyl dimethylammonium chloride (PDDAM, 10 wt %) were obtained from Sigma Aldrich (St. MO, USA). All chemicals were used without further purification or treatment. 1.2. Preparation of Hierarchical Mn3O4/ZSM-5 Hierarchical ZSM-5 zeolite synthesis was carried out following the procedure reported by Krisnandi et al. [6]. The mixture was prepared following the molar ratio of previous studies with a composition of 0.29 g NaAlO2, 27.15 g 98% tetraethyl orthosilicate, 25.94 g TPAOH 40%, 111.83 mL H2O, and 1.0 g PDDAM. All mixtures were put into a teflon-coated stainless steel autoclave with volume c.a.250 ml. The autoclave was then put into the oven at 170 oC for 144 h and after that, the precipitate was filtered to obtain ZSM-5 hierarchical white powder.The hierarchical Mn3O4/ZSM-5 catalyst was prepared using the wet impregnation method by spraying Mn(II) from 2 mL of 0.214 M MnCl2.4H2O solution onto 1.0 g of the hierarchical ZSM-5 to obtain 2 wt % of Mn. The mixture was stirred to form a paste, dried, and then calcined at 550 °C for 8 h to obtain hierarchical Mn3O4/ZSM-5. 1.3. Characterization of the catalysts The as-prepared hierarchical ZSM-5 and Mn3O4/ZSM-5 were characterized using several techniques. The Powder X-ray diffraction (XRD) patterns were investigated on PANanalytical: X’Pert Pro 2318 diffractometer using Cu-Kα radiation (λ =1.54184 Å) as the incident beam with 2θ ranging from 5 to 50°. The Si/Al ratios were obtained by X-ray Fluorescence in XRD Orbis EDAX, 100 kV, 40 mA, and 100 scans. Analysis of functional groups on zeolites was carried out using an Alpha-Bruker FTIR spectrometer, by measuring the KBr-pellet of the sample (20:1) with 128 scans at 4000-400 range of wavenumber. Surface area analysis was performed in Quantachrom-Evo Surface Area and Pore Analyzer instrumentation, in which the pore size distribution was determined using Barrett-Joyner-Halenda (BJH) desorption curve and Horvath Kawazoe (H-K) plot methods. The morphology and mesopores information of the catalysts was analyzed on Jeol JIB-4610F Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS), while the Mn content was seen using the Atomic Absorption Spectrometer (AAS) Shimadzu AA-625-0. 1.4. Conversion of biomass to LA Conventional heating method [6,7] . The conversion of the substrate was carried out by adding 0.5 g substrate (delignified rice husk, cellobiose, or glucose), 0.05 g of hierarchical Mn3O4/ZSM-5 catalyst, 10 ml H3PO4 (40%, v/v), and 2 drops of H2O2 (30%, v/v) into a 20 mL-vial glass which then was immersed into oil-bath at 130 °C. After 2, 4, 6, and 8 h reaction time, the sample was immediately cooled in an ice bath to quench the reaction, then the filtrate was separated from the solid for product analysis. Microwave-assisted method. For conversions using a household microwave (Samsung ME731K), the same mixture was placed into a 50-mL porcelain crucible and heated using 300 W, 450 W, and 600 W of power for 60 s. Prior to the reaction, the microwave heat was calibrated (the data is available in the SI.2, Table S2). Then, further reactions were performed at 30 s and 180 s reaction times, with the power that gave the best results. As a control, a reaction without a catalyst was also carried out.To evaluate the reusability and stability of the Mn3O4/ZSM-5 catalyst, the conversion of glucose as the model was carried out by adding 0.5 g glucose, 0.05 g of the catalyst, 10 ml H3PO4 (40%, v/v), and 2 drops of H2O2 (30%, v/v) into a 50-mL porcelain crucible using a microwave-assisted method and heated using 600 W of power for 180 s. The used Mn3O4/ZSM-5 catalyst was collected by filtration, washed thoroughly with deionized water to attain neutral pH, and dried at 75 °C for 30 minutes. The dried catalyst sample was then calcined at 550 °C for 3 hours. The recycled catalyst was reused according to the conversion experiment mentioned above until three times cycles. The products were analyzed using HPLC to determine the yield of LA. The used catalyst was characterized with FTIR, XRD, and SEM-EDS after four times use.Product analysis from the conversion reaction was carried out using High-Performance Liquid Chromatography (HPLC) at Welizer LC200 PG Instrument, and Nuclear Magnetic Resonance (NMR) at Agilent 500 MHz NMR spectrometer with DD2 console system. HPLC measurements were carried out to see the results of conversion products. Analysis was performed using HPLC Welizer LC200 Quarantine Isocratic Pump with UV detector. The column used is ODS with 0.1% HClO4 as eluent. The final eluent rate was adjusted to 1.0 mL/min at 40˚C. The wavelength used is 285 nm to identify compounds such as LA and 5-HMF with a retention time of 15 min. The conversion and yield of products were calculated using the equation (1) and (2). Details on the calculation to determine % conversion and % yield are available in the SI.3. (1) % c o n v e r s i o n = i n i t i a l s u b s t r a t e m a s s g - r e s i d u a l m a s s g i n i t i a l s u b s t r a t e m a s s g x 100 % (2) % y i e l d L A o r 5 - H M F = t o t a l c o n c e n t r a t i o n mg L L A o r 5 - H M F X s o l u t i o n v o l u m e ( L ) substratemass ( m g ) x 100 % NMR analysis. Prior to the NMR test, the organic phase of the product was extracted from the aqueous phase using 8 mL of ethyl acetate [30]. The separated organic phase was then added with MgSO4 to remove the remaining water [30]. Furthermore, the ethyl acetate was evaporated from the organic phase using a rotary evaporator at a temperature of 60 oC for 10 minutes. The result is a brown viscous liquid, this liquid is then analyzed using 1H-NMR and 13C-NMR. NMR characterization used Agilent 500 MHz NMR for 1H and 125 MHz for 13C with DD2 console system and D2O solvent. 3. Result and discussion In this work, a hierarchical ZSM-5 zeolite was synthesized and then modified with Mn3O4 2.14 wt% to reconstruct the best catalyst that was used in the conversion of delignified cellulose to LA [7]. For brevity, the characterization results from FT-IR analysis are available in SI. 4, Fig S2, while others are discussed below. XRD. Fig. 1 shows the characteristic powder XRD pattern of the hierarchical ZSM-5 catalyst that is similar to the previous results [13] and having high crystallinity [6]. In addition, the existence of Mn3O4 in the ZSM-5 catalysts was shown by a peak in 30–35o and JCPDS: 24-0734 [31]. The structure of the catalyst did not significantly alter after being added with Mn3O4. SEM. Fig. 1 also shows that the as-synthesized hierarchical ZSM-5 forms a coffin-like shape, a typical MFI morphology [6], and after the impregnation process the shape is unchanged, which indicates that the impregnation does not damage the structure. SAA. Analysis of the surface area and pore size of the catalyst was presented in Fig. 2 . The adsorption-desorption isotherm curves (Fig. 2 (a)) contain hysteresis loops in the range 0.4-0.9 P/Po, indicating the presence of mesoporosity, although the size pore distribution, calculated from BJH desorption curve (Fig. 2 (b)) and H-K plot (Fig. 2 (c)) show that the predominant pores are micropores that have an average pore diameter of 1.84 nm. However, it was also seen in Fig. 2 (b), that there was a mesopore distribution present at an average of 2.7 nm. Therefore, this as-synthesized ZSM-5 is labeled a hierarchical ZSM-5 because there are two types of pores that are micro (d < 2 nm) and meso (2 nm < d < 50 nm) in one structure [32]. Table 2 summarizes the results of SAA of both hierarchical ZSM-5 and Mn3O4/ZSM-5.Conversion of biomass to LA has been thoroughly studied [6,7,11]. This conversion reaction uses phosphoric acid solvent (H3PO4 40% (v/v)) which serves to break the inter- and intramolecular hydrogen bonds belonging to cellulose [11]. In addition, 30% H2O2 solvent is also added that will react with the Mn2+ and Mn3+ from Mn3O4/ZSM-5 through a Fenton-like reaction as shown by equations (3) and (4) as reported by Pratama et al. [7].Mn2+ + H2O2 → Mn3+ + HO• + OH−(3)Mn3+ + H2O2 → Mn2+ + HO2• + H+(4)The reaction in equation (3) will produce hydroxyl radicals to break the β-(1,4)-glycosidic bonds of cellulose and cellobiose so that they can be degraded into glucose [11]. The isomerized glucose will then be converted into LA and by-products such as formic acid with an intermediate compound in the form of 5-HMF [33]. General reaction scheme of the conversion of the cellulosic compounds to 5-HMF and Levulinic acid is depicted in Figure 3 , while the detailed proposed mechanism available in SI.5. Conventional heated reaction. Reaction conversion was carried out with the conventional method, equipped with an oil bath as a heating source. The results are displayed in Fig. 4 , the current experiment was tested using delignified cellulose [6,7], glucose, and cellobiose as the substrate, and the results were obtained from measurements using HPLC and gave the same trend as shown in Fig. 4. From the graph we can see that for all substrates the longer the reaction time, the percentage of conversion tends to be higher. There was a slight decrease during the 8-h reaction although it was not significant because of the decrease in catalyst activity after the reaction lasted for 8 h. This result is consistent with the results reported in the previous studies [6,7], that using hierarchical Mn3O4/ZSM-5 as catalysts, it took quite some time to reach a high yield of LA. During the reaction time, the yield of 5-HMF is decreased simultaneously, following the increase of the yield of LA, because 5-HMF is an intermediate product that will be converted to LA (illustration in Fig. 3). Microwave-assisted reaction. With the purpose of the reaction time, biomass conversion was carried out by using a household microwave. First, to find out the most optimal power required, the power used was varied at 300, 450, and 600 W, and the conversion was carried out for 60 s, using the three substrates, and the results are shown in Fig 5 . When reaction control using a reaction mixture without Mn3O4/ZSM-5 catalyst was carried out, thick black charcoal was produced so it is difficult to identify and quantify (for brevity the results are not shown). This confirms that the reaction using the microwave also requires a heterogeneous catalyst. Fig. 5 (a) shows an increasing trend of % conversion along with the increment of power used. The best percentage yield is obtained when 600 W of power is applied. Therefore, in the next experiments, the power used was set to 600 W. Second, the reaction time was set to 30 s and 180 s (Fig. 5 (b)). It can be seen that the conversion continues to increase with a longer reaction time. However, the conversion for 240 and 300 s resulted in a charred solution so that the LA yield could not be determined. This is because household microwaves have limitations, there is no temperature regulation as well as stirring.The highest yield for LA was obtained when using a 600 W microwave for 180 s. Glucose gave the best percentage yield of 9.57%, followed by cellobiose at 6.12% and delignified cellulose at 4.33%. This shows that the longer the reaction time used, the more LA is produced. Interestingly, the percentage yield of the 5-HMF intermediate was rather low given by the three substrates (for brevity, the results are available in SI.3, Table S6). Glucose has the highest percentage yield because it has a shorter pathway for conversion to 5-HMF and LA. Meanwhile, delignified cellulose and cellobiose must face the β-(1,4)-glycosidic bond-breaking step first, as shown in Fig 3. In addition, delignified cellulose still contains some retained lignin which may inhibit the conversion process so delignified cellulose has the lowest percentage yield of LA, compared to cellobiose and glucose.Further look at the results, it shows that the percentage yield of LA from biomass conversion using the microwave-assisted reaction in 180 s, 600 W (Fig 5.(b)) was comparable with those obtained by using the conventional heating method at 130 oC for 4 h. This shows that the microwave method has the potential to convert in a short time so it is more efficient to use compared to conventional methods. The HPLC chromatograms of the microwave-assisted reaction in 180 s and the conventional heating at 130 oC for 4 h were compared (available in SI.6, Fig. S6), and it can be seen that, besides LA and 5-HMF, there are peaks that correspond to by-products such as formic acid [34]. However, the intensity of the peak from the microwave method is weaker compared to that of conventional methods. Furthermore, the 1H and 13C NMR measurements on the isolated LA also show that the product obtained with the microwave assisted method has higher purity than that of the conventional heating method. The analysis by NMR (SI 6., Fig. S7, S8) showed the presence of by-products in the form of acetic acid and formic acid [35,36] . The intensity of by-product converted using a microwave was lower than the conversion using conventional methods.As depicted in Fig. 6 (a), the glucose conversion was still maintained at more than 45% after 4 times of use (3 cycles). Furthermore, the weight loss of the catalyst is illustrated in Fig. 4 (a), which was around thirty percent at the first use until after the fourth use. In addition, for the first cycle, the yield of LA as a product was 10.57% and decreased gradually to 7.00% after the 2rd cycle then tends to stay on the 3th cycle. The results confirmed that the catalyst had good reusability. The above analysis results show that Mn3O4/ZSM-5 has sufficient stability in the conversion of glucose to LA.To find out the properties of the reused catalyst, some characterization was carried out, i.e FTIR, powder XRD and SEM-EDS. As shown in Fig. 6 (b), the FTIR spectra of Mn3O4/ZSM-5 before and after use,. the intensity of the absorption band at 3500 cm-1, which is the absorption band of the silanol group (Si-OH) on the zeolite surface, was increased, after the catalyst was used and calcined several times. Furthermore, the spectra at 700-1250 cm-1 that was attributed to Si-O and Al-O vibrations of the aluminosilicate framework [38] remain. This suggests that the reuse of Mn3O4/ZSM-5 catalyst did not cause any damage to the zeolite framework. This was also supported by the PXRD pattern of Mn3O4/ZSM-5 (Fig. 6 (c)) after four times of use. The catalysts still had sharp PXRD diffraction peaks, and no new peaks appeared in contrast to the fresh catalysts, with the characteristics of the Mn peak remaining at the same intensity and position with no shifting compared to the fresh catalyst. It confirmed the structural stability of Mn3O4/ZSM-5 under catalytic reaction in the microwave-assisted method. The SEM images of the used catalysts in Fig. 6 (d) shows the after used catalysts which is similar to the Mn3O4/ZSM-5 catalyst before reaction. Furthermore, the Si/Al ratio of the used catalyst decreases to 28.4 and the Mn content decreases to 1.6%. This provided evidence that Mn (II) was removed from the Mn3O4/ZSM-5 structure during the reaction. This procedure suggests that the interaction of solid Mn3O4 as the active site in ZSM-5 and aqueous Mn2+ ions in the solution with H2O2 to create HO• radicals in a Fenton-like system took place in conversion using Mn3O4/ZSM-5 zeolites as a catalyst. This indicates that the reusability test of the catalyst in the microwave system does not damage the structure, however the active species of the catalyst such as Mn content and the Si/Al ratio on zeolite surface were calculated decrease because it has been used 4 times cycles reaction. The last cycle reaction shows results that are comparable with the 4 h reaction using the conventional heating methods in our previous work [6] identified that the EDX characterization for after used catalyst was (Si 12.28%, Al 0.11%, Mn 0.69%).This catalyst reusability test under the microwave-assisted system shows its excellence as confirmed by the overall conversion results, and its selectivity to LA (proof by HPLC and NMR measurement). In microwave synthesis, it has also been observed that the relatively lengthy heating and times result in an equivalent reaction temperature than attained for oil bath or conventional heating [37]. In addition, the stability of the catalyst under optimal conditions in this system is remarkable compared to that of the catalysts used in conventional heating systems as reported in our previous studies [6], in which the catalyst has undergone structural changes since the reaction started at 100 oC at 0 h, and removal of the active sites, Mn3O4, since 3 h reaction time. 4. Conclusion The hierarchical Mn3O4/ZSM-5 catalyst was successfully synthesized and used in the conversion of biomass to LA. It is confirmed that the conventional heating method at 130 oC for 8 h reaction time gives the highest conversion and LA yield. On the other hand, the reaction with a 600 W microwave for 180 s shows results that are comparable with the 4 h reaction using the conventional heating methods, with better purity of LA. In addition, the used catalyst after reactivation could be applied for 3 cycles of reaction without losing too much of its activity. This shows that the conversion using a microwave has the potential to be explored in the future to achieve cleaner reaction conditions in a very short reaction time.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was funded by BOPTN Research Fund number NKB-322/UN2.RST/HKP.05.00/2021 from the Ministry of Research and Technology Indonesia.

Conversion of delignified cellulose from rice husk biomass, and model compounds of cellobiose and glucose to levulinic acid (LA) over hierarchical Mn3O4/ZSM-5 catalyst was carried out using a household microwave method, and then compared to the established conventional thermos-reaction method. The hierarchical ZSM-5 was prepared using a double template method, aiming for micro and mesoporous systems developed in the structure. The as-prepared ZSM-5 were modified with Mn3O4 through incipient wetness impregnation with Mn2+ solution followed by calcination at 550 oC. The catalysts were characterized using various techniques such as powder XRD, SEM, BET, AAS, and FT-IR which indicated the hierarchical structure of MFI zeolite (Si/Al of 30-34) with Mn loading of 2.14 wt%. The conversion products were analyzed using HPLC, 1H-NMR, and 13C-NMR instruments. The microwave-assisted reaction using 600 W for 180 s using delignified cellulose, cellobiose, and glucose gave conversion of 37.27%, 46.35%, and 54.29%, respectively which is close to the conversion given by the conventional reaction carried out at 130 oC for 4 h (36.75%, 55.62%, and 60.9%, respectively). Interestingly, the LA yield from the microwave-assisted reaction (4.33 %, 6.12 %, and 9.57 %) is higher than the yield from the conventional reaction, which only produced 5.2%, 4.88%, and 6.93% respectively. The microwave-assisted method is also shown to give less by-products compared to the thermochemical reaction. Therefore, it could be considered an alternative method for converting cellulose to LA.

With the increasing energy consumption and the challenge of the global climate problem, it is urgent to develop novel and efficient energy conversion and storage technologies [1–3]. In recent years, a lot of electrochemical technologies, which convert sustainable electric energy into chemical energy, have been developed. The typical electrochemical systems include water splitting [4–6], carbon dioxide reduction (CO2R) [7,8], nitrate reduction reaction [9,10], fuel cells [11], and metal-air batteries [12,13]. Oxygen evolution reaction (OER) is a key half-reaction in these technologies [14,15]. However, OER is a complex four-proton-coupled electron transfer process, and a large overpotential is required due to its sluggish kinetics [16–19]. Therefore, it is of great significance to develop an efficient catalyst with high durability and catalytic activity to reduce the overpotential of OER. Currently, commercial OER catalysts are Ir/Ru-based compounds, but the scarcity and high cost limit their industrial applications [4,20].Earth-abundant transition metals and their compounds show great potential as OER catalysts due to their decent performance and low cost [21–25]. Among them, Ni-based catalysts have attracted extensive attention [26,27]. However, these catalysts are still far from commercial applications due to their unsatisfied activity and poor stability. High-valent Ni compounds show much better OER performance, which are considered to be the active phase [28–32]. Recently, doping strategies have been used to promote the formation of high-valent Ni sites [30,33–38]. High-valence metallic dopants, such as Mo and V, do not exhibit satisfactory OER activity but can induce electronic effect to improve the catalytic activity of Ni-based catalysts [39]. Besides, Fe dopants have been widely used as a critical role in enhancing the OER activity of Ni-based catalysts [40]. Therefore, the doping strategy is a promising way to develop highly efficient OER catalysts. However, due to the complexity of the chemical structure, designing catalysts with multiple metallic dopants is still a great challenge.Based on Fe doped Ni catalysts (Fe–Ni nanoparticles), we introduce multiple metallic dopants using a simple oil phase strategy in this study. The Fe–Ni nanoparticles, with the other 3 dopants of Mn, Mo, and V, show an overpotential of 220 ​mV at 10 ​mA ​cm−2 and a 200 ​h long-term electrochemical stability. Experimental results show that Mo and V dopants introduce high-valence Ni sites and promote the formation of NiOOH at a lower potential to enhance their OER performance. Meanwhile, Mn dopants increase the electrochemical active surface area (ECSA) and exposed active sites, further boosting the OER activity.Iron (III) acetylacetonate (Fe(acac)3, 97%) and (1-Hexadecyl) trimethylammonium chloride (CTAC, 96%) were purchased from Shanghai Aladdin Reagent Co., Ltd. Nafion solution (5 ​wt%) was supplied by Sigma-Aldrich. Molybdenum hexacarbonyl (Mo(CO)6, 98%), manganese(II) acetylacetonate (Mn(acac)3, 97%), vanadium(IV) oxy acetylacetonate (VO(acac)2, 95%), oleylamine (OAm, >70%), molybdenyl acetylacetonate (MoO2(acac)2, 95%), ruthenium oxide (RuO2, 99.95%) and glucose were bought from Heowns Biochem Technologies, LLC, Tianjin. Nickel (II) acetylacetonate hydrate (Ni(acac)2·2H2O, 95%) was supplied by Meryer (Shanghai) Chemical Technology Co., Ltd. Other reagents were purchased from Tianjin Yuanli Chemical Co., Ltd.CTAC (0.5 ​g) was added into oleylamine (50 ​mL) in a 100 ​mL three-necked flask. After sonication for 30 ​min, Fe(acac)3 (10 ​mg), Ni(acac)2·2H2O (6.4 ​mg), Mn(acac)3 (8.9 ​mg), MoO2(acac)2 (8.8 ​mg), VO(acac)2 (6.5 ​mg), glucose (0.6 ​g), and Mo(CO)6 (165 ​mg) were added into the three-necked flask. And the mixture was sonicated for 30 ​min and heated to 60 ​°C to obtain a homogeneous solution. The solution was heated to 220 ​°C and then kept for 2 ​h under magnetic stirring. The products were collected by centrifugation, then washed three times with an ethanol/cyclohexane mixture, and twice with 0.5 ​mol/L acetic acids (ethanol solution). Finally, the products were dried in the vacuum oven overnight. The other multiple metal doped nickel nanoparticles were synthesized by the same method.The catalyst ink was prepared by mixing 10 ​mg prepared products, 5 ​mg carbon black, 500 ​μL isopropanol, 500 ​μL Milli-Q ultrapure water, and 20 ​μL 5% Nafion solution for at least 30 ​min. The homogeneous ink was dropped on a nickel foam and dried at room temperature to form a 0.25 ​cm2 effective catalytic area. The catalyst loading was about 8 ​mg/cm2.The morphologies and elemental distribution of the prepared samples were characterized by field emission scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEM-2100F) equipped with an energy-dispersive spectroscopy (EDS) detector. The crystalline structure of the catalysts was investigated by the powder X-ray diffractometer (XRD, Bruker D8) with Cu Kα radiation. X-ray photoelectron spectrometer (XPS, Thermo ESCALAB 250XI) was used to analyze the surface elemental composition and chemical states.All electrochemical tests were carried out at room temperature with a standard three-electrode system using an electrochemical workstation (Autolab PGSTAT302N). The nickel foam with catalyst, Ag/AgCl electrode with saturated KCl as the filling solution, and platinum foil were used as the working electrode, reference electrode, and counter electrode, respectively. The electrolyte was 1 ​M KOH solution (pH ​= ​14). All applied potentials against the Ag/AgCl reference were calibrated to the reversible hydrogen electrode (RHE) through the equation: E R H E = E A g / A g C l + 0.197 + 0.0591 × p H Linear sweep voltammetry (LSV) was performed in the potential range from 1.0 to 1.8 ​V (vs RHE) at a scan rate of 1 ​mV ​s−1 to evaluate the OER activity. Before LSV measurement, 20 cycles of cyclic voltammetry (CV) scans were carried out in the potential range from 1 to 1.8 ​V (vs RHE) at a sweep rate of 20 ​mV ​s−1. Tafel slopes were derived from the linear region of the LSV polarization curves. To correct the i-R drop and analyze the electrode kinetics during the reaction, electrochemical impedance spectroscopy (EIS) measurement was performed in frequencies ranging from 0.1 ​Hz to 100 ​kHz with an AC amplitude of 5 ​mV. The electrochemical double-layer capacitance (C dl) was carried out to determine ECSA. CV scans were carried out in a non-Faradic potential region at various scan rates ranging from 20 to 120 ​mV ​s−1 at a potential window of 0.92–1.02 ​V (vs RHE). The long-term durability of the catalysts was evaluated by a chronopotentiometry experiment at 1 ​A ​cm−2. The cyclic stability was conducted by 2000 cycles of CV measurements in the potential range from 1.0 to 1.8 ​V (vs RHE). All the potentials were corrected by 85% iR correction.The FeMnMoV–Ni catalysts were synthesized via a simple oil phase strategy, as shown in Fig. 1 a. Fe(acac)3, Ni(acac)2, Mn(acac)2, MoO2(acac)2, and VO(acac)3 were dissolved into oleylamine. At 220 ​°C, all metal precursors were reduced. Various catalysts, such as FeMoV–Ni, FeMnV–Ni, FeMnMo–Ni, and Fe–Ni, were synthesized by the same method. The XRD patterns of synthesized catalysts are shown in Fig. 1b. The peaks at 44.5°, 51.9°, and 66.5° are related to Ni (111), (200), and (220) planes (PDF#89–7128) [41,42]. The high-resolution transmission electron microscopy (HRTEM) image shows lattice fringes with an interplanar distance of 0.208 ​nm related to the Ni (111) plane. The selected area electron diffraction (SAED) pattern (Fig. S1) of the catalyst displays (111), (002), (022), (222), and (133) diffraction rings of Ni, which suggests that the main phase of the catalyst is Ni phase. As shown in Fig. 1c and Fig. S2, TEM images and SEM images illustrate that the obtained products are irregular particles with an average size of ∼105 ​nm. The EDS elemental mapping images (Fig. 1e, S3) show that Ni, Fe, Mn, Mo, and V are uniformly distributed in the products, which indicates that the catalyst was multiple metal doped nickel nanoparticles.The OER performance of multiple metal doped nickel nanoparticles was evaluated by CV and LSV measurements on a typical three-electrode configuration in 1 ​M KOH solution. The LSV polarization curves for all samples are shown in Fig. 2 a, and Fig. 2b displays the overpotentials of all catalysts. The FeMnMoV–Ni catalyst exhibits the best OER performance with an overpotential of 220 ​mV at 10 ​mA ​cm−2 current density (η 10 ​= ​220 ​mV), which is superior to that of FeMoV–Ni (η 10 ​= ​240 ​mV), RuO2 (η 10 ​= ​251 ​mV), FeMnV–Ni (η 10 ​= ​287 ​mV), FeMnMo–Ni (η 10 ​= ​295 ​mV), and FeNi (η 10 ​= ​363 ​mV). Meanwhile, the Tafel slope (Fig. 2c) of FeMnMoV–Ni is 50.9 mV/dec, which is also lower than that of FeMoV–Ni (59.2 mV/dec), RuO2 (83.2 mV/dec), FeMnV–Ni (87.6 mV/dec), FeMnMo–Ni (88.1 mV/dec), and Fe–Ni (106.6 mV/dec). These results indicate that the FeMnMoV–Ni exhibits faster kinetics and excellent catalytic activity for OER. EIS measurements were performed to evaluate the electrode kinetics for all catalysts during the OER process [43], and the results are shown in Fig. 2d. The semicircles in the EIS spectra represent the charge-transfer resistance (R ct) at the electrode/electrolyte interfaces. The R ct of FeMnMoV–Ni (4.7 ​Ω) is lower than that of the other samples, demonstrating a rapid charge transfer during OER. Moreover, ECSA was assessed to evaluate the intrinsic catalytic activities of all catalysts. As shown in Fig. S4, the C dl value of the FeMnMoV–Ni catalyst is 8.8 ​mF ​cm−2, which is the highest among all samples. The results indicate that Mn, Mo, and V dopants increase the ECSA and then promote OER performance. In addition, the LSV curves normalized by ECSA are presented in Fig. S5, which represent the intrinsic OER activity of catalysts. It shows that FeMnMoV–Ni and FeMoV–Ni exhibit similar current densities, suggesting that Mn dopants do not promote the intrinsic OER activity. And FeMnV–Ni and FeMnMo–Ni also show similar standardized current density at the same overpotential. Compared with FeMnV–Ni and FeMnMo–Ni, FeMnMoV–Ni exhibits a lower overpotential at the normalized current density of 0.2 ​mA ​cm−2, indicating that the Mo and V dopants increase the intrinsic OER activity. According to the ECSA results, Mn dopants only increase the ECSA, while Mo and V increased both the ECSA as well as the intrinsic OER activity.High stability is important for practical applications. Fig. 2e exhibits the LSV polarization curves before and after 2000 cycles of CV measurements with a scan rate of 50 ​mV ​s−1. There is no obvious difference between these two curves, indicating the superior durability of the FeMnMoV–Ni catalyst. Moreover, multiple current steps for the chronopotentiometry test are used to evaluate the stability of the catalyst. From Fig. S6, the current density increased from 5 to 50 ​mA ​cm−2 with a step of 5 ​mA ​cm−2 per 500 ​s. Furthermore, a chronopotentiometry measurement was performed to evaluate electrocatalytic stability. As shown in Fig. 2f, a long-term test was employed at 1 ​A ​cm−2 over 250 ​h, and there is no significant increase in voltage, suggesting excellent electrocatalytic stability for the FeMnMoV–Ni catalyst. Table 1 presents a comparison with previous reported transition metal doped catalysts, demonstrating the remarkable OER performance of the FeMnMoV–Ni catalyst.We performed in situ Raman to identify the active site in the catalyst and confirm the promotion of different elemental dopants for the formation of active species. As shown in Fig. 3 , the in situ Raman spectra were recorded at applied potentials ranging from 1.1 V to 1.6 ​V vs RHE. At low applied potential, there are no obvious Raman peaks. With the increase of applied potential, two peaks located at ∼476 ​cm−1 and 555 ​cm−1, which are attributed to the Ni–O bond vibration in NiOOH [48,50,51], appeared and strengthened. Raman spectra indicate that the Ni phase transformed into NiOOH during the OER process. Noted that no other MOOH peaks appear, NiOOH is expected to be the real active site. The characteristic peaks of NiOOH first appear in the FeMnMoV–Ni and FeMoV–Ni at 1.3 ​V vs RHE and further strengthen with the increasing potential. The peaks of the FeMnMoV–Ni catalyst are significantly stronger than that of the FeMoV–Ni, indicating that Mn dopants facilitate the formation of NiOOH species. In comparison, the peaks of NiOOH appear until the potential reaches 1.4 ​V and 1.5 ​V in FeMnMo–Ni and FeMnV–Ni samples, respectively, indicating that the Mo dopants and V dopants promote the formation of NiOOH. EDS results show that all elements still distribute homogeneously in the sample after the OER performance test (Fig. S7 and Fig. S8).To understand the effect of the synergistic interaction of different elements of the catalyst on the electronic structure, the XPS tests were performed on the different catalysts after the OER performance test, as shown in Fig. 4 . The survey XPS spectra shown in Fig. S9 display the presence of Ni, Fe, Mn, Mo, V, and O elements, which are consistent with the EDS results (Fig. S7 and Fig. S8). Fig. 4a shows the high-resolution XPS spectra of Ni 2p for the different catalysts after OER. Compared with FeMnV–Ni and FeMnMo–Ni, the ratio of Ni3+/Ni2+ in FeMnMoV–Ni is much higher (Table S1). In comparison to FeMnMo–Ni and FeMnV–Ni, the ratio of Ni3+/Ni2+ in FeMnMoV–Ni increased significantly, suggesting that the Mo dopants and V dopants could promote the formation of Ni3+ sites. Fig. 4b and Table S2 show the XPS results of Fe 2p. The Fe 2p peak of the samples can be fitted to four peaks [52,53]. The XPS spectra of Mn, Mo, and V are shown in Fig. 4c–e and Tables S3–S5. The peaks at 642.1 and 653.3 ​eV belong to Mn4+, and the peaks at 232.8 ​eV and 235.9 ​eV are attributed to Mo6+ in different samples [54]. After the OER test, V is divided into two valence states, V5+ and V4+ [55,56]. The chemical states of Fe, Mn, Mo, and V are similar in different samples, indicating that there is no obvious interaction between Mo, V, Mn, and Fe dopants. The O 1s maps are shown in Fig. 4f, and the peaks located at ∼536 ​eV, ∼533 ​eV, and ∼531 ​eV are attributed to adsorbed H2O, M ​− ​OH, and M ​− ​O, respectively [57]. It is noteworthy that the M ​− ​O signal related to NiOOH increases visibly in FeMnMoV–Ni and FeMoV–Ni catalysts, revealing that Mo and V dopants favor the formation of NiOOH [31,57,58].Here, we designed multiple metal doped nickel nanoparticles via a simple oil phase strategy. The nanoparticles with an average size of ∼105 ​nm show exceptional OER performance and excellent stability. The FeMnMoV–Ni catalysts exhibit superior OER activity with a low overpotential of 220 ​mV at of 10 ​mA ​cm−2, a Tafel slope of 50.9 ​mV dec−1, and a 200 ​h stability at 1 ​A ​cm−2. Mn, Mo and V dopants expose more active sites to boost the OER. In situ Raman analysis indicate that multiple metallic dopants facilitate the formation of NiOOH. Furthermore, XPS analysis show that the Mo and V dopants facilitate the formation of high-valence active sites. This work provides a facile strategy to develop multiple metal doped catalysts and opens a new window for catalyst design and various applications.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Natural Science Foundation of China (NSFC No. 51771132 and 52204320).The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.pnsc.2023.03.002.

Developing efficient oxygen evolution reaction (OER) electrocatalysts is of great importance for sustainable energy conversion and storage. Ni-based catalysts have shown great potential as OER electrocatalysts, but their performance still needs to be improved. Herein, we report the multiple metal doped nickel nanoparticles synthesized via a simple oil phase strategy as efficient OER catalysts. The FeMnMoV–Ni exhibits superior OER performance with an overpotential of 220 ​mV at 10 ​mA ​cm−2 and a long-term stability of 250 ​h in 1 ​M KOH solution. In situ Raman analysis shows that the NiOOH site works as the active center and multiple metallic dopants facilitate the formation of NiOOH. Mo and V dopants promote the formation of high-valence state of Ni sites, and Mn dopants increase the electrochemical active surface area and expose more active sites. This work provides a novel strategy for catalyst design, which is critical for developing multiple metal doped catalysts.

In the past 170 years, human activities have been responsible for almost all of the rapid increase in CO2 emissions, which has been linked to global climate change and ocean acidification (Figure 1 A). 1 , 2 Strategies such as CO2 capture and catalytic conversion have been proposed to mitigate CO2 emissions. Among the latter strategy, thermocatalytic CO2 hydrogenation 3 , 4 provides a wide variety of products, such as CO, methane, methanol, olefins, ethers, and aromatics, whereas the CO2 mitigation efficiency depends on the source of H2; electrocatalytic CO2 reduction using water-derived protons is the subject of many recent studies due to its promise in the production of value-added chemicals, 5–7 such as syngas (CO and H2), formate, alkanes, olefins, and alcohols, although it is still far from competing with conventional processes. 8 Over the past 2 decades, the revolution in shale gas—which contains both carbon and hydrogen resources—has transformed the world's energy landscape. 15 After methane, ethane comprises the second-largest (3%–12%) hydrocarbon shale gas fraction, and the discovery of large global reserves of shale gas (ca. 16.1 quadrillion cubic feet) has boosted its supply significantly. 16 The ethane surplus far exceeds its demand in conventional petrochemical processes despite its low price, especially in the United States (Figure 1B). 10 , 17 Recently, the strategy of simultaneously upgrading this underutilized ethane from shale gas with the greenhouse gas CO2 has introduced opportunities for synthesizing value-added gaseous (syngas 18–21 and ethylene 18–20 , 22 , 23 ) and liquid (aromatics 24 , 25 and C3 oxygenates 26 ) chemicals. As shown in Figure 1C, these products are among the most essential feedstocks for the petrochemical, agricultural, cosmetic, and pharmaceutical industries all over the world. Global market reports forecast an increasing market value for these chemicals, primarily driven by the rapidly growing demand for synthetic chemicals, fertilizers, cosmetics, and pharmaceuticals.The key to the simultaneous upgrading of CO2 and ethane (SU-CO2Et) involves cleavage of the C=O bond in CO2 along with competitive cleavage of the C–C (dry re-forming) and C–H (oxidative dehydrogenation) bonds in ethane. 18 , 19 , 23 Such requirements pose challenges for catalyst materials engineering to develop not only controllable catalyst selectivity and activity for desired pathways but also high resistance to deactivation at elevated temperatures (e.g., due to metal sintering and coke deposition). However, catalysts that demonstrate these properties generally consist of precious platinum-group metals. Therefore, it is crucial to develop advanced catalysts using environmentally friendly and earth-abundant non-precious materials with improved activity and stability while lowering the utilization of expensive and scarce precious metals. 27 Bimetallic materials typically present electronic and chemical properties that are distinct from those of the corresponding parent metals due to ligand and strain effects, 28 providing opportunities for designing new catalytic materials with enhanced catalytic performance. 29 The terminology “bimetallic-derived catalyst” used herein is a more general concept that refers to catalysts containing two metallic elements supported on a substrate, even though one or both of them may be oxidized under catalytic conditions. Thus, the bimetallic-derived catalysts include not only conventional bimetallic structures, such as core-shell, segregated, ordered, or random homogeneous alloys, but also metal oxide (MO)/metal and MO/MO interfacial structures.Bimetallic-derived catalysts have garnered considerable academic and commercial interest since the early 1960s, and Sinfelt introduced the term “bimetallic clusters” to refer to highly dispersed bimetallic entities present on the surface of a support. 30 An understanding of structure-function relationships is required to guide the design of bimetallic-derived catalysts with desired catalytic performance. These relationships can reveal key factors or descriptors 31 , 32 that control the reaction pathways as well as the binding and activation energies of critical reaction intermediates. Density functional theory (DFT) calculations in combination with machine learning have been shown to help the discovery of promising bimetallic-derived catalysts from millions of candidates. 33–36 However, the identification of catalyst structure-function relationships for SU-CO2Et is commonly restricted by the structural complexity of the supported powder catalysts (e.g., reduction/oxidation properties, alloying/dealloying, compositional ordering, site isolation, interfacial interactions, and other dynamic restructuring) and further complicated by the effects of temperature, pressure, and reactant composition. Thus, the atomically precise synthesis of well-defined and uniform bimetallic structures, combined with in situ bulk- and surface-sensitive techniques, is crucial to unravel structure-function relationships and to guide the design of bimetallic-derived catalysts for SU-CO2Et.Given the promising properties and applications of bimetallic-derived catalysts, this review focuses on precious and non-precious bimetallic materials for SU-CO2Et into value-added products. Drawing from the recent literature, we illustrate how to identify, control, and synthesize distinct types of active sites, including selecting different bimetallic compositions and utilizing a variety of MO supports. First, we highlight the importance of thermodynamics and kinetics in understanding bimetallic materials synthesis, and we provide an overview of the state-of-the-art synthesis methods and in situ/ex situ characterization techniques for bimetallic-derived catalysts. We then focus on how to apply distinct types of active sites to upgrade CO2 and ethane into select value-added chemicals (syngas and ethylene), and we illustrate how to gain insight into the structure-function relationships by using combined experimental and theoretical studies. The review concludes by highlighting challenges and opportunities in order to stimulate future fundamental investigations and potential commercial applications of bimetallic-derived catalysts for upgrading abundant shale gas and CO2.In the following discussions, we establish benchmarks for the investigation of bimetallic-derived catalysts in a bottom-up manner. Even though some of the general principles and state-of-the-art synthesis methods have not yet been applied to the synthesis of catalysts for SU-CO2Et, they provide guidance on the synthesis of bimetallic-derived catalysts with desired structures.Understanding the thermodynamics and kinetics of nucleation, growth, and phase formation in bimetallic-derived catalysts provides fundamental guidance on targeting synthesis for tailored applications in catalytic reactions. According to the second law of thermodynamics, a negative value of the change in Gibbs (or Helmholtz) free energy is necessary for a process to be spontaneous. The chemical equilibrium condition (dG = 0) for materials synthesis at a specific temperature sets the thermodynamically stable structure of bimetallic-derived catalysts. Despite the importance of thermodynamics in circumscribing and predicting final structures, it is frequently ignored in favor of a trial-and-error or empirical materials synthesis process.Although the growth stage of a nanoparticle (NP) can be regulated via both thermodynamic and kinetic controls, the process of homogeneous nuclei formation can be understood thermodynamically by looking at the total Gibbs free energy change (ΔG). The ΔG of homogeneous nucleation (Figure 2 A) is defined as the sum of two opposite contributions, i.e., a bulk term (free-energy lowering) and a surface term (free-energy increasing) as shown in 1: 37 (Equation 1) Δ G = 4 π r 2 γ + 4 3 π r 3 ( − k B T ln ( S ) ν ) , where r is the radius of a spherical nucleus, γ is the free energy increase per unit surface area of the nucleus, ν is the molar volume of the nucleus, k B is the Boltzmann constant, T is the temperature, and S is the supersaturation of solution. As shown in Figure 2A, a critical radius (r∗) can be obtained when dΔG/dr = 0, which gives a critical free energy: 37 (Equation 2) Δ G Homo ∗ = 16 π γ 3 ν 2 3 ( k B T ln S ) 2 . The r∗ refers to the minimum size in a metastable equilibrium at which nuclei can continue to grow spontaneously without being redissolved. Accordingly, the ΔG∗Homo essentially represents the energy barrier that must be overcome for continuous growth to occur. Thus, the nucleation rate (J) can be written as follows: 37 (Equation 3) J = A ⋅ exp ( − Δ G Homo ∗ k B T ) = A ⋅ exp [ − 16 π γ 3 ν 2 3 k B 3 T 3 ( ln S ) 2 ] . If the nucleation occurs in the presence of heterogeneous structures such as seeds or supports (Figure 2A), then the energy barrier ΔG∗ for this heterogeneous nucleation would be decreased to: 38 (Equation 4) Δ G Hetero ∗ = ( 2 − 3 cos θ + cos 3 θ 4 ) Δ G Homo ∗ , where θ is the contact angle and correlates with the characteristics of the nucleus and support (tentatively ignoring the charge effect on the surface). Equations 2, 3, and 4 provide a basic outline of how to regulate cluster formation within solutions by carefully controlling the key factors: synthesis temperature, precursor concentration, surface free energy, and contact angle (for heterogeneous nucleation). As illustrated by Figure 2B, in contrast to facile nuclei formation at room temperature, the nucleation of reduced metal atoms at low temperatures (e.g., −60°C) can be inhibited owing to the retarded kinetics. Therefore, under controlled temperature, metal atoms can be highly (and even atomically) dispersed on supports. 39 , 45 , 46 Equations 2, 3, and 4 also indicate that both homogeneous and heterogeneous nucleation will occur simultaneously as the precursor(s) concentration exceeds the threshold of homogeneous nucleation, which in turn leads to non-uniform catalyst sizes, shapes, and morphologies. Thus, a straightforward way to achieve high and uniform dispersion on the support is to keep the precursor(s) concentration between the homogeneous and the heterogeneous thresholds. As illustrated in Figure 2C, the homogeneous nucleation can be minimized by precisely controlling the injection rate in order to maintain the upper concentration limit (C up) below the critical supersaturation for the homogeneous nucleation. 38 , 47 Surface free energy γ also affects the preferred growth of crystallites or facets, and it can be regulated with different capping agents (e.g., inorganic ions, organic polymers, and biomolecules) or surface stabilizers. 48 Moreover, the selective chemisorption of the capping agent on a specific type of facet can act as a physical barrier, imposing additional kinetic control over the deposition of metal atoms on this facet.In practice, the surface of supports (such as those in Figure 2D) is typically positively or negatively charged, which dictates the electrostatic interaction of the precursor cations or anions with the support. A thermodynamic model accounting for the support surface charge effect was developed by Yu and colleagues to elucidate the cluster evolution on supports. 49 As shown in Equation 5, the total free energy of the supported cluster can be expressed as follows: (Equation 5) Δ G = Δ H N P + Δ H N P / sup − T Δ S M , where ΔH NP and ΔH NP/sup are the enthalpy changes due to the metal dispersion and the creation of the metal-support interface, respectively, and ΔS M is the entropy change due to the mixing of metal clusters and their adsorption on the surface. Among these parameters, ΔH NP/sup is the most important item to describe the metal-support interface and can be further represented by a key parameter, γ ss , which denotes the interfacial energy with the support. As indicated in Equation 6, 49 γ ss reflects the dependence of interfacial energy on the electrostatic interactions of NPs with supports, which is in turn related to the point of zero charge (PZC) of the support and the pH of solution: (Equation 6) γ s s = γ s s 0 + γ s s e l e c t r o s t a t i c = γ s s 0 + γ s s 0 λ ( p H − P Z C P Z C ) = γ s s 0 ( 1 + λ Δ z ) . As illustrated in Figure 2E, the surface of supports can be positively (protonated) or negatively (deprotonated) charged by carefully tuning the pH of solution to below or above the PZC value, respectively. As a result, the oppositely charged metal precursor ions can be anchored onto the support in a highly dispersed manner via the strong electrostatic adsorption (SEA). The PZC values of the most commonly used oxide supports are tabulated in Figure 2E.Likewise, the thermodynamically stable cluster size (r) for given precursors, supports (PZC), and pH values of a solution can also be estimated by taking dΔG/dr = 0 to obtain the following correlation: 49 (Equation 7) r 3 + 3 ( γ s s 0 − β R T ) γ s s 0 π λ Δ z r + 2 4 α γ Ω γ s s 0 π λ Δ z = 0 , where β is a dimensionless factor that depends on the ratio of the surface concentration of clusters to that of adsorption sites, Ω denotes the atomic volume of the bulk metal, and λ is the independent proportionality constant of the cluster size. This relation can be used to predict the final stable NP cluster size for the given conditions.As illustrated in Figure 3 , multiple configurations (e.g., core-shell, segregated, ordered or random alloys, and MO/metal and M(O)/support interfaces) might be generated in bimetallic-derived catalysts. Many phase diagrams for binary alloys have been constructed by experimentally determining the equilibrated alloys of the constituents or by calculating the theoretical minimum free energy of the relevant phases. The phase diagram provides a straightforward way to identify the thermodynamically favorable phases that exist under equilibrium at a given temperature, pressure, and composition, which is crucial for designing and predicting the properties of bimetallic-derived catalysts. Subject to minimizing the total free energy, the most probable structures of the bimetallic NPs are core-shell, segregated, ordered (intermetallic compound), or random alloy. The alloying ability of bimetallic NPs can be approximated by the corresponding enthalpy of formation as follows: 50 (Equation 8) H p f = H b f ( 1 − α n - 1 / 3 ) − α n - 1 / 3 [ x A E b A ( 1 − x A - 1 / 3 ) + x B E b B ( 1 − x B - 1 / 3 ) ] , where H p f and H b f are the enthalpies of formation of alloy NPs and bulk alloys at 0 K, respectively; α is a shape factor; x A and x B are the atomic fractions of the elements A and B, respectively; n denotes the total number of atoms in the NP, correlating with the particle size; and E b refers to the cohesive energy of NPs. As indicated in Equation 8, the alloy formation enthalpy is a monotonically decreasing function of n or NP size, and thereby there should be a critical n or NP size below which H p f would become negative and lead to spontaneous alloying. Thus, reducing the size of NPs should promote the alloying between heterometals, especially those with poor miscibility in the bulk phase.It is noted that Equation 8 cannot provide details about the thermodynamic surface composition. In practice, surface segregation of alloys is a common phenomenon in bimetallic-derived catalysts mainly due to the difference in surface free energy between the two constituents. A Langmuir-McLean model was proposed to approximate the surface fractions of the components (x surf ) under the equilibrium of segregation: 56 , 57 (Equation 9) x A s u r f x B s u r f = x A b u l k x B b u l k exp ( Δ H s e g R T ) = x A b u l k x B b u l k exp ( α ( γ B − γ A ) + Δ H s e g m i x i n g + Δ H s e g s t r a i n R T ) , where Δ H s e g is the driving force of segregation and is composed of the contributions from the difference in surface free energy (γ B − γ A ), the heat of mixing (associated with the relative strength of A–A, B–B, and A–B bonds), and the change in lattice strain energy (owing to the mismatch in atomic size). As a consequence, the final surface configuration depends on the net result from the following contributions: (1) the surface energy of the two metals, with the metal showing lower surface energy tending to move to the surface of the NP and forming a shell; (2) the relative bond strength (cohesive energy) between the two different metal atoms compared with that of the same parent metals, where weaker heterometallic bonds favor segregation; (3) the relative atomic sizes, with the smaller atoms tending to occupy the core; and (4) the synthesis or pre-treatment temperature, which is critical to overcome the diffusion barrier of segregation. 56 , 58 , 59 Based on a principal component analysis, Wang and colleagues 53 revealed that the cohesive energy and atomic size were the primary independent factors controlling surface segregation, as indicated by the color matrix in Figure 3B; furthermore, a strong correlation was obtained between the segregation energy and the core-shell structure preference. This provides a "design map" to predict or tune the surface configurations of bimetallic NPs.In addition to the above parameters, the adsorption of species (e.g., CO, O, H, OH, NO, and H2S) during synthesis or reaction also plays a significant role in dictating the surface composition of bimetallic-derived catalysts. The driving force of surface segregation between two metals can be modified due to their different binding strengths to a given adsorbate. 57 Using DFT calculations, Menning and co-workers reported an effective way to approximate the adsorbate-induced segregation of subsurface metal atoms to the surface in bimetallic systems. 54 Within a wide range of bimetallic systems, as shown in Figure 3B, the driving force of segregation was predicted to be proportional to the difference in occupied d-band center (Δεd) between the subsurface and the surface configurations, but with different slopes for various adsorbates. Thus, adsorbate-induced segregation offers an additional way to vary the surface composition of bimetallic NPs and to explain the dynamic restructuring behavior that occurs during catalytic reactions.In addition, the catalyst support is more than just an inert carrier used to disperse metals on a large-area surface. Instead, the support typically interacts with NPs via chemical bonding and creates metal-support interfaces. These interactions may have a profound effect on the geometric and electronic properties of NPs and in turn on the catalytic performance. Specifically, as shown in Figure 3C, the metal-support interactions could lead to the following effects: (1) charge transfer leading to electron redistribution and even variation of chemical state at the interfacial region; (2) change in NP morphology resulting from the strong interaction of certain facets of NPs with the support; (3) modification of surface composition with one metal segregating from the alloy NP toward the surface or into the lattice of the support due to preferential interactions; (4) metal ions in the support (e.g., oxides of Si, Al, Ga, In, Ti, V, and Nb) being reduced and incorporated into metal NPs to generate new alloy structures; and (5) strong metal-support interactions (SMSIs) resulting in a partial or complete encapsulation of the NPs by the MO layers, typically under reductive conditions. As will be demonstrated in the catalytic reactions of CO2 and ethane, these metal-support interactions and the resultant interfacial sites modify the adsorption of reactants, intermediates, and products, leading to unique catalytic properties. More details about the metal-support interactions can be found in previous reviews. 55 , 56 The synthesis of supported bimetallic-derived catalysts involves complicated precursor-precursor/solute interactions in solution and metal-metal/support interactions in solid that vary under different temperatures, pressures, and atmospheric compositions. The controlled synthesis of supported bimetallic-derived catalysts with well-defined, uniform, and tunable structures remains challenging. A comprehensive discussion of synthesis methods of bimetallic NPs has been reviewed previously. 59–62 The following discussion briefly describes the conventional synthesis protocols and focuses on the state-of-the-art methods for the synthesis of supported bimetallic-derived catalysts.Co-impregnation, sequential-impregnation, and deposition-precipitation methods are the most common approaches used in the large-scale synthesis of supported bimetallic-derived catalysts. Although these methods are simple and economical, they usually lead to NPs with broad size distributions and compositional heterogeneity, which prevents rigorous correlation of the catalytic performance (i.e., activity, selectivity, and stability) with a well-defined structure. Colloidal methods may improve the uniformity but often require structure-stabilizing/directing ligands to cap catalytic surfaces. In practice, it is difficult to completely remove all the ligands, which would reduce the number of exposed metal sites available for catalysis. The colloidal metallic NPs mainly are used as model materials to compare the properties of the alloy with those of its monometallic parent NPs. Metal infiltration is a solvent-free method in which metal precursor salts, such as nitrate hydrate, are physically mixed with the support and subsequently heated above the melting temperatures of the metal salts. However, a non-uniform structure such as a core-shell pattern may be formed if the two metal precursors have very different decomposition temperatures. Thermal decomposition of bimetallic organometallic complexes provides an alternative way to form ultrafine bimetallic NPs. However, its application is limited by the high cost of organic precursors.Atomic layer deposition (ALD) is an efficient chemical vapor deposition method for the preparation of supported bimetallic NPs and MO-decorated NPs. 52 The self-limiting deposition reaction between the precursor vapors and the surface allows for an atomically homogeneous distribution, even on highly porous and heterogeneous supports. As illustrated in Figure 4 A, Lu and colleagues reported a strategy to selectively grow the secondary metal only on the primary metal surface while avoiding growth on the support. By judicious selection of the co-reactant, deposition temperature, and pulsing sequence, monometallic NP formation was prevented successfully. 63 , 64 In addition to secondary metals, MOs (e.g., Al2O3 and FeOx) also can be precisely deposited on primary metal NPs such as Pd and Pt with controllable thickness and deposition position. 65–67 ALD of MOs is also a powerful tool to manipulate the highly and lowly coordinated sites (HCSs and LCSs) of the NPs without changing the particle size. This strategy has been successfully applied to disentangle the size-dependent geometric and electronic effects of Pd/Al2O3 catalysts by selectively blocking the HCSs and LCSs of Pd NPs with the ALD of FeOx and Al2O3, respectively. 68 As such, ALD provides opportunities to explore directly the structure-function relationships on MO/metal interfaces in supported catalysts by synthesizing model interface structures.Adsorption or deposition of metal precursor ions typically relies on the electrostatic interactions, van der Waals interactions, and/or polar bonds between the precursor and the support. As illustrated in Figure 2E, SEAs occur between the oppositely charged metal precursor ions and the support. Since the initial study by Brunelle in 1978, 74 the SEA method has drawn extensive attention for the preparation of uniform and ultrasmall supported mono/bimetallic NPs. Wong and colleagues 75 reported a simple and generalizable simultaneous SEA method using a common silica support with a variety of precious and non-precious metal (Pt, Pd, Co, Ni, and Cu) ammine precursor pairs. The obtained bimetallic NPs were ultrasmall and homogeneously alloyed with a narrow metal size distribution (0.9–1.4 nm). Ding and co-workers 69 recently developed a sequential SEA protocol with good size control of supported alloy NPs (1–3 nm) by regulating the sequential uptake of target cations (e.g., [Pd(NH3)4]2+) and anions (e.g., [PtCl4]2−) from heterometallic double complex salts onto silica (Figure 4B). Owing to its facile operation, precise synthesis, and low cost, the SEA method is promising for large-scale synthesis applications.The concept of reactive metal-support interaction (RMSI) is related to the chemical interaction between a metal and the support that induces the formation of bimetallic structures. 55 , 76 Xu and co-workers 77 reported an RMSI method to synthesize sub-2-nm bimetallic NPs (PtCo, RhCo, and IrCo) on mesoporous sulfur-doped carbon supports via the strong chemical interaction between metals and the sulfur atoms that are doped in the carbon substrate. Perovskite oxides (POs) with the nominal chemical formula ABO3 are versatile templates for the RMSI synthesis of supported bimetallic-derived catalysts due to their compositional flexibility, structural stability, and relatively low cost. 78 By substituting transition metals (M and M′) into the B site of a PO with a moderate A-site deficiency (A1-xB1-y-zMyM′zO3-δ), uniform bimetallic NPs (e.g., PtNi and CoNi) can be achieved on the PO support via thermal reduction, during which in situ exsolution of metal NPs would occur. 79 , 80 In recent years, MXenes—two-dimensional transition metal carbide materials with well-defined structures and widely tunable compositions—have been explored as supports for bimetallic-derived catalysts. Li and colleagues 70 , 76 successfully applied the RMSI method between Pt NPs and Ti3C2Tx and Nb2CTx (T = F, O, or OH) MXenes to synthesize MXene-supported well-defined Pt3Ti (6.0 ± 3.2 nm) and Pt3Nb (2.6 ± 0.7 nm) NPs (Figure 4C). The RMSI method offers an opportunity to obtain supported bimetallic-derived catalysts with tunable chemical and structural properties.Conventional synthesis methods exhibit limited capability to produce homogeneously alloyed bimetallic NPs since they are typically constrained by the thermodynamic immiscibility of the constituents. Certain bimetallic-derived catalysts may possess promising unique electronic and geometric properties despite their inherent immiscibility, such as Cu-X (X = Ag, Ni, Sn) NPs. To overcome the thermodynamic hindrance in bimetallic systems, Yang and colleagues 71 recently proposed a non-equilibrium synthesis strategy using a pulse thermal shock method, as illustrated in Figure 4D. For each synthesis, metal precursor solutions were mixed well and dispersed on the substrates before being charged by the Joule heating pulses induced by current pulses (0.2 s). The metal precursors (e.g., metal nitrates) were instantly decomposed during the thermal shock, and then the resultant metals were mixed and kinetically trapped into bimetallic NPs following rapid quenching. This type of synthesis strategy sets an example for achieving bimetallic-derived catalysts using metals characterized by thermodynamic immiscibility.In recent years, the development of the concept of “single-atomic-site catalysis” has spurred the emergence of single-atom alloy (SAA) and hetero-pair metal dimer catalysts, which expand the family of supported bimetallic-derived catalysts. SAAs contain isolated metal atoms on the matrix of another metal and can be synthesized using the galvanic replacement (GR) method, which enables the production of bimetallic and hollow NPs displaying ultrathin walls and even atomic dispersion on a primary metal substrate. 81 , 82 The bimetallic NPs obtained can also be deposited over solid supports by complementary wet-impregnation techniques, opening up the possibility of achieving supported SAA catalysts. As shown in Figure 4E, the GR method was used with an aqueous suspension of Cu/metal/MO and a solution of Pt precursor to synthesize a mixed metal (MgAl) oxide-supported PtCu SAA catalyst. 72 The synthesis of hetero-pair metal dimers remains challenging, and the state-of-the-art examples are predominantly limited to C(N)-supported systems that use metal-organic framework (MOF) materials as templates. 83 As illustrated in Figure 4F, a diatomic metal-nitrogen catalyst (Ni/Fe-N-C) was synthesized by an ion-exchange strategy based on the pyrolysis of a Zn/Ni/Fe MOF; in this method, Ni ions were exchanged with Zn nodes in the framework of an Fe-doped ZIF-8, and afterward residual Zn nodes were evaporated by a thermal treatment at 1,000°C. 73 The characterization of supported bimetallic-derived catalysts is complicated by their wide range of sizes (1–10 nm), shapes (polyhedral, spherical, nanorod, tripod), morphologies (octahedral, tetrahedral, cubic), compositional ordering (core-shell, segregated, random, intermetallic), metal-support interfacial interactions, and reaction-driven restructuring. Although surface science techniques using well-defined crystal surfaces have yielded some structure-function relationships, the extension of such conclusions to practical supported catalysts is hindered by the “pressure gap” and “materials gap.” 29 Advancements in in situ/operando techniques have enabled atomic-level and time-resolved insights into the diverse geometric and electronic structures of supported bimetallic-derived catalysts under reaction conditions. 84 , 85 This section highlights some of the most common and powerful characterization techniques spanning different length scales from atoms to hundreds of nanometers (Figure 5 A) in order to demonstrate how to achieve comprehensive characterization of supported bimetallic-derived catalysts.Time-resolved in situ X-ray diffraction (XRD) is a prominent technique used to track the structural evolution of the long-range order of bimetallic-derived catalysts under reaction conditions. A significant amount of structural information is contained in the position, intensity, and shape of XRD peaks. 95 Phase identification—the most common application—can be achieved simply by comparing the sample diffraction peaks with those of known standards in a database. 95 By using the empirical Vegard's law or Zen's law, XRD can be used to estimate the lattice parameter/volume or composition of alloy NPs within the same class of unit cells. 96 , 97 Solving the crystal structure via Rietveld refinement of full XRD patterns provides more information about the lattice structure, crystallite size, particle composition, and atomic arrangement. However, XRD can be applied only to systems with long-range periodic atomic arrangement since it counts the Bragg diffraction signals. It is typically difficult to characterize diffraction peaks for particle sizes smaller than 2 nm due to the low crystallinity or significant disordering, limiting the applicability for NP characterization.In the past decade, advances in synchrotron source and detector technology have enabled a time-resolved total scattering analysis technique, pair distribution function analysis (PDF), to probe supported NP catalysts. 98 , 99 As illustrated in Figure 5B, PDF counts the diffuse scattering in addition to Bragg diffraction and thus should be able to depict the probability of finding pairs of atoms separated by a distance r within a local atomic arrangement. 100 After a real-space Rietveld refinement, one can also obtain structural information such as the local bonding, lattice strain, and crystalline core size. It should be noted that in order to isolate the PDF information that is associated with metal NPs, differential PDFs should be used for supported catalysts for which contributions from the support can be subtracted from the raw PDF pattern. 99 Advantageously, the structural insights are not limited to the immediate coordination environment, enabling PDF to bridge the very important blind range (5–30 Å) that exists among the other X-ray techniques, as shown in Figure 5A. 86 In situ X-ray absorption spectroscopy (XAS) is a powerful tool for providing information on the electronic and geometric properties of bimetallic-derived catalysts. As illustrated in Figure 5B, XAS measurements consist of the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) signals. In the XANES region, the appearance of a K or L pre-edge feature, if present, primarily arises from the dipole transition from the 1s or 2p state to the hybridized orbitals of nd − (n + 1)p. 101 The variation in the edge position or the area intensity of the “white line” offers information regarding the oxidation state of the element of interest. Within a similar ligand family, the edge position can be used to approximate the chemical state based on the formal oxidation state of standard reference compounds. Moreover, the linear combination fitting of XANES spectra combined with principal-component analysis offers a way to quantify and track the fraction of different compositions in heterogeneous samples under various reduction or reaction conditions. 102 The ΔXANES technique (Figure 5B) utilizes XANES signals relative to a reference sample (such as a substrate that is free of adsorbates). With the help of full multiple scattering ab initio calculations, the difference between these signals can reveal the signature of the adsorbates and the binding sites. 88 The oscillatory part in the extended region results from the modulation by the scattering of the outgoing electron off the absorber atom itself (AXAFS) and the backscattering off the neighboring atoms (EXAFS). 88 , 103 Fitting the Fourier transformed EXAFS signal (FT-EXAFS, see Figure 5B) reveals structural parameters such as the bond identity, coordination number, bond length, and mean-square disorder deviation (σ2). Additional correlation constraints on these parameters in bimetallic systems offer a way of validating the presence of metal-metal and metal-support bonding as well as estimating the particle size, shape, and configuration. 104 As a complement, the wavelet transformed EXAFS (WT-EXAFS, see Figure 5B) is applicable to differentiate the equidistant heavier (e.g., transition metals) and lighter (e.g., C, N, O) backscattering atoms from the central atom, due to its better resolution of wave number dependence of the scattering. 105 , 106 AXAFS (Figure 5B), which is employed less frequently than EXAFS, contains information about the interatomic potential (bonding electrons and/or oxidation state) and provides insight into the electronic interactions of the support with small- to medium-sized metallic and oxidic clusters (d < 2 nm). 88 , 103 The AXAFS signal can be isolated by properly subtracting the double-electron excitation and EXAFS contributions. In recent years, time-resolved dynamic structural and mechanistic changes have been revealed simultaneously using state-of-the-art operando techniques, which couple quick EXAFS or energy dispersive XAS with XRD, three-dimensional tomography, vibrational (IR, UV-Vis, and Raman) spectroscopy, and online gas analysis (micro-gas chromatography and residual gas analysis/mass spectrometry) techniques. 107 The increasing application of such cutting-edge combinations will shed light on attaining a more convincing understanding of structure-mechanism-function relationships for bimetallic-derived catalysts.X-ray photoelectron spectroscopy (XPS)—based on the photoelectric effect—is a surface-sensitive technique for qualitatively or semiquantitatively analyzing the elemental and chemical compositions within the near-surface region. 108 Conventional XPS requires high-vacuum conditions due to the significant scattering of photoelectrons by gas molecules on the way to the detector, limiting its application under practical reaction conditions. Remarkable improvements in X-ray sources and differential pumping have enabled the application of XPS for unraveling electronic structures and surface compositions under elevated temperature and reaction atmosphere (up to a few tens of torrs). 109 For supported bimetallic-derived catalysts, metal-metal or metal-support electron transfers can be elucidated by examining the core-level shift of the metal relative to control samples (monometallic catalysts or inert-material-supported catalysts) or tabulated values. Moreover, a careful examination of the core level of C1s (Figure 5C) and O1s provides information regarding reaction intermediates, such as the types of carbonaceous species (e.g., CO2 δ−, CO, carbonates, formates, carbides, coke) and oxygen features (e.g., lattice oxygen, oxygen vacancy, OH). 89 Notably, the synchrotron light source offers an opportunity to obtain a depth profile, such as for core-shell configurations over bimetallic-derived catalysts, by varying the incident photon energy. 108 Despite the excellent capabilities of ambient-pressure XPS (AP-XPS), the signal-to-noise ratio may be significantly reduced for insulated materials and highly diluted catalytic systems.Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) accomplishes useful surface spectroscopic measurements. By exploiting the characteristic shifts in vibrational frequencies of isotopes, this vibrational spectroscopic technique enables operando analysis of surface intermediates. DRIFTS also can be used with specific probe molecules in order to determine the surface termination and binding sites. For instance, the bonding of a CO molecule to a transition metal atom (Figure 5C) involves the donation and back-donation of electrons between the σ-π∗ orbitals of CO and the d-orbitals of transition metals. Consequently, characteristic CO stretching features are observed on different metal surfaces and binding sites. DRIFTS measurements following CO adsorption often can be used to determine the surface termination of a bimetallic system by comparing the vibrational frequencies to those of the corresponding monometallic surfaces. However, this strategy is restricted in cases where carbonyl complex formation, CO decomposition, CO-induced segregation, and weak chemisorption (e.g., on Cu and Ag) may occur. In these cases, other probe molecules should be identified and used in the DRIFTS measurements.Compared with the above-mentioned techniques, scanning transmission electron microscopy (STEM) provides an intuitive view at an atomic scale for insights into the structural properties of supported bimetallic-derived catalysts. STEM is a process wherein pre-specimen lenses focus the beam into a small probe that is scanned in a raster pattern across the sample. A variety of signals can be emitted from the sample due to excitation by the high-energy electron probe. High-angle annular dark-field (HAADF) images (Figure 5D) are obtained by collecting the electrons scattered to high angles with an annular detector, where the signal features a Z-contrast (Zα, α = 1–2) dependence. 110 HAADF-STEM provides structural information on supported bimetallic-derived catalysts regarding the size (NPs, clusters, and even single atoms), morphology, lattice fringe, epitaxial growth, and metal-support interfaces. However, the image contrast may not be sufficient to differentiate the Z-similar elements or lighter elements on a heavy support, e.g., Fe and Ni on a CeO2 support. In such cases, HAADF-STEM combined with simultaneous spectroscopic imaging, such as energy-dispersive spectroscopy (EDS) or electron energy loss spectroscopy (EELS) (Figure 5D), can be exploited to elucidate the spatial distribution of elements. The characteristic X-ray emissions from each element following beam irradiation can be collected by an energy-dispersive detector, allowing the analysis of elemental composition and distribution on supported bimetallic-derived catalysts. The characteristic losses in the energy of the transmitted electrons are related to the chemical state and electronic structure of the excited elements. Compared with EDS, EELS provides improved signal, spatial resolution (down to 1 nm), and sensitivity to the lower-atomic-number elements, such as C, N, and O. For heavier elements (e.g., Pt and Pd), EDS is typically preferred despite its lower spatial resolution. Improved spatial (sub-0.1 nm) and energy resolution (sub-0.1 eV) of electron microscopy (Figure 5D) has been achieved with the advent of spherical and chromatic aberration correctors. Nowadays, atomic-lattice-resolution environmental (S)TEM has led to unprecedented insights into the dynamic restructuring that occurs under reaction conditions, such as metal sintering, phase transformation, oxidation-reduction processes, layered MO encapsulation, and MO interfacial behavior. 111 It should be noted that results from STEM imaging of particularly small areas may not be representative of the entire sample. Thus, STEM should be used in conjunction with other techniques such as those detailed above.In parallel with the aforementioned characterization techniques, macroscopic reactor studies constitute an important method to deduce microscopic structural and mechanistic information. Flow-reactor experiments (Figure 5E) have been applied widely to evaluate the potential synergistic properties of bimetallic-derived catalysts, where the catalytic performance (activity, selectivity, and stability) of the bimetallic-derived catalysts can be compared with that of the sum of the corresponding monometallic-derived catalysts as well as of their physical mixtures. To obtain measurements of the intrinsic catalytic reaction kinetics, two typical approaches are used to eliminate the heat and mass transport limitations inside the catalyst particles and the reactor: (1) enhancing convection by increasing the flow rate at a constant space velocity, and improving diffusion by reducing particle size, and (2) mitigating temperature and concentration gradients using intraparticle and interparticle dilution of active sites with inert materials such as SiO2 powders and acid-purified quartz granules, respectively. 112 Multiple criteria (e.g., Kubota, Mears, and Kubota-Yamanaka criteria) can also be adopted to further diagnose the potential transport limitations within the intraparticle, interparticle, and reactor. 20 In addition, for steady-state measurements of intrinsic kinetic information such as activation barriers, activation entropy changes, and reaction orders, the overall conversions typically should be kept below 15% (even <5%) so as to enable the application of the differential plug-flow reactor model as well as to alleviate temperature and concentration gradients. It remains challenging to accurately measure activation barriers over catalysts with fast deactivation. In this scenario, a stepwise cooling-heating method might be used to approximate the activation barrier by means of averaging techniques. 113 The reaction mechanism can be deduced by examining the kinetic response to the partial pressure of reactants and/or products.In addition to structure characterizations and catalytic performance evaluations, the elucidation of structure-function relationships requires exploration of the surface species adsorbed on the catalyst under reaction conditions. However, even though certain chemisorbed species may be observed by the aforementioned spectroscopic approaches during a reaction, their identification does not necessarily confirm that the overall reaction proceeds via these surface intermediates, since they could be spectators, merely occupying and even poisoning the active sites. In contrast, isotopic labeling provides a more unambiguous means of studying the reaction mechanism. The deuterium kinetic isotope effect is based on a zero-point energy difference that occurs during deuterium substitution in a reaction. A primary isotope effect is observed when the substitution involves the dissociating bond (e.g., C–D in alkanes/alkenes 114 , 115 and D–D in dihydrogen) 116 in the rate-determining or kinetically relevant step(s). When the substitution involves neighboring (ɑ) or remote (β) bonds relative to the reaction center (e.g., –CD3 substitution in methanol relative to the hydroxyl group), a secondary isotope effect affecting the reaction rate may occur due to the variations in electronic interactions, such as by hybridization or hyperconjugation. 117 As illustrated in Figure 5E, steady-state isotopic-transient kinetic analysis (SSITKA) and temporal analysis of products (TAP) are among the most powerful transient techniques to obtain kinetic and mechanistic information with isotopic labeling. SSITKA detects the response of reactor effluent species to a step switch of isotopic labels, and the response pattern provides a variety of in situ kinetic information regarding the abundance of surface intermediates, mean surface-residence time, reactivity, adsorbate surface coverage, and distribution of site heterogeneity. 118 TAP, introduced by Gleaves and colleagues 119 during the 1980s, is another transient pulse response technique operated in the well-defined Knudsen diffusion regime with sub-millisecond time resolution. The transient response pattern at a given temperature is a sensitive function of gas-solid interactions, and thereby it offers a unique opportunity to differentiate among the reaction sequences, probe the lifetime of surface intermediates, and isolate the role of gas diffusion inside porous materials such as zeolites. 94 , 120 , 121 Despite its excellent performance in kinetic and mechanistic studies, the applications of SSITKA and TAP are still limited by expensive facilities and isotopes as well as by complicated mathematical modeling. However, coupling these methods with other in situ spectroscopic techniques should expand insights into reaction intermediates and mechanisms over bimetallic-derived catalysts.The previous sections provided a survey of the general principles underlying materials synthesis, the state-of-the-art synthesis methods, and the essential characterization techniques for supported bimetallic-derived catalysts. In the current section, several representative bimetallic-derived catalysts with precious and non-precious metals are adopted to illustrate SU-CO2Et into different value-added products. In particular, we demonstrate how the combined in situ and ex situ techniques, coupled with DFT calculations, are utilized to identify distinct types of active sites and establish structure-function relationships.Dry re-forming of ethane (DRE) with CO2—involving the scission of both the C–H and the C–C bonds of ethane—is a promising approach to produce syngas, an important industrial feedstock for methanol synthesis and Fischer-Tropsch reactions. Ni-based catalysts are widely used for dry re-forming, although rapid deactivation owing to severe coke deposition and metal sintering restricts its applications. Extensive studies have been performed to improve the catalytic activity and stability by controlling bimetallic formation, metal dispersion (or size), oxygen storage capacity, support reducibility, and support acidity and basicity. 122 , 123 Yan and co-workers 21 synthesized CeO2-supported PtNi bimetallic-derived catalysts for the DRE reaction with CO2 to produce syngas. It is noted that both the conversion (Figure 6 A) and the turnover frequency (TOF, not shown here) of reactants indicated that the PtNi/CeO2 catalyst was more active than the sum of the corresponding parent catalysts, revealing the synergistic effect from the formation of the Pt-Ni bimetallic bond. The in situ XRD experiments (Figure 6B) revealed the phase evolution of the metals and the CeO2 support with temperature, showing that a PtNi alloy phase (2θ = 8.5°) was formed above 533 K. Simultaneously, a refinement of the CeO2 lattice constant suggested enhanced reducibility of CeO2 (Ce4+ → Ce3+) in the presence of the PtNi alloy above 533 K, which in turn promoted its CO2 activation properties via more oxygen vacancies. The XANES spectra of the Pt L3-edge and Ni K-edge of the PtNi/CeO2 catalyst revealed that Pt and Ni remained metallic under reaction conditions; and the EXAFS fittings of both edges (Figure 6D) demonstrated the formation of Pt-Ni intermetallic bonds. The TEM images of the reduced and spent PtNi/CeO2 catalysts (Figure 6E) indicated negligible metal agglomeration (2.3 and 2.5 nm, respectively) after the DRE reaction. To understand the active sites, it was crucial to determine the surface termination of the PtNi alloy configurations. As shown in Figure 6C, comparison of the DRIFTS of CO adsorption over the bimetallic-derived catalyst and the corresponding monometallic catalysts illustrated that the CO vibrational features on PtNi/CeO2 resembled those on Pt/CeO2, indicating a Pt-terminated PtNi bimetallic surface. Therefore, DFT calculations were performed for the C–C bond cleavage of ethane over a Pt-terminated-PtNi-Pt(111) model as well as over Pt(111), Ni(111), and mixed-PtNi-Pt(111) models (Figure 6F). The DFT-calculated binding strengths of all the O- and C-bound intermediates over the Pt-terminated-PtNi-Pt(111) surface generally were weaker than those over the Pt(111), Ni(111), and mixed-PtNi-Pt(111) surfaces. Moreover, comparison of the energy changes along the re-forming of ethane (C–C bond cleavage) revealed that the re-forming reaction on the Pt-terminated-PtNi-Pt(111) surface was the most energetically favorable, as illustrated in Figure 6G. The combined results from characterization and DFT calculations provided the important insights that the Pt-rich PtNi bimetallic surface structure weakened the binding of surface oxygenate/carbon species and reduced the activation barrier for C−C bond scission, leading to enhanced DRE activity as well as stability to produce syngas. This approach demonstrates how robust characterization of active sites can empower computational modeling to reveal structure-function relationships; in turn, these results establish structural targets for the synthesis of catalysts with improved properties.The oxidative dehydrogenation of ethane (ODHE) by CO2 leads to the formation of valuable ethylene with simultaneous consumption of CO2. However, compared with the C–H bond (C2H5–H, 415 kJ mol−1) of ethane, cleavage of the C–C bond (CH3–CH3, 368 kJ mol−1) is thermodynamically favored, thereby imposing a challenge on promoting the product selectivity toward ethylene, which requires breaking the C–H bond while retaining the formed C=C bond. Recent advances in modified Cr/Zr-based catalysts, 124–126 metal carbides, 18 , 127 MOs, 128 , 129 and especially bimetallic-derived catalysts 19 , 23 , 125 , 128 , 130 have demonstrated the feasibility of enhancing ethylene selectivity. However, few fundamental strategies are available to guide the development of ODHE catalysts due to their structural complexity under reaction conditions. Recent studies have suggested that the electronic and geometric properties of oxygen species may play a critical role in the selective cleavage of the C–C/C–H bonds of ethane. 23 , 127–129 In addition, different metal-oxide or oxide-oxide interfacial configurations may tune the selective cleavage of C–H/C–C bonds by controlling the electronic and geometric features of oxygen species.Bimetallic Ni-Al mixed oxides derived from layered double hydroxides (LDHs) have been applied to ODHE. 131 Compared with the NiO/Al2O3 catalyst, Ni-Al mixed oxides are more selective to ethylene (selectivity 70%) due to the partial isolation of the electrophilic oxygen species. Recently, Zhou et al. 128 reported a sulfate-modified Ni-Al mixed oxide catalyst and highlighted the role of the sulfate ion modifier in enhancing the ethylene selectivity by regulating the proportion of adjacent and isolated oxygen species. As illustrated in Figure 7 A, the Ni-Al mixed oxide catalysts were synthesized using Ni-Al hydrotalcite precursors interacting with sodium dodecyl sulfate (SDS) via a co-precipitation method. As the SDS amount increased from 0 to 0.05 mol, the layer thickness of the derived LDHs decreased from ∼7.1 to ∼2.0 nm (Figure 7B). The XRD patterns in Figure 7C indicated that the diffraction peaks on the Ni-Al mixed oxide catalysts were shifted slightly relative to those of pure NiO due to the doping effect of Al cations. The FT-EXAFS spectra in Figure 7D demonstrated a significant reduction in the peak intensity associated with Ni-Ni bonding (in NiO) on the NiAl-S1 catalyst. The XPS results indicated a dominant coordination of sulfate with Ni atoms and the co-existence of Ni2+ and Ni3+, and the surface atomic ratio of Ni3+/Ni2+ (Figure 7E) increased with the doping amount of S. Accordingly, the DRIFT spectra of linearly adsorbed CO on the partial positively charged Ni sites exhibited a blue shift with increasing sulfate concentration. The sulfate modifier can also impose a steric effect on the oxygen species, as indicated by a probe reaction of oxygen species coordination from N2O decomposition. The linear correlations in Figure 7E suggested that the sulfate modifier interaction with the Ni3+ sites decreased the adjacent electrophilic oxygen sites and in turn increased the ethylene selectivity. Figure 7F showed that NiAl-S1—possessing the largest proportion of isolated oxygen species (∼100%)—exhibited the highest ethylene selectivity (∼100%), confirming the importance of controlling the nature of interfacial sites and oxygen species for enhancing ethylene selectivity (Figures 7G and 7H). Similar effects were also observed recently in the ODHE by CO2 over CeO2-supported PdFe bimetallic-derived catalysts, in which the electron-enriched oxygen in the FeOx/Pd interface enhances the selective scission of C–H bond to yield ethylene. 23 Bimetallic-derived catalysts of given metal pairs typically show the capability of promoting reactions of CO2 and ethane toward only one pathway, i.e., either DRE or ODHE. Recent studies by Yan and colleagues 19 reported that changing the atomic ratio of Fe and Ni in NixFey/CeO2 bimetallic-derived catalysts enabled tunable selectivity toward either DRE or ODHE.As shown in Figure 8 A, introducing a small amount of Fe (Ni3Fe1/CeO2) promoted the DRE pathway (99% CO selectivity), while an increased amount of Fe (Ni1Fe3/CeO2) significantly shifted toward the ODHE pathway (78% C2H4 selectivity). Thus, distinct active sites selective to the DRE or ODHE reaction must exist on the NiFe/CeO2 catalysts, which can be regulated accordingly by changing the atomic Ni/Fe ratios. In situ XRD experiments (Figure 8B) revealed slightly larger lattice constants for Ni1Fe3/CeO2 (3.575 Å) and Ni3Fe1/CeO2 (3.570 Å) compared with fcc metallic Ni (3.558 Å), suggesting the formation of a Ni-rich NiFe alloy with the fcc structure on both catalysts. The XANES results showed that Ni in both catalysts remained metallic, while the Fe species in both catalysts exhibited similar near-edge features between Fe(II)O and Fe(II, III)3O4. The EXAFS fitting results (Figure 8C) demonstrated a slight increase in the Ni-Ni(Fe) bond length over both catalysts relative to Ni3/CeO2, in accordance with the formation of a Ni-rich NiFe alloy as suggested by XRD. The Fe-Ce bonding at 3.6–3.7 Å was observed on both catalysts, demonstrating the strong interaction between Fe and the CeO2 support. Combined with the small coordination numbers of the Fe-O bond, it was suggested that the oxidized Fe species tended to form thin layers on CeO2. A STEM-EELS line-scan analysis (Figure 8D) of Fe species indeed revealed the presence of a thin layer of FeOx on CeO2 particles in Ni1Fe3/CeO2, while such a layer was absent in Ni3Fe1/CeO2.DFT calculations (Figure 8E) showed that, compared with the Ni(111), Ni-terminated-Ni3Fe(111), and bulk-terminated-Ni3Fe(111) models, the FeOx/Ni(111) interface promoted the stability of ∗CH3CH2 and therefore the dehydrogenation pathway, while hindering the formation of ∗CH3CH2O and its subsequent dehydrogenation and C–C bond cleavage reactions via ∗CH3CO. In addition, the DFT-calculated activation barrier of the ODHE pathway (oxygen-assisted C–H bond cleavage) was lower than that of DRE (C–C bond scission) on FeOx/Ni(111), consistent with the experimental results, which identified FeOx/Ni interfaces as the most likely active sites to promote selective C–H bond cleavage in ethane. This work offers a better understanding of the structure-function relationships between interfacial active sites and catalytic performance; furthermore, it highlights the feasibility of tuning the product selectivity of the CO2-ethane reaction by using this understanding to control the active sites of versatile and non-precious NiFe bimetallic-derived catalysts.In addition to the metal effects, support effects or metal-support interfacial interactions also play a significant role in the activation of CO2 and ethane. Specifically, the oxide supports not only act as carriers to disperse catalytically active sites and hence improve conversion, but also exert enormous influence on the electronic and geometric properties of active sites through metal-support interactions, including the modification of oxygen storage/release/transfer, reaction pathways, deactivation resistance, and catalytic performance. 20 , 125 , 130 , 132 Xie and co-workers 20 investigated PtNi bimetallic-derived catalysts on reducible (CeO2 and TiO2) and irreducible (γ-Al2O3 and SiO2) oxide supports. The results of initial activity (both conversion and TOF, not shown here) indicated that PtNi catalysts supported on reducible oxides (CeO2 and TiO2) were generally more active than those supported on irreducible oxides (SiO2 and γ-Al2O3). However, PtNi/TiO2 and PtNi/γ-Al2O3 exhibited a rapid decay in the activity. PtNi/CeO2 and PtNi/SiO2 remained stable, with the former showing the highest CO yield (Figure 9 A) at the pseudo-steady state. Thermogravimetric analysis (TGA) indicated negligible coke deposition on the spent PtNi/TiO2 catalyst, while TEM imaging revealed an encapsulation layer blocking the metal active sites, due to the SMSI effects. On the spent PtNi/γ-Al2O3 catalyst, the TEM (Figure 9B), TGA (Figure 9C), and Raman spectroscopy (Figure 9D) results illustrated that the metal sites underwent noticeable agglomeration from 2.9 to 7.5 nm, and that the metal ensembles were encapsulated by graphic carbon. PtNi/CeO2 showed large amounts of carbonaceous species on the spent sample, but since they were identified primarily as disordered/amorphous in morphology, they could be removed readily as active intermediates during the re-forming reaction. Flow-reactor experiments revealed different kinetic behaviors for the DRE reaction on the irreducible SiO2- and reducible CeO2-supported PtNi catalysts.Pulse-reactor experiments using CO2 pulses indicated that the SiO2 support acted as a spectator, while the metal sites could weakly activate CO2. In situ DRIFTS studies validated that CO2 activation on PtNi/SiO2 proceeded not only with direct decomposition (CO2 + 2∗ → CO∗ + O∗) on metal sites but also with an H-assisted pathway to formate species (CO2 + H∗ → formates). In contrast, the pulse-reactor experiments and the ceria lattice parameter derived from the Rietveld refinement of in situ XRD patterns (Figure 9E) revealed that the reducibility of CeO2 was considerably enhanced in the presence of the PtNi alloy. The partially reduced CeO2 could provide additional sites (oxygen vacancies) on the support or on metal-support interfaces for the activation of CO2 (primarily CO2 + 2# → CO# + O#), and the formed active oxygen species subsequently promoted ethane conversion. As illustrated in Figure 9F, the effects of the reducibility of oxide supports should play a significant role in the catalytic performance, reaction kinetics, and reaction mechanisms during the DRE reaction. Similar effects were observed for the ODHE reaction over NiFe bimetallic-derived catalysts supported on reducible (CeO2) and irreducible (SiO2) oxides. 132 Catalytic conversion of CO2 and ethane into value-added gaseous products (syngas and ethylene) via the SU-CO2Et strategy represents a versatile strategy to mitigate CO2 emissions while utilizing underutilized fractions of abundant shale gas reserves. As summarized in this review, bimetallic-derived materials show promising catalytic activity, product selectivity, and stability for SU-CO2Et, as demonstrated through combined experimental and theoretical efforts in synthesis, reactor studies, and characterization. This section discusses some of the challenges and opportunities to further improve bimetallic-derived catalysts for the reactions of CO2 and ethane.The key to selectively upgrading ethane and CO2 into gaseous and liquid products is to finely control the extent of DRE and ODHE contributions. Based on the aforementioned studies, an illustration is provided in Figure 10 by considering the most relevant key intermediates and reaction pathways [DRE, ODHE, RWGS (reverse water-gas shift), and hydrogenolysis] for different products (C2H4, H2, and CO) and by-products (CH4, H2O, formates, carboxylates, and bi-/carbonates). As implied in Figure 10, designing a catalyst with targeted properties poses significant challenges since it requires the improvement of at least five important reaction steps: (1) CO2 activation, (2) oxygen transfer, (3) activation of C–H/C–C/C=C bonds, (4) formation of the C–O bond, and (5) desorption of C2H4 and CO. Significantly, the failure to balance these five processes would lead to poor activity, selectivity, and/or stability. Thus, delicate design strategies, such as exploring the various synthesis methods described under Synthesis Methods of Supported Bimetallic-Derived Catalysts, need to be considered in order to achieve the desired active sites.For DRE catalysts (e.g., Ni-based catalysts) the efficient non-selective rupture of C–H/C–C/C=C bonds can be achieved easily, while the subsequent oxidation of the derived carbonaceous species is usually more difficult. Thus, these processes may be balanced by (1) enhancing carbon gasification as CO by weakening the binding of carbon and oxygen via alloying or modification with a precious metal with a relatively lower d-band center (e.g., Pd or Pt for Ni); (2) alleviating the rapid non-selective cleavage by hindering the highly mobile electrophilic oxygen species by decreasing large metal NPs into smaller entities; (3) enriching the availability of surface oxygen by introducing an oxophilic element (e.g., Fe, Co, or Mo for Ni) and controlling the CO2/ethane partial pressure ratios to achieve the desired oxygen coverage; or (4) promoting CO2 activation and mobility of resultant oxygen from the bulk to the surface or interface by doping a lower-valence metal into the oxide support bulk lattice, forming a solid solution (e.g., Ce-Zr). 133 , 134 In contrast, the ODHE reaction is more likely to be limited by the C–H bond activation (activity) and the desorption of ethylene (selectivity) than by the availability of oxygen from CO2 activation, since the reaction usually exhibits a zero-order kinetic response to the partial pressure of CO2. In this case, catalytic performance for ODHE may be improved by (1) choosing primary metals with considerable C–H bond activation, less activity to hydrogenolysis (e.g., Pd), 135 and mild interaction with ethylene; (2) weakening the electrophilic nature of oxygen by introducing a secondary metal with stronger M–O bonding 136 to form new MO/metal interfaces (e.g., FeOx/Ni, FeOx/Pd, and SnOx/Pt) on the metal ensembles; (3) isolating adjacent electrophilic oxygen adatoms or selectively blocking the highly active sites (such as steps and kinks) for C–C/C=C bond breaking using strongly binding modifiers (e.g., C, S, SiO2, and AlOx); (4) alleviating the supply of oxygen to prevent further oxidation by moderately suppressing the formation of oxygen vacancies on the support and retarding oxygen transfer via robust interfaces (e.g., FeOx/CeO2 interface); (5) maximizing the desired interfacial structures by reducing the size of the active phase or forming core-shell structures; (6) isolating active metal atoms (e.g., Ni or Pd) by forming well-defined intermetallic compounds 137 with less active metals (e.g., Zn, Ga, In, and Sn) to promote ethylene desorption by relatively downshifting the d-band center; or (7) breaking the trade-off relationship between activity and selectivity with SAA catalysts, anchoring single atoms of an active metal (e.g., Pd) onto a less active metal host (e.g., Cu, Ag, and Au).The synthesis of bimetallic-derived catalysts with the above-mentioned active-site configurations requires careful and precise synthesis strategies. For example, to obtain uniform and ultrasmall supported random alloy or intermetallic compound NPs, SEA with heterometallic double complex salts, RMSI with appropriate metal and support combinations, or ALD with appropriate gaseous precursors can be used. In addition, well-defined MO/metal interfacial structures can also be synthesized using the ALD method. To obtain the SAA catalysts, GR is among the most popular methods, involving an electroredox process between the sacrificial host metal NP template and the metal ions in solution. In addition, for bimetallic structures that cannot be synthesized by conventional thermal methods due to bulk thermodynamic instability, the pulse thermal shock method can be used to synthesize the desired alloy structures.Improving the SU-CO2Et processes using supported bimetallic-derived catalysts requires an understanding of a wide range of structural and chemical features, such as sizes, shapes, morphologies, compositional ordering, surface terminations, metal-support interfacial interactions, reaction-induced reconstructions, chemical states, surface intermediates, and reaction mechanisms. Thus, a definitive structural and mechanistic determination, especially under reaction conditions, is the premise to establish a reliable structure-function relationship. For a well-defined catalyst with a single type of active sites, the bulk electronic and geometric structural information of the bimetallic system can be elucidated relatively easily using combined characterization results from XANES, FT/WT-EXAFS, XRD-PDF, and HAADF/EDS/EELS-(E)STEM under reaction conditions. In contrast, the identification of the surface or interfacial features (compositional, structural, and chemical) in powder bimetallic-derived catalysts remains challenging owing to the interference by the bulk signals, the severe scattering of the signal on the way to the detector, and the short lifetime of the active intermediates. Intuitively, decreasing the size of metal ensembles would amplify the surface or interfacial contributions, enabling XANES, EXAFS, and AXAFS to probe the surface chemical state, interfacial bond formation, and interfacial electronic interaction, respectively. Moreover, it should be feasible to derive the chemical state and the signature of the adsorbate and its binding site using the ΔXANES technique coupled with ab initio calculations.The surface properties of the bimetallic-derived catalysts can be revealed directly by surface-sensitive analytical techniques, such as low-energy ion scattering (LEIS), DRIFTS, XPS, and electron microscopy. LEIS (Figure 5C) is an extremely surface-sensitive technique used to quantify the composition of the topmost surface, although it is restricted to ex situ studies under ultrahigh vacuum conditions. 90 DRIFTS used with appropriate probe molecules such as CO is a useful approach to identify the composition of a bimetallic surface (overlayer, random/ordering alloy, or atom-isolated configurations, etc.). In addition, vibrational spectroscopies such as DRIFTS and Raman are useful techniques to identify key intermediates (e.g., carbonyl, formates, carboxylates, bi-/carbonates, and ethoxy) during some relatively low-temperature processes. 5–7 , 138 However, special care has to be taken when applying these techniques to the high-temperature reaction of SU-CO2Et, since the real intermediates might be too transient and weakly binding to be detected, while the observed species might be spectators. As a complementary technique, the SSITKA and TAP techniques coupled with isotope switching (e.g., 13CO2, C18O2, 13C2H6, or C2D6) and kinetic analysis should be employed to explore the key intermediates and reaction pathways. The advancements of differential pumping and microreactors have enabled the AP-XPS technique to identify the near-surface structural compositions and reaction intermediates over bimetallic-derived catalysts at elevated temperatures and in the presence of CO2 and light alkanes. Structural properties (e.g., size, shape, facets, morphology, defects, strain, alloying, and segregation) generally have been measured by ex situ (S)TEM. In recent years, both layered encapsulation- and RMSI-induced interfaces have attracted extensive attention due to their superior performance during reactions, while their properties are typically sensitive to distinct temperatures and chemical atmospheres. Emerging in situ (or semi-in situ) (S)TEM techniques 139–141 with atomic resolution have demonstrated the feasibility of directly tracking surface encapsulations by MO clusters or layers (e.g., FeOx/Ni [or Pd]) and metal-support dynamic interactions (e.g., PtNi/CeO2) in the presence of CO2 and ethane at reaction temperatures.Equally as important as the metallic components, it is noted that the nature of the oxygen species is critical in determining the bond cleavage of ethane and its derivatives. Certain types of electrophilic oxygen species may exhibit unpaired electrons, which are usually too active and transient to be tracked by the above-mentioned techniques. In this situation, electron spin resonance spectroscopy is an appropriate approach to explore the evolution of these transient oxygen species, due to its extreme sensitivity to paramagnetic features. 142 , 143 At a steady state, the type and chemical state of oxygen species under reaction conditions can be elucidated by performing AP-XPS measurements of the O 1s feature. To gain a deeper understanding of the variations in oxygen reactivity due to electronic, geometric, and kinetic modifications at a specific orbital level, the total density of state (DOS) is usually calculated and projected to the surface atoms of interest (e.g., O and neighboring metals) using DFT calculations. Experimentally, resonant inelastic X-ray scattering (RIXS) spectroscopy—essentially containing XAS and X-ray emission spectroscopy (XES)—makes it feasible to probe the electron orbitals. The XAS and XES spectra provide information on the unoccupied and occupied electronic states, respectively; these results enable projection of the electronic structure onto the excited atom and can be compared directly with the linear combination of atomic orbitals approach used in DFT calculations. Therefore, RIXS should be a powerful tool to investigate intramolecular bonds and adsorption of surface species during the reactions of CO2 and ethane on bimetallic-derived catalysts. In the soft X-ray region (such as around the O and C K-edge), XAS and XES are dominated by dipole transitions, together with the symmetry character of the core level; thus, they can provide important information regarding the chemical bonding in an oxygen (2p)-projected manner. 144 , 145 RIXS also has been applied successfully for metal edges in the hard X-ray region to map out the metal (d)-projected DOS. 146 , 147 By combining the structural and chemical information elucidated by the RIXS- and DFT-derived site-projected DOS—together with information obtained from the other techniques mentioned above—it would be possible to determine the precise nature of the active sites responsible for the various pathways in the reaction of CO2 with ethane.As shown in Figure 10, the CO2-assisted selective cleavage of the C–H and/or C–C bonds of ethane produces a variety of gaseous chemicals, including CO, H2, C2H4, CH4, and H2O. Among these, C2H4, CO, and H2 are important industrial feedstocks, which further can be upgraded into liquid products, such as via the aromatization reaction of C2H4 to aromatics 24 or the hydroformylation reaction of C2H4, CO, and H2 to C3 oxygenates. 26 These gas-phase heterogeneous reactions produce higher-value liquid products (e.g., aromatics and oxygenates) and reduce the costs associated with the separation of products from the gaseous reaction stream or from conventional liquid-phase homogeneous reactions (e.g., hydroformylation). However, the aromatization and hydroformylation reactions require additional types of catalytic active sites. In practical applications, it would be desirable to combine the CO2-ethane reactions (DRE and/or ODHE) with further upgrading (aromatization or hydroformylation), although this would result in additional challenges for the design of bimetallic-derived catalysts containing multifunctional active sites for different reactions. This prospect of achieving SU-CO2Et to higher-value products requires further advances in the synthesis and characterization of promising bimetallic-derived catalysts.This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Catalysis Science Program, under contract DE-SC0012704. L.R.W. acknowledges the US National Science Foundation Graduate Research Fellowship Program grant DGE 16-44869.Conceptualization, Z.X. and J.G.C.; Writing – Original Draft, Z.X.; Writing – Review & Editing, Z.X., L.R.W., and J.G.C.; Supervision, Z.X. and J.G.C.

Bimetallic-derived catalysts have played a pivotal role in many industrially important catalytic processes. In recent years, simultaneously upgrading the greenhouse gas CO2 and underutilized ethane (SU-CO2Et) has represented a promising route for producing value-added chemicals, such as synthesis gas and olefin. This review focuses on the synthesis and characterization of precious and non-precious bimetallic-derived catalysts and their application in SU-CO2Et. We discuss general principles and state-of-the-art strategies for catalyst synthesis, as well as in situ techniques for structural characterization and mechanistic insights. We then illustrate how to design and apply versatile bimetallic-derived catalysts for SU-CO2Et and to establish structure-function relationships by using combined experimental and theoretical approaches. We conclude the review by highlighting challenges and opportunities in active-site design and synthesis, in situ structural and mechanistic identification, and development of multifunctional bimetallic-derived catalysts for further upgrading abundant CO2 and shale gas into valuable aromatics and oxygenates.

According to statistics reported by International Energy Agency (IEA) and the American Energy Information Administration (EIA), the global energy consumption increases by 50 quadrillion Btu every year [1]. From another perspective, the intensive dependence on fossil fuels for electricity generation has absolutely altered our ecosystem [2–8]. Hence, research has been oriented to offer clean, abundant, and renewable sources for energy that suffice the global consumption and maintain a safe environment.In this context, fuel cells (FCs) appeared promising as advanced electrochemical energy converters of featured environmental flexibility, and high efficiency at low operating temperatures [9–12]. Technically, FCs convert directly and efficiently (up to 60%) the chemical energy into electricity with a tremendous (up to 90%) reduction in major pollutants [13]. The polymer electrolyte membrane FCs (PEMFCs) that belongs to the low-temperature FCs that utilize solid polymeric electrolyte for the ionic conduction has attracted a special attention due to their robustness, high power density, and low operational temperature. Of these low-temperature PEMFCs, the direct methanol FCs (DMFCs) and hydrogen FCs (HFCs) offered, recently, a comprehensive interest for electric vehicles and portable electronics [6,14,15]. However, their fast commercialization has been restricted by the huge cost of H2 containers, the prospective risks during H2 transportation and the toxicity and crossover of methanol [2,5,16]. Hence, the DFAFCs appeared more attractive owing to their higher theoretical open circuit potential (1.45 V compared to 1.23 V of HFCs and 1.21 V of DMFCs), lower toxicity, non-flammability and lower crossover through the Nafion membrane [17–20].Till now, the most common anodic catalysts for DFAFCs are mainly based on noble Pt and Pd metals [11,21–24]. However, Pt revealed a higher stability than Pd due to its higher dissolution resistance in harsh reaction conditions [25,26]. However, unfortunately, Pt can easily be poisoned by some reaction intermediates, such as carbon monoxide (CO) [27–29]. Several investigations reported that FAOR proceeds at Pt surfaces with a double-pathway mechanism [2,16,30–33]. The direct pathway which involves the direct oxidation of FA to CO2 (Eq. (1)) and the indirect pathway which involves the non-faradaic adsorption of poisoning CO at the Pt surface (Eq. (2)) followed by its subsequent oxidation at a high overvoltage (Eq. (3)). o Direct dehydrogenation pathway: (1) HCOOH → CO2 + 2H+ + 2e− o Indirect dehydration pathway: (2) HCOOH → COads + H2O (3) COads + H2O → CO2 + 2H+ + 2e− Direct dehydrogenation pathway:Indirect dehydration pathway:Considering the huge cost of Pt catalysts, it became crucial to amend it with a cheaper modifier to increase its utilization efficiency and catalytic performance [34,35]. In fact, previous modifications of Pt with metals (e.g. Au [32], Pd [36], Bi [37] and/or metal oxides (e.g., oxides of Ni [30], Mn [2], Cu [38], and Fe [39])) could greatly sustain better performance toward FAOR with enhanced structural and/or electronic properties.Although the difficultness in controlling size, shape and distribution of the catalyst component [20], electrodeposition represents a facile, fast and economic technique for assembling metal and metal oxide modified catalysts; ensuring a controlled production of a smooth surface with strong bonding with the substrate and offering opportunities for alloys and composite coatings with high hardness [40].In this study, a binary catalyst composed of Pt and Cu was fabricated onto the GC (a typical substrate for the deposition of nanoparticles that can be used for a simple investigation) and proved competent for FAOR. The catalyst was synthesized by the “simultaneous co-electrodeposition” protocol that ensured a convenient homogeneity of the catalytic constituents that were added in minute loadings (relatively to other procedures as the layer-by-layer approach) [2,32]. The molar Pt4+/Cu2+ ratio of the electrolyte during the catalyst's deposition was optimized to attain the highest catalytic activity toward FAOR. Furthermore, the catalyst’s morphology, surface composition, and molecular structure were inspected to address the remarkable enhancement of these catalysts toward FAOR.Copper (II) sulfate pentahydrate (CuSO4·5H2O, 99%), sodium hydroxide-pellets (NaOH), sodium sulfate anhydrous (Na2SO4), and FA (HCOOH, 98%) were purchased from Alfa Aesar while dihydrogen hexachloroplatinate (IV) hydrate (Premion®, H2PtCl6·6H2O, 99.9%, metals basis) and sulfuric acid (AR, H2SO4, 98%) were purchased from Sigma Aldrich. All chemicals were of high purity and were used as received without further purification. A three-electrode electrochemical cell was used for the catalyst's preparation and electrochemical and catalytic inspections. A cleaned (by mechanical polishing with aqueous slurries of successively finer alumina powder (down to 0.06 mm) followed by a thorough washing with second distilled water) pristine and modified glassy carbon (GC) electrode (5.0 mm diameter) of a geometric area of ca. 0.196 cm2 was used as the working electrode, a spiral Pt wire was used as the counter electrode and an Ag/AgCl/NaCl (3 M) electrode was used as the reference electrode. All potentials were measured relative to this Ag/AgCl/NaCl (3 M) reference electrode.The “simultaneous co-electrodeposition” technique was employed to prepare PtxCuy catalysts onto the GC electrode surface with several molar ratios (starting from 1:0 till 1:4) [2,32]. The electrolyte of electrodeposition was 0.1 M Na2SO4 aqueous solution containing 2.0 mM H2PtCl6·6H2O and 2.0 mM CuSO4·5H2O For all catalysts, the electrodeposition of Pt and Cu onto the GC electrode surface was carried out potentiostatically at −0.2 V permitting the passage of only 9.4 mC.For a simple recognition of the electrodes' preparation, an abbreviation of PtxCuy was assigned to recognize the molar ratio of Pt4+ to Cu2+ in the deposition electrolyte, where x and y referred to the molar ratios of Pt4+ and Cu2+ ions, respectively. For example, the catalyst denoted as Pt1Cu4 correspond to a mole ratio of 1:4 for Pt4+ to Cu2+ ions in the deposition electrolyte.All electrochemical experiments were tested at room temperature (ca. 25 ± 1 °C) in aqueous solutions using a Bio-Logic SAS Potentiostat (model SP-150) operated with EC-Lab software. The electrocatalytic performance of the PtxCuy catalysts toward FAOR was inspected in aqueous solutions containing 0.3 M FA (pH ∼3.5). The pH was adjusted by a dilute solution of NaOH. Current densities were always calculated on the basis of real Pt surface areas of the working electrodes (as Fig. S1 shows) employing a reference value of 420 μC cm−2 [41].The morphology and elemental composition of PtxCuy catalysts were evaluated using a field-emission scanning electron microscope (FE-SEM, Quattro S, Thermo Fisher Scientific USA) whose accelerating voltage extended from 200 V to 30 kV with a magnification range from 6 to 2500000x that equipped with an energy dispersive X-ray spectrometer (EDS, AMETEK USA Element Detector). The crystallographic information of PtxCuy catalysts was obtained using a high resolution X-ray diffractometer (XRD-PANalytical X’Pert Pro powder) that operated with a Cu anode, wavelength 0.154 nm, maximum 2.2 kW, and 60 kV. The inductively coupled plasma mass spectrometry, ICP-MS, (8800 ICP-MS, Agilent Technologies) was employed to assess the dissolution (loss) of Pt and Cu from the catalysts after stability measurements. Fig. 1 a (Pt1Cu0 catalyst) displays the characteristic performance of a poly-Pt electrode in an acidic medium. This demonstrated the oxidation of Pt which extended over a potential range between ca. 0.6 and 1.2 V and coupled with the subsequent PtO reduction at ca. 0.46 V. Furthermore, the peaks that appeared in the potential range between 0 and −0.2 V were assigned to the hydrogen adsorption/desorption (Hads/des) at the Pt surface [25]. For Fig. 1b–e, the current intensities of the Hads/des and PtO reduction peaks were gradually decreased with a parallel decrease in the intensity of the PtO reduction peak. This resulted from the distribution of the deposition charge between Pt and Cu which further indicated the successful deposition of Cu. As a result of Cu deposition, a new redox couple corresponding to the Cu oxidation (at ca. 0.45 V) and its subsequent reduction (at ca. 0.25 V) was developed [42–44]. With the further increase of Cu2+ in the solution (Fig. 1d and 1e), the current intensities of the Hads/des and PtO reduction peaks continued decreasing significantly concurrently with an observable increase in the current intensity of the Cu oxidation peak which appeared split [45,46] in two peaks; at 0.05 V and 0.45 V (almost similar to those obtained at the Pt0Cu1 catalyst (Fig. S2A)) to infer about a possible phase transformation. The calculated Pt surface area for all proposed catalysts was additionally calculated, based on the procedures in Fig. S1, as appeared in Table S1.The investigation is directed to elucidate the surface morphology, elemental composition, and molecular structure of the proposed PtxCuy catalysts. Fig. 2 displays FE-SEM micrographs of the Pt1Cu0, Pt1Cu1, Pt1Cu2, Pt1Cu3, and Pt1Cu4 catalysts. As obviously seen in Fig. 2A, the deposition of Pt (Pt1Cu0 catalyst) occurred in spherical shape (ca. 110 nm in average diameter) with intensive aggregations (∼500 nm in diameter each). This morphology was retained for Pt with the deposition of Cu in starfish and/or intersected laminar structures (almost similar to the Cu morphology for the Pt0Cu1 catalyst appeared in Fig. S2B) in the Pt1Cu1, Pt1Cu2, Pt1Cu3, and Pt1Cu4 catalysts (Fig. 2B-E).Moreover, the EDS spectra of the Pt1Cu0, Pt1Cu1, Pt1Cu2, Pt1Cu3, and Pt1Cu4 catalysts (Fig. 3 a-e, respectively) provided a confirmation for the deposition of catalytic ingredients in the different catalysts where all peaks of C, O, Pt, and Cu appeared in their expected positions [47–49]. The elemental mapping of the Pt1Cu0 and Pt1Cu4 catalysts demonstrated the homogeneous distribution of C, O, Pt, and Cu elements in the proposed catalysts (Figs. S3A and B, respectively).Furthermore, the crystal structures of the different PtxCuy catalysts were examined by XRD (Fig. 4 a-e) where several diffraction peaks were identified for all catalysts at ca. 25°, 43° and 79° corresponding, respectively, to the (002), (100), and (110) planes of the hexagonal carbon structure (JCPDS card No. 075-1621) [50]. Also, the diffraction peaks identified at ca. 38.6°, 44.6°, 65.4°, and 78.7° for all catalysts were assigned, respectively, to the (111), (200), (220), and (311) planes of the face-centered cubic (fcc) Pt lattice (JCPDS card No. 96-101-1112) [32,51]. The Pt1Cu1, Pt1Cu2, Pt1Cu3, and Pt1Cu4 catalysts showed three diffraction peaks at 2θ of ca. 43.1°, 50.2°, and 73.7°, consistent with those observed for the Pt0Cu1 catalyst (Fig. S2C), which were assigned to the (111), (200), and (220) facets of metallic Cu (JCPDS card No. 96-901-3016) [52–54]. The very small positive shift (ca. 0.05°) in the Pt diffractions in the Cu-modified catalysts might account for the composition change of the catalyst surface and/or Pt-Cu alloying [55]. Fig. 5 (for easy comparison, all subfigures in Fig. 5 were grouped in a single figure - Fig. S4) represented the influence of Pt4+ and Cu2+ relative molar ratio in the deposition electrolyte on the catalytic activity of the proposed PtxCuy catalysts toward FAOR. First, it worth to point the inactivity of the unmodified GC electrode [56] and Cu [57] toward FAOR (see Fig. S2D). A blank test was carried out in FA-free solution (FAFS) that has the same pH (3.5) with the same measuring conditions to confirm our interpretation (will be mentioned later in text) for the peaks associated with FAOR. Fig. S5 confirmed that in FAFS, no peaks were detected at the same potentials like in case of the solution contacting FA. In Fig. 5A (Pt1Cu0 catalyst), two oxidation peaks were observed in the forward (anodic-going) scan at 0.34 V and 0.8 V. The first peak (at ca. 0.34 V) was assigned to the direct (preferred, because of its lower anodic overpotential that turns the output voltage of DFAFCs higher) oxidation of FA to CO2 (Eq. (1)). The current density of this peak will be abbreviated as I p d. The second peak (ca. 0.8 V) was assigned to the oxidation of the COads to CO2 (Eq. (3)) after the hydroxylation of the Pt surface at a potential of ca. 0.7 V. The current density of this peak will be abbreviated as I p ind [58]. In reality, the core challenge of assigning Pt-based catalyst for FAOR is related to the adsorption of CO (COads) which occurs spontaneously from the non-faradaic dissociation of FA at open circuit potentials (Eq. (2)). This deactivates the Pt surface and prompts a potential poisoning for a significant number of Pt active sites, which, in turns, impede the direct “preferred” pathway of FAOR. Balandin proposed the “Multiplet theory” that investigated the simultaneous adsorption of reacting species to a group of active atoms of a given catalyst [59]. He proposed a correspondence between the geometry of active centers and the energies of forming and breaking chemical bonds of the adsorbate/adsorbent clusters. According to this theory, the adsorption of CO on the Pt surface requires the presence of three neighboring Pt active sites with a certain geometry. If this contiguity was disturbed, the Pt–CO bonding will not form; as will be elaborated below. In the backward (cathodic-going) scan, the Pt surface became clean (free of poisoning COads) after the oxidation of poisoning COads and that boosted FAOR as shown from the high peak current density of the backward scan (I p b).For the Pt1Cu1, Pt1Cu2, Pt1Cu3, and Pt1Cu4 catalysts, critical changes appeared and influenced the relative ratios of I p d, I p ind, and I p b which denote important catalytic information. Generally, the I p d/I p ind ratio evaluates the enhancement in the catalytic activity in the favorable direct oxidation pathway. On the other hand, the I p d /I p b ratio estimates the catalytic tolerance of the catalyst for poisoning CO species. These changes in current densities can associate a further change in the onset potential of the direct FAOR, E onset (measured at a constant current density of ca. 0.2 mA cm−2), that reflects the capability of the catalyst to overcome unnecessary overpotentials (particularly of charge transfer) that normally detracts the voltage output of the cell. The I p d/I p ind ratio of the Pt1Cu0 catalyst was ca. 0.65 (see Table 1 ), which is low to permit the movement for DFAFCs into a real commercialization. The increase of this I p d/I p ind ratio of the catalyst is highly important for a commercial purpose. Fascinatingly, the CVs in Fig. 5B-E and the catalytic data in Table 1 demonstrated the importance of adding Cu to the catalysts and elaborated the influence of varying the (Pt4+ and Cu2+) molar ratios in the deposition electrolyte on the catalytic activity toward FAOR. Interestingly, both of I p d/I p ind and I p d /I p b increased with the increase in the Cu2+ molar ratio. This reflected the critical role of Cu to direct FAOR in the direct pathway and to mitigate the CO poisoning. In addition, with the increase in Cu2+ molar ratio, a regular negative shift in E onset was observed. The best catalytic data was obtained for the Pt1Cu4 catalyst whose I p d/I p ind was 3.58 (i.e., 6-times as that of the Pt1Cu0 catalyst). Its I p d /I p b was also the highest (0.73, i.e., 4-times as that (0.18) of the Pt1Cu0 catalyst). The negative shift in E onset of this Pt1Cu4 catalyst was as well the largest (ca. 336 mV). Fig. 6 represents graphically these catalytic data. Also, several other parameters such as the potentials at I p d, I p ind, and I p b (E p d, E p ind, and E p b, respectively) were monitored and tabulated in Table S2. The potential changes associated with FAOR may relate to the surface composition change of the proposed catalysts.It is important to mention that the catalytic activities of the Pt1Cu5 and Pt1Cu6 (see Figs. S6A and B and data in Table S3) catalysts toward FAOR were lower than that of the Pt1Cu4, presumably due to the too low loading of the active (Pt) component in the catalysts. Hence, the Pt1Cu4 represented the best catalyst for FAOR among all the inspected catalysts in this investigation. Interestingly, this activity surpassed many of the reported activities for FAOR in literature (see Table S4 in the supplementary data file).Another important measurement besides the catalyst’s activity is related to the catalyst’s stability. Herein, the catalyst’s modification with Cu was proposed not only to promote the catalytic activity but also to enhance the stability of the catalyst, which quickly deteriorates during continuous electrolysis. The stability of the entire set of our proposed catalysts were assessed by chronoamperometric measurements (see Fig. 7 ) for 3600 s at a constant potential of 0.2 V. Fig. 7 (a-e, respectively) displays the current transients (i-t curves) of the Pt1Cu0, Pt1Cu1, Pt1Cu2, Pt1Cu3, and Pt1Cu4 catalysts in aqueous solution of 0.3 M FA (pH ∼3.5) at 0.2 V. As obviously seen in Fig. 7a and the attached inset, the current density of the Pt1Cu0 catalyst decayed rapidly due to the accumulation of poisoning CO on the Pt surface. This decay diminished largely for the Pt1Cu1, Pt1Cu2, Pt1Cu3, and Pt1Cu4 catalysts (see Fig. 7b-e). Interestingly, the highest stability was also recorded for the Pt1Cu4 catalyst (20% loss in the catalytic activity compared to 35% for the Pt1Cu0 catalyst, see Fig. 7a and e). This represented an additional merit for Cu in boosting the catalytic tolerance of the PtxCuy catalysts against CO poisoning during FAOR.The ICP-MS was employed to assess the loss in the catalytic ingredients (Pt & Cu) after the electrochemical stability inspection. As expected, a loss in Cu was observed, which was expected for Cu at high potentials. This loss in Cu increased with the molar ratio of Cu2+ ions in the deposition electrolyte (Table 2 ). On contrary, the loss of the active and precious material (Pt) in the Pt1Cu0, Pt1Cu1, Pt1Cu2, Pt1Cu3, and Pt1Cu4 catalysts was relatively minor and got decreased with the Cu2+ molar ratio (Table 2). This reinforced the role of Cu in boosting the durability of the PtxCuy catalysts and in ranking the Pt1Cu4 catalyst the best for FAOR.Electrochemical impedance spectroscopy (EIS) was used to interpret the catalysis of FAOR on the PtxCuy catalysts. The charge transfer resistance (R ct) of the proposed PtxCuy catalysts was correlated to their catalytic performance toward FAOR [13,60,61]. Fig. 8 (a-e, respectively) represents the Nyquist plots for all catalysts in aqueous solution of 0.3 M FA (pH ∼3.5) at a potential of 0.2 V in the frequency range between 10 mHz and 100 kHz. The data fitting was carried out using the EC-Lab software and the equivalent circuit of this system was displayed in the inset of Fig. 8. Over there, R s and C dl referred to the solution resistance and double layer capacitance, respectively, of the electrochemical system. Analysis of R ct for the whole set of catalysts grouped them in two categories; one of a lower and another of a higher R ct than that (0.21 kΩ) of the unmodified Pt1Cu0 catalyst (Fig. 8a). The Pt1Cu1 and Pt1Cu2 catalysts recorded 0.14 and 0.15 kΩ, respectively, for R ct as obviously seen from their smaller semicircle diameters (Fig. 8b and c). Such a decrease in R ct inferred the existence of an electronic element in the catalytic enhancement [5,13]. This electronic enhancement could possibly result from the Pt-Cu alloying that might affect the Pt–FA, Pt–CO2 and/or Pt–CO bonding or perhaps from the participation of Cu with its higher electrical conductivity than Pt [62,63] in the reaction mechanism of FAOR in the way facilitating the kinetics of charge transfer. This might associate a structural influence that could synergistically boost the catalytic enhancement. Surprisingly, the Pt1Cu3 and Pt1Cu4 catalysts owned higher (0.27 and 0.43 kΩ, respectively) R ct than that of the Pt1Cu0 catalyst with larger semicircle diameters (Fig. 8d and e). This came consistent with diminishing the active Pt surface and the appearance of redox pair for copper (recall the splitting of the Cu peak that appeared only in Fig. 1d and e) that probably deactivated Pt electronically toward FAOR. This electronic deactivation was not equivalent to the geometrical (structural, third body) influence that Cu added to Pt which boosted synergistically the catalytic activity of the Pt1Cu3 and Pt1Cu4 catalysts toward FAOR. Table 3 summarizes the electrochemical data (R s and R ct) obtained from Fig. 8.To precisely confirm this claim, CO was allowed to be adsorbed at open circuit potential for 10 min and then stripped oxidatively in CO-free electrolyte containing 0.5 M Na2SO4 (pH ∼3.5) at the Pt1Cu0, Pt1Cu1, Pt1Cu2, Pt1Cu3, and Pt1Cu4 catalysts (Fig. 9 a-e, respectively). The Pt1Cu0 catalyst (Fig. 9a) showed (in the anodic scan) a zero current (Pt surface was blocked) up to ca. 0.73 V where CO started to desorb [64,65]. The charge (Q CO) consumed in the CO stripping is proportional to the poisoning level of COads and the onset potential of CO desorption (Eonset/ CO) assess the minimum energy required for this desorption, which also accounts for the electronic properties of the Pt surface. Fortunately, the data of Fig. 9 agreed with the hypotheses of Fig. 8 in suggesting prevailing the electronic element in the catalytic enhancement of the Pt1Cu1 and Pt1Cu2 catalysts. This was obvious in the increased negative shift of their Eonset/ CO (Fig. 9b and c). However, the amount of Cu in the Pt1Cu1 catalyst was not sufficient to provide an overall (electronic and geometric) enhancement for CO adsorption. The behavior of the Pt1Cu2 catalyst was much better in terms of Q CO and Eonset/ CO. Interestingly, regardless the approximate agreement in their Eonset/ CO, the Pt1Cu3 and Pt1Cu4 catalysts retained much lower Q CO than that of the Pt1Cu0 catalyst which highlighted the geometrical influence in retarding the adsorption of poisoning CO at the Pt surface. Table 4 summarizes the data obtained from Fig. 9 which confirmed ca. 25, 60 and 75% improvement in the CO tolerance on the Pt surface of the Pt1Cu2, Pt1Cu3 and Pt1Cu4 catalysts, respectively. This confirmed the structural (third body) enhancement for the Pt1Cu2, Pt1Cu3, and Pt1Cu4 catalysts. Besides, Cu could also facilitate the oxidative removal of CO at lower potentials (electronic impact) at the Pt1Cu1 and Pt1Cu2 catalysts as respectively shown from the −0.17 and −0.22 V shift E onset/ CO. Lastly, it is important to mention that although the Pt1Cu4 catalyst did not show any electronic enhancement, it acquired the highest activity and stability toward FAOR that originated solely from its structural (third body) enhancement effect.A PtxCuy binary catalyst was endorsed for efficient FAOR. The molar ratio of Pt4+ and Cu2+ ions in the deposition bath influenced, to a high degree, the catalytic performance and the enhancement mechanism toward FAOR. The Pt1Cu4 catalyst retained the highest catalytic activity (with up to ca. 6 times increase in the I p d/I p ind index, 4 times increase in the I p d /I p b index and −336 mV shift in E onset) of FAOR. This associated critical improvement in the catalytic stability that appeared in maintaining the highest current density and lowest current decay during prolonged electrolysis at 0.2 V, comparing to all other inspected catalysts. Based on the EIS and the CO stripping measurements, the catalytic enhancement of the Pt1Cu4 catalyst arose principally from a structural (third body) effect.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Supplementary data to this article can be found online at https://doi.org/10.1016/j.jscs.2022.101437.The following are the Supplementary data to this article: Supplementary data 1

A propitious binary catalyst composed of Pt and Cu which were electrodeposited simultaneously onto a glassy carbon (GC) substrate was recommended for the formic acid (FA) electro-oxidation reaction (FAOR); the principal anodic reaction in the direct FA fuel cells (DFAFCs). The simultaneous co-electrodeposition of Pt and Cu in the catalyst provided an opportunity to tune the geometric functionality of the catalyst to resist the adsorption of poisoning CO at the Pt surface that represented the major impediment for DFAFCs marketing. The catalytic activity of the catalyst toward FAOR was significantly influenced by the (Pt4+/Cu2+) molar ratio of the electrolyte during electrodeposition, which also affected the surface coverage of Pt and Cu in the catalyst. Interestingly, with a molar (Pt4+/Cu2+) ratio of (1:4), the catalyst sustained superior (3.58 compared to 0.65 obtained at the pristine Pt/GC catalyst) activity for FAOR, concurrently with up to four-times (0.73 compared to 0.18 obtained at the pristine Pt/GC catalyst) improvement in the catalytic tolerance against CO poisoning. This associated, surprisingly, a negative shift of ca. 336 mV in the onset potential of FAOR, in an indication for the competitiveness of the catalyst to minimize superfluous polarizations in DFAFCs. Furthermore, it offered a better (ended up with 20% loss in the activity) stability for continuous (1 h) electrolysis than pristine Pt/GC catalyst (the loss reached 35%). The impedance and CO stripping measurements together excluded the electronic element but confirmed the geometrical influence in the catalytic enhancement.

Subject Chemical Engineering: Catalysis Specific subject area Development of catalyst for green transformation of waste feedstock's to valuable chemicals Type of data Figures How the data were acquired XRD (Bruker D8 Discover), TGA (SDT Q600, T.A. Instruments), NH3–TPD (Micromeritics Autochem-2920), Pyridine diffuse reflectance infrared Fourier transform spectroscopy (DRIFT, Thermofisher Scientific NICOLET iS50 FTIR spectrometer), XPS (PHI5000 Version Probe III (ULVAC-PHI)), HRSEM (Hitachi S-4800) and HRTEM (JEM-2100 Plus, JEOL, Japan). Raw profiles and images were collected for the biochar, activated biochar and metal loaded activated biochar. Data format Raw and processed data Description of data collection X-ray diffraction profiles were collected in 2θ range of 10–90°, TGA in N2 ambience at a heating rate of 5 °C min−1, acidity of the catalysts was measured using NH3–TPD up to 800 °C using a thermal conductivity detector (TCD).Pyridine-DRIFT spectrum was recorded at 240 °C. EDS images were obtained using HRTEM JEM-2100 Plus instrument operated at 200 kV. Data source location Indian Institute of Technology Madras, Chennai, India. Data accessibility Repository name: Mendeley DataData identification number (doi): http://dx.doi.org/10.17632/v38nbb9rtp.3 Direct URL to data: http://dx.doi.org/10.17632/v38nbb9rtp.3 Related research article Authors’ names: L. Gurrala, M. M. Kumar, A. Yerrayya, P. Kandasamy, P. Castaño, T. Raja, G. Pilloni, C. Paek, R. VinuTitle: Unraveling the reaction mechanism of selective C9 monomeric phenols formation from lignin using Pd-Al2O3-activated biochar catalystJournal: Bioresource Technology, http://dx.doi.org/10.1016/j.biortech.2021.126204 • The biochar obtained from biomass pyrolysis has a significant potential to be used as a catalyst support. Furthermore, the development of composite catalysts with Lewis acidic metal oxide supported on renewable carbon is vital for the production of chemicals and fuel molecules via hydrogenolysis and hydrodeoxygenation. • The detailed characterization methodology of biochar-derived catalyst (Pd-Al/ABC) is essential for researchers working in the field of green chemistry, renewable energy, and catalysis for the production of fine chemicals. • The present data can be used to gain fundamental insights on the properties of metal loaded activated biochar catalyst. The structural understanding can be used to probe the elementary reactions occurring on the catalyst active sites at different reaction conditions, and to develop structure-activity relationships. The biochar obtained from biomass pyrolysis has a significant potential to be used as a catalyst support. Furthermore, the development of composite catalysts with Lewis acidic metal oxide supported on renewable carbon is vital for the production of chemicals and fuel molecules via hydrogenolysis and hydrodeoxygenation.The detailed characterization methodology of biochar-derived catalyst (Pd-Al/ABC) is essential for researchers working in the field of green chemistry, renewable energy, and catalysis for the production of fine chemicals.The present data can be used to gain fundamental insights on the properties of metal loaded activated biochar catalyst. The structural understanding can be used to probe the elementary reactions occurring on the catalyst active sites at different reaction conditions, and to develop structure-activity relationships.The chemically activated biochar (ABC) was prepared from biochar, which was obtained as a by-product from co-pyrolysis of biomass and waste plastic [2–4]. Al and Pd metals were loaded on ABC sequentially using wetness impregnation method [5]. The synthesized catalyst was analyzed to gain morphological and structural information. The raw and processed data of 2θ vs intensity values are reported in Mendeley data [6]. Fig. 1 shows the powder XRD profiles of the untreated biochar, ABC and metal loaded ABC catalysts. The XRD profile of untreated biochar shows crystalline diffractions due to the inorganic constituents in it, while after chemical activation most of the diffractions are absent for ABC. Only one sharp crystalline diffraction was observed at 2θ 26.6°. For Pd loaded catalyst, diffraction at 2θ 39.9, 46.4, 68.0 and 81.8° were observed. The TG mass loss and differential mass loss profiles of ABC and Pd loaded ABC in presence of nitrogen are shown in Fig. 2 , and the associated data is presented in Mendeley data [6]. The major mass loss regimes were observed at 300-450 °C and 500-700 °C in the derivative mass loss profiles of ABC, 2Pd/ABC and 2Pd-Al/ABC. The mass loss at 850 °C follows the trend: 2Pd/ABC (19.2%) ∼ 2Pd-Al/ABC (19.1%) > ABC (15.2%). Fig. 3 a shows the XPS survey spectrum of reduced 2Pd-5Al/ABC catalyst. The carbon 1s peak at 284.5 eV (CI) was taken as the reference [7]. The deconvoluted C1s spectrum shows different carbons corresponding to binding energies (BE) ∼285.0, ∼286.5, ∼287.6 and ∼289.1 eV, and these are labelled as C1, C2, C3 and C4, respectively. The relative area % of each of these deconvoluted peaks corresponding to different carbons are CI (10.8%), C1 (78.1%), C2 (5.4%), C3 (2.6%), C4 (3.1%). Similarly, the XPS of Pd3d was further deconvoluted to 5/2 and 3/2 of Pd0 and Pd2+, respectively. The BE values for Pd3d are 341.2, 342.9, 335.4, and 336.0 eV. The XPS data of binding energy vs intensity for the survey spectrum, C 1s and Pd 3d are available in Mendeley data [6]. Fig. 4 shows the HRTEM images of the ABC, Pd/ABC, and 2Pd-5Al/ABC catalysts at different magnification. The lattice fringes corresponding to Pd and Al2O3 are also displayed. Fig. 5 depicts the STEM image of 2Pd-5Al/ABC and elemental mapping images of C, Al, O, and Pd. The EDS spectrum shows the elemental composition of the loaded Al and Pd metals, which is ∼5% and ∼2%, respectively. Fig. 6 shows the TCD signal of NH3 evolved from temperature programed desorption from ABC and metal loaded ABC. The actual data of temperature vs TCD signal for the different catalysts are presented in Mendeley Data [6]. Three desorption peaks in the range of 100–250 °C (D1), 250-500/550 °C (D2) and >500/550 °C (D3) are observed. TCD signal of ABC support without adsorbing NH3 (dashed line) using NH3-TPD shows desorption peak above 500 °C. The pyridine-DRIFT analysis of ABC and Pd-loaded catalysts are shown in Fig. 7. A broad peak was observed at 1440 cm−1 in all pyridine-DRIFT profiles. The processed FTIR data of wavenumber vs transmittance are given in Mendeley data [6]. A Bruker D8 Discover diffractometer was used to collect powder X-ray diffraction (XRD) patterns. An integrated multi-mode EIGER2 detector with Ni filter and CuKα (λ = 1.5406 Å) operated at 40 kV, 30 mA was used. XRD profiles were recorded at a scan speed of 0.3° s−1 with a step size 0.02°. The 2θ range was 10–90°. The mass loss profiles of the catalysts were recorded using T.A. Instruments SDT Q600 thermogravimetric analyzer equipped with internal air cooling unit and horizontal sample holder. The measurement was carried out in N2 ambience with 100 mL min−1 flow rate. Typically, 5 ± 0.4 mg of the catalyst sample was taken in an alumina cup, and heated up to 850 °C at a heating rate of 5 °C min−1. An empty sample cup was loaded in the reference pan. The mass loss was continuously monitored, and the derivative mass loss was calculated to understand the regimes of major decomposition and the temperature corresponding to maximum mass loss rate.A Micromeritics Autochem-2920 instrument was used to determine the acidity of the catalysts using ammonia temperature programmed desorption (NH3–TPD) method. The typical steps involved in the TPD measurement include: (a) activation of the catalyst at 300 °C in He ambience (40 mL min−1) for 60 min, (b) decrease in temperature to 50 °C, (c) adsorption of ammonia gas of 10% concentration in He (30 mL min−1) for 30 min at 50 °C, (d) evacuation of the physisorbed ammonia by flushing with He (30 mL min−1) for 60 min at 100 °C, and (e) monitoring the release of chemisorbed ammonia by increasing the temperature to 750 °C at 10 °C min−1 in continuous He flow (40 mL min−1). The concentration of the desorbed ammonia was measured using a thermal conductivity detector (TCD).A Thermofisher Scientific NICOLET iS50 FTIR spectrometer was used to conduct pyridine diffuse reflectance infrared Fourier transform (DRIFT) study to assess the type of acid sites in the catalyst. The catalyst sample was taken in a high vacuum cell provided by Harrick Scientific Products, and assembled with the spectrometer. The collection of a DRIFT spectrum involved the following steps: (a) initial activation of the sample in N2 ambience (20 mL min−1) at 300 °C followed by cooling it to 100 °C, (b) recording a baseline spectrum before pyridine adsorption at 100 °C, (c) injection of 30 µL pyridine while cooling down the sample cell from 100 to 50 °C, (d) evacuating the physisorbed pyridine by increasing the temperature to 100 °C and maintaining it for 60 min, and (e) raising the temperature further to 240 °C, and recording the DRIFT spectrum after 15 min at 240 °C. The final pyridine adsorption spectra was obtained by subtracting the baseline spectrum obtained at 100 °C from that obtained at 240 °C.High-resolution scanning electron microscopy (HRSEM) images of the catalysts were obtained using Hitachi S-4800 HRSEM, which was operated in the voltage range of 0.5–30 kV and current of 10 µA. A JEM-2100 Plus instrument (JEOL, Japan) operated at 200 kV was used to record high-resolution transmission electron microscopy (HRTEM) images of the catalysts. Typically, ∼1 mg of sample was finely dispersed in isopropyl alcohol of 30 mL using ultrasonication. This highly dispersed sample was drop casted on a carbon-coated copper grid. Finally, isopropyl alcohol was dried at room temperature for 48 h. X-ray photoelectron spectroscopy (XPS) measurements were performed using PHI5000 Version Probe III (ULVAC-PHI). X-ray core level spectra were recorded using Al Kα radiation (hʋ = 1486.6 eV). The carbon 1s peak at 284.5 eV was taken as the reference to determine the binding energy values of various elements in the catalyst.Not applicable. Lakshmiprasad Gurrala: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Visualization, Writing – original draft, Writing – review & editing; M. Midhun Kumar: Methodology, Validation, Formal analysis; Changyub Paek: Methodology, Visualization, Funding; R. Vinu: Conceptualization, Methodology, Formal analysis, Visualization, Writing – original draft, Writing – review & editing, Resources, Supervision, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The corresponding author thanks ExxonMobil Research and Engineering (EMRE), U.S.A., for the research funding to IIT Madras. The National Center for Combustion Research and Development at IIT Madras is funded by Department of Science and Technology, Govt. of India.

This article presents experimental data on the techniques used for the characterization of Pd-Al2O3 supported on activated biochar (2Pd-5Al/ABC) catalyst. The reported data is collected as a part of the research on the 2Pd-5Al/ABC catalyst used for lignin hydrogenolysis [1]. The data on X-ray powder diffraction, ammonia-temperature programmed desorption, pyridine diffuse reflectance infrared Fourier transform spectroscopy, and high-resolution scanning electron microscopy of various catalysts are valuable to study the changes in surface morphology and acidity upon metal loading. The data from thermogravimetric analysis, X-ray photoelectron spectroscopy, and scanning electron microscopy-energy dispersive X-ray spectroscopy are also provided to understand the thermal stability, ionic state of various metals and elemental composition of the catalyst, respectively. The data provided can be used for developing novel catalysts from renewable biochar, and the characterization of noble metal-metal oxide loaded catalysts can aid researchers to design composite catalytic materials for various applications.

Access to safe and clean drinking water has become a major source of contention and concern across the world. People in most parts of the world are having difficulty getting sufficient drinkable water. Water contamination has become more and more of a concern. Water pollution is a serious issue in developing countries with the majority of water in rivers and streams polluted [1]. Contamination and lack of drinkable water caused by microbiological and chemical pollution are major issues in rural areas all over the world. According to the WHO, the majority of people of developing nations suffer from health complications linked to microbiologically or chemically polluting potable water. The primary issue is generally the microbiological purity of drinkable water. The term “groundwater” refers to an important supply of drinking water. Water pollution has resulted in the spread of waterborne diseases [2,3].Water pollution due to chemicals is either caused by organic or inorganic compounds. The presence of organic compounds is the primary cause of major drinking water supply issues. Synthetic chemical compounds such as dyes and pigments, insecticides, herbicides, nitroarenes and chlorinated compounds such as trihalomethans and their derivatives are examples of organic compounds [4]. Water could be polluted due to soluble organic compounds or the presence of too many inorganic ions such as sulfates, nitrates, mercury, cadmium, arsenic, lead, and other heavy metal ions. The mixture of mono-aromatic and di-aromatic benzene based hydrocarbon compounds is the common organic contaminant of natural waters [5].Nitrophenols and dyes are considered as water contaminants, which have high-quality biological and chemical stability and elevated toxicity [6]. Consequently, numerous techniques were developed for their oxidative and/or reduction. For instance, for the oxidative degradation, Fenton reactions are generally used for nitrophenols and dyes using hydrogen peroxide [7,8]. Currently, the catalytic reduction of nitrophenols and dyes in aqueous media using borohydride as a source of reducing agent is considered as a green and alternative facile method to resolve such ecological issue and gains increasing consideration. Moreover, the product of 4-nitrophenol reduction (aminophenol) is a fundamental intermediate in the synthesis of plasticizers, drugs, organic dyes, and hair dyeing agents [7,9].Aromatic amino group compounds and their precursors are important chemical raw materials that are used in the production of medicines and other fine products [10]. China produces 200,000 tons of 4-aminophenol per year with an annual demand increase of 8%. The pharmaceutical sector consumes roughly 85% of 4-aminophenol, which is used to synthesize benorilate (4-acetamidophenyl-2-acetoxybenzoate), clofibrate, and paracetamol (4-acetamidophenol). Benorilate and paracetamol, in particular, are potential antipyretic and analgesic replacements that do not have the same side effects as aspirin and phenacetin. As a result, they are an important component of non-steroidal anti-inflammatory medications all over the world. The catalytic reduction of nitro derivatives is the commercial approach for their manufacture. The most important easy way is the 4-nitrophenol reduction, which is for the most part accomplished by metal catalyst and reducing agent [11].Besides, 4-NP also acts as a pollutant, so it is a squeezing requirement for developing compelling and eco-friendly way to eliminate 4-NP from wastewater [12]. Conventional strategies include adsorption [13], photocatalytic and microbial degradation; [14], the electro-coagulation [15], electro-Fenton process [16], catalytic reduction by metal nanoparticles [17], and electrochemical treatment. However, all the above systems have their disservices. Among them, the metal nano-catalyst easily assists 4-nitrophenol reduction into 4-aminophenol because of its effective, practical and green eco-friendly properties [18]. Additionally, the reduction of nitro-aromatic to amines, such as 4-NP to 4-aminophenol, is a crucial step in synthetic organic and medicinal chemistry and the manufacture of numerous industrially vital chemicals [19].Metallic nanoparticles supported catalysts [20–22] have effectively left their promise in various fields of technological and scientific research. Presently, the world of catalysis including metallic nanoparticles has been revolutionized and is anticipating an oceanic change. Bimetallic nanomaterials are investigated widely in terms of their finer electronic, optical and catalytic properties over monometallic partners, because of the synergistic impacts between the two individual metals, alongside the across the extensive applications in sensors [23,24], plasmonics [25], catalysis [26,27], biomedical, and drug delivery [28]. Besides, the characteristics of the bimetals are firmly determined by the shape, size, types, and structure of the nanoparticles. Recently graphene oxide supported porous PtAu alloyed nanoflowers nanocomposites has been used for the catalytic reduction of 4-NP to 4-aminophenol (4-AP) by sodium borohydride [29]. Similarly, branched Pt@Ag core–shell nanoparticles showed improved catalytic activity for the reduction of 4-nitrophenol [30]. Considerable attention has been given to zerovalent metal nanoparticles (like Au, Ni, Pd, Ag and Cu) due to their high stability and distinctive localized surface Plasmon resonance (SPR). Applications of noble metal nanoparticles in catalysis, optoelectronics, medicine, sensors and optics are known [31,32]. However, the remediation of toxic organic pollutants and their elimination from the aqueous environment is increasingly demanded day by day.Although smaller nanoparticles have better catalytic effectiveness in general, they agglomerate quickly due to their elevated surface energy owing to a reduction in catalytic activity. To address this problem, the reasonable way is to separate/isolate the metallic nanoparticles from one another by a synthetic substrate or physical obstruction, so they can't be in contact with one another straightforwardly. In the colloidal framework, metallic nanoparticles (NPs) are stabilized by steric stabilizers or electrostatic forces; noble metallic NPs can likewise be bolstered on chemically active oxides (for example titania, or ceria) [19,33].Here, in this work, the well-synthesized Ag and Cu-NPs on taro plant powder (Ag-Cu/TP) were fabricated. The synthesized nano-catalysts were used effectively in the catalytic reduction and hydrogenation of nitrophenols and organic dyes pollutants. Also, the study of the recyclability and stability of the prepared nano-catalysts were carried out for MO reduction.Taro-rhizomes were collected at a local market, rinsed in distilled water, then dried, and processed into powder by using a blender. Precursor's of silver nitrate and copper sulfate, as well as reducing agent (sodium borohydride) were from Merck Chemicals. Acid red 27 or Amaranth (AM), 2-nitrophenol (2-NP), 2,4,6-trinitrophenol (TNP), and 4-nitrophenol (4-NP), 4-nitroaniline (4-NA), congo red (CR), methyl red (MR), methyl orange (MO), and Rhodamine B (RhB) were from BDH Chemicals, England.All of the compounds/chemicals purchased were analytical pure grade and were utilized directly without any additional purification.Bimetallic taro supported Ag-Cu nanocatalyst was synthesized by a simple adsorption method. The catalyst was prepared by using the following two-steps.Firstly, distilled water washed air dried 1 g powder was put in a 0.2 M solution of AgNO3 and CuSO4 mixture (50 mL each solution). Then, the powder mixture and salt solution were left for about 7 h at constant stirring in order to saturate the sites of adsorption of plant powder. Powders were removed through filtration and to remove the free ions, they were rinsed three times with distilled water (DW) and then filtered again and dried.In the second step, in order to reduce the metal ions into corresponding nanoparticles, 100 mg of dried treated powders was added to 50 mL of the reducing agent (sodium borohydride) solution (0.5 M). The color changed from brown to dark black after the treatment with NaBH4 confirming the reduction process and nanoparticles formation. To convert all ions into respective nanoparticles (Cu and Ag), the powders were left in the borohydride solution for about 1 h. The nano-catalyst (Ag-Cu/TP) after washing with DW was dried and stored for the next step.The prepared taro powder supported bimetallic silver-copper nano-catalysts (Ag-Cu/TP) were tested in eight different reactions. The different reactions include catalytic hydrogenation/reduction of nitrophenols (4-NP and 2-NP) into aminophenol, in picric acid reduction, and azo dyes MR, Acid Red 27, MO, CR and methylxanthene family RhB dye.For the nitroarenes hydrogenation, 1 mM aqueous solution of nitroarenes (2-NP, 4-NP and TNP) and 0.5 M solution of NaBH4 were prepared in a volumetric flask. In reaction, 3 mL of nitroarenes (NP or picric acid) was put in a reaction vessel (a quartz cuvette was used for analysis as a reaction vessel) to carry out its UV–Vis spectra. After that, 0.5 mL of the reducing agent was added to study the variation in their UV–Vis spectra. Usually, sodium borohydride addition increases the solution pH and forms nitrophenolate ions, due to which the variations in spectra occur. After NaBH4 addition, the catalyst was added (10 mg) into the reaction mixture. The decrease in λ max started once Ag-Cu/TP catalyst was added, which indicates the reduction of nitroarenes. The variation in λ max was noted and recorded (for TNP at 393 nm; for 4-NP at 401 nm, and for 2-NP at 418 nm) [34].The concentration of dyes employed for dye reduction was 0.05 mM. To reduce the dye, three mL of the chosen solution of dye was placed in a 3 mL UV cuvette and 0.5 mL reducing agent solution was added to record the spectra. After NaBH4 addition, the catalyst was added (10 mg) into the cuvette. The variation in λ max position and intensity at λ max after Ag-Cu/TP inclusion was recorded (i.e., for CR at 493 nm, for AM at 521 nm, for MO at 466 nm, for MR at 428 nm and for RhB at 555 nm).The reduction of the mixture of nitroarene was carried out by combining 1.5 mL of each reactant solution and recording the spectra. Then, 0.5 mL of reducing agent was added, and recorded its spectra. The variances in both intensity and λ max were recorded and monitored on a regular basis after the addition of the catalyst (10 mg).In the reduction reaction of MO, the reusability or recyclability of the Ag-Cu/TP catalyst was investigated. To investigate reusability, the Ag-Cu/TP was rinsed with distilled water three times and utilized for the next reaction at the same time.Stability study of Ag-Cu/TP catalyst was carried out at different intervals of time. The stability study was carried out on the reductive hydrogenation of 4-NP into 4-AP for up to 98 h.A chromatography glass column was developed for the dye reduction by packing with Ag-Cu/TP catalyst. The catalyst was mixed with a fixed amount of sand and then used for the reduction of dyes. For column packing, 2 g of Ag-Cu/TP was homogeneously mixed with 20 g of sea sand and then the column was packed through wet method, with cotton supports and sand at the bottom of the prepared column. The sea sand used was washed with 1 M HCl for 30 min, and then washed with 1 M NaOH, followed by washing four times with distilled water. Then, the washed sand was dried in a furnace at 400 °C for 4 h. The column top was filled for 1 cm with sea sand. The packed column was washed twice with distilled water before the reaction. Then, freshly prepared solutions of dyes mixed with sodium borohydride were fed continuously throughout the column. UV–Vis spectra of the mixture eluted were carried out for each reaction using UV–Vis spectrophotometer. The filled column without catalyst was used as a negative control to observe the adsorption capability of pure coarse sand.Different analysis techniques were used to characterize the Ag-Cu/TP catalyst, including SEM, FTIR, TEM, EDX, and XRD.The structural characterization of synthesized nanoparticles was investigated by using SEM. The synthesized Ag-Cu/TP was structurally characterized by using an SEM (JSM-5910, Joel) (Japan). Similarly, the elemental analysis was also carried out by using a JEOL-JSM-5910, (Japan), EDX system coupled with the SEM. For the TEM images, JEOL JEM-2100 electron microscope at 200 kV was used.Powder XRD was conducted by using a CuK source on a JEOL JDX-9C XRD analytical diffractometer (Japan). The Scherrer equation as shown below was used to figure out the average size of the catalyst crystallite. (i) D = K λ / β 1 / 2 cos θ In the above equation, k represents a constant which has unity, λ is the CuKα source wavelength and equals to 1.5418 Å, β1/2 represents the full width at half maximum on 2θ scale, and θ is the Bragg's angle.A UV–visible Shimadzu-1800 spectrophotometer was used to conduct catalytic reduction tests on nitrophenols and dyes.The visual representation of the taro supported Ag-Cu/TP catalyst is presented in Fig. 1 . Metal ions treated powders look dark brown. The ions were absorbed by the taro powder because of the electrostatic contacts between positive metal ions and hydroxyl groups of phytochemicals present in the plant part. Besides, metal ions were absorbed by the pores of the plant powder. Because the –OH of plant polyphenols reduced silver ions easily, the treated taro sample was dark brown in color [35]. To get rid of the ions that are weakly bound, which will cause leaching problems in the reduction process, the powder after metal ion treatment was washed three to four times with DW and then dried for further use at room temperature.The color of the sample immediately turned to dark black when treated with NaBH4 solution (Fig. 1), indicating the reduction of metal ions to their nanoparticles and the formation of a catalyst. Metal ion reduction to their corresponding (Ag0 and Cu0) nanoparticles is shown in the given chemical reactions. (ii) 2 A g + + 2 B H 4 ¯ + 6 H 2 O → 2 A g 0 + 7 H 2 + 2 B ( O H ) 3 (iii) 2 C u 2 + + 4 B H 4 ¯ + 12 H 2 O → 2 C u 0 + 14 H 2 + 4 B ( O H ) 3 Active catalyst was prepared by treatment with NaBH4 as a reducing agent.Since catalytic activities of nanomaterials are primarily dependent on their crystal structure, shape, order, size, stability, and reusability. Materials with scattered and ordered nanoparticles frameworks have gained significant interest due to their fascinating catalytic and biological properties.The shape and surface morphology taro supported catalyst were evaluated using TEM and SEM analyses. For Ag-Cu/TP catalyst, both SEM and TEM were used for the surface morphology. As shown in Fig. 2 , the Ag-Cu/TP nano-catalyst was poly-dispersed size regularly distribution of nanoparticles on plant surfaces. The excellent catalytic effectiveness of Ag-Cu/TP was because of the uniform distribution of Ag and Cu nanoparticles to the powder surface. In addition to monodispersed particles, certain nanostructure clusters were found in Ag-Cu/TP SEM pictures, which are due to the accumulation of high metal ion concentration. Generally, the electronic and optical properties of nanomaterials/catalysts as reported previously in the literature are greatly dependent on the size, shape and nanoparticle distribution [36].As described in the catalytic reduction of nitrophenols and dyes, the excellent catalytic performance of the prepared catalyst is due to the uniform distribution of nanoparticles on the support surface.TEM images of Ag-Cu/TP showed the polydispersed and spherical-shaped smaller size nanoparticle formation on the surface of support taro. The average calculated size of the Ag-Cu/TP nanoparticles from TEM images was 2–33 nm [mean size ≈ 20 ± 13 nm] as presented in Fig. 3 .An EDX investigation was carried out for the elemental characterization of the Ag-Cu/TP catalyst. EDX is a technique which determines the composition of elements in a nanoparticle sample by detecting specific elements in the framework of a catalyst and quantifies their amount also. For the EDX pattern, the crystalline nature of the sample was clarified.EDX investigation of Ag-Cu/TP catalyst showed three distinct peaks at 0.5, 1.0, and 3.0 keV as presented in Fig. 2(d). In the EDX spectrum, the peak at the 1 keV confirms the presence of copper [37], while the peak at 3 keV represents silver [38] in the Ag-Cu/TP catalyst. The weight proportion of the particular elements in the Ag-Cu/TP catalyst were 32.3% carbon, 34.28% oxygen, 13.17% copper, and 20.25% silver, as illustrated in Fig. 2(d).Crystalline nature and the average size of prepared catalyst were evaluated by powder XRD pattern. The pattern of the powder XRD of Ag-Cu/TP catalyst is presented in Fig. 4 . The XRD of the prepared Ag-Cu/TP catalyst demonstrated five different diffraction peaks at a 2θ position of 38.58°, 43.67°, 50.67°, 64.69° and 75.8° as depicting in Fig. 4. Peaks at 38.58, 64.69, and 75.8 correlate to Brag peaks [111], [220], and [311], respectively, which coincide with face centered cubic (fcc) structural features of supported Ag nano-catalyst [39,40]. The CuNPs in Ag-Cu/GP were identified at 43.67 and 50.67, correspond to Bragg's diffraction (111) and (200), respectively, and correlate to Cu nanoparticles' fcc structure [37].After the catalytic hydrogenation/reduction of 4-NP, an XRD study of Ag-Cu/TP was also performed as presented in Fig. 4 (XRD pattern after catalysis). In the XRD pattern of Ag-Cu/TP, the most intense peak appears at 35.42° which is related to the Bragg's index of (−111) of CuO nanoparticles. This confirmed the pure monoclinic phase of CuO formation [41,42]. The intensities of XRD peaks of Ag and Cu nanoparticles decreased after catalysis, which showed the formation of oxides after five cycle reduction of MO dye. Some unassigned (∗) peaks were also observed, which might be because of the biomolecules crystallization.As demonstrated in Fig. 5 , the functional groups and chemical nature of the produced Ag–Cu/TP catalyst were determined using the FT–IR spectra. FT–IR analysis of the pure taro powder was also carried out. Absorption bands at 3320, 2917, 3320, 2917, 16510, 1470, 1187 and 1015 cm−1 have been seen in the FT–IR spectrum of pure taro powder (Fig. 5), which correspond to hydroxyl (O–H) stretching vibration, asymmetrical stretching of methylene (C–H), the primary amine stretching vibration (N–H), asymmetrical stretching of ether (C–O–C) group, bending of –OH, C–H and C–OH side group vibrations [43–45].In the spectrum of Ag–Cu/TP catalyst, the typical taro peaks were observed with a slight shift along with unique peak at 608 cm−1. The new peak observed at 608 cm−1 corresponds to the metal–oxygen (M−O) bond and confirms that Cu–Ag nanoparticles were synthesized on the taro support [26]. Furthermore, when the pure taro powder and the supported Ag–Cu/TP catalyst were examined, the bands of carboxyl, hydroxyl, and amine in the support catalyst appeared broad and slightly shifted in the catalyst, demonstrating the interaction between the metallic component and the taro powder. These findings reveal that Cu–Ag nanoparticles were successfully produced over the taro powder surface.Animals, birds, plants, and marine life are all at risk from nitroaromatic molecules because of their toxicity. Due to its challenging decomposition at such trace levels, 4-NP poses harm to the ecosystem even at lower quantities. Catalytic reduction/hydrogenation is the best approach to transfer 4-nitrophenol into the beneficial product 4-aminophenol, which is then used in the manufacturing of antipyretic and analgesic medicines including acetaminophenol, acetanilide, and phenacetin [46]. As a result, reducing 4-NP is important in order to fulfill the needs for 4-aminophenol.The reducing agent sodium borohydride has been reported in researches to be unable to reduce 4-NP completely. Because borohydride ions function as powerful reducing species in the aqueous phase (E for H3BO3/BH4 = −1.33 V), 4-NP reduction by NaBH4 is thermodynamically favored (E for 4-NP/4-AP is −0.76 V). However, resulting in significant difference and kinetic restriction between the borohydride and nitrophenolate ions, the reduction is very slower kinetically, and the chance of this reaction decreases. To surpass the kinetic barrier, nanomaterials catalyzed/promote the hydrogenation process of 4-NP by allowing electron transport from the donor (borohydride ions) to the acceptor (i.e., nitrophenolate ions) [47]. Catalytic reduction of 4-NP to 4-AP followed a pseudo-first-order kinetic. When both borohydride and nitrophenolate ions are adsorbed on the surface of the active site of the catalyst, then electron transfer starts from negative borohydride to nitrophenolate ions. As a result, Ag and Cu nanoparticles boost up the reduction reaction by reducing the energy of activation and acting as a catalyst. In the presence of NaBH4 reducing agent, the transformation of NP to AP is a six-electron transfer process, as indicated in Scheme 1 . As a result, the 4-NP reduction could not be achieved effectively without the use of a catalyst, as demonstrated experimentally by utilizing sodium borohydride alone in the previous study [35]. Accordingly, the reductive conversion of 4-NP required the use of an active catalyst. The chemical pathway for the transformation of NP to AP by sodium borohydride by the use of Ag-Cu/TP catalyst is shown in Scheme 1.The red curve in the spectrum shows that the solution of 4-NP has a significant absorption maxima (λ max) at 316 nm in the ultraviolet spectrum, as shown in Fig. 6 (a). With the addition of 0.5 mL sodium borohydride, the sharp peak of 4-NP shifted from 316 to 401 nm. The formation of phenolate ions from 4-nitrophenol caused the peak position of 4-NP to shift since NaBH4 raised the solution pH, causing nitrophenolate ions to form. Due to the existence of nitrophenolate, the peak intensity and position at 401 nm remained static even after 20 min without the use of a catalyst. After catalyst addition, however, the height of the peak at 401 nm began to drop. The new peak in the reaction appeared at 302 nm (Fig. 6(a)). The synthesis of 4-AP was confirmed due to the drop in peak intensity at 401 nm and a rise in the intensity at 302 nm [48,49].The same Ag-Cu/TP catalyst was also utilized in the hydrogenation of 2-NP to 2-AP. The UV–Vis spectrum of a dilute 2-NP have two prominent peaks at 356 nm and 273 nm, as shown by the red curve (Fig. 6(b)). With the introduction of NaBH4 to the sample vial containing 3 mL of 2-NP, a change in the peak position of 2-NP was noticed from 356 nm with a rising intensity to 418 nm (Fig. 6(b)). Until the catalyst was added, both intensity and position at 273 and 418 nm remained constant. Once Ag-Cu/TP catalyst was introduced, the peak at 418 nm began to decline. During this time, the peak in the UV area shifted from 273 to 294 nm. Thus, the reduction of 2-NP and generation of 2-aminophenolate ions are confirmed by the emergence of a second peak at 294 nm and a drop in the height of the main peak at 419 nm [50,51].From the peak of the UV–Vis spectrum, the percent reduction of the NP reaction was measured using equation (iv) (iv) Percent Reduction = 100 − ( A t × 100 / A 0 ) where A 0 signifies the absorbance maximum at time zero, and A t denotes the absorbance at a specific interval of time of reading.4-NP and 2-NP reduction processes by NaBH4 follow pseudo first order kinetic reaction. For the computation of the apparent rate constant (k app ), the graph of reduction time (in minutes) versus ln (A t /A 0 ) was used (Fig. 6(d)).The equation for the pseudo-first order is given below (v) r = l n C t / C 0 = d c / d t = − K a p p t where the reduction rate is represented by r; time of reaction is represented by t; while concentration by c; C 0 represents initial concentration at time t = 0; C t stands for the concentration at specific interval t.Since here in this case, both reactants (4-NP and 2-NP) have a bright color and provide a maximum peak in the visual region, the reaction rate can be presented in comparative absorption intensity. Thus, the pseudo-first order in terms of adsorption can be expressed as: (vi) r = l n C t / C 0 = l n A t / A 0 = − K a p p t The kinetic data for 4-NP and 2-NP and the graph between time versus ln(A t /A 0 ) are presented in Fig. 6(d), reduced by NaBH4 and catalyzed by Ag-Cu/TP. The Ag-Cu/TP catalyst showed outstanding performance and 99.4% of 4-NP was reduced just in 5 min, while 97 percent reduction took 17 min with 2-NP (Fig. 6(c)). The calculated rate constant for Ag-Cu/TP at room temperature was 1.01 × 10−3s−1 for 2-NP reaction and 5.24 × 10−3s−1 for 4-NP as presented in Table 1 . Impressively, the catalytic effectiveness of the prepared catalyst (Ag-Cu/TP) showed higher catalytic performance than recently reported bimetallic platinum-rhodium alloyed nano-multipods, where the reduction efficiency of 4-NP reaches to 94.02% in 10 min [52].The number of moles of nitrophenols (reactant) used in a known specific reaction per mol of catalyst (Ag-Cu/TP) per unit of time is called the Turnover frequency (TOF) as expressed by Eq. (vii) [53]. The computed TOF for the reaction of nitrophenols (4-NP and 2-NP) is revealed in Table 1. (vii) TOF = No of mole of Reactant ( NP ) No of mole of Catalyst × Y i e l d T i m e The higher TOF may be due to the dispersion of homogeneous and uniform silver and copper nanoparticles on taro support. The excellent catalytic performance of our catalyst (Ag-Cu/TP) was due to the uniform dispersion of metallic nanoparticles on the surface of support.Picric acid or TNP is a highly explosive and unstable chemical. Picric acid, which is ignited by flames, sparks or even heat, is commonly employed in military explosives and also has a rich history of nonmilitary and military applications. When subjected to friction or shock, heat or flame dried-out picric acid could burst, and it should be considered as an explosive material. Toxicity of picric acid is mostly acute; nevertheless, any long-term consequences, such as mutagenicity, have been described. Picric acid is a cutaneous sensitizer and a potent eye irritant in animals [60]. Nitro-substances like TNP have a very deficient -electron network resulting in greater xenobiotic characteristics due to their peculiar electron-withdrawing nature. Biodegradation is difficult for nitro-aromatic and other nitrogen-containing molecules. Because of the low electron density of the nitro group, electrophilic assault in the benzene ring of nitroarenes has become more difficult, that is the first phase in the reduction/degradation. As a result, di-nitroaromatic and tri-nitroaromatic molecules, such as picric acid, undergo the first reductive process.For the reductive hydrogenation of TNP, the same Ag-Cu/TP catalyst was utilized. TNP's aqueous solution has a yellowish color. The main absorption peak for TNP is illustrated at 357 nm as revealed in Fig. 7 (a). After adding NaBH4 solution, the peak location was shifted from 357 to 393 nm, with increase in intensity. It was observed that with the addition of NaBH4, the color turned from light yellowish to dark yellow. The presence of phenolate ions in the reaction cuvette is evidenced by a change in peak λ max from 357 to 393 nm after adding NaBH4. The height of a peak λ max at 393 nm for TNP stayed unchanged for an hour without catalyst. This demonstrates that the reducing agent NaBH4 could not reduce TNP on its own. As a result, complete TNP reduction without a specific catalyst is incredibly challenging. The peak λ max at 393 nm began to decline after adding a catalyst to the cuvette. However, around 234 nm, a new peak appeared. With time, the unique peak that appeared at 234 nm grew in height. The catalytic reduction of TNP caused nitrophenolate ions to drop in intensity at 393 nm and aminophenolate ions to increase in intensity at 234 nm (Fig. 7(a)). The synthesis of aromatic amine (aminophenolate) is responsible for novel peak formation in the UV region at 234 nm [61]. TNP reduction was assisted by the Ag-Cu/TP, which occured at its surface by allowing the transfer of electrons from borohydride to the phenolate ions to cross the barrier of kinetic. TNP reduction follows a pseudo first order with regard to the substrate since it is independent of NaBH4 amount. A graph of ln (A t /A 0 ) versus time of reduction was used to determine the rate constant (in second) for the pseudo-first-order kinetic of TNP reduction using Eq. (vi).Ag-Cu/TP catalyst demonstrated excellent activity and reduced 92.5% of TNP in 7 min as presented in Fig. 7(c). The k app calculated for TNP by Ag-Cu/TP from the slope was 5.62 × 10−3 s−1 (Fig. 7(d)). Ag-Cu/TP has a greater k app than our latest published Ag@CAF, which has a k app of 8.97 10−4 s−1 for TNP [62].Dyes are currently commonly used and synthesized in a wide range of industries, including textiles, cosmetics, paper and pulp, printing, and so on. However, it was reported that about 10–15% of dye chemicals are lost into the aquatic environment, posing a threat to organisms as well as public health. Organic synthetic dyes are resistant to sunlight, base and acid, and because of the benzene ring in their molecules they have the potential to be genotoxic as well as detrimental to marine life. It produces an ecological imbalance by accumulation in water [63]. As a result, dyes must be removed and eliminated from the aquatic environment.The traditionally Amaranthus plant was used to produce AM dye. In recent years, it has been chemically manufactured as a trisodium salt on an industrial scale. Food Red 9 and Acid Red 27 are common names for the chemically manufactured AM dye. The carcinogenic impact of AM dye on albino rats has been studied in the literature. AM dye at 10 times the Acceptable Daily Intake causes lethal abnormalities, and levels of 47 mg kg−1 by bodyweight of AM dye can affect liver function [64].The aqueous AM dyes produced strong single peak at 521 nm and smaller peaks at 330, 282, and 242 nm in the UV and visible regions of the UV–Vis spectrum, as illustrated by the red curve (Fig. 7(b)). After adding the reducing agent sodium borohydride, no change was observed in peak position and the peak remained at 521 nm. Even in the absence of a catalyst, the height and position of the λ max at 521 nm remained unchanged. However, after the catalyst was added to the cuvette, the intensity of the peak began to fall steadily. The prominent λ max at 521 nm vanished after the catalyst was introduced, while simultaneously a new peak formed at 267 nm (Fig. 7(b)). The beginning of AM reduction was confirmed by the gradual peak decline and vanishing at 521 nm, and novel peak formation at 267 nm. Since the reduction of AM is independent of NaHB4, so the reaction is pseudo-first order. Ag-Cu/TP catalyst reduced 76.54% AM dye in 9 min as shown (Fig. 7(c)). The k app calculated for AM by Ag-Cu/TP was 9.87 × 10−4 s−1 (Fig. 7(d)). Hence, Ag-Cu/TP displayed good action for AM reduction.The disintegration of MO dye is of major interest because it poses sever environmental and health concerns. MO could be reduced to smaller organic molecules by NaBH4 as a reducing agent. However, the reduction of MO by NaBH4 is very slow and time-consuming. Without a catalyst, the reduction ofMO by the reducing agent sodium borohydride is kinetically unfavorable but feasible thermodynamically. Noble metal nanoparticles like Au and Ag due to their distinctive SPR character and increased specific area are now widely used as efficient catalysts for the reductive degradation of MO dye.Firstly, 3 mL of aqueous dye solution was taken in a cuvette. After that, 0.5 mL of aqueous NaBH4 was added and their spectra in the 200–800 nm range was then measured. Maximum absorption was observed at 466 and 280 nm for MO solution. Even after 20 min of NaBH4 addition, the absorbance at 466 nm remained unchanged. However, after adding Ag-Cu/TP, the absorbance at both 466 and 280 nm began to change (Fig. 8 (a)). With increased intensity, the peak at 466 nm consistently reduced, whereas the λ max at 280 nm shifted and reached 252 nm to some extent (Fig. 8(a)). MO reduction was related to the steady decline and eventual vanishing of peak intensity at 466 nm. The primary amino group molecule (hydrazine derivative) formed during MO reduction resulted in the new peak appearance at 252 nm [65,66]. NaBH4 degrades MO at the azo (–N=N–) site by forming smaller amine derivative molecules in the presence of the prepared effective catalyst. The absorption bands of the –NH2 group of the resultant products are attributed to the peak at 252 nm that was generated during the MO reaction. The possible process given in Scheme 2 is most likely to be used in the reductive breakdown of MO on the Ag-Cu/TP catalyst. MO reduction requires electron transport from the source borohydride ions to the recipient dye molecule. Both the MO molecules and borohydride ions are initially adsorbed on the catalytic surface (Ag and Cu nanocatalysts). In this case, metal NPs serve as electron relays enabling electron transport between nucleophilic borohydride ions and electrophilic MO molecules [67]. Previous studies have found that metallic NPs in water accelerate the passage of electrons from borohydride ions to the dye resulting in formation of BO3 3− ions [68]. These electron transfers might continue indefinitely resulting in further dye molecule degradation [69].The high reactivity of the prepared catalyst could be due to the high surface area and irregular morphology of the nano-catalyst (as shown in SEM images), which facilitate charge transfer and allow the depletion mechanism to overcome kinetic barriers. The reduction of MO by the reducing agent NaHB4 follows a pseudo-first-order kinetic.Eq. (iv) was used to calculate the percent degradation of MO using Ag-Cu/TP catalyst. Here in this report, as presented in Fig. 8(c), the prepared catalyst (Ag-Cu/TP) showed 96% MO reduction in just 9 min. The calculated pseudo-first-order reaction rate constant for this reaction from the slope was 2.91 × 10−3s−1 for Ag-Cu/TP at room temperature (Fig. 8(d)).MR is used as an indicator marker in microbiology to detect bacteria that produces stable acids from glucose using mixed acid fermentation processes. Besides that, it's been discovered to be a great booster for the sonochemical disintegration of polychlorinated. When inhaled or ingested, MR produces sensitivity in the hair, skin, and eyes, as well as intestinal inflammation. Because of the breakdown of its molecule into toxic (carcinogenic and mutagenic) aromatic amines, the presence of MR in the aquatic system causes aesthetic issues that have a detrimental impact on public health [70]. MR produces visual and skin sensitivity if it is inhaled or ingested as well as digestive tract irritation.Our prepared catalyst was also used in the catalytic degradation of MR. Fig. 8(b) shows the absorption maxima of aqueous MR solution at 428 nm. With the introduction of NaBH4, the peak height at 428 nm remained constant. With Ag-Cu/TP addition, the intensity of the absorption peak declined steadily. However, as seen in Fig. 8(b), the additional two peaks at 307 and 246 nm appeared with time. The beginning of MR reduction at the azo region by producing smaller aromatic amines resulted in a drop in λ max at 428 nm and the raising of two separate peaks at 307 and 246 nm.Here, Ag-Cu/TP showed 97% MR reduction in just 9 min as presented in Fig. 8(c). The pseudo-first order reaction rate constant was calculated through Eq. (vi). The rate constant of MR at normal temperature evaluated from the slope was 4.126 × 10−3s−1 for Ag-Cu/TP. The plausible mechanism for reductive degradation of MR by Ag-Cu/TP is presented in Scheme 2.CR containing wastewater is brightly colored; its release into the aquatic environment inhibits photosynthesis by reducing light penetration, and is also cytotoxic to a wide range of aquatic creatures. As a result, the elimination of CR from aquatic environments is a major concern.Two major peaks were observed at 493 nm and 346 nm in an aqueous solution of CR. Without any active catalyst, the absorption peaks remained unchanged. After Ag-Cu/TP addition, the intensities at both points (493 and 346 nm) slowly decreased with time. After the catalyst was added, a novel peak at 250 nm appeared, as seen in Fig. 9 (a). CR reduction was initiated by the fall in intensities at both positions of 493 and 346 nm as well as the rise of a novel peak maxima at 250 nm. With the addition of a catalyst (Ag-Cu/TP), sodium borohydride degraded the CR molecules by producing amines of the aromatic nature with a small molecular weight. As a result, the peak at 250 nm that increased in height with time was due to the reducing of CR molecules [71]. This demonstrates that the presence of the prepared catalyst caused the deterioration of CR by NaBH4.An Eq. (iv) was used to calculate the percent reduction. Ag-Cu/TP degraded 89 percent of CR molecules in solution in 8 min, as shown in Fig. 9(c). Since NaBH4 reduction of CR is a pseudo-first order [37], therefore, the rate of reaction of CR was 2.73 × 10−3s−1 for Ag-Cu/TP catalyst (Fig. 9(d)).In this study, the degradation rate of CR dye was higher than the earlier reported reduction of CR using filter paper coated Chitosan loaded CuNPs, which took 13 min for 0.012 mM CR reduction [47]. The reductive degradation of CR on silver and copper-decorated catalyst is probably presented by the plausible mechanism presented in Scheme 2.Rhodamine B is a fabric dye that belongs to the methyl-xanthine family. RhB is commonly employed in cell luminescence as a coloring agent; a tracer reagent in water quality studies, and a color tracer in the coloring of wool, cotton, leather, colored glass, jute, silk, and sprays. RhB is employed as a fluorescent sensing and colorimetric reagent in possible metal ions diagnosis due to some advantageous chemical characteristics [72,73]. If inhaled, RhB has harmful consequences for human health. Carcinogenicity, acute and neurological toxicity, development and reproduction toxicity have all been reported in humans and animals in the laboratory. This dye has the potential to cause tissue melanoma. Aside from these, it causes eye and skin burning, nasal itching, vomiting, throat incinerating, and chest pain. Some countries around the globe have banned and prohibited its usage in food goods due to its numerous negative health impacts. However, because of its low cost, great stability, and color, RhB is still being used illegally in food and other areas [59,74]. Fig. 9(b), shows the UV–Vis reduction spectrums of RhB by using Ag-Cu/TP catalyst and reducing agent NaBH4. The same experimental technique was employed to reduce RhB as it was for other dyes. In the visible region, an aqueous RhB solution showed a maximum peak centered at 555 nm. The sharp absorption peak stayed stable at 555 nm for a long period without an appropriate catalyst. It has also been reported that without an appropriate catalyst, NaBH4 could not effectively degrade/reduce RhB [75]. However, after 10 mg of Ag-Cu/TP catalyst addition, maximum absorbance at 555 nm was obtained to steadily decline with time. The reduction dye may be seen in the gradual drop of the major peak of RhB, and a novel peak appearance at 242 nm. As stated previously, RhB de-ethylation is caused by the utilization of a NaBH4 with an active catalyst. As a result, the decline in λ max at 555 nm is because of RhB reduction [76]. Ag-Cu/TP reduced RhB dye by 71.2 percent in 7 min, as shown in (Fig. 9(c)). This reduction by sodium borohydride follows a pseudo-first reaction model [75]. The kinetic data and graph of time (t) versus -ln (A t /A 0 ) of RhB reduction is shown in Fig. 9(d). As a result, the rate constant for RhB was 2.05 × 10−3s−1. The plausible reduction mechanism of RhB dye of Ag-Cu/TP catalyst is presented in Scheme 2.For further study, a particular substratewas used to study the catalytic deterioration and reduction processes of the catalyst. The catalytic degradation/hydrogenation of the catalyst was tested in this work against a solution of nitroarenes.As seen in Fig. 10 (a), the mixture of TNP and 2-NP had the highest absorption at 359 nm. The λ max was shifted to 393 nm after NaBH4 addition, as shown in Fig. 10(a). The production of phenolate ions caused the peak to move from 359 to 393 nm. With catalyst introduction, the peak intensity at 393 nm began to drop. The conversion of nitrophenols to corresponding aminophenols was demonstrated by the progressive drop in λ max at 393 nm and the generation of two additional peaks at 308 and 235 nm. The complete reduction of TNP and 2-NP mixture took 6 min using Ag-Cu/TP catalyst.The mixture of TNP and 4-NA showed a λ max at 358 nm (Fig. 10(b)). The peak maxima was shifted from 358 nm to 392 nm after NaBH4 addition, as presented in Fig. 10(b). The change in peak was because of phenolate ions formation in a solution. Once the catalyst was added, the peak height at 392 nm began to drop. The transformation of nitrophenols to aminophenols was demonstrated by the progressive drop in λ max at 392 nm and the development of a novel peak at 299 nm. The complete reduction of TNP and 4-NA mixture took 9 min using Ag-Cu/TP catalyst.As demonstrated in Fig. 10(c), the mixture of 4-NP and 4-NA exhibited three different absorptions at 215, 281 and 404 nm. After adding NaBH4, the position of the peaks was unchanged, but the strength of the maximum (λ max) at 404 nm rose due to the generation of phenolate ions, as seen in Fig. 10(c). The catalytic reduction/hydrogenation was demonstrated by a drop in both positions at 215 and 404 nm and the transfer in peak position from 281 to 295 nm with catalyst addition. Using Ag-Cu/TP, the full reduction of mixtures of 4-NP and 4-NA took 9 min.As demonstrated in Fig. 10(d), the combination of 4-NP, 4-NA, and 2-NP exhibited three strong absorbance peaks at 403, 280, and 240 nm. After adding a reducing agent, the location of the peaks was unchanged, but the strength of the λ max at 403 nm rose due to phenolate ion production (Fig. 10(d)). Once the catalyst was added, the absorption maxima at 403 and 240 nm dropped, while the third peak shifted from 280 to 293 nm. This fall in λ max at 403 and 240mn indicates catalytic hydrogenation of nitroarenes. Using Ag-Cu/TP, the full reduction of the three nitroarenes combination took just 8 min.As a result of the above findings, it was concluded that the Ag-Cu/TP catalyst is not only efficient in degradation of a particular signal pollutant, but also in the remediation of a combination of different pollutant molecules.When designing a wastewater treatment system, the cost of the process must be considered. A large proportion of the under-created and underdeveloped nation's enterprises could not pay the costs of usual sewage treatment plants since they are excessive to develop and supervise. Moreover, attention must be given to its operating costs. Many companies in under-developed and third world countries couldn't even afford to build and operate modern sewage treatment plants. Therefore, it is important to develop a low cost treatment method.Here in the current study, a novel column reactor for dyes remediation was developed as shown in Fig. 11 (a). A glass chromatographic column (Fig. 11(b)) was filled with Ag-Cu/TP catalyst through the wet method by mixing it with the appropriate amount. The freshly prepared solutions of dye mixed with sodium borohydride were constantly passed through the developed column. Eluted solutions from the column were collected to carry out their UV–Vis spectra for each reaction. The filled column without catalyst was used as a negative control to observe the adsorption capability of pure coarse sand.Reaction optimization for the development of columns was carried out for the reduction of MO dye. In order to study the catalyst amount influence on reduction using column through batch study, three different types of reactions were carried out. In the first experiment, 0.5 g of Ag-Cu/TP catalyst was mixed with 10 g of sand and then used for the column development. In the second experiment, 1 g of Ag-Cu/TP catalyst was mixed with 10 g of sand. In the third experiment, 2 g of Ag-Cu/TP catalyst was mixed with 10 g of sand and used for the column development. The three developed columns contain different amounts of catalyst. These columns were used for selective reduction of MO as a model pollutant. The flow rate of dye solutions passing the column was 2 mL/min and 5 mL/min.From the batch experiment, it was observed from the first experiment that 0.5 g of Ag-Cu/TP catalyst was able to reduce 37.7% of 0.05 mM of MO dye with a flow rate of 2 mL/min; however, its reduction efficiency declined to 22.6% with increasing the flow rate to 5 mL/min (as presented in Table 2 ). Results from the second experiment showed that 1 g of catalyst was able to reduce 67.4% of MO dye from aqueous solution with a flow rate of 2 mL/min, but its efficiency lowered to 49.2% by increasing the flow rate to 5 mL/min. Similarly, the column of 2 g of catalyst per 10 g of sand showed 72.8% with a flow rate of 2 mL/min, and 55.5% of MO reduction with the flow rate of 5 mL/min. From the results given in Table 2, it was concluded that 1 g of catalyst mixed with 10 g of sand showed the optimal catalyst amount as compared to 0.5 and 2 g of catalyst in 10 g of sand.The results of the catalytic reduction of azo dyes by column method are presented in Table 3 . It was observed that the Ag-Cu/TP catalyst showed a good activity and reduced 68.7% and 68.0% of the CR and RhB dyes, respectively, with a flow rate of 2 mL/min. Similarly, it reduced 64.2% and 67.4% of MR and MO dyes, respectively, with a flow rate of 2 mL/min. Also, Ag-Cu/TP catalyst reduced 46.9% and 52.3% of the CR and RhB dyes, respectively, and 49.2% of MO and 46.4% of MR with a flow rate of 5 mL/min. The column was also used for the reduction of mixtures of dye solution, and it showed an excellent activity by reducing 69.0% of MO and CR solution. Besides catalyst, pure taro powder and sand were used to check their adsorption capabilities.The reusability and recycling of the effective catalyst are a major concern in today's catalysis field. Most catalysts are only used for a first or second cycle before being deactivated. Aside from catalytic efficiency, photocorrosion, durability, recyclability, and stability are also key factors to be considered when evaluating catalysts' performance because they can drastically reduce the method's cost. Catalysts in the type of colloidal nanoparticles have been successfully described in the literature as they exhibit photocatalytic activity; however, the issue is their recovering from reaction medium and their reusability for subsequent use. As a result, stable and completely separable catalysts are critical for the recyclability and reusability processes.The recyclability of synthesized Ag-Cu/TP catalyst was investigated using MO and 4-NP in this study. Unlike other pollutants, MO and 4-NP are easily reduced by sodium borohydride and Ag-Cu/TP. As a result, MO and 4-NP were chosen for a more detailed analysis of recyclability. For a 100 percent reduction of both compounds, the recyclability of the Ag-Cu/TP was evaluated for consecutive five cycles. The recyclability test of Ag-Cu/TP was tested after rinsing it three to four times in double DW when it was used for the reduction and utilized again in the reduction reaction at the same time. The disappearance and formation of peaks were observed periodically. Fig.12 shows the recyclability study and the degradation of MO by using Ag-Cu/TP for five cycles. It was shown that Ag-Cu/TP took 9 min for the first and second use for MO reduction, while it took 10 and 11 min for 3rd and 4th uses, respectively. However, it took 14 min for 5th use of Ag-Cu/TP for MO degradation/reduction.However, in the 4-NP reaction, it took 5 and 6 min for the first and second uses, repectively For the 3rd and 4th uses, it took 8 and 9 min, respectively. While for the 5th use of 4-NP reduction, it took 12 min using Ag-Cu/TP (Fig. 12(b)). The stability of Ag-Cu/TP, on the other hand, was assessed after 1, 18, and 36 h after preparation of catalyst. As demonstrated in Fig. 12(b), the first usage took 5 min and 30 s, whereas the complete reduction required 7 min after 18 h. After 36 h of synthesis, however, the complete reduction of 4-NP took only 8 min (Fig. 13 ).Because of the loss of nano-material during the recycling and washing process, the catalyst reduction performance in the continuous cycle reduced slightly. In fact, during the reduction phase, the reaction product might potentially block the active sites of the catalyst. However, after five cycles of turnover rates, Ag-Cu/TP was found to maintain strong catalytic activity. The reusability study indicated that the catalyst possesses excellent activity and stability and can be utilized many times in different reactions. The exceptional efficacy of the catalyst during the reusability study could be attributed to uniform dispersion of silver and copper nanoparticles on the support.Bimetallic Ag and Cu-based support catalyst was successfully synthesized by taro-rhizome through a low-cost method. SEM, EDX, and TEM pictures showed that Ag and Cu NPs were successfully synthesized on the plant powder surface. Powder XRD confirmed the crystalline nature and the fcc structure of synthesized catalyst.The synthesized Ag-Cu/TP nanomaterials have an outstanding catalytic activity for nitroarenes and dyes. The synthesized catalyst remained stable for several days, with exciting efficiency for 48 h, and could be simply recycled and utilized five times for 100 percent reduction of 4-NP and MO dye with very small changes in its catalytic activity. The catalyst that had not been treated with NaBH4 remained stable for a year without any change in catalytic properties. It was found that the catalyst has the best catalytic reduction activity not just for a particular substrate, but also for the reduction of a mixture of nitrophenols. As a result, this catalyst promises to evaluate the overall safety of diverse water-based systems by reducing nitrophenols and organic dyes using the continuous column reactor. Because of their low cost and environmentally benign plant support, these catalysts might be produced in large quantities due to their superior metal ion absorption, high surface distribution, extraordinary recyclability, and stability.Not applicable.Not applicable.The authors confirm that the data supporting the findings of this study are available within the article.The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project number (IFPRC-061-247-2020) and King Abdulaziz University, DSR, Jeddah, Saudi Arabia.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project number (IFPRC-061-247-2020) and King Abdulaziz University, DSR, Jeddah, Saudi Arabia.

Every day, the risk of water pollution grows. According to the World Health Organization (WHO), approximately half of the population of poor countries suffers from health problems linked to chemically or microbially contaminating drinkable water. Here in the present study, powder of taro-rhizome was used for preparation of supported bimetallic catalyst (Ag-Cu/TP). Characterization techniques used for analysis were XRD, TEM, SEM, FTIR and EDS. The prepared catalyst was then used in the catalytic reduction of chemical pollutants, including 4-nitrophenol (4-NP), 2,4,6-trinitrophenol (TNP), 2-nitrophenol (2-NP), methyl orange (MO), Acid Red 27 or amaranth (AM), Congo red (CR), Rhodamine B (RhB) and methyl red (MR). Turnover frequency was calculated for nitrophenol reductions, which were 0.98min−1 for 2-NP and 3.45min−1 for 4-NP. The Ag-Cu/TP catalyst shows outstanding performance and 99.4% of 4-NP was reduced just in 5 min with pseudo-first order constant of 5.24 × 10−3s−1, while 97% reduction of 2-NP took 17 min with the pseudo-first order constant of 1.01 × 10−3s−1. The prepared Ag-Cu/TP catalyst was packed into a glass column and organic dyes containing water were filtered through it to test the cleaning effectiveness of water from dyes. Excellent dye reduction was achieved. The prepared column reactor works excellent in the remediation of dyes and reduces about 68.7% CR. In addition, stability and reusability study showed exceptional reusability up to 5 times with very small loss of activity and was stable for up to 10 days.

Polyhydroquinolines are an imperative cluster of 1, 4-dihydropyridine (1,4-DHP) nucleus containing heterocycles, that have fascinated much consideration because of their potent pharmacological and biological activities [1–6]. Polyhydroquinolines are also distinguished as calcium channel antagonists and platelet aggregation inhibitors. Indeed, a few 1,4-dihydropyridine derived commercial drugs constitute a major class of ligands for L-type Ca2+ channel (LTCC) blockers and are extensively used in the treatment for the therapy of the cardiovascular and Alzheimer's diseases [7–9]. Owing to the above noted most valuable therapeutic applications of the Polyhydroquinolines, synthetic researcher had developed number of methods for the synthesis of this valuable framework using several metal and halogen containing catalytic systems [10–30].In addition, heterocycles containing pyrimidine moieties occupy an outstanding position in medicinal chemistry due to its abundant therapeutic applications and that pyrimidine containing commercial drugs encloses the anticancer and antiviral activities [31,32]. Recently, many dihydropyrimidinone-functionalized scaffolds have been discovered antihypertensive and calcium channel modulating activities [33–36]. Certain developed protocols for the synthesis of dihydropyrimidinone compounds have been reported in the last decades by employing highly reactive enaminones as a precursor using different acid catalysts [37–39]. Most of the above reported methodologies for synthesis of polyhydroquinoline and dihydropyrimidinone compounds have been suffered with use of halogen containing acid catalysts, organic solvents and drastic conditions which are not environment friendly. Therefore, there is extreme necessity to develop a sustainable method to construct these compounds.Nowadays, ionic liquids have emerged as an excellent material due to their unique properties and have been extensively used in the industries as a catalysts as well as solvents [40]. The properties of ionic liquids such as low vapor pressure, wide liquid range, good conductivity, thermal, mechanical, and chemical stability, and modification of chemical structures of their ions offer a new improved technology [41,42]. Ionic liquids have already been found valuable in several fields of chemistry and it can be a replacement for volatile organic compounds in organic processes [43,44]. High efficiency is the most significant feature of green catalysis, and >90% processes worldwide depend on the fast-growing field of catalysis [45,46]. The ionic liquids catalysts are extremely important as they are environment friendly and leads to minimize the waste and cater sustainability [47,48].Currently, acidic ionic liquids (AILs) used as ecologically important reaction medium for the organic synthesis and has established extensive attention in organic transformations as an efficient catalyst [49–55]. Number of reviews discloses the importance of acidic ionic liquids encompassing a green catalyst as well as their application as a solvent in biodiesel production and industrial applications [56–59].It is well recognized point that the sustainability of chemical processes prominently depends on selection of renewable raw materials, use of benign solvents, low generation of hazardous waste, and reduction of energy consumption during manufacturing [60–62].Keeping in mind sustainable aspects and to introduce a new green protocol in terms of efficient reusable catalyst, herein in this endeavor we had demonstrated the synthesis of novel acidic ionic liquid [CEMIM][MSA]. The novel acidic ionic liquid ([CEMIM][MSA]) is synthesized for the first time and confirmed by various spectroscopic techniques such as IR, 1H NMR, 13C NMR, DEPT-135, Mass, HSQC, TGA and Elemental analysis. The acidic ionic liquid [CEMIM][MSA] was first time implemented as a catalyst for the green synthesis of polyhydroquinoline and 6-unsubstituted dihydropyrimidinone derivatives in eco-friendly media. Furthermore, we had established the sustainable, cost-effective, work up less and purification free process to isolate pure polyhydroquinolines and 6-unsubstituted dihydropyrimidinones with high yield in the very short span of time. The acidic ionic liquid catalyst [CEMIM][MSA] was recycled and reused five time without much loss in its catalytic activity. The plausible mechanism for the synthesis of polyhydroquinoline and 6-unsubstituted dihydropyrimidinone derivatives over [CEMIM][MSA] is discussed in detail. Molecular structure and hydrogen bonding study of polyhydroquinolines and 6-unsubstituted dihydropyrimidinone were carried out using a single crystal X-ray analysis.The main objective of our current research work is to design, the route of synthesis of acidic ionic liquid 3-(2-Carboxyethyl) -1-methyl-imidazole mesylate [CEMIM][MSA] using economically feasible raw materials. The synthesis is outlined in Scheme 1 . 1 equivalent of N-methyl imidazole and 1 equivalent of 3-chloro propionic acid compounds were treated at neat condition to obtain halogenated ionic liquid 3-(2- carboxyethyl) -1-methyl-imidazole chloride [CEMIM][Cl] with 96% yield. Further, the [CEMIM][Cl] treated in stoichiometric amount with methane sulfonic acid at ambient temperature in ethanol. The halide free acidic ionic liquid 3-(2-carboxyethyl)-1-methyl-imidazole mesylate [CEMIM][MSA] is obtained with 98% yield.The formation of acidic ionic liquid [CEMIM][MSA] was confirmed by spectral characterization techniques including Fourier transform infrared (FTIR), Nuclear magnetic resonance (1H NMR, 13C NMR, DEPT-135 and HSQC), Mass spectrometry, Elemental analysis and the spectral data is included in ESI (Figs. S1-S7). The thermal stability of acidic ionic liquid [CEMIM] [MSA] was determined by thermogravimetric analysis (TG-DSC and TG-DTG) under N2 atmosphere at 25 to 1000 °C with the heating rate of 10 °C /min. The TG-DSC analysis plot is depicted in Fig. S8, and it showed two-stage decomposition. There is very slight weight loss occurred at 130 °C which correspond to the evaporation of water and volatile impurities. However, 31% weight loss observed at 284 °C reflects that the fragmentation of acidic ionic liquid [CEMIM] [MSA] has started followed by the main weight loss at 405 °C. Thus, complete decomposition of acidic ionic liquid take place at 405 °C. Moreover, DSC analysis results revealed an exothermal shift at temperatures 271 °C and 400 °C.These results pointed out that the acidic ionic liquid [CEMIM] [MSA] is a binary system catalyst and stable at high temperature reaction as well (TG-DSC and TG-DTG Figs. S8-S10 respectively included in the ESI).After confirmation of the structure of an ideal ionic liquid, the catalytic efficiency and applicability of acidic ionic liquid [CEMIM][MSA] as a catalyst was evaluated for the multicomponent synthesis of polyhydroquinolines and 6-unsubstitured dihydropyrimidinones.To optimize reaction condition for the synthesis of polyhydroquinolines ( Scheme 2 ), we have chosen the dimedone (1 mmol), 4-Cl benzaldehyde (1 mmol), ethyl acetoacetate (1 mmol), and ammonium acetate (1.5 mmol) as the reactants with 5 ml solvent volume for the model reaction.The initial experiments were carried in ecological solvents such as water and ethanol with 10 mol% [CEMIM][MSA] catalyst at ambient temperature. The reaction results, low yield of product and the time taken to complete the reaction is also quite long (Table 1 , Entries 1,2). To evaluate activity of catalyst at higher temperature, same reactions were screened in Water, EtOH, it results in improvement in the yield of product (up to 85%) with reducing the reaction time (Table 1, Entries 3,4). Next step was testing of different ionic liquid catalysts were by changing catalyst loading concentration in EtOH medium at reflux temperature. It results in the poor to satisfactory yield of product (Table 1, Entries 5–8). However, model reaction performed at 100 °C in different ionic liquids as a reaction medium had achieved maximum yield (80%) of isolated polyhydroquinoline for ionic liquid [CEMIM][MSA] (Table 1, Entries 9–12). Also, the reaction progress was inspected under solvent free condition at higher temperature with 40 mol% [CEMIM][MSA] catalyst and it results in the satisfactory yield of product (Table 1, Entry 13). Inspired by the results of Water and EtOH at higher temperature with 20 mol% [CEMIM][MSA] catalyst, we have worked an experiment with model reactants in H2O: EtOH (1:1) solvent at 75 °C by charging 20 mol% [CEMIM][MSA] catalyst and astonishingly, the reaction completed in very less time and the yield of desired product obtained was 93% (Table 1, Entry 14). Then, the counter ion effect of synthesized ionic liquids [CEMIM][Cl] & [CEMIM][MSA] on the model reaction was evaluated on basis of experimental results, tabled in Table 1. The reaction performed by 20 mol% [CEMIM][Cl] in ethanol was observed 70% yield of product (Table 1, Entry 5), whereas the equal amount of [CEMIM][MSA] used in ethanol and H2O: EtOH solvents results, 80% and 93% yields of the product respectively as well as the reaction completed in short time (Table 1, Entries 4 &14). From the above results we can conclude that, the catalytic activity of ionic liquid gets enhanced when the chloride anion exchanged by mesylate anion. The ionic liquid [CEMIM][MSA] is more acidic in nature and thereby it perceived high catalytic performance compared to [CEMIM][Cl].Further the effect of solvents were investigated on the model reaction by employing 20 mol% [CEMIM][MSA] catalyst at various temperatures and results are depicted in Table 2 . The reaction doesn't progress in non-polar solvent like toluene, o-xylene, and cyclohexane even when it was stirred for longer period (Table 2, Entries 1–3). Even by using the chlorinated and etheral solvents such as MDC, CHCl3, di isopropyl ether, methyl tert-butyl ether, the trace amount of product was obtained (Table 2, Entries 4–7). Then, the slightly polar aprotic solvents acetone and ethyl acetate, yielded up to 40% product while the low boiling polar aprotic solvents THF and acetonitrile were afforded 65% and 75% of polyhydroquinoline (Table 2, Entries 8–11). On using polar protic solvents such as water, MeOH, EtOH, IPA, H2O: EtOH (1:1), trifluoroethanol and ethylene glycol it had enhanced good to excellent yield of product (Table 2, Entries 12–18). However, satisfactory yield of product 10a was obtained by employing reaction at 100 °C for 1–1.5 h in high boiling polar aprotic solvents like DMSO, DMF, NMP and N, N-dimethyl acetamide (Table 2, Entries 19–22). In addition, trace amount of product was observed at 80 °C in neat condition after 1.5 h (Table 2, Entry 23). It is important to note that, the catalytic efficiency of catalyst enhanced in H2O: EtOH (1:1) polar protic medium at 75 °C and reaction had marched successfully to yield, 93% of product. The general investigation of solvent effect on product yield is tabled in (Table 2). It was observed that the ecofriendly solvent system H2O: EtOH (1:1) had high yield of product formation attributed owing to solubility of all reactant and intermediate product at optimized temperature as well as the low solubility of product at isolation temperature in reaction media.The loading of catalyst amount is a crucial factor in terms of reaction efficiency. To optimize catalyst loading, a set of experiment for model reaction at 75 °C in 5 ml of H2O: EtOH (1:1) with [CEMIM][MSA] catalyst (10 to 25 mol%) are displayed in Fig. 1 . The results reflects that the yield of polyhydroquinoline 10a was maximum by loading 20 mol% of [CEMIM][MSA] catalyst. Further excess in the catalyst mol% did not affect the yield and time of reaction.Moreover, the effect of temperature for the synthesis of targeted molecule was scrutinized on the model reaction with 20 mol% of [CEMIM][MSA] catalyst and 5 ml of H2O: EtOH (1:1) solvent as shown in Fig. 2 . The results signifies that the reaction performed at 25 °C and 50 °C was perceived satisfactory yield of product. Further, the yield of polyhydroquinoline 10a was raised up to 93% in 15 min when reaction implemented at 75 °C. On increasing the temperature up to 80 °C there was neither improvement in the yield of product 10a nor reduction in the time for the reaction to go to completion.Having the suitable condition in hand, the adaptability of the methodology was examined by changing different substituents in the reaction. The reaction of substituted aryl aldehydes was performed with dimedone, ethyl acetoacetate and ammonium acetate in H2O: EtOH (1;1) medium at 75 °C. The reaction results in 90–93% yield of polyhydroquinolines (10a-10v) in 15–30 min (Table 3 , entries 1–22).Normally it is observed that the reactions of aromatic aldehydes containing electron withdrawing group such as Cl, Br, F, NO2 at different positions demonstrated high yield of the products than the reactions of aldehydes containing electron donating group such as –CH3, -OH, -N(CH3)2, -OCH3, -OC2H5. The synthesized polyhydroquinolines (10a-10v) were crystalline compounds. These compounds were characterized based on their melting points, spectroscopic analysis (IR, 1H NMR, 13C NMR, DEPT-135 and LC-Mass) and were matched with those of literature authentic sample [25,63–68].Furthermore, to validate the formation of synthesized polyhydroquinolines the single crystal X- ray diffraction analysis of compounds 10j, 10v were carried out and their molecular structures were confirmed (Figs. 3 & 4 ). The compound 10j crystallized in monoclinic crystal system with the space group P 21/n, whereas the compound 10v crystallized in orthorhombic system with space group P bca. Based on the Single crystal XRD analysis results, the conformations of fused cyclohexenone and quinoline ring (10j and 10v), were identified as chair and boat conformations. The crystal data and structure refinement details for compounds 10j and 10v is tabled in Table S1. The intra-molecular hydrogen bonding details for compounds 10j and 10v are tabled in Tables S2 & S3. It discloses the H bonding interaction between O-atom of carbonyl and H-atom of amino for 10j with a distance 2.11 Å and an angle of 172.8°. Whereas H bonding interaction between O-atom of carbonyl and H-atom of amino for 10v with a distance 2.06 Å and an angle of 174°. Then, the intermolecular packing interaction of molecules (10j and 10v) are displayed in Figs. S11 & S12 (Tables S2-S3 & Figs. S10-S11 are included in the ESI).Purity of compound is the most important entity in chemical reaction and to highlight this prominent point, we have checked the HPLC purity of typical compounds (10a and 10v). The purity of the synthesized polyhydroquinolines (without purification) was found to be >99.0%, which makes the process highly cost-effective.A plausible mechanism studies of acidic ionic liquid [CEMIM][MSA] employed as a catalyst for multicomponent transformation of polyhydroquinoline (10) under optimized condition is accredited in Scheme 3 . The acidic ionic liquid [CEMIM][MSA] contains the imidazolium cation moiety and carboxylic acid group as active catalytic centers. Initially, the aryl aldehyde (7) will be activated by imidazolium cation moiety and carboxylic acid group. Afterward, the mesylate anion abstract proton from active methylene compounds (6, 8) to generate carbanion which will rapidly react with activated aryl aldehyde to afford the Knovenagel intermediate (C, D) via elimination of water molecule and regeneration of ionic liquid. On the other side, the imidazolium cation moiety and carboxylic acid group activated the active methylene compounds (6, 8) to convert their enol form, which will further react with ammonium acetate (9) to give an enaminone compounds (A, B). Further, the mesylate anion abstract the proton from enaminone (A, B), the generated anion will react with intermediate adduct (C, D) to afford polyhydroquinolines (10) with elimination of water molecule and regeneration of catalyst. The hydrogen bonding effect of acidic ionic liquid [CEMIM][MSA] with aryl aldehydes (7) and active methylene compounds (6, 8) in polar media enhance their electrophilic character. Therefore, the acidic ionic liquid [CEMIM][MSA] could increase the rate of construction of polyhydroquinolines (10). The projected mechanism explain the formation Knovenagel compounds by the Knovenagel reaction of aryl aldehydes with active methylene compounds. It is followed by Michael addition reaction, which occurs between Knovenagel compounds and enaminone, followed by intramolecular cyclization and dehydration to polyhydroquinolines. The versatility along with catalytic effectiveness of this catalyst for the three component one pot synthesis of dihydropyrimidinones ( Scheme 4 ) was explored. The reaction was carried out using 4-chlorobenzaldehyde, enaminones, and urea with 1:1:1.2 M ratio as the model substrates using 5 ml of solvent.Initially, the catalytic potency of acidic ionic liquid [CEMIM][MSA] as a catalyst was evaluated in toluene, methanol, and water solvents with minimum charging of catalyst quantity for the synthesis of dihydropyrimidinone 14a. However, the results were not satisfactory in terms of reaction time and the yield of product (Table 4 , Entries 1–3). Consequently, the model reaction was obtained satisfactory to excellent yield of dihydropyrimidinone 14a, when it was performed with ionic liquid [CEMIM][MSA] and [CEMIM][Cl] as a catalyst in ethanol, isopropanol, and H2O: EtOH (1:1) solvents along with neat condition by fluctuating the temperature and catalyst concentration parameter (Table 4, Entries 4–12). However, the ionic liquids [CEMIM][MSA] and [CEMIM][Cl] were scrutinized as solvent and reaction placed at 120 °C for 180 min it results in the yield of product 55% and 40% respectively (Table 4, Entries 13–14). Next step is to study, the impact of other acid catalysts such as HCOOH, H3PO4, L-Proline, PTSA, Amberlyst-15 and NH2SO3H, using protic and aprotic solvents at boiling temperatures and for longer duration. It afforded poor yield of isolated dihydropyrimidinone 14a (Table 4, Entries 15–20). From the results of Table 4 , we can conclude that 25 mol% [CEMIM][MSA] catalyst in H2O: EtOH (1:1) solvent at the temperature of 75 °C is found to be the superior optimum condition in relative to all other completed experiments with respect to yield and time.For exploring the robustness of the reaction using this typical condition, we further synthesized a series of 6-unsubstituted dihydropyrimidinones by treating various aryl aldehydes, different enaminones and urea ( Table 5 ). The survey of the reveled protocol specified that the reaction of urea with aromatic aldehydes and various enaminones gave excellent yields of dihydropyrimidinones. The synthesized 6-unsubstituted dihydropyrimidinones were confirmed by spectroscopic techniques and physical results were compared with those reported in the literature [37–39].One of the illustrative derivative such as 4-(4-Methoxyphenyl)-5-phenylmethanone-yl - 3, 4 -dihydropyrimidin-2(1H)-one (14c) was thoroughly explored by spectroscopic data. The FTIR spectrum of compound 14c showed absorption peaks for NH stretching at 3336 and 3204 cm−1 while the >C=O stretching vibrations absorption peaks assigned at 1691 and 1654 cm−1. 1H NMR analysis revealed that the OCH3 proton observed singlet at δ 3.73 ppm, whereas the characteristic methine proton signal appeared doublet at δ 5.39 ppm. The aromatic proton signals were observed in the range of at δ 6.89–7.28 ppm while two NH protons showed downfield signals at δ 7.78 and δ 9.25 ppm respectively. Next, the 13C NMR spectra was assigned the signals at δ 39.72 ppm and δ 55.54 ppm for OCH3 and methine carbon. The carbonyl C signals were observed at δ 159.07 and δ 192.05 ppm while aromatic C signals appeared in the range at δ 113.31–151.72 ppm. The GC–MS analysis disclosed the molecular ion mass (m/z) at 308.20, which is like calculated mass. All the above interpreted spectroscopic results defined the formation of desired dihydropyrimidinone derivative 14c (All the spectra are included in the ESI as entry 14c).Moreover, in supporting to the spectroscopic results, the crystal structure of compound 14c was determined in detail by single crystal X-ray diffraction analysis. The compound 14c crystallized in monoclinic crystal system with P 21/c space group and the unit cell dimensions are; a = 9.4018 (6) Å, α = 90°, b = 8.1698 (5) Å, β = 101.575 (3)° and c = 20.6085 (13) Å, γ = 90°. ORTEP of 14c with confirmation of pyrimidine ring is as shown in Fig. 5 and the crystal structure refinement details is expressed in Table S4. The geometry of intramolecular hydrogen bonding for compound 14c is represented in Table S5. The proton attached nitrogen N1 forms an H bonds with the Oxygen O2 (>C=O) such that the N1….H1A….O2 distance is 2.789 Ǻ while that of the N2….H2A ….O1 (>C=O) distance is 2.958 Ǻ. Also, the packing interactions of molecules (14c) along different axes is outlined as in Fig. S13 It is significant to remark that this is a sustainable process to prepare ten novel compounds (Table 5, Entries 4–13) not listed in the literature derived from dihydropyrimidinones while the four compounds (Table 5, entries 1–3 & 14) are known derivatives. (Tables S4-S5 & Fig. S13 are included in the ESI).A proposed mechanism for the developed transformation is projected in Scheme 5 . Initially the catalytic active centers of acidic ionic liquid [CEMIM][MSA] will activate the aryl aldehyde 11 and enaminone 12. Then urea 13 directly reacted with activated aryl aldehyde 11 to form the N-acyliminium intermediate E. Subsequently, the activated enol tautomer 12A of enaminone 12 will reacted with N-acyliminium intermediate E to produce an intermediate G via adduct F. The intermediate G underwent intramolecular cyclization to form hexahydropyrimidine compound H, which on further removal of dimethyl amine as by product to afford the dihydropyrimidinone 14.Atom economy is an essential aspect of green chemistry, and the efficiency of reactions were established by using the percent atom economy. The percent atom economy for the reaction of polyhydroquinolines (10a-10v) and dihydropyrimidinones (14a-14n) is calculated by using the formula, % Atom economy = (MW of product / ∑ MW of reactants) x 100 and the results implied that, both the reaction have high percent atom economy (Fig. 6 ) which generated minimum amount of waste product. Then, the percent carbon economy exposes the percentage carbon in the reactants that remain in the ideal product and was evaluated for the polyhydroquinoline 10a and dihydropyrimidinone 14a by applying the formula,% Carbon economy = (Number of carbon in product / ∑ number of carbon in reactants) x 100.The compounds 10a and 14a had shown 91.3% and 89.5% carbon economy which persisted maximum number of carbons.Reusability and recovery of the acidic ionic liquid catalyst [CEMIM][MSA] was examined for the synthesis of polyhydroquinoline (10a) and dihydropyrimidinone (14a) under the optimized conditions ( Fig. 7 ). After completion of the reaction, the catalyst was recovered from the filtrate and applied for repeated reactions for the same model reaction under the same reaction conditions. Based on recyclable results, it was observed that the [CEMIM][MSA] exhibited as highly efficient and recoverable for a five consecutive times with insignificant loss in its catalytic efficacy. The physicochemical transformation of recovered catalyst after fifth runs was inspected by employing NMR and Mass analysis (Figs. S16-S20). There is no remarkable transformation in the structure of recovered catalyst as referred by NMR (1H NMR, 13C NMR, and DEPT-135) and Mass in comparison with fresh catalyst. These results suggest that the catalyst was greatly stable up to fifth recycle and suitably proficient for its reuse under typical reaction conditions. (Figs. S16-S20 are included in the ESI).To explore the merits of the existing catalyst in association with other reported catalyst in literature for similar reaction. We have tabulated the performance of acidic ionic liquid [CEMIM][MSA] amongst other catalysts for the synthesis of polyhydroquinolines (10a-10v) and dihydropyrimidinones (14-14n) are revealed in Table 6 . As it is marked from the results, acidic ionic liquid [CEMIM][MSA] can accomplished as highly proficient catalyst with respect to reaction times and yield of products. Furthermore, to highlight the noteworthy advantages of the developed method, we have matched our result with literature reported methods ( Table 6 ). As described in Table 6 , the developed strategies are superior as compared with defined approaches in terms of i) use of benign media and halogen free catalysts, ii) avoidance of drastic reaction condition, iii) workup less safe process, iv) easy recovery of catalysts, v) evasion of column and recrystallization methods to purify product, vi) short reaction time, and vii) excellent yields of product.All the chemical reactions were performed in round bottom single neck glass flask. The chemicals and solvents were purchased from Loba Chemicals, Sigma Aldrich and S. D. fine chemicals and used as received without further purification. All the melting points were measured by a Labstar melting point apparatus and were uncorrected. All the reactions were monitored by the thin layer chromatography (TLC) using silica gel coated plates. Infrared spectra were documented on a Perkin- Elmer, FTIR-1600 and Bruker, 3000 hyperion microscope with vertex 80 spectrophotometer in KBr and expressed in cm−1. Analysis of 1H NMR, 13C NMR, DEPT-135and HSQC spectra were determined on a Bruker Avance (300, and 400 MHz) spectrometer as D2O, DMSO‑d 6 solutions, using tetramethyl silane (TMS) as the internal standard. Chemical shifts (δ) were expressed in ppm. Mass spectra were recorded with a Waters Q-TOF micromass spectrometer by electrospray ionization (ESI). Thermogravimetric analysis (TGA) was performed on a Netzsch Sta 449 F3 Jupiter analyzer under a nitrogen atmosphere. Purity of compounds were analyzed by the HPLC on an Agilent 1200 system.X-ray diffraction data for compounds 10j, 10v and 14c were collected at T = 297 (2) K on a Bruker APEXII CCD diffractometer with graphite monochromated Mo Kα (λ = 0.71073 A°) radiation. Table S1 and Table S4 data are exhibited the unit cell parameters and other crystallographic details. Table S2, Table S3 and Table S5 data represented the hydrogen bonding details. The structures were solved using the direct methods of program SHELXS 2018 and refined anisotropically by full-matrix least-square on F2 with the program SHELXL 2018.Acidic ionic liquid [CEMIM][MSA] was synthesized based on the approach termed in preceding studies with some modifications [69–72]. In a 100 ml single neck flask, charged 1- methyl imidazole (1) (20 mmol) and 2- chloro propionic acid (2) (20 mmol) at 25–30 °C. The reaction mixture was agitated at 45 °C till completion of reaction (viscous mass). After that reaction mixture was washed and decanted with 3 × 50 ml ethyl acetate at 25–30 °C to remove unreacted raw materials. Then the oily mass degassed under vacuum at 70–75 °C to give an intermediate ionic liquid [CEMIM][Cl] (3) as viscous white semisolid in 96.0% yield. Next, in same reaction flask [CEMIM][Cl] (3) (20 mmol) and 40 ml ethanol were charged, and reaction mixture stirred well for 15 min at 25–30 °C to observed homogeneous solution. To the intermediate ionic liquid solution, methane sulfonic acid (4) (21 mmol) was slowly added with vigorous stirring at 25–30 °C and mixture was stirred further 4–5 h on the same condition to carry anion metathesis reaction. Furthermore, the reaction mixture was distilled under vacuum at 50–55 °C to obtain viscous oil. The viscous oil washed and decanted with 3 × 50 ml ethyl acetate to remove traces of acid. Finally, the oily residue degassed well under vacuum at 70–75 °C, afforded viscous a light green colored desired acidic ionic liquid [CEMIM][MSA] (5) with 98.0% yield.Charged dimedone 6 (1 mmol), aldehyde 7 (1 mmol), ethyl acetoacetate 8 (1 mmol), ammonium acetate 9 (1.5 mmol) and H2O: EtOH (1:1, 5 ml) in a flask. Charged acidic ionic liquid [CEMIM][MSA] (20 mol%) catalyst and the reaction mixture was agitated at 75 °C till completion of reaction. After completion of reaction, the mixture was cooled to 0–5 °C and stirred for 1 h. The precipitated solid was filtered and washed with 2 × 2.5 ml cooled aqueous ethanol (1:1) to get a pure product in excellent yield.A three-component mixture of aldehyde 11 (1 mmol), enaminones 12 (1 mmol) and urea 13 (1.2 mmol) was treated with [CEMIM][MSA] (25 mol%) in a flask at 75 °C in H2O: EtOH (1:1, 5 ml) After completion of reaction, the crude mixture was cooled to 25–30 °C and stirred for 30 min. The precipitated solid was filtered out and washed with 2 × 2.5 ml aqueous ethanol (1:1), which results in excellent yield of pure dihydropyrimidinones 14.The recovery of the catalyst [CEMIM][MSA] was evaluated for polyhydroquinoline synthesis on the model reaction dimedone (1 mmol), 4-Cl benzaldehyde (1 mmol), ethyl acetoacetate (1 mmol), ammonium acetate (1.5 mmol) and 20 mol% of [CEMIM][MSA] in aq. media at 75 °C. After filtration of product, the filtrate was concentrated on rotary evaporator to give crude residue containing ionic liquid and organic impurities. The residue was washed with ethyl acetate (3 × 15 ml) to remove organic impurities. After that the catalyst was dried at 70 °C under reduced pressure and reused in consequent reaction for next run.White semisolid; Yield: 96.0%; 1H NMR (500 MHz, D2O): δ 2.93–2.95(t, 2H, CO-CH2), 3.82 (s, 3H, N-CH3), 4.40–4.43 (t, 2H, N-CH2), 7.36–7.38(m, 1H, Ar- H), 7.45–7.48(m, 1H, Ar- H), 8.71 (s, 1H, Ar- H);ESI-MS (+ve ion) (m/z): 155.09(M+);Anal. Calcd. For C7H11N2O2Cl (190.627): C, 44.10; H, 5.82; N, 14.70 Found: C, 44.07; H, 5.78; N, 14.76.Light green viscous oil; Yield: 98.0%;IR (KBr): 3483, 3110, 1725, 1574, 1413, 1190, 1051 cm−1; 1H NMR (400 MHz, D2O): δ 2.66 (s, 3H, S-CH3), 2.85–2.88 (t, 2H, CO-CH2), 3.74 (s, 3H, N-CH3), 4.32–4.35 (t, 2H, N-CH2), 7.28–7.30(d, 1H, Ar- H), 7.37–7.38(d, 1H, Ar- H), 8.63 (s, 1H, Ar- H); 13C NMR (100 MHz, D2O): δ 33.84, 35.54, 38.30, 44.70, 122.18, 123.43, 136.44, 174.16;DEPT-135 (100 MHz, D2O): δ 33.83, 35.57, 38.29, 44.70, 122.18, 123.43, 136.43;ESI-MS (+ve ion) (m/z): 155.08(M+);ESI-MS (− ve ion) (m/z): 96.06(M+);Anal. Calcd. For C8H14N2O5S (250.272): C, 38.39; H, 5.64; N, 11.19. Found: C, 38.34; H, 5.61; N, 11.25.Light green viscous oil; Yield: 94.0%; 1H NMR (400 MHz, D2O): δ 2.66 (s, 3H, S-CH3), 2.83–2.87 (t, 2H, CO-CH2), 3.75 (s, 3H, N-CH3), 4.30–4.34 (t, 2H, N-CH2), 7.27–7.28(d, 1H, Ar- H), 7.34–7.35(d, 1H, Ar- H), 8.61 (s, 1H, Ar- H); 13C NMR (100 MHz, D2O): δ 33.75, 35.58, 38.28, 44.67, 122.18, 123.44, 136.42, 174.07;DEPT-135 (100 MHz, D2O): δ 33.76, 35.50, 38.29, 44.61, 122.17, 123.42, 136.41;ESI-MS (+ve ion) (m/z): 155.09(M+);ESI-MS (− ve ion) (m/z): 96.06(M+);Light yellow solid; mp: 249–250 °C; Yield: 94.0%;IR (KBr): 3274, 3205, 3077, 29,591,706, 1648, 1604, 1491, 1382, 1279, 1214, 1071 cm−1; 1H NMR(400 MHz, DMSO‑d 6): δ 0.81 (s, 3H, CH3), 0.98 (s, 3H, CH3), 1.08–1.12 (t, 3H, CH3), 1.92–2.11 (m, 2H, CH2), 2.20 (s, 3H,CH3), 2.24–2.33 (s, 2H,CH2), 3.90–3.95 (q, 2H,CH2), 4.81 (s, 1H, CH), 7.06–7.13 (m, 4H, ArH), 8.83 (s, 1H, NH); 13C NMR (100 MHz, DMSO‑d 6): δ14.45, 18.66, 26.89, 29.52, 32.49, 36.04, 50.60, 59.52, 103.86, 110.16, 127.86, 129.66, 130.73, 145.65, 146.82, 150.24, 167.17, 195.12;DEPT-135 (100 MHz, DMSO‑d 6): δ 14.43, 18.64, 26.86, 29.50, 36.02, 39.85, 50.56, 59.55, 127.84, 127.87, 129.65;ESI-MS (m/z): 374.15 (M + H)+;HPLC purity: 99.09%;Yellow solid; mp: 256–258 °C; Yield:91.0%;IR (KBr): 3227, 3206, 3077, 2958, 1701, 1648, 1495, 1380, 1280, 1216, 1168, 1072, 1030 cm−1; 1H NMR(400 MHz, DMSO‑d 6): δ 0.84 (s, 3H, CH3), 0.99 (s, 3H, CH3), 1.12–1.14 (t, 3H, CH3), 1.92–2.06 (m, 2H, CH2), 2.20 (s, 3H,CH3), 2.10–2.33 (m, 2H,CH2), 3.63 (s, 3H, CH3), 3.90–3.96 (q, 2H,CH2), 4.76 (s, 1H, CH), 6.59–6.63 (m, 2H, ArH), 7.03–7.05 (m, 2H, ArH), 8.73 (s, 1H, NH); 13C NMR (100 MHz, DMSO‑d 6): δ 14.49, 18.68, 27.03, 27.94, 29.64, 31.51, 32.49, 35.42, 50.79, 55.08, 59.33, 104.67, 110.81, 113.17, 128.86, 128.96, 140.44, 144.81, 149.76, 157.56, 167.41, 194.95;DEPT-135 (100 MHz, DMSO‑d 6): δ 14.48, 18.69, 27.03, 29.63, 35.42, 40.04, 50.79, 55.07, 59.32, 113.15, 128.86;ESI-MS (m/z): 392.18 (M + Na)+;Light yellow solid; mp:216–218 °C; Yield:90.0%;IR (KBr): 3399, 3294, 3207, 3096, 2951, 1658, 1589, 1510, 1311, 1272, 1168, 1031 cm−1; 1H NMR(400 MHz, DMSO‑d 6): δ 0.87 (s, 3H, CH3), 1.00 (s, 3H, CH3), 1.13–1.17 (t, 3H, CH3), 1.94–2.13 (m, 2H, CH2), 2.23 (s, 3H,CH3), 2.26–2.37 (m, 2H,CH2), 3.67 (s, 3H, OCH3), 3.96–3.98 (m, 2H,CH2), 4.73 (s, 1H, CH), 6.48–6.55 (m, 2H, ArH), 6.69 (s, 1H, ArH), 8.47 (s, 1H, OH), 8.84 (s, 1H, NH); 13C NMR (100 MHz, DMSO‑d 6): δ 14.60, 18.62, 26.81, 29.68, 32.50, 35.54, 50.70, 55.78, 59.46, 104.72, 110.65, 112.26, 115.21, 119.97, 139.51, 144.72, 144.77, 147.07, 15.03, 167.56, 195.22;DEPT-135 (100 MHz, DMSO‑d 6): δ 14.60, 18.63, 26.81, 29.68, 53.54, 39.91, 50.70, 55.77, 59.46, 112.25, 115.21, 119.97;ESI-MS (m/z): 408.18 (M + Na)+;Light yellow solid; mp: 230–232 °C; Yield: 92.0%;IR (KBr): 3280, 3212, 3077, 2966, 2937, 1708, 1645, 1605, 1527, 1489, 1381, 1278, 1211, 1151, 1108, 1072, 1030 cm−1; 1H NMR(400 MHz, DMSO‑d 6): δ 0.83 (s, 3H, CH3), 0.99 (s, 3H, CH3), 1.09–1.13 (t, 3H, CH3), 1.93–2.13 (m, 2H, CH2), 2.29 (s, 3H,CH3), 2.40 (s, 3H, CH3), 2.25–2.49 (m, 2H,CH2), 3.91–3.96 (m, 2H,CH2), 4.87 (s, 1H, CH), 7.18–7.20 (d, 1H, ArH), 7.34–7.36 (m, 1H, ArH), 7.70–7.71 (d, 1H, ArH), 9.00 (s, 1H, NH); 13C NMR (100 MHz, DMSO‑d 6): δ 14.38, 18.70, 19.83, 26.84, 29.52, 32.54, 36.25, 50.49, 59.66, 103.36, 109.75, 123.61, 130.70, 132.64, 132.95, 146.19, 147.52, 148.60, 150.64, 166.94, 195.07;DEPT-135 (100 MHz, DMSO‑d 6): δ 14.38, 18.70, 19.83, 26.84, 29.52, 36.25, 39.81, 50.49, 59.66, 123.62, 123.65, 132.95;ESI-MS (m/z): 399.20 (M + H)+;HPLC purity: 99.03%;Yellow solid; mp: 230–232 °C; Yield: 91.0%;IR (KBr): 3336, 3204, 3083, 2897, 1691, 1654, 1609, 1510, 1435, 1370, 1325, 1250, 1215, 1157, 1028 cm−1; 1H NMR (300 MHz, DMSO‑d 6): δ 3.73(s, 3H, OCH3), 5.39–5.40 (d, 1H, CH), 6.89–6.92 (d, 2H,Ar-H), 6.99–7.01 (d, 1H, CH), 7.25–7.28 (d,1H, ArH), 7.45–7.53 (m, 6H, ArH), 7.78 (s,1H,NH), 9.25–9.27 (d,1H, NH); 13C NMR (75 MHz, DMSO‑d 6): δ 39.72, 55.54, 113.31, 114.30, 128.08, 128.44, 128.80, 136.76, 139.11, 141.86, 151.72, 159.07, 192.05;GC–MS (m/z): 308.20 (M)+;Yellow solid; mp: 284–285 °C; Yield: 93.0%;IR (KBr): 3313, 3211, 3084, 2911, 1697, 1653, 1611, 1499, 1376, 1325, 1253, 1216, 1162 cm−1; 1H NMR (300 MHz, DMSO‑d 6): δ 5.44 (s, 1H, CH), 7.06–7.08 (d, 1H, CH), 7.31–7.51(m, 9H, ArH), 7.92 (s, 1H, NH), 9.44 (s, 1H, NH); 13C NMR (75 MHz, DMSO‑d 6): δ 53.49, 112.24, 125.53, 126.88, 127.91, 128.49, 128.86, 131.04, 131.51, 133.51, 138.78, 142.68, 146.81, 151.55, 192.05;ESI-MS (m/z): 313.1 (M + H)+;Yellow solid; mp: 292–295 °C; Yield: 92.0%;IR (KBr): 3289, 3074, 2896, 1698, 1657, 1614, 1447, 1378, 1328, 1224, 1163, 1134 cm−1; 1H NMR (300 MHz, DMSO‑d 6): δ 5.49 (s, 1H, CH), 7.10–7.22 (m, 4H, CH & ArH), 7.49 (s, 6H, ArH), 7.95 (s, 1H, NH), 9.43 (s, 1H, NH); 13C NMR (75 MHz, DMSO‑d 6): δ 53.42, 112.35, 113.54, 113.82, 114.61, 114.88, 122.86, 122.90, 128.50, 128.87, 131.04, 131.14, 131.51, 138.83, 142.64, 147.18, 147.26, 151.64, 161.05, 164.28, 192.11;ESI-MS (m/z): 297.1 (M + H)+;Yellow solid; mp: 282–284 °C; Yield: 94.0%;IR (KBr): 3268, 2964, 1682, 1592, 1571, 1508, 1445, 1371, 1326, 1245, 1200, 1151, 1088 cm−1; 1H NMR (300 MHz, DMSO‑d 6): δ 5.45–5.46 (d, 1H, CH), 7.03–7.14 (m, 3H, CH & ArH), 7.35–7.50 (m, 6H, ArH), 7.86–7.87 (d, 1H, ArH), 8.21 (s, 1H, NH), 9.38 (s, 1H, NH); 13C NMR (75 MHz, DMSO‑d 6): δ 53.23, 112.97, 115.70, 128.40, 128.72, 138.90, 140.65, 140.69, 142.15, 151.64, 160.27, 163.50, 192.04;ESI-MS (m/z): 297.1 (M + H)+;Yellow solid; mp: 258–260 °C; Yield: 90.0%;IR (KBr): 3327, 3206, 3087, 2923, 1687, 1652, 1620, 1439, 1369, 1320, 1250, 1210, 1180, 1157, 1078 cm−1; 1H NMR (300 MHz, DMSO‑d 6): δ 2.17 (s, 3H, CH3), 2.20 (s, 3H, CH3), 5.36 (s, 1H, CH), 7.01–7.09 (m, 4H, CH & ArH), 7.47–7.50 (d, 5H, ArH), 7.78 (s, 1H, NH), 9.28–9.29 (d, 1H, NH); 13C NMR (75 MHz, DMSO‑d 6): δ 19.45, 19.98, 53.52, 113.19, 124.22, 128.03, 128.45, 128.85, 130.06, 131.42, 135.82, 136.55, 139.00, 141.95, 157.75, 192.12;ESI-MS (m/z): 307.1 (M + H)+;White solid; mp: 278–279 °C; Yield: 92.0%;IR (KBr): 3273, 3142, 2924, 1703, 1651, 1612, 1575, 1446, 1369, 1329, 1263, 1247, 1199, 1125, 1074 cm−1; 1H NMR (300 MHz, DMSO‑d 6): δ 5.54 (s, 1H, CH), 7.00–7.11 (d, 1H, CH), 7.47–7.65 (m, 9H, ArH), 7.99 (s, 1H, NH), 9.50 (s, 1H, NH); 13C NMR (75 MHz, DMSO‑d 6): δ 53.63, 112.06, 123.63, 124.78, 128.48, 128.88, 129.32, 129.75, 130.31, 131.02, 131.52, 138.79, 142.88, 145.74, 151.43, 192.04;ESI-MS (m/z): 347.1 (M + H)+;White solid; mp: 257–258 °C; Yield: 92.0%;IR (KBr): 3286, 2923, 1702, 1673, 1650, 1597, 1440, 1369, 1324, 1256, 1200, 1178, 1089 cm−1; 1H NMR (300 MHz, DMSO‑d 6): δ 2.34 (s, 3H, CH3), 5.43–5.44 (d, 1H, CH), 7.08 (s, 1H, CH), 7.25–7.42 (m, 8H, ArH), 7.92 (s, 1H, NH), 9.42 (s, 1H, NH); 13C NMR (75 MHz, DMSO‑d 6): δ 21.43, 53.53, 112.23, 125.54, 126.88, 127.89, 128.69, 129.40, 131.05, 133.48, 136.03, 141.61, 142.18, 146.88, 151.60, 191.80;ESI-MS (m/z): 327.1 (M + H)+;Light yellow solid; mp: 270–272 °C; Yield: 91.0%;IR (KBr): 3316, 3087, 2900, 1702, 1655, 1615, 1602, 1487, 1444, 1373, 1323, 1225, 1183, 1160 cm−1; 1H NMR (300 MHz, DMSO‑d 6): δ 2.34 (s, 3H, CH3), 5.48–5.49 (d, 1H, CH), 7.02–7.11 (m, 3H, CH & ArH), 7.17–7.39 (m, 6H, ArH), 7.89 (s, 1H, NH), 9.38–9.39 (d, 1H, NH); 13C NMR (75 MHz, DMSO‑d 6): δ 21.45, 53.47, 112.51, 113.51, 113.79, 114.40, 114.68, 122.70, 122.73, 128.58, 129.24, 130.69, 130.80, 136.06, 141.45, 141.89, 147.19, 147.28, 151.78, 161.03, 164.27, 191.82;ESI-MS (m/z): 311.1 (M + H)+;Light yellow solid; mp: 291–293 °C; Yield: 92.0%;IR (KBr): 3327, 3199, 3096, 2916, 1700, 1658, 1617, 1602, 1507, 1438, 1372, 1322, 1225, 1182, 1157 cm−1; 1H NMR (300 MHz, DMSO‑d 6): δ 2.33 (s, 3H, CH3), 5.45–5.46 (d, 1H, CH), 7.03–7.14 (d, 1H, CH), 7.09–7.11 (m, 2H, ArH), 7.22–7.24 (d, 2H, ArH), 7.35–7.39 (m, 4H, ArH), 7.85 (s, 1H, NH), 9.34–9.36 (d, 1H, NH); 13C NMR (75 MHz, DMSO‑d 6): δ 21.43, 53.29, 112.91, 115.41, 115.70, 128.59, 128.80, 128.91, 129.26, 136.13, 140.71, 140.74, 141.45, 141.64, 151.73, 160.26, 163.49, 191.83;ESI-MS (m/z): 311.1 (M + H)+;Yellow solid; mp: 300–302 °C; Yield: 90.0%;IR (KBr): 3334, 3208, 3089, 2933, 1691, 1649, 1615, 1438, 1368, 1320, 1252, 1209, 1179 cm−1; 1H NMR (300 MHz, DMSO‑d 6): δ 2.17 (s, 3H, CH3), 2.19 (s, 3H, CH3), 2.34 (s, 3H, CH3), 5.35 (s, 1H, CH), 7.00–7.08 (m, 4H, CH & ArH), 7.24–7.27 (d, 2H, ArH), 7.37–7.40 (d, 2H, ArH), 7.78 (s, 1H, NH), 9.27 (s, 1H, NH); 13C NMR (75 MHz, DMSO‑d 6): δ 19.47, 20.01, 21.43, 53.56, 113.18, 124.23, 128.04, 128.64, 129.36, 130.03, 135.77, 136.24, 136.51, 141.41, 141.47, 142.03, 151.79, 191.85;ESI-MS (m/z): 321.2 (M + H)+;White solid; mp: 262–264 °C; Yield: 92.0%;IR (KBr): 3292, 2923, 1702, 1667, 1651, 1614, 1601, 1567, 1453, 1370, 1328, 1263, 1248, 1201, 1180, 1123, 1099, 1072 cm−1; 1H NMR (300 MHz, DMSO‑d 6): δ 2.29 (s, 3H, CH3), 5.63 (s, 1H, CH), 7.04 (s, 1H, CH), 7.10–7.12 (d, 2H, ArH), 7.20 (s, 1H, ArH), 7.28–7.30 (d, 2H, ArH), 7.42–7.44 (d, 3H, ArH), 7.60 (s, 1H, NH), 9.19 (s, 1H, NH); 13C NMR (75 MHz, DMSO‑d 6): δ 21.45, 53.88, 112.60, 123.59, 124.27, 129.36, 130.56, 135.85, 141.44, 141.67, 145.42, 151.89, 192.04;ESI-MS (m/z): 361.1 (M + H)+;In summary, we have successfully established a sustainable strategy for the synthesis of polyhydroquinoline and 6-unsubstituted dihydropyrimidinone derivatives using novel acidic ionic liquid [CEMIM][MSA] as recoverable and reusable catalyst in a single operation. The remarkable features of explored protocol are high catalytic efficacy of acidic ionic liquid [CEMIM][MSA] catalyst, minimum catalyst loading, green reaction profile, step and atom economic methods, shorter time period of reactions, economically feasible starting materials, workup less procedures, low generation of waste materials and purification free isolation of pure products with excellent yield. Priyanka Patil: Formal analysis, Methodology, Investigation. Suresh Kadam: Supervision, Software, Project administration. Dayanand Patil: Resources, Data curation, Validation. Paresh More: Conceptualization, Writing – review & editing, Writing – original draft, Supervision.None.The authors thank the SARTHI of Maharashtra for the funding provided through CSMNRF-2021/2021-22/896. The authors are also grateful to Solapur University, Punjab University, IIT Bombay and IIT Madras for spectral measurements. Further authors are thankful to the Principal and the Management of the V. G. Vaze College Autonomous for the laboratory facilities. Supplementary material Image 87 Crystallographic data for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as Supplementary Publication No. CCDC 2107726, 2,107,727, 2,107,728. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (www.ccdc.cam.ac.uk/data_request/cif or e-mail: [email protected]).

Carboxylic acid functionalized imidazolium based novel acidic ionic liquid [CEMIM][MSA] was synthesized using economically feasible raw materials under very mild condition. The cost effective and sustainable synthesis of polyhydroquinoline and 6-unsustituted dihydropyrimidinone derivatives in green media was first time carried out very effectively using [CEMIM][MSA] catalyst. Acidic ionic liquid catalyst [CEMIM][MSA] with carboxylic acid group was easily separated and reused up to five cycles without much loss in its catalytic activity and stability. The introduced catalyst had showed a better catalytic performance in aqueous medium to obtain described compounds with excellent yields in shorter time. Purification free isolation of pure polyhydroquinolines and 6-unsustituted dihydropyrimidinones derivative makes the revealed protocol attractive and ecological.

The Ni–MoO2 heterostructure was synthesized in suit on porous bulk NiMo alloy by a facile powder metallurgy and hydrothermal method. The results of field emission scanning electron microscopy (SEM), field emission transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) reveal that the as-prepared electrode possesses the heterostructure and a layer of Ni(OH)2 nanosheets is formed on the surface of Ni–MoO2 electrode simultaneously after hydrothermal treatment, which provides abundant interface and much active sites, as well as much active specific surface area. The results of hydrogen evolution reaction indicate that the Ni–MoO2 heterostructure electrode exhibits excellent catalytic performance, requiring only 41 mV overpotential to reach the current density of 10 mA/cm2. It also possesses a small Tafel slope of 52.7 mV/dec and long-term stability of electrolysis in alkaline medium.

Electrochemistry plays an essential role in the revolution from the current fossil-fuel-dominated energy market to a green, affordable, and sustainable energy economy because of its great potential to convert and store electricity from renewable energy sources [1]. For instance, hydrogen generation by electricity provided from renewable resources in conjunction with fuel cells and rechargeable batteries represent promising technologies for our future energy transformation. Catalysis is at the core of the processes taking place in these electrochemical devices. Their large-scale application is yet limited by low catalytic efficiency and high costs of electrode materials [2]. To address these problems, computational researchers have spent considerable efforts on the fundamental understanding of reaction mechanisms because a profound understanding of the elementary processes on the atomic scale may ultimately aid the rational design of multi-functional catalysts for the application in practice.Nanostructured catalysts have attracted tremendous interests for various electrochemical processes, such as water splitting [3,4], nitrogen fixation [5], carbon dioxide reduction [6], or rechargeable batteries [7,8], due to their large specific surface area, flexible structure, and unique electronic properties. To maximize these advantages, different nanostructures have been developed and designed, including zero-dimensional (0D) nanocages [9–11] and nanodots [12–14], one-dimensional (1D) nanotubes [15] and nanowires [7], two-dimensional (2D) nanosheets [16], three-dimensional (3D) nanocups [17], or mixed combinations thereof [18,19]. The precise control of structure and composition of catalysts can greatly boost their catalytic performances. Heterostructure is a typical nanostructure, which couples two or more low-dimensional nanostructures through van der Waals interactions [20]. This kind of architecture has attracted great interest in the catalysis community owing to their extraordinary chemical and physical properties [21]. Well-defined composites can work synergically at the interface to effectively tune the binding strength of reaction intermediates, and this may lead to an improved catalytic performance [22,23]. Another widely developed nanocomposite refers to the core-shell structure, which also has shown excellent advantages in many energy conversion applications [24–26]. This type of structure consists of two different constituent phases, and one of them serves as the core while the other acts as the shell layer to cover the core. A smartly constructed core-shell structure can lead to specific features at the interface. For example, Raza et al. [27] reported a ceria-based nanocomposite in which ceria forms the core and a salt acts as the shell for the application as a low-temperature solid-oxide fuel cell. This functional nanocomposite exhibited extremely fast ion transport at around 300 °C with ionic conductivities exceeding 0.1 S/cm at the interfaces. Apart from the combination of different composites, nanostructured catalysts with diverse morphologies have reported to reveal outstanding performances in electrochemical devices. Liu et al. [28] proposed a tetragonal VO2 hollow nanosphere as a cathode material for aqueous zinc-ion batteries. These hollow nanospheres not only facilitate Zn ion migration but can also enhance the stability and conductivity of the electrode. Moreover, a newly developed nanocatalyst with flower-shaped structure has been reported [29]. The high surface-to-volume ratio and enhanced surface sensitivity and stability allows wide applications in the area of nanotechnology [30]. Another example refers to the work by Li et al., who synthesized a trimetallic carbon nanoflower electrocatalyst with adjustable Co2+/Fe2+/Ni2+ ratio within a metal–organic framework. The synergistic electronic effects between the different components result in an excellent overall water-splitting activity [31].The abovementioned nanostructured catalysts share the common advantage of high surface area, which can provide abundant active sites, and this may result in a larger current density for the electrocatalytic process at a fixed electrode potential. According to the Butler–Volmer equation (cf. Eq. (1)) [32,33], an ‘ideal’ nanostructured catalyst should reveal large number of active sites, Γact, but at the same time also a small free energy of activation, G rds #. In Eqs. (1) and (2), j, j 0 (cf. Fig. 2), and η indicate current density, exchange current density, and applied overpotential, respectively. Otherwise, e, k B, T, (γ + rrds α rds), h, and z denote the elementary charge, Boltzmann's constant, absolute temperature in K, the number of electrons transferred before the transition state with highest free energy, Planck's constant, and the overall number of electrons transferred in the reaction, respectively. The catalytic properties of an electrode materials are mainly captured by the terms Γact and G rds # (vide supra) and to a practically negligible extent also in the term (γ + rrds α rds). (1) j ( η ) = j 0 { exp ( ( γ + r r d s α r d s ) e η k B T ) − exp ( − ( z − γ − r r d s α r d s ) e η k B T ) } (2) j 0 = k B T z e Г a c t h exp ( − G r d s # k B T ) Despite various types of nanostructured catalysts have been developed and synthesized, it is still a formidable task to precisely describe their structure–performance relationship under reaction conditions, as well as to steer Γact and G rds # for the rational design of highly efficient electrocatalysts.Currently, theory-based catalyst discovery takes an increasing impact on the development of electrode materials for experimental investigations [34]. Electronic structure calculations in the density functional theory (DFT) approximation combined with thermodynamic analyses of binding energies have been well accepted as a rational tool to suggest electrode compositions to experimentalists. However, there are still many gaps between theory and experiment, on the one hand referring to the understanding of reaction mechanisms and on the other hand referring to the accurate prediction of catalytic activity [35,36]. In this perspective, we briefly discuss pitfalls and challenges of theoretical modeling for the description of nanostructured catalysts with application in energy conversion and storage, and we point out potential solutions to fill these gaps.DFT is currently the most used approach to study electrochemical interfaces [37,38]. The first and probably the most important step to accurately predict the properties of electrocatalysts is to build a model that is as close as possible to the actual structure encountered in experiments. Given that DFT can cope with a few hundred atoms only, the theoretically constructed models are yet too simple to reflect the real structure under operando conditions. This finding is even more pronounced when considering that typical nanoparticles (NPs) in experiments contain several thousand atoms, which is clearly beyond the scope of DFT calculations. Recalling that the computational costs increase exponentially as soon as the number of atoms rises, so far there is a complex trade-off between theoretical models and the real structure in experiments, especially when calculating electronic properties such as band structures.Calle-Vallejo et al. [39] theoretically investigated finite-size effects of adsorption energies on Pt NPs with different sizes. The authors reported that the adsorption energies of intermediates in the oxygen reduction reaction (ORR) could differ by about 0.5 eV for different NP sizes and extended surface as depicted in Fig. 1 a [40]. Such large deviations, however, can lead to unreliable and even completely wrong guidelines to devise new catalysts. To address this issue, Calle-Vallejo et al. developed a ‘coordination-activity plot’ method to predict trends of adsorption energies and resolve the geometric structure of optimum active sites. The effects of first-nearest neighbors and second-nearest neighbors are considered both by applying different weights, summarized in the concept of generalized coordination numbers ( C N ¯ ). As shown in Fig. 1b, the volcano map is constructed by compiling the coordination numbers and potential-determining steps of the reaction. On the left side of the volcano, low coordination is accompanied by too strong bonding whereas high coordination results in too weak bonding at the right volcano leg. The optimum binding strength is located around C N ¯ = 8 , and this coordination outperforms the Pt(111) surface ( C N ¯ = 7.5 ). In summary, the concept of coordination provides a path in heterogeneous catalysis for the design of optimum surface sites at affordable computational costs.The concept of generalized coordination numbers can also be used to describe the relationship between catalytic activity and various morphologies of NPs [41]. Fig. 1c indicates four different types of shapes, including convex, concave, frame, and cross-shaped NPs. The most active site on these NPs are identified and compared with the flat Pt(111) surface. It turns out that the convex sites are less active than the concave sites for the ORR due to their smaller coordination number, culminating in a volcano trend that qualitatively explains the different catalytic performance of Pt nanostructures (cf. Fig. 1d).Electrochemical experiments on the laboratory scale take place at 25 °C in an aqueous electrolyte with a well-defined pH value and a well-defined electrode potential. Quite in contrast, DFT calculations are commonly performed at 0 K under vacuum conditions, and the effects of pH and potential are often neglected in the computations. However, changes in pH and potential can greatly influence the structure and composition of electrocatalysts.Operando conditions in experiments are challenging for ab initio theory, particularly since the elementary reaction steps may take place on several surface facets simultaneously [42]. It is a common approximation in theory to determine the energetically favored surface facet in terms of surface energy and to build a slab model for this surface termination [43]. A potential opportunity to consider the presence of several surface facets refers to the Wulff construction [44]. While relying on surface energy calculations, this method is regarded as an effective tool to understand and predict the shape of NPs and its accuracy has been further improved recently [45,46]. In this context, we would like to point out the work of Opalka et al. [47] as a prototypical example. While the authors made use of this advanced approach, even the prediction of NP shapes by the Wulff construction is accompanied with pitfalls, as outlined in the following.Opalka et al. [47] systematically investigated the shape transformation of IrO2 NPs under application of an electrode potential when moving toward the potential regime of oxygen evolution reaction (OER). Typically, NP models are constructed at U = 0 V vs. SHE (standard hydrogen electrode). Opalka et al., however, reported that the equilibrium shape of a IrO2 NP is strongly dependent on the applied electrode potential. While for U = 0 V vs. SHE the IrO2 NP exhibits two apical facets, namely the (100) and (110) terminations, at potentials exceeding U = 0.9 V vs. SHE, the (111) facet becomes thermodynamically stable. When increasing the potential further to U = 1.3 V vs. SHE, the entire NP forms (111) facets only. The reason why the preferred surface facet changes at different potentials is related to the binding strengths of the intermediate species on the electrode surface. The (111) facet of IrO2 NP is more favorable than the (110) facet at U = 1.3 V vs. SHE because the (111) facet binds surface oxygen, ∗O, stronger than the (110) facet, and therefore, the surface energy of the (111) facet excels that of the (110) facet (cf. Fig. 2). Consequently, the OER over IrO2 NPs commences from the (111) facet rather than on the (110) facet under experimental conditions. In contrast, in ab initio theory the reaction mechanism is commonly studied for the (110) facet [48–50], which can result in erroneous conclusions on mechanism and activity when comparing the outcome to experimental studies of nanosized systems.The above finding causes severe efforts for theoreticians to cover the following factors into the analysis of nanosized systems: a) a variety of surface facets need to be considered in the calculations, and even facets that are not stable at U = 0 V vs. SHE have to be taken into account because they can possibly be stable for elevated electrode potentials; b) for all these surface facets, a sufficient number of representative surface structures with different adsorbates (e.g. OH, O, and OOH when moving toward anodic potential conditions) need to be calculated. Therefore, the resulting parameter space is enormous, and thus the corresponding computations can easily surmount the level of feasibility.In the following, we want to point out a methodical aspect in the calculation of surface structures with applications in energy conversion and storage. In most of the cases, the entire slab model is treated as a charge neutral system by using the concept of computational hydrogen electrode (CHE), developed by Nørskov and co-workers [51]. The CHE method regards that a proton-electron pair will transfer from the charge-neutral system into the electrode reservoir in each elementary step, but the description of charged systems or calculations under applied bias are not possible by this method. Consequently, scientists have made considerable efforts to move from a constant-charge description, as encountered with the CHE, to a constant-potential formalism, also denoted as grand canonical approach [52]. For instance, Gao et al. [53] proposed a ‘fixed potential’ method to overcome this problem, in which the electrode potential (Fermi energy) is fixed while the total number of electrons can vary at the atomic level. They found that different electrode potentials will greatly influence the local electronic structures and binding environments, and this may alter catalytic activity in agreement with experimental studies [54]. Given that the binding energies obtained by the constant-charge and the fixed-potential methods can differ up to 0.50 eV [54], it appears that the conventional CHE approach is outdated and too inaccurate.Another important aspect that the CHE formulism does not address refers to the electric double layer (EDL) at the interface, which plays a vital role in heterogeneous catalysis. Modeling the EDL is a quite complex and challenging task given that not only the effect of solvent and electrolyte solutions but also the electrode potential needs to be considered [55]. Currently, ab initio molecular dynamics (AIMD) simulations are widely used to simulate the structures and dielectric properties of the EDL. Cheng et al. used this approach to investigate electrified metal/water interfaces, such as the Pt(111)/water and Ag(111)/water interfaces, to unravel structure and capacitive behavior [56–58]. However, by the inclusion of ions at the interface the computational costs for AIMD increase, and accuracy may decrease given that this method is unable to consider long-range electrostatic interaction of charged electrolytes and electrodes [59]. In recent works, Zhang and coworkers connected the AIMD approach with machine learning (ML) models to speed up the convergence of the polarization, P, which lowers computational costs for the explicit modeling of electrified interfaces [60–63]. These efforts contribute to move the theoretical description of electrified interfaces closer to the experimental situation under operando conditions.Despite the critical discussion on the CHE method because of its simplicity, it is noteworthy to refer to the work of Hörmann et al. [64]. The authors reported that the CHE approach is a first-order approximation to a fully grand canonical approach, and the results obtained by the CHE method are in qualitative agreement to a grand canonical ensemble, except for adsorbates with large dipole moments such as halides. This pinpoints that the CHE approach is yet valid for most electrocatalytic systems including the hydrogen and oxygen electrocatalysis, which are of high importance to energy storage and conversion. In summary, the community does not have a unified view on the application of computationally cost-effective charge-neutral methods (such as the CHE approach) and computationally more demanding grand canonical schemes, and there is a trade-off between the application of DFT to tackle the energetics of catalytic processes or AIMD to describe the electrochemical interface accurately [59–62].Another challenge refers to the description of the aqueous electrolyte in DFT calculations. While in experiments, the catalytic processes take place at the solid/liquid interface, which is prone to alter under operational conditions, it is still a common consensus to apply gas-phase DFT calculations to approximate the complex solid/liquid junction. One reason for this is that there is no unified method available of how to treat the solvent in ab initio electrochemistry. The solvent can be modeled by adding one or two solvent layers over the catalyst surface or by embedding a solvent environment on the whole reaction system, and these two different approaches refer to an explicit or implicit description, respectively. Apparently, the explicit solvent method provides a more accurate description of the reaction system at the expense of higher computational costs [65,66]. On the other hand, the dynamic nature of the solvent when modeled explicitly is not accounted for by DFT, and this may lead to artifacts because formally, averaging over all possible water configurations need to be carried out. The latter can only be achieved by computationally demanding AIMD simulations [67].Due to their lower computational costs, implicit solvent models are very popular in the computational electrochemistry community. A recent review by Ringe et al. summarizes the application of implicit solvation models for the incorporation of solvent effects with applications in catalysis [38]. As shown in Fig. 3 , the solid–liquid interface (SLI) can be simplified from a fully explicit quantum mechanical description to a fully implicit model by parametrization of each coarse-grained level. The implicit solvation approach accounts for the surrounding liquid electrolyte on the level of a continuous polarizable medium. This method can also mimic polarization of the electrode's electronic density under applied potential conditions and the concomitant capacitive charging of the entire double layer. Therefore, the implicit solvation scheme allows electronic structure calculations at the SLI at moderate computational costs compared with the accurate treatment of the solvent by AIMD. Despite its computational efficiency, the implicit solvation approach also reveals drawbacks that should be pointed out. These concerns address the extensive parametrization needed and the issue of transferability from one to another system, thus causing erroneous results in the worst case.An intimate interplay of experiment and theory may spur our understanding of electrochemical processes [68], and this is key for the development of catalysts with applications in energy storage and conversion. Synergy effects between theory and experiment can greatly accelerate the time-consuming process of catalyst exploration. Yet, to take the full advantage of theoretical approaches, the apparent size gap between theory and experiment needs to be carefully addressed.One key point is the complex trade-off between accuracy and computational costs. As discussed in the above, DFT is the method of choice for the investigation of catalytic processes at electrified solid/liquid interfaces, but it reaches its limit as soon as a few hundred atoms are part of the calculations. To address systems of larger size, the density functional tight binding method (DFTB) method is widely used. This approach is two to three orders of magnitude faster than DFT [69,70] due to relying on a semi-empirical approach that can handle thousands of atoms [71,72]. While the DFTB method may be a solution to model real NPs encountered in experiments, and thus, to close the gap between experiment and theory, it lacks accuracy compared with DFT, recalling that DFT can also only describe trends qualitatively but barely quantitatively [73].To our opinion, one promising opportunity to approach the gap between theory and experiment relating to nanocatalysts is the incorporation of ML techniques (cf. Scheme 1 ). ML is a branch of artificial intelligence. While experiments and theory both can easily generate tons of data, the intrinsic correlations between data points often remain unknown. This is where ML comes into play because this method of data analysis is able to identify patterns, mine valuable information from data sets, and can train models, thus allowing to speed up research activities as discussed in the following.While relying on the DFT level to maintain reasonable accuracy, ML may contribute to expand the number of calculations conducted with the caveat that theoretical models need to be sufficiently trained by reference data. More precisely, a thorough assessment of the solvent contribution by the training of implicit solvation models (cf. Fig. 3), the training of canonical or grand canonical approaches to assess their impact on the energetics of adsorbate species, and the extension of calculations to higher index surface facets for an accurate NP shape prediction under applied bias (cf. Fig. 2) are of importance and relevance for nanocatalysts. Activity trends can then be derived by concepts such as the generalized coordination number (cf. Fig. 1) or advanced activity descriptors that go beyond the prototypical thermodynamic reasoning [36,74,75]. Taking all these subtleties into account will be beneficial in that theoretical predictions and models are closer to the real systems in experiments [76–78].Nanocatalysts are auspicious for various energy storage and conversion applications and have already led to some achievements by merging computational and experimental methodologies. In this perspective, we have focused on the current gaps between computational modeling and real experimental conditions, and we have identified three main pitfalls for theoreticians. This comprises discrepancies between theoretical models and real structures, deviations between computational and experimental conditions, and the simplicity of computational protocols in terms of the electrode potential and solvation. If realistic models are studied under conditions that refer to the experimental ones by accounting for applied bias and solvation in an electrochemical environment, with the currently available computational resources we do not have a chance to tackle all the outlined pitfalls. To our opinion, machine learning (ML) techniques are encountered as a gamer changer in this regard because they exhibit great potential to overcome the associated gaps between theoretical and experimental studies. Yet, one should pay attention to that fact that ML strongly depends on the reference data derived from density functional theory(DFT) calculations and ab initio molecular dynamics(AIMD) simulations. Therefore, success of this approach is only guaranteed for reliable and sufficiently large data sets. To foster further breakthroughs to fill the gap between theory and experiment, we suggest that the development of new methods for the modeling of electrochemical systems by DFT or AIMD should go hand in hand with the ML approach.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.KSE acknowledges funding by the Ministry of Culture and Science of the Federal State of North Rhine-Westphalia (NRW Return Grant). KSE is associated with the CRC/TRR247: “Heterogeneous Oxidation Catalysis in the Liquid Phase” (Project number 388390466-TRR 247), the RESOLV Cluster of Excellence, funded by the Deutsche Forschungsgemeinschaft under Germany's Excellence Strategy – EXC 2033–390677874 – RESOLV, and the Center for Nanointegration (CENIDE). This article is based on the work from COST Action 18234, supported by COST (European Cooperation in Science and Technology).

Computational approaches to describe catalysts under electrochemical conditions are steadily increasing. Yet, particularly the theoretical description of nanostructured catalysts, which have the advantage of a high surface area or unique electronic properties through refined synthetic protocols, is still hampered by the occurrence of pitfalls that need to be circumvented. In this perspective, we aim to introduce the reader to common pitfalls in the modeling of nanostructured catalysts with applications in energy conversion and storage, and we discuss the application of machine learning techniques as a potential solution to overcome the associated gaps.

No data was used for the research described in the article.CN and CC coupling reaction has gained immense interest recently as they find importance in synthesis of fine chemicals, natural products, pharmaceuticals, agrochemicals and functional materials [1]. In drug discovery, CN and CC coupling reactions are generally palladium catalyzed. As these heterocyclic products are found in various biologically active compounds & natural products there is a need to develop a facile method for CN bond forming reaction. Traditionally, palladium catalyzed CN coupling is more efficient & used widely [2–5], There are some limitations of this method such as high cost of the catalyst & availability of palladium. Use of palladium catalyst in the synthesis of API’s can lead to contamination of drug substances. Hence, there is a need to find alternative catalytic method for CN coupling. Thus, use of non-noble transition metals such as copper [6–9], nickel [10–11], iron [12], cobalt [13] have found importance in such type of reactions.[14–24]. Yousefi et al. [21] reported Fe3O4@PVA/CuCl catalyst for CN coupling giving 90 % yield of the product in DMF at 110 °C whereas Sardarian et al. [22] reported Fe3O4@SiO2/Schiff base-Cu(II) NPs catalyst for CN cross coupling reaction for CN coupling reaction which gave 86 % yield at 100 °C in DMF. Mallick et al. [19] reported Cu-gCN catalyst in toluene at 100 °C giving 80 % yield of CN cross coupled product. Rout et al. [6] reported CuMoO4 nano-catalyst for CN coupling in DMSO at 90 °C giving 82 % yield of the product. Bazgir et al. [15] reported Cu@Cu2O nanoparticles on reduced graphene oxide and their catalytic activities in N-arylation of N-heterocycles giving 90 % yield in DMSO at 110 °C. Addition of various ligands increases the rate of the reaction but the disadvantage of using ligands is increase in additional cost for CN coupling reaction [25–27]. However, there are various disadvantages of these methods such as use of stoichiometric amount of metal reagents & use of expensive ligands which restricts its synthetic utility. Although reports are available for CN coupling reaction, it is still important to develop a ligand free, sustainable, economical & environmentally friendly method. Use of carbon-based heterogeneous catalysts is advantageous over homogenous catalysts such as ease of preparation, ability to incorporate various functionalities on the surface of the catalyst, separation, reusability, less cost, low toxicity etc.There are also reports for synthesis of various metal nanoparticles for number of applications. Metal nanoparticles are prepared from various sources such as flower extract, marine debris, fungus, vegetable waste, different species collected from sea coast, marine algae, plant extract, leaf extract, fruit extract & different types of biomass. Metals such as Cu, Fe, Ag, Pd, Au, Ni, Zr, etc were then loaded on it to form metal nanoparticles of various shapes & sizes. These nanoparticles have been used for various biomedical applications, which include antibacterial, antioxidant, free radical scavenging, antifungal, anticancer, larvicidal activity and showed biocompatibility properties. Other applications of metal nanoparticles include photocatalytic degradation of azo dyes & degradation of tetracycline and ibuprofen molecules [28–44].In continuation of our work, in the area of heterogeneous catalysis [45,46], we reported a ligand free approach towards CN coupling. Cobalt is found to be stable, non-toxic metal and can be considered as an effective alternative to palladium for CN coupling. There are very few reports available in literature for CN coupling using cobalt based heterogeneous catalysts, hence it’s important to develop cobalt-based heterogeneous catalyst and study its utility for CN coupling reaction [47,48].In this work, we synthesized cobalt immobilized carbon-based nano-catalyst Co@CC by carbonization of glucose, its functionalization followed by immobilization of cobalt. The catalyst has been characterized using various characterization techniques. Cobalt based catalysts Co@CC was evaluated for CN coupling of several amines & aryl halides. Co@CC11, Co@CC12, Co@CC13 & Co@CC14 catalyst were synthesized by varying ratio of CC and CoCl2 ratio from 1:1 to 1:4 and were denoted as Co@CC11, Co@CC12, Co@CC13 & Co@CC14 respectively.Carbonaceous catalyst has gained lot of importance now-a-days as they can be prepared easily. Due to high density of oxygen functional groups present on surface of the catalyst, it is easy to immobilize metal on it. Carbon-based catalysts are highly stable, reusable, economical and eco-friendly. In this work, we synthesized cobalt immobilized carbonaceous catalyst and studied its catalytic activity for CN coupling of aryl halides & amines. Cobalt based carbonaceous solid acid catalyst was prepared using readily available cheaper source glucose. Glucose was heated with p-TSA to form carbonaceous material containing sulphonic acid (-SO3H), phenolic (–OH) and carboxyl acid (–COOH) functional groups. It was further treated with 3-aminopropyltrimethylsilane to introduce -OSi-CH2CH2CH2NH2 groups followed by treating with CoCl2 for immobilization of cobalt. The Co@CC12 catalyst was characterized with FT-IR (Spectrum 400), P-XRD (Rigaku MiniFlex 600) Power: 100–240 VAC 1ɸ 15A 50/60 Hz, EDAX (Bruker), SEM (Quanta scanning electron microscope), HR-TEM (Jeol JEM F200), 13CP-MASS (Jeol ECX 400), XPS (Thermo K α) & BET (Quantachrome) techniques.As seen from FT-IR (Fig. 1 a), CO bonding peak at 1030 cm−1 and OS = O stretching vibration peak at 1157 cm−1 was observed. CH bending vibration peak for alkyl groups was observed at 1438 cm−1 & CC stretching vibration peak was observed at 1630 cm−1 for aromatic carbons. CO peak at 1703 cm−1 was observed for acid functionality. Strong peaks were observed for –NH groups at 1582 cm−1 & for –OH groups at 3205 cm−1. P-XRD (Fig. 1b) displayed broad peak at 2θ = 15–30⁰ for amorphous carbon with random orientation of aromatic carbon sheets and peaks at 2θ of 64.9⁰, 58.9⁰, 51.3⁰, 39.7⁰, and 29.0⁰ were ascribed to the cobalt species. EDAX analysis (Fig. 1c) showed elemental composition of C, O, N, Si, S and Co to be 59.5 %, 23.9 %, 8.1 %, 3.3 %, 0.1 % and 5.1 % respectively. SEM analysis (Fig. 1d) showed heavy crumpling features of cobalt based nano-catalyst. HR-TEM analysis (Fig. 1e) showed well dispersed nanoparticles of particle size of 25–50 nm. 13CP/MAS NMR (Fig. 1f) showed signals for N-propyl groups from 11 ppm to 61 ppm. Broad signals for polycyclic aromatic carbon atoms, –OH and –COOH groups were seen at 129, 151, 172 & 209 ppm respectively.As seen from Fig. 2 , HR-TEM elemental mapping data showed deposition of cobalt over carbon based support.XPS analysis (Fig. 3 ) showed binding energy peaks at 533 eV & 285 eV for O and C for Co@CC12. Presence of N and incorporation of 3-amino propyl trimethoxy group was confirmed from binding energy peak at 400 eV. Presence of Cl, S & Si was confirmed from peaks at 201, 167.5 and 103 eV respectively. Cobalt showed two spin orbital doublets at 797.1 eV in Co2p1/2 and 781.5 eV in Co2p3/2 confirming presence of Co3+ and Co2+ species. ICP analysis showed 3.92 % of cobalt in the catalyst. Surface area from BET analysis was found to be 15.5 m2/g.Co@CC12 was studied for CN coupling of indole & bromo-benzene using potassium hydroxide as a base. Various solvents like sulfolane, dimethylsulfoxide, water, tetrahydrofuran, γ-valerolactone, toluene, acetonitrile & dimethylformamide were studied for the reaction (Table 1 ). The reaction was found to proceed only in sulfonated solvents sulfolane and dimethylsulfoxide giving 64 % and 35 % yield respectively. The reaction did not proceed in remaining solvents. Sulfolane gave higher yield of cross-coupled product. Sulfolane being thermally stable can be operated within a wide range of reaction conditions and reaction temperatures. The reaction when carried out under homogenous conditions using CoCl2 as a catalyst in sulfolane as a solvent showed no formation of cross coupled product.We also carried out reaction using CC catalyst (without cobalt) and it was found that only traces of product were observed during this reaction. We then studied the effect of different bases and their concentration for CN coupling of indole & bromo-benzene in sulfolane as a solvent using 20 wt% of Co@CC12 catalyst at 120 °C. We tried bases such as potassium tert-butoxide (KO t Bu), potassium triphosphate (K3PO4), lithium hexamethyldisilazane (LiHMDS) and also studied various concentrations of base KOH (Table 2 ).It was found that the reaction proceeded in KO t Bu base giving 41 % yield whereas the reaction did not proceed at-all in bases K3PO4 and LiHMDS. KOH was studied at different concentrations of 1 mmol to 4 mmol and it was observed that 4 mmol of KOH was found to give highest yield of cross-coupled product. After optimizing the solvent and concentration of base we then focused on optimizing various catalysts and catalyst concentration. The Co@CC12 catalyst was screened for various catalyst concentrations i.e. 10, 20, 30 & 40 wt% for reaction of indole & bromo-benzene using KOH as a base in sulfolane as a solvent (Table 3 ). The results showed that as the catalyst concentration was increased, yield of product also increased from 59 % to 74 % (Table 3, entries 1–3). At catalyst concentration of 40 wt% using Co@CC12 catalyst the reaction did not proceed at all at rt, whereas the yield of product was increased from 67 % to 91 % as the temperature was raised from 80 to 150 °C in 35 h (Table 3, entries 5–7). The starting was completely consumed at 150 °C in 35 h to give 91 % yield. The Co@CC11, Co@CC13, Co@CC14 catalysts were screened using optimized reaction conditions used for Co@CC12 catalyst.It was observed that at 40 wt% concentration of Co@CC11 catalyst using KOH in sulfolane at 150 °C the yield was 65 % after 35 h (Table 3, entry 8). The yield of product using Co@CC13 & Co@CC14 catalysts was similar to that of Co@CC12 catalyst (Table 3, entries 9–10). Hence Co@CC12 catalyst gave maximum yield of CN cross coupled product at 150 °C after 35 h.Various different indoles and aryl halides were reacted using Co@CC12 catalyst to form respective CN cross coupled product (Table 4 ). Indole was reacted with various aryl halides such as chloro-benzene, bromo-benzene, iodo-benzene, 4-ethyl-iodo-benzene, 4-isopropyl-iodo-benzene. It was observed that indole on reaction with chloro-benzene gave 65 % cross coupled product in 35 h (Table 4, entry 1), whereas in case of reaction of indole with bromo-benzene and iodo-benzene, starting was completely consumed in nearly 35 h giving 91 % yield (Table 4, entries 2 & 3). Indole on reaction with 4-ethyl-iodo-benzene & 4-isopropyl-iodo-benzene after 35 h gave 55 & 81 % yield respectively (Table 4, entries 4 & 5). Next, we studied reaction of 3-methyl-indole with aryl halides such as bromo-benzene, 4-ethyl-iodo-benzene & 4-isopropyl-iodo-benzene (Table 4, entries 6–8). It was observed that in case of 3-methyl-indole and bromo-benzene, starting was completely consumed in 24 h giving 85 % yield of the product, whereas in case of 4-ethyl-iodo-benzene & 4-isopropyl-iodo-benzene after 35 h, 70 & 75 % yield of cross coupled product was observed. Reaction of 6-bromo-indole with bromo-benzene gave 88 % yield whereas the yield in case of 4-ethyl-iodo-benzene & 4-isopropyl-iodo-benzene (Table 4, 9-11) was less. 2-methyl-indole, 8-methyl-indole and 2-phenyl-indole were also reacted with bromo-benzene, but the reaction was found to be slow giving 69, 66 & 65 % yield of the product (Table 4, entries 12, 14, 15) whereas reaction of 2-methyl-indole and 2-phenyl-indole with 4-ethyl-iodo-benzene gave 62 & 60 % yield respectively (Table 4, entries 13 & 16).The products were purified and characterized using NMR and HRMS. It was observed that electronic properties of the precursors did not have any impact on the yield of the products. The catalyst was easily prepared from readily available natural source glucose and showed good catalytic activity for CN coupling reaction. The catalytic activity of Co@CC12 was compared with other metal based catalysts reported in literature for CN coupling (Table 5 ). Co@CC12 catalyst showed good yield with respect to the reported ones.The catalytic process is considered efficient and economical only when a catalyst can be recycled and reused. Co@CC12 catalyst was evaluated for its recyclability. After first use, the catalyst was filtered, washed and dried for further recycles. We studied it’ s reusability up-to five cycles for cross coupling of indole and bromo-benzene and it was observed to give consistent yield of the CN cross coupled product even after 5 recycles (Fig. 4 ). FT-IR, P-XRD analysis of the recycled catalyst is provided in the ESI (Fig 58, 59). In FT-IR, we observed peaks for reused catalyst for functionalities such as CO, -SO3H, –OH & –NH groups whereas P-XRD also displayed broad peaks in the range of 2θ = 15-30⁰ for amorphous carbon with random orientation of aromatic carbon sheets and peaks for cobalt species. There were no significant changes observed in fresh and recovered Co@CC12 catalyst. ICP analysis of reused catalyst showed only 0.04 % of cobalt has been leached.In conclusion, we developed a cobalt based Co@CC12 nano-catalyst and studied its catalytic activity for CN coupling reaction. It was found to be highly efficient for CN cross coupling of various amines and aryl halides giving yields in the range of 60–91 % in sulfolane as a solvent at 150 °C. The catalyst was studied for 16 different CN cross coupling reactions using various amines & aryl halides. The catalyst was easy to recycle and reused up to five cycles with consistency in yield of the products. The cobalt based Co@CC12 catalyst is reusable, economical and environment friendly compared to much expensive palladium and toxic copper-based catalyst and the method developed for CN coupling reaction is noble metal-free & ligand-free.All commercially available reagents were used without further purification unless otherwise stated. Column chromatography was performed on silica gel (60–120 mesh). 1H and 13C NMR spectra were recorded on a Bruker spectrometer using CDCl3 as a solvent and TMS as an internal standard. NMR data are reported as follows: chemical shift, multiplicity (s = singlet, bs = broad singlet, d = doublet, dd = doublet of doublet, t = triplet, td = triplet of doublet, m = multiplet), coupling constants (Hz), and integration. High-resolution mass spectra (HRMS) were recorded using electron spray ionization (ESI) with a time-off-flight mass analyzer.Glucose (5 g, 27.7 mmol) and p-toluene sulfonic acid (10 g, 58.1 mmol) were taken in a clean and dry round bottom flask and stirred at 180 ℃ for 24 h under nitrogen. After 24 h, a black solid mass was obtained which was then washed with millipore water and then with ethanol to obtain a black powder. This black powder was then dried in the oven at 100 ℃ for 5 h. After drying to this black powder was added 20 mL of ethanol and 3-amino propyl triethoxy silane (APTES) (6 mL), sonicated for 30 min and further refluxed at 80 ℃ for 8 h. The solid was collected by filtration, washed with water, followed by ethanol, and dried in the oven at 100 ℃ for 5 h. This black powder was denoted as CC. After drying, to this black powder CC (0.5 g) was added 10 mL ethanol, CoCl2·H2O (0.5 g) and further refluxed 80 ℃ for 12 h. The solid was collected by filtration, washed with water, followed by ethanol, and dried in the oven at 100 ℃ for 5–6 h. This black powder was denoted as Co/CC11 catalyst.Black powder (0.5 g) (CC) was added 10 mL ethanol, CoCl2·H2O (1 g) and further refluxed 80 ℃ for 12 h. The solid was collected by filtration, washed with water, followed by ethanol, and dried in the oven at 100 ℃ for 5–6 h. This black powder was denoted as Co/CC12 catalyst.Black powder (0.5 g) (CC) was added 10 mL ethanol, CoCl2·H2O (1.5 g) and further refluxed 80 ℃ for 12 h. The solid was collected by filtration, washed with water, followed by ethanol, and dried in the oven at 100 ℃ for 5–6 h. This black powder was denoted as Co/CC13 catalyst.Black powder (0.5 g) (CC) was added 10 mL ethanol, CoCl2·H2O (2 g) and further refluxed 80 ℃ for 12 h. The solid was collected by filtration, washed with water, followed by ethanol, and dried in the oven at 100 ℃ for 5–6 h. This black powder was denoted as Co/CC14 catalyst.Aryl amine (1 mmol) & aryl halides (1.5 mmol) were taken in a round bottom flask along with solvent sulfolane. Catalyst Co@CC12 (40 wt%) & base KOH (4 mmol) was then added to it and the reaction mixture was heated to desired temperature. After completion of the reaction from TLC, water was added to the reaction and the reaction mixture was extracted in ethyl acetate and the organic layer was evaporated on rota-vac & the crude product was purified using column chromatography. Shubham R. Bankar: Methodology, Investigation. Swapnali P. Kirdant: Methodology, Data curation. Vrushali H. Jadhav: Supervision, Conceptualization, Funding acquisition.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Vrushali Jadhav reports financial support was provided by National Chemical Laboratory CSIR. Jadhav Vrushali reports a relationship with National Chemical Laboratory CSIR that includes: employment.The remaining authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.S.R.B thanks UGC-New Delhi 276/(CSIR-UGC NET DEC.2018) & S.P.K. thanks CSIR-New Delhi 31/011(1151)2020-EMR-I for providing research fellowship. Dr. V. H. Jadhav thanks CSIR-NCL for providing the start-up fund (MLP036926) & all the facilities.Supplementary data to this article can be found online at https://doi.org/10.1016/j.rechem.2022.100682.The following are the Supplementary data to this article: Supplementary data 1

CN cross coupling reaction is very important in synthesis of pharmaceuticals, natural products, agrochemicals, fine chemicals and functional materials. Traditionally, palladium or copper metals are used for CN coupling reaction. As palladium is expensive, we developed cobalt immobilized carbon-based nano-catalyst Co@CC for CN coupling. In this work, we synthesized non-noble metal-based Co@CC nano-catalyst by carbonization of glucose, it’s functionalization followed by immobilization of cobalt on the surface of the catalyst. The catalyst was well characterized. The CN cross coupling reaction of various aryl halides & amines using Co@CC nano-catalyst was optimized for solvent, reaction temperature & catalyst concentration conditions. The catalyst showed high catalytic activity for CN coupling of various aryl halides & amines to form aryl amines in good to excellent yield up to 91 % in sulfolane as a solvent at 150 °C. The catalyst showed recyclability up to 5 times. The method developed for CN coupling reaction was noble metal free, ligand free, recyclable, sustainable, economical & environmentally friendly.

Cellulose, the most abundant polymer in nature, has vital importance for modern industry [1]. Cellulose is biocompatible, biodegradable, bioadhesive, and nontoxic properties using in many applications such as food, cosmetics, detergents, textile, and pharmaceutical areas [2]. On the other hand, chitosan deriving from chitin is the second abundant polysaccharide in nature [3]. It should be noted that chitosan has remarkable antimicrobial properties extracted from shrimp and other crustaceans. It is a natural biocompatible polymer that has been extensively investigated in pharmaceutical research and the. Recently, composite cellulose/chitosan was prepared by chemical modification including chemical grafting or physical treatments [4,5]. In recent years, natural composites including transition metal oxide, have a broad range of applications in gas sensors, supercapacitors, lithium-ion batteries, solar cells, electrochromic coatings, composite anodes for fuel cells, dye-sensitized photocathodes, as a contrasting agent for magnetic resonance.The pharmaceutical compounds have played essential roles in the human life [6]. Drugs are widely used not only to treat and prevent disease in humans and animals but also to improve the growth rate of agricultural products [7,8]. In addition to all the benefits of medicinal compounds, they are able to seriously pollute the environment, because their residues in the aquatic media have the potential to create resistance to environmental bacteria, making diseases difficult to treat and a great threat to public health [9,10]. Ciprofloxacin is a widely used antibiotic owing to its good performance in treating diseases [11]. Ciprofloxacin is released inactive forms due to partial metabolism and high structural stability after consumption [9]. The presence of antibiotics in the water cycle has caused serious concern because of their destructive impacts on human health and permanent damage to aquatic ecosystems [12]. They can disrupt the natural life cycle of native microbes. Therefore, many efforts have been made to find efficient methods that can ultimately convert ciprofloxacin into small biodegradable molecules or directly decompose it into carbon dioxide and water [10].Photocatalysis processes are considered as one of the efficient environmentally friendly methods to overcome the above-mentioned problems of antibiotics [13]. Photocatalytic degradation has the advantages such as energy saving, simplicity, mild reaction conditions and cost-effectiveness in the removal of pharmaceutical contaminants [14]. It has opened up a landscape in order to protect the environment through the design of novel high-performance photocatalysts for the removal of the toxic organic pollutants [15]. The electrons and holes are generated in the presence of photocatalyst and light source that can activate H2O and O2 to produce reactive oxygen species. These active species degrade antibiotics into substances with lower or even non-toxic biotoxicity [16]. Until now, prominent researches have been performed on the fabrication of various nanostructures with high photocatalytic properties [17]. However, there is still a need to design highly efficient photocatalysts with significant photoinduced charge transfer capabilities and a special electron-hole separation system simultaneously [18].Nickel oxide (a p-type semiconductor with antiferromagnetic behavior) and nickel (conducting ferromagnetic metal) possess unique electrical, optical properties, low cost, and high stability and can be used in catalysis, rechargeable batteries, and fuel cells [14]. They have impressive electrical conductivity, high electrochromic efficiency, high electro-activity, high effective surface area, cheap and straightforward synthesis procedures, and a wide modulation range [19]. They have been widely used in photocatalysis, supercapacitors, lithium-ion batteries, dye sensitizer in solar cells, transparent electrodes, biosensors, etc.[16,20]One of the major problems in photocatalytic processes is high recombination rate of photo generated electron-hole pairs which reduces the amount of reactive species for the degradation of organic compounds. The supporting the photocatalysts onto the suitable supports is an efficient solution for decreasing recombination of electron–hole pairs [21].Here, in continuing our works, micro-crystalline aldehyde cellulose and nano chitosan were extracted from wastes of barely and shrimp wastes, respectively [22,23]. Cellulose and chitosan can attach covalently via condensation reaction and also Ni/NiO nanocomposite was synthesized using Calotropis procera. Finally, by adding Ni/NiO to cellulose/chitosan, nano-biocomposite Chit-Cell@Ni/NiO was designed. The biosynthesized nanocomposite has been utilized for the photocatalytic degradation of ciprofloxacin under sunlight. This study provides a green cost-effective strategy for degradation of ciprofloxacin without the use of excipients such as peroximonosulfate, H2O2, and NaBH4. Parameters affecting the degradation efficiency such as ciprofloxacin concentration, catalyst dosage, and pH value have been investigated and optimized. The mechanism and pathways of ciprofloxacin degradation were proposed through identification of intermediates.Extraction of nano chitosan by chemical method [24]:10 g samples of raw shrimp shell waste, after washing and drying, was added to sodium hydroxide (2.0 M) in the ratio of 1:16 (w/v) and stirred for 2 day at room temperature (pH must be 11–13). Then, the samples were washed with water to obtain pH = 6.5–8 after filtering. The sample was dried at 80 °C for 16 h.The sample from the first step was added to HCl (1.0 M) in the ratio of 1:16 (w/v) and stirred at pH = 1–2.5 at 25 °C. After 24 h and filtering, the obtained sample was washed for having pH = 6.5–8.0. Chitin was obtained after drying at 75 °C for 15 h.The obtained chitin was added to 50% NaOH in the ratio (1:10) (w/v) at room temperature. After 48 h, chitosan was obtained after filtering and washing to gain pH = 6.5–8.0.Chitosan was added to 1 M of NaOH (1% (w/v)) to have pH = 7, the precipitate was then washed several times with water and centrifuged at 5000 rpm for 30 min and dried. The obtained sample was dissolved in 2% acetic acid with 1:15 (w/v) and refluxed at 80 °C within 30 min. Then, chitosan nanoparticles were achieved after washing several times.4 g of C. procera wood powder was added to 200 mL of distilled water at 90 °C for 10 min. A KOH solution (0.01 M) was then added drop-wise (drop rate 1 mL min–1) at room temperature to reach the reaction pH to 9.170 mL of the obtained extraction and 20 mL of NiSO4 solution (0.05 M) were added to 130 mL of distilled water. The mixture of reaction was heated to 80 °C for 2.5 h. The resulting Ni/NiO nanoparticles was separated by centrifuge, washed with water, and dried in oven under vacuum. The as-synthesized sample was heated by the furnace at 500 °C for 4 h.10 g of barely wastes was treated with 1 g of NaClO2 salt in 50 mL of water at pH value 4.0 at 75 °C for 2 h, then filtered and the residue was washed with distilled water and ethanol (95%), and dried in an oven at 50 °C for 13 h. Then, dried residues were extracted with KOH 10% at room temperature for 8 h. After filtration, residue was washed until them neutral, and then washed with ethanol (95%). Finally, the samples were dried in an oven at 50 °C for 20 h (Tajik et al. [23]).Sodium metaperiodate (0.27 g) was added to 0.5 g of microcrystalline cellulose suspended in 60 mL of distilled water. The mixture was stirred at 50 °C in the dark for 9 h. After this, the remaining NaIO4 was decomposed by adding of glycerol. Finally, the product was washed with distilled water and dried at room temperature for 24 h [22].The 0.03 g of aldehyde cellulose and 6 g of nano chitosan were dispersed in water for 8 h at 80 °C. Then, 0.5 g of Ni/NiO nanocomposite was added and refluxed for 3 h at 80 °C.The degradation tests of ciprofloxacin were carried out under direct sunlight on sunny days with radiation intensity of 260–280 Klux from 11 p.m. to 2 p.m. The Chit-Cell@Ni/NiO nano-biocomposite was dispersed in 50 mL of the aqueous solution of ciprofloxacin in darkness conditions for 30 min. Subsequently, they were exposed to direct sunlight and stirred for 20 min. The initial and final concentration of ciprofloxacin was measured by a UV–Vis spectrophotometer (JENWAY) with the maximum absorbance intensity at 275 nm [25] using the corresponding calibration plot. The efficiency of degradation of ciprofloxacin was calculated using the following equation: Degradation efficiency ( % ) = C 0 − C t C 0 × 100 C 0 and C t (mg/L) are initial and final concentrations of ciprofloxacin, respectively.First, cellulose aldehyde and chitosan were extracted from barely and shrimp wastes and attached together through condensation reaction between amine group of chitosan and C=O group of cellulose and the formation of Schiff base ligand. Then green synthesis of Ni/NiO was performed from C. procera as a reductant and stabilizing agent. Finally, the Chit-Cell@Ni/NiO nano-biocomposite was gained by adding Cellulose/chitosan to Ni/NiO.XRD of X-ray diffraction studies of Chit-Cell@Ni/NiO nano-biocomposite exhibit some peaks at 2θ = 9.8 °and 2θ = 20° relating to chitosan (Fig. 1a) [26]. The two well-defined crystalline peaks observed around 2θ = 22° and 35° were typical of cellulose indicating the success of present of cellulose [27]. Amount of Ni and NiO in the sample is low and this matter confirming by EDAX (Fig. S1). The Williamson-Hall (W–H) method was used to estimate the crystallite size of the as-synthesized Ni/NiO nanocomposite. In the following W–H equation, λ is the Cu-kα radiation wavelength, d the crystallite size, β the peak with at the half maximum, θ the brag angle, the terms η shows the strain in the crystallites while the term d shows the size of the crystallites [28]. The constant k (k = 1/d) commonly is about 1 and varies in the range from 0.8 to 1.39. In the case of η = 0 the W–H equation reduces to the Scherer equation. The crystallite size varies as 1/cos θ and stain varies as tan θ from the peak width. By construction of the plot of (β cos θ) versus (η sin θ) a straight line will be got with slope η and intercept (kλ/d) [29]. The intercept of plot was used to estimate the crystallite size, about 45 nm. This plot was drowning (line equation: β cos θ=(kλ/d) (ղ Sin θ) The structure changes of nano chitosan and Chit-Cell@Ni/NiO nano-biocomposite were considered by FT-IR (Fig. 1b). In nano chitosan spectrum exhibited the peaks at 3434, 2994 cm−1 were assigned the OH and CH, respectively [24]. 1655, 1312, and 1023 cm−1 corresponded to acetylated amino groups, C=N stretching, and C–O–C stretching, respectively. In Chit-Cell@Ni/NiO nano-biocomposite spectrum, along with the peaks relating to nano chitosan, other peaks have confirmed the presence of cellulose and Ni/NiO nanoparticles. Typical absorption peaks of cellulose around 3448 and 2912 cm−1 are visible due to, respectively, –OH and –CH stretching vibrations (Fig. S2). Stretching vibration mode of pure NiO nanocrystals has reported in the regions 430–490 cm−1 and 600–700 cm−1 which showed blue shift with respect to bulk NiO due to quantum size effect (Fig. S1),[30 ].EDAX of nanochitosan shows C, N, and O in its structure (Fig. 2 a). The presence of Ni, C, O, and N in Chit-Cell@Ni/NiO nano-biocomposite was confirmed its successful synthesis (Fig. 2b). Also. tables illustrate the elemental analysis of Chit-Cell@Ni/NiO nano-biocomposite and 52.33, 5.59, 24.97, and 17.12 of C, N, O, and Ni were obtained.TGA, DTG, and DTA were used to evaluate the extracted nano chitosan and Chit-Cell@Ni/NiO nano-biocomposite thermal behaviors. TGA graph of nano chitosan is also depicted in Fig. 3 a, which demonstrates two degradation steps. The evaporation of adsorbed water was shown at the first step with a 5.8% weight loss at 50–100 °C. It seems that the nature of chitosan is hydrophilic [31]. The second step involved the 63.49% weight loss at 200–410 °C attributing the cleavage of glycosidic linkages via dehydration and the decomposition of chitosan polysaccharides. TGA of Chit-Cell@Ni/NiO nano-biocomposite shows three steps. First, one was related to moisture evaporation (6.3% loss weight), and the second one corresponded to thermal degradation of cellulose, and chitosan (59.86% loss weight) and the third step was attributed to Ni/NiO nanocomposite synthesizing from natural plants (12.74% loss weight). It is important to note that the amount of the hydrogen-bonded water molecule in chitosan and the coordination biopolymers is of the order: nano Chitosan < Chit-Cell@Ni/NiO nano-biocomposite. Fig. 3b and c were the DGA and DTA of nano chitosan and Chit-Cell@Ni/NiO nano-biocomposite confirming the two and three-step thermal behavior of nano chitosan and Chit-Cell@Ni/NiO nano-biocomposite thermal behaviors, respectively. It reveals that with increased amounts of Ni/NiO into the chitosan matrix lead to enhanced thermal stability. In the DTA curve of chitosan at least four thermal events, endothermic and exothermic, were observed up to 630 °C. Those thermal events were associated with the dehydration and thermal degradation processes, including depolymerization and decomposition stages of nano chitosan and Chit-Cell@Ni/NiO nano-biocomposite [32]. The exothermic effect on the differential thermal analysis (DTA) curves presented in the inset of Fig. 3b, can be linked to weight loss due to the evaporation of physically adsorbed water. In the case of nano chitosan, that peak presented a maximam at 100 °C, while a second peak with a higher mass loss was found at 380 °C, caused by the decomposition of chitosan containing hydroxyl and amino groups [33]. Chit-Cell@Ni/NiO nano-biocomposite displayed a broad exothermic peak around 270 °C that can be attributed to the decomposition of nano chitosan and also cellulose [34] while the weight loss in the range 450–500 °C should be assigned to the further decomposition of nano chitosan residues [33], whereas the band in the range of 600–900 °C can be attributed to the reduction of Ni/NiO nanocomposite by reaction with residual carbon.The morphology of Chit-Cell@Ni/NiO nano-biocomposite was investigated by SEM (Fig. 4 ). Fibril shape of attached cellulose to chitosan and randomly shaped shape of Ni/NiO were observed very well. The size of Chit-Cell@Ni/NiO nano-biocomposite was evaluated by TEM [35] (Fig. 5 ). TEM image of Ni/NiO also shows these kind of shape confirming the presence of Ni/NiO in the Chit-Cell@Ni/NiO nano-biocomposite without any change in the morphology of Ni/NiO (Fig. S1) The BET isotherms results showed that the Ni/NiO had a pore size of 11.3 nm with a BET area of 13.8 m2 g−1. While, Chit-Cell@Ni/NiO nano-biocomposite had a pore size of 16.087 nm with a BET area of 15.539 m2 g−1.The photo-absorption behavior of the Chit-Cell@Ni/NiO nano-biocomposite was evaluated by UV–Vis DRS [36]. Fig. 6 a displays the strong absorption edge at 200 nm–600 nm. The bandgap (Eg) value of Chit-Cell@Ni/NiO was estimated using Kubelka–Munk equation [36]: α h υ = A ( h υ − E g ) n where, A is the absorption constant, h υ is the photon energy, E g is the energy band gap and n is 1/2 or 2 for direct or indirect optical transition, respectively. As shown in Fig. 6b, direct E g of Chit-Cell@Ni/NiO was found at about 3.30 eV from plotting ( α h υ ) 2 vs. h υ (Tauc plot) [37]. According to the obtained result, Chit-Cell@Ni/NiO could be an excellent photocatalyst for the efficient degradation of drug containments [21].The photocatalytic performance of Ni/NiO and Chit-Cell@Ni/NiO nanocatalysts were studied by degrading ciprofloxacin antibiotics. Before photocatalytic experiments, the adsorption potential of nanocatalysts were tested in darkness for 30 min. The adsorption efficiency of Ni/NiO and Chit-Cell@Ni/NiO for ciprofloxacin was 4.6% and 7.87%, respectively. The photodegradation of ciprofloxacin in the presence of Ni/NiO and Chit-Cell@Ni/NiO reached 59.05% and 91.88%, respectively, under direct sunlight exposure. Therefore, the effect of adsorption by these nanocatalysts could be ignored. On the other hand, the better photocatalytic performance of Chit-Cell@Ni/NiO than Ni/NiO could be attributed to the unique nanostructure of nanocomposite and synergistic effects of nickel oxides and metallic nickel with nano chitosan and cellulose. The degradation of ciprofloxacin follows pseudo-first-order kinetics (Eq. (1)) [38]. The rate constant for Chit-Cell@Ni/NiO (kapp = 0.125542 min−1) was 2.8 times Ni/NiO (kapp = 0.044641 min−1). (1) − ln ( C C 0 ) = k a p p t where C is the concentration of ciprofloxacin at reaction time t, C 0 is the initial concentration of ciprofloxacin, and k is the rate constant (min−1).The effects of the initial pH value, the concentration of antibiotic, and dosage of photocatalyst on the degradation of ciprofloxacin in Chit-Cell@Ni/NiO system were studied. At first, the effect of pH variation on the degradation efficiency of ciprofloxacin was examined (Figs 7 a and b). The degradation efficiency of ciprofloxacin was obtained 58.49%, 73.19%, 91.88%, 84.16% and 77.07% in pHs 4, 5, 6, 7 and 8, respectively. In this study, K app of ciprofloxacin increased to 0.04396 min−1, 0.06582 min−1 and 0.12554 min−1 as the solution pH increased from 4 to 6 and then decreased to 0.09213 min−1 and 0.07363 min−1 at pHs 7 and 8, respectively. In general, almost neutral pH (pH = 6) was more useful for the degradation of ciprofloxacin than acidic and alkaline pHs. On the other hand, it was known that pKa value for ciprofloxacin is 5.9 and 8.89 [39]. It means that between these pHs, ciprofloxacin can be found in the form of zwitterion [40]. For further investigation, the isoelectric point of Chit-Cell@Ni/NiO was determined to be about pH 6.2 according to the zeta potential analysis (Inset of Fig. 7a). Because the charge on the surface of the catalyst is positive and negative in highly acidic and alkaline conditions, respectively. The degradation of ciprofloxacin is minimal due to strong repulsion between Chit-Cell@Ni/NiO and ciprofloxacin molecules at acidic pHs [36]. On the other, the presence of more –OH may inactivate the •OH radicals, therefore decreased degradation efficiencies at pH higher than 6 can also be due to high concentration of –OH [41]. It is worth noting that the working pH of Chit-Cell @Ni/NiO is close to neutral pH range, which completely covers the ambient pH of the wastewater [42]. Therefore, Chit-Cell@Ni/NiO can be introduced as an effective photocatalyst for the degradation of ciprofloxacin in wastewater.In the next step, the effect of the initial concentration of ciprofloxacin was examined and the obtained results were displayed in Fig. 7c and d. As seen in Fig. 7c, initially when the concentration of ciprofloxacin was increased the degradation efficiency of Chit-Cell@Ni/NiO was enhanced up to concentration of 10 mg/L, then decreased. The degradation percentages at the ciprofloxacin concentrations of 5, 10, 25, and 50 mg/L were 86.65%, 91.88%, 78.88%, and 67.10%, respectively, in a reaction time of 20 min. Furthermore, the Kapp for photodegradation of ciprofloxacin increases from 0.10068 min−1 at 5 mg/L to 0.12554 min−1 at 10 mg/L and then decreases to 0.05558 min−1 at 50 mg/L. The reason for the initial increase can be short lifetimes of radicals, because they can only react where they were generated. Increasing the quantity of ciprofloxacin molecules per volume unit increased the probability of collision between ciprofloxacin and active species that leads to increased degradation efficiency [43]. In case of decreasing efficiency with further increase of pollutant concentration, one reason may be that high initial concentrations of ciprofloxacin prevent light from reaching the surfaces of Chit-Cell@Ni/NiO photocatalyst and limit the formation of photogenic species responsible for the photocatalytic reaction. Also, more intermediates were produced at high ciprofloxacin concentrations. The generated intermediates compete with ciprofloxacin molecules for contact with active sites of Chit-Cell@Ni/NiO, resulting in a reduced percentage of degradation [44].Also, the Langmuir–Hinshelwood kinetics model (Eq. (2)) can be used to describe this phenomenon [45]: (2) r = − d C d t = k K C 1 + K C (3) ln C 0 C + k a p p ( C 0 − C ) = k K t = k a p p t where r is the reaction rate for degradation of ciprofloxacin, C 0 is initial concentration of ciprofloxacin, C is the final concentration of ciprofloxacin, k is the specific reaction rate constant, and K is the equilibrium constant of the reactant. The logarithmic form of the Langmuir-Hinshelwood equation was exhibited by Eq. (3), [46]. As shown in Fig. 7d, the rate constant of 0.12554 min−1 was obtained for the degradation of ciprofloxacin using Chit-Cell@Ni/NiO (Catalyst dosage: 0.2 g/L, ciprofloxacin concentration: 10 mg/L and pH: 6). The amount of 0.988 for correlation coefficient ( R 2 ) confirmed a good fitting of the data by the Langmuir–Hinshelwood kinetics model.The catalytic degradation of ciprofloxacin was evaluated by varying the amount of Chit-Cell@Ni/NiO at pH = 6 under direct sunlight. As exhibited in Figs. 7e and f, the degradation of ciprofloxacin increases with an increase in the amounts of catalyst from 0.1 to 0.2 g/L. In further studies, it was found that when the photocatalyst doses were more than 0.2 g/L, Chit-Cell@Ni/NiO is not able to disperse well in solution and the degradation of ciprofloxacin changes insignificant. A slight decrease in degradation efficiency is due to that high doses of Chit-Cell@Ni/NiO prevent sunlight from penetrating the solution because of the turbidity of the solution [47]. Also, Kapp of ciprofloxacin increased from 0.05837 min−1 to 0.12554 min−1 as the photocatalyst doses increased from 0.1 to 0.2 g/L and then decreased to 0.11453 min−1 and 0.10278 min−1 at photocatalyst doses 0.5 and 1 g/L, respectively. Based on this experiment, the optimal catalyst dose was 0.2 g/L.For better understanding, effect of Operational variables on the degradation of ciprofloxacin was showed in Table 1 .The photogenerated h+, •OH, and O 2 • − as reactive species were responsible for the degradation of different pollutants and their intermediates [48,49]. To determine the active species in photodegradation of ciprofloxacin, radical trapping studies were carried out using different scavengers [50]. The iso-Propanol (10 mM, IPA), benzoquinone (10 mM, BQ), and ammonium oxalate (10 mM, AO) were applied as scavengers, which indicate quenchers of •OH and O 2 • − radicals, and h+, respectively (Fig. 8 ). The maximum degradation of the ciprofloxacin was obtained without any scavenger species. This result emphasizes the photocatalytic degradation of ciprofloxacin in the Chit-Cell@Ni/NiO system. The photodegradation efficiency of ciprofloxacin decreases from 91.88% (without scavenger) to 58.34%, 79.12%, and 38.69 in the presence of IPA, BQ, and AO, respectively. These results show that although •OH and O 2 • − were reactive species involved in the degradation process of ciprofloxacin, h+ plays a significant role in this reaction.In order to describe the possible mechanism of ciprofloxacin photodegradation under sunlight, the conduction bond (ECB) and valence bond (EVB) energy levels of Chit-Cell@Ni/NiO were measured using the following equations: (4) E C B = χ −  E C − 0.5 E g (5) E VB = E CB +  E g where, is the energy of free electrons (∼4.5 eV) on the hydrogen scale and χ is absolute electronegativity of semiconductor and it was calculated by the following equation [51]: (6) χ = [ x A a x B b x C c ] 1 ( a + b + c ) (7) x = E I E + E E A 2 where, χ A , χ B and χ C are electronegativity of atoms and a, b and c are the number of atoms in the compound. E I E and E E A are the electro affinity and the first ionization energy of atoms, respectively. In this way, χ for Chit-Cell@Ni/NiO were found 5.26 eV. The E g value of Chit-Cell@Ni/NiO is obtained 3.30 eV from Tauc plot. Therefore, the ECB and EVB values of Chit-Cell@Ni/NiO were calculated to be −0.805 eV and +2.41 eV versus normal hydrogen electrode (NHE), respectively. Fig. 9 presents the photocatalytic mechanism using Chit-Cell@Ni/NiO. When the Chit-Cell@Ni/NiO is exposed to sunlight, an electronic transfer occurs that takes an electron from a lower valence level to a higher conductivity level, creating an electron–hole pair. e − and h + represent for an electron in the conduction band (CB) and the deficiency in the valence band (VB), respectively. e − and h + transfer to the photocatalyst surface, where the redox reaction occurs. Since the VB energy level of Chit-Cell@Ni/NiO is more positive than the H2O/•OH standard redox potential (E0 = +2.34 eV vs. NHE), •OH radicals were formed by the reaction of h + with H2O in VB. Also, due to the fact that the energy level of CB of Chit-Cell@Ni/NiO is more negative than the O2/ O 2 • − standard redox-potential (E0 = −0.33 eV vs. NHE), O 2 • − radicals were formed by a combination of e − with O2 in CB [52]. The generated radicals degrade ciprofloxacin into simple harmless molecules. In addition, immobilizing of Ni/NiO on the chitosan-cellulose surface due to increasing e − and h + separation and the wide surface area of the photocatalyst can lead to enhancing ciprofloxacin degradation efficiency. As the surface area increases, there will be more active sites for adsorption of pollutant molecules, which can increase the likelihood of interaction between reactive species and ciprofloxacin molecules. Also, the bandgap of the Chit-Cell@Ni/NiO has been reduced compared to Ni/NiO (3.8 eV) [53], which provides the conditions for the photocatalytic process in the presence of natural sunlight.Based on LC-MS results as shown in Supporting Fig. S3, the degradation ways of ciprofloxacin were proposed in the presence of Chit-Cell@Ni/NiO. As shown in Scheme 1 , the photodegradation of ciprofloxacin generally underwent two pathways, including the degradation of the piperazine ring and the cleavage of the quinolone ring [54]. In pathway 1, at first, the piperazine ring of ciprofloxacin is oxidized to produce intermediate I (m/z 362) through the piperazine breaking, followed by the losing the carbonyl group to form intermediate II (m/z 306). The intermediate II could be converted into intermediate III (m/z 262) via the losing of ethylamine. In the following, the cyclopropyl group is removed from III to achieve intermediate IV (m/z 221) by cleavage. Then, the ring-opening of the quinolone occurs and then diketone VI (m/z 181) is produced through decarboxylation and closing ring. In pathway 2, the •OH radicals can attack ciprofloxacin to generate intermediate VII (m/z 288) by decarboxylation. On the other hand, the piperazine and quinolone rings were destroyed and the intermediate V (m/z 270) is generated. Eventually, the intermediates could be degraded to safe lower mass intermediates.From a practical view, one of the most essential properties of a catalyst is its recyclability and durability in long-term use. The stability studies of Chit-Cell@Ni/NiO were done with fresh ciprofloxacin solution under sunlight in three consecutive cycles. When each cycle was finished, Chit-Cell@Ni/NiO was separated by filter paper, washed with water several times, dried, and reused in the next run. The degradation efficiency of ciprofloxacin for these three repeated applications was exhibited in Fig. S4. The obtained results show that the catalytic performance of Chit-Cell@Ni/NiO was only slightly decreased after three consecutive cycles of photodegradation. TEM and FT-IR of the recycled nanocomposite were considered and structure, size, and morphology were not changed after three times of reusability (Fig. 10 ).To show the merit of Chit-Cell@Ni/NiO photocatalytic performance in the comparison with other photocatalysts toward the degradation of ciprofloxacin, some of the previous reports were compared in Table 2 . The current protocol was compared with the data in the literature based on the ciprofloxacin concentration, catalyst dosage, degradation time and efficiency. As shown Table 2, Chit-Cell@Ni/NiO exhibited relatively higher activity for photodegradation of ciprofloxacin in a shorter time without any auxiliary agent.Biosynthesis fabrication of Ni/NiO and aldehyde cellulose was performed using barely wastes. Nano chitosan was extracted after deproteination, demineralization and deacetylation of shrimp waste. By incorporating the nano chitosan and Ni/NiO nanocomposite to aldehyde cellulose, Chit-Cell@Ni/NiO nano-biocomposite from organisms was fabricated. The catalytic performance of the Chit-Cell@Ni/NiO was evaluated for the photodegradation of ciprofloxacin under direct sunlight. The maximum degradation efficiency of 10 mg/L ciprofloxacin was obtained at neutral pH (pH = 6) and Chit-Cell@Ni/NiO dosage of 0.2 g/L. About 92% of ciprofloxacin was degraded within 20 min, while the degradation efficiency for the Ni/NiO was 59.05% only. Radicals quenching experiments were illustrated that h+ was played a dominant role in degradation process of ciprofloxacin. We believe which this study offers a novel perspective for practical photocatalytic degradation to effective reduce the number of antibiotics in the water system.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors gratefully acknowledge the Birjand University of Technology and the University of Jiroft for the support of this work.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmrt.2022.04.046.

A novel nanocomposite including cellulose, chitosan and Ni/NiO was fabricated from barely wastes, shrimp wastes, and Calotropis procera, respectively. It was characterized by TEM, SEM, TGA, DTA, FT-IR, BET, EDAX, and elemental analysis. 5–25 nm of Ni/NiO were dispersed on chitosan and cellulose. The BET isotherms results showed that the Ni/NiO had a pore size of 11.3 nm with a BET area of 13.8 m2 g−1. While, Chit-Cell@Ni/NiO nano-biocomposite had a pore size of 16.087 nm with a BET area of 15.539 m2 g−1. Then, Chit-Cell@Ni/NiO bio-nanocomposite was applied to the photodegradation of ciprofloxacin under sunlight. About 92% of ciprofloxacin could be efficiently degraded within 20 min. Radical quenching experiments confirmed the contribution of active species was in descending order of h+> •OH > •O2 − in the Chit-Cell@Ni/NiO system. The possible ciprofloxacin degradation pathway has been proposed according to the intermediates detected by LC-MS. Also, Chit-Cell@Ni/NiO showed high durability and stability after three-cycle ciprofloxacin degradation. In short, this study offers an efficient green methodology to decrease the number of antibiotics in the water system.

Core-shell particles is a class of particles containing a core and a shell, which can be either the same or different materials that can be separately identified (Hayes et al., 2014). Their properties can be adjusted by modification of a material with a variety of another materials through surface coating techniques, such as chemical grafting and microemulsion, in order to form layer-by-layer core-shell structures. Chemical grafting is a surface modification method mostly used to adjust the surface properties of the resulting material. This technique involves the use of an initiator that can undergo a copolymerization reaction on the surface of a substrate. Different types of initiators have been employed such as potassium permanganate, ammonium peroxydisulfate, benzoyl peroxide, etc. in order to achieve the desired properties of the materials (Abidi, 2009). Moreover, micro-emulsion is one of the simplest and most effective methods for the preparation of nano-sized particles that can be immobilized on the support materials. A formation of microemulsion basically involves a water, hydrocarbon, and surfactant, which can be divided into two types; that are, the water-in-oil type of micro-emulsion and the oil-in-water type of micro-emulsion (Hanaoka et al., 2015).Owing to their adaptable properties, core-shell structured materials have been employed in various potential applications; for instance, the catalytic pyrolysis of lignin to valuable monoaromatics (Xue et al., 2020), the Fischer-Tropsch reaction (Chen et al., 2020) and the catalytic reduction of NOx (Liu et al., 2018). Certainly, like other catalyst types, the core-shell catalysts were found to have catalytic activity varied, depending on several characteristics such as shell/core thickness, exposed active sites, the ratio of components, and most importantly, the order and/or location of active components in the structure. For examples, the effect of the shell thickness of a Co@C@SiO2 core-shell catalyst was studied on the Fischer-Tropsch synthesis (Chen et al., 2020). The results indicated that the CO conversion decreased with the increasing SiO2 shell thickness because a larger thickness obstructed CO from entering to the Co active sites. Therefore, the sequence of the materials on a core-shell structure exposed to the reaction mixture is very crucial for the catalytic activity. Morever, the catalytic reduction of NOx was also examined over a Fe/Beta@SBA-15 core-shell catalyst (Liu et al., 2018). The SBA-15 shell thickness, modified by varing the ratio of Si/Beta, was found to affect the acidic and redox properties of the catalyst. The optimum thickness (10 ​nm) improved the performance of the catalyst whereas a too thick shell (30 ​nm) decreased the activity. Additionally, the preparation of a core-shell Raney Fe@HZSM-5 catalyst was accomplished using one-pot hydrothermal synthesis method, for gasoline production via Fischer–Tropsch reaction (Sun et al., 2010). They found that the core-shell catalyst provided a higher CO conversion and C5–C11 selectivity than the physical mixture of Fe and HZSM-5 catalyst. So, it can be implied that the synergy between the two components of the core-shell catalyst helped promote the performance of the catalyst. In addition, the core-shell form of a catalyst was also constructed in order to prevent the sintering of active metals for high temperature reaction. For example, the core-shell structure of a Ni-yolk@Ni@SiO2 catalyst was developed for the CO2 reforming of methane (Li et al. (2016). The TEM analysis suggested that the Ni nanoparticles was trapped inside the silica shell that prevented Ni from sintering; as a result, the catalyst with the core-shell structure gave a higher methane turnover frequency than the SiO2-supported Ni catalyst.1,3-Butadiene is one of the most important chemicals widely used as a building block in the production of polymers such as polybutadiene, styrene-butadiene rubber, and polycholoprene (Fedotov et al., 2019). It can be commercially produced by three main processes, including steam cracking of paraffinic hydrocarbons, catalytic dehydrogenation of n-butane and n-butene, and oxidative dehydrogenation of n-butene (White, 2007). However, due to the depletion of the petroleum sources and environmetal issues, the use of a renewable resource becomes more attractive, especially bio-ethanol that is considered as a renewable feedstock for the production of several industrial products such as light hydrocarbons (Chinniyomphanich et al., 2016) or heavy hydrocarbons (Choopun and Jitkarnka, 2016) and oxygenate compounds (Sujeerakulkai and Jitkarnka, 2016). Not only is it used as the main feedstock, but bio-ethanol is also used as a hydrogen donor for other feedstocks such as glycerol in order to produce oxygenates, like 1,2-propanediol, acetaldehyde (Kumpradit and Jitkarnka, 2019), co-produced with ethyl lactate (Kuljiraseth and Jitkarnka, 2019). Among ethanol-derived oxigenates, 1,3-butadiene is one of the most important chemicals expected to be produced from the catalytic conversion of bio-ethanol through four principal steps; that are, (1) ethanol dehydrogenation to acetaldehyde, (2) the condensation of acetaldehyde to crotonaldehyde via the dehydration of 3-hydroxyl butanal, (3) the reduction of crotonaldehyde to crotyl alcohol via Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reduction using an alcohol as a hydrogen donor, and (4) butadiene formation via the dehydration of crotyl alcohol (Zhao et al., 2020). Therefore, a multi-functional catalyst with an optimum acid and base ratio is needed in order to achieve a high 1,3-butadiene selectivity. Especially, since these are sets of consecutive reactions that need different types of catalysts with multi-functions to drive, a catalyst with a core-shell structure that contains different materials layer by layer were believed to be beneficial to such reactions because they can be driven consecutively on consecutive layers, and selectively geared layer by layer, using various designed materials, until the desired product is formed (Namchot and Jitkarnka, 2016).Based on several research articles, Cu catalysts provided a good efficiency in ethanol dehydrogenation. The effect of Cu content on the ethanol dehydrogenation over Cu/ZrO2 catalyst was examined (Freitas et al., 2014). The % content of Cu was varried from 5% to 20%, and the reaction was carried out in a continuous-flow tubular fixed-bed reactor at the temperature range of 473–548 ​K and atmospheric pressure. The results indicated that the highest acetaldehyde formation rate was obtained over the 5Cu/ZrO2 catalyst. According to the results from XPS analysis, 5Cu/ZrO2 provided highly-dispersed Cu+1 species selective for the formation of acetaldehyde, the product from dehydrogenation of ethanol. Moreover, the different morphologies of copper-based catalysts, including urchin-like (CuO-UC), fiber-like (CuO-FB) and nanorod (CuO-NR) catalysts, prepared by a microwave-assisted method, were studied on the dehydrogenation of ethanol (Sato et al., 2012). The results indicated that the catalyst with the highest amount of Cu+1 species from the urchin-like morphology exhibited the highest activity of ethanol dehydrogenation to acetaldehyde among all catalysts. Besides the metal content and morphology, thermal treatment condition also influenced the copper species (Cu0/Cu1+) and then the activity on the bio-ethanol dehydrogenation to acetaldehyde over CuMgAl mixed oxide (Campisano et al., 2018), and it also had the impacts on the acid-base properties and catalytic activity of ZrO2/Nano-SiO2 catalyst for 1,3-butadiene production from the mixture of ethanol and acetaldehyde (Gao et al., 2018).For the acetaldehyde condensation reaction, the roles of acid and base sites on the acetaldehyde condensation were investigated over MgO/SiO2 and ZrO2/SiO2 catalysts (Ordomsky et al., 2010). It was found that the selectivity of crotonaldehyde was high on both magnesium oxide and zirconium oxide supported on silica catalyst. In order to investigate the role of active sites, CO2 and pyridine were doped as the acid or base molecular probe in order to poison the corresponding active sites. The conversion of acetaldehyde was found to decrease when pyridine was co-fed with acetaldehyde, suggesting that the lewis acid site played an important role on the condensation of acetaldehyde to crotonaldehyde. In addition, ZrO2 was also studied as a catalyst for MPVO reduction. For example, the effects of various oxides, including Nb2O5, TiO2, and ZrO2, on the MPVO reduction of cyclohexanone to cyclohexanol were investigated with using 2-propanol as a hydrogen donor (Komanoya et al., 2015). They found that although ZrO2 had a lower lewis acid density than Nb2O5 and TiO2, ZrO2 was the most effective catalyst for the MPVO reduction of cyclohexanone due to the highest density of base sites provided by the hydroxyl groups on the surface of ZrO2. Such a base site type was very important for the evolution of six-membered ring intermediates on the lewis acid site of the catalyst.Layered double hydroxides (LDHs) belong to a group of anionic clay materials. Their structure consists of positively-charged brucite-like layers of mixed hydroxides and the exchangeable charge compensating anions in the interlayer space. The general formula of an LDH is [M2+ 1-xM3+ x(OH)2][(An−) x/n·mH2O], where M2+ and M3+ are divalent and trivalent cations, respectively. An− is mainly an inorganic or organic interlayer anion (Pšenička et al., 2020). It has received attention as a catalyst for the conversion of ethanol due to its high surface area, acid-base properties, and thermal stability (Mishra et al., 2018). For instance, the effects of preparation methods on the catalytic condensation of ethanol were determined using Mg–Al mixed oxide catalysts (Leon et al., 2011). The reaction was performed at the temperature range of 473–723 ​°C. They found that the preparation methods strongly affected the acid-base properties of the catalysts. Moreover, the results also indicated that ethylene and acetaldehyde were observed as two primary products from ethanol conversion over Mg–Al catalysts via dehydration and dehydrogenation reactions, respectively. In addition, the effect of adding a tetravalent metal, Tin, into the hydrotalcite structure was evaluated on the MPVO reduction of aldehyde and ketone with 2-propanol as a hydrogen donor (Jiménez-Sanchidrián and Ruiz, 2014). It was found that the catalyst containing Mg/Sn/Al mixed oxides showed a higher activity and selectivity than Mg/Al and MgO catalysts. The better performance of tin-containing catalysts can be ascribed to the Lewis acid site of Sn+ ion where 2-propanol can be adsorbed more efficiently.Previously, our research group studied some sets of core-shell catalysts composed of a metal oxide shell and a MgAl-LDO-based core promoted with a dot-coated metal oxide promoter (Sricheun, 2018). A variety of core-shell catalysts prepared with various metal oxide shells (MgO, CuO and ZnO), and different MgAl-LDO cores, synthesized by partial substitution of various transition metals (Cu, Zn, Mn and Fe) in the core’s structure promoted with two dot-coated metal oxide promoters (HfO2 and ZrO2), were prepared and subsequently studied for ethanol conversion to 1,3-butadiene. It was found that the catalysts that consisted of FeMgAl-LDO core, promoted by dot-coated ZrO2, and then coated with CuO shell, denoted as “CuO@ZrO2/FeMgAl-LDO” shell@core catalyst, gave the highest 1,3-butadiene yield due to the synergistic effect of all components. Based on the best combination of this shell@core catalyst, further investigation shall be done on preparation techniques that might impact the 1,3-butadiene yield as well. Recently, our preliminary detailed work on grafted ZrO2/FeMgAl-LDO catalyst has confirmed that the presence of grafted ZrO2 on the catalyst core helped promote the condensation reaction of acetaldehyde to crotonaldehyde and Meerwein-Ponndorf-Verley (MPV) reduction of crotonaldehyde to crotyl alcohol, which are the important steps for 1,3-butadiene production (Suwansawat and Jitkarnka, 2020a). As earlier mentioned, since the preparation method can influence the exposure of active components in the core-shell structure and then the catalytic activity, the next step of our catalyst development was therefore to investigate the effect of the shapes (powder and granule) of FeMgAl-LDO core and the method of ZrO2 deposition on the core in order to determine the appropriate exposure and synergy of all active components, which would lead to the highest yield of 1,3-butadiene. Based on this concept, the preparation of CuO shell was fixed for all prepared samples of the core-shell catalyst in order to investigate the catalyst behavior through the alterations of the core and its promoter only. Therefore, in this work, the effects of preparation methods were investigated in two steps; that are, Step 1 : granular formation of the FeMgAl-LDO core with α-Al2O3 binder by pelletization, and then Step 2 : ZrO2 deposition on the core using either chemical grafting or micro-emulsion. Based on the two methods of two different steps of preparation, the catalysts, prepared and used in this work, were CuO@m-ZrO2/p-FeMgAl-LDO, CuO@gf-ZrO2/p-FeMgAl-LDO, CuO@m-ZrO2/g-FeMgAl-LDO, and CuO@gf-ZrO2/g-FeMgAl-LDO, where p is powder, g is granular, gf is grafting, and m is micro-emulsion.The FeMgAl-LDO core was prepared using the co-precipitation method. A solution of Fe(NO3)3⋅9H2O, Mg(NO3)2⋅6H2O and Al(NO3)3⋅9H2O was mixed into the three-neck flask, containing a 700 ​ml of 0.25 ​M sodium carbonate solution with constant stirring. The pH was controlled at 10 using a 5 ​M NaOH solution. After aging for 16 ​h, the precipitate was separated by filtration, and washed with deionized water until pH 7 was reached. The precipitate FeMgAl-LDH was next dried in an oven at 65 ​°C for 12 ​h, and then calcined at 500 ​°C for 5 ​h in order to obtain the p-FeMgAl-LDO catalyst, where p stands for “powder”.The calcined p-FeMgAl-LDO catalyst was mixed with 20 ​wt% Al2O3 binder, and then formed into a granular shape using a hydraulic press machine. The palletized catalyst was then ground, and subsequently sieved to obtain 0.8 ​mm diameter granules using a mesh No.20 stainless steel wire sieve. Next, the sieved catalyst was calcined at 500 ​°C for 5 ​h to obtain g-FeMgAl-LDO, where g stands for “granular”.The p-FeMgAl-LDO catalyst obtained from the first step (2.1.1) was promoted with a ZrO2 promoter via the micro-emulsion method. 1.246 ​g of Zr(NO3)2⋅xH2O precursor was added into a two-neck flask containing 130 ​ml of deionized water, 10.4 ​ml of octane, and 0.146 ​g of L-arginine. The mixture was stirred for 4 ​h, followed by the addition of sieved p-FeMgAl-LDO catalyst. After 20 ​h of aging, the solid product was filtered and washed with ethanol and deionized water, respectively. Next, the solid was dried in an oven at 65 ​°C overnight, followed by calcination at 500 ​°C for 5 ​h to obtain m-ZrO2/p-FeMgAl-LDO catalyst, where m stands for micro-emulsion.The g-FeMgAl-LDO was also promoted with a ZrO2 promoter via the micro-emulsion method as stated in 2.1.3.The ZrO2 promoter was also deposited on the p-FeMgAl-LDO via the chemical grafting method. First, 3 ​g of p-FeMgAl-LDO was dispersed in 100 ​ml of anhydrous toluene at constant stirring, followed by the slow addition of 2.4 ​g of zirconium sec-butoxide. The resulting slurry was stirred at room temperature for 6 ​h. After that unreacted zirconia in the solution was removed by centrifugation, and the resulting solid was then maintained in deionized water for 6 ​h. Next, the solid product was isolated by filtration, followed by drying at 65 ​°C overnight and calcination at 500 ​°C for 5 ​h to yield the gf-ZrO2/p-FeMgAl-LDO catalyst, where gf stands for grafting.The g-FeMgAl-LDO was also promoted with ZrO2 using the chemical grafting method as described in 2.1.5.CuO that was encapsulated on ZrO2/FeMgAl-LDO samples was prepared using the sol-gel method. First, 2 ​g of a ZrO2/FeMgAl-LDO was dispersed in a mixed solution of 800 ​ml of deionized water, 600 ​ml of ethanol and 3 ​g of hexadecyltrimethyl ammonium bromide at constant stirring for 2 ​h. The pH was controlled at 11 using NH4OH. After that, 0.84 ​g of Cu(NO3)2⋅3H2O precursor was added into the mixed solution at constant stirring, and then aged for 24 ​h. The resulting product was then filtrated and washed with deionized water until pH 7 was reached, followed by washing with 800 ​ml of ethanol. Next, the precipitate was dried in an over at 65 ​°C overnight, and calcined at 500 ​°C for 5 ​h to obtain CuO@ZrO2/FeMgAl-LDO catalysts.The crystalline structure of the catalysts was investigated using Rikagu SmartLab X-Ray Diffractometer (XRD) equipped with CuKα radiation (1.5405). The diffraction patterns were collected in the 2θ range of 5°–70° using the scan speed of 0.02 ​× ​(2θ)/0.6 ​s. X-Ray fluorescence spectrometer (XRF) was used to identify the elemental composition of the catalysts using Best Detection-Vacuum method. The temperature programmed desorption of ammonia (NH3-TPD) and carbon dioxide (CO2-TPD) was employed to study the acid-base properties of the catalyst using a Temperature Program Desorption/Reduction/Oxidation analyzer (TPDRO), BELCAT II. To determine the specific surface area, total pore volume, and pore size of catalysts, the Brunauer-Emmett-Teller (BET) technique was employed using Surface Area Analyzer (Quantachrome, Autosorb-1MP) with Multipoint nitrogen adsorption and desorption isotherm plots. A Field Emission Scanning Electron Microscope (FE-SEM) was used to investigate the morphology of the catalyst at high magnification. The catalysts were placed on carbon tape and then coated with platinum by sputtering. The catalysts were then analyzed on Hitachi/S-4800 (accelerating voltage 10.0 ​kV) Transmission electron microscopy (TEM) to investigate the morphology and elemental composition of the catalysts. The TEM measurements were done on Thermo Scientific TALOS F200X equipped with EDS system.The activity testing was carried out in a continuous U-tube fixed bed reactor under atmospheric pressure. 1 g of a catalyst was loaded into the reactor using quartz wool as a bed supporter. After that the ethanol was fed into the reactor at the rate of 3.2 ​ml/h. The liquid product was collected in the cooling condensing flask while the gas product was passed from the condensing flask to an online gas chromatograph (GC) equipped with a flame ionization detector with a Plot alumina column. The liquid product from the condensing flask was extracted by CS2, and then the obtained non-aqueous products were analyzed for their composition using a LECO Pegasas 1D-mode Gas Chromatograph equipped with a mass spectrometer of Time-of-Flight Type (GC-TOFMS) using a capillary column, Rxi-PAH (60 ​m ​× ​0.25mmID and 0.10 μm film thicknesses). The product selectivity and yields were calculated using Equations (1)-(2) (1) S e l e c t i v i t y i ( % ) = C o n c e n t r a t i o n i ∑ i n C o n c e n t r a t i o n i × 100 (2) y i e l d = s e l e c t i v i t y ( % ) × c o n v e r s i o n ( % ) 100 The XRD patterns of CuO@m-ZrO2/p-FeMgAl-LDO, CuO@gf-ZrO2/p-FeMgAl-LDO, CuO@m-ZrO2/g-FeMgAl-LDO, and CuO@gf-ZrO2/g-FeMgAl-LDO catalysts are shown in Fig. 1 . All the XRD patterns of catalysts show the diffraction peaks at 2θ ​= ​25.44°, 35.02°, 37.64°, 43.22°, 52.42°, 57.36°, 61.18°, 66.38°, and 68.08°, which correspond to the Al2O3 phase (Li et al., 2017). Moreover, they also show the diffraction peaks of MgO phases at 2θ ​= ​43.2°, and 62.7° (Mishra et al., 2018). Furthermore, the characteristic diffraction peaks of MgAl2O4 spinel at 2θ ​= ​44.86°, and 65.30° (Kuljiraseth et al., 2019) are also observed over in all catalysts. It can be concluded that all the layered double oxide-based catalysts were successfully synthesized. No diffraction peak of ZrO2 is observed over all the catalysts, indicating that ZrO2 is very well dispersed or formed as an amorphous phase. For the CuO@m-ZrO2/p-FeMgAl-LDO catalyst, the small diffraction peak of CuO is located at 2θ ​= ​38.9° (Zhu et al., 2018) while the XRD of CuO@gf-ZrO2/p-FeMgAl-LDO catalyst shows the sharp reflection peak of CuO at 35.7° and 38.9°. Additionally, no diffraction peaks of CuO can be detected over CuO@m-ZrO2/g-FeMgAl-LDO and CuO@gf-ZrO2/g-FeMgAl-LDO granular catalysts, possibly because the high peak intensity of crystalline Al2O3 obstructs the amorphous peaks of amorphous CuO from appearance. CuO was also possibly formed as an amorphous phase or very well-dispersed.TEM-EDX analysis was employed to study the morphology of the powder catalysts. The HR-TEM images (Fig. 2 a) suggest that the powder p-FeMgAl-LDO core is composed of a group of the agglomerated stacks of the FeMgAl layered double oxide as can be depicted in Fig. 2 ci. Moreover, the elemental mapping also reveals that the Fe (yellow), Mg (red), and Al (green) particles are homogeneously distributed (Fig. 2b).After the deposition of ZrO2 using two different methods, including micro-emulsion and chemical grafting, the elemental mapping shows that the Zr particles (purple) prepared from both micro-emulsion (Fig. 3 A b) and chemical grafting (Fig. 3B b) methods are homogeneously dispersed on the surface of the p-FeMgAl-LDO core. In addition, the elemental mapping also shows that the promoted sample prepared by the chemical grafting method provides a higher density of Zr particles than the one prepared by the micro-emulsion method as shown in Fig. 3A d-v and Fig. 3B d-v.After the encapsulation with CuO using the sol-gel method, the HR-TEM images of CuO@m-ZrO2/p-FeMgAl-LDO catalyst show the agglomeration of several core-shell particles. The agglomerated core is encapsulated by CuO as shown in Fig. 4 A a. Moreover, the elemental mapping also suggests that Cu particles are mainly located at the outer part of the promoted core (Fig. 4A b). For the CuO@gf-ZrO2/p-FeMgAl-LDO catalyst, the result from elemental mapping (Fig. 4B b) reveals that the large particle of cores are surrounded by CuO particles in a crystalline form as observed from the XRD patterns. So, it can be concluded that the core-shell catalysts were successfully synthesized.The SEM-EDX technique was adopted to study the morphology of the granular catalysts. Fig. 5 illustrates the results obtained from the SEM analysis of the g-FeMgAl-LDO granular core. The elemental mapping (Fig. 5a) shows that the FeMgAl-LDO core is composed of Mg (purple), Al (blue), O (White), and Fe (red) particles that are homogeneously distrubuted. After the deposition of the ZrO2 promoter using micro-emulsion and chemical grafting methods, the elemental mapping images reveal that ZrO2 particles (yellow) are homogeneously dispersed on the surface of the g-FeMgAl-LDO core as shown in Figs 6A b and Fig. 6 B b. Furthermore, after the encapsulation of CuO, the images from elemental mapping (Fig. 7 A b and Fig. 7B b) suggest that the Cu particles (green) are mainly populated at the outer part of the CuO@m-ZrO2/g-FeMgAl-LDO and CuO@gf-ZrO2/g-FeMgAl-LDO catalysts.The physical properties of all catalysts are summarized in Table 1 . For both core-shell catalysts with the powder core, the surface area, pore volume and pore diameter of the CuO@m-ZrO2/p-FeMgAl-LDO catalyst are 117.3 ​m2/g, 0.362 ​cm3/g, and 60.69 ​Å, respectively while the surface area, pore volume and pore diameter of the CuO@gf-ZrO2/p-FeMgAl-LDO are 141.7 ​m2/g, 0.148 ​cm3/g, 35.94 ​Å. In case of the granular shape catalysts, the surface area is significantly decreased with the addition of Al2O3 binder (Suwansawat and Jitkarnka, 2020b). The surface area, pore volume and pore diameter of the CuO@m-ZrO2/g-FeMgAl-LDO catalyst are 107.1 ​m2/g, 0.199 ​cm3/g, and 36.16 ​Å while the surface area, pore volume and pore diameter of the CuO@gf-ZrO2/g-FeMgAl-LDO catalyst are 106.8 ​m2/g, 0.198 ​cm3/g, and 36.16 ​Å. The elemental composition of the catalysts is also shown in Table 1.The acid and base properties of all catalysts are stated in Table 1. Moreover, the results also indicate that the total acidity and total basicity of the catalysts are suppressed with the presence of Al2O3 binder.The ethanol conversion was performed over CuO@m-ZrO2/p-FeMgAl-LDO and CuO@gf-ZrO2/p-FeMgAl-LDO catalysts in order to study the effect of the ZrO2 deposition method on 1,3-butadiene production. The results suggest that ethanol can almost entirely be converted over both catalysts as shown in Table 2 . Moreover, Fig. 8 A indicates that ethylene is formed as a primary product over both catalysts with the yield of 25.1% and 25.8%. Additionally, the yield of 1,3-butadiene is also not significantly different. In addition, the relative yield of dehydration and dehydrogenation products (Fig. 8B) also exhibits that the core-shell catalyst with ZrO2 prepared by micro-mulsion method provides 42.7% yield of dehydrogenation products and 57.3% yield of dehydration products while the one with grafted ZrO2 gives 36.7% yield of dehydorgenation products and 63.5% of dehydration products. According to the results from XRF and TEM analyses, the catalyst with ZrO2 prepared by the chemical grafting method gives a higher Zr content. Based on the results from the previous study, the presence of ZrO2 promoted the dehydrogenation pathway. So, the higher Zr content of CuO@gf-ZrO2/p-FeMgAl-LDO resulted in the higher yield of dehydration pathway. Hence, it can be concluded that, in the case of powder core samples, the ZrO2 deposition method did not significantly affect the formation of 1,3-butadiene. Nevertheless, the catalyst with ZrO2 deposited by the micro-emulsion method is better because it gives higher yield of dehydrogenation products.In the case of the granular core catalyst samples, the ethanol conversion was performed in order to also investigate the effect of the ZrO2 deposition method on the catalytic activity of CuO@m-ZrO2/g-FeMgAl-LDO and CuO@gf-ZrO2/g-FeMgAl-LDO. The results indicate that the conversion of ethanol obtained from both catalysts is the same at 99.9% (Table 3 ) In addition, the relative yield of the products in Fig. 9 A also suggest that ethylene is produced as a main product over CuO@m-ZrO2/g-FeMgAl-LDO and CuO@gf-ZrO2/g-FeMgAl-LDO catalysts with the yield of 21.6% and 23.8%, respectively. Moreover, the relative yield indicates that the catalyst with ZrO2 prepared by the micro-emulsion method gives 31.6% of dehydrogenation products and 68.4% of dehydration products whereas the chemical grafting one provides 42.3% of dehydrogenation products and 57.7% dehydration products. The higher yield of dehydrogenation products of the chemical grafted ZrO2 catalyst sample results in higher yield of 1,3-butadiene (10.2%). Based on the results from XRF and TEM analyses, the catalyst with ZrO2 prepared by the chemical grafting method provides a higher content of Cu, which helps convert more ethanol to acetaldehyde, resulting in higher yield of 1,3-butadiene.The ethanol conversion was performed over the core-shell catalysts, where the ZrO2 promoter was deposited on the cores using the different methods in order to investigate the impact of the deposition method on the conversion of ethanol to 1,3-butadiene. In the case of the powder catalysts, the method of ZrO2 deposition did not significantly alter the formation of 1,3-butadiene. However, the catalyst with micro-emulsioned ZrO2 exhibited a better activity on ethanol dehydrogenation. In case of the granular catalysts, the highest 1,3-butadiene yield was found over the catalyst that had ZrO2 prepared by the chemical grafting method. Therefore, it can be concluded that the conversion of ethanol to 1,3-butadiene is a set of surface sensitive reactions. Therefore, the specific catalyst preparation method was required to achieve a high 1,3-butadiene yield. Among all catalysts, the granular one with grafted ZrO2 provided the best activity for 1,3-butadiene production, due to its optimum acid and base ratio together with the Cu content that helps promote the formation of acetaldehyde, resulting in the highest yield of 1,3-butadiene.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to acknowledge the financial supports from IRPC Public Company Limited, Center of Excellent on Petrochemical and Materials Technology (PETROMAT), Thailand, and The Petroleum and Petrochemical College, Chulalongkorn University, Thailand. GC-TOFMS Gas Chromatography-equipped with a Mass Spectrometry of Time-of-Fight Type LDHs Layered Double Hydroxides LDOs Layered Double Oxides TPD Temperature Programmed Desorption TPDRO Temperature Programmed Desorption/Reduction/Oxidation. XRD X-Ray Diffraction XRF X-Ray Fluorescence Spectrometer SEM Scanning Electron Microscope TEM Transmission electron microscopy BET BELCAT II. The Brunauer-Emmett-Teller MPV Meerwein–Ponndorf–Verley Gas Chromatography-equipped with a Mass Spectrometry of Time-of-Fight TypeLayered Double HydroxidesLayered Double OxidesTemperature Programmed DesorptionTemperature Programmed Desorption/Reduction/Oxidation.X-Ray DiffractionX-Ray Fluorescence SpectrometerScanning Electron MicroscopeTransmission electron microscopyBELCAT II. The Brunauer-Emmett-TellerMeerwein–Ponndorf–VerleyThe following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.clet.2021.100088.

Core-shell particles is a type of materials that consist of an inner core structure and an outer shell made from different components. They have been employed as a catalyst due to their unique properties, arising from the combination of the core and shell materials. In addtion, the properties of the core-shell particles can be designed using several suface modification techniques in order to improve the activity and stability of the catalyst. Chemical grafting is a surface modification method that involves the reaction between a metal alkoxide precursor and the surface hydroxyl group of a support. This technique has been reported to be one of the most interesting surface modification techniques that provide a well-dispersed metal oxide on the surface of a support. Micro-emulsion is also considered as one of the simplest and effective methods for the preparation of nano-sized particles with a narrow size distribution, which can be immobilized on the surface of a support. In this work, the impact of preparation methods on the physical and chemical properties of the CuO@ZrO2/FeMgAl-LDO shell@core catalyst and 1,3-butadiene production were investigated. The catalyst samples were characterized using several techniques, including XRD, XRF, BET, NH3-TPD, CO2-TPD, TEM-EDX, and SEM-EDX. The activity of catalyst samples on ethanol conversion was performed in a continuous U-tube fixed-bed reactor at 400 ​°C and atmospheric pressure. It was found that the conversion of ethanol to 1,3-butadiene was a surface sensitive reaction. Therefore, the specific catalyst preparation method was required in order to achieve a high 1,3-butadiene yield. Particularly, in this case, the catalyst that provided the highest yield of 1,3-butadiene was in the form of ZrO2-grafted granular catalyst (CuO@gf-ZrO2/g-FeMgAl-LDO one). Based on the characterization results, the grafted granular core-shell catalyst was the one that had the highest ratio of total base/acid sites.

In the current energy scenario, the production of heat, power and biofuels from biomass has become of major interest. In this line, gasification is a key technology for the large-scale exploitation of biomass. However, the development of biomass gasification is conditioned by the efficient conversion of the feed and the formation of troublesome by-products, such as tars [1–6].The tar is a complex mixture of high molecular weight aromatic hydrocarbons, which cause fouling, corrosion and blocking of downstream equipment, leading to unacceptable level of maintenance for engines and turbines. Apart from the total concentration of the tar, its nature (mainly its dew point) also determines the problems associated with this matter. Nevertheless, the tar contains a significant amount of energy that could be transformed into syngas by acting on the operating conditions, reactor design and gas conditioning systems [7–12].The design of conventional conical spouted bed reactors has recently been modified to optimize reactor performance, especially for biomass steam gasification. Conventional spouted bed reactors are characterized by short gas residence times, which is an advantage for pyrolysis processes, but a severe drawback for gasification ones because tar cracking/reforming reactions are avoided [13,14]. Thus, the fountain confined spouted bed reactor has been developed to overcome these problems and improve the overall process efficiency [15–17]. Moreover, this novel technology widens the applicability range of conventional spouted beds, as it may handle very fine particles without elutriation from the bed by confining in the fountain the gases produced in the bed, and therefore lengthening their path. Therefore, the gas residence time is increased and the gas-solid contact improved, which is even better under the fountain enhanced regime. The latter regime is characterized by a great expansion of the fountain region, which significantly improves the contact between the gas and the solid, and therefore tar conversion [18–21].Catalytic gas cleaning methods for tar removal also entail an additional increase in H2 and gas productions, as they promote tar cracking and steam reforming reactions. These catalysts may be used as primary catalysts directly in the gasifier, or as secondary catalysts in downstream catalytic processes. Thus, in the case of fluidized bed reactors, the use of an active and appropriate in-bed material as primary catalyst is a promising strategy to decrease the tar content in comparison with the use of a more expensive secondary catalytic reactor downstream [22–28].A large number of materials with significant activity for tar cracking and reforming have been used as primary catalysts. Natural minerals, such as dolomite and olivine, have attracted most of the attention because, apart from being active for tar cracking and reforming, they are inexpensive and abundant. Although the activity of dolomite is reported to overcome that of olivine, it is very fragile and undergoes severe attrition when used in fluidized beds. Furthermore, olivine has higher mechanical strength, comparable to that of sand [14,29–36]. However, the catalytic activity of these primary materials for tar conversion leaves room for improvement by metal phase addition.Ni based catalysts are more effective for converting tar into hydrogen-rich gas, but they undergo a rapid deactivation by coke deposition and are toxic [37–44]. Recently, iron based catalysts have gained considerable attention among the catalysts for tar removal. Compared to nickel, the use of iron reduces the catalyst cost and lowers its toxicity [45–55]. Apart from the well-known activity of metallic iron for tar reforming and cracking, magnetite (Fe3O4) has also been proven to be active for the WGS reaction [56–58]. Therefore, impregnation of natural minerals with iron seems to be an interesting alternative to synthesize primary catalysts.The novelty of this paper is associated with the proposal of a novel and efficient gasification technology. Thus, an original gasification technology based on the fountain confined spouted bed reactor has been developed. This reactor is able to operate under a vigorous fluidization regime (enhanced fountain regime), which greatly improves the gas-solid contact, and therefore the catalyst efficiency. Accordingly, this paper assesses the potential benefits of the fountain confined conical spouted bed for reducing the tar produced during biomass gasification, and the potential to improve the overall process efficiency by using Fe/olivine catalysts. Furthermore, the novelty is also related to the role of active iron species and their behaviour in biomass steam gasification. Thus, this paper analyses the performance and stability of an Fe/catalyst and relate its activity for biomass steam gasification with its physical and metallic properties. A detailed characterization (BET surface area, X-ray fluorescence (XRF), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and oxidation (TPO)) of the fresh and deactivated catalysts was carried out to ascertain the cause of the deactivation.The biomass feedstock used in this study was crushed and sieved forest pinewood sawdust with an average particle size of 1–2mm and dried to a moisture content below 10wt%. The ultimate and proximate analyses were conducted in a LECO CHNS-932 elemental analyzer and in a TGA Q5000IR thermogravimetric analyzer, respectively. Moreover, the higher heating value (HHV) was measured in a Parr 1356 isoperibolic bomb calorimeter. The main features of the biomass used are listed in Table 1 .Olivine supplied by Minerals Sibelco was used in this study as catalyst support and primary catalyst. Based on previous studies [19], olivine in the 90–150μm particle size range was used in order to operate in the fountain enhanced regime. The performance of the Fe/olivine catalyst was compared with that of olivine, which was calcined in situ at 850°C.An Fe/olivine catalyst with 5wt% iron was prepared by wet impregnation of the support with an aqueous solution of Fe(NO3)3·9H2O (Panreac AppliChem, 98%), by means of a rotavapor, which allows evaporating the solution under reduced pressure and moderate temperatures. The rotavapor used was a Büchi rotavapor R-114, which operates under vacuum at 70°C. A relatively low metallic load (5wt%) was used, as the physical properties of olivine hinder a suitable dispersion of the metal due to its non-porous nature. The iron solution was added to the support and the water excess was evaporated at 70°C and under vacuum environment. The samples were dried in an oven at 100°C for a couple of days and calcined in a muffle oven at 1000°C for 4h.The physical properties of the catalyst (specific surface area, pore volume and average pore size) were determined by N2 adsorption–desorption in a Micromeritics ASAP 2010 apparatus. Before each analysis, the samples were degassed under vacuum at 150°C overnight. Surface area was calculated based on the BET equation, whereas the pore size distribution was determined by BJH method.X-ray fluorescence (XRF) spectrometry was used to measure the chemical composition (wt%) of both the calcined olivine and the synthesized catalyst. The chemical analysis of the particles was carried out under vacuum atmosphere using a sequential wavelength dispersion X-ray fluorescence (WDXRF) spectrometer (Axios 2005, PANalytical) equipped with a Rh tube, and three detectors (gaseous flow, scintillation and Xe sealing). The calibration lines were determined by means of well-characterized international patterns of rocks and minerals.X-ray powder diffraction (XRD) patterns were obtained in a Bruker D8 Advance using CuKα radiation equipped with a Germanium primary monochromator and Sol-X dispersive energy detector in order to analyze the crystalline structure of both the olivine and the reduced catalyst. The spectra were obtained in a 2θ range of 20–90°. The diffraction spectra were indexed by comparing with JCPDS files (Joint Committee on Powder Diffraction Standards).X-ray photoelectron spectroscopy (XPS) analysis was carried out to record in detail the elements making up the surface, and quantify and analyze their oxidation states. XPS measurements were conducted in a SPECS system equipped with a Phoibos 150 1D-DLD analyzer and a monochromatic Al-Kα radiation source (1486.7eV). Prior to the analysis, the spectrometer was calibrated with Ag (Ag 3d5/2368.26eV).The reducibility of the materials was determined by Temperature Programmed Reduction (TPR) in an AutoChem II 2920 Micromeritics. The tests were carried out on a 200mg sample, through which a flow of 10vol% hydrogen in argon was circulated. Prior to the reduction experiments, the catalyst was thermally treated under He stream at 200°C in order to remove water or any impurities. The temperature was increased at a rate of 10°Cmin−1 from room temperature to 900°C. A thermal conductivity detector (TCD) was used to analyze the hydrogen consumption of the samples and its signal recorded continuously.Carbon deposition on the catalyst was ascertained by temperature programmed oxidation (TPO) in a Thermobalance (TGA Q5000 TA Instruments) coupled in-line to a mass spectrometer (Thermostar Balzers Instrument). This device allows recording the signals at 14, 18, 28 and 44 atomic numbers, corresponding to N2, H2O, CO and CO2, respectively. The coke content was determined based on CO2 signal. Once the signal was stabilized under N2 stream (50mLmin−1) at 100°C, oxidation of the sample with air was carried out by increasing temperature to 800°C using a ramp of 5°Cmin−1 and keeping the final temperature for 30min to ensure total carbon combustion.The biomass steam gasification experiments were performed in a bench scale unit based on the conical spouted bed technology (Fig. 1 ). This reactor was designed for biomass valorization processes, and specifically fine-tuned and optimized for the pyrolysis and gasification of different solid wastes [59–63].The main element of the plant is a fountain confined conical spouted bed, which is also provided with a non-porous draft tube (internal diameter 5.5mm and entrainment zone height 15mm). It enables operating in a wide range of conditions and improving the hydrodynamic performance of the reactor. This reactor may also operate in the conventional spouting regime by using a lid without confiner. The main dimensions of the reactor are as follows: cylindrical section diameter 95mm, height of the conical section 150mm, cone included angle 30°, length of the fountain confiner 330mm, and total height of the reactor 430mm. The cone base diameter is 20mm, and the internal diameter of the fountain confiner 54mm, with its volume being of around 0.8L. The height from the reactor base to the lower end of the confiner is 105mm. The fountain confiner is a tube welded to the lid of the reactor, whose lower end is close to the bed surface and confines the gases generated in the bed and force them to circulate upwards through the core of the fountain and downwards through its periphery. This device increases the residence time, narrows its distribution and enhances gas-solid contact in the fountain region [17]. Furthermore, the draft tube makes operation feasible in a much wider range of gas flowrates and improves bed stability [64]. More details about the reactor, fountain confinement technology and draft tube have been reported elsewhere [18,19]. A gas preheater is located below the reactor to heat the gases to reaction conditions. A radiant oven made up of two independent sections heats the gasifier (the lower section heats the gas preheater and the upper section the fountain confined spouted bed reactor). The temperature in each section is controlled by two thermocouples, one placed in the bed annulus and the other one at the inlet of the gaseous stream.All the unit elements, i.e., the reactor, the interconnection pipes, the cyclone and the filter, are located inside a forced convection oven, which is 1830×1950×1000mm stainless steel box and features 100mm insulation of quartz wool with fiberglass reinforcement fabric, kept at 300°C to prevent tar condensation before the condensation system. The high-efficiency cyclone and 5μm sintered steel filter retain the char and catalyst fine particles entrained from the bed.The biomass was fed by means of a piston dispenser. This system consists of a cylindrical vessel equipped with a vertical shaft connected to a piston placed below the material bed. When the piston rises, the biomass is pushed towards the top of the feeding system and drops into the reactor through a tube cooled with tap water at the same time as the whole system is vibrating by means of an electric engine to prevent biomass agglomeration. Moreover, a very small nitrogen flow is introduced from the top of the feeding vessel in order to ease the solid flow into the reactor and avoid the condensation of steam in the dispenser. A detailed description of the functioning of this device to feed the solid has been reported elsewhere [20]. The water flowrate was measured by an ASI 521 pump and directed to an evaporator to produce steam before entering the reactor. The plant is also provided with three mass flow meters for N2, air and H2, with N2 being used as fluidizing agent during the heating process and H2 for the reduction of Fe catalyst prior to the runs.The volatile condensation system is located after the convection oven and consists of a double-shell tube condenser cooled by tap water, a 1L vessel, a stainless steel filter (60μm) and a coalescence filter, with the latter ensuring total recovery of the tars.Continuous gasification experiments were performed at 850°C using steam as fluidizing gas. Thus, a water flow rate of 1.5mLmin−1 was employed (1.86 NL min−1 of steam) and biomass was fed at a rate of 0.75gmin−1, which corresponded to a steam/biomass (S/B) ratio of 2. This high S/B ratio is needed to guarantee vigorous spouting conditions (enhanced fountain regime), as steam not only acts as gasifying agent, but also as fluidizing gas.The bed consisted of 100g either calcined olivine or Fe/olivine catalyst, with their particle size being in the 90–150μm range. Prior to the reactions, the iron catalyst was subjected to an in situ reduction process at 850°C for 4h with a stream containing 10vol% of H2 to ensure complete reduction to Fe0 phase.The experiments were performed in continuous regime and the gas chromatography (GC) and micro GC analyses were conducted once several minutes of operation had elapsed to ensure steady state conditions. Moreover, the runs were repeated several times (at least three) under the same conditions in order to guarantee reproducibility of results.Certain limitations in the experimental unit for operation and product monitoring had to be addressed. Thus, the feeder must be refilled after 40min operation and the GC analysis of the tars lasts about 20min. Accordingly, successive 10min reactions were carried out in order to ensure suitable product analysis throughout continuous operation and overcome these limitations.Samples of the volatile stream leaving the reactor were analysed on-line by means of a GC Agilent 7890 outfitted with a HP-Pona column (50m long, 0.2mm in internal diameter and a coating thickness of 0.5mm) a flame ionization detector (FID). The sample was injected into the GC through a line thermostated at 280°C to avoid the condensation of the tars. The temperature programme used in the GC is as follow: 2min at 40°C in order to attain a good separation; a sequence of 25°Cmin−1 up to 320°C and 7min at this temperature to ensure that all products were outside the column. Furthermore, the non-condensable gaseous stream was also analysed online in a Varian 4900 micro GC equipped with three modules (molecular sieve, Porapak (PPQ) and plot alumina) and thermal conductivity detectors (TCD), which allow both identification and quantification of gaseous products previously calibrated. The conditions for the analysis were the same for the modules: column temperature 90°C, injector temperature 100°C and pressure 20psi. In this case, the sampling point was located after the condensation and filtering sections. Moreover, the tars retained in the condensation system were identified by Shimadzu UP-2010S GC/MS once they had been dissolved in acetone.In order to assess the gasification performance the following reaction parameters were considered: • Gas production (P gas , Nm3 kg−1) based on the mass unit of biomass fed into the gasification process: (1) P g a s = Q g a s m 0 where Q gas is the volumetric flow rate of the gas produced and m 0 is the mass flow rate of biomass fed into the process. • H2 production ( P H 2 , wt%) by mass unit of the biomass fed into the reactor, which is calculated as follows: (2) P H 2 = m H 2 m 0 ⋅ 100 where m H 2 and m 0 are the mass flow rates of the H2 produced and biomass fed into the reactor, respectively. • Tar concentration determined as the amount of tar (in mass) per m3 of syngas: (3) Tar concentration :   m t a r Q g a s •Carbon conversion efficiency defined as the ratio between the moles of C in the gaseous product and those entering the reactor. (4) X = C g a s C b i o m a s s ⋅ 100 Gas production (P gas , Nm3 kg−1) based on the mass unit of biomass fed into the gasification process: (1) P g a s = Q g a s m 0 where Q gas is the volumetric flow rate of the gas produced and m 0 is the mass flow rate of biomass fed into the process.H2 production ( P H 2 , wt%) by mass unit of the biomass fed into the reactor, which is calculated as follows: (2) P H 2 = m H 2 m 0 ⋅ 100 where m H 2 and m 0 are the mass flow rates of the H2 produced and biomass fed into the reactor, respectively.Tar concentration determined as the amount of tar (in mass) per m3 of syngas: (3) Tar concentration :   m t a r Q g a s •Carbon conversion efficiency defined as the ratio between the moles of C in the gaseous product and those entering the reactor.(4) X = C g a s C b i o m a s s ⋅ 100 Table 2 shows the specific surface area, pore volume and average pore size. As observed, the specific surface area of the calcined olivine was as low as 1.92 m2 g−1 and the pore volume 0.0023 cm3 g−1, which are evidences of its non-porous structure. Regarding the synthesized Fe/olivine catalyst, olivine physical properties were improved by iron impregnation. Thus, pore volume and average pore size became larger, which was due to the collapse of the inter-pore structure of olivine. Likewise, the specific area also increased, which may be attributed to the deposition of Fe on the external surface. This trend has also been reported for Ni impregnation on low porosity surfaces [65,66].The chemical compositions of the calcined olivine and the prepared catalyst are summarized in Table 3 . The content of Fe in the olivine was of around 5.2wt%. After impregnation, Fe content in the catalyst increased significantly, 10.2wt%, which confirmed that the metal content was that corresponding to the impregnation (5wt%) plus that in the original olivine. Fig. 2 shows the diffractograms of the calcined olivine and fresh and reduced Fe/olivine. In the case of the calcined olivine, the XRD data revealed the main diffraction lines were characteristic to the olivine structure ((Mg1.81·Fe0.19)·(SiO4)). Additional peaks corresponding to secondary crystalline phases may also be observed, such as enstatite (MgSiO3) and quartz (SiO2). According to Michel et al. [67,68] and Świerczyński et al. [69], numerous phases of iron oxide may appear subsequent to olivine calcination, as are γ-Fe2O3, α-Fe2O3, Fe3O4 and MgFe2O4. The presence of these iron oxides is explained by the migration of the iron Fe2+ located within the internal structure of the olivine to its surface due to oxidation (Eq. (5)) [69,70]. However, none of these phases were detected in this study. It should be noted that the calcination temperature used for the natural olivine was rather low (850°C) compared to other studies in the literature, in which they were over 1100°C. Kuhn et al. [71] performed XRD analysis to olivine calcined at 900°C during 2h and they neither observed free Fe oxide phases. These oxide phases diffract in the same main lines as the olivine structure, but they were not strong enough to be detected and so inferred their presence. For the fresh and reduced Fe/olivine catalysts, the main crystalline forms were still those corresponding to olivine structure and MgSiO3 enstatite phase, even though the olivine was subjected to iron impregnation, calcination and reduction. However, significant changes in the relative intensity of olivine structure and MgSiO3 enstatite phases were noticed at 2θ =21°, 31° and 36°, which indicated certain modifications in the crystallinity of the samples due to iron impregnation. In fact, the higher intensity of the diffraction lines in the reduced catalyst is evidence of its greater crystallinity compared to the calcined olivine or fresh catalyst, which was due to iron reincorporation into the olivine structure. In addition, hematite (α-Fe2O3) peak appeared at 2θ =24° in the fresh catalyst, whereas for the reduced catalyst the presence of an intense peak of the metallic iron phase was observed at 2θ =44° and a smaller one at 2θ =65°. Iron oxide phases were not detected in the reduced sample, which is evidence of their full reduction. Other authors reported the same main lines for this catalyst [47,48,72]. The SiO2 lines detected in the support disappeared in the catalyst. Michel et al. [67] stated that olivine phase reacts with quartz at 1000°C to form enstatite phase: (5) M g , F e 2 S i O 4 + O 2 → x M g 2 S i O 4 + 1 − x S i O 2 + 2 x − 1 F e 2 O 3 α + 2 1 − x M g F e 2 O 4 Fig. 3 shows the XPS spectra for the samples in different binding energy ranges. This analysis revealed the main components on the surface of the samples, which were Si, Mg, Fe and O. No significant changes were observed in Si after iron impregnation and catalyst reduction, whereas more pronounced changes were detected in the peaks corresponding to Mg and Fe. In the case of Fe, its oxidation states are analyzed below in the paper. These variations are also visible in Tables 4 and 5 . Furthermore, peaks of other trace elements, previously detected by XRF, were not observed, which is evidence that they were not located on the surface. Table 4 shows the surface composition of the samples. The quantification of each element was carried out by integrating the intensities of Si 2p, Mg 2p, O 1s and Fe 2p using Scofield sensitivity factors. As observed, after iron impregnation, the amount of iron on the catalyst surface increased (from 6.2 to 8%), which suggests that part of the impregnated iron was deposited on the surface of the catalyst, as evidenced by the increase in the BET surface of the catalyst (Table 2). However, the amount of Mg on the surface decreased (from 17.5 to 14.2%) after iron loading. According to Frekdisson et al. [73], after the oxidizing treatments, the surface is enriched in Fe at the expense of Mg. Furthermore, catalyst reduction with H2 led to a decrease in the amount of Fe to 4.4% and an increase in that of Mg to 22.1% on the surface. Under reducing conditions, Fe clustered into large particles and incorporated into the olivine structure [71]. Regarding oxygen concentration, its oscillations on the surface of the catalyst were related to the oxidation state of iron.XPS spectra in the 700–750eV binding energy range of the samples were analyzed to further understand the valence state of the iron in the calcined olivine and fresh and reduced Fe/olivine catalysts (Fig. 3b and c). Accordingly, Fe 2p lines were used instead of Fe 3p because they were stronger. Moreover, Table 5 shows the iron distribution on the surface of the samples. Yamashita and Hayes [74] reported that Fe 2p3/2 peak at 711eV with satellite peak at 719eV and Fe 2p1/2 peak at 725eV with satellite peak at 732eV are characteristic of Fe3+, whereas Fe 2p3/2 peak at 709eV with satellite peak at 714eV and Fe 2p1/2 peak at 723eV with satellite peak at 728eV correspond to Fe2+. In Fig. 3c, the positions of these peaks are marked with dashed lines. Iron in Fe3+ state corresponds to Fe2O3 and MgFe2O4 compounds, whereas Fe2+ state is characteristic of iron in the olivine structure and FeO. In the calcined olivine, most of the Fe was as Fe3+ and doubled the amount of Fe as Fe2+, which is evidence that a higher amount of iron led to free oxides on the surface than those remained within the olivine structure. The presence of free iron oxide phases (Fe3+) stemmed from Fe migration from the olivine structure (Fe2+) during the calcination process [69,73], although none of these compounds were detected by XRD analysis. Regarding iron distribution, the fresh Fe/olivine catalyst followed the same trend as the calcined olivine. However, when the former is compared with the calcined olivine, the amount of Fe2+ in the fresh catalyst increased (from 32. 89 to 35.42%), whereas that of Fe3+ decreased (from 67.11 to 64.54%), although Fe2+/Fe3+ ratio remained approximately constant. These results suggest that, after impregnation, the iron within the olivine structure was preferably in the metallic state rather than forming free oxides. After reduction, a weak peak of metallic Fe appeared at 707eV, which cannot be quantified due to its very small size. It seems that the metallic iron on the catalyst surface was oxidized due to its contact with air, but the iron inside the olivine remained in the metallic form, as was revealed by the XRD analysis (Fig. 2). Moreover, the Fe2+/Fe3+ ratio in the reduced catalyst was higher than that in the fresh one, with the amount of Fe2+ and Fe3+ being almost the same. Thus, the oxidation state of the iron located on the surface changed from a Fe3+ dominating state after oxidation to Fe2+ state after reduction [73]. Meng et al. [72] observed the same trend for the iron distribution on the surface of the catalyst.H2-TPR experiments for the bed materials were carried out prior to their use in the reaction environment. The TPR profile of the catalysts enables determining the temperature needed for their reduction [75]. As well-known, the profile depends not only on the nature of the metallic species, but also on the metal-support interactions. Moreover, as the metallic iron is supposed to be the active phase for hydrocarbon cracking, the reducibility of the catalysts is of great relevance [76].The TPR profiles of the calcined olivine and synthesized catalyst are shown in Fig. 4 . In the case of the calcined olivine, two small peaks are observed between 350 and 550°C. A third peak is also observed at a reduction temperature above 600°C. In the case of the first two peaks, their low reduction temperature is evidence that these species are easy to reduce. Thus, these peaks are attributed to the reduction of iron oxides on the olivine surface [77]. According to the XPS analysis (Table 5), the surface of the calcined olivine is presumably made up of Fe2O3 and/or MgFe2O4, which migrated from the internal olivine structure during the calcination [48,69,71,72]. Thus, the peak at 350°C is assigned to the reduction of Fe2O3 and the peak at 550°C to the reduction of Fe3O4, as the reduction of Fe2O3 to metallic Fe occurs in two steps (Fe2O3 →Fe3O4 →Fe0) [46,48]. The peak that might appear at higher temperatures is associated with the reduction of iron phases inside the olivine grain, in which reduction is more difficult. The TPR profile of the Fe/olivine catalyst shows a broad reduction zone covering the range from 300 to 700°C. Three main peaks may be observed, with the first two being associated with the two-step oxidation of Fe2O3 on the olivine surface and the peak above 600°C to the Fe atoms that migrated into the olivine support to form a very stable MgFe2O4 spinel phase [78]. In the case of the Fe/olivine catalyst, the reduction of iron phases inside the olivine grain is not observed due to the high stability of the olivine structure, i.e., higher temperatures are required for its reduction.The effect of Fe/olivine catalyst on the steam gasification process parameters (H2 and gas productions, gas composition, carbon conversion and tar concentration and composition) was assessed and compared with that of calcined olivine. The performance of these in-bed materials was analyzed based on the following reactions inside the gasifier: (6) Biomass pyrolysis :   B i o m a s s → G a s e s C O , C O 2 , C H 4 , H 2 , … + o x y g e n a t e s + c h a r (7) Steam gasification of the char :   C ( s ) + H 2 O → C O + H 2 (8) CO 2 gasification of the char :   C ( s ) + C O 2 → 2 C O (9) Tar cracking :   T a r → C O + H 2 + C O 2 + C + C H 4 + ⋯ (10) Tar steam reforming :   T a r + H 2 O → C O + H 2 (11) Water gas-shift ( WGS ) :   C O + H 2 O ⇔ C O 2 + H 2 (12) Methane ( or hydrocarbon ) steam reforming :   C H 4 + H 2 O ⇔ C O + 3 H 2 As observed in Fig. 5 , all representative gasification parameters were significantly improved on the Fe/olivine catalyst. An increase in gas and hydrogen productions and a decrease in tar concentration was noticeable when 5wt%Fe/olivine was used instead of olivine (Fig. 5a and b). Thus, gas production increased from 1.30 to 1.46Nm3 kg−1 and so did the hydrogen production, from around 5 wt% on the olivine to 6.25wt% on the iron impregnated catalyst. Fig. 6 illustrates the product gas composition for the runs using 5 wt%Fe/olivine catalyst and calcined olivine. Iron impregnation led to an increase in H2 concentration from 43.2 to 48.2vol% and a reduction in that of CO, which implies that H2/CO ratio increased from 1.41 for olivine to 3.26 for the iron catalyst. Consequently, CO2 concentration increased to 28.2vol%. From these results, it could be deduced that the addition of iron to olivine enhances the WGS reaction (Eq. (10)), as well as light hydrocarbon steam reforming and cracking reactions (Eqs. (8) and (11)). Consequently, tar concentration was reduced approximately to half, from 20.6 to 10.4gNm−3, and carbon conversion efficiency accounted for 87.6% (Fig. 5c and d). Likewise, the heating value of the gas increased from 2.44MJm−3 with calcined olivine to 8.66MJm−3 with 5 wt%Fe/olivine catalyst. According to several authors [73,76,79], the metallic Fe on the reduced catalyst enhances tar decomposition reactions. Moreover, the BET surface area (Table 2) and XPS analyses (Table 4) revealed that Fe was mainly located on the external surface of the catalyst, and was therefore easily accessible to the volatiles and promotes tar cracking and reforming reactions (Eqs. (8) and (9)).Although there are many studies dealing with steam reforming of biomass tar model compounds using a wide variety of supported metal catalyst [80–89], those dealing with the effect of metal impregnated in situ catalysts on the biomass steam gasification are scarce, especially those carried out in laboratory pilot plants. Several authors reported the same trend as that obtained in this study for iron impregnated olivine and compared its activity with that of raw olivine using different gasification technologies [45,49,50,90,91]. Thus, Rapagnà et al. [49] studied the performance of 10 wt%Fe/olivine catalysts in the biomass steam gasification at 820°C in a fluidized bed gasifier and obtained slightly higher reaction indices than in this study. They observed that H2 and gas productions increased from 3.5 to 6.6 wt% (the gaseous stream contained 53 vol% of H2) and from 1 to 1.4Nm3 kg−1, respectively when an Fe/olivine catalyst was used instead of raw olivine, whereas tar concentration was reduced by approximately 62%, with the value being 2.25gNm−3 with the catalyst. Carbon conversion efficiency reached a value of 80%, which was similar to that obtained with raw olivine. Virginie et al. [50] used the same catalyst as the previous authors, but they used a dual fluidized bed. They reported that tar reduction was more notable in the presence of Fe/olivine in the bed than in the run with raw olivine (5.1 and 2.6gNm−3 of tar content for olivine and Fe/olivine at 850°C). In addition, Barisano et al. [90,91] evaluated the performance of 10 wt%Fe/olivine catalyst in the biomass steam/O2 gasification at 890°C in an internal circulating bubbling fluidized bed (ICBFB) and they reported 1.2Nm3 kg−1 and 3wt% for the gas and H2 productions, respectively. They also reported a reduction in the total tar content by 38% (from 10.1 to 6.2gNm−3), and 98% of carbon conversion efficiency was therefore attained. However, Pan et al. [45] used a lower Fe load in the catalyst (5wt%Fe/olivine) for the steam co-gasification of pine sawdust and bituminous coal in a pyrolysis-reforming-combustion decoupled triple bed system (DTBG) at 850°C. All the studied reaction indices were improved, but the differences were not as remarkable as those observed for the biomass steam gasification. Thus, they obtained gas and H2 productions of 0.66Nm3 kg−1 and 2.49wt% (10% higher in both cases) and a tar content as low as 4.87gNm−3 (17% reduction).Ni loading to olivine also enhances tar reforming activity in the biomass steam gasification, with the performance being even better than that of Fe/olivine catalyst. Thus, Pfeifer et al. [92] studied tar removal activity of Ni/olivine catalyst in a 100kWth dual fluidized bed reactor. After adding 20% of 5wt%Ni/olivine catalyst to a bed of olivine, the tar concentration was reduced by half and gas and H2 productions increased to 1Nm3 kg−1 and 3.93wt%, respectively at 850°C. Michel et al. [93] used in situ 3.9wt%Ni/olivine catalyst in the biomass steam gasification carried out in fluidized bed at 800°C and reported a higher efficiency of the catalyst compared to raw olivine. Thus, they obtained H2 and gas productions of 7.56wt% and 1.7Nm3 kg−1, instead of 3.36wt% and 1Nm3 kg−1 with olivine, and less than 1wt% of tar. More recently, Tursun et al. [94] used 5wt%Ni/olivine catalyst in a decoupled triple bed gasification system consisting of a pyrolyzer, reformer and combustor, and reported that the catalyst not only improved tar removal, but also enhanced H2 and gas productions. Their results were slightly better than those obtained by Michel et al. [93], but the Ni loading was also slightly higher. They reported a gas production of 1.59Nm3 kg−1, with H2 concentration being 56.1vol% (H2 production of 7.96wt%) and tar content as low as 0.6gNm−3.The tar fraction is a mixture containing polycyclic aromatic compounds larger than benzene, and is commonly studied by dividing into four lumps [7], as are: light aromatics (monoaromatic compounds), heterocycles (aromatic rings with heteroatoms), light polyaromatics (PAHs with up to 3 rings) and heavy polyaromatics (PAHs with more than 3 rings). Apart from the total concentration of the tar, its nature (mainly its dew point) is of high relevance, as it is responsible of problems related to fouling and sooting. Fig. 7 shows a significant reduction in the amount of heterocycles and heavy PAHs using Fe/olivine catalyst. In fact, the mass fraction of those lumps was reduced from 10.33 and 10.20 to 7.43 and 5.05wt%, respectively. However, the percentage of light aromatics and PAHs in the total tar amount increased from 14.22 and 62.09 to 19.91 and 65.20wt%. It is noteworthy that the Fe/olivine catalyst managed to reduce significantly the concentration of all tar families, as shown in Table 6 . Based on the tar formation and PAH growth mechanisms [95], the Fe/olivine catalyst seems to hinder the growth of light PAHs into heavier ones, and the amount of the light PAHs was therefore higher. Furthermore, Diels–Alder reactions involving light alkenes in the permanent gases and phenols may produce light aromatics, and therefore its amount was increased [96–98]. Table 6 provides a detailed composition of the tar obtained with raw olivine and Fe/olivine catalyst. Naphthalene was the most abundant tar molecule for calcined olivine and Fe/olivine catalyst, although its concentration was reduced by 42% approximately with the iron enrich catalyst. Barisano et al. [91] reported a higher naphthalene reduction (of around 58%) in the biomass steam/O2 gasification. Moreover, compounds such as phenol, methyl phenol, 1-methyl naphthalene, dibenzofuran, 1-H phenalene, 2-phenyl naphthalene and pirene were significantly removed, as the catalyst managed to reduce their content beyond 60%. Thus, it is clear that metallic iron is active for CC and CH bond breakdown [76,99]. The results in Table 6 also show the more stable tar compounds, which are those that are more difficult to remove. Using the Fe/olivine catalyst the concentration of toluene, naphthalene and anthracene was reduced, but their amounts were still rather high, as they are refractory to reforming/cracking reactions [49]. Therefore, all the efforts in the development of supported metal catalysts should be directed towards their capacity for removing the most stable tar compounds.The evolution of the gasification performance (Fig. 8 ) and gas and tar compositions (Figs. 9 and 10 ) were monitored for Fe/olivine with time on stream. In the case of the calcined olivine, gasification performance remained stable after 140min on stream. The main properties of the Fe/olivine catalyst and their role on the biomass steam gasification explain these results. Fig. 8 illustrates the evolution of the reaction indexes as a function of time on stream for Fe/olivine catalyst. Even though the performance of the calcined olivine remained stable after 140min on stream, that of Fe/olivine catalyst underwent deactivation and the efficiency of the gasification process decreased with time on stream. Catalyst deactivation was especially evident by tar concentration, which increased by around 90%, from 10.4 to 19.8gNm−3, as shown in Fig. 8c. After 140min on stream, the amount of tar produced with the Fe/olivine catalyst reached almost that obtained with the calcined olivine (20.6gNm−3). Other reaction indexes also showed the deterioration of the catalyst. Thus, gas and H2 productions declined from 1.46Nm3 kg−1 and 6.25wt% to 1.35Nm3 kg−1 and 5.44wt%, respectively (Fig. 8a and b). However, the gas and H2 productions were still above those obtained with calcined olivine, which suggests that although the catalyst was not able to maintain its original tar elimination capacity, it was still active in the WGS reaction. Likewise, a similar trend was observed in the evolution of the gas composition (Fig. 9). H2 concentration slightly decreased from 48.2 to 45.5vol%, whereas that of CO increased from 14.3 to 20.2vol%. CO2 concentration remained almost stable at 24.9vol%. A comparison of this performance with the stable calcined olivine shows that higher H2 and CO2 concentrations were obtained, whereas the value of CO was lower due to the enhancement of the WGS reaction. Concerning CH4 and C2–C4 light hydrocarbons, they showed a slightly upward trend. In the case of the deactivated catalyst, CH4 concentration was even lower (6.4vol%) than that obtained with the calcined olivine and C2–C4 concentration reached a similar value as that with the calcined olivine (2.7vol%). The latter results are evidence that the Fe/olivine catalyst was still active for steam reforming of CH4 subsequent to 140min on stream.The evolution of tar lumps with time on stream is shown in Fig. 10. As the Fe/olivine catalyst was deactivated, the amount of each tar family was similar to that obtained with the calcined olivine. Thus, the amount of light aromatics and PAHs declined from 19.91 and 64.36 to 15.24 and 57.47wt%, whereas that of heterocycles and heavy PAHs increased from 7.43 and 5.05 to 10.82 and 11.63wt% after 140min on stream. Small differences were observed in the amount of light PAHs between the value with the calcined olivine and that with the deactivated Fe/olivine catalyst, which are related to the amount of unidentified compounds (there were more unidentified compounds with the deactivated catalyst). When the deactivation of the catalyst was not considerable, the Fe/olivine catalyst seemed to hinder the growth of light PAHs into heavier ones, and the amount of the light PAHs was therefore higher than for the calcined olivine. Moreover, Diels–Alder reactions involving light alkenes in the permanent gases and heterocyclic compounds may also have produced light aromatics, which led to an increase in their amount [96–98].The prevention and attenuation of catalyst deactivation is a challenging task. Thus, most catalytic processes undergo catalyst deactivation, and therefore understanding the deactivation mechanisms is vital. In the biomass gasification processes, deactivation is mainly caused by sulphur and chlorine poisoning or carbon deposition. However, catalyst physical changes, such as sintering, phase change and attrition may also lead to catalyst deactivation. The deactivated catalyst was characterized in detail in order to understand the main causes of catalyst activity decay. Table 7 shows the values of the physical properties for the fresh and deactivated Fe/olivine catalysts. After 140min on stream, the specific surface area of the Fe/olivine catalyst was significantly lower, with the reduction being even more noticeable in the pore volume and size, which underwent a more severe decrease. Therefore, the pores of the catalyst were partially blocked, which led to a decrease in the total surface area, as well as pore volume and size. The deactivated Fe/olivine catalyst had still a higher surface area and pore volume than the calcined olivine. However, the pore size was higher in the calcined olivine.In order to assess the changes in the metallic structure of the Fe/olivine catalyst after the reaction, Fig. 11 shows the XRD patterns of the reduced and deactivated catalysts. After the reaction, the main crystalline structures were still the olivine structure and the MgSiO3 enstatite phase, although more diffraction lines corresponding to MgSiO3 phase appeared in the deactivated catalyst. The most significant differences between both spectra are related to the iron phases. In the spectrum of the deactivated catalyst, there is no evidence of the presence of metallic iron, neither in 2θ =45° nor 2θ =65° diffraction lines. However, multiple lines of Fe3O4 or MgFe2O4 spinel phase were noticeable, which are evidence of a loss of active phase by oxidation of the metallic iron under reaction conditions. Virginie et al. [50] also reported the presence of intense diffraction lines corresponding to Fe3O4 or MgFe2O4 spinel phase after reaction.XPS analysis of the deactivated catalyst was carried out to determine the components located on the surface of the catalyst after reaction. The XPS spectra of the reduced and deactivated samples in different binding energy ranges are shown in Fig. 12 . This analysis revealed that, after the reaction, the main components on the surface of the samples were still Si, Mg, Fe and O (Fig. 12b). However, the presence of K and Ca was also observed, although the amount of the latter on the surface could not be quantified because it was very small. Their existence is probably due to the biomass ashes. As shown in Fig. 12c, metallic iron was not detected on the catalyst surface in the deactivated catalyst, which is consistent with the previous XRD results (Fig. 11).The surface composition and iron distribution in the reduced and deactivated catalysts are shown in Table 8 . As observed, after the reaction there were small differences in the amount of Mg and Fe on the catalyst surface. The amount of iron slightly increased from 4.4 to 5.1% at the expense of Mg, which decreased from 22.1 to 18.4%. However, the iron distribution remained constant (the amount of Fe2+ and Fe3+ compounds was the same), which is an indication that iron migration from the olivine structure into the surface happened. A comparison of this catalyst with the calcined olivine shows that the deactivated catalyst had more iron in the olivine structure and a higher amount of Fe2+ compounds on its surface. Iron migration from the inside to the surface or vice versa occurs in order to reach iron equilibrium in the structure [50,73]. Regarding the amount of K on the deactivated catalyst surface (1.4%), its origin is attributed to biomass ashes. Alvarez et al. [100] reported the chemical analysis of the ashes of the same biomass used in this work and the amount of K2O was 11.3wt%. Moreover, at 850°C, potassium salts melt and they might have formed deposits on the deactivated catalyst surface. Fig. 13 shows the TPR curve of the fresh and spent catalysts. A single peak at 500°C with a small shoulder at a slightly higher temperature (590°C) was observed for the deactivated catalyst, which is evidence that the iron in the Fe/olivine catalyst was oxidized during the gasification process. As the XRD revealed, this peak should be attributed to the Fe3O4 or MgFe2O4 spinel phase detected. According to Meng et al. [72], the difficulty for reducing the possible iron oxides is as follows: MgFe2O4 >FeO>Fe3O4 >Fe2O3. However, the low reduction temperature suggests that this specie was easy to reduce, i.e., it was probably Fe3O4. Furthermore, the shoulder at 590°C is attributed to the reduction of a small amount of MgFe2O4 spinel phase. In fact, it seems that most of the MgFe2O4 spinel phase did not undergo oxidization during the reaction, as it is a very stable compound.As carbon deposition may cause catalyst deactivation, temperature programmed oxidation (TPO) was conducted on the spent Fe/olivine catalyst to quantify the amount of carbon settled. The total amount of coke and its composition depend on the operating conditions, mainly temperature and S/B ratio, as carbon deposition is a consequence of a balance between its formation and removal by gasification [101]. The TPO analysis revealed that a negligible amount of coke (0.11wt%) was deposited on the catalyst after the reaction, which is evidence that high temperatures and steam promoted the oxidation of almost all the carbon that may have formed. Fig. 14 shows the TPO profile of the deactivated catalyst. Two different peaks are observed, which is an indication of the heterogeneous nature of the coke. According to the literature [102–105], the coke combustion temperature on supported metal catalysts is related to its location on the catalyst and composition. Low combustion temperatures are attributed to the coke deposited on the metallic sites (encapsulating coke), which may catalyse coke combustion, whereas higher combustion temperatures indicate that the coke is deposited on the support, which prevents coke combustion by metallic sites. Furthermore, even if the coke is deposited on similar locations, its combustion temperature is higher as the condensation degree is higher, i.e., more organized structures with lower H/C ratios. Fig. 14 reveals the heterogeneity of the coke. Thus, two different carbon species were detected, with their combustion temperatures being 530 and 606°C. The peak at 530°C is attributed to the amorphous coke and the shoulder at 606°C to a coke with a slightly more condensate structure. It seems that the severe reaction conditions prevent coke formation from the evolving compounds to more condensed ones due to the in situ gasification of the amorphous coke. Virginie et al. [50] observed a similar TPO profile after biomass steam gasification experiments, although their carbon oxidation temperatures were slightly higher than those obtained in this work (585 and 630°C). As the coke content was very low (0.11wt%), it cannot be stated that coke deposition caused catalyst deactivation. Table 9 shows the chemical composition of the fresh and deactivated Fe/olivine catalysts. XRF analysis revealed that there was no any iron loss due to attrition phenomena, which was also checked by sieving the deactivated catalyst (it had the same size range (90–150μm) as prior to the runs). Claude et al. [51] and Meng et al. [106] reported that the olivine catalysts synthesized by wet impregnation may undergo attrition, since the metallic species were mainly placed on the surface, and therefore their interaction with the support was rather weak. Some other authors studied this aspect. Thus, Virginie et al. [50] reported an iron loss of 32% after 12h gasification in a dual fluidized bed and Rapagnà et al. [49] about 5wt% during 320min operation in a fluidized bed reactor.The gas composition in the gasifier plays a crucial role on the oxidation state of the iron located on the catalyst. During biomass steam gasification, the metallic Fe0 was oxidized as detected by XRD, XPS and TPR analyses. The operating methodology used in this study had some limitations, which may have contributed to the catalyst oxidation, as explained in the experimental section. These limitations caused changes in the reaction environment, as the fluidizing agent had to be changed from N2 to steam and ensure suitable fluidization regime prior to starting biomass feed. Thus, the presence of steam may have induced partial oxidation of the metallic phase at the beginning of the reactions. However, as biomass was fed into the reactor, the reaction environment shifted from oxidizing to reducing due to the high hydrogen concentration, and therefore the iron oxidized under steam atmosphere was reduced again. It should be noted that this problem can be avoided in full scale operation with continuous biomass feed.A similar catalyst deactivation cause was observed in the in-line steam reforming of biomass fast pyrolysis volatiles using 10wt%Co/Al2O3 catalyst by Santamaria et al. [107]. Nordgreen et al. [99] reported that, when the oxygen concentration in the reaction environment is too high, it would oxidize the metallic iron to wustite (FeO) and subsequently to magnetite (Fe3O4), since some locations favour these transformations. Based on the results obtained, when Fe was in the metallic state in the Fe/olivine catalyst, it showed a higher activity for reducing tar than when it was in the oxidized state (Fig. 8c). Changes in tar removal capacity of the Fe/olivine catalyst with time on stream may also be related to the distribution of iron oxides. Nordgreen et al. [99] also stated that the catalyst with metallic iron was capable of reducing the tar concentration above 60%, whereas the catalyst with the oxidized iron only had a capacity of 18%. The catalytic activity of iron oxides species increases with their reduction state (Fe2O3 <Fe3O4 <FeO<Fe0) [51]. After 140min on stream, the tar concentration obtained with the deactivated Fe/olivine catalyst and that obtained with the calcined olivine were almost the same, which suggests the presence of iron oxides leads to the same tar removal performance as calcined olivine. The same trend was observed in the evolution of tar lumps. As the Fe/olivine catalyst was deactivated, the amount of each tar family was similar to that obtained with the calcined olivine. Undoubtedly, the oxidation of metallic Fe0 sites led to their decrease, and therefore caused catalyst deactivation, as was revealed by the characterization techniques.Several studies pointed out that different Fe-phases may catalyze different reactions. Thus, Fe2O3 is reported to catalyze shoot and NOx conversion, Fe3O4 to be active in the WGS reaction and metallic Fe to catalyze Boudard reaction and tar removal reactions. Therefore, changes in the oxidation state of iron will drastically influence catalytic properties [73]. However, it seems that, after 140min on stream, the Fe/olivine catalyst was still active for WGS and CH4 steam reforming reactions, as shown in Figures 8a and 8b, which may be attributed to Fe3O4 or MgFe2O4 spinel phase detected on the deactivated catalyst (Fig. 11). However, the TPR analysis of the deactivated catalyst was more conclusive than XRD and XPS analyses, which allows inferring that Fe3O4 is the responsible, as the reduction temperature was rather low. Some authors proved Fe3O4 was active in the WGS reaction [57,58,108–110].The fountain confined conical spouted bed performs well in the biomass steam gasification with primary catalysts. In fact, this reactor allows enhancing the gas-solid contact, and therefore the catalytic activity by avoiding the elutriation of fine catalyst particles.Iron incorporation to olivine proved to be beneficial in the biomass steam gasification at zero time on stream, as it allowed not only reducing tar formation, but also improving syngas production and composition. Thus, gas production was increased from 1.30Nm3 kg−1 with calcined olivine to 1.46Nm3 kg−1 with Fe/olivine catalyst, and similarly did the hydrogen production, with the value being 6.25wt%. Likewise, tar concentration was reduced approximately to half, from 20.6 to 11.4gNm3. This was explained by the positive effect of metallic iron, which greatly favours WGS and light hydrocarbon steam reforming and cracking reactions. At zero time on stream, naphthalene was the most abundant tar compound for both the calcined olivine and Fe/olivine catalyst, although its concentration decreased to 42wt% with the latter.The evolution of the gasification performance and gas and tar compositions with time on stream was also monitored. The stability of the Fe/olivine catalyst was lower than that of calcined olivine, which was still stable after 140min on stream. Catalyst deactivation was especially evident based on the tar concentration, which increased from 10.4 to 19.9gNm−3, i.e., it almost reached the value obtained on the calcined olivine (20.6gNm−3). Other reaction indexes also showed the deterioration of the catalyst. Thus, gas and H2 productions declined from 1.46Nm3 kg−1 and 6.25wt% to 1.35Nm3 kg−1 and 5.44wt%, respectively, but still remained above those obtained with the calcined olivine, suggesting the activity of the deactivated Fe/olivine catalyst for WGS and steam reforming of CH4.The characterization techniques revealed that the catalyst deactivation was due to the oxidation of the metallic iron into Fe3O4. The presence of steam in the reactor for a few minutes before starting biomass feed may have induced the partial oxidation of the metallic phase at the beginning of the reactions. However, as biomass feed started, the reaction environment shifted from oxidizing to reducing conditions, and the iron that may have oxidized became reduced again. These changes in the iron oxidation state had a great influence on the catalytic properties of the Fe/olivine, and therefore on the evolution of tar removal, as well on WGS and light hydrocarbon reforming reactions.Although the experimental unit used in this study involves certain limitations for the operation with metallic catalysts during the start-up period, the results obtained shed light on the biomass steam gasification using Fe/olivine as primary catalyst in large-scale plants.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors report no declarations of interest.This work was carried out with financial support from the Spain's Ministries of Science, Innovation and Universities (<GN1>RTI2018-098283-J-I00</GN10> (MCIU/AEI/FEDER, UE) and Science and Innovation (PID2019-107357RB-I00 (MCI/AEI/FEDER, UE), the Basque Government (IT1218-19 and KK-2020/00107), and the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 823745. The authors also thank the technical and human support provided by SGIker from UPV/EHU and European funding (ERDF and ESF).

The performance of Fe/olivine catalysts was tested in the continuous steam gasification of sawdust in a bench scale plant provided with a fountain confined conical spouted bed reactor at 850°C. Olivine was used as catalyst support and loaded with 5wt%Fe. The activity and stability of the catalyst was monitored by nitrogen adsorption-desorption, X-ray fluorescence spectroscopy, temperature programmed reduction, X-ray diffraction and X-ray photoelectron spectroscopy techniques, which were conducted before and after the runs. The fountain confined conical spouted bed performs well in the biomass steam gasification with primary catalysts. In fact, this reactor allows enhancing the gas-solid contact, and therefore the catalytic activity by avoiding the elutriation of fine catalyst particles. The uncatalysed efficiency of the gasification process, assessed based on the gas production and composition, H2 production, tar concentration and composition, and carbon conversion efficiency, was consideraby improved on the Fe/olivine catalyst, with tar reduction being especially remarkable (to 10.4gNm−3). After 140min on stream, catalyst deactivation was particularly evident, as tar concentration increased to 19.9gNm−3 (90% of that without catalyst). However, Fe/olivine catalyst was still active for WGS and CH4 steam reforming reactions, with gas and H2 productions being 1.35Nm3 kg−1 and 5.44wt%, respectively. Metal iron oxidation to Fe3O4 caused catalyst deactivation, as the reaction environment shifted from oxidizing to reducing conditions due to operational limitations.

Single-atom catalysts (SACs), created by decorating isolated metal atoms or mononuclear metal complexes on appropriate supports, have received tremendous and continuous attention in chemistry and materials science. 1–9 In principle, SACs provide 100% metal dispersion on the surface and thus maximize the metal utilization, which is a special and ideal feature for developing efficient and low-cost heterogeneous catalysts, especially significant for the effective utilization of noble metals (e.g., Pt, Pd, Ru, and Ir). 10–12 Furthermore, the enormous possibilities of agilely pairing the single-atom metal centers with the host materials and the fine control of their local coordination environment give the great potentials to actualize many highly efficient SACs. 7 , 13 , 14 As an admitted bridge between homo- and heterogeneous catalysis, SACs render favorable features of homogeneous catalysts, including the uniformity of active sites and the tunable interactions with ligands while inheriting the high durability and excellent recoverability as heterogeneous catalysts. 15–17 Benefiting from these attributes, SACs have afforded us spectacular opportunities to develop efficient heterogeneous catalysts with competitive activity and selectivity to homogeneous counterparts and, importantly, to understand the fundamental mechanism of heterogeneous catalysis at the atomic and molecular levels. 5 , 18 , 19 After a decade of rapid progresses and breakthroughs, SACs have been engaged as promising platforms for chemical transformation and energy-related catalysis, including photocatalytic energy conversion to produce sustainable fuels and chemicals. 20–23 For the utilization of infinite and freely accessible solar energy, artificial photocatalytic energy conversion provides a promising strategy to overcome the global energy crisis and combat the increasingly erratic climate by reducing greenhouse gas emissions. 24 , 25 For example, photocatalytic water splitting is a technologically straightforward and cost-competitive avenue toward sustainable clean H2 fuel production, 26–28 while photocatalytic CO2 reduction represents a deployable and highly attractive strategy to convert inert CO2 into value-added chemicals and definitively close down the carbon cycle. 11 , 24 , 29–31 Nevertheless, the performance of the traditional photocatalytic systems, which greatly rely upon the energy band configuration and surface structure of the catalysts, is still far from satisfactory because of the sluggish separation of electron–hole pairs and limited surface-active sites. 32 , 33 Despite tremendous efforts, the heterogeneous photocatalysts heretofore still suffer from many deficiencies such as the fast charge carrier recombination and inefficient molecular activation, 34–37 thus significantly depressing the charge transfer from catalyst surface to reactant species and further transformation of molecules.In such circumstances, SACs have emerged as new options and notions for the design and fabrication of cost-effective and high-efficiency photocatalysts and cocatalysts. Advantageously, the isolated reactive centers in a single-atom photocatalytic system can not only create an increased number of active sites for photocatalytic reactions but also broaden the light-harvesting range and elevate the charge separation/transfer efficiency. 38–40 The as-constructed single-atom photocatalysts are highly configurable, providing sufficient light traps and fine modification of the surface structure to adsorb and activate molecules. 33 , 39 Furthermore, the structural simplicity of single-atom photocatalysts allows us to draw more precise structure-performance correlations, enabling the improved understanding of the fundamental mechanism of photocatalysis and thus promoting the rational design of targeted photocatalysts. Albeit research efforts on SACs applications in photocatalytic systems are still at their nascent stage, these advantages have made them up-and-coming candidates to facilitate the photocatalytic reactions. 30 , 35 , 41–43 In order to promote such a fledgling yet rapidly evolving field, timely overviews on the recent progress of single-atom photocatalysis are highly desirable to not only reveal the primary working mechanism but also inspire future research directions. As far as we know, although some excellent reviews with the emphasis on the background and synthetic strategies of SACs, as well as their unique advantages for photocatalysis, have emerged in the literature, the fundamental principles that are highly relevant to the energy band engineering and energy transfer route in single-atom photocatalysis have not been covered in the reported review articles. 33 , 39–41 In this review article, we herein focus on extracting the key principles from literature for the single-atom photocatalysis to understand the working mechanism comprehensively and thus promote the rational design and fabrication of more efficient single-atom photocatalysts. The review begins with a short presentation on the achievements and features of the photocatalytic application of SACs. Then, the synthetic strategies and related structural characterization methods for confirming the successful construction of single-atom photocatalysts have been concisely overviewed. More importantly, as shown in Figure 1 , we dedicatedly disclose the mechanism for the surface charge separation/transfer accelerated by isolated metal centers and also the adsorption and activation of molecules in single-atom photocatalysis by illustrating some representative examples. The applications of SACs for well known, and emerging photocatalytic reactions are also introduced with the most recent developments. Finally, we present some challenges and perspectives for the future development direction of SACs in photocatalytic energy conversion. It is highly expected that this review would deliver some new insights toward the understanding and engineering of SACs in photocatalysis and further accelerate the development of this important emerging research area.An irrefragable witness of the ever-growing interest in the topic of single-atom photocatalysis could be expressed by the exponentially increased number of publications and related citations from 2014 to 2020 and its sustained growth trend by 2021 (Figure 2 A). During the 7-year research period, the potential of SACs has been explored in plenty of photocatalytic applications, especially in sustainable energy conversion and small-molecule activation (Figure 2B). Figure 3 depicts the timeline for the key achievements and features related to single-atom photocatalysis. In 2014, Yang and coworkers have reported the first use of an SAC (0.2 Pt/TiO2) for photocatalytic hydrogen evolution, which exhibits better performance than those of metallic nanoparticles or clusters. 44 2 years later, Zhang and coworkers have demonstrated that the anchoring of single Co atoms in the metal-organic framework (MOF) could remarkably promote the photoexcited carrier separation in the MOF-525-Co photocatalyst and enhance the oriented migration of photoexcited electrons from support to metal centers, which also represents the first report for the use of SACs in the highly efficient photocatalytic CO2 reduction. 45 In 2016, Wu and coworkers have directly evidenced that a longer lifetime of photoexcited electrons can be provided from isolated Pt sites decorated g-C3N4 to improve the performance of photocatalytic hydrogen evolution significantly. 46 In 2017, Wei and coworkers have further discovered that the generation of mid-gap states in an isolated Co1-P4 site anchored g-C3N4 photocatalyst can increase the light-harvesting ability and provide separation centers to depress charge recombination and elongate carrier lifetime. 47 These pioneering achievements have greatly promoted and triggered the booming development of single-atom photocatalysis.Subsequently, researchers have demonstrated the bridging function of single-atom photocatalysts among heterogeneous, homogeneous, and even enzymatic photocatalysis for the photoactivation of molecules, for example, through the well-designed study of Co1-G 48 and Cu/TiO2 49 photocatalysts for the activation of CO2 and H2O, respectively. Meanwhile, a plenty of new strategies have also been discovered to expand the family members and related applications of single-atom photocatalysts, such as single-tungsten-atom-oxide (STAO) for photocatalytic pollutant degradation (2019) 50 and plasmonic Cu nanoparticle supported single-atom Ru for photothermal methane dry reforming (2020). 51 Furthermore, in 2020, Lou and coworkers have investigated the dynamic changes in chemical valence and coordination environment of isolated metal centers in their multi-edged TiO2 supported single-atom Ru photocatalyst through in situ extended X-ray absorption fine structure (EXAFS) technique. 52 This work opens new avenues toward the observation and interpretation of the intrinsic working mechanism for single-atom photocatalysis. Based on these great achievements in the past seven years, single-atom photocatalysts have been proved as excellent candidates for various energy conversion processes with high activity and selectivity (Table 1 ), including H2 evolution, 44 , 71–73 CO2 reduction, 45 , 62 , 74–76 N2 reduction, 63 , 77 , 78 C–O coupling, 65 pollutant degradation, 79 , 80 CH4 conversion, 68 H2O2 production, 69 as well as photocatalytic sensing. 70 , 81 , 82 As various synthetic strategies for SACs have been well summarized in other excellent reviews, 7 , 14 , 83 we here mainly focus on the preparation of semiconductive supports-based SACs for photocatalysis. Due to the high surface energy of the isolated atoms in SACs, the active sites in SACs are rather vulnerable to relocating and aggregating into clusters and nanoparticles, which brings additional difficulties in stabilizing the active sites in high loading contents. Some innovative strategies for fabricating single-atom photocatalysts to combat these challenges have been briefly introduced.To realize the homogeneous dispersion of isolated reactive centers on the semiconductive supports, creating sufficient distance and excellent distribution among every species of the metal precursor is an easy and efficient solution, which prevents the agglomeration of metal sites after post-treatment to form single-atom photocatalysts with a specific geometric environment. Moreover, surface modification with functionalities for the semiconductive supports before integrating with the metal precursor is quite necessary, providing adequate anchoring sites for stabilizing the single atoms. Based on these progresses, we have summarized some detailed approaches for the preparation of single-atom photocatalysts.Wet-chemical synthesis has been recognized as a facile and repeatable strategy for the synthesis of single-atom photocatalysts with well-defined mono-dispersity and deployable scale-up. In a typical process, the metal precursor (e.g., metal salt or mononuclear metal complex) is added to the semiconductive supports, followed by the reduction or activation steps to realize the decoration of isolated metal sites onto the semiconductor. For example, Chen et al. have decorated isolated Pt atoms (loading: 0.02 wt %) onto the defect-rich TiO2 support via the wet-deposition approach (Figure 4 A). 53 The Pt precursor (H2PtCl6) has been first added to the suspension containing sodium titanate nanotubes, followed by the calcination of the as-prepared mixture at 400°C to obtain the Pt single-atom photocatalyst. A plenty of transition-metal-based single photocatalysts (e.g., Pt, 46 Pd, 87 Ru, 52 Cu, 61 Ni, 65 and Co 88 ) have also been synthesized via the nearly identical strategy. However, the loading contents for the isolated metal atoms are commonly lower than 2 wt %, which should be attributed to the limited control of the distribution of active sites on the support surface, especially under the effect of capillary forces during the drying process. 7 The pyrolysis method is an essential strategy to obtain single-atom photocatalysts via the pyrolysis of the mixed precursors of metal and organic semiconductors (e.g., g-C3N4 89 ; MOFs 90 ) at relatively high temperatures. In general, a strong coordination bond between the metal sites and the organic precursor is required to prevent isolated metal sites from agglomeration during pyrolysis. For example, Jiang et al. have introduced Ga (0.014 ∼ 0.045 wt %) into g-C3N4 by direct pyrolysis of the mixture of GaCl3 and urea at 550°C (Figure 4B). 84 Through pyrolysis of the supramolecular precursor assembled by melamine, cyanuric acid, and silver citrate, Zou et al. have synthesized a Ag-N2C2/g-C3N4 photocatalyst with a high content of isolated Ag metal (3.7 wt %). 56 Coordination confinement has been believed as another effective approach for constructing single-atom photocatalysts in which the encapsulation of suitable mononuclear metal precursors with semiconductive supports can thus be realized. Due to the strong interaction between mononuclear metal and anchor sites from the porous supports, the agglomeration of isolated metal sites is largely avoided. Porous MOFs with suitable anchor sites have been developed as ideal semiconductive supports for the synthesis of single-atom photocatalysts via the coordination confinement strategy. Zhang et al. have directly incorporated coordinatively unsaturated Co atoms into a porphyrin-based MOF (MOF-525) to prepare a MOF-525-Co single-atom photocatalyst with the Co loading content of 6.01 wt % (Figure 4C). 45 Wang et al. have further stabilized a series of isolated metal atoms (i.e., Ir, 1.41 wt %; Pt, 2.74 wt %; Ru, 1.92 wt %; Au, 1.18 wt %; Pd, 3.68 wt %) on zirconium-porphyrinic MOF hollow nanotubes by the coordination between porphyrin units and metal atoms. 91 Moreover, Zuo et al. have achieved an ultrahigh loading content (12.0 wt %) of isolated Pt atoms on the ultrathin MOF nanosheets by assembling the linker of PtII tetrakis(4-carboxyphenyl)porphyrin (PtTCPP) and the metal nodes of Cu2(COO)4 paddle-wheel clusters (Figure 4D). 57 Apart from the above-mentioned approaches, some other specific avenues have also been developed to synthesize the single-atom photocatalysts. Atomic layer deposition (ALD) could be a controllable approach to synthesize single-atom photocatalysts via the layer-by-layer deposition of isolated metal sites onto the semiconductors. Cao et al. have decorated isolated Co metal sites (Co1-N4; loading: 1.0 wt %) onto the g-C3N4 by an ALD method. By using bis(cyclopentadienyl)cobalt as the ALD reagent and further removing the surface cyclopentadienyl ligand through O3 treatment, isolated Co atoms decorated g-C3N4 has been thus developed (Figure 4E). 85 By providing sufficient ionized anions and cations with a strong polarizing force for breaking the covalent, ionic, or metallic bonds in the liquid environment of melt salts, the molten-salt method (MSM) could be another powerful strategy for preparing single-atom photocatalysts. Xiao et al. have realized the decoration of isolated Ni atoms on the TiO2 photocatalyst through a controllable MSM (Figure 4F). 86 The molten salts (a mixture of LiCl and KCl) have provided liquid conditions and space confinement for the isolated dispersion of Ni species and promoted the generation of strong Ni–O bonds on TiO2. Furthermore, chemical etching can also be used to construct single-atom photocatalysts with relatively high metal loading. Zhang et al. have further provided an etching method to decorate isolated Ni atoms on the defect-rich zirconia support (Figure 4G). They have prepared Ni-Zr hydroxide from the Ni-Zr sol-gel. Subsequently, the porous Ni-Zr hydroxide has been calcinated and etched with dilute hydrochloric acid to obtain the single-atom photocatalysts with the nickel content varied from 2 to 8 wt %. 59 In addition to those main traditional characterizations for elucidating the physical properties (e.g, optical absorption and crystal properties), chemical composition, and band structures for photocatalysts, the dispersion of isolated reactive centers in single-atom photocatalysts requires a whole toolbox of complementary techniques. 32 , 92 Here, we mainly focus on the overview of structural characterizations of isolated reactive centers in single-atom photocatalysts by advanced techniques in electron microscopy and spectroscopy.Transmission electron microscopy (TEM) in atomic resolution has been developed as an effective approach for studying the detailed structural information of the isolated reactive centers, as well as their interactions with the supports. In particular, by using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM), we can easily confirm the existence of isolated reactive centers over the supports, provided the metal atom of interest shows a much higher atomic number than the support element. In addition, the attached energy-dispersive X-ray (EDX) detector of STEM could further provide the elemental mapping elucidating the atomic dispersion of the metal atoms on the semiconductive supports. 49 , 53 Although electron microscopy images provide us with effective information for identifying the structural information of the catalyst, we also need to notice that there are some limitations for its applications in structural characterizations. Electron microscopy can only image the structure in the local area and fail to provide overall structural information for the whole catalyst. Furthermore, due to the limited electron penetrability of microscopic techniques, isolated metal atoms decorated in the bulk phase or cavities instead of the surface structure are difficult to be observed. 28 Consequently, some additional spectroscopic methods are necessary to provide complementary data and underpin the presence of isolated metal sites in single-atom photocatalysts.X-ray absorption spectroscopy (XAS) is one of the most heavily used and powerful tools to characterize single-atom photocatalysts, including X-ray absorption near edge structure (XANES) spectroscopy and EXAFS. The XANES can deliver the local electronic state information for the probed elements, while the EXAFS provides notable details on the coordination environment and local geometric structure of isolated metal sites in high resolution. Combined with the density functional theory (DFT) simulations, well-defined structures of single-atom photocatalysts could be assigned through XAS observations. Cao et al. have forcefully demonstrated the formation of “Co-N4” in their isolated Co decorated g-C3N4 catalyst. 85 The relaxed bond lengths determined by the fitted EXAFS spectrum are extraordinarily close to those calculated by DFT from geometry optimization of simulated “Co1-N4” model in a 2 × 2 × 1 supercell of g-C3N4. As a site-specific characterization technique, Fourier-transform infrared (FTIR) spectroscopy has also been widely applied to evaluate the existence of isolated metal sites according to their significant shift compared with clusters or nanoparticles. 38 , 93 X-ray photoelectron spectroscopy (XPS) is another widely used technique to reveal the surface valence structure of single-atom photocatalysts. A distinct shift in binding energy compared with pure metal references may elucidate the oxidation state of isolated metal atoms and exclude the existence of nanoparticles. 7 , 94 Furthermore, Mössbauer spectroscopy could become another forceful tool to analyze the chemical state and coordination environment of Mössbauer-active elements (e.g., Fe and Sn). Noteworthily, Wu et al. have validated the existence of Fe(IV) species in a single-atom Fe-modified TiO2-SiO2 photocatalyst by employing the Mössbauer spectroscopy. 77 The initial step in the photocatalytic process is to harvest the incident photons with a specific frequency (hν) by the component of the semiconductor for the generation of photoexcited electron (e−) and hole (h+). 95 The energy band structures of single-atom photocatalysts, including the bandgap size and positions of the conduction band (CB) and valence band (VB), are the dominant elements to tune the light-harvesting ability and drive the redox reactions. 32 The introduction of isolated metal atoms could not only adjust the band structure to enhance the light absorption capability of semiconductive supports but also provide electron pumps to boost the transfer of photoexcited electrons, thus highly accelerating the surface charge separation/transfer in the single-atom photocatalytic systems.To fully utilize solar energy, it is of great importance to narrow down the required bandgap of photocatalysts for the generation of electron–hole pairs under visible-light excitation. Furthermore, the CB minimum (CBM) and VB maximum (VBM) positions of photocatalysts should well match the redox potentials of specific reactions to trigger the proceeding of the photocatalytic process. 32 , 39 Experimental and theoretical results have clearly revealed that the introduction of isolated metal sites could significantly modulate the energy band configuration of semiconductive host materials to highlight the light absorption ability and adjust their redox potentials.In principle, when the loading contents of isolated metal sites are in trace amount, limited effect on the inherent band structure of host semiconductors will emerge while some new electronic states are inclined to be generated to boost the light-harvesting ability of the entire photocatalyst (Figure 5 A). Xiong and coworkers have confirmed that the band structure of the isolated Pt (Pt loading: 0.18 wt %) decorated g-C3N4 photocatalyst is rather similar to that of pristine g-C3N4 by the Mott-Schottky plots measurement and ultraviolet photoelectron spectroscopy investigation (Figure 5B). 38 They have further demonstrated the generation of new hybrid states over the surface of the catalyst due to the metal-to-ligand charge transfer (MLCT) between the isolated atoms and the host organic semiconductor. The first-principle simulations further confirm the up-shift of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) positions in MLCT-induced hybrid state (Figure 5C). Under the simultaneous contributions of HOMO-to-LUMO transition in local g-C3N4 and MLCT in g-C3N4-Pt units, the single-atom photocatalyst exhibits enhanced broad-spectrum light-harvesting ability. This tentative conclusion has been further verified by other reports combined with the DFT calculations. 39 , 87 , 96 , 97 For example, Lou and coworkers have confirmed that the isolated Ru decoration can lead to the generation of new energetic states corresponding to the 4d orbitals of Ru near the Fermi levels of TiO2 (Figure 5D), which significantly improves the light-harvesting capability of the single-atom photocatalyst under visible-light irradiation. 52 With the increase of the doped metal contents to a relatively high amount, the band structure configuration of host semiconductor materials can be significantly changed (Figure 5A). For instance, Zhang and coworkers have reported that the inclusion of 2 ∼ 5 wt % of isolated Ni atoms in ZrO2 photocatalyst would directly narrow the bandgap of the as-synthesized Ni-SA-x/ZrO2 photocatalyst by an up-shifted VB level and a down-shifted CB level as demonstrated by both the experimental results and theoretical calculations (Figure 5E). 59 As revealed by diffuse reflectance spectroscopy, the Ni-SA-x/ZrO2 photocatalyst herein presents additional light absorption peaks in the range from 400 to 700 nm compared with the pristine ZrO2 (p-ZrO2) sample (Figure 5F). Furthermore, the modified CB potential of Ni-SA-5/ZrO2 highly satisfies the potential of CO2 reduction into CO reaction, making it an excellent SAC for selective CO2 photoreduction. This phenomenon has also been demonstrated by Wang and coworkers. 58 They have validated that the bandgap of zincblende cadmium-zinc sulfide quantum dots (ZCS QDs) could be gradually narrowed by the increase of Ni species in the Ni x ZCS QDs photocatalyst. Both CBM and VBM positions of the single-atom photocatalyst are tailored significantly by the decoration of Ni atoms (Figure 5G), which triggers excellent charge separation efficiency and photocatalytic activity for the hydrogen evolution. The modification of band structures for semiconductors by the decoration of isolated metal sites in high concentration is similar to those of traditional doping avenues with 3d-transition elements or nonmetal elements 32 ; however, the type of metal has limited influence on the modification, possibly due to the atomically isolated distribution of metals in single-atom photocatalysts. 98 To date, the mechanism of isolated metal sites decoration on modulating the band alignments of host semiconductor materials is increasingly evident but still exists contentions. Therefore, further experimental and theoretical investigations are highly demanded to reveal such a role of isolated metal sites, thus providing the guidance of rational design of single-atom photocatalysts for targeted reactions.The photogenerated electrons transfer from VB to unoccupied CB in the photo-excitation process, while the equivalent holes stay in the VB. The following migration of photoexcited carriers from the photocatalyst surface to the reactant molecules is crucial for redox reactions. Nevertheless, a fraction of electron–hole pairs recombines on the surface or in volume with the release of light or heat. 27 In this case, decorating the isolated metal atoms on the semiconductor supports could serve as electron pumps to accelerate the photogenerated electron–hole transfer, thus greatly speeding up the whole reaction process.Note that the Schottky barrier in a traditional particulate cocatalyst/semiconductor system can perform as an electron trap for photoreactions and prevent the photoexcited electrons from traveling back to the host semiconductor from the cocatalyst; however, it also blocks the extraction of photoexcited electrons and lowers down the efficiency of electron transfer and injection. 99 Distinctively, the covalent coordination between the isolated metal sites and host semiconductor materials makes the single-atom photocatalyst Schottky-barrier free, enabling the electron transfer to be unrestricted and more effective. 100 Benefiting from the modern characterization techniques, several electron pump models and local electron trap states induced by isolated metal sites have been found to reveal the charge separation/transfer manner in single-atom photocatalysts.For instance, Wu and coworkers have confirmed that the newly generated electron trap states near the CBM of g-C3N4 induced by MLCT between isolated Pt atoms and g-C3N4 could be the electron transfer and injection channels (Figure 6 A). 46 The isolated Pt sites coordinated with C/N atoms on g-C3N4 can function as electron pumps to elevate the charge separation/transfer efficiency (Figure 6B), resulting in longer-lived photogenerated electrons than those of Pt nanoparticles decorated g-C3N4, as revealed by an ultrafast transient absorption spectroscopy (Figure 6C). The as-synthesized catalysts thus provide more photoexcited electrons to engage in the reduction of H+. This typical electron pump model and unique MLCT-induced electron trap state have been demonstrated as general cases for the single-atom photocatalysts. 57 , 93 , 96 , 102–104 Furthermore, Zhang and coworkers have demonstrated that there would be an MLCT-induced electron trap state and an additional vacancy-related state in the single-atom photocatalyst with a defect-rich host semiconductor (Figure 6D). 100 These newly generated electron states could collectively and significantly promote the electron trapping capability and extend the lifetime of photogenerated electrons according to the results of photoluminescence emission spectra (Figures 6E and 6F). Moreover, the decoration of isolated metal sites could also form mid-gap states on the host semiconductor to promote charge separation and transfer. 39 , 105 , 106 For example, Wei and coworkers have verified the generation of mid-gap states by restricting isolated Co1-P4 sites on g-C3N4 (Figures 6G and 6H). 47 The presence of mid-gap states below the CB position improves the light-harvesting capability of the single-atom photocatalyst, which also provides charge separation centers to suppress the electron–hole recombination and prolong the photocarrier lifetimes, as demonstrated by transient open-circuit voltage decay measurements (Figure 6I).Moreover, the strength and efficiency of electron pumps can be further enhanced via the decoration of isolated metal sites onto the dyadic semiconductor composites such as the Z-scheme photocatalyst. 101 , 107 Recently, Peng and coworkers have demonstrated the first example of isolated PtN4 sites on a Z-scheme photocatalyst composed of Zn-/Pt-porphyrin conjugated polymer (ZnPtP-CP) and BiVO4 semiconductors (Figures 6J–6L). 101 The isolated Pt metal sites can perform as the strengthened electron pumps to trap more photogenerated electrons migration from Z-scheme of the ZnPtP-CP/BiVO4 composite and further transfer them to the reactant of H+. Femtosecond time-resolved fluorescence spectra further reveal that the charge separation efficiency is significantly enhanced over this kind of single-atom-metal-Z-scheme composite photocatalyst (Figure 6L).Upon the construction of enhanced electron pumps induced by isolated metal sites, the photogenerated electron–hole could be efficiently separated and transferred, also significantly promoting photocatalytic oxidation reactions (e.g., pollutant degradation and oxidation of organic compounds). For example, Yang et al. have implanted isolated Co atoms in the polymeric carbon nitride (pCN) for the efficient oxytetracycline degradation under visible light. 108 The introduction of isolated Co sites has significantly extended the light absorption area and electron density for the pCN, thus accelerating the charge separation/transfer and the degradation of oxytetracycline. The generation of reactive species (i.e., h+, 1O2, ⋅O2 −, and ⋅OH) facilitated by isolated Co sites triggers the catalyst to exhibit an apparent rate of 0.038 min−1 for the oxytetracycline degradation, outpacing the pristine pCN by about 3.7 times. Xiao et al. have further reported a promoted charge separation/transfer system for photocatalytic hydrogen evolution and benzene oxidation by introducing isolated Cu atoms to the C3N4 layers. 109 The isolated Cu sites coordinated with the N atoms in C3N4 have constructed efficient Cu-N x charge-transfer channels to promote in-plane and interlayer transfers of photoexcited electrons and holes. Consequently, the as-formed single-atom photocatalyst exhibits excellent performance for the hydrogen evolution as well as the oxidation of benzene into phenol with a conversion rate of 92.3% and the selectivity of 99.9% under visible light.It is evident that the decoration of isolated metal sites could improve the light-harvesting ability and accelerate the charge separation/transfer for photocatalytic systems, which offers us a fascinating strategy to elevate the utilization efficiency of photogenerated carriers. Thereafter, adsorption and activation of molecules over the single-atom photocatalyst hold the keys for driving the entire photocatalytic performance. Advantageously, the coordinatively isolated metal atoms have provided adequate unsaturated active sites on the catalyst for photocatalytic reactions.According to the Sabatier principle, the ideal catalytic site should have moderate bonding strength to the key intermediate in the reaction, which is not only strong to activate the intermediate but also facilitates the desorption of the product. 110 , 111 This principle is essentially related to the electronic structure of the catalysts. 112 In a single-atom photocatalyst, control of unsaturated active sites with diverse coordination environments to modulate the electronic structure supplies numerous opportunities to tune the adsorption energy of molecules or intermediates onto the catalyst surface. Combined with the progress of DFT calculations, researchers could further forecast the catalytic activity of particular active sites and thereby realize the theory-guided synthesis of efficient single-atom photocatalysts.As a typical example, Ma and coworkers have quantitatively assessed the adsorption energy of hydrogen over isolated Pt sites supported on carbon nitride with six coordination environments by following the DFT calculation (Figures 7A and 7B). 113 The theoretical calculations directly indicate that the coordination environments of unsaturated Pt atoms can modulate the local electronic configuration of the catalyst and thus influence the adsorption energy of hydrogen on the catalyst surface. DFT simulations further demonstrate that the hydrogen can be absorbed by the unsaturated Pt active sites and neighboring C atoms (Figure 7C). As the Pt-C4 moiety exhibits moderate bonding strength between hydrogen and unsaturated Pt sites, the as-constructed Pt-C4 cocatalyst doped CuS photocatalyst has well-balanced active sites for the proton adsorption and final desorption of H2, thus significantly upgrading the performance of photocatalytic H2 production from water (Figure 7D). This work also emphasizes the fact that the modulation of the coordination environment is crucial for stabilizing unsaturated metal sites and tuning the adsorption energy to achieve the desired photocatalytic activity.Moreover, the coordination environment modulation of the unsaturated active sites also significantly affects the electron density of the entire catalyst surface to improve the adsorption capacity of single-atom photocatalysts. Zhou and coworkers have demonstrated that adjusting the coordination environment of isolated Au atoms through changing the vacancy types in CdS could enhance the surface electron density of Au-anchored CdS photocatalyst (Figure 7E). 114 Higher electron density can be achieved on the surface of as-constructed Au/Cd1-x S photocatalyst because of the electron accumulation at Cd vacancies instead of unsaturated Au active sites. The bonding of CO2 molecules on the catalyst surface thus converts from physical adsorption into chemical adsorption, which significantly promotes the following photocatalytic CO2 reduction.Since the adsorption process of molecules or intermediates onto the catalyst surface is crucial for the single-atom photocatalysis, researchers have begun to evidence the enhancement in adsorption capacity induced by unsaturated active sites through the related adsorption-desorption experiments. Ozin and coworkers have investigated the effect of unsaturated Bi3+ active sites substitution on the adsorption capacity of In2O3 photocatalyst for CO2 molecules via the CO2 temperature-programmed desorption (TPD) analysis and in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). 115 CO2-TPD clearly reveals that the typical desorption peaks for CO2 and other related surface species move to higher temperatures after introducing unsaturated Bi3+ sites (Figure 7F), indicating stronger binding of these surface species over the Bi3+-anchored In2O3 photocatalysts (Bi x In 2–x O3). In situ DRIFTS has been further applied to observe the transition evolvement of surface species during the adsorption of CO2 molecules over Bi x In 2–x O3 and pure In2O3 (Figure 7G). The enhanced peak intensities for all surface species over Bi x In 2–x O3 compared with those over In2O3 imply the empowered adsorption-bonding-activating capacity for CO2 molecules after incorporating the unsaturated Bi active sites. Strikingly, these pieces of experimental evidence further confirm that the unsaturated active sites can serve as efficient promoters to elevate the adsorption capacity for molecules or intermediates over the single-atom photocatalysts.Upon the moderate adsorption of molecules onto the surface of the single-atom photocatalyst, the activation of reactant molecules over the unsaturated active sites by using the photogenerated carriers is the ultimate goal for the entire photocatalytic system. For the traditional heterogeneous photocatalytic system, the molecular activation mechanism is rather complicated due to the diversity of active centers and varied reaction paths. 99 Single-atom photocatalysis has provided us with a streamlined model to investigate the photoactivation process for reactant molecules, attributed to the structural simplicity of active centers. 116 That is to say, the initial activation process can be traced in the whole single-atom photocatalytic process, including photoexcited carriers separation/transfer, molecular adsorption, intermediates formation, and the final product desorption (Figure 8 A). On the basis of the above-mentioned advantages of emerging single-atom photocatalysts in promoting the key principles of the photocatalytic process, researchers have drawn several convincing photoactivation cycles, thus evidently proving the high activation capacity of unsaturated active sites for reactant molecules.For instance, Sun and coworkers have proposed a compelling photoactivation process for hydrogen evolution from water over the unsaturated GaN4 active sites supported on carbon nitride (Figure 8B). 84 As an efficient electron pump, GaN4 active sites capture and store the photogenerated electrons under light illumination, resulting in the negatively charged GaN4 sites preferring to absorb the positive protons. The absorbed protons further accept electrons from the charged GaN4 site and form the H∗, while the GaN4 sites return back to the ground state after releasing the captured electron. H∗ tends to desorb from the GaN4 site and generate H2 with other H∗ in the system. Upon the generation and release of the H2 product, the free GaN4 active site moves to the next reaction cycle (Figure 8B). For a more complex photocatalytic system like reduction of CO2 into CO, Ye and coworkers have also revealed the photoactivation cycle over unsaturated Co active sites decorated in covalent organic frameworks (sp2c-COFsdpy) by combining the in situ DRIFTS measurements and DFT calculations (Figures 8C–8E). 117 In situ DRIFTS study confirms the formation of absorbed CO2 (CO2∗) and COOH∗ as the surface species during the CO2 photoreduction (Figure 8C). DFT calculations directly reveal that the unsaturated Co active sites significantly decrease the Gibbs free energy of the rate-limiting reduction step of CO2∗ into COOH∗ to activate the CO2 molecules while depressing the process of hydrogen evolution to elevate the selectivity of CO2 photoreduction (Figure 8D). In the photoactivation cycle of CO2 reduction, the unsaturated Co active sites undergo the circulation between initial Co(II) and Co(I) to transport the photogenerated electron flow, thus upholding the selective reduction of CO2 into CO product through a proton-electron coupling path (Figure 8E). These reports demonstrate the structural advantages of unsaturated active sites in delivering the photoexcited carriers for the photoactivation of molecules.It is worth pointing out that the local coordination environment and chemical state of the unsaturated active sites would be dynamically changed in the photoactivation cycle. The related study can gain deep insight for us to access the intrinsic activation mechanism of single-atom photocatalysis. Hyeon and coworkers have evidenced that the valence state of unsaturated Cu active sites on TiO2 is changed after accepting the photoexcited electrons during the photoactivation process, which also triggers the activation of TiO2 support. 49 They have herein identified a reversible and cooperative photoactivation cycle at the atomic level for hydrogen evolution from water over the Cu/TiO2 photocatalyst (Figure 8F). That is, under the light excitation, the Cu/TiO2 is transferred from the initial resting state (CT0) to a photoexcited state (CT1) following by the generation of electrons and holes. The flow of photoexcited electrons from the CB of TiO2 to the d orbital of Cu results in the valence change of the Cu active site (CT2). The electron localization at Cu further triggers a polarization field, causing lattice distortion in adjacent TiO2 (CT3). CT3 as an active state manifests enhanced photocatalytic activity for H2 generation. Furthermore, the exposure of CT3 to O2 can revert it to the initial CT0 state and thus complete the photoactivation cycle. Such a systematic work further implies that the activation of water molecules over unsaturated active sites in single-atom photocatalysts could share the fundamental principles resembling enzymes and homogeneous catalysts. Moreover, Reisner and coworkers have recently observed that the reactivity for organic C–O coupling reaction over isolated Ni sites deposited on mesoporous carbon nitride (mpg-CN x ) shows a similar trend to a homogeneous catalytic system (Figure 8G). 65 The unsaturated Ni2+ active sites are converted into NiI species after receiving photogenerated electrons from mpg-CN x , thus providing coupling reaction centers in an activated state for the whole photoactivation cycle.Artificial photocatalytic hydrogen evolution is a feasible way to convert the infinite sunlight into the green and clean energy carrier of H2, providing a sustainable solution to overcome the global energy dilemmas and environmental problems. 26 , 27 , 52 As one of the “holy grail” reactions, photocatalytic water splitting has great potential to achieve economically practicable and scalable solar H2 production. 26 Although plenty of efforts are being dedicated to realizing excellent performance for the H2 evolution from water splitting, the desirable solar-to-hydrogen efficiency for viable application still suffers from the fast recombination of photogenerated electron–hole pairs and the inadequacy of surface-active sites. 37 , 118 To overcome these issues, noble-metal materials, like Pt nanoparticles, have been used as cocatalysts on host semiconductor photocatalysts to accelerate the charge separation/transfer and create sufficient active sites for the reaction, thus remarkably improving the photocatalytic performance. 27 , 95 However, the low atomic efficiency, high cost, and scarcity still hinder the practical use of noble-metal-particle-based cocatalysts. In this case, the development of single-atom photocatalysts could reduce the utilization of noble metal and offer new possibilities for enhancing the performance of photocatalytic water splitting. Statistical results for the published articles clearly demonstrate that ∼40% of single-atom photocatalysts have been applied in hydrogen evolution (Figure 2). Among them, host semiconducting materials such as TiO2 and g-C3N4 are most widely used as the supports, showing impressive improvement for the photocatalytic activity of hydrogen evolution.Since the discovery of water splitting on a TiO2 photoelectrode by Fujishima and Honda in 1972, semiconducting TiO2 has become the commonly used photocatalyst and still received continuous attention. 119–121 Yang and coworkers have first decorated the isolated Pt atoms onto TiO2 for H2 production from water splitting. 44 The isolated Pt atom-based TiO2 exhibits reduced H∗ adsorption energy and boosted active sites, thus giving better performance for photocatalytic H2 evolution than the Pt nanoparticle-based TiO2 photocatalyst. Subsequently, Liu and coworkers have further validated that the selective decoration of isolated Pt sites on (101) facet of TiO2 could achieve higher activity and more considerable stability for photocatalytic hydrogen evolution compared with the traditional Pt nanoparticle- or cluster-decorated TiO2 photocatalysts. 122 The defects and morphology structure of TiO2 have been finely engineered to enhance the intrinsic photocatalytic activity of TiO2-based single-atom photocatalysts. 90 , 104 , 123–125 Li and coworkers have fabricated a high-performance photocatalyst by assembling isolated Pt sites on a defect-rich anatase TiO2 support (Pt1/def-TiO2). A Pt–O–Ti3+ atomic interface is constructed after the decoration of isolated Pt atoms, which significantly promotes the charge separation/transfer and suppresses the recombination of electron–hole pairs. 53 The unique Pt1/def-TiO2 delivers record-high photocatalytic H2 evolution activity with an extremely high turnover frequency (TOF) of 51,423 h−1, outpacing the traditional particle-based Pt/TiO2 catalyst by 591 times. Schmuki and coworkers have further demonstrated that the high density of Ti3+-Ov defects on TiO2 nanotubes favors the stabilization of the isolated Ir atoms. 126 The resulting isolated Ir sites decorated TiO2 photocatalyst exhibits higher activity for H2 evolution than single atoms supported on other nanostructures in their system. Lou and coworkers have further decorated isolated Ru sites over multi-edged TiO2 spheres (ME-TiO2@Ru) for photocatalytic H2 production (Figures 9A–9C). 52 The sharp edges of TiO2 (Figure 9B), as revealed by STEM observations, have been confirmed to be conducive for the migration of photoexcited electrons from TiO2 to Ru sites. Consequently, the ME-TiO2@Ru shows a highly enhanced hydrogen evolution rate (Figure 9C), exceeding the TiO2 nanoparticles stabilized isolated Ru catalyst (TiO2@Ru) by 2.2 times. This work further highlights the importance of controlling the morphology of host semiconductor supports for improving the performance of single-atom photocatalysts.Moreover, non-noble transition metal atoms have also been introduced to fabricate TiO2-based single-atom photocatalysts, 86 , 127–129 which not only reduce the utilization of noble metals but also break the ultimate activity limit of noble-metal atom sites. Li and coworkers have successfully anchored isolated Co and Pt atoms on the surface of TiO2, which contains 13.4% oxygen-coordinated Co-O-Pt dimers (Figure 9D). 54 The dual-single-atom photocatalyst exhibits much better performance for H2 evolution than Pt single-atom and cluster-based photocatalysts (Figure 9E). They have confirmed that the Co-O-Pt dimer coupling enables the mutual optimization of electronic structure for Pt and Co centers to weaken the binding of H∗, which is the prime factor for breaking the ultimate activity ceiling of the Pt atom.As a representative organic semiconductor, two-dimensional g-C3N4 has also been used as an excellent host support for fabricating SACs, attributing to its abundant surface trapping sites for isolated atoms. Many types of isolated metal sites, including noble metal (e.g., Pt, 38 , 46 , 94 , 130–134 Pd, 55 , 87 , 102 Rh, 135 Au, 136 and Ag 56 , 137 ) and non-noble metal (e.g., Fe, 138 Co, 47 , 85 , 139 Ni, 97 , 140 Cu, 141 , 142 and Ga 84 ), have been decorated on g-C3N4 to synthesize single-atom photocatalysts for hydrogen evolution. Wu and coworkers have decorated isolated Pt atoms on g-C3N4 via a liquid-phase reaction of H2PtCl6 and g-C3N4 followed by the low-temperature annealing. The as-synthesized Pt-CN single-atom photocatalyst presents a longer lifetime of photoexcited electrons by the formation of surface Pt-N/C bonds, which results in boosted photocatalytic performance for H2 production, outpacing Pt nanoparticles and pristine g-C3N4 by 8.6 and 50 times, respectively. 46 Yu and coworkers have further demonstrated that the single-atom photocatalyst of Pd/g-C3N4 exhibits enhanced charge separation/transfer due to the introduction of isolated Pd sites, thus giving much better H2 evolution activity than the benchmarked Pt/g-C3N4. 87 During the rational design of g-C3N4-based single-atom photocatalysts for hydrogen evolution, researchers pay much attention to modulate the coordination configuration between unsaturated metal active sites and C/N atoms to enhance the light response, charge separation, and adsorption/activation processes, thus significantly elevating the photocatalytic performance. 55 , 56 , 134 , 140 Zou and coworkers have synthesized a single-atom photocatalyst of isolated Ag sites decorated g-C3N4 with a unique coordination configuration of Ag-N2C2 (Ag-N2C2/CN) through a facile supramolecular approach for H2 evolution (Figure 9F). 56 DFT simulations indicate that the C and N co-coordinated Ag sites exhibit better charge distribution than Ag nanoparticles (AgNPs) and isolated Ag-N4 sites (Figure 9G), which renders a faster transfer of photogenerated electrons from C3N4 to Ag and significantly reduces the energy barrier of H2 evolution. The Ag-N2C2/CN thus delivers much better photocatalytic activity than AgNP-, Ag-N4-, and Pt nanoparticle-based C3N4 photocatalysts (Figure 9H).Non-noble metals have also been applied to construct the g-C3N4-based single-atom photocatalysts for hydrogen evolution with impressive performance. Wei and coworkers have grafted isolated Co1-N4 sites on g-C3N4 nanosheets via ALD, and the formed photocatalyst delivers a superior H2 evolution rate of more than 10.8 μmol h−1, surpassing the pristine g-C3N4 by 11 times. 85 Theoretical investigations validate that the coordinated donor N atoms improve the electron density of the unsaturated Co active center and accelerate the formation of the key intermediate of Co hydride, hereby promoting the H–H coupling to speed up the H2 production. They have further fabricated a Co1-phosphide/PCN photocatalyst of isolated Co1-P4 sites decorated g-C3N4 by a simple phosphidation strategy, elevating the H2 production rate over 410.3 μmol h−1 g−1. 47 The newly generated Co1-P4 configuration leads to the generation of mid-gap state in Co1-phosphide/PCN, forcefully improving the light-harvesting capacity and inhibiting the recombination of electron–hole pairs. The unsaturated Co active sites coordinated with P atoms are more stable and favorable for the adsorption/activation of water molecules than the N sites in pristine g-C3N4. Moreover, the electric conductivity of g-C3N4 has been enhanced after P doping. These positive factors markedly promote the photocatalytic activity of Co1-phosphide/PCN for the hydrogen evolution from water splitting.Moreover, researchers have also developed various single-atom photocatalysts based on other semiconductor supports (i.e., metal sulfide 58 , 72 , 96 , 103 , 107 , 113 , 143–147 and MOFs 57 , 71 , 91 , 93 , 148–150 ) for hydrogen evolution. Wang and coworkers have decorated isolated Ni atoms onto the zinc-blende Cd–Zn sulfide quantum dots (ZCS QDs) with the finely tuned concentrations of Ni sites for photocatalytic H2 evolution (Figures 9I–K). 58 HAADF-STEM observations demonstrate that the shapes of as-synthesized Ni x ZCS QDs (x = 0.015625, 0.03125, 0.0625, and 0.125) vary from tetrahedra to truncated tetrahedra with the increase of Ni contents (Figure 9I). Atomically resolved HAADF-STEM investigations further demonstrate that the (111) and (110) facets are the only existed facets for Ni x ZCS QDs in the presence of Ni. The as-constructed surface junctions between the anisotropic (111) and (110) crystal plane in the same phase of Ni x ZCS QDs significantly reinforce the charge carrier separation/transfer and result in enhanced electronic conductivity by the built-in electric field (Figure 9J), leading to optimized surface hydrogen adsorption thermodynamics. The best-in-class Ni0.03125ZCS QDs catalyst herein exhibits an ultrahigh H2 production rate of 18.87 mmol h−1 g−1 under sunlight (Figure 9K). Jiang and coworkers have anchored isolated Pt atoms into the Al-based porphyrinic MOF of ((AlOH)2H2TCPP (H2TCPP = 4,4′,4″,4′″-(porphyrin-5,10,15,20-tetrayl)tetrabenzoate), simplified as Al-TCPP) for photocatalytic hydrogen evolution. 93 The Al-TCPP has affluent coordination sites for the implantation of Pt atoms, thus providing efficient channels for the electron transfer from the MOF photosensitizer to the unsaturated Pt active sites. The as-synthesized Al-TCPP-Pt photocatalyst herein exhibits outstanding activity for H2 evolution with a TOF of 35 h−1, outperforming the Pt nanoparticle catalyst stabilized by the same MOF by ∼30 times.Photocatalytic reduction of CO2 into fuels and value-added chemicals by harvesting solar energy has delivered one of the ideal blueprints for reducing the concentration of atmospheric CO2 greenhouse gas in an ecofriendly manner. 11 , 24 Major ongoing research of photocatalytic CO2 reduction is to create efficient catalysts for overcoming the sluggish kinetics and speeding up the conversion rate of the linear CO2 molecule. 29 , 114 Due to the distinctive advantages for adsorption and activation of molecules, single-atom photocatalysts have been engaged as promising candidates for photocatalytic CO2 reduction. 48 , 89 , 151–155 Ye and coworkers have first applied SACs for photocatalytic CO2 reduction in 2016. 45 By introducing the isolated Co atoms into the porphyrin units of the MOF-525, SACs with unsaturated Co sites (MOF-525-Co) have thus been constructed. The as-synthesized catalyst exhibits a CO production rate of 200.6 μmol g−1 h−1 and a CH4 evolution rate of 36.67 μmol g−1 h−1, exceeding that of the parent MOF by 3.13 and 5.93-fold, respectively. The authors have attributed the greatly improved photocatalytic activity of MOF-525-Co to the boosted separation efficiency of electron–hole pairs in porphyrin units after the Co introduction. Since then, many research works of photocatalytic CO2 reduction into CO or CH4 fuel gas have been reported over various single-atom photocatalysts along with the production of H2, where H2O provides the hydrogen resource. 59 , 74–76 , 114 , 155–166 Li et al. have created a unique Fe-N4O site supported on nitrogen-rich carbon for photocatalytic CO2 reduction via a facile top-down strategy (Figures 10A–10C). 167 The EXAFS observation reveals that the isolated Fe sites exist in high valence due to the construction of a coordinated Fe-N4O configuration (Figure 10B). These optimized unsaturated Fe active sites significantly facilitate the adsorption of CO2 molecules and accelerate the generation of key intermediate COOH∗, which promotes the photocatalytic system rendering a stable turnover number of 1,494 within 1 h and an excellent selectivity of 86.7% for CO generation (Figure 10C).Lou and coworkers have recently decorated isolated Co atoms onto the W18O49 ultrathin nanowires (W18O49@Co) via surface modification engineering (Figure 10D). 88 The inclusion of Co active sites has modulated the band structure of the W18O49 support, thus enhancing the redox capability of photogenerated electrons and promoting the charge separation/transfer for the CO2 reduction system. Furthermore, DFT calculations confirm that the anchored Co sites have modified the surface of W18O49@Co to enhance the adsorption of CO2 molecules (Figure 10E). Benefiting from these positive factors, the unsaturated Co sites could work as reaction switches in a tandem system of photocatalytic reduction with [Ru(bpy)3]Cl2·6H2O (bpy = 2,2′-bipyridine) serving as the photosensitizer, which results in high activity with a considerable CO production rate of 21.18 mmol g−1 h−1 and a hydrogen evolution rate of 6.49 mmol g−1 h−1 (Figure 10F).In addition to producing the fuel gases, the photocatalytic CO2 reduction over single-atom photocatalysts can also achieve the generation of value-added chemicals, such as methanol and ethanol. 60 , 61 , 168 Yu and coworkers have presented that the isolated Cu atoms-modified g-C3N4 can provide unsaturated C-Cu-N2 active sites for the reduction of CO2 into various C1 products, including CH4, CO, and methanol. 60 Zbořil and coworkers have also constructed a g-C3N4-based single-atom photocatalyst with Ru-N/C active sites for reusable reduction of CO2 into methanol, which can yield 1,500 μmol of methanol per gram of catalyst upon six hours of the photocatalytic reaction. 168 Wang and coworkers have decorated the isolated Cu sites into the UiO-66-NH2 MOF through a photoinduction strategy (Figure 10G). The as-synthesized catalyst could trigger the solar-driven reduction of CO2 into methanol with a yield rate of 5.33 μmol h−1 g−1 and ethanol with a generation rate of 4.22 μmol h−1 g−1 (Figure 10H). 61 DFT calculations directly unravel that the inclusion of isolated Cu atoms on the UiO-66-NH2 significantly reduces the formation barrier of both CHO∗ and CO∗ during the CO2 reduction (Figure 10I), thus resulting in impressive selectivity for methanol and ethanol. Moreover, photocatalytic conversion of CO2 into C2+ product over single-atom photocatalyst has also been reported recently. By introducing isolated Mo atoms into 2,2′-bipyridine-based COF, Kou et al. have constructed a Mo-COF single-atom photocatalyst with a coordination configuration of MoN2, which could reduce CO2 into C2H4 product with a yield rate of 3.57 μmol h−1 g−1 and the selectivity of 42.92% under visible light. 62 In situ FTIR investigation and theoretical calculation have directly demonstrated that the inclusion of isolated MoN2 sites in COF could enhance the adsorption/activation of CO2 molecules and further promote the hydrogenation process for the generation of C2H4.Furthermore, researchers have also achieved the gas-phase CO2 photoreduction over single-atom photocatalysts, with H2 serving as a hydrogen source. Ozin and coworkers have constructed surface frustrated Lewis pairs (SFLPs) on defective In2O3 through the isomorphic substitution of isolated Bi sites for Lewis acidic In3+ sites in In2O3, enabling enhanced activity for gas-phase CO2 photocatalysis. 115 The isolated Bi3+ sites (Bi’), oxygen vacancies ([O]v), coordinately unsaturated In sites (In’) below the CB of In2O3, and the oxygen states (O′) above the VB of In2O3 could serve as mid-gap states (comprising SFLPs) for trapping photoexcited electrons and holes, respectively, greatly inhibiting the recombination of electron–hole pairs and driving the reaction between H2 and CO2 (Figure 10J). Herein, benefiting from the enhanced charge separation/transfer and also the adsorption capacity, the as-constructed Bi x In2-x O3 photocatalyst shows an excellent CO evolution rate higher than that of pristine In2O3 by three orders of magnitude (Figure 10K), while also renders remarkable reactivity toward photocatalytic methanol production (Figure 10L). This work further exemplifies the distinct advantage of decoration of isolated metal sites for the atom-scale photocatalyst engineering toward the adsorption and activation of inert molecules.The ammonia (NH3) synthesis through the well-known Haber-Bosch process has artificially provided essential nitrogen sources for various N-containing reactions relating to human development, including the production of fertilizer, drugs, and fine chemicals. 169 However, this process requires harsh temperature (400°C–500°C) and pressure (10–30 Mpa) due to the great activation barrier of nonpolar N≡N in N2 molecule, which consumes ∼2% of the world’s energy production and generates ∼3% of the CO2 emissions worldwide per year. 170 It is thus highly demanded for researchers to explore alternative pathways to produce NH3 via the fixation of N2. Recently, artificial photocatalytic N2 reduction under ambient conditions has offered a more sustainable approach beyond the traditional Haber-Bosch process. 171 Single-atom photocatalysts have thus become emerging platforms for the photocatalytic N2 reduction reaction. 63 , 78 , 172 , 173 Xie and coworkers have first reported a g-C3N4-based single Cu photocatalyst (Cu–CN) for N2 reduction, which achieves an NH3 yield rate of 186 μmol g−1 h−1 and quantum efficiency of 1.01% under light illumination at 420 nm. 63 The coordination between isolated Cu atoms and edged N atoms in g-C3N4 results in extra active lone-pair electrons on the g-C3N4 support and enhanced adsorption capacity of unsaturated Cu active sites, thus leading to an impressive performance for the activation of N2 molecules. Zhong and coworkers have anchored isolated Pt atoms in the covalent triazine framework (CTF) nanosheets with a coordinated configuration of Pt-N3 for photocatalytic N2 reduction. 64 Due to the introduction of isolated Pt sites, the modified CB position of CTF is higher than the electrode potential of N2/NH3, enabling the single-atom photocatalyst more thermodynamically feasible to accelerate the reaction (Figure 11 A). The as-constructed Pt-SACs/CTF photocatalyst thus exhibits a high NH3 yield rate of 171.4 μmol g−1 h−1 without the addition of sacrificial agents (Figure 11B). The isotopic labeling experiments further confirm the formation of NH3 from photocatalytic N2 reduction (Figure 11C).Apart from organic semiconductor materials, inorganic semiconductors have also been applied to design single-atom photocatalysts for photocatalytic N2 reduction. 77 , 100 , 171 , 174 For example, Niu et al. have decorated isolated Ru atoms on defect-rich TiO2 nanotubes for N2 fixation (Figure 11D). 100 The typical charge-transfer state induced by ligand (TiO2)-to-metal (Ru) has greatly promoted the transfer of photogenerated electrons and turned the Ru active site into a strong electron pump (Figure 11E). As a result, the single-atom photocatalyst exhibits higher efficiency than anchored Ru nanoparticles and pristine TiO2 nanotubes (Figure 11F). Recently, Wu et al. have further reported the elevated photocatalytic activity over macro/mesoporous TiO2-SiO2 supported Fe sites for N2 reduction (Figure 11G), which gives an NH3 yield rate of 32 μmol g−1 h−1 in the absence of any sacrificial agents and cocatalysts (Figure 11H). 77 Mössbauer spectroscopy directly confirms that the generation of photoexcited hole-trapping polarons around the Fe active sites has resulted in the formation of high-valent Fe(IV) species (Figure 11I), which significantly promotes the N2 reduction on the adjacent oxygen vacancy. Moreover, Zhang and coworkers have developed a single-atom photocatalyst of electron-rich Cu δ+ sites and oxygen vacancies co-decorated Zn-Al layered double hydroxide nanosheets via a simple coprecipitation method (Figure 11J). 174 The unsaturated Cu δ+ active sites and oxygen vacancies greatly enhance the charge separation/transfer and the adsorption of N2 molecules (Figure 11K), thus resulting in optimized activity for photocatalytic N2 reduction with a durable NH3 yield rate of 110 μmol g−1 h−1 in pure water under UV-vis excitation (Figure 11L).Solar-driven organic synthesis over homogeneous catalysts (e.g., metal complexes and organic photosensitizers) represents one of the most sustainable and economic strategies to produce valuable organic compounds. 175 However, the high cost and nonrecyclability of homogeneous catalysts greatly hamper the viable application of photocatalytic organic reactions. To this end, the construction of efficient heterogeneous catalysts for photocatalytic organic synthesis has received wide attention from chemists. 65 , 176 Considering the bridging function between homo- and heterogeneous catalysis, single-atom photocatalysts have been regarded as the promising candidates to achieve selective and recyclable photocatalytic organic synthesis. 16 , 175 , 177 Wang and coworkers have confirmed that the isolated Ag sites can serve as effective photocatalytic active centers in their AgF/visible-light system for selective hydrodehalogenation and dehalogenation-arylation of various organic halides without any organic additives (Figure 12 A). 178 The photolysis of AgF results in the in situ generation of both AgNPs and single-atom Ag (SAAg) sites. Under the visible-light excitation, AgNPs can serve as light-harvesting units attributed to the localized surface plasmon resonance, while SAAg provides surface-active sites for the anchoring of activated halides and driving the photocatalytic reactions. Based on the robust synergy between AgNPs and SAAg, the TOF of deiodination-phenylation reaction over isolated Ag center can reach 6,000 h−1 under mild conditions. Yang et al. have loaded isolated Ni atoms onto TiO2 supports for selective sulfonation of enamides into amidosulfones with yields of 99% for 33 examples (Figure 12B). 176 The as-synthesized Ni/TiO2 photocatalyst can selectively realize the formation of α-amidosulfones and β-propionamidosulfones with considerable recyclability and a high turnover number over 18,963, which also exhibits high tolerance of functional groups and enables easy gram-scale reaction with considerable efficiency.Organic semiconductor supports have also been applied to construct single-atom photocatalysts for organic synthesis. 109 , 175 , 179 , 182 Wang et al. have anchored isolated Co atoms on carbon quantum dots (CoSAS@CD) with a coordinated Co-N4 structure through straightforward hydrolysis of vitamin B12 in sodium hydroxide solution. The carbon dots serve as both the photosensitizer and the support for isolated Co sites. The elevated visible-light adsorption capacity and charge separation/transfer by the synergy between the Co atoms and carbon quantum dots enable the CoSAS@CD with excellent oxidation ability, exhibiting an oxygen evolution rate over 168 μmol h−1 g−1 for water oxidation and imine synthesis with a high conversion of ∼90% and selectivity over 99% (Figure 12C). 179 Song and coworkers have assembled isolated Ni active sites onto the g-C3N4 with imidazole auxiliary ligand for C–O cross-coupling reactions under visible light, which could efficiently realize the etherification of various aryl bromides with alcohols/water, giving high turnover numbers up to 500 (Figure 12D). 180 The dispersion of isolated Ni sites can be well maintained without aggregation after the photocatalytic reaction and be reused with sustained activity, indicating the excellent stability and recyclability of SACs in photocatalytic organic synthesis. Reisner and coworkers have further emphasized that the single-atom photocatalyst of Ni active sites deposited on mesoporous g-C3N4 is a robust, earth-abundant, low-cost, and heterogeneous easily recyclable platform for achieving C–O coupling reactions between aryl halides and aliphatic alcohols under the mild condition without the addition of external ligands, which exhibits a reactivity trend paralleling to that of homogeneous catalysts (Figure 12E). 65 In addition, the use of single-atom photocatalysts for biomass reforming to produce value-added organic chemicals has emerged recently. 66 , 181 Guo and coworkers have demonstrated that the single-atom Pt-loaded commercial P25-TiO2 (Figure 12F) exhibits superior activity for photocatalytic reforming of acetone into highly value-added 2,5-hexanedione (HDN) and H2 with a selectivity of 93%. 162 The HDN production rate can be optimized to 3.87 mmol g−1 h−1 over the as-synthesized Pt/P25-TiO2 photocatalyst, outpacing those of other precious isolated metals (Ru, Rh, and Ir) or Pt nanoparticle-loaded photocatalysts by at least 13 times (Figure 12G). Furthermore, they have developed partially reduced Pd-P3 active sites on CdS nanorods (PdPSA-CdS) for solar-driven reforming of bioethanol into hydrogen and a C6 compound of 1,1-diethoxyethane, exhibiting a photocatalytic generation rate of 35.1 mmol g−1 h−1 and a selectivity of nearly 100% (Figure 12H). 66 By the investigations of in situ attenuated total reflection-infrared spectra combined with the theoretical simulation of reaction pathway, the reinforced electron transfer from ethanol molecule to the unoccupied P 3p and Pd 4d states has been demonstrated as the origin for the highly effective activation of C–H and O–H bonds over the PdPSA-CdS photocatalyst.Organic pollutants, including industrial dyes, agrochemicals, and pharmaceuticals, have undesirable hazards on the water environment and potentially severe danger to the creatures. 183–185 It is urgent to explore efficient approaches for removing these contaminants, thus achieving a sustainable planet. The traditional removal of organic pollutants mainly relies on biodegradation and solid-adsorption separation process, which still suffer from kinetic inertness and low processing capacity for trace contaminants. 186 Photocatalytic degradation has provided an intriguing avenue to dispose of these pollutants in trace concentration by harvesting solar energy and promoting the generation of radical oxygen species. 108 , 187 , 188 Single-atom photocatalysts have also been demonstrated as efficient candidates for photocatalytic pollutant degradation due to the sufficient surface-active sites and accelerated charge separation/transfer.Zhao and coworkers have decorated isolated Ag atoms onto the mesoporous g-C3N4 for the photocatalytic degradation of bisphenol A (BPA) with the addition of peroxymonosulfate (PMS) under visible light (Figures 13A–13C). 189 The introduction of Ag active sites promotes the capture of photons for g-C3N4 in the visible-light range by narrowing the bandgap of g-C3N4. The suitable match of energy level between Ag and g-C3N4 results in a quick transfer of photoexcited electrons while the addition of PMS accelerates the separation of electron–hole pairs (Figure 13A), leading to a 100% degradation of BPA over 0.1 g L−1 of photocatalysts within 1 h (Figure 13B). Electron spin resonance spectra (Figure 13C) and quenched experiments for free radicals directly confirm that the construction of SAAg/g-C3N4 photocatalyst could promote the generation of radical oxygen species (i.e., sulfate radicals, superoxide radicals, and photogenerated holes), significantly accelerating the photocatalytic degradation of BPA.Dong et al. have developed laminar COF-909(Cu) nanorods for the efficient solar-driven degradation of sulfamethoxazole (Figures 13D and 13E). 190 The decorated Cu active sites enhance the light absorption capability of COF and elevate the separation of electron–hole pairs (Figure 13D). The COF-909(Cu) further provides abundant binding sites for adsorbing target molecules in the channels of COF, which delivers state-of-the-art photocatalytic activity for the sulfamethoxazole degradation with a high kinetic constant of 0.133 min−1, exceeding the pristine COF-909 and commercial TiO2-P25 by 27 and 40 times, respectively (Figure 13E).Furthermore, Wang et al. have demonstrated the construction of a STAO with W6+ and W5+ sites for the visible-light photocatalytic degradation of various dyes (e.g., dimethyl yellow and methyl red). 50 Spherical aberration (Cs) corrected STEM-HAADF observation directly reveals the monodispersed nature of STAO (Figure 13F). The combinations of the XPS, EXAFS, and electron energy loss spectroscopy clearly confirm the distorted octahedron structure of STAO with W6+ and W5+ deriving from the coordinated configuration of W-O6 (Figure 13G). The as-synthesized photocatalyst exhibits an impressive and stable photocatalytic degradation rate of 0.24 s−1, outpacing various photocatalysts by two orders of magnitudes (Figure 13H). Based on the systematical experimental results and theoretical calculations, the unsaturated W5+ sites in STAO are demonstrated as the real active centers, which enable the efficient transfer of photogenerated electrons from HOMO to LUMO +1 state to drive the entire photocatalytic degradation reaction (Figure 13I). The as-synthesized STAO has also enriched the family members of single-atom photocatalysts.Duo to the highly configurable structure, single-atom photocatalysts have also shown great application potentials in some other photocatalytic systems involving NO x removal, 67 , 191–193 CH4 conversion, 51 , 68 , 194 H2O2 production, 69 , 98 , 195 , 196 as well as photocatalytic sensing. 70 , 81 By sharing the accelerated charge separation/transfer and enhanced capacity for adsorption/activation of reactants, these single-atom photocatalytic reactions also show appealing activity, selectivity, and durability.Liu et al. have developed Pd-N3 active sites by the decoration of isolated Pd atoms onto the g-C3N4 support with tunable carbon defects (Figure 14 A), which exhibit outstanding and stable activity for photocatalytic NO removal, exceeding the pristine g-C3N4 by 4.4 times (Figure 14B). 67 Steady-state PL measurements confirm the accelerated separation/transfer of photoexcited carriers after the introduction of isolated Pd metal sites (Figure 14C). The as-constructed Pd-Cv-CN catalyst thus provides sufficient photoelectrons for the formation of ·O2 − and OH−, resulting in highly selective conversion of NO gas into NO3 −.In the case of photocatalytic CH4 conversion, Chen et al. have recently loaded isolated Nb atoms into hierarchical macro-mesoporous TiO2-SiO2 for constructing a single-atom photocatalyst (Figure 14D), which exhibits an optimal conversion rate of 3.57 μmol g−1 h−1 with considerable recyclability for non-oxidative coupling of CH4 (Figure 14E). 68 The dopant of isolated Nb atoms has facilitated the charge separation and elevated the electron mobility, which is significantly beneficial to the activation of methane and the desorption of ethane product. They have further emphasized that the Nb, Mo, W, Ta dopants on TiO2-SiO2 exhibit much better activity for CH4 conversion than Cu, Ga, Fe dopants, as shown in Figure 14F.The applications of SACs have been further extended for photocatalytic H2O2 production. Ohno and coworkers have developed an efficient single-atom photocatalyst of isolated Sb atoms decorated g-C3N4 (Sb-SAPC) through a wet-chemical method from the precursor of NaSbF6 and melamine. 69 The fitted EXAFS spectrum and the DFT simulation reveal that each Sb atom on average coordinates with 3.3N atoms in a typical Sb-SAPC15 catalyst (Figure 14G), directly indicating the isolated dispersion of unsaturated Sb sites over g-C3N4. A solar-to-chemical conversion efficiency up to 0.61% (Figure 14H) can be achieved over an optimized Sb-SAPC15 catalyst for photocatalytic H2O2 production from a two-electron oxygen reduction reaction (ORR). The isolated Sb active sites can deeply trap the photogenerated electrons, serving as the photoreduction centers for two-electron ORR (Figure 14I). Meanwhile, the adjacent N atoms enable the accumulation of holes, which significantly facilitates the kinetics of water oxidation. The collaborative effect between isolated Sb sites and coordinated N atoms thus dramatically accelerates the overall photocatalytic reaction.Guo and coworkers have recently applied the isolated Pd atoms decorated TiO2 catalyst (Pd/TiO2) as a photocatalytic sensing platform for the highly selective detection of the organophosphorus pesticide chlorpyrifos. 70 The inhibition effect of chlorpyrifos on the activity of photocatalytic hydrogen evolution over Pd/TiO2 has served as the stable detection sensor for the chlorpyrifos molecules (Figures 14J and 14K), which results in an extremely low detection limit of 0.01 ng mL−1 (Figure 14L). This work opens up new insights for the development of biosensing strategies and extends the application of single-atom photocatalysts.We have summarized the key principles of SACs and their versatile applications for photocatalysis. It has no doubt that single-atom photocatalysts are excellent brand-new candidates to construct efficient photocatalytic systems due to the accelerated charge separation/transfer efficiency and enhanced molecular adsorption/activation capacity. The advantages of single-atom photocatalysts are highly conducive to promoting photocatalytic activities of the photocatalytic reactions and thus covering a greater application range. Apart from the previous outstanding achievements, there are still many challenges in the exploration and practical uses of single-atom photocatalysts, including how to achieve the long-term stability and high loading of isolated reactive centers. Some future research directions and deployable solutions to overcome the ongoing challenges are also proposed as follows: (1) The relatively low stability of single-atom photocatalysts is one obvious drawback for photocatalysis. 197 The poisoning effect derived from the strong bonding of the reaction intermediate or by-product onto the isolated metal sites may deactivate the single-atom photocatalysts. Additionally, the photogenerated electrons induced conversion of isolated reactive centers into zero-valence metal atoms may cause the aggregation of isolated metal sites and the formation of clusters or nanoparticles. 40 Enhancing the metal-support interaction and optimizing photocatalytic reaction under ideal conditions may somewhat prevent the aggregation of isolated metal sites during the reaction. Moreover, the preparation of nonmetal-based SACs for photocatalysis with low cost and effective modulation for substrates may provide another intriguing solution to realize the improvement of both reactivity and stability. Although there are no related reports yet, it is highly expected that the synergetic interaction between nonmetal atoms and semiconductive supports may significantly prevent the migration of active sites and thus further explore the intrinsic activity of the single-atom photocatalysts. (2) Although the energy conversion efficiency over single-atom photocatalysts has shown a distinguished advantage for the average reactive centers, their overall performances are still far from satisfactory because of the low densities of unsaturated active sites. Developing single-atom photocatalysts with high loading of metal atoms favors elevating the densities of both photoexcited electron pumps and photocatalytic active sites. However, due to the high surface energy, isolated metal sites are rather apt to relocate and aggregate into clusters or even nanoparticles during the synthetic process. 14 And thus, the surface structure of the host semiconductive supports should be further modified to boost the meta-support interaction for increasing their loading amount. In addition to the creation of cation/anion vacancies in the surface modification process, introducing sufficient anchor sites (e.g., N, P, and S) or grafting specific functional groups (e.g., pyridine and −NH2) could also be applied to provide abundant binding sites for stabilizing the isolated atoms. (3) The precise control of the coordination geometry of the reactive centers and the number of isolated atoms is crucial to tune the activity and selectivity of single-atom photocatalysts but still remains as one great challenge. Accurate control of the interactions between the isolated reactive centers and semiconductive supports during the synthetic process is necessary for constructing the desired configuration, including the invention of a modular strategy to guarantee the geometry of implanted reactive centers. Furthermore, developing synthesis strategies based on the theoretical predictions of the structure-performance relationship may also provide a feasible approach. Furthermore, as the current XAS investigation still exhibits a significant error (∼20%) in confirming the coordination number for isolated atoms, 17 it is highly desirable to improve the accuracy of the characterization strategies to clarify the coordination configurations of single-atom photocatalysts in practice. (4) Exploring the synergetic interaction between two neighboring monomers has great potential to manipulate the catalytic properties and deepen the mechanistic understanding of heterogeneous catalysis. The controllable synthesis of photocatalysts with dual-single-atoms or multi-single-atoms should be thus considered. Due to the synergistic effect among adjacent reactive centers in the dual or multisingle-atom photocatalysts, the reaction paths for the reactant may be greatly modified, and it may thus result in the significantly reduced reaction barrier energy and obviously improved catalytic performance. Moreover, the construction of dual- or multi-SACs will further enrich the family members of single-atom photocatalysts. (5) Photocatalytic and enzymic systems in nature have provided a delicate blueprint for solar-to-chemical energy conversion, which generally exhibit prominent activity and selectivity under ambient conditions. From a structural aspect, isolated atom sites in SACs are highly analogous to the simplest active sites in enzymes. By mimicking the configurations of active centers such as enzyme catalytic pockets in natural photosynthetic reactions, the higher-level biomimetic design may be achieved to further release the overall potential of SACs for photocatalytic energy conversion. The integrations of terminal functional ligands or additional active sites around the isolated metal sites could be deployable strategies for developing these bioinspired single-atom photocatalysts. The related investigation will also gain a deep understanding of natural photosynthetic processes. (6) Large numbers of single-atom photocatalysts have been developed based on defect-laden semiconductor materials containing abundant surface binding sites for the coordinated metal atoms, thus achieving the enhanced charge separation/transfer. However, a high concentration of defect may deteriorate the crystallinity of host semiconductor materials and thus increase the massive recombination of electron–hole pairs in the volume or on the surface of single-atom photocatalysts. Plenty of research has focused on the distribution and coordination configuration of single-atom active sites while ignoring the structural control of semiconductor supports. Therefore, the structures (e.g., crystallinity, defects) of host semiconductors in single-atom photocatalysts are suggested to be controlled meticulously due to the above trade-off, their contribution to the whole reaction process should be further identified, thereby realizing the innovation in catalyst design and a fresh round of elevation for the photocatalytic activity. (7) The full interpretation of the charge separation/transfer process in single-atom photocatalysts induced by isolated metal sites is still challenging. Ultrafast transient absorption spectroscopy, particularly combined with electrochemical or microscopy techniques, is a powerful strategy to study the energy transfer and trapping processes in photocatalytic reactions. Developing real-time ultrafast transient absorption techniques to track the dynamics of photoexcited carriers in photocatalysis will further advance the understanding of electron pump models and electron trap states induced by the isolated metal sites, thus elucidating the deeply intrinsic mechanism of charge transfer and energy conversion in the photocatalytic systems. (8) Tracking the structural evolvements of active sites during the photocatalytic reactions provides not only the deep insight into the single-atom photoactivation process but also the guidelines for the rational design of effective photocatalysts. However, the related experimental pieces of evidence are still highly limited to understand the molecular adsorption/activation mechanism over SACs in photocatalytic systems. In situ or operando investigations combining with various techniques such as Raman spectroscopy, XAS, and XPS could be excellent approaches to detecting the dynamic changes of chemical state and coordinated environment of the isolated metal sites during the reactions. The development and application of these technologies in single-atom photocatalysis may provide more holistic trails for structural evolution, which are highly associated with the adsorption and activation of molecules over the unsaturated metal active sites. (9) Theoretical calculation combining with experimental results has become a robust strategy to investigate the electronic structure of catalysts and the molecular adsorption/activation in the photocatalytic process, forcefully revealing the working mechanism of single-atom photocatalysis on the atomic scale. However, the theoretical modeling of dynamic changes of active sites over the photocatalytic reactions is rather limited to achieve a rational cognition of the single-atom photoactivation cycle. The combinations of different simulation methods, such as time-dependent DFT and molecular dynamics, may provide deployable and reasonable approaches for exploring the evolution of single-atom photocatalyst during the reactions and revealing the photocatalytic activation mechanisms. (10) As one of the most forceful parts of artificial intelligence, machine learning based on computer algorithms enables fast and reliable predictions through data mining, which shows great potential in exploring high-efficiency catalysts. The application of SACs has covered a wide range of photocatalytic reactions, while the possibility of undiscovered single-atom photocatalysts is boundless, resulting in infinite combinations of structure–activity relationship and great challenges in the rational design of single-atom photocatalysts. It is thus highly desirable to explore appropriate machine learning methods integrating with theoretical calculation data to predict the catalytic performance of single-atom photocatalysts and figure out the ideal configuration for targeted reactions. The implementation of machine learning will also construct reliant structure-performance relationships for the photocatalytic reactions, thereby enhancing the understanding of single-atom photocatalysis and promoting the rational development of efficient single-atom photocatalysts with highly applicable potential. The relatively low stability of single-atom photocatalysts is one obvious drawback for photocatalysis. 197 The poisoning effect derived from the strong bonding of the reaction intermediate or by-product onto the isolated metal sites may deactivate the single-atom photocatalysts. Additionally, the photogenerated electrons induced conversion of isolated reactive centers into zero-valence metal atoms may cause the aggregation of isolated metal sites and the formation of clusters or nanoparticles. 40 Enhancing the metal-support interaction and optimizing photocatalytic reaction under ideal conditions may somewhat prevent the aggregation of isolated metal sites during the reaction. Moreover, the preparation of nonmetal-based SACs for photocatalysis with low cost and effective modulation for substrates may provide another intriguing solution to realize the improvement of both reactivity and stability. Although there are no related reports yet, it is highly expected that the synergetic interaction between nonmetal atoms and semiconductive supports may significantly prevent the migration of active sites and thus further explore the intrinsic activity of the single-atom photocatalysts.Although the energy conversion efficiency over single-atom photocatalysts has shown a distinguished advantage for the average reactive centers, their overall performances are still far from satisfactory because of the low densities of unsaturated active sites. Developing single-atom photocatalysts with high loading of metal atoms favors elevating the densities of both photoexcited electron pumps and photocatalytic active sites. However, due to the high surface energy, isolated metal sites are rather apt to relocate and aggregate into clusters or even nanoparticles during the synthetic process. 14 And thus, the surface structure of the host semiconductive supports should be further modified to boost the meta-support interaction for increasing their loading amount. In addition to the creation of cation/anion vacancies in the surface modification process, introducing sufficient anchor sites (e.g., N, P, and S) or grafting specific functional groups (e.g., pyridine and −NH2) could also be applied to provide abundant binding sites for stabilizing the isolated atoms.The precise control of the coordination geometry of the reactive centers and the number of isolated atoms is crucial to tune the activity and selectivity of single-atom photocatalysts but still remains as one great challenge. Accurate control of the interactions between the isolated reactive centers and semiconductive supports during the synthetic process is necessary for constructing the desired configuration, including the invention of a modular strategy to guarantee the geometry of implanted reactive centers. Furthermore, developing synthesis strategies based on the theoretical predictions of the structure-performance relationship may also provide a feasible approach. Furthermore, as the current XAS investigation still exhibits a significant error (∼20%) in confirming the coordination number for isolated atoms, 17 it is highly desirable to improve the accuracy of the characterization strategies to clarify the coordination configurations of single-atom photocatalysts in practice.Exploring the synergetic interaction between two neighboring monomers has great potential to manipulate the catalytic properties and deepen the mechanistic understanding of heterogeneous catalysis. The controllable synthesis of photocatalysts with dual-single-atoms or multi-single-atoms should be thus considered. Due to the synergistic effect among adjacent reactive centers in the dual or multisingle-atom photocatalysts, the reaction paths for the reactant may be greatly modified, and it may thus result in the significantly reduced reaction barrier energy and obviously improved catalytic performance. Moreover, the construction of dual- or multi-SACs will further enrich the family members of single-atom photocatalysts.Photocatalytic and enzymic systems in nature have provided a delicate blueprint for solar-to-chemical energy conversion, which generally exhibit prominent activity and selectivity under ambient conditions. From a structural aspect, isolated atom sites in SACs are highly analogous to the simplest active sites in enzymes. By mimicking the configurations of active centers such as enzyme catalytic pockets in natural photosynthetic reactions, the higher-level biomimetic design may be achieved to further release the overall potential of SACs for photocatalytic energy conversion. The integrations of terminal functional ligands or additional active sites around the isolated metal sites could be deployable strategies for developing these bioinspired single-atom photocatalysts. The related investigation will also gain a deep understanding of natural photosynthetic processes.Large numbers of single-atom photocatalysts have been developed based on defect-laden semiconductor materials containing abundant surface binding sites for the coordinated metal atoms, thus achieving the enhanced charge separation/transfer. However, a high concentration of defect may deteriorate the crystallinity of host semiconductor materials and thus increase the massive recombination of electron–hole pairs in the volume or on the surface of single-atom photocatalysts. Plenty of research has focused on the distribution and coordination configuration of single-atom active sites while ignoring the structural control of semiconductor supports. Therefore, the structures (e.g., crystallinity, defects) of host semiconductors in single-atom photocatalysts are suggested to be controlled meticulously due to the above trade-off, their contribution to the whole reaction process should be further identified, thereby realizing the innovation in catalyst design and a fresh round of elevation for the photocatalytic activity.The full interpretation of the charge separation/transfer process in single-atom photocatalysts induced by isolated metal sites is still challenging. Ultrafast transient absorption spectroscopy, particularly combined with electrochemical or microscopy techniques, is a powerful strategy to study the energy transfer and trapping processes in photocatalytic reactions. Developing real-time ultrafast transient absorption techniques to track the dynamics of photoexcited carriers in photocatalysis will further advance the understanding of electron pump models and electron trap states induced by the isolated metal sites, thus elucidating the deeply intrinsic mechanism of charge transfer and energy conversion in the photocatalytic systems.Tracking the structural evolvements of active sites during the photocatalytic reactions provides not only the deep insight into the single-atom photoactivation process but also the guidelines for the rational design of effective photocatalysts. However, the related experimental pieces of evidence are still highly limited to understand the molecular adsorption/activation mechanism over SACs in photocatalytic systems. In situ or operando investigations combining with various techniques such as Raman spectroscopy, XAS, and XPS could be excellent approaches to detecting the dynamic changes of chemical state and coordinated environment of the isolated metal sites during the reactions. The development and application of these technologies in single-atom photocatalysis may provide more holistic trails for structural evolution, which are highly associated with the adsorption and activation of molecules over the unsaturated metal active sites.Theoretical calculation combining with experimental results has become a robust strategy to investigate the electronic structure of catalysts and the molecular adsorption/activation in the photocatalytic process, forcefully revealing the working mechanism of single-atom photocatalysis on the atomic scale. However, the theoretical modeling of dynamic changes of active sites over the photocatalytic reactions is rather limited to achieve a rational cognition of the single-atom photoactivation cycle. The combinations of different simulation methods, such as time-dependent DFT and molecular dynamics, may provide deployable and reasonable approaches for exploring the evolution of single-atom photocatalyst during the reactions and revealing the photocatalytic activation mechanisms.As one of the most forceful parts of artificial intelligence, machine learning based on computer algorithms enables fast and reliable predictions through data mining, which shows great potential in exploring high-efficiency catalysts. The application of SACs has covered a wide range of photocatalytic reactions, while the possibility of undiscovered single-atom photocatalysts is boundless, resulting in infinite combinations of structure–activity relationship and great challenges in the rational design of single-atom photocatalysts. It is thus highly desirable to explore appropriate machine learning methods integrating with theoretical calculation data to predict the catalytic performance of single-atom photocatalysts and figure out the ideal configuration for targeted reactions. The implementation of machine learning will also construct reliant structure-performance relationships for the photocatalytic reactions, thereby enhancing the understanding of single-atom photocatalysis and promoting the rational development of efficient single-atom photocatalysts with highly applicable potential.This work received financial support from the King Abdullah University of Science and Technology (KAUST). X.W.L. acknowledges funding support from the Ministry of Education of Singapore via the Academic Research Fund (AcRF) Tier-2 grant (MOE2019-T2-2-049).The authors declare no competing interests.

Artificial photocatalytic energy conversion represents a highly intriguing strategy for solving the energy crisis and environmental problems by directly harvesting solar energy. The development of efficient photocatalysts is the central task for pushing the real-world application of photocatalytic reactions. Due to the maximum atomic utilization efficiency and distinct advantages of outstanding catalytic activity, single-atom catalysts (SACs) have emerged as promising candidates for photocatalysts. In the current review, recent progresses and challenges on SACs for photocatalytic energy conversion systems are presented. Fundamental principles focusing on charge separation/transfer and molecular adsorption/activation for the single-atom photocatalysis are systemically explored. We outline how the isolated reactive sites facilitate the photogenerated electron–hole transfer and promote the construction of efficient photoactivation cycles. The widespread adoption of SACs in diverse photocatalytic reactions is also comprehensively introduced. By presenting these advances and addressing some future challenges with potential solutions related to the integral development of photocatalysis over SACs, we expect to shed some light on the forthcoming research of SACs for photocatalytic energy conversion.

With the growing attachment to the environmental issues, the production of clean gasoline has become a major assignment in petroleum refining industry. To this end, the strict gasoline quality standards have been formulated by countries all over the world, limiting the contents of olefins and aromatics in gasoline. The decrease in the contents of high-octane olefins and aromatics will unavoidably cause the great reduction of gasoline octane number. n-alkane hydroisomerization is a process that converts low-octane straight chain paraffin into their high-octane branched alkanes (Qin et al., 2011; Samad et al., 2016; Kim et al., 2018; Jaroszewska et al., 2019). In this process, bifunctional catalysts containing metal sites and acid sites are often adopted in alkane hydroisomerization, and the metal sites provide the (de)hydrogenation active sites and acid sites provide skeletal isomerization active sites (Kim et al., 2013; Liu et al., 2015; Yu et al., 2020). In general, the metal sites are usually supplied by precious metals (such as platinum and palladium) (Lv et al., 2018; Oenema et al., 2020.), non-precious metals (such as nickel and MoO x ) (Yang et al., 2019; Harmel et al., 2020) and bimetallic nanoparticles (such as Pt–Ni and Pt–Ni2P) (Eswaramoorthi and Lingappan, 2003; Yao et al., 2015), and the acid sites are normally offered by silica-alumina zeolites (such as ZSM-22, Beta, ZSM-5 and Y) (Chi et al., 2013; Jin et al., 2009; Lee et al., 2013; Sazama et al., 2018), silicoaluminophosphate (SAPO-n) (Zhao et al., 2020; Guo et al., 2021; Wen et al., 2021), metal oxides (such as SO4 2−/ZrO2) (Kimura, 2003) and mesoporous materials (such as AlSBA-15 and Al-TUD-1) (Kang et al., 2021; Vedachalam et al., 2021). Among them, Pt has an excellent hydrogenation/dehydrogenation activity, and SAPO-11 possesses mild acidity and an appropriate pore structure for different processes (Sánchez-Contador et al., 2018). Hence, the Pt/SAPO-11 catalysts are extensively used on alkane hydroisomerization (Zhang et al., 2018a). However, mono-branched (Mo) isomers instead of multi-branched (Mu) isomers are the principal products for alkane hydroisomerization over Pt/SAPO-11 due to the small micropore size (0.39 × 0.63 nm) and external surface area (ESA) of conventional SAPO-11. The Mu isomers with higher octane numbers than the Mo isomers are optimal components to improve gasoline octane number (Liu et al., 2014). Researches show that the hierarchical SAPO-11 is able to effectively improve the selectivity to Mu isomers (Nandan et al., 2014; Zhao et al., 2019). Thus, it is of great significance to synthesize SAPO-11 molecular sieve with a hierarchical structure.There are many methods to synthesize hierarchical molecular sieves, such as desilication or dealumination, hard template strategy and soft template strategy (Jacobsen et al., 2000; Fan et al., 2012; Xi et al., 2014; Chen et al., 2016). However, in desilication or dealumination, the amount of removed silicon or aluminum cannot be controlled, and the excessive removal of framework silicon or aluminum will affect the crystallinity and stability of molecular sieves (Cartlidge et al., 1989; Groen et al., 2004). The soft template strategy is limited because the soft templates have a competition with the micropore templates to degrade the crystallization of molecular sieves and cause more environmental pollution in the process of removing soft templates. For the synthesis of hierarchical molecular sieves with a hard template strategy, the process is usually simple and the hard templates used do not interfere with the micropore templates. Thereby, the synthesis of hierarchical molecular sieves using hard templates has aroused extensive attention. Carbon materials, as a kind of hard templates, are commonly applied in the synthesis of hierarchical molecular sieves (Jacobsen et al., 2000; Christensen et al., 2005). However, the phase separation, which exists between carbon materials and synthesis precursor gel of hierarchical molecular sieves, is easy to occur, and which is attributed to the weak hydrophilicity of carbon materials (Machoke et al., 2015). To improve the hydrophilicity of carbon materials. Zhao et al. (2019) prepared carbon nanoparticles with abundant -C-O-C- groups by calcining a mixture of urea and polyethylene oxide in N2, and the -C-O-C- groups were converted into the -C-O-H groups in the alkali synthetic solution of hierarchical ZSM-5 molecular sieve. The existence of -C-O-H groups enhances the interaction between the carbon nanoparticles and ZSM-5 synthesis gel, and thus the hierarchical ZSM-5 with large mesopore size was successfully obtained. Bértolo et al. (2014) adopted a new process to obtain the hierarchical SAPO-11 through the use of commercial Merck carbon and Merck carbon with the treatment of nitric acid as mesoporogen, respectively. Merck carbon treated with nitric acid has more amount of oxygenate surface groups compared to the commercial Merck carbon, but the ESA and mesopore volume of the hierarchical SAPO-11 using Merck carbon with the treatment of nitric acid as the template are almost the same as those of the hierarchical SAPO-11 obtained by employing Merck carbon as mesoporogen.Metal-organic frameworks (MOFs) have aroused considerable attention because of their diversity of pore structures and high specific surface areas (Yang et al., 2022; Zhang et al., 2022) over recent years. The composites with the uniform mixture of carbon and metal oxides can be prepared through the heat treatment of MOFs (Li et al., 2022; Liu et al., 2022). Al-MOF-96, a typical porous aluminum based metal organic framework material, has the characteristics of simple synthesis and large-scale production (Deng and Peng, 2019), which can be used to prepare the composites with the uniform mixture of carbon and Al2O3. Additionally, Al2O3 can interact with phosphoric acid, thereby forming the aluminophosphate layers (Zhang et al., 2018b). Thus, compared with conventional carbon materials, the Al2O3/C composite has better hydrophilicity in the phosphoric acid solution. However, the hierarchical molecular sieves prepared by using the metal oxide/carbon composite derived from MOFs as the mesoporogen has not been reported.In this work, a novel Al2O3/C composite derived from Al-MOF-96 was used to synthesize hierarchical SAPO-11. The above synthesized hierarchical SAPO-11 has more mesopores, a greater ESA and a higher number of medium Brønsted acid centers (MBAC) than the conventional SAPO-11 (S-11) and the hierarchical SAPO-11 obtained employing activated carbon as the mesoporogen (CS-11). As a result, its corresponding catalyst displays enhanced selectivity to branched C10 isomers and low cracking selectivity in the n-decane hydroisomerization.Pseudoboehmite (73.0 wt% PB) was purchased from Shandong Aluminum Plant. Tetraethoxysilane (99.0 wt% TEOS), trimesic acid (98 wt% C6H3(CO2H)3), di-n-propylamine (99.5 wt% DPA), phosphoric acid (85.0 wt% H3PO4) and activated carbon (AR 200 mesh) were purchased from Aladdin. n-decane (98 wt% C10H22), aluminum nitrate (99 wt% Al(NO3)3·9H2O) and chloroplatinic acid (37.0 wt% H2PtCl6·6H2O) were provided by Innochem. Deionized water (H2O) was made in the laboratory. All reagents were employed directly as purchased.The synthesis procedures of conventional SAPO-11 were consistent with literature (Wen et al., 2021), which is shown as detailed below: firstly, 12.2 g of H3PO4 and 40.0 g of H2O were mixed thoroughly, followed by adding 8.1 g of PB and evenly stirring for 3 h. Subsequently, 4.7 g of TEOS and 6.8 g of DPA were added to the suspension in turn and stirred for 3 h to afford the mixture with the composition of 40 H2O: 0.95 P2O5: 1.2 DPA: 0.4 TEOS: 1.0 Al2O3. Finally, the above synthesis mixture was poured into a 100 mL autoclave to crystallize at 200 °C for 24 h. The resultant sample was obtained after washing, drying and calcination at 600 °C for 6 h for the removal of DPA, and it was denoted as S-11.The preparation procedures of hierarchical SAPO-11 with Al2O3/C as mesoporogen were as follows. First, Al-MOF-96 was synthesized through the method described in literature (Liu et al., 2015), and then the Al2O3/C composite was obtained after the calcination of Al-MOF-96 at 600 °C for 2 h in N2. Subsequently, 1.5 g of Al2O3/C composite was added to the mixture with the composition of 12.2 g of H3PO4 and 40.0 g of H2O and mixed thoroughly for 4 h. Afterwards, 7.0 g of PB and 4.7 g of TEOS were added in turn and evenly mixed for 5 h. Whereafter, 6.8 g of DPA was dropwise added to the solution and mixed for 1 h. Finally, the resulting mixture was put into a 100 mL autoclave and heated at 200 °C for 24 h. The as-synthesized sample was washed, dried and calcined at 600 °C for 6 h to remove DPA and mesoporogen, and it was denoted as ACS-11.The preparation procedures of hierarchical SAPO-11 employing activated carbon as mesoporogen were same as that of ACS-11 except that the activated carbon was utilized as mesoporogen rather than Al2O3/C. The obtained hierarchical SAPO-11 was named CS-11.Pt/SAPO-11 catalysts with the platinum mass fraction of 0.5% were obtained via incipient wetness impregnation. First, S-11, CS-11 and ACS-11 were pressed, and then they were sieved to 20–40 mesh. Subsequently, these catalysts were prepared by adding a solution of H2PtCl6 to the shaped SAPO-11 samples. Finally, these catalysts were got after drying and calcination at 450 °C for 4 h.A D8 Advance X-ray diffractometer (XRD) produced by Bruker AXS was used to analysis the sample structure (a Cu Kα radiation, 40 kV and 40 mA). A scanning electron microscopy (SEM) was performed to analyze the sizes and morphological properties using a SU8010. An ASAP 2420 physical adsorption equipment was adopted to perform the N2 adsorption-desorption measurement. The micropore volume (V micro) and ESA of the sample were obtained based on the t-plot method (Lippens and Boer, 1965). The specific surface area (S BET) and the pore size distribution of the sample were obtained according to the Brunauer-Emmett-Teller method and the Barrett-Joyner-Halenda method individually (Barrett et al., 1951).A F20 transmission electron microscopy (TEM) was adopted to obtain the TEM photos and the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) photos of samples. A NICOLE-750 infrared instrument was adopted to measure the acid properties of the samples. The disk sample with 0.02 g was heated at 400 °C for 1 h, then the disk absorbed pyridine vapor for 0.25 h at room temperature. Afterwards, the vacuum desorption of pyridine was carried out at 200 °C and then 300 °C. Eventually, the pyridine infrared (Py-IR) spectrums at 200 and 300 °C were obtained at room temperature. X-ray fluorescence spectroscopy (XRF) was carried out on a ZSX-100e spectrometer to detect the elemental compositions of samples. An Avance III spectrometer with a 4 mm probe head was employed to analyze the Si coordinated environment of the synthesized samples, and the 29Si spectra were recorded at 79.5 MHz and a recycle delay of 3 s. The 29Si shifts are referenced to 3-(trimethylsilyl)-1-propanesulfonic acid sodium salt. H2 chemical adsorption was carried out to analyze the Pt dispersions of the catalysts by the Auto Chem II 2920 equipment.n-decane hydroisomerization was performed in a fixed-bed microreactor. First, the catalysts were reduced at 400 °C, 2.0 MPa and a H2 flow rate of 50 mL/min for 4 h. Afterwards, n-decane hydroisomerization was carried out at 340 °C, 2.0 MPa, a H2/n-decane volume ratio (HDVR) of 400 and different weight hourly space velocities (WHSVs) of 2.0–25 h−1. The products were qualitatively confirmed using a Trace 1310 mass spectrometry. What is more, the n-decane hydroisomerization products were analyzed adopting an SP3420 gas chromatograph, which contains an HP-PONA column (50 m × 0.25 mm) and an FID detector.The n-decane conversion, the selectivity to total branched C10 isomers (S T), the selectivity to multi-branched C10 isomers (S Mu), the cracking selectivity (S C), the total branched C10 isomers yield (Y T) and the multi-branched C10 isomers yield (Y Mu) were calculated as follows: (1) C o n v e r s i o n = C r - C P C r × 100 % (2) S T = C T C r - C P × 100 % (3) S Mu = C Mu C r - C P × 100 % (4) S C = C C C r - C P × 100 % (5) Y T = C T C r × 100 % (6) Y Mu = C Mu C r × 100 % where C r represents the concentration of n-decane in the feedstock; C P and C T represent the n-decane concentration and total branched C10 isomers concentration in the products individually; C Mu represents the multi-branched C10 isomers concentrations in products, and C C is cracking products concentration in products.n-decane hydroisomerization was considered as a pseudo-first-order reaction (Wen et al., 2017). The rate constant (k) was calculated as detailed below: (7) k = F q ln ( 1 1 − c ) where F represents the n-decane feed rate in mol s−1, q and c represent the quality of catalyst (g) and n-decane conversion individually.The formula adopted to calculate the catalyst turnover frequency (TOF) was as follows (Guo et al., 2013): (8) T O F = f C B where f refers to the amount of reacted n-decane per gram of catalyst per second (mol/g/s). C B represents the numbers of MSBAC in the per catalyst quality (mol/g). Fig. 1 displays the XRD spectra of S-11, CS-11 and ACS-11. All samples appear characteristic peaks at 2θ = 8.1°, 9.5°, 13.1°, 15.7°, 20.4°, 21.1° and 22.2–23.3°, which are ascribed to the AEL topology of SAPO-11 (Phienluphon et al., 2015; Wen et al., 2017). The XRD results show that the addition of activated carbon or Al2O3/C in the synthesis of SAPO-11 does not change the crystal structure of SAPO-11. Additionally, the XRD patterns of the synthesized SAPO-11 samples appear a weak peak at 2θ = 6.6° attributed to SAPO-41, which is commonly observed in SAPO-11 (Guo et al., 2013; Wen et al., 2021). In the synthesis of SAPO-11, the aluminum phosphate (AlPO4) precursors are first produced, and then Si substitution in the AlPO4 framework occurs. During this process, the slow formation rate of AlPO4 precursors and the fast releasing rate of silicon species lead to a change in the composition of partial liquid phases, which results in the formation of SAPO-41 (Ren et al., 1991; López et al., 1997). Fig. 2 presents the nitrogen adsorption-desorption isotherms of S-11, CS-11 and ACS-11. CS-11 and ACS-11 exhibit obvious hysteresis loops while S-11 presents a small hysteresis loop in the range of relative pressure (P/P 0) = 0.4–0.9, showing that there are more mesopores in CS-11 and ACS-11. According to the SEM results (Fig. S1), the addition of activated carbon or Al2O3/C could hinder the aggregation between SAPO-11 crystals, so CS-11 and ACS-11 have more intercrystal mesopores in comparison to S-11.The pore diameter distributions of S-11, CS-11 and ACS-11 are given in Fig. 3 . All samples present a peak at 3.8 nm, which is caused by the tensile strength effect (Danilina et al., 2010). In addition, CS-11 and ACS-11 present high intensity peaks at 6.0 and 6.5 nm, respectively, and S-11 presents a low intensity peak at 5.5 nm, indicating that CS-11 and ACS-11 have abundant mesopores while S-11 has a relatively low amount of mesopores. Additionally, the texture properties of S-11, CS-11 and ACS-11 are exhibited in Table 1 . The mesopore volume and ESA of these samples rise in the order S-11 < CS-11 < ACS-11. In addition, as shown in the TEM images (Fig. S2), ACS-11 has much more mesopores than S-11 and CS-11, which is in consistence with the nitrogen adsorption-desorption result.In order to explain the Al2O3/C function in the synthesis of hierarchical ACS-11, the Al2O3/C composite derived from Al-MOF-96 was treated with phosphoric acid according to the following procedures: Al2O3/C (1.5 g) was added to the solution of H2O (40 g) and phosphoric acid (12.2 g) and stirred for 4 h. Afterwards, the mixture was put into the autoclave and maintained at 200 °C for 24 h. The sample named H3PO4–Al2O3/C was collected by washing and drying. And the H3PO4–Al2O3/C and Al2O3/C were characterized by XRD. Fig. 4 displays the XRD spectra of H3PO4–Al2O3/C and Al2O3/C. Al2O3/C shows no diffraction peaks, while H3PO4–Al2O3/C presents several characteristic peaks at 2θ = 20–50°, which is in line with the PDF standard card of AlPO4 (PDF # 71–1041). The above results indicate that Al2O3 has an interaction with phosphoric acid, forming the AlPO4 structure in the process of treating Al2O3/C with phosphoric acid. Additionally, Fig. S3 displays the HADDF-STEM result of Al2O3/C, Al2O3 is uniformly doped in carbon material.SAPO-11 crystals are easy to aggregate with each other due to the absence of mesoporogen in the synthesis of S-11. As a result, SAPO-11 with large crystallites is obtained (Liu et al., 2014). For CS-11, activated carbon as mesoporogen is dispersed between SAPO-11 crystals, which reduces the contact of these crystals. As a consequence, CS-11 has smaller crystallites and more mesopores than S-11 (Yu et al., 2021). However, activated carbon has weak hydrophilicity, leading to the phase separation between SAPO-11 gel and activated carbon, and thus activated carbon plays an inferior role as mesoporogen. During the preparation of hierarchical ACS-11, Al2O3 doped in carbon material has an interaction with phosphoric acid, forming AlPO4 structure. Thereby, the phase separation between Al2O3/C and SAPO-11 gel is avoided in the synthesis of ACS-11, and Al2O3/C effectively prevents the contact between SAPO-11 crystals. As a consequence, SAPO-11 with small crystallites is obtained. Thereby, ACS-11 has smaller crystallites and more mesopores than S-11 and CS-11. Fig. 5 displays the Py-IR spectrums of S-11, CS-11 and ACS-11. The bands at 1455 and 1545 cm−1 correspondingly represent the Lewis acid centers (LAC) and Brønsted acid centers (BAC), and the band at 1490 cm−1 can be ascribed to the cooperative action of LAC and BAC (Wen et al., 2021). All samples show bands at 1545 and 1455 cm−1, indicating that they all have BAC and LAC.The amount of total B/L acid centers (TB/LAC) and the amount of medium B/L acid centers (MB/LAC) are calculated at 200 and 300 °C individually. The calculation formula is as follows (Fan et al., 2006): (9) C B / L = A S / m ε where C B/L and A are the amount of LAC or BAC per quality of samples (μmol g−1) and the absorbance (cm−1), respectively; S and m represent the cross-sectional area (cm2) and the mass (g) of samples, respectively; and ε is the extinction coefficient (cm μmol−1) (Datka, 1981). As shown in Table 2 , the amount of TBAC and the amount of MBAC over the SAPO-11 samples follow the order ACS-11 > CS-11 > S-11. To explain these results, the contents and coordination environment of Si in S-11, CS-11 and ACS-11 are analyzed by XRF and 29Si MAS NMR.According to the results of XRF (Table S1), S-11, CS-11 and ACS-11 show similar Si contents. Consequently, the acidities of these prepared SAPO-11 samples are mainly related to the coordination environment of Si atoms (Barthomeuf, 1994). The acidity of SAPO molecular sieves is generated by the substitution of the neutral AlPO4 framework by Si atoms. According to the different formats of Si substitution, it can be divided into SM2 and SM3. In the SM2 substitution method, Si(4Al) is formed through the replacement of one Si for one phosphorus. The silicon islands are formed by the replacement of an adjacent aluminum and phosphorus by two Si atoms in the SM3 substitution method, and Si(nAl,4-nSi) (0 < n < 4) is formed at the borders of silicon islands (Barthomeuf, 1994). Small silicon islands are conductive to improve the acidity of SAPO-11, thereby enhancing the activity for alkane hydroisomerization (Yang et al., 2017). Fig. 6 presents the 29Si MAS NMR results of S-11, CS-11 and ACS-11. All SAPO-11 samples exhibit five resonance peaks in the range of −80 to −115 ppm. The structures of Si(4Al), Si(3Al,1Si), Si(2Al,2Si), Si(1Al,3Si) and Si(4Si) are centered at −86, −95, −101, −106 and −112 ppm, respectively (Fan et al., 2012).The proportions of different Si species in SAPO-11 are shown in Table 3 . The size of silicon islands decreases following the sequence S-11 > CS-11 > ACS-11. Consequently, the amount of MBAC of ACS-11 is the maximum among these samples, which corresponds to the results of Py-IR. According to the results of Py-IR and 29Si MAS-NMR, the addition of activated carbon or Al2O3/C increases the amount of MBAC of SAPO-11. This is because Si atoms are promoted to enter the SAPO-11 framework with the existence of activated carbon, which improves Si distribution in the SAPO-11 framework (Yu et al., 2021). As a result, the amount of MBAC of CS-11 is larger than that of S-11. Compared with activated carbon, Al2O3/C is better dispersed between SAPO-11 crystals and thus further improves Si distribution in SAPO-11 framework. Consequently, ACS-11 possesses the largest amount of MBAC among these samples.The n-decane hydroisomerization over Pt/S-11, Pt/CS-11 and Pt/ACS-11 are evaluated, and the evaluation results over these catalysts at 300–360 °C, 2.0 MPa, a weight hourly space velocity (WHSV) of 2.0 h−1 and a HDVR of 400 are presented in Fig. S4, which shows that 340 °C is considered as the optimal reaction temperature for n-decane hydroisomerization over these Pt/SAPO-11 catalysts. Additionally, the n-decane hydroisomerization performances over these catalysts at 340 °C, different WHSVs of 2.0–25 h−1, 2.0 MPa and a HDVR of 400 are displayed in Fig. 7 . The n-decane conversion of these catalysts decreases in the wake of the increase of WHSV (Fig. 7(a)). As the n-decane conversion rises, S T decreases (Fig. 7(b)) and both S Mu and S C increase for all catalysts (Fig. 7(c) and (d)). The n-decane conversion at the same WHSV follows this order Pt/ACS-11 > Pt/CS-11 > Pt/S-11; S T and S Mu at the same n-decane conversion decrease in the same sequence, and the S C increases following the sequence Pt/ACS-11 < Pt/CS-11 < Pt/S-11 in the whole range of n-decane conversion. However, the difference of S T and S C over these catalysts is small under the low n-decane conversion, while the gap of them gradually rises with the increasing n-decane conversion. According to the typical bifunctional reaction mechanism of n-alkane hydroisomerization, the n-decane isomerization process can be expressed as: n-decane → mono-branched C10 isomers → multi-branched C10 isomers, and the isomerization process is accompanied by cracking reactions (Deldari, 2005). In this work, the low n-decane conversion is due to the high WHSV, which results in the short residence time of reactants at the active sites of catalysts (Singh et al., 2014). As a result, the cracking selectivity is low and the C10 isomers selectivity is high over the three catalysts. Therefore, there is a little difference in the S T and S C over the three catalysts under the low n-decane conversion (Yang et al., 2019; Wen et al., 2020). However, with the decrease in the WHSV, the n-decane conversion rises, and the difference in the S T and S C over the three catalysts gradually increases. ACS-11 has more mesopores, smaller crystallite size and larger ESA than S-11 and CS-11, which promote the formation and diffusion of isomerized intermediates and suppress cracking reactions. Therefore, there is an obvious difference in S T and S C between Pt/ACS-11 and the counterparts under the high n-decane conversion.The n-decane hydroisomerization results over the Pt/SAPO-11 catalysts are shown in Table 4 . The primary n-decane hydroisomerization products on Pt/S-11, Pt/CS-11 and Pt/ACS-11 are 2-methylnonane (2-MC9), 3-ethyloctane (3-EC8), 3-methylnonane (3-MC9), 4-ethyloctane (4-EC8), 4-methylnonane (4-MC9), 5-methylnonane (5-MC9), 2,5-dimethyloctane (2,5-DMC8), 3,5-dimethyloctane (3,5-DMC8), 4,5-dimethyloctane (4,5-DMC8), 2,6-dimethyloctane (2,6-DMC8), 3,6-dimethyloctane (3,6-DMC8), 2,7-dimethyloctane (2,7-DMC8), 4,4-dimethyloctane (4,4-DMC8), 3,3-dimethyloctane (3,3-DMC8), 2-methyl-3-ethylheptane (2-M-3-EC7), 3-methyl-3-ethylheptane (3-M-3-EC7) and 2-methyl-5-ethylheptane (2-M-5-EC7). S Mu of Pt/ACS-11 is 23.28% at the n-decane conversion of approximately 92%, which is higher than those of Pt/S-11 (18.38%) and Pt/CS-11 (19.53%), and S C of Pt/ACS-11 (15.83%) is lower than those of Pt/S-11 (22.95%) and Pt/CS-11 (19.09%). Additionally, Pt/ACS-11 has a superior stability for n-decane hydroisomerization, which is presented in Fig. S5. The rate constant (k) and turnover frequency (TOF) values of Pt/ACS-11 obtained at the n-decane conversion of 20% are 15.25 × 10−6 mol g−1 s−1 and 25.69 × 10−1 s−1 individually, which is higher than those of Pt/S-11 (7.84 × 10−6 mol g−1 s−1 and 20.43 × 10−1 s−1) and Pt/CS-11(10.19 × 10−6 mol g−1 s−1 and 22.84 × 10−1 s−1).The n-decane hydroisomerization over Pt/SAPO-11 catalysts follows a typical bifunctional reaction mechanism (Martens et al., 1986; Deldari, 2005; Zhang et al., 2019), and the details are displayed in Fig. S6. Firstly, n-decane is dehydrogenated over platinum to produce corresponding n-decene intermediates; then, these intermediates are quickly transferred to the BAC inside the pore of SAPO-11 and occur a skeletal rearrangement reaction to form mono-branched C10 intermediates. Based on the theory of “pore mouth/key-lock” (Zhang et al., 2018a; Liu et al., 2020), one side of the mono-branched C10 intermediates can be adsorbed on the pore mouth of SAPO-11, and another side is adsorbed on the BAC of the adjacent pore mouth to undergo skeletal isomerization and generate multi-branched C10 intermediates. These n-decane branched intermediates can also be cracked on the BAC of SAPO-11. Finally, these branched n-decane intermediates are transferred on the Pt metal sites for hydrogenation to produce C10 isomers.The loadings amount of Pt in these Pt/SAPO-11 catalysts are 0.5 wt%, which meet the hydrogenation-dehydrogenation requirements of alkanes hydroisomerization (Wen et al., 2021). Fig. 8 presents the TEM pictures and Pt particle size distributions of Pt/S-11, Pt/CS-11 and Pt/ACS-11. The average Pt particle size and dispersions over Pt/S-11 and Pt/CS-11 are both 4.3 nm and 56%, respectively, and which over Pt/ACS-11 are 4.2 nm and 57% individually. The nearly same particle size and dispersion of Pt suggests that the performance of n-decane hydroisomerization over these catalysts chiefly depends on the physicochemical properties of S-11, CS-11 and ACS-11. Fig. 9 exhibits the forming schematics of the of branched n-decane isomers over Pt/SAPO-11 with different crystallites. SAPO-11 with a small amount of MBAC and large crystallites offers few active sites and long diffusion path in n-decane hydroisomerization, which is disadvantageous for the generation and diffusion of isomerized intermediates and products (Noh et al., 2018; Oenema et al., 2020). S-11 with less MBAC and larger crystallites than CS-11 and ACS-11 provides a smaller number of hydroisomerization sites and a longer residence time for isomerized intermediates and products over Pt/S-11 than Pt/CS-11 and Pt/ACS-11 (Fig. 9(a)). Consequently, Pt/S-11 presents low S T and high S C. Fig. 9(b) shows that SAPO-11 with small crystallites and a large amount of MBAC provides a great amount of hydroisomerization sites and a short diffusion path in n-decane hydroisomerization, which enhances the generation of branched isomers and reduces the cracking side reactions. Pt/ACS-11 presents the maximum n-decane conversion in n-decane hydroisomerization among these catalysts. This can be attributed to the fact that ACS-11 has the largest amount of MBAC among these SAPO-11 samples, which provides a large number of active sites for the n-decane hydroisomerization. Additionally, Pt/ACS-11 shows the maximum S T and the minimum S C among these catalysts. This is because ACS-11 has the smallest crystallites and the largest mesopore volume among these samples, which makes the isomerized C10 intermediates easy to diffuse out the pores of ACS-11 and thereby reduces the cracking reactions and enhances the production of C10 isomers. Furthermore, based on the “pore mouth/key-lock” mechanism, the multi-branched alkane isomers are generated at the ESA rather than in the SAPO-11 channels due to their larger diameters than in the pore openings of SAPO-11 (Claude and Martens, 2000), and a high ESA for SAPO-11 favors the formation of multi-branched C10 intermediates. Thus, ACS-11 with higher ESA than S-11 and CS-11 endows the corresponding catalyst with the maximum S Mu among the three catalysts. Additionally, the maximum Y T and Y Mu of Pt/ACS-11 are 77.85% and 21.44% individually, which is higher than those of Pt/S-11 (73.21% and 16.75%) and Pt/CS-11(73.85% and 17.19%) (Fig. S7), and also higher than those of the reported catalysts in literatures (Table S2).Hierarchical SAPO-11 molecular sieve was prepared using the Al2O3/C composite derived from Al-based metal-organic framework as mesoporogen. The Al2O3/C reacts with phosphoric acid to generate the AlPO4/C structure during the synthesis of hierarchical SAPO-11, which efficiently disperses the Al2O3/C in the synthesis gel of SAPO-11 and thereby inhibits the aggregation of SAPO-11 crystals. Consequently, the optimal hierarchical SAPO-11 is obtained with smaller crystallites, a bigger mesopore volume (0.13 cm3 g−1) and a greater amount of MBAC (26.6 μmol g−1) than the conventional SAPO-11 and the hierarchical SAPO-11 employing activated carbon as mesoporogen. Additionally, its corresponding catalyst displays the maximum selectivity to multi-branched C10 isomers (23.28%), the minimum cracking selectivity (15.83%) and a superior stability in n-decane hydroisomerization among the prepared catalysts. This work provides a new route for preparing hierarchical silicoaluminophosphate molecular sieve-based catalysts with superior alkane hydroisomerization performances.The authors gratefully acknowledge the financial support of Science Foundation of China University of Petroleum, Beijing (Grant No. KYJJ2012-03-03).The following are the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Multimedia component 2 Multimedia component 2 Supplementary data to this article can be found online at https://doi.org/10.1016/j.petsci.2022.06.003.

Hierarchical SAPO-11 molecular sieve (ACS-11) was successfully synthesized employing the Al2O3/carbon (Al2O3/C) composite obtained through the pyrolysis of Al-based metal-organic framework (Al-MOF-96) as mesoporogen. Unlike other carbon-based mesoporogens with strong hydrophobicity, the Al2O3/C interacts with phosphoric acid and generates the AlPO4/C structure, which promotes the Al2O3/C dispersion in the synthesis gel of SAPO-11 and avoids the phase separation between them. The Al2O3/C as mesoporogen decreases the crystallite size of SAPO-11 via preventing the aggregation of SAPO-11 crystals. Additionally, the addition of Al2O3/C improves the Si distribution in the ACS-11 framework. Consequently, ACS-11 has smaller crystallites, more mesopores, and a greater amount of medium Brønsted acid centers than the conventional microporous SAPO-11 and the SAPO-11 synthesized using activated carbon as mesoporogen. The corresponding Pt/ACS-11 catalyst exhibits the maximal selectivity to multi-branched C10 isomers (23.28%) and the minimal cracking selectivity (15.83%) in n-decane hydroisomerization among these catalysts. This research provides a new approach for preparing hierarchical silicoaluminophosphate molecular sieve-based catalysts to produce high-quality fuels.

Second generation biomass is a renewable feedstock with a lower carbon footprint than fossil crudes and is thus a promising alternative raw material to produce fuels and chemicals [1–3]. One of the interesting biomass-derived platform molecules is furfural (Scheme 1 ), which can be produced on an industrial scale by dehydration of hemicellulose from agricultural waste and forest residue [4–7] and has been identified as one of the numerous oxygenated compounds in pyrolysis bio-oils [8–14].Currently, the most relevant use of furfural as a chemical feedstock is in the production of furfuryl alcohol (FOL) and tetrahydrofurfuryl alcohol (THFOL) from selective hydrogenation, which have significant applications in furan resins industry as solvents or chemical intermediates [5,15–21]. Besides, it is also feasible to obtain furan and tetrahydrofuran (THF) via decarbonation and hydrogenation as an environmental benign industrial solvent in the synthesis of plastics, pharmaceuticals and agrochemicals [5,22,23]. Furthermore, the production of biofuels from furfural has received extensive attention during the last decades. Aldol-condensation of furfural with acetone, self-condensation of furfural, or condensation of intermediate products (e.g. furan, 2-methylfuran) with other substrates are promising approaches to produce higher alkanes (C8-C15) for diesel fuel applications [20,24–26]. Selective HDO of furfural, on the other hand, yields 2-methylfuran (MF) or tetrahydro-2-methylfuran (THMF) which are useful blending components for gasoline [6,27,28].In the present study, we focus on the production of MF from furfural HDO, because the involved hydrogen consumption is lower, while the carbon yield is higher compared with products like THMF, THFOL, and furan [5,6]. Moreover, MF has favorable properties in fuel applications. Cu-based catalysts have been found being active in the MF production [29–32]. However, Cu-Cr catalysts [29] used in the industrial MF production are highly toxic, while Cu supported on SiO2, Al2O3 or ZnO [30] suffers from sintering during long reaction periods. Group 9–10 metal catalysts (Ni [5,33], Co [33], Pt [13,34] and Pd [4,5,35]) are also active in the MF production. However, the strong interaction between the furan ring and the transition metal surface leads to undesired ring-hydrogenation or ring-opening reactions; the unstable η2(C, O) adsorbate configuration tends to turn into an η1(C) configuration at high temperature enabling undesired decarbonylation reactions [5].To optimize MF production over pure transition metals (e.g. Ni, Pt), more oxophilic metals (e.g. Fe, Zn) have been added to design alloy catalysts (e.g. PtZn [7] and NiFe [6,27]) for furfural HDO. The addition of more oxophilic elements provides more stable η2(C, O) configurations to the catalyst surface, in which the furan ring is tilted away from the catalyst surface and the carbonyl group is bonded stronger to the catalyst surface by the additional interaction of the carbonyl O to the oxophilic metal [6,7]. Such η2(C, O) configurations suppress furan-ring hydrogenation or ring-opening, and enhance the conversion of the carbonyl group by weakening the C1O1 bond (Scheme 1) [6,7]. By adjusting the Fe loading from 0 to 5 wt% in FeNi/SiO2 catalysts at a constant Ni loading of 5wt%, the MF yield was improved from 20% for Ni/SiO2 to 80% for 5/5 wt% NiFe/SiO2 [6]. Besides alloying metals, ceramic materials like Mo2C [36–39] have also been explored for the furfural to MF conversion. Mo2C shows a high MF selectivity of 70% [36–38], but suffers from rapid deactivation with the furfural conversion decreasing from 90% to 18% over a period of 3 h [38]. Metal borides like NiB, CoB and NiPB [14,19,40,41] have also been considered as catalysts in the furfural conversion. The electron-deficient B can attract the O atom of the carbonyl group, which weakens the CO bond and thus promotes its hydrogenation to the alcohol [14,19]. Nevertheless, only FOL, instead of MF, has been obtained as main products on these metal boride catalysts.Metal phosphides have extensively been investigated in hydrodesulfurization (HDS) [42–44] and hydrodenitrogenation (HDN) [44–48] reactions in recent years. Due to their promising catalytic performance, they are also increasingly applied in hydrodeoxygenation (HDO) reactions [49–52]. So far, there was a focus on investigating HDO model compounds[53], such as phenol [50,54–56], anisole [2,56], or guaiacol [1,57–62], furfural [63–65] and some aliphatic model compounds [51,66–69]. However, studies of the furfural HDO over metal phosphides were limited to Ni2P-based catalysts. Hence, we decided to explore the catalytic performance of different metal phosphides in the furfural HDO reaction with a special interest in obtaining MF as the most desired product. In addition to changing the metal component, we varied the phosphorus/metal ratios in our samples and explored catalytic properties in response to the changed P/M stoichiometries for both Ni and Co. Previous literature reports the influence of the P/Ni ratio on the performance of nickel phosphides in HDS, HDN and HDO [70–73]. However, only the relationship between the active nickel phosphide phase and the catalytic performance has been established, while the presence of P is considered to only determine or maintain the composition of the active phases. In the present work, we provide deeper insight into how the P/Ni ratio influences in each of the active phases the catalytic performance during furfural HDO, by means of furfural-IR and CO-IR spectroscopy.Commercial silica (silica gel Davisil, particle size 90–125 um, SBET = 305 m2/g) was used as support material. H3PO3 (Aesar, 98%), Ni(NO3)2·6H2O (Aldrich, 97%), (NH4)6Mo7O24·4H2O (VWR, 99%), (NH4)W12O39·H2O (Aldrich, 99%), Co(NO3)2·6H2O (Aldrich, 99%), Fe(NO3)2·9H2O (Aldrich, 99%), Cu(NO3)2·3H2O (Aldrich, 99%) were used to prepare catalyst samples of Ni2P, MoP, WP, Co2P, Fe2P or Cu3P.Metal and phosphorus species were loaded on silica by impregnation. For NiO-P/SiO2, MoO3-P/SiO2, WO3-P/SiO2, Fe2O3-P/SiO2, Co3O4-P/SiO2, and CuO-P/SiO2 samples, metal precursors were first loaded on silica support by incipient wetness impregnation. After drying overnight at 110 °C in air and calcination at 550 °C for 1 h, the silica supported metal oxides were impregnated with H3PO3 by incipient wetness impregnation and then air-dried at 110 °C overnight. Before reaction or characterization, the prepared samples were reduced for 2 h in hydrogen at the desired reduction temperature as determined by temperature programmed reduction measurements (TPR) with a heating rate of 5 °C/min.The metal loading is 1.5 mmol/g SiO2; the atomic ratios of P/M (phosphorus to metal) are 2 for Ni2P; 1 for MoP; 1.5 for WP; 0.50 for Fe2P, Co2P and Cu3P, which were empirically found to be needed for the formation of the respective phases. The unreduced precursors are referred to as NiO-P/SiO2, MoO3-P/SiO2, WO3-P/SiO2, Fe2O3-P/SiO2, Co3O4-P/SiO2, and CuO-P/SiO2. A H3PO3/SiO2 sample with a phosphorus loading of 3 mmol/g was prepared as reference sample. NixMoyP/SiO2 catalysts with atomic Ni/Mo ratios of 1:1 and 2:1 were prepared similarly by co-impregnation with a solution containing both Ni and Mo. The loading of Ni/Mo/P on the NixMoyP/SiO2 samples was 1.5/1.5/3 or 1.5/0.75/3 mmol (for the elements Ni/Mo/P, respectively) per gram of SiO2.NiP(x)/SiO2 and CoP(x)/SiO2 were prepared by incipient wetness impregnation as in our previous work [42]. Catalysts were prepared by a one-step impregnation of metal nitrates and H3PO3 followed by direct reduction of as-prepared precursors. The molar P/Ni ratio was varied from 0 to 2 (0, 0.33, 0.5, 1.0, 2.0) in the precursors. Catalyst with different P/Ni ratio were labeled as Ni/SiO2, NiP(0.33)/SiO2, NiP(0.5)/SiO2, NiP(1)/SiO2, and NiP(2)/SiO2, respectively. Molar ratios of P/Co were varied from 0 to 2 (0, 0.5, 1.0, 1.5, 2.0) in the catalyst precursors. Catalysts are denoted as Co/SiO2, CoP(0.5)/SiO2, CoP(1.0)/SiO2, CoP(1.5)/SiO2, and CoP(2.0)/SiO2, respectively.The metal loading was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis performed on a Spectro Blue apparatus. Prior to analysis, samples were dissolved in an HF/HNO3/H2O solution with a volumetric ratio of 1:1:1. HF (VWR, 40%) and HNO3 (VWR, 65%) were used to prepare the acidic solvent for ICP measurements.X-ray diffraction (XRD) patterns were acquired on a Bruker D2 Phaser powder diffraction system using Cu Kα radiation (1.5406 Å). Scans were taken at a rate of 1°/min in the range of 10° ≤ 2θ ≤ 80°.BET surface area measurements were performed on a Micromeritics ASAP3020 Tristar system and a nitrogen stream was applied for N2 physisorption at −196 °C. The specific surface area was determined by the Brunauer-Emmett-Teller (BET) equation using the adsorption data.X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific K-Alpha XPS apparatus equipped with a monochromatic Al Kα X-ray source. Samples were reduced and sealed in a tubular quartz reactor, transferred into a glovebox without exposure to air, then loaded in an airtight transfer vessel and introduced into the XPS analysis chamber for analysis. The background pressure prior to analysis was 2 × 10−9 mbar. Survey scans were collected at constant pass energy of 200 eV, and region scans at 50 eV. The spectra were calibrated to the Si 2p line at 103.3 eV and fitted with the CasaXPS program.Temperature programmed reduction measurements in H2 (H2-TPR) were carried out between 50 °C and 700 °C. Typically, 15 mg catalyst precursor was heated in a 20 ml/min H2 flow from 50 °C to 700 °C at a heating rate of 5 °C /min. The effluent gas composition was analyzed by an online mass spectrometer. The mass signals of 2 (H2), 18 (H2O), 34 (PH3), and 30 (NO) were monitored.Temperature programmed desorption of NH3 (NH3-TPD) was conducted between 25 °C and 400 °C. Typically, 15 mg catalyst precursor was reduced at 550 °C for 1 h and cooled down to 25 °C in a 20 ml/min flow of H2. Afterwards, the reduced samples were first flushed with He for 30 min to remove physisorbed species, then with NH3/He (10 v/v%) for 30 min to adsorb NH3, followed by He for 30 min to remove weakly absorbed NH3. Subsequently, the pretreated samples were heated from 25 °C to 400 °C in a 20 ml/min He flow with a heating rate of 5 °C /min. The effluent gas flow was monitored by a TCD detector.Infrared (IR) spectroscopy of CO absorbed samples were carried out with a Bruker Vertex V70v Fourier Transform IR spectrometer equipped with a DTGS detector. The catalysts were pressed into a self-supporting wafer with a diameter of 13 mm and then mounted in an in-situ cell equipped with CaF2 windows. Prior to CO adsorption, the samples were reduced at 500 °C for 2 h, applying a heating rate of 10 °C/min, followed by evacuating the cell to a pressure below 2 × 10−6 mbar for 0.5 h at 500 °C. Afterwards, the reduced sample was cooled down to 25 °C under evacuation. CO was introduced into the cell with a metering valve until the CO pressure reached 5 mbar. The catalyst was exposed to CO for 5 min, subsequently evacuated for 5 min to 2 × 10−6 mbar, followed by heating to 425 °C at a rate of 10 °C /min under evacuation. The IR spectrum was recorded by accumulating 64 scans at a resolution of 2 cm−1. Spectra of the freshly reduced as well as CO saturated catalysts were recorded during the heating process.In case of furfural-IR, catalyst pretreatment and experimental parameters were the same as those of the CO-IR experiments. Liquid furfural was expanded into the evacuated cell at a pressure of 2 × 10−3 mbar for 5 min at 25 °C and then evaporated until a pressure of 2 × 10−6 mbar was reached. Afterwards, the catalyst was heated to 300 °C at a rate of 10 °C/min at 2 × 10−6 mbar. Spectra of the furfural-saturated catalysts were recorded during the heating process. Spectra of freshly reduced catalysts were taken to use as background correction reference.The particle size of active Ni-phosphide phase was investigated with a FEI Tecnai 20 transmission electron microscopy (TEM).The catalytic performance of synthesized materials was evaluated in a plug flow fixed-bed reactor with 4 mm internal diameter. Furfural was fed by a syringe pump (Hewlett Packard 1050) and gasified at 170 °C using H2 as carrier gas.Prior to reaction, catalysts were reduced in-situ at the desired temperature in a hydrogen flow for 2 h. After reduction, the reactor was cooled to reaction temperature, then fed with a furfural/H2 flow (H2/furfural molar ratio = 74). The products were analyzed by an in-line gas chromatograph using a flame ionization detector (GC-FID) and a DB-1 (30 m, 0.32 mm, 1.00 μm) column.The reaction was carried out at atmospheric pressure between 120 °C and 200 °C. In a typical run, the H2 flow, the feeding rate of furfural, H2 and the loading amount of catalysts were 20 ml/min, 0.001 ml/min in liquid phase, and 60 mg, respectively. This corresponds to a weight hourly space velocity (WHSV) of ca. 3 h−1. All activity measurements were carried out in duplicate to verify reproducibility. The product gas stream was analyzed after 2 h on stream.Conversion and selectivity were calculated as follows: C o n v e r s i o n ( % ) = mol o f t h e p r o d u c t s mol o f f u r f u r a l f e d × 100 Selectivity ( % ) = mol o f o n e p r o d u c t mol o f a l l p r o d u c t s × 100 The metal and phosphorus content of unreduced precursors as determined with ICP and the textural properties are listed in Table S1. Each precursor shows a P/metal molar ratio close to the target value. BET surface areas are between 150 m2/g and 250 m2/g, which are all lower than the surface area of bare SiO2 (305 m2/g) and decrease with increasing P content due to pore blocking [72].Temperature programmed reduction (TPR) was used (Figure S1) to determine the right reduction temperature for each catalyst. As revealed by Figure S1, the NiO-P/SiO2, WO3-P/SiO2 and MoO3-P/SiO2 samples are reduced at 500 °C, 600 °C and 550 °C, respectively. A higher reduction temperature (650 °C) is required to reduce Co3O4-P/SiO2 and CuO-P/SiO2, while Fe2O3-P/SiO2 reduces at 680 °C.In case of the NiP(x)/SiO2 catalysts (Figure S1b), the reduction temperature is determined as 450 °C for Ni/SiO2, and 550 °C for NiP(x) (x = 0.33, 0.5, 1, 2). Although the TPR profiles of NiP(1) and NiP(2) show the most significant signals at 650 °C as a result of PO4 3− reduction, we have chosen 550 °C as reduction temperature since P species formed by reduction of excessive H3PO3 suffice for nickel phosphide formation.In the case of CoP(x)/SiO2 catalysts (Figure S1c), Co/SiO2 is reduced at 400 °C, while a high temperature of 650 °C is required for CoP(x) reduction.X-ray diffraction (XRD) reveals that each catalyst formed crystalline phases after reduction (Fig. 1 ). All catalysts show a broad diffraction peak in the 2θ range of 15–35°, typical of amorphous silica [60,74,75]. In Fig. 1a, all reduced samples contain pure phases of the corresponding metal phosphides (i.e. Ni2P, MoP, Co2P, Fe2P, and Cu3P) except WP/SiO2, which displays reflections of both WP and unphosphided W0 phases. The sharp diffraction peaks of WP/SiO2, Fe2P/SiO2, Co2P/SiO2 and Cu3P/SiO2 indicate that the metal phosphides consist of relatively large particles due to the rather high reduction temperature. For MoP/SiO2 and Ni2P/SiO2, milder phosphidation conditions were applied and consistently the XRD patterns show broad diffraction peaks, suggesting smaller particles. Fig. 1b shows XRD patterns of reduced NiP(x)/SiO2 (x = 0, 0.33, 0.5, 1.0, and 2.0). Pure metallic Ni0 is formed in Ni/SiO2. Ni3P is obtained on NiP(0.33), Ni12P5 on NiP(0.5), and Ni2P on NiP(1) and NiP(2). FWHM indicates comparable particle sizes. Fig. 1c shows that reduced CoP(x)/SiO2 (x = 0, 0.5, 1.0, 1.5, and 2.0) contains a pure Co0 phase on reduced Co/SiO2. As the P/Co ratio increases, the active phase shifts from Co2P to CoP with Co2P present in CoP(0.5), CoP, CoP(1.5), and CoP(2.0), and both phases in CoP(1).Different from NixPy (Ni2P, Ni3P, Ni12P5) and CoxPy (Co2P, CoP), only one single phosphide phase has been obtained for Mo, Fe, Cu, and W. Hence we explored the effect of metal/P ratio on the catalytic performance only for nickel and cobalt phosphides.Reduced Ni, NiP(0.5), NiP(1) and NiP(2) samples were also characterized by TEM to determine the particle size distributions (Fig. 2 ). Besides some large particles observed in the Ni catalyst, all catalyst show small nanoparticles of a comparable size of ~ 5 nm, consistent with the XRD data for the NiP(x) catalysts.The XPS results reveal the type and amount of chemical species on the surface of reduced catalysts, which confirm the formation of metal phosphides. Fig. 3 and Table S2 show the results of the reduced NiP(x) (x = 0, 0.33, 0.5, 1, and 2) catalysts. The contribution at 852.2 eV in Ni 2p spectra is assigned to Ni0 in metallic nickel, the contribution at 852.7 eV is assigned to Niδ+ (0 < δ < 1) in nickel phosphide phases (Ni2P, Ni5P2 and Ni3P), and the signal at 856.4 eV to Ni2+ species [42]. Concerning the P 2p spectra, the three peaks at 134.4 eV, 133.5 eV and 129.1 eV are due to PO4 3−, PO3 3− and Pδ− (0 < δ < 1) species in nickel phosphides [42], respectively. PO3 3− originates from unreduced H3PO3, while PO4 3− forms from the disproportionation or reduction of H3PO3 [42]. The presence of Niδ+ (0 < δ < 1) and Pδ− (0 < δ < 1) in nickel phosphide phases points to an interphase Ni → P transfer of electron density as a consequence of the electron withdrawing nature of P [76]. The electron density on Niδ+ is expected to decline at higher P content due to the electron withdrawing nature of P atoms. Note that the presence of Ni0 species in NiP(2), NiP(1), NiP(0.5) and NiP(0.33) catalysts cannot be excluded, since the P/Ni ratios of these samples (Table S2) are all lower than the theoretical P/Ni ratios of Ni2P, Ni5P2 and Ni3P phases (0.5, 0.4, and 0.33, respectively). Figure S2 displays the Co 2p and P 2p core-level spectra of reduced CoP(x) catalysts. Peaks at 777.4–777.9 eV are assigned to the Co0 or Coδ+ (0 < δ < 1) sites in catalysts, while peaks at 129.0 eV are the fingerprint of Pδ− (0 < δ < 1) in cobalt phosphides. Similar to NiP(x) catalysts, the presence of Coδ+ (0 < δ < 1) and Pδ− (0 < δ < 1) in Co2P and CoP phases points to an interphase Co → P transfer of electron density as a consequence of the electron withdrawing nature of P [76]. The binding energy of Coδ+ is slightly higher than that of Co0 due to the slight positive charge it bears. Different from NiP(x) catalysts, the P/Co molar ratios of the reduced phases are higher than the elemental analysis in the respective phases (i.e. 0.5 in Co2P and 1 in CoP), indicating a P rich surface of Co2P and CoP phases.NH3-TPD is used to characterize acid properties of the catalysts. Fig. 4 shows the NH3-TPD profiles of reduced Ni/SiO2 and NiP(x)/ SiO2 (x = 0.33, 0.5, 1, 2). The TPD profiles consist of two peaks: one at low temperature in the range between 100 °C and 300 °C and another at high temperature between 500 °C and 650 °C. The low temperature peak in the profile of H3PO3/SiO2 corresponds to the POH groups with weak Brønsted acidity, while the high temperature peak in the profile of Ni/SiO2 might be attributed to Lewis acid sites stemming from Niδ+ species bearing a small positive charge or unreduced Ni2+ species [2]. Both Brønsted acid and Lewis acid sites are present on NiP(x)/SiO2 samples due to the POH groups and Niδ+/Ni2+ species, respectively. The amount of Brønsted acid sites evidently increases as the P content increases.To get insight into how P influences the nature of active sites on the catalyst surface, further characterization of the reduced NiP(x) catalysts by CO-IR (Fig. 5 ) and furfural-IR (Fig. 6 ) was performed. The strength of the CO adsorption is directly related to the transfer of electrons from the metal to the π* orbital of the CO molecule. More back-donation of electrons to the π* orbital results in a stronger metal-CO bond and a weaker CO bond. The latter is reflected in a lower frequency of the CO stretching vibration. CO-IR can thus be used to probe the CO bond strength after adsorption as well as the electron density of the adsorption site. Fig. 5 shows the effect of varying P content on the chemical nature of catalysts by CO-IR. The bands at 3745 and 3670 cm−1 are due to the ν(OH) stretching vibrations of SiOH and POH surface groups [58]. The intensity of the ν(POH) vibration (3670 cm−1) increases with the P content, indicating a higher amount of Brønsted acid sites on catalyst surface. This is in alignment with the NH3-TPD results shown above.For the Ni/SiO2 catalyst, the band at 2045 cm−1 is assigned to CO molecules bonded to a single Ni atom [77,78]; the band at 1962 cm−1 is attributed to CO molecules bonded to two Ni atoms (bridge-CO species) [77,78]. The band at 2086 cm−1 is assigned to coordination of multiple CO molecules to a single coordinatively unsaturated Ni0 site.The infrared spectra of NiP(x)/SiO2 catalysts show the strong bands between 2096 and 2086 cm−1 (linear-CO bonded on Ni sites [57,78–80]), NiP(0.33) and NiP(0.5) exhibit a weak band at 1905 cm−1 (bridge-CO species [77,81]), and NiP(1) and NiP(2) a very weak band at 2202 cm−1 (PCO surface species) [77,80]. The peak broadening at 2096 cm−1 is observed on NiP(0.33) and NiP(0.5) samples likely due to the dipole-dipole interaction of absorbed CO since the XPS results (Table S2) reveal that more neighboring Ni sites (i.e. lower P/Ni ratios) are available on NiP(0.33) and NiP(0.5) surfaces than on NiP(1) and NiP(2) surfaces [82,83].The absorption band at 2086 cm−1 can tentatively be assigned to CO adsorbed on a single Ni in a phosphide matrix. As nickel in NiP(x) is positively charged according to XPS results (see Fig. 3), less back donation of electrons will occur creating a blue shift for the vibrational frequency of linearly adsorbed CO from 2045 cm−1 to 2086 cm−1. If the P content is reduced, more adjacent Ni will be available, increasing the dipole interaction and thus shifting and broadening the peak towards 2096 cm−1. Since both adsorption on positively charged nickel as adsorption of multiple CO on unsaturated nickel atoms lead to less back donation, it is not unexpected that the adsorption band of CO adsorbed on Ni in NiP(x) has a frequency (Fig. 5b colored lines) similar to that of multiple CO adsorbed on unsaturated sites of pure nickel (Fig. 5b, black line). The frequency of bridging-CO bands in NiP(x)/SiO2 (1905 cm−1) is lower than those in the Ni catalyst (1962 cm−1), indicating the presence of stronger bonded bridge-CO species in NiP(x)/SiO2. This allows us to conclude that the addition of P alters the properties of the adsorption site, apparently leading to a stronger bridge-CO adsorption. The intensity of the bridge-CO bands (1962 and 1905 cm−1) decreases significantly with increasing P content, indicating that P lowers the amount of bridge-CO species (Fig. 5b). This is expected since P could block the availability of adjacent Ni atoms, which would act as adsorption sites for bridge-CO [77,78]. Since adjacent Ni sites are required for a planar adsorption of furan, the planar adsorption of the furan ring is also expected to decrease with increasing P content and so do the furan-ring hydrogenation or the ring-opening reaction in the furfural HDO.The strength of CO adsorption on the catalyst surface is also explored by IR spectra obtained during temperature programmed desorption (Figure S3). As temperature increases, the intensity of the linear-CO band declines due to CO desorption. Increasing P content facilitates the desorption of linear-CO (Figure S3f), indicating a weaker CO adsorption on Ni sites at higher P content. This confirms the conclusions drawn from Fig. 5b that the adsorption band of linearly absorbed CO shifts to higher frequencies at higher P content as the adsorption is weaker due to the lower electron density on Niδ+. Based on this understanding, the increasing P content is also expected to weaken the furan adsorption on the catalyst surface from an electronic point of view, because it contributes to a lower electron density on Niδ+ sites providing a lower back donation of electrons from to the d-band of Ni to the π* bond of the furan ring.The adsorption bands (Fig. 5b) as well as the CO desorption behaviors (Figure S3f) are similar for NiP(0.33) and NiP(0.5) catalysts, as well as for NiP(1) and NiP(2), which is consistent with the XPS results (Table S2) showing that NiP(0.33) and NiP(0.5) have a similar Ni reduction degree of 0.67 and 0.68, respectively, and NiP(1) and NiP(2) of 0.74 and 0.75, respectively. This suggests that NiP(0.33) and NiP(0.5), as well as NiP(1) and NiP(2), possess active sites with similar geometrical and electronical properties.In summary, P addition decreases the amount of adjacent Ni sites and the electron density on Ni sites, which is expected to suppress the planar adsorption of furan ring from both geometrical and electronical view.IR spectra of adsorbed furfural on silica supported Ni/SiO2 and NiP(x)/SiO2 are shown in Fig. 6. The bands at 1675 and 1695 cm−1 are assigned to the ν (CO) stretching vibration of the carbonyl group; bands at 1570, 1465, and 1475 cm−1 are due to ν(CC) stretching vibration of the furan-ring [84–86]. The broad bands between 1400 and 1300 cm−1 are a result of the background correction.As the P/Ni molar ratio increases, the ν(CO) bands gradually shift to lower frequency from 1675 cm−1 of Ni/SiO2 to 1655 cm−1 for NiP(2)/SiO2, which suggests that the interaction between the carbonyl group and catalyst surface is notably enhanced by P addition. Furthermore, new bands at 1625 cm−1 and 1622 cm−1 are observed on NiP(0.33) and NiP(0.5), indicating the formation of a second adsorption mode of the carbonyl group. In case of NiP(1) and NiP(2), the shoulder at 1635 cm−1 may also result from this new adsorption configuration of carbonyl groups.Based on geometric and electronic considerations, the tendency of planar furan ring adsorption is expected to decrease with increasing P content. Nevertheless, no frequency shift of the ν(CC) band (at 1465 and 1475 cm−1) is observed (Fig. 6). Likely because the interaction between the furan-ring and the catalyst surface is strong and the weakening effect of P on the furan-catalyst interaction insufficient to cause a frequency shift of IR bands. The intensity of the ν(CC) IR bands, however, clearly decrease with increasing P content. We therefore calculated the intensity ratios of ν(CO)/ν(CC) during a temperature programmed heating process to further investigate the adsorption situation of carbonyl groups and furan-rings on the catalyst surface. As temperature increases above 150 °C, a significant decrease is observed in the ν(CO)/ν(CC) ratios on Ni/SiO2 and NiP(0.5)/SiO2, while only a slight decrease is detected on NiP(1)/SiO2 and NiP(2)/SiO2. This does again confirm that NiP(1)/SiO2 and NiP(2)/SiO2 with higher P content exhibit a stronger carbonyl adsorption than Ni/SiO2 and NiP(0.5)/SiO2 do.The CO-IR investigation of CoP(x) was also conducted (Figure S4). As in the case of P in the NiP(x) samples, the P addition does suppress the presence of adjacent Co sites, i.e. bridging-CO adsorption sites. As argued above, P atoms do likely “dilute” Co atoms at the surface hence suppressing the bridge-CO adsorption [77]. Besides, the intensity of the 2066 cm−1 band assigned to multiple CO adsorbed on unsaturated sites, declines with increasing P content, suggesting saturation of coordinated unsaturated Co sites by P. Furthermore, a decrease in electron density of Coδ+ is expected at higher P content due to the electron withdrawing nature of P, in line with the declining adsorption strength of linear-CO at higher P content (Figure S5f). Since adjacent Co sites are necessary for the planar adsorption of the furan ring and a lowered electron density of Coδ+ contributes to weaker adsorption of furan ring, the P addition exerts a suppressing effect on the planar adsorption of furan rings on CoP(x) surface for reasons of geometric constrains and electronic structure, which is very similar to the situation in NiP(x).The activity of different metal phosphides was tested in furfural HDO at a WHSV of 3 h−1, a pressure of 1 bar and a temperature of 200 °C (Fig. 7 a). MoP, Ni2P, Co2P and WP show promising furfural conversions in the order Ni2P ≈ MoP > Co2P ≈ WP with values around 90%, 91%, 53%, and 50%, respectively. Cu3P and Fe2P show almost no activity and are excluded from our study. Subsequently, Ni2P, MoP, Co2P and WP were further tested at different contact times to explore the reaction mechanism of furfural HDO over these catalysts (Fig. 7b,c,d,e).According to previous research employed pure metal catalysts [5,6,21,35], mainly two reaction pathways are proposed for furfural HDO (Scheme 2 ). One is a hydrogenation pathway (pathway 1), in which the carbonyl group is first hydrogenated to an alcoholic hydroxyl group, which is then removed by hydrogenolysis, generating furfuryl alcohol (FOL) and subsequently 2-methylfuran (MF) as products. The other one is decarbonylation (pathway 2) yielding furan as a product. If applied catalysts (e.g., Ni, Co, Pd [5,33]) have a high hydrogenation capacity, furfuryl alcohol (FOL), 2-methylfuran (MF) and furan can be further hydrogenated to tetrahydrofurfuryl alcohol (THFOL), tetrahydro-2-methylfuran (THMF) and tetrahydrofuran (THF), respectively. If catalysts possess high hydrogenolysis capability, the formation of ring-opening products (e.g. butanol, butane, pentanol, pentane, etc.) would be obtained.In our cases, metal phosphides show a promising performance in the furfural HDO with activity orders of Ni2P ≈ MoP > Co2P ≈ WP and a desirable product selectivity towards 2-methylfuran. Product distributions differ across the series of metal phosphides significantly. WP and MoP show highest selectivity towards MF (>90%), whereas Co2P mainly produces the less-desired furan product (>40%). This shows that each metal phosphide sample favors the hydrogenation and decarbonylation routes in a different way.At decreasing furfural conversions (attained by increasing WHSV), all catalysts showed increasing furfural alcohol selectivity at the expense of 2-methylfuran production, revealing the hydrogenation mechanism (pathway 1) of metal phosphide catalysts. The constant furan selectivity revealed a decarbonylation pathway (pathway 2), which is independent from the hydrogenation pathway. Small amounts of THFOL and THMF, and no trace of ring-opening products were detected, indicating that furan-ring hydrogenation and ring-opening reactions are successfully suppressed over these catalysts. Accordingly, these catalysts can contribute to lower hydrogen consumption and lower light-product formation in comparison to traditional transition metal catalysts (i.e. Ni [5,6] or Pd [5]).For MF production, indirect and direct reaction pathways have been reported over metal and metal alloy surfaces (e.g., PtZn, NiFe, and Mo2C) [6,7,20,37]. The indirect reaction pathway consists of hydrogenation of η2(C, O) surface adsorbed species to FOL, followed by conversion into MF via FOL hydrogenolysis [6,20]. The direct reaction pathway involves the conversion of the η2(C, O) species into C4H3O-CH2 or C4H3O-CH intermediates, which are anticipated to directly produce MF in a H2-rich environment [7,20,37]. The presence of FOL at high WHSV over Ni2P, MoP, Co2P, and WP catalysts (Fig. 7) confirms the indirect MF formation mechanism. Yet, the direct MF formation pathways cannot be excluded, especially for MoP and WP, where high MF production can be still obtained at low conversion (Fig. 7).As shown above, Ni2P/SiO2 is practically the most active catalyst among our metal phosphides with a promising product distribution. We therefore decided to study this catalyst in more detail: Nickel phosphide samples were prepared by varying the P/Ni molar ratios NiP(x) (with x = 0.33, 0.5, 1, 2) and were tested in the furfural HDO (Fig. 8 ). Since the focus of our work is to optimize selectivity and to develop a mechanistic explanation of the furfural HDO reaction, instead of optimizing reaction rates, TOF investigations at low conversion are not included here. Nevertheless, upper bound estimates of TOFs based on the given data are provided in Table S3.Significant yields of FOL and THFOL are produced over metallic Ni, demonstrating the high hydrogenation ability, i.e. strong interaction between furfural and the Ni metal surface. Higher amounts of MF and THMF are produced at the expense of FOL and THFOL over NiP(0.33) and NiP(0.5), implying an enhanced C1O1 (Scheme 1) hydrogenolysis ability of these catalysts, i.e. a stronger interaction between the carbonyl group and the Ni-P catalyst surface. The decreasing production of THMF over NiP(0.33) and NiP(0.5), as well as the absence of THMF over NiP(1) and NiP(2) reveals a suppressing effect of P on the furan-ring hydrogenation, i.e. a weaker interaction between furan-ring and catalyst surface. High reaction temperatures contribute to high production of undesired decarbonylation and ring-opening reactions, consistent with the effect of reaction temperature on furfural HDO over pure metal catalysts [5,6,33]. The NiP(1) and NiP(2) catalysts show similar catalytic performance, which can be attributed to the fact that these catalysts contain the same active (Ni2P) phase (XRD). In line with this, their surface characteristics as probed for XPS, CO-IR and furfural-IR spectroscopy are also similar. The behaviors of the NiP(0.33) and NiP(0.5) catalysts are clearly different from those of NiP(1) and NiP(2), most likely due to the different active phases of Ni3P and Ni12P5. Reasons of different catalytic behavior of the Ni3P, Ni12P5 and Ni2P active phases are discussed below.The interaction between the furan-ring and the catalyst surface (labeled below as the furan-ring/Ni interaction) is essentially the interaction between the d-band of Ni and the π* bonding of the furan-ring. As the Ni/SiO2 catalyst accommodates the highest electron density on Ni sites and the largest concentration of bridge-CO sites among all of our catalysts, the furan-ring/Ni interaction is expected to be the strongest here, contributing to ring-hydrogenation and ring-opening reactions in the furfural conversion. Therefore, it is plausible that substantial amounts of THFOL are obtained on a Ni catalyst.As the P content increases, the number of adjacent Ni sites, required to form bridge-CO species, is substantially suppressed (Fig. 5) and the electron density on Ni sites is gradually reduced due to the electron withdrawing nature of phosphorus. From a geometric viewpoint, the furan-ring/Ni interaction is expected to decrease due to the decreasing amount of adjacent Ni sites. From an electronic structural point of view, the interaction between the Ni d-band and the furan π-system is expected to decline at higher P content due to the lower electron density on Niδ+ sites. Therefore, the furan-ring/Ni interaction as well as the ring-hydrogenation capacity of the catalysts are expected to decline as the P content increases, which is consistent with our activity results that the product distribution shifts from THFOL for Ni/SiO2 to THMF for NiP(0.33), then to MF for NiP(0.5), NiP(1), and NiP(2).The interaction between the carbonyl group and the catalyst surface, which can be characterized by furfural-IR, is another key factor influencing furfural HDO performance.According to literature, the carbonyl group adopts an η2(C, O) configuration on the Ni surface, which is unstable and tends to rearrange into an η1(C) configuration at higher temperature (Scheme 4c) [5]. As the η1(C) configuration is most likely the precursor for decarbonylation, an increasing furan production is observed as reaction temperature increases [5]. Our results are consistent with those published in literature [5]: the ν(CO)/ν(CC) ratio of the adsorbed species decreases at higher temperature (Fig. 6f) confirming the η2(C, O) → η1(C) conversion at higher temperature. In accordance with the formation of the η1(C) configuration, more furan is produced at higher temperature (Fig. 8).After P addition, the ν(CO) stretching vibration of adsorbed furfural shifts to lower frequency and the downward shift tends to be more prominent when the P content increases (Fig. 6), indicating a stronger carbonyl/catalyst interaction at higher P content. It is likely that electron-deficient Niδ+ binds to the lone pairs of the carbonyl O, while electron-rich Pδ− donates electrons to the anti-bonding orbitals of the CO moiety, contributing to a stronger carbonyl/catalyst interaction (i.e., a more stable η2(C, O) configuration, Scheme 4 d). Such stronger carbonyl/catalyst interaction weakens the C1O1 bond (Scheme 1) in the carbonyl group. Consequently, the carbonyl hydrogenation and subsequent C1O1 (Scheme 1) hydrogenolysis of FOL could be enhanced, which is consistent with the product distribution shift from FOL and THFOL of Ni catalyst to MF and THMF of NiP(0.33) catalyst.Besides, the MF yields are relatively stable for NiP(1) and NiP(2) at low or high furfural conversions. Likely that the Ni2P surface exposes sites with the appropriate geometry and electronic properties that enable a direct MF formation from furfural deoxygenation via C4H3OCH [7] or C4H3OCH2O [37] intermediates (Scheme 3 ).Another explanation for the changing catalytic performance by P addition is based on the role of Brønsted acid sites: In the hydrodeoxygenation of phenolic compounds, a Brønsted acid site adjacent to a metal site is considered to facilitate CO hydrogenolysis by protonating the oxygen in OH or OCH3 groups [87,88]. Since the P species in NiP(x) catalysts could also act as Brønsted acid sites adjacent to Ni sites, i.e. POH (Figs. 3 and 4), we infer that the increasing P content in the catalyst could also offer a promoting effect on C1O1 (Scheme 1) hydrogenolysis in FOL and THFOL, contributing to enhanced production of MF and THMF. Scheme 5 illustrates above findings and possible explanation: in the metal phosphides (in contrast to the pure metal) the weakened CC adsorption leads to less furan-ring hydrogenation products, and the strengthened CO adsorption contributes to easier C1O1 (Scheme 1) conversion. Besides, phosphorus could act as Brønsted acid which would facilitate the C1O1 (Scheme 1) hydrogenolysis in FOL and thus contribute to a higher production of MF.Since Co2P is the third active catalyst in furfural HDO (Fig. 6) and the active phases of cobalt phosphide are adjustable by varying the molar P/Co ratio, we investigated the effect of the P stoichiometry in CoP(x) phases on the furfural HDO as well (Figure S6). The conversion decreases with increasing P content, indicating the decreasing activity in the order CoP < Co2P < Co.Similar to NiP(x), the ring-opening products are suppressed with increasing P content. This is understandable, since as in the case of the nickel phosphides, P suppresses the formation of adjacent Co sites required for the planar adsorption of the furan ring, and lowers the electron density of Coδ+ sites leading to a weaker interaction between furan ring and catalyst surface. In addition, the MF production is enhanced at the expense of the FOL production by increasing the P content as a result of Brønsted acid sites facilitating the C1O1 (Scheme 1) bond breaking.A higher furan production is found on CoP(x) than on NiP(x); a possible explanation for this observation is as follows. The η2(C, O) configurations on the catalyst surface are stabilized by the carbonyl O donating electrons to electron-deficient Coδ+ or Niδ+ sites and the antibonding orbitals of CO accepting electron density from electron-rich Pδ−. Since the density of Coδ+ sites on CoP(x) surfaces is lower than that of Niδ+ on NiP(x) (P/Co > P/Ni, Table S2), the binding of carbonyl O to the catalyst surface will be lower on CoP(x) surfaces, which contributes to an easier η2(C, O) → η1(C) transformation and, consequently to a higher furan formation rate on CoP(x) catalysts.Overall, we summarize that increasing the P/Co stoichiometry in CoP(x) samples suppresses the ring-opening reactions but does not suppress the undesired decarbonylation. In comparison with NiP(x), CoP(x) catalysts are not desirable in furfural conversion due to the higher furan production and the lower MF yield (furan has a lower C yield than MF in furfural conversion). Our characterization and activity results of CoP(x) and NiP(x) demonstrate that the type of metal phosphide and the phosphorus/metal ratio plays a crucial role in determining the furfural adsorption on the catalyst surface, as well as the catalytic performance in furfural HDO.A series of transition metal phosphides was evaluated in the furfural HDO reaction, showing the following activity trend: Ni2P ≈ MoP > Co2P ≈ WP ≫ Cu3P > Fe2P. In comparison to traditional transition metal catalysts (i.e. Ni or Pd), metal phosphides like Ni2P, MoP, Co2P and WP are promising catalysts for furfural HDO as they produce 2-methylfuran and furan as major products and contribute to lower hydrogen consumption and lower light-product formation.By varying the P/Ni molar ratios in NiP(x) precursors, the effect of P stoichiometry on catalyst properties and performance are investigated in depth. A higher P content weakens the furan-ring/catalyst interaction contributing to lower furan ring-hydrogenation and ring-opening reaction. On the other hand, it provides a stronger carbonyl/catalyst interaction by providing a more stable η2(C, O) configuration with the electron-deficient Niδ+ binding to the lone pairs of carbonyl O and the electron-rich Pδ− donating electrons to the antibonding orbitals of CO. Such enhanced carbonyl/catalyst interaction weakens the CO bond in the carbonyl group promoting its hydrogenation and further conversion. Moreover, P species could also act as Brønsted acid sites that facilitate hydrogenolysis of FOL and THFOL, contributing to a higher production of MF and THMF. A comparable role of P was observed in CoP(x) samples, except that the undesired decarbonylation is suppressed to a lesser extent. This is likely because the density of available Coδ+ sites on CoP(x) surfaces is lower than that of Niδ+ on NiP(x). This reduces the binding of the carbonyl O to the catalyst surface, thereby contributing to an easier η2(C, O) → η1(C) transformation and consequently a higher furan formation rate on CoP catalysts.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. We consulted the author's guide when submitting the manuscript. Manuscripts are prepared in accordance with publishing ethics policies described in the Author's Guide.The China Scholarship Council (CSC) is acknowledged for financial support. We thank Tiny Verhoeven for performing XPS measurements and Adelheid Elemans for the ICP measurements.Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2021.01.031.The following are the Supplementary data to this article: Supplementary data 1

The gas-phase hydrodeoxygenation (HDO) of furfural, a model compound for bio-based conversion, was investigated over transition metal phosphide catalysts. The HDO activity decreases in the order Ni2P ≈ MoP > Co2P ≈ WP ≫ Cu3P > Fe2P. Nickel phosphide phases (e.g., Ni2P, Ni12P5, Ni3P) are the most promising catalysts in the furfural HDO. Their selectivity to the gasoline additives 2-methylfuran and tetrahydro-2-methylfuran can be adjusted by varying the P/Ni ratio. The effect of P on catalyst properties as well as on the reaction mechanism of furfural HDO were investigated in depth for the first time. An increase of the P stoichiometry weakens the furan-ring/catalyst interaction, which contributes to a lower ring-opening and ring-hydrogenation activity. On the other hand, an increasing P content does lead to a stronger carbonyl/catalyst interaction, i.e., to a stronger η2(C, O) adsorption configuration, which weakens the C1O1 bond (Scheme 1) in the carbonyl group and enhances the carbonyl conversion. Phosphorus species can also act as Brønsted acid sites promoting C1O1 (Scheme 1) hydrogenolysis of furfuryl alcohol, hence contributing to higher production of 2-methylfuran.

Data will be made available on request.Supercritical water gasification (SCWG) has drawn considerable attention in recent years as a clean and renewable synthetic natural gas (bio-SNG) production technology. As water acts as the solvent and reactant in these conditions, no drying step is required to convert biomass feeds of high moisture content, leading to higher gas production efficiencies ( ≈ 70%) compared to conventional conversion technologies [1]. Combustible gases (CH4 and H2) can easily be produced from the catalytic conversion of wet biomass at moderate SCWG temperatures (375–450 ∘C) at which CH4 formation is favoured. For this however, an active gasification and methanation catalyst is required [2–7]. A lot of work has been performed on SCWG catalysts in order to guarantee high activity and long lifetimes, [6,8,9] as well as to understand the main deactivation mechanisms, namely leaching, [10–13] sintering, [14,15] poisoning [16–18,15] and coking [15,19,20].Many studies already showed the superiority of ruthenium-based catalysts for SCWG, be it in terms of gasification/methanation activity or in terms of stability towards leaching and sintering [8,10,16,21,22]. However, Ru is known to exhibit structure sensitivity in several catalytic reactions such as Fischer-Tropsch synthesis, [23,24] ammonia synthesis [25,26] and ammonia decomposition [27,28]. This is also the case for the methanation reaction, where size sensitivity was observed for Ru/TiO2, [29,30] Ru/C [31] and single crystals [32]. The reason for this size sensitivity is thought to arise from B 5 sites, introduced by van Hardeveld and van Montfoort, [33] which are a combination of five under-coordinated “step-edge” atoms creating a three-dimensional adsorption site for reactive species (i.e. CO or N2). For the methanation reaction, CO dissociation seems to be the rate-limiting step and occurs preferentially at under-coordinated sites, as shown over a Ni surface [34]. On Ru, ab initio studies at 400 ∘C reported lower free energy barriers during CO methanation at stepped Ru surfaces through multiple hydrogen transfer steps leading to an easier C-O bond cleavage and the subsequent formation of H2C. and water [35]. The computational study of Shetty et al. [36] showed a lower CO dissociation energy barrier at “hollow” Ru sites (allowing high coordination of CO) than on stepped surfaces, indicating that specific sites (such as B 5 sites) are highly active for CO activation. These very active sites have high probabilities of being found at defined Ru nanoparticle (NP) diameters, as they are purely geometrical features. Based on the work of van Hardeveld and van Montfoort, [33] Jacobsen et al. [25] showed that there was a high concentration of B 5 sites on Ru NPs of 1–3 nm in diameter. They suggested that the increase in ammonia synthesis activity was due to the disappearance of the smallest NPs (< 1 nm) due to sintering, which led to larger crystals containing more B 5 sites. Indeed, Ru NPs smaller than 0.8 nm exhibit very few B 5 sites, as there are not enough atoms available to form these special ensembles. Czekaj et al. [37] performed DFT calculations of Ru clusters of different sizes supported on graphitic carbon layers. They showed that Ru clusters only stabilised in given geometries on graphite, and that 1.5 nm Ru NPs contained more B 5 sites (i.e. 12) than 1.0 nm Ru NPs (i.e. 6), which is in line with the high activity observed with Ru/C catalysts composed of small Ru NPs (1.2–1.4 nm) [17].Most fundamental studies on Ru-based SCWG catalysts were performed with activated carbon (AC) as support, as it possesses a high specific surface area and exhibits good mechanical stability in supercritical water (SCW). Unfortunately, using AC as catalyst support is far from optimal for several reasons. On the one hand, the high surface area of AC mainly arises from micropores, which are often too small ( < 1 nm) to welcome Ru NPs, and are prone to rapid surface area losses and pore blockage by coke deposits, eventually leading to mass transfer limitation and catalyst deactivation [15,38,39]. On the other hand, the heterogeneity, density and intrinsic activity of AC make it difficult to precisely evaluate the different deactivation mechanisms, e.g. coking or sintering.De Vlieger et al. showed that carbon nanofibers (CNF) could be used in continuous SCWG, highlighting the good stability of unsupported and supported CNF [19]. They followed up with another study using Ru/CNF for aqueous-phase reforming of acetic acid, proving the good gasification activity and deactivation resistance of this material in SCW [40]. CNF were also shown to be an ideal support for particle size effect studies due to their high pore volume, mostly open porosity (micropore free), high specific surface area, as well as their purity and inertness [41]. Furthermore, the CNF structure makes it an ideal support for the analysis of supported metal NPs, particularly in transmission electron microscopy (TEM) [42].The geometry of a particle is known to greatly depend on the atmosphere it is exposed to, even at low partial pressures [43]. Hence, the effect of particle size on Ru activity in SCWG may therefore greatly differ from the theory. Despite a few preliminary studies, [44,45] to the best of our knowledge, the effect of the Ru NP size on the gasification activity has never been thoroughly studied in SCWG conditions. In this paper, we elucidated the particle size effect in a model SCWG system, using Ru/CNF catalysts of different Ru NP sizes to gasify aqueous glycerol solutions to CH4.Commercial carbon nanofibers (CNF, NC7000, Nanocyl) were used as catalyst support. They were first sieved to 0.50–0.80 mm, then purified in 1 M KOH (2 h, reflux), washed in deionised water (DI H2O) until the filtrate was neutral, dried overnight (110 ∘C in air), and sieved again to the fraction of interest (0.50–0.80 mm). The purified support was then impregnated with RuNO(NO3)3 (31.3% Ru, Alfa Aesar) or RuCl3 ⋅ xH2O (38% Ru, Alfa Aesar) dissolved in DI H2O with the incipient wetness method (IWI). The solution concentration was adapted to reach the desired catalyst loading (xRu) based on the support pore volume (Vp = 3.6 cm3 g−1), determined by addition of water to mimic the synthesis method. The impregnated CNF were dried at 110 ∘C (air, 15 h) before being reduced in a quartz reactor (i.d. = 45 mm, L = 600 mm, with a fritted disc in the middle) for 4 h at 300 ∘C (5 ∘C min−1) in H2/N2 (5:95 vol/vol, 150 mL min−1). After, the reactor was cooled down to room temperature and the catalysts were passivated by letting air diffuse through the quartz reactor. This procedure allowed the formation of a RuO2 oxide layer at the surface of Ru(0) particles in a controlled and reproducible way, which will be readily reduced back under reactive conditions [17]. The Ru/CNF catalysts were eventually sieved again to 0.50–0.80 mm before being loaded in the catalytic reactor. At this stage, the catalysts were referred to as “fresh”. High-purity glycerol (≤ 99.7%, Carl Roth GmbH & Co. KG) was diluted in DI H2O to reach glycerol concentrations ranging from 6 wt% to 20 wt% and was used as biomass model feed for the SCWG experiments.The catalytic performance was investigated on a SCWG setup used in a previous study [10] (Konti-I, P&ID shown in Fig. S1). A high-pressure pump (Knauer 80 P) fed the aqueous glycerol (6–20 wt%) into the system at 28.5 MPa. A series of three heaters was used to bring the feed up to 405–410 ∘C at the beginning of the catalyst bed. A 316 L stainless steel tube (SITEC-Sieber Engineering AG) was used as fixed-bed plug-flow reactor (L = 460 mm, i.d. = 8 mm, o.d. = 14.3 mm), the flow configuration was top to bottom. The Ru/CNF catalyst bed was situated in the middle of the reactor (260 mm from reactor entry), held in place by three sizes of stainless steel wire mesh – 0.08, 0.25 and 0.50 mm placed on top of a hollow stainless steel rod. The rest of the catalyst bed was filled with α-Al2O3 beads (0.8 mm diameter, 0.03 cm3 g−1 porosity, Alfa Aesar), which was used as inert filling material. A heat exchanger was located at the exit of the catalytic reactor to cool down the effluent. A back pressure regulator (BPR, Tescom), protected by a 15 μm frit, maintained the system at the desired pressure (28.5 MPa). After the BPR, the effluent entered a phase separator from where the liquid and gas phases exited the setup. The latter was cooled through a Peltier element (1–4 ∘C) to remove the water before being analysed online with a μGC (Inficon). An automated sampler located between the BPR and the phase separator was used to collect the liquid effluent at defined times on stream (TOS). Samples (2 min sampling time, 10–15 mL) were taken every 30 min to monitor the carbon and ruthenium concentrations. A target WHSVgRu of 4000 gorg g Ru − 1 h−1 was selected to ensure a final carbon conversion below 50% and thus monitor the catalyst activity (i.e. turnover frequency, TOF) in the kinetic regime.The absence of internal mass transfer limitation was verified through the Weisz-Prater criterion [46], which was in an acceptable range (0.03 ≪ 0.3) for all catalytic tests performed in this study.The gas produced from the catalytic experiments was analysed online with a μGC 3000 series (Inficon) having two different columns (Molsieve, 10 m x 320 μm x 30 μm and PLOTQ, 8 m x 320 μm x 10 μm) with TCD detectors. The former analysed H2, O2, N2, CO, and CH4 in He as carrier gas at 120 ∘C, 25 psi. The latter analysed CO2 and C2,3 in Ar as carrier gas at 70 ∘C, 20 psi. The carbon content of the unfiltered liquid effluent was analysed on a Dimatoc2000 (DIMATEC) total (TC), total inorganic (TIC) and total organic (TOC) carbon analyser. The instrument determined the TC by oxidising the carbon into CO2 at 850 ∘C in a quartz reactor containing a Pt/SiO2 catalyst. The TIC was determined by converting the carbonates to CO2 at 160 ∘C with addition of H3PO4 (42.5%) in a quartz reactor filled with porous silica gel beads. The TOC was eventually determined by subtraction (TOC = TC – TIC).The carbon gasification efficiency GE C was determined by Equation 1. (1) G E C ( % ) = n ˙ C , g a s n ˙ C , f e e d ⋅ 100 % by knowing the flow of carbon in the produced gas ( n ˙ C , g a s ) and the amount of carbon entering the system ( n ˙ C , f e e d ) per unit of time.The carbon conversion (X C ) was calculated with Equation 2. (2) X C ( % ) = T O C f e e d − T O C o u t T O C f e e d ⋅ 100 % where TOC feed and TOC out are the amounts of organic carbon present in the feed and the process waters, respectively.The specific surface area (SSA) and the pore volume (Vp) were measured by N2 physisorption (77 K) on an Autosorb-1 (Quantachrome). The samples were outgassed in dynamic vacuum (10−6 bar) for a minimum of 3 h at 300 ∘C. The SSA was calculated according to the Brunauer-Emmett-Teller (BET) model, the total pore volume was determined at a relative pressure p p 0 − 1 ≥ 0.99 .After synthesis, the catalyst loading was verified by calcination in static air (900 ∘C, 10 ∘C min−1, 12 h) in a muffle oven (Nabertherm). The ash content of the support was then subtracted and the residual ash content was corrected, as the ruthenium was completely oxidised (RuO2). The loading determination through calcination was in good agreement with the calculated loading from the impregnation step (error ≤ 10%), validating this characterisation method.The Ru dispersion (DTEM) was determined from TEM micrographs acquired with a JEOL JEM 2010 microscope operated at 200 keV and equipped with a LaB6 cathode. Images were recorded by a slow scan CCD camera (4008 × 2672 pixels, Orius Gatan Inc.). High-resolution TEM images were acquired on a probe-corrected JEOL JEM-ARM200F (NeoARM) microscope equipped with a cold-field emission gun operated at 200 keV and a Gatan OneView camera. The instrument could be operated in TEM or STEM modes. Samples were prepared on lacey carbon grids (Ted Pella Inc.) using ethanol to disperse the ground catalyst. For each catalyst sample, a thorough qualitative analysis was performed and representative micrographs were carefully selected to determine a particle size distribution (PSD). The minimum sample size for each analysed catalyst was 160 particles, except for the fresh 30%Ru/CNF and both spent 1%Ru/CNF catalysts (sample size ≈ 100). The histogram bin size for the PSD was selected by following the guidelines of Alxneit [47] to ensure a statistically representative particle size determination. The Ru NP diameters were corrected for the formation of the oxide passivation layer in contact with air, which was reported to reach 0.6 nm [48]. For Ru NPs smaller than 1.2 nm, the size was corrected by the ratio of the Ru(0) and RuO2 bulk densities, as performed in other studies [49,50]. The dispersion was then calculated according to Equation 3, (3) D T E M ( % ) = ∑ i R u s f c , i ∑ i R u t o t , i ⋅ 100 % where Rusfc,i and Rutot,i are the amount of surface and total Ru atoms in the ith NP, calculated from the geometrical equations published by van Hardeveld and Hartog [51], linking the particle diameter to the number of atoms for a truncated bipyramid (see Fig. S2 and Table S1). The detailed calculation steps can be found in the supporting information (SI). The reported Ru NP diameters refer to the main mode obtained from the PSDs (Figs. S3-S6).The turnover frequency (TOF) was used to compare the activity of the different catalysts and was calculated with Equation 4, (4) T O F m o l C m o l R u , s f c ⋅ m i n = n ˙ C , f e d ⋅ X C n R u ⋅ D T E M where n ˙ C , f e d is the mole flow rate of carbon into the system, n Ru is the moles of Ru in the catalyst bed and D TEM is the Ru dispersion. Thus, n Ru , sfc = n Ru ⋅ D TEM . The initial activity (TOF 30 min ) was calculated with the fresh catalyst dispersion, while the steady-state activity (TOF ∞ ) was calculated with the dispersion of the spent catalyst.The reaction rate (r) was used to evaluate the impact of dispersion loss on the activity of the catalyst and was calculated with Equation 5. (5) r m o l C m o l R u , t o t ⋅ m i n = n ˙ C , f e d ⋅ X C n R u Thermogravimetric analyses (TGA, Mettler Toledo TGA/SDTA 851e) were performed on the CNF support as well as on selected catalyst samples (fresh and spent). Approximately 10 mg of sample were loaded in an Al2O3 crucible. The samples were first heated up to 110 ∘C in air (5 ∘C min−1, 30 min hold) to get rid of the moisture. The temperature was then increased to 900 ∘C (5 ∘C min−1). Analyses were performed with air as reactive gas (10 mL min−1) and Ar (10 mL min−1) as protective gas.The Ru concentration in process waters was analysed using an Agilent 7700x ICP-MS with the following parameters: RF power 1350 W, sampling depth 10 mm, carrier gas (Ar) flow rate 0.93 L min−1. Each sample was acidified to 1% HNO3 using TraceSELECT™ nitric acid before analysis. Two isotopes (99Ru and 101Ru) were monitored at an integration time of 0.20 s in transient analysis mode. External calibration with commercial standards from Inorganic Ventures (ICP Precious Metals Std) was done using five standard points with concentrations of 0, 0.01, 0.1, 1, and 10 μg L−1.The main support and catalyst properties are summarised in Table 1. When comparing the as-received support (CNF_AR) and the purified one (CNF), one can see that treatment in KOH did not alter the support properties, as the SSA and pore volume remained similar. The catalyst synthesis did not affect the SSA too much either, with values ranging from 297 ± 14 m2g−1 (1%Ru/CNF_2) to 226 m2g−1 (30%Ru/CNF). However, the pore volume decreased linearly with increasing Ru loading. The decrease in SSA with increasing metal loading was explained by the increase in material density (Fig. S7, right). However, the decrease in pore volume was not fully explained by the density (Fig. S7, left). The volume occupied by the Ru NPs is the most likely explanation for the pore volume decrease, as the 30%Ru/CNF catalyst exhibited the most pronounced loss in Vp.By preparing the catalysts by incipient wetness impregnation, the following hypothesis can be made: 1) 100% of the support pore volume was filled by the Ru precursor solution, 2) the solution was distributed homogeneously throughout the pore volume. Hence, the presence of Ru salts inside the CNF are expected to remain there upon thermal treatment, as observed with other metallic salts in SBA-15 [52,53]. Statistically, 12 ± 7% of Ru should be located on the inside of the fibres used in this work. This is in line with results from Winter et al., who proved that Co and Pd NPs were located inside carbon nanotubes (up to 15% and 34%, respectively) after synthesis [54].The catalyst synthesis method applied in this work yielded very small Ru NPs, with DTEM reaching 67–71% for the 1%Ru/CNF and 5%Ru/CNF catalysts (dp = 0.9–1.1 nm, Fig. 1). Another 5%Ru/CNF catalyst not shown in this work, synthesised with the same technique, yielded Ru NPs of the same size (dp = 1.0 nm, DTEM = 69%). These figures also highlight the high reproducibility of the synthesis method. The use of a chloride salt instead of nitrosyl nitrate led to a slightly higher dispersion.Very small Ru NPs (≤ 1.1 nm, DTEM ≥ 67%) were achieved with catalyst loadings up to 5%, while higher Ru contents led to a decrease in DTEM, from 59% for 10%Ru/CNF down to 35% for 30%Ru/CNF. However, even the high loading of the latter catalyst yielded a main Ru NP diameter mode at 2.0 nm, which remains relatively small. The evolution of dp and DTEM as function of Ru loading is shown in Fig. 2 for the fresh catalysts. These results show that small Ru NPs, i.e. high metal dispersion, can easily be achieved on CNF with a facile synthesis method, even at high loadings.In Fig. 3, an example of a catalytic SCWG experiment is shown with the 10%Ru/CNF catalyst (see Figs. S8-S14 for all other experiments). The carbon gasification efficiency (GEC) and conversion (XC) overlapped, indicating that all the converted carbon ended up in the gas phase (the discrepancy observed for the first GEC data point is explained by the significant variation in gas flow at the start of an experiment). 10%Ru/CNF exhibited a high initial activity, before stabilising towards 30–35% conversion. Because steady state was not reached for all catalysts, an extrapolation was performed in order to have a more accurate estimation of the steady-state values for XC and TOF and ensure a better comparison between the different catalysts. Both parameters were hence fitted with an exponential decay function (optimised through a Levenberg-Marquardt iteration algorithm). An example is shown for the 10%Ru/CNF catalyst in Fig. S15.The carbon conversion was reported for all experiments (see Fig. S16), where two distinct trends were observed. The catalysts exhibiting a lower activity (15%Ru/CNF, 20%Ru/CNF, 30%Ru/CNF) had initial conversions in the range 50–65%, stabilising towards ≈ 20% at steady state. On the contrary, catalysts of lower loadings and smaller Ru NP diameters (1%Ru/CNF_2, 5%Ru/CNF_Cl, 5%Ru/CNF_1, 10%Ru/CNF) showed initial conversions higher than 65%, stabilising at steady-state values around 40–50%. Both 1%Ru/CNF catalysts exhibited high initial conversions, but suffered from drastic deactivation. At the evaluated conditions, full conversion was initially achieved for the 5%Ru/CNF_Cl and 1%Ru/CNF_1 catalysts, preventing accurate estimations of an initial TOF.To understand the implications of the different deactivation mechanisms, the effects of leaching, sintering and coking were assessed for this set of experiments.We showed in a previous work that Ru leaching was negligible from activated carbon supports, with concentrations in the range 0.01–0.2 μg L−1, being close to thermodynamic models [10]. Ru NPs may exhibit a decreased metal-support interaction on CNF, as this support is known to be more inert than activated carbon due to its well-defined structure [55,56]. To investigate the effect of Ru loss, time-resolved ICP-MS was used to quantify the Ru loss from the Ru/CNF catalysts. The acquired data (Figs. S17 & S18) shows that the final Ru concentrations in the process waters were in the same range as for Ru/AC. Most catalysts exhibited a similar Ru loss trend i.e. higher amounts at the start before stabilising towards 0.06–0.12 μg L−1. The measured concentrations showed that the higher inertness of the support (compared to activated carbon) did not alter the metal-support interaction. Hence, leaching is thought to have a negligible effect on catalyst deactivation.In the case of the 1%Ru/CNF_1 catalyst, it is interesting to note that the Ru concentration in the process waters suddenly increased (8-fold) after 3 h TOS, which coincided with the rapid decrease in XC (Fig. S19). The reason for this Ru loss behaviour remains unclear. Changes in solvent properties around the Ru NPs could be the reason for this increase, as the density and the chemical composition of the medium rapidly changed due to the rapid loss in XC. With a rapid change in XC from 100% to 10%, the gaseous products were replaced mainly by glycerol and its degradation products. However, more experimental data is required to further conclude on the effect of rapid activity loss on the Ru loss increase.As Ru leaching was shown to be negligible, the effect of sintering and coking were investigated together due to the impossibility of disentangling both effects in SCWG conditions. To do so, the SCWG activity (TOF) of the different Ru/CNF catalysts was compared in Fig. 4, where the top graph regroups the experiments performed at WHSVgRu ≈ 4000 gorg g Ru − 1 h−1 and the bottom graph shows the 1%Ru/CNF catalysts tested at different WHSVgRu (3000 and 9000 gorg g Ru − 1 h−1). In the top graph of Fig. 4, all catalysts showed a similar trend in TOF throughout the experiments. The highest activity was recorded for 5%Ru/CNF_Cl, which also exhibited a very high dispersion (69%). Note however that this catalyst reached high initial conversions, close to the thermodynamic value, explaining the plateau observed initially. The other 5%Ru/CNF_1 catalyst was less active at the beginning of the experiment, but eventually stabilised at a steady-state TOF (TOF ∞ ) in the same range as the former catalyst (TOF ∞ = 55 and 47 min−1, respectively). The activity of 10%Ru/CNF was slightly lower than both 5%Ru/CNF catalysts, but still significantly higher than the higher-loading catalysts. For the 15%, 20% and 30%Ru/CNF catalysts, the loss in activity was more pronounced and their TOF ∞ were consequently lower (26–28 min−1). Looking at the 1%Ru/CNF catalysts (Fig. 4, bottom), the initial activity was much higher for 1%Ru/CNF_2 than for the other catalysts and reached TOF = 152 min−1 at XC = 64%), while 5%Ru/CNF_Cl was limited at 105 min−1 because of the high conversion (XC ≈ 100%). Due to the relatively high initial conversion, the initial TOF of the 1%Ru/CNF_2 experiment might be underestimated. However, the TOF loss was very rapid in both 1%Ru/CNF experiments, leading to low final TOF values overlapping after TOS = 4 h and stabilising at 11 min−1. The complete data set discussed here can be found in Table S2.In Fig. 5, the initial TOF taken at 30 min TOS (TOF30 min), is presented as a function of catalyst dispersion. Note that the initial conversions of the 5%Ru/CNF_1 and 10%Ru/CNF catalysts being above 60% (70% and 76%, respectively), the TOF 30 min may be slightly underestimated (marked with an asterisk in Fig. 5). For catalyst dispersions between 35% and 60%, the TOF 30 min remained relatively stable in the range 80–100 min−1. However, very small Ru NPs (DTEM > 65%) presented a higher initial TOF than larger ones. This clearly evidences a particle size effect in SCWG over Ru-based catalysts and highlights the benefit of working with small Ru NPs around 1.0 nm. After the experiments, the spent catalysts were analysed by TEM and N2 physisorption and the results are reported in Table 2. All catalysts suffered from dispersion loss, but to different extents as it can be observed in Fig. 6. Globally, the catalysts exhibiting the most significant dispersion losses were those with the highest initial values. The Ru NP size of all spent catalysts stabilised towards 2–3 nm, independent of the initial dispersion and loading. These results could suggest a thermodynamically-stable Ru NP size, as reported by Parker and Campbell [57] for gold NPs supported on TiO2. The pore volume generally increased by 5–10%, whereas the surface area decreased by 10–20%. Overall however, all catalysts exhibited excellent stability, which demonstrates the compatibility of the Ru/CNF catalytic system with supercritical water conditions.The TOF ∞ of all catalysts are presented in Fig. 5. As described previously, the dispersion of all tested catalyst decreased to values ranging from 30% to 50% after the gasification experiments. In contrast with the initial TOF values, TOF ∞ were in disarray with values varying by a 2-to-3-fold factor for catalysts with final dispersions around 40–45%. Interestingly, the three high-loading catalysts (15%, 20%, 30%) showed similar TOF ∞ around 32 min−1, both 5% and the 10%Ru/CNF catalysts around 60–80 min−1, while both 1%Ru/CNF catalysts exhibited very low TOF ∞ values ( ≈ 15 min−1). These results suggest that the catalyst dispersion alone is not the only parameter influencing the steady-state catalyst activity.In Fig. 6, the rate loss was reported as a function of the loss in DTEM. The dashed line representing "rate loss = DTEM loss" is an indication of the level at which the rate loss would be entirely caused by the loss of Ru dispersion. Since all catalysts lie above that line, another phenomenon than Ru NP sintering contributed to the observed deactivation. The impact of Ru loss by leaching or loss of catalyst debris being negligible, the other cause of catalyst deactivation in this study can only be coking. However, the impact of coking on mass transfer limitation to the inside of the CNF cannot explain the observed difference between the catalysts, assuming a constant fraction of Ru NPs anchored on the inside of the fibers (see Section 3.1). The share of deactivation caused by coking does not appear to be the same for all catalysts. Indeed, the two catalysts with the highest TOF ∞ in Fig. 4 exhibited the smallest difference between the rate loss and the reference line, indicating that coke had a lower impact on deactivation.One could have expected the catalysts of higher loadings to sinter more significantly, because of the lower inter-particle distance. Indeed, Yin et al. [58] showed that a critical particle distance existed, up to which significant NP sintering occurred. In their study, Pt sintering could be avoided up to 900 ∘C through higher spacing between the NPs, either by using lower metal loadings or high surface area supports. However, the lowest DTEM losses occurred for the higher-loading catalysts (15%, 20% and 30%Ru/CNF). The limited sintering data presented in this work suggests that, for all catalysts, the inter-particle distance must have been below this critical particle distance due to the high surface area of CNF.Peng et al. [59] reported that a 5%Ru/AC catalysts exhibited an increased stability compared to its 2% analogue during isopropanol conversion (450 ∘C, 30 MPa), even though the dispersion of the former was lower. He showed that the coke formation rate was lower on the catalyst containing a higher fraction of Ru, although the Ru NPs were larger (5 nm vs. 3 nm). This is in line with the results presented here, showing that the dispersion was not the only factor affecting the stability of the catalyst. One reason for the 3-times higher activity and good stability of both 5%Ru/CNF catalysts with regard to other catalysts of same dispersion could be due to the density of active sites. Indeed, the metal loading can significantly vary between catalysts of similar dispersions.To verify this hypothesis, TOF 30 min and TOF ∞ were plotted as function of the surface density of active Ru (Rusfc) in the fresh and spent materials ( Fig. 7). As described previously, the TOF 30 min of 1%Ru/CNF_2 clearly stood out due to its very high initial activity at high space velocity. For the other catalysts, TOF 30 min decreased before stabilising at higher Rusfc surface density ( ≈ 1.5 atomsRu,sfc nm−2). The smallest Ru NPs (0.9–1.2 nm) were clearly responsible for the high initial activity with Ru surface densities lower than 0.8 atomsRu,sfc nm−2. However, the trend changed when looking at the steady-state data (TOF ∞ ), with an optimum Ru surface density appearing in the range 0.4–0.7 atomsRu,sfc nm−2, corresponding to a catalyst of Ru NP diameter around 2.1 nm with a 4–8 wt%Ru loading and 260 m2g−1 surface area. At higher Ru surface density, the steady-state trend matched the initial activity one, remaining constant with increasing Rusfc surface density. This is a strong indication that the steady-state catalyst activity can be increased by having the optimal amount of active sites at the catalyst surface. It is important to keep in mind that the mentioned Rusfc surface densities are average densities, and do not necessarily represent the reality when looking at the local surface density, especially for the low-loading catalysts (1%Ru/CNF). A catalyst synthesis method yielding high catalyst homogeneity and narrow particle size distribution as used here (Fig. 1) is likely of great importance.As mentioned previously, coking may be one of the deactivation mechanisms leading to the rapid loss in activity for the 1%Ru/CNF catalysts. The extent of coking investigated by HR-TEM revealed that very little coke deposited on all spent catalysts. Except for the slight increase in Ru NP size, hardly any difference could be observed between the fresh and spent catalysts. However, in the case of the 1%Ru/CNF_2 catalyst treated at high space velocity, very thin carbon deposits could be observed at the surface of Ru NPs, as it can be seen from representative images in Fig. 8. Note that these images were acquired at very low beam intensity and exposure to limit carbon deposition from the environment. The fact that this observation could only be made on the catalyst that suffered from the most severe deactivation supports coking as the source of deactivation together with sintering. This also suggests that deactivation by coking only occurred by the deposition of a thin layer of C at the surface of the metal.To further investigate coke deposition, TGA was performed on the neat support, the 1%Ru/CNF and 5%Ru/CNF_Cl catalysts ( Figs. 9 and 10). For the neat CNF support, a sharp weight loss was observed with a maximum at 590 ∘C. When Ru was loaded onto the CNF (1 and 5 wt%), the weight loss occurred at lower temperatures and over a longer range, with maximum rates at 570 and 550∘C, respectively. The weight loss patterns were different for the fresh and both spent (1%Ru/CNF_1 and 1%Ru/CNF_2) catalysts. The former underwent constant weight loss, with one clear contribution in the DTG curve close to 600 ∘C. The spent catalysts exhibited broader differential profiles, with the weight-loss offset higher in temperature for 1%Ru/CNF_2 ( ≈ 510 ∘C vs. ≈ 460 ∘C). This indicated an altered/decreased activity of ruthenium for oxidising the support (especially for 1%Ru/CNF_2), which could be ascribed to Ru NP size increase and/or coke deposits. The catalyst treated at lower WHSVgRu exhibited a similar initial oxidation activity to the fresh 1%Ru/CNF catalyst, although it required higher temperatures to fully oxidise the support and the carbon deposits. For 1%Ru/CNF_2 treated at higher WHSVgRu, the initial weight-loss phase was similar to the neat CNF, showing that the Ru NPs had a low activity towards support oxidation. Only at temperatures above 600 ∘C could the support and coke be completely oxidised. This is in phase with HR-TEM results and supports that Ru was largely covered and blocked by coke.TGA performed on the best-performing catalyst (5%Ru/CNF_Cl, Fig. 10) showed a completely different weight-loss trend. Indeed, the offset was shifted to a lower temperature for the spent catalyst ( ≈ 300 ∘C) compared to the fresh one ( ≈ 380 ∘C), while the differential profile looked similar. The maximum weight-loss rate of the spent catalyst was shifted to a slightly lower temperature (520 vs. 550 ∘C). The significant shift in temperature observed for the 5%Ru/CNF_Cl catalyst cannot be explained by deposited coke precursors of lower thermal stability than CNF because of the very small difference in the differential weight-loss curve. The reason for this shift in temperature remains unclear. For both loadings (1% and 5%), the spent catalysts lost slightly more weight than the fresh ones, indicating either a slight decrease in ash content due to the SCW conditions and/or an increase in the carbon fraction i.e. by coke deposition. The observed difference being small, it did not allow us to exclude one of the possibilities and we were not able to conclude on the impact of metal loading on coke deposition.As discussed previously, a higher Ru NP density at the CNF surface seemed beneficial to maintain a high gasification activity (5%Ru/CNF vs. 1%Ru/CNF) and avoid catalyst deactivation by carbon deposits rapidly obstructing the access to the active sites, most probably by covering Ru NPs with nanometric layers of carbon.We showed that Ru/CNF catalysts can successfully be used in continuous SCWG systems and exhibit high gasification activity and stability. Structure sensitivity of Ru was shown for the first time in SCWG conditions, with Ru NP diameters smaller than 1.2 nm leading to the highest initial activity. However, a combination of high initial dispersion and optimal surface ruthenium density was shown to be crucial to maintain a high steady-state activity. The optimal surface Ru density was found to be around 0.4–0.7 atomRu,sfc nm−2, which is thought to help in delaying/suppressing coke formation by limiting carbon deposition on large Ru-free carbon surfaces. The loss of active phase from Ru/CNF catalysts being negligible, the observed catalyst deactivation was linked to a combination of coking and Ru NP sintering, systematically stabilising to 2–3 nm. Coking was found to occur in very small amounts and to be limited to nanometric layers on top of Ru NPs. With an optimal particle size and surface density identified, efforts must now go into improving the stability of Ru nanoparticles towards coking, for instance through doping of the carbon support or ruthenium to limit adsorption of unsaturated compounds.Evaluating deactivation mechanisms in SCWG conditions is not straightforward, nevertheless Ru/CNF proved to be an ideal system to perform in-depth catalytic studies in SCWG conditions. This is mainly due to the well-defined CNF structure and high contrasts generated in TEM, as well as the large CNF pore volume (micropore-free) and surface area allowing high Ru loadings. The good stability of Ru/CNF catalysts gives new opportunities in the field of catalytic SCWG as it allows the preparation of highly dispersed and homogeneously distributed Ru nanoparticles. Its large open (non-microporous) surface allows a much higher loading than activated carbon at optimal Ru surface density, with optimised catalyst stability towards coking. Consequently, the volume of the catalyst fixed bed can be reduced, which should in turn reduce investment costs for SCWG processes, the latter being closely linked with the volume of a vessel at high pressures. The feasibility of using this catalyst on larger scales should however be assessed. Christopher Hunston: Investigation, Methodology, Validation, Formal analysis, Writing – original draft. David Baudouin: Conceptualization, Methodology, Validation, Writing – review & editing, Supervision. Leo Koning: Investigation, Formal analysis. Ayush Agarwal: Investigation, Validation, Formal analysis. Oliver Kröcher: Resources, Writing – review & editing, Supervision. Frédéric Vogel: Methodology, Writing – review & editing, Funding acquisition, Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This project was financially supported by the Swiss National Science Foundation grants 200021_172624/1 and 200021_184817, as well as by the Swiss Innovation Agency Innosuisse and is part of the Swiss Competence Centre for Energy Research SCCER BIOSWEET. The authors would like to thank Erich De Boni and Pascal Unverricht for designing, optimising and keeping the continuous SCWG test rig running smoothly. We also thank Andrea Testino for helping with TGA measurements and Ivo Alxneit for some HR-TEM images. The Swiss National Science Foundation (R′Equip Project 206021_177020) is kindly acknowledged for the co-funding of the electron microscope.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2022.121956. Supplementary material . Supplementary material .

Ru/CNF catalysts of different Ru nanoparticle (NP) sizes (0.9–2.7 nm) were assessed for their performance in continuous supercritical water gasification (SCWG) of glycerol. A structure sensitivity of Ru was demonstrated, with high initial turnover frequencies (TOF) for Ru NPs smaller than 1.2 nm. Deactivation happens essentially through: 1) coking, which occurs more readily at low Ru surface densities and is limited to nanometric layers, and 2) sintering, which led to final NP sizes ranging from 2 to 3 nm independent of the catalyst loading (1–30 wt% Ru). A correlation between TOF and the density of surface Ru atoms was found, with an optimal surface density (0.4–0.7 atomRu,sfc nm−2) resulting in high initial and high steady-state TOFs, allowing longer lifetimes through the delay of coke formation. CNF as a support enables high metal loadings with optimal performance, which could help in decreasing the volume of high-pressure vessels and hence the cost of SCWG plants, making this technology even more attractive.

Data will be made available on request.As the only renewable carbon source, the development of biomass energy is conducive to reducing fossil-fuel energy depletion, mitigating greenhouse gas emissions, and promoting sustainable development [1,2]. Gasification is one of the most effective mechanisms for the utilization of biomass [3,4]. However, the presence of tar not only reduces the gasification efficiency but also leads to the blockage and corrosion of downstream pipelines [5–7]. Therefore, tar removal is crucial for improving biomass gasification.Thermal catalytic reforming is an effective method for treating biomass tar, which not only removes tar but also increases the gas yield [8–10]. Ni-based catalysts are widely used for tar reforming owing to their low cost, easy preparation, and high activity for tar cracking and hydrocarbon conversion [11–13]. However, such processes are energy-intensive given that the thermal catalytic reforming of tar is conducted at high temperatures. Moreover, Ni-based catalysts are easily deactivated by coke deposition, sintering, or oxidation [14,15].Photocatalysis can decompose organic pollutants under mild conditions by promoting relevant redox reactions. Among the various photocatalytic materials, TiO2 has been widely studied as a suitable semiconductor material owing to its high catalytic activity, high stability, non-toxicity, and low cost [16–18]. However, its applicability is constrained by its limited photocatalytic reaction rate [19].In contrast to the limitations of thermal catalysis and photocatalysis, photothermocatalysis can simultaneously apply light and heat to the catalyst to promote thermochemical reactions at lower temperatures with a higher efficiency [20–22]. For example, Li et al. [22] prepared Pt/TiO2 catalysts and conducted the catalytic oxidation of benzene under combined UV irradiation and thermal energy. Their results showed that the photothermocatalytic oxidation of benzene is more effective than photocatalytic and thermal catalytic oxidation individually. Ren et al. [23] found that the photothermocatalytic activity of the TiO2 nanosheets under mercury lamp irradiation at 240 °C or 290 °C was much higher than its cumulative photocatalytic activity under UV irradiation with the same intensity and thermal catalytic activity at the same reaction temperature. These studies demonstrate the feasibility of photothermocatalysis for the treatment of volatile organic compounds (VOCs), attributing to the synergistic photothermal effect. In addition, Mao et al. [24] prepared a Pt/CeO2 catalyst that was effective for the catalytic oxidation of benzene under UV-, visible-, and infrared-light irradiation. Li et al. [25] conducted photothermocatalytic experiments on typical VOCs and CO2 using a Ag3PO4/Ag/GdCrO3 catalyst. At 363 K and under visible-light irradiation, the catalyst achieved nearly 100% conversion of toluene. Wei et al. [26] reduced CO2 by constructing In-Em In2O3 nanoflake with metallic In embedded, which exhibited higher intrinsic activity of CO2 reduction than In2O3 under the light condition. Some studies demonstrate that the excellent performance of the synergistic photothermocatalysis results from the introduction of a plasmonic metal through the local surface plasmon resonance (LSPR) effect, which further improves the catalytic performance [24–27].Currently, most researches on the photothermal catalysts are focused on noble metals, including Pt [22,24,28], Ag [29,30], Au [31,32]. Experiments have shown that transition metals (Ni, Co) also exhibit a photocatalytic activity. Song et al. [33] investigated the photothermocatalytic activity of Ni2P/TiO2 nanoparticles for hydrogen generation from methanol. The photothermocatalytic activity of Ni2P/TiO2 was approximately 3.6 times the sum of activities associated with the photocatalytic and thermocatalytic reactions, exhibiting a synergistic photothermal effect. Shi et al. [34] prepared Co3O4/TiO2 nanocomposites possessing high catalytic activity and stability. A synergistic effect between the photo-assisted thermal catalysis of Co3O4 and UV photocatalysis of TiO2 could be observed, accelerating the oxidation of benzene on Co3O4. Fiorenza et al. [35] found that CoO-CuO supported on the TiO2-3% CeO2 gave the best results of CO2 conversion. The energy synergism between the thermocatalytic and photocatalytic mechanisms increased the CO2 conversion and favored efficient e-/h+ transfers.Photothermocatalysis is predominantly applied to promote the oxidation of VOCs to CO2 and H2O, or in the CO2 reduction with methane (CRM). Researchers have prepared and explored various catalysts for CRM to produce syngas and reduce greenhouse gas emissions [36,37]. Liu et al. [38] prepared Ni/Al2O3 catalysts and applied them to CO2 reduction by CH4 to produce syngas. As a result of light irradiation, the Ni/Al2O3 activity increased significantly and the syngas yield was doubled. Tan et al. [39] found that Ni/Mg-Al2O3 demonstrated high production rates of H2 and CO for photothermocatalytic CRM under light irradiation, which also showed better photothermocatalytic durability in comparison with Ni/Al2O3. Xie et al. [40] prepared Ni/TiO2 catalysts and applied them to the dry reforming of CH4 at a high temperature. With the increase in lighting intensity, the formation rates of CO/H2 were enhanced correspondingly. In addition, some new photoactive nanomaterials such as Ru/CeO2 [41], MCM-Ni/Ni-MgO [42], NiCo/Co-Al2O3 [43], Co/Co-Al2O3 [44], and plasmonic photocatalyst Cu-Ru [45] were reported for enhancing the high light-to-fuel efficiencies of CRM and the excellent photothermocatalytic durability. These studies demonstrate that the photothermocatalytic effect could significantly increase the yields of H2 and CO through the improved catalytic performance of CO2 reforming. However, photothermocatalysis has rarely been utilized in tar treatment. Chen et al. [46] conducted experiments on the photothermal steam reforming of toluene using Ni/TiO2 catalyst. At 600 °C, the toluene conversion was 90.1%, which was 16.3% higher than that achieved by thermal steam reforming alone. Sun et al. [47] applied photocatalysis to the plasma-steam reforming of toluene, which showed that the conversion of toluene could be increased by 4–5% through the introduction of UV.This study aims to effectively convert tar (contained in biomass syngas) to syngas by CO2 reforming under mild conditions. Therefore, the photothermal catalysts were prepared using Ni as the active component, TiO2 as the photo-responsive material, and Al2O3 as the carrier. Moreover, the experimental photothermocatalytic CO2 reforming (PTCR) of tar model compounds was carried out to explore the synergistic photothermal catalytic effect.Ni/TiO2-Al2O3 was prepared in this study as a photothermal catalyst using various impregnation methods. Before catalyst preparation, the Al2O3 carrier (Nanjing Aotai Catalyst Carrier Co., Ltd., China) was calcined at 600 °C for 4 h to facilitate the formation of γ -Al2O3.For the first impregnation method (M1), 10 g of a mixture of TiO2 powder (10 nm, Nanjing Hongde Nanomaterials Co., Ltd., China) and Al2O3 particles were placed in a beaker with a predefined mass ratio, followed by the addition of 20 ml deionized water. Next, the mixture was uniformly blended using an electromagnetic stirrer at 45 °C for 30 min. Ni(NO3)2·6 H2O (analytical reagent (AR) 98.0%, Sinopharm Ltd., China) was dissolved in deionized water to obtain Ni(NO3)2 aqueous solution, in which the ratio of Ni(NO3)2·6 H2O to water was adjusted according to the desired NiO content in the catalyst. Then, 30 ml of the Ni(NO3)2 aqueous solution was added to the mixture and stirred continuously until the water evaporated completely, yielding the precursor of the Ni/TiO2-Al2O3(M1) catalyst.In contrast to M1, the second method of catalyst preparation (M2) involved the addition of TiO2 after the Ni(NO3)2 aqueous solution was impregnated into Al2O3. The Ni/TiO2-Al2O3(M2) catalyst was prepared according to the following methodology. 10 g of a mixture of TiO2 powder and Al2O3 particles with a predefined mass ratio was prepared. Al2O3 particles were placed in a beaker, following which 50 ml Ni(NO3)2 aqueous solution was impregnated into the Al2O3 consistent with the desired NiO content in the catalyst. TiO2 powder was added following constant stirring of the solution at 45 °C for 30 min and the stirring was continued until the water evaporated completely to obtain the Ni/TiO2-Al2O3(M2) catalyst precursor.The semi-finished catalysts were dried at 115 °C for 12 h and calcined in a tube furnace for 6 h to obtain the finished Ni/TiO2-Al2O3 catalysts, which were ground to the sizes of 0.125–0.25 mm for the experiments. For the two preparation methods, the catalysts calcined at 450 °C under a nitrogen atmosphere were denoted as mNi/xTiyAl(M1) and mNi/xTiyAl(M2), where m denotes the mass fraction of NiO in the catalysts; x and y denote the mass ratios of TiO2 and Al2O3, respectively.As shown in Fig. 1, the experimental equipment consists of a fixed bed reactor, an electric heating furnace, a vapor generator, a gas mixer, a pipe preheater, a light source system, a cooling and collecting system, and a control system.The fixed bed reactor was made of quartz with an internal diameter of 10 mm. First, the catalyst (1.2 ml) was loaded into the fixed bed reactor before the experiment. The reaction zone was heated to the desired temperature (450 °C) using an electric heating furnace with a light window diameter of 15 mm and the whole system was purged with nitrogen during the preheating process. Following stabilization of the reaction zone temperature, CO2 (36 ml/min) as a reforming agent and N2 (54 ml/min) as carrier gas were injected into the pipeline and preheated to 150 °C by the pipe preheater. Meanwhile, benzene (analytical reagent (AR) ≥ 99.5%, Shanghai Aladdin Biochemical Technology Co., Ltd., China), as the tar model compound (0.6 ml/h), was fed into the system through a syringe pump and heated to 150 °C by a vapor generator. Next, the gas mixture entered the fixed-bed reactor after the benzene vapor was mixed with CO2 and N2. During the experiment, the reaction zone was heated using an electric heating furnace and illuminated with a xenon lamp (50 W, Beijing Zhongjiaojinyuan Technology Co., Ltd., China). The reaction bed temperature was monitored in real-time by the thermocouple and maintained at 450 °C. After that, the produced gas was cooled by the condenser and then purified and dried using an absorption device equipped with adsorbent resin and silica gel. Finally, the clean non-condensable gas product was collected in bags with a collection time of 15 min per bag. The reactant gas flow rate and reaction temperature were adjusted using a suitable control system. The benzene reforming reaction lasted for 2 h in this study.The specific surface area and pore volume of the catalyst were measured by the physical absorption instrument (ASAP2020, MAC instrument, USA).The chemical composition of the catalyst was analyzed via an X-ray diffractometer (XRD) (Ultima IV, Rigaku Corporation, Japan). The X-ray tube was a Cu target with a tube voltage of 40 kV and a tube current of 40 mA, and the diffraction angle 2θ was scanned from 10° to 80° at a rate of 1°/min with a step of 0.02°.The functional group analysis was investigated by Fourier transform infrared (FTIR) spectrometer (Thermo Nicolet iS5). All test measurements were performed according to KBr disc technique.Temperature-programmed reduction (TPR) (AutoChem II 2920, Micromeritics, USA) was performed to investigate the influences of catalyst preparation conditions on the catalytic property. The characterization procedure was as follows. Fresh catalysts were pretreated at 200 °C for 30 min in 30 ml/min of Ar and then cooled below 50 °C and purged with Ar at 50 °C for 60 min. Finally, a constant flow of 10% H2/Ar was injected as the temperature ramped at 10 °C/min to 800 °C.The surface topography of catalysts was characterized by scanning electron microscope (SEM) (Ultra Plus, Zeiss, Germany). The accelerating voltage is 15 kV. And transmission electron microscopy (TEM) (TALOS-F200X, Thermo Fisher Scientific, USA) was conducted to investigate the morphology of catalysts. The accelerating voltage was 200 kV.All the gaseous products produced in the experiment were analyzed by GC9800 gas chromatograph (Shanghai Kechuang, China) to obtain the volume fractions.The components of the gas products were CO2, N2, H2, CO and CH4. Because N2 did not participate in the reaction process, the volume flow rate of N2 was constant before and after the reaction. Therefore, the volume flow rate of each component can be calculated by Eq. 1. (1) V i = C i × V N 2 C N 2 Where Vi and Ci represent the volume flow rate (ml/min) and volume concentration (%) of gas component i (CO2, N2, H2, CO and CH4), respectively.Since all H atoms in the gas products came from benzene, the tar conversion rate (Xc, %) can be calculated as shown in Eq. 2. (2) X c = V H 2 × 2 + V CH 4 × 4 × A 22400 × V Benz × n × ρ Where A represents the molecular weight of benzene, VBenz represents the inlet volume flow rate of benzene (ml/min), n represents the number of H atoms in benzene, ρ represents the density of benzene (g/ml).The main gas products of the photo-assisted thermal catalytic reforming of tar are H2 and CO. The syngas yield (Yg, mol/kg) can be calculated by Eq. 3. The reaction rate of CO2 (rCO2, mmol min−1 g−1) and benzene (rBenz, mmol min−1 g−1) can be calculated by Eq.4 and Eq.5, respectively. (3) Y g = V H 2 + V CO × 1000 22400 × V × ρ (4) r CO 2 = V CO 2 − in − V CO 2 − out × 1000 22400 × m (5) r Benz = X c × V Benz × ρ × 1000 m × A Where m represents catalyst quality, VCO2-in represents inlet CO2 volume flow rate (ml/min), VCO2-out represents outlet CO2 volume flow rate (ml/min).The Xc and Yg were calculated based on the average values during the stable reaction process.The Brunauer-Emmett-Teller (BET) surface areas, pore volumes, and pore sizes of the Ni/TiO2-Al2O3 catalysts prepared by different methods are listed in Table 1. It can be found that the pore sizes of the catalysts synthesized by the two impregnation methods are mainly in the mesoporous range. However, different preparation conditions influence the physical structures of the catalysts. Compared to the 0.2Ni/0.5Ti0.5Al(M1) catalyst calcined in a nitrogen atmosphere, the 0.2Ni/0.5Ti0.5Al(M1)-Air catalyst calcined in air exhibits a decrease in the BET surface area and an increase in the pore size. This change might be caused by the interaction between the catalyst components and oxygen in the air. Moreover, the 0.2Ni/0.5Ti0.5Al(M1)-T550 catalyst calcined at 550 °C exhibits a significant decrease in the BET surface area, pore volume, and pore size. It implies that a high calcination temperature is apt to cause the growth and aggregation of crystal grains and the collapse of mesopores, destroying the physical structure of the catalyst.In addition, a comparison between the two different impregnation methods (with the same component content) exhibits a demonstrable difference in the pore structure. In M1, given that the TiO2 is loaded before NiO, TiO2 covers the surface of the Al2O3 carrier to form the stacking pores, hindering the impregnation of NiO into Al2O3. When the Ni/TiO2-Al2O3 catalysts are prepared by M2, as NiO is loaded onto the Al2O3 carrier before TiO2, NiO is apt to impregnate Al2O3, which results in a decrease in the surface area of the catalyst. Compared to the 0.25Ni/0.5Ti0.5Al(M2) catalyst, the 0.25Ni/0.3Ti0.7Al(M2) catalyst (with identical Ni content) possesses an increased BET surface area and pore volume, while the 0.25Ni/0.7Ti0.3Al(M2) catalyst exhibits the opposite result. Moreover, followed by an increase in the NiO content to 30%, the 0.3Ni/0.3Ti0.7Al(M2) catalyst possesses a smaller BET surface area and pore volume than the 0.25Ni/0.3Ti0.7Al(M2) catalyst. Therefore, it could be concluded that a NiO content of more than 25% and a ratio of TiO2 to Al2O3 of more than 50% are disadvantageous to the physical structure of the catalyst.The XRD patterns of the fresh catalysts are shown in Fig. 2. TiO2 exhibits the anatase phase (PDF#71–1166) in all catalysts, and the diffraction peaks of NiO are found at 2θ= 37.2°, 43.3°, 62.8°, 75.4° and 79.4° (PDF#71–1179). A diffraction peak of Al2TiO5 is detected near 2θ= 26.8° for both 0.2Ni/0.5Ti0.5Al(M1)-Air and 0.25Ni/0.5Ti0.5Al(M1)-T550 catalysts. It implies that a new Al2TiO5 phase can be formed by the interaction between TiO2 and Al2O3 when the catalyst is calcined in air or at a high temperature, which may negatively impact the catalytic activity. Compared to the diffraction peaks observed for the catalysts prepared using M1, those for the NiO and TiO2 catalysts prepared using M2 are observed at higher positions, indicating enhanced crystallinity. Fig. 3 shows XRD patterns of the spent catalysts. TiO2 is still in the anatase phase, while the Ni diffraction peaks are observed at 2θ= 44.6°, 51.8° and 76.4°, indicating the transformation of NiO to Ni during the reaction. Moreover, a diffraction peak of graphitic carbon (2θ=26.5°) is detected for all the spent catalysts, indicating coke deposition. Fig. 4 presents the FTIR spectra of the fresh and spent catalysts with different preparation methods. It can be concluded that the preparation conditions have no effects on the functional groups on the catalyst surface. Both the fresh 0.25Ni/0.5Ti0.5Al(M1) and 0.25Ni/0.3Ti0.7Al(M2) catalysts exhibit the typical FTIR spectra of TiO2-based materials, with absorption vibrational peaks at around 1640 cm−1 and 3400 cm−1, which were caused by H2O deformation vibrations and -OH stretching vibrations, respectively [35]. These bands may be due to adsorbed water molecules in the air and Ti-OH bonds. The broad band at around 600 cm−1 is caused by Ti-O-Ti vibrations [35]. Comparing the fresh and spent catalysts, it can be found that new peaks appear near 2900 cm−1 and 1430 cm−1, corresponding to -CH stretching vibration, -CH2 stretching vibration and -CH asymmetric vibration. The existence of the foregoing functional groups indicates that there are some hydrocarbon compounds on the catalyst surface produced after benzene cracking. Moreover, the spent catalyst shows a small signal at 1100 cm−1, corresponding to the C-OH compound [48], which implies that the alcohol intermediates may be generated during the reaction process. Consequently, a small amount of H2O adsorbed was found on the desiccant.Ni/TiO2-Al2O3 catalysts prepared by various methods were tested by temperature-programmed reduction (TPR) ( Fig. 5). According to previous studies [49], conventional NiO/Al2O3 catalysts calcined at high temperatures (900 °C) are dominated by the NiAl2O4 reduction peaks, which are observed at approximately 800 °C. Compared to the NiO/Al2O3 catalysts, the Ni/TiO2-Al2O3 catalysts exhibit reduction peaks at temperatures below 500 °C that are associated with NiO, having improved low-temperature reducibility. It indicates that the Ni/TiO2-Al2O3 catalysts are reduced during PTCR.Compared to the reduction temperature of the 0.2Ni/0.5Ti0.5Al(M1) catalyst, that of the 0.25Ni/0.5Ti0.5Al(M1) catalyst decreases, while the H2 consumption increases by 24.4%, indicating that an increase in the NiO content can increase the number of the active sites and enhance the catalyst performance. When the calcination temperature is 550 °C, the temperature of the NiO reduction peak is shifted to approximately 500 °C. Furthermore, relative to the 0.2Ni/0.5Ti0.5Al(M1) catalyst, the H2 consumption of the 0.2Ni/0.5Ti0.5Al(M1)-T550 catalyst is reduced by 12.3%. It implies that an increase in the calcination temperature weakens the properties of the active metal and reduces the number of active sites. When the catalysts are calcinated at 450 °C, the NiO reduction peaks are observed at approximately 430 °C for all catalysts. However, multiple NiO reduction peaks are observed at less than 340 °C for the catalysts prepared using M2. These low-temperature reduction peaks may be the result of the interactions between NiO and Al2O3 [50].The TEM images of the two catalysts prepared using the various methods are presented in Fig. 6. From the particle-size distributions shown in Fig. 6(a) and 6(b), the mean particle sizes of NiO in the 0.25Ni/0.3Ti0.7Al(M1) and 0.25Ni/0.3Ti0.7Al(M2) catalysts are 17.48 nm and 14.13 nm, respectively. These values indicate that the catalyst prepared by M2 can achieve a smaller size and more uniform distribution of the crystalline particles than that prepared with M1.The high-resolution images of the selected regions in Fig. 6(a) and 6(b) are presented in Fig. 6(c) and 6(d), respectively. It can be observed from the images that TiO2 and Al2O3 in the two catalysts possess lattice spacings of 0.352 nm (belonging to the {101} facets) and 0.198 nm (belonging to the {400} facets), respectively. However, the NiO components in the 0.25Ni/0.3Ti0.7Al(M1) and 0.25Ni/0.3Ti0.7Al(M2) catalysts exhibit the different dominated lattice spacings of 0.209 nm (belonging to the {012} facets) and 0.241 nm (belonging to the {111} facets), respectively. It is suggested that the different preparation methods influence the catalytic activity of NiO.The experiments on the CO2 reforming of benzene using 0.25Ni/0.3Ti0.7Al(M2) catalyst were conducted under different reaction conditions, including the thermocatalytic CO2 reforming (TCR) at 450 °C without light irradiation, the photocatalytic CO2 reforming (PCR) at room temperature, and photothermocatalytic CO2 reforming (PTCR) at 450 °C under light irradiation.The different conditions have dramatic effects on the catalyst activity ( Fig. 7). The Xc is only 4.36%, while the catalyst is observed to remain nearly unchanged following the TCR experiment. By contrast, Xc is approximately zero under the PCR because H2 and CO cannot be detected in the gas product, while the CO2 concentration remains unchanged, indicating the lack of catalytic activity in the absence of thermal effects. However, under the PTCR, Xc reaches 73.45%, which is approximately 17 times higher than that under TCR. In addition, the catalyst color changes from grey to black over the course of the reaction, indicating the transformation of NiO to Ni or coke deposition, as verified by the XRD, TPR, and SEM characterizations. Therefore, it could be concluded that the 0.25Ni/0.3Ti0.7Al(M2) catalyst exhibits a strong photo-assisted thermal catalytic effect.To illustrate the function of Ni, the CO2 reforming experiments of benzene were conducted under PTCR using Ni/TiO2 and TiO2 as shown in Fig. 7(a). When TiO2 is used as the catalyst, H2 and CO are not detected in the gaseous product, indicating that the reaction fails to occur without Ni. It demonstrates that metallic Ni is the major active site for promoting CO2 reforming of benzene.As shown in Fig. 7(b), the Xc using Ni/TiO2 is about 70% at the beginning of the reaction, but decreases rapidly to about 40% after 1 h. Compared to Ni/TiO2 and TiO2, the addition of Al2O3 can significantly improve the BET surface area of catalysts (based on Section 3.1 BET analysis) and the structural strength of the catalyst (based on 3.9 SEM analysis). Therefore, the performance of the Ni/TiO2-Al2O3 catalyst is enhanced due to the introduction of Al2O3.To verify the role of TiO2, the CO2 reforming experiment of benzene using conventional NiO/Al2O3 (25% NiO content and calcined at 450 °C) as the catalyst was performed under a PTCR. The results indicate that H2 and CO are not detected in the gaseous product and so the reaction barely progresses. The formation of Ni-TiO2 structure can enhance the separation of e−/h+ pairs under the condition of illumination. Therefore, the introduction of TiO2 plays a significant role in photocatalysis, as demonstrated by the comparative benzene conversion performance using NiO/Al2O3 and 0.25Ni/0.3Ti0.7Al(M2) catalysts.To further investigate the photothermal catalytic performance of the Ni/TiO2-Al2O3 catalysts, the CO2 reforming experiments with benzene under variable photothermal reaction conditions were conducted. Xc achieves approximately 75% at the beginning of the experiment, which is under a PTCR for 45 min (Fig. 7(b)). For the experiment conducted with the PTCR, Xc is maintained at approximately 75% for more than 60 min. However, when the reforming reaction is altered to a TCR (by turning off the light source), Xc decreases rapidly to approximately 25% after 30 min, which is only one-third of that under PTCR. The XRD characterization of the spent catalyst after 45 min of the reaction indicates that the Ni-containing phases detected on the catalyst were all metallic Ni monomers, indicating that NiO on the catalyst is reduced after 45 min. Moreover, the photothermal catalytic activity of the reduced 0.25Ni/0.3Ti0.7Al(M2) catalyst for the CO2 reforming of benzene is significantly higher than only its thermal catalytic activity. However, a small amount of CH4 can be detected in the gas product, indicating that the catalytic cracking of benzene also occurs.The conversion efficiencies and syngas yield of PTCR for benzene based on the catalysts prepared under various calcination atmospheres and temperatures are shown in Fig. 8. The Xc and Yg values obtained using the 0.2Ni/0.5Ti0.5Al(M1) catalyst are 53.2% and 55.7 mol/kg, respectively, which are 12.5% and 5.5% higher than those obtained using the 0.2Ni/0.5Ti0.5Al(M1)-Air catalyst calcined in air. Moreover, the Xc obtained using the 0.25Ni/0.5Ti0.5Al(M1) catalyst is 59.2%, which is 7.2% higher than that using the 0.25Ni/0.5Ti0.5Al(M1)-T550 catalyst calcined at 550 °C, while the yields of H2 and CO are 21.4 and 35.8 mol/kg, respectively (increasing by 1.6 and 3.2 mol/kg, respectively). The rCO2 and rBenz of these catalysts are 0.300–0.487 and 0.082–0.102 mmol min−1 g−1, respectively, where 0.25Ni/0.5Ti0.5Al(M1) catalyst achieves the maximum reaction rate. It indicates that the nitrogen atmosphere and the calcination temperature of 450 °C are beneficial for the catalytic activity, given the formation of Al2TiO5 due to calcination in air or at high temperatures (as verified by XRD characterisation (Fig. 3)). Moreover, the TPR analysis (Fig. 5) indicates that the reducibility of the catalyst calcined at 450 °C is more favorable than that calcined at higher temperatures.The effect of the impregnation method on Xc and Yg is shown in Fig. 9. Using 0.25Ni/0.5Ti0.5Al(M2) and 0.25Ni/0.3Ti0.7Al(M2) catalysts, Xc can reach 66.8% and 73.5%, respectively, which are 10.0% and 12.8% higher than those obtained using 0.25Ni/0.5Ti0.5 Al (M1) and 0.25Ni/0.3Ti0.7 Al (M1) catalysts, respectively. Moreover, the yields of H2 and CO are higher for the catalyst prepared by M2 than that prepared by M1. It could be observed that when the 0.25Ni/0.3Ti0.7Al(M2) catalyst is used, the yields of H2 and CO can achieve 25.6 and 50.3 mol/kg, respectively (representing an increase of 4.5 and 4.7 mol/kg, respectively, compared to the yields obtained using 0.25Ni/0.3Ti0.7Al(M1) catalyst). The rCO2 of these catalysts are in the range of 0.487–0.545 mmol min−1 g−1 and rBenz are 0.102–0.129 mmol min−1 g−1, where 0.25Ni/0.3Ti0.7Al(M2) catalyst has better performance. BET and TPR characterizations showed that the catalysts prepared using M2 possess a superior structure and more active sites than those prepared using M1.Experiments on the photothermal catalytic feasibility of Ni/TiO2-Al2O3 catalysts show that the introduction of TiO2 is necessary for the NiO/Al2O3 catalysts to possess photocatalytic properties. However, BET analysis indicates that the introduction of TiO2 may cover the Al2O3 surface, which has a negative impact on the physical structure of the catalyst.Therefore, the influence of the TiO2 to Al2O3 ratios on Xc is investigated to identify the optimal value. Xc and Yg for various TiO2/Al2O3 ratios are shown in Fig. 10. When the TiO2 to Al2O3 ratio is 3:7, Xc attains an optimal value of 73.5%, while the yields of H2 and CO are 25.6 and 50.3 mol/kg, respectively. As the ratio of TiO2 to Al2O3 is further increased, Xc and the Yg decline owing to the reduction in the specific surface area and pore blockage caused by excessive TiO2 coverage on the catalyst surface, which hinders the interaction between the active site and reactants. When the ratio decreases to less than the optimal value, Xc and Yg also decline because of the decrease in TiO2 photocatalysis. The maximum rCO2 and rBenz of this catalyst group is also the 0.25Ni/0.3Ti0.7Al (M2) catalyst (0.545 and 0.129 mmol min−1 g−1, respectively).The effect of NiO content on the PTCR is shown in Fig. 11. When the NiO content is 20%, Xc and the yields of H2 and CO are 57.7%, 23.3 and 40.1 mol/kg, respectively, representing a decrease of 27.3%, 9.8%, and 25.4%, respectively, compared to those obtained by the catalyst with 25% NiO content. The increase in NiO content increases the number of active sites on the catalyst surface, thus enhancing the CO2 reforming of benzene. However, when the NiO content is increased to 30%, compared to the 0.25Ni/0.3Ti0.7Al(M2) catalyst, the Xc decreases by almost 20%, while the yields of H2 and CO are reduced by 6.1 and 16.7 mol/kg, respectively. This may be explained by the fact that excessive NiO leads to pore blockage when the NiO content exceeds 25%, which weakens the physical structure of the catalyst, thereby decreasing photothermal catalytic performance. Moreover, more NiO might cover TiO2 reducing the light absorption, thus leading to the degradation of the catalyst performance. The rCO2 and rBenz of these catalysts are 0.475–0.545 and 0.090–0.129 mmol min−1 g−1, respectively. Also, the 0.25Ni/0.3Ti0.7Al(M2) catalyst achieves the maximum reaction rate.From the comparison of the above catalysts, it can be found that best performance is achieved by the 0.25Ni/0.3Ti0.7Al(M2) catalyst. The space velocity (SV) directly affects the contact time between the reactants and catalysts. Experiments on the effects of SV were conducted by adjusting the total flow rates while keeping the concentrations of benzene and CO2 constant, using the 0.25Ni/0.3Ti0.7Al(M2) catalyst. As shown in Fig. 12, when the SV is reduced from 4500 to 3000 h−1, Xc increases significantly from 73.45% to 88.24%. When the SV is reduced from 4500 to 3750 h−1, the H2 yield increases by 2.9 mol/kg, while the CO yield remains essentially unchanged. When the SV is further reduced to 3000 h−1, the H2 yield remains nearly unchanged while the CO yield increases by 0.8 mol/kg relative to that associated with the SV of 3750 h−1. On the one hand, the molar ratios of H2 to CO are in the range of 0.5–0.56, which is higher than the theoretical value for the CO2 reforming of benzene (C6H6 + 6CO2 → 3 H2 + 12CO). As SV decreases, the H2 to CO ratio exhibits an increasing trend. On the other hand, Xc increases dramatically owing to the increase in CH4 yield. This indicates that the decrease in SV is more important for enhancing catalytic benzene cracking than for reforming.To investigate the causes of catalyst deactivation, SEM analysis was performed on the fresh and spent 0.25Ni/0.5Ti0.5Al(M2) and 0.25Ni/0.3Ti0.7Al(M2) catalysts.As shown in Fig. 13(a) and (d), the morphologies of the two fresh catalysts are similar. Following the photothermal CO2 reforming reaction, the surface morphology of the 0.25Ni/0.5Ti0.5Al(M2) catalyst is severely damaged and many granular substances appear on the surface (Fig. 13(b)-(c)). By contrast, the spent 0.25Ni/0.3Ti0.7Al(M2) catalyst (Fig. 13(d)-(e)) retains its original surface morphology, while granular and flocculent substances also appear. It can be inferred that the proportion of TiO2 has a specific influence on the physical structure of the catalyst, and the catalyst stability is weakened when the proportion of TiO2 exceeds 50% because the light-induced thermal effect causes sintering.Moreover, further magnified images of the sintered and unsintered surfaces of the spent 0.25Ni/0.5Ti0.5Al(M2) catalyst (Fig. 13(g)-(h)) distinctly show clusters and granular substances. Through EDS analysis, the new element on the spent catalyst is identified as carbon, indicating coke deposition occurs. It implies that the main reason for the deactivation of the Ni/TiO2-Al2O3 catalysts is deposited carbon. Therefore, further research is needed to reduce the amount of coke deposition to improve the catalyst stability.In this study, experiments on the photothermocatalytic CO2 reforming of benzene as a tar model compound were conducted using the prepared Ni/TiO2-Al2O3 catalysts. The main conclusions drawn are as follows. (1) The introduction of TiO2 can convert Ni2+ to Ni during mild photothermal reactions, which enhances the availability of holes for benzene oxidation. The Ni-TiO2 structure brings a better separation of e−/h+ pairs under the light condition, significantly enhancing the photothermocatalytic activity of the Ni/Al2O3 catalyst at low temperatures. (2) The catalyst preparation method involving the loading of NiO before TiO2 is beneficial to the catalyst performance. However, excessive TiO2 content caused a decrease in the specific surface area and pore volume of the catalysts and their structural stability, thereby decreasing the catalyst activity. (3) The experimental results of PTCR show that the appropriate preparation parameter for the Ni/TiO2-Al2O3 catalyst is 25% NiO content with a TiO2/Al2O3 ratio of 3:7 at a 450 °C calcination temperature in a nitrogen atmosphere. When the PTCR of benzene is conducted using the 0.25Ni/0.3Ti0.7Al(M2) catalyst, a maximum Xc of 88.2% was achieved here. (4) The main cause of catalyst deactivation is coke deposition on the catalyst surface, which is mainly due to the thermal cracking of benzene. The introduction of TiO2 can convert Ni2+ to Ni during mild photothermal reactions, which enhances the availability of holes for benzene oxidation. The Ni-TiO2 structure brings a better separation of e−/h+ pairs under the light condition, significantly enhancing the photothermocatalytic activity of the Ni/Al2O3 catalyst at low temperatures.The catalyst preparation method involving the loading of NiO before TiO2 is beneficial to the catalyst performance. However, excessive TiO2 content caused a decrease in the specific surface area and pore volume of the catalysts and their structural stability, thereby decreasing the catalyst activity.The experimental results of PTCR show that the appropriate preparation parameter for the Ni/TiO2-Al2O3 catalyst is 25% NiO content with a TiO2/Al2O3 ratio of 3:7 at a 450 °C calcination temperature in a nitrogen atmosphere. When the PTCR of benzene is conducted using the 0.25Ni/0.3Ti0.7Al(M2) catalyst, a maximum Xc of 88.2% was achieved here.The main cause of catalyst deactivation is coke deposition on the catalyst surface, which is mainly due to the thermal cracking of benzene. Yutong Shen: Investigation, Methodology, Data analysis, Writing – original draft and Editing. Jun Xiao: Funding acquisition, Methodology, Writing- Reviewing, Supervision. Qijing Wu: Writing – review & editing. Jingting Su: Investigation. Li Zhu: Methodology.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Natural Science Foundation of China (51576047).

To improve the conversion efficiency and reduce the CO2 emissions associated with the reforming of biomass tar to syngas, photothermal catalysts were prepared in this study through various impregnation methods. Photothermocatalytic CO2 reforming (PTCR) of benzene as a tar model compound, was conducted using the prepared Ni/TiO2-Al2O3 catalysts. The results show that the introduction of TiO2 facilitates the conversion of NiO to Ni during the photothermal reaction, while the photocatalysis of TiO2 significantly enhances the photothermocatalytic activity of the Ni/Al2O3 catalysts at low temperatures. Moreover, the methodology for catalyst preparation involving the loading of NiO before TiO2 is also beneficial to catalyst performance. However, when the mass ratio of TiO2 to Al2O3 exceeds 1, the specific surface area, pore volume, and structural stability of the catalysts are reduced. The results of PTCR experiment show the following optimal Ni/TiO2-Al2O3 catalyst preparation parameters: 25% NiO content and a TiO2/Al2O3 ratio of 3:7; 450 °C calcination temperature and a nitrogen atmosphere. The maximum benzene conversion achieved through PTCR with the 0.25Ni/0.3Ti0.7Al(M2) catalyst is 88.2%. However, a large amount of coke is deposited onto the spent catalyst surface, contributing significantly to catalyst deactivation.

Fuel cells, which directly convert the chemical energy stored in fuels to electrical energy, have been widely investigated and applied in many variations since the first hydrogen fuel cell was invented in 1838. Direct ethanol fuel cells (DEFCs) are promising candidates for portable power applications. Ethanol has an energy density of 8.03 kW h·kg–1, which is lower than that of hydrogen (32.8 kW h·kg–1). Regarding the density of the fuel, however, the volumetric energy density of ethanol (6.28 kW h·L–1) is much higher than that of hydrogen gas compressed at 20 MPa (0.18 kW h·L–1). Even if we consider realistic operation conditions where a DEFC operates at 0.5 V and a current density of 100 mA·cm–2 with the complete oxidation of ethanol to CO2 via a 12-electron transfer, the thermodynamic efficiency of the DEFC will be approximately 40%, which is comparable to that of a conventional diesel engine (Lamy, Coutanceau, & Leger, 2009). In addition, ethanol has a low toxicity, is easy to store and transport, and can be derived from biomass. Therefore, ethanol is a promising feedstock for low-temperature fuel cells.In a DEFC, the anode reaction is an ethanol oxidation reaction (EOR), which involves a 12-electron transfer for the complete oxidation of ethanol to CO2. However, the kinetics are slow and incomplete oxidation is an issue, thus reducing the current density and hindering the application of DEFCs. The reaction pathways of EOR have been investigated by many techniques coupled with cyclic voltammetry and/or chronoamperometry, including in situ infrared spectroscopy (IR) (Chang, Leung, & Weaver, 1990; Vigier, Coutanceau, Hahn, Belgsir, & Lamy, 2004), in situ Raman spectroscopy (De Souza, Neto et al., 2011; Lai, Kleyn, Rosca, & Koper, 2008), in situ sum-frequency generation (SFG) spectroscopy (Kutz, Braunschweig, Mukherjee, Dlott, & Wieckowski, 2011, 2011b), differential electrochemical mass spectrometry (DEMS) (Wang, Jusys, & Behm, 2004, 2007; Willsau & Heitbaum, 1985), and in situ nuclear magnetic resonance (NMR) spectroscopy (Huang, Sorte, Sun, & Tong, 2015). Recently, Wang et al. thoroughly discussed the reaction mechanism of EOR (Wang & Cai, 2015). This is briefly introduced in this review. Generally, two reaction pathways, namely, the C1-path and C2-path, are recognized, corresponding to the complete oxidation to CO2 with 12-electron transfer and incomplete oxidation to acetaldehyde and acetic acid with 2- and 4-electron transfer, respectively. CO2 and acetic acid are the final products of the reaction, while acetaldehyde can desorb to the bulk electrolyte and can subsequently be re-adsorbed onto the catalysts, followed by being further oxidized to CO2 or acetic acid, as shown in Fig. 1 (a). However, the acetic acid is difficult to oxidize further and therefore becomes dissolved in the electrolyte. Fig. 1(b) presents a schematic illustration of the pathway to complete oxidation. COad acting as the intermediate followed by CC bond cleavage strongly adsorbs onto the catalyst surface and cannot be removed oxidatively at low potential, thus inhibiting further adsorption of the reactants and consequently suppressing the current density. Till to the high applied potential, COad can be oxidized by the *OH group from water dissociation, producing CO2. To date, low selectivity to CO2 and low current of the EOR in the low potential range remain fundamental issues. Mechanistically, the target catalyst should be able to easily break the CC bond together while also providing sufficient *OH oxidant adjacent to the C1 species to contribute to complete oxidation. In this scenario, the active sites can be renewed and thus adsorb the reactants.Pt was the earliest-reported single-component catalyst and remains the best, making it the most intensively investigated for EOR (Marie, 1929; Marinkovic, Li, & Adzic, 2019; Rizo, Pérez-Rodríguez, & García, 2019; Zhou et al., 2003). The mechanism of EOR has been well studied on a well-defined Pt surface (Colmati et al., 2009a, 2009b; Ferre-Vilaplana, Buso-Rogero, Feliu, & Herrero, 2016; Lai & Koper, 2009, 2010; Tarnowski & Korzeniewski, 1997; Xia, Liess, & Iwasita, 1997), with the finding that a surface with steps facilitates CC bond splitting. Pt nanocrystals with different shapes, such as nanocubes, nanowires, multi-pods, tetra-hexahedrons, and other polyhedrons, have been synthesized and studied for EOR (Han, Song, Lee, Kim, & Park, 2008; Huang, Zhao, Fan, Tan, & Zheng, 2011; Liu et al., 2016; Tian, Zhou, Sun, Ding, & Wang, 2007; Zhou, Huang et al., 2010, 2011). These studies have shown that Pt nanocrystals enclosed by high-index surfaces improve the current density. In addition to Pt metal, Pd (Bianchini & Shen, 2009; Cui, Song, Shen, Kowal, & Bianchini, 2009; Liang, Zhao, Xu, & Zhu, 2009; Tian, Zhou, Yu, Wang, & Sun, 2010; Yang et al., 2014; Zhang, Zhou, & Zhou, 2016; Zhou, Wang, Lin, Tian, & Sun, 2010), Ir (de Tacconi, Lezna, Beden, Hahn, & Lamy, 1994; Du, Wang, Saxner et al., 2011; Zhu et al., 2017), Au (Cao, Li et al., 2020, 2020b; Rodriguez, Kwon, & Koper, 2012; Tremiliosi-Filho et al., 1998), and Rh (Caram & Gutiérrez, 1992; Li, Fan et al., 2019, 2017; Méndez, Rodríguez, Arévalo, & Pastor, 2002; Suo & Hsing, 2011; Zhang et al., 2018; Zhu, Lan, Wei, Wang, & Yang, 2019) single-component catalysts have also been applied to EOR. Although each exhibits a low catalytic activity relative to Pt, the current density of the Pt catalyst can be enhanced together with a second component. Various Pt-based binary catalysts have been developed to improve the current density of EOR by either promoting water dissociation or facilitating CC bond cleavage. The enhancement of the current density in the cases of PtRh (Almeida et al., 2019; Bergamaski, Gonzalez, & Nart, 2008; Leão, Giz, Camara, & Maia, 2011; Li, Liu et al., 2019; Rao, Jiang, Zhang, Cai, & Sun, 2014; Zhu, Bu, Shao, & Huang, 2019, 2018), PtIr (Chang et al., 2019), PtPd (Miao et al., 2020), PtAu (Li, Liu, & Adzic, 2012), PtCo (Zhang et al., 2017), PtBi (Zhang, Lai et al., 2019), PtNi (Altarawneh, Brueckner, Chen, & Pickup, 2018; Sulaiman, Zhu, Xing, Chang, & Shao, 2017) core-shell, or alloy nanoparticles has been attributed to the prompted capability of breaking the CC bond due to the electronic effect. The incorporation of metal oxides. such as TiOx (Corchado-García, Morais, Alonso-Vante, & Cabrera, 2017; Song et al., 2007), CeOx (Bai et al., 2007; Corchado-García et al., 2015; Menéndez et al., 2014; Murphin Kumar et al., 2017; Xu & Shen, 2005; Xu, Zeng, Shen, & Wei, 2005), WOx (Ganesan & Lee, 2006; Wu, Liu, & Wu, 2010; Zhang, Ma et al., 2006), or NiO (Comignani, Sieben, Brigante, & Duarte, 2015), increases the current density by increasing the capability to oxidize the intermediates, owing to the synergistic effect between the oxides and Pt nanoparticles. To date, the most widely investigated binary-system catalysts have been Pt-Ru (Camara, de Lima, & Iwasita, 2004; Dong, Gari, Li, Craig, & Hou, 2010; Fujiwara, Friedrich, & Stimming, 1999; Hu, Zhu, Zhang, & Liu, 2016; Schmidt, Ianniello, Pastor, & González, 1996; Şen, Şen, & Gökağaç, 2011; Spinacé, Neto, Vasconcelos, & Linardi, 2004; Zhao et al., 2017) and Pt-Sn (Antolini, Colmati, & Gonzalez, 2009; Colmati, Antolini, & Gonzalez, 2006; Du et al., 2014; Godoi, Perez, & Villullas, 2010; Jiang et al., 2005; Lamy, Rousseau, Belgsir, Coutanceau, & Léger, 2004; López-Suárez, Carvalho-Filho et al., 2015; Silva et al., 2010, 2011; Wang et al., 2007; Zhang, Liu et al., 2019; Zhu, Bu, Shao, & Huang, 2020), given their apparent ability to improve the current density over pure Pt metal, which can be in the form of a single phase (as a metallic/intermetallic alloy) or a mixed phase (as a core–shell or segregated grains) (Şen et al., 2011). Moreover, the chemical states of Ru and Sn vary depending on the synthesis method, making the roles of Ru and Sn species in EOR more complicated, as has been reviewed by (Antolini, 2013) and Lamy et al. (2004), respectively. Since the recognition that the current can be enhanced by the introduction of a second component in a binary catalyst, ternary catalysts have attracted the interest of many scientists, who have attempted not only to introduce a second component to supply more oxidants, but also to introduce a third component to facilitate CC bond splitting. Various structural configurations and component combinations for ternary materials have been reported for EOR, which have been partly summarized in recent reviews (Antolini (2013); Bai, Liu, Yang, & Chen, 2019; Marinkovic et al., 2019; Wang & Cai, 2015). However, no review has focused on ternary catalysts for EOR, which offer the promise of overcoming the issues mentioned for single and binary catalysts. In previous reviews, the role of the third species added to a binary system has been discussed in detail. In addition to the type of the third component, the architecture of the nanoparticles (either homogeneous or segregated phases) plays a decisive role in the EOR performance. For example, triphasic PtRhOx–SnO2 catalysts with partially oxidized Pt and Rh cores and SnO2 shells showed a 2.5-fold increase in the CO2 generation rate for EOR, compared with biphasic PtRh-SnO2 catalysts with a metallic PtRh alloy core (Yang, Frenkel, Su, & Teng, 2016). Table 1 summarizes the structure, components, and composition of existing ternary electrocatalysts. We classified the reported ternary catalysts into single-phase and segregated-phase materials according to their structures. The influences of the components and composition are discussed considering that they had the same structure.Adding a third component to the binary catalysts not only reduces the amount of platinum but also may promote CC bond cleavage and provide more oxidants to completely oxidize intermediates to CO2. To date, many elements have been used as auxiliary components in ternary catalysts to promote the current density for EOR, as shown in Fig. 2 and summarized in Table 1. Generally, d-block metals (the d-block elements are located in the middle of the periodic table and include elements from groups 3–12) were the most frequently used for ternary catalysts, such as Pt-noble metal-nonnoble metal (Pt-Au-Ni (Co and Cu) (Cui et al., 2020; Dutta & Ouyang, 2015; Li, Jilani et al., 2019; Wang et al., 2015), Pt-Ir-Cu (Ni) (Ahmad et al., 2019; Wang et al., 2011), Pt-Pd-Cu(Ni) (Arroyo-Gómez et al., 2019; Hu et al., 2012; Ma, Wang, Fan, Zhang, & Li, 2015; Ren, Zhang, Liang, Wu, & Shen, 2020; Wang, Xue et al., 2018; Wang, An, & Zhang, 2020; Wang, Zhang et al., 2020), Pt-Rh-Ni(Co, Fe, Cu) (Almeida et al., 2020; Han, Liu, Chen, Jiang, & Chen, 2018; Chen et al., 2019; Erini, Rudi et al., 2015, 2017; Han et al., 2019; Liu et al., 2017; Shen, Zhang, Xiao, & Xi, 2014; Wang, Du, Sriphathoorat, & Shen, 2018; Zhang, Huang et al., 2019; Wang et al., 2017), and Pt-Ru-Co(Ni, Mo, and W) (García et al., 2012; Li, Kang, & Wang, 2011; Wang, Yin et al., 2007; Oliveira Neto, Franco, Aricó, Linardi, & Gonzalez, 2003; Ribadeneira & Hoyos, 2008; Tanaka et al., 2005; Wang, Yin, Zhang, Sun, & Shi, 2016, 2019c)); Pt-noble metal-noble metal (Pt-Au-Pd (Chen et al., 2015; Dutta, Mahapatra, & Datta, 2011; Dutta, Ray, Sasmal, Negishi, & Pal, 2016; Zhang, Wu, & Xu, 2012), Pt-Au-Ir (da Silva et al., 2017; Liang et al., 2019), Pt-Ir-Rh (Liu, Chia, Cheng, & Lee, 2011), Pt-Rh-Pd (Huang et al., 2015; Zhu et al., 2015), Pt-Rh-Ru (Lima & Gonzalez, 2008; Nakagawa, Kaneda, Wagatsuma, & Tsujiguchi, 2012; Salazar-Banda, Suffredini, Calegaro, Tanimoto, & Avaca, 2006; Xiong et al., 2020) and Pt-Ru-Re (Choudhary & Pramanik, 2020)); and Pt-nonnoble metal-nonnoble metal (Pt-Ni-Mo (Mao et al., 2017) and Pt-Ni-Cu (Castagna, Sieben, Alvarez, Sanchez, & Duarte, 2020; Hong, Lee, Kim, & Choi, 2019; Huang, Liu et al., 2019; Imanzadeh & Habibi, 2020; Jilani et al., 2020)). In addition, some p-block elements (p-block elements are located on the right side of the standard periodic table and include elements from groups 13–18) can be added to form a unique structure to improve the electrocatalytic performance. Examples include Pt-Sn-noble metal (Pt-Sn-Rh (Bach Delpeuch, Maillard, Chatenet, Soudant, & Cremers, 2016; Colmati, Antolini, & Gonzalez, 2008; Dai, Wang et al., 2018; de Souza, Giz, Camara, Antolini, & Passos, 2014; Du, Wang, LaScala et al., 2011; Erini, Krause et al., 2015, 2014; Fan et al., 2019; García-Rodríguez et al., 2011; Higuchi, Takase, Chiku, & Inoue, 2014; Jiang, Bu, Wang, Guo, & Huang, 2015; Kowal, Gojković, Lee, Olszewski, & Sung, 2009, 2009b; Li, Marinkovic, & Sasaki, 2012; Li, Zhou, Marinkovic, Sasaki, & Adzic, 2013; Li et al., 2010; López-Suárez, Perez-Cadenas et al., 2015; Mai, Chiku, Higuchi, & Inoue, 2015; Mai, Chiku, Higuchi, & Inoue, 2017; Silva, Camara, & Giz, 2019; Silva-Junior et al., 2013; Soares, Morais, Napporn, Kokoh, & Olivi, 2016; Song et al., 2012; Spinacé, Dias, Brandalise, Linardi, & Neto, 2010; Yang et al., 2016; Yang, Namin, Aaron Deskins, & Teng, 2017), Pt-Sn-Ag (Dai, Huang et al., 2018), Pt-Sn-Ir (Li, Cullen et al., 2013; Ribeiro et al., 2007; Silva et al., 2012, 2013; Tayal, Rawat, & Basu, 2011; Thilaga, Durga, Selvarani, Kiruthika, & Muthukumaran, 2018; Zhao, Mitsushima, Ishihara, Matsuzawa, & Ota, 2011), Pt-Sn-Pd (Lee, Park, & Manthiram, 2010; Wang et al., 2013, 2016), Pt-Sn-Ru (Chang, Liu, Wei, & Wang, 2009; Chu & Shul, 2010; Cunha, Ribeiro, Kokoh, & de Andrade, 2011; Hang, Altarawneh, Brueckner, & Pickup, 2020; Huang, Zheng et al., 2019; Liu, Chang, Wei, & Wang, 2011; Neto, Dias, Tusi, Linardi, & Spinacé, 2007; Thepkaew, Therdthianwong, Therdthianwong, Kucernak, & Wongyao, 2013; Wu, Swaidan, & Cui, 2007; Xia, Zhang, Zhao, & Li, 2017), and Pt-Sn-Au (Dai et al., 2020; Shakibi Nia et al., 2020)); Pt-Sn-nonnoble metal(Pt-Sn-Mo (Lee, Murthy, & Manthiram, 2011), Pt-Sn-Ni (Beyhan, Léger, & Kadırgan, 2013; HUANG, Lian-Hua ZHAO, & Shi-Gang, 2017; Parreira et al., 2013; Ponmani, Kiruthika, & Muthukumaran, 2015; Spinacé, Linardi, & Neto, 2005), Pt-Sn-V (Jin, Sun, Huang, & Zhao, 2014; Sun, Zhao, & Yu, 2013), Pt-Sn-W (Anjos, Hahn, Léger, Kokoh, & Tremiliosi-Filho, 2008; Ribeiro et al., 2008), Pt-Sn-Cu (Huang, Wu, Wu, & Guan, 2015), Pt-Sn-Ce (De Souza, Parreira et al., 2011; Jacob, Corradini, Antolini, Santos, & Perez, 2015), Pt-Sn-Fe (Almeida, Van Wassen, VanDover, de Andrade, & Abruña, 2015), Pt-Sn-Pb (Santos, Almeida, Tremiliosi-Filho, Eguiluz, & Salazar-Banda, 2020), and Pt-Sn-In (Chu et al., 2012; Zhu, Sun, Yan, Li, & Xin, 2009)); and Pt-Bi-Ru(Pb) (Brandalise et al., 2009; Huang, Cai, Liu, & Guo, 2012; Huang, Cai, & Guo, 2013). Theoretical and experimental research has shown that the composition of a hybrid catalyst plays a crucial role in EOR.Generally, oxophilic metals, such as Sn, Ru, and Ni, primarily facilitate water dissociation to form adsorbed oxidant or surface oxides, which oxidize the surface-adsorbed C-containing species. Noble metals, such as Rh, Pd, and Ir, primarily enhance the capability of breaking CC bonds to increase the selectivity of CO2 generation. However, the roles of the components of a ternary catalyst vary dramatically depending on the structure of the catalyst. For example, in PtSn and PtRu binary systems, the degree of alloying, the oxidized state of the Sn/Ru, and the arrangement of the Sn/Ru oxides and Pt nanoparticles plays a decisive role in the EOR performance (Camara et al., 2004; Du et al., 2014; Gupta, Singh, & Datta, 2009; Hu et al., 2016; Liu et al., 2018; Pires, Corradini, Paganin, Antolini, & Perez, 2013; Silva et al., 2011; Zhang, Liu et al., 2019; Zhao et al., 2017; Zhu et al., 2020). In most cases, ternary catalysts have more than one phase, making them considerably more complicated than binary catalysts. Therefore, validating the structure is the first step and critical in investigating the mechanism of EOR. Given that there are numerous ternary catalysts, we classified them into two groups in terms of their structure: those with single-phase and those with segregated-phase structures, as shown in Fig. 3 . The former includes random alloys with random ratios of components, intermetallic alloys with a particular ratio of components, and metal surface-rich alloys with a concentration gradient of a certain component. The latter includes bi-phase and tri-phase nanoparticles, which are elaborated in the following context. Consequently, it is difficult to attain improved catalytic activity simply by adding one or two factors. In reality, multiple factors interact to produce a beneficial effect. Thus, we will review and discuss the structures of the materials in the following sections.Many researchers have shown that the use of Pt-M binary alloys improves the kinetics of EOR compared with pure Pt. Incorporating a third metal into a binary system has been assumed to be an effective means of further improving the electrocatalytic activity of EOR. Relative to Pt, PtRh alloys are much better able to break the CC bond and attain complete oxidation (Almeida et al., 2019; Rao et al., 2014; Zhu, Bu et al., 2019). However, they have an issue of poisoning resulting from the adsorption of large amounts of CO after CC bond splitting. Introducing a third oxophilic metal, such as Ni, Co, Cu, or Fe, to form a random ternary alloy would alleviate this poisoning (Han et al., 2018, 2019; Han et al., 2019; Wang et al., 2017). Using Cu2O nanocubes as a template, Han et al. synthesized PtRhCu cubic nanoboxes via a galvanic reaction. The X-ray diffraction (XRD) pattern illustrated in Fig. 4 (a) shows a single face-centered cubic phase, suggesting the formation of PtRhCu alloy (Han et al., 2018). When this is combined with the uniform distribution of elemental Pt, Rh, and Cu, as revealed by energy-dispersive X-ray (EDX) spectroscopy, we can conclude that a ternary random alloy has formed. In a 1 M ethanol/1 KOH solution, Pt54Rh4Cu42 cubic nanoboxes exhibit a remarkably higher EOR current density than Pt58Cu42 cubic nanoboxes. For instance, Pt54Rh4Cu42 cubic nanoboxes have a peak current density of 4090 mA mg−1, which is double that of the Pt58Cu42 cubic nanoboxes. They contribute to the improvement of both the facilitated CC cleavage due to the synergistic effect between the elemental Pt, Rh, and Cu, as well as the high oxidation activity of the COads intermediate in the presence of Cu, which favors the formation of OH species. Similarly, PtCuRh-alloyed nanowires were found to have a higher current density than PtCu nanowires because of their outstanding anti−CO-poisoning properties (Chen et al., 2019).In addition, PtNiRh nanowires with an average diameter of 1.63 nm (Fig. 5 ) were synthesized by reducing the metal acetylacetonate with glucose and oleylamine in the presence of the structure-direct agents of tungsten hexacarbonyl, dodecyl trimethyl, and ammonium chloride (Zhang, Huang et al., 2019). High-resolution transmission electron microscopy (HRTEM) images revealed that the thickness of an individual wire is about eight atom layers, as shown in Fig. 5(d). The uniform distribution of Pt, Rh, and Ni in Fig. 5(e) and the single phase, as shown in the XRD pattern in Fig. 5(g), prove the formation of a PtRhNi random alloy. For the alloy, the electron of the Pt tends to transfer to Rh or Ni, leading to less CO being adsorbed onto the Pt. These results reveal the effects of Rh on EOR and show that Pt69Ni16Rh15 nanowires have a considerably lower onset potential than Pt, as well as a mass-normalized current that is 3.26 times higher than that of Pt/C in 0.1 M HClO4/0.5 M ethanol. They contributed to the alleviation of CO poisoning due to the bifunctional mechanism of Rh and the electronic effect among the three metal elements. DFT calculations showed that, with the association of Ni 3d and Rh 4d synergetic d-orbital interplay, the Pt 5d band has been integrally pinned at the higher band center close to the Fermi level EF, thus balancing the reactivity and anti-poisoning and exhibiting long-term stability. The advantages of alloying with Rh and Ni have also been reported for other PtRhNi alloyed nanoparticles (Liu et al., 2017; Shen et al., 2014).PtRhM ternary alloys derived from PtRh binary alloys doped with M transition metal have proven effective for enhancing the current density of EOR (Erini, Rudi et al., 2015; Han et al., 2019; Shen et al., 2014; Wang et al., 2017). To elucidate the role of the third metal, Dai et al. synthesized a series of PtRhM (M = Fe, Co, Ni, Cu, In, Sn, or Pb) ternary alloys with a fixed ratio of 3:1 between Pt and Rh by using the dimethylformamide solvothermal method (Dai, Wang et al., 2018). With negligible differences in their structures and a similar particle size, as shown in Fig. 6 (a), they investigated the composition–reactivity relationship for EOR in a 0.5 mol CH3CH2OH/0.1 mol HClO4 solution. At 0.45 V vs. RHE, the specific activity occurred in the order of: Pt3RhSn/C > Pt3RhIn/C > Pt3RhGa/C > Pt3RhPb/C > Pt3RhCu/C > Pt3RhCo/C > Pt3RhNi/C > Pt3RhFe/C > Pt3Rh/C > Pt/C (size of ca. 3.5 nm), as shown in Fig. 6(b), suggesting that the PtRh alloy can be modified by the transition metals. With the help of DFT calculations, they stated that the Rh and M control the adsorption configuration of the key intermediates and thus change the barrier in the rate-determining step. The M further influences the adsorption and dissociation energies of water and tunes the d-band centers of Pt and Rh, thus modulating the catalytic activity.In addition to the improvement in the composition, Pt-based ternary materials can generate more edges with low-coordination sites in the presence of other metals. Candied haws-shaped AuPtNi alloy (Cui et al., 2020), Pt-Mo-Ni nanowires (Mao et al., 2017), ultrathin 2D PdPtCu nano-rings (Wang, Zhang et al., 2020), PtRhTe nanotubes (Jin et al., 2020), and PtPdCu nanodendrites (Wang, An et al., 2020) have numerous unsaturated edge sites that facilitate CC bond scission and CO oxidative removal, thereby enhancing the current density of EOR.Among the ternary alloys, the surfaces of some alloys are enriched by a metal forming a concentration gradient, which has a well-tuned geometric structure and modified electronic structures, resulting in an enhanced current density of EOR. Zhu et al. synthesized (111)-terminated Pt-Pd-Rh nanotruncated-octahedrons (NTOs) with a Rh-rich surface, as determined by extended X-ray absorption fine structure (EXAFS) analysis.(Zhu et al., 2015) Because Rh has the minimum standard reduction potential from RhCl6 3−, relative to Pd from PdCl4 2- and Pt from PtCl6 2-, Rh prefers to segregate on the surface of the particle after the reduction of PdCl4 2- and PtCl6 2-. The formed unique surface facilitates CC bond cleavage, and Pd on the surface provides OH for COad oxidation. Erini et al. prepared octahedral PtNiRh FCC alloy nanoparticles with Ni-rich facets and Pt-rich frames in terms of the EDX mapping depicted in Fig. 7 (Erini et al., 2017) By varying the concentration of Rh, they were able to tune the surface composition, revealing that a material with a greater amount of Rh facilitates EOR. They found that the electronic effect facilitates CC bond cleavage to form C1 products on PtNiRh with a low Rh content. In contrast, C2 was the main product of a PtNiRh alloy with a high Rh content, due to the formation of an Rh shell outside the PtNi alloy core.In most cases, ternary catalysts tend to undergo phase segregation because of the differences in the surface free energy and chemical stability. Bi-phase heterostructures include a core-shell structure and a mixture of an alloyed phase patched by the other phase. Chen et al. synthesized a Ni core-PbPt alloy shell that exhibited significantly superior current density and stability compared with PtRu and Pt catalysts, resulting from the unique core-shell structure with the tuned d-band center of Pt. This weakened the CO binding to Pt and the fast electron transport in the presence of the Ni metal core (Chen et al., 2013). Dog-bone-shaped Au core-PtPd alloy shell nanoparticles were also synthesized to improve the performance of EOR (Dutta et al., 2016). Liang et al. prepared a Au core-PtIr monolayer core-shell catalyst for ethanol electrooxidation, which exhibited a current density that was six orders of magnitude greater than that of the AuPtIr random alloy, as shown in Fig. 8 (Liang et al., 2019). In the case of the core-shell structure, the monatomic steps within the PtIr layer and the Au-induced tensile strain on the PtIr layer facilitate CC bond cleavage via ethanol dissociative adsorption. Meanwhile, Ir together with Au can dehydrate ethanol at very low potentials. Therefore, at the peak potential, the current for generating CO2 accounts for 57%, which is the highest selectivity towards complete oxidation so far. In addition to pure metal cores, binary alloy cores can also be used. Li et al. developed ternary CoPtAu nanoparticles with an intermetallic PtCo L10-tetragonal structure core-PtAu shell, which exhibited a higher current density and better stability than commercial PtRu and Pt catalysts in a 2 CH3CH2OH/0.1 M HClO4 electrolyte (Li, Jilani et al., 2019). The stabilized Co inside the core induces surface compression on the PtAu shell, as revealed by HR-STEM and EXAFS analyses, thus facilitating the CC bond scission.In addition to the core-shell structure, two segregated phases are frequently formed in ternary systems: a binary alloy with another phase and a ternary alloy with another phase. Kowal et al. developed a cation-adsorption-reduction-galvanic displacement synthetic method to prepare the ternary Pt/Rh/SnO2 electrocatalyst, which consisted of a PtRh quasi-alloy and a SnO2 patch, which produced more CO2 than PtSnO2 (Kowal, Li et al., 2009). Assisted by DFT calculations for RhPt/SnO2(110), they concluded that Rh facilitates CC scission while SnO2 facilitated the dissociation of water to form an OH oxidant for oxidizing the C1 species. A similar phenomenon was observed for PtRh nanowires patched by SnO2 (Fan et al., 2019). Li et al. alloyed Pt with Ir on SnO2 nanoparticles and compared them with PtRh-alloy-patched SnO2 nanoparticles, as shown in Fig. 9 (Li, Cullen et al., 2013). The PtIr/SnO2/C catalyst, with the highest Ir content (Pt/Ir/Sn = 1:1:1), exhibited the most negative EOR onset potential together with a greatly improved CC bond-splitting capability. This was attributed to both the ensemble and ligand effects.For a PtRhSn ternary system, some researchers synthesized the structure in the form of a PtRhSn nanoalloy decorated with SnOx. Colmati et al. created a PtSnRh fcc structure with the Pt:Sn:Rh atomic ratio from 1:1:0.3 to 1:1:1 by reduction of the metal precursors with formic acid (Colmati et al., 2008), while Erini et al. obtained a PtRhSn alloy of Niggliite-phase-decorated SnOx with a Pt:Sn:Rh atomic ratio of 41:50:9, as shown in Fig. 10 (a) and (b). Compared with the Pt/Rh/SnO2 nanoparticle reported by Kowal, Li et al. (2009), PtRhSn nanoparticles with a Niggliite phase decorated by SnOx exhibited current densities that were four times higher, at 0.45 V, relative to RHE. This high current density was attributed to the active surface structure, where the PtRhSn sites interact strongly with the SnO2 moieties nearby, thus promoting CC bond cleavage and further oxidation to CO2. Du et al. synthesized ternary PtSnRh material to obtain a high current density for EOR (Du, Wang, LaScala et al., 2011). An EXAFS analysis revealed the coexistence of a homogeneous Pt/Sn/Rh random alloy and non-alloyed SnO2 throughout the catalyst. This led to the superior electronic effect of the Pt/Sn/Rh random alloy. Similarly, a Pt2SnCu alloy patched with SnO2 was developed to improve the current density owing to the synergistic catalytic effects of the Pt defects and SnO2 (Huang, Wu et al., 2015).Among all the ternary systems, the PtRhSn system has been the subject of the most intensive studies. The architecture of the PtRhSn system varies from that produced by the synthesis approach. Yang et al. prepared a PtRhSn material by reducing Pt and Rh precursors in ethylene glycol using SnO2 nanoparticles as seeds. A scanning electron energy loss spectroscopy (STEM-EELS) line scan revealed the formation of a PtRh core-Sn-rich shell (Yang et al., 2016). They further analyzed the core using EXAFS, as shown in Fig. 11 (a), where it can be seen that the Pt and Rh are partially oxidized without Pt-Rh coordination. They concluded that the material is a triphasic PtRhOx–SnO2 catalyst with a partially oxidized Pt and Rh core and a SnO2 shell, which realized a 2.5-fold increase in the CO2 generation rate for EOR compared with biphasic PtRhSnO2 catalysts with a metallic PtRh alloy core. This improved selectivity to CO2 can be attributed to the co-existence of metallic and oxidized Pt and Rh on the surface, whereas the metallic phase provides a large and available site for the dissociative adsorption of ethanol by CC splitting, while the oxidized Pt and Rh phases provide mobile O atoms for the oxidation of reaction intermediates, such as CO and CHx. Higuchi et al. synthesized a PtRhSn catalyst with segregated Pt, Rh, and SnO2 nanoparticles, which are partially in contact with each other, as shown in Fig. 11(b) (Higuchi et al., 2014). On the catalyst, SnO2 provides OH species from water dissociation to oxidize the dissociated CO at the Rh sites, while the Pt sites facilitate ethanol dehydrogenation. Thus, both SnO2 and Rh are necessary for an active electrocatalyst. Given its ability to break the CC bond of Re in the reforming process, researchers have attempted to use Re to replace Rh. Generally, a Pt/Re/Sn system has three segregated phases, Pt, Re, and SnO2, that are in physical contact with each other, as shown in Fig. 12 (Drzymała et al., 2018, 2020; Parlinska-Wojtan et al., 2019). This synergistic effect is a result of the assembly, enabling the ethanol molecule to be accessible to the three components, which are in direct contact with each other, with each playing an important role in the oxidation pathway. In addition, the interfaces between the particles are active for EOR.Dutta et al. synthesized NiAuPt nanoparticles on reduced graphene oxide nanosheets (NGs) consisting of tightly coupled nanostructures of Ni, Au, and Pt, which have neither an alloy nor a core-shell structure, as shown in Fig. 13 (Dutta & Ouyang, 2015). NiAuPt-NGs with this unique architecture produce an 8, 4, and 2 times higher EOR current than is the case with monometallic Pt-NGs, bimetallic NiPt-NGs, and bimetallic AuPt-NGs, respectively, in an alkaline electrolyte. This can be attributed to the synergetic effect of the three nanostructured metals.Practically, if portable DEFCs are to be commercialized, it is essential that ethanol be completely oxidized to CO2 without partial oxidation to acetaldehyde and acetic acid. Although much effort has been expended on the synthesis of Pt-based ternary catalysts, the use of noble metals (Rh, Ir, and Pd) is essential to enhance the ability to break CC bonds with high CO2 selectivity. However, noble metals are a rare resource and, therefore, their cost prevents their widespread implementation. To remove COad or CHx,ad oxidatively, an oxophilic metal is required. However, these metals suffer from severe leaching during electrochemical oxidation over the long term, leading to damage to the structure and dramatically reducing the current density.First, the commercialization of DEFCs will demand that the cost of the EOR catalyst be reduced. To reduce the required amount of noble metals, the adoption of the core–shell structure appears to have great potential. Technically, particles with a core of cheap metal and a shell fabricated from a thin layer of noble metals, configured in a manner that is highly active for EOR, have shown substantial promise. Second, maintaining stability after cycling within a large potential window and over long-term operation at a constant potential is a serious challenge for ternary catalysts. In most cases, the catalysts are degraded by the rearrangement of the bulk and/or surface structure, the agglomeration of particles, and the loss of the active components, specifically in the case of non-noble metals. To mitigate the etching of cheap metal, the metal atoms can be anchored at structural defects through strong interaction with the noble metal within the alloyed shell. In addition, alloying a second non-noble metal with the initial non-noble metal in a binary catalyst is an effective means of preventing the leaching of the non-noble metal. For example, adding Cu atoms to PtNi nanoparticles can stabilize the PtNi nanoparticles significantly, because the Cu atoms have a large orbital overlap with the Ni atoms. Last, increasing the ability to break the CC bond to achieve complete oxidation is the main focus. There are numerous possibilities for the structure of Pt-based ternary catalysts, which should exhibit appropriate selectivity for breaking the CC bond together with fast kinetics for oxidizing the C1 fragments at low potentials. With progress in surface/interface engineering and techniques enabling the precise control of synthesis at the atomic level, we believe that researchers will attain a breakthrough in this field in the near future.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the National Natural Science Foundation of China (grant No. 21373091), Guangdong Basic and Applied Basic Research Foundation (grant No. 2019A1515110035), and the State Key Laboratory of Pulp and Paper Engineering (grant No. 202013).

Direct ethanol fuel cell (DEFC) as a promising device for converting chemical energy to electricity has been paid ever-increasing attention. However, the slow kinetics of ethanol electrooxidation at an anode hinders the application of DEFCs. Although Pt is the best catalyst among all the pure metal catalysts, it still has a relatively poor ability to break the CC bond, is deactivated by the accumulated COad intermediates, and undergoes unwanted desired structure change over long-term operation. In recent years, the addition of other metals to form binary, ternary, and quaternary catalysts have significantly improved electroactivity and stability. Ternary catalysts can have numerous element combinations and complicated architectures and, therefore, have been the subject of considerable research. In this review, most of the reported ternary catalysts will be summarized and categorized according to their structure while discussing the essence of the role of each component.

No data was used for the research described in the article.Friedel–Crafts alkylations are important reactions for attaching alkyl chains to an aromatic ring and hence is of considerable industrial and pharmaceutical significance [1]. It proceeds through the electrophilic attack of an aromatic ring by an alkylating agent, such as olefins, alkyl halides, or alcohols [2–4]. Mineral acids like sulphuric acid and Friedel–Crafts catalysts like AlCl3 and BF3 are used in the synthesis of many fine chemicals and pharmaceutical intermediates, though they have inherent drawbacks such as tedious catalyst recovery post reaction and large amounts of acid waste generation [5]. Hence solid acid catalysts such as zeolites, clays, metal oxides, metal organic frameworks have been thoroughly researched as potential catalysts for Friedel-Crafts alkylation reactions [6–9].Alkylation of benzene with isopropanol to yield cumene is a vital Friedel Craft’s reaction as the global production of phenol and acetone is largely founded on the cumene process. Cumene also known as isopropyl benzene is a colorless liquid, having high antiknock value. It is also important in the production of cymene and polyalkylated benzenes [10]. Other industrial uses of cumene include the production of phenolic resins, bisphenol A, and caprolactam. During the isopropylation of benzene to cumene, 5–10 wt% diisopropylbenzene (DIPB) isomers are produced always as low-value byproduct. These can be recycled for cumene production, making the production process more economical [11]. A tremendous increase in cumene and phenol production has recently been reported worldwide [12]. In the last decades, several zeolite catalysts like zeolite UZM-8, silica supported beta zeolite, zeolites BEA and MWW catalysts have been reported in cumene synthesis [13–15]. Though cumene selectivity increased over beta-zeolite catalyst, benzene conversion dropped during alkylation of benzene with isopropanol. The decrease in benzene conversion was attributed to catalyst deactivation due to the presence of by products like ethylbenzene, p-xylene and 1-ethyl-3-(1-methyl) benzene [16]. In a similar study, Zou et al. achieved cumene production over nano-sized beta zeolite in a submerged ceramic membrane reactor, where the benzene isopropylation and the in-situ catalyst separation could be done [17].Spinel ferrites are important solid acid catalysts for various reactions like nitrogen fixation [18], CO hydrogenation [19], thermolysis of perchlorate [20], waste water treatment [21–22], synthesis of pyrazolopyridine derivatives [23] etc owing to their high activity and stability. As they exhibit admirable magnetic properties, these materials have the added advantage that they are magnetically recoverable, post reaction. Among them, cobalt ferrite that exists in a partially inverse spinel structure in which both sites (A and B) contain a fraction of Co2+ and Fe3+ cations are found to be highly active catalysts [24,25]. The catalytic and magnetic properties of cobalt ferrites can be modified by substitution of specific cations such as Cd, Ni, Zn, Sr, Zr and Cr etc as well as non-metals like N and S in the metal sites [26–27]. Elements that have not been prior selected were chosen for the present study. Though zirconia is of much technical importance it has not been used in ferrite doping. Also, non metal such as S, N and C have been used as dopants in metal oxides such as titania with excellent results, it has not been done in ferrites. In the present study, we report the liquid-phase isopropylation of benzene using isopropyl alcohol (IPA) as alkylating agent over Zr/N/S doped cobalt nanoferrites. The catalysts were prepared by wet impregnation and coprecipitation techniques. The effect of reaction parameters like catalyst concentration, reaction temperature, and the mole ratio of reactants on reaction rate were analyzed to determine the feasibility of the catalyzed reaction. The structural stability and reusability of these catalysts also were studied in detail. Zirconium doped cobalt ferrite nanoparticles Co1-xZrxFe2O4 (x = 0, 0.25, 0.5, 0.75 and1) and the N/S doped analogues were synthesized by co-precipitation method as reported in detail in our previous publications [28–29]. Required amounts of Fe(NO3)3·9H2O, Co(NO3)2·6H2O and ZrO(NO3)2·H2O were dissolved in deionized water and homogenized. 5 M NaOH solution was added drop wise with continuous stirring at 80 °C. The precipitate was washed repeatedly with deionised water to remove excess NaOH. It was dried for 24 h in an air oven at 110 °C and subsequently calcined for four hours at 700 °C. The prepared five ferrite compositions were designated as CoFe2O4, Co0.75Zr0.25Fe2O4, Co0.5Zr0.5Fe2O4, Co0.25Zr0.75Fe2O4 and ZrFe2O4. To assess the effect of non metal doping, N/S co doped CoFe2O4 and Zr/N/S co doped CoFe2O4 were synthesised as detailed before, the only difference being the addition of 0.1 M thiourea and 0.1 M urea solutions during homogenisation. In order to study the effect of non metal doping after the formation of spinel structure, samples were prepared by wet impregnation of 0.1 M thiourea and 0.1 M urea solutions. The samples were heated at 300 °C in a muffle furnace for six hours. The prepared samples were denoted as CF (CoFe2O4), CFTD (S/N co doped CoFe2O4), CFTA (S/N doped after CoFe2O4 formation), CZFTD (N/S/Zr co doped CoFe2O4) and CZFTA (S/N doped after Co/ZrFe2O4 formation) respectively. The N doped samples prepared using urea precursor were denoted as CFUD (N co doped CoFe2O4), CFUA (N doped after CoFe2O4 formation), CZFUD (N/Zr co doped CoFe2O4) and CZFUA (N doped after Co/ZrFe2O4 formation) respectively. The crystallite structures of the synthesised particles were evaluated by Rigaku Miniflex 600 X-ray diffractometer (CuKα as the radiation source). The particle size and morphology were evaluated using JEM 2100 model High Resolution Transmission Electron Microscope. A Bruker-S4-Pioneer model spectroscope was used to analyse the composition of the samples. Fourier transform infrared spectroscopic analyses were done in Thermo Nicolet Avatar 370 spectrometer in the range 400–4000 cm−1 with a resolution of 4 cm−1. Raman spectra were recorded in the spectral range 50–4000 cm−1 on a Bruker: RFS 27 Raman spectrometer using a laser source Nd: YAG 1064 nm. X-ray photoelectron spectra were obtained on Axis Ultra DLD instrument of KRATOS using monochromatic AlKα radiation. Magnetic properties of the samples were characterized by a Lakeshore 7410 vibrating sample magnetometer at room temperature.Isopropylation of benzene was conducted in a 500 mL three-necked round bottom flask with a condenser under constant magnetic stirring at atmospheric pressure and 420 rpm of stirring speed. The alkylation reaction was studied under different conditions of reaction time, temperature, benzene/IPA ratio, and catalyst loading. The products were characterized by gas chromatography (GC) and GC–MS analysis. GC used was Perkin Elmer Clarus 580 model Gas Chromatograph equipped with an Elite-5 capillary column and Flame Ionisation detector. The injector temperature was 250  ℃ and detector temperature was 275 ℃. The GC–MS used is a Varian 1200 Single Quadrupole mass spectrometer with helium as the carrier gas.Reusability of the prepared catalysts was checked for four consecutive catalytic runs by magnetic separation of the catalysts on the completion of each run. The spent catalysts were washed with acetone, followed by drying in an air oven at 200 ℃.The prepared doped nano ferrites were characterised by standard techniques and the results have been detailed in our previous publications [28–29]. The obtained X-ray diffraction profile could be indexed to single-phased cubic spinel systems with average sizes in the range of 16–26 nm. Fourier Transform Infrared and Raman spectra confirmed the presence of stable cubic spinel ferrite structure. Nearly spherical particles were seen on TEM images. X-ray photoelectron spectroscopy indicated the substitution of zirconium ions mostly into octahedral sites, suggesting a partially inverse spinel structure. Interestingly, the sulphur and nitrogen were inserted into the lattice structure as cations. Decrease in saturation magnetisation on doping was observed by magnetic measurements at room temperature whereas coercivity increased with reduction in particle size.In this work, the efficiency of the synthesized Zr/N/S doped cobalt ferrite magnetic nanocatalysts for benzene isopropylation is evaluated. Optimization of various reaction parameters such as temperature, molar ratio of substrates, time and catalyst dosage was done by changing the variable while keeping all other parameters constant. Results of the optimization studies of reaction conditions done by taking CZFTD nanocomposite as the reference catalyst are discussed in the subsequent sections. Fig. 1 depicts the effect of temperature on isopropylation of benzene at four different temperatures viz 130, 140, 150 and 160 °C keeping catalyst amount as 0.5 g, and benzene to isopropanol mole ratio as 1:3. It can be noted from the figure that the reaction temperature has significant effect on reaction rate. The % conversion of IPA increases from 24.9 % to 77.2 % with increase in temperature. The major products found are cumene and di/tri isopropyl benzene. However, the selectivity to cumene, which is high at lower temperatures, decreases gradually with increase in temperature to 160 °C. This may be due to subsequent alkylation to di/tri isopropyl benzene, which takes place at high temperatures [30]. Maximum cumene selectivity of 80.8 % with the good conversion of IPA (70.8 %) is obtained at 150 °C. From the results, considering the reaction rate, selectivity and energy cost, 150 °C is chosen as the optimum temperature for further studies.Reactant molar ratio plays an important role in determining reactant conversion and product yield. The effect of mole ratio of reactants on conversion and product selectivity were studied by taking benzene: isopropanol ratio as 1:1, 1:3, 1:5, and 1:7 at 150 °C with catalyst weight 0.3 g for 2 h. The results are shown in Fig. 2 . Conversion of isopropanol increases as expected, on increasing the mole ratio of the reactants up to 1:5. Further increase causes reduction in conversion due to the blocking of active surface sites with excess reactants [31]. Selectivity to cumene also increases from 65.7 % to 83.6 % with increase in the mole ratio from 1 to 5. Cumene selectivity then decreases to 70.6 % on increasing the mole ratio to 1:7. The decrease in selectivity of di and tri substituted products at lower molar ratio is due to the transalkylation of these products, attributed to the increased availability of alkyl groups. From this study, it can be deduced that high concentration of IPA is not preferred and hence a minimum ratio 1:5 was maintained for further studies. Fig. 3 shows the effect of catalyst concentration on benzene isopropylation. As the catalyst concentration increases from 0.3 g to 1.5 g/L, there is a noticeable increase in IPA conversion from 58.9 % to 90.3 % and decrease in cumene selectivity from 83.6 % to 63.7 %. A possible explanation for these observations is that an increase in the number of active sites accelerates the reaction rate with concomitant enhancement in the rate of disproportionation, which could adversely affect the cumene selectivity [32]. The % conversion of di/tri-substituted benzene also increases slightly with increase in catalyst concentration. Considering the results in Fig. 3, the catalyst concentration of 0.5 g/L was employed for further studies.The optimized reaction conditions are presented in Table 1 .The catalytic activities of all the prepared catalysts for isopropylation of benzene were evaluated under the optimized reaction conditions and the results are given in Table 2. . Cobalt ferrite shows 60.2 % conversion of IPA with a cumene selectivity of 63.90 %. Zr doping increases the catalytic activity. Among the different Zr doped catalysts, Co0.5Zr0.5Fe2O4 shows maximum conversion of 73.5 % with selectivity of 65.8 %. The high selectivity of Co0.25Zr0.75Fe2O4 and ZrFe2O4 maybe due to the presence of lattice imperfections that provides catalytically active sites. Further doping with nonmetals such as N and S increases the reaction rate. In Zr/N doped samples, CFUD shows a maximum conversion of 80.5 %. Cumene is the major product over all the catalytic systems. In Zr/N/S doped series, CZFTD shows a maximum conversion of 82.7 % with selectivity of 83.6 %. The catalytic efficiency of doped cobalt ferrite nanocomposites can be attributed to two reasons. One is the increased surface area; Zr/N/S substitution increases the surface area of cobalt ferrite nanocomposites in a noticeable manner. Another reason is the improved surface sites due to the presence of Co3+/Co2+, Fe3+/Fe2+, Zr2+/Zr4+, and non-metal cationic species, which is directly related to the catalytic efficiency towards alkylation reactions [33]. Thus, it can be concluded that the prepared catalysts effectively catalyze the isopropylation of benzene at relatively low temperature and catalyst load.The schematic diagram of the reaction pathway is depicted in Fig. 4 . Along with cumene, di/tri substituted products also is formed, which then gets converted to the monosubstituted product viz cumene.Friedel-Crafts isopropylation of benzene with isopropanol is a well-known aromatic substitution reaction and it involves a complex reaction network. This reaction is sometimes associated with several side reactions which result in the formation of various products through transalkylation, dispropotionation, dealkylation etc [34]. The formation of n- propylbenzene is always accompanied by the production of cumene, which is formed either by the isomerisation reaction of cumene or from the primary n-propyl cation formation before isopropylation of benzene. From GC–MS analysis, cumene was found to be the major component whereas di/tri isopropyl benzene was the minor product along with minor quantities of propene, n-propyl benzene, and other aliphatic products. Similar observations were also made by other researchers [35]. Based on this a suitable mechanism is suggested for the isopropylation of benzene. Water is a byproduct formed in the isopropylation reaction. Due to the electron deficient dopant cations of Zr, N and S, the catalyst surface becomes highly hydrophilic as evident by the FT-IR analysis. Therefore, the catalysts easily become proton donors by the abstraction of water molecules. The isopropanol is converted into propene by dehydration followed by the proton abstraction from the catalysts sites resulting in the formation of a carbocation. Isopropyl substitution on the aromatic ring is possible by the attack of the carbocation after the removal of H+ back to the catalyst. The main reaction steps can be represented as follows:The reusability of the spent catalyst was studied by separating it from the reaction mixture by magnetic separation on the completion of a catalytic run. The catalyst removed from the reaction mixture was washed thoroughly with acetone to remove organic matter, dried, calcined, and reused for another three catalytic runs under the same reaction conditions. The results of the reaction in 2 h are shown in Fig. 5 . No remarkable fall in the activity was observed till two consecutive runs whereupon it slightly reduces in the third and fourth cycles. The selectivity of the catalyst towards cumene remains almost constant till its fourth cycle. The reduction in the activity can be explained by two reasons; the deactivation of the catalyst by structural and surface changes due to its interaction with substrate molecules and the deposition of organic matters, especially the byproduct DIPB [36] The structural stability of the spent catalyst was analyzed by recording the XRD pattern and surface area of CZFTD catalyst. PXRD pattern of the fresh and reused catalyst remains the same, indicating the phase stability of the prepared nanoferrites (Fig. 6 ). Average crystallite size increases due to agglomeration of particles. Surface area decreases significantly due to the incorporation of particles into the surface sites [37] (Table 3 ) (see Fig. 7 ).Zr/N/S doped nanoferrite catalysts efficiently catalyze the isopropylation of benzene offering an alternate process for the selective cumene preparation at low temperature and catalyst loading. Comparatively high activity was observed with nitrogen and nitrogen/zirconium doped samples. The critical role played by the different reaction parameters on the catalytic activity and selectivity is noteworthy. Monoalkylated product is the major product along with negligible formation of polyalkylated products. The isopropylation activity of the catalyst is found to be dependent on the number and strength of active surface sites, which in turn depends on the nature and concentration of dopant. The prepared catalytic systems are found to be reusable and resistant to rapid deactivation. The remarkable feature of this catalytic system is that it can be readily isolated from the reaction mixture by employing an external magnet.All authors contributed to the study conception and design, material preparation, data collection and analysis. Both authors contributed equally towards the data analysis and writing. All authors read and approved the final manuscript.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Nitrogen, sulphur and zirconium doped cobalt ferrites prepared by coprecipitation method were evaluated as catalysts for the production of cumene from benzene and isopropyl alcohol. The catalytic efficiency of cobalt ferrite towards the reaction increased due to the doping by zirconium and non-metals. The isopropylation activity is decidedly dependent on the number and strength of active surface sites that in turn depends on the nature and concentration of dopant. Reaction variables like temperature, reactant ratio and catalyst load affected the reaction rate considerably. On successive uses, the catalysts were found to be magnetically recoverable and stable.

In the current transition period towards green energy, the implementation of carbon capture and utilization (CCU) strategies is one of the biggest challenges to achieve the goal of decarbonization and tackle climate change. In this scenario, the technological development of efficient routes for the large scale conversion of CO2 into value-added products is imperative to offset the cost of its capture and storage (Hepburn et al., 2019; Kamkeng et al., 2021; Zhang et al., 2020).The catalytic processes for the conversion of CO2 into fuels and raw materials (as olefins and aromatics) receive great attention for their implementation in the refineries of the future (Garba et al., 2021; Leonzio, 2018; Ye et al., 2019) and are complemented by other initiatives aimed at intensifying the recovery of oil and natural gas (Alabdullah et al., 2020; Palos et al., 2021). As an alternative to the well-developed two-stage hydrocarbon production technologies from CO2 hydrogenation to methanol/dimethyl ether (DME) (Sehested, 2019) and its selective conversion into olefins, gasoline or aromatics (Tian et al., 2015), hydrocarbon synthesis routes in one stage (modified Fischer Tropsch (MFT) or with oxygenates, methanol/DME, as intermediates), through cascade reactions and with tandem catalyst (Ma and Porosoff, 2019; Wei et al., 2021), have the attraction of lower equipment cost and higher CO2 conversion. The thermodynamic equilibrium of methanol synthesis is displaced by the in situ conversion of oxygenates into hydrocarbons, which allows the integrated process to be carried out at higher temperatures and at moderate pressure (15–30 bar) compared with the conventional methanol synthesis. It is remarkable that this facilitates the supply of H2, using commercial PEM electrolyzers.The CO2 hydrogenation route with oxygenates as intermediates is more attractive than the MFT reaction (based on Fe and Co catalysts) for the selective production of hydrocarbons, as it is not conditioned by the Anderson-Schulz-Flory (ASF) distribution. Using appropriate zeolites (SAPO-34, HZSM-5, Hbeta or HY, the most studied), the oxygenate intermediate route can be selectively addressed towards the production of olefins, aromatics or gasoline. The acidity and appropriate shape selectivity are key features of the catalyst for this purpose (Ramirez et al., 2019; Wei et al., 2017). Comparing the deactivation of the catalysts used in oxygenates conversion into olefins, that is, MTO (methanol to olefins) and DTO (DME to olefins) processes, the high partial pressure of H2 in the integrated CO2 to olefins process contributes to minimizing the formation of coke on the acid catalyst (Nieskens et al., 2018), which is a relevant feature that conditions the feasibility of the overall process and the configuration of the reaction equipment (Cordero-Lanzac et al., 2020b; Tian et al., 2015).Although conventional Cu based catalysts (with of Cu/Zn, Cu–ZnO–Al2O3, Cu–ZrO2 and Cu–ZnO–ZrO2 configurations, among others), are very active and selective for the synthesis of methanol from syngas, in the hydrogenation of CO2 (with high H2O concentration in the medium) and especially in the conditions required for the synthesis of hydrocarbons (above 300 °C), suffer severe deactivation by sintering and are particularly active for the rWGS reaction (Marcos et al., 2022). The adequate activity of In2O3 for the synthesis of methanol under these conditions, especially from pure CO2 source, is accepted in the literature (Araújo et al., 2021a; Martin et al., 2016). Numerous experimental and theoretical studies delve into the reaction mechanism of In2O3, whose activity is attributed to its CO2 adsorption capacity in the superficial oxygen vacancies and H2 dissociation (Frei et al., 2018; Wang et al., 2021; Ye et al., 2013), favoring the advance of the reaction mechanism with formate ions as intermediates (Chen et al., 2019). CO2 is successively hydrogenated: CO2* → HCOO* (formate) → H2COO* (dioxymethylene) → H3CO* (methoxy) → CH3OH (Chen et al., 2019; Gao et al., 2017; Ye et al., 2013, 2014).However, at the temperature required for the direct synthesis of hydrocarbons as well, In2O3 presents limitations due to: i) Thermodynamics, because secondary endothermic reactions (rWGS and methanation) are favored; ii) partial sintering, favored by the required higher temperature and the presence of H2O and CO (Wang et al., 2021). To increase the hydrogenation activity and the selectivity to methanol of In2O3, and reduce sintering deactivation, different strategies have been used (Wang et al., 2021): i) Improving the dispersion of In2O3 and increasing the oxygen vacancies, ii) promoting the dissociative H2 adsorption and spillover; iii) promoting the activation of CO2; iv) stabilizing key reaction intermediates, and v) generating new types of active sites.In these strategies, the use of supports and promoters has been of great importance. From studies on the synthesis of methanol (Araújo et al., 2021a; Zhang et al., 2018) and olefins in one stage (Gao et al., 2018), the role of ZrO2 as carrier in favoring the dispersion of In2O3 and generating new vacancies by the formation of epitaxially-grown In2O3 or solid In2O3–ZrO2 solutions is well established (Nieskens et al., 2018; Ramirez et al., 2019), with the consequent increase in the adsorption capacity of CO2 and to attenuate the sintering of In2O3 (Alabdullah et al., 2020; Garba et al., 2021). Additionally, In2O3 can be combined with other metals, active for hydrogenation reactions, such as Zn (Palos et al., 2021), Ni (Araújo et al., 2021b; Jia et al., 2020), Co (Bavykina et al., 2019; Pustovarenko et al., 2020), Au (Rui et al., 2020), Rh (Li et al., 2020), Pt (Han et al., 2021) or Pd (Araújo et al., 2021b; Frei et al., 2018; Snider et al., 2019), to increase CO2 conversion and methanol selectivity. Various studies in the literature compare In2O3 based catalysts either for methanol production from syngas (Su et al., 2018) or by CO2 hydrogenation, nonetheless, the literature studying the joint hydrogenation on CO2+CO mixtures is scarce (Araújo et al., 2021b). The co-feeding of syngas together with CO2 is interesting from various perspectives: i) It allows the joint valorization (avoiding separation costs) of streams derived from the gasification of biomass or wastes of the consumer society (where CO2 and syngas are present) (Couto et al., 2013; Lopez et al., 2015), ii) considers the real need to recirculate the stream of unreacted gases in the synthesis of methanol and direct synthesis of olefins (Araújo et al., 2021b), and; iii) contributes to the necessary supply of H2. Araújo et al. (2021a) have verified the capacity of different catalysts (In2O3–ZrO2, Cu–ZnO–Al2O3 and ZnO–ZrO2) for the hydrogenation of mixtures of CO and CO2, under suitable conditions for the synthesis of methanol, verifying the favorable effect of the presence of CO on the controlled formation of surplus oxygen vacancies in In2O3. However, the effect on the yield and selectivity of methanol and on the deactivation of the catalyst by sintering is complex, since CO acts as a reducing agent (which favors the sintering of In2O3 by over-reduction). Another factor influencing the deactivation of the In2O3 catalyst for these feedstocks is the concentration of H2O, which increases as CO2 conversion increases. Several authors (Frei et al., 2018; Ye et al., 2014) have verified in the synthesis of methanol that a limited concentration of H2O favors the formation of methoxy ions, increasing the yield of methanol. However, an excess of H2O leads to annihilate the oxygen vacancies, to the aggregation of In species, and decrease of In0 species, affecting the dissociation capacity of H2, and so, the overall performance of the catalyst for CO2 hydrogenation.The aforementioned results in the literature show the need to progress in the knowledge and improvement of catalysts for the synthesis of methanol from CO2 under the reaction conditions required for the integrated process (CO2 to hydrocarbons), and especially when CO is co-fed given the interplay of CO/H2O. The reaction conditions for maximizing CO2 conversion and hydrocarbon production while limiting catalyst deactivation by sintering must also be optimized, since the CO2 to hydrocarbons process is conducted at a higher temperature than the individual stage of methanol formation and at higher concentration of H2 than oxygenates to hydrocarbons conversion. As to contribute filling this shortage, in this work, the effect of the Zr/In ratio on the In2O3–ZrO2 catalyst has been studied in the synthesis of methanol from CO2/syngas mixtures. Moreover, the selected In2O3–ZrO2 catalyst (based on its good kinetic performance) has been used to conform a In2O3–ZrO2/SAPO-34 tandem catalyst, and its performance has been further studied (in terms activity-selectivity-stability) in the direct synthesis of olefins. SAPO-34 has been selected (Dang et al., 2019) as acid catalyst for this purpose due to its well-known suitable behavior (highly selective) in the conversion of methanol (and/or DME) into olefins. The results are explained according to the properties of the catalysts determined by different analysis techniques.The In2O3–ZrO2 catalysts have been synthesized following a co-precipitation method (Sánchez-Contador et al., 2018). Metal nitrates solutions, In(NO3)3 (Sigma-Aldrich) and Zr(NO3)4 (Panreac) with the desired Zr/In atomic ratio (0 (In2O3), 1:3, 1:2, 1:1, and ZrO2) and total metal concentration of 1 M were co-precipitated over 20 mL of deionized water under stirring, with ammonium carbonate (Panreac, 1M), at 70 °C and neutral pH. The mixtures were aged for 2 h to ensure the complete co-precipitation, and then filtered and cleaned with deionized water until neutral supernate was obtained. Finally, the resulting powders were dried and calcined at 500 °C for 1 h and pelletized, crushed and sieved to the desired particle size (125–250 μm).SAPO-34 acid catalyst (ACS Material) was calcined at 550 °C for 5 h, pelletized, crushed and sieved to the desired particle size (300–450 μm). The tandem catalyst was composed by physical mixture of pelletized In2O3–ZrO2 metallic catalyst and SAPO-34 acid catalyst in a 2/1 mass ratio. The different particle size of both functions allowed analyzing the spent catalysts independently.The physical properties of the catalysts (BET specific surface area and pore volume) have been determined by N2 temperature programmed adsorption-desorption (N2-TPD) analyses (Micromeritics ASAP 2010) at −196 °C. The procedure consists on a previous conditioning stage of the sample, on which degassing is carried out at 150 °C under vacuum (10−3 mmHg) for 8 h to eliminate impurities and remove the H2O adsorbed on the surface of the catalyst sample, facilitating N2 sorption. Subsequently, serial equilibrium stages of N2 adsorption-desorption are carried out until the complete saturation of the sample at cryogenic temperature of liquid N2. The pore volume is calculated with the BJH method using the adsorption branch of the isotherm.The chemical composition has been quantified and qualified by X-Ray fluorescence (PANalyticalAxios) and the structure by means of X-Ray diffraction (PANalyticalXpert PRO) and XRD vs temperature analyses. For determining the metallic properties, H2 temperature programmed reduction (H2-TPR) and CO-TPR analyses were carried out (MicromeriticsAutochem 2920). Briefly, 100 mg of sample were first swept with He to eliminate impurities and adsorbed H2O. After stabilizing the catalyst in the corresponding mixture (10% H2 or CO, in Ar), the samples were heated up to 800 °C at a 2 °C min−1 rate, and reference and analyzed streams were compared.The same equipment was used for CO2-TPD analyses and for acidity measurements (TPD-NH3). For CO2-TPD the following steps were used: i) 30 min of He sweeping (160 mL min−1) at 550 °C, for eliminating possible impurities and adsorbed H2O; ii) stabilization at 150 °C with He (20 mL min−1); iii) sample saturation by CO2 injection (5 mL min−1) at 50 °C; iv) He sweeping (20 mL min−1) to remove the physisorbed adsorbate; and, v) desorption by heating the sample with a controlled temperature ramp (5 °C min−1) from 50 to 400 °C, the operating reaction temperature.Analogous technique was used for NH3-TPD analyses, using 50 μL min−1 NH3 injections at 150 °C for the saturation of the sample and 5 °C min−1 temperature ramp for the desorption step, up to 550 °C.The reaction runs have been carried out in an isothermal PID Eng&Tech fixed bed reactor. The reactor dimensions are: 9 mm internal diameter and 10 cm of effective length and is made of 316 stainless steel. The equipment can operate up to 700 °C, 100 bar and with catalyst loadings up to 5 g. The catalyst was mixed with an inert (SiC) to ensure isothermal conditions and to avoid preferential pathways. The reactor outlet stream was heated up to 110 °C to avoid products condensation, and analyzed online in a gas-chromatograph (microGC Varian CP4900). For this analysis three modules were used: i) Molecular sieve (MS-5) to quantify H2, N2, O2 and CO; ii) Porapak Q (PPQ) for CO2, water, C1–C4 hydrocarbons and MeOH/DME; and iii) 5CB column (CPSiL) for higher hydrocarbons. Typically, the reaction runs were carried out at 400 °C, 30 bar, H2/COX ratio of 3 and with 125 mg of catalyst. These conditions were established in a previous work as suitable for the joint valorization of CO2 and syngas into olefins (Gao et al., 2018; Portillo et al., 2021). H2, CO and CO2 flowrates were adjusted to get a 3.35 gcat h molC −1 space-time value with the corresponding CO2/COX ratio (between 0, corresponding to 100% CO; and 1, corresponding to 100% of CO2). The reaction system has been described in more detail elsewhere (Portillo et al., 2021).In order to quantify the obtained results, the following reaction indices have been defined. The conversion of CO and CO2: (1) X C O x = F C O x 0 − F C O x F C O x 0 · 100 where F C O x 0 is inlet molar flowrate in content C atoms, and F C O x its analogous at the reactor outlet stream.Similarly, CO2 conversion, X C O 2 has been defined as: (2) X C O 2 = F C O 2 0 − F C O 2 F C O 2 0 · 100 where F C O 2 0 and F C O 2 are the CO2 molar flowrates at the inlet and outlet of the reactor, respectively. Carbonaceous products yields (Y i ) and selectivities (S i ) (except for CO and CO2) have been defined according to Eqs. (3) and (4), respectively, by grouping the products into the following lumps: methane, C2–C4 olefins, C2–C4 paraffins, oxygenates (MeOH and DME) by the use of the following expressions: (3) Y i = n i · F i F C O x 0 · 100 (4) S i = n i · F i ∑ i ( n i · F i ) · 100 being ni the number of C atoms in a molecule of component i and F i the molar flowrate of the component i at the reactor outlet stream.The carbon balance in all experiments was closed over 99%.In this section, first, the effect of the Zr loading on the properties of the In2O3–ZrO2 catalyst has been studied. Second, a comparison of the performance of the catalysts for CO2/CO mixtures hydrogenation has been carried out in order to select the most suitable Zr/In ratio to favor methanol production. The reaction runs have been carried out under the reaction conditions required for the direct olefins synthesis process, pursuing to use the selected In2O3–ZrO2 catalyst in a In2O3–ZrO2/SAPO-34 tandem for this reaction. Catalyst screening has been carried out attending to activity, selectivity to methanol and stability criteria. Finally, the performance of the In2O3–ZrO2/SAPO-34 tandem catalyst has been assessed for the direct synthesis of olefins from CO2/CO mixtures.Attending to the physical properties of the catalysts (Table 1 ) determined by N2-TPD analyses, the pore volume of In2O3 is higher (0.25 cm3 g−1) than that of ZrO2 (0.16 cm3 g−1). Consequently, the pore volume of the composite In2O3–ZrO2 catalysts decreases with increasing Zr loading in the catalyst. As to the BET specific surface area (SBET) regards, being 53 m2 g−1 for In2O3 and 96 m2 g−1 for ZrO2, the SBET of In2O3–ZrO2 catalysts increases upon increasing Zr/In ratio, in agreement with the results reported by Frei et al. (2020). Indeed, higher SBET than expected from the Zr/In ratio has been obtained for the In2O3–ZrO2 catalysts.As to the chemical and metallic properties characterization regards, XRF analyses have been carried out to ascertain the co-precipitation of the metals in the desired Zr/In ratio. In Table 2 , the nominal and measured metal ratios are listed.As to analyze the morphology of the catalysts, XRD patterns for the different catalysts are depicted in Fig. 1 . According to these spectra, pure In2O3 and ZrO2 catalysts show the typical peaks for these structures. At 2 θ  = 21.68°, 30.74°, 35.61°, 37.88°, 40°, 42.03°, 43.99°, 45.86,° 51.14°, 52.84°, 56.14°, 59.245°, 60.784°, 62.33° and 63.79° for In2O3, and; at 2 θ  = 30.6876°, 35.5315°, 51.0336°, 60.6445°, 63.0967°, 74.9399° for ZrO2. For the composite catalysts, that is, for the mixture of In2O3 with ZrO2, a combination of those in good agreement with the Zr/In ratio reported in Table 2 is observed. Moreover, the results evidence that ZrO2 coexists in its monoclinic (most favored thermodynamically) (Martin et al., 2016) and tetragonal structure, whereas it changes completely into its tetragonal polymorph with the incorporation of In2O3 in the In2O3–ZrO2 catalysts as determined in the literature (Frei et al., 2019). Moreover, from further XRD vs temperature measurements carried out, the structure of the In2O3–ZrO2 catalysts is expected to remain stable under the reaction temperatures used in the direct CO2/syngas to olefins process. Additionally, the Rietveld calculations carried out evidenced the presence of Zr atoms in the In2O3 structure and of In atoms integrated in the ZrO2 structure, which is consistent with previous findings (Artamonova et al., 2006; Frei et al., 2020; Portillo et al., 2021). For the 1Zr–1In catalyst, 42.6% of In2O3 structure and 57.4% of ZrO2 structures were determined. The metal content within the In2O3 structure, is divided into 81.2% of In, and 18.8% of Zr. Likewise, within in the ZrO2 structure, 76.2% stands for Zr and 23.8% for In. These results are consistent with the suggestion in the literature that indium-zirconium composite oxides are not simple mechanical mixtures but generate active composite In1-xZrxOy oxides (Dang et al., 2018).The reducibility of the catalysts has been studied by H2-TPR and CO-TPR analyses (Fig. 2 , where the TCD signals have been normalized for In2O3 mass). With this approach, gathering information on the H2 splitting activity (H2 desorption) and on the reducibility of the catalyst in the reaction medium is pursued. Prior to both H2 and CO-TPR, the samples were swept with He at 200 °C for 1 h to eliminate impurities and adsorbed water. Later on, the samples were cooled down to 30 °C, the inlet gas changed to H2/CO and temperature increased after attaining a stable baseline at 30 °C.As expected, ZrO2 is not reduced under the studied TPR conditions and so, it is not expected either under the not so severe reaction temperature used. As observed, the combination of In2O3 and ZrO2 incurs peaks at higher temperatures for In2O3–ZrO2 catalyst compared to In2O3 and ZrO2, for both reducing agents. Comparing H2-TPR (Fig. 2a) and CO-TPR (Fig. 2b), CO presents higher reduction capacity, which is in accordance with the previous results (Chen et al., 2019; Dang et al., 2018; Frei et al., 2018; Martin et al., 2016). The results also indicate a favorable effect of the addition of ZrO2 on the number of In2O3 sites accessible to H2 and CO (greater area under the curve per unit mass of In2O3). It is noteworthy that the presence of ZrO2 favors the reduction of In2O3 with CO at low temperature, as a peak is observed with a maximum between 275 and 300 °C for composite In2O3–ZrO2 catalysts. The effect of CO as vacancy generator (Martin et al., 2016; Wang et al., 2021) will contribute to this result, also favoring its adsorption and that of CO2. Fig. 2 also shows a greater resistance to reduction of In2O3 sites due to the presence of ZrO2. This lower reducibility is consistent with the larger crystal size of In2O3 observed with increasing Zr/In ratio (Table 3 ). In addition, it is well established in the literature that the presence of ZrO2 hinders the sintering of In2O3 (Araújo et al., 2021b), which is consistent with the lower reducibility observed in Fig. 2 for moderate ZrO2 contents in the composite catalyst.CO2-TPD analysis have been carried out for all the catalysts to quantify their CO2 adsorption capacity, since this is a key feature for their activity for oxygenates synthesis. The results are plotted in Fig. 3 . According to these profiles, In2O3–ZrO2 catalysts outperform significantly the CO2 adsorption capacity of the parent In2O3 and ZrO2 catalysts.The responsibility of the oxygen vacancies in In2O3 on the CO2 adsorption capacity and activity for hydrogenation to methanol is well established (Martin et al., 2016; Sun et al., 2015). Consequently, the higher CO2 adsorption capacity of the In2O3–ZrO2 catalyst than of In2O3 observed in Fig. 6 is attributable to its higher density of oxygen vacancies. Dang et al. (2018) determine by X-ray photoelectron spectroscopy (XPS), CO2-TPD analysis, and periodic DFT calculations that the incorporation of ZrO2 into In2O3 generates interactions in the electronic structure, the formation of In1-xZrxOy mixed oxide and the formation of additional oxygen vacancies, increasing CO2 conversion.Regarding the characterization of the SAPO-34 acid function: an specific surface area BET of 652 m2g-1, micropore volume of 0.2192 cm3 g−1, and total pore volume of 0.23 cm2 g−1were determined (Fig. S1). In the NH3-TPD analysis (Fig. S2) 777.6 mmolNH3 gcat −1 were measured for SAPO-34, and from the profile two types of acid sites were identified, with peaks at 180 °C (14%) and 375 °C (86%), related to weak and strong acid sites, respectively.The performance of the In2O3–ZrO2 catalysts has been studied in CO2 (Fig. 4) and CO2/CO mixtures hydrogenation (Fig. 5) under the reaction conditions (described in Section 2.3). It is observed in Fig. 4, that for CO2 hydrogenation, parent catalysts (thus, In2O3 and ZrO2) reach slightly higher COX conversion values than combined metal oxides, and even higher oxygenates selectivities. However, all CO2 conversion values are significantly enhanced in the In2O3–ZrO2 catalysts. For the catalysts with a Zr/In ratio of 1/3 and 1/2 similar performance is observed, thus, more than doubling the value of XCO2 with respect to that obtained with In2O3 and upgrading that of ZrO2 over 50–60%. However, the selectivity of oxygenates decreases from 60% to 55% for In2O3–ZrO2 catalysts, as a consequence of the increase in the formation of methane and, to a lesser extent, of olefins. It should be noted that the results in Fig. 6 evidence that the weakly acidic sites (Lewis sites) of ZrO2 in the In2O3–ZrO2 catalysts (Dang et al., 2018) are sufficient to activate the dual cycle mechanism, with a reduced conversion of methanol into olefins, justifying the upturning olefins yield when increasing the Zr content. However, ZrO2 itself is not sufficient for the formation of olefins, because the presence of In2O3 is required for an efficient CO2 adsorption and H2 splitting as the first steps for methanol formation.Based on these results, a Zr/In ratio in the range between 1/3 and 1/2 is considered adequate to maximize both pursued targets in the hydrogenation of CO2, thus, CO2 conversion and oxygenate yield. This upgrade in CO2 conversion for the In2O3–ZrO2 catalysts at 400 °C is consistent with the characterization results observed in the H2-TPR (Fig. 2a) and CO2-TPD of (Fig. 3) analyses. Both results explain the synergy in the conversion of CO2 by the improvement of the CO2 adsorption capacity by ZrO2 and H2 dissociation on In2O3 sites, due to the proximity of these sites. Frei et al. (2020) already observed this phenomenon in the usual conditions of methanol synthesis from CO2 (300 °C) with In2O3–ZrO2 catalysts.Comparing the results in Figs. 4 and 5, a significant effect of the feed composition over the performance of the catalysts is evidenced. In both cases, the high CH4 selectivity is noteworthy, being higher for CO2/CO mixture hydrogenation (Fig. 5). For this feed, higher Zr/In ratio in the tandem catalysts leads to upturn CH4 selectivity, at the expense of olefins and oxygenates yields. This significant CH4 formation, is consequence of the fact that the endothermic methanation reaction is favored at such high reaction temperature of 400 °C required in the direct CO2 to olefins process. However, this reaction, which also occurs with methoxy ions as intermediates, as oxygenates formation does (Solis-Garcia et al., 2017), will be suppressed in the direct synthesis of olefins, by the in situ conversion of these ions into olefins, by means of the very fast dual cycle mechanism over the SAPO-34 catalyst (Cordero-Lanzac et al., 2020a) (subsequent section 3.4). The greater CO2 conversion attained with the catalyst with Zr/In ratio of 1/2 (Fig. 4) is interesting for its use in the direct synthesis of olefins by CO2 and CO2/CO hydrogenation.For further studying the effect that adding ZrO2 might have in the deactivation of In2O3–ZrO2 catalysts, the evolution of CO2 conversion with time on stream is compared in Fig. 6 for CO2/CO mixture hydrogenation. It is noticeable that deactivation is only observed for pure In2O3. These trends evidence that ZrO2 addition improves the stability of the parent In2O3 catalyst. In accordance with the results in Figs. 4 and 5, 1Zr–2In and 1Zr–3In catalysts have also similar performance for the evolution of CO2 conversion with time on stream.This high stability of the catalyst in presence of ZrO2 is also evident in the evolution of products distribution with time on stream for the tested 24 h. As an example, in Fig. 7 , the evolution of products yield with time on stream is depicted for the In2O3–ZrO2 catalyst with Zr/In of 1/2 for the hydrogenation of a CO2/CO mixture with a CO2/COX ratio of 0.5. A high CO2 conversion with ZrO2 catalyst is also observed in Fig. 6. However, this result is a consequence of an undesired high CH4 formation (Fig. 5). In addition, Figs. 4–6 show that for a Zr/In ratio of 1/1 the conversion of CO2 is remarkably lower, which does not correspond to the CO2 adsorption capacity (Fig. 3). This result is explained because with this Zr/In ratio the amount of In2O3 is not enough for the dissociation of the H2 amount required for the methanol and CH4 formation reactions.All in all, considering CO2 conversion, oxygenates selectivity, catalyst stability and the influence of the CO2/CO composition in the feed, the composite catalyst with Zr/In ratio of 1/2 is considered to give the best balanced results, and so, the best prospects for conforming tandem In2O3–ZrO2/SAPO-34 catalysts for the direct olefins synthesis.As it is sensed by comparing the results in Figs. 4 and 5, the composition of the CO2/COX mixture in the feed has a great effect on products distribution in the synthesis of methanol using In2O3–ZrO2 catalysts. In Fig. 8 , product yield values after 16 h TOS corresponding to the catalysts with Zr/In ratios of 1/2 (Figs. 8a) and 1/3 (Fig. 8b) are shown. It is observed that in both cases CH4 formation decays sharply when co-feeding CO2 together with syngas. It is also noteworthy that methanol yield passes through a maximum for feed compositions with equal concentration of CO and CO2. Likewise, the results in Fig. 8b contribute to further standing out the greater production of methane the 1Zr–3In catalyst leads to, being it especially relevant for pure syngas feeds (CO2/COX, 0). Probably due to the over-reduction of the catalyst, in good agreement with the highest reducibility under H2 and CO atmospheres reported in section 3.1.Accordingly, these results reaffirm the selection of 1Zr–2In as the In2O3–ZrO2 catalyst with best prospects to be used in combination with SAPO-34 in the direct olefins formation from CO2 and syngas mixtures whatever CO2/CO composition.The suitability of the selected In2O3–ZrO2 catalyst (1Zr–2In) in the direct synthesis of olefins process has been addressed in this section. Fig. 9 illustrates the products yields obtained with the tandem In2O3–ZrO2/SAPO-34 catalyst for feeds with different composition. The results evidence the good performance of the tandem catalyst, given almost all the oxygenated compounds formed as intermediates are converted into hydrocarbons, the selectively to olefins, being paraffins the only by-products, and the absence of CH4.Comparing the results of the direct synthesis of olefins (Fig. 9) with those in Fig. 8 of the first stage of the process (oxygenates formation), various features are to be highlighted: i) The upgrade of the overall conversion obtained in the direct synthesis. Thus, COX conversion increases from 0.88% to 4.28% for feeds with CO2/COX ratio of 0.5, and the conversion of the targeted products from 0.51% (oxygenates yield in methanol formation) to 3.11% (olefins yield). ii) The suppression of the undesired methane formation pathway, being it almost undetectable.The mechanism of hydrocarbon formation directly by hydrogenation of CO2 and CO on the In2O3–ZrO2/SAPO-34 tandem catalyst is the cascade combination of the mechanisms of methanol/DME synthesis and the in situ conversion of these oxygenates into hydrocarbons. It is well established in the literature (Frei et al., 2018; Tan et al., 2019; Ye et al., 2012, 2013) that in methanol synthesis over In2O3–ZrO2 catalysts, CO2 is adsorbed on In2O3 oxygen vacancies and on additional oxygen vacancies formed by the presence of ZrO2 and the formation of stable In1-xZrOxOy mixed oxide. The H2 dissociation capacity of the In2O3 in In2O3–ZrO2 facilitates the hydrogenation of adsorbed CO2 to form formate species (HCOO*). The next steps consists of the reaction of these species with H* ions to produce H2COO* species, and the hydrogenation of the latter to methoxy species (H3CO*), which will hydrogenate to form methanol. The presence of these intermediates has been determined by means of experimental and theoretical studies (Dang et al., 2018; Wang et al., 2021). The effect of the presence of CO in this reaction mechanism is controversial. Besides disfavoring the rWGS reaction, it is well established (Martin et al., 2016) that a moderate concentration of CO increases the density of oxygen vacancies, favoring therefore CO2 adsorption and the extent of methanol formation. However, due to its strong reducing character (verified in Fig. 2b), a high concentration of CO can favor the over-reduction of In2O3 and its sintering (Araújo et al., 2021b). The results in Fig. 9 are consistent with the commented effect of CO concentration, giving rise to higher olefins yield for the CO2/COx ratio of 0.5 in the feed than for the hydrogenation of CO (CO2/COx of 0), as a consequence of the higher methanol yield.The formation of the C–C bonds of the light olefins from methanol/DME is a consequence of the activity of the acid sites of SAPO-34 (Gayubo et al., 2000; Pérez-Uriarte et al., 2016). The reaction proceeds through the dual cycle mechanism, with two related routes, with polyalkyl benzenes and olefins as intermediates (Gao et al., 2019). The severe shape selectivity of SAPO-34, with CHA topology (cavities of 10 × 6.7 Å connected by 3.8 × 3.8 Å 8-ring cages) (Hemelsoet et al., 2013), is suitable for the selective formation of ethylene and propylene when used in the tandem In2O3–ZrO2/SAPO-34 catalyst (Dang et al., 2019; Portillo et al., 2021).The evolution of products yield with time on stream obtained in the direct synthesis of olefins is shown in Fig. 10. These results, corresponding to the most severe deactivation conditions studied, that is, for syngas feeds, evidences that after an initial activity decay taking place in the first 4 h of reaction, an pseudo-steady state is achieved (in this case and for all the studied feed compositions). From characterization analyses through temperature programmed oxidation (TPO) with air (results not shown), this deactivation is attributed to coke deposition in the acid function (coke content of 4.9 wt% on the acid catalyst and 0.5 wt% on the metallic catalyst of the tandem catalyst, for the reaction conditions in Fig. 10). Coke formation takes place fast in the first reaction hours over SAPO-34, reaching almost the maximum reported value in 2 h TOS (4.6 wt% and 4.9 wt% at pseudo-stable conditions). Once at that point, coke formation rate is residual, suppressed by the hydrogenation of the intermediates, and does not lead to further activity decay.The addition of Zr to In2O3 catalysts improves the performance (activity, oxygenates selectivity and stability) for the hydrogenation of CO2/CO mixtures to methanol under the suitable operation conditions for the direct CO2 to olefins process (400 °C, 30 bar). This is a key feature for the configuration of tandem catalysts for the direct conversion of CO2 (and syngas) into olefins via the route with oxygenates as intermediates. The behavior of the In2O3–ZrO2 catalysts is a consequence of its properties. The loading of ZrO2 with a Zr/In ratio between 1/3 and 1/2 increases the yield of oxygenates compared to that obtained with parent In2O3 and ZrO2 catalysts, improves stability, and is suitable for attaining an outstanding conversion co-feeding CO (syngas) together with CO2, albeit with high formation of CH4 (favored with increasing Zr/In ratio).Accordingly, the In2O3–ZrO2 catalyst with Zr/In of 1/2 has been selected as suitable for its use in the In2O3–ZrO2/SAPO-34 tandem catalyst for the direct synthesis of olefins. The results obtained with this catalyst offer a good balance between CO2 and COX conversion, olefins yield and selectivity, and catalyst stability at 400 °C and 30 bar, for different CO2/COX composition feeds. The in situ conversion of the formed oxygenates into olefins, displaces the thermodynamic equilibrium of the methanol formation reactions, and as a result, olefins yield 6 folds compared to the oxygenates yield of the first stage, and methane formation is negligible.The results (obtained at low space time values, to work in demanding conditions for the stability of the tandem catalyst) are encouraging to progress towards the optimization of the operating conditions for the joint valorization of CO2 and syngas, which is an interesting strategy to mitigate climate change. A. Portillo: Conceptualization, Methodology, Investigation, Validation, Data curation, Writing – original draft, Writing – review & editing. A. Ateka: Conceptualization, Methodology, Investigation, Validation, Data curation, Writing – original draft, Writing – review & editing. J. Ereña: Data curation, Writing – original draft. J. Bilbao: Conceptualization, Methodology, Investigation, Data curation, Writing – original draft, Writing – review & editing, Supervision. A.T. Aguayo: Project administration, Conceptualization, Supervision, Validation, Data curation.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work has been carried out with the financial support of the Ministry of Science, Innovation and Universities of the Spanish Government (PID2019-108448RB-100); the Basque Government (Project IT1645-22), the European Regional Development Funds (ERDF) and the European Commission (HORIZON H2020-MSCA RISE-2018. Contract No. 823745). A. Portillo is grateful for the grateful for the Ph.D. grant from the Ministry of Science, Innovation and Universities of the Spanish Government (BES2017-081135). The authors thank for technical and human support provided by SGIker (UPV/EHU).The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.jenvman.2022.115329.

The effect of the ZrO2 content on the performance (activity, selectivity, stability) of In2O3–ZrO2 catalyst has been studied on the hydrogenation of CO2/CO mixtures. This effect is a key feature for the viability of using In2O3–ZrO2/SAPO-34 tandem catalysts for the direct conversion of CO2 and syngas into olefins via oxygenates as intermediates. The interest of co-feeding syngas together with CO2 resides in jointly valorizing syngas derived from biomass or wastes (via gasification) and supplying the required H2. The experiments of methanol synthesis and direct synthesis of olefins, with In2O3–ZrO2 and In2O3–ZrO2/SAPO-34 catalysts, respectively, have been carried out under the appropriate conditions for the direct olefins synthesis (400 °C, 30 bar, H2/COX ratio = 3) in an isothermal fixed bed reactor at low space time values (kinetic conditions) to evaluate the behavior and deactivation of the catalysts. The Zr/In ratio of 1/2 favors the conversion of CO2 and COX, attaining good oxygenates selectivity, and prevents the sintering attributable to the over-reduction of the In2O3 (more significant for syngas feeds). The improvement is more remarkable in the direct olefins synthesis, where the thermodynamic equilibrium of methanol formation is displaced, and methanation suppressed (in a greater extent for feeds with high CO content). With the In2O3–ZrO2/SAPO-34 tandem catalysts, the conversion of COx almost 5 folds respect oxygenates synthesis with In2O3–ZrO2 catalyst, meaning the yield of the target products boosts from ∼0.5% of oxygenates to >3% of olefins (selectivity >70%) for mixtures of CO2/COX of 0.5, where an optimum performance has been obtained.

The huge emission of carbon dioxide (CO2) due to the excessive consumption of fossils has caused severe problems, so the CO2 issue has attracted much attention all over the world [1–3]. As a renewable and environmentally friendly C1 resource, the capture and utilization of CO2 has attracted tremendous interest because it is not only an efficient way to alleviate greenhouse effect but also can provide green routes to synthesize chemicals [4–7]. By constructing C-X (X = N, O, C, H) bonds or direct CO2 hydrogenation [8], CO2 has been applied in the synthesis of urea [9,10], carbonate derivatives [11–13], carbonyl derivatives [14,15] or fuels [16–18]. However, CO2 is a thermal stable and dynamic sluggish apolar molecule, which makes its utilization be challenging, and its transformation is generally more difficult than other C1 resources (e.g., CO) [19]. In the past decades, the chemical transformation of CO2 has been widely investigated via thermo-, or electro-, or photocatalytic conversion, with focus on exploring new reaction routes and developing catalysts with high efficiency [20–23].As an emerging material platform, porous organic polymers (POPs), featuring in tailorable functionalization, large surface areas, adjustable porosity, versatile polymerizations, good physicochemical and thermal stability, have attracted considerable scientific interest [24–27] and showed promising applications in adsorption and separation [28–30], heterogeneous catalysis [31–33], energy storage [31,34] and so on. Especially, the task-specific design of POPs via selecting functional monomers, polymerization protocols, and modification strategies endows them promising candidates for CO2 capture and transformation, and various efficient POPs-derived catalysts have been developed [35–37]. In these reported POPs, CO2-philic heteroatoms (e.g., N, F, O, P) [38–40] and/or ionic sites [41–43] have been incorporated into the skeletons of POPs, which can achieve efficient adsorption and activation of CO2. Moreover, metal species as active catalytic components (e.g., Zn, Cu, Ru, Ag ions or nanoparticles) [44–46] can be immobilized onto POPs, achieving metalated POPs catalysts for CO2 transformation. In addition, POPs can be designed with conjugated structures to serve as photocatalysts for CO2 photoreduction to value-added chemicals [47,48].Our group has paid much attention on design and preparation of task-specific POPs for CO2 capture and utilization via introducing CO2-philic groups into the skeleton, constructing conjugated structures, modification with metal active species (i.e., metalated POPs) and so on, and a series of POPs with functional groups, such as, azo, Tröger's base, fluorine, phenolic –OH, N-containing sites, have been prepared with high catalytic performance. In this article, we will introduce our recent work on synthesis of POPs catalysts for CO2 transformation, which include POPs-based catalysts for cycloaddition reactions of epoxides and propargylic alcohols with CO2, reductive conversion of CO2 with H2, photocatalytic/electrocatalytic conversion of CO2. Finally, the challenges on POP-based catalysts for CO2 capture and conversion are discussed.Cycloaddition reactions of epoxides or propargylic alcohols with CO2 are economically favorable and environment-friendly routes to access cyclic carbonates that are widely applied as raw substrates for polycarbonates and polyurethanes, aprotic polar solvents, electrolytes of batteries and so on [49–51]. Developing catalysts with high efficiency, good stability and low cost is of significance for this kind of reactions. Besides the ability to activate CO2, the catalysts are also required to be capable of activating epoxides or propargylic alcohols [52–54]. In our work, we designed and synthesized POPs-based metal-free and metalated catalysts, which efficiently realize the transformation of CO2 under mild conditions.For the cycloaddition of epoxides with CO2, various catalysts that are capable of activating the epoxides and CO2 simultaneously have been reported, including homogeneous [55] and heterogeneous catalysts [56], metal-based [57,58] and metal-free catalysts [59]. Among the reported catalysts, POPs-based catalysts with or without metal components have been widely investigated, and have shown promising application potentials due to their unique performances [60–62]. In our work, we designed metal-free and metalated POPs catalysts for this reaction, and achieved the reaction under mild conditions.Lewis acid metal sites (e.g., Zn2+, Co2+, Ni2+) have been proved to be efficient for the cycloaddition of epoxides with CO2 due to their coordination ability with O atom of epoxides, and various metalated POPs with coordination unites (e.g., porphyrin, azo, salen, carbene, pyridine) have been synthesized over the past decades [63–65]. For example, Deng's group [66,67] reported a series of metalated salen-based POPs (M-CMPs, M = Zn, Co, Al) for this kind of reactions, which were synthesized by polymerization of metal-salen or post metalation. M-CMPs showed good CO2 adsorption capacity, and could achieve cycloaddition of epoxides with CO2 under ambient conditions. Especially, the resultant Zn-salen-POPs showed excellent catalytic performance for the cycloaddition of propylene oxide with CO2, and achieved an ultrahigh turnover frequency (TOF) up to 11,600 h−1 at 120 °C and 3 MPa with good stability [67]. Yang and coworkers [68] synthesized zinc porphyrin-based frameworks (P-POF-Zn) using hydroxyl-containing monomers (e.g., 1,3,5-tris(30-tert-butyl-40-hydroxy-50-formylphenyl)benzene) by two-step metallization, and the as-prepared Zn-P-POF displayed high catalytic performance for the cycloaddition of epoxides. It was demonstrated that the presence of massive –OH groups was beneficial for CO2 adsorption and activation. Though the above reported POPs-based catalysts showed high performance for cycloaddition of epoxides with CO2, the high cost of the monomers and complicated synthetic procedures limit their applications.We developed a simple strategy to synthesize o-hydroxyazobenzene POPs (HAzo-POPs) based on the diazo-coupling reaction of aryl tri/diamines with tri/diphenols in aqueous solution without any template or metal catalysts under ambient condition, as illustrated in Fig. 1 [69]. In contrast, the feedstocks used in this protocol are cheap and easily available, and the product yield reached up to 90%. The resultant HAzo-POPs displayed mesoporous structures with BET surface area up to ∼ 600 m2 g−1. They possess massive phenolic hydroxyl and azo groups in their skeletons, which are favorable to adsorption of CO2 and to coordination of metal ions. HAzo-POPs exhibited high metal adsorption capacity (e.g., 26.24 wt% for Cu2+ by HAzo-POPs-1) and excellent CO2 adsorption capacity (e.g., 7.5 wt% at 273 K and 1.0 bar) and high separation selectivity of CO2/N2 (106). Especially, Zn2+ complexed HAzo-POP-1 (Zn/HAzo-POP-1) exhibited excellent performances for cycloaddition of propylene oxide (PO) and CO2 in the presence of tetrabutylammonium bromide (TBAB), which could realize this reaction under ambient conditions, and showed 10 times higher activity than Zn-CMPs that was reported to be the state of art of the POP catalysts at that time. The good CO2 adsorption ability, mesoporous architecture, and excellent dispersity of metal sites are believed to be responsible for their high performance for catalyzing the cycloaddition of PO with CO2.Subsequently, the aqueous azo-coupling reactions were applied in the synthesis of porous polymers for CO2 capture [70], wastewater treatment [71], sensors [72], organic battery [73] and so on. For example, Chen and coworkers [70] prepared metalloporphyrin-based o-hydroxy azo-hierarchical POPs (ZnTPP/QA-azo-PiPs) by azo-coupling of tetra(4-aminophenyl) porphyrin zinc-derived diazonium salts with multihydroxy benzene in aqueous solution. The resultant ZnTPP/QA-azo-PiP1 not only showed excellent catalytic performance for the cycloaddition of various epoxides under mild conditions, but also it could catalyze the CO2-involved synthesis of oxazolidinones and N-formylated amines under diluted CO2 (15% CO2 in 85% N2, v/v).From the viewpoint of green chemistry, using renewable feedstocks and adopting simple synthetic procedures to synthesize porous polymers with low-cost are valuable. Utilizing renewable feedstocks including chitosan and phytic acid, we prepared a kind of porous metal–organic hybrids (MOH) based on the complexation of metal ions (e.g., Zn2+, Cu2+, Ni2+) with the interaction sites in the organic feedstocks, as illustrated in Fig. 2 [74]. As an example, the resultant MOH-Zn exhibited mesoporous structures with a BET surface area of 90 m2 g−1 and a pore volume of 0.40 cm3 g−1. Due to the presence of plenty of –OH, –NH2, and –PO4 groups, MOH-Zn displays excellent activity for CO2 activation, which can make cycloaddition of epoxides with CO2 proceed in the presence of TBAB under ambient conditions, affording a high turnover frequency of 7.8 h−1. This work provides a facile protocol for the synthesis of low-cost POP-based catalysts from renewable biomass feedstocks, which may have practical application potential. In a recent work, Zn-modified N-doped carbon (Zn/NC-X) was prepared via carbonization of the chitosan-derived MOHs-Zn, and the obtained Zn/NC-950 exhibited excellent performance for the aerobic oxidative cleavage of C(CO)–C bonds in acetophenone derivates at 100 °C [75].In most cases, TBAB is required as a co-catalyst for the cycloaddition of epoxides with CO2, which is unfavorable to the purification of product and increases the practical cost. To avoid the use of TBAB, Ding's group [76] developed a Zn2+ and Br− co-modified porous imidazolium ionic polymer containing P coordination sites for the cycloaddition of PO with CO2, achieving a high TOF of 6022 h−1 owing to the high BET surface areas, excellent CO2 uptake and synergistic effect of Zn2+ and Br−.Though great progress has been achieved in the synthesis of efficient metalated POPs for the cycloaddition of epoxides with CO2, the leakage or deactivation of active metal sites is still a big challenge. Therefore, developing metal-free POPs based catalyst with excellent catalytic performance and stability is interesting.Fluorine (F) element has the highest electronegativity and a small radius of 71 pm, which endow it with many special properties and coordination ability [39]. F-containing materials have CO2-philic property, and fluorinated POPs have been intensively studied, which indicated that F modification in POPs can obviously enhance the CO2 adsorption capacity [77–81]. For example, Han's group [82] reported that fluorinated-covalent triazine frameworks (CTFs) afforded a much higher CO2 adsorption capacity (5.53 mmol g−1) than that of non-fluorinated-CTFs (3.82 mmol g−1) due to the presence of massive C–F bonds. Besides of F modification, integrating nucleophilic ions (Br−, Cl−) into the skeleton of POPs has been proved to be an effective way to activate the epoxides [83]. For instance, quaternary phosphonium-functionalized POPs prepared via the polycondensation of tetrakis(4-chlorophenyl)phosphonium bromide showed intrinsic catalytic activity for the cycloaddition of epoxides with CO2 under 1 atm [84].Our group synthesized a series of fluorine and nucleophilic ions (Br− or Cl−) co-modified imidazolium based polymeric ionic liquids (denoted as PILs-X, X = Cl, Br) by an one-pot ionic polymerization based on the coupling reactions as illustrated in (Fig. 3 ) [85]. It was indicated that all the resultant PILs-X samples were effective for catalyzing cycloaddition of styrene oxide with CO2, and the F content in the catalysts showed positive correlation with their catalytic performance, following the order: PIL-Br < F0.5-PIL-Br < F-PIL-Br. Notably, F-PIL-Br showed three times higher efficiency for the cycloaddition of styrene oxide than that of the polymers without F modification (PIL-Br) under 1 MPa and 120 °C, together with broad scope of the reactants, excellent product yields (93%–99%), high stability and easy recyclability. In addition, F-PIL-Br showed much higher activity than F-PIL-Cl, which should be caused by the superior leaving ability and nucleophilicity of Br− over Cl−.F and ion co-modified POP catalysts have been reported to be efficient for other CO2 involved reactions as well. For example, Yan's group [86] synthesized frustrated Lewis pair (FLP) functionalized polymeric ionic liquids (PILs) using 4-styryl-di(pentafluorophenyl)borane. Because of the synergistic effect between two complementary Lewis acidic and basic sites, the as-prepared FLP-polymer exhibited ultrafast response for CO2 adsorption at about 20 s, and it also showed high catalytic performance for N-formylation of amines with CO2 in the presence of PhSiH3, affording a high TON of 14,800 at room temperature.The carboxylative cyclization of propargylic alcohols with CO2 is a green approach to synthesize α-alkylidene cyclic carbonates that have potential bioactivity with a broad range of applications as intermediates in organic synthesis. Transition metal catalysts (e.g., Ag, Cu, Co) have been reported to be active for this kind of reactions [49,87,88]. To achieve the cycloaddition under mild conditions, we developed several Ag-metalated POPs for this reaction. As illustrated in Fig. 4 , we integrated the fluorine-containing component and phenanthroline ligand into the backbone of POP via direct Sonogashira-Hagihara cross-coupling of tetrakis(4-ethynylphenyl)methane with perfluorinated aromatic bromides and bromo-substituted phenanthroline, and prepared fluorinated POP with phenanthroline sites (F-MOP) [89]. F-MOPs showed microporous structures with high BET surface area and much higher CO2 adsorption capacity (223 mg g−1) than nonfluorous MOPs (98 mg g−1). Due to the presence of phenanthroline sites, F-MOP could be metalated with Ag(I) to form Ag-metalated F-POP (denoted as F-MOP-Ag). The resultant F-MOP-Ag was capable of catalyzing the cyclization of propargylic alcohol with CO2 at room temperature, and showed much higher catalytic activity than that of MOP-Ag without F modification.Lately, we prepared rose bengal (RB)-functionalized POP (RB-POP) via direct Sonogashira-Hagihara cross-coupling of RB with 1,4-diethynylbenzene (Fig. 5 a) [90]. The resultant RB-POP had a high BET surface area of 562 m2 g−1, and could adsorb CO2 with capacity of 72 mg g−1 at 1 bar at 273 K. RB-POP could immobilize Ag nanoparticles with uniform distribution and size around 3.0 nm (Ag@RB-POPs). Importantly, Ag@RB-POPs exhibited extraordinary activity for the carboxylative cyclization of propargyl alcohols with CO2 under mild conditions, achieving a high TOF of 5000 h−1, with a wide substrate scope, high stability, and easy recyclability.Recently, He's group [87] developed a Ag(0) nanoparticles modified reduced graphene oxide (Ag-rGO) with massive O sites to serve as catalyst, which can not only catalyze the cyclization of propargylic alcohols with atmospheric CO2 with high efficiency, but also can catalyze synthesis of other value-added chemicals (β-oxopropylcarbamates and 2-oxazolidinones) from CO2 under ambient conditions.The reductive transformation of CO2 with H2 to chemicals is an important way for CO2 utilization, and can also provide green routes for chemical synthesis [91]. This kind of reactions generally require catalysts that can simultaneously activate CO2 and H2, and transition metal (e.g., Rh, Ru, Pd, Ir, Ni) decorated POPs catalysts can be designed to meet this requirement [92–95]. In our work, we integrated the CO2-philic group and ligands that can coordinate with metal species into the skeletons, and designed various POPs-based metal catalysts, which have realized reductive transformation of CO2 with H2 including CO2 hydrogenation to formic acid and N-formylation of amines with CO2/H2. Different from organic molecule ligands with unique structures, the polymers that can coordinate with metal species are only required to have coordination sites, which provide more opportunities to develop new catalysts. To get high-efficiency catalysts for reductive transformation of CO2 with H2, we integrated N-containing sites into the polymer skeletons and prepared various metalated POPs catalysts via post metallization strategy.Tröger's base (TB)-type ligand is one of the most versatile ligands applied in coordination chemistry. We introduced the TB unit into the skeletons of polymers via the reaction of a planar rigid building block (tris(4-aminophenyl)amine) with dimethoxymethane in trifluoroacetic acid at room temperature (Fig. 6 ), and TB-based POP was obtained [96]. The resultant TB-MOP showed microporous structure with a BET surface area up to 802 m2 g−1, which could adsorb CO2 with an uptake of 169 mg g−1 at 1 bar and 273 K. As expected, TB-MOP could coordinate with Ru(III) complex via the interaction between the TB ligand and the metal ions. The resultant material (TB-MOP-Ru) showed declined CO2 adsorption capacity (127 mg g−1 at 273 K) compared to TB-MOP, but it could efficiently adsorb H2 with a capacity of 9.5 mg g−1 at 77 K. As a result, serving as an efficient catalyst TB-MOP-Ru realized the CO2 hydrogenation to formic acid in Et3N with a TON of 2254 at 40 °C.Han and coworkers [46] reported a Ru-metalated N,P-co-functionalized POP for CO2 hydrogenation, which afforded an ultrahigh TON of 25,400. It was discovered that the electron-rich Ru3+ accelerated the H2 dissociation and N-functionalized architecture enhanced the CO2 adsorption. Yoon et al. [97] reported Ru modified dipyridyl functionalized POP for CO2 hydrogenation in a fixed bed reactor, which showed substantial catalytic performance with the highest productivity of 669.0 gform. gcat. −1 d−1. To avoid the purification process from formate to pure formic acid, they further developed a Ru metalated pyridine and imidazole co-functionalized POPs, affording a high TON of 2197 for the generation of methyl formate with methanol as solvent in the presence of Et3N [45].The imine-type ligands in the polymer skeleton have good ability to complex with metal species. We presented a novel approach to prepare imine-based POP (Imine-POP) via the reaction of aryl ammonium salt with aromatic aldehyde in water without any catalyst or template, which could metalated with Pd2+ by mixing with Pd(OAc)2 in CH2Cl2 and ethanol solution at room temperature (Fig. 7 a) [98]. It was indicated that Imine-POP possessed micro- and mesoporous structures (Fig. 7b), dominated with mesopores with BET surface area around 200 m2 g−1. Since the imine bond can chelate with metal species, the Pd nanoparticles with mean size of 2.84 nm were uniformly distributed onto the surface of Imine-POP (Fig. 7c). The resultant mesoporous Imine-POP@Pd was applied in catalyzing formylation of amines with CO2/H2 at 100 °C, which was tolerant to primary and secondary aliphatic amines, affording a series of formamides in moderate to excellent yields (48%–97%). Mechanism investigation indicated that the N-formylation of amines underwent the Imine-POP@Pd-catalyzed CO2 hydrogenation to HCOOH and the subsequent reaction of HCOOH with amine.In another work for formylation of amines with CO2/H2, we prepared pyridine-functionalized POPs (CarPy-CMP) via radical oxidative coupling polymerization of 2,6-di(9H-carbazol-9-yl)pyridine catalyzed by FeCl3 in chloroform, which was decorated with Ru nanoparticles to form a Ru catalyst (CarPy-CMP@Ru) [99]. The resultant CarPy-CMP exhibited micro- and mesoporous structures with BET surface area ∼1000 m2 g−1, and it showed excellent CO2 uptake capacity (up to 63 and 171 mg g−1 at 0.1 bar and 1 bar at 273 K). CarPy-CMP@Ru showed high catalytic activity for the reaction of secondary amines with CO2/H2, affording a series of formyamides in high yields (89%–93%), together with high stability and easy recyclability.The azo-type ligands within the polymer skeletons also show good coordinating ability for metal species. We presented a simple and efficient method for the synthesis of azo connected POPs through oxidative polymerization of aromatic multi-amines catalyzed by t BuOCl/NaI at room temperature (e.g., 25 °C, 1 h), and 4 samples (Azo-MOP-N, N = 1∼4) were prepared using the corresponding amine monomers (including tetrakis(4-aminophenyl)methane, A-1; 2,6,14-triaminotriptycene, A-2; 1,3,5-tris(4-aminophenyl)benzene, A-3; and tris(4-aminophenyl)amine, A-4) as illustrated in Fig. 8 [100]. The resultant Azo-MOPs displayed high thermal stability up to 400 °C, high BET surface areas up to 706 m2 g−1 and high adsorption capacity to CO2 of 134.8 mg g−1 (273 K, 1 bar). Furthermore, the Azo-MOPs could coordinate with Ru(III) complex to form Azo-MOP-N-Ru with homogeneous ruthenium distribution without detectable ruthenium clusters or nanoparticles, and the Ru content in each sample was almost identical around 4.70 wt%. All the Azo-MOP-N-Ru samples could adsorb CO2 and catalyze the methylation of amines with CO2 and phenylsilane with high efficiency under low pressure (0.5 MPa). Especially, Azo-MOP-3-Ru showed the best performance, together with broad scope of amines (including N-methylanilines with both electron-donating and electron-withdrawing groups, dialkylamines), excellent product yields (93%–99%), high stability and easy recyclability.Triphenylphosphine (PPh3) is an organic ligand widely applied in organic chemistry, and the PPh3-based POPs can combine the advantages of PPh3 and POP together to generate new functions. In our work, we integrated PPh3 unit and CO2-philic group (e.g., azo group) into the skeletons of polymers, and prepared poly(PPh3) connected with azo bonds (poly(PPh3)-azo) via oxidative polymerization of phosphine-containing aromatic amines, P(m-NH2Ph)3, in the presence of tBuOCl/NaI at 25 °C (Fig. 9 a) [101]. The resultant poly(PPh3)-azo showed a BET surface area and of 118 m2 g−1 dominated with mesopores (Fig. 9b). Treating poly(PPh3)-azo with AgBF4 in tetrahydrofuran under refluxing conditions resulted in the decoration of small amount (0.17 wt%) of Ag nanoparticles with size around 3.0 nm onto the surface of the polymer (Fig. 9c, poly(PPh3)-azo-Ag). Similarly, treating the polymer with RuCl3 in ethanol realized the complexation of Ru(III) with the ligand sites in the polymer as illustrated in Fig. 9d, and no Ru particles were observed in the resultant poly(PPh3)-azo-Ru. Moreover, the Ru content reached a higher value of 3.72 wt%. The resultant poly(PPh3)-azo-Ag could catalyze the carboxylative cyclization of propargylic alcohols with CO2 at room temperature, affording more than 400 times higher site-time-yield (STY) compared with the best heterogeneous catalytic system reported. Poly(PPh3)-azo-Ru exhibited extraordinary activity for the methylation of amines with CO2 under low pressure. The high performances of the catalysts were originated from the cooperative effects between the polymer and the metal species. Moreover, both poly(PPh3)-azo-Ag and poly(PPh3)-azo-Ru showed good stability and easy recyclability, thus demonstrating great potential for practical utilization in catalysis.Recently, Dai's group [38] prepared a phosphabenzene functionalized POPs (Phos-POPs) by substitution of pyrylium-based POPs (Py-POPs) with P(Me3Si)3. After partially fluorinated and subsequently metalated with ruthenium, the resultant Ru/F-Phos-POP-2 exhibited excellent catalytic performance for the N-formylation of amines at 100 °C, affording an ultrahigh TOF of 204 h−1.Because of the high cost of the POPs metalated with noble metals, developing efficient POPs modified with earth-rich metals is more interesting. We developed zinc-metalated and fluoro-functionalized porous N-heterocyclic carbene polymer (F–PNHC–Zn) via ionic polymerization and post metalation [102]. F–PNHC–Zn showed excellent catalytic performance for both formylation and methylation of various N-methylanilines with both electron-withdrawing or electron-donating groups. Remarkably, even under low CO2 pressure (0.05 MPa), F–PNHC–Zn can catalyze the reaction efficiently.Photocatalytic reduction of CO2 (especially with H2O) to value-added chemicals is a promising and ideal way for CO2 transformation, and has attracted tremendous attention, considering its double benefits for CO2 utilization and conversion of solar energy [103–107]. Generally speaking, the photoreduction of CO2 over photocatalysts undergoes three steps: (i) light absorption by the catalyst to generate photoexcited electron–hole pairs; (ii) charge carrier separation and transfer to active sites; (iii) surficial reduction of the absorbed CO2 to chemicals (Fig. 10 ). The photocatalyst is the key to realize the CO2 photoreduction, thus extensive efforts have been dedicated to developing efficient catalysts. To date, various photocatalysts have been reported for photoreduction of CO2, including inorganic semiconductors [22,108] (e.g., TiO2 [109,110], CdS [111,112], perovskite [113–115], g-C3N4 [116–119]), metal/organic hybrids [120] (e.g., heterojunctions [121–123], MOFs [124,125]), and conjugated POPs [126–128]. In particular, conjugated POPs showed promising applications in CO2 photoreduction because of their good CO2 adsorption ability, high surface areas, and tailorable functionalization [47,48].Up to now, great progress has been achieved in CO2 photoreduction over metalated POPs catalysts (e.g., Re [129], Zn [128], Co [130,131]) though they suffer from metal leakage, poor product selectivity, and high cost. Metal-free conjugated POPs have attracted attention due to their excellent CO2 adsorption capacity and tailorable structures [132–134].Integrating CO2-philic elements N, O, and P into the skeleton of polymer, we prepared a N,O,P-containing COP (NOP–COP) via condensation of hexachlorocyclo-triphosphazene with barbituric acid (BA) as illustrated in Fig. 11 [134]. This sample can effectively capture CO2 with a capacity up to 7.21 wt% under ambient conditions, and exhibits appropriate energy band structure (E g = 2.14 eV, CB = −0.81 eV). It was indicated that the incorporation of phosphorus in the skeleton of NOP–COP promoted the visible light absorption, improved the separation efficiency for photoinduced electron-hole pairs, increased the lifetime of photoexcited charge carriers and reinforced the redox ability, compared to those of N,O-containing COP (NO–COP) prepared via condensation of cyanuric chloride with BA. As a result, NOP–COP showed high activity for photoreduction of CO2 in the presence of sacrifice agent TEOA, achieving a selectivity of 90.2% towards CH4 as the sole carbonaceous product with a rate of 22.5 μmol gcat. −1 h−1 under visible light irradiation. This is the first example of a COP metal-free photocatalyst for CO2 reduction to CH4 under visible light irradiation. However, sacrifice agent TEOA was required.To achieve photocatalytic CO2 reduction with H2O as an proton donor, specific POPs catalysts are required, especially in solid-gas system [135]. We contributed to the synthesis of metal-free POP photocatalysts for photoreduction of CO2 with H2O. Eosin Y is a dye, which can absorb visible light. We introduced Eosin Y unit into the skeleton of polymer, and designed Eosin Y-functionalized COPs (PEosinY-N, N = 1∼3) via direct Sonogashira-Hagihara cross-coupling of Eosin Y with aromatic alkynes (Fig. 12 a, A-1, A-2 and A-3) [136]. The resultant materials possessed porous structures with BET surface areas up to 610 m2 g−1, and could adsorb and activate CO2 and H2O simultaneously, confirmed by in situ diffuse reflectance infrared Fourier transform spectroscopy analysis and DFT calculations. PEosinY-N could absorb visible light with suitable band structures as illustrated in Fig. 12c, which make them be capable of catalyzing the photoreduction of CO2 to CO and the H2O oxidation to O2. As expected, CO was obtained as the sole carbonaceous product under the visible-light-drawn catalysis over PEosinY-N, and PEosinY-1 showed the best performance, affording a CO production rate of 33 μmol g−1 h−1 and a selectivity of 92%. However, no O2 was detected in the gas products, while H2O2 was instead detected in the aqueous solution. We found that the generation of H2O2 resulted from the photoreduction of O2 originated from the H2O photooxidation. For this kind of POP photocatalysts, the Eosin Y units and conjugated structure of PEosinY-N were found to be responsible for the light absorption as well as efficient electron/hole separation, which cooperatively realized the photoreduction of CO2 with H2O, generating CO and H2O2.We synthesized a series of pyrene-based POPs using the nickel-catalyzed Yamamoto protocol as shown in Fig. 13 [137]. To capture CO2 from air and realize its photoreduction, we constructed an ionic liquid (IL)-assisted catalytic system using pyrene-based POPs as the photocatalysts. The task-specific IL ([P4444][p-2-O]) can chemically capture CO2 and absorb H2O via hydrogen bonding from air. In combination with the IL, the pyrene-based CP realized the capture of CO2 from air and further photoreduction to CO under visible light irradiation, affording a CO production rate up to 47.37 μmol g−1 h−1 with a high selectivity of 98.3%. It was demonstrated that the IL enhanced the CO2 photoreduction to CO and suppressed H2 evolution.The natural photosynthesis of CO2 with H2O produces biomass with O2 release and no H2 generation. However, in the artificial photocatalytic process of CO2 with H2O the generation of H2 is hardly avoided due to the presence of competing reaction of proton reduction. Therefore, the rational design of POPs based catalysts is important. In our recent work, we prepared a kind of amide-bridged conjugated POPs via self-condensation of amino nitriles (i.e., diaminomaleonitrile, DAMN; 2,3-diaminobut-2-ene-1,4-dinitrile, DAEN; 3,4-diaminobenzonitrile, 34AB) in combination with subsequent hydrolysis, as illustrated in Fig. 14 a [138]. These COPs are full of CO2-philic groups (CO, C–NH-), thus exhibiting high CO2 adsorption capacities (up to 32.7 mg g−1) in spite of their low BET surface areas. Meanwhile, they could absorb visible light efficiently, displaying suitable energy band structures for reducing CO2 and oxidizing H2O (Fig. 14d). Therefore, they realized the photoreduction of CO2 with H2O without any photosensitizer and sacrifice reagent under visible light irradiation. Interestingly, CO was obtained as the sole carbonaceous product and no H2 was generated. Among the resultant catalysts, Amide-DAMN with energy band structure (E g = 2.19 eV, CB = −0.75 eV, VB = 1.44 eV) displayed the highest activity, affording CO with a production rate of 20.6 μmol g−1 h−1, much better than the most reported metal-free catalysts. DFT calculations indicate that the adjacent redox sites of this kind of photocatalysts makes the CO2 photoreduction couple well with H2O photooxidation, inhibiting the generation of H2 (Fig. 14e).Besides amide-bridged conjugated POPs, other POPs with unique chemical structures also exhibited excellent selectivity towards target reduction product in CO2 photoreduction with H2O. For example, 2,5-diphenyl-1,3,4-oxadiazole derived CMPs (OXD-TPA) achieved CO2 photoreduction with H2O to access CO as the sole carbonaceous product in a selectivity nearly 100% under visible light irradiation [139]. In general, the photocatalysts with electron donor and acceptor (D-A) structures can enhance the light-excited charge carrier separation and transmission, thus resulting in better photocatalytic performance. The imine-linked COF with D-A structure (CT-COF) from 9-ethyl-9H-carbazole-2,7-dicarboxaldehyde and tris-(4-aminophenyl)triazine realized photoreduction of CO2 with H2O to CO in a selectivity of 100% with an evolution rate of 102.7 μmol g−1 h−1 under visible light irradiation [140].Compared to conjugated organic polymer semiconductors, inorganic semiconductors possess better photosensitivity, but suffer from high band gap energy and low CO2 adsorption capacity, thus lowering its performances for CO2 photoreduction [110]. The combination of POPs with inorganic semiconductors can overcome this shortcoming, showing enhanced activity [120]. For example, Tan's group [141] decorated porous hyper-crosslinked polymers onto the TiO2 particles (HCP–TiO2-FG), which significantly improved the CO2 adsorption capacity (12.87 wt%) and exhibited high activity for catalyzing photoreduction of CO2 with H2O, generating CH4 with a production rate of 27.62 μmol g−1 h−1. Constructing heterojunction between POPs and inorganic semiconductors is an efficient strategy to prepare efficient catalysts with high performance for CO2 photoreduction. For example, Dai and coworkers [142] developed a series of hybrid catalysts derived from conducting polymers (including polyaniline, polypyrrole, and polythiophene) and Bi2WO6 hierarchical hollow microspheres via in situ deposition oxidative polymerization. The resultant polythiophene/Bi2WO6 exhibited the best CO2 photoreduction activity, affording a methanol production rate of 14.1 μmol g−1 h−1 and an ethanol production rate of 5.1 μmol g−1 h−1. It was found that the improved photocatalytic activity resulted from decrease in the recombination of photogenerated electron–hole pairs caused by the forming of Z-type heterojunction.To construct a Z-scheme photocatalytic system composed of POP and SnS2 nanoparticles, we designed sulfur-bridged CTFs (S-CTFs) nanospheres via nucleophilic substitution coupling of cyanuric chloride and trithiocyanuric acid (Fig. 15 ) [143]. The resultant S-CTFs could adsorb CO2 with capacity of 2.6 wt% and showed energy gap of 2.73 eV with CB at −0.93 eV. Treating the S-CTFs sample in SnCl2 ethanol solution at 180 °C resulted in the formation of SnS2 nanoparticles on the surface of S-CTFs, forming SnS2/S-CTFs composites, which exhibited energy gap of 2.40 eV with CB at −0.27 eV. It was found that under light irradiation the transfer of the photo-induced electron-hole pairs in SnS2/S-CTFs follows Z-scheme mechanism, thus achieving effective separation of photo-generated carries. As a result, SnS2/S-CTFs displayed high efficiency for catalyzing CO2 photoreduction using TEOA as a sacrifice reagent under visible-light irradiation, yielding CO and CH4 with evolution rates of 123.6 μmol g−1 h−1 and 43.4 μmol g−1 h−1, respectively, much higher than those obtained over pristine S-CFTs and other reported photocatalysts. Notably, the SnS2/S-CTFs hybrid displayed excellent stability for photoreduction of CO2.Along with the substantial reduction in the cost of sustainable electricity, electrocatalytic CO2 reduction reaction (CO2RR) to value-added chemicals has attracted much attention in recent years [144,145], and the studies on POPs-based electrocatalysts (e.g., COFs [146,147] and CTFs [148]) are burgeoning. In 2015, Lin and coworkers [149] applied cobalt porphyrin-derived COFs as the electrocatalysts for CO2RR, affording a high FEco of 90% and ultrahigh turnover numbers of 290,000 with an overpotential of −0.55 V. After that, kinds of metal porphyrin and phthalocyanine derived COFs have been developed and applied as electrocatalysts for CO2RR [150–153]. For example, a series of 2D cobalt(II)-phthalocyanine based COFs with high conductivity were developed by Wang and coworkers which exhibited excellent CO faradaic efficiency (FEco) of 87%–97% at the potentials of −0.6∼–0.9 V (vs. RHE) [150]. Lan's group [151] developed metallo-porphyrin-tetrathiafulvalene-based COFs with electron-rich tetrathiafulvalene to serve as electrocatalyst, which showed an ultrahigh FEco nearly 100% at the potential −0.8 V (vs. RHE). Though great progress has been achieved, developing POP electrocatalysts with high catalytic performance and good long-term stability is still challenging. Just recently, we have developed perfluorinated CTFs (F-CTFs) by polymerization of tetrafluoroterephthalonitrile over Lewis superacids (e.g., Zn(NTf2)2), as shown in Fig. 16 [154]. Serving as an electrocatalyst in a three-electrode flow-cell system with gas diffusion electrode (GDE), the resultant F-CTF-1-275 showed excellent catalytic performance, affording a high FECO of 95.7% at −0.8 V (vs. RHE) with a high current density of 114 mA m−2. It was found that the high hydrophobic of F-CTF-1-275 caused by the high fluorine content (31 wt%) inhibited the HER competing reaction and improved the CO2 adsorption ability. The low-temperature ionothermal strategy great extend the applications of CTFs in the field of electrochemistry.In this review article, we have described our recent work on POPs catalysts for CO2 transformation, with focus on the design strategies for various functional POPs based catalysts as illustrated in Fig. 17 . For the synthesis of POPs-based catalysts, we introduced CO2-philic groups (such as azo, Tröger's base, fluorine, phenolic –OH) and ligands that can chelate with metal species into the skeletons of the polymers via various coupling reactions, with subsequent immoblization of metal active species onto the surfaces of POPs. A series of POPs-based catalysts for cycloaddition reactions of epoxides or propargylic alcohols with CO2, reductive conversion of CO2 with H2, and photocatalytic/electrocatalytic reduction of CO2 have been prepared, which have shown high efficiency for CO2 transformation. Our research work demonstrates that POPs-based catalysts have promising application potentials in CO2 capture and transformation. To achieve their commercial applications, the following issues are suggested to be considered and paid much attention in the future.Firstly, low-cost and environmental-friendly protocols should be developed. The most used approaches suffer from uses of expensive monomers, massive organic solvents, and noble metals catalysts, together with low product yields, which generally result in high cost and pollution. Biomass-derived feedstocks with multiple reactive sites are easily available and environmentally friendly, which are good candidates of the monomers of POPs. Therefore, biomass-derived POPs catalysts should be explored. Secondly, high-performance POPs catalysts that can achieve CO2 transformation under mild conditions should be explored. Due to its inherent nature, the transformation of CO2 under mild conditions is still challenging. The rational design on POPs that can capture and activate CO2 under mild conditions is highly required, which may achieve efficient conversion of CO2 under mild conditions. Especially, photocatalysis provides an ideal way to transform CO2 to chemicals, thus developing POPs-based photocatalysts with high efficiency is an interesting and important topic, which should be paid much attention. Electrocatalytic CO2 conversion has regard as the most promising approach to achieve CO2 transformation, however, the low conductivity of POPs-based catalsyts limits their applications as electrocatalysts and carbon black is still necessary in most cases. Therefore, developing POPs-based electrocatalysts with high conductivity are highly desirable. In addition, the catalytic stability is a key factor to determine the practical applications of the POPs catalysts. The POPs catalysts should be designed with high stability. Thirdly, the catalytic mechanism should be investigated. Comparing to homogeneous catalysts, the POPs-based catalsyts can combine multiple functions together and play multiple roles in chemical transformation of CO2. Understanding the catalytic mechanism is very important for developing efficient catalysts for CO2 transformation. In all, the study on POPs-based catalysts is still in its infancy, and the related research work is of significance for promoting the utilization of CO2.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors thank the National Natural Science Foundation of China (22121002, 21773266), Chinese Academy of Sciences (121111KYSB20200057) for the financial support.

The transformation of carbon dioxide (CO2) into fuels and chemicals is an interesting topic, which has been paid much attention in recent years. The materials with specific functionalities are highly required for CO2 capture and conversion, which have been widely investigated. As an emerging material platform, porous organic polymers (POPs) have attracted considerable scientific interest due to their distinctive properties such as tailorable functionalization, large surface areas, adjustable porosity, versatile polymerizations, good physicochemical and thermal stability. Our group focuses on designing and synthesizing POPs via introducing CO2-philic groups and organic ligands into the skeletons of the polymers and immobilizing metal active species onto their surface, and a series of POPs with functional groups, such as azo, Tröger's base, fluorine, phenolic –OH, have been prepared for CO2 transformation. In this review article, we mainly introduce our recent work on design of POPs-based catalysts for CO2 transformation, which include POPs-based catalysts for cycloaddition reactions of epoxides and propargylic alcohols with CO2, for reductive transformation of CO2 with H2, for photocatalytic/electrocatalytic reduction of CO2. In addition, the perspectives of the POP-based catalysts for CO2 transformation will be discussed as well.

Azo dyes are identified with one or more azo bond linkage (-N=N-) having the most prominent use of 70% among over 2,000 species of synthetic dyes [1–4,70]. Several studies have been carried out on the properties of these dyes which reveal their unique simplicity in synthesis, excellent fastness rating, high solubility, and uptake by the substrate. These properties give it a preferential choice of use among other dyes in textile industries (Oyetade et al. [173]). However, these dyes are commonly present in discharged textile wastewater which results in environmental pollution, limiting the quality of life ([9,100]). An estimate of 50,000 tonnes of textile wastewater is annually discharged with 10–30% concertation of unfixed dyes from dyeing, printing, pigmentation, and bleeding of the textile substrate [13], (Adetuyi et al. [175]; Oyetade et al. [174]). The toxic impacts of these dyes in wastewater are based on high resistance to microbial degradation, conventional treatments and chemical transformation of the dye molecules in the effluent to more toxic pollutants (benzidine) [13,27,56]. Although various conventional treatment techniques have been reported. However, limitations of toxic sludge formation, cost and low dye removal efficiency of azo dyes necessitated the use of the improved photocatalytic technique in nanotechnology (10−9) for dye remediation [13,100,142,174]. The process of dye degradation/decolorization via the use of photon-active nanomaterials as adsorbents is called photocatalysis [27,56,124]. Studies have shown the effectiveness of these techniques in azo dye degradation among other processes of advanced oxidation processes (AOPS) (Fig. 1 ) (Rauf and Ashraf [176]; Pandey et al. [156]). The whole process initiates a redox reaction which spontaneously generates radicals (e.g hydroxides, peroxides, and superoxides) while forming electron-hole pairs. The radicals combine with the organic pollutants (e.g. dyes in wastewater) and mineralize them into compounds such as C O 2 a n d H 2 O (Shindhal et al. [150] Pandey et al. [156]).Although there exists a wide spectrum of nano-photocatalyst materials such as bio-based, metal oxide-based, polymer-based, carbon-based and inorganic-based materials [35,101]. However, the challenges corresponding to their uses as photocatalyst includes photo-corrosion, frequent electron-hole recombination, large band gap, low photon detection and capturing propensity and agglomeration of powdered nano particles (NPs) [6,123,127,130]. These challenges gave rise to the investigation of polymer-based nano-composite from conducting polyaniline (PANI). Conducting polymers such as polyaniline (PANI), polythiophene, polypyrrole, etc., have been known for multiple varieties of applications in electronic devices, sensors, anticorrosive coatings, energy storage devices, catalysis, etc [101].Interestingly, out of these varieties of conducting polymers, PANI stands out with a unique applicable potential in photocatalysis [42]. The polymer commonly synthesized by oxidative polymerization exhibits incredible charge transport dynamics which supports its high photon capturing activity and lower band gap [19,72]. Furthermore, it exhibits other unique potentials such as high adsorptive capacity, ability to form polymeric support for nano powder materials with enhanced morphology, tuneability of bandgap on fabrication with other nano materials as composites and high solubility based on its amphiphilicity [59,112,141]. The multifunctional potentials of this polymer in composite fabrication necessitated the study of improved functional potentials and performance evaluation of the polymeric nano composite catalyst. This present paper evaluates the photodegradation performance of PANI nano composite applied to textile wastewater laden with azo dyes as compared to the conventional catalyst as reported in literature.Azo dyes are the largest class of synthetic dyes which are commonly synthesized by diazotization and coupling reactions via several chemical routes [16]. Although they can equally be synthesized by Gewald reaction (Fig. 2 ) however, the most adopted synthetic pathway for these industrial dyes is by diazotization and coupling reaction to give more brilliant shades, optimum yield, desired particle size and improved dispersibility [14,31,47,114].The process involves the diazotization of a primary aromatic amine, before coupling with electron-rich nucleophiles as described in Fig. 2 Azo dyes structurally consist of one or more azo (−N=N−) chromophoric group(s) and mostly water-soluble sulfonic ( S O 3 − ) (Fig. 3 a) group which justifies their affinity for water and excellent fastness rating (Sudha et al., [177,90]. These structural features also explain their increasing industrial demands and presence in most textile wastewater). Furthermore, auxochromes linking to phenyl or naphthyl rings such as amine, chloro, hydroxyl, methyl, nitro, sulfonate may be present in their structure which contributes to bathochromic shift thereby enhancing colour intensity (Sudha et al. [177]).Generally, dyes are classified into two broad categories concerning their application or chemical structure. Classification based on applications separates dyes into groups such as reactive, acid, basic, sulfur dye, mordant, direct, disperse, pigment, vat, and azo dyes (Popoola [90]). While structurally, they can be categorized as, indamine, diphenylmethane, xanthene sulfur, carotenoid, acridine, quinoline, anthraquinone, nitro, azo, indigoid, amino- and hydroxy ketone, phthalocyanine, inorganic pigment, etc. Although based on their charge they may be either anionic, non-ionic, or cationic dyes . However, based on their chemical reactivity they are classed as either acid, basic, reactive, direct or disperse azo dyes [128]. Furthermore, classification concerning azo constituents could be mono azo (one azo group), diazo (two azo groups), triazo (three azo), tretrakisazo (four azo) or more (poly azo) [13]. However, based on their colour index in Table 1 , they are classed monozo, diazo, triazo, polyazo and azoic dyes [102]. These varieties of classification, their aromatic constituent(s) and the auxochromic substituent(s) account for their vast application in the textile industries and determine the bonding arrangement with cellulosic, protein, regenerated, and synthetic polymers [90,102]. Popoola [90] and Oyetade et al. [174] added that these dyes have strong dye-fiber interaction (Fig. 3b), which accounts for their excellent fastness rating and resistance to bleeding, crocking and generally running off from the substrate applied.Statistically, they have 70% use among other industrial dyes with applications in paint, paper and most especially textile industries [102]. Despite their vast use, azo dyes remain an industrial dye of global threat. Among the assortment of azo dyes, the most widely used reactive azo dyes are associated with the toxic environmental impact on disposal, and difficulty in the treatment of their corresponding effluents (Jagadeesan et al., 2021). The toxicity of azo dyes stems from the hydrolysis reaction of loose dyes present in textile wastewater after textile substrate application. On discharge, the dye molecules become persistent in the environment and can bio-transform into aromatic amine products with acute mutagenic and carcinogenic effects on the organism (Bruna et al. [178]). Textile effluents laden with azo dyes, especially the commonly used Congo red, methylene blue and green, benzidine may predispose a man to various impaired health effects and hormonal dysfunction such as mutagenesis, quadriplegia, jaundice, cyanosis, tissue necrosis, vomiting chromosomal fractures, carcinogenesis, and respiratory toxicity (Vinothkannan et al. [179]). The toxicological impacts of these dyes are connected to the substituent position and the nature of the dyes (Bruna et al. [178]). Some of these dyes are less toxic but their presence in wastewater after dyeing, printing, or pigmentation processes may cause a reduction in the azo bond, imparting them with strong mutagenic action (Vinothkannan et al. [179]). For instance, direct black 38 (azodisalecylate) gives a breakdown product like benzidine (Fig. 4 ) and other derivates such as anilines nitro semis, and dimethyl amines which have carcinogenic inducing effects in humans and animals [100,111]. Furthermore, the presence of these dyes discharged into water bodies constitute a significant reduction of light penetration thereby producing different amine with higher genotoxic and mutagenic effect (Ventura et al. [180]; Bruna et al. [178]).Literature has reported and recommended the use of various conventional treatment processes for textile wastewater laden with azo dyes and dyeing auxiliaries. These processes are functionally described as physical, chemical, or biological treatment processes (Table 3) ([148] Ventura et al. [180]; Bruna et al. [178]; Siani, 2017) [111]. Table 2 describes these conventional treatment approaches and their corresponding merits and limitations. Among the treatment technologies, recent studies have emphasized the use of adsorption and biological treatment process which are classed as secondary treatments given to textile wastewater. However, the problem of disposal of sludge and the recovery of materials used for the treatment process are fundamental challenges of these technologies [85,111]. On the other hand, the use of advanced oxidation processes in recent times has appreciable advantages of faster reaction kinetics and the generation of little or no toxic sludge. However, the sophistication of some instruments for the treatment brought about the study of high dye remediating photocatalysts using which gives provision for the use of various nanomaterials in their pure form and as composites with the possibility of recovery [45,106]. These evolving limitations gave rise to the evolution of the photocatalytic treatment approach for textile wastewater using nano polymeric composites (Jangid et al. [181]).The uses of nano technological materials as adsorbents have grain prominence and applicability to environmental science. This thematic field studies the use of nano scale materials (10−9) characterized by novel, versatile and multiple functionalized properties for the treatment of toxic dye pollutants [17]. The transition in structural, functional and reactivity of nanomaterials from bulk scale to nano-size offers its desirable usage as a catalyst in photocatalysis [37], Mishra [75]. Although, these materials are generally classed as conductors, semiconductors and insulators. However, this classification is a function of the value of their respective bandgap, which is pivotal for the photocatalytic process [8,130]. Nano adsorbent materials are broadly referred to as either organic or inorganic (Fig. 4) with distinguishing features of photosensitivity, high surface area, high thermal chemical and mechanical stability, appreciable electrostatic features, compressibility, tunability of pore volume and bandgap, high magnetic and adsorptive capacity and enhanced solubility properties due to short intra-particle diffusion [36,124].These distinguishing features account for their application in the photocatalysis of recalcitrant dye molecules in industrial effluents [13]. The use of the photocatalytic technique in nanotechnology is described as a photon-induced molecular transformation that occurs at the surface of exciting photoactive nanomaterial adsorbing organic pollutants (e.g dye molecules) from the wastewater [86]. The mechanism of the degradation process starts with the capturing of photon energy from light by the photocatalyst leading to the excitation of the electrons from the valence band (VB) to the conduction band (CB), forming oxidizing and reducing sites (Fig. 6 ) (Antonio et al., 2019) [86]. For electronic excitation to occur, the energy of the photon captured by the material must be equal to or greater than the energy of its bandgap. This excitation leads to the generation of hydroxyl radicals (•OH), superoxide radical anions (•O2–), and hydroperoxyl radicals (•OOH), which are oxidizing species (Xing et al., 2018). Dyes adsorbed already by the nano adsorbents from the wastewater combine with the electrons in the conduction band resulting in the formation of dye radical anions and consequently degradation of the dye molecules (Hossain et al. [205]; Sioni et al., 2020). For instance, in the photodegradation process of azo dyes present in textile wastewater, the energy of photons captured via UV or sunlight generates electron-hole pairs (e− and h+) which migrate to the surface and site of the adsorbed azo dyes.This migration set-up a redox reaction (Fig. 6), which produces oxidizing radicals that attacks the adsorbed azo dye molecules and degrade them into non-toxic substances such as H 2 O a n d C O 2 (Comparelli et al. [182]; Zhu et al. [183] 2013; Soltani and Entezari [184]; Ullah et al. [185]). Photocatalyst nano-adsorbents used for this process can be bio-based, metal-organic frameworks (MOFs), carbon-based, polymeric-based or inorganic as described in Fig. 4 However, commonly used nano adsorbents in photocatalysis are the inorganic metal oxides (Table 3 ) [85]. This is due to the promising potentials such as photon detection and capturing, electronic structure, carrier transportation and band gap [30,77,110].The study by Dutta et al. (2021) reveals that metal-oxides-based adsorbents exist as magnetic or non-magnetic. An investigative study by Mohamed et al. [79] on one of the magnetic classes of the nano-adsorbents (Fe3O4-Nps) has an adsorption capacity of 150–600 mg g−1 for rhodamine dye within 30 min, while the fabricated composites such as Fe3O4/CeO2, Fe2O3–Al2O3 have a notable adsorption capacity of six-times higher than the pure metal oxides nanomaterial in dye remediation [73]. This action is due to their surface-to-volume ratio and pore size, which is consequent to bandgap tuneability for improved photocatalytic effect [33,36,112,132]. On the other hand, the non-magnetic metal-based oxides ZnO, TiO2, MgO are often fabricated as a hybridized composite with notable adsorption efficiency [36,85].Examples of these composites with their corresponding adsorption capacity include Co/Cr-co doped ZnO with 1057.9 mg g−1 for methyl orange, ZnO–Al2O3 nano adsorbents for Congo red and Ni-MgO having maximum adsorption of 397 mg g−1 [62,64,79]. Dutta et al. (2021) revealed that apart from their appreciable specific surface area, the charge on the adsorbent surface gives room for electrostatic dye-adsorbent interaction. Also, the polymorphic nature of the composite nano adsorbent creates a more active site for the binding of dye molecules. Although the challenges of toxicity and frequent recombination notably exist for the metal oxides photocatalyst in Table 4 ,which greatly limits their performance in dye degradation [8,42]. Additionally, ceramic nanoparticles are another class of adsorbent chemically existing as oxide, phosphate and carbonates (silica, alumina, titania, zirconia), with high chemical inertness and heat-resistance [36,58]. However, the limitation of the large bandgap of these materials incited the study of carbon-based nanomaterials with unique structural features fit for adsorption and composite fabrication. Carbon-based nanomaterials generally exist as either carbon-nanotubes (single-walled or double-walled) or carbon-fullerenes [103]. One of the unique examples of carbon-nanotubes (CNTs) is graphene sheet which can be rolled in the form of tubes, having characteristic features of thermally-conductive, less toxic photoactive, bandgap tunability and synergistic composite forming potential [38,42,131].Bezerra de Araujo et al. [18], Oni and Sanni [88] and Sivakumar et al. [121]discussed the high adsorptive performance of GO for toxic dye pollutants such as Direct Red 81 and Indosol SFGL direct blue, crystal violet and methyl orange, and methylene blue respectively at pH 6-7. Sivakumar et al. [121] added that the efficiency of GO is due to the interaction of the hydroxyl and carboxylic groups with the auxochormic substituents (functional groups) on the dye molecules. Furthermore, Zheng et al. [140] related that the hierarchically unique orientation of GO–NiFe-LDH nano composites enhance its adsorption efficiency for Congo red and methyl orange at 489 and 438 mg/g respectively. Similarly, the composite of GO with metal oxides (GO/MgO) studied by Heidarizad and Şengör [49]notable adsorption of 833 mg/l within a contact time of 60 based on the multiple functional sites available for binding. Although GO is thermally, mechanically and chemically stable, however, the need for the use of photosensitizers in place of toxic metal and metal oxide NPs justifies the investigation of p-type conductive polymers especially the novel polyaniline and its composite forming mechanism with other materials with appreciable advantageThe beneficial attributes of high effective surface area, high selectivity and absorptivity, appreciable doping/de-doping technique, effective electrical transport characteristics, well-established binding affinities, and unique textural properties offer polyaniline a leading advantage [71,118]. Mu et al. [84] reported a high adsorption capacity of 248.76 mg g−1 for Congo red dye graphene/polyaniline and PANI/Fe3O4 respectively. Although in its purest form Smita et al. [122] reveal that PANI exhibit an adsorption efficiency of 92% for methyl orange due to the structural versatility and the presence of amine and imine active group. However, higher adsorption and reduced recombination enhance photodegradation when hybridized composites of PANI are formed with other nanomaterials [103]. This also improves the surface area and enhances photocatalytic reaction sites thereby inducing electron-hole separation [92] In photocatalysis, vital parameters such as pH, initial dye concentration, temperature, nature and dosage of nano adsorbent, contact time, irradiation time, and irradiation intensity play a significant role in the adsorption-degradation process as described in Table 3 [15,93,96]. As the initial dye concentration increases the adsorption and degradation increase up to the point where the binding sites on the adsorbent are saturated, beyond this point desorption occurs [35,93,99]. The pH on the other hand plays a very crucial role in the determination of adsorption and consequent degradation of dye molecules adsorbed onto the surface of a nano photocatalyst [95]. Research by Salleh et al. [104] reveals that low and high pH enhances the adsorption of anionic and cationic dyes, respectively. This claim was affirmed by Daneshvar et al. [29] and Phoemphoonthanyakit et al. [89] in their research where higher adsorption of 1093 mg g−1 at pH 2 for Acid Blue 25 and 600 mg g−1 at pH 7 for Rhodamine 6G at pH of 2 and 7 respectively.Furthermore, photocatalytic decomposition of dye molecules in wastewater has been studied at pH values ranging from 3 (acidic) to 13 (alkaline) for anionic, cationic and neutral dyes in wastewater [69]. The pH value required for the reaction is suggestive of the kind of charge on the surface of the nano adsorbent. This is because at pH <pHzpc, pH>pHzpcor pH=pHzpc it implies that the surface charge is positive, negative and neutral respectively [15]. Hence, cationic dyes are adsorbed more in an alkaline medium to establish electrostatic interaction resulting in increased degradation efficiency [15,53]. Alakhras et al., [5] added that at low pH there is a notable reduction in the production of hydroxyl radicals by the positively charged surface which is needful for hydroxyl radical formation. Also, temperature requirement quantitatively determines whether the process is endothermic or exothermic which influences the adsorbent-adsorbate interaction prior to photocatalysis [35].Generally, physical properties such as high selectivity, high absorption capacity, durability, reusability, cost-effectiveness, surface area, optoelectrical properties, crystallite size and distribution, dispersibility, and mechanical and thermal stability are vital properties that determine the choice of nano adsorbents materials in photocatalysis [41,42]. Table 4 describes the physical properties of the commonly used conventional catalyst with metal oxides taking the highest frequency of use [37]. Although the frequent use of these is based on their polymorphic nature. However, electron (e−)- hole (h+) recombination, agglomeration and photo-corrosion of some of the metal oxide nanoparticles limits their performance, recovery and reuse (Meng et al. [186]). Another limiting challenge of this conventional catalyst is that their photo capturing propensity is only within the ultraviolent region Beyond this region to the visible, the nanomaterials exhibit low photon detection and capturing potential which lowers the generation of radical species needful for dye degradation [25]. Furthermore, from Table 4, semiconductors such as TiO2,h-2DBN, Nb2O5, ZrO2, and ZnO2 have appreciably high band-gap which limits their photon capturing potentials, especially when irradiated within the visible region [112,130]. Additionally, the use of these semiconductors in their pure form as nano-adsorbent has toxicological impacts and reduced photocatalytic performance [13,136]. Xu et al. [187] suggest that the reduction in adsorption capacity during adsorbent-adsorbate contact is due to the saturation of the adsorbent surface and charges present on synthetic dye molecules, especially azo dyes. For instance, cationic azo dyes easily undergo adsorption and degradation when compared to the anionic azo dyes such as Eriochrome Black T due to the lack of electrostatic interaction [74] (Xu et al. [187]). This is due to the charge present on the nano-adsorbent, the difference in sorption mechanisms and the value of the pH which is greater than the pHpzc, favouring the preferential adsorption of cationic dyes compared to its anionic counterpart [15,116]. This action justifies the report on the faster adsorption-degradation kinetics of cationic azo dye using T i O 2 when compared to the anionic quinizarin, having a low adsorption efficiency of 21.8% with the same catalyst (Abuabara et al. [188]; Pereira et al. [189]).Also, the challenges of recovery and reuse of conventional catalyst after the process of photocatalysis has limited their commercial acceptance in the treatment of wastewater laden with recalcitrant dyes from textile industries (Muhd et al. [190]). Hence, tackling the attendant limitations of these nano-adsorbent requires the fabrication of nanocomposite by combining these semiconductors with a photon sensitizing conducting polymer (PANI) characterized by high resistance to corrosion, high adsorptive capacity, chemical and thermal stability and versatile surface area [54,112]. The fabrication of composite mix form using these materials enhances surface modification and bandgap tunability necessary for improved performance (lowering the bandgap- Fig. 7 ) [30,112].There are various types of π-conjugated conducting polymers with appreciable conductivity and novelty. These emerging conducting polymers include polyaniline (PANI), polythiophene (PTH), polypyrrole (PPY) poly(para-phenylene) (PPP), poly(phenylenevinylene) (PPV) and polyfuran (PF) [17,22,44]. However, the most prevalent among these varieties is polyaniline (PANI). This is based on unique properties such as exceptional electronic and optoelectronic features, cost-effectiveness, ease of synthesis, environmental stability, high electrical conductivity, high thermal power and unique structural ordering [19,65,87,110]. Various approaches such as interfacial polymerization, seeding polymerization, vapor phase self-assembling polymerization, photo-induced polymerization, plasma polymerization, sonochemical synthesis, and electrochemical synthesis can be used in the synthesis of polyaniline [19,109]. However, the solution technique via oxidative polymerization of aniline with ammonium persulphate is commonly used due to its faster polymerization rate and excellent yield (Fig. 7). The resultant products of this polymerization process can be either leucoemeraldine (fully reduced state), emeraldine salt (half oxidized state) or pernigraniline (fully oxidized state) which are described in Fig. 7 [17,19]. Out of these products, emeraldine salt stands out with unique structural characteristics of two benzoid units and one alternating quinoid unit which results in high thermal stability and nanocomposite forming potential with inorganic nanoparticles such as CeO2, TiO2, ZrO2, Fe2O3, Fe3O4, ZnO and TiO2 thereby generating nanostructures such as nanofibers, nano tubes, nano sphere and nano flowers [17,19]These unique structural features and versatility account for its vast application in corrosion protective coatings, energy devices, sensors, water purification and as photocatalyst [100,111]. Current research reveals that PANI exhibits incredible photoelectronic properties and the composite resulting from its incorporation with other materials has appreciable synergic properties higher than the use of polymers [87,109,137]. Furthermore, PANI is amphiphilic based on its conjugated organic part (Quiniond) and–NH•+ formed during protonation, the excellent dispersion and ability to bind with organic and water-soluble dyes in effluent account for its choice among other polymeric adsorbents [19,20,66,141]. Also, the structural ordering of the polymer chain and its high conjugation give it dynamic electrical properties associated with high carrier transport mechanism which is vital for the photocatalytic process [19,109]. The dynamics of electrical transport in polyaniline are described by its intra-chain or inter-chain transport processes [109]. The inter-chain transport process depends on the carrier delocalization of the polymer chain with appreciable conjugation length [63]. On the other hand, the inter-chain charge transport process is dependent on the hopping mechanism, based on the molecular crystalline packing of the polymer matrix [30,109]. The unique molecular orientation of PANI accounts for its improved charge mobility properties which enhances its photosensitivity when existing in pure form or as composites with other semiconductor materials [83]. Apart from the hopping of charge carriers along and between polymer chains, other incredible conductive mechanisms of polyaniline can be tunnelling between high-conductive crystallites embedded into the amorphous matrix or electron-phonon interaction [30].Composite fabricated from PANI is aimed at tackling the inefficiency of conventional catalyst and treatment process for recalcitrant dyes. Various cost-effective and environmentally friendly approaches have been investigated by literature on the fabrication of polymeric nanocomposite catalysts with polyaniline (Liu et al. [204]). Studies show that the fabrication of these novel nanocomposites is mostly by ex situ or in situ polymerization (Vargas et al. [191]). The ex-situ polymerization involves the mixing under sonication of the semiconductor's particles with already synthesized polyaniline (Cruz et al., 2017). However, the commonly used composite fabrication technique (in situ polymerization) involves homogenous dispersion of the nano-grade semiconductors or other nano-size materials of choice during the polymerization process of polyaniline (Fig. 8 ). This method of fabricating nanocomposite is more appreciable than the former because the former exhibits the formation of low‐density and non‐uniform coverage of nanostructures of polyaniline by the material semi-conductor (Vargas et al. [191]). The in-situ polymerization of polyaniline with semiconducting material such as T i O 2 , Z n O 2 , forms coupled nano polymeric composites (Figs. 8 and 9 ) (Tai et al. [192]; Liu et al. [204])During composite fabrication, PANI acts as a photosensitizer while interacting with the bandgap of the semi-conductors in Fig. 9. This consequently lowers the bandgap of the The presence of the semiconductor provides the composite with appreciable morphological structure for dye adsorption [101]; Jangid [181]). Consequently, composite mix improved the subsequent performance of the photocatalyst by preventing frequent recombination of electron-hole pairs during photocatalysis (Jangid [181]). Furthermore, during photocatalysis, the advantages of high mobility charge carriers, stability, and the strong coupling effect existing between the polymer and the semiconductors, make the nanocomposites function effectively in the degradation of dye molecules within the visible region. (Bingham and Daoud [193]; Riaz et al. [99]; Jadoun et al. [194]). Ansari et al. [195] and Hao et al. [196] added that the tuned morphological property of the composite proffer higher performance in photocatalysis compared to the conventional catalyst. Morphologically, the presence of PANI enhances the high hole transporting ability and stability of the material during the remediation process (Vikas et al. [197]). Jangid [181] and Shahabuddin et al. [112] revealed that the presence of the conducting polymer in the composite mix accounts for the photocatalytic stability and activeness of the material under visible light after five consecutive runs. Studies by Saha et al. [101], Shahabuddin et al. [112] and Ameen et al. [10] reveal that PANI exhibited well-defined tubular morphology, however, incorporation of h-BN and graphene nanosheet correspond to the formation of the granular polymeric network. Consequently, the surface modification improved the adsorption capacity by enhancing the availability of the binding sites of the composite for adsorption [39,101]. Furthermore, in Fig. 9, the adsorption of dye molecules unto the surface of the nano composite of polyaniline is higher as a result of electrostatic interactions and π–π* conjugation among the aromatic rings of dye molecules [109,110]. In the fabricated composite, PANI act as a photosensitizer via photon capturing and excitation of valence electrons in HOMO, jumping to LUMO through π–π* transitions [19,76,112]. Also, the incorporation of semiconductor material into the PANI polymeric chains by in situ polymerization prevents recombination such that, as the positively charged holes (h+) are returning to HOMO to recombine, the empty conduction band of the semiconductor intercepts the recombination thereby improving it photocatalytic efficiency [8,42,112].The current research scope focuses on the vital need to improve composite fabricated from polyaniline to enhance effective industrial performance, recovery and reuse [85,120,135]. Recent studies discussed the use of immobilization techniques to enhance the recovery and improve the performance of these catalysts for several runs [97,120]. The study suggested the fabrication of nano composite photocatalyst via immobilization of the semiconductors either on the surface or in the PANI polymer matrix [117]. The technology involving the immobilization of the photocatalyst nanoparticle on the surface of the polymer matrix could be via dip-coating, chemical vapour deposition, grafting or plasma treatment followed by UV irradiation (Fig. 10 ) [7,21,66,135].Alhaji et al. [7] and Lin et al. [66] added that this method enhances the reduction of agglomeration, creation of more active sites, surface modification and its cost-effectiveness. Although the method is disadvantaged by wide particle size distribution. On the other hand, the technology involving the immobilization in the support matrix enhances the reduction in leaching at lower energy and higher catalyst recovery propensity, although may be limited by agglomeration inside the matrix [117,135]. One of the most effective processes in immobilization in the polymer matrix is the in-situ process. This process is characterized by considerable advantages of lower bandgap and higher dye-degrading efficiency as highlighted in Table 5 . The in-situ process can either be by in situ polymerization (described in Fig. 8: incorporation of TiO2 on PANI nano rods), or the sol-gel method [54].The in-situ polymerization involves a controlled blending of selected semiconductor nano material with the neat monomer before subsequent polymerization [117]. However, the sol-gel approach is a two-stage technique that involves hydrolysis where bond cleavage occurs between the organic matrix and the semiconductor nano powder before condensation which involves bond formation alongside small leaving groups [3,34]. The use of the in-situ process enhances the homogenous dispersion of the semiconductors on the PANI matrix, and reduces possible agglomeration [3,34]. Examples of other processes are mixing, hydrothermal method catalyst deposition, galvanostatic method and plasma-irradiation treatment which have the challenges of nano powder agglomeration and its energy-intensive [46]. Studies carried out by Jangid et al. [54] and Shahabuddin et al. [112] on the fabrication of catalyst nano composite via in situ polymerization to generate hybridized h-BN/PANI and TiO2/PANI nano rod exhibited excellent photodegradation efficiency of more than 90% and with over four runs due to the immobilization of the nano powder in the polymer matrix which enhances the recovery and reuse. Similarly, Mondal & Sharma [81] and Singh et al. [119] also suggested the use of the immobilization method in the polymer matrix to improve the adsorption capacity enhance their photocatalytic water splitting potential and boost the possibility of recovery of titania.Also, Lin et al. [66] discussed three possible approaches to improve the functional features of the polyaniline-based nanocomposite. Firstly, the solute-solvent interaction of PANI can be enhanced via the use of Graphene oxides (GO), and CNTs as additives which limit fouling, enhance the rate of flux recovery and improve the hydrophilicity of the conducting polymer. Secondly, the use of secondary amine with molecular width < 4.53 Å and a pKa>7 to facilitate gelation inhibition. This also provides needful solvent-polyaniline interaction for the enhanced adsorptive potential of the nanocomposite. Thirdly, the use of zwitterions to form zwitterionic polyaniline. The zwitterions are characterized by charge functional groups of both positive and negative. This enhances the Pickering emulsions at the interface between organic solvents and aqueous solution leading to improved solubility of the polymer matrix [30]. Other techniques for the production of high profile multiple functionalized PANI based composites are photo-induced polymerization, interfacial polymerization, electrochemical polymerization, solution polymerization, seeding polymerization, emulsion polymerization, vapor phase self-assembling polymerization, plasma polymerization, and sonochemical synthesis process techniques [109] Table 6 shows the polyaniline nanocomposites with their respective degradation time and percentage, while Table 7 reveals the degradation time and percentage of conventional catalysts used in photocatalytic remediation of industrial azo dyes in wastewater. From Table 6, degradation percentage of 97% and 94.35% degradation of methylene blue at 150 min and 30 min respectively was recorded when the composite fabricated with PANI was used in the presence of UV radiation. Also, for methyl orange 94%,81.3%, and 95% degradation were obtained within 30, 125, and 90 min of photocatalysis. The observed time variation for the reaction is a function of the nature and composition of the composite and dyes, pH, dosage of the catalyst, contact time, and temperature (Reze et al., 2017). Furthermore, the composites of PANI used under influence of sunlight energy show a high percentage performance of 92%, 95% and 97% for the degradation of azo in Table 6 at the rate of 45,45 and 180 min respectively. However, in Table 7 when sunlight irradiates the photocatalytic system using conventional catalyst low degradation percentage of 25%, 21.8%, and 56% for the industrial azo dyes at the longer duration of 120, 180 and 180 min was recorded. Ameen et al. [213] and Vargas et al. [191]) added that the faster photocatalytic reaction of PANI composite to sunlight energy was because of the conduction band of the semi-conductors used and the LUMO level of the PANI are well matched for the charge transfer. This consequently, promotes the electrons p–p* absorption band of the outside PANI film upon irradiation with sunlight light leading to faster reaction kinetics (Sadia et al., 2012; Rita et al., 2017). Also, when the same dyestuff present in the wastewater was comparatively assessed, at 90 min in Table 6, the degradation efficiency of methylene blue azo dye was 93% and 99.6% while in Table 7, 97%, 92%, and 90% were recorded at longer duration time of 120, 350, 180 min respectively for the same dye. Others include acid blue having percentage degradation of 95% and 73% at 45 min in Tables 6 and 7, respectively and Rose Bengal having 97% in 150 min in Table 2, while it has 81% in 180 min in Table 7. However, the low percentage of degradation of 56% from Table 2 of the same dye may be due to the adopted method of composite fabrication (Vargas et al. [191]).Furthermore, the photosensitizing action of PANI when coupled with other semiconductors enhances the photocatalytic performance of the nanocomposites within a shorter duration as compared to its conventional counterpart (Muhd et al. [190]; Yang et al. [198]). This action is based on the narrow bandgap and enhanced charge mobility of PANI resulting in photon response as the energy level of PANI is incorporated into the semi-conductors [61,133]. Yang et al. [198] added that the synergistic effect between PANI and semiconductors such as TiO2, makes it act as sensitizer for its large bandgap (3.2 eV), resulting in the reduction of the bandgap during photon excitation (Yang et al. [198]). The surface of the negatively charged TiO2(rod) undergoes electrostatic interaction with positively charged anilium ion from the PANI forming a composite with improved the photocatalytic degradation of phenol in the azo at strong adsorption of light in the visible region (Yaseen and Scholz [214]). Furthermore, the use of PANI as a coating in in-situ polymerization of nanomaterial forestalls the possibility of recombination rate of electron-hole (e– – h+) by contributing to the bathochromic shift of the absorption band (Wang [207]; Yang et al. [198]; Jiang et al. [206]). Although, the use of metals as doping as a nanomaterial for the degradation of dyes comes with high dye degradation efficiency in Table 7 (92.6% and 92%). However, this technique is not cost-effective, not reproducible, and has a problem with catalyst recovery, hence it becomes imperative to use the alternative of nanocomposite developed from polyaniline (Yang et al. [199]; Abd El-Rady et al. [200]; Sood et al. [201]). Table 8 shows various microorganism which serves as bio-catalysts used in the degradation of industrial azo dyes, their degradation time, and percentage. From the report in the Table, the microbial degradation of azo dye molecules from textile wastewater is characterized by a longer degradation time when compared to the photocatalyst used in Tables 6 and 7. The extended time makes the technique inappreciable for industrial commercialization owing to the slow dye decolorization rate and consequent generation of more toxic substances during continuous industrial dyeing and pigmentation processes. (Bruna et al. [178]) [55]. Also, from Table 8, only fungal laccase bio-catalyst has the lowest degradation time of 12 h to degrade 99% of methyl orange. This efficiency is based on its high redox potential (400 to 800 mV) resulting in the generation of intermediates via two distinct pathways [134]. However, one major setback of this bio-catalyst is that it is unstable at elevated temperatures and alkaline conditions, with which most effluent emerges after industrial usage (Shahzad and Burhan, [202]). This challenge, however, is an advantage to polymeric composites of polyaniline (Shahzad and Burhan, [202]). Other organisms from the Table exhibiting degradation at a duration lesser than 2 days are fungal Aspergillus niger having 98%, 94% and 99% for Acid red 151, Orange II, and Congo red respectively, and Pseudomonas sp having 83% dye degradation at 24-33h for reactive black 5. It is also worthy of note to add that some of these microorganisms from the table have low degradation efficiency over an extended time. Examples of these are Mutant Bacillus sp. ACT2 with a degradation efficiency of 12–30% for Congo red at 37–48 h, Bacillus cereus having a degradation efficiency of 67% for Cibacron black PSG and Cibacron Red P4B during 5days. Although these bacteria cleave –N=N– bonds reductively and utilize amines as the source of carbon and energy for their growth. However, their stability to high pH, temperature, and the recalcitrant nature of some azo dyes remains a major challenge (Saratale et al. [203]; Mohammed and Burhan, 2014). Furthermore, the report by Buitrón et al [208] reveals that aerobic microbes cannot reduce azo linkages, their ability to destroy dye chromogens is lower when compared to the anaerobic bacterium. This justifies the low degradation of some bacterial microorganisms in Table 8. Other limiting tendencies of this bio-catalyst are problems of early saturation, making the nano-polymeric photocatalyst of greater industrial acceptance than the microorganism (Vikas et al., 2013; Shindhal et al. [150]).The use of nano photoactive polymeric composites in photocatalysis incredibly proves to be most effective in performance during adsorption and degradation of toxic azo dyes appreciably present in the effluent. The study was able to establish the stability, flexibility, versatility cost-effectiveness, high performance and the possibility of recovery and reuse of the catalyst composite for treatment of pilot and field-scale wastewater. Additionally, the fabrication of this composite form polyaniline lowers the band gap and creates photon capturing possibility within the visible region, prevents agglomeration of Nps, lowers recombination of electron-hole pair and creates a more active site for binding of the dye molecules via tunability of the morphological properties of the NPs semiconductors used. Although the performance and the integrity of these nanomaterials are a function of the pathways of composite fabrication which is most efficient by the in-situ process. This process involves the incorporation of powered catalyst NPs into an immobilized conducting polymer matrix as stated by this review. However, factors such as irradiation intensity and source, contact time for adsorption-desorption equilibrium before photons are incident on the system, pH, temperature and initial dye concentration also play a vital role in the effective performance of the process. The review also establishes the fact that among the varieties of semi-conductors used carbon-based nanomaterials stands due to their low toxicity, higher adsorption capacity and the excellent synergic effect when coupled with polyaniline to form composites. Hence, continued research should focus on the fabrication of nanocomposite using PANI coupled with carbon-based-nano materials, especially the multi-walled carbon nanotube (MWCNT), having unique morphological features fit for the adsorption and photodegradation process. Additionally, it is imperative to investigate more the possibility of recycling and recuse of the spent catalyst and to quantitatively determine the point of saturation of the fabricated composites during their reuses. It is worthy of note to add that the efficient primary treatment process must be given to textile effluent laden with dyes to remove the extraneous materials to limit the challenges of the photocatalytic process. Furthermore, the challenges of quantitative determination of initial dye concentration before photocatalysis limits the use of this treatment process due to oversaturation of the nano-catalyst at extremely high concentration dye molecules. Hence there is a need to create a robust industrial system to monitor the dye concertation fed into the photocatalytic reactor for effective use. Also, a future outlook on a hyphenated system of biological-photocatalytic techniques should be developed where biological microorganism such as laccase microbial enzymes associated with faster dye decolorizing speed is primarily used for the treatment of effluent before photocatalysis.The authors declare an absence of competing financial interests in personal relationships that could influence the work reported in this paperThis work was supported and funded by Regional Scholarship for Innovation Fund (RSIF) a flagship program of the Partnership for Skills in Applied Sciences, Engineering and Technology (PASET).

Azo dyes in industrial textile and dye effluent (5–30%) have become irresistibly recalcitrant and toxic to both treatments and the environment respectively. Global concerns about the persistent nature of these dyes and the limitation of the conventional treatment currently in place have led to this critical analysis and evaluation of the photocatalytic approach using nano-technology. The review of literature has indicated that although this approach is effective, however, the limitation of frequent electron-hole recombination during the process coupled with challenges of agglomeration of nano particle powder, photo-corrosion and photosensitivity of the various nano-materials are still challenges associated with the development of polymeric based nano composite catalyst of polyaniline (PANI). The unique features of incredible charge transport properties, surface morphology and enhanced functional properties gave PANI the choice of use among other conductive polymers for composite fabrication with materials such T i O 2 , a n d Z n O 2 , G r a p h e n e o x i d e s , C N T s . Photoactive properties, conductivity mechanical, thermal and chemical stability equally offers the polymer the propensity of bandgap tunability when in composites with other materials. Consequently, effective recovery and reuse of the composite catalyst for more than four runs with efficiency > 90% becomes obtainable. These appreciable advantages offer fabricated nano composite polymeric-based catalysts an effective outlook of use in the remediation of toxic azo dyes industrially as compared to the bio-catalyst and pure nano adsorbent materials. Therefore, the review discusses the treatment process for azo dyes, fabrication and performance evaluation of improved composite catalyst of PANI as an alternative to the conventional catalyst in wastewater and recommends for further investigation in PANI to enhance treatability of azo dyes.

The oxidation of carbon monoxide (CO) is a key reaction in automotive and industrial pollution control (Wu et al., 2001). Due to its relative simplicity, it has been the subject of numerous fundamental studies into heterogeneous catalytic processes (Szanyi et al., 1994). The reaction is traditionally carried out over noble metals such as platinum, palladium and rhodium. These materials exhibit high activity and selectivity toward total oxidation of CO as well as other volatile organic compounds (VOCs). However, high cost and limited availability of these materials have discouraged their extensive application in larger scale processes (Agula et al., 2011). Transition metal oxides function as suitable alternatives to the noble metals for oxidation of CO and are particularly attractive due to their relatively lower cost and good stability under cycled conditions (Yu-Yao and Kummer, 1977). Supported oxides of cobalt, iron, copper, manganese and nickel have all shown good performance in CO oxidation (Gravelle and Teichner, 1969). Nickel-based catalysts have been employed in numerous experimental studies of supported and single crystal catalytic processes, including mixed oxide catalytic systems (Parravano, 1953; Choi and Kim, 1974; Royer and Duprez, 2011; Agula et al., 2011). Having excellent activity also for reforming and catalytic partial oxidation of methane, nickel-based catalysts are considered promising candidates for processes involving a combination of these basic C1 transformations (Raju et al., 2009).The aim of the present work was to investigate the kinetics of heterogeneous CO oxidation over a commercial alumina supported nickel oxide (NiO/Al2O3) catalyst (Süd-Chemie) and over a temperature range between 180 and 210 °C. The influences of the concentrations of CO and oxygen on the reaction rate were studied to derive a rate expression that can be used over the specific range of operating conditions for large scale reactor design.Consensus has not yet been reached in literature with respect to the mechanism by which the catalytic oxidation of CO takes place. Therefore, prior to kinetic model development, it was deemed necessary to identify potential reactions mechanisms describing the kinetics over the specified operating regime. This section outlines the mechanisms proposed.Research conducted by Yu-Yao and Kummer (1977) proposed that CO and oxygen are initially chemisorbed on catalytic centres before the surface reaction, after which carbon dioxide is released in two successive steps. These researchers proposed that catalytic oxidation of CO over nickel oxide could be represented by a Langmuir-Hinshelwood (L-H) type reaction mechanism with competitive adsorption of oxygen and CO onto active catalyst surface sites (see Fig. 1 below).However, earlier work undertaken by Dell and Stone (1954) to investigate the adsorption of gasses such as CO on thin nickel oxide films concluded that at low temperatures ( 20 ∘ C ) , the amount of CO which could be adsorbed was very much less than the amount of oxygen which could be adsorbed at corresponding conditions. Similarly, Gravelle and Teichner (1969) noted that the adsorption of CO decreases with temperature and Gandhi and Shelef (1972) found CO adsorption negligible above 140 °C. As a result, an Eley-Rideal (E-R) type reaction mechanism could also be proposed. In this proposal, CO directly attacks a chemisorbed oxygen complex before carbon dioxide release (see Fig. 2 ).In more recent research conducted by Conner and Bennet (1976), it was proposed that there are three types of site onto which oxygen can adsorb. Two sites participate in the CO oxidation reaction at 180 ∘ C . The first type of active catalyst surface site ( S 1 ) is hypothesised to be a nickel atom onto which removable oxygen atoms are chemisorbed. Carbon dioxide generated during the reaction then exclusively adsorbs onto the second type of site ( S 2 ) which strongly bond to oxygen atoms such as found in a lattice, and this oxygen is proposed to be in the O2− state. In addition, experimental results concluded that no CO adsorbs and that the presence of such would be due only to reaction.Based on these observations, Conner and Bennet (1976) compiled a probable sequence of reaction steps (see Fig. 3 ) describing the CO oxidation reaction pathway over nickel oxide. This proposal provides a plausible alternative to the reaction schemes listed above.This section will present the formulation of initial reaction rate expressions based on the different reaction mechanisms presented above. Given that isotope or other spectroscopy measurements were not possible within the experimental system, possible rate-controlling steps within each mechanism could not be identified a-priori. Therefore, in the case of the proposed l-H and E-R mechanisms, different rate expressions were developed based on the assumption that each possible forward reaction step can be rate-controlling. Given that the conversion of CO in the reaction system was limited to <10%, it was assumed that the reverse reactions do no occur and thus are not possible rate-controlling steps. In the case of the alternative reaction mechanism put forward by Conner and Bennett (1976), isotope studies were conducted by the authors to ascertain the possible rate-controlling step. Therefore, the findings of these studies were used to develop a reaction rate expression from the proposed mechanism.There are four steps in this sequence viz. the adsorption steps for oxygen and CO, a reaction step between the adsorbed species and a product desorption step. In this development, focus on initial rate expressions where reactions involving products are considered negligible in their rate.For the case of the adsorption of CO being rate-controlling viz. forward step (1a), the net rate can be described as that shown in Eq. (1). (1) − R CO , 0 = k CO P CO C V …where C V is the total number of vacant active sites on the catalyst surface.Applying the Pseudo-Steady State Approximation (PSSA), (2) d [ CO . S ] dt ≅ k CO P CO C V − k ′ CO [ CO . S ] − k S [ C O . S ] [ O . S ] ≅ 0 ∴ [ CO . S ] ≅ ( k C O k S ) P C O C v [ O . S ] + k C O ′ k S As forward step (1a) is rate-controlling, it can be assumed that: k CO < < k S and ( k CO k S ) ≅ 0 ∴ [ C O . S ] ≅ 0 Similarly, (3) d [ O . S ] dt ≅ k O 2 P O 2 C V 2 − k ′ O 2 [ O . S ] 2 − k S [ C O . S ] [ O . S ] ≅ 0 But considering that [ CO . S ] ≅ 0 , it can be assumed that k S [ C O . S ] [ O . S ] ≅ 0 ∴ [ O . S ] ≅ ( K O 2 P O 2 ) 0 . 5 C V The concentration of vacant sites can be calculated as follows: (4) C V = C T − [ CO . S ] − [ O . S ] where C T represents the total number of active sites on the catalyst surface.Then, C V = C T − ( K O 2 P O 2 ) 0 . 5 C V ∴ C V = C T 1 + ( K O 2 P O 2 ) 0 . 5 Finally, the rate expression could be obtained in Eq. (1) as: (5) − R CO , 0 = k CO P CO C T 1 + ( K O 2 P O 2 ) 0 . 5 In a similar way, in the case of the adsorption of oxygen being rate-controlling viz. forward step (2a), the net reaction rate can be expressed as: (6) − R C O , 0 = k O 2 P O 2 C V 2 It was shown that in the case where the adsorption of CO is rate-controlling, the concentration of CO adsorbed onto active catalyst sites is negligible ( [ C O . S ] ≅ 0 ) . This makes sense considering that the adsorption rate of CO onto catalyst sites was proposed to be the slowest reaction step. As a result, in the case where the adsorption of oxygen is the rate-controlling step, it is assumed that the concentration of oxygen adsorbed onto catalyst sites is also negligible ( [ O . S ] ≅ 0 ) .Then, [ CO . S ] ≅ K CO P CO C V C V = C T 1 + K CO P CO As a result, the reaction rate expression in Eq. (6) can be reduced to: (7) − R CO , 0 = k O 2 P O 2 C T 2 ( 1 + K CO P CO ) 2 In the case for the reaction between the adsorbed reactant species being rate controlling viz. forward step (3a), then the net reaction rate can be expressed as follows: (8) − R CO , 0 = k S [ C O . S ] [ O . S ] Here, [ CO . S ] ≅ K CO P CO C V k S k C O ′ [ O . S ] + 1 As the forward step (3a) is rate-controlling, it can be assumed that k S < < k C O ′ such that k S k C O ′ ≅ 0 ∴ [ CO . S ] ≅ K CO P CO C V In addition, d [ O . S ] dt ≅ k O 2 P O 2 C V 2 − k ′ O 2 [ O . S ] 2 − k S [ C O . S ] [ O . S ] ≅ 0 K O 2 P O 2 C V 2 − [ O . S ] 2 − k S k O 2 ′ [ CO . S ] [ O . S ] ≅ 0 As the forward step (3a) is rate-controlling, it can be assumed that k S < < k O 2 ′ such that k S k O 2 ′ ≅ 0 ∴ [ O . S ] ≅ ( K O 2 P O 2 ) 0.5 C V Then, C V = C T 1 + K CO P CO + ( K O 2 P O 2 ) 0 . 5 Therefore, the reaction rate expression in Eq. (8) can be reduced to: (9) − R CO , 0 = K S K CO P CO ( K O 2 P O 2 ) 0.5 C T 2 ( 1 + K C O P C O + ( K O 2 P O 2 ) 0 . 5 ) 2 In the case where the adsorption of a reactant species viz. CO or oxygen, is rate-controlling, the driving force for the reaction depends only on the concentration of that reactant species (see Eq. (5) and Eq. (7) above). In the case where the surface reaction viz. CO oxidation, is rate-controlling, the driving force for the reaction depends on all reactant concentrations (see Eq. (9) above).This mechanism differs from reaction mechanism (RM) 1 in that adsorbed oxygen is proposed to only react with CO from the bulk gas to generate carbon dioxide. The modified step is represented by step (2b) above and if this step is assumed to be rate-controlling then: (10) − R CO , 0 = k s P co [ O . S ] Then (11) d [ O . S ] dt ≅ k O 2 P O 2 C V 2 − k ′ O 2 [ O . S ] 2 − k S P C O [ O . S ] ≅ 0 K O 2 P O 2 C V 2 − [ O . S ] 2 − k S k O 2 ′ P C O [ O . S ] ≅ 0 As the forward step (2b) is rate-controlling, it can be assumed that k S < < k O 2 ′ such that k S k O 2 ′ ≅ 0 ∴ [ O . S ] ≅ ( K O 2 P O 2 ) 0.5 C V Here, (12) C V = C T − [ O . S ] ∴ C V = C T 1 + ( K O 2 P O 2 ) 0 . 5 Therefore, reducing the rate expression presented in Eq. (10) gives: (13) − R CO , 0 = k S P CO ( K O 2 P O 2 ) 0.5 C T 1 + ( K O 2 P O 2 ) 0 . 5 In contrast to RM 1, this rate expression describes a linearly dependence on the partial pressure of CO.To obtain the rate expression formulation, the following propositions regarding the probable reaction scheme for CO oxidation put forward by Conner and Bennett (1976) were adopted: - At 180 °C, the predominant surface species on S 1 sites would be the O . S 1 surface intermediate. This was assumed to be the most abundant reaction intermediation on S 1 sites. - At 180 °C, step (3c) is pushed towards the production of C O 2 O . S 2 such that the concentration of C O 2 O . S 1 is very low. Therefore, it is assumed that the reverse reaction of step (3c) is unlikely to occur. - Isotope studies conducted at 180 °C suggest that although carbon dioxide can be generated via step (4c), bulk of the carbon dioxide would be generated from subsequent steps (5c) and (6c). Therefore, step (4c) will be neglected in the formulation. - The two-carbon surface intermediate C 2 O 3 O . S 2 has low concentration such that C O 2 O . S 2 is found to be the most abundant surface intermediate on S 2 sites. Therefore, the forward reaction of step (5c), which describes the consumption of C O 2 O . S 2 species to produce C 2 O 3 O . S 2 , is assumed to be the rate-controlling step. Applying these assumptions, (14) − R CO , 0 = k 4 [ C O 2 O . S 2 ] P C O Further applying the PSSA to determine [ C O 2 O . S 2 ] : (15) d [ C O 2 O . S ] dt ≅ k 4 [ C O 2 O . S 2 ] + k 2 [ C O 2 O . S 2 ] [ O . S 2 ] ≅ 0 (16) ∴ [ C O 2 O . S 2 ] ≅ k 2 [ C O 2 . S 1 ] [ O . S 2 ] k 4 P CO (17) d [ C O 2 . S 1 ] dt ≅ − k 2 [ C O 2 . S 1 ] [ O . S 2 ] + k 1 [ O . S 1 ] P CO ≅ 0 (18) ∴ [ C O 2 O . S 2 ] ≅ k 1 [ O . S 1 ] P C O k 2 [ O . S 2 ] Substituting Eq. (18) into Eq. (16), (19) [ C O 2 O . S 2 ] ≅ k 1 k 4 [ O . S 1 ] (20) d [ O . S 1 ] dt ≅ k O 2 P O 2 C V S 1 2 − k 1 [ O . S 1 ] P CO ≅ 0 Where C V S 1 represents the concentration of vacant S 1 sites on the catalyst surface. (21) ∴ [ O . S 1 ] ≅ k O 2 P O 2 C V S 1 2 k 1 P CO Here, (22) C V S 1 = C T S 1 − [ O . S 1 ] Where C T S 1 represents the total number of active α sites on the catalyst surface.At 180 °C, the predominant surface species on S 1 sites would be the O . S 1 surface intermediate. This was assumed to be the most abundant reaction intermediation on S 1 sites.At 180 °C, step (3c) is pushed towards the production of C O 2 O . S 2 such that the concentration of C O 2 O . S 1 is very low. Therefore, it is assumed that the reverse reaction of step (3c) is unlikely to occur.Isotope studies conducted at 180 °C suggest that although carbon dioxide can be generated via step (4c), bulk of the carbon dioxide would be generated from subsequent steps (5c) and (6c). Therefore, step (4c) will be neglected in the formulation.The two-carbon surface intermediate C 2 O 3 O . S 2 has low concentration such that C O 2 O . S 2 is found to be the most abundant surface intermediate on S 2 sites. Therefore, the forward reaction of step (5c), which describes the consumption of C O 2 O . S 2 species to produce C 2 O 3 O . S 2 , is assumed to be the rate-controlling step.There are three expressions. viz. Eq. (19), (21) and (22) and three unknown variables viz. [ C O 2 O . S 2 ] , [ O . S 1 ] and C V S 1 . Solving simultaneously gives: (23) − R CO , 0 = k O 2 k 1 P CO P O 2 C T S 1 2 2 k O 2 P O 2 C T S 1 + k 1 P CO This rate expression is fundamentally different from those obtained from RMs 1 and 2 in that the reaction rate is a function that is linearly dependent on both the partial pressures of oxygen and CO. In addition, this rate expression does not contain adsorption equilibrium constants. A summary of the rate expressions obtained is presented in Table 1 below.Given that the list of probable kinetic models describing the oxidation of CO over nickel oxide has been outlined (see Table 1), the model parameters can be estimated by fitting these models to experimental data. The experimental procedure used to perform kinetic measurements is outlined in the section below.A commercial G-65 steam reforming catalyst (Süd-Chemie) was used for all experiments. The catalyst is also a common methanation catalyst. Van Herwijnen et al. (1973) carried out X-ray diffraction measurements on a sample of the catalyst and deduced that the carrier material is γ-alumina. No further information was elucidated. Since the catalyst was used as supplied by the manufacturer, for the purposes of this study only the surface and morphological properties were measured. The textural properties of the catalyst (BET surface area and average pore size) were determined using a Micrometrics ASAP 2020 gas adsorption analyser. Measurements were performed with nitrogen as the adsorbate at −196 °C. The samples were degassed under nitrogen at 200 °C for 18 h prior to analysis. The catalyst was found to have a specific surface area of 42.4 m2 g − 1 with an average pore diameter of 100 A ˙ . Energy dispersive X-ray (EDX) analysis was performed using a ZEISS Ultra Plus scanning electron microscope (SEM) instrument, and the results were used to determine the amount of active material present on the surface. The catalyst was found to contain 33.6 wt.% nickel. The commercial pellets were ground and sieved to recover a fraction in the size range of 250–350 µm, which were used for all subsequent kinetic tests.Experiments were carried out in a conventional laboratory-scale gas-phase fixed bed reactor (as shown in Fig. 4 ) at atmospheric pressure. The reactor was fabricated from a ½ inch nominal sized stainless steel tube (i.d. 9.2 mm, length 400 mm) and was placed in a vertical, electrically heated tube furnace. The catalyst was placed on a stainless steel grid attached to a sheathed type K thermocouple mounted through the center of the tube. CO (Afrox, 99.9%), oxygen and nitrogen (Afrox, 99.999%) were supplied in standard cylinders and metered using precision flow control valves. The product stream was sampled and analysed using gas-solid chromatography and the flowrate was measured using a conventional gas flowmeter.In each experiment, a catalyst sample of 1 g was loaded into the reactor tube. The catalyst was pretreated in situ by heating at 500 °C under a stream of nitrogen for two hours, then cooled to reaction temperature. The feed gas was then opened to the reactor and the reaction was allowed to proceed until steady state was achieved (approximately 1 hour, determined from preliminary tests by monitoring the change in the exit composition of the product gas). Samples of the gas stream were analysed after this period of stabilization. Analyses of the product gas were performed on a Shimadzu 2014 gas chromatograph (GC) equipped with a packed Restek ShinCarbon ST column (80/100 mesh, 2 m long, 1/8 inch o.d.) and a thermal conductivity detector (TCD) using helium as the carrier gas (25 ml•min−1). The temperature program for gas samples was 40 °C for 3 min, 40 °C to 220 °C (at 8 °C min−1) and 220 °C for 10 min.Kinetic data was collected at four different temperatures (180, 190, 200 and 210 °C) using feed mixtures of CO, oxygen and nitrogen. Each isothermal dataset consisted of 11 experiments. In the first six experiments, the feed concentration of oxygen was varied whilst maintaining the concentration of CO approximately constant. In the last five experiments, the feed concentration of CO was varied whilst maintaining the concentration of oxygen approximately constant. Nitrogen was used as the dilution gas, with the flowrate adjusted to maintain a constant space time in the reactor across all experiments. The total inlet molar flowrate across all experiments ranged between 2.42–2.65 × 10−4 mol s − 1. The partial pressure of O2 ranged between 7 and 20 kPa and that of CO ranged between 7 and 25 kPa, corresponding to inlet concentrations of 2–5 mol m − 3 and 2–7 mol m − 3 for O2 and CO, respectively.The conversion of CO was kept below 10% to ensure differential operation of the reactor in addition to the use of limited quantities of catalyst and high gas flow rates. The modified Reynolds number of the gas flow through the reactor was estimated to be ∼100 which is close to fully turbulent flow through the packed bed (Chhabra and Basavaraj, 2019). Therefore, plug flow is assumed through the reactor and the initial rate could be calculated directly using Eq. (24) (24) − R CO , 0 = F CO , in − F CO , out W cat The Weisz-Prater criterion (CWP ) was used to check whether the reaction is internal diffusion limited. This criterion was calculated using the following equation: (25) C WP = − R CO , 0 · ρ s · R p 2 D e · C AS , 0 The effect of internal diffusion on the observed CO oxidation reaction rate is considered negligible when CWP is <<1. The parameters used in Eq. (25) are presented in the table below.Using the values in Table 2 , the CWP value was estimated using Eq. (25) as follows: C WP = ( 2 . 95 e − 06 · 6 . 67 · 0 . 015 ) 2 ( 1 . 10 e − 04 · 4 . 87 e − 04 ) ≅ 0.0021 < < 1 Therefore, it can be assumed that internal diffusion within the supported NiO catalyst has negligible effect on the observed CO oxidation reaction rate.The catalyst internal effectiveness factor ( η ) also indicates the effects of the internal mass transfer and is used to evaluate the performance of a catalytic reactor. The effectiveness factor ( η ) is defined as the ratio of the observed rate of reaction to the hypothetical rate in the absence of mass transfer limitations. Therefore, as the effectiveness factor approaches 1, the effects of internal mass transfer becomes more negligible.The effectiveness factor ( η ) was calculated using the initial reaction rate constant (kCO,0 ). Assuming that at the start of the reaction (t= 0) the surface CO concentration is the same as the concentration across the catalyst particle i.e. CAS, 0 = CA, 0, the initial reaction rate constant can be estimated by using the measured initial reaction rate value and initial CO concentration at the surface i.e. -RCO, 0= kCO, 0 CAS, 0.For a first-order reaction and assuming spherical catalyst particles, the internal effective ( η ) can be calculated using the Thiele modulus ( ϕ ) , which correlates the catalyst activity ( k c 0 , 0 ) and radius of the catalyst particle (R) as follows: (26) ϕ = R k c 0 , 0 D e (27) η = 3 ϕ 2 ( ϕ coth ϕ − 1 ) Based on the Eqns. (26) and 27, the Thiele modulus and internal effectiveness factor was estimated as follows: ϕ = ( 0.015 ) 6.06 e − 03 1.1 e − 04 = 0.0081 η = 3 ( 0 . 0081 ) 2 ( 0 . 0081 · coth ( 0 . 0081 ) − 1 ) ≅ 1 Given that the catalyst internal effectiveness factor ≅ 1 , the internal mass transfer effects are considered negligible within the catalytic reactor.The Mears criterion is used to check whether the reaction is limited by external diffusion. This criterion is calculated using the following equation: (28) C mears = − R CO , 0 · ρ b · R p · n k c · C AS , 0 The effect of external diffusion on the observed CO oxidation reaction rate is considered negligible when Cmears  < 0.15.Here, the external mass transfer coefficient (kc ) for CO in the bulk diffusing through air is calculated by: (29) k c = Sh · D A B d p Here, Sh is the Sherwood number and is calculated as: (30) Sh = 2 + 0 . 6 R e 0 . 5 S c 1 / 3 where Sc and Re are the Schmidt and Reynolds numbers for CO in the bulk diffusing through air, respectively. The Sherwood and Reynolds numbers are found as follows: (31) S c = V D AB (32) Re = ρ · D 2 · u μ The Mears criterion was then calculated as follows: C mears = ( 2 . 95 e − 06 ) · ( 25 . 1 ) · ( 0 . 15 ) · ( 1 ) ( 92 . 59 ) · ( 4 . 87 e − 04 ) ≅ 2.46 e − 05 < < 0.15 Therefore, it can be assumed that external diffusion within the bulk CO-air layer has negligible effect on the observed CO oxidation reaction rate at the catalyst surface.The average relative deviation on TCD calibration for CO and oxygen was 2.3%. Relative uncertainty in the gas flowrate measurements averaged 3%, based on the manufacturers’ specifications for the precision rotameters and the laboratory calibration of the rotameters. The catalyst mass was determined using a digital scale having an accuracy of 0.1 mg and hence the uncertainty in measurement was considered negligible. Consequently, the average relative uncertainty on observed reaction rates was found to be low (3.7%) and was not accounted for in the rate measurements presented in the results section below.Kinetic measurements were performed to determine the influence of the partial pressure of CO and oxygen on the reaction rate with increase in temperature from 180 to 210 °C. These kinetic rate measurements allow for the estimation of model parameters contained within the proposed reaction rate expressions. In addition, assessing the influence of these parameters on the reaction rate can allow for preliminary discrimination between the reaction rate expressions prior to the estimation of model parameters. Plots of the relationship between the reaction rate and the partial pressure of oxygen and the partial pressure of CO are shown in Figs. 5 and 6 below, respectively.The experimental data presented in Figs. 5 and 6 both reveal that an increase in the partial pressure of oxygen and CO respectively results in an increase in the reaction rate. As the temperature increases, these observed trends become more pronounced with increase in the reactant partial pressures. As a result, the overall reaction rate must be driven by both the partial pressures of oxygen and CO. This is not represented by reaction rate expressions (1) and (2). As a result, these expressions could be ruled out from the list of possible reaction rate expressions. Given that rate expressions (3), (4) and (5) were still potential kinetic models for the system, further model discrimination was made by measuring the degree of fit between these rate expressions and the experimental data measured at 180 °C. The experimental data was regressed to the rate expressions using the Levenberg-Marquardt algorithm based on MATLAB software.The graphical representations of the model fits regressed to the experimental data measured at 180 °C and non-linear regression outputs from MATLAB are presented in Fig. 7 and Table 4, respectively. Analysis of Fig. 7, as well as, the coefficient of determination (R 2) values presented in Table 4 reveals that the experimental data measured at 180 °C best fits rate expression (5).The model variances ( σ 2 ) presented in Table 4 measures the magnitude of deviation between the experimental data measured at 180 °C and the model outputs. These variances reflect on the R 2 values and can be used to conduct a statistical F-test to assess whether the fit between the experimental data measured at 180 °C and rate expression (5) differs significantly from the model fits made with rate expressions (3) and (4). The F-test value represents the ratio of the model variances ( σ 1 2 σ 2 2 ) where σ 1 2 > σ 2 2 . For the null hypothesis viz. that there is significant statistical difference between the model fits to be accepted, the F-test value must be greater than the tabulated critical F-test value. At the 95% confidence interval, the tabulated critical F-test values for rate expression (3) and (4) with regards to rate expression (5) are 3.44 and 3.39 respectively. The calculated F-test values for rate expression (3) and (4) with regards to rate expression (5) are 1.74 and 1.25 respectively. Therefore, the alternate hypothesis is accepted in both cases viz. that there is no significant statistical difference between the closer model fit made with rate expression (5) and those made with rate expressions (3) and (4).However, given the better fit to the experimental data measured at 180 °C, rate expression (5) was selected from the list of probable rate expressions as the most reliable description of the kinetics of CO oxidation within the specified operating region. It is important to note that the lack of agreement between the experimental data measured at 180 °C and rate expression (5) can be accounted to possible factors such as mass and thermal diffusion resistances, side reactions and deviation from the plug flow reactor (PFR) model. Figs. 5 and 6 also both reveal that the degree of influence of the partial pressure of oxygen and CO respectively on the reaction rate increases with temperature. Given that the reaction rate is strongly influenced by temperature, rate expression (5) should also be a function of the reaction temperature. The model parameters contained with rate expression (5) can be related to the reaction temperature through the Arrhenius expression as follows: (33) k = A exp ( − E a R g T ) In this form, values for the pre-exponential factor (A) and apparent activation energy (E a) could be estimated by regressing the non-isothermal experimental data set to rate expression (5) using MATLAB software. Given that these constants can be appreciably sensitive to even small changes in the temperature due to the exponential form of the Arrhenius expression presented in Eq. (33), caution was taken when specifying initial guesses for A and E a required to initiate the MATLAB regression loop.Linearization of Eq. (33) gives: (34) ln k = − E a RT + lnA Plotting ln k against the inverse of the temperature (1/T) gives rough estimation of the Arrhenius constants A and E a. These plot estimates could then be specified as the initial guesses used in the regression procedure.However, prior to the construction of Arrhenius plots, non-linear regression on MATLAB software was carried out to estimate the model parameters for the remaining isothermal data sets measured at T= 190 °C, 200 °C and 210 °CGiven that the overall rate of the CO oxidation reaction increases with temperature as shown in Figs. 5 and 6, the model parameter estimates should also increase with temperature. This trend is seen in Table 5 with exception of k 2 ′ at T= 210 °C. The range of R 2 values presented in Table 3 reveal that with increase in temperature, there is better agreement between the experimental data and reaction rate expression (5). However, given that the range of R 2 values lies between 0.665 and 0.802, there is still reasonably close agreement between the experimental data and this kinetic model.Arrhenius plots could then be constructed as follows: Figs. 8 (a), (b) and (c) reveal that in all cases, reasonably good linear fits are obtained. As a result, the pre-exponential factors (A) and apparent activation energies (E a) shown in Table 6 could be obtained confidently from the y-intercept and slope of the straight-line presented in Figs. 8(a), (b) and (c), respectively.Regression for the Arrhenius constants could then be carried out on MATLAB software:The MATLAB output for the coefficient of determination (R2) when fitting the consolidated non-isothermal data set to rate expression (5) was found to be 0.717. This result represents the best agreement that could be obtained between the experimental data and rate expression (5) over the range of operating temperaturesFigure 9. The apparent activation energies (E a), and pre-exponential factors (A) shown in Table 7 for rate expression (5) are found kinetically realizable given that these Arrhenius constants are both positive and finite quantities. Comparison of Figs. 5 and 6 shows that the reaction rate is more sensitive to changes in the partial pressure of CO with increase in temperature. This could be related to the temperature dependence of the constants k 2 ′ and k 3 ′ in rate expression (5) given the activation energy associated with k 2 ′ (the rate coefficient coupled with the partial pressure of CO in rate expression (5)) is higher than the activation energy associated with k 3 ′ (the rate coefficient coupled with the partial pressure of oxygen in rate expression (5)). The model most representative of the kinetics associated with the oxidation of CO over NiO/Al2O3 catalyst, among those proposed in this study, is found to be a reaction rate expression based on a mechanism whereby non-competitive adsorption of reactant and product species occur at low temperature. The model based on this mechanism is observed to best fit the experimental data collected at temperatures between 180 °C and 210°C despite the lack of significant statistical difference between the model fit results. However, the agreement between the best fitting model and the experimental data appears to deteriorate with increase in temperature. This is possibly due to the occurrence of side reactions and deviation from the proposed reaction mechanism at higher temperatures. Despite this observation, the identified rate expression fits bulk of the experimental data measured within the range of operating temperatures, as indicated by a coefficient of determination (R 2) value of 0.717. Therefore, the mechanistic model developed and identified in this study provides a useful description of the kinetics of the NiO catalysed CO oxidation system at low temperature. A [mol gcat−1 bar−1 s − 1] Arrhenius pre-exponential constant C [mol cm−3] Reaction rate constant CAS [mol cm−3] Surface concentration of species A CWP [-] Weisz-Prater criterion Cmears [-] Mears criterion D [cm/m] Diameter DAB [cm2/s] Diffusion coefficient De [cm2/s] Effective diffusivity E a [kJ mol−1] Apparent activation energy F [mol s-1] Molar flow rate k [mol gcat−1 s − 1] Reaction rate constant kc [cm/s] Mass transfer coefficient K [mol gcat−1 s − 1] Equilibrium adsorption rate constant N [-] Reaction order P [bar] Partial Pressure R [mol s − 1 gcat−1]/[cm] Methane consumption reaction rate/radius Re [-] Reynolds number R g 8. 314 [J mol−1 K − 1] Ideal gas constant S [-] Active surface catalyst site Sc [-] Schmidt number Sh [-] Sherwood number T [s] Reaction time T [ °C] Reaction temperature U [m/s] Velocity V [cm2/s] Kinematic viscosity W [gcat] Mass σ [-] Standard deviation ϕ [-] Thiele modulus η [-] Catalyst effectiveness factor ρ [g/cm3] Density μ [kg s − 1 m − 1] Dynamic viscosity B Bulk Cat Catalyst CO Carbon monoxide In Inlet O2 Oxygen 0 Initial Out Outlet P Particle S Solid catalyst S Surface T Total V Vacant CO Carbon monoxide E-R Eley-Rideal GC Gas chromatograph L-H Langmuir-Hinshelwood NiO/Al2O3 Alumina supported nickel oxide catalyst PSSA Pseudo steady state approximation RM Reaction mechanism TCD Temperature controlled decomposition VOCS Volatile organic compounds The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors are grateful to the University of Kwa-Zulu Natal for financial support of the study and Ms S. Naicker for laboratory assistance in the experimental work. This work is based on the research supported by the National Research Foundation of South Africa.

The kinetics of carbon monoxide oxidation over alumina supported nickel oxide was studied using a mechanistic approach to model development and identification. Langmuir-Hinshelwood and Eley-Rideal reaction schemes, together with an alternative scheme were investigated, using low temperature, differential rate measurements. Experimental results were found to be most consistent with the alternative scheme, supporting a reaction mechanism based on non-competitive adsorption of carbon monoxide and oxygen onto different catalyst sites without mutual displacement. The resulting kinetic model was observed to have reasonable agreement with the experimental findings and was found representative of the system.

The oxidation of alcohols is considered a benchmark reaction for the development of new catalysts [1]. Besides this, the oxidation of primary and secondary alcohols to their respective aldehydes and ketones is a common laboratory procedure. These reactions traditionally employ toxic oxidants such as chromium VI salts (dichromate, chromic acid, and chromium trioxide), potassium permanganate [2,3], and pyridinium chlorochromate [4] that, although selective to aldehyde and ketones, generally require an excess to ensure better conversions. However, the procedure drawback is the generation of toxic waste [5]. At the same time, these reactions attract attention mainly because aldehydes and ketones are intermediates of many products used in fine chemistry [6]. The oxidation of benzyl alcohol to benzaldehyde is an important example since benzaldehyde is industrially the most important aromatic aldehyde and one of the main aromatic compounds used in the pharmaceutical, cosmetic, food, and perfumery industries [7–9].In this context, several works have sought catalysts and nanocatalysts [10,11] that increase the production of intermediate products, i.e. benzaldehyde [9]. Some of these new procedures have applied catalysts with or without an oxidant, highlighting the application of metallic nanoparticles, including, for instance, Au [6,12], Pt [13], Au-Pd [14], Ag [15]; Ag [8,16], Co [17], and Pd [18] in association with O2; and Cu associated with hydrogen peroxide [19] or TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) [1,20]. The oxidant choice is also important, being suitable to the catalyst (e.g. Cu and TEMPO), as well as for the process as a whole, relatively stable and, if possible, having a low cost, as molecular oxygen, hydrogen peroxide, and sodium/calcium hypochlorite.The application of sodium and calcium hypochlorite as an oxidant in organic synthesis was extensively reported in the 1980s. Stevens and co-workers applied “Swimming Pool Chlorine” (an inexpensive commercial sodium hypochlorite solution) to oxidize secondary alcohols to ketones [21] and diols/aldehydes to their respective ketones and methyl esters [22]. At the same time, calcium hypochlorite was applied to oxidize secondary methyl ethers into ketones [23], aldehydes to their corresponding carboxylic acids [24], secondary alcohols to ketones, primary alcohols to esters, and ethers to esters [25]. All these applications report hypochlorite as a versatile, effective, safer, and potential substitute for traditional chromium VI salts [3] and pyridine-based [4] oxidants in organic synthesis. Besides this, some alcohol oxidation using sodium hypochlorite has been recognized as environmentally benign and/or a greener process [26–30], when compared with that described by Stevens [21].Germanophosphate glasses containing self-supported nickel-based nanoparticles as catalysts were recently described as a new protocol for benzyl alcohol oxidation using sodium hypochlorite as an oxidant [31]. In this study, good conversions were achieved (≈75 mol%), with high benzaldehyde selectivity (>99%), employing mild reaction conditions (20 °C, acetonitrile as solvent). In this sense, the glass acts as an active substrate for the synthesis of self-supported Ni-based catalysts. The employment of glass materials in catalysis is practically an unexplored topic [32], and the major application of this material is like a sort of support for catalysts [33–38].However, Matzkeit et al. [32] reported the use of borophosphate glass as a catalyst for bioactive bis(indolyl)methanes molecules (BIMs) synthesis under solvent-free conditions, achieving high yields. Phosphate-based glass materials can be obtained from simple raw chemicals (e.g., KH2PO4, P2O5, NaPO3, and NaH2PO4). Therefore, phosphate-based glasses could represent an unconventional method that moves towards the development of new heterogeneous catalysts.In this sense, considering the relevance of the development of new catalysts and processes that look for chemical compounds of laboratory and industrial interest in an effective and selective way, and the use of less harmful chemical reagents, this work applied a simple and easy to produce borophosphate glass as a catalyst for the oxidation of benzyl alcohol and 1-phenylethanol by sodium hypochlorite 11 wt%, in an organic-aqueous biphasic system. To optimize the alcohol conversion and ensure good aldehyde selectivity, some experimental parameters of the reaction such as temperature, oxidant amount, catalyst mass and particle size, and reaction media were evaluated and monitored by High-Performance Liquid Chromatography (HPLC) analysis. HPLC analysis of both organic and aqueous phases was also used to propose the reaction mechanism. The glass catalyst was characterized by Raman spectroscopy, X-ray diffraction, density, and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES).Borophosphate glass was prepared by melting-quenching technique [34,35] with high-purity reagents (Sigma–Aldrich®) using NaH2PO4, H3BO3, and Al2O3 as precursors. The NaH2PO4/H3BO3 molar proportion was set as 2 and Al2O3 was added in proportions of 0, 3, 5, 7, and 10 mol%. In a typical synthesis, 5 g of the aforementioned compounds in the predetermined proportions were weighted and homogenized in an agate mortar for 10 min, transferred to a covered Pt crucible, and then melted at 1050 °C in a resistive preheated oven for 1 h. The glass sample was obtained by quenching the molten mixture on a graphite mold at room temperature. Posteriorly, the glass was grounded in an agate mortar and sieved through a 150 to <400 mesh range, and stored under a vacuum desiccator until analysis.The glass density (ρ) was measured with the bulk glass samples by the Archimedes method using a density module mounted on a Mettler Toledo ME240/M analytical balance and ethanol as immersion solvent.The total content of aluminum, boron, sodium, and phosphorus on borophosphate glasses was determined by ICP-OES using a Thermo Scientific® iCap 6000 Series Spectrometer. The ICP solutions were prepared in triplicate with 100 mg of sample 100-time diluted by ultrapure 1% (v/v) HNO3 aqueous solution in an ultrasonic bath. The analytical standard curves (0.1–10 mg L−1) were prepared using a multi-element standard solution (Fluka®) diluted by the aforementioned solvent. All samples were prepared in plastic materials to avoid borosilicate glass interference. Emission lines used for quantification in axial view: Al 396.152 nm; B 249.678 nm; Na 589.592 nm, and P 185.942 nm [35].Raman spectrum of the glass sample was recorded using a micro-Raman Renishaw InVia®, laser power 8 mW, 633 nm excitation wavelength, and CCD (Charge Coupled Device) detector. The powder glass sample was measured without any additional treatment. Deconvolution analysis of the Raman spectrum was carried out by Voigt functions using the Fityk program (version 1.3.1).The amorphous nature of the borophosphate glass powders was assessed by X-ray powder diffraction (XRD) measurements, using a Rigaku SmartLab SE Diffractometer equipped with the Cu Kα radiation (λ = 1.5418 Å), and at angles between 15° and 80° (θ – 2θ).First of all, the concentration of the NaOCl solution was determined as 11 wt% by iodometric titrimetric analysis (Supplementary Information, Section 2).Benzyl alcohol (BnOH) and 1-phenylethanol (BnEtOH) oxidations were conducted using NaOCl 11 wt% as oxidant and acetonitrile (ACN) as a solvent, based on our previous paper [32]. Initially, the reaction parameters oxidant and catalyst amount, temperature, and catalyst mesh were tested to optimize the reaction conditions, as reported in Table 1 . All reactions were carried out under constant stirring. Thus, the glass catalyst was first dispersed in 10 mL ACN for 2 min, followed by the addition of 0.75 mmol of the aforementioned alcohols. The oxidant, NaOCl, was added in 4 equal portions (total amount tested divided by 4) in the beginning and within 30, 60, and 90 min of the reaction, being the reaction time adjusted according to the optimization of reaction conditions [28].After the evaluation of oxidant amount, temperature, catalyst mass, and particle size, the standard reaction conditions applied for additional tests (Supplementary Material), the effect of pH, and catalyst reuse were: 75 mg and 100 mg glass-catalyst for benzyl alcohol and 1-phenylethanol reactions, respectively, first dispersed in 10 mL ACN for 2 min, followed by the addition of 0.75 mmol of the alcohols. NaOCl was added in 4 equal portions of 1 mL (total 4 mL, 6.4 mmol) in the beginning and for 30, 60, and 90 min. The reactions were carried out under constant stirring at 50 °C for 3 or 5 h.For catalyst reuse, the reaction media, containing the catalyst, was centrifuged at 3400 rpm for 10 min. The sedimented catalyst was separated from the liquid, washed with 5 mL of absolute ethanol, and the mixture centrifuged at 3400 rpm for 5 min. This procedure was repeated 3 times. Then, the freshly washed catalyst was dried at 80 °C for 4 h and reused.The unconverted BnOH, BnEtOH, and the oxidation products benzaldehyde (BnCHO), benzoic acid (BnCOOH), and acetophenone (BnCOCH) were determined using HPLC (Thermo Scientific ® Ultimate 3000). The separation was performed at 30 °C using an octadecylsilane C18 column (Ace ltd.®), a flow rate of 1 mL min−1 in gradient elution, with the mobile phase composed of the mixture of acidified water (0.01% v/v phosphoric acid, pH 2.75 ± 0.05) and acetonitrile (ACN, J.T. Baker® HPLC grade): initial 30% ACN → to 60% ACN in 10 min, keeping this condition until 15 min. The detection was made by a diode-array detector (DAD) at 210 nm. Aliquots of the reaction media were collected at the beginning, during, and at the end of the reaction, diluted 10 times with the mobile phase, and filtered in a 0.22 μm hydrophilic PVDF syringe filter [31]. Analytical standards (Sigma Aldrich®, Supelco®) were used as a reference for sample concentration determination.To verify the distribution of compounds (mmol) in the biphasic formed phases, the organic reaction media was centrifuged (3400 rpm, 6 min), and a fraction of both organic and aqueous phases were collected for HPLC analysis.The selectivity, linear range, quantification limit, precision, and accuracy of the HPLC method were previously tested to ensure the correct determination of the reaction compounds. BnOH and BnEtOH conversions (C), product yields (Y), and BnCHO selectivity (S) were calculated, respectively, according to Eqs. (1)–(3). n refers to the number of mmol calculated for each compound. (1) C a l c o h o l ( m o l % ) = n a l c o h o l i n i t i a l − n a l c o h o l t i m e n a l c o h o l i n i t i a l x 100 (2) Y p r o d u c t ( m o l % ) = n p r o d u c t t i m e n p r o d u c t t h e o r e t i c a l x 100 (3) S B n C H O ( % ) = n B n C H O e n d ∑ n e n d B n C H O , B n C O O H x 100 All pH measurements were performed by an MS Tecnopon® digital pHmether previously calibrated with standard solutions (pH 4–10). pH adjustment, when needed, was made with HCl 10 wt% and NaOH 10 wt% solutions. Table 2 shows the molar composition of P, Na, B, and Al as their respective oxides, determined by ICP-OES [35], for borophosphate glass doped with 10 mol% Al2O3 used as a catalyst for benzyl alcohol and 1-phenylethanol oxidation by NaOCl.Figure S1 shows the x-ray diffraction patterns for a borophosphate glass series with an increase of Al2O3 content from 0 to 10 mol%. The absence of crystallization peaks and the presence of the broad amorphous regions (halo) indicate the glassy characteristic of the materials. Moisture-resistant borophosphate glasses (molar ratio P/B = 2) can be obtained with the addition of Al2O3 (10 mol%) at a relatively low fusion temperature (e.g., 1050 °C) [32,35]. The successive addition of Al3+ ions reduces the moisture absorption of borophosphate glasses and increases the glass transition temperature (Tg). However, concentrations above 12.5 mol% Al2O3 crystallize when fused at 1050 °C [35].The addition of Al2O3 improves the chemical resistance of borophosphate glasses due to the depolymerization of the phosphate network and the formation of P–O–Al bonds [35,39]. The depolymerization of phosphate glass structure can be observed through Raman spectroscopy, Fig. 1 . The band at ≈ 330 cm−1 is associated with symmetric stretching of the O–P–O bond in metaphosphate structures (Q2, based on Qn terminology, where n represents the number of bridging oxygen that links one tetrahedron to another) [35,39–41]. Borophosphate B–O–P band is observed at ≈ 630 cm−1 [41], and the symmetric stretching of P–O–P at ≈ 700 cm−1 [32,35,40,42]. With the addition of 10 mol% Al2O3, Al–O–Al bending modes can be noticed at ≈ 540 cm−1 [32,35].The band at ≈ 930 cm−1 (inset graph, Fig. 1) is associated with isolated orthophosphate groups (Q0) and asymmetric stretching of P–O–P in Q2 structures [35,40]. The main effect of Al3+ addition to the borophosphate glass network is the depolymerization process, e.g. the reduction of metaphosphate structures (Q2) and the increase of pyrophosphate groups (Q1). This effect can be observed in the 1000–1250 cm−1 region of the Raman spectrum: pyrophosphate band νs P–O occurs at ≈ 1030 cm−1 [41], whilst metaphosphate band νs PO2 occurs at ≈ 1100 cm−1 [41], and both overlap to form a single band with a maximum at 1070 cm−1, as highlighted in the inset graph of Fig. 1. The asymmetric stretching νas PO2 occurs at ≈ 1235 cm−1 [35,41].The density of borophosphate glass rises with Al2O3 addition up to 7.5 mol% Al2O3 and decreases with further addition of Al2O3 (10 mol%) (Fig. 2 ). The variations in density indicate that aluminum addition changes the O/P ratios determined from the ICP-OES analysis: for glasses with Al2O3 amount below 7.5 mol%, O/P < 3.5, and metaphosphate structures are predominant (Q2). On the other hand, 10 mol% Al2O3 borophosphate glass has O/P > 3.6, and pyrophosphate groups are predominant [43,44]. These variations in O/P ratios also change the predominant Al groups. In glasses with O/P < 3.5, the Al2O3 addition tends to replace P–O–P and PO-Na+ bonds by cross-linked POAl(6)-phosphate chains, increasing the glass density, for instance. However, further aluminum additions change O/P > 3.5, with the replacement of POAl(6) groups for more open POAl(4) structures, reducing the glass density [43,44].The oxidation of benzyl alcohol (primary alcohol) and 1-phenylethanol (secondary alcohol) to benzaldehyde and acetophenone, respectively, were carried out using acetonitrile as the solvent, NaClO as oxidant, and borophosphate glass (10 mol% Al2O3) as the catalyst. After the addition of NaOCl to the reaction, a biphasic organic-aqueous system was formed, and its role is properly discussed in section 3.2.1. The reaction conditions were optimized to determine the best alcohol conversion and aldehyde selectivity.Sodium hypochlorite was added to the reaction at 20 °C [31] varying its amount between 1.6 mmol (1 mL) and 6.4 mmol (4 mL). Fig. 3 shows the effect of oxidant over alcohol oxidation. When added 1.6 and 3.2 mmol (oxidant: alcohol molar ratio 2.1:1 and 4.2:1, respectively) in both reactions, BnOH (Fig. 3(a)) and BnEtOH (Fig. 3(b)) achieved a constant conversion, indicating that the oxidant is the limiting reactant. Increasing the NaClO amount to 4.8 and 6.4 mmol (oxidant: alcohol molar ratio 6.4:1 and 8.5:1, respectively), the conversions rise for both reactions. Applying 6.4 mmol (4 mL) of NaClO, BnOH conversion after 10 h of reaction was 58.0 mol% (Fig. 3(a))) and BnEtOH conversion was 70.4 mol% (Fig. 3(b)). Thus, 6.4 mmol of NaClO was set as the standard amount for further reactions.The reaction using NaOCl as oxidant tends to be strongly affected by temperature, and the addition of this compound leads to an exothermic process, requiring, in some cases, the reactions to be carried out in ice baths or under mild temperatures [21,22,45]. For instance, Mombarg et al. [46] have reported that 2,3-butanediol oxidation by NaClO catalyzed by NiSO4·6H2O was slow in temperatures above 20 °C, whereas an exothermic reaction occurs at 30 °C. However, the formation of the organic-aqueous biphasic system allows us to investigate the effect of ambient to 50 °C over the alcohol conversion. In this sense, Fig. 4 shows the effect of temperature on BnOH and BnEtOH oxidation at 20, 30, and 50 °C. The increase of the temperature from 20 to 30 and 50 °C provides higher conversions. At 50 °C, the maximum conversion of BnOH (77.2 mol%, Fig. 4 (a)) and BnEtOH (75.7 mol%, Fig. 4 (b)) were achieved after 5 h. Thus, the temperature rise to 50 °C allows a reduction of reaction time by half without loss in alcohol conversion. Fig. 5 shows the effect of catalyst mass on the reaction, where the glass catalyst plays a fundamental role mainly in BnOH oxidation. Without the glass catalyst, conversions reached only 12.4 mol% for BnOH (Fig. 5(a)) and 0.6 mol% for BnEtOH (Fig. 5(b)). Nevertheless, for BnOH oxidation, the increase of glass mass from 25 mg to 75 mg results in conversions between 72.7 and 78.5 mol%. Additional mass, 100 mg, does not result in higher conversions. On the other hand, BnEtOH conversion showed to be more affected by glass-catalyst mass (Fig. 5(b)). The increase from 25 mg to 75 mg of glass-catalyst led to conversions between 56.2 and 80.3 mol%. For further mass increases (100–150 mg), the conversion levels are between 85 and 87 mol%. These low conversions achieved in uncatalyzed reactions indicate that the glass catalyst and the biphasic organic-aqueous system play an important role in the process, even being NaOCl readily active in the oxidation of primary and secondary alcohols in other reports [5,21,22,45].At least, the glass-catalyst particle size effect in the reaction was evaluated. The previous evaluations for oxidant, temperature, and catalyst mass were carried out with glass particle sizes between 325 and 400 (Tyler) (44–37 μm). As a general trend, the reduction of particle size of the catalyst or the use of nanocatalysts tends to increase the surface area and, consequently, the conversion [38,47,48]. So, the glass catalyst sieved with a lower Tyler scale (higher glass particle, 150–325 mesh range) results in reduced yields (67–72 mol% for benzyl alcohol and 68–70 mol% for 1-phenylethanol), whereas higher Tyler sieves (325–400 and <400 mesh) rise the conversion, Fig. 6 . However, the lowest particle glass catalyst (<400 mesh) shows an opposite behavior with a reduced conversion (72.3 mol% for benzyl alcohol and 77.1 mol% for 1-phenylethanol) in relation to the 400–325 range (higher glass particle size, 78.2 mol% for benzyl alcohol and 85.6 mol% for 1-phenylethanol) due to its agglomeration and adhesion to the reaction vessel.The “salting-out” [49] or “sugaring-out” [50,51] is a well-established analytical procedure used to extract organic solutes from water [49], allowing its quantitative analysis [52]. The process is characterized by the formation of a biphasic organic-aqueous system when a salt is added to a water-acetonitrile mixture, for instance. Thus, the addition of aqueous NaClO 1.6 mmol mL−1 to acetonitrile results in a biphasic organic-aqueous system [31]. The catalyzed reaction proceeds in the aqueous phase and the oxidized products are transferred to the organic phase. At the end of the procedure, the reaction system was centrifuged to separate the organic from the aqueous phase and to determine the concentration of the reactants and products, Table 3 . Benzyl alcohol and benzaldehyde are present mainly in the organic phase, whereas benzoic acid was only observed in the aqueous phase in a small amount. The maximum percentage of BnOH mmols present in the aqueous phase related to the organic phase was 1.3%, whilst for BnCHO it was 0.5%. The same pattern occurs with BnEtOH and BnCOCH, where the maximum percentage of BnEtOH mmols present in the aqueous phase was 0.5% related to the organic phase, whilst for BnCOCH it was 0.2%.Acetophenone is the only expected product of 1-phenylethanol oxidation (secondary alcohol), whilst two products can be obtained by benzyl alcohol oxidation—benzaldehyde and benzoic acid. Benzaldehyde is one of the most important aromatic molecules applied in the cosmetic, perfumery, food, and pharmaceutical industries [8]. Thus, the development of selective routes for aldehyde synthesis using low-cost and greener reagents is highly desirable. The biphasic system enables a high selectivity for benzaldehyde under a wide range of experimental conditions (Table 4 ). The catalytic reaction proceeds in the organic phase, and the benzaldehyde is transferred to the organic phase avoiding further oxidation by sodium hypochlorite, a strong oxidant.Grill and co-workers [5] used commercial bleach (≈5% aqueous sodium hypochlorite) and a nickel salt to convert benzyl alcohol directly to benzoic acid achieving 89% yield, with or without an organic solvent (dichloromethane). On the other hand, the oxidation of 1-phenylethanol reached only 33% after 4 h of reaction using dichloromethane as solvent. Mirafzal and Lozeva [53], using ethyl acetate as the solvent, achieved a 93% yield for the aldehyde when tetrabutylammonium bromide was applied as a phase transfer catalyst (PTC) to improve benzaldehyde selectivity. The authors concluded that in the absence of the quaternary ammonium salt, little or no reaction was evident. Okada et al. [54] have used NaOCl⋅5H2O crystals as the oxidant to convert benzyl alcohol to benzaldehyde with a 99% yield. However, the reaction was performed in dichloromethane using 1 mol% TEMPO and 5 mol% Bu4NHSO4. Similar results were also obtained by Abramovici et al. [55], and Lee et al. [56,57] using sodium hypochlorite as oxidant, a water-immiscible solvent, and a phase transfer catalyst (PTC); Vitaku and Christie [26] used bleach as oxidant, ethyl acetate as the solvent, and NaHSO4 as an acid source, and Fukuda et al. [45] using NaOCl with an imide compound-nitroxyl radical catalyst system. Fig. 7 shows the proposed process for benzyl alcohol and 1-phenylethanol oxidation by NaOCl using the borophosphate glass as the catalyst. Based on the compound distribution in the organic and aqueous phases (Table 3), the pathway for the catalytic process is composed of six steps. First, benzyl alcohol or 1-phenylethanol dissolves in acetonitrile (1). After the oxidant (NaClO) addition and the formation of the biphasic system, a small amount of alcohol is transferred to the aqueous phase (2). During the reaction, the glass catalyst is dispersed in the aqueous phase where BnOH (3) and BnEtOH (5) oxidation occurs. The products are transferred back to the organic phase (6). As a consequence of a small solubility of benzaldehyde in the aqueous phase, benzoic acid is formed only as a by-product of benzaldehyde oxidation by NaOCl (4).To test the proposed mechanism, considering that benzaldehyde is more reactive (it undergoes autoxidation under exposure to air at room temperature) [9] and susceptible to oxidation than benzyl alcohol [1] (that is considered an intermediary on benzyl alcohol oxidation by NaOCl to benzoic acid) [5], the BnCHO susceptibility to oxidation was evaluated applying the standard reaction conditions described in Fig. 8 . Initially, benzaldehyde is dissolved in acetonitrile (1). After the addition of NaOCl and the formation of the biphasic system, a small amount of BnCHO is transferred to the aqueous phase (2). where its oxidation to benzoic acid took place (3). Even being more reactive than BnOH, BnCHO conversion achieved only 24.4 mol%. At the end of the reaction, the medium was centrifuged, and compound distribution was determined in both phases (values % described in Fig. 8). Benzaldehyde is present mainly in the organic phase, whereas 0.5% (3.27 × 10−3 mmol) is present in the aqueous phase. The concentration of benzoic acid in the organic phase is 17.4% (2.68 × 10−2 mmol), whereas 82.6% (1.28 × 10−1 mmol) of benzoic acid is present in the aqueous phase.In this sense, whilst the solubility of BnOH in water is 40 g L−1 (25 °C) [7], the solubility of BnCHO is 4 g L−1 (20 °C). Thus, the BnOH is transferred to the aqueous phase to react with −OCl, and the BnCHO, once formed, is transferred to the organic phase (ACN), avoiding further oxidation by −OCl, which results in high selectivity. Following the same pattern, the water solubility of 1-phenylethanol and acetophenone at 25 °C is 1.95 g L−1 and 6.1 g L−1, respectively [58]. Therefore, we can infer that even with ketone being more water-soluble than the secondary alcohol, the reaction occurred with satisfactory yields, and acetophenone is transferred to ACN once formed, Table 3.The application of sodium hypochlorite as the oxidant for aldehydes/alcohols/amines conversion using phase transfer catalysis (PTC) is performed with a water-insoluble solvent (usually a chlorinated solvent) [53,55,56,59] and a quaternary ammonium salt, whose function is transporting −OCl to the organic phase [56]. In these cases, the oxidation mostly occurs in the organic phase, whereas in the mechanism of Fig. 7 the reaction takes place in the aqueous phase without quaternary ammonium salt for phase transference. In our reaction condition, no trace of −OCl was determined by iodometric titration (Supplementary Information Section 2) in the corresponding organic phase, supporting this statement.The effectiveness of −OCl in the reaction is pH-dependent, being less effective and slower whereas the pH increases from 8 to 13 [3,46,55,59]. At the same time, pH lower than 7 has the opposite trend due to the fast decomposition of the hypochlorite solution [55]. Borophosphate glass reduces the pH of the solution from ≈13 (the original pH of NaOCl) to ≈ 9. The pH decreasing enables the formation of HOCl and, consequently, the oxidation [3,45]. Catalytic reactions were evaluated without the glass catalyst adjusting the pH of NaOCl solution to 9.00 ± 0.05 with HCl and NaOH. Fig. 9 shows the conversions obtained by glass-catalyzed reactions and uncatalyzed reactions (without and with pH adjustment). The pH decrease of NaOCl solution from 13 to 9 increases the conversion from 0.6 mol% to 12.4 mol% to 50.5 mol% and 44.7 mol% for BnEtOH and BnOH, respectively, whereas the glass-catalyzed reactions reach conversions of 87.0 mol% for BnEtOH and 78.5 mol% for BnOH.The pH reduction of NaOCl solution in the glass-catalyst reactions occurred due to the controlled release of the catalyst during the reaction. The ICP-OES analysis (Supplementary Information, Table S2) demonstrated the presence of P in the aqueous phase, with values varying between 6 and 15 wt% depending on the test. The in situ release of phosphate groups reduces the pH and activates the oxidant, i.e. NaOCl. The Al3+ ion released to the aqueous phase was negligible (maximum value 0.14 wt%). Furthermore, the role of phosphate-based glasses for catalytic purposes [32,35] is demonstrated by the ineffectiveness of four commercial silica-based glasses evaluated (Supplementary Materials Table S3 entry 1–4).To evaluate the efficacy of glass-catalyst recycling, the catalyst was recovered by centrifugation, cleaned, dried, and applied again as a catalyst for benzyl alcohol and 1-phenylethanol oxidation by NaOCl. Fig. 10 shows the glass-based catalyst performance after three cycles for both reactions.The reuse of the glass-based catalyst can be accomplished without a significant decrease in the conversions, mainly between the first and the second cycle of reaction. The conversions reduce just 14.5% and 25.8% for BnOH and BnEtOH, respectively, in the third cycle. In this sense, even with a partial glass release during the process, the catalysis remains with high selectivity.In addition to the glass-catalyst recycling, we have tested acetonitrile recovery by applying two daily common and simple laboratory procedures of solvent recovery—fractioned distillation and vacuum distillation. The recovered acetonitrile was applied as the solvent in a new cycle of BnOH and BnEtOH oxidation, Supplementary Information Table S5. The reaction where ACN recovered by fractioned distillation was used as solvent achieved good conversions after 3 h of reaction (57.6 mol% for BnOH and 58.8 mol% for BnEtOH). In addition, the acetonitrile recovered by fractioned distillation presented only traces (below the HPLC quantification limit) of both alcohols and the reaction products (Supplementary Information Table S4). These results demonstrated that not only the solvent can be easily recovered by a simple method and recycled in a new reaction cycle, but also the reaction products can be easily isolated from the solvent.Besides the high benzaldehyde selectivity, relatively mild reaction conditions, and the possibility of catalyst and solvent recovery, our proposed reaction also allowed the simultaneous oxidation of benzyl alcohol and 1-phenylethanol, as can be seen in Supplementary Information Fig. S1. The oxidation of both alcohols was concomitant and allowed good conversions (BnEtOH 79.6 mol% and BnOH 71.2 mol%), in addition to exempting the need for isolated reactions if a mixture of aldehyde and ketone is required as the product.Some common organic solvents were combined with NaOCl 11 wt% to investigate the salting-out formation: acetone, dimethyl sulfoxide (DMSO), and isopropyl alcohol do not form the biphasic system; on the other hand, it was formed with ethyl acetate and dimethyl carbonate (DMC). Based on this, ethyl acetate was tested as a solvent (see Supplementary Information Table S3 entries 5–6) instead of acetonitrile, but lower conversions were achieved for both alcohols—BnOH 28.8 mol% and BnEtOH 17.3 mol%—compared to reactions that have used ACN as solvent. The same trend was observed for DMC, resulting in similar conversions—BnOH 28.7 mol% and BnEtOH 13.8 mol% (see Supplementary Information Table S3 entry 7).The depolymerization of phosphate chains with the addition of 10 mol% Al2O3 in borophosphate glass (NaH2PO4/H3BO3 ratio = 2) is associated with the formation of phosphate-aluminum structures that cross-link to each other, enhancing the glass-network strength, increasing the chemical resistance, and making this glass moisture resistant, which makes this material attractive for applications in various fields. In this sense, borophosphate glass effectively catalyzed benzyl alcohol and 1-phenylethanol oxidation by aqueous 11 wt% sodium hypochlorite using acetonitrile as a solvent, under mild conditions. The reaction conditions oxidant amount, temperature, mass, and particle size of the catalyst were screened to achieve high alcohol conversions (87.0 mol% for 1-phenylethanol and 79.4 mol% for benzyl alcohol) and benzaldehyde selectivity above 95%. The addition of NaOCl to the reaction results in a biphasic organic-aqueous system. HPLC analysis allowed us to infer that a small amount of the alcohol is transferred to the aqueous phase, where it is oxidized. Once formed, benzaldehyde and acetophenone are transferred back to the organic phase. The formation of the biphasic system prevents benzaldehyde oxidation, even employing a strong oxidant, and can be an interesting option for processes that look for intermediary compounds. Furthermore, the proposed biphasic system exempted the use of PTC. The ICP-OES analysis allowed us to infer that the catalytic activity of borophosphate glass occurs due to partial liberation of phosphate-based groups to the aqueous oxidant, reducing the pH of NaOCl from 13 to 9, which enables the formation of HOCl and, consequently, the oxidation. Based on this, the use of phosphate-based glasses as the catalyst is something unexplored and promising: it is easily and quickly produced with low-cost raw material, and its chemical properties can be modified according to the application required, i.e. control the dissolution during the reaction, it can be used as a host for several metal ions, and as an active material for supported nanoparticles applied in catalysis.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Ricardo Schneider would like to acknowledge Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for funding (grant 422774/2018–9). Jorlandio F. Felix acknowledges the CNPq (grant number: 430470/2018–5 and 309610/2021-4) and Fundação de Apoio a Pesquisa do Distrito Federal(FAPDF) (grant number: 193.001.757/2017), for financial support.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmrt.2022.05.105.

The oxidation of primary and secondary alcohols to their respective aldehydes/ketones is one of the most important reactions in fine chemistry due to the industrial application of these products. Based on this, a large number of new catalysts and oxidants have been tested using this reaction as a catalytic model, mainly looking for a process that ensures high aldehyde selectivity. In this paper, we have used moisture stable borophosphate glass doped with 10 mol% Al2O3 as a heterogeneous catalyst in the oxidation of sodium hypochlorite, an effective, greener, and low-cost oxidant, using acetonitrile as solvent under mild conditions. The glass catalyst mass and the particle size were evaluated, as were the reaction temperature and oxidant amount, to determine the ideal reaction conditions where the conversions achieved 87.0 mol% for 1-phenylethanol to acetophenone and 79.4 mol% for benzyl alcohol to benzaldehyde, with benzaldehyde selectivity above 95%. Although sodium hypochlorite is a strong oxidant, benzaldehyde was the main product of the oxidation of benzyl alcohol due to the formation of a biphasic organic-aqueous system that protects the aldehyde from oxidation and allows the reaction to occur without the use of a phase transfer catalyst (PTC). HPLC analysis of both phases showed that alcohols, aldehyde, and ketone were mostly present in the organic phase (concentrations above 98.7%). During the reaction, a small amount of alcohol is transferred to the aqueous phase, where the oxidation took place. Once formed, the products are transferred back to the organic phase. ICP-OES analysis indicates that borophosphate glass acts in the reaction by partially releasing phosphate-based groups, reducing the pH of hypochlorite to 9. In this sense, borophosphate glasses prove to be a simple and inexpensive alternative for the development of new catalysts.

catalyst to oil weight ratiofinal boiling pointfluid catalytic crackingheavy cycle oilinitial boiling pointlight cycle oilliquefied petroleum gasesmunicipal solid wasteplastic pyrolysis oilmean squared errorvacuum gasoilcorrelation coefficientmolar concentration (mol cm-3)apparent activation energy (kJ mol-1)certain lumpcertain reactionapparent kinetic parameter (m6 kgcat -1 kmol-1 s-1 / m3 kgcat -1 s-1)catalyst deactivation kinetic parameter (s-1)reaction ordermolecular weight of HCO lump (g mol-1)mass of catalyst (g)mass of PPO (g)number of experimentsnumber of lumpsnumber of parameterscertain experimentreaction rateideal gas constant (8.314 J mol-1 K-1)contact time (s)reaction temperature (°C)reference temperature (°C)volume of the reactor (cm3)weight fractionlevel of significanceactivity termstoichiometric coefficientdegrees of freedomThe development and wellness of the humankind implies an increase of global pollution. One of the consequences is the increasing presence of waste plastics in the municipal solid wastes (MSW), which overflows the management capability of both public and private entities. Consequently, an unacceptable amount of these wastes ends up landfilled, causing the contamination of the soils and the aquifers [1]. In order to solve these problems, it is well established the interest on tertiary recycling by means of thermochemical processes, i.e. pyrolysis and gasification [2].The fast pyrolysis of plastics is performed at low temperature, using high heating rates and short residence time for the volatiles. Moreover, it can be carried out in simple and versatile units equipped with different types of reactors (rotary kilns, screw reactors, fluidized or spouted beds, etc.) entailing a reduced environmental impact and with the possibility of tuning the operating conditions to adapt the production to the type of plastic fed [3,4]. The ideal goal of pyrolysis processes is the monomer recovery, which can be done with high yields in the pyrolysis of polystyrene [5] and polymethyl-methacrylate [6]. On the other hand, for polyolefinic plastics, which constitute two thirds of the plastic fraction found in the MSW [7], is of great interest the production of plastic pyrolysis oil (PPO) because of its possibilities to be used as an alternative fuel [8].Based on its properties, the PPO has been considered as a potential fuel for diesel engines feeding it neatly or blended with commercial diesel [9]. Nevertheless, the PPO does not meet the tough requirements of commercial fuels and requires of physicochemical treatments to adapt its composition [10]. This situation has led to the proposal of integrating the fast pyrolysis of waste plastics with the upgrading of the PPO in refinery units (Waste-Refinery) [11]. The interest of the proposal lays on the capacity of refinery units for valorizing the PPO, either in ad hoc catalytic units or in already existing industrial units. The fluid catalytic cracking (FCC) units are the most appropriate ones in the short term, given their high capacity and versatility to manage unconventional feeds, such as diverse secondary refinery streams [12,13] or bio-oil [14]. Indeed, the chemical composition of the PPO (highly olefinic and free of aromatics) makes it appropriate to be fed to catalytic cracking units with the aim of producing fuels free of sulfur and nitrogen [15]. Furthermore, within the facilities available in the refineries, there are the required fractionation and conditioning units to obtain fuels similar to conventional ones. Among the advantages that the Waste–Refinery strategy offers, the following ones must be highlighted: (i) the recycling of petroleum-derived products with the subsequent savings of raw materials; (ii) the removal of economic barriers that entails the design and construction of new units, which correspond to the high cost of the equipment and of the marketing of non-conventional fuels that would compete against the conventional fuels; and, (iii) the rational organization of the plastics recycling, carrying out the pyrolysis process in a delocalized way in units located nearby of the waste plastics collection and segregation points. The PPO would be afterwards transported from different geographical areas to centralized refineries for its large-scale valorization. Feeding a liquid stream, such as the PPO, into a cracking unit, entails less technical difficulties that the feeding of pure polyolefins, the cracking of which has been also studied [16–18]. Nonetheless, these initiatives will require a rigorous control of the feeds, since their composition can be easily contaminated by the presence of different plastics and of additives and pollutants in the waste plastics.In a previous work it has been studied the effect of the properties of different FCC equilibrium catalysts on the production of fuel from PPO, operating at 500–560 °C and using a riser simulator reactor [19]. Interestingly, the yields of naphtha (highly olefinic and with a high octane rating) and light olefins were superior to 40 and 12 wt%, respectively. The proposed initiative is similar to that of cracking wax from Fischer-Tropsch process with the aim of producing high octane gasoline and light olefins [20,21].Both for the simulation and optimization of an ad hoc designed reactor and for the feeding of the PPO to an industrial FCC unit, it is required a kinetic model capable of quantifying the products distribution. Traditionally, the efforts in the kinetic modeling have been focused on the cracking of vacuum gasoil (VGO). Moustafa and Froment [22] were pioneers in taking into account the heterogeneous composition of the VGO and they proposed a kinetic model with a complex reaction scheme that described the individual reactions involved and the formation of coke by means of elementary steps. This type of molecular-level kinetic models has been also applied for the cracking of wax from Fischer-Tropsch process [21]. Nevertheless, most of the works have established lump-based kinetic models that simplify the computing and their posterior use in the design of the reactor [23–25]. Apart from the complexity of the reaction scheme, an additional difficulty for obtaining kinetic models is the extremely fast deactivation of the catalyst caused by coke deposition [26,27]. Kinetic models for the catalytic cracking of VGO consider between 3 and 17 lumps and have been collected by different authors [28,29]. These models assume that kinetic parameters are apparent values as a consequence of the diffusional restrictions caused by the components of the VGO (especially the heavier ones) [30].In this work, it has been established a six-lump based kinetic model for the cracking of PPO obtained in the pyrolysis of high-density polyethylene from the experimental data obtained in a previous work [19]. The aim of the work is to provide a tool for quantifying the effects of the operating conditions on the yields of products of interest, such as fuels (gasoline and diesel) and commodities (light olefins). In the modeling, it has been taking into account the catalyst deactivation by coke deposition, which is extremely significant in cracking reactions. Moreover, the analysis of the kinetic parameters obtained for three different FCC equilibrium catalysts with different acidity and porous structure allows for assessing the effect of these properties on the different catalytic steps and on the formation of coke.The plastic pyrolysis oil (PPO) has been obtained at 500 °C under fast pyrolysis conditions by feeding virgin high-density polyethylene (HDPE) to a fountain confined conical spouted bed reactor [31].Three different commercial equilibrium FCC catalysts (ECAT-1, ECAT-2 and ECAT-3) have been used in the work. The catalysts have been collected from the catalyst purge stream of industrial FCC units, specifically ECAT-1 from Petronor Refinery (Spain) and the other catalysts from Petrobras Refinery (Brazil). Consequently, they are equilibrium catalysts, since they have been submitted to numerous cycles composed of reaction, stripping and regeneration steps in their corresponding FCC units [32].The catalytic cracking runs have been performed on a laboratory scale micro-riser reactor, specifically designed to mimic the conditions of the riser reactor of industrial FCC units [33]. A schematic representation of the experimental unit together with an explanation of the experimental procedure can be found elsewhere [34]. The operating conditions of riser simulator reactor have been: temperature, 500, 530 and 560 °C; catalyst to oil weight ratio (C/O), 3–7 gcat gPPO -1; and contact time, 1.5–6 s.The catalytic cracking of the PPO has been described by means of a six-lump reaction network. The six lumps are heavy cycle oil (HCO, C20+), light cycle oil (LCO, C13-C20), naphtha (C5-C12), liquefied petroleum gases (LPG, C3-C4), dry gas (C1-C2) and coke (carbonaceous material deposited on the catalyst). The reaction network in Fig. 1 a corresponds to parent reaction network and it accounts for sixteen kinetic parameters, to which it must be added a parameter for catalyst deactivation.The kinetic modeling methodology used is based on the one developed by Toch et al. [35] for catalytic processes with complex pathways and on that proposed by Cordero-Lanzac et al. [36,37], since they included the catalyst deactivation on it. Furthermore, the methodology has been adapted for handling the experimental data obtained in a batch reactor. Likewise, a molar balance to the micro riser reactor has been also required in order to properly describe the behavior of the different lumps [38]. According to the reaction network in Fig. 1a, the reaction rate equations that describe the evolution with contact time of the different lumps are listed below. (1) dy HCO dt = - φ m PPO M HCO V k 1 + k 2 + k 3 + k 4 + k 5 y HCO 2 m cat V (2) dy LCO dt = φ m PPO M HCO V k 1 y HCO 2 - k 6 + k 7 + k 8 + k 9 y LCO m cat V (3) dy Naphtha dt = φ m PPO M HCO V k 2 y HCO 2 + k 6 y LCO - k 10 + k 11 + k 12 y Naphtha m cat V (4) dy LPG dt = φ m PPO M HCO V k 3 y HCO 2 + k 7 y LCO + k 10 y Naphtha - k 13 + k 14 y LPG m cat V (5) dy Dry Gas dt = φ m PPO M HCO V k 4 y HCO 2 + k 8 y LCO + k 11 y Naphtha + k 13 y LPG - k 15 y DryGas m cat V (6) dy Coke dt = φ m PPO M HCO V k 5 y HCO 2 + k 9 y LCO + k 12 y Naphtha + k 14 y LPG + k 15 y DryGas m cat V being yi the weight fraction of lump i, t the contact time between the reactants and the catalyst in the reactor, MHCO the molecular weight of the lump HCO, V the volume of the reactor, kj the apparent rate constant of reaction j, mPPO the mass of PPO fed and mcat the mass of catalyst used.One should observe that the catalytic cracking of the different lumps has been described using irreversible first-order reactions, with the exception of the cracking of HCO lump, which has been considered as an irreversible second-order reaction [28,39]. Additionally, it has been assumed that cracking reactions are non-selectively affected by catalyst deactivation and it has been quantified in Eqs. (1)-(6) by using the same activity term (φ), which has been defined as: (7) φ = ( - r j ) ( - r j ) 0 = exp - k d t where (−rj) and (−rj)0 are the reaction rates of each step of the reaction network at t time and zero time, respectively, and kd is the deactivation parameter.The equation proposed for explaining the deactivation kinetics corresponds to a first-order exponential function, which is effective for describing the activity decay in the cracking reactions where a notably deactivation occurs for short contact times (<20 s) [40].For computing the kinetic parameters, they have been expressed as a function of temperature by means of the reparameterized Arrhenius equation in order to avoid the regression issues derived from the strong correlation between the activation energy and the pre-exponential factor. (8) k j = k j * exp - E j R 1 T - 1 T * being kj* the kinetic reaction rate of the j reaction step at the reference temperature T* (500 °C), Ej the corresponding apparent activation energy and R the universal gas constant.The system of differential equations that describes the catalytic cracking of PPO, Eqs. (1)-(6), has been solved using an in-house written MATLAB code. The code estimated the required kinetic rate constants and activation energies to fit the weight fraction of the different parameters to those experimentally obtained. To find the best fitting values, a loss function in which the mean squared error is minimized has been employed. (9) Loss Function = 1 n e · ∑ 1 n l ∑ 1 p y i,p cal - y i,p exp 2 where yip is the weight fraction of lump i for experiment p, nl is the total number of lumps and ne the total number of experiments. Moreover, superscripts “cal” and “exp” denote the calculated and experimentally determined weight fractions, respectively.The discrimination between the different models proposed has been performed by means of a statistical significance test based on Fisher’s method. The procedure is well explained in the literature [41]. In brief, for two kinetic models with different degrees of freedom (νA ≠ νB) if model B shows a smaller mean squared error than model A (SSEB < SSEA), the improvement offered by model B with respect to model A will be statistically significant when the following condition is fulfilled: (10) F A - B = SSE A - SSE B SSE B ν A - ν B ν B > F 1 - α ν A - ν B , ν B being F1-α the critical value of the Fischer distribution function for a level of significance of 95% (α = 0.05). The degrees of freedom have been computed according to the following equation by taking into account the number of experiments (ne), number of lumps (nl) and number of parameters (np): (11) ν = n e · n l - n p The main properties of the PPO are provided in Table 1 . It consists of a mixture of hydrocarbons with a broad distillation range that can be divided into 82.0 wt% of HCO (heavy cycle oil), 12.5 wt% of LCO (light cycle oil) and 5.5 wt% of naphtha. These fractions have been defined according to the usual criteria followed by oil refiners: naphtha (C5–C12), LCO (C13–C20) and HCO (C21+) [42]. Additionally, the chemical composition of the PPO obtained by chromatographic means has been already reported in our previous work [43]. Briefly, they are composed of 67.6 wt% of olefins and 32.4 wt% of paraffins.Even though a descriptive characterization of the catalysts has been already reported in a previous work [44], their main properties are shown in Table 2 . In order to compare the properties of these industrial catalysts it must be taken into account its complex configuration, which is composed of an ultrastable Y zeolite (USY) embedded in a meso- and macroporous matrix (consisting of a mixture of clay, silica and alumina) [45]. The highest content of zeolite of ECAT-2 (21 wt%) is in concordance with its high micropore surface area (139 m2 g-1). ECAT-3, in turn, has the highest matrix/USY zeolite ratio, which turns into the highest mesopore surface area and mesopore volume (111 m2 g-1 and 172 cm3 g-1, respectively). This way, its wide porous structure increases the accessibility of the NH3 to the acid sites, making ECAT-3 the catalyst with the highest total acidity (124 μmolNH3 g-1), acid strength (130 kJ molNH3 -1) and Brønsted/Lewis acid sites ratio (1.56). Furthermore, the high mesopore volume of the matrix will reduce diffusional constraints of the long chains of PPO, easing its access to the external crystal surface of zeolites and, consequently, its posterior cracking on the channels of the zeolite.It is also remarkable the presence of rare earths in the catalysts, especially for ECAT-1 (2.50 wt%), since these elements increase the selectivity to naphtha lump [46]. The presence of P2O5 (with a maximum value of 0.62 wt% for ECAT-1) aims the formation of light olefins. Metals, such as V, Fe and Ni, are irreversibly deposited on FCC catalysts in the successive reaction-regeneration cycles acting as poisons and causing a reduction in throughput by increasing coke formation [47].In order to validate the model proposed, the results obtained with ECAT-1 for the parent reaction network (Fig. 1a) are shown below. The values of the apparent kinetic rate constants and activation energies that have minimized the mean squared error of the loss function (Eq. (9)) have been collected in Table 3 . The values of the parameters provide a large amount of information about the relevancy of the different catalytic steps. This way, it can be seen that the steps that govern the catalytic cracking of PPO are those in which the cracking of HCO fraction is involved (steps #1 to #5 in Fig. 1a). The highest crackability of the compounds within the HCO fraction is a common result obtained in the catalytic cracking of hydrocarbon streams and it is coherent with the higher crackability of the high-molecular weight olefins [25,48]. However, the values of some of the kinetic rate constants, in particular those corresponding to steps #8 to #12, are so small that the contribution of these kinetic steps can be considered negligible.Therefore, three alternative reaction networks have been proposed (Fig. 1b, c and d) in which various simplifications have been made based on the results collected in Table 3. This way, in alternative network 2 (Fig. 1b) the naphtha fraction has been considered as a final product, i.e. steps #10, #11 and #12 have been removed from the parent network (Fig. 1a). In alternative network 3 (Fig. 1c), in turn, the steps removed have been those in which LCO fraction is converted into coke and into dry gas fractions (steps #8 and #9). Finally, alternative network 4, which is the simplest one from all the proposed, ignores all the routes removed in alternative networks 2 and 3. Thus, in alternative network 4 (Fig. 1d) steps #8 to #12 have been removed from the parent one (Fig. 1a).Consequently, the fitting of the experimental data obtained for the catalytic cracking of PPO with ECAT-1 has been also performed for the alternative reaction networks. Overall, good fitting results have been obtained for all of them. Hence, in order to perform an appropriate discrimination between the four networks, the statistical significance test based on Fisher’s method described in Section 3.3 has been applied. The results obtained have been tabulated in Table 4 . Attending to the statistical parameters, it can be seen that the number of experiments and lumps is the same for all the networks, but the number of parameters varies following the trend: np,1 > np,3 > np,2 > np,4. Consequently, the degrees of freedom of the different reaction networks follows just the opposite order. On the other hand, the lowest value for the sum of squared errors has been obtained for scheme 3 (5.844 10-3), whereas the values obtained for the other networks are slightly higher and follow the trend: SSE2 < SSE1 < SSE4. Therefore, since alternative reaction network 4 is the simplest one (less amount of parameters) and the worst fitting has been obtained with it, this one has been taken as reference for the statistical comparison. Thus, it has been assessed if the addition of more catalytic steps is statistically significant. It has been obtained that F4–1 < F1−α (0.841 < 2.259), F4–2 < F1−α (1.569 < 3.040) and F4−3 < F1−α (1.569 < 2.649), meaning that neither parent network nor alternatives 2 and 3 improved in a statistically significant way the fitting of alternative scheme 4.Based on all the previous, the alternative reaction network 4 has been also used for the fitting of the data obtained with both ECAT-2 and ECAT-3. The goodness of fit has been evaluated using parity plots (Fig. S1 in the Supplementary Material) by evaluating the final fit of calculated data for the three catalysts against raw experimental data. As it can be seen, almost a perfect fit between the calculated and the experimental weight fraction has been obtained for all the catalysts, with the exception of the scattering of some points, especially for ECAT-3. Nevertheless, those deviations do not exceed the 5% as they remain inside the region delimited by the dashed lines.In Table 5 have been collected the values computed for the apparent kinetic parameters and the activation energies of the kinetic steps involved in the catalytic cracking of PPO by the three catalysts. Overall, small differences have been obtained in the values of the kinetic parameters with all the catalysts. These differences lie in the properties of the catalysts, considering the effect of the acidity and porous structure on the activity, selectivity and deactivation of the catalysts [49]. This way, ECAT-2 has the highest value for the deactivation parameter (0.030 s-1) because of its moderate mesopore surface area (50 m2 g-1) and mesopore volume (147 cm3 g-1), which are not enough for easing the diffusion of coke precursors towards the external surface of catalyst particles. Likewise, the confinement of the precursors will block the micropores of the zeolite resulting in the ineffectiveness of its high content of zeolite [50]. It is well-established the role as coke precursors of light olefins in cracking processes, as they undergo oligomerization, aromatization and condensation reactions that are catalyzed by strong acid sites [51,52]. In the same line, do stand out the rate constants of the reactions that form coke from LPG and dry gas fractions (2.8 10-3 and 1.0 10-4 m3 kgcat -1 s-1, respectively) using ECAT-2.In contrast, the lower zeolite/matrix ratio of ECAT-3 that entails a higher mesopore surface area (111 m2 g-1) and a higher mesopore volume (172 cm3 g-1), will improve the diffusion of coke precursors, attenuating their confinement and, consequently, the blockage of the micropores. Moreover, the enhanced accessibility and diffusion of PPO chains to the active sites in ECAT-3 are in concordance with the high values of the kinetic parameters for the reactions that convert the components within the HCO lump into dry gases (1.16 m6 kgcat -1 kmol-1 s-1) and the components within the LCO lump into naphtha (1.0 10-3 m3 kgcat -1 s-1). The similarities among the rest of the kinetic parameters for the different catalysts lie in the synergistic and parallel effects of the porosity and acidity that boost the extent of the cracking reactions.The values computed for the apparent activation energy of the different catalytic steps have been collected in Table 6 . Unlike the kinetic parameters (Table 5), significant differences are observed between the activation energy required in some of the catalytic steps for the different catalysts. Likewise, the energy barrier that must be overcome for the deactivation step is very different depending on the catalyst used. The lower activation energy of the deactivation stage (79.2 kJ mol-1), added to the high value of the deactivation kinetic parameter obtained (3.0 10-2 s-1 in Table 5) expose the high tendency of ECAT-2 to be deactivated. Equally, the high amount of acid sites on ECAT-3 explains the low activation energy required for the steps of formation of dry gas from HCO and LPG lumps (17.1 and 40.0 kJ mol-1, respectively). In addition, the high matrix mesoporosity of ECAT-3, which is the other key feature of the catalysts, reduces the activation energy of the steps limited by the diffusivity of the components. This way, this catalyst reduces the energy involved in the steps that convert the HCO in LCO, LCO in naphtha and LCO in LPG (60.5, 42.5 and 58.3 kJ mol-1, respectively), as well as in the formation of coke from LPG and dry gas (4.4 and 40.7 kJ mol-1, respectively).The evolution of the yields of products with high commercial interest as fuels (LCO and naphtha) and with high content of olefins (LPG) has been obtained for the three catalysts by computing the kinetic model previously described and using the kinetic parameters collected in Tables 5 and 6. One should note that the PPO fed to the reactor has a content of 12.5 wt% of LCO and a 5.5 wt% of naphtha, as it has been previously detailed in Section 4.1. Those contents have not been taken into account for depicting the evolution of the yields, in order to assess the formation of these lumps in its real magnitude. On the other hand, in order to fully understand the obtained results, it should be taken into account that conversion has been defined as the ratio of mass of HCO converted to lighter products and to coke to the mass of HCO fed: (12) Conversion = ( m HCO ) PPO - ( m HCO ) Products ( m HCO ) PPO 100 The yield of each i lump of products has been defined as the mass of lump i referred to the total mass of lump HCO fed: (13) Yield i = m i m HCO 100 Therefore, Figs. 2-4 compare the evolution with conversion of the yield of LCO, naphtha and LPG, respectively, obtained with the three catalysts at different temperatures. In a previous work [19] it has been detailed the composition of these lumps, stressing out the interest of the naphtha lump (research octane number up to 105) for being added to the stream of gasoline in refinery. It is also remarkable the propylene-rich LPG lump produced.The trend of the curves in Fig. 2 exposes the character of the LCO lump as an intermediate in the reaction network [53], as they go through a maximum at values of conversion of ca. 80 wt%. Furthermore, it can be seen that high temperatures promote the cracking reactions that convert the molecules within this lump into lighter molecules, resulting in lower yields of LCO. Comparing the results obtained with the three catalysts, similarities are observed between the results obtained. This way, with ECAT-1 and ECAT-2 higher values than with ECAT-3 are obtained, yielding up to 46.5 wt% with the former catalysts at 500 °C.Attending to the evolution of the yields of naphtha and LPG lumps (Figs. 3 and 4, respectively), both are end-products in the reaction network since their yield increases continuously with the extent of conversion. Nonetheless, in spite of the evolution obtained for LPG lump (Fig. 4), the molecules within this lump are cracked to dry gas and condensed to coke as it has been previously obtained in the reaction network (Fig. 1d). With regard to the evolution of the yield of naphtha (Fig. 3), high temperatures promote the production of this lump, since the cracking of molecules within HCO and LCO lumps is boosted. Furthermore, higher yields of naphtha have been obtained with ECAT-3, yielding up to 33.6 wt% at 560 °C. However, the maximum values obtained with ECAT-2 and ECAT-1 have been slightly inferiors (31.1 and 29.6 wt%, respectively).The evolution of the different yields (Figs. 2-4) strongly depends on the properties of the catalyst used (Table 2) and can be correlated with the values of the apparent kinetic parameters reported on Table 5. This way, ECAT-3 is by far the catalyst with the highest and strongest acidity (124 μmolNH3 g−1 and 130 kJ molNH3 -1, respectively), which turns into the catalyst with the highest cracking activity. Moreover, it is the catalyst with the highest mesoporosity that eases the access of the bulky molecules within the HCO and LCO lumps to the acid sites located in the inside of the porous structure of the catalyst. Therefore, the highest yields of naphtha and LPG, together with the lowest yield of LCO should be expected using this catalyst.Even though ECAT-2 possesses a lower amount of acid sites available (81 μmolNH3 g−1), their strength is quite remarkable (126 kJ molNH3 -1) making a priori this catalyst a serious candidate for maximizing the yield of naphtha and LPG lumps. However, its microporous nature and its, subsequent, shortness in mesopores are unsuitable for boosting the access of the heavy molecules to inner acid sites. Consequently, the behavior of ECAT-2 is only comparable with ECAT-3 at 560 °C as an increase in temperature increases the diffusivity [54]. Nonetheless, ECAT-2 promotes the formation of LPG instead of naphtha, which can be attributed to the overcracking reaction that takes place within the micropores of the zeolite as a consequence of the higher residence time of the reactants. Its low content of rare earths will also presumably contribute to obtain the aforementioned results [46]. Furthermore, the narrower porous structure of ECAT-2 will lead to a faster activity decay of the catalyst.Finally, the configuration and composition of ECAT-1 are the less favorable ones to promote the cracking reactions. Indeed, ECAT-1 has the lowest superficial area (124 m2 g−1), the lowest acidity (40 μmolNH3 g−1) and the weakest acid strength (100 kJ molNH3 -1). In addition, the high concentration of impurity metals detected on ECAT-1, especially of vanadium (3335 ppm), will also contribute to deteriorate the properties of the catalyst. Consequently, slightly lower yields of both naphtha and LPG lumps (Figs. 3 and 4) have been obtained with ECAT-1.The selectivity to each lump i has been defined as the mass of lump i formed respect to that of all the products: (14) Selectivity i = m i m LCO + m Naphtha + m LPG + m DryGas + m Coke 100 Taking into account that naphtha and LPG lumps are the ones with the highest commercial interest, the evolution with conversion of the selectivity to them has been depicted in Fig. 5 . Overall, it can be seen how different the selectivity to each lump is. This way, the selectivity to naphtha lump is barely affected by the extent of conversion. The trend followed by the selectivity to naphtha curves depends on the catalyst. This way, it can be seen that with ECAT-1, which is the less active catalysts, the selectivity to naphtha remains almost steady for values of conversion below 80%, to increase exponentially at higher values. For ECAT-3, in turn, the growth can be noticed for values of conversion above 55%. Furthermore, the differences between the performances of the catalysts are more evident at high temperatures and high values of conversion. This way, the selectivity to naphtha has been maximized at 560 °C with ECAT-3 reaching a value of 35.8 wt%, whereas under the same conditions a selectivity of 31.5 wt% has been obtained with ECAT-1. ECAT-2, in turn, offers an intermediate result and a selectivity to naphtha of 32.8 wt%.In contrast, selectivity to LPG lump grows exponentially with conversion since the very beginning of the reaction. The effect of the temperature is less marked but the opposite in the case of the LPG. Likewise, an increase from 500 to 560 °C for a fixed value of conversion entails just a reduction of the selectivity to LPG of ca. 2.5 wt%. Focusing on the performance of the catalysts, ECAT-2 offers the highest selectivity at 560 °C but also the lowest at 530 and 500 °C. This result is characteristic of a partially deactivated catalyst, in which thermal cracking plays a more important role that in the case of ECAT-1 and ECAT-3.Attending to the results collected on Figs. 2-5, operating under the conditions that allow for reaching conversions levels within the range 60–80% would be the optimal considering the possibility of varying the reaction temperature between 500 and 560 °C. This way, the conversion of HCO would be promoted keeping under control the overcracking reactions that would lead to obtain too much dry gas. Furthermore, ECAT-3 should be the selected one for turning the production to LPG and naphtha lumps, whereas ECAT-1 and ECAT-2 would increase the yield of LCO lump in detriment to the yield of naphtha.Since catalyst deactivation has a notable impact on the results collected in Sections 4.4 and 4.5 about the yields and selectivity, the evolution of the activity term (φ) of the three catalysts with contact time at 500 °C has been plotted on Fig. 6 . One should note that these curves have been obtained by applying the previously proposed deactivation equation (Eq. (7)) and using the corresponding kinetic parameters (Tables 5 and 6). It can be seen that for a contact time of 6 s, all the catalysts maintain good activity levels as they are above of the 83% of the initial activity. This result is very different to that obtained in the cracking of VGO (benchmark feed in FCC) [55], where the catalyst was totally deactivated. This result exposes the crucial role that the composition of the stream fed to cracking reactor plays in catalyst deactivation and, therefore, in products yield and distribution. This way, the heterogeneity of the VGO, with high contents of aromatics and the presence of polyaromatics, is more prone to the formation of coke than the olefins that predominate in the composition of the PPO [43]. This low deactivation is an interesting result for adopting different cracking strategies for the PPO, such as being co-fed with other refinery streams that deactivate the catalysts in a large extent.Comparing the evolution followed by the catalysts, ECAT-1 and ECAT-3 show almost identical curves of activity vs. time as a difference of<1% for a contact time of 6 s (ca. 87%) has been obtained. However, ECAT-2 suffers from a higher and more severe activity decay since the very beginning of the reaction. Indeed, the final value for activity obtained for this latter catalyst is of 83.5%. Undoubtedly, the deactivation suffered by ECAT-2 lies in the porous structure of the catalyst, which is by far more microporous (Table 2) than the structure of the other catalysts. Consequently, the coke formed during the reaction will more easily block the channels of the zeolite reducing the accessibility of hydrocarbon species to the catalyst inner micropore network [56].To offer another perspective of the deactivation results, Fig. 7 depicts the evolution of the activity of the three catalysts with the content of coke deposited. Clearly, the amount of coke deposited on ECAT-2 is higher than that deposited on ECAT-1 or ECAT-3. Consequently, ECAT-2 suffers from a higher activity decay than the other catalysts. In spite of that, attending to the accelerated decrease of the activity obtained for all the catalysts, it can be concluded that the deactivation mechanism is highly affected by the micropore blocking caused by coke deposition. This phenomenon will also restrict the access of the reactants to the acid sites located in the inner crystals of the zeolite. This result is in concordance with the hypothesis of the key role of the matrix mesopores for attenuating the catalyst deactivation by delaying the aforementioned phenomenon.A lumped kinetic modeling method has been applied to the experimental data of the catalytic cracking of plastic pyrolysis oil (PPO) over three commercial FCC equilibrium catalysts. By means of a statistical data analysis, it has been obtained that from the four different reaction networks proposed, the simplest one was the most appropriate for describing the process. From the kinetic parameters obtained in the fitting of the results, it has been obtained that both total acidity and acid strength rule the cracking process, boosting the extent of the different reaction steps and modifying the distribution of the lumps of products. Furthermore, the mesoporous structure of the matrix is a key feature for reducing the diffusional restrictions and, subsequently, for maximizing the formation of the naphtha and LPG lumps. This way, the maximum yield and selectivity to naphtha of 33.6 and 35.8 wt%, respectively, have been obtained with ECAT-3 for a conversion value of 94%. In contrast, ECAT-1 and ECAT-2 promote the formation of LCO instead of naphtha.The deactivation of the three catalysts in the cracking of the PPO is by far lower than that obtained in the cracking of VGO (benchmark feedstock of FCC unit), because of the absence of aromatics in the PPO. Likewise, for a contact time of 6 s the catalysts keep a residual activity above the 80%. The lowest deactivation of ECAT-3 (kd = 2.3 10-2 s−1) has been related to the high mesoporosity of its matrix, which is appropriate for promoting the internal diffusion of coke precursors, attenuating the catalyst deactivation. This way, for this catalyst, the apparent activation energies of the conversion of heavy cycle oil (HCO) into light cycle oil (LCO), LCO into naphtha, and LCO into liquefied petroleum gases (LPG) are 60.5 42.5 and 58.3 kJ mol−1, respectively. In addition, those of the formation of coke from HCO, LPG and dry gas are 129.0, 4.4 and 40.7 kJ mol−1, respectively.The kinetic model proposed is an interesting tool for facing the manufacturing of reactors designed ad hoc for the catalytic cracking of PPO. Additionally, obtained results could also encourage the future co-feeding of this alternative and waste-derived feedstock to industrial FCC units commonly available in oil refineries. Nevertheless, the kinetic results could be modified by the presence of additives and pollutants in the waste plastics. Roberto Palos: Formal analysis, Conceptualization, Writing – original draft. Elena Rodríguez: Investigation, Formal analysis. Alazne Gutiérrez: Supervision, Methodology, Visualization. Javier Bilbao: Conceptualization, Writing – review & editing, Supervision, Project administration, Funding acquisition. José M. Arandes: Software, Resources, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work has been carried out with the financial support of the Ministry of Science, Innovation and Universities (MICIU) of the Spanish Government (grant RTI2018-096981-B-I00), the European Union’s ERDF funds and Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Actions (grant No 823745) and the Basque Government (grant IT1218-19). Dr. Roberto Palos thanks the University of the Basque Country UPV/EHU for his postdoctoral grant (UPV/EHU 2019). The authors also acknowledge Petronor Refinery for providing with the catalyst used in this work.Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2022.123341.The following are the Supplementary data to this article: Supplementary data 1

The kinetics of the catalytic cracking of plastic pyrolysis oil (PPO) over three FCC (fluid catalytic cracking) equilibrium commercial catalysts has been modeled. The PPO comes from the fast pyrolysis of high-density polyethylene (HDPE). The cracking runs have been carried out in a laboratory-scale reactor under FCC conditions: 500–560 °C; catalyst/oil weight ratio of 5 gcat gPPO -1; and contact time of 1.5–6 s. Four different reaction schemes composed of six lumps have been compared and it has been obtained by statistical means that the simplest one is the most appropriate for describing the process. The differences in the kinetic parameters have been related to the properties of the catalysts. Among them, total acidity and mesoporous structure have a key role. The former for promoting the cracking reactions and the latter for limiting the diffusional restrictions of both the bulky compounds within the PPO and the formed coke precursors. This way, ECAT-3 that is the most acid and most mesoporous catalyst, maximizes the yields of naphtha (33.6 wt%) and liquefied petroleum gases (LPG) (18.9 wt%). In contrast, ECAT-1 and ECAT-2 should be chosen for producing light cycle oil (LCO). For ECAT-3, the apparent activation energies of the conversion of heavy cycle oil (HCO) into light cycle oil (LCO), LCO into naphtha, and LCO into LPG are 60.5 42.5 and 58.3 kJ mol-1, respectively. In addition, those of the formation of coke from HCO, LPG and dry gas are 129.0, 4.4 and 40.7 kJ mol-1, respectively.

As energy crisis and environmental pollution are two major enemies for mankind, developing new energy sources and realizing carbon neutrality have become an urgent task in the 21st century. Hydrogen is considered to be the ultimate form of energy in the future because of its advantages such as high calorific value, pollution-free, and wide sources [1,2], but how to store and use it safely has always troubled everyone.Solid hydrogen storage, which is a promising hydrogen storage technology, owns the advantages of high density of hydrogen storage in weight and volume, moderate operating pressure and high energy efficiency [3,4], compared to high-pressure gas hydrogen storage with low hydrogen storage efficiency, poor safety and to low-temperature liquid hydrogen storage with high cost and large energy consumption [5]. In recent decades, many solid hydrogen storage materials have been discovered and studied, which can be divided into chemical hydrogen storage and physical hydrogen storage according to different hydrogen storage methods. Metal hydrides belong to chemical hydrogen storage, which can store hydrogen in the form of hydrogen atoms in solid hydrogen storage materials, and exhibit excellent safety and cycle performance. Magnesium based hydrogen storage alloys have been extensively studied in the field of hydrogen storage [2,6,7] due to their high hydrogen storage capacity (7.6 wt.% for MgH2), low cost and abundant resources. Nevertheless, the poor thermodynamic and kinetic barrier of Mg-based hydrogen storage alloys lead to high working temperature and slow kinetics, which limits its practical application in the field of hydrogen energy [8,9]. The modification methods, such as alloying [10], nano-crystallization [11,12] and catalyst addition [13,14], have been used to improve the performance of Mg-based hydrogen storage materials. Noteworthy, alloying can effectively lower the thermodynamic and kinetic barriers. Among various alloying elements, transition metal elements (TM) promote the dissociation of H2 molecules and play a synergistic catalytic role in improving the hydrogen absorption and desorption performance of magnesium- based hydrogen storage alloys [15–18]. Rare earth elements (RE) can also promote the hydrogen absorption and desorption properties of the alloys [19–22].In recent years, Mg-Ni-Y hydrogen storage alloys have been widely studied. The transition metal element Ni in the alloys can reduce the activation energy of H2 decomposition during the process of hydrogen absorption [23], and Mg2Ni generated by reaction with Mg can be used as a stable catalyst during hydrogen ab- and desorption reaction [24–26]. YH2 produced by rare earth element Y and H2 can also catalyze the reaction of hydrogenation [27–29]. Remarkably, long period ordered stacking phase (LPSO phase) can be formed in Mg-Ni-Y hydrogen storage alloys [30]. After hydrogen absorption, the LPSO phase decomposes and forms nano-sized Mg2NiHx, MgH2 and YHX, which can reduce the hydrogen diffusion path and promote the activation and the kinetics of hydrogen ab- and desorption in Mg-Ni-Y hydrogen storage alloys [31–33]. Whereas, the influence of size, distribution and type of LPSO phase on hydrogen storage performance remains not very clear.In this work, Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys were prepared by melting and ball milling. The microstructure and hydrogen ab- and desorption properties of the alloys were analyzed. The effect of the size, distribution of LPSO phase and ternary eutectic microstructure in the alloys with different contents of Ni and Y on hydrogen uptake and release properties were studied in detail. The hydrogen absorption mechanism of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys with LPSO structure is also discussed in this work. It is found that the catalytic effect is better when the catalytic phases are more evenly dispersed in the magnesium matrix. The enhancement of in-situ decomposition of coarse LPSO phases may be lower than that of fine Mg-Mg2Ni eutectic structure. So the size and distribution of phases and the number of phase boundaries may be more conducive to improve the hydrogen storage performance of the alloy. It is believed that this paper can provide us with an idea that the improving effect of LPSO phases is limited, reducing the size of LPSO phases and eutectic structure can be beneficial to further enhance the hydrogen storage performance of Mg-Ni-Y alloy.The character of raw materials is shown in Table 1 . The Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 casting ingots are prepared by melting in induction furnace in SF6 atmosphere and cooling in a salt solution. The casting ingots were then broken into powder, and further refined in the ball mill (pulverisette5 planetary ball mill, Feichi company, Germany). The rotation speed of the ball mill is 250 rpm (positive and negative rotation cycle), the mass ratio of grinding ball to material is 20:1, and the ball grinding time lasts 7 h.The phase structures of the as-cast, ball milled, hydrogenated and dehydrogenated alloys were determined by an X-ray diffractometer (XRD, RIGAKUD/max250pc) with Cu Kα radiation, at a diffraction angle (2θ) from 10° to 90° at 4° min−1. Scanning electron microscopy and corresponding elemental analysis were performed on a field emission scanning electron microscope (JEOL JSM-7800F) equipped with an energy dispersive X-ray spectrometer (EDS), which perform microanalysis of specific areas. LPSO phases and hydrides were identified via transmission electron microscopy (TEM, Talos F200s, Czech).Non-isothermal dehydrogenation behaviors of hydrides were performed using differential scanning calorimetric (DSC, STA449F3, NETZSCH, Germany) at the heating rate of 5 °C/min, 10 °C/min and 15 °C/min ranging from room temperature to 500 °C. The hydrogen absorption kinetics curves and PCT curves at different temperatures were measured by high pressure gas adsorption instrument (Beth company 3h-2000 pH) at the initial pressure 3 MPa. Figure 1 shows the XRD patterns of as-cast, ball milled, hydrogenated and dehydrogenated Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys. The as-cast alloys consist of α-Mg, Mg2Ni, LPSO [27,32,34] according to Fig. 1. Due to the increasing Y content in Mg92.8Ni2.4Y4.8 alloys, Mg24Y5 phase is formed while Mg2Ni disappears compared with Mg91.4Ni7Y1.6 alloys. After ball milling, the diffraction peaks of various phases are broadened, which indicates that the plastic deformation of the alloy results in lattice distortion and grain size refinement. The refinement of grain size is beneficial to enhance the hydrogen absorption and desorption properties. In addition, the types of phases in the alloy do not transform, and the LPSO phases still exist after ball milling. After hydrogen absorption, five kinds of hydrogen absorbing phases can be found, namely MgH2, Mg2NiH4, YH3, YH2 and Mg2NiH0.3. After releasing hydrogen, the XRD analysis shows that the dehydrogenated alloys are mainly composed of Mg, Mg2Ni and YH2. The LPSO phases and Mg24Y5 phases do not form any more, indicating that the decomposition of LPSO phase and Mg24Y5 phases are irreversible. Figure 2 shows the SEM images of as-cast Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys with different magnification. In the Mg91.4Ni7.1Y1.6 alloy, the black elliptical pre-precipitated phase and lamellar eutectic structure can be clearly observed. When the atomic ratio of Ni to Y is greater than 0.5, there are three phases of α-Mg, LPSO and Mg2Ni in Mg-Ni-Y alloy, and eutectic reaction occurs during solidification:L→α-Mg+LPSO+Mg2Ni [29]. So occurs the eutectic reaction in Mg91.4Ni7Y1.6 alloys with Ni/Y atomic ratio of 4.4. In addition, based on the XRD data and the SEM microstructure in Fig. 2(a), the Mg91.4Ni7Y1.6 alloys consist of black Mg matrix and a large number of gray eutectic structures, which is composed of fine lamellar LPSO phases, Mg2Ni and α-Mg alternating layers. The TEM patterns of LPSO phase regions in Mg91.4Ni7Y1.6 alloy is observed in Fig. 3 . The results of selected area electron diffraction and HRTEM micrograph analysis show that the lamellar LPSO phase in Mg91.4Ni7Y1.6 alloys is 14H-type LPSO structure. The microstructure of the Mg92.8Ni2.4Y4.8 alloy observed by Scanning Electron Microscopy is shown in Fig. 2(b). The Mg92.8Ni2.4Y4.8 alloy is made up of black Mg matrix, a large number of bulk LPSO phases and gray eutectic structure, and the LPSO phases, Mg24Y5 and α-Mg are distributed alternatively in the small eutectic structure. The TEM patterns of LPSO phase regions in Mg92.8Ni2.4Y4.8 alloy is observed in Fig. 4 . The results of selected area electron diffraction and HRTEM micrograph analysis confirms the bulk LPSO phase in Mg92.8Ni2.4Y4.8 alloys belong to 18R-type LPSO structure. In Mg92.8Ni2.4Y4.8 there are less eutectic phase and more bulk 18R-LPSO, whose width is more about 10 µm or even wider. Whereas the lamellar 14H-LPSO phase is more and finer, its width is about 200–500 nm, and the content of ternary eutectic areas is more in Mg91.4Ni7Y1.6 alloy.In summary, there will be more grain boundaries and phase boundary where it is easier to diffuse for hydrogen atoms. Therefore, kinetic properties of hydrogen absorption and desorption of Mg91.4Ni7Y1.6 alloys will be more excellent. Fig. 5 shows the SEM pictures of the particle size and surface morphology of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys after ball milling under the same conditions. According to Fig. 5, the particles of Mg91.4Ni7Y1.6 alloy are finer and there are more cracks and other defects on the surface compared with the ones of Mg92.8Ni2.4Y4.8 alloy, which can provide surface where H2 can be adhered and increase the diffusion rate of hydrogen atoms. Figire 6 and Fig. 7 show the SEM images of Mg91.4Ni7Y1.6 alloys and Mg92.8Ni2.4Y4.8 alloys after hydrogenation and dehydrogenation, respectively. It is found in Fig. 6 and Fig. 7 that the particle size of Mg91.4Ni7Y1.6 alloy is significantly smaller than that of Mg92.8Ni2.4Y4.8 alloy. Moreover, the particle surfaces of Mg92.8Ni2.4Y4.8 alloys after hydrogen absorption is relatively dense, and there are fewer fine particles on the alloy surfaces after hydrogenation and dehydrogenation. However, lots of flocculating particle clusters appear on the surfaces of Mg91.4Ni7Y1.6 alloys after hydrogen adsorption. These nano-sized particles can improve the hydrogen absorption and desorption dynamics of the alloy because it shortens the diffusion distance of H atoms. The TEM, energy spectrum, high resolution electron microscope and selected area electron diffraction pattern of a hydrogenated particle can be observed in Fig. 8 and Fig. 9 . The EDS results show that Mg, Ni and Y elements are slightly evenly distributed in the alloy particles after hydrogen absorption. The calibration results of electron diffraction pattern and high resolution electron microscope data confirm also that MgH2, Mg2NiH4, Mg2NiH0.3 and YH3 phases are uniformly distributed in the alloy particles after hydrogen absorption. After hydrogen desorption, Nano-sized compounds, such as Mg2Ni and YH2, are evenly dispersed on the surface of the alloy particles. According to Nobuko, Hanada et al. [35], transition metal and Rare earth metal nano-sized compounds uniformly dispersed on the surface of MgH2 can modify the surface condition of the particle and greatly reduce the activation energy of hydrogen desorption on the alloy surface. Therefore, Nano-sized hydride on the surface of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys can not only absorb hydrogen, but also play the important role of surface modification, leading to better kinetic properties of hydrogenation and dehydrogenation of the alloys.The DSC curves of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 during hydrogen desorption at various heating rates are presented in Fig. 10 . As one can see, three endothermic peaks of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys can be found. The hydrogen desorption temperature of Mg2NiH4 is lower than that of MgH2 and YH3 [36–38]. In the hydrogen desorption process, Mg2NiH4 first desorbs hydrogen to Mg2NiH0.3 and undergoes a significant volume contraction, causing a contraction strain on MgH2 around it, facilitating hydrogen desorption of MgH2. Hence, the desorption processes of primary MgH2 and Mg2NiH4 are not isolated, the synergistic reaction of them displays the lowest desorption peak temperature: Mg2NiH4 ↔ Mg2NiH0.3 + 2H2, MgH2↔Mg+H2. The dehydrogenation of Mg2NiH0.3 into Mg2Ni when the decomposition of MgH2 is complete, so the dehydrogenation process of Mg2NiH0.3 will occur at a higher temperature: Mg2NiH0.3↔Mg2Ni+H2. The dehydrogenation temperature of YH2 and YH3 is generally around 790 °C and 400 °C, respectively [36,38]. Therefore, DSC curve shows no heat absorption peak of YH2 and the highest desorption peak temperatures is YH3↔YH2+H2. When the heating rate is 5 ℃/min, the onset and peak decomposition temperatures of Mg91.4Ni7Y1.6 hydride are only 210.3 °C and 237.7 °C, both of which are lower than that of Mg92.8Ni2.4Y4.8 hydride: 227.9 °C and 248.2 °C, which indicates that more Ni doped in the alloys would contribute to enhancing thermodynamic property. Figure 11 show the fitted Kissinger curves of the Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys. The DSC curves of the synergistic catalytic de-/hydrogenation reaction between MgH2 and Mg2NiH4 at different heating rates were analyzed. The endothermic peak temperatures of Mg91.4Ni7Y1.6 alloy are 230.7 ℃, 248.3 °C and 262 °C, and that of Mg92.8Ni2.4Y4.8 alloy are 248.2 °C, 266.6 °C and 273.1 °C, respectively. The activation energies of hydrogen desorption of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys were calculated by fitting the straight line ln(β/Tm 2)vs.1/Tm through Kissinger equation [39]. As shown in Fig. 10, the hydrogen desorption activation energies of Mg91.4Ni7Y1.6 alloy and Mg92.8Ni2.4Y4.8 alloy are 87.7 and 112.4 kJ/mol H2, respectively, which are lower than that of [40] ball-milled pure MgH2, 250 kJ/mol H2. Combined with SEM and TEM analysis, the synergistic catalysis of uniformly distributed YH2, YH3, Mg2NiH0.3 and Mg2NiH4, which are generated by hydrogen absorption of cluster-parallel LPSO phases, gives rise to in-situ element catalysis and nano-scale effect [41]. Thus, these kinds of effect would greatly reduce the activation energy of hydrogen release and improve dehydrogenation performance.The experimental results are consistent with the analysis results of microstructure characteristics observed by SEM, that is, the more eutectic phase, the finer and more dispersed LPSO phase, leading to better in-situ catalytic effect after hydrogen absorption.The de-/hydrogenation process of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys includes three chemical reactions: Mg + H 2 ⇌ Mg H 2 M g 2 N i + 2 H 2 ⇌ M g 2 N i H 4 2 Y H 2 + H 2 ⇌ 2 Y H 3 The respective de-/hydrogenation plateau pressures of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys at different temperatures can be obtained from the PCT curves in Fig. 12 (a) and (b). The low- plateau pressures corresponds to the transformation of Mg/MgH2 while the high-plateau pressures relates to the transformation of Mg2Ni/Mg2NiHx. Due to the low content of Ni element in Mg92.8Ni2.4Y4.8 alloy, the synergistic catalytic effect is weak, resulting part of the hydrogen is still retained in Mg92.8Ni2.4Y4.8 alloy at 330 °C, indicating that its dehydrogenation temperature under 3 MPa pressure is higher than 330 °C. Moreover, there is only one de-/hydrogenation platform, and the higher Mg2Ni platform was not obvious. In addition, we found that the reversible hydrogen storage capacity of the two alloys at 380 °C is slightly less than the hydrogen storage capacity at 350 °C and 330 °C. Combined with the analysis of the hydrogen absorption kinetic mechanism on page 15 of the article, with the increase of absorption temperature, the hydride nucleation rate decreases. There are more hydride grains with large sizes and a thicker hydride layer [42] during the hydrogenation of the Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys at 380 ℃, which are not conducive to the diffusion of hydrogen atoms, resulting in a lower hydrogen storage capacity. Wenjie Song [41] also reported the similar situation.The corresponding van't Hoff plots for both hydrogen absorption and desorption are shown in Fig. 13 (a) and (b). According to the fitting result, the formation enthalpy changes (ΔH) of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 hydride are calculated to be −60.6 and −68.6 kJ/mol H2, and the formation entropy changes (ΔS) are 105.5 and 116.2 J/K/mol H2, respectively. According to Fig. 12, The Mg92.8Ni2.4Y4.8 alloy release hydrogen incompletely at 330 °C, and the complete dehydrogenation PCT curve was not obtained. It is difficult to distinguish the hydrogen release platform. Therefore, it is difficult to calculate the enthalpy change and entropy change of dehydrogenation through van't Hoff plots. In addition, the dehydrogenation capacity of the Mg91.4Ni7Y1.6 alloy is more significant, so the dehydrogenation enthalpy change and entropy change of the Mg92.8Ni2.4Y4.8 alloy can be omitted. The decomposition enthalpy and entropy changes of Mg91.4Ni7Y1.6 alloys are 56.9 kJ/mol H2 and 97.9 J/K/mol H2, separately.These values are all lower than that of MgH2 theoretical values [43], which indicates that the addition of Y and Ni reduce the thermodynamic stability of MgH2 and show better enhancement effects than just adding Ni or Y. Because of the large atomic radius of Ni and Y elements, the Mg-H bond distance is increased, which reduces the binding energy, thus reducing the stability. In addition, the activation energy and enthalpy of the hydrogen storage alloy will be reduced due to the nanostructure, which will greatly improve the hydrogen storage performance of the alloy [44]. In Mg91.4Ni7Y1.6 alloy, the LPSO phase is decomposed to form the finer and more dispersed nanoscale catalytic phases, and the value of hydrogen absorption enthalpy decreases more than that of Mg92.8Ni2.4Y4.8 alloy. This indicated that the Mg91.4Ni7Y1.6 alloy presents better hydrogen absorption performance and lower de-/hydrogenation temperature. Figure 14 gives the hydrogen absorption kinetics curves of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys. The amount of hydrogen absorbed and the rate of hydrogen absorption reaction increase with the rising temperature. The hydrogen absorption kinetic curve of the alloy is composed of two parts, which are fast hydrogen absorption and stable hydrogen absorption. The hydrogen absorption rate of the alloy rises steeply in the first 5 min, which is the rapid hydrogen absorption. However, the hydrogen absorption of the alloy shows a slow upward trend in the stable hydrogen absorption stage. The reason is as follows: after the hydride layer is formed on the surface of the particles, further hydrogenation of the alloy requires H atoms to penetrate through the hydride layer and diffuse into the alloy particles. However, the diffusion rate of H atoms in the hydride layer is lower than that in metallic magnesium, which causes the hydrogen absorption rate of the alloy to continuously decrease until it can no longer continue to absorb hydrogen when the size of the hydride layer grows to a certain thickness. Therefore, the hydrogen storage capacity of the alloy is determined by the rapid hydrogen absorption stage, and the high hydrogen storage capacity in the rapid hydrogen absorption stage results in a high total hydrogen storage capacity of the alloy. Representative hydrogen absorption data for Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys are summarized in Table 2 . The data shows that Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloy can absorb 5.78 wt.% and 3.84 wt.% hydrogen respectively under an initial hydrogen pressure of 3 MPa at 350 °C in 5 min, reaching 90% and 78% of the maximum hydrogen absorption capacity. Apparently, the theoretical hydrogen uptake of Mg91.4Ni7Y1.6 alloy is lower than that of Mg92.8Ni2.4Y4.8 alloy, but Mg91.4Ni7Y1.6 has a higher maximum hydrogen uptake and a faster hydrogen absorption rate.To explain the hydrogen absorption kinetic mechanism of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model are adopted to analyze the evolution of kinetics [45]: [ − l n ( 1 − α ) ] 1 / n = k t Equation transformation: α = 1 − e x p ( − k t n ) where α is the reaction fraction of the hydrogen storage material converted to hydride corresponding to time, k is the reaction rate constant, n (commonly between 0∼3) is the Avrami exponent of reaction order. When the value of 0.5<n<1.5, the nucleation and growth mode of the alloy is three-dimensional growth. After nucleation, the core takes a spherical shape and grows in a three-dimensional manner. When the spherical hydrides grow up and collide with each other, a continuous hydride layer is formed. In contrast to the value of n<0.5, the nucleation and growth method of the alloy is one-dimensional growth, and the core-shell structure will be formed in Mg-MgH2 transition, and the rate of hydrogenation is one to two orders of magnitude slower [31].The hydrogenation curves of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloy at 100 °C, 200 °C and 300 °C under 3 MPa are analyzed by JMAK model, which is shown in Fig. 15 (a-c) and Fig. 16 (a-c). The fitted curve is in good agreement with the experimental data (R2>0.99). At 100 ℃, the hydride nucleation and growth of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys are all three-dimensional diffusion-controlled methods. However, at 200 ℃, Mg91.4Ni7Y1.6 alloy forms a cored shell structure while Mg92.8Ni2.4Y4.8 alloy still grows at the three-dimensional manner, and the cored shell structure in Mg91.4Ni7Y1.6 alloy is formed at a lower temperature, indicating the nucleation rate of the alloy is very fast at a lower temperature. The rapid nucleation promotes the alloy to absorb a large amount of hydrogen during the rapid hydrogen absorption stage, and the hydride core quickly grows to form a hydride layer.Temperature greatly affects the hydrogen absorption reaction rate. When the temperature is low, the reaction rate is not fast enough to form cored shell structure. After rising to a certain temperature, nucleation and growth rate of alloy is accelerated, the core-shell structure is formed, and this gives rise to reducing the rate of reaction of materials. As grain boundaries and formed compounds act as pathways for hydrogen diffusion [46], interfacial diffusion can be maintained at higher temperatures. Therefore, although the Mg91.4Ni7Y1.6 alloy has a high nucleation rate and is easy to form a core-shell structure, its powder particles are fine, and a large number of fine LPSO phases and phase interfaces are contained in the alloy, which can provide diffusion path for H atoms and reduce the diffusion distance of H atoms, thereby improving the hydrogen storage performance of Mg91.4Ni7Y1.6 alloy.As we all know, hydrogen storage alloys need to overcome a certain energy barrier in the hydrogenation process in order to absorb hydrogen. This energy barrier is called the activation energy, which can be obtained by the Arrhenius equation. According to the Arrhenius equation, the activation energy of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys can be calculated by lnk vs. 1000/RT fitting, their hydrogen absorption activation energy was reduced from 100 kJ/mol H2 of pure Mg [47] to 25.4 kJ/mol H2 and 28.6 kJ/mol H2, respectively, as shown in Fig. 15(d) and Fig. 16(d). Through the analysis of hydrogen absorption kinetics curve and activation energy, Mg91.4Ni7Y1.6 alloy has a better hydrogen absorption kinetics performance. Adding different amount of Ni and Y element, Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys have different structure after smelting. Compared with the Mg92.8Ni2.4Y4.8, the Mg91.4Ni7Y1.6 alloy has finer LPSO phases and more eutectic structures, and the hydrogenated YH2/YH3 and Mg2NiHx phases are more dispersed, contributing to better synergistic catalytic effect and kinetics performance.Based on the analysis to XRD, SEM and TEM, it can be seen that there are a lot of LPSO phases in Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys, as shown in Figs. 1-4. The LPSO phase is a long-period stacking ordered phase containing Ni and Y atoms. Moreover, the LPSO structure with the periodic distribution of Y and Ni has an orderly composition and an orderly stacking in the entire LPSO phase. Combined with Fig. 1 and Fig. 9, the alloys after hydrogen absorption consist of hydrides such as MgH2, Mg2NiH4, YH3, YH2 and Mg2NiH0.3. This means that the hydrogen induced decomposition of the LPSO phase occurs upon initial hydrogen absorption, which can be expressed as: LPSO + H 2 → Mg H 2 + Y H 2 + M g 2 Ni H 0.3 where YH2 and Mg2NiH0.3 are further hydrogenated into YH3 and Mg2NiH4, respectively: Y H 2 + H 2 → Y H 3 M g 2 Ni H 0.2 + H 2 → M g 2 Ni H 4 After releasing hydrogen, the XRD analysis shows that the dehydrogenated alloys are mainly composed of Mg, Mg2Ni and YH2. Since the decomposition temperature of YH2 is as high as 790 ℃ [36], YH2 cannot release hydrogen at the experimental temperature. The dehydrogenation reaction can be described as follow: M g 2 Ni H 4 → M g 2 Ni H 0.3 + H 2 → M g 2 Ni + H 2 Mg H 2 → Mg + H 2 Y H 3 → Y H 2 + H 2 The LPSO phases do not form any more, indicating that the decomposition of LPSO phase is irreversible.A large number of uniformly distributed nano-hydrides formed during hydrogen absorption have a significant catalytic effect on the hydrogen absorption and desorption performance of the alloy [25,48], which is attributed to the nanosizing and in-situ catalyzing effects. In the process of hydrogen absorption, Ni can promote the decomposition of H2 molecules into H atoms, and the ability to form YHx hydrides is better than that of Mg2NiHx and MgH2 [49,50]. The formation of YHx will causes significant lattice distortion and then promote the formation of Mg2NiHx and MgH2. In the hydrogen desorption process, Mg2NiH4 first desorbs hydrogen to Mg2NiH0.3 and undergoes a significant volume contraction, causing a contraction strain on MgH2 around it, facilitating hydrogen desorption of MgH2. In addition, the multi-phase nanostructure can prevent the growth of Mg grains during the hydrogen absorption and desorption cycle, which can shorten the diffusion distance of H atoms.Although plenty of LPSO phases are contained in Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys, the hydrogen storage performance of Mg91.4Ni7Y1.6 is better than that of Mg92.8Ni2.4Y4.8 alloy. The reason is that the fine eutectic structure in Mg91.4Ni7Y1.6 alloy and many fine particles formed after activation are conducive to increasing the specific surface area, providing many reaction nucleation and diffusion interfaces, and enhancing the diffusion ability of H atoms in the alloy. The schematic diagram of the hydrogen absorption process of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys is shown in Fig. 17. According to the results of SEM and TEM analysis, there are a large number of bulky 18R-LPSO phases and a small amount of phase interfaces in the Mg92.8Ni2.4Y4.8 alloy, which causes most of the hydrides to nucleate only on the surface of the alloy. The growth of hydride requires H atoms to penetrate through the hydride layer and diffuse into the alloy. The growth of the hydride layer stops when it grows to a certain thickness, which results in part of the alloy structure not participating in the hydrogenation reaction, thereby reducing the maximum hydrogen absorption capacity and slowing down the hydrogen absorption rate of the alloy. On the contrary, Mg91.4Ni7Y1.6 alloy contains a large amount of Mg+Mg2Ni+14H-LPSO ternary eutectic, the size of LPSO phase is small, and there are many phase interfaces in the alloy. The hydride can not only nucleate on the surface of alloy particles, but also nucleate at the phase interface. In addition, the diffusion speed of H atoms in the LPSO phase is faster than that in the Mg phase, and the particles size of the Mg91.4Ni7Y1.6 alloy is smaller than that of the Mg92.8Ni2.4Y4.8 alloy, which makes the diffusion distance of the H atoms shorter. Therefore, the hydrogen absorption rate and total hydrogen absorption of Mg91.4Ni7Y1.6 alloy are higher than that of Mg92.8Ni2.4Y4.8 alloy under the same reaction conditions.In this paper, Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys were prepared by smelting and ball grinding. The microstructure and hydrogen storage properties are studied in detail. The catalytic mechanisms of alloying Ni and Y are elaborated. The results are as follows: (a) There are three phases in Mg91.4Ni7Y1.6 alloys, including Mg, Mg2Ni and abundant lamellar 14H-LPSO, while the massive bulk 18R-LPSO phases appear in the Mg92.8Ni2.4Y4.8 alloy. With the increase of the relative content of Y and the decrease of Ni, Mg24Y5 are formed in the Mg92.8Ni2.4Y4.8 alloy, and Ni element is completely distributed in the LPSO phases, inducing Mg2Ni phases disappears. (b) During the hydrogen absorption, LPSO phases decompose, causing hydride Mg2NiHx and YH2/YH3 to generate in situ. These phases distributed more uniformly in the alloy can play a better role of in-situ catalysis. In addition to the better dispersion of these catalysts, Mg91.4Ni7Y1.6 alloys also have more phase boundaries, so hydrogen atoms can diffuse better and improve the dynamic properties of the material. (c) A large number of fine particles are contained in Mg91.4Ni7Y1.6 alloys, which exposes more second-phase hydrides to the alloy surface and shortens the diffusion distance of H atoms. Not only can the maximum amount of hydrogen absorption of the material be increased, but also the activation energy of hydrogen absorption can be reduced, and the rate of hydrogen absorption of the material can be improved. (d) Mg91.4Ni7Y1.6 alloys have better kinetic and thermodynamic properties. Under the conditions of 300 ℃, 350 ℃ and 3 MPa, the hydrogen absorption contents of Mg91.4Ni7Y1.6 alloys reach 4.64 wt.% and 5.78 wt.% in 5 min, respectively. The activation energy of hydrogen absorption was 25.4 kJ/mol H2, and the enthalpy and entropy of hydrogen absorption were −60.6 kJ/mol H2 and 105.5 J/K/mol H2, separately. The alloy begins to dehydrogenate at 210 ℃, with the dehydrogenation activation energy of 87.7 kJ/mol H2, and the ΔH and ΔS of dehydrogenation are 56.9 kJ/mol H2 and 97.9 J/K/mol H2, respectively. There are three phases in Mg91.4Ni7Y1.6 alloys, including Mg, Mg2Ni and abundant lamellar 14H-LPSO, while the massive bulk 18R-LPSO phases appear in the Mg92.8Ni2.4Y4.8 alloy. With the increase of the relative content of Y and the decrease of Ni, Mg24Y5 are formed in the Mg92.8Ni2.4Y4.8 alloy, and Ni element is completely distributed in the LPSO phases, inducing Mg2Ni phases disappears.During the hydrogen absorption, LPSO phases decompose, causing hydride Mg2NiHx and YH2/YH3 to generate in situ. These phases distributed more uniformly in the alloy can play a better role of in-situ catalysis. In addition to the better dispersion of these catalysts, Mg91.4Ni7Y1.6 alloys also have more phase boundaries, so hydrogen atoms can diffuse better and improve the dynamic properties of the material.A large number of fine particles are contained in Mg91.4Ni7Y1.6 alloys, which exposes more second-phase hydrides to the alloy surface and shortens the diffusion distance of H atoms. Not only can the maximum amount of hydrogen absorption of the material be increased, but also the activation energy of hydrogen absorption can be reduced, and the rate of hydrogen absorption of the material can be improved.Mg91.4Ni7Y1.6 alloys have better kinetic and thermodynamic properties. Under the conditions of 300 ℃, 350 ℃ and 3 MPa, the hydrogen absorption contents of Mg91.4Ni7Y1.6 alloys reach 4.64 wt.% and 5.78 wt.% in 5 min, respectively. The activation energy of hydrogen absorption was 25.4 kJ/mol H2, and the enthalpy and entropy of hydrogen absorption were −60.6 kJ/mol H2 and 105.5 J/K/mol H2, separately. The alloy begins to dehydrogenate at 210 ℃, with the dehydrogenation activation energy of 87.7 kJ/mol H2, and the ΔH and ΔS of dehydrogenation are 56.9 kJ/mol H2 and 97.9 J/K/mol H2, respectively.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by Chongqing Special Key Project of Technology Innovation and Application Development, China (Grant No. cstc2019jscx-dxwtB0029)

Magnesium-based hydrogen storage materials are considered as one of the most promising candidates for solid state hydrogen storage due to their advantages of high hydrogen capacity, excellent reversibility and low cost. In this paper, Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys were prepared by melting and ball milling. Their microstructures and phases were characterized by X-ray diffraction, scanning electron microscope and transmission electron microscope, and hydrogen absorbing and desorbing properties were tested by the high pressure gas adsorption apparatus and differential scanning calorimetry (DSC). In order to estimate the activation energy and growth mechanism of alloy hydride, the JMAK, Arrhenius and Kissinger methods were applied for calculation. The hydrogen absorption content of Mg92.8Ni2.4Y4.8 alloy reaches 3.84 wt.% within 5 min under 350 ℃, 3 MPa, and the maximum hydrogen capacity of the alloy is 4.89 wt.% in same condition. However, the hydrogen absorption of Mg91.4Ni7Y1.6 alloy reaches 5.78 wt.% within 5 min, and the maximum hydrogen absorption of the alloy is 6.44 wt.% at 350 ℃ and 3 MPa. The hydrogenation activation energy of Mg91.4Ni7Y1.6 alloy is 25.4 kJ/mol H2, and the enthalpy and entropy of hydrogen absorption are -60.6 kJ/mol H2 and 105.5 J/K/mol H2, separately. The alloy begins to dehydrogenate at 210 ℃, with the dehydrogenation activation energy of 87.7 kJ/mol H2. By altering the addition amount of Ni and Y elements, the 14H-LPSO phase with smaller size and ternary eutectic areas with high volume fraction are obtained, which provides more phase boundaries and catalysts with better dispersion, and there are a lot of fine particles in the alloy, these structures are beneficial to enhance the hydrogen storage performance of the alloys.

Data available on request from the interested.In this era, it is becoming imperative to make a transition towards renewable energies. However, most renewable energy sources have an intermittent nature, precluding their adoption and utilization at their full potential [1]. One energy vector that has the potential to overcome this issue is hydrogen. This carrier can store energy for longer periods of time compared to alternative electrochemical energy storage, e.g. batteries, and can alleviate intermittency issues for daily or even seasonal variations [2,3]. In order to accomplish this, water is electrochemically split into its components, oxygen and hydrogen, both products that can be used in a wide range of processes [4].The study of catalytic surfaces has concentrated mainly on the development of materials that are very catalytically active, but are not very dependent on stability, the majority being analyzed in very short times of less than 20 h, and in low currents of 10 mA cm−2, [5–7]. Some methods use cyclic voltammetry of the materials in a very narrow potential window [8,9], preventing study of the corrosion of the materials in process. There are several works in which the cathodic corrosion process is analyzed during the off periods for the formation of reverse current in the cathode and the electrode is subject to degradation. These findings were obtained using electrochemical impedance spectroscopy (EIS) in the open circuit potential (OCP) and during the off processes [10–13]. Hence, there is a lack of studies on the stability of electrodes during the oxygen evolution reaction (OER) in more realistic operating conditions, i.e. high density currents and long operation times. Some research works use techniques such as inductively coupled plasma mass spectrometry (ICP-MS) and electrochemical quartz crystal microbalance (EQCM) to measure the loss of material during the electrolysis operation [14]. Acceleration tests have been proposed to study the degradation of electrodes during water splitting. These tests use a series of techniques applied consecutively, such as constant current density, then a constant potential and finally cyclic voltammetry to simulate the inverse current in the off process [15]. Nevertheless, these studies are few in number and, to the best of our knowledge, do not use additional transient techniques to measure or detect the changes on the surface of the electrodes due to the water splitting reactions. Transient and in-situ techniques are powerful tools capable of discerning small changes in the surface of the electrode that allow observation of the degradation of the electrode and the formation of new compounds on the electrode surface.Traditional corrosion studies performed on different stainless steels and nickel alloys immersed in aerated and hot concentrate alkaline media have demonstrated that metals such as Cr, Mo, Fe and Ni dissolve and leave a porous layer of nickel oxides and nickel hydroxides [16,17]. Formation of these nickel oxide compounds depend on the temperature, and it has was found that at 120 °C the main compound formed was Ni(OH)2, while at 180 °C this was NiO [18]. When nickel was used as catalytic active substrate, a small amount of iron was incorporated, finding that the iron is beneficial because it forms a layer of NiFe2O3 that protects the Ni against corrosion [18]. Studies performed on an AISI-SAE 316 stainless steel (SS) in NaOH at 90 °C over 4 months found the formation of a black layer on top of the electrode. The layer was constituted by iron oxyhydroxide, nickel oxide and nickel hydroxide [19]. In the ASI-SAE 304 SS at different concentrations of NaOH and 150 °C, stress corrosion cracking in the samples and a depletion of Fe and Cr were found, leaving the Ni on the surface and a under layer of oxides such as NaMO2, where M represents the different transition metals [20].Recently, AISI-SAE 304 SS plates were used in the structure of an electrolyzer, operating with NaOH electrolyte at a pH of 13 and 60 °C [21]. The SS plates were subject to electrolysis at low potential for several hours, and subsequently analyzed by XPS and Raman spectroscopy. The electrolyte was analyzed after electrolysis by ICP. Iron dissolution was found during the first hours of electrolysis, as well as the formation of different hydroxides and oxyhydroxides on the surface of electrodes, such as FeOOH, CrOOH, and Ni(OH)2. Depth profiles of composition performed by XPS analysis showed an increase in oxygen and depletion of the different metals, which suggests the formation of oxide compounds on the electrode surface.There are some contradictory studies about the role of phosphorous in the catalytic layer of the electrode for water splitting. Some of these have found that the coating of NiFeP in alkaline media acts as a passive layer that protect the electrode against corrosion [22]. In agreement with that work, Huang et al. found that an increase in phosphorus in the catalytic layer changes the properties of the coating, which become more amorphous, and facilitates the formation of a passive layer [23], creating a coating with fewer phase boundaries and crystalline defects. However, Safizadeh et al., evaluated the behavior of FeMoP catalytic layer in 1 M KOH and found that the phosphorous in the layer makes it more susceptible to corrosion [24]. This was demonstrated considering the open circuit potential (OCP) measurements after cathodic polarization. The OCP values of the FeMo layer changed to a more noble value with P incorporation; but the FeMoP layer stayed at a corrosion active state during the experiment, while, similarly, the corrosion current was higher with the addition of phosphorus.The current work aims to fill some gaps in the knowledge of the behavior of the NiFeP catalytic layer deposited on stainless-steel electrodes used as anode in the water splitting process, i.e., during the oxygen evolution reaction (OER). Similar conditions to the real operation of an electrolyzer were proposed in the study, considering the application of a constant anodic current density of 400 mA cm−2 over2h and the open circuit potential (OCP) monitoring for 2.5 h, in which time the system stabilizes. At the same time, the condition of intermittence of the electrolysis process that currently occurs with the use of renewable sources is evaluated. Electrochemical transient technique, such as EIS measurements, and in-situ Raman spectroscopy, were also performed in order to evaluate the changes in the electrode surface during the electrolysis. All of these were complemented with other characterization techniques to get a clearer understanding of what happens on the electrode during the OER.The working electrodes were AISI 304 stainless-steel square plates of 0.7 × 0.7 cm2 (approx. exposed area of 0.5 cm−2). The electrodes were polished with sandpaper up to P1500 grit, followed by a final polish with cloth covered with alumina (particle size of 1 µm). Non-exposed areas of the electrodes, laterals and back (where the electrodes were connected) sides were isolated, covering with epoxy resin and leaving 0.5 cm−2 of area expose to the electrolyte.In the current work we look for deep information about of the behavior against corrosion of the NiFeP catalytic layer deposited on stainless-steel electrodes used as anode in the water splitting process, i.e., during the oxygen evolution reaction (OER). Accordingly, stainless steel samples with and without anodic treatment and deposited catalytic layer were tested in anodic exigent conditions. The anodic treatment and the nature of catalytic layer and the electrodeposition conditions were selected from the best results of water splitting previously obtained [25] Polished stainless steel substrates were anodized following the procedure reported by Bervian [26]. Briefly, an aqueous solution with 10% v/v sulfuric acid (J.T. Baker® 95.9%) and 10% v/v glycerol (Pharmaceutical Grade) was prepared as electrolyte using type III water (conductivity ≤ 2.5 µS cm−1). A current density of 1 A cm−2 was applied for 600 s between the contra (platinum plate) and working (stainless steel) electrodes. The electrodes were separate 2 cm, and a constant potential around 3 V was observed. Finally, the working electrode was washed with abundant type III water.The catalytic coatings of NiFeP were electrodeposited either on polished or on anodically treated stainless steel, following the procedure described by Sridharan et al. [27]. Before coating application, the electrodes were thoroughly cleaned in an ultrasonic bath by immersion in ethanol followed by distillate water. The electrodeposition of NiFeP coatings was performed using a Pt foil as a counter electrode and applying a current density of − 30 mA cm−2 for 2400 s. The electrolytic bath composition was: 0.114 M NiS O 4 . 6 H 2 O (EMSURE® MERK, ≥ 99.0%), 0.108 M FeS O 4 . 7 H 2 O (CARLO ERBA, ≥ 99.5%), 0.129 M CH3COOK (MERK) and 0.094 M Na H 2 P O 2 . H 2 O (ALDRICH, ≥ 99 % ) using type III water. The pH of the bath was adjusted to 2 with sulfuric acid addition. The bath was kept at 40 °C under vigorous agitation in order to promote good adhesion of the coatings. Finally, the electrodes were cleaned with abundant type III water.Three kinds of samples were studied, namely: stainless-steel with anodic treatment (SS-AT); stainless-steel with anodic treatment and coated with NiFeP (NiFeP/SS-AT); and stainless-steel without anodic treatment and coated with NiFeP (NiFeP/SS).Surface morphology of NiFeP/SS, NiFeP/SS-AT and SS-AT samples were characterized before and after performing the oxygen evolution reaction (OER) process in 1 M NaOH electrolyte. Scanning electron microscopy (SEM) and electron dispersive X-ray spectroscopy (EDS) were performed using a JEOL-JSM 6490LV equipment, with acquisition parameter following the ASTM E1508–12 standard, as follow: acceleration voltage 20 kV, spot size 50, working distance 10 mm, dead time 11%, number of counts that arrive to the detector of 1.2kcps. SEM images were obtained using a secondary electron’s detector, after capture the imagen was not modified. EDS elemental analysis was semiquantitative because sense the photons, also the microprobe only carry information of 10 µm of the sample and the identification percent is 0.1% in weight. The algorithm use was INCAEnergy. Crystal structures were characterized by X-ray diffraction (XRD) Miniflex600 Rigaku with Cu K α ( λ = 1.54059 Å ) as source and operated with source powers of 40 kV and 15 mA, with a step size (°2θ) of 0.02 and dwell time (s) of 1. The scans were made at angles °2θ of 10–80. The data were analyzed using the high score plus program, with an automatic background determination and subtraction, and a smooth using quintic the program and a convolution range of 7. The peaks were indexed using several data bases as COD, PDF2, ICSD and information taken from different papers which concern similar materials. Raman spectra of the surfaces were taken in a Horiba Jobin Yvon (Labram HR) Nikon (BX41) microscope, with a CCD detector (Wright 1024 × 256 pixels), with a laser of 625 nm. In situ Raman measurements were performed using a homemade cell and the same Raman equipment already mentioned, using 6 repetitions and 50 s of capturing without filter, the range of the capture was from 100 to 3000 cm−1 with an objective of x50WD, a slit of 600 µm, a hole of 800 µm.Electrochemical characterization for OER was performed using a Zhaner IM6e potentiostat. A typical three-electrode cell was used for all measurements, a Hg=HgO electrode made in-house was used as reference electrode and a Pt mesh was used as counter electrode. The potentials were converted to reversible hydrogen electrode (RHE) according to literature reports [4,28–30], as follows: (1) E RHE V = E Hg / HgO + 0.098 V + 0.059 V × pH Where, E Hg / HgO is the experimental measured potential, 0.098 V factor is the relation between E Hg / HgO and the E NHE V , the factor of 0.059 V × pH is the correction given for the pH of the solution. All the potentials were corrected by the ohmic drop using R u × I , where R u is the resistance of the electrolyte obtained by electrochemical impedance spectroscopy (EIS), and I is the current. The overpotentials of the oxygen evolution reaction (OER) were calculated using the RHE potentials early calculated and an equilibrium potential of 1.23 V (2) η = E RHE V − 1.23 V The electrochemical performance of the working electrode and its corrosion resistance were evaluated by the following procedure: (i) a period of stabilization in the solution of 1 M NaOH for 2 h, (ii) electrochemical impedance spectroscopy (EIS) performed at the open circuit potential (OCP); next (iii) a linear sweep voltammetry (LSV) at a scan rate of 5 mV s−1 caried out to measure the initial activity of the material to oxygen evolution; then, (iv) cycles of 2 h of electrolysis (OER) at 400 mA cm−2 followed by two and a half hours without pass of current and a final EIS measurement at the OCP. This procedure was repeated 12 times. Finally, a LSV measurement performed at 5 mV s−1 was conducted to evaluate possible changes in the catalytic activity of the samples after electrolysis test. All experiments were conducted at room temperature and using 1 M NaOH as electrolyte, prepare with type III water.The EIS spectra were analyzed using both distribution of relaxation times (DRT) calculations and fit with electrical equivalent circuit, using R s ( R 1 CPE 1 ) ( R 2 CPE 2 ) ( R 3 CPE 3 ) and R s ( R 1 CPE 1 ) ( R 2 CPE 2 ) ( R 3 − W CPE 3 ) circuits for experimental impedance without and with diffusion, respectively (see details of electrical equivalent circuits in Fig. S-7 in the supporting information). In the electrical equivalent circuits Rs is the electrolyte resistance, R 1 and CPE 1 are, respectively, the resistance and the constant-phase element associated with the passive layer formed on the surface of the electrode. Similarly, R 2 and CPE 2 are associated with a second passive layer formed on the catalytic coating. R 3 is the charge transfer resistance and CPE 3 is the constant-phase element associated with the double layer capacitance, and W is the Warburg element of the oxygen diffusion. The electrical equivalent circuits were analyzed with Gamry Chem Analyst and the DRT calculations were performed using the algorithm implemented by Wan et al. [31], using a regularization parameter of 0.005 and a gaussian discretization. Capacitances and resistances were comparable for both methods. The capacitance (C eff ) values were calculated from Brug’s relationship [32], using the values of CPE and considering the approximation performed by Hirschorn et al. [33], where the Ohmic resistance is included in CPE behavior associated with surface distributions, following the Eq. (3). This approximation has been used in similar conditions [34,35]. (3) C eff = Q 1 α R e − 1 + R t − 1 ( α − 1 ) / α Where C eff is the effective capacitance, R e is the Ohmic resistance and R t , Q, and α are the current properties of the CPE. For the DRT spectra, the peak position was used directly as the effective time constants τ. Fig. 1 shows the scanning electron microscopy (SEM) of the samples before and after different cycles of electrolysis (OER) performed in 1 M NaOH. There is a kind of micro-texture formation on the surface of NiFeP/SS sample after OER (Fig. 1(a) and 1(b)), meaning that a generalized attack occurs on the catalytic layer of NiFeP after OER. The products of the corrosive attack are well adhered to the surface. On the other hand, at the surfaces of anodically treated samples, NiFeP/SS-AT and SS-AT, Fig. 1(c) and (e), some material dissolution was observed during the anodic treatment. The dissolution process occurs mainly around the grain boundaries [36], causing the formation of micro-structured features, like faceted/granular appearance, of varied sizes between 10 and 50 µm. The dissolution process was controlled in part with the addition of glycerin that diminishes the conductivity of the media, enabling better process control, in which the porous structure is formed. This is desirable, because this causes an increase of the area of the electrodes making it more suitable for water electrolysis. These coatings did not experience appreciable changes after electrolysis, as can be seen in Fig. 1(c) to (f). The surfaces of those samples exhibited similar texture, indicating that there was not severe corrosive attack after anodic polarization during OER. The roughness of the surface achieved by the initial anodizing process performed on the stainless-steel samples was preserved. It was also observed that the catalyst NiFeP coating (NiFeP/SS-AT sample) perfectly copied the texture of the surface initially generated by the anodic pretreatment process (SS-AT sample). Due to the anodic treatment, the NiFeP/SS-AT sample is expected to have a larger surface area than the NiFeP/SS sample. Consequently, lower current density during the NiFeP-catalyst electrodeposition and lower thickness of the layer for the NiFeP/SS-AT sample could be expected, because the catalyst electrodeposition was performed a constant current.The EDS mapping analysis shows surface homogeneity of the elemental composition of the NiFeP coating before and after the OER process (see Figure S-1 and Table S-1 in the supporting information). There was only an appreciable change in the iron composition after electrolysis. The amount of Fe in the catalyst coating diminished during the OER process by around 50%, resulting in a relative increase in Ni content in the surface of the coated samples. In the case of the sample SS-AT, there was only a slight reduction in the iron content. A dissolution process after the electrolysis that only involved the iron in the samples was corroborated, this result is in agreement with that reported by Santarini [20]. In addition, an increasing in the amount of oxygen in the surface of the samples could be expected due to the formation of different oxides [21,37,38], as it will be shown in the next section during the Raman results analysis. Fig. 2 shows the XRD patterns of different samples before and after the electrolysis process. SS-AT and NiFeP/SS-AT samples exhibited peaks located at 2θ = 43.79°, 50.91°, and 74.83°, corresponding to crystalline planes {111}, {200}, and {220} respectively, of the austenite phase of the stainless-steel substrate [39]. This because the thickness of the coatings is very thin, as was suggested by SEM analysis, making evident the diffraction peaks of the substrate. No diffraction peaks associated to the NiFeP catalytic coating were observed, which suggests that the coatings are amorphous in nature. The amorphization of the coating is due to the presence of phosphorus, which causes a distortion in the base structure of the coating, which is also dependent on the amount of phosphorus present in the deposit [40,41]. The XRD patterns obtained after the electrolysis process ( Fig. 3(b)) did not show changes in the diffraction peaks of the samples and the peaks related to stainless steel substrate remain in the patterns, meaning that the amorphous nature of the coating was preserved. Even though there was a dissolution of Fe and a probable enrichment of nickel on the surfaces (EDS analysis) the amorphous nature of the coating and the formed nickel compounds (Raman analysis will confirm further the formation of nickel oxides) is preserved after several cycles of electrolysis. The formation of complete amorphous structures after the electrochemical activation treatment in a basic electrolyte has been confirmed by other researchers [42]. This is convenient in terms of catalytic activity to oxygen evolution, that because electrocatalysts with an amorphous structure exhibit better OER activity than electrocatalysts with a crystalline structure [42,43].Raman spectroscopy is a technique commonly used for surface characterization, mainly for oxides and salt compounds materials. The main ventage of this techniques respect to others are that it allows a rapid characterization of the problem sample material without further preparation, enabling to characterize the actual surface without the interferences or surface changes that can be introduced by sample preparation. In addition, Raman spectroscopy allows to perform in-situ experiments during the electrochemical measurements in aqueous media, allowing to detect changes of the surface sample during the anodic or cathodic processes without water interferences.The ex-situ Raman spectra of the samples before the electrolysis process is shown in Fig. S-2 in the supporting information. There are no appreciable bands in the Raman spectra related with metal-oxide compounds formed on the surface of the samples before the electrolysis process. These results indicate either that the surfaces of the anodically treated samples and the NiFeP catalytic layer preserve their metal character, or that there were not enough oxides to be detectable with this technique. Raman results indicate no metal oxide formation in the samples in the condition “before electrolysis” and, as expected, not oxygen was detected in the EDX analysis, see Table S-1 in the supporting information. Conversely, in the condition “after electrolysis” oxygen content was detected in the samples, which can be related to the formation of NiOOH compound, as will be seen further by in situ Raman spectroscopy measurements.Raman in-situ spectra of the samples were recorded during the anodic polarization at different potential for OER. The Raman in-situ measurements were done in a home-made cell in which the surface of the samples was covered by 1 M NaOH electrolyte. The objective of the microscope was located very close to the surface of the electrolyte to prevent losses in the signal. Raman spectra were measured at different anodic potentials. The polarization potential values were chosen considering the potential reached by the OCP after the on-period of electrolysis at 400 mA cm−2, to identify possible species formed. The polarization potentials for in-situ Raman measurements were 0.81 V, 1.05 V and 1.39 V vs RHE, and finally a current density of 10 mA cm−2 was applied to complete the polarization of the surface at the OER. At this last stage, the number of oxygen bubbles was very large. Following this, a return to the polarization potential was applied, i.e., 1.39 V, 1.05 V, 0.81 V vs RHE. Each polarization potential was applied for 30 min in order to obtain a stationary state and Raman spectrum was then recorded. At least three measurements were performed for each sample, in order to verify reproducibility of the results. In-situ Raman spectra of the samples recorded before and after all the cycles of electrolysis at 400 mA cm−2 are shown in Fig. 3. In the case of the NiFeP/SS sample, Fig. 3(a) and (b), two bands at 475 cm−1 and 555 cm−1, associated with Ni−O vibration of the NiOOH compound [44–46], begin to emerge with the anodic polarization. The two allotropic compounds γ-NiOOH and β-NiOOH exhibit a pair of bands at these wavenumbers, but with different relative intensities of the bands at 475 and 555 cm −1. The ratio of the intensity of the bands I475 cm−1 /I555 cm−1 of the β-NiOOH compound is lower than for γ-NiOOH [47,48]. However, it was demonstrated that the presence of Fe in the deposition leads to a decrease in the number of redox electrons, so the formation of γ-NiOOH could be inhibited [49]. Table S-2 shows the ratio of the intensities of the bands at 475 cm−1 and 555 cm−1 (I475 cm−1 /I555 cm−1) observed in the in-situ Raman spectra of the samples performed at different polarization potentials. In the NiFeP/SS sample the bands of NiOOH are clearly visible at the anodic potential where a high rate of oxygen bubbles occurs, i.e., 1.5 V RHE (10 mA cm−1), and the presence of the bands remains even after the return of the anodic polarization to low values. With the return of anodic polarization to lower values, the intensities of the bands at 475 and 555 cm −1 tend to decrease, indicating a probable reduction process of the compounds. These features occur for both situations, before and after electrolysis at 400 mA cm−2. However, after all cycles of electrolysis it was observed that the ratio of the intensities of the bands at 475 and 555 cm −1 is different in some respects to that observed before electrolysis, being larger after electrolysis cycles. This is probably due to the previously detected iron dissolution that occurs during electrolysis. According to the study by Bell [44] the intensity of the 555 cm−1 band exhibited by the NiOOH layer increases with Fe content in the Ni-Fe alloys. Conversely, if iron content in the Ni-Fe coating diminishes, the intensity of the 555 cm−1 band also decreases and the I475 cm−1 /I555 cm−1 ratio increases after electrolysis, which is consistent with what occurs in the current study. Due to iron dissolution from the coating, a stable and more catalytic layer of γ-NiOOH can be formed. No more bands were detected in the in-situ Raman spectra, which indicates that no additional Ni or Fe species were formed on the surface of the sample.In the case of the anodic-treated (SS-AT) sample, Fig. 4(c) and (d), no Raman signals associated with Ni or Fe compounds were observed on the sample before electrolysis, with only a broad band at 1000 cm−1 associated with OH deformation being observed before the electrolysis process. However, after electrolysis the characteristic bands at 475 and 555 cm−1 of the NiOOH layer appeared at high anodic polarization where a high rate of oxygen bubbles occurs, i.e., 1.5 V (10 mA cm−1). This result indicates that there are changes in the elemental composition of the surface of SS-AT sample, i.e. local dissolution of Fe and increase in Ni due to the corrosion process. This situation induces the formation of the NiOOH layer on anodically treated stainless steel after electrolysis. However, in this sample, the Raman bands of the NiOOH (475 and 555 cm−1) are less intense than those of the samples with NiFeP catalytic layer and they disappear with the return of anodic polarization, see Fig. 3(d). This is probably due to the lower thickness of the NiOOH layer formed in the SS-AT sample after electrolysis, which experiences a complete reduction process with the return of anodic polarization to lower values.In-situ Raman spectra of sample NiFeP/SS-AT, Fig. 4(e) and (f), exhibited a similar behavior to the NiFeP/SS sample. As a result of the presence of the catalytic layer of NiFeP, the two bands at 475 and 555 cm −1, associated with Ni−O vibration in NiOOH compound, are present both before and after the electrolysis process. Before the electrolysis the bands associated with NiOOH are clearly visible at the anodic potential where high rate of oxygen bubbles occurs, i.e., 1.5 V RHE (10 mA cm−1), and the presence of the bands remains even when the anodic polarization was returned to low values. In addition, after the electrolysis process the NiOOH bands are detected even at potentials lower than that where a high rate of oxygen bubbles occurs (1.5 V vs. RHE).According to the surface characterization and in-situ Raman results shown above, the mechanism that best explains the corrosion and transformation of the surfaces of the samples during OER is that proposed by Jerkiewicz [50], which involves nickel dissolution followed by formation of several nickel species in an alkaline electrolyte, reactions (1) to (3). According to this reaction model, the OH- of the media is adsorbed in the surface, reaction (1), followed by a rearrangement of Ni, in turn creating a 3D lattice, reaction (2). Subsequently, nickel dissolution occurs upon the formation of adsorbed α-Ni(OH)2, reaction (3). (1) Ni + O H − → Ni − O H ad + e − fast (2) Ni − O H ad → OH − Ni quiasi − 3 D lattice rate determining step (3) OH − Ni quiasi − 3 D lattice + O H − → Ni OH 2 , ad + e − ( fast ) The α-Ni(OH)2 can be then converted into a more stable form with the application of an anodic polarization potential, in which it loses water, passing to β-Ni(OH)2 [51]. The new compound is better arranged compared with the previous α-Ni(OH)2, and the reaction is irreversible when a high enough voltage is applied. Upon further anodic polarization the β-Ni(OH)2 passes to β-NiOOH, involving the nickel oxidation from 2+ to 3+. If the anodic polarization increases, allotropic transformation of the nickel oxyhydroxide occurs, forming γ-NiOOH, in which nickel has an oxidation state of 3.2. Fig. 4 shows the results of the electrochemical evaluation of the NiFeP/SS sample after electrolysis process (OER). The procedure was described in the experimental part and consisted of a period of stabilization in the electrolyte, EIS measurement, LSV, on/off cycles, electrolysis process (2 h of electrolysis (OER) at 400 mA cm−2), OCP measurement and EIS, and finally LSV. Fig. 4a) shows the Nyquist diagrams of impedance measurements performed at the different times of electrolysis. EIS measurements were performed during the off period, after the OCP stabilization, of each electrolysis cycle. After the first electrolysis processes (0–12 h) two coupled capacitive loops were observed in the Nyquist diagram (for more detail see the Bode diagram of EIS, Fig. 4(c) and Figure S-3 in supporting information). DRT analysis (Figure S-3c in supporting information) clearly indicates that three time-constants should be considered in the impedance response after the first electrolysis cycles. The flattened capacitive loop at high frequency (HF) is related to the previously formed Ni(OH)2 layer, which gradually transforms to NiO oxide [37] and catalytic NiOOH layer; while the low frequency (LF) loop could be related to the parallel combination between double-layer capacitance and the charge-transfer resistance of the coating/electrolyte interface. The formation of hydroxide layer has been pointed out in theoretical studies, which demonstrate that it is not thermodynamically stable materials during the oxygen evolution reaction and propose the dissolution and the formation of a hydrous amorphous layer over the bulk metal, through which the oxygen anion diffuses [52]. Also, the formation of Ni oxide – hydroxide layer in alkaline media has been evidenced by TEM images, where dense layer of NiO and NiOOH was found on the surface [37,38].After successive electrolysis cycles, both HF and LF capacitive loops decouple and a straight line at 45° at the limit of the low frequency region begins to emerge, see Fig. 4(a)-(c) and Figure S-3 in supporting information. Similar feature has been observed in passive layer formed on steel immerse in alkaline media [53]. More detail of the features of EIS results can be observed in Figure S-3, where bode plots and DRT analysis of the impedance measurements performed during the on/off cycles of electrolysis process are presented. The parameter values calculated from DRT analysis are shown in Table S-3, while Figure S-7 shows those values as a function of time, for comparison purpose. From the DRT plots of impedance, it was possible to determined that three processes took place. Some of these were coupled at initial times of electrolysis; however, they decoupled at larger cycle numbers and a diffusion process also begins at the limit of low frequency, this probably due to the formation of the dense layer of Ni oxides and the oxygen diffusion [37]. From DRT analysis an increase of the peak located at the HF region can be seen, indicating the increase of the coverage of the surface by the NiOOH layer during the cycles of electrolysis. On the other hand, the peak at middle frequency region (MF) diminishes, indicating the reduction of the early Ni(OH)2 layer [41]. Likewise, the peak at LF diminishes, indicating that the charge-transfer resistance is reduced over time, due to the formation of NiOOH catalytic layer. The straight line at 45°, observed at the zero-limit of the LF region, could be related to the diffusion process of oxygen away from the electrode surface. The formation of the NiOOH layer consolidates after successive cycles of electrolysis. Consequently, the impedance diagrams are very similar. The layer formed are expected to become more stable, as has been shown in other studies in which the surface composition stop changing after the initials time of polarization [21].As can be seen in the Tables S-2 and S3, in the supporting information, the values of capacitance, in some cases, are in the order of the mF, which are larger and no typical of films and double layer. The reason for this is that due to the difficulty of accurately calculating the actual area of the electrodes, which is larger than a planar electrode, because the high roughness of the surface, we use the geometric area instead of the real area in order to calculate the capacitances. In a recent published paper [25] we obtained that the geometric area is about two orders of magnitude inferior to the real area of this kind electrodes. Because of that the capacitance values appears one or two orders of magnitude larger than typical values of capacitance for films and double layer. The same situation has been evidenced by other researcher works concerning the capacitance of electrodes for oxygen evolution reaction [34,54], and in tests in alkaline media of steel and iron [55,56]. Furthermore, if we consider that deviation of capacitance values, we estimate that real capacitances of the samples are in the range of 10–100 μF cm−2, and the thickness the oxide films will be in the range of 100 – 1000 nm. Which is typical of this kind of films [37,38].EIS fitting using electrical equivalent circuits was also performed. Figure S-6, in the supporting information, shows the electrical equivalent circuit configurations used to fit EIS experimental results, Figure S-6a for the first electrolysis cycles where no diffusion process was observed and Figure S-6b for the last electrolysis cycles, also considering Warburg impedance to model the diffusion process. The parameter values obtained after EIS fitting with electrical equivalent circuits are presented in Table S-4. The results of fitting using electrical equivalent circuits are in accordance with those obtained by DRT analysis, see also Figure S-7. The reduction in the resistance of catalytic layers during on/off electrolysis process is evident and the straight line of 45° observed at the limit of the low frequency of the EIS diagrams performed at the end of electrolysis cycles can be associated with the diffusion of oxygen away from the electrode surface. It is supposed that a large amount of O2 remains on the catalytic layer of the electrode after OER during each cycle of electrolysis, which is then released from the electrode during the off-electrolysis period. As a result, overpotential jumps of the electrode are observed during the on/off cycles of electrolysis, as can be seen in Fig. 4(d).The red vertical lines drawn in Fig. 4(d) indicate the time when the electrolysis process is stopped and the OCP is recorded. A gradual increase in the overpotential during the on period of electrolysis is attributed to accumulation of oxygen bubbles on the catalytic layers of the electrode [57]. Then, during the off period of electrolysis, a gradual decrease in the overpotential is observed due to the release of the oxygen away from the electrode surface. The observed noisy signal was attributed to the excessive production of bubbles that accumulate in the surface, partially blocking the active area of the electrode [14]. These experiments were made at quiescent conditions, so the adsorption of oxygen bubbles on the electrode surface takes on a large importance. The evolution of the overpotential appears to reach a maximum after the fourth on/off cycle of electrolysis, after which a decrease in the overpotential signal is observed, probably due to changes in the nature of the catalytic layer of the electrode or changes on the morphology of the compound formed on the electrode surface, see Fig. 1(b). According to the in-situ Raman and EIS analyses, the catalytic layer NiFeP transforms to a more active NiOOH [37] compound on the electrode surface, and partial iron dissolution occurs during the electrolysis process. This change in the catalytic layer increases the activity of Ni cations for OER [44]. The OCP of the electrode shifts to more positive values after each on/off period of electrolysis (see Fig. 4(e)) as a consequence of the formation of NiOOH layer on the electrode surface. In addition, the formation of NiOOH layer causes a gradual reduction in the size of the OCP jump after each on/off cycle. The rest time in each potentials is longer in each cycle indicating an increase in the thickness of the oxide layer in agreement with the impedance results and other related investigations [37,58]. In addition, the steady state of OCP values are reached in more positive potentials at the end of the cycling process, and an easier release of oxygen bubbles away from the electrode is achieved. Fig. 4(f) shows the results of the LSV performed at 5 mV s−1 before the first and after the last electrolysis cycles. After the electrolysis cycles, reductions of the overpotential at 10 mA cm−2 and 100 mA cm−2 by 13 and 18 mV respectively are observed, indicating improvement of the catalytic activity of the surface after electrolysis. Fig. 5 shows the electrochemical evaluation of the naked anodic-treated stainless-steel (SS-AT) sample during on/off electrolysis (OER) cycling process. It is interesting to highlight that, despite the fact that the SS-AT sample lacks a NiFeP catalytic layer, the main features of its electrochemical response are almost identical to those observed for the NiFeP/SS sample. The in-situ Raman spectra of the SS-AT sample recorded after water electrolysis showed that the OER also induces the formation of the NiOOH layer in this kind of sample. Accordingly, the naked anodic-treated stainless-steel sample can develop a catalytic layer through OER, which could improve its catalytic properties during the on/off electrolysis cycling. This agrees with the activation procedures, which has been used to develop a layer of NiFe over the surface of an austenitic stainless steel. During the activation procedure it was observe the formation of a layer of oxide that began to grow and then pass to a process of densification [37]. The thickness of the layer was 40 nm and the active surface area was around 14 times of a plane electrode. It has been demonstrated by high resolution TEM images that the main oxide in the surface is a Ni-rich Ni-Fe hydroxides and a buffer layer of NiO that reduce the lattice mismatch [37]. This is in accordance with the detected structures of Ni(OH)2, which changes to NiOOH during electrolysis and reduces again. Other studies with successive potentiometric cycles has found similar results with an interlayer of 850 nm of thickness [38].In the SS-AT sample the electrochemical impedance diagrams also show coupled HF and LF capacitive loops and a straight line of 45° at the limit of the low frequency region after successive electrolysis cycles, see Fig. 5(a)-(b) and Figure S-4 in the supporting information. The high frequency (HF) loop is related to the NiOOH layer formed after electrolysis. The low frequency (LF) loop is related to the parallel combination between double-layer capacitance and the charge-transfer resistance of metal/electrolyte interface, and the straight line of 45° is associated with the diffusion of oxygen away from the electrode surface. The charge-transfer resistance of the sample diminishes considerably after successive electrolysis cycles, see Table S-3 and Figure S-7 in the supporting information. However, the overall resistance values are much higher than those of the samples with NiFeP catalytic layer. According to DRT analysis (Figure S-4c), it is clear that the passive layer was growing after every cycle of electrolysis. The overpotential exhibited by the SS-AT after each cycle of electrolysis rises from 0.7 V, until reaching a stable value of 1.0 V at the 16° cycle, see Fig. 5(d). This indicates that oxygen bubbles are accumulated on the surface [57]; but, unlike what was observed for electrodes with NiFeP catalytic layer, the NiOOH formed on the stainless-steel surface does not favor the release of the oxygen bubbles away from the electrode, probably due to incipient formation of the NiOOH compound on the SS substrate. Similarly, the evolution of the OCP values during the on/off electrolysis cycles shows an increase until reaching a stable value, see Fig. 5(e), which also indicates the accumulation of oxygen bubbles on the surface. Similarly, to what was observed for electrodes with NiFeP catalytic layer, the stainless steel undergoes a period of activation due to NiOOH formation, showing a considerable reduction in the overpotential at 10 mA cm−2 and 100 mA cm−2 by 56 and 60 mV, respectively after the electrolysis cycles, see Fig. 5(f). As has been shown by other studies, the products of the corrosion of stainless-steel in alkaline media are majority metal hydroxides [19,21], which is coherent with what was observed by in-situ Raman spectroscopy, where the formation of NiOOH on the SS surface during electrolysis process at the OER potentials was corroborated. This compound is much more catalytically active that the normal oxide of Cr2O3 present in the surface of stainless steels. Fig. 6 shows the results of the electrochemical evaluation of the NiFeP/SS-AT sample after electrolysis process (OER). Due to the presence of the catalytic layer of NiFeP on the anodic-treated stainless-steel, this electrode exhibited similar electrochemical behavior to that observed for the NiFeP/SS electrode during the on/off cycles of electrolysis. Fig. 6(a) shows the Nyquist diagrams of impedance measurements performed at the different times of electrolysis. After the first electrolysis process, two coupled capacitive loops were observed in the Nyquist diagram (see for more detail the Bode diagram of EIS in Fig. 6(c) and Figure S-5). Similarly to what was observed for the NiFeP/SS sample, the flattened capacitive loop at high frequency (HF) of the NiFeP/SS-AT electrode is related to the previously formed Ni(OH)2 layer, which gradually transforms to catalytic NiOOH layer; while the low frequency (LF) loop is related to the parallel combination between double-layer capacitance and the charge-transfer resistance of coating/electrolyte interface. After successive electrolysis cycles, both HF and LF capacitive loops continue to be coupled, but with a low associated impedance, and a straight line of 45° at the limit of the low frequency region, associated with the diffusion of oxygen away from the electrode surface, begins to emerge, see Fig. 6(a)-(c). More detail of the features of EIS results can be observed in Figure S-5, where Bode plots and DRT analysis of the impedance measurements performed during the on/off cycles of electrolysis are presented. The reduction in the resistance of catalytic layers during on/off electrolysis process was corroborated, see Figure S-7. This sample exhibited the lowest values of impedance during the on/off cycles of electrolysis, as can be seen in Table S-3, Table S-4 and Figure S-7. In addition, the lowest values of overpotential jumps and more stable OCP values after each on/off cycle of electrolysis were observed in this sample, see Fig. 6(d)-(e). This indicates that the O2 accumulated on the electrode surface after OER during each cycle of electrolysis is more easily released away from the surface than what was observed for the other evaluated electrodes. As was shown in the in-situ Raman analysis performed on the NiFeP/SS-AT sample before and after electrolysis process, the bands at 475 and 555 cm −1 associated with the NiOOH were clearly visible even at potentials lower than that where a high rate of oxygen bubbles occurs, i.e., 1.5 V (10 mA cm−1), and the bands continued to be present when the anodic polarization was returned to low values. The best condition for the formation of the active layer of NiOOH was that exhibited by the NiFeP/SS-AT electrode, showing the lowest overpotential values at 10 mA cm−2 and 100 mA cm−2 in the LSV curves performed before and after the electrolysis cycles, Fig. 6(f). Although there was little change in the LSV curves observed for the NiFeP/SS-AT electrode, this demonstrates that the changes undergone by this electrode in catalytic activity after on/off cycles are very low, and that the catalytic layer becomes more stable over time.The catalytic NiFeP layer electro-deposited on stainless-steel and naked anodic-treated stainless-steel experiences selective dissolution of iron after the oxygen evolution reaction during the electrolysis of water. The relative content of nickel in the surface of electrodes increases and a second amorphous catalytic layer of nickel oxyhydroxide (NiOOH) is formed as a corrosion product after the oxygen evolution reaction. The formation of the NiOOH layer after electrolysis, improve the catalytic response of the surface to OER. The formed NiOOH layer becomes more stable over electrolysis time. In addition, the presence of the NiOOH layer causes a gradual diminution in the size of the OCP jump after each on/off cycle, indicating that steady OCP values are reached more quickly at the end of the cycling process, and an easier release of oxygen bubbles from the electrode is achieved.Selective dissolution of a catalyst could be implemented as a key strategy to improve de catalytic behavior to OER and to improve the release of bubbles, which must be considered to future works. S. Cartagena: Investigation, Validation, Writing - original draft. J. A. Calderón: Conceptualization, Methodology, Funding acquisition, Writing – review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors would like to thank Colombian Ministry of Science, Technology and Innovation “Minciencias” for financial support through the Colombia Scientific Program (Contract No FP44842-218-2018).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.corsci.2022.110437. Supplementary material .

Green-hydrogen generation has become a focus for research due to its promising future as an energy vector. In this regard, one topic that has not been explored in depth, is the corrosion of catalytic layers under on/off operation in alkaline media during oxygen evolution reaction (OER) on the anode. Here, we studied the corrosion of, and changes to, the catalytic layer on stainless steel (SS) electrodes under different surface treatments. The results showed that there was formation of a passive and catalytic layer of NiOOH during the anodic polarization concomitantly with iron dissolution.

Steelmaking, one of the world’s key industrial sectors, is highly energy-, and thus, carbon-intensive. The iron and steel sector directly accounts for 2.6 Gt of CO2 emissions annually, corresponding to about 7% of the global CO2 emissions [1]. Modern installations are reaching their limits in terms of efficiency improvement and energy and emission reduction. Further significant CO2 mitigation in the steel sector can only be realized with the introduction of breakthrough technologies, such as carbon capture, utilization and storage (CCUS). According to the International Energy Agency (IEA), CCUS technologies will play a central role to the industry decarbonization portfolio, contributing by about 15% to the required emissions reductions [2].While the removal of CO2 from the blast furnace gases via physical or chemical adsorption/absorption has been extensively studied ([3] and references therein), CCS has not been yet widely deployed in the steel industry, hampered by the high energy consumption of these capture technologies. In that context, CCU, i.e. the valorization of the carbonaceous steel-work off-gases for the production of useful products, holds more promise. The first commercial steel facility integrated with emissions conversion was launched in China in 2018, employing LanzaTech’s low-temperature gas fermentation-to-ethanol technology with a capacity of 60,567 m3/y [4]. A similar plant is currently being constructed in ArcelorMittal’s steel mill in Ghent, with support from the EU via the project STEELANOL [5]. Carbon Recycling International (CRI), the company that operates the first CO2-to-methanol production facility with a current capacity of 5 million liters of methanol per year, has achieved production of methanol at industrial scale using blast furnace gas from the SSAB steel mill in Lulea, Sweden, in the frame of the EU-funded project FReSMe [6]. CRI’s technology is based on catalytic CO2 hydrogenation over CuO/ZnO/Al2O3-based catalyst under similar operating conditions to syngas-based methanol production, after careful removal of gas impurities [7].Despite the enormous progress, CO2 reduction remains a grand challenge, as commercial CuO/ZnO/Al2O3 catalysts have been reported to have unsatisfactory CO2 hydrogenation performance and low stability due to the negative “flooding” effect of H2O and the use of a hydrophilic alumina promoter component [8]. CO2 conversion becomes even more demanding when the CO2 source is industrial flue gases due to the co-presence of several other gases and impurities (e.g. CO, O2, N2, S- and N-compounds, inorganics, metals, etc.). Commercial Cu-Zn oxide catalysts have been recognized, early on, to be susceptible to sulfur, nitrogen, chlorine, phosphorus and Fe or Ni metal carbonyls [9 –11]. Ma et al. showed that Cu/ZnO deactivates sharply in syngas containing 3 ppm H2S, with CO conversion decreasing from 24.5% to lower than 1% within 7 h of reaction [12]. Deactivation was attributed mainly to the formation of ZnS and CuS, which destroyed the synergetic effect between Cu and ZnO. The work of Wood et al. [13] revealed the importance of the sulfur source, as the rate of deactivation was similar for thiophene and H2S, but negligible for COS.Although there is vast literature on the design and development of heterogeneous catalysts for direct CO2 hydrogenation to methanol for CCU applications (see recent reviews [7,14–18], and references therein), little attention has been paid to the effect of flue-gas impurities on catalyst performance and stability. This is especially important for the economic viability, and thus wide-scale deployment of such processes, as low catalyst poison tolerance necessitates the use of advanced and cost-intensive purification methods. In a recent study, Schühle et al. [19] addressed the performance of In2O3/ZrO2 and commercial CuO/ZnO/Al2O3 for the hydrogenation of CO2 to methanol in the presence of typical impurities of industrial CO2 feed gas streams, i.e. SO2, H2S, NO2, NH3 and hydrocarbons (C1 – C3). The Cu-based catalyst was more resilient to sulfur, but still methanol productivity decreased by about 36% under severe sulfur poisoning by SO2 and H2S. The negative effect caused by nitrogen (either NO2 or NH3) was much less, in the order of ∼ 10%, while the presence of hydrocarbons caused a more than 50% decrease in methanol productivity. The focus of that work was however the In-based system. The poison-induced changes in the rate of deactivation under prolonged reaction times were also not considered.In this work, we investigate in a systematic way the deactivation and stability of a commercial CuO/ZnO/Al2O3 methanol synthesis catalyst, poisoned with typical contaminants present in steel-work off-gases, in the hydrogenation of CO2 to methanol. To accurately control the concentration of impurities on the catalyst surface, we poison the catalyst ex-situ with known amounts of contaminants. Besides the classical nonmetal impurities, i.e. H2S and NH3, we extend our study by considering the effect of Na, Ca and Fe. Fe in oxidic form is the major inorganic component of the dust that is produced in blast furnace and basic oxygen furnace steelmaking, followed by CaO/CaCO3 [20,21]. Na, although present in smaller amounts [20], is strongly basic and could potentially have adverse effects on the reaction. The poisoned and untreated catalyst samples are characterized and tested under CO2/H2 mixture at industrially relevant reaction conditions as a function of time on stream.The catalyst used in this study was a commercial CuO/ZnO/Al2O3-based catalyst provided by Clariant. The catalyst was supplied in pellet form (6*4 mm). Prior to poisoning, the catalyst was ball milled and sieved to particle size 250 μm – 350 μm.Poisoning with sulfur and nitrogen was performed ex-situ with H2S and NH3, respectively, to protect the high-pressure testing unit from corrosion. A Linseis STA PT-1750 thermogravimetric analyzer (TGA), offering precise temperature and gas flow control, was used for this purpose. The samples, each comprising 5 g of catalyst, were placed in a crucible in the TGA. Prior to the poisoning process, initial reduction took place by slowly ramping temperature and H2 content to finally 5% H2/N2 at 250 °C and holding this point for 4 h. The system was then purged with N2 for 1 h. For the sulfur poisoning, the catalyst was exposed at 250 °C for 5 h to a gas mixture of 0.3 vol% H2S/He diluted with an appropriate amount of N2 to achieve a final concentration of 400 ppm H2S in the feed gas. For the nitrogen poisoning, the catalyst was exposed at 250 °C for 10 h to a gas mixture of 500 ppm NH3/CH4 diluted with N2 to achieve final concentrations of 400 ppm NH3 in the feed gas. In both cases, the system was then purged again with N2 for 1 h at 250 °C and cooled down in N2. Before removing the sample from the setup, it was carefully passivated at ambient temperature step-wise with increasing concentrations of O2/N2 from 0.1 to 21 vol%.Poisoning with the Na, Ca and Fe metals was performed via incipient wetness impregnation of the catalyst with aqueous solutions of 1.5 mg/ml Na2CO3, 2.9 mg/ml Ca(NO3)2*4H2O and 3.6 mg/ml Fe(NO3)3*9H2O. After impregnation, the samples were dried in a compartment dryer at 80 °C for 18 h and subsequently calcined at 550 °C (ramp 100 °C/h) for 12 h in a muffle furnace.The catalysts are denoted as CuZnAl and CuZnAl_X (where X  = S, N, Na, Ca and Fe) and refer to the untreated and treated samples, respectively.The treated samples were subjected to physiochemical characterization to determine the actual concentration of the impurities, both pre- and post-reaction. The S content was determined via tube furnace combustion, according to the ASTM D4239 method. The N content was measured with the Kjeldahl method, following a three-step process completed by titration for nitrogen. Finally, the Na, Ca and Fe content was determined by Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES) on a spectrophotometer Optima 4300 DV (PerkinElmer).X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Analytical AXIS UltraDLD spectrometer equipped with a monochromatic Al Kα excitation source (λKa = 1486.6 eV), under ultra-high vacuum conditions (10−9 Torr). Binding energy (BE) referencing was employed using the adventitious carbon peak at 284.6 eV. Survey scans were recorded for surface elemental analysis (pass energy 160 eV) at an X-ray power of 180 W, while high resolution spectra were recorded at 20 eV pass energy with a step of 0.1 eV and an X-ray power of 225 W. Deconvolution of the peaks was performed with Kratos Vision software (version 2.2.1) using Shirley background subtraction and mixed Gaussian (70%) – Lorentzian (30%) functions. Auger electron spectroscopy (AES) was performed both via X-ray excitation under the same conditions as described above and via electron beam accelerated with a voltage of 10 kV.The catalytic performance of the materials in CO2 hydrogenation was evaluated in a continuous high-pressure dual fixed-bed reactor unit (Microactivity Effi-PID) with an electronic feed control system for both gaseous and liquid feeds. The unit’s feeding system consists of three gas lines, each equipped with high accuracy mass flow controllers, and one liquid feed line, using a high precision pump. The stainless steel reactors (ID: 9.1 mm) are fitted with porous plates to ensure that the catalyst bed is located in the furnace’s isothermal temperature zone. The reaction temperature is monitored with a thermocouple inserted in the catalytic bed. The exit stream of the reactors is sent to a liquid–gas separator for separation and collection of the liquid and gaseous products.In a typical experiment, the required catalyst amount was diluted with equal amount of SiC (dp: 200 μm) and was introduced into the reactor. Prior to the measurements, the catalyst was pre-reduced in-situ by slowly ramping the temperature and hydrogen content to 5% H2/N2 at 250 °C and holding for 2 h. The reaction was conducted at temperature 250 °C, pressure 70 bar, GHSV 6500 h−1 and H2/CO2 feed molar ratio 3. Both liquid and gaseous products were analyzed with a GC 7890 gas chromatograph (Agilent Technologies) equipped with a dual FID/TCD detector. Mass and carbon balances typically closed to within 5%. All the gases used (H2, N2, CO2) were of purity above 99.999%. The molar conversion of the reactants and the carbon-based molar selectivity of the products was calculated as follows: (1) Conversio n i mol % = n i , i n - n i . o u t n i , i n ∗ 100 where n i , i n and n i , o u t are the molar flows of reactant i at the inlet and outlet of the reactor. (2) Selectivit y i C - m o l % = c n i . o u t c n C O x , i n - c n C O x , o u t ∗ 100 where c n i , o u t is the C-molar flow of product i and c n C O x , i n and c n C O x , o u t are the C-molar flow of CO2 at the inlet and outlet of the reactor.The ex-situ poisoned CuO/ZnO/Al2O3 samples were characterized to determine the concentration of the impurities on the catalytic surface. Table 1 shows the nominal (calculated based on the poisoning experimental parameters) and the experimentally-determined concentration of the different impurities before and after reaction. The contamination of the catalyst with sulfur from gas phase H2S leads to high S concentration (∼65% of the nominal), confirming the known affinity of the catalyst components to sulfur. Several studies report the formation of sulfide phases, including ZnS, CuS, Cu2S, and CuSO4 [12,22]. The results are very different for nitrogen, where the actual deposition is more than an order of magnitude lower than the nominal value. This suggests that gaseous nitrogen compounds, such as NH3, do not interact strongly with the catalyst components. Schühle et al. [19] attained similar results and report the accumulation of only 0.02 wt% N on a commercial CuO/ZnO/Al2O3 after treatment in 500 ppm NH3 for 4 h. In the case of Na, Ca and Fe, the actual content of the metals is in all cases very close - as expected - to the nominal one, due to the impregnation procedure that was followed. It should be noted that the untreated catalyst also contains a small amount of residual Na (0.05 wt%), probably originating from Na2CO3 typically used as precipitation agent in the industrial preparation of CuO/ZnO/Al2O3 catalysts.The poisoned catalysts were also characterized post-reaction to check whether the contaminants are removed under prolonged exposure to the reaction conditions (see following section on catalyst testing). As shown in Table 1, there is a prominent reduction in the nitrogen content, and a milder, yet significant, decrease in sulfur. This suggests partial reversibility and/or weak binding of the sulfur and nitrogen species on the surface of the CuO/ZnO/Al2O3 catalyst. The contaminants are probably removed from the surface in the form of H2S and NH3 respectively under the high H2 pressure conditions of the reaction. Schühle et al. [19] also showed substantial decrease in the sulfur levels of H2S- and SO2- poisoned In2O3/ZrO2 catalysts after the CO2 hydrogenation reaction. The metals appear to be more resilient; their concentration varies within experimental error before and after reaction.X-ray photoelectron spectroscopy (XPS) was employed to probe the catalyst surface and obtain insight on the surface elemental states. The Cu 2p, Zn 2p, Al 2 s and O 1 s core-level spectra of the ex-situ poisoned and non-treated CuO/ZnO/Al2O3 samples are shown in Fig. 1 . The Cu 2p3/2 transition (Fig. 1, top left) demonstrates a main peak centered at ∼ 933.3 eV, with a satellite peak at 941–943 eV characteristic of Cu2+ species [23]. The Cu 2p3/2 peak is strongly asymmetric in the samples poisoned with nitrogen and sulfur. Via spectral deconvolution, three different Cu species can be identified on the catalysts’ surface. In all samples, the main copper species is CuO at 933.2 – 933.4 eV [24]. A second well-defined peak appears at ∼ 935 eV. This high binding energy peak has been attributed to basic copper carbonate, Cu2(OH)2CO3, which forms either due to the exposure of the samples in atmospheric air and/or from the precursor salts used during the preparation of CuO/ZnO/Al2O3 [25].The presence of metal–oxygen and metal-carbonate bonds is also confirmed by the detection of the respective oxygen species in the O 1 s core-level spectra (Fig. 1, bottom right). The S- and N-poisoned samples exhibit in addition a third copper peak at 932.2 eV that corresponds to reduced Cu+/Cu0 species [26]. This is reasonable, considering that both these samples were poisoned in the gas phase in hydrogen atmosphere. Further chemical state differentiation between Cu+ and Cu0 is difficult with XPS alone, as the XP binding energies are practically the same [26], but is possible with Auger spectroscopy. The respective Cu LMM Auger spectra show peaks at 569.4 – 569.6 eV (calculated Auger parameter 1849.6 – 1849.7 eV) attributed to Cu+ species and at 568.2 – 568.3 eV (Auger parameter 1850.7 – 1850.8 eV) corresponding to metallic Cu. In both S- and N-poisoned samples, the majority of the reduced copper species are in + 1 valence state (greater than 80%).The Zn 2p3/2 core-level spectra (Fig. 1, top right) consist of a well-resolved peak at 1021.3 eV, originating from ZnO or Zn in close interaction with Cu [27]. The Al 2 s transition (Fig. 1, bottom left) also has one main peak at 118.7 eV associated with Al2O3 [28]. The commonly used Al 2p line is not considered, as there is strong overlap with the Cu 3p transition. Overall, the binding energies of the main catalyst components, Cu, Zn and Al, do not show appreciable shifts, suggesting that the poisons (at the investigated concentrations) do not affect in a significant manner the nature of the main species on the CuO/ZnO/Al2O3 catalyst.The XP spectra of the poison elements are presented in Fig. 2 . Despite the high background noise, which is a result of the low contaminants’ concentration, some useful information can be deduced. The S 2p core-level spectrum (Fig. 2, top left) demonstrates a single peak centered at 168.9 eV, characteristic of sulfates. The binding energy of sulfur in both copper and zinc sulfate species is within the range of the recorded value [29]. Cu and Zn are known to react with H2S, forming sulfide species. ZnS formation is thermodynamically more favorable and thus, ZnO serves also as a sulfur trap, improving the S-poisoning tolerance of Cu/ZnO catalysts [30]. In fact, sulfur adsorption on ZnO is promoted in the presence of Cu [10]. Still, both ZnS and CuS have been previously detected on sulfur-deactivated catalysts [12,22,31]. Furthermore, CuSO4, ZnSO4, Zn3O(SO4)2 and Cu1.5ZnSO4(OH)3 phases were identified on Cu/Zn/Al2O3 catalysts poisoned under commercial and laboratory conditions [31]. It was suggested that in a commercial reactor, the conversion is so high that the gas phase becomes less reducing near the reactor exit, leading to sulfate formation [9]. Moreover, CuSO4 has been previously observed on Cu/ZnO catalysts, formed as a result of the thermal decomposition of CuS in O2-containing atmosphere during passivation [22,32,33]. Previous reports show that CuS is easily and irreversibly oxidized to CuSO4 by oxygen in humid environments [34]. It can thus be postulated that the poisoning procedure with H2S followed in our study possibly leads to the formation of zinc and copper sulfides on the surface that readily oxidize to sulfates during the passivation process and the subsequent exposure to atmospheric air.With regards to nitrogen, the surface concentration is below the detection limit (Fig. 2, top right) and therefore no information on the nitrogen compounds can be derived. Both Na and Ca exist on the surface in their respective carbonate form (Na2CO3 and CaCO3), as evidenced by the Na 1 s peak at 1071.6 eV [29] (Fig. 2, middle left) and the Ca 2p3/2 and Ca 2p1/2 doublet at 346.9 eV and 351.1 eV [29] (Fig. 2, middle right). Previtali et al. also detected the formation of CaCO3 by XRD in CuZnAl catalysts doped with 1% Ca [35]. The Fe 2p core-level spectrum (Fig. 2, bottom left) is complex, with multiplet splitting and satellite peaks. Due to the low resolution, unambiguous identification of the iron species is difficult, however the binding energy values and the strong satellites point to the presence of Fe2O3 [36].Besides information on the oxidation state and the chemical environment of the different elements, XPS also allows the analysis of the elemental composition of the outer catalyst surface layers. Fig. 3 (left) shows the surface composition of the poisoned and un-treated samples. The main component on the surface of the CuO/ZnO/Al2O3 catalyst is Cu, which is also the active site for the methanol synthesis reaction, with surface coverage of greater than 40 wt%. There is also high Zn surface coverage and substantial amount of carbon and oxygen that originate from the respective metal carbonates, metal oxides and air contamination. The controlled poisoning of the catalyst with the different contaminants causes a clear decrease of the Cu surface exposure, with a concurrent surface segregation of Zn. This possibly occurs due to partial sintering of the CuO/Cu particles during the different heating and calcination steps of the poisoning procedure. It also suggests preferential deposition of the contaminants on the Cu sites. The extent of surface Cu depletion varies between the different poisons, with the smallest effect caused by S and N and the largest by Ca and Fe. Wood et al. [13] showed by means of Auger electron spectroscopy and XRD that sulfiding at low levels of H2S (<4 ppm) favors the formation of surface sulfur adspecies on copper, thus decreasing the surface concentration of Cu, in agreement with our results. This trend was however reversed at higher concentrations of H2S, with the surface concentration of copper increasing beyond that of the fresh catalyst, as the ZnO phase started gradually being converted to ZnS. Previous studies on the effect of Na species in Cu/ZnO and Cu/ZnO/Al2O3 catalysts also showed lower Cu surface area due to blocking of the Cu sites by Na+ [37,38]. Fig. 3 (right) shows on smaller scale the concentration of the contaminants on the surface. The bulk concentration (nominal and experimentally-determined) is also plotted for comparison reasons. It is interesting to note that the non-metallic elements, S and N, have less surface exposure than the bulk, as opposed to the three metallic elements, Na, Ca and Fe, that preferentially segregate on the catalyst’s surface. The most striking difference is observed for Fe that has more than an order of magnitude higher surface concentration compared to the bulk. In multicomponent systems, the surface rearranges thermodynamically to the most stable configuration, i.e., the one with the lowest surface energy. Therefore, the surface becomes enriched in the constituent possessing the lowest surface tension. Experimental surface energy data is scarce and more often, theoretical values, based on DFT calculations, are reported. For CuO, the most stable crystal plane is CuO(111) with a calculated surface energy of 0.76 J/m2 [39]. The respective values for the most stable crystal planes of CaCO3 (the calcium compound detected by XPS on the surface of the Ca-poisoned sample) are comparable, in the range of 0.73 – 0.84 J/m2 [40]. Unfortunately, no data could be retrieved for Na2CO3 detected in the Na-contaminated catalyst. On the other hand, the surface energy of FeOx phases is reported to be about half compared to CuO, ca. 0.36 – 0.40 J/m2 [41]. Therefore, the Fe-enriched surface in the CuZnAl_Fe sample could be possibly attributed to the lower surface energy of FeOx compared to CuO, which provides the thermodynamic driving force for the formation of an iron oxide capping layer on copper. Alternatively, the very high surface concentration of Fe might also originate from gradients of Fe over the macroscopic grains measured in XPS.The catalytic performance of the ex-situ poisoned catalysts was assessed as a function of time-on-stream at constant experimental conditions (T = 250 °C, P = 70 bar, GHSV = 6500 h−1, H2/CO2 molar ratio = 3) to determine the impurities’ effect both on the initial catalytic performance and the deactivation rate. The non-treated catalyst was also tested for comparison reasons. Measurements were taken after ∼ 24 h time-on-stream to ensure that the system reached steady-state. Over all samples, the carbon-containing reaction products consist only of CH3OH, produced via CO2 hydrogenation, and CO, produced via the reverse-water–gas-shift (RWGS) reaction. No other products are detected even in ppm levels, confirming, in accord to the XPS findings, that the poisons do not alter the basic nature of the active site and thus do not catalyze other side reactions (at least at the investigated concentrations and conditions). The normalized conversion of CO2 with time and the activity losses in initial conversion and after 100 h TOS are shown in Fig. 4 . The respective results for the normalized CH3OH carbon-based selectivity and the selectivity losses are presented in Fig. 5 . The conversion and selectivity values were normalized over the initial value (first measured data point) recorded for the non-poisoned reference catalyst.The untreated catalyst exhibits relatively good stability, with about 5–6% loss in activity and selectivity after 150 h time-on-stream. It is widely accepted that the main deactivation process of Cu-based catalysts in syngas methanol synthesis is thermal sintering via a surface migration process and the growth of the Cu crystallites [9,42]. More recent studies show that ZnO plays a major role in the sintering process. Lunkenbein et al. [43] claimed that deactivation mainly occurs from changes in the ZnO moiety. The disruption of the Cu/ZnO synergy caused by the formation of Zn-Al mixed phases has also been suggested by Ficht et al. [44]. The presence of water, which is generated in abundance in methanol production from CO2, significantly accelerates deactivation. Liang et al. [45] investigated in detail the deactivation behavior of a CuZnAl catalyst in CO2 hydrogenation to methanol during 720 h TOS and showed that the agglomeration of ZnO species and the oxidation of metallic Cu are the main reasons for catalyst deactivation.The presence of impurities clearly degrades the performance of the CuO/ZnO/Al2O3 commercial catalyst. Depending on the contaminant’s nature and concentration, there is decrease in both the initial activity and selectivity and the rate of deactivation as a function of time-on-stream. Nitrogen (which has the lowest concentration on the catalyst) has the smallest impact; it causes an ∼ 8% decrease in initial activity compared to the benchmark and negligible effect on the deactivation rate (in the same range as that of the un-treated sample). This agrees very well with the results of Schühle et al. [19], who report that the treatment of a Cu/ZnO/Al2O3 catalyst for a period of 4 h with NH3 (leading to a N content of 0.02 wt%) decreases methanol productivity at 250 °C from 0.64 to 0.58 gMeOH/g Cu −1h−1 (9.4% reduction). Furthermore, they report no further deactivation at higher N concentrations.Sulfur, despite its much higher concentration than nitrogen, is moderately worse, with ∼ 10% lower initial CO2 conversion and CH3OH selectivity than the untreated sample. It accelerates however deactivation, as evidenced by the more negative slope of the performance curves as a function of time-on-stream in both Figs. 4 and 5. The activity and methanol selectivity drop by about 10% after 100 h time-on-stream, suggesting that the S-poisoned catalyst exhibits twice the deactivation rate of the non-treated and NH3-treated samples. In the study of Schühle et al. [19], a 17% drop of methanol productivity at 250 °C is reported after 4 h of H2S-treatment leading to a sulfur content of 0.3 wt%. This discrepancy with our data could be due to differences in the composition of the employed CuZnAl catalyst. Data on the poisoned catalysts’ stability are unfortunately not reported in the aforementioned study. Older works [31,46] examining the poisoning of Cu-based catalysts in H2S and thiophene-contaminated syngas showed that a Cu/ZnO/Al2O3 catalyst loses about 20% of its methanol synthesis activity with an average sulfur accumulation of 2%. This loss increases to 75% when the sulfur content increase to 12%.The contamination of the catalyst with metals has more pronounced impact on the CO2 hydrogenation performance. The ex-situ poisoning procedure followed for the metals involved the calcination of the catalyst at 550 °C for 12 h after impregnation to decompose the respective precursors. This higher temperature treatment could induce modifications in the structure and active phase dispersion of the catalyst and thus contribute to the observed deactivation. To address this, we calcined the untreated catalyst to these high-temperature conditions and re-measured its catalytic performance under identical experimental conditions. The normalized CO2 conversion and CH3OH selectivity, shown as open black symbols in Figs. 4 and 5 respectively, denote that the catalyst performance does not deteriorate as a result of the exposure to 550 °C. Therefore, the pronounced deactivation observed for the metal-poisoned samples is solely attributable to the impact of these elements on the Cu/ZnO/Al2O3 catalyst. Among the investigated metals, Na has the most detrimental effect. Sodium contamination reduces initial conversion and selectivity by ∼ 20–24% compared to the benchmark. It also accelerates the deactivation rate, with conversion and selectivity dropping additionally by 16% and 23% respectively after 100 h operation. In the reviews by Kung [9] and Twigg and Spencer [42], alkaline impurities appear to promote the production of higher alcohols and hydrocarbons from syngas; these high molecular weight deposits can eventually block the pores and reduce the activity. No formation of higher alcohols or hydrocarbons is detected here, in agreement with other studies on Na-doped Cu/ZnO and Cu/ZnO/Al2O3 studies in CO2 hydrogenation [37,38], underlining the differences between CO and CO2 conversion. Both these aforementioned studies report a strong decrease in CO2 conversion and methanol selectivity with increasing sodium content.Iron poisoning decreases the initial values of conversion and selectivity by ∼ 18%; the respective decrease for calcium is lower (∼12%). The order is reverse for the deactivation rate. Calcium demonstrates a higher deactivation rate than iron, so that the Fe- and Ca-poisoned samples present a very similar CO2 hydrogenation performance after 100 h time-on-stream. Literature on the poisoning effect of alkaline earth metals is scarce. Previtali et al. doped a commercial CuZnAl with 1 wt% Ca and reported lower CO2 conversion and CH3OH selectivity than the undoped catalyst [35]. More studies exist on the effect of iron, focused however on methanol production from syngas. These studies, summarized in the review of Chinchen et al. [46], report a decrease in methanol selectivity and total conversion in iron-doped catalysts, with an increase in the selectivity to methane, higher hydrocarbons and higher alcohols (not detected in this study). In general, the effects of a given level of iron impurity in a catalyst are strongly dependent on its form, oxidation state and distribution [42]. The addition of Fe was reported to enhance stability by preventing the sintering of Cu nanoparticles and inhibiting the oxidation of copper surfaces in CuO-ZnO-ZrO2-Al2O3/HZSM-5 [47] and CuO-Fe2O3-CeO2/HZSM-5 [48] bifunctional catalysts for CO2 hydrogenation to dimethyl ether, and in Fe-Cu/SiO2 catalysts for the high temperature reverse water gas shift reaction [49]. On the other hand, transition metal oxides, such as the Fe2O3 species formed on the surface of the Fe-poisoned catalyst in this study, can inhibit the formation of methanol, probably by surface coverage of the copper crystallites [42].To verify this hypothesis, we plot in Fig. 6 the normalized initial conversion of CO2 over the poisoned and un-treated CuO/ZnO/Al2O3 samples as a function of the surface copper exposure, determined from the XPS analysis. The data reveal that the catalytic activity is closely related to the exposed Cu surface area. This observation fully supports the XPS findings and confirms that the poisons deactivate the catalyst by blocking part of the Cu active sites, either through association with Cu and formation of new phases (as in the case of sulfur) or mere deposition and coverage of the Cu sites (e.g., iron). Therefore, the poisons do not appear to modify the nature, but rather the number of the active sites that convert CO2. The only catalyst that deviates from this general trend is the Na-poisoned sample, which suffers more severe activity losses than expected based on the measured Cu surface area. We postulate that this stems from the strong basicity of sodium. Sodium contamination probably leads to the generation of basic sites on the catalyst surface that not only block active Cu sites, but also strongly bind the acidic CO2 and thus cause a considerable reduction in activity. Kondrat et al. [37] also observed discernable changes in the activities and selectivities of different loading Na+ doped catalysts with however comparable Cu surface areas. They thus concluded that Na+ acts directly as a poison, through increasing surface basicity, by blocking active sites or by inducing phase separation between Cu and ZnO.Besides the reduction in CO2 conversion, all samples, including the un-treated CuZnAl catalyst, demonstrate a progressive decrease in methanol selectivity with time-on-stream. The plot of CH3OH selectivity versus CO2 conversion for all catalysts and different times-on-stream reveals a very interesting trend. It is evident from the data in Fig. 7 that, despite the differences in the initial CH3OH selectivity induced by the different poisons (see Fig. 5), the further decrease of selectivity is independent of the contaminant type and relies only on the extent of the activity reduction with time due to thermal sintering. As aforementioned, thermal sintering disrupts the Cu/ZnO synergy which is more crucial for methanol synthesis, but less important for the RWGS reaction [50]. This explains why deactivation reduces not only activity, but also methanol selectivity. The above reinforce the finding that the poisons block or render inactive a part of the active sites, but do not modify their intrinsic nature.In this work, we show that typical impurities present in steel-work off-gases, namely S, N, Na, Ca and Fe, can influence the performance and deactivation of a commercial CuO/ZnO/Al2O3 catalyst in the production of methanol from CO2. The exposure of the catalyst to gaseous H2S leads to high sulfur accumulation, confirming the known affinity of the catalyst components to sulfur. The interaction with NH3 is much weaker, resulting in low concentration of weakly-bound nitrogen species on the catalyst surface. The poisoning of the catalyst with Na, Ca and Fe via impregnation from the respective carbonate and nitrate salts leads to almost quantitative deposition of the metals. XPS characterization reveals that the contaminants preferentially block the Cu sites, as they cause a clear decrease of the Cu surface exposure, with concurrent Zn surface segregation. The extent of surface Cu depletion varies between the different poisons, with the smallest effect caused by S and N and the largest by Ca and Fe. However, XPS shows that the contaminants do not affect in a significant manner the oxidation state or the chemical environment of the main Cu, Zn and Al catalyst species.This is fully supported by the results of the catalyst testing in CO2 hydrogenation at 250 °C and 70 bar for more than 120 h time-on-stream. All impurities decrease the initial CO2 conversion; the extent of the activity loss is directly proportional to the poison-induced decrease in the Cu surface area, confirming that the poisons do not modify the nature, but rather the number of the active sites. The Na-contaminated catalyst deviates from this general trend, due to the strong basicity of sodium that creates basic sites on the catalyst surface that not only block active Cu sites, but also strongly bind the acidic CO2 and thus cause a more severe reduction in activity. Moreover, all catalysts, including the un-treated sample, suffer a progressive decrease in CH3OH selectivity with time, which is independent of the contaminant. We postulate that the preferential formation of CO with time-on-stream is due to thermal sintering that disrupts the Cu/ZnO synergy, which is more crucial for methanol synthesis but less important for the RWGS reaction.Overall, this study demonstrates that industrial CO2 off-gases can be exploited for the production of methanol over classical Cu/Zn/Al methanol synthesis catalysts after appropriate cleaning. The relatively high poison concentration used to accelerate deactivation and mimic the state of the catalyst after several months of continuous operation allows to extend the main findings of the study to real industrial operation, where the impurities would have a permanent presence in the gas feed stream and would accrue on the catalyst surface over time. Moreover, the irreversibility/partial reversibility of the contaminants under reaction conditions suggests that also in real in-situ poisoning, the reactants would compete with the poisons for the catalytic active sites, rendering the results relevant to industrial operation. Regarding flue gas cleaning, purification should primarily target the removal of metal impurities, such as alkali, alkaline earth and transition metals, that cause severe blocking of the active Cu sites. Sulfur and especially nitrogen removal are less critical, due to weaker and partially reversible binding on the catalyst surface.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was funded by the European Union through the Research Fund for Coal and Steel (RFCS), within the project entitled “i3upgrade: Integrated and intelligent upgrade of carbon sources through hydrogen addition for the steel industry”, Grant Agreement No 800659. The authors thank Clariant for providing the commercial methanol synthesis catalyst.

The use of steel-work off-gases as CO2 source for methanol production is a promising strategy for the decarbonization of the steel industry. In this work, we show that typical impurities, namely S, N, Na, Ca and Fe, reduce the activity of a commercial CuO/ZnO/Al2O3 catalyst by blocking part of the Cu active sites. The extent of the activity loss is proportional to the poison-induced decrease in the exposed Cu surface area, suggesting that the poisons do not modify the nature, but rather the number of active sites. The only deviation is observed for Na, as its strong basicity leads to stronger binding of CO2 on the catalyst surface. Moreover, all catalysts suffer from methanol selectivity losses with time, due to thermal sintering that occurs similarly over the untreated and poisoned samples. Industrial CO2 off-gases can thus be exploited for methanol production after appropriate purification targeted primarily at the removal of metal impurities. The presence of sulfur and nitrogen is less critical to the catalyst performance.

The effects of global warming have prompted the search for technological alternatives to mitigate its impact and thus avoid the increase in greenhouse gas emissions, with CO2 and CH4 representing an important part of the total amount emitted into the atmosphere. Among these alternatives, the dry reforming of methane (DRM; CO2 + CH4 ⇌ 2 CO + 2H2) has been positioned as a technology that can reduce pollution while also acting as an important energy, source, thus allowing the development of a comprehensive system for capturing greenhouse gases [1].Developing efficient catalysts that allow the application of DRM on an industrial scale is essential for the implementation of this technology and to produce synthesis gas that can be used to obtain synthetic gasoline [2]. As the support plays a key role in the catalytic activity, it must therefore be carefully selected to allow full advantage to be taken of its physical and chemical properties, such as texture, thermal stability, redox properties, storage capacity, oxygen, and surface acidity-basicity [3,4]. This improves the metal-support interaction and increases the dispersion of active metal particles, thus minimizing the effects of C deposition [5,6]. Noble metals such as Ir, Rh, Ru, Pt, and Pd have a higher resistance to coke deposition than non-noble metals. Given that noble metals are more expensive than non-noble metals, an inexpensive way to prevent coke formation involves the use of multi-metal formulations of non-noble metals such as Ni, Co, and Fe with noble metals [7]. These formulations facilitate metal dispersion and generate more active metallic centers [8]. Ni is the only transition metal that exhibits catalytic properties comparable to those of precious metals. However, Ni-based catalysts tend to generate carbon deposits on the catalyst surface and, subsequently, a loss of catalyst activity. The resulting poor stability limits the commercial use of Ni-based catalysts for DRM reactions and, therefore, Ni-based catalysts must be modified—in terms of the nature of the support and the preparation method—to improve their performance and resistance to carbon deposition [4,9,10].Hexaaluminates are an excellent choice as DRM catalytic supports due to their thermal stability [11]—they exhibit high thermal resistance above 1873 K. The general formula for hexaaluminates is ABxAl12-xO19, where AB represents a large cation such as Ba, La, Na, etc. and a transition (Co, Cu, Fe, Mn, Ni, etc.) or noble metal (Ir, Pd, Rh, Ru, etc.). These materials have been used as catalysts for high temperature applications, superionic conductors and luminescent laser materials, ceramics, and matrices for immobilization of radioactive elements, amongst others [12,13]. In recent years, several synthetic methods for the production of hexaaluminates have been developed, including lyophilization, nitrate decomposition, solid-state reaction, sol-gel, coprecipitation, inverse microemulsion, and hydrothermal synthesis [11]. The most widely used synthetic method is coprecipitation, in which the precursors are homogeneously mixed in the form of ions and precipitated simultaneously. In recent years, this method has been the object of studies aimed at improving the textural properties of the materials synthesized. In this regard, unconventional drying methods, as well as the use of nonconventional sources of raw materials, such as industrial inorganic waste, have been explored.Wastes known as aluminum saline slags are generated during aluminum recycling. These slags contain metallic Al, various oxides, and flux brines as main components, with variations in the percentages thereof depending on the nature of the material to be recycled [14,15]. Due to their limitations for final disposal in controlled landfills, these slags have been used recently to synthesize various materials, including alumina, calcium aluminate, layered double hydroxides, molecular sieves, microporous aluminophosphate, zeolites, pillared clays, and hexaaluminates [15–34]. The objective of these works is to synthesize materials with an application and that, therefore, can contribute to the recovery of industrial waste. The objective of the work would be framed within the so-called Circular Economy [38]. Logically, it is necessary to compare these new materials with the materials that are being used in these applications, to reduce and control the new emissions of pollutants generated and, if they can be applied, to analyze economically this route of recovery of inorganic industrial waste.In this work, a hibonite-type Ni/La-hexaaluminate synthesized from an industrial waste is used and compared as catalyst in DRM. The structure, catalytic behavior, and stability during a run time of at least 50 h of three Ni-catalysts obtained from two commercial supports and two preparation methods were used for comparison.Lanthanum(III) chloride heptahydrate (99.9%, Sigma-Aldrich), nickel(II) nitrate hexahydrate (99% Panreac), polyethylene glycol 400 (Merck), polyethylene glycol monolaurate 400 (PegMn400, Aldrich), and methanol (99.8%, ACS) were used as materials and reagents for the synthesis of the hexaaluminate and supported catalysts. Carbon dioxide (99.996%, Praxair), helium (99.999%, Praxair), hydrogen (99.999%, Praxair), methane (99.5%, Praxair), and nitrogen (99.999%, Praxair) were also used in the characterization and catalytic-performance studies.Aluminum was extracted from the saline slag using a previously reported procedure [33]; briefly, 50 g of saline slag was added to 750 mL of an aqueous reagent solution (HCl 2 mol/dm3) in a reflux system consisting of a 1000 cm3 Erlenmeyer flask with tube condenser, thus avoiding volume losses. The slurry was heated to 373 K and kept at that temperature for 2 h. The solution was then allowed to cool and separated by centrifugation. The most important constituents of the filtered solution were determined by ICP-OES using a VARIAN ICP-OES VISTA MPX with radial vision: Al (9.40 g/dm3), Ca (1.19 g/dm3), Fe (1.03 g/dm3) and Si (0.33 g/dm3).The synthesis of La-hexaaluminate-support was performed with a La/Al molar ratio of 1:11 using a previously reported and optimized method [33]. The slag solution was concentrated to one third of its initial volume to obtain a yellow liquor. A microelmulsion was then prepared using Methanol/Peg400/PegMn400/Al solution in a volumetric ratio of 1/0.8/0.4/0.6. The lanthanum chloride was mixed with the aluminum solution at 353 K, with vigorous stirring. After 10 min, the methanol was added slowly, the mixture stirred for a further 10 min, then Peg400 and PegMn400 were added and the temperature increased to 373 K. This mixture was kept under these conditions for 20 min prior to digestion in the autoclave. The resulting final mixture was heated in a stainless steel autoclave at 493 K for 16 h, drying in an oven until the liquid matrix had been eliminated, then calcined at 673 K for 1 h and 1473 K for 2 h, in both cases using a heating ramp of 10 K/min (Ni/LHA). Wet impregnation of the La-hexaaluminate support synthesized was carried out using 10 wt% of NiO, then the catalyst with the impregnated metallic phase was calcined at 673 K for 2 h. For comparison, the same method is used to prepare the Ni/LHA catalyst but using aluminum nitrate nonahydrate (≥98%, sigma-Aldrich) as aluminum source [34]. Three reference catalysts were prepared using two methods, namely wet impregnation (I) and precipitation-deposition (PD), starting from two commercial oxides, γ-Al2O3 (Rhône-Poulenc) and SiO2 (AF125, Kali Chemie), as supports (Ni–I/Al2O3, Ni–I/SiO2 and Ni-PD/SiO2) [39].The structural phases were analyzed using an X-ray diffractometer (model Siemens D5000) equipped with a Ni-filtered CuKα radiation source (λ = 0.1548 nm). The main textural properties of the solids were determined by nitrogen adsorption at 77 K using two Micromeritics ASAP (2010 and 2020 Plus) adsorption analyzers. Prior to the adsorption measurements, 0.3 g of sample was degassed at 473 K for 2 h at pressures lower than 0.133 Pa. The BET surface area (SBET) was calculated from the adsorption data obtained over the relative pressure range 0.05–0.20. The total pore volume (VP) was calculated from the amount of nitrogen adsorbed at a relative pressure of 0.99. Temperature-programmed reduction (TPR) studies were performed using a Micromeritics TPR/TPD 2900 equipment instrument. TPR tests were then performed from room temperature to 1273 K under a total flow of 30 mL/min (5% H2 in Ar, Praxair). Finally, the morphological analysis and chemical composition of the samples were carried out using a SEM Phenom XL desktop (Mode: 15 kV - Map, Detector: BSD Full) and HR-TEM (JEOL JEM 2100F, Accelerating voltage: 200 kV, Detector: X-Max).DRM was carried out at 973 K using an automated bench-scale catalytic unit (Microactivity Reference, PID Eng&Tech). The reactor was a tubular, fixed-bed, downflow type, with an internal diameter of 0.9 cm and a length of 30 cm. Catalyst samples (25 mg) were mixed with an inert material (SiC, VWV Chemicals-Prolabo) to dilute the catalyst bed and avoid hot-spot formation. The reaction mixture consisted of CH4 and CO2 with a molar ratio of 1:1 (concentration of 12% in the feed), with helium as equilibrium gas up to a total feed flow of 40 cm3/min, thus achieving a gas hourly space velocity (GHSV) of 9.6·104 cm3/g h. Before the reaction, the catalyst was reduced in situ using 30 cm3/min of H2 at 973 K for 2 h. The reagent and product streams were analyzed using an Agilent 6890 gas chromatography system.The nitrogen adsorption isotherms for the supports and catalysts were of type II and IV in the BDDT classification [40] (see Fig. 1 ). The specific surface area (S BET ) and total pore volume (Vp) derived from the experimental adsorption results are summarized in Table 1 . A decrease in textural parameters for the supported catalysts with respect to their corresponding support can be seen. In the case of the precipitation-deposition preparation method, reduction of the textural properties compared to the properties of the catalyst obtained by impregnation was not so important, and can be related to the formation of a talc-like nickel phyllosilicate structure during synthesis of the catalyst. This structure that is formed has been previously reported by our research group [41]. The incorporation of nickel through the wet impregnation method, and its subsequent drying and calcination to obtain NiO, causes the clogging of the porous structure. Under these conditions, the textural properties of the catalyst, specific surface area and pore volume are reduced, with a decrease from 304 to 225 m2/g and from 0.840 to 0.615 cm3/g. In the case of the precipitation-deposition preparation method, the textural properties are practically maintained to those corresponding to those of the support precisely due to the formation of the talc-like nickel phyllosilicate structure.The XRD patterns of the supported nickel catalysts are presented in Fig. 2 . In the case of the hexaaluminate, a very complex diffractogram was obtained. Based on the synthesis methods used and the presence of La and Ca, the most probable hexaaluminate structure appears to be magnetoplumbite (hibonite-Ca, pattern # 00-007-0785). The hexaaluminate obtained presents crystalline characteristics and different phases, as indicated in Fig. 2. The patterns of the samples prepared by wet impregnation reveal the presence of NiO. In the case of the hexaaluminate, the structure of the support remains perfectly stable, without modification. Logically, supports with a higher specific surface area will favor the dispersion of NiO, which can favor the catalytic behavior of these materials. The pattern of the sample prepared by the precipitation-deposition method corresponds mainly to that of the silica support. The possible formation of talc-like nickel phyllosilicate compounds has been reported previously [39,41]. The crystallite sizes of NiO determined using the Scherrer equation can be found in Table 1.The TPR analysis provides information about the interaction between NiO and the support. Depending on the reduction temperatures, the degree of interaction of the NiO species can be classified into four different types: α , β1 , β2 and γ [42]. The TPR patterns and TCD curves of the supported NiO catalysts are included and compared in Fig. 3 . A comparison between the TPR data and the main regions of this classification has also been included in Fig. 3 A). The maximum peak temperatures and fraction of the total area represented by each can be found in Table 2 . Fig. 3 also shows the deconvolution of overlapping peaks using a Gaussian fit for determination. In the case of Ni/LHA, a single peak appears and then undergoes a complete reduction in the region of weak or poor NiO/support intercations, representing 100% of the total peak area. The peak found is not totally symmetric (maximum reduction temperatures at 612 and 652 K), thus suggesting that two types of NiO particles are present and that they may interact with a different surface that makes their reduction rate slightly different. A reduction behavior similar to the previous one can be seen in the case of Ni–I/SiO2, but in this case it also encompasses the next interaction stage ( β1 ). This behavior could be related to NiO particles dispersed on the surface of the easily reducible support (651 K) and other particles occluded in the porous structure, which are more difficult to reduce. The area of the first peak corresponds to 44%, whereas the other peak accounts for 56% of the total. A shift of the TPR peaks to higher temperatures is observed for the samples prepared by the precipitation-deposition method (NiO-PD/SiO2) and considering Al2O3 as support. In these two cases the temperatures of the reduction maxima are shifted to 871 and 981–1044 K, respectively. These catalysts exhibit four peaks with maximum peak area percentages of 71% (Ni-PD/SiO2) and 74% (Ni–I/Al2O3) in the interaction stages β1 and β2 . The main difference between the two catalysts is that the reduction temperatures found for the Ni–I/Al2O3 catalyst are shifted to a higher temperature. Indeed, the highest temperature of one of the reduction peaks in this case is 1044 K. These results confirm the findings of the X-ray structural analyses, namely that the formation of various nickel compounds makes it more difficult to reduce than the NiO metallic oxide. These results are in according to the degree of interaction/reaction of nickel with the surface of the SiO2 and Al2O3 supports, aspects previously referenced in the literature [41]. The structure of γ-alumina and the size of Ni2+ allow the formation of Ni–Al spinel, especially if it favors temperature. Under these conditions, the reduction temperature of nickel increases considerably when compared to the reduction of NiO particles dispersed on a support. This is the situation observed in Ni–I/LHA and Ni–I/SiO2. When the preparation method is modified so that the degree of nickel interaction with the support surface (Ni-PD/SiO2) increases, the reduction temperature also increases, precisely because of the presence of this strong interaction.The DRM reaction (CO2 + CH4 ⇌ 2 CO + 2H2) is affected by several parallel reactions that occur during the catalytic process: methane decomposition (CH4 ⇌ C + 2H2), the reverse water-gas shift reaction (RWGS; CO2 + H2 ⇌ CO + H2O), the Boudouard reaction (2 CO ⇌ C + CO2), CO2 hydrogenation (CO2 + 2H2 ⇌ C + 2H2O), CO hydrogenation (CO + H2 ⇌ C + H2O), and steam reforming (CH4 + H2O ⇌ CO + 3H2). The conversion and ratio of CO2 and CH4, as well as the selectivity with respect to hydrogen (H2/CO), can give an idea of the prevalence of these reactions during DRM.The conversion (CO2–CH4), selectivity (H2/CO), and carbon balance (CB) results obtained for the catalysts during a long 50 h catalytic test are presented in Fig. 4 . The carbon dioxide and methane conversions [X]i, selectivity [H2/CO] and carbon balance (CB) were calculated using the following equations: Equation 1 [ X ] C H 4 = [ C H 4 ] i n − [ C H 4 ] o u t [ C H 4 ] i n Equation 2 [ X ] C O 2 = [ C O 2 ] i n − [ C O 2 ] o u t [ C O 2 ] i n Equation 3 S e l e c t i v i t y [ H 2 C O ] = [ H 2 ] o u t 2 ∗ [ C H 4 ] i n [ C O ] o u t [ C H 4 ] i n + [ C O 2 ] i n Equation 4 C B = [ C O 2 ] o u t + [ C O ] o u t + [ C H 4 ] o u t [ C H 4 ] i n + [ C O 2 ] i n In the case of Ni–I/SiO2, this catalyst showed a very different thermodynamic behavior from the rest, with the conversion of CH4 and CO2 and selectivity decreasing during the DRM reaction, with slopes of Δ X C O 2 ≈ − 6 % , Δ X C H 4 ≈ − 14 % , Δ X C H 4 ≈ − 7 % (see Fig. 4 A) B) C)) and, therefore, an increasing slope for the CB during the first 35 h (3%), subsequently stabilizing to a mean value of 98%. This behavior can be related to a high deposition of coke, which explains the low catalytic performance as a result of deactivation of the catalyst at a constant rate, practically during the entire test run. The blocking effect of the active sites could be attributed to the morphological transformations that the support and metallic phase undergo during the reduction stage, as an effect of a weak interaction (type α) in which the nickel nanoparticles are practically free and/or weakly fixed on SiO2. This situation causes a high rate of diffusive migration on the surface of the catalyst, thereby greatly benefiting the sintering and growth of the NiO grains. This effect is induced by the thermal gradient and the differences between the calcination and reduction temperature before the DRM test, as well as the impregnation method used for deposition of the metallic phase. In the case of Ni-PD/SiO2 and Ni–I/Al2O3, the catalytic behavior of these catalysts is much more stable than for Ni–I/SiO2, with a slight increase in the case of Ni-PD/SiO2 with respect to Ni–I/Al2O3. The CO2 conversion presents mean values of between 73% and 75% (see Fig. 4 A)). The CH4 conversion also maintains the same thermodynamic regime for both catalysts with average values of between 82% and 85%. With regard to the H2/CO selectivity (see Fig. 4C)), Ni-PD/SiO2 presents an average value of 98% versus 94% for Ni–I/Al2O3, and in the case of the CB (see Fig. 4 D)), in the first 40 h they show different behaviors, with Ni–I/Al2O3 presenting an increasing behavior, stopping at 85% and stabilizing at around 95%, whereas Ni-PD/SiO2 remains at around a mean value of 98%. Over the last 10 h, both catalysts essentially stabilize in the same thermodynamic regime. The precipitation-deposition method (PD) allows a significant improvement in the catalytic performance for Ni-PD/SiO2 compared to Ni–I/SiO2. These results seem to indicate that the strong interaction of nickel with the support favors the stability of the DRM reaction. The hexaaluminate catalyst (Ni/LHA) exhibited the best catalytic performance in terms of yield and stability, with CO2 conversion values of 80% at the beginning of the reaction and 75% at the end ( Δ X  = −2%). CH4 (opposite behavior to that of CO2), in turn, presents an increasing slope of 4% up to a final value of 85%, a behavior which is better than that of the aforementioned catalysts at all times (see Fig. 4 A), B)). With regard to H2/CO selectivity and CB (see Fig. 4C), D)), Ni/LHA continues to show a consistent thermodynamic behavior, with mean H2/CO selectivity values of 99% and a CB of 76%, thus exhibiting the best H2/CO ratio and the lowest rate of deactivation by coke deposition. Similarly to the strong interaction of nickel with the support, the presence of Al in the catalyst also seems to favor the stability of the reaction.SEM, TEM, and TEM-HAADF images for NiO/LHA are shown in Fig. 5 before the reduction stage and after the catalytic test. The morphology of the catalyst (see Fig. 5 A) and B)) shows spherical agglomerates of NiO/LHA for the fresh catalyst and a rosette-like morphology for the used catalyst. The carbonaceous deposits generated during DRM, which were identified as filamentous carbon and carbon nanotubes (see Fig. 5 B) E)) that displace the Ni0 grains (distribution in the range dp ≈ 10–50 nm, see Fig. 5 F)) from the catalyst surface, are also found. Although these forms of coke still block the active metallic sites, they are the least harmful, thus allowing this catalyst to maintain great stability and excellent performance in DRM at 973 K. Another important aspect to highlight is that the amount of carbon produced is much lower than the critical concentration necessary to completely exhaust the catalyst and cause it to lose its catalytic ability compared to other catalysts, which in turn exhibit better textural properties.A Ni/La-hexaaluminate catalyst has been synthesized using an aluminum saline slag—a hazardous waste generated in aluminum recycling—as aluminum source in the synthesis of the La-hexaaluminate used as catalytic support. The catalysts were synthesized by impregnation. This method generated two types of morphologies, namely rosettes and clusters of amorphous tables, which allow a good distribution of the metallic nanoparticles on the support, as determined by SEM and HR-TEM. The catalyst showed excellent stability after 50 h of DRM reaction at 973 K. The conversion of CH4 is higher than CO2 and the H2/CO selectivity is about 99%, thus suggesting the predominance of the Boudouard reaction over the RWGS reaction. The behavior of this catalyst is comparable to that of Ni–I/Al2O3 and Ni-PD/SiO2, which is related to both the Ni-support interaction and the presence of alumina.All the authors conceived, designed, and performed the experiments, analyzed the data, and drafted the manuscript.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The authors are grateful for financial support from the Spanish Ministry of Science and Innovation (MCIN/AdslEI/10.13039/501100011033) through project PID2020-112656RB-C21. JJTH thanks Universidad Pública de Navarra for a postdoctoral grant. AG also thanks Santander Bank for funding via the Research Intensification Program.

In this work, a hibonite-type Ni/La-hexaaluminate (Ni/LHA) synthesized from an industrial waste is used and compared as catalyst in the dry reforming of methane (DRM) at 973 K. The structure, catalytic behavior, and stability during a run time of at least 50 h of three Ni-catalysts obtained from two commercial supports and two preparation methods were used for comparison. An aluminum solution (9.40 g/L) obtained from an aluminum saline slag waste by acid extraction was used to synthesize the hexaaluminate by mixing with a stoichiometric amount of lanthanum nitrate and methanol/Peg400/PegMn400 under hydrothermal conditions at 493 K for 16 h. The Ni/LHA catalyst (10 wt% NiO) was obtained by impregnation of the synthesized support, calcined previously at 1473 K for 2 h. The resulting solids were characterized by several techniques as: X-ray diffraction (XRD), N2 adsorption at 77 K, temperature-programmed reduction (TPR), scanning electron microscopy (SEM) and transmission electron microscopy (HR-TEM). In order to compare the catalytic behavior and properties of the Ni/LHA catalyst, three Ni catalysts obtained from two commercial supports (γ-Al2O3 and SiO2) and two preparation methods (wet impregnation (I) and precipitation-deposition (PD)) were synthesized. Analysis of the TPR patterns for the catalysts allowed the type of metal support interaction and NiO species to be determined, with a weak interaction with the support being observed in Ni/LHA and Ni–I/SiO2. The NiO species observed, with crystallite sizes between 9.7 and 40.4 nm, confirm the X-ray structural analyses. The Ni/LHA catalyst was found to be active and very stable in the DRM reaction after 50 h. The catalytic behavior was evaluated from the CO2 and CH4 conversions, as well as the H2/CO selectivity, with values of 99% over almost all the time range evaluated. The behavior of this catalyst is comparable to that of Ni–I/Al2O3 and Ni-PD/SiO2. The results found indicating that the strong interaction of nickel with the support favors the stability of the catalysts in the DRM reaction.

With the rapid development of the automobile industry, the pollution of the environment by automobile exhaust is becoming increasingly serious [1,2]. SOx generated by incomplete combustion of sulfur compounds in gasoline not only is the main source of acid rain but also can significantly reduce the conversion efficiency of vehicle exhaust to NOx, incomplete combustion hydrocarbons and particulate matter and aggravate environmental pollution [3–5]. The sulfur content of gasoline has been strictly regulated worldwide [6,7]. China fully implemented the national V gasoline standard in 2017, which requires the sulfur content to be reduced to 10 μg/g [8,9]. The main sulfur compounds in gasoline include mercaptans, sulfides, disulfides and thiophenes [10]. Mercaptans and thioethers are easily removed due to their high reactivity, while thiophene sulfides are the most difficult to remove due to their low reactivity [11–16]. The petroleum fraction contains thiophene sulfides. To achieve deep desulfurization, the pressure of the hydrogenation reaction should not be less than 2.0 MPa, and the temperature should not less than 300 °C [17,18]. Moreover, thiophene sulfides account for more than 85% of the total sulfur content in gasoline. Therefore, the conditions for deep desulfurization of gasoline by the hydrogenation method are very strict. Generally, the operating pressure is 3–5 MPa, the temperature is 300–450 °C [19,20], and the high-pressure method must reach 10 MPa, which correspondingly increases the material requirements of the equipment, requires materials resistant to high temperature and high pressure, and greatly increases the operating costs as well as the risk factors related to high-temperature and high-pressure operation [21,22]. As an emerging desulfurization technology, photocatalytic oxidation has attracted increasing attention due to its mild reaction conditions and low cost [22–26]. Some materials can be excited only by ultraviolet light, such as Ti, which greatly limits the use of visible light. Therefore, the development of a catalyst with visible light response and its application in the field of photocatalytic oxidative desulfurization has important significance and potential.NiO is a typical p-type semiconductor material and is considered one of the most promising photocatalysts because there are many vacancies in the 3d orbital of Ni2+, which can promote electron migration, resulting in the existence of more holes in the crystal [27,28]. Due to the wide band gap (3.6 eV) of NiO, the absorption efficiency of visible light is reduced. By combining NiO with an n-type semiconductor, the absorption range of NiO can be changed, and the utilization of visible light can be improved, thus improving the photocatalytic desulfurization performance. Dong et al. [29] successfully prepared NiO/g-C3N4 heterojunction photocatalysts by the ammonia evaporation method and investigated the catalytic performance of the composite photocatalyst under visible light. Compared with that of g-C3N4, the catalytic activity of the NiO/g-C3N4 heterojunction photocatalyst has been greatly improved. The research shows that the main reason for this improvement is that the larger specific surface area and heterojunction structure inhibit the recombination of photogenerated electrons and holes. In recent years, bismuth-based semiconductors have received extensive attention from researchers, and Bi2WO6 has been studied in-depth due to its narrow band gap, large visible light response range, and strong oxidation ability. Zhang et al. [30] investigated the effects of different hydrothermal treatment conditions on the particle size, crystal form and morphology of Bi2WO6 and studied its light absorption characteristics. Research shows that prepared Bi2WO6 has strong ultraviolet-visible absorption characteristics and luminous intensity and has good photocatalytic performance. In this paper, NiO and Bi2WO6 were synthesized to prepare new p-n heterojunction semiconductors by hydrothermal and high-temperature calcination methods. The advantages of NiO and Bi2WO6 are complementary to maximize the performance of photocatalytic oxidation desulfurization and achieve the deep desulfurization of gasoline.To meet the national fuel standards and reduce the air pollution caused by automobile exhaust gas, a photocatalyst with strong light absorption performance and high efficiency of photogenerated electron–hole separation was prepared in this paper, which exhibited high photocatalytic oxidation performance and was applied to gasoline desulfurization. Using Na2WO4·2H2O, Bi(NO3)3·5H2O and Ni(NO3)2·6H2O as raw materials, the NiO-Bi2WO6 composite photocatalyst was prepared by hydrothermal and high-temperature calcination methods and applied to the study of benzothiophene (BT) desulfurization in gasoline. The photocatalyst was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectroscopy (UV–Vis-DRS) and other means, providing sufficient evidence for the synthesis of NiO-Bi2WO6 photocatalytic composite materials. In the application of photocatalytic oxidation desulfurization, BT is oxidized into the highly polar benzothiophene sulfone (BTO2) through continuous optimization of the process conditions, which is then extracted by acetonitrile to achieve deep desulfurization.Na2WO4·2H2O, Bi(NO3)3·5H2O, Ni(NO3)2·6H2O, Na2C2O4, C3H8O, C6H4O2, dilute nitric acid, NaOH, thiophene (Th), benzothiophene (BT), dibenzothiophene (DBT), n-octane, acetonitrile, deionized water and absolute ethanol (all the above are analytical pure reagents).The morphology and lattice structure of the catalyst were observed by SEM with a Zeiss Merlin Compact system and by TEM with a Talos F200X system. An Ultimate IV X-ray powder diffraction analyzer (Rigaku Corporation) was used to determine the crystal structure of the catalyst. Cu-Ka rays were used as the laser light source in the experiment. The scanning range was 10°–80°, and the scanning speed was 5°/min. The visible light absorption performance of the catalyst was characterized by a U3900H UV–Vis diffuse reflectance absorption spectrometer (Hitachi Company, Japan), and the wavelength measurement range was 200–800 nm. The elemental composition of the catalyst surface was analyzed by a Thermo Fisher Scientific k-alpha+ XPS system. Electrochemical testing was carried out by an electrochemical workstation (Zahner PP211, Germany). The test conditions were Ag/AgCl as the reference electrode and Pt wire as the counter electrode [31–33]. To prepare a working electrode, NiO/Bi2WO6 powder was deposited on conductive glass with a frequency of 0.1 * 106 Hz and a 150 W cold light source. Organic sulfur in BT was identified by a gas chromatography (GC) system coupled with a flame ionization detector (Agilent 7820A), which was equipped with a capillary column (HP-5, 25 mm × 0.25 mm × 0.25 mm). Heating program: The initial temperature was 120 °C, held for 1 min, increased to 180 °C at a rate of 10 °C/min, held for 1 min, and then increased to 250 °C at a speed of 20 °C/min for 5 min [34,35].First, 0.01 mol of Bi(NO3)3·5H2O was added to 50 m of 0.4 mol/L nitric acid solution and stirred continuously. Then, 0.005 mol of Na2WO4·2H2O was added into 50 ml of deionized water and slowly added to the above solution. After stirring for 1 h, the mixed solution was transferred into a stainless steel autoclave. The solution temperature was maintained at 150 °C for 24 h. After the reaction, the solution was naturally cooled for 2 h to room temperature. The precipitates were centrifugally separated, washed with deionized water and absolute ethanol 3 times, and dried at 60 °C for 12 h to obtain the Bi2WO6 sample.First, 0.006 mol of Ni(NO3)2·6H2O and 0.0003 mol of NaOH were separately dissolved in 20 ml of deionized water, and the latter solution was transferred dropwise to the former solution. After adjusting the pH to 8 with nitric acid, the proper amount of Bi2WO6 was added, and the mixture was stirred for 1 h, ultrasonicated for 15 min, and then placed into a hydrothermal kettle at 180 °C for 16 h. After the reaction, the precipitate was filtered, washed with water and with alcohol 3 times each, and dried at 180 °C for 2 h. Then, the precipitate was placed in a muffle furnace and heated at 350 °C for 2 h, yielding NiO-Bi2WO6. According to the above method, NiO-Bi2WO6 samples with different NiO loadings of 10%, 20%, 30%, 40%, 50%, and 100% were sequentially prepared.First, 0.2097 g of BT was dissolved in 250 ml of n-octane solution and then mixed evenly to prepare a model oil solution with a sulfur content of 200 mg/L. Then, 40 ml of model oil solution and an appropriate amount of catalyst were added into a quartz tube and placed in the photochemical reactor. The reaction solution was first reacted in the dark for 30 min to reach adsorption-desorption equilibrium. Next, the light source was turned on, a 5 ml sample was taken every 30 min, and the extract was extracted with acetonitrile. Then, the upper solution was taken, and the sulfur content was determined with a WK-2D microcoulomb analyzer. By comparing the sulfur content results with that of the original solution, the desulfurization rate was obtained, and the desulfurization effect was analyzed. The desulfurization rate is calculated according to the following formula, where η is the desulfurization rate, C0 is the sulfur content of the solution before the reaction, and Ct is the sulfur content of the solution after the reaction. η = C 0 − C t C 0 × 100 % Fig. 1 shows the SEM images of the sample, where (a) (b) presents the morphology of pure Bi2WO6, and panels (c) and (d) depict the morphology of the NiO-Bi2WO6 composite photocatalyst. As shown in Fig. 1 (a) (b), the pure Bi2WO6 sample has a spherical shape, which is composed of flakes and has a hydrangea-like structure resembling a bird's nest. Fig. 1 (c) (d) shows that the NiO-Bi2WO6 composite still maintains a complete spherical structure. Fig. 1 (d) shows that there are deposits on the surface of Bi2WO6, and the dispersion is good, with no agglomeration. The supported material was thus preliminarily determined to be NiO.To study the lattice structure of NiO-Bi2WO6 in depth, the samples were evaluated by TEM. As shown in Fig. 2 (a), the composite material exhibits a spherical structure. Fig. 2 (c) presents a local enlarged view of the red box shown in Fig. 2 (b). In panel (c), the lattice spacing of 2.59 nm corresponds to the (002) crystal planes of Bi2WO6 (JCPDS No. 39-0256), and 2.09 nm corresponds to the (200) crystal planes of NiO (JCPDS No. 65-2901), indicating that the prepared material was NiO-Bi2WO6. In addition, the EDS images of the corresponding elements in Fig. 2 (d–h) show that the composite was composed of four elements: Bi, W, O and Ni. The elemental mappings of Bi, W and O were consistent with the shape of the composite, and Ni was evenly distributed on the composite. Fig. 2 (i) presents the content diagram of each element in the composite. The atom number ratios are Bi:W:O = 2:1:6 and Ni:O = 1:1, confirming the successful synthesis of NiO-Bi2WO6 composites.To study the phase composition and crystal structure of NiO-Bi2WO6, the samples were characterized by XRD, as shown in Fig. 3 . In Fig. 3 (a), characteristic peaks appeared at 28.4°, 32.9°, 47.2°, 56.0°, 58.6°, 75.7° and 78.2°, corresponding to the (131), (002), (202), (133), (262), (391), and (460) crystal planes of Bi2WO6 (JCPDS No. 39-0256) [36], respectively. The diffraction peaks at 37.2°, 43.3°, and 62.8° corresponded to the (111), (200), and (220) crystal planes of NiO (JCPDS No. 65-2901), respectively. There were no heteropeaks in the figure, and the diffraction peaks were sharp and obvious. This result confirms that we successfully synthesized a NiO-Bi2WO6 catalyst with increased purity and improved crystallinity. Fig. 2 (b) presents the XRD comparison diagram of the photocatalyst before and after the reaction. The figure shows that the catalyst maintains an obvious characteristic peak at the same diffraction angle before and after the reaction. The diffraction peak intensity of the catalyst was only slightly reduced after the reaction, illustrating the excellent stability of the catalyst.The elemental composition and chemical state of the NiO-Bi2WO6 catalyst were further analyzed by XPS. As shown in Fig. 4 (a), the catalyst was composed of Bi, W, O, and Ni, consistent with the EDS test results. Fig. 4 (b) shows two peaks of Bi 4f at 159.0 eV and 164.3 eV after peak fitting, corresponding to Bi3+ in Bi2WO6 [37]. The peaks of W 4f at 35.3 eV and 37.4 eV in Fig. 4 (c) were attributed to W6+ in Bi2WO6. The O 1 s peak fitting results are shown in Fig. 4 (d); three peaks appeared after splitting. Two peaks at 530.2 eV and 531.5 eV were attributed to the O atom of BiO in Bi2WO6, while the peak at 529.3 eV was attributed to the O atom of NiO in NiO34. Fig. 4 (e) shows the characteristic peaks of Ni 2p. The peaks located at 855.6 eV and 860.6 eV were assigned to Ni 2p3/2, while those at 872.0 eV and 879.0 eV were assigned to Ni 2p1/2 [38,39]. These results indicate that the NiO-Bi2WO6 catalyst was successfully synthesized through hydrothermal and calcination methods.The light absorption performance of the photocatalysts was studied by UV–Vis-DRS under the same scale, as shown in Fig. 5 . As observed, pure Bi2WO6 has strong visible absorption in the ultraviolet region from 200 to 400 nm, with the absorption edge at 464.3 nm. However, the NiO-Bi2WO6 composite materials absorb strongly in the visible light region from 400 to 750 nm, with an absorption edge of 704.1 nm, and a redshift occurred. The visible region accounts for approximately 50% of the whole solar radiation energy, while the ultraviolet region only accounts for approximately 7%. Therefore, NiO-Bi2WO6 composite materials broaden the response to light, which is beneficial for improving the photocatalytic oxidation activity. According to the formula of band gap and band edge absorption (Eg = 1239.8/λg, where λg is the absorption edge), the band gaps of Bi2WO6 and NiO-Bi2WO6 are 2.67 eV and 1.76 eV, respectively. In summary, the combination of NiO and Bi2WO6 reduces the band gap, which is conducive to generating photogenerated electrons and holes, thereby further improving the photocatalytic oxidation performance of NiO-Bi2WO6.To evaluate the photocatalytic oxidative desulfurization performance of NiO-Bi2WO6, desulfurization of BT model oil was carried out. As shown in Fig. 6 (a), initially, the model oil exhibited almost no degradation in the absence of light and catalyst. Then, different photocatalysts (NiO, pure Bi2WO6, NiO-Bi2WO6) were separately added to the model oil. After reacting for 30 min in the dark, reaching adsorption-desorption equilibrium, desulfurization was carried out under light conditions. Fig. 6 (a) shows that under dark conditions, although different catalysts exhibited a certain desulfurization rate, the rate was relatively low. When the light source was turned on, the desulfurization rate was significantly increased. The results show that the desulfurization rate of the NiO-Bi2WO6 composite was higher than that of pure Bi2WO6 and NiO because light contributes to the generation of photogenerated electron-hole pairs and the combination of NiO and Bi2WO6 reduces the band gap of the catalyst and photogenerated electron-hole pair recombination, thereby improving the photocatalytic oxidation activity. Fig. 6 (b) further depicts the effect of NiO loading on the desulfurization performance. As the NiO loading increased from 10% to 30%, the desulfurization rate increased from 74.53% to 93.32%. However, when the NiO loading reached 100%, the desulfurization rate unexpectedly decreased to 58.89%. The reason was that with increasing NiO loading, the number of redox-active photogenerated electron-hole pairs increased, increasing the desulfurization rate. As the NiO loading continued to increase, the active sites on the catalyst surface became covered, which unfortunately decreased the desulfurization rate of NiO-Bi2WO6. The experiments show that the ratio of NiO and Bi2WO6 affects the desulfurization effect. When the NiO loading was 30%, the desulfurization effect reached 93.32%.The effect of different catalyst dosages on the desulfurization performance was studied to further improve the desulfurization rate. As shown in Fig. 7 , when there was no catalyst in the system, the desulfurization rate was almost zero. As the amount of catalyst increased from 0.3 g/L to 1.2 g/L, the desulfurization rate increased from 81.36% to 95.37%, reaching the highest value at this time. As the catalyst dosage increased to 1.8 g/L, the desulfurization rate decreased to 86.84%. When the amount of catalyst added to the reaction system was inadequate, the amount of photogenerated electron holes was insufficient, resulting in poor catalytic effects and a low desulfurization rate. As the amount of NiO-Bi2WO6 increased, the desulfurization rate gradually increased. However, an excessive amount of photocatalyst can decrease the light transmittance of the model oil solution and cause light scattering, which will reduce the light absorption and utilization rate as well as the generation of excited states in the reaction system, leading to a lowered desulfurization rate. The experimental results show that the appropriate amount of catalyst has a positive effect on the photocatalytic oxidation desulfurization performance of NiO-Bi2WO6.To explore whether NiO-Bi2WO6 has wide application prospects for the removal of sulfur-containing compounds from gasoline, the desulfurization performance of different substrates, namely, Th, BT and DBT, which are present at high concentrations and difficult to remove, was studied. Fig. 8 shows that NiO-Bi2WO6 exhibited the highest removal rate for DBT, followed by BT and then Th; the desulfurization rates were 98.2%, 95.37% and 85.8%, respectively. The efficiency was affected by the electron cloud density of the S atom. The lower the density of the s electron cloud, the more difficult it is to oxidize. The s electron cloud densities of DBT, BT, and Th were 5.758, 5.696 and 5.639, respectively, so the removal efficiency showed a downward trend. The above results show that the NiO-Bi2WO6 photocatalyst has a good degradation effect on various sulfur-containing model compounds and has a wide range of practical applications.To study the mechanism of the NiO-Bi2WO6 photocatalyst for photocatalytic oxidative desulfurization under light conditions, analyze the electronic conduction and transfer capabilities of the material, and determine the separation efficiency of electrons and holes, the transient photocurrent responses and electrochemical impedance spectra were measured. As shown in Fig. 9 (a), compared with that of Bi2WO6, the transient photocurrent density of the NiO-Bi2WO6 photocatalyst was significantly increased, indicating that as more electrons were excited under light irradiation, the charge transfer performance increased [40]. Thus, the separation efficiency of photogenerated electrons and holes was improved. Fig. 9 (b) shows the electrochemical impedance values of the Bi2WO6 and NiO-Bi2WO6 samples. The resistance value of the NiO-Bi2WO6 samples was much lower than that of the Bi2WO6 samples. A decreased resistance value will accelerate electron transfer, which is consistent with the transient photocurrent response test results. Undoubtedly, the combination of NiO and Bi2WO6 effectively improves the charge transfer capability, reduces the recombination rate of photogenerated electrons and holes, and is beneficial for improving the photocatalytic oxidation performance.To thoroughly study the active species of the NiO-Bi2WO6 photocatalyst during the degradation of BT, an active radical trapping experiment was carried out. In the experiment, isopropanol (IPA), sodium oxalate (Na2C2O4), and p-benzoquinone (1,4-BQ) were used as radical scavengers for h+, ·OH, and ·O2−, respectively. The experimental results are shown in Fig. 10 . IPA, Na2C2O4 and 1,4-BQ have little effect on the degradation of BT. When NiO-Bi2WO6 and different active radical capture agents were added to the BT model oil, the degradation rate changed. The system containing 1,4-BQ changed the most, and the desulfurization rate decreased from 95.31% to 25.83%, followed by Na2C2O4 and IPA, which decreased the rate to 65.37% and 79.44%, respectively. The experimental results show that when the ·O2− in the system was captured by 1,4-BQ, the desulfurization rate was significantly reduced; when h+ was captured, the desulfurization rate decreased slightly; and when ·OH was captured, the desulfurization rate decreased slightly. These results indicate that the active species from NiO-Bi2WO6 that played a major role in the photocatalytic oxidation degradation of BT was ·O2−, followed by h+, and ·OH was not the main active species. The existence of ·O2− was because light irradiation excited NiO-Bi2WO6 to generate photogenerated electrons and holes, and molecular oxygen was reduced by the electrons to generate superoxide radicals. Therefore, the higher the separation efficiency of photogenerated electrons and holes was, the stronger the photocatalytic activity. This result was consistent with the electrochemical impedance spectra.The substance of BT model oil before and after reaction was identified by GC analysis. As shown in Fig. 11 , before the photocatalytic oxidation of the model oil, the only substance present was BT. After the reaction, a large amount of BTO2 existed in the system, while only minimal BT remained. When the reacted model oil was extracted with acetonitrile, it contained only a small amount of BT. The results showed that the superoxide radicals and holes produced by NiO-Bi2WO6 oxidized BT to the highly polar species BTO2 under light irradiation, which was then extracted by acetonitrile to achieve deep desulfurization.Based on the analysis of the above experiments and characterization results, a schematic of NiO-Bi2WO6 photocatalytic oxidation is proposed, as shown in Fig. 12 . The valence band (EVB) and conduction band (ECB) positions of NiO and Bi2WO6 were calculated from empirical formulas (E VB  = X − E e  + 0.5E g and η = C 0 − C t C 0 × 100 % ), and the calculation results are also shown in Fig. 12. Under light excitation, the electrons produced by the NiO/Bi2WO6 photocatalyst were transferred from the valence band to the conduction band, leaving holes in the conduction band and thereby generating photogenerated electrons and holes. Fig. 12 (a) shows that due to the Schottky barrier at the interface, the conduction band position of NiO (−0.54 eV) is more negative than that of Bi2WO6 (+0.55 eV), and the valence band position of Bi2WO6 (+3.22 eV) is more positive than that of NiO (+3.06 eV). Therefore, the photogenerated electrons in the conduction band of NiO easily transition to the conduction band of Bi2WO6, and the holes in the Bi2WO6 valence band easily flow into the NiO valence band, thereby realizing the separation of photogenerated electrons and holes. However, since the standard oxidation-reduction potential of O2/·O2− (−0.33 eV) is more negative than that of Bi2WO6 (+0.55 eV), the conversion cannot be completed [41]. Active radical experiments show that ·O2− is essential to the degradation process; thus, the schematic shown in Fig. 12 (a) is not the mechanism of NiO-Bi2WO6. The energy band structure presented in Fig. 12 (b) can well explain the mechanism of NiO-Bi2WO6. The electrons in the conduction band of Bi2WO6 first recombine with the holes in the valence band of NiO so that the photogenerated electron-hole pairs on Bi2WO6 and NiO are separated. Then, the electrons on the conduction band of NiO are captured by O2, producing the oxidants ·O2−, ·O2− and h+, which can oxidize BT to BTO2. Since the position of the valence band (+3.22 eV) of Bi2WO6 is more positive than that of ·OH/OH− (+2.40 eV) [42], a portion of h+ oxidizes H2O to form ·OH and participates in the reaction. According to the above analysis and the active radical capture experiment, three active substances, ·O2−, h+ and ·OH, participate in the photocatalytic oxidation reaction, and ·O2− plays a major role in the reaction.Catalyst stability and reusability is an important indicator to measure whether a catalyst has application value. Accordingly, the activity of the NiO-Bi2WO6 composite material was studied through a catalyst recycling experiment. Fig. 13 shows the relationship between the desulfurization rate and time when NiO-Bi2WO6 was used for six cycles. The figure shows that after six cycles with NiO-Bi2WO6, the desulfurization rate was only reduced from 95.37% to 88.96%, and the catalyst still exhibited high catalytic activity, indicating that it has good recycling performance and stability. This result is consistent with the XRD measurement results of NiO-Bi2WO6 before and after the reaction, as shown in panel (b). Based on the cycle experiment and XRD measurement results, the catalyst has excellent recycling performance and great stability.In summary, based on the matching band structure between NiO and Bi2WO6, a heterojunction was formed, and this structure facilitated the generation and separation of photogenerated electron-hole pairs. Therefore, a new flower-like NiO-Bi2WO6 composite photocatalyst was successfully synthesized by a hydrothermal method and a high-temperature calcination method. This result was confirmed though various characterization methods, such as SEM, TEM, EDS, XRD and XPS. Under visible light irradiation, the desulfurization rate of the NiO-Bi2WO6 composite material for BT was much higher than that of pure NiO and Bi2WO6. In particular, when the NiO loading was 30% and the catalyst dosage was 1.2 g/L, the desulfurization rate was highest (95.31%). According to the ultraviolet-visible diffuse reflectance absorption spectrum and the electrochemical impedance diagram, this optimal result is because the combination of NiO and Bi2WO6 reduces the band gap, causes a redshift, and exhibits an increased electron-hole separation efficiency, thereby improving the photocatalytic performance. NiO-Bi2WO6 showed a good removal effect on many different thiophene-type sulfides. Moreover, the active radical capture experiments and GC results showed that three active species ·O2−, h+ and ·OH participate in the photocatalytic oxidation reaction, oxidizing BT to BTO2 for removal, and that ·O2− plays a major role in the reaction. Furthermore, the high stability and excellent recycling performance of the NiO-Bi2WO6 photocatalyst indicate that it has good application value.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was financially supported by the National Natural Science Foundation of China (Grant No. 21276156).

In this paper, a hydrangea-like Bi2WO6 material was first synthesized by a hydrothermal method. Then, as a carrier, a NiO-Bi2WO6 composite photocatalyst was successfully synthesized by the hydrothermal method and high-temperature calcination and applied to studying the removal of benzothiophene (BT) from fluid catalytic cracking (FCC) gasoline. The morphology, crystal structure and elemental composition of the catalyst were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS); ultraviolet-visible diffuse reflectance spectroscopy (UV–Vis-DRS) and electronic impedance spectroscopy (EIS) were used to characterize the light absorption and charge transfer ability of the catalyst. The characterization confirmed that the NiO-Bi2WO6 composite photocatalyst was successfully prepared. Moreover, the effects of catalyst dosage, NiO loading and different substrates on the desulfurization rate were investigated by photocatalytic oxidation desulfurization experiments. The experimental results showed that under the conditions of a catalyst dosage of 1.2 g/L and NiO loading of 30%, the highest desulfurization rate of BT was 95.37%. The active species in the reaction process were studied through active radical capture experiments and qualitative analysis of the substances before and after the reaction was carried out via gas chromatography (GC), and the reaction mechanism of the catalyst was explored in depth. The results indicated that the catalyst mainly oxidized BT to benzothiophene sulfone (BTO2) under the oxidation of superoxide radicals to achieve deep desulfurization. Cycling experiments demonstrated that the catalyst still had high stability and catalytic activity after six cycles.

The widespread presence of organic pollutants in wastewater poses a serious threat to ecosystems and public health, many micropollutants are extremely toxic even at low concentrations and require effective removal [1–3]. However, many difficult-to-remove organic pollutants, especially antibiotic aromatic compounds, are difficult to remove by conventional water treatment techniques [4,5]. The Electro-Fenton (EF) process, which oxidizes harmful pollutants by strongly oxidizing OH, is particularly suitable for the degradation of such stubborn compounds [6,7]. The generation of active components in traditional EF generally includes two processes: (1) anodic oxidation to generate oxygen and hydroxyl radicals (·OH yield of the anticathode is much lower than that of the negative electrode) [8]; (2) dissolved oxygen (produced by pumping in air or oxygen) undergoes an oxygen reduction process at the cathode to generate hydrogen peroxide, followed by homogeneous or heterogeneous reactions to generate hydroxyl radicals [9]. Traditional EF anodes are dominated by BDD, Pb, Ti/SnO2, and Au. Although they have higher oxygen evolution overpotential, the expensive cost and high energy consumption limit its application in practical water treatment [8,10–12]. If these energy-intensive anodes can be replaced by excellent oxygen evolution catalysts, it may help to reduce energy consumption while achieving self-oxygenation of the system, thus avoiding the need for additional air or oxygen. Recent developments of EF cathode catalysts have been mostly focused on the composites of transition metals and carbon materials [2,3,13–16], in which carbon defects and heteroatom dopants are exploited as the active sites for preparation of H2O2 by electrosynthesis [17,18]. In addition, some transition metals (such as Co, Fe, Ni, and Cu) have become promising active components for EF processes due to their cheap cost and good property in activating H2O2 [19–23]. However, highly loaded atomically dispersed carbon materials generally require complex preparation process [24], moreover, obvious challenges still remain in the rational regulation of dopants or defects with atomic-level accuracy [25]. Thus, it is of significance to develop bifunctional transition metal catalysts with efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) activity for improving energy efficiency and cathode electrocatalytic activity in conventional water treatment processes.Generally, high efficiency of electron transfer near the catalytic site and suitable free energy of adsorption for OER intermediates at the catalytic site are crucial for the fabrication of highly active OER and ORR catalysts [26,27]. For a long time, electrocatalysts with high oxygen evolution activity are usually noble metal-based materials. However, due to their rarity, more researches currently focus on transition metal-based catalysts, such as Co3O4/CeO2, NiFe LDH, CoSe2, RuO2, etc [28–31]. Among them, CoSe2, as a typical transition metal selenide, has attracted a great deal of interest in energy consumption and conversion, including water separation, Li-ion cells and zinc-air cells, due to its availability in abundance on Earth, acceptable low cost, intrinsic excellent electrical properties and remarkable stability ​[32–34]. Dong et al. [35] homogeneously immobilised CoSe2 nanoparticles on carbon fibre paper by pyrolysis and selenisation of ZIF-67, resulting in a catalyst (CoSe2/CF) with good long-term stability and an over-potential of 297 mV in the OER process (10 mA/cm2). In addition, orthorhombic marcasite-type CoSe2 (o-CoSe2) has been demonstrated to exhibit excellent H2O2 accumulated concentration and selectivity (more than 80%) in 2e− ORR due to the ability that the adjacent Co active sites to inhibit the breaking of O–O bonds, which comparable to that of noble metals in acidic electrolytes [19]. However, when o-CoSe2 is used as the cathode of the EF process, it is usually necessary to add Fe2+ to the solution to activate H2O2 due to tepid fenton activity of CoSe2, which may lead to the formation of iron sludge [22,23]. Iron (Fe), as a typical catalyst, exhibits outstanding catalytic activity in the degradation process of organic wastewater [7,36], and our previous work has also demonstrated the co-operative interaction of Co and Fe in a Fenton-like process [37], Therefore, combining CoSe2 with Fe-based metals may be an efficacious method to improve the Fenton property of CoSe2. The electronic structure projection of doped Mo can facilitate electron transport and synergistic interactions between various elements, affect the sorption/desorption energy of active substance in electro-catalytic reactions and adjust the catalytic capacity [38–40]. Meanwhile, Mo doping also can leads to electronic rearrangements and lattice flaws in CoSe2, which contributes to the electron transfer capacity and catalytic activity of CoSe2 [41]. Therefore, the adjustment of the electronic structure at the CoSe2 surface by the action of Mo and Fe may help to tune the adsorption energy of intermediate ∗OOH and enhance deprotonation of adsorbed water molecules in the OER and ORR process, thereby enhancing its EF performance.Here, we developed Mo-doped FeCo–Se aerogels to couple OER and EF reactions. Because of the superiority of three-dimensional frame structure and large surface area, the aerogel structure could improve the electrochemical behavior of OER and EF through shortened distance diffusion pathways and accelerated mass transfer. The Mo-doped FeCo–Se anode effectively replaces the common anodes such as Pt, BDD, PbO2, SnO2, RuO2, and TiO2. The doping effect of Mo can significantly increase the carrier density of the anode and lower the reaction free energy of the OER process. Under the current intensity of 10 mA/cm2, the overpotential is only 235 mV, which is significantly higher than that of RuO2. The excellent OER performance can provide sufficient oxygen for the oxygen reduction process of the cathode, unlike the general electro-Fenton process, which requires continuous introduction of air or oxygen. Furthermore, on the electro-Fenton side, Mo doping CoSe2 exhibits excellent H2O2 selectivity (ca. 85% at the region of 0.1–0.6 V). Due to synergistic effect of the bimetals, Fe metal effectively activates H2O2 generated on the surface of CoSe2, compared to the CoSe2/Fe2+ system, the production rate of active components is higher, which helps to further obtain the ideal degradation effect, The best Mo0 . 3Fe1Co3–Se catalyst can remove 97.7% of sulfamethazine (SMT) within 60 min (SMT content: 10 mg/L, current intensity: 10 mA/cm2). This work provided a novel perspective for the development of transition metal-based electro-catalysts for EF process.Sodium borohydride (NaBH4, 99.5%), selenium powder (Se, AR), ferric chloride anhydrous (FeCl3, AR), sodium molybdate (Na2MoO4, AR), cobalt (II) chloride hexahydrate (CoCl2‧6H2O, 99%), were bought from Inno-Chem Science & Technology Co., Ltd (Beijing).CoFe metal aerogels (MAs) were prepared by a typical NaBH4-induced gelation process. NaBH4 (0.1 M) was added in the mixture CoCl2‧6H2O (0.05 M) and FeCl3 (0.05 M), then Na2MoO4 (0.05 M) was added to aqueous solution. The obtained solution was left for about 8 h to form a hydrogel. Samples with different metal ratios were achieved by varying the volume ratio of the FeCl3 and CoCl2‧6H2O solutions. In a typical preparation of FeCo–Se, the selenium powder and CoFe aerogels were positioned upside and downside of the tubular furnace, respectively, and the CoFe–Se aerogel was obtained by selenization at 400 °C for 3 h.Scanning electron microscopes (SEM, Hitachi, SU8010), transmission electron microscope (TEM, F20), and electrochemical tests were performed using a CHI 760D electrochemical workstation (CH Instruments, Shanghai) with incorporates a three-electrode system. Oxygen reducing property-related electrochemical tests collected by rotating disc electrodes (PINE, CPR1 and Wavenow)A catalyst suspension was obtained by mixing 20 mg of aerogels sample with 200 μL of ethanol, 200 μL of deionised water, and 50 μL of poly tetra fluoroethylene (PTFE) and subjected to ultrasonic treatment for 30 min. Then, The catalyst suspension was dipcoated in carbon fabric and dried overnight at 60 °C to make a working cathode and anode. 10 mg/L SMT solutions (0.1 M Na2SO4) was used as a model for the target pollutants. Use the organic filter to extract 1 mL of the sample solution at set intervals. The micro-pollutants concentration during the degradation was measured by the high performance liquid chromatography (HPLC) (Shimadzu, LC-10ADVP) fitted with a C18 column (particles size: 1.9 μm, 2.1 × 150 mm). The mobile phase solution for the SMT detection was a mixture of 0.1% acetonitrile and formic acid solution in a ratio of 1:4.DFT calculation was conducted by DMol3 as implemented in materials studio (MS). All energy variations were processed by the generalised gradient approximation of the Perdew-Burke-Ernzerhof (GGA-PBE) function. The K point is set to 4 × 4 × 1 and the size of the vacuum zone has a size of 15 Å. The Mo doping Co–Fe model was formed by cubic cell (Im-3m 229) consisting of elementary particles with the molar ratio of Co: Fe: Mo = 3:1:0.3, where the crystal surface (110) was cleaved as the research object. Meanwhile, the Mo doping CoSe2–FeSe2 model was constructed by a modified CoSe2 cell (Pa-3 (205)), where the Co element particles were replaced by elementary particles with the molar ratio of Co: Fe: Mo = 3:1:0.3 and the crystal surface (110) was cleaved as the research object. The surfaces were modified by U × 2 and V × 2 super cell. The adsorption energy of the intermediate state in the OER process is calculated according to Eq. (1): (1) Δ G = Δ E + Δ Z P E − T Δ S Where ΔS, ΔZPE, and ΔE were the entropy change, zero-point energy change, and binding energy for the adsorption process, respectively.Synthesis of Mo-doped FeCo–Se MAs including the fabrication of CoFe MAs and the subsequent selenization process (Fig. 1 a). The primary self-assemble aggregates first rapidly bind to form nanowire structures when sodium borohydride is used as a reducing and gelling agent, followed by cross-linking of the nanowires to form hydrogels with a three-dimensional porous network feature in the presence of H2 template. After drying, the CoFe MAs were selenized at 400 °C for 3 h by upstream and downstream methods.It can be seen that Mo0 . 3Fe1Co3–Se MAs exhibits an obvious nanowire cross-linked skeleton structure in Fig. 1b–d, and further increase of Mo content may lead to clogging of aerogel pore structure (Fig. S1), which is detrimental to the mass transfer of the catalytic process. Compared with the Mo-doped CoFe MAs without selenization (Fig. S2), the cross-linked framework structure did not collapse and deform significantly after the selenization treatment at 400 °C. This inter-connected framework structure helps accelerate mass transfer and shorten the distance diffusion pathway, which is thought to help enhance electrocatalytic performance. As can be seen in Fig. 1d, the aerogel nanowires have small protrusions on their surface, which may help to further increase the number of active sites available to the catalyst and thus improve its catalytic performance. We performed the corresponding TEM and EDS characterizations to further reveal its morphological features. The nanowires are formed by self-assembly of small primary aggregate units, the diameter of the aerogel nanowires is about 50–60 nm with the core-shell structure features (Fig. 1e and f). We speculate that the internal composition of the nanowires may be Fe (0) according to the different activities of Fe and Co in the reaction process with sodium borohydride, and the lattice diffraction fringes of the surface layer well matched the (111) crystal plane of CoSe2 after the selenization treatment. Fe, Co, Mo, and Se are uniformly dispersed in the aerogel structure. Notably, Se elements are mostly dispersed on surface layers of the aerogel nanowires in the elements mapping (Fig. 1g).The Fe1Co3 MAs, Mo0 . 3Fe1Co3 MAs and Mo0 . 3Fe1Co3–Se MAs samples are described by XRD (Fig. 2 a), the only peak at 2θ = 44.9° are indexed to (110) planes of the Co7Fe3 (PDF#50–0795). In the XRD of Mo0 . 3Fe1Co3 MAs, only the features of Co7Fe3 (2θ = 45.1°) can be observed, suggesting the existing form of Mo is doped or amorphous. After the calcining of Mo0 . 3Fe1Co3 MAs in a Se atmosphere, the resulting of Mo0 . 3Fe1Co3–Se MAs exhibits a marked difference. The different peaks at 2θ = 30.7°, 34.5°, 35.9°and 47.7° correspond to the (101), (111), (120), and (211) of CoSe2 (PDF#53–0449) [19,41], The additional peak at 2θ = 45.2° is indexed to (110) planes of the Co7Fe3. Note that a small positive shift (0.1°) is observed on the characteristic peaks of Mo0 . 3Fe1Co3 MAs and Mo0 . 3Fe1Co3–Se MAs, further confirming that the incorporation of Mo does not produce new crystalline phases in the high temperature heating process (oxidation) [39,40,42]. The above results fully demonstrate the successful preparation of Mo0 . 3Fe1Co3–Se MAs. The specific surface areas of aerogel samples were presented in Fig. 2b and Table S1. It can be seen that the specific surface area of Fe1Co3 MAs, Mo0 . 3Fe1Co3 MAs and Mo0 . 3Fe1Co3–Se MAs are 16.25, 16.70, and 26.13 m2/g, respectively. The significant increase in surface area of the Mo0 . 3Fe1Co3–Se MAs is due to the increased surface roughness of the nanowires as a result of selenisation. Corresponding results can also be seen from the TEM images, with raised CoSe2 crystals on the surface of the aerogel nanowire structure. Besides, Mo0 . 3Fe1Co3–Se MAs also exhibits the larger pore volume and pore width (Table S1). This can provide a sufficient mass transition pathways and active sites number to improve kinetics of reactions.Next, the XPS spectra of Fe1Co3 MAs, Mo0 . 3Fe1Co3 MAs, and Mo0 . 3Fe1Co3–Se MAs are studied to investigate their elemental interrelationships (Fig. S3). The characteristic peaks at ∼712 and 780 eV in the XPS spectra of the different catalysts correspond to Fe 2p and Co 2p [37]. Note that for the Mo0 . 3Fe1Co3 MAs and Mo0 . 3Fe1Co3–Se MAs, an additional peak attributable to Mo 3d occurs at ∼230 eV, implying successful adulteration of Mo elements [39]. In addition, it was noticed that the selenized samples showed more pronounced Se peaks at 55 eV, indicating the successful selenization of Mo0 . 3Fe1Co3 MAs [41]. The hi-resolution spectra of Fe 2p, Co 2p, Mo 3d, and Se 3d are displayed in Fig. 3 . The spectra of Fe 2p can be decomposed into six peaks. Two peaks at 719.9 and 706.6 eV are ascribed to Fe (0), and the other four peaks at 724.7, 722.8, 715.9, and 711.8 eV are ascribed to Fe2+ and Fe3+, respectively [43–45], which is caused by partial oxidation of the aerogel when being exposed to solution and air. Six characteristic peaks were obtained by fitting the Co 2p spectra. The two peaks at 778.5 and 797.2 eV are assigned to Co2+ [40], and another one at 793.6 eV is assigned to Co3+ [39,40]. The two peaks at about 230.6 eV and 228.9 eV on the Mo 3d spectra of Mo0 . 3Fe1Co3 MAs and Mo0 . 3Fe1Co3–Se MAs are assigned to Mo4+ 3d5/2 and Mo 3d5/2, confirming the existence of Mo4+ in the two catalysts, which is attributed to the reduction of NaBH4 [34]. The two peaks at 55.0 eV and 54.3 eV are attributed to Se 3d3/2 and 3d5/2, respectively (Fig. 3c), while another one at 59.4 eV is assigned to SeO x [46], which may be due to surface oxide layer and adsorbed oxygen in air. It is noted that two new peaks at 235.5 and 232.4 eV appears after selenization, which indicates the existence of the Mo6+. The change from Mo4+ to Mo6+ indicates that some of the electrons are captured by the nearby Fe and Co atoms, and this electron transfer phenomenon may be conducive for electrocatalytic behavior [42].The EF catalytic property of the fabricated materials was assessed in a model two-electrode device configured using carbon cloth coated with catalyst as the cathode in the electrolyte. (SMT = 10 mg/L, 0.1 M Na2SO4). Firstly, different metal ratios were examined, where the material with the molar ratio of Mo: Fe: Co is 0.3:1:3 and annealed at 400 °C showed the best catalytic ability (Fig. 4 a). Adsorption removal of SMT is negligible in this process. The removal rate of SMT is close to 100% within 90 min, and the proposed first-order reaction rate of up to 0.084 min−1 (Fig. S4a), which is better than the performance of some recently reported EF cathodes materials (Table S2). Besides, it is noted that the EF performance of Mo0 . 5Fe1Co3–Se MAs becomes worse with higher Mo content. This result may be due to that the increase of sodium molybdate leads to formation of part of molybdate, which is wrapped on the aerogel framework structure (Fig. S1b), thereby decreasing the number of available active sites and leading to poor performance. To determine the dominant role of EF cathode in the degradation process, the corresponding comparative experiments are shown in Fig. S1c. First, Mo0 . 3Fe1Co3–Se MAs/Carbon fabric (CF) is made as the cathode and anode, and the removal of SMT by adsorption is negligible under the condition of no current. CF without catalyst as the cathode and anode exhibits about 16% SMT attenuation performance at the same current density (10 mA/cm2). CF without catalyst as the cathode, Mo0 . 3Fe1Co3–Se MAs/CF as the anode, approximately 34.8% of SMT was removed within 90 min, and when Mo0 . 3Fe1Co3–Se MAs/CF served as the cathode and anode, SMT attenuation efficiency close to 100%. Although the small amount of ‧OH produced by anode will also contribute to the removal of SMT (in addition to the OER process, the anode can contribute to the formation of adsorbed ·OH under the action of oxidation potential), the main contribution to the SMT removal process comes from the EF cathode. In addition, the same volume of 25 mg/L methyl blue (MB) was placed in the cathode chamber and the anode chamber, respectively. Take out 1 mL of the solution at regular intervals for ultravioletand visible (UV) absorbance testing. As shown in Fig. S1d, with the increase of time, the absorption peak of cathode chamber solution gradually decreases and approaches 0, and its fading process is shown in Fig. S1f. The change of UV absorbance of MB solution in the anode chamber is shown in Fig. S1e, and the absorption peak of MB only decreased slightly, and the decoloration of the solution is almost imperceptible (Fig. S1f). In addition, the fading process of MB was similar to the above results when MB indicator was added to the anode chamber (Fig. S1g) or the cathode chamber (Fig. S1h) alone. As the reaction proceeds, the fading of the anode chamber MB solution was almost negligible, while the decolorization of MB in the cathode chamber was almost complete. Given the above comparative experiments, it is not difficult to find that the cathode catalytic process plays a leading role in the degradation process.In general, pH has an impact on the reaction process of EF that involve iron-based catalysts. The percentage degradation decreases with increasing initial pH at any selected electrolysis time (Fig. 4c). The fastest degradation was achieved at a pH of 3.0, achieving complete decay within 75 min, which can be due to the fact that the dissolution of iron can promote a homogeneous Fenton reaction to produce ·OH. None of the other pH conditions resulted in complete elimination of SMT within 90 min, reaching removal rates of 99.8%, 69%, and 61.1% at pH 4.0, 7.0, and 9.0, respectively. The evaluation of degradation performance at various cathode currents is shown in Fig. 4d, only 64.5% SMT was removed at 5 mA/cm2, while the current intensity reaches 10 mA/cm2, improved removal performance of SMT with increasing current is related to that larger current helps to promote the production of H2O2. Besides, note that the applied current intensity does not take a key factor in heterogeneous EF with further increase in current, since similar removal rate could be obtained. At 90 min, 99.8% and 100% SMT decay was determined at 10 and 15 mA/cm2. The removal performance of different SMT concentrations in this EF system is shown in Fig. S3b, when the concentration of SMT is 5 and 20 mg/L, the degradation performance can reach more than 97% and 55% within 90 min, which indicates that Mo0 . 3Fe1Co3–Se MAs still has good catalytic removal performance for SMT below 20 mg/L concentration. This means that Mo0 . 3Fe1Co3–Se MAs is suitable for industrial high-concentration, complex antibiotic removal.To evaluate the catalytic ability of Mo-doped FeCo MAs for EF anode, The OER properties of Mo0 . 3Fe1Co3–Se MAs materials were determined by performing LSVs tests in 1 M KOH with a scan rate of 10 mV/s. Mo0.3Fe x Co y -Se MAs catalysts with different Fe and Co contents were measured to verify the influence of the metal ratio on the OER behavior. Based on the electrochemical data obtained for the different materials (Fig. 5 a and b), the doping of Mo contributes to the OER properties of FeCo aerogels, and the OER properties is further improved after selenization. Moreover, the Mo-doped selenide samples displayed the best catalytic properties, providing the lowest overpotential of 235 mV at 10 mA/cm2 current intensity and the lowest Tafel slope of 73 mV/dec in all samples tested (Fig. 5c), which outperforms many similar materials (Table S3). The prepared sample shows higher Tafel slope compared to RuO2, which indicates that the OER performance of RuO2 is slightly better than Mo0 . 3Fe1Co3–Se MAs at lower current densities, however, the open circuit potential of RuO2 in the OER process is much higher than that of Mo0 . 3Fe1Co3–Se MAs, therefore, Mo0 . 3Fe1Co3–Se MAs is more advantageous from the point of actual energy consumption. In addition to this, the Tafel slope of the samples at higher current densities was shown in Fig. S5, in the current interval of 1–1.65 (log j (mA/cm2)), Mo0 . 3Fe1Co3–Se MAs (84 mV/dec) exhibits a lower Tafel slope than RuO2, which indicates that as the current increases, the Tafel slope as well as the open circuit potential of Mo0 . 3Fe1Co3–Se MAs sample are better than RuO2. Considering the possible industrial catalytic processes, the persistence of Mo0 . 3Fe1Co3–Se MAs for OER is evaluated by measuring the LSVs profiles of the materials after and before 1000 continuous cycles and the current-time curve in 1 M KOH with a scan rate of 100 mV/s (10 mA/cm2). Mo0.3Fe1Co3–Se MAs displayed almost ignorable decay after 1000 continuous cycles (Fig. 5d), and no obvious current decay was seen and the current maintained 95.3% of the initial current intensity after 12 h of chrono-voltage measurements (Fig. 7c). Nyquist plot of aerogels samples were shown in Fig. 5e, and the fitted resistance value (Rct) of Mo0 . 3Fe1Co3–Se MAs is 24.58 Ω, which is much lower than that of Mo0 . 3Fe1Co3 MAs (41.45 Ω) and Fe1Co3 MAs (70.41 Ω), indicating its excellent electron transfer capability.To further illustrate the oxygen reduction activity of Mo0 . 3Fe1Co3–Se MAs, CV test of different aerogels in oxygen saturated solution (0.1 M KOH) is shown in Fig. 5f. Mo0 . 3Fe1Co3–Se MAs displays a stronger oxygen reduction characteristic peak (−0.17 V vs. SCE), comparing with Fe1Co3 MAs (−0.2 V) and Mo0 . 3Fe1Co3 MAs (−0.18 V), and a larger positive potential and peak intensity indicate that Mo0 . 3Fe1Co3–Se MAs has better ORR activity. The peaks at −0.61 V may be attributed to the oxidation process of Co or Fe. The ORR properties of FeCo aerogels were further investigated by testing continuous RDE and RRDE scans in O2-saturated electrolyte (0.1 M KOH) simultaneously changing the rotation speed between 400 and 1600 r/min in sequence. RDE tests of Fe1Co3 MAs at different rotational speeds is displayed in Fig. 6 a. The current intensity corresponding to the voltage from 0.15 to 0.3 V vs. RHE with different speeds was used to calculate the electron transfer number according to K-L equation (Eqs. S1 and S2). The calculated electron transfer number (n) of Fe1Co3 MAs is about 2.9, indicating that it tends to occur 2e− ORR process. To further illustrate the selectivity of the prepared CoFe aerogels, the corresponding RRDE test results are presented in Fig. 6b and c. Fig. 6b shows that i ring and i disk intensities increase with increasing rotational speed, and the n value at 1600 r/min is calculated to be ca. 2.7 according to ​Eq. S3, which is close to the calculated result (2.9) of the K-L equation. LSVs of the prepared materials at 1600 r/min is shown in Fig. 6d, the positive onset potentials of Fe1Co3 MAs, Mo0 . 3Fe1Co3 MAs and Mo0 . 3Fe1Co3–Se MAs are identified as 0.67, 0.68 and 0.69 V vs. RHE, and the potential of Mo0 . 3Fe1Co3–Se MAs is close to the thermo-dynamic limitations of the 2e− oxygen reduction process, which indicates that it has better oxygen reduction activity. Besides, the Tafel slope of Mo0 . 3Fe1Co3–Se MAs (Fig. S6), is 53.6 mV/dec in the corresponding voltage range, lower than that of Mo0 . 3Fe1Co3 MAs (60.9 mV/dec), confirming the more effective electron transfer kinetics, which is consistent with its best ORR performance. Among the three samples, Mo0 . 3Fe1Co3–Se MAs has the largest i ring and i disk intensities, especially, this dramatically increased i ring is an index of low electron transfer number and high H2O2 selectivity. Fig. 6e shows the H2O2 selectivity of the different aerogel catalysts. The selectivity of Mo0 . 3Fe1Co3–Se MAs for H2O2 can be up to more than 85% and stable over a wide voltage range, and this performance is significantly better than that of Fe1Co3 MAs (ca. 65%) and Mo0 . 3Fe1Co3 MAs (ca. 69%). Correspondingly, the number of electron transfers of Mo0 . 3Fe1Co3–Se MAs can be as low as ca. 2.2, while the electron transfer numbers of Fe1Co3 MAs and Mo0 . 3Fe1Co3 MAs are around 2.7 and 2.5 (Fig. 6f), respectively. It should be noted that the ORR performance of H2O2 synthesis of Mo0 . 3Fe1Co3–Se MAs is encouraging and it exceeds many of other electrocatalysts reported (Table S4), which provides a sufficient guarantee for the subsequent activation of H2O2 to generate ·OH.The materials structural reusability of Mo0 . 3Fe1Co3–Se MAs was confirmed after OER and stability testing, and the morphological features and XPS spectra of the catalyst after OER are displayed in Fig. S7 and Fig. S8a. The original nanowire cross-linked skeleton structure morphology of the Mo0 . 3Fe1Co3–Se MAs is well preserved without the phenomenon of structural damage. Co 2p spectra (Fig. 7a) shows Co–Se bond is still well preserved, and the Co–Co bond disappears, which demonstrates that the Co–Se bond is fully exposed in the electrochemical catalytic process, at the same time, the internal active site is fully exposed, facilitating the adsorption and reaction of intermediates in the catalytic process. Additionally, for the Se 3d, the strength of Se–O bonds increased obviously after cyclic decay, and the strength of Se–Se has been weakened, which may be related to the oxidation of Se due to the electrochemical reconfiguration process (Fig. 7b). For the spectra of Mo 3d and Fe 2p, there is no obvious variation before and after OER process as displayed in Fig. S8b and c, which further indicates that Mo0 . 3Fe1Co3–Se MAs has satisfactory stability in OER process. Fig. 7c shows the i-t of the Mo0 . 3Fe1Co3–Se MAs electrode in OER process for about 12 h. The results show that the current intensity decreased, but the variation is not obvious (maintained at more than 90% of initial current density) compared to the Fe1Co3 MAs (current density attenuation over 40%), indicating the excellent stability of Mo0 . 3Fe1Co3–Se MAs catalyst. To demonstrate the stability, Mo0 . 3Fe1Co3–Se MAs after stability testing was tested by XRD (Fig. 7d). The XRD pattern still has the typical characteristic peaks of CoSe2 (PDF#53–0449). In contrast to the fresh sample, the characteristic peak of Mo0 . 3Fe1Co3–Se MAs located at 45.1° decreases, which is attributed to the reconstruction of the catalyst during the OER process. Moreover, the occurrence of the remodeling behavior is generally favorable for the improvement of the catalytic performance in OER. After i-t stability test, the Mo0 . 3Fe1Co3–Se MAs sample still maintains the 3D network structure of aerogel, and there is no significant change compared with the fresh sample except for a small amount of nanowire agglomeration caused by the Nafion binder (Fig. S9).Cycling tests were used to determine the durability of the material during the EF process (Fig. S10). After 5 cycles, the degradation rate of SMT was still maintained at about 88%, and the decrease in catalytic performance could be attributed to the shedding of catalyst and SMT adsorption on surface active sites during the cycle. In addition, Fig. 7d also shows the crystal phase structure of the Mo0 . 3Fe1Co3–Se MAs cathode after EF, except for the weakened peak intensity, the crystalline features can be better matched with CoSe2 (PDF#53–0449), which is basically unchanged compared with the fresh sample. In addition, the v-t and i-t curves in SMT degradation are shown in Fig. S11, when the current is set to 5, 10, 15 mA/cm2, the voltage of the entire SMT degradation process is basically kept stable, indicating that the degradation process is stable (Fig. S11a). At the current density of 15 mA/cm2, slight fluctuation in the v-t curve may be attributed to the acceleration generation of oxygen at anode, but the overall trend is still stable. In addition, the stability of the degradation process at different pH values was also investigated, as shown in Fig. S11b. At the current density of 10 mA/cm2, the voltage value shows an increasing trend with the increase of pH, but when the pH is 3.0, 4.0, 7.0, and 9.0, the voltage window is stable in the SMT degradation process, and the voltage is respectively stable at about 2.89, 2.94, 3.17, and 3.57 V. Based on the above results, electrochemical v-t tests under all conditions indicate the stability of the SMT degradation process. The i-t stability tests were performed using an electrochemical workstation with a three-electrode system (CHI 760E). The initial current density was adjusted to 10 mA/cm2 by setting a voltage of 2.9 V in the i-t program, the current density may fluctuate or decay as the reaction proceeds at this set voltage, so the stability of the reaction process is determined by monitoring the fluctuation of current density. As shown in Fig. S11c, the overall trend remained stable after 90 min of degradation, and the final current density is maintained above 98% of the initial current density. The above encouraging results indicate that Mo0 . 3Fe1Co3–Se MAs has excellent stability, which can be used as a coupled system to generate oxygen and decay SMT.EPR spectra were conducted to determine the free radical active species using DMPO as the trap agent in the EF-catalytic process. Fig. 7e indicates that a classic 4-fold peak is found in the EPR spectrum of Mo0 . 3Fe1Co3–Se MAs cathode and the ratio of peak intensities is about 1:2:2:1, which is a standard feature of DMPO-·OH adduct. In addition, with the accumulation of time increase, the signal peak intensity is gradually increasing, demonstrating that ·OH is the major active free radical, and the corresponding generation path is shown in Eqs. (2)–(9) [9,20,22], moreover, the metal of the high valence state will gain electrons at the cathode and be reduced to the low valence state, which contributes to the continuous activation of H2O2 [14,17]. In this process, clearly defined DMPO-SO4 ·- (six hyperfine lines; 1:1:1:1:1:1) also was observed, which may be caused by the activation of electrolyte SO4 2− in the solution (Eq. (10)). To investigate the contribution of individual active free radical to SMT degradation in Mo0 . 3Fe1Co3–Se MAs system, TBA, MeOH, and BQ (p-benzoquinone) were acted as the quenching agent of ·OH, SO4 ·- and O2 ·-, respectively. Fig. 7f shows that in the absence of TBA, the degradation performance of SMT decreased from 99.6% (without scavenger) in the control group to 53.8%. However, degradation performance was only reduced to 71.2% and 80.5% when MeOH and BQ was added into the electrolyte solution, suggesting that ·OH played the key roles and SO4 ·- and O2 ·- played secondary roles for SMT decay in this EF catalysis, which is agreement with the results of EPR experiments. (2) M + H 2 O → H + + e − + M · OH (3) O 2 + e − → O 2 · − (4) O 2 · − + H + → H O 2 · (5) O 2 + 2 e − + 2 H + → H 2 O 2 (6) F e 2 + + H 2 O 2 → F e 3 + + O H − + · OH (7) C o 2 + + H 2 O 2 → C o 3 + + O H − + · OH (8) F e 3 + + e − → F e 2 + (9) C o 3 + + e − → C o 2 + (10) S O 4 2 − + · O H → S O 4 · − + O H − The DFT calculations were utilized to deeply explain the catalytic process of Mo0 . 3Fe1Co3–Se MAs towards OER. According to the above results, the electron transfer mechanism of the four-proton coupling is related to the OER process as the ​Eqs.11-14. Fig. 8 presents the free energy at each reaction stage, which suggested that the transformation of O∗ to OOH∗ intermediates on the surface of Mo0 . 3Fe1Co3–Se MAs is the decisive step in the OER process, while the adsorption of OH− on the surface of Fe1Co3 MAs and Mo0 . 3Fe1Co3 MAs is the rate-control step, because Mo0 . 3Fe1Co3–Se MAs exhibits the lowest adsorption energy (1.58 eV) of OH−, which is much lower than that of Fe1Co3 MAs (1.96 eV) and Mo0 . 3Fe1Co3 MAs (2.11 eV). The difference in rate-limiting step is attributed to the fact that Fe1Co3 MAs and Mo0 . 3Fe1Co3 MAs have properties similar to high-entropy alloys (HEA), in which the low electron density state of the active site of Co near the Fermi energy level leads to its low electronegativity, which is not favorable for OH− adsorption [47]. After selenization of Mo0 . 3Fe1Co3 MAs, the O2 formation on the CoSe2 surface is a peculiar mechanism of adsorbate evolution. OH− can be readily adsorbed on the surface of the metal Co site and then desorbed to form O∗, resulting in the third step becoming limiting. For the entire OER process, the reaction energy barrier was reduced from 2.11 ​eV (Fe1Co3 MAs) to 1.96 eV (Mo0 . 3Fe1Co3 MAs) and 1.58 eV (Mo0 . 3Fe1Co3–Se MAs), indicating that the Mo doping and formation of selenide greatly decreases the adsorption of OH−, while promoting the transformation of ∗OOH, which facilitates the production of O2. Based on the above DFT calculations results and the theory [48–50], if the influence of electric potential is not considered, the ORR process can be considered as the reversal process of OER. It can be seen from the step diagram that the energy barrier of OOH∗ to O2 step is appropriate for Mo0 . 3Fe1Co3–Se MAs (from 1.53 eV to 1.27 eV), which implies that 2e− ORR is more possible because the larger anion increases the separation between adjacent Co active sites in CoSe2 [19]. (11) M ​+ ​OH− ​= ​M·OH∗ ​+ ​e− (12) M·OH∗ ​+ ​OH− ​= ​M·O∗ ​+ ​H2O ​+ ​e− (13) M·O∗ ​+ ​OH− ​= ​M·OOH∗ ​+ ​e− (14) M·OOH∗ ​+ ​OH− ​= ​M ​+ ​O2 ​+ ​H2O ​+ ​e− In this work, we prepared bimetallic Mo-doped CoFe aerogels by a simple sodium borohydride template method and selenized their surfaces by vapor deposition. Mo-doped FeCo–Se aerogels were used as anode and cathode for electro-Fenton. The experimental results show that the optimal Mo0 . 3Fe1Co3–Se MAs catalyst can remove 97.7% of SMT (10 mg/L) within 60 min at a current intensity of 10 mA/cm2, and the overpotential is 235 mV under the current intensity of 10 mA/cm2. The superior performance is due to that the unique porous cross-linked structure of aerogel endowed the catalyst with enriched active sites and efficient mass transmission paths. Mo doping can lead to the lattice contraction and metallic character of CoSe2, which is beneficial to accelerate electron transfer. In addition, DFT calculations indicated that the selenization treatment lowered the reaction energy barriers for the OER and ORR processes, thereby optimizing the reaction kinetics. RRDE test indicated that Mo0 . 3Fe1Co3–Se has excellent 2e− ORR activity with H2O2 selectivity up to 88%. Fe active sites can effectively activate H2O2 to generate ‧OH. The excellent 2e− ORR and Fenton-like activity ensure its excellent EF performance. This work provided a novel perspective on the exploration of transition metal-based materials for EF process.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Thanks for the support of the National Natural Science Foundation of China (No.21776308) in this work.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.gce.2022.11.003.

Antibiotic pollution in aqueous solutions seriously endangers the natural environment and public health. In this work, Mo-doped transition metal FeCo–Se metal aerogels (MAs) were investigated as bifunctional catalysts for the removal of sulfamethazine (SMT) in solution. The optimal Mo0 . 3Fe1Co3–Se catalyst can remove 97.7% of SMT within 60 min (SMT content: 10 mg/L, current intensity: 10 mA/cm2). The unique porous cross-linked structure of aerogel confered the catalyst sufficient active sites and efficient mass transfer channels. For the anode, Mo0 . 3Fe1Co3–Se MAs exhibits superior oxygen evolution reaction (OER) property, with an overpotential of only 235 mV (10 mA/cm2). Compared with Fe1Co3 MAs or Mo0 . 3Fe1Co3 MAs, density functional theory (DFT) demonstrated that the better catalytic capacity of Mo0 . 3Fe1Co3–Se MAs is attributed to the doping of Mo species and selenization lowers the energy barrier for the ∗OOH to O2 step in the OER process. Excellent OER performance ensures the self-oxygenation in this system, avoiding the addition of air or oxygen in the traditional electro-Fenton process. For the cathode, Mo doping can lead to the lattice contraction and metallic character of CoSe2, which is beneficial to accelerate electron transfer. The adjacent Co active sites effectively adsorb ∗OOH and inhibit the breakage of the O–O bond. Rotating ring disk electrode (RRDE) test indicated that Mo0 . 3Fe1Co3–Se MAs has excellent 2e− ORR activity with H2O2 selectivity up to 88%, and the generated H2O2 is activated by the adjacent Fe site through heterogeneous Fenton process to generate ·OH.

Biomass gasification has attracted considerable attention as a technology to reduce environmental pollution and to face the steady increase in the world heat and power consumption. Gasification converts solid biomass into biosyngas, a gaseous mixture of hydrogen, carbon monoxide, methane, carbon dioxide, alongside nitrogen and water vapour, whose presence and concentrations are highly dependent on the gasification agent used. However, biosyngas also contains minor concentrations of species that might damage downstream equipment. These contaminants include nitrogen compounds (NH3 and HCN), sulfur compounds (H2S), halides (HCl), particulate matter (carbon and solid metals), and tar (e.g., toluene and naphthalene). Among biosyngas contaminants, tar requires particular attention.A common definition of tar is that of organic hydrocarbons with a molecular weight (MW) higher than benzene [1]. Otherwise, Milne et al. defined tar as the organic compounds, largely aromatic, released during pyrolysis or gasification of any organic material [2]. The same authors developed a tar classification method based on the maturation of tar from their initial release during the pyrolysis step. The first step starts at 200–500 °C, with the conversion of cellulose, hemicellulose and lignin to oxygenated hydrocarbons called primary tar. Above 500 °C, primary tar is converted into phenolics and olefins compounds known as secondary tar. When the temperature is increased above 800 °C, part of the secondary tar converts to polycyclic aromatic compounds, also called tertiary [2].Being mostly composed of hydrogen and carbon, tar is potentially a fuel. However, tar compounds can cause various problems such as condensation in cold spots, polymerization to form complex structures that condense even at high temperature, and formation of carbonaceous deposits which can deactivate catalysts or plug filter pores, gas lines and heat exchangers. The term carbonaceous deposit is used in this work to indicate a deposit mostly consisting of carbon, that might have originated from thermal cracking, catalytic reactions, or polymerization [3]. To avoid the aforementioned issues, tar is often removed from the gas stream at low or intermediate temperature (below 300 °C) via physical methods, or converted into useable gases at high temperature (above 1000 °C) via processes such as reforming.Reforming consists in converting tar into CO and H2 using CO2 (dry reforming) or H2O (steam reforming). The process temperature can be lowered by using a catalyst, that can be added inside the gasifier (primary methods) or in a downstream reactor (secondary methods). Catalysts can be classified in several different ways and the most important distinction can be made between metal-based, or synthetic, catalyst (e.g., Ni and Fe based catalysts) and naturally-occurring, or mineral, catalysts (e.g., dolomite and olivine). Generally, mineral catalysts are less catalytically active, but also less expensive. The latter is a significant advantage since the use of catalyst increases the overall system cost [4]. Moreover, naturally occurring catalyst could exhibit higher tolerance to impurities, such as H2S and HCl, or simply be replaced in case of deactivation due to carbon deposition [5]. Therefore, despite the lower activity, mineral catalysts, such as dolomite and olivine, are thoroughly studied [6–9].Most of the research on biosyngas tar reforming has focused on secondary and tertiary tar, since the majority of the gasifiers are operated in the temperature interval 700–1000 °C (downdraft and fluidized bed gasifiers) [10]. Selected representative compounds or real tar have been used in these studies. As an example, Ren et al. used toluene as model compound to study the effect on the reforming activity of different Ni loading methods and oxidation degree of the char support [11]. Recently, Ashok et al. reviewed the progress in the development of catalysts for steam reforming of biomass tar using toluene, benzene and naphthalene as tar model compounds [12]. Various catalysts have been compared by Simell et al. on the ability to reform tar generated in a full-scale updraft gasifier [13,14]. This research found that silicon carbide is an inert material and the activity of the catalysts decreases in the order Ni on Al2O3 > dolomite > activated alumina > silica-alumina [14]. Other authors tested different materials, as Fuente-Cano et al., who studied the conversion of tar generated in a steam-blown fluidized-bed gasifier using wood char as catalyst [15]. Furthermore, innovative system configurations have been investigated by Savuto et al. who achieved tar reforming by placing metal-based catalysts inside the ceramic filter candles in the freeboard of a fluidized bed gasifier [16]. Nonetheless, updraft gasifiers present some advantages over downdraft and fluidized bed gasifiers: operability with a broad spectrum of feedstock (in terms of ash, moisture and size), very low particulate matter entrainment (both dust and ash), simplicity of construction, technology maturity and robustness and high efficiency [17]. On the other hand, updraft gasifiers have the disadvantage of producing a larger amount of tar, mostly belonging to the primary tar group.Studies on biosyngas primary tar reforming are limited in literature. Most of primary tar have a molecular weight lower than that of benzene; therefore, they are often not considered as tar. Moreover, problems with tar such as condensation at high temperature and formation of carbonaceous deposits are promoted by aromatic hydrocarbons (secondary and tertiary tar). Conversely, primary tar are volatile compounds (e.g., formaldehyde, furan) or light condensable compounds (e.g., acetic acid, hydroxyacetone) with low condensation temperatures. Nonetheless, also primary tar represent a threat for downstream equipment due to their corrosiveness [18] and, if temperature is increased, their tendency to form carbonaceous deposits and harmful secondary/tertiary tar compounds via cracking or polymerization [19,20]. Furthermore, the wastewater generated by low temperature physical removal methods, such as scrubbing, has to be treated to prevent environmental issues caused by these organic molecules [21,22].Some information on primary tar reforming can be derived from studies on catalytic steam reforming of bio-oil and its representative compounds. In fact, bio-oil is a product of biomass pyrolysis and consists predominantly of a mixture of primary tar as aldehydes, alcohols and acids derived from the carbohydrate fraction of biomass, and phenolics derived from lignin [23]. Detailed reviews on catalytic reforming of bio-oil and its representative compounds can be found in literature [24–27]. In general, the activity of transition metal catalyst towards the steam reforming of acetic acid is Ni > Co > Fe > Cu [28]. Ni and Co are much more catalytically active than Fe and Cu, but less active than noble metals [29]. The majority of studies focus on metal-based catalyst, whereas a very limited amount of research has investigated natural catalysts, such as dolomite and olivine [30,31]. As with most tar compounds, one of the main reasons for catalyst deactivation in studies with bio-oil and its representative compounds is the formation of carbonaceous deposits. Studying the catalytic steam reforming of acetic acid on Ni/γ-Al2O3, An et al. concluded that the carbonaceous deposits originated from the catalytic cracking reactions and the CO disproportionation reaction [32]. With Pt/ZrO2 and Pt/CeO2 catalysts, the formation reaction of acetone via acetic acid condensation/dehydration was identified as the carbonaceous deposits formation mechanism [33]. Additionally, since the oxygenates are often thermally unstable, thermal cracking can also lead to carbonaceous deposits formation [34,35].Although the studies on bio-oil and its representatives compounds (e.g., acetic acid) provide some insights, most studies use humidified N2, Ar or He as gas carrier [36–38]. However, the presence of CO, CO2 and CH4 in biosyngas can have a significant impact on the catalytic reforming of tar compounds [39]. In fact, CO2 can increase tar conversion via dry reforming and gasify carbonaceous deposits, H2 and CO can shift the reforming reaction equilibrium towards the reactants, and tar and biosyngas components might compete for catalysts active sites [40]. Therefore, this study evaluates the ability of three different catalysts to reform acetic acid, selected as main primary tar compound from updraft gasification, using simulated biosyngas as gas carrier. Particular attention was given to one of the catalysts tested, dolomite, owing to the promising results obtained and the opportunity this natural catalyst offers to decrease system costs. To the best of the authors knowledge, this represent one of the few, if not the first study focusing on acetic acid reforming under updraft gasification representative conditions. The insights presented are expected to help the further development of high temperature gas cleaning, and the commercialisation of energy conversion systems based on updraft biomass gasifiers.In this study, the ability of one metal-based catalyst and two mineral catalysts (i.e., dolomite and olivine) to reform acetic acid was compared using simulated biosyngas as gas carrier. The results are compared with a benchmark test performed using activated alumina. This material was selected since it is often used as support for metal-based catalyst and it shows limited catalytic activity. Biosyngas was used as gas carrier instead of a simple carrier (e.g., humidified N2 and He) as it can have a significant impact on the reforming process. Moreover, the use of biosyngas gives the possibility to evaluate the catalyst activity towards methane reforming and Water-Gas-Shift (WGS) reactions, thus providing additional useful information for the design of tar reformers for updraft biomass gasifiers based systems.Acetic acid was selected as model compound since it is the main component of primary tar, that is the largest group of tar generated in updraft gasifiers [2]. Above 500 °C primary tar undergoes the maturation process studied by Milne et al. [2]. Therefore, when the gas is heated up, some of the acetic acid might be converted into different compounds such as higher molecular weight compounds and carbonaceous particles even before reaching the catalyst bed, as observed also by Matas Güell et al. or by Boot-Handford et al. [33,41]. This conversion process is affected by the heating up conditions (e.g., surfaces available, residence time and temperature). Using acetic acid as initial compound was considered a more interesting option as compared to using only a secondary tar compound, as it provides conditions representative of an updraft gasifier based system, where primary tar compounds, together with higher molecular weight compounds and carbonaceous particles generated when heating up the biosyngas containing primary tar compounds, reach the reformer. Therefore, in the experiments performed, the three catalysts are evaluated on the ability to convert residual acetic acid and higher molecular weight compounds, and to withstand the deposition of carbonaceous particles generated while heating up the gas mixture. All catalysts were tested under the same operating conditions and, in the case of dolomite, additional tests were performed: one at lower temperature (400 °C), and one at higher temperature (900 °C) and higher steam content (50.1 vol%). The low-temperature test was performed to investigate the influence of dolomite on the process of heating up biosyngas containing acetic acid. Other catalysts (i.e., Ni based catalysts) are in fact reported to promote the ketonization reaction already at 350 °C [42]. The high-temperature and high steam content test was performed to investigate the possibility to completely suppress accumulation of carbonaceous deposits. The results of these additional tests provide a useful direction for future research on using dolomite for reforming tar from biomass updraft gasifiers without the formation of carbonaceous deposits.The tests were carried out in a bench-scale unit consisting of a quartz reactor mounted inside an electric furnace. The Process and Instrumentation Diagram (P&ID) of the unit is illustrated in Fig. 1 . The electric furnace is an insulated ceramic hollow cylinder with a heating coil wrapped along its length. A thermocouple is placed at the middle of the cylinder height, on the inner cylinder surface, to control the temperature of the heating coil. The quartz reactor has an inner diameter of 2 cm and the catalyst bed, kept in place thanks to a quartz frit, has a height of 5.5 cm. A thermocouple is placed next to the top part of the catalyst bed to monitor the temperature during the tests. Before starting the experimental campaign, this thermocouple was used to measure the temperature of the furnace along its length. The furnace showed a temperature gradient and, as a consequence, the catalyst bed was not kept at a uniform temperature: the bottom part was at 680 °C, while the top part at 750 °C. The top of the catalyst bed was placed at the highest temperature in the furnace. Fig. 2 shows the furnace and the design of the quartz reactor next to the temperature profile over the setup.A gas flow rate of 380 NmL/min simulated biosyngas composed of 133.1 NmL/min H2, 8.9 NmL/min CO, 74.1 NmL/min CO2, 13.7 NmL/min CH4, 91.1 NmL/min H2 and 59.1 NNmL/min N2 (35.0 vol% H2O, 2.3 vol% CO, 19.5 vol% CO2, 3.6 vol% CH4, 24.0 vol% H2 and 15.6 vol% N2) was used. The volume percentages correspond to the measured values during the first “FlexiFuel-SOFC” project experimental campaign. This EU funded project aimed at the development of a micro scale combined heat and power system composed of an updraft biomass gasifier, a high temperature gas cleaning unit and a solid oxide fuel cell (SOFC) [43,44]. The gas flow rates were regulated using mass flow controllers Bronkhorst EL-FLOW (Bronkhorst, The Netherlands). Steam was added to the fuel gas stream by bubbling the gas mixture (except CO2) in a temperature controlled water bath (humidifier). It was assumed that the gas in the water bath was constantly in equilibrium with the liquid phase. Therefore, the steam content is a function of the liquid temperature, according to Antoine's equation. The piping after the humidifier and the bottom part of the quartz reactor were trace heated and kept at 150 °C.The tar concentration at the inlet was 40 g/Nm3, and was selected based on the results of tar sampling from the “FlexiFuel-SOFC” project updraft gasifier that used wood chips as feedstock and humidified air as gasifying agent. The analysis was performed following the tar protocol (CEN TC BT/TF 143 WI CSC 03002.4) [1]. Liquid acetic acid of 99.7% purity (Sigma Aldrich, USA) was injected at the entrance of the quartz reactor using a peristaltic pump BT100-2 J (Longer Precision Pump Co., China). The trace heating temperature assured the continuous evaporation of the acetic acid. With the steam and acetic acid flow rates selected, the steam-to-carbon (S/C) ratio results equal to 1.3 and to 1.8 for the tests performed with dolomite at 900 °C and different gas composition.At the reactor outlet, a three way valve directed the gas flow either to a series of impinger bottles for tar sampling, or to a microGC to monitor the outlet gas composition. An Agilent 490 microGC with thermal conductivity detector and a CP-COX column for measuring CO, H2, N2, CH4 and CO2 (Agilent, USA) was used. Before reaching the microGC, the gas was passed through a condenser and a desiccator containing silica-gel to remove the moisture contained in the gas. The outlet flow rate was back-calculated from the inlet N2 flow rate and the N2 outlet concentration that was measured with the microGC. This was then used to calculate the flow rates of H2, CO, CO2 and CH4. The outlet tar concentration was measured by bubbling the outlet gas flow in a series of 4 impinger bottles, the first one kept empty and acting as moisture collector, two containing isopropanol at room temperature, and a last one containing isopropanol at 0 °C. The isopropanol and water samples were analyzed with a Varian 430 GC-FID (Agilent, USA) equipped with a Rtx-1 column (Restek, USA).The catalyst used were all in the form of particles with a diameter of 2–3 mm. The metal-based catalyst was a commercially available catalyst called TARGET™ developed specifically for tar reforming by the company Nexceris and consisted of Pt/MNS (MgO, NiO, SiO2). The olivine used is produced by the manufacturer specifically for biomass gasifiers bed, and the material is reported to have a catalytic activity close to that of calcined dolomite [45]. The dolomite was supplied in partially calcined form (CaCO3·MgO), that is with MgCO3 already converted into MgO and with Ca still in the form of CaCO3. An additional calcination process was performed at 900 °C to increase the dolomite catalytic activity. However, the catalyst became too brittle; consequently, the calcination temperature was lowered to 800 °C but the dolomite mechanical resistance was still significantly affected. Therefore, the dolomite used in the tests was in the partially calcined form as received from the supplier. The chemical composition of the partially-calcined dolomite and of the olivine is shown in Table 1 .After having positioned the quartz reactor in the furnace, the temperature was increased to the set value with a ramp of 50 °C/h. A nitrogen flow of 100 NmL/min was passed through the catalyst bed during the heating up stage. The gas flow rate was then changed to simulated biosyngas and the gas composition was monitored for 12 h before adding acetic acid. In the case of the metal-based catalyst, acetic acid was injected after 18 h. The longer time was used to assure full reduction of the catalyst. The gas composition at the outlet of the reactor was monitored during this time to determine the catalyst activity towards WGS and methane reforming and to be sure that the catalyst was not undergoing any change affecting its catalytic activity.Acetic acid was successively injected and the experiment was kept running for three days. During the day the outlet gas composition was monitored with the microGC while during the night the outlet gas was led through the sampling bottles to measure the residual tar content with the GC. The gas composition results presented are the average values recorded over time. After the three days operation, the setup was cooled down with a ramp rate of 50 °C/h and a gas flow rate of 100 NmL/min N2 passing through the catalyst bed. The catalyst was then visually inspected for carbonaceous deposits. An elemental and morphological analysis of the deposit was not performed since the scope of this work was limited to the performance comparison of the different catalysts, and to the identification of a suitable material and conditions to be used in an integrated biomass gasifier SOFC microCHP system. Table 2 summarises the tests performed and the relevant parameters.Before proceeding with the experimental tests, thermodynamic equilibrium calculations were performed using the software FactSage version 5.4.1 (Thermfact/CRCT, Montreal, Canada and GTT-Technologies, Aachen, Germany) to assure the catalysts were operated outside the theoretical solid carbon formation region [46]. The results of the calculations also provide an indication of the expected gas composition at the outlet of the catalyst bed if thermodynamic equilibrium conditions are reached. The software calculates the concentrations of chemical species when specified elements or compounds react to reach a state of chemical equilibrium. The users specifies the mass of the reactants, process temperature and pressure, and the software solves a Gibbs minimization algorithm based on three constraints: the equilibrium product amounts are positive, the mass balance with respect to the system components is satisfied and correspond to the lowest possible Gibbs energy for the possible products. A detailed explanation on the method followed for the calculation can be found on the specific software webpage [47]. Thermodynamic equilibrium calculations only give an indication of the possibility for solid carbon formation since equilibrium might not be reached during the test. Moreover, the software database used contains only thermodynamic properties of solid carbon in the form of graphite. Fig. 3 shows the carbon-hydrogen-oxygen ternary diagram calculated using the software FactSage. A specific mixture containing these elements is represented by a point in the diagram. The operating points without tar (triangle) and with tar (dot) were calculated taking into account all the biosyngas compounds containing hydrogen, oxygen and carbon. In the diagram, two regions can be distinguished, one where solid carbon is formed and one where the mixture completely remains in the gas phase. The two regions are delimitated by a line whose position depends on the temperature. If the operating point falls on the left of the line, then solid carbon is formed at equilibrium. The results indicate that, if equilibrium conditions are reached, no carbonaceous deposits should be present in the whole temperature interval in which the catalyst was operated. Both operating points in fact fall on the right of the continuous and of the dotted line, representing the operating temperatures of 750 °C and 680 °C, respectively.The software was also used to calculate the equilibrium gas composition of clean biosyngas at 750 °C with and without acetic acid, as illustrated in Table 3 . If equilibrium is reached, methane is almost completely reformed and an increase in the carbon monoxide flow rate is expected because of the reverse water gas shift (RWGS) reaction occurring in the reactor. Despite hydrogen is converted into water by the RWGS reaction, the outlet flow rate is higher than the inlet flow rate due to methane being reformed. Similarly, the flow rate of water remains almost constant since the amount produced via the RWGS is balanced by the amount consumed via methane reforming. When acetic acid is added to biosyngas, its conversion leads to an increase in H2 and CO, and to a minor extent of CO2 flow rates.A pre-test was performed without filling the reactor with any catalyst to determine whether reactions towards equilibrium take place at 750 °C even without any catalyst. The gas composition measured at the outlet of the furnace, shown in Fig. 4 , indicates that the residence time was not sufficient to allow any reaction to a noticeable extent. The minor differences between the set and measured inlet values were probably caused by small inaccuracies in the mass flow controllers’ calibration. During this test, the inlet concentration of acetic acid was also measured by wet sampling and the results show an inlet concentration of 37–41 g/Nm3, corresponding to a gas volume flowrate of 3.4–3.8 NmL/min.The reference test with activated alumina beads in the reactor showed that alumina interacts with the gaseous species but it is not significantly catalytically active. Fig. 5 shows the gas composition at the inlet and at the outlet of the reactor when biosyngas with and without acetic acid was passing through the alumina bed. While the flow rate of CH4 remained unchanged, H2 and CO2 reacted and formed CO via the reverse water gas shift reaction. When acetic acid was added to the biosyngas, there was an increase in the methane outlet flowrate, while the other compounds remained almost unchanged. This shows that part of the acetic acid undergoes cracking to CH4 rather than catalytic reforming leading to CO, H2 and CO2, as observed by Basagiannis et al. [42]. Unfortunately, the microGC was not calibrated for any hydrocarbon other than methane nor for oxygen. Moreover, the water condensed at the outlet of the reactor and the amount and composition of the carbonaceous deposits were not measured. It is therefore not possible to give details on the acetic acid thermal decomposition pathway. The study of acetic acid decomposition mechanism is however considered beyond the scope of this paper.No acetic acid was detected at the reactor outlet by wet sampling. However, an amount of hydroxyacetone between 0.07 and 0.45 g/Nm3 was measured. In this test, the first bottle of the sampling train was filled with 30 ml of isopropanol. The mix of isopropanol and condensed water contained in the first bottle turned slightly yellow and had a distinct odour typical of aromatic compounds. This might indicate that part of the acetic acid underwent the maturation process described in Ref. [2]. Nonetheless, the analysis with GC-FID did not show any tar compound. This could have been due to the inability of the column to separate the compounds, or to the high dilution caused by the water condensed in the first sampling bottle. Gravimetric analysis could have been performed on the liquid samples to confirm the presence and quantify the total amount of compounds that were not being detected by GC-FID analysis. To avoid dilution in the successive tests, the first bottle was kept empty and served as moisture collector.At the end of the test, the alumina beads were fully covered with carbonaceous deposits. In literature it is reported that acidic supports as Al2O3 have the tendency to cause a larger amount of carbonaceous deposits as opposed to basic supports which enhance water adsorption [29,42]. The deposit was also present on the reactor walls where the temperature was above 400 °C, that is even before the catalyst bed. Therefore, the carbonaceous deposits might have formed directly in the bed, and/or it might have formed during the heating up process, after which the carbonaceous particles accumulated in the bed by a filtering effect. The formation of carbonaceous deposits on the reactor walls might have occurred due to a radial temperature gradient resulting in higher temperature near the surface, or to acetic acid reacting on the reactor surface. From the experiment performed it is not possible to know if the conversion of acetic acid was complete even before reaching the alumina bed. To better understand the thermal cracking of acetic acid and causes for carbon deposits on the reactor wall and catalytic bed, a test with an empty reactor could be performed. Nonetheless, the understanding of acetic acid thermal cracking behaviour was considered outside the scope of this study and the results of the tests with alumina represent the base case for the comparison of the three catalysts tested. Fig. 6 presents the gas composition when olivine was used as catalyst. The catalyst showed a very limited catalytic activity towards the reverse water gas shift reaction and did not significantly catalyse methane reforming. Moreover, when acetic acid was added to biosyngas, there was an increase in the methane outlet flowrate. However, also the flow rates of CO, H2 and CO2 slightly increased which might indicate the occurrence of acetic acid catalytic reforming.The wet sampling showed no acetic acid at the reactor outlet and no other compound was detected during the first two days of measurement. Nonetheless, the third day of sampling 0.02–0.19 g/Nm3 of hydroxyacetone were measured, which might indicate that the catalytic activity was reducing over time. For this reason, the test was extended for an extra day and during the next sampling the amount of hydroxyacetone increased to 0.14–0.30 g/Nm3, thus confirming a decreased activity of the catalyst.After having cooled down the furnace, carbonaceous deposits considerably covered the catalyst bed. The acetic acid was therefore completely converted into non-condensable gases and carbonaceous deposits, at least during the first two days of operation. Also in this case, the deposits were found also on the reactor walls. It is not known if the carbonaceous deposits on the catalyst originated from reactions in the catalytic bed and/or might have formed before the catalyst, carried by the gas and then filtered by the catalyst bed.The results in Fig. 7 show that roughly 10 NmL/min of H2 reacted with CO2 to form CO, thus indicating that dolomite is catalytically active for the reverse water gas shift reaction. The outlet CO flow rate is still far from the value expected at equilibrium, partially due to the lack of catalytic activity of dolomite towards methane reforming, and partially to the reverse water gas shift reaction not reaching equilibrium. When acetic acid was added to the biosyngas, methane outlet flow rate increased while the flow rates of the other compounds remained almost unchanged.Tar sampling indicated that all acetic acid was converted, and no other compounds were found at the reactor outlet. At the end of the test, carbonaceous deposits were present on the reactor walls and on the catalyst. Therefore, acetic acid was converted into non-condensable gases and carbonaceous deposits. Interestingly, the deposits were found only in the first 2.5 cm of the catalyst bed, while the top part was clean. This might have been due to the conversion of all acetic acid and intermediates taking place before this top section or to the ability of the catalyst to convert acetic acid without carbonaceous deposits formation. Another explanation might be that from 2.5 cm onwards, the bed temperature was sufficiently high to allow the gasification of the carbonaceous deposits filtered. However, it seems unlikely that the carbonaceous deposits accumulated on the catalyst by filtration were oxidized again under the testing conditions, that is a reducing atmosphere and moderate temperatures. Therefore, it is more likely that the catalyst was active towards acetic acid conversion without carbonaceous deposits formation. Fig. 8 shows a schematic of the process described.An additional test was performed keeping the top of the catalyst bed at 400 °C to investigate the influence of dolomite on the process of heating up biosyngas containing acetic acid. Moreover, considering that dolomite appeared not to suffer any deposition at temperatures above roughly 730 °C, an additional test was performed to verify the possibility to suppress carbonaceous deposits accumulation completely. In this second test, a different gas composition was used, with a water flow rate of 190 NmL/min, corresponding to 50% vol, and the top of the bed kept at 900 °C. Fig. 9 compares the measured flow rates with the calculated equilibrium flow rates. At both the temperatures tested, the gas composition was far from equilibrium. At low temperature, that is 400 °C, while CO should be absent and the H2 content is supposed to be significantly lower than the measured values, the CH4 flow rate should be higher than the inlet value due to methane formation reaction. However, dolomite is not catalytically active towards methane formation or reforming reactions, as visible also from the outlet methane flowrate measured at 900 °C, that is almost equal to the inlet value and higher than the expected equilibrium amount. Fig. 10 and Fig. 11 show the gas composition measured at the inlet and outlet of the reactor in the two tests. The test at low temperature clearly showed that dolomite had no catalytic activity at this temperature. This was confirmed by the sampling at the outlet which resulted in an amount of acetic acid equal to 35.5–39.4 g/Nm3, corresponding to 3.3–3.6 NmL/min. At the end of the test, the dolomite at the top of the bed was light grey, indicating a very minor presence of carbonaceous deposits. At this temperature also thermal cracking of acetic acid almost did not take place, in accordance with the observations of An et al. [32]. In the test at high temperature, that is 900 °C, the water gas shift reaction occurred while the methane content remained almost unchanged. Interestingly, a slightly more evident increase in H2 and CO flow rates was noticed as compared to the tests at lower temperatures, suggesting the occurring of acetic acid catalytic reforming. No acetic acid or other tar compound was measured at the reactor outlet and no carbonaceous deposits were observed.The results obtained might have been due to the higher temperature or the higher steam content in the gas since both variables are expected to suppress the formation of carbonaceous deposits. The results therefore provide only a preliminary indication of the possibility to suppress carbonaceous deposits accumulation. However, as experienced during the pre-calcination tests at 800 °C and 900 °C, the catalyst became excessively brittle, with some of the catalyst beads at the top of the bed losing their structure. The loss of mechanical strength of dolomite might be explained by the occurrence of secondary calcination, converting the partially-calcined dolomite (MgO–CaCO3) to full-calcined dolomite (MgO–CaO). Half-calcined dolomite is a rigid and strong material whereas full-calcined dolomite can be very fragile [48]. The secondary calcination reaction depends on process temperature and CO2 partial pressure; upon checking the secondary calcination temperature with thermodynamic equilibrium calculations, it can be noticed that secondary calcination is expected to occur approximately at 750–760 °C. Therefore, further tests maintaining the dolomite temperature below 750 °C and with high amounts of steam are suggested to identify under which circumstances carbonaceous deposit accumulation can be suppressed without compromising the catalyst attrition resistance.The biosyngas composition changed significantly when passing through the metal-based catalyst bed. The outlet flow rates of CO2 decreased while that of CO increased due to the reverse water gas shift reaction. The H2 outlet flow rate increased due to almost complete reforming of CH4. The gas composition is very close to the expected equilibrium composition presented in Table 3, for both clean biosyngas and acetic acid containing biosyngas. The increase in H2 and CO flow rates when acetic acid was injected indicates the occurring of reforming of acetic acid but also the reforming of the CH4 generated during acetic acid thermal decomposition during the heating up process. Fig. 12 presents the gas composition measured at the outlet of the reactor with and without acetic acid.Wet sampling showed no traces of any tar compounds, thus indicating the complete conversion of acetic acid into non-condensable gases and a very minor amount of carbonaceous deposits. Carbonaceous deposits were found on the reactor walls before the bed, and only a very minor amount was found on the first 0.5 cm of the catalyst bed; this deposit might have been filtered or formed in this section of the bed. Irrespectively from the formation mechanism, it can be concluded that at this temperature the metal based catalyst was able to almost completely suppress the accumulation of carbonaceous deposits. Table 4 summarises the results previously discussed to facilitate a comparison between the different catalysts. Fig. 13 compares the gas composition measured at the reactor outlet with the different catalysts tested when biosyngas containing acetic acid was passed through the bed. It can be seen that in terms of catalytic activity towards the reverse water gas shift reaction and methane reforming, the metal-based catalyst has the best performances, with the outlet gas composition being almost equal to the expected equilibrium composition. Both dolomite and olivine showed some catalytic activity towards the reverse water gas shift reaction, with dolomite being more active than olivine. Neither dolomite nor olivine showed activity towards methane reforming and only some activity towards acetic acid reforming, with the majority of the primary tar being converted into methane and carbonaceous deposits. The result are in good agreement with literature, where it is often stated that metal-based catalysts outperform naturally-occurring catalysts [14].The metal-based catalyst also appeared as the most capable of suppressing carbonaceous deposits accumulation. The catalysts can be sorted as olivine > dolomite > metal-based from the highest to the lowest amount of deposits accumulated during the test. From the experiment performed, it is not possible to know if the carbonaceous deposits found on the catalysts were formed in the bed or before it. However, irrespectively from the formation mechanism, it is clear that olivine similarly to alumina was not able to suppress carbonaceous deposits accumulation, while dolomite above roughly 730 °C and the metal-based catalyst already around 680 °C did not suffer carbonaceous deposits accumulation. Moreover, while dolomite and metal-based catalyst completely converted acetic acid into non-condensable gases for the whole duration of the test, the performances of olivine appeared to decrease in time and, at the end of the third and the additional fourth testing day, small amounts of hydroxyacetone were found. Hydroxyacetone is a common intermediate product in the conversion of carboxylic acids, such as acetic acid, into ketones via ketonization reaction [35,49].The olivine used in this study is reported to have a catalytic activity close to that of calcined dolomite [45]. However, according to the manufacturer information, the material was sintered in a rotary kiln at 1600 °C for 3 h and, according to Corella et al. the sintering process strongly decreases the catalyst pore structure [50]. This might explain the performance of olivine observed in this study. Moreover, also Quan et al. observed a decrease in the catalyst performance of olivine due to calcination at higher temperatures [31]. However, since there are studies indicating that calcination might improve the performance of raw olivine [51], further investigation of the performances of raw and calcined olivine are suggested as future work. In case of dolomite, the catalyst used had a low iron content and literature indicates the iron promotes catalytic tar reforming and the WGS reaction [52]. Most importantly, dolomite was used in partially calcined form with Mg being in oxide form but Ca in the carbonate form. An additional test was attempted using dolomite that was pre-calcined at 800 °C. However, the gas flow rate was shortly stopped due to dolomite becoming powder and clogging the reactor. Moreover, the biosyngas atmosphere in our tests contained a high concentration of CO2 and upon checking with equilibrium calculations, it is possible that CO2 reacted with CaO and formed CaCO3 at temperatures lower than 800 °C. This form of Ca is believed not to be significantly catalytically active [53]. Nonetheless, the tests in this study showed that dolomite can almost completely convert the acetic acid and intermediate compounds into non-condensable gases at 680–750 °C, with the carbonaceous deposits present only in the bottom part of the bed. Dolomite friability and release of fines might limit its use as tar reforming catalyst. However, this issue could be mitigated by adding a particle filtration system downstream the catalytic bed. Alternatively, more resistant catalysts should be developed, as studied by Miao et al. who prepared catalysts based on dolomite, clay and carboxyl methyl cellulose with increased strength resistance and higher porosity [30].The metal-based catalyst appeared to outperform the naturally-occurring catalysts with regard to reforming and resistance to carbonaceous deposits accumulation. However, it might suffer poisoning from other contaminants usually present in biosyngas, as chlorine and sulfur, and it has the disadvantage of higher cost [4]. Moreover, the lack of catalytic activity of olivine and dolomite towards methane reforming is not necessarily a drawback. This compound can remain in biosyngas and be used directly in downstream processes without causing issues in some applications for heat and electricity production. With some conversion technologies, such as solid oxide fuel cells, the presence of methane in biosyngas even increases the system efficiency owing to the cooling effect of direct internal methane reforming [54].Despite the differences in the operating conditions (temperature, space velocity, gas composition, tar concentration, and steam-to-carbon ratio), the tar abatement efficiency of the catalysts tested in this study can be compared with results found in literature. A short overview is illustrated in Table 5 , where the catalyst tested and the abatement efficiencies are summarized. The most effective catalyst reported in literature are noble (i.e. Rh-, Pd-, Pt-) and transition (predominantly nickel) metal-based catalyst, followed by calcined dolomite, olivine, biochar, and ash. Ferrous metal oxides, clay, activated alumina, and fluid cracking catalyst are generally less effective. Several studies comparing catalyst with tertiary tar indicate a similar order of the catalyst based on their activity [55,56]. The high tar abatement efficiency of the commercial nickel catalyst (100%) found in this work agrees well with figures commonly found in literature on acetic acid reforming with Ni/Al2O3 [28,42,56–58] and several noble metal-based catalyst [59,60]. Furthermore, in this study the complete conversion of acetic acid was observed with dolomite and such abatement efficiencies have been previously reported [30,37]. The olivine in this work initially achieves a complete conversion of acetic acid as reported by Kechagiopoulos et al. [61,62]; however, with time the formation of carbonaceous deposits seemed to reduce the catalytic activity. The alumina catalyst studied in this work displayed a lower catalytic activity than the nickel, dolomite and olivine, which agrees with the lower tar abatement efficiencies of alumina supports reported in literature [29,63]. Although most catalyst in Table 5 obtain lower efficiencies than transition/noble metal based catalyst, these compounds are often used as support materials.The formation of carbonaceous deposits on the catalyst and sulfide poisoning can severely limit the tar abatement efficiency and lifetime of catalyst. Therefore, regeneration techniques such as air oxidation and application H2, H2O and CO2 at elevated temperature are developed to remove carbonaceous deposits on spent catalyst and recover the activity of the catalyst [76]. For example, after oxidizing the carbonaceous deposit on nickel catalyst at 750 °C catalytic activity could be completely regained [77]. Regeneration techniques are also very promising for less active catalyst or catalyst more prone to deactivation by carbonaceous deposits as it allows using less expensive catalyst for tar removal [78]. Lind et al. applied air to a catalyst regeneration unit located next to the tar reformer and continuously replaced older catalyst, namely ilmenite (FeTiO3) with regenerated catalyst, thereby continuously removing carbonaceous deposits and maintaining a tar conversion of 35% [78]. Although catalyst regeneration is promising, the cyclic high temperature processing can for example lead to nickel sintering, phase transformations and volatilization [79] and recent studies show that regeneration is still an important topic of research [76].Tar represents one of the biggest bottlenecks in biomass conversion via gasification. Therefore, the goal of this study was to evaluate in a lab-scale reactor the ability of olivine, dolomite and a metal-based catalysts to reform acetic acid and its thermal decomposition products using simulated biosyngas as gas carrier. Upon heating biosyngas containing acetic acid above 400 °C, the tar is at least partially converted to higher molecular weight compounds and carbonaceous particles that will reach the reforming catalyst where they can affect the reactor performance. The tests performed assess the reforming ability of the catalysts under conditions representative of the high temperature gas cleaning unit designed by TU Delft to connect a 50 kW updraft gasifier with a solid oxide fuel cell in the EU funded project “FlexiFuel-SOFC”. The catalyst performance was evaluated assessing the outlet gas composition, the outlet tar concentration, and presence of carbonaceous deposits on the catalyst.Initially olivine appeared able to completely convert acetic acid into non-condensable gases. However, a considerable amount of carbonaceous deposits was found on the catalyst, and the catalytic activity of olivine decreased in time, as indicated by the increasing amounts of hydroxyacetone measured. Dolomite showed promising performances at 680–750 °C since acetic acid was completely converted into non-condensable gases and only a minor amount of carbonaceous deposits was found on the low temperature part of the bed. The carbonaceous deposits accumulation was suppressed by increasing the operating temperature and the steam flow rate. However, operating dolomite at 900 °C, or pre-treating dolomite by calcination at 800 °C, causes the catalyst to become excessively brittle, which leads to powder entrainment in the gas flow, or even reactor clogging. The metal-based catalyst out-performed the naturally-occurring catalysts since it completely reformed acetic acid and suffered almost no carbonaceous deposits accumulation. The metal-based catalyst showed good catalytic activity towards the reverse water gas shift reaction and methane reforming, while both dolomite and olivine showed some minor catalytic activity towards the reverse water gas shift reaction, but neither dolomite nor olivine showed activity towards methane reforming. The results presented are expected to assist in the development of systems based on biomass gasification, such as biomass gasifier SOFC systems.This research was partially supported by the project “FlexiFuel-SOFC”. The project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 641229.The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.biombioe.2021.105982.

Tar compounds have been defined as Achilles’ heel in biomass gasification. Catalytic reforming solves problems caused by tar by converting them into H2 and CO. Most of the research has focused on secondary and tertiary tar reforming while some information on primary tar can be derived from bio-oil reforming. However, these studies use humidified N2, Ar or He as gas carrier. Therefore, in this work, three catalysts are compared for reforming 40 g/Nm3 acetic acid as main primary tar compound from biomass updraft gasification using simulated biosyngas as gas carrier. The catalysts were tested over a 72 h period between 680 and 750 °C with a gas composition of 35.0 vol% H2O, 2.3 vol% CO, 19.5 vol% CO2, 3.6 vol% CH4, 24.0 vol% H2 and 15.6 vol% N2. Olivine completely converted acetic acid, but a considerable amount of carbonaceous deposits was found on the catalyst and the catalytic activity decreased over time with 0.2 g/Nm3 hydroxyacetone measured in the last day of testing. Dolomite showed promising performances by completely converting acetic acid and accumulating carbonaceous deposits only in the low temperature part of the catalyst bed. The carbonaceous deposits could be suppressed increasing the steam content to 50.1 vol% and the temperature to 900 °C. However, the catalyst became excessively brittle. The metal-based catalyst out-performed the naturally-occurring catalysts by completely converting acetic acid with almost no carbonaceous deposits accumulation. These results are expected to help the further development of tar reformers, and the commercialisation of biomass updraft gasifiers based systems.

Over the past few decades, heterogeneous catalysis in industrial processes has received more attention due to the many advantages related to green chemistry principles. The reusability and the ease of separation and saving catalytic process efficiencies are some of the advantages of the improved ecotechnological process in industrial applications; this includes sustainable energy-related processes such as biodiesel production (Ciriminna et al., 2021). In addition, biodiesel is potentially renewable energy that aligns with some of the requirements regarding green chemistry principles; these requirements include biodegradability, low toxicity, combustion efficiency, availability, and renewability (Ramos et al., 2019).The use of heterogeneous catalysis in biodiesel conversion is associated with the replacement of basic or acid homogeneous catalysts, particularly those with some basic/solid catalysts, and this brings about greater challenges from the sustainable development perspective (Yusuff and Owolabi, 2019). The high level of activity, reusability, recoverability, and low cost are some of the catalyst properties considered for the economical and sustainable process. The physicochemical properties—such as surface area, surface basicity/acidity, pore distributions, chemical stability—are among the influencing characters of the catalyst (Rossa et al., 2017). Some studies have revealed the effectiveness of Lewis acid catalysts in the transesterification mechanism; some transition metal salts suggest that the strength of the Lewis acid sites is Sn2+ >> Zn2+ > Al3+ (Einloft et al., 2008).Considering the high cost and the various steps required for the synthesis of some solid supports—such as mesoporous silica (MCM-41) and synthetic zeolite (TUD-1) is of great importance when considering the cost of the entire process. As such, there is a need to develop easily recyclable heterogeneous catalysts with low-cost and sustainable materials. In this regard of adding economic reasonability, low-cost materials such as CaO-based materials, SiO2-based materials, and clay-based materials were widely developed (Fattah et al., 2020). Biogenic silica is one of the reasonable catalysts for this purpose, and this is due to its ease of production and modifiability (Mazaheri et al., 2021). Within this scheme, previous works revealed the use of biogenic silica from agricultural waste—such as rice husk ash, bamboo leaf ash, and other cellulose-biomass sources—as the support for ZnO, ZrO2, and SrO. From previous work, salacca leaf ash has been seen to contain high silica content (Triawan et al., 2021). As opposed to many other agricultural wastes such as bamboo and rice husk ashes, the biogenic silica from salacca leaves is presumably a good source to support the catalyst of biodiesel conversion (Adam et al., 2012).To the authors’ knowledge, this is the first case study to utilize biogenic silica from the salacca leaf for the synthesis of ZnO/SiO2. The study on the effect of Zn content on the physicochemical characteristics of ZnO/SiO2 was performed using instrumental analysis. The use of salacca leaf ash for supporting biodiesel production will give the enhancement in clean production and sustainable production in term of renewable energy. Additionally, the optimum condition for biodiesel conversion application was statistically determined. Rice bran oil (RBO) was chosen as the biodiesel source due to its low-cost; RBO is a co-product and—in countries such as Indonesia—it is also a byproduct of rice milling (Trirahayu, 2020). Since RBO is also a non-food oil, the use of RBO for biodiesel production is classified into the second generation of biodiesel; this means that its production will not compete with food consumption, and it can potentially be developed with improved economic value and sustainable production (Ju and Vali, 2005). The use of non-edible resources—such as biomass feedstock—eliminates the debate of food and energy scarcity, and it much less demanding on land used to provide feedstocks (Ramos et al., 2019). In addition, the convertible oil content in RBO is quite high, totaling approximately 15–25% wt. from rice bran (Einloft et al., 2008).Salacca leaves were collected from plants grown in a domestic cultivation area of the Sleman District, Yogyakarta Province, Indonesia. The salacca leaf ash (SLA) was used as the main source of silica, and it was obtained by calcining the dried salacca leaves at 600 °C for 1 hour in a Memmert muffle furnace (Germany). Rice bran oil was purchased from Oryza Grace (Jakarta, Indonesia). Chemicals consisted of zinc acetate dihydrate, NaOH, methanol, HCl, n-butylamine, and pyridine; these chemicals were obtained from Merck-Millipore (Darmstadt, Germany) and utilized without further purification.The extraction of silica from SLA was performed using a procedure similar to that of the previously published bamboo leaf ash silica extraction procedure (Fatimah et al., 2019b). The SLA was incorporated into a mixture with 4 M of NaOH solution, and it was subsequently refluxed using a round-bottom flask on an electric heating mantle for 4 hours. The filtrate from the reflux was slowly titrated with HCl 0.5 M until a pH of 8 was reached and the gel formed from the silica. The silica gel was separated from the filtrate by decantation, and the gel was neutralized using distilled water before it was dried in an oven at 60 °C.The composite of ZnO/SiO2 was synthesized using the obtained silica gel. The precursor solution of zinc acetate dihydrate was dissolved in water. It was then stirred into the silica gel to form a colloidal solution. The colloid obtained was then kept at 150 °C in a Teflon-lined autoclave overnight under hydrothermal conditions. The colloid was then dried in an oven at 80°C before being calcined at 500°C for 2 hours. To study the effect of zinc on the properties of the composite, the zinc content was varied at 20, 25, and 30 wt.%, and the obtained samples were encoded as ZnO/SiO2-20, ZnO/SiO2-25, and ZnO/SiO2-30, respectively. Fig. 1 represents the method for the ZnO/SiO2 synthesis.The ZnO/SiO2 nanocomposites were characterized by X-ray diffraction (XRD), gas sorption analysis, scanning electron microscopy/energy dispersive X-ray (SEM-EDX), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). XRD analysis was performed using a Shimadzu X-6000 diffractometer. Measurements were carried out from 2θ = 10° to 80° at a rate of 0.4°/min using Ni-filtered Cu Kα X-rays. For gas sorption analysis, a NOVA 1200 instrument was employed. For each analysis, the samples were degassed at 90°C for 4 hours beforehand. The Brunauer–Emmett–Teller (BET) specific surface area, pore volume, and pore radius were calculated from N2 adsorption/desorption data. The SEM-EDX analysis was performed using a JEM-ARM200F (JEOL) equipped with a 50-mm2 Si (Li) detector (JED-2300, JEOL) for EDX analysis. Additionally, the FTIR spectroscopy analysis was performed on the Perkin Elmer FTIR instrument (Singapore). TEM analysis was performed using the JEOL instrument. XPS analysis was performed on a V.G. Scientific ESKALAB MKII instrument using monochromatic Al K α radiation with a photon energy of 1486.6 ± 0.2 eV. The samples (0.2 mg) were gently pressed into a small pellet with a 15 mm diameter and mounted onto the sample holder. The sample was initially degassed for 4 hours prior to analysis in order to achieve a dynamic vacuum below 10−8 Pa.Surface acidity of the catalyst samples was determined using the n-butylamine titration method and the pyridine Fourier transform infrared (FTIR) analysis method. With the titration method, approximately 0.25 g of ZnO/SiO2 powder was mixed with 10 mL of n-butylamine 0.01 M followed by stirring overnight. The mixture was filtered, and the filtrate was titrated with HCl in order to measure the unadsorbed amount of n-butylamine. The acidity was determined by the amount of adsorbed n-butylamine (mmol) per gram of the 0.2 ZnO/SiO2 sample. The surface acidity measurement using the pyridine method was conducted by weighing the ZnO/SiO2 sample after it was dried at 100 °C for 2 hours; this was followed by placing the sample in a vacuum desiccator. The pyridine vapor fled into the desiccator overnight. After the desiccator was opened at room temperature, the pyridine-adsorbing sample was analyzed by using FTIR spectrophotometry analysis. The spectrum recorded from the analysis contains the peak of pyridine bound to the sample surface by both the Lewis acid-base interaction and the Bronsted acid-base interaction.All reactions were carried out in a batch reactor. For each reaction, 20 mL of RBO was mixed with methanol and 4 g of catalyst. The quantitative analysis of the produced biodiesel sample was performed on a Shimadzu GCMS instrument equipped with a spilt injector and a FID detector. The HP-5MS 5% (phenyl methyl siloxane) capillary column (30 m x 250 um i.d., 0.25 um film thickness) was employed as the separating phase, and helium was used as the gas carrier with an average velocity of 37 cm/sec at 200 °C. The analyses were set at the temperature of 100 °C, with a maximum increase of up to 370 °C at 15 °C/min, and held for 16 min. The varied reaction condition was set up in reference to the Box-Behnken Design (BBD) of the Response Surface Methodology (RSM). This was done in order to identify the affecting parameters and the optimum conditions of catalytic conversion. The varied parameters selected for optimization were as follows: Zn content in catalyst, catalyst dose, methanol-to-RBO volume ratio time, and time of reaction.The yield (%) was selected as the response for the reaction. The parameters were selected following optimization on a wider range of variables. In addition, temperature was not selected as a crucial factor due to the initial optimization; the results demonstrated that a higher yield was obtained as the temperature increased within the range of 60–110 °C, and it reached a maximum at 100–110 °C. Therefore, all experiments were conducted at 100 °C. The low, middle, and high levels for all of the independent variables are shown in Table 1 . Each variable was coded using a coding scheme to denote the level of the factor among three potential levels; -1 indicated the lower level, 0 indicated the medium level, and +1 indicated the higher level. Minitab 16 software (version 6.0) was utilized to carry out the regression analysis and to analyze the constructed data obtained from the preliminary experiments. Optimization via the evaluation of interactions among different parameters was thus performed.Reusability of the catalyst was tested by recycling the spent catalyst and utilizing it for further cycles. The recycling procedure was performed by filtering out the solids, washing them in methanol under stirring the mixture for 1 hour, and drying them in an oven at 100 °C overnight.The XRD pattern of the materials are presented in Fig. 2 . The SiO2 sample shows a broad reflection ranging between 22–25o. This is characteristic of amorphous silica. The pattern is similar with silica extracted from rice husk ash, bamboo leaf ash, and other biogenic sources. Moreover, after modification with ZnO, the ZnO signal in the composite was observed (Aneesh et al., 2007). The peaks at 2θ = 31.64°, 34.45°, 36.23°, 47.61°—which are indexed as (100), (002), (101), and (102) planes, respectively—are associated with JCPDS card no. 29-1487 (Garcia-Sotelo et al., 2019). In regard to the ZnO phase, the refinement of the pattern was obtained using the Rietica software. As can be seen from the ZnO/SiO2 reflection, the broad peak associated with amorphous SiO2 disappeared along with the formation of the crystalline ZnO. This pattern is similar to that reported in other ZnO/SiO2 synthesis utilizing other silica precursors (Galedari et al., 2017). From the varied Zn content ranging from 20-30 %wt. and using the Scherrer's equation (Eq. 1): (1) D = 0.9 β c o s θ where λ is a wave length of the X-ray (0.154186 nm) and β is the measured FWHM, it can be seen that the Zn content contributed insignificantly to the crystallite size, as listed in Table 2 .The higher Zn content produced a higher crystallite size, and this is similar to the trend found from the synthesis of ZnO/SiO2 using bamboo leaf ash as the silica precursor (Fatimah et al., 2019). The higher Zn concentration influenced the precipitation rate, which affects the growth of the crystallite size. The higher precursor concentration also accelerates the sol-gel reaction as well as the distribution of ZnO. It is influenced by factors such as the precipitation rate, interaction between the Zn precursor and the support, and the calcination temperature (Liu et al., 2014). On the basis of the crystallite sizes obtained from the XRD data, the growth of ZnO particles is influenced by the amount of the Zn precursor, which also affects the ZnO distribution in the composite. The formation of ZnO particles also affected the surface characters of the specific surface area, pore volume, and pore radius, as listed in Table 3 . As seen in Table 3, the formation of ZnO particles lead to an increase in the specific surface area of the material. The SiO2 specific surface area is 45.2 m2/g, and it increased to 80.3; 82.8, and 87.1 m2/g for ZnO/SiO2-20; ZnO/SiO2-25, and ZnO/SiO2-30, respectively. The trend of the increased specific surface area along with the ZnO formation is similar with previous reports of synthesis of composites using bamboo leaf ash as well as the synthesis using tetraethyl ortho silicate (Somoghi et al., 2021). However, compared to the composites produced by using the bamboo leaf ash, the specific surface area values in this study are lower than the comparable ZnO content. The enhanced specific surface area with the presence of ZnO on the surface also affected the surface acidity, and this is exhibited by the data in Table 3. The surface acidity represents the capability of the surface to interact with the base, n-butylamine, in the case of this analytical method. Moreover, the acid sites of the surface play a role in conducting surface interaction with both the triglyceride and the alcohol within the transesterification mechanism, as schematically represented in Fig. 3 .The presence of ZnO on the catalyst surface reflects the Lewis acid site contributing to the adsorption site of the fatty acid via the Lewis acid-base interaction; this occurs with π-bonding of the double bond of the carbonyl functional group (Abedin et al., 2020). The chemisorption produced surface-bound carbocation that was further attacked by hydroxyl from methanol as a nucleophile. The methyl shift, proton migration, and rearrangement breakdown of bonding between the reactant and the Zn surface was followed by the desorption of methyl ester and alcohol/glycerol as product (Boonyuen et al., 2018).The presence of surface acidity is also identified by FTIR analysis of the pyridine-adsorbed samples (Fig. 4 ).From previous literature on surface acidity identification using pyridine as a probe molecule, the interaction of pyridine with the surface acid sites is identified by the Bronsted acid and hydrogen-bonded pyridine at around 1540–1600 cm−1; with Bronsted and the Lewis acid-bound pyridine at 1450–1490 cm−1, while the Bronsted acid-bound pyridinium cation did so at around 1540 cm−1 and 1640 cm−1 (Reddy et al., 2009; Ali-Dahmane et al., 2019). From the measurement, adsorbed pyridine is represented by peaks at 3067 cm−1 and 2925 cm−1 corresponding to the presence of C-H stretching from pyridine structure. The pyridine-coordinated through the aromatic π electrons at around 1607 cm−1, while the peaks at 1465 cm−1 corresponds to the Lewis acid and Bronsted acid sites, respectively (Larina et al., 2019; Pham et al., 2021). Furthermore, on comparison of the intensity of the absorption, the Lewis-to-Bronsted acid ratio (L/B) was calculated using the following equation (Eq. 2): (2) L B = I 1450 − 1459 I 1540 − 1560 where I1450–1455 is the intensity of the band at 1450-1455 cm–1 and I1540–1560 is intensity of the band at 1540-1560 cm–1. The L/B values for each sample can be seen in Table 3. The increasing Zn content represented the increasing L/B, which indicated that the Lewis acid-base interaction between the surface and the reactant is predominantly created by the increasing amount of ZnO on the surface (Abedin et al., 2020).Aside from the peaks representing the surface acidity, some peaks correspond to the presence of ZnO and SiO2 in the composite, such as the absorption spectrum seen at 501 cm−1, 803 cm−1, 1105 cm−1, and 3445 cm−1. The band at 501 cm−1 comes as a result of the stretching of the Zn-O. Additionally, the intense peaks at 803 cm−1 and 1105 cm−1 can be attributed to the symmetric stretching and the asymmetric stretching vibrational mode of the Si-O-Si, respectively. The band at 3445 cm−1 is due to the O-H bending modes of the hydroxyl groups and the adsorbed water in the powder (Galedari et al., 2017).The evolution of the composite surface can be seen by the change of specific surface area, and it has also been identified by SEM and TEM analysis. As shown in Fig. 5 , flower-like morphologies are shown by the ZnO/SiO2 samples, with rod-like forms appearing at the higher magnification. The morphologies are similar to that of ZnO-based composites reported by various methods, and this includes the ZnO/SiO2 prepared using bamboo leaf ash. With varied Zn content, it can be seen that the higher Zn content leads to the formation of larger clods. The pattern suggests the crystal growth in the hydrothermal process as the growth of ZnO to Zn(OH)4 2−; this occurred as a result of the systematic pattern of ZnO growth influenced by Zn as crystal nuclei (Shi et al., 2013). The TEM profiles depicted in Fig. 6 are in line with the theoretical approach of crystal growth; this is influenced by the Zn content, as the larger particle size is represented at the higher Zn content. From the HTEM image in Fig. 6b, it can be seen that the ZnO particle size is approximately 20–30 nm; this is in line with the calculated crystallite size from the XRD measurement. Moreover, the HRTEM images exhibit the lattice fringes of the attached ZnO nanoparticles, and the estimated d-spacing value between the two adjacent of the fringes was found to be 2.49 Å; this corresponds to the (101) planes of hexagonal wurtzite ZnO (Fig. 6e). Table 4 presents the average composition of fatty acid methyl ester (FAME) produced from the processes, and one of the GCMS analysis results is presented in Fig. 7 . It can be seen that the dominant components are the methyl esters from the palmitic acid, oleic acid, linoleic acid, linolenic acid. The stearic acid with the palmitic acid and the stearic acid methyl ester are the minor components. The composition is similar to that found in previous investigations on RBO transesterification with reference to the fatty acid composition (Einloft et al., 2008; Zaidel et al., 2019).The optimization of catalytic activity was conducted based on RSM using BBD for selected influential parameters. With reference to the catalytic mechanism, the parameters of the ZnO content, catalyst dosage, methanol-to-oil ratio, and time of reaction were chosen. Table 5 lists the yield values at the varied conditions. Table 5. The yield values at the varied conditions.The analysis of variance (ANOVA) results of the response surface model are shown in Table 6 . The statistical parameters demonstrate that the second-order regression model is significant at the 95% confidence level with the predictability of the model.The determination coefficient, or R2, of 0.9891 indicates that the model could be used as a predictor of the response. In addition, a relatively high value of the coefficient of variation demonstrated the high reliability of the experiments. Based on the multiple regression analysis, the following equation is the model fit for prediction of the experimental results: Yield ( % ) = − 133.740 + 4.373 x 1 + 14.470 x 2 + 7.320 x 3 + 0.054 x 1 2 − − 2.089 x 2 2 + 1.515 x 3 2 − 5.435 x 4 2 + 0.175 x 1 . x 2 − 1.127 x 1 . x 3 − 0.986 x 1 . x 4 + 1.627 x 2 . x 3 − 0.570 x 2 . x 4 + 0.788 x 3 . x 4 The predicted responses were in good agreement with the experimentally obtained response (R2 = 0.972 and Adj R2 = 0.936). Furthermore, the model's F-value of 27.94 and the P-value of 0.00 implies that the model is significant. From the P-values of the effect of varied parameters, it can be concluded that—with the exception of the catalyst dose—other parameters of ZnO content, such as the methanol-to-oil ratio and time of reaction, influence the conversion significantly. It can be confirmed that, at the range of catalyst doses, the effect of the surface provided for the surface mechanism is similar. However, the effect ZnO content is significant. Considering that the ZnO content is in line with the provided surface acidity and the catalyst dose is related with the specific surface area of the catalyst, it can be concluded that surface acidity is more dominant in governing the conversion.The plot in Fig. 8 was created to ensure the adequacy of the proposed regression model; an R2 of 0.964 was obtained, representing the fitness of the model. The contour plot in Fig. 9 a and 9b demonstrate that the higher value of conversion can be achieved by a high catalyst dose and time of reaction. The plots and the model concluded that the optimum condition for producing biodiesel is a catalyst dose of 6 g/L, a time of reaction of 3 h, and methanol-to-oil ratio of 6. Zn content within the range of 20–30 % will give a similar yield, as it does not significantly influence the conversion.The contour plots represent that the higher conversion favored an increase of ZnO content and time of reaction; the conversion, however, reached an optimum level at the methanol-to-oil ratio of 6:1 (Fig. 8c). This indicates the excess methanol tends to reduce the surface equilibrium for the transesterification; this subsequently hinders the completion of FFA and triglycerides being protonated at the active sites, leading to a lower FAME yield (Zhang et al., 2013) . A similar effect of the methanol-to-oil ratio was reported by previous work (Anwar et al., 2018). However, the Zn content generally has no significant effect on the yield at along the methanol-to-oil ratio. In general, the activity of the catalysts resulting from this research was comparable with other silica-based catalysts used for biodiesel conversion. In previous reported works, the CaO/SiO2 catalyst gave yields of 98.5% and 85.6% (Moradi et al., 2014). With ZrO2/SiO2, a yield of 96.2% was obtained from the oil-methanol-catalyst ratio of 100:400:2.4 for 3 hours (Faria et al., 2009). Similarly, the yield ranging from 60–80% was reported with the use of alkali earth metal oxides (CaO, MgO, and BaO) supported by SiO2 (Mohadesi et al., 2014).Catalyst reusability is an important characteristic to consider for application purposes. The catalyst reusability was identified by the change of yield on reaction cycles. The catalyst was recycled by being washed in ethanol and water and dried at 100 °C. As shown in Fig. 10 , the catalysts showed good reusability until the third cycle, as the maintained yield at the reduced value was no more than 10% from the fresh catalyst; the yield decreased significantly during further cycles (fourth and fifth cycles).This suggested that the composite could not accommodate the surface reaction in the mechanism due to the loss of catalytic properties. XRD analysis was conducted to evaluate the structural change of the catalyst. The XRD pattern is presented in Fig. 11 .From the diffraction pattern, it can be seen that there are new phases identified and reflected by the change of ZnO into Zn(OH)2 as well as the presence of silica in the cristobalite and the quartz. The orthorhombic Zn(OH)2 (O) peaks are observed by strong peaks at 32.93°, 42.8°, and 49.5° corresponding to the (100), (201), and (102), respectively (Ghaedi et al., 2016; Mousavi-Kamazani et al., 2020; Wang et al., 2013). The adsorbed hydrophobic reactant on the catalyst surface also reduced the catalyst porosity. Further investigation on this was performed using catalytic refreshment; this was conducted by calcining the used catalyst at the temperature of 400 °C for 2 hours. The comparison of specific surface area and the pore distribution of the refreshed catalyst are presented in Fig. 12 . It can be seen that the refreshment successfully exhibited the remaining catalytic activity, and this was comparable to that of the fresh catalyst. The data suggests that the catalyst is recyclable, and it can be recycled using a simple method. This suggests that the material is potentially a developed, low-cost catalyst from a sustainable resource. The production of ZnO/SiO2 catalyst by using salacca leaf ash for biodiesel production represented the potential usage of waste into valuable material in renewable energy.The results of the present studies show that ZnO/SiO2 was successfully prepared using silica derived from salacca leaf ash. The samples exhibit efficient catalytic activity for biodiesel conversion from rice bran oil. The use of the response surface methodology—which is based on the Box-Behnken design—was applied for the optimization of the following Zn content parameters: catalyst dose, methanol-to-oil ratio, and the time of reaction. With the exception of Zn content, the statistical parameters imply the strong influence of the tested parameters. The catalyst is recyclable, and it can be recycled using the simple procedure of heating at 400 °C for 2 hours. This suggests that the material is potentially developed for biodiesel conversion.The authors declare that they have no known competing financial interests or personal relationships that cdocould have appeared to influence the work reported in this paper.The authors would like to express appreciation for the support from the Ministry of Ministry of Education, Culture, Research, and Technology through the World Class Professor Program in 2021.

This paper reports on the first case of the recyclable catalysts of ZnO/SiO2 prepared from salacca leaf ash as a source of silica for biodiesel production. The catalysts were prepared using a hydrothermal synthesis method, and the catalyst was utilized for biodiesel conversion from rice bran oil. Physicochemical properties of the catalysts were studied by multiple instrumental analyses consisting of XRD, SEM, TEM, and surface acidity measurements on pyridine adsorption followed by FTIR analysis. The study focuses on the effect of Zn content on the physicochemical character. As such, a varied Zn content of 20, 25, and 30 % wt. was applied. In order to evaluate the influencing parameters for the catalytic process, a response surface methodology based on the Box-Behnken design was applied in optimization. The selected parameters of catalysis included the Zn content, catalyst dose, methanol and oil ratio, and the time of reaction. It was concluded that all tested parameters—with the exception of Zn content—significantly influence the yield of the reaction. The catalyst demonstrated a reusable feature, as there was an insignificant yield value of catalytic activity until the fifth cycle during the simple procedure of recycling. This suggests that the material was potentially developed for biodiesel conversion.

Data will be made available on request.It is widely accepted that hydrogen, a clean and sustainable energy vector, is considered the ideal alternative of fossil fuels. Within the actual framework of the socioeconomical and environmental situation, green hydrogen will play a crucial role for a clean energy transition process [1]. Since many years, photocatalytic water splitting towards hydrogen evolution has attracted numerous attentions because of its green sustainability [2,3]. In spite of the huge efforts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for large scale plants [4,5]. The challenge is to develop stable and efficient catalysts that can harvest solar light, using co-catalyst alternative to precious materials that would allow the scale-up and practical applications [6,7]. In this sense, promising studies at pilot plant scale have been recently reported [8–11].Within this frame, green H2 production from alcohol photocatalytic reforming reaction appears as a hot topic in the field of photocatalysis [12,13]. From the enormous literature on this field, TiO2 based systems have provided the best performances [14]. However, some factors such as rapid recombination, the occurrence of backward reaction or even the deactivation by the formed intermediates hindered the development of H2 production at large scale [15]. The use of metal co-catalysts, used as charge trapping sites, have demonstrated necessary in order to enhance the efficiency of the photocatalytic reaction by avoiding the electron–hole recombination processes [16–18]. It has been found that the addition of these noble metals could have different effects on the photocatalytic activity which is also strongly affected not only by the nature of metal but also by other parameters from sample history and metal features [19]. Alternatively to noble metals, copper-based catalysts have been extensively considered as a cheaper option [13,20–23].In addition to this traditional strategy, the combination of heat and light has been recently suggested as a novel approach pursuing the improving efficiency of the photocatalytic process [24–26]. Thus, by the combination of classical thermo- and photo-catalytic processes, interesting synergistic effects have been reported that drastically enhances the hydrogen production through photoreforming reaction [25].Due to the mechanistic complexity of this multicatalytic approach, which would involve different simultaneous reactions, the origin of this cooperative effect is still unclear. In principle, the synergistic improvement would be conditioned by the primary importance of single thermo- and photo- contributions on the kinetic behavior and reaction mechanism [27,28].Moreover, most of the reported results do not cover the influence of the co-catalyst loading on such complex mechanism and the catalytic performance [27,28]. In the present paper we present an interesting study on methanol reforming through a multicatalytic approach, by combining heating and photonic excitation. For this scope, we use a Cu/TiO2 system with different Cu loading that could serve to explore how the mechanism is influenced by metal content and therefore condition the overall reaction performance.TiO2 (Evonik P90) was used as photoactive support (Table 1 in Supporting Information). TiO2 Evonik P90 exhibits higher surface area than widely used Evonik P25. Since the aim of this study is to see the effect of the amount of Cu loading on the catalytic activity, we decided to use this high surface area anatase to avoid the effect of metal agglomeration at increasing contents. Metal loading was performed through chemical reduction deposition procedure using copper nitrate as metal source. In this case, 0.5 g of support was suspended in 100 mL of water containing the stoichiometric amount of Cu precursor for a nominal loading between 1.0 and 5.0 wt%. Chemical reduction of metal precursor was achieved by using NaBH4 as reducing agent. After adding the reducing agent at certain excess (1:20 molar ratio with respect to Cu), the suspension was stirred at room temperature for 30 min. The effectiveness of the deposition was always checked by adding more NaBH4 to the filtrated liquid. We did not appreciate in any case any turbidity or color changes due to the presence of Cu. The obtained systems were filtered, thoroughly washed with distilled water and finally dried at 90 ºC. As prepared catalysts were labeled as CunP90, being n the nominal metal loading.BET surface area measurements were carried out by N2 adsorption using a Micromeritics 2000 instrument.X-ray diffraction (XRD) patterns were obtained using a Siemens D-501 diffractometer with Ni filter and graphite monochromator, using the Cu Kα radiation as X-ray source., We have calculated the anatase fraction as well the mean crystallite size by Rietveld fitting using HighScore-Plus software, using the line broadening of corresponding diffraction peaks.Diffuse reflectance UV–vis spectroscopy was performed using a Cary 300 instrument. Spectra were recorded in the diffuse reflectance mode using Spectralon® as white standard. Scans range was 240–800 nm.XPS data were recorded on pellets using a customized system incorporating a hemispherical analyser (SPECS Phoibos 100), a non-monochromatized X-ray source (Al Kα; 1486.6 eV, Mg Kα, 1253.6 eV). The analyser was operated at a fixed transmission and 50 eV pass energy with an energy step of 0.1 eV. Binding energies were calibrated using C 1 s (284.6 eV) as an internal reference. Prior to each analysis, the samples were evacuated to 10−9 mbar at room temperature. In a typical experiment, the sample was initially placed in the sample holder and transferred to the spectrometer chamber where XPS spectra were acquired. In-situ FTIR studies were performed in a Harricks praying mantis cell. The spectra were recorded on a Nicolet FT-IR spectrometer at reaction conditions with a resolution of 4 cm−1. Before reaction, catalysts were pretreated in N2 at 50 ºC for 30 minTemperature programmed reduction (H2-TPR) was performed using a Chemstar instrument (Quantachrome). About 30 mg of the catalysts previously degassed at 50 °C for 30 min under Ar flow. The TPR spectra were collected in 10 mL/min mixture of 5% H2/Ar from 50 ºC to 700 °C with a heating rate of 10 °C/min.Transmission electron microscopy were performed by using a FEI Tecnai F30 microscope in STEM mode operated at 300 kV equipped with a Gatan GIF Quantum 963 energy filter. The samples were directly dropped on a nickel grid.Gas phase photocatalytic H2 production tests were performed in a flow cell ( Fig. 1). The powder photocatalysts (50 mg) were placed in the cell and then degassed with N2 at 50 ºC. Methanol steam flow (CH3OH = 20% v/v) at 15 mL/min was passed through the sample for 60 min before reaction. After which, the lamp (200 W lamp housing) was switched on and the sample was illuminated through the quartz top-window of the cell. Effluent gases were analysed to quantify H2 production by gas chromatography (Agilent microGC) using a thermal conductivity detector. Only CO and CO2 were detected as side products. CO selectivity (S CO ) were calculated by considering all products detected: S CO % = CO ( H 2 + CO + CO 2 ) × 100 Thermo-photocatalytic runs were performed at 200 ºC. As we have evidenced in a previous study, at this temperature the synergistic effect for thermo-photocatalytic process is maximum [29]. As reference, thermo-catalytic runs were performed in the absence of light under similar reaction conditions.The apparent quantum efficiency for the H2 evolution reaction has been determined from the reaction rate and the flux of incoming photons (calculated for the irradiation wavelengths of 365 nm) [20,30]. AQE = rate mol · s − 1 rate of incident photons mol · s − 1 The synergetic effect of the thermal contribution on thermo-photocatalytic process was evaluated by considering the difference rate with single process rates [31]. rate syn = rate thermo − photo − rate photo + rate thermo From this rate syn value we have calculated the synergetic efficiency (AQE syn ) which specifically accounts the efficiency attributable to the synergy of both thermo- and photo-catalytic processes.Copper deposition over TiO2 by chemical reduction leads to well dispersed Cu nanoparticles over the range of studied systems. As it can be observed from STEM images ( Fig. 2), homogeneous dispersion of Cu nanoparticles of ca. 2 nm is obtained. Moreover, no aggregation of Cu clusters is noticed as metal loading increases.From diffuse reflectance spectra ( Fig. 3), important absorption bands can be found at 350–450 nm and 750 nm that can be attributed to Cu clusters at the surface. These new bands progressively increase as copper loading increases, specially bands at 410 nm and 750 nm. Thus, bands within the range 350–450 nm have been ascribed to (Cu–O–Cu)2+ clusters in a highly dispersed state as well as to three-dimensional Cu1+ clusters. [32,33] On the other hand, the band at 750–850 nm has been assigned to 2 E g → 2 T 2 g transitions of Cu2+ located in the distorted or perfect octahedral symmetry. [34] From these results, we could expect that Cu species would consist on highly dispersed CuO and surface doped Cu2+/1+. Both bands progress similarly as metal content increases (Fig. 3 .b).Reduction profile also denotes the evolution of copper clusters as loading increases ( Fig. 4). In all cases a single TPR peak ca. 180 ºC is found for all catalysts. This is a rather lower reduction temperature compared with values reported in the literature, and would be attributed to highly dispersed CuO species at the nanoscale. [35–37] Moreover, from the deconvolution of TPR peaks it is possible to envisage four Cu species, denoted as α to δ. The presence of different reduction peaks in the TPR profile of oxide-supported Cu samples has been extensively reported. [38,39] Thus, it is accepted that large and crystallized CuO particles would be reduced at higher temperature than smaller CuO clusters or highly dispersed Cu2+ species in strong interaction with the oxide support. [40,41] In our case, the reduction temperature around 180 ºC clearly points out the absence of bulk CuO, as was stated above from TEM images. For all samples, species reduced at slightly higher temperature (γ + δ) are prominent. However, as metal loading increases, low reduction temperature species (α + β) become more evident (Fig. 4 .b). Such low reduction temperature species would be associated to finely disperse copper species as support Cu2+ doped and would be hardly detected from TEM analysis. So, only small CuO nanoclusters are noticed. Therefore, for higher Cu content catalyst, Cu species distribution seems to be composed equally by small finely disperse CuO nanoclusters and surface doped Cu2+ species. Quantitative analysis from H2 consumption clearly denotes the efficiency of deposition method. The amount of reduced Cu follows a clear linear evolution being in all cases close to the nominal values (Fig. 4.c).From XPS analysis, the observed broad peaks as well as the presence of a shake-up satellite would denote the coexistence of Cu2+ and Cu+/Cu species ( Fig. 5 .a). Thus, peaks at 932.0 eV and 951.9 eV would correspond to Cu 2p 3/2 and Cu 2p 1/2 spin-orbit split of Cu/Cu2O species. [42] On the other hand, a second contribution at 934.2 eV and 954.1 eV with satellite shakeup at ca. 942 eV, in all the synthesized products should correspond to CuO species. It is also worthy to mention that for lower Cu contents, partially reduced fraction seems to be slightly higher ( Table 1). This fact could be explaining considering a higher dispersion in these samples. Such low dimensional CuO clusters should be easily reduced under XPS conditions.In agreement with TPR analysis, the surface Cu/Ti ratios calculated from XPS signals also follows a linear progression with nominal values, indicating the homogeneous distribution of Cu species over TiO2 surface even at large metal loadings (Fig. 5.b). The Cu/Ti ratios obtained for samples after thermo-catalytic or thermo-photocatalytic runs at 200 ºC do not show a significant variation from fresh sample (Table 1). Therefore, no aggregation should be expected after reaction by effect of temperature.Moreover, as Cu content increases the ratio of oxidized Cu seems to be slightly increased (Table 1), similarly as α + β family evolution from TPR experiment. This can be also observed from satellite relative intensity (I sat /I Cu2p3/2 ), showing the highest value for Cu4P90 catalyst. Indeed, α + β family has been previously associated to small CuO nanoclusters. The higher partial reduced fraction in low Cu content catalysts could therefore be ascribed to highly disperse Cu2+ entities (γ + δ species) that would easily reduce under XPS conditions. Fig. 6 shows the H2 production rates and apparent efficiencies and H2 yields of Cu/TiO2 P90 systems. Thus, the obtained rates for photo-, thermo-, and thermo-photocatalytic runs under UV–vis light illumination for 4 h are summarized in Table 2. As it can be observed, the performance for thermo-photocatalytic H2 production are notably higher in all cases with respect to single thermo- and photocatalytic processes (Fig. S1).The apparent quantum efficiencies denote that for photocatalytic performance the maximum value is attained for Cu2P90 catalyst, showing an H2 production of 12 mmol/h·g after 4 h (Fig. 6 .a). A higher Cu loading negatively affects to the photoreforming activity. In principle, the higher photoactivity exhibited by Cu2P90 would be associated to the finely dispersion and stronger interaction with support observed from TPR. [43] As metal loading increases, recombination processes would hinder the photocatalytic performance.It can be also stated that thermo-catalytic process also leads to significant production of H2. As we already discussed, H2 formation under these conditions could be explained considering methanol thermal decomposition (Eq. i) [44,45]. Eq. i C H 3 OH → 2 H 2 + CO ⁢ methanol decomposition In this case, AQE values lineally increases in the whole range of Cu content, reaching an H2 yield of ca 30 mmol/h·g for Cu5P90 catalyst (Fig. 6.a). Therefore, in this case and within the range of metal content studied, thermo-catalytic activity is clearly favoured by the increasing surface Cu over TiO2.Finally, the combined thermo-photoreforming leads to notable values of efficiency, with a maximum yield of ca. 60 mmol/h·g also for Cu5P90 catalyst. However, in this case the photocatalytic performance of Cu5P90 is close to that exhibited by Cu4P90 denoting certain exhausting effect in the thermo-photocatalytic activity with increasing metal loading. Thus, at this Cu loading the optimum metal content could be considered. (Fig. 6.a).Since calculated rate syn always show positive values, the combined process cannot be explained as the sum of both single ones, and a certain synergistic effect can be envisaged for all catalysts. Moreover, the calculated AQE syn clearly denote that the combination of thermo- and photo- processes notably affects to the overall reaction performance and appear clearly affected by metal loading (Fig. 6.b).In this sense, it is also interesting to highlight the evolution of the synergy with Cu loading (Fig. 6.b). For the lowest Cu content, since the photocatalytic performance is rather low, thermal contribution appears important. Minimum AQE syn is found for Cu2P90, for which the photocatalytic performance is maximum. Thermal-photocatalytic synergy reaches a minimum for Cu2P90 catalyst, for which photocatalytic performance is maximum and thermo-catalytic activity is still low. Catalysts with metal contents higher than 2 showed outstanding thermo-photocatalytic AQE that progressively increases with Cu content. However, though thermal reforming lineally grows with Cu, a maximum in the AQE syn is found for Cu4P90 catalysts. This fact would point out a process that would hinder the thermo-photocatalytic performance for Cu5P90.In order to understand the evolution showed by AQE and AQE syn in different studied photoreforming processes, we followed CO and CO2 as interesting intermediates from side reactions that would simultaneously take place ( Fig. 7). Thus, CO can be derived from side reactions that would proceed simultaneously with photoreforming; that is, methanol decomposition as well as formic acid dehydration (Eqs. i and ii) [45]. Eq. ii HCOOH → H 2 O + CO formic acid dehydration From the evolution of CO selectivity, the single photocatalytic process clearly denotes a concomitant formation of CO. The observed CO formation progressively decreases as Cu loading increases. For the thermocatalytic process, low amounts of copper do not lead to CO formation, starting at Cu loading higher than 2 wt%. It is worthy to notice that the combine thermo-photocatalytic process shows a similar trend as photocatalytic one, but with much lower CO selectivity. Thus, for higher Cu loading S CO values are similar to that obtained by thermocatalysis.The marked lower S CO values attained for thermo- and thermo-photocatalytic processes would denote an important CO consumption (probably associated with a higher H2 yield).Thus, two probable processes could be involved to explain the lower CO formation and higher H2 production. On one hand, formic acid dehydrogenation as alternative way to decompose formic acid instead of dehydration; and water gas shift reactions (Eqs. iii and iv). Indeed, the occurrence of water-gas-shift reaction has been previously argued within H2 photoreforming reaction [29,44]. Eq. iii HCOOH → H 2 + C O 2 dehydrogenation Eq. iv CO + H 2 O → H 2 + C O 2 water gas shift ( WGS ) We may also point out that the presence of CO during photoreforming reaction would probably affect to the photocatalytic activity during the reaction time by CO poisoning of the metal active sites [46,47].In Fig. 8 we depict the FTIR spectra during in-situ photoreforming reaction. From these spectra, two important regions can be envisaged. In the higher wavenumber region, C–H stretching modes can be found. Thus, C–H contributions at 2920/2820 cm−1 have been associated to metoxy species [45,48]. The appearance of these bands is accompanied by a negative broad peak at around 3620 cm–1 due to the disappearance of surface hydroxyl groups through the dissociative adsorption of methanol. As described in the literature, the C–O stretching region located in the region 1800–700 cm–1 also shows the characteristics bands associated to methanol. Thus, at room temperature it can be notice a sharp band at around 1060 and 1135 cm–1 that can be ascribed to C–O stretching modes of methoxy group. The evolution of bands in this region during different experimental conditions is similar for all catalysts, varying the relative intensity of certain bands. Thus, during photocatalytic experiment at 50 ºC, new intense bands appear within this region at 1580 cm–1 and 1360 cm–1. These new bands can be ascribed respectively to the υas(COO) and υs(COO) for bidentate formate ions adsorbed on TiO2 surface. The intensity of these bands remains almost similar as Cu loading increases for Cu contents higher than 2 wt% (Fig. S2). It is worthy to mention that these bands do not appear during thermal treatment. In this case, only the band at 1135 cm–1 seems to be exalted (Fig. S2).Concerning the thermo-photocatalytic process, as temperature increases, bands associated to formates become notably marked. However, at 200 ºC these bands completely disappear. At 120 ºC, such bands are noticeable, showing exponential growing intensities as Cu content increases (Fig. S2). Therefore, the formation of formates seems to be clearly favoured by the increasing metal sites. As previously stated, the presence of formates would determine the different evolution of CO.[29] Thus, for photocatalytic runs we have already showed that S CO values were particularly lower with respect to those found for thermo- and thermo-photocatalytic experiments (Fig. 8). These two facts can be correlated by considering formic acid dehydration reaction which leads to the formation of CO and H2O. As above discussed, during thermo-photocatalytic experiment, at moderate temperatures the formation of formates was exalted and increases with Cu content. It is clear that formate formation is favoured by temperature.The absence of formate bands at reaction temperature clearly indicates that they were completely transformed into CO through dehydration. At the same time, we have also stated that S CO drastically decays. Therefore, during thermo-catalytic reforming at 200 ºC the outstanding H2 formation can be ascribable to the enhanced transformation of methanol into formate which proceed to CO and then through WGS reaction to H2 formation. From both facts it would be inferred that WGS reaction is playing an important role. In this sense, the higher the Cu load, the more important the WGS reaction becomes in the overall process.Thus, it can be argued that the efficiency of the thermo-photocatalytic reaction over Cu/TiO2 systems would be improved with Cu loading. Thus, the different optimum Cu loading attained for photocatalytic and thermo-photocatalytic reforming was conditioned by the thermal contribution through WGS reaction. From these results, it seems that this later process would be favoured by increasing Cu loadings. Thus, while for photocatalytic reforming Cu2P90 showed the optimum value, for the combined thermo-photocatalytic process Cu5P90 would be the best catalyst.Therefore, we evidenced a clear dependency of the mechanism on the metal content. Moreover, as we stated from TPR and XPS Cu species present in each case is different. Thus, while for Cu2P90 Cu2+ with strong interaction with support are the predominant, for Cu5P90 CuO dispersed nanoclusters would be also present. The influence of cluster size on the photocatalytic performance of H2 photoreforming was studied by Zheng et al. which stated that small clusters usually possess higher charge separation efficiency [49]. In this sense, Li et al. showed that through ligand assisted method, ultrafine size clusters of Cu species (with about 1 nm), dispersed on the surface of TiO2 were obtained [50]. These authors reported that the superior H2 evolution performance of CuO/TiO2 mainly originates from the cluster size and unique interfacial structure formed. On the other hand, CuO clusters modulated in the range 4–8 nm by controlling the loading amount of Cu and calcining temperature were obtained for photocatalytic WGS reaction [51]. Therefore, it is clear that different process would require not only a particular metal loading but also an optimum cluster size. Moreover, different copper species distribution could be also correlated with the better performance as temperature increases.Copper supported TiO2 with different metal loadings was used as catalyst for the gas phase methanol thermal photoreforming. From the wide characterization of the catalyst, we have shown that copper metal co-catalyst has been efficiently deposited and well dispersed on the support surface within the studied range of metal loading. We have stated the important synergistic effect in the thermal-photocatalytic reaction. Thus, ca 60 mmol/h·gcat was obtained after 4 h at 200 ºC upon illumination. Different optimum metal loading can be envisaged for different methanol reforming experiments. Moreover, as increasing metal loading, the nature of Cu species seems to be modified, subsequently conditioning the catalytic performance. Thus, while for the photocatalytic reforming Cu2P90 was the best catalyst, the thermo-catalytic reforming performance follows a lineal evolution with Cu content in the range of the studied metal content. For thermo-photocatalytic reforming, the optimum value would be Cu5P90 for which the synergistic improvement started to decay. During thermo-photocatalytic reaction different thermo- and photo-processes are involved, resulting in different optimal catalyst formulation. We have shown that while for photocatalytic reaction low co-catalyst loading leads to the better performance, for thermo-photocatalytic process higher loading is needed. These results clearly highlight the necessity of metal loading optimization due to the occurrence of different catalytic mechanism that simultaneously takes place during the thermo-photocatalytic reaction. F. Platero: Methodology, Investigation, Writing – original draft. A. Caballero: Conceptualization, Methodology, Supervision. G. Colón: Conceptualization, Methodology, Writing – original draft, Writing – review & editing, Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Authors acknowledge the financial support from the EU FEDER and Junta de Andalucía under I+D+i Project P20-00156. We also acknowledge the financial support from PID2020-119946RB-I00 project funded by MCIN/AEI/ 10.13039/501100011033 and, as appropriate, by “ERDF A way of making Europe”, by the “European Union”.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcata.2022.118804. Supplementary material .

We have optimized the H2 production by methanol thermo-photocatalytic reforming in the gas phase using Cu/TiO2 catalyst by tuning metal loading. Metal co-catalyst has been deposited by means of chemical reduction deposition. We have stated that thermo- and thermo-photocatalytic process leads to a notable H2 production at 200 ºC. By in-situ FTIR studies we evidenced that formate formation follows a different evolution depending on the reforming experiment. These surface formate would lead to CO formation through dehydration reaction. At higher Cu content the low CO selectivity denote that water-gas-shift reaction would predominate and exalt H2 yield. Thus, different optimum Cu content is found for each reforming experiment. While for the photocatalytic reforming Cu/TiO2 (2 wt%) is the best catalyst of the series, we should increase the Cu content to Cu/TiO2 (5 wt%) to achieve the optimum performance for thermo-photocatalytic reforming of methanol.

In order to deal with the increasing global energy shortage and serious climate issues caused by the usage of fossil fuels, technologies for production of renewable and clean energy on the terawatt scale are urgently required in the near future, with a required minimum production of 10 terawatt (TW) of renewable energy needed by 2050. 1–3 One of the promising solutions is to use “green hydrogen”, hydrogen produced by water electrolysis using renewable electricity. 3–5 As a consequence, further development of water electrolysis technologies is required to support the terawatt-scale production of H2 fuel.Proton-exchange membrane (PEM) electrolysis, in which proton-conducting polymer thin membranes are used as solid electrolyte instead of liquid electrolyte, was developed earlier in 1950s for hydrogen and oxygen generation under anaerobic environments. 6 , 7 Compared to the conventional alkaline electrolysis, the PEM electrolysis shows plenty of superiorities, such as a small footprint, fast response time, capability of reaching high current densities above 2 A/cm2, high Faradaic efficiency, enabling safe differential pressure operation, and requiring only pure water in place of corrosive electrolytes. These advantages avoid most of the shortcomings of alkaline electrolysis and can facilitate the future coupling with solar electricity grid. It is time to rekindle the research on PEM electrolyzers to overcome the long-existing disadvantages of PEM electrolyzer. One of the biggest disadvantages is that only noble metal materials can currently be used as working electrodes, for instance, Pt group metals (PGM) as cathode and Ir-based oxides or metallic Ir as anode. 6 The reason for this current limitation is the acidic local environment within the polymer membrane, even though only pure water is used as reactant solution. The high local acidity can lead to fast corrosion of non-PGM inorganic materials under electrolytic conditions, and concurrently the catalytic efficiencies of non-PGM materials are far from matching catalytic efficiencies achieved by PGM-based catalysts.A convictive fact is that most molecular water oxidation catalysts 8 , 9 (WOCs) and hydrogen evolution reaction (HER) catalysts 10 , 11 were initially developed, evaluated, and deeply studied under acidic conditions. Considering the chemical nature of H+ and OH−, in contrast to non-PGM inorganic materials which are more resistant to alkaline conditions, metal complexes generally are more resistive to acidic conditions, especially in unbuffered pure water systems. In the preliminary investigations by Millet and co-workers, several molecular WOCs and HER catalysts evaluated in PEM electrolyzers displayed demonstrable performances (can reach A/cm2 current density), sufficient stability (1,000 h of operation without significant degradation for the hydrogen evolution catalyst), and expectable lower sensitivity to metal impurities in the system. 12–15 For an example, the [Ru(tpy)(bpy)(OH)] (tpy = 2,2′:6′,2′'-terpyridine and bpy is 2,2′-bipyridine) tested in PEM electrolyzer achieved 1 mA/cm2 current density at about 1.9 V applied potential, which is only 100 and 200 mV higher than when the state-of-the-art catalysts Ir and RuO2 were used under the same test conditions, respectively. 14 It has been seen that molecular catalysts may bring new opportunities to replace noble metal catalysts in PEM electrolyzers. 16 , 17 Moreover, molecular catalysts have many advantages over inorganic materials, as they are atom economical and their discernible active sites and relatively clear catalytic mechanisms allow finely structural design to obtain high intrinsic activities. 1 , 8 , 9 , 18–21 During recent decades, numerous efficient molecular catalysts with outstanding catalytic performances and well-studied catalytic mechanisms have been developed for water oxidation, 8 , 9 , 22 proton reduction, 11 and CO2 reduction reactions (CO2RR) 23 (several representative series are shown in Figure 1 A). However, these molecular catalysts are initially designed to be combined with photosensitizers for (photo)electrocatalytic systems, and have been rarely considered in view of their application in PEM electrolyzers.The idea of applying molecular catalysts in PEM eletrolyzers 6 and anion exchange membrane (AEM) electrolyzers, 25 , 26 which together are known as solid polymer electrolyte membrane (SPE) electrolyzers, deserves wide consideration. The development of SPE electrolysis technologies can potentially be taken in a completely new direction and high efficacies by the application of these ready molecular catalysts, 16–18 as indicated by the successful precedent reported recently by Berlinguette and co-workers. 24 Through implementation of a commercially available cobalt phthalocyanine (CoPc) CO2RR catalyst in a AEM electrolyzer (Figure 1B), production of CO from CO2 reduction can be achieved with >95% selectivity at current densities of 150 mA/cm2, and catalytic current density at 150 mA/cm2 can be maintained for >100 h. Turnover numbers (TON) of 4,000 and a turnover frequency (TOF) of 216 h−1 was achieved over 8 h of electrolysis at 50 mA/cm2 with a Faradaic efficiency for CO production >90%. Although an obvious efficiency drop was observed for operation at high current density, further detailed investigations displayed that the rapid drop is primarily due to the lowered proton inventory inside the membrane-electrodes assembly (MEA) and not due the CoPc decomposition. These fresh and excellent results challenge the widely accepted dogma that there is no chance to employ molecular catalysts in commercially applicable electrolysis techniques. 24 Efficient CO2RR molecular catalysts with good selectivity and stability can benefit the production of fuels from CO2 reduction by AEM electrolyzer.With abundant well-developed molecular catalysts in hand, another remaining challenge to efficiently utilize molecular catalysts in PEM electrolysis is the effective preparation of molecular catalysts modified electrodes. Considering the modification stability, conductivity, and homogeneous exposure of catalytic sites, immobilization of molecular catalysts is not a straightforward process. Both adequate conductive supports and well-designed immobilization methods are required to assemble a finally efficient molecular catalyst incorporated MEA. Carbon materials are cost-effective, environmentally friendly, are highly electrically conductive with wide potential windows and display good chemical stability and rich surface chemistry to facilitate their modification with molecular catalysts. 27 Moreover, carbon materials are commonly employed as substrates for loading Pt catalysts and as cathode current collectors in PEM electrolyzers, indicating their promising stability as substrates for loading reduction catalysts under acidic and electrochemical conditions. 6 The currently available carbon materials also have enough stability to be used as anode substrate for short-term operation at high voltages, a time-frame suitable for electrocatalyst characterization purposes (See details about carbon-material stability in Stability of Carbon Materials).In this perspective review, we highlight advances of molecular catalysts immobilized on carbon materials as a valuable approach to launch the investigations on application of molecular catalysts in PEM electrolyzers. After the introduction, carbon materials, catalyst immobilization, and characterizations of molecular catalyst on carbon materials are described. Then we provide an overview of molecular catalysts immobilized on carbon materials for electrochemical water oxidation, proton reduction, and CO2 reduction reactions, which may be directly employed for the investigations of application of molecular catalysts in PEM and AEM electrolyzers. Concerns about the stability of molecular catalysts are discussed in a separate section. To conclude, we discuss future scientific perspectives and challenges to advance this promising, but currently underdeveloped, technology for solar fuel production through integration of SPE electrolysis with molecular catalysts.Depending on the type and organization of C–C bonds in the structure, carbon materials can comprise different allotropes with different electrochemical properties, e.g., amorphous carbon, diamond, graphite, graphene, carbon nanotubes (CNTs) and fullerenes (Figure 2 ). Amorphous carbon, the simplest structure amongst these, has poor stability under positive potentials. Diamond, containing sp3-hybridized C–C bonds, possesses excellent mechanical properties but low conductivity. Graphite has high electrical conductivity due to the stacking of aromatic planes consisting of sp2-hybridized C–C bonds with delocalized π electrons. An important variation of graphite is carbon fibers, which can be used to fabricate conductive carbon papers and carbon cloth. Graphene, CNTs, and fullerenes are advanced carbon materials derived from a single aromatic plane of graphite. Therefore, their intriguing structures lead to unique physical, chemical, and electrochemical properties. 28 , 29 A well-known variant based on these structures is non-graphitizing glassy carbon (GC, also known as vitreous carbon), which is widely used as an inert electrode material in electrochemical studies.GC electrodes are commonly used for studying catalytic mechanisms and the kinetics of molecular catalysts because of their high conductivity, extreme resistance to chemicals, and control over the active surface area. Besides GC, CNTs, and graphene have been widely employed to immobilize molecular catalysts. CNTs and graphene are usually processed as thin films, porous sheets, and porous foams, providing super-conductive substrates with large electrochemically active surface areas. Not only do these materials increase the surface loading of the molecular catalysts, the carbon surfaces can also easily be modified with −OH and −COOH functionalities to facilitate reactant and proton transfer. 28 , 29 Commercial carbon materials, such as carbon fiber cloth, carbon fiber paper, graphite blocks, and GC electrodes (e.g. GC plate and GC foam) can be directly used as electrodes. However, many commercially available carbon materials, including CNTs and graphene, are marketed as powder. Hence, the powdery carbon materials must first be deposited on the current collecting substrate to form an electrode. Figure 3 shows three preparative procedures widely employed to fabricate carbon electrodes, i.e., drop-casting, 30 electrophoresis, 31 and vacuum filtration. 32 These three procedures have the advantage of simple and mild operating conditions. However, many other approaches can be adopted to prepare carbon electrodes, e.g., chemical vapor deposition (CVD), Langmuir–Blodgett film deposition, layer-by-layer self-assembly and electro-polymerization. 33 To exemplify, interesting architectures such as three-dimensional (3D) self-supporting graphene can be produced based on the CVD method. 34 Development of more advanced porous carbon electrodes will indisputably facilitate the investigation of artificial photosynthetic (AP) devices based on molecular catalysts. However, the ideal methodology for electrode production for future applicable electrolyzers should be cost-effective and scalable.In the view of future practical application, the stability of carbon materials should be considered. Carbon materials are commonly employed as substrates for loading Pt catalysts and as cathode current collectors in PEM electrolyzers, indicating their promising stability as substrates for loading reduction catalysts under acidic and electrochemical conditions. 6 However, the use of carbon materials at the anodic side is limited due to the ability of the material to undergo electrochemical oxidation under the local stringent conditions. Carbon is thermodynamically easy to be oxidized to carbon dioxide or carbon monoxide (C + 2H2O → CO2 + 4H+ + 4e−, E0 = 0.207 V versus normal hydrogen electrode (NHE); C + H2O → CO + 2H+ + 2e−, E0 = 0.518 V versus NHE). 35 Under typical conditions applied in PEM electrolyzers, the high oxidative potential, high oxygen concentration, and acidic aqueous solution, corrosion of common carbon materials is rapid. Therefore, they are not suitable for use in the anode of PEM electrolyzer for long-term operation.Athough most carbon materials suffer from this oxidative instability, these carbon materials can still be applied for the purposes of electrocatalyst characterization and performance testing of WOCs. 6 , 14 Fortunately, the field of carbon materials is still under continuous development, 36–38 providing carbon materials of increasing stability such as multi-walled carbon nanotubes (MWCNTs). 35 , 39 , 40 MWCNTs, which are widely used for modification of molecular catalysts, already have better stability under the highly oxidative and acidic conditions owing to their unique structure. Yi et al. report that after an initial electrochemical oxidation processes, a passivating oxide layer forms on the MWCNTs, effectively protecting the inner graphitic layers from further electrochemical oxidation. 35 Moreover, different from the direct application of carbon materials as current collectors, carbon materials used as substrates are covered by molecular catalysts. The coverage of the carbon material with WOCs and the fast catalysis can also limit the corrosion of carbon materials. Considering the abundant advantages of carbon materials, they offer excellent opportunity for initial stages of investigating molecular catalyst based electrolyzers and further development of carbon materials may even boost applicability of these materials in future practical PEM electrolyzers.In the view of future practical application, common carbon materials, such as carbon black, carbon cloth, and carbon paper, cannot resist the high oxidation potentials, oxygen environments, and acidic conditions for long time. More advanced carbon materials, such as MWCNTs, are more stable; however, further research is certainly necessary to understand how MWCNTs and other new carbon materials would perform under realistic conditions. Corrosive resistant oxides, such as TiO2, SnO2, and Ta2O5, can be considered to replace carbon materials for loading molecular catalysts. 6 However, these materials generally have lower electron conductivities and smaller surface area than carbon materials. To realize the practical application of molecular catalyst based PEM electrolyzers, development of more advanced conductive substrates with high surface area and long-term stability under acidic and oxidative conditions is essential.A great advantage of molecular catalysts is that each active site is in principle not restricted, and all can participate in catalysis. To retain this fundamental advantage, the approach for loading a molecular catalyst on an electrode must be compatible. 41–43 Two different approaches for the immobilization of a molecular catalyst are demonstrated in Figure 4 A. With immobilization via self-assembly, the catalyst is integrated “one-by-one” on the surface of the highly conductive electrode. In comparison, in bulk immobilization catalysts aggregate on the electrode to form a solid-solid contact interface, resulting in the loss of both surface features as well as molecular properties. Immobilization through self-assembly facilitates the electron transfer between the catalyst and the supporting electrode, accessibility of active sites by the reactants, and efficient proton transfer between the catalyst and electrolyte, i.e., it often promotes efficient operation of catalyst molecules. Many simple and conventional methods for the preparation of hybrid electrodes that have emerged do not meet this demand (e.g., drop casting, spin coating, and Nafion conglutination) and do not utilize the intrinsic activity of every catalyst molecule introduced. To achieve one-by-one anchoring of the catalyst on the substrate carbon material, the connection between the molecule and carbon substrate should be established in situ. In other words, the catalyst should be anchored in a molecular fashion via self-assembly from its homogeneous solution.Self-assembly of a molecular catalyst on the surface of carbon materials can be accomplished via various approaches, generally classified based on the type of interaction between the catalyst and the surface of carbon substrate, i.e., covalent or non-covalent immobilizations. 27 , 42 To achieve covalent attachment to the substrate surface, the molecular catalyst needs to be modified with a reactive group that can be reacted with the pretreated carbon electrode surface to form a covalent linkage. Various synthetic procedures have been developed for achieving this covalent grafting of molecules on the carbon electrode surface, e.g., amidation, nucleophilic substitution, and 1,3-dipolar cycloaddition reactions. 27 The attachment of molecules by covalent linkage provides highly stable connections, however, the pre-modification of molecular catalysts and carbon surfaces is generally complex and time-consuming. Based on the simplicity of molecular modification, mild reaction conditions, and stability of the target covalent bond, the most widely occurring immobilization reactions used for electrode functionalization are the diazonium ion reduction and the copper catalyzed alkyne-azide “click” reaction (Figure 4B). 44 In contrast, non-covalent immobilization of molecular catalysts involves van der Waals interactions, π–π stacking, or electrostatic interactions between the functional groups in the catalyst and the carbon material (Figure 4B). 45 Organometallic compounds with aromatic rings in their structure, can physically adsorb onto porous carbon materials, but often molecule immobilization through physical adsorption has relatively low stability and loading amount. 27 Generally, decoration with large conjugated π systems, as can be found in porphyrines, phthalocyanines, and pyrene, is required for the adsorbed complex to stably adhere to the surface of CNTs and graphene. 46–48 The molecular catalysts can also be immobilized on the carbon material through electrostatic interactions. Ion-pairing can be achieved by functionalizing the target molecule and the surface of the carbon material with opposite electrical charges. 49 Another acceptable method is in situ polymerization of the modified molecular catalyst to form a polymer chain or a thin polymer film of catalysts onto carbon electrodes, 50 although the optimization of polymerization conditions and the characterization of the final polymer provides additional challenges associated with this method. Further details and examples of these approaches will be discussed in the following sections.One of the challenges associated with molecular catalyst immobilized on carbon materials is their characterization. Due to the trace loading amount, the sub-nano scale size of the molecules, and the strong UV-visible absorption of carbon materials, some conventional characterization methods, such as UV-visible spectroscopy, X-ray diffraction, and scanning electron microscopy are not capable of analyzing molecular catalyst modified carbon materials. Electrochemical measurements, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), resonance Raman spectroscopy, and infrared (IR) spectroscopy have to be used instead for characterization. This section provides a summary of these approaches for the characterization of molecular catalyst modified carbon electrodes.Electrochemical measurements can be used to detail the redox processes of the catalyst. For example, Figure 5 A shows the cyclic voltammogram (CV) of a cobalt-catalyst-modified CNT electrode for proton reduction (cf. Figure 16D for structural details). 51 By comparing the obtained CV with that recorded for the bulk solution of the cobalt catalyst, it was observed that the functionalized CNT electrode showed similar electrochemical behavior, displaying a distinct reversible redox character at −1.08 V versus ferrocenium/ferrocene (Fc+/Fc), attributed to the CoII/CoI couple of the cobalt catalyst. Observation of this redox event provides a clear indication that the cobalt complex was successfully modified on the CNT electrode. Since the currents of both anodic and cathodic peaks of the CoII/CoI redox vary linearly with the scan rates (Figures 5B and 5C), the modification of the cobalt catalyst on the surface of the carbon electrode could be confirmed. Furthermore, the loading amount (4.5 × 10−9 mol·cm−2) of the molecular catalyst can be estimated from the coulomb value associated with the single-electron redox process by integration of the redox wave in Figure 5A.To obtain more detailed information of the electronic structure and coordination environment of the metal center, XPS and XAS can be used. XPS is a very sensitive technique for surface characterization, especially for metal atoms, making this technique a powerful tool for the characterization of immobilized molecules on the surfaces of carbon materials. Revisiting the cobalt-modified CNT example, the XPS spectrum of the Co-CNT sample (blue line in Figures 6A and 6B) shows an increased N 1s peak and two distinct peaks at 781.7 and 796.6 eV for the Co 2p3/2 and Co 2p1/2 levels, respectively. 51 The splitting of the peaks with an energy difference of 14.9 eV and absence of any distinct satellite on the Co 2p core-level signals demonstrates the presence of the CoIII ion, i.e., the immobilized cobalt catalyst.XAS is especially useful for the characterization of molecular catalysts modified on carbon materials, as it can provide information of oxidation states and electronic structure from X-ray absorption near edge structure (XANES) spectrum, and local environment from extended X-Ray absorption fine structure (EXAFS) spectrum. 53 , 54 Under the fluorescence detection mode, XAS is highly surface sensitive. For example, XAS analysis was achieved for a copper catalyst immobilized graphene with surface loading of 0.050 nmol/cm2 (cf. Figure 10E for structural details). 52 The XANES spectrum of the copper catalyst-graphene was found similar as the XANES spectrum of the individual copper molecules in frozen MeCN, indicating the successful modification (Figure 6C). The interactions between the copper molecules and the graphene interfaces were also identified from some minor changes in the XANES spectrum of the immobilized copper catalyst. Samples after bulk electrolysis showed identical XANES features to those characterized before test. Decomposition of the copper catalyst into CuO could be excluded by comparing the obtained XANES and EXAFS spectra (Figure 6D) of the copper catalyst after catalysis and the contrasting CuO. Moreover, it is relatively easy to realize in-situ investigations by XAS measurements, which can benefit a lot for studies of catalytic mechanism. 53 , 54 For instance, XAS techniques were employed to successfully track the key 7-coordinate RuV=O intermediate of a Ru WOC modified on electrode surface. 55 The third approach to directly observe the molecular catalyst on the surface of carbon materials, involves advanced TEM techniques, e.g., scanning TEM (STEM) and high-resolution TEM (HRTEM). Figure 7 shows STEM and HRTEM images of the Ru4POM modified CNT (cf. Figure 9 for structural details). 49 The STEM images illustrate the electron-dense catalysts (bright spots, diameter 1–2 nm), indicating individually separated Ru4POM molecules on the CNT surface (Figures 7A and 7B). The corresponding energy-dispersive X-ray spectroscopy (EDX) analysis showed the distribution of elemental Ru and W in the color-coded elemental map (Figure 7C). The blue and red colors were assigned to the carbon Kα and tungsten Lα emissions, respectively. The catalytic ruthenium cores were barely visible around 19 keV because they are embedded in the large tungsten based polyoxymetalate ligands. The HRTEM images show a 2 nm thick layer of catalyst coated on the surface of the CNT walls (Figure 7D). In a separate study, where the Ru4POM was modified on graphene, the Ru4POM molecules were identified with atomic resolution, as shown in its HRTEM images (Figure 7E). 56 These examples illustrate that it is feasible to observe the immobilization of molecular catalysts on the surface of carbon-materials. However, Ru4POM catalysts have numerous heavy metal atoms in the ligands, making their identification easier relative to catalysts with pure organic ligands. For carbon materials modified by molecular catalysts with organic ligands, center metals of molecular catalysts generally can be identified by TEM techniques. 57–59 By the more advanced single-molecule atomic-resolution real-time (SMART) TEM technique, 60–62 it is also possible to visualize the shape and movement of single molecules on CNT (Figure 7F), but this technique is not widely available and requires special and strict sample preparation.Resonance Raman spectroscopy is a simple and efficient way to probe the immobilization of molecular catalyst on the surface of carbon materials. For example, the characteristic bands of both Ru4POM (Ru–H2O stretching modes, 250–500 cm−1, and Ru4 core vibrations, 450–500 cm−1) and graphene (typical D, G, and G' bands at 1,350, 1,600, and 2,700 cm−1) were observed in the Raman spectra of the Ru4POM modified graphene (Figure 8 A), confirming the successful immobilization. 56 It should be noted that some vibrational features of the molecular catalyst may not be observable after it is modified on the surface of carbon material, such as the peak at ∼770 cm−1 of Ru4POM in Figure 8A, because some of the vibrational modes of the molecule could be prohibited due to the electronic interactions between the modified molecule and the surface of carbon material. 59 Raman characterization not only show the existence of molecules on the carbon surface but can also provide information regarding the interactions between immobilized molecule and carbon surface. The Raman D, G, and G' bands of graphitic materials, originating from the presence of sp2-sp3 hybridization in the hexagonal framework, are sensitive to changes of surface structure and perturbations to the π-electronic structure. 63 For the covalent modification of graphitic materials, sp2 hybridization in the hexagonal framework is decreased due to the formation of anchoring bonds of molecular catalysts, usually leading to the observation of increased D and G band ratios (I D /I G ). 58 , 64 Taking the Ni pyridine complex (Ni-bpy) modified CNTs as an example (cf. Figure 14C for structural details), the I D /I G ratio of ∼1.4 for the pristine CNTs is increased to 1.9 for the bpy-CNTs (Figure 8B). 58 Moreover, a shift of the G' band was also observed, indicating charge transfer between CNTs and the modified bpy moiety. In contrast, the noncovalent modification of graphitic materials does not directly change the surface structure of graphitic materials. The influence of graphitic materials to the Raman spectra depended on the strength of noncovalent interaction between molecular layer and the carbon surface. Adsorption of planar molecules, such as phthalocyanines, porphyrins, and tetrapyrroles, on graphitic materials via π–π stacking can induce 2–4 cm−1 shifts of the G band and changes of the I D /I G ratio, due to the strong charge transfer between the adsorbed molecular layer and the carbon surface. 47 , 65 When the phthalocyanine with steric hindrance groups adsorbed on CNTs, the change of I D /I G ratio disappeared, indicating less close interaction between the molecular layer and the carbon surface. 65 When the distance between the molecular layer and carbon surface is large, Raman spectra of graphitic materials showed no obvious changes, indicating minor interaction between the molecular layer and the carbon surface. 56 , 57 Raman spectroscopy is generally recognized as a qualitative characterization method. Less known is that Raman spectroscopy, in principle, can also be used for quantitative analysis. 66 Nonetheless, it is challenging to make Raman analysis quantitative because the intensity of a Raman spectrum depends not only on the nature of sample, but also on instrumental parameters and operation factors. Advanced Raman spectrometers, smart experimental design, and highly skilled operation are required to realize Raman quantitative analysis. Considering the merits of Raman spectroscopy and the lack of methods for quantifying molecular catalysts modified on carbon materials, it is advantageous to develop Raman quantitative analysis to support future developments in this field.IR spectroscopy, which is complementary to the Raman spectroscopy, is another important and effective method to characterize molecular catalysts modified on carbon materials, especially many HER catalysts and CO2RR catalysts that have CO ligands in their structures. 67–70 IR can also be used to sensitively detect trace amounts of in situ generated CO products during electrocatalytic CO2 reduction. 71 For the covalent immobilization via “click” reaction, the characteristic stretching vibration near 2,110 cm−1 of the azido group can be used to monitor the grafting process (Figure 8C, cf. Figure 13 for structural details). 46 , 71 Importantly, both IR spectroscopy 69 , 72 and Raman spectroscopy 73 can be combined with electrochemical systems to establish spectroelectrochemistry investigations for tracking catalytic intermediates and deeply studying catalytic mechanisms.In artificial photosynthesis, water oxidation (2H2O → O2 + 4e− + 4H+, E o = 1.23 V versus NHE at pH = 0, NHE = normal hydrogen electrode) is considered the ideal way to provide abundant electrons and protons for the reduction reactions (e.g., proton reduction and CO2 reduction). However, this reaction proves to be a bottleneck due to the high potential requirements caused by the high reaction barrier and kinetic complexity of the process. To increase water oxidation efficiencies, molecular WOCs have been investigated during the past 3 decades, providing knowledge and insight into establishing efficient WOCs. 8 , 9 , 19 , 20 , 74–76 With a wide variety of WOCs established, the essential fabrication of electrolyzers by grafting molecular WOCs on electrode materials has become more attractive. As a promising strategy, molecular WOC modified carbon materials have been studied in electrochemical water oxidation.A pioneering work on noncovalent modification of WOCs on carbon materials was reported by Bonchio and co-workers in 2010. A Ru4-POM (polyoxometalate) complex was assembled on a conductive support of MWCNTs. 49 Figure 9 represents catalyst adsorption on the MWCNTs via electrostatic interactions, achieved by decorating the MWCNTs with polyamidoamine ammonium dendrons. The Ru4-POM@MWCNTs hybrid composite was characterized via resonant Raman spectroscopy, small-angle X-ray scattering (SAXS) analysis, STEM, EDX-spectroscopy, and HRTEM to reveal the local distribution of catalysts (Figures 7A–7D). Electrochemical water oxidation using this Ru4-POM@MWCNTs composite material was studied by drop-casting the material on an indium tin oxide (ITO) electrode. Using a Pt counter electrode, catalytic water splitting at pH 7 was observed with a TOF in the range 36–306 h−1 (η = 0.35–0.6 V) for oxygen evolution. An analogous anode fabricated by mixing Ru4-POM with amorphous carbon showed only very poor catalytic performance, indicating the importance of molecular interfaces in controlling and promoting electron-transfer at heterogeneous surfaces.Bonchio’s group further studied the behavior of Ru4-POM on graphene-based electrodes. Graphene is considered more attractive than MWCNTs due to its high stability and conductivity, as well as larger surface area-to-volume ratio. Following the same functionalization strategy, indeed a 2-fold enhancement in electrocatalytic performance compared to that of the MWCNTs analogue could be recorded when graphene was utilized as the support material. 56 The resulting electrode displayed catalytic activity for oxygen evolution at pH 7 with an overpotential of 0.3 V and the activity was maintained after 4 h of testing. A similar strategy has been reported involving confining the catalyst in a highly porous wet graphene film. 77 In 2011, the groups of Li and Sun developed an approach to decorate MWCNTs with pyrene-functionalized WOCs via noncovalent π–π stacking interactions. The Ru(bda)(pic)2 (pic = 4-picoline) catalyst was functionalized with two pyrene anchoring groups, allowing the molecule to adsorb on MWCNT modified ITO electrodes (Figure 10 A). 48 The obtained electrodes displayed catalytic oxygen evolution under neutral conditions at a low overpotential of 0.28 V. The molecular nature of the Ru-bda catalyst was retained on the surface of MWCNTs electrode as confirmed by observation of the two distinct peaks at 0.59 and 0.91 V corresponding to the RuII/RuIII and RuIII/RuIV redox couples, respectively. An average TOF of 0.3 s−1 and TON of 11,000 were observed for oxygen evolution in a neutral solution under long-term (> 10 h) electrolysis. After long-term electrolysis, the redox peaks of the Ru-bda catalyst were still clearly discernible indicating the promising stability of this system.Meyer et al. found that on metal oxide surfaces an electron transfer mediator, such as [Ru(bpy)3]2+, is required for the oxidation of the Ru-bda catalysts. 79 , 80 These results clearly indicate that the highly conductive CNTs are superior for the electrochemical water oxidation by Ru-bda catalysts. The catalytic behavior on the CNT surface was further detailed by Ahlquist and co-workers through simulation of catalyst dynamics using the empirical valence bond method (Figure 10B). 78 It was found that not only that the pyrene “arm” attached to the axial ligand tightly adsorbs to the surface of CNTs, but also that the “body” of the Ru-bda catalysts adsorbs as well. This significantly retards O−O bond formation due to the large distortion required for the catalyst to reach the transition state geometry.Recently, Llobet and co-workers reported the Ru(tda)L2 (tda2- = [2,2′:6′,2′’-terpyridine]-6,6′’-dicarboxylato) WOC modified MWCNT anode obtained following the noncovalent π–π stacking modification strategy (Figure 10C). 81 According to the authors’ calculations based on the foot-of-the-wave analysis, more than a million TONs at pH 7 with an Eapp = 1.45 V versus NHE were achieved by these hybrid solid state materials. The oxidative stability of the working electrode was monitored by electrochemical techniques and XAS spectroscopy before, during, and after catalysis. After electrocatalysis for 1 h, the Ru molecule still can be identified on the electrode, and no obvious evidences indicate the presence of RuO2. With this Ru-tda/MWCNT material, the groups of Llobet and Lewis assembled an n-Si/TiO2/C/CNT/Ru-tda photoanode for photoelectrochemical water oxidation by binding the Ru-tda/MWCNT on TiO2-protected Si photoanode (Figure 11 A). 82 Photocurrent densities of 1 mA cm−2 could be achieved with this hybrid device at pH = 7 under three Sun illumination. Application of the Ru-tda/MWCNT material to a multilayered hetero-structured WO3/BiVO4 semiconductor photoanode (Figure 11B) further enhanced the overall photoelectrochemical performance. 83 Non-ruthenium centered catalysts have also been immobilized using the π–π stacking strategy of pyrene. For example, the Cao group reported the immobilization of a bi-functional cobalt corrole catalyst onto MWCNTs (Figure 10D). 84 The corrole functionalized carbon MWCNTs were tested for both electrochemical water oxidation and oxygen reduction. For the water oxidation reaction, the fabricated electrodes achieved high electrocatalytic performance and durability under neutral aqueous conditions with an onset overpotential of 0.33 V. To illustrate the benefit of the π–π stacking using pyrene, MWCNTs loaded with pyrene-free cobalt corrole were prepared as control samples. The pyrene-free electrodes displayed much lower performances for water oxidation, clearly demonstrating the importance of strong noncovalent π–π interactions between the pyrene moiety and the MWCNTs in facilitating the immobilization and electron transfer processes.Interestingly, the pyrene has been shown to not only function as anchor for molecular catalysts. The electronic π-delocalization effect of pyrene on the electrocatalytic performance was revealed recently by Llobet and co-workers. 52 They synthesized two copper complexes with the general formula of [(L)CuII]2− (L is 4-pyrenyl-1,2-phenylenebis(oxamidate) or pyrene free control o-phenylenebis(oxamidate), Figure 10E). In addition to the anchoring function of the pyrene group, it provided electronic perturbation to the system. Consequently, the overpotential for water oxidation was lowered by 150 mV, in the homogeneous system in the presence of the pyrene functionality, accompanied with a dramatic increase of the TOF from 6 to 128 s−1. After assembly of the catalysts on the graphene sheets, the π-delocalization effect, provided by the support, boosts the catalytic activity of both catalysts. Due to the combined effect of both the pyrene group and the graphene substrate, the pyrene functionalized catalyst displays an electrocatalytic activity with an overpotential of 538 mV, a TOF of 540 s−1 and TONs as high as 5,300.The π–π stacking effect has been widely adopted to modify molecular catalysts on CNTs and graphene. 48 , 52 , 81 , 82 , 84–86 The success of this strategy highlights the ease and advantages of noncovalent modification. As a result, different noncovalent modification strategies for the fabrication of electrodes were explored. For example, a Ru-pdc (pdc2- = 2,6-pyridine dicarboxylate) based WOC was immobilized on MWCNTs using the hydrophobic effect. By pre-functionalization of the catalyst with a long alkyl chain Sun and co-workers (Figure 12 A) show catalyst aggregation with the MWCNT support. 87 The obtained electrodes maintain a catalytic current density of 2.2 mA cm−2 at an overpotential of 480 mV after 1 h of bulk electrolysis, and a high TOF of 7.6 s−1 was observed. This immobilization strategy proved successful for cobalt based WOCs on carbon black as well (Figure 12B). 88 The resulting material displayed good catalytic activity for water oxidation under basic conditions, with an onset overpotential of 0.32 V, and current densities of 10 mA cm−2 could be achieved at 0.37 V.Good electrocatalytic performance requires not only a well-designed molecular WOC with high intrinsic activity but also a suitable immobilization method. The effects of different non-covalent immobilization methods on catalysis were systematically investigated and highlighted by Cao and co-workers. They described the modification of cobalt corroles, with four different anchoring groups, on CNTs and compared the performance of these functional carbon materials for H2 and O2 evolution (Figure 13 ). 46 The CNTs decorated with cobalt corroles with short conjugated linkers displayed the highest electrocatalytic activity for both H2 and O2 evolution reactions in pH 0–14 aqueous solutions. The increased performance obtained with shorter conjugated linkers was attributed to (1) the fast electron transfer ability and (2) the high stability of intermediates under strong basic or acidic conditions due to the stronger coordination of the corroles.To prevent catalyst leaching a more stable linkage between the catalyst and the substrate could offer a solution. In 2012, the Sun group reported a strategy to covalently attach a RuII(pdc)(pic)3 (pdc = 2,6-pyridinedicarboxylate, pic = 4-picoline) catalyst on carbon electrodes, using a diazonium ion electro-reduction protocol to introduce alkyne functionalities to the support, followed by an alkyne-azide “click” reaction to immobilize the catalyst (Figure 14 A). 44 This study provided a universal method to covalently attach WOCs on conductive carbon surfaces. Lin and co-workers showed that diazonium grafting can also be achieved directly using the catalyst molecules. In their study, covalent immobilization of molecular Ir complexes onto a carbon electrode was achieved through direct C–C bond linkage of the bipyridine. 89 The obtained electrode exhibits a TOF of 3.3 s−1 and TON of 644 during the first hour of electrolysis (Figure 14B). Compared to chemically driven water oxidation with the corresponding Ir molecular catalysts, electrochemical water oxidation with modified catalysts gave increased rates and stability. The authors suggested that this strategy can be utilized as an alternative way to systematically evaluate catalysts under tunable conditions.Based on these results, the Lin group later studied catalytic performances of various less inert transition metal ions (i.e., Fe3+, Co2+, Ni2+, Cu2+) by a simple electrochemical approach (Figure 14C). 64 In the first step, an amino-bpy was bound on the surface of CNT electrodes by diazonium grafting. Various metal ions were coordinated to this functionalized electrode and the resulting electrodes were assessed for water oxidation catalysis. Results showed that the Co-complex immobilized electrode possessed the highest catalytic activity, and is capable of oxidizing water with a TOF of 14 s−1 at an overpotential of 0.834 V. Recently, the Ni-bpy modified CNT electrodes were further investigated under basic conditions by Laasonen and co-workers. 58 They show that the Ni-bpy modified electrodes are capable of generating a current density of 10 mA cm−2 at an overpotential of 0.31 and 0.29 V in 0.1 and 1 M NaOH, respectively.The last discussed immobilization strategy of WOCs on carbon materials is through the polymerization of catalyst monomers. An in situ polymerization of catalyst monomers has been carried out by Sun and co-workers to decorate the surface of graphitic carbon (Figure 15 A). 90 In this work, in situ polymerization of pyrrole functionalized Ru-bda catalysts was employed to enrich the graphite electrode surface with catalysts. The obtained electrode showed a high initial TOF of 10.47 s−1 at an overpotential of 700 mV, and a TON of 31,600 after 1 h of electrolysis in a pH 7.2 aqueous solution. The advantages of catalyst monomer polymerization were further illustrated by Du and co-workers. Through the Glaser coupling reaction, multi-layer covalent cobalt porphyrin frameworks were built on the MWCNTs template to form a hybrid material for electrochemical water oxidation (Figure 15B). This hybrid material performed significantly better than its analogue where the cobalt porphyrin monomer analogue was immobilized. Overall, the hybrid materials achieved a catalytic current density of 1.0 mA/cm2 under an overpotential of 0.29 V in a pH 13.6 solution. 50 An additional novel modification procedure has been reported by Heumann and co-workers. They constructed a cobalt-bridged ionic liquid polymer on a CNT for efficient oxygen evolution (Figure 15C). 57 In their material, the ionic liquid polymer can act as the counter ion to increase the stability of the Co2+ ion and adjust the electron structure of the atomically dispersed Co creating a favorable environment for an oxygen evolution reaction.Hydrogen has been considered a clean and efficient next generation energy-carrier. Numerous molecular transition metal catalysts have been developed for water oxidation coupled hydrogen production, i.e., overall water splitting. 10 , 11 , 91 , 92 Immobilization of HER catalysts on carbon material has been widely investigated for future practical utilization in industry.Artero and co-workers reported nanomaterials decorated with noble metal-free molecular catalysts for hydrogen production and uptake. 42 , 51 , 68 , 93–96 A nickel bisdiphosphine based mimic ([Ni(P2N2)2]2+) of the catalytic center of hydrogenase enzymes was covalently attached to MWCNTs, as illustrated in Figure 16 A. 96 For this grafting, an activated phthalimide ester functionalized Ni(P2N2)2 complex was prepared and anchored to the amino-modified MWCNTs-electrode material via amide bond formation. The obtained electrodes showed excellent hydrogen evolution activity in aqueous sulfuric acid conditions with a low overpotential of 200 mV and exceptional stability achieving >105 turnovers. To increase practical applicability, they prepared a membrane-electrode assembly of the Ni(P2N2)2-functionalized MWCNTs. With this assembly, a current density of 4 mA cm−2 could be obtained at an overpotential of 300 mV, and a TON of 105 (±30%) was achieved within 10 h stability tests. The authors also evaluated capability of the electrode to perform the reverse reaction, i.e., hydrogen oxidation. Under 1 atmosphere pressure of H2, an overpotential of 300 mV and TON of 3.5 × 104 (±30%) was observed for 10-h bulk electrolysis.The synthetic procedure was later improved by Artero and co-workers. 93 Similarly, the MWCNTs were firstly functionalized with amine groups by a diazonium reduction. Now, the ligand PCy 2NR1 2 was initially covalently immobilized on the surface of the electrode via amide coupling, followed by separate introduction of the Ni active sites (Figure 16B). The obtained hybrid CNTs were further processed into the MEA, and were evaluated for H2 production and H2 oxidation under typically operational conditions in PEM electrolyzers or fuel cells. At 85°C, the MEA with Ni-PCy 2NR1 2 catalysts showed a current density of 38.3 mA/cm2 at −100 mV versus standard hydrogen electrode (SHE) for hydrogen evolution and 16.8 mA/cm2 at +100 mV for hydrogen oxidation, and the corresponding performances achieved with platinum MEA (commercial Pt/C containing 46% Pt deposit on the gas diffusion layer) are 32.2 and 26.6 mA/cm2, respectively. Under technologically relevant conditions, the performance of platinum-based MEA is likely to be matched by this Ni-PCy 2NR1 2 modified MEA. However, it must be noted that although all the electrodes were evaluated under the same conditions in this work, these tests were carried out under half-cell configuration, which is significantly different from the real PEM cells. In addition, the performances of Pt-based electrodes can be influenced by the MEA preparation procedure over several orders of magnitude. Nevertheless, to some extent, these studies and results demonstrate possibilities of applying molecular HER catalysts in the PEM cells.To achieve straightforward and highly convenient attachment of the [Ni(P2N2)2]2+ catalyst on carbon materials, Artero and co-workers further explored non-covalent immobilization of the catalyst on CNT via π–π stacking interactions. 94 This was achieved by functionalization of the [Ni(P2N2)2]2+catalyst with pyrene substituents, which was subsequently physisorbed on MWCNTs through the formation of π–π stacking between the pyrene groups and graphene substrates (Figure 16C). After deposition of the active MWCNT material on a gas diffusion layer electrode, an outstanding electrocatalytic hydrogen production activity was observed in 0.5 M aqueous sulfuric acid electrolyte in the presence of CO. Stability tests performed at −300 mV versus NHE showed no obvious loss of activity after 6 h resulting in 8.5 × 104 turnovers.In addition to Ni based electrocatalysts, Artero and co-workers also grafted a diimine-dioxime cobalt catalyst on functionalized MWCNTs, as shown in Figure 16D. 51 , 97 , 98 Cobalt diimine-dioxime complexes were functionalized with an azido-group, and then attached on cyclooctyne-functionalized MWCNTs by a copper free click reaction. The obtained electrode showed impressive activity towards H2 generation under acidic aqueous conditions, with onset of the reduction at an overpotential of 350 mV and a TOF of 8,000 h−1 per cobalt site. Over the course of 7 h electrolysis at a potential of −0.59 V versus RHE, 5.5 × 104 turnovers were obtained. No change of the anchored complex could be observed after long-term electrolysis, indicating the remarkable stability of the electrode. Afterwards, they investigated the oxygen tolerance of this Co based cathode for hydrogen production using GC instead of a gas-diffusion layer as electrode. 99 The electrode retained its excellent activity for H2 evolution using an O2-saturated aqueous acetate buffer as electrolyte, promoting the potential application of such cathodes in water-splitting devices.Direct comparison between an HER catalyst immobilized on a carbon surface and its homogeneously dissolved counterpart has been provided by Roberts and co-workers. To achieve immobilization, the GC surface was pre-functionalized with triazolyllithium groups using azide alkyne chemistry, which were subsequently reacted with an active NHS-ester-functionalized [Ni(P2N2)2]n+ electrocatalyst (Figure 17 ). 100 The coupling could be performed for both Ni0 and Ni2+ complexes to achieve surface densities of 1.3 × 10−10 and 6.7 × 10−11 mol cm−2, respectively. The coupling reaction proved highly successful for the Ni0 species producing a densely packed layer, however, coupling of the Ni2+-species was less clean. Interestingly, the onset potential for hydrogen evolution was unaffected by the immobilization of the catalyst. Loss of activity of their catalyst assembly was rather caused by the decomposition of the surface-confined [Ni(P2N2)2]2+ complex. This decomposition was shown to occur more rapidly under more acidic conditions, suggesting increased lability of the P2N2-ligands through protonation.Non-covalent immobilization has also been used as successful strategy for the immobilization of planar hydrogen evolving complexes, by direct adsorption onto sp2-hybridized carbon materials, e.g., CNTs and graphene. For example, Peters and co-workers reported the preparation of hydrogen evolving electrode by simple adsorption of cobalt tetraimine complexes on electrode surfaces. 101 Under controlled potential electrolysis and in the presence of p-toluenesulfonic acid, Co(dmgBF2)2(MeCN)2 could be directly adsorbed on GC electrodes (Figure 18 B). The resulting electrode showed a catalytic onset overpotential of 100 mV in acetate buffer electrolyte at pH < 4.5. Furthermore, the high electrocatalytic stability of the electrodes was shown by bulk electrolysis for 16 h at 1 mA cm−2, achieving a TON of 5 × 106 and Faradaic efficiency of 75 ± 10%.Lehnert and co-workers also reported a facile adsorption of a cobalt catalyst on several carbon materials by a facile soaking process (Figure 18B). 102 , 103 Initially, graphene electrodeposited on fluorine doped tin oxide (FTO) plated glass was used as an adsorption substrate for the Co-based molecular catalyst. In a weakly acidic aqueous electrolyte (pH > 3), this electrode showed low overpotentials of 0.37 V versus Pt and TOF > 1,000 s−1 for hydrogen production without significant degradation. 93 In later works, a variety of carbon materials including bulk highly ordered pyrolytic graphite, graphite, pencil graphite, graphite powder in Nafion films, graphene, and GC electrodes were targeted as substrate for molecular catalyst adsorption. All the electrodes displayed similar activity towards hydrogen production, achieving high activity in moderately acidic aqueous solutions (pH < 4), moderate overpotentials of 0.42 V versus Pt and high initial TOFs of 96 s−1. Their work highlights that adsorption is a facile strategy for the immobilization of catalysts. However, it should be noted that interaction of the catalyst with GC and single-layer graphene surfaces was too weak to provide sustainable catalytic currents. 103 Pyrene functionalization, as reported for WOCs, was likewise used for immobilizing hydrogen evolution catalysts, as shown for example by Brunschwig, Gray and co-workers. 104 In their report a [Cp∗Rh(L)Cl]+ catalyst (L is a pyrene functionalized bipyridine ligand) for hydrogen production is immobilized on a high surface area carbon black support (Figure 19 A). Appearance of redox peaks in the CVs confirmed loading of the complex on the electrode surface. With p-toluenesulfonic acid as proton source in the electrolyte, the electrode exhibited a hydrogen production activity with steady-state current density about 2 mA cm−2, TOF of 0.95 s−1 and TON of 206 over 75 min electrolysis. After 75 min electrolysis, the current density gradually decreased approximately 30% of activity in every hour, which is indicative of the release of the catalytically active species from the electrode.To investigate if the stability of the immobilized species can be increased, Reisner and co-workers fabricated a poly(cobaloxime)/CNT electrode and compared its H2 production performance with its analogue monomeric catalyst immobilized on the electrode surface (Figure 19B). 67 For the monomeric catalyst, the complex was directly anchored on MWCNTs using π-π interactions with a pyrene substituted pyridine as a ligand coordinating to the catalyst. For the polymeric complex, the monomeric catalyst precursor was placed through ligand substitution on the polymer, consisting of pyrene and pyridine substituted monomers. The pyrene groups of the polymer allowed for immobilization of the polymeric catalyst on MWCNTs. The stability of the two electrodes was probed at near-neutral pH, where the polymer-substituted electrode showed TON of 420 towards H2 production, four times higher compared to the 80 turnovers achieved by the monomer-substituted electrode. These results suggested that the polymer structure can enhance the stability of the assembly and the catalysts incorporated therein, leading to enhancement of the electrode lifetime.The importance of the π–π interactions of the catalyst with the electrode surface is made clearly apparent by the work of Cao and co-workers. They adsorbed three different cobalt corrole complexes for hydrogen production on graphene. The complexes bear a pyrene functionality and two axial pyridine ligands (1-py), a pyrene functionality and triphenylphosphine axial ligand (1-PPh3), and a complex that only has the two axial pyridine ligands and lacks the pyrene functionality (2-py) (Figure 19C). 105 Overall, 1-PPh3/G showed better activity as compared to its analogues, indicating the significant influence of both axial and equatorial ligands on the π-π interactions between the cobalt corrole and graphene. This catalyst achieved H2 production under the full pH range (pH 0 to 14) with high efficiency and stability. A similar conclusion could also be drawn in the study of molecular catalyst modified carbon electrodes for water oxidation fabricated based on π-π interaction. 52 The Lee group systematically studied catalytic activity and charge transfer behavior of (metal-)porphyrin monolayers on inactive graphene (Figure 19D). 47 The interfacial charge transfer process was analyzed based on the performance operations of the graphene field effect transistor. They found that the two-dimensional porphyrin-based monolayers induced homogeneous active sites on graphene, showed electrochemical stability and enhanced charge transfer at the electrolyte/graphene interface. In addition, the electronegative pristine porphyrin and Pt-porphyrin monolayers displayed higher HER performance than Ni-, or Zn-porphyrin monolayers, due to their increased interfacial charge transfers. Monolayers exhibiting intermolecular hydrogen bonding also showed better electrocatalytic effects, indicating that surface hydration potentially plays an important role. Overall, this work indicates that the introduction of surface electronegativity, induced by either porphyrin or metal-porphyrin immobilization, plays an important role in the electrocatalytic HER.In addition to the strategy of molecular catalysts engineered carbon materials, which is the main focus of this review, we wish to note that metal organic compounds have also emerged as a promising support for molecular catalysts in recent years. 106–111 For example, Marinescu and co-workers successfully integrated the cobalt dithiolene catalyst into a metal-organic surface (MOS) (Figure 20 ). By layering an acetonitrile solution of [Co(MeCN)6][BF4]2 on the top of an aqueous solution of sodium benzenehexathiolate (C6S6Na6), they obtained a film with molecular formula (CoC4S4Na)n. 112 They placed the film on GC by simply immersing the GC surface into the reaction solution. The formed electrodes MOS1 and MOS2 achieved a current density of 10 mA cm−2 for H2 production at overpotential of 0.34 and 0.53 V under fully aqueous conditions at pH 1.3, respectively. A faradaic efficiency of 97 ± 3% for MOS1 was determined for 2 h of electrolysis.Recently, Downes and Marinescu also reported a cobalt dithiolene polymer MOS-based electrode. 106 By the reaction of a cobalt (II) salt and benzene-1,2,4,5-tetrathiol in a basic solution the MOS forms and can be immobilized by simply immersing the GC electrode in the reaction solution. The resulting electrode displays a current density of 10 mA cm−2 at −0.56V versus RHE for hydrogen production at pH 1.3. A Faradaic efficiency of 97 ± 3% was determined for 2 h electrolysis, and the electrodes displayed moderate stability after a 10-h electrolysis at 0.55 V versus SHE. The decrease in activity was largely due to delamination of the catalysts. In their work, they also immobilize the MOS on p-type Si electrodes for achieving photoelectrochemical hydrogen production. The assembled photocathode displayed a current density of 3.8 mA cm−2 at 0 V versus RHE under simulated AM 1.5 G sunlight illumination.The catalytic conversion of CO2 to liquid fuel or fuel precursors such as carbon monoxide, formic acid, ethanol, methanol, and methane is viewed as a potential source of renewable energy as well as a strategy to fix atmospheric CO2. 23 , 113 The tremendous challenges posed by CO2 fixation are great as CO2 is a thermodynamically and kinetically stable molecule, however, the rewards gained can be enormous. Catalytic mechanisms of inorganic and organometallic CO2 reduction catalysts, offering excellent activity and selectivity under electrochemical or photochemical conditions, have been extensively studied in this field. However, development of active, selective, and robust catalysts still requires huge efforts. As detailed earlier for WOCs and HER catalysts, immobilization on carbon surfaces can offer an attractive approach for increasing catalyst stability, decreasing required catalyst loading, avoiding bimolecular decomposition pathways, and facilitating catalyst lifetime determination. In this part of the review, we describe immobilization of molecular CO2 reductions catalysts on widely explored carbon electrode surfaces. To slightly expand the scope of immobilization techniques, this section will also introduce some immobilization techniques utilizing other useful materials that are doped with carbon nanomaterials.Similar to the previously described catalysts, both covalent and non-covalent techniques have been used for immobilization of CO2 reduction catalysts on carbon supports. For non-covalent immobilization, a similar pyrene substitution strategy is commonly employed as for example shown by Brunschwig, Gray, and co-workers. In their report they immobilized pyrene modified CO2 reduction catalyst Re(P)-(CO)3Cl on graphitic carbon electrode surfaces via π-π stacking interactions (Figure 21 A). 104 The functionalized electrode was prepared by soaking pyrolytic graphite in a CH2Cl2 solution containing the Re catalyst for 12 h. The surface-immobilized Re catalyst reduces CO2 to CO with 70% faradaic efficiency and 58 TON at −2.3 V versus Cp2Fe+/0. This strategy proved useful for different catalysts. For example, Brookhart, Meyer, and co-workers reported immobilization of a pyrene appended iridium pincer catalyst on MWCNTs coated gas diffusion electrode (Figure 21B). 114 The prepared electrode was highly efficient, selective, and stable for electrocatalytic reduction of CO2 to formate. For this engineered electrode, the polyethylene glycol coating played a critical role for the overall stability of the electrode. Under optimized electrocatalytic conditions at −1.4 V versus NHE a current density of 15.6 mA/cm2, with TON 54,200 and TOF 15.1 s−1 was obtained. The assembly was shown to be stable for over 1 h, converting CO2 to formate with 83% selectivity.Manganese catalyst [Mn(bpy)(CO)3Br] has been reported as highly efficient catalyst for homogenous CO2 reduction to CO, achieving quantitative selectivity for CO over H2 at −1.35 V versus Ag/AgCl. 115 The [Mn(bpy)(CO)3Br] catalyst was initially not immobilized on carbon materials, but was casted in a nafion membrane, where addition of MWCNT leads to great current enhancements, providing stable CO:H2 yields at −1.4 V versus Ag/AgCl under pH 7 conditions. 116 However, this strategy is not always successful as Nafion/MWCNT electrodes functionalized with [Mn(bpy(COOH)2)(CO)3Br] and [Mn(bpy(OH)2)(CO)3Br] catalysts were found to be inactive for CO2 reduction in aqueous conditions. 117 Recently, Reisner and co-workers reported that the product selectivity, to CO or formate, can be tuned by altering the catalyst loading of pyrene-anchored Mn catalyst (Mnpyr) on MWCNT (Figure 21C). 69 Immersion of the MWCNT electrode in high concentration solutions of Mnpyr (10 and 20 mM in DMF) gave high surface loading (>30 nmol cm−2), whereas lower concentrations of Mnpyr (0.5 and 1 mM in DMF) gave low surface loading (<20 nmol cm−2). With higher surface catalyst loadings a dimeric Mn0 species is formed, preferentially leading to CO as major reaction product, whereas lower surface catalyst loading prefer formation of Mn-hydride species leading to enhanced formate production.The dinuclear rhenium(I) complex [ReCl(CO)3(μ-tptzH)Re(CO)3] (tptz-H = 2,4,6-tri(pyridine-2-yl)-2H-1,3,5-triazine-1-ide) was examined as an homogeneous and heterogeneous electrocatalyst for CO2 reduction. For both homogenous and heterogeneous systems addition of methanol enhances the catalytic process and lowers the onset potential. For heterogeneous catalysis, the dinuclear rhenium complex casted on carboxylated MWCNTs-PGE shows a higher cathodic current and about 650 mV lower overpotential compared to homogeneous catalysis. 118 Similar results were observed for the mononuclear Re(I) complex, [ReCl(CO)3(phen-dione)] (phen-dione = 1,10-Phenanthroline-5,6-dione), investigated under homogeneous and heterogeneous CO2 reduction conditions. 119 In these two cases, the Re complexes were adsorbed on the surface of the electrodes via interaction of the functional groups of the ligand coordinated to Re and the carboxyl groups of the activated MWCNTs.Covalent immobilization of CO2 reduction catalysts has also been elaborately explored. As a first example, the functionalization of a GC electrode by electro-grafting of terpyridine ligands and later modification of the electrode by, e.g., cobalt was described by Fontecave and co-workers. The electro-grafting of diazotized terpyridine ligand (tpy-Ph-N2 + BF4 ‒) on GC electrodes was achieved by performing cyclic voltammetry (+0.80 to −0.40 V, at 50 mV s‒1 10 cycles). During the cyclic voltammetry experiment the reduction of the diazonium group liberates N2 generating the free radical, which reacts with the GC electrode yielding a functionalized surface. The modified electrode can be metallated by immersion in a solution of CoCl2 in DMF for 3 h at room temperature to furnish the functionalized working electrode (Figure 22 A). This cobalt-modified electrode shows excellent stability for proton and CO2 reduction into hydrogen and CO in both organic and aqueous media. 120 Another work where the carbon material is not directly utilized for immobilization of the catalyst is provided by Meyer and co-workers. Herein, the properties of CNT are exploited to lower the barrier for electron transfer. The hybrid electrode was realized by anchoring a phosphonate anchoring group containing Ru(II) polypyridial carbene complex on the surface of thin layered TiO2/CNT/TiO2 on FTO glass electrodes (Figure 22B). 121 The derived electrode shows short-term activity for CO2 reduction in 0.5 M NaHCO3 to produce syngas with H2/CO2 ratios varying from 1.5 at −0.96 V to 5.6 at −1.16 V versus NHE with observed maximum TONs of 308 for CO and 597 for H2 for 15 min of electrolysis. The long-term stability of the electrode is however hampered by the decomposition and partial detachment of the catalyst from the electrode. Detachment of catalysts from electrodes, to achieve longer lifetimes of electrodes, can be prevented by utilizing the stronger adherence of polymers to materials. Recently, Sato, Arai, and co-workers polymerized pyrrole functionalized mononuclear ruthenium catalyst on carbon paper coated MWCNT. The modified electrode selectively produces formate as the product of the CO2 reduction reaction in an aqueous solution, with a current density 0.9 mA cm−2 at −0.15 V (versus RHE) for 24 h. 122 Peptide coupling chemistry can also be used to immobilized catalysts bearing carboxylic acid functionalities on CNT, as detailed by Maurin and Robert. In their work, amine functionalized MWCNTs were suspended overnight with CATCO2H in DMF in the presence of HBTU and DIPEA at room temperature (Figure 22C). The modified MWCNTs were then deposited on GC electrodes by the commonly used drop casting method. The prepared electrode led to efficient electro-reduction of CO2 to CO in water (pH 7.3) with 90% catalytic selectivity and 95% faradaic efficiency at 0.5 V overpotential. 123 An illustrative example that covalent immobilization is not always the best or only solution is provided by the same group, through immobilization of a similar iron porphyrin catalyst on MWCNTs through non-covalent interactions. Iron porphyrins functionalized with a pyrene group (CATpyr) were immobilized on MWCNT via noncovalent interactions and deposited on the GC electrode. Bulk electrolysis performed by using this surface immobilized iron porphyrin on GC electrode shows high catalytic activity (97% faradaic efficiency, 432 TON and 144 h−1 TOF), selectivity (CO/H2 is 25) and durability for CO2 reduction to CO in pH 7.3 water. 124 Typical iron and cobalt tetraphenyl porphyrin were immobilized on electrodes with MWCNTs. Comparative investigation under similar conditions revealed that the iron porphyrin-MWCNT modified electrode always performed better than the cobalt porphyrin-MWCNT modified electrode. 125 The collective works performed on cobalt catalysts for CO2 reduction provide more detail on some of the potentials and pitfalls of catalyst immobilization on carbon materials. Kramer and McCrory reported that the edge-plane graphite (EPG) electrodes modified with cobalt–phthalocyanine immobilized in poly-4-vinylpridine (P4VP) film shows 90% faradaic efficiency for CO2 reduction to CO, with a TOF of 4.8 s−1 at —0.75 V versus RHE. In comparison, the modified EPG electrode of cobalt–phthalocyanine without P4VP shows only modest activity and generates mixtures of H2 and CO. The improved activity and selectivity was attributed to the formation of pyridine coordinated cobalt complexes, and to secondary effects associated with uncoordinated pyridine ligands throughout the film. 126 Hybrid electrodes with CoPc molecules, anchored on a CNT (Figure 23 A) were later devised by combining nanoscale and molecular level approaches. Through ultra-sonication and mechanical stirring, the catalyst could be uniformly distributed on the CNT surface, enabling a high degree of active site exposure. The electrode obtained thus showed excellent electrocatalytic activity, improved selectivity, and enhanced durability for CO2 reduction to CO. Bulk electrolysis with the prepared hybrid electrode in near neutral aqueous solution exhibits >95% faradaic efficiency for CO at 0.52 V overpotential, with 15.0 mA cm−2 current density and a TOF of 4.1 s−1. 59 Another approach for immobilization of the CoPc catalyst is the formation of organic-inorganic hybrid scaffold electrodes by a polymerization reaction. These electrodes were prepared by microwave irradiation, polymerizing 1,2,4,5-tetracyanobenzene together with Co2+ ions on the surface of pre-oxidized MWCNT. The resulting electrodes exhibit high catalytic activity (90% faradaic efficiency and 4,900 h−1 TOF at 0.5 V) and improved physical and chemical robustness compared to molecular phthalocyanine electrodes. 127 Daasbjerg and co-workers reported a comparative electrocatalytic CO2 reduction evaluation of cobalt meso-tetraphenylporphyrin (CoTPP) under both homogeneous and heterogeneous conditions (Figure 23B). 129 In homogenous catalysis, CoTPP performs poorly with a low faradaic efficiency displaying a low product selectivity and requiring high overpotentials. Interestingly, through a straightforward immobilization which includes, sonication, drop casting and drying a relatively efficient and stable electrode could be obtained. Remarkable enhancement in the catalysts CO2 reduction ability is seen with CO (>90%) as major product at low overpotential under aqueous conditions. A CoTPP derivative could also be covalently anchored to a boron-doped, p-type conductive diamond, as shown by Hamers, Berry and co-workers. Through copper catalysed click chemistry, an azide-functionalized diamond surface could be reacted with a CoTPP derivative containing peripheral acetylene groups (Figure 23C). The electrocatalytic CO2 reduction activity of this assembly was promising, displaying good stability and catalytic activity, with an overall 0.8 s−1 TOF observed for 16-h controlled potential electrolysis at −1.8 V for CO2 reduction to CO in acetonitrile solutions. 71 These works demonstrate how the method of modification of molecular catalysts can significantly influence the performance of the devices.A [Co(qpy)]Cl2 complex appended at the surface of MWCNT was analyzed for CO2 reduction in water at pH 7.3, giving 100% catalytic selectivity and 100% Faradaic efficiency for CO production at reduced overpotential (Figure 23D). The electrodes were prepared by drop-casting GC and carbon paper with the MWCNTs suspension in 1:1 ethylene glycol-ethanol mixtures with addition of the catalyst and a small amount of Nafion (commonly utilized as PEM and stabilizing agent). The prepared electrodes achieved a current density of 9.3 mA/cm2 at only 340 mV overpotential with outstanding stability (TON of 89,095 in 4.5 h). This hybrid material retained the high selectivity of the homogeneous molecular catalyst whilst introducing the robustness of heterogeneous materials. 130 A structurally simple 1,10-phenanthroline-Cu complex (phen-Cu) applied on mesostructured graphene electrode provided efficient and selective CO2 reduction, with a TOF of approximately 45 s−1 at −1 V versus RHE. Although the phen-Cu has no anchoring groups in the structure, it could be observed that the Cu complex can be reversibly accumulated close to the graphene surface by controlling the potential. This indicates the significance of interaction between molecular catalysts and carbon materials. 72 Fukuzumi and co-workers explored a Co(II) chlorin complex as an efficient electrocatalytic CO2 reduction catalyst. The Co(II) chlorin complex was adsorbed on MWCNT and through sonication and drop-casting placed on a GC electrode (Figure 23E). The resulting electrode efficiently reduces CO2 to CO in H2O (pH = 4.6) at an applied potential of —1.1 V versus NHE with a 89% faradaic efficiency for CO formation, with the remaining electrons consumed for H2 evolution. 128 The same technology was efficiently extended to photoelectrocatalytic reduction by using cobalt(II) chlorin complexes adsorbed on MWCNT as CO2 reduction catalyst, and introduction of [RuII(Me2phen)3]2+ as photocatalyst. This photoelectrochemical system yielded CO and H2 with a ratio of 2.4:1 and provided a high TON of 710 in acetonitrile solution containing 5 v/v%water. 131 The cathodes with cobalt(II) chlorin on MWCNT have also been applied in devices with the surface-modified FeO(OH)/BiVO4/FTO photoanode. 132 The photocathode in this device showed a selective photoelectrocatalytic reduction of CO2 to CO in H2O (pH 4.6) with 83% faradaic efficiency at −1.3 V bias. These works provided a unique strategy for the production of syngas from CO2 and H2O under (photo)electrochemical conditions.The last example discussed for the immobilization of CO2 reduction catalysts is a report that clearly underlines one of the challenges in employing π–π interactions as method of immobilization. Flat catalysts like the CoPc can utilize their extended π-systems to interact with the carbon support, but they can also interact with each other. For example, the highly soluble and sterically hindered cobalt(II) octaalkoxyphthalocyanine (CoPc-A) shows enhanced catalytic activity, compared to its analogous CoPc, after being successfully immobilized on graphene via π–π stacking (Figure 23F). The alkoxy substitutions on the heterocycle periphery in CoPc-A help to suppress molecular aggregation on the graphene surface, leading to increased accessibility of active sites and a significantly enhanced CO conversion catalytic activity (∼5 s−1 at 480 mV overpotential) compared to its analogue CoPc (∼2 s−1 at 590 mV overpotential). The introduction of alkyl groups also increased the activity and long-term stability for CO production of the CoPc-A electrode, providing a ∼6 s−1 TOF and 6.7 × 105 TON for CO evolution over 30 h of electrolysis. 65 The misunderstanding of the poor stability of molecular catalysts is mainly due to the short lifetime of photocatalysis, photoelectrochemical cells and other lab-scale electrodes currently developed with molecular catalysts. Obviously this is an inappropriate conclusion, because stability of molecular catalysts does not necessarily translate into the stability of devices (Intrinsic stability of molecular catalysts ≠ Stability of devices). 1 The stability of (photo)electrochemical devices has many influencing factors (e.g., detachment of electrode films, decomposition of light absorbers, desorption of molecules, mass transfer problems, and selection of electrolyte solutions), and no systematic in-depth research has proved that catalyst decomposition commonly leads to the performance loss of devices. To the best of our knowledge, there is no specific theory or fundamental that indicates molecular catalysts are less stable than inorganic materials. Actually, many molecular catalysts have been reported with lower overpotential, and much higher TOFs and TONs, 11 , 21 , 23 which are scientifically reasonable indicators that represent the true intrinsic activity and stability of the catalysts. 133 In fact, the stability of molecular catalysts has been seriously misunderstood and underestimated at present. It is an accepted dogma that molecular catalysts have no chance to be employed in commercially applicable electrolysis techniques due to their low stability. 24 It must be also noted that the observed stability of a catalyst depends on the measurement methods, even for inorganic catalysts. For example, large losses in water-oxidation activity of Ir nanoparticles were observed when they were evaluated in rotating disk electrode (RDE) half-cells (It is also common system for lab-scale testing), 134 but as known the durability of Ir catalysts is not an issue when they were used in PEM electrolyzer. 6 In our opinion the less-developed testing systems and cell-designs in lab-scale studies could be an essential reason that induce the decomposition of the modified molecular catalysts (cf. our recent review paper 1 for more in-depth discussions and examples). Indeed, research on developing molecular catalysts and testing molecular catalysts on electrodes are just at beginning stages; much more studies are required before the arrival of an efficient and applicable device.In addition, the current stability comparison between molecular catalysts engineered electrodes and inorganic materials based electrodes is unequal, if one considers the loading amount of catalysts. Under the commonly employed evaluation system, the loading amount of molecular catalyst is in a scale of nmol·cm−2. 48 , 49 , 81 , 96 , 104 In contrast, it is usually in a scale of μmol·cm−2 for inorganic catalyst, which is three orders of magnitude higher. 135–137 The far higher catalyst loadings may mask durability losses for a relatively long period, e.g., a few hours. 133 , 134 To exemplify, detachment or decomposition of 1 nmol of catalyst has far greater implications for systems with lower loading than for electrodes with 1 μmol of catalyst on the surface where this loss only accounts for 0.1%. Actually, there are already examples of molecular catalysts based electrodes with only several nmol·cm−2 catalyst loading that reach the stability of a few hours, which is common time-frame for evaluating stability of electrodes on laboratory scale. 48 , 96 , 138 Therefore, with consideration of loading amounts of catalysts, the observed stability of electrodes may not be a real disadvantage and molecular catalysts have a great chance to be superior. The high variability of molecular catalysts in terms of structural design is also advantageous for regulating and further improving stability.Compared with traditional alkaline water electrolysis, PEM electrolysis has many principle advantages. But as a comparably young technology, PEM technology has many aspects that need further development. Noble metal catalysts, irreplaceable PEM, and corrosion-resistant current collection plates primarily contribute to the high price of PEM electrolyzers. With the increase of market demand and commercialization process, the degree of patent opening, we believe the replacement of proton exchange membranes and other components for the reduction of costs are expectable. However, the most unavoidable and most challenging part is the replacement of rare noble metal catalysts. We discussed in this review that molecular catalysts have a potential in breaking this bottleneck. We summarized the reported molecular catalyst immobilized on carbon materials for catalytic water oxidation, hydrogen evolution and CO2 reduction, as these materials and the involved immobilization strategies as well as characterization techniques can facilitate the advance of a promising but underdeveloped technology, the molecular catalysts integrated PEM/AEM electrolyzers for solar fuel production. From the blueprint to practical implementation of this technology, the following research roadmap and tasks are proposed:First, evaluate state-of-the-art WOCs, HER, and CO2RR catalysts in PEM/AEM electrolyzer setup. The possibilities and advantages of using molecular catalysts in PEM/AEM electrolyzers should be explored and further verified. Molecular catalyst modified carbon materials summarized in this review can be directly tested by replacing noble metal catalysts in the construction of PEM/AEM electrolyzers. The introduced concepts and strategies for modifying molecular catalysts on carbon materials can also be used to design molecular catalyst based PEM/AEM electrolyzers. Based on the preliminary investigations discussed in the introduction, it is expectable to realize the aim in this stage, achieving current density in A/cm2 with comparable or lower overpotential compared to electrolyzers based on noble metal catalysts.Second, increase catalyst loading for higher current density. Most of the reported molecular catalysts modified materials are in the form of single molecule layer. These methods have very limited catalyst loading. More reliable methods are required to integrate molecular catalysts into bulk materials to increase catalyst loading, e.g., molecular catalyst engineered polymers, metal–organic frameworks (MOFs), and covalent organic frameworks (COFs). Exposure of catalysts, charge, and mass transports should be considered in the material design. A proper amount of catalyst loading is important for obtaining higher activity. In addition, the coating of the carbon material with the catalyst assembly may prove advantageous to improve overall stability under the harsh oxidative conditions commonly found at the anodic site in PEM electrolyzers.Third, challenge the long-term stability. Stability of an electrolyzer depends on many factors, including the configuration of the cell, stability of current collectors, stability of the MEA, stability of catalyst loading, and the intrinsic stability of catalysts. These factors may also influence each other. Although the intrinsic stability of molecular catalyst has been demonstrated above, this does not necessarily represent good stability of the constructed electrolyzer. Catalytic behaviors and deactivation pathways of molecular catalyst based MEA should be deeply studied to find strategies for obtaining good stability of molecular catalysts based electrolyzers. For long-term operation of the electrolyzer, the stability of substrates for catalyst loading is also important, requiring special considerations. Carbon based materials discussed in this review are promising for loading reduction catalysts. For anodes, common carbon materials display long term-stability to high oxidation potentials, oxygen environments, and acidic conditions. More advanced conductive substrates showing long-term stability under acidic and oxidative conditions, should be developed for loading WOCs. Sintered Ti particles, which form current collectors for PEM electrolyzer, pre-oxidized MWCNTs, and special conductive metal oxides, e.g., TiO2, SnO2, and Ta2O5, can be potential substrates for supporting molecular catalysts.At last, more intrinsically efficient and low-cost molecular WOCs and HER catalysts operating under acidic conditions should be developed. In addition, for CO2RR catalysts high selectivity and efficiency for production of carbon-based fuels are required. Even though the application of molecular catalysts potentially can reduce the overall high resource requirements due to lower loadings, the terawatt-scale requirement of renewable fuels would still only be reachable with low-cost and earth-abundant-based catalysts as ultimate ideal catalysts for real practical electrolyzers.We acknowledge financial support for this work from the Swedish Research Council (2017-00935), Swedish Energy Agency, Knut and Alice Wallenberg Foundation, and National Basic Research Program of China (973 program, 2014CB239402). L.F., T.L., and Q.M. also thank the China Scholarship Council for a special scholarship award.B.Z. and L.S. proposed the topic of the manuscript. B.Z., L.F., R.B.A., and T.L. wrote the manuscript. B.Z., B.J.J.T., and L.S. revised and edited the original draft. All authors discussed and revised the final manuscript.

Molecular catalysts possess numerous advantages over conventional heterogeneous catalysts in precise structure regulation, in-depth mechanism understanding, and efficient metal utilization. Various molecular catalysts have been reported that efficiently catalyze reactions involved in artificial photosynthesis, however, these catalysts have been rarely considered in view of practical applications. With this review, firstly we demonstrate in the introduction that molecular catalysts can bring new opportunities to proton exchange membrane (PEM) electrolyzers. In the following parts, we provide an overview of molecular catalyst modified carbon materials developed for electrochemical water oxidation, proton reduction, and CO2 reduction reactions. These materials and the involved immobilization strategies as well as characterization techniques may be directly employed in the investigations of application of molecular catalysts in PEM electrolyzers. The future scientific perspectives and challenges to advance this promising, yet underdeveloped technology for solar fuel production, integrating PEM electrolyzer with molecular-level catalysis, are discussed in the conclusions.

Data will be made available on request.Nanomaterials based on oxides (nanostructure and nano dispersed) are a diverse class of materials in terms of electronic structure, physical, chemical, and electromagnetic properties. The application of metal oxide nanomaterials and nano composition based on them is becoming increasingly popular in applied ecology especially; where they can be used as absorbents and photocatalysts as well as a material for the manufacture of environmental monitoring devices. The nano-sized metal oxide-based absorption materials have a large specific affinity for various contaminants. The metal oxide nanomaterials and their nano compositions are based on TiO2, ZnO, SnO2, ZrO2, and Fe3O4 for environmental application [1].Titanium dioxide (TiO2) is considered to be one of the most attractive semiconductor photocatalysts owing to its long-term stability, nontoxicity, and excellent photocatalytic property. On one hand, the wide band gap nature of titania (3.2 eV for the anatase structure or 3.0 eV for the rutile structure) makes it absorb only ultraviolet (UV) light, which limits the effective usage of solar light. In spite of this, a low quantum yield of TiO2 is present as a result of its high recombination rate of photogenerated electrons and holes [2]. Doping TiO2 with foreign ions is a promising approach to extend its response to the visible-light region. Furthermore, doping TiO2 with multiple dopants has become an effective and promising approach to improve the photocatalytic performance of TiO2 [3]. As one of the earliest studied n-type semiconductor photocatalysts, TiO2 has been widely used in environmental purification, self-cleaning, H2 production, photosynthesis, CO2 reduction, organic synthesis, solar cells, etc. Therefore, the number of publications on TiO2 has increased exponentially in recent decades [4].TiO2 has also been used as a gas sensor, which helps to sense the amount of oxygen (or reducing gas) present in the atmosphere. Another application is found in car engines for controlling fuel consumption and environmental pollution [5–8].In order to use solar energy more efficiently, most of the investigations have been focused on preparing TiO2 sensitive to visible light during the past several years. Many ionic dopants in different valence states have been investigated, including both metallic (e.g) Ca2+, Sr2+, Ba2+, Al3+, Ga3+, Cr3+, Fe3+, Co3+, Ce3+, Sn4+ and non-metallic ions (e.g) N3+, C4+, S4+, F and particularly TiO2 doped with Sn, has proved to be an effective method and has been widely studied [9,10]. SnO2 is an important n-type wide band gap semiconductor and SnO2-based nanostructures act themselves as one of the most important classes due to their various tunable physicochemical properties [9]. In some of the reports, sn-doped TiO2 was prepared by the vapour transport method of water molecules [11]. It would effectively control the rate of hydrolysis of Ti4+ by adjusting the flow speed of vapour. Sn-doped TiO2 particles with different concentrations and photocatalytic test show all the sn-doped TiO2 have higher photocatalytic activity than that of pure TiO2 [11].Although the mono-doped non-metal (or) metal atoms can obviously enhance photocatalytic performance of TiO2, they always act as the recombination centre because of the partially occupied impurity bands. The passivity of the impurity band while co-doping two or more foreign atoms have been recognized theoretically and this decreases the formation of recombination centres by increasing the solubility limit of dopants. Furthermore, co-doping can also modulate the charge equilibrium. Consequently, co-doping can effectively enhance photocatalytic activity. Based on the research on the doping effect, two (or) more elements are introduced into the TiO2 lattice to check the chance of electronic structure and band gap energy, including non-metal [12–14] and non-metal and metal atoms [15–17], and metal and metal atoms [18–20]. The modified N-doped TiO2 usually shows a favourable effect on improving the activity in the range of visible light compared to C-doped TiO2 [21,22], (C, F) co-doping [23], and (C, S) co-doping [24–27]. Recently, many researchers found that tri doping in TiO2 crystals is an effective supplemental tool [28–32]. This implies that (C, F) co-doping might be a kind of efficient way to improve the photocatalytic activity of TiO2 [33].Although mono-metal doping can improve the band gap structure, the serious recombination centres also deteriorate carrier transport due to their partially occupied impurity bands. Hence many research want to improve carrier transport through co-doping with two different metal elements such as (Ti, Ni) co-doping [34], (Ti, Fe) co-doping [35], (Ag, W) co-doping(Ag, or) co-doping [36], (Cu, V) co-doping [37] and (Fe, Co) co-doping [38]. Recently S-metal co-doping has also been widely investigated, such as (S, Fe) co-doping [39] and (S, Cu) co-doping [40]. Compared to undoped TiO2, (S, Fe) co-doped TiO2 showed a higher photocatalytic activity under both UV and visible light irradiation, and the optimal methyl blue degradation level was about 96.92 % [29,41].Azo dyes are the largest class of dyes used in industry. These compounds are characterized by the presence of one (or) more Azo bonds (–NN–) in their molecule that are associated with one (or) more aromatics structures. The presence of Azo dyes in textile effluents makes them particularly harmful to the environment and human health. In fact, their release into aquatic ecosystems may lead to a reduction of sunlight penetration and dissolved oxygen concentration, with deleterious effects on local flora and fauna [42].Herein, we report that the Ni-co-doped SnO2-TiO2 powders were prepared and characterized by means of XRD, SEM-EDX, HRTEM, XPS, XANES and photocatalytic abilities of Ni-deposited SnO2-TiO2 nanocomposite was evaluated. The photocatalytic activity of TiO2, SnO2 and SnO2-TiO2 has been enhanced by Ni complex deposited on SnO2-TiO2.NiCl2·6H2O, EtOH, Imidazole, Titanium isopropoxide (TTIP, 98 %), Hydrogen peroxide (H2O2, 30 wt%), Double distilled water, Azo dye, Methyl Orange, Whatman filter paper-41. All AR-grade chemicals were purchased from Qualigens.The IR spectra of the complex and prepared nanocompiste were recorded on a Thermo Nicollet −6700 FT-IR instrument (KBr pellet technique). The UV–Visible spectra were undertaken in an aqueous medium at room temperature on Shimadzu model UV-2450 double beam spectrophotometer with a quartz cell. A crystallography study was performed on the prepared nanocomposite by rotating anode diffractometer (XRD-PHILIPSPANALYTICAL, Netherland) using Cu-kα irradiation. The HRTEM images were taken on JEOL JEM-2000EX microscope. X-ray photoelectron spectra (XPS) were measured at the beam line-14, Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India (Ref. IBR/3850/2022–05-07/INDUS-2/BL-14 XPS) and the binding energy (B.E.) was calibrated using contaminated carbon as an internal standard (C 1s B.E. 284.6 eV). Ni K-edge X-ray absorption fine structure (XAFS) spectra (Ref. IBR/3823/2022-04-07/INDUS-2/BL-9 Scanning EXAFS) were taken at the beam line-09, RRCAT, Indore, India. Ni K-edge XAFS of Ni-foil and Ni-SnO2-TiO2 measured by fluorescence mode, while Ni K-edge XAFS of NiO and Ni-precursor complex measured by transmittance mode. Steady state fluorescence emission spectra were recorded on Spex FluoroLog-3 spectrofluorometer (Jobin-Yvon Inc.) using 450 W xenon lamp and equipped with a Hamamatsu R928 photomultiplier tube. The photocatalytic activity of the samples were evaluated by photodegradation of MO using HEBER Visible Annular Type Photoreactor, model HVAR1234 (Heber Scientific, India), under visible light irradiation using 300 W Tungsten lamp as a light source.Ni(II)-imidazole complex was prepared by the reported method [43]. Dissolve 6.0 g of NiCl2·6H2O in 3 mL. of EtOH. A little warming improves the rate of dissolution. Cool the solution in ice while adding 5.0 g (5.6 mL) of imidazole in EtOH. Add the imidazole slowly because the reaction is quite exothermic. Cool and add 15 mL of cold ethanol to initiate crystallization. Keep cold for 10 min and collect the product on a Buchner funnel and wash it with two 5 mL portions of ethanol. Dry in the air.The synthesis of TiO2 nanoparticles was reported and characterization details are given in reference [44]. The reported synthesis method is as follows: titanium isopropoxide (TTIP, 98 %) and an aqueous solution of hydrogen peroxide (H2O2, 30 wt%) were used as the starting materials. In a typical experiment, TTIP was added to H2O2 in a 500 mL beaker with vigorous magnetic stirring at a 70 °C water bath for 15 min in a basic medium (adjust pH using ammonia solution). The solution was made concentrated and the resultant yellow gel was put into a porcelain crucible with a cap and dried at 80 °C and then TiO2 nanoparticles were obtained.The preparation process was reported in our group’s previous paper. The same material has been taken for further nickel co-doping process [43].In a typical preparation, the prepared sample of the SnO2-TiO2 nanoparticle (200 mg) was dispersed in 80 mL of water in a 250 mL beaker. The prepared Ni(II)-imidazole complex was dissolved in 20 mL of distilled water it become (10-3 M) concentration and the basic medium was maintained. The latter solution was added to the SnO2-TiO2 suspension, and the mixture solution was stirred for one hour and put the suspension in a water bath for one hour at 80 °C. The resultant suspension was filtered and washed thoroughly using water and ethanol, followed by acetone. The resultant solid sample was collected and dried at 100 °C in the oven for two hours. (i) Typically Azo dye was added to 1000 mL of double distilled water and used as a stock solution. About 10 mg of TiO2, SnO2-TiO2 or Ni-SnO2-TiO2 samples were added to 100 mL of Azo dye solution. A control was also maintained without the addition of catalyst. Before exposure to irradiation, the reaction suspension was magnetically stirred well for about 30 min to make up the equilibrium of the working solution. Afterward, the dispersion was put under the sunlight and monitored from morning to evening sunset. At specific time intervals, aliquots of 2–3 mL suspension were filtered and used to evaluate the photocatalytic degradation of dye. The absorbance spectrum of the supernatant was subsequently measured using a UV–Visible spectrophotometer. The concentration of dye during degradation was calculated by the absorbance value at 660 nm. (ii) In a typical process, 80 mL of aqueous solution of methyl orange (MO) with initial concentration 2.5 × 10-6 M and 100 mg of catalyst were taken in a cylindrical-shaped glass reactor at room temperature in air and at ∼ 7 pH conditions. The flow rate of air was kept at a constant value of 80 mL min−1. The mixture solutions were kept in dark for 30 min before irradiation. Furthermore, prior to irradiation, the mixture solution were continuously aerated by a pump to provide oxygen and for complete mixing. The samples (4 mL) were taken out every 10 min, and were analyzed by UV–vis spectrophotometer. The photodegradation efficiencies (PDE) were calculated via the formula PDE = (A0 - At/A0) × 100 %, where A0 is the absorbance of initial MO solution and At is the absorbance of MO solution measured at various irradiation time at 465 nm. Typically Azo dye was added to 1000 mL of double distilled water and used as a stock solution. About 10 mg of TiO2, SnO2-TiO2 or Ni-SnO2-TiO2 samples were added to 100 mL of Azo dye solution. A control was also maintained without the addition of catalyst. Before exposure to irradiation, the reaction suspension was magnetically stirred well for about 30 min to make up the equilibrium of the working solution. Afterward, the dispersion was put under the sunlight and monitored from morning to evening sunset. At specific time intervals, aliquots of 2–3 mL suspension were filtered and used to evaluate the photocatalytic degradation of dye. The absorbance spectrum of the supernatant was subsequently measured using a UV–Visible spectrophotometer. The concentration of dye during degradation was calculated by the absorbance value at 660 nm.In a typical process, 80 mL of aqueous solution of methyl orange (MO) with initial concentration 2.5 × 10-6 M and 100 mg of catalyst were taken in a cylindrical-shaped glass reactor at room temperature in air and at ∼ 7 pH conditions. The flow rate of air was kept at a constant value of 80 mL min−1. The mixture solutions were kept in dark for 30 min before irradiation. Furthermore, prior to irradiation, the mixture solution were continuously aerated by a pump to provide oxygen and for complete mixing. The samples (4 mL) were taken out every 10 min, and were analyzed by UV–vis spectrophotometer. The photodegradation efficiencies (PDE) were calculated via the formula PDE = (A0 - At/A0) × 100 %, where A0 is the absorbance of initial MO solution and At is the absorbance of MO solution measured at various irradiation time at 465 nm.The theoretical calculation to determine the electronic density of states for dopant induced changes in Ni-Sn-TiO2 based on the Density Functional Theory (DFT) were carried out using the Vienna ab initio Simulation Package (VASP) with projector augmented wave (PAW) pseudopotentials and Generalized Gradient Approximation (GGA) to Perdew, Burke and Ernzerhof (PBE) as exchange correlation energy. The geometrical structure relaxation performed for pristine Sn-, Ni-doped TiO2 using the conjugate-gradient algorithm (CGA). Monkhorst-Pack method was used to obtain the plane wave cutoff and the k-point density. Fig. 1 shows XRD patterns of prepared sample. The narrow and intense peaks at 2θ = 26.9°, 38.1°, 53.8°, and 55.4° are the standard X-ray diffraction peaks of anatase TiO2 (Table 1 ), while the narrow and intense peaks at 2θ = 27.4°, 34.9°, and 52.7° demonstrate that the phase structure is rutile TiO2. The diffraction pattern for pure SnO2 show two main peaks at 2θ = 27.2 and 34.4 ° that referred to SnO2 with rutile (cassiterite tetragonal) structure (ICDD card No. 41–1445) of space group P42/mnm. Moreover, there is a diffraction peak corresponding to SnO2 in patterns of the prepared sample. The XRD reflection indicates the presence of anatase phase more predominantly, the peak at 27.4° 2θ corresponds to a rutile (110) reflection near that of pure TiO2 (27.4° 2θ). The rutile (110) reflection appears at ∼27° as 2θ, which is intermediate to that of the pure TiO2 (27.4°, 2θ) and pure SnO2 (26.6°2θ) positions [44]. The impurity phase of NiO is not observed, it may be because of very limited Ni loaded or Ni substituted in lattice site. XRD result indicates that cassiterite of SnO2 coupled with anatase–rutile of TiO2. Fig. 2 depicts FTIR spectrum of Ni-SnO2-TiO2 sample. The entire spectrum display-two characteristics broad band centred at 3408 and 1627 cm−1 which are assigned to the stretching and bending modes of vibrations of physical adsorbed water on titania surface or to hydroxyl groups exist on the surface of the oxides, respectively. A remarkable broad band in the region 680–400 cm−1 is associated with the stretching modes of vibrations of bridged sn-O-Sn, Ti-O-Ti, Ni-O-Ti, and Ti-O-Sn bonds. It should be emphasizing to notice two weak bands at 1389 and 1041 cm−1 which are assigned to the hetero Ti-O-Sn bonds. FTIR spectrum confirmed that SnO2 coupled with TiO2.The UV–vis measurements of TiO2, SnO2-TiO2, and Ni-SnO2-TiO2 are shown in Fig. 3 . The absorption spectrum of TiO2 consists of a single broad intense absorption around 400 nm due to the charge transfer from the valence band (mainly formed by 2p orbital of the oxide anions) to the conduction band (mainly formed by 3d t2g orbitals of the Ti4+ cations). The adsorbed TiO2 shows absorbance in the shorter wavelength region while Ni-SnO2-TiO2 and the UV–vis results show a red shift in the absorption onset value in the case of Ni-added tin-titania composite. The adsorption of various transitional metal ions into TiO2 shifts its optical absorption edge from UV into visible light range, but no prominent change in the TiO2 band gap is observed.The SEM images of Ni-SnO2-TiO2 particles are presented in Fig. 4 . The micrographs show that most of the particles are spherical with various size distributions. The Ni implanted particles are found to be highly agglomerated and some are non-spherical in characteristics. These differences in morphology, particle shape, and size indicate the presence of Ni and SnO2 species in TiO2 nanoparticles. From SEM images, it can be presumed that nickel and SnO2 deposited titania samples consist of very fine free individual grains plus other granular aggregates: very large distributions of shape and dimensions of the particles are observed, with an average diameter of 110 nm. It is evident that the growths of particles are highly restricted due to Ni deposition, which is of significant importance not only for the design of surface properties but also for tuning the electronic structure of the semiconductor. Ni and Sn were loaded in titania, which were observed from the Energy X-ray Diffraction study. EDX spectra are shown in Fig. 5 and Table 2 presents the concentration of Ni and Sn loaded on the solid. Sn is present in a considerable amount of TiO2. Table 2 exhibits the relationship between the density of Ni and Sn in atomic percent (at %) in titania, in which Ni is loaded 0.39 at % and Sn is loaded at 13.11 at% in TiO2. Ti and Sn are present in the nearly same ratio in prepared samples, which means that nearly 13 at% of each Ti and Sn is present in a prepared sample. It indicates that nearly 50 % of SnO2 and 50 % of TiO2 are present in the prepared nanocomposite. The EDX confirms the existence of nickel in the SnO2-TiO2 powder. Fig. 6 (a) shows the TEM images with a diameter of 5–10 nm were observed in Ni-SnO2-TiO2 sample. The selected area electron diffraction (SAED) on prepared sample (inset Fig. 6(a)) shows the strong Debye-Scherrer rings and additionally complicated bright spots were observed, indicating the coexistence of polycrystalline anatase–rutile or rutile-casseterite crystallites. This is in well agreement with the aforementioned conclusion that the Sn4+ ions doped in TiO2 can promote the formation of new phases. Fig. 6(b) displays the HRTEM images of prepared sample. For the anatase structure of TiO2, the fringe spacing (d) of (101) crystallographic plane is determined to be 3.55 Å [45]. Furthermore, a fringe spacing of ∼3.33 Å corresponding to the (110) planes of rutile TiO2 are observed in prepared sample. The crystallographic planes of SnO2 cassiterite (101) fringe spacing observed at 2.64 Å, which were further confirmed that SnO2 was coupled with TiO2 in prepared sample. No fringe spacing of NiO or Ni-precursor complex observed but amorphous phase can observed surface site of prepared sample.XPS analysis was performed to further study the chemical states of Sn in TiO2. Fig. 7 shows the survey, Ti 2p, and Sn 3d XPS spectrum of prepared sample. It can be seen that XPS peak positions of Ti 2p3/2 locate at 448.31, 449.36, 454.31, and 453.53 eV, which indicates that Ti element mainly existed as the chemical states of Ti4+. The doublet peaks observed at 476.38 and 484.70 eV in the Sn 3d XPS spectrum, it is ascribed to Sn 3d5 / 2 and Sn 3d3 / 2 of the substitutional Sn4+ dopants in the lattice, since the peak position of Sn 3d5 / 2 (484.70 eV) is located between that of SnO2 (486.6 eV) and metallic Sn (484.0 eV) [38]. The XPS analysis proved that the Sn4+ easy to replace Ti4+ in the lattice of TiO2 [46]. From survey XPS spectrum indicates that very minimum amount of Ni species (0.40 at%) presented on surface of our prepared sample.To further study the coordination structures of Ni over SnO2-TiO2 nanocomposites by XANES study of our sample. The XANES spectra of the pure NiO, Ni-precursor complex and our prepared nanocomposite are shown in Fig. 8 . Their spectra revealed the well-defined pre-edge peaks at 8346.31 eV, the main characteristic shoulder features for NiO and Ni-precursor complex at 8392.56 eV, which are attributed to Ni2+, while little shift was observed at 8398.28 eV for Ni-SnO2-TiO2 sample. This indicates that Ni is not just deposited in the form of NiO or Ni-precursor complex in Ni-SnO2-TiO2 sample. This result clearly indicated that the ionic state for Ni in Ni-SnO2-TiO2 is + 2. Moreover, it could be seen that the spectral shape and features of Ni-SnO2-TiO2 is intermediate to that of NiO and Ni-precursor complex suggesting that small amount of Ni species surrounded with O, N, or C is coupled with SnO2-TiO2 nanocomposite. The structural, chemical and morphological characterizations performed by XRD, XPS, XANES, SEM-EDX, and HRTEM microscopes clearly confirm the octahedral substitution of Sn4+ for Ti4+ and additionally SnO2 and small Ni2+ species are coupled with TiO2 in Ni-SnO2-TiO2 nanocomposite.The aim is to increase in the electron deficiency and a breakdown of the azo molecules. Ni(II) loaded photocatalyst was used to study the photodegradation of Azo dye in an aqueous medium under sunlight irradiation. The effectiveness of the degradation of this substrate by the prepared sample is compared with the photodegradation yields of the substrate by pristine TiO2. From Figs. S1-S2 the UV–vis spectra of azo dye at before and after irradiation finds that a gradual decrease in the absorption bands of an azo dye, when the irradiation time increases, is due to the decomposition of azo molecules. The predominant peak is observed at 583 nm for azo dye before irradiation (Table S1) but this peak completely disappears after irradiation of dye solution using Ni-SnO2-TiO2 as photocatalyst (Table S2). This indicates an increase in electron deficiency and a breakdown of the azo molecules. The PDE of Azo dye versus irradiation times is shown in Fig. 9 (Tables 3-5 ) using pure TiO2, SnO2-TiO2, and Ni-SnO2-TiO2, respectively. TiO2 presents a fair activity for the degradation of azo under sunlight irradiation. The photodegradation rate is been determined for each experiment and the highest values are observed for SnO2-TiO2 and Ni-SnO2-TiO2. Comparatively, the photocatalyst Ni-doped SnO2-TiO2, which is prepared by us, has very highest photocatalytic efficiency. The complete mineralization was confirmed by total organic carbon (TOC) analysis, and COD measurement. This indicates that Ni loaded has improved the photocatalytic behavior of TiO2 as well as SnO2-TiO2 photocatalysts under sunlight irradiation. Fig. 10 shows the UV–vis spectra of photodegradation of MO with the Ni-SnO2-TiO2 sample suspended in water at different irradiation times. It can be found that gradually decreases the absorption bands at 268 and 464 nm, when the irradiation time increases due to oxidation of the MO molecules, which indicates an increase in the electron deficiency and a break-down of the MO molecules. The photocatalytic activities of the pure TiO2, SnO2, SnO2-TiO2, and prepared Ni-SnO2-TiO2 nanocomposites were evaluated by the degradation of MO under visible light irradiation, and the photoefficiencies were shown in Fig. 11 . Present reported nanocomposites showed the better photocatalytic performances than above reference samples under visible light irradiation. The photocatalytic mechanism of SnO2-TiO2 was reported in our previous works [45,46]. similar mechanism could work in Ni-SnO2-TiO2 nanocomposite, in addition Ni species influence the photoefficiency of SnO2-TiO2 nanocomposite.The photoluminescence (PL) spectra of TiO2 and Ni-SnO2-TiO2 samples with excitations at 330 nm are presented in Fig. 12 . Broad emission in the spectral range from 350 to 600 nm was observed as well as the presence of well-resolved peaks/ shoulders at 468, 483, 494 and 560 nm. A addition of Sn and Ni in TiO2 was increase the intensity or changes the shape and peak position of the PL spectra compared to spectrum of pure TiO2 sample. It was found that the steady state emission spectra contain a narrower UV emission located near the position of 390 nm (3.18 eV) and a widened emission range from 450 to 491 nm [45–48]. The strong green emissions were observed in both TiO2 and Ni-SnO2-TiO2 at 560 nm (2.22 eV) in Fig. 12. Therefore, emissions likely originate from surface defects, such as ionizable oxygen vacancies and the recombination of self-trapped excitons (STEs) localized within TiO6 octahedra [44]. The green emission increase in Ni-SnO2-TiO2 sample as compare to TiO2, it indicates that our prepared sample contain more surface defect sites. The defect concentrations and life times of photoexcited species plays an important role in photocatalysis, investigation of photocatalysts through PL spectroscopy is important to obtain critical reasons behind the enhanced photocatalytic activity. Moreover, the contributions of charges from the dopants to the electronic states are clearly observed from the density functional theory calculations demonstrated in Fig. 13 . These effects are occurring in nanocomposite system as a consequence of charge trapping on surfaces or the inter phases between the two oxide phases [46]. Hence, the PL and DFT studies supported for photocataltic activities enhanced through Ni and Sn presented in TiO2.A first attempt was made to synthesize of Ni-SnO2-TiO2 nanocomposite. [Ni(Im)6]Cl2 is preferably adsorbed on the solid surface forming surface: complex adduct, simultaneously it becomes surface species; SnO2-TiO2:Ni(II). The surface species was identified by SEM-EDX, HRTEM, XPS and XANES measurements. From the DRS spectra, we could conclude that the absorption of composite particles expands to visible region that are able to be excited by visible light. The organic model pollutant of azo dye degradation property of TiO2 photocatalyst under visible light irradiation has been greatly enhanced after the introduction of Ni into the SnO2-TiO2 surface matrix. The present work considered that the low-level Ni-loaded anatase–rutile- cassiterite (SnO2-TiO2) induced photocatalytic behaviours due to the creation of surface synergetic effect and defect sites. Various spectroscopic measurements indicated the presence of structural, optical, and surface composition in the Ni-Sn–Ti system. The photocatalytic properties of Ni-SnO2-TiO2 sample was tested, and the results showed that a small amount of nickel co-doped SnO2-TiO2 which preferably formed anatase–rutile-cassiterite mixed phases had improved photocatalytic activity due to the more efficient separation of photoinduced electrons and holes on its surface. The major important finding in this report: (i) the formation of sn-doped and Ni complex deposited in surface of anatase–rutile-cassiterite mixed phases, no pure NiO was found, (ii) the enhancement of light absorption property in the visible region and shift of absorption edge to the long wavelength side, and (iii) Ni content in SnO2-TiO2 assisted optical character and improve the photocatalytic property. This study could point out a potential way to develop new and more active nickel complex deposited and tin doped titania photocatalysts for wastewater treatments.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Dr. ASG is thankful to National College (Autonomous), Tiruchirappalli, Tamil Nadu for financial support through a college minor research project scheme (No. NCT/SEC/010/2022-2023/19-07-2022). Authors are grateful to Raja Ramanna Centre for Advanced Technology (RRCAT) (Ref.: IBR/3850/2022-05-07/INDUS-2/BL-14 XPS and IBR/3823/2022-04-07/INDUS-2/BL-9 Scanning EXAFS) and thanks to Dr. S. N. Jha, Dr. R. K. Sharma, and Dr. Jaspreet Singh at RRCAT, Indore, India and Dr. D. Bhattacharyya,  Bhabha Atomic Research Centre (BARC), Mumbai, India for supported EXAFS and XPS studies.Supplementary data to this article can be found online at https://doi.org/10.1016/j.rechem.2022.100557.The following are the Supplementary data to this article: Supplementary data 1

A first attempt was made to synthesize a Ni-SnO2-TiO2 nanocomposite by a one-pot simple synthetic method. A [Ni(im)6]Cl2 precursor complex is preferably adsorbed on the solid surface forming nanocomposite: complex adduct, it simultaneously becomes a surface species (Ni-SnO2-TiO2). In this investigation, it has been found that surface interaction of nickel complex ions lead to the formation of surface species that are identified by XRD, FTIR, UV–vis DRS, SEM, EDX, HRTEM, XPS, and XANES analyses. The metal-loaded metal oxide coupled semiconductor solids find their applications as catalysts and in advanced electronics. We demonstrate the dopant induced changes in electronic density of states using DFT VASP calculations. The photocatalytic property of Ni-SnO2-TiO2 sample was tested, and the results showed that a small amount of nickel surface co-doped SnO2-TiO2, formed an anatase–rutile-cassiterite mixed phases with surface defects or oxygen vacancies, and had improved the photocatalytic activity. The organic model pollutant of Azo dye and methyl orange degradation property of TiO2 photocatalyst under visible light irradiation has greatly enhanced after the introduction of Ni into the SnO2-TiO2 surface matrix. This study points out a potential way to develop new and more active tin and nickel co-doped titania photocatalysts for organic pollutant degradation in water systems.

The hydrogenation of aromatic nitro compounds is one of the most important applications of precious metal powder catalysts. Amongst these, one of the most used processes is the hydrogenation of dinitrotoluene (DNT) to toluenediamine (TDA), with more than 1 million tons produced globally on an annual basis. TDA is an intermediate in the synthesis of toluene diisocyanate (TDI), a compound predominantly used in the preparation of flexible polyurethane foams and elastomers [1]. Carbon, silica, and alumina-supported transition metal catalysts such as Pd, Pt, Ru, and Ir have been extensively studied in the catalytic hydrogenation of dinitrotoluene [2–9]. However, it is difficult to fully separate powder form catalysts from the reaction medium because of the small particle size and the degree of polarity of the surface. The separation can be achieved to a certain extent in some cases by filtration [10] or centrifugation [11], and the presence of small residual amounts of the catalyst particles can be ignored. But there are applications when the catalyst loss and product contamination due to incomplete separation is unacceptable [12]. A solution to this problem is the application of catalyst supports with magnetic properties, and thus, the separation can be carried out easily by using a magnetic field. The use of magnetic nanocatalysts, such as spinel ferrites, is becoming increasingly important, especially in heterogeneous catalysis [13–16]. The crystal structure of the spinel allows various metal ions to be introduced into the system without significantly altering it, but with this, the magnetic, electrical, and dielectric properties can be influenced [17]. Ferrites can be prepared in a number of ways, including hydrothermal [18,19], sonochemical [20,21], sol-gel [22,23], precipitation [24,25], microemulsion [26,27], and even mechanical alloying [28,29] methods. In our previous study [30], a unique method that combines combustion and sonochemical treatment has been developed and proved to be suitable for the efficient synthesis of magnetic metal oxide nanoparticles. Therefore, this method has been used to prepare magnetic Ni, Co, and Cu ferrite supported Pd catalysts, and their applicability has been compared in the hydrogenation of DNT to TDA.During the synthesis of the magnetic spinel nanoparticles cobalt(II) nitrate hexahydrate, (Co(NO3)2 ∙ 6H2O), copper(II) nitrate dihydrate (Cu(NO3)2 ∙ 2H2O), nickel(II) nitrate hexahydrate (Ni(NO3)2 ∙ 6H2O), and iron(III) nitrate nonahydrate (Fe(NO3)3 ∙ 9H2O) from Reanal Ltd., have been used. Polyethylene glycol (PEG 400, Mw: ~400 g/mol, Macrogola 400 from Molar Chemicals Ltd.) was applied as a reducing agent and dispersion media. In the final catalyst preparation step, palladium(II) nitrate dihydrate (Pd(NO3)2 ∙ 2H2O, Merck Ltd.) was utilized as a precursor and Patosolv (mixture of aliphatic alcohols: 90 vol% ethanol, 10 vol% isopropanol; from Molar Chemicals Ltd.) as reaction media.Hielscher UIP100 hdT tip homogenizer (1,000 W, 20 kHz) was used to deposit palladium nanoparticles onto the surface of the ferrite crystals. Bs4d22 ultrasonic block sonotrode (D = 22 mm) was applied to initiate the formation of metal hydroxides in PEG 400 dispersion.All three synthesized spinel magnetic nanoparticle samples were examined by high-resolution transmission electron microscopy (HRTEM, FEI Technai G2 electron microscope, 200 kV). The morphology of the nanoparticles has been characterized, and particle size distributions have been measured. The samples were prepared by dropping the aqueous suspension of the nanoparticles on 300 mesh copper grids (Ted Pella Inc.). X-ray diffraction (XRD) measurements were used to identify and quantify the crystalline phases. Bruker D8 Advance diffractometer (Cu-Kα source, 40 kV and 40 mA) in parallel beam geometry (Göbel mirror) with Vantec detector was applied to perform the XRD measurements. The average crystallite size of the domains calculated by the mean column length calibrated method using full width at half maximum (lowest) and the width of the Lorentzian component (highest) of the fitted profiles. The palladium contents of the prepared magnetic catalysts have been measured by using a Varian 720 ES inductively coupled optical emission spectrometer (ICP-OES). For the ICP-EOS measurements, the samples have been prepared by placing them into aqua regia. The specific surface area of the catalysts was measured by nitrogen adsorption-desorption experiments at 77 K using Micromeritics ASAP 2020 sorptometer based on the Brauner-Emmett-Teller (BET) method. The carbon content of the spinel ferrite samples was determined by using a Vario Macro CHNS element analyzer with a certified phenanthrene standard (C: 93.538%, H: 5.629%, N: 0.179%, S: 0.453%; from Carlo Erba Inc.) and the samples were loaded into tin foils. The carrier gas was helium (99.9990%), while oxygen (99.995%) was used for oxidation during the measurements.Cobalt, copper, and nickel-containing ferrite nanoparticles were synthesized by using a two-step process that includes sonochemical treatment and combustion (Fig. 1 ). In the first step, iron(III) nitrate nonahydrate and one of the precursors (Table 1 ) were dissolved in 20 g polyethylene glycol, and then, the solutions were treated by using a Hielscher UIP1000 hdT tip homogenizer for 3 min (130 W, 19 kHz).The color of the dispersions has deepened and changed to brownish red, during which metal hydroxides formed from their nitrate salts.In the second step, the PEG 400 was burned by using a Bunsen burner in air and the metal hydroxide phase remained. After that, the samples were heated for another 30 min to fully oxidize the carbon content.The palladium nitrate dihydrate precursor (0.25 g) was dissolved in 50 ml Patosolv, and 2.00 g ferrite (Co, Ni, or Cu ferrite) was also added to the solution. The dispersions were sonicated by using the homogenizer (130 W) for 2 min. The sonication initiates the continuous formation and collapse of vapor microbubbles in the solution and will lead to intense local heating, high pressure, enormous local heating and cooling rates, and liquid jet streams. The area around the microbubbles is full of energy, and thus, chemical reactions, such as the reduction of palladium ions to Pd metal nanoparticles in the presence of alcohol as reducing agent, could take place easily. During this process, the Pd nanoparticles are deposited onto the surface of the magnetic spinel supports, and the final catalysts are formed. Then, the catalyst samples were removed from the dispersion with an Nd magnet, washed with Patosolv, and dried at 105 °C overnight. The final palladium content of the magnetic catalysts was determined by ICP-OES measurements.Dinitrotoluene hydrogenation was carried out in a methanolic solution (c = 0.05 mol ∙ dm3) at four different temperatures (303 K, 313 K, 323 K, and 333 K) and by applying 20 bar hydrogen pressure. The reactions were performed in a Büchi Uster picoclave reactor system (SI Fig. 1) (volume: 200 cm3) with continuous mixing (1,000 rpm). The sampling took place after the beginning of the hydrogenation at 0, 5, 10, 15, 40, 20, 30, 40, 60, 80, 120, 180, and 240 min. The quantitative analysis of the samples was carried by Agilent 7,890 A gas chromatograph coupled with Agilent 5975C Mass Selective detector. During the measurements, a Restek Rxi-1MS column was used (30 m × 0.25 mm × 0.25 mm). Three analytical standards, 2,4-diaminotoluene, 2,4-dinitrotoluene, and 2-methyl-5-nitroaniline (Sigma Aldrich Ltd.), have been used in the analysis of the samples.The efficiency of the catalyst was determined by calculating the conversion (X%) of DNT based on the following equation (Eqn.1): X % = c o n s u m e d n D N T i n i t i a l n D N T x 100 The DNT hydrogenation can be seen as a first-order reaction [31–33]. Thus, the rate constant of the reaction (k) at different temperatures can be calculated by using non-linear regression as the following equation is considered (Eqn. 2): c k = c 0 · exp ( − k · t ) where c 0 and c k (mol/dm3) are the initial and final dinitrotoluene concentrations, respectively.The activation energy (Ea) was calculated based on the Arrhenius equation (Eqn. 3): k = A × exp [ − ( E a R × T ) ] where k is the reaction rate coefficient, A is the pre-exponential factor, T is temperature, and R is the universal gas constant.The yield (Y%) of the product (TDA) was also calculated as follows (Eqn. 4): Y % = n T D A n T D A t h e o r i t i c a l · 100 where n TDA is the real amount of the formed TDA, while n TDA theoretical is the amount, which can be formed theoretically.The PEG was eliminated from the samples by combustion, and thus, carbon is expected to remain in the system. This was also confirmed by FTIR measurements (Fig. 2 ). The absorption bands, which are visible at 1,630/cm, 2,858/cm, and 2,919/cm can be attributed to the stretching of CC and to the antisymmetric and symmetric stretching vibration modes of –CH2, respectively (Fig. 1. A). The ferrite particles are covered with a carbon layer that is visible on the HRTEM images (Fig. 2. B). The carbon content of the samples varies between 0.15 wt% and 0.64 wt% and the cobalt ferrite-based sample contains the least, while the nickel ferrite the most. The copper-containing sample’s carbon content (0.17 wt%) is just slightly higher than in the case of the cobalt ferrite-based one.Two more bands have been located around 580/cm and 3,435/cm on the spectra, which can be associated with the vibration mode of the metal-oxygen (νM-O) stretching and the surface hydroxyl groups and adsorbed water, respectively. The –OH groups are beneficial for the catalyst preparation because they promote the adsorption of Pd2+ ions on the catalyst support. The adsorption mechanism is a complex process, which involves physical adsorption, electrostatic interaction, ion exchange, and surface complexation. Hydroxyl groups can be deprotonated depending on the pH, and thus, the surface of the nanoparticles can be negative, which will further promote the adsorption of the metal ions. The strength of the interaction between the precursor ions and the support (with electrostatic interaction or ion exchange) influences the rate of nucleation and growth of palladium nanoparticles on the surface of catalyst support, and thus, smaller nanoparticles can form.In order to identify the different oxide phases in the magnetic spinel-based catalyst supports, XRD measurements have been carried out. In the case of the cobalt-containing spinel, seven reflexion peaks have been identified at 18.3° (101), 30.1° (200), 35.5° (211), 43.1° (220), 53.6° (312), 57.2° (303), and 62.7° (224) two Theta degrees (ICDD card number: 22-1086) each of which corresponds only one metal oxide phase, CoFe2O4, and thus, it is a pure cobalt ferrite sample (Fig. 3 . A).Three metal oxide phases have been identified in the copper-containing sample (Fig. 2. B). The reflexions at 18.3° (111), 30.2° (220), 35.7° (311), 43.3° (400), 54° (422), 57.2° (511), and 62.7° (440) 2Θ degrees can be associated with the presence of the CuFe2O4 phase that is the main component (78.9 wt%, Table 2 ) of the system (ICDD card number: 77-0010). CuO (tenorite) and iron(III) oxide (hematite) phases are also located and can be identified as by-products (11.73 wt% and 9.97 wt%, respectively). The low-intensity peaks at 35.5° (111), 38.7 (022), 48.8 (202), 58.3 (202), 61.5 (113), 66.1 (022), and 68.1 (220) (ICDD card number: 00-001-1117) corresponds to the CuO phase. The hematite phase in the sample is represented by peaks at 24.1 (012), 33.1 (104), 40.9 (113), 49.6 (024), 54.0 (116), 62.5 (214), and 63.9 (300) (ICDD card number: 33-0664).In the nickel-containing sample besides NiFe2O4 (66.3 wt%), nickel(II) oxide (NiO, bunsenite, 30.01 wt%) and FeNi3 (awaruite, 3.75 wt%) have also been identified (Fig. 3. C, Table 2). On the diffractogram, high-intensity peaks appeared at 18.4° (111), 30.2° (220), 35.3° (311) 37.3 (222); 43.4° (400), 53.6° (422), 57.4° (511), and 63.1° (440) 2Θ degrees which correspond to NiFe2O4 (ICDD card number: 54-0964). The presence of NiO has been confirmed as peaks at 37.3° (111), 43.2° (200), and 62.9° (220), and two theta degrees have been located (ICDD card number: 47-1049). Furthermore, low-intensity reflexions have been identified at 44.1° (111) and 51.3° (200) 2Θ degrees that correspond to FeNi3 (ICDD card number: 38-419).The Pd/ferrite catalysts have also been examined (Fig. 3 D-F). In each case, peaks at 40.0°, 46.6°, and 68.2° two Theta degrees have been identified and associated with Pd(111), Pd (200), and Pd (220) reflexions (ICDD card number 046–1043), respectively. Thus, the palladium is in the elemental state in the catalytic systems.Specific surface areas (SSA) of the magnetic palladium catalysts have also been measured, and it was found that the Pd/CuFe2O4 sample has the largest (38.6 m2/g), and it is more than two times the case of Pd/CoFe2O4 (18.2 m2/g). The SSA of Pd/NiFe2O4 is 21.1 m2/g, which is also larger than in the case of Pd/CoFe2O4. The palladium content of the catalyst has also been determined, and the Pd/CuFe2O4 contains the most (4.34 wt%), and it is followed by Pd/CoFe2O4 (3.98 wt%) and Pd/NiFe2O4 (3.82 wt%)The size of the nanoparticles of the supports and the corresponding catalytic systems have also been analyzed (Table 3 ). The average particle size of the main phases (ferrites) remained the same for CoFe2O4 and CuFe2O4 before and after the sonochemical treatment of the samples, which led to the palladium deposition. However, in the case of NiFe2O4, the particle size slightly increased from 21 to 23 nm. The palladium particles are found in a finely dispersed form, and their average diameters are 6 ± 1 nm, 4 ± 1 nm, and 5 ± 2 nm in the case of Pd/CoFe2O4, Pd/CuFe2O4, and Pd/NiFe2O4, respectively.The electron microscopic analysis of the magnetic catalysts shows that each of them contains highly dispersed nanoparticles (Fig. 4 A- C). However, the different phases are inseparable in the images.Before the catalyst’s preparation, the palladium-free ferrite supports were tested in DNT hydrogenation. The non-loaded ferrites showed varying degrees of catalytic activity by using the NiFe2O4, 69.89 n/n% DNT conversion, and 18.23% TDA yield was achieved after 4 h of hydrogenation at 333 K and 20 bar pressure (SI Fig. 2 A). In the case of the CoFe2O4, 64.17 n/n% DNT conversion was reached, and 18.23 n/n% TDA yield (SI Fig. 2 B). The copper ferrite sample was the least active, and only 56.12% DNT conversion and 15.61% TDA yield were measured. Although the supports were active, only low TDA yield and DNT conversion were achieved, and thus, the involvement of palladium is essential to reach the desirable high activity. Then, the activity of the three synthesized magnetic Pd catalysts has also been compared in dinitrotoluene hydrogenation. The change of DNT concentration with time was measured, and the experiments were carried out at four different temperatures to achieve TDA. The conversion was excellent in each case. The Pd/CoFe2O4 and Pd/NiFe2O4 were able to achieve full conversion after 40 min at 333 K and 20 bar hydrogen pressure (Fig. 5 A and C). By using the Pd/CuFe2O4 sample, the reaction was slower, which shows that it is slightly less active than its counterparts, but after two hours of hydrogenation at 333 K and 20 bar, it was still able to reach 99.97 n/n% conversion (Fig. 5 B). The lower activity of the Pd/CuFe2O4 catalyst can be explained by the relatively high CuO content (11.74 wt%) of the sample. CuO is used to improve the selectivity of precious metal-containing catalysts in hydrogenation [34,35]. However, above a certain copper content, the activity of the catalyst decreases (catalyst poison effect), and a similar phenomenon is experienced in the present case.The corresponding reaction rate coefficients (k) were also calculated (Table 4 ). The k values are similar in the case of the Pd/CoFe2O4 and Pd/NiFe2O4 catalysts. The Pd/CuFe2O4 sample was less efficient, and its reaction rate was an order of magnitude lower compared to the other two catalysts. The activation energies (Ea) were determined based on the Arrhenius plots (Fig. 5, Table 4 and SI Fig. 3), and it was found that they are in a range between 32 and 39 kJ/mol, which is similar to other Pd-containing catalysts [36,37].Next to the main-product (TDA), two semi-hydrogenated intermediates, 4-amino-2-nitrotoluene (4A2NT) and 2-amino-4-nitrotoluene (2A4NT) were identified (Fig. 6 and Fig. 7 ). Furthermore, three additional larger intermediates (Fig. 6, see species 1, 2, and 3) have also been identified, which could be formed through side reactions. Based on the results of the catalytic tests and previous findings, a possible reaction mechanism, including the detected species, has been envisaged (Fig. 6). As DNT contains two nitro functional groups, two main reaction pathways can be proposed. These channels lead to TDA (Fig. 6, middle section) going through six consecutive hydrogenation steps. In the first step, a nitroso-nitrotoluene is formed, which is followed by a hydroxylamino-nitrotoluene formation. The formation of hydroxylamino-nitrotoluene was confirmed earlier, but additional molecules or condensed derivates were not reported [36,37]. Thereafter, one of the above-mentioned semi-hydrogenated compounds, depending on which nitro group is hydrogenated first, 2A4NT or 4A2NT will be formed. Then, if the reaction will go further, the other nitro group will be hydrogenated, and the corresponding nitroso and hydroxylamino species will be produced. In the last step, both pathways will reach the main product, TDA. The detected three larger side-products (1, 2, 3) are the results of condensation reaction steps, which can be explained with the enhanced reactivity of the nitroso groups. Janssen et al. detected similar condensed species, which formed from 4-nitroso-2-nitrotoluene and 4-hydroxylamino-2-nitrotoluene in the reaction media [38]. In the studied system, 1 could form by the condensation of 2-hydroxy-4-nitrotoluene and 2-nitroso-4-nitrotoluene (Fig. 6). The formation of 2 can be explained with a possible reaction between 4-methyl-resorcinol (which can be formed from 2-hydroxy-4-nitrotolune in methanol) and 2-nitroso-4-nitrotolune. For explaining the presence of the third condensed side-product (3), the formation and reaction of N-2,5-xylylhydroxylamine and 4-nitrotoluene have been assumed.The partially hydrogenated compounds (2A4NT and 4A2NT) converted to toluenediamine above 323 K by using the Pd/CoFe2O4 and Pd/NiFe2O4 catalysts (Fig. 7 A,B and E,F). However, regardless of temperature, the less active Pd/CuFe2O4 sample is not able to convert all of the intermediates (Fig. 7C,D). In this case, even after 2 h, there are still some semi-hydrogenated compounds that remained and were not converted to TDA. Thus, the Pd-copper ferrite nanocomposite was less active in the catalytic hydrogenation of nitro compounds than the other catalysts. This phenomenon can be explained by the structure of Pd/CuO nanocomposite, where the active Pd particles are coated with CuO layers, and thus, prevent them from being involved in the catalytic process [39]. As a relatively high amount of CuO is present in the developed Pd/CuFe2O4 catalytic system, a significant portion of the Pd particles can be covered with copper oxide. On the other hand, due to the inhibitory effect, copper and CuO-containing palladium catalysts are very well suited to semi-hydrogenate alkynes, as the interaction between the intermediate species and the bimetallic surface is weakened because of the encapsulation of the active nanoparticles [40–45]. This is the so-called ‘ensemble effect’ that may have contributed to the decreased activity of Pd/CuFe2O4, and thus, the lasting presence of 2A4NT and 4A2NT (Fig. 7C,D).In the case of the Pd/NiFe2O4 and Pd/CoFe2O4 catalysts, the TDA yield was the highest at 333 K and 20 bar H2 pressure and reached 99.8 n/n% and 84.2 n/n%, respectively (Fig. 8 A). The Pd/CuFe2O4 catalyst underperformed its counterparts and achieved only 54.2 n/n% yield. The Pd/CoFe2O4 sample can be separated easily by a neodymium magnet because its support is a pure magnetic ferrite phase (Fig. 8 B). The magnetic separation is not as efficient in the case of the Pd/CuFe2O4 catalyst. The non-magnetic CuO particles dispersed into the reaction media that is shown by its dark green color. (Fig. 8C). Thus, total catalyst recovery is not possible by using magnetic separation. Despite the high NiO content (30 wt%), the Pd/NiFe2O4 catalyst was easy to separate by using a magnetic field (Fig. 8 D). Most probably, it is due to the adsorption of the NiO on the surface of the nickel ferrite particles.For comparing the catalytic activity of the different ferrite supported catalysts, the corresponding turnover numbers (TON) have been calculated as follows: T O N = n T D A n P d where n TDA is the amount of TDA formed during a 60 min reaction.Pd/NiFe2O4 catalyst was the most effective as 110.99 mol TDA was produced at 333 K and 20 bar pressure (Table 5 ). The TON was lower (90.84) when the Pd/CoFe2O4 catalyst was used, while in the case of the Pd/CuFe2O4 sample, only 27.18 mol TDA formed. The turnover numbers confirmed that the activity of the cobalt ferrite and nickel ferrite supported Pd catalysts are excellent, as shown by the corresponding reaction rates (Table 4).The reusability of the most active Pd/NiFe2O4 catalyst was also tested without regeneration (SI Fig. 4, A and B). DNT conversion continuously decreased after each cycle, and the deviation was 14.3% compared to the first and second cycles. The decrease in TDA yields was even more dramatic, as it decreased from 99.8 n/n% to 38.91 n/n%. Thus, regeneration is inevitable after each cycle to preserve the efficiency and functionality of the catalysts.CoFe2O4, CuFe2O4, and NiFe2O4 spinel nanoparticles have been successfully synthesized by using a combination of sonochemical treatment and combustion. These magnetic nanoparticles have been used as supports to prepare palladium-containing catalysts for hydrogenation reactions. The previously developed fast and easy catalyst preparation method has been applied within which post-treatments such as calcination and reduction in a hydrogen atmosphere at high temperature are not necessary. The formation of palladium nanoparticles was initiated by sonication, and they were deposited onto the surface of the synthesized ferrite supports. The final Pd/ferrite catalysts are in an active form and ready to use from the beginning and can be easily separated from the liquid phase with a magnet as the main component of the supports is magnetic. Despite their relatively low specific surface areas (18.2 m2/g and 21.1 m2/g), Pd/CoFe2O4 and Pd/NiFe2O4 were highly active during the DNT hydrogenation reactions. Reaction rate constants (k) were similar at 333 K and 20 bar hydrogen pressure (2.6∙10−3 ± 2.3∙10−4 and 2.3∙10−3 ± 4.4∙10−5/s). The TON was also calculated (90.84 and 110.99) and confirmed the high catalytic activity of these catalysts. The activity of the Pd/CuFe2O4 catalyst was much lower and reached only 54.2 n/n% TDA yield at 333 K (TON: 27.18 mol TDA/mol Pd), which can be explained by the inhibitor effect of the high CuO content (CuO: 11.73 wt%) of the support. Moreover, the copper ferrite catalyst contains a significant amount of hematite (Fe2O3: 9.97 wt%), which is not magnetic, and thus, the magnetic catalyst separation is inefficient. Possible reaction mechanism, including the detected species, has been envisaged based on the results. The TDA yield was the highest (>99 n/n% at 333 K and 20 bar hydrogen pressure) in the case of the Pd/NiFe2O4 catalyst. In addition, despite the presence of non-magnetic phases in the support (NiO: 30.00 wt%), it was well separable by using magnets. The magnetic separability of the catalyst can be explained by the adsorption interaction between the NiO and the spinel particles. The TDA yield was lower (84.2 n/n%) in the case of the cobalt ferrite-based Pd catalyst. However, it is still very well suitable for the hydrogenation of DNT or other aromatic nitro compounds because the cobalt ferrite-based support contains only one pure, magnetic phase, which ensures an easy and complete recovery of the catalyst from the reaction medium.Viktória Hajdu Methodology, Writing- Original draft preparation.Miklós Varga Methodology, Visualization.Gábor Muránszky Methodology, Data curation.Gábor Karacs Methodology.Ferenc Kristály Methodology.Béla Fiser Writing- Reviewing and Editing.Béla Viskolcz Validation, Funding acquisition.László Vanyorek: Conceptualization, Supervision.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The described study was carried out as part of the EFOP-3.6.1-16-2016-00011 ‘Younger and Renewing University – Innovative Knowledge City – institutional development of the University of Miskolc aiming at intelligent specialization’ project implemented in the framework of the Széchenyi 2020 program. The realization of this project is supported by the European Union, cofinanced by the European Social Fund. Further support has been given by the National Talent Program (HU), National Young Talents Scholarship, Ministry of Human Capacities (HU), Human Capacities Grant Management Office (EMET) (contract number: NTP-NFTÖ-20-B-0062).The following is the supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtchem.2021.100470.

Cobalt, copper, and nickel ferrite spinel nanoparticles have been synthesized by using a combination of sonochemical treatment and combustion. The magnetic nanoparticles have been used as supports to prepare ~4 wt% palladium catalysts. The ferrites were dispersed in an ethanolic solution of Pd(II) nitrate by ultrasonication. The palladium ions were reduced to metallic Pd nanoparticles, which were then attached to the surface of the different metal oxide supports. Thus, three different catalysts (Pd/CoFe2O4, Pd/CuFe2O4, Pd/NiFe2O4) were made and tested in the hydrogenation of 2,4-dinitrotoluene (DNT). A possible reaction mechanism, including the detected species, has been envisaged based on the results. The highest 2,4-diaminotoluene (TDA) yield (99 n/n%) has been achieved by using the Pd/NiFe2O4 catalyst. Furthermore, the TDA yield was also reasonable (84.2 n/n%) when the Pd/CoFe2O4 catalyst was used. In this case, complete and easy recovery of the catalyst from the reaction medium is ensured, as the ferrite support is fully magnetic. Thus, the catalyst is very well suited for applicationy in the hydrogenation of DNT or other aromatic nitro compounds.

As one of the 100 most important chemicals in the world, hydrogen peroxide (H2O2) is a valuable and environmentally friendly oxidizing agent with a wide range of applications, ranging from the provision of clean water to the synthesis of valuable chemicals as well as being a potential energy carrier. 1–3 The current industrial synthesis of H2O2 involves an energy-intensive anthraquinone oxidation-reduction step, which requires elaborate and large-scale equipment and at the same time generates substantial waste. 3 , 4 An attractive alternative route for direct on-site production of H2O2 is through an electrochemical process in a fuel cell setup (anode: H2 → 2e − + 2H+; cathode: O2 + 2e − + 2H+ → H2O2, E 0 = 0.695 V), where the oxygen reduction reaction (ORR) occurs via a two-electron pathway. Substantial efforts devoted in recent years to this fuel cell concept have aimed at efficiently generating electricity with a simultaneous high-yield production of H2O2 in basic media. 5–8 Indeed, recent results have suggested negligible room for further improvements in the activity and selectivity of carbon-based materials for H2O2 synthesis in basic media. 3 , 6 , 7 , 9 , 10 Unfortunately, the production of H2O2 in basic media has several drawbacks: (1) H2O2 is less stable and can self-decompose in bases (especially at pH > 9) 11 ; (2) there is no commercially competitive anion exchange membrane with comparable stability and conductivity to that of the proton exchange membrane (PEM) for device development 3 ; and (3) H2O2 is more widely used in acidic media with stronger oxidation ability than in basic media. For example, Fenton's reagent, which is mostly applied in organic synthesis and effluent treatment, has an optimal pH range of 2.5–3.5. 12 Therefore, there is a great industrial motivation to improve H2O2 catalysis in acidic media, more specifically using PEM-type apparatus. 3 , 13–15 Previously, mercury-alloyed platinum or palladium 16 , 17 nanoparticles supported on carbon, as state-of-the-art catalysts, have been investigated for H2O2 synthesis via ORR in acidic media. However, these catalysts contain precious noble metals and toxic mercury, thus limiting their potential applications in H2O2 production. Although homogeneous molecular catalysts such as cobalt macrocycles are highly selective for H2O2 production via ORR, 18 the low activity and poor stability hinder their possible applications. Transition metals such as cobalt particles 19 or manganese species 20 loaded on nitrogenated carbon materials can also be used to produce H2O2 but lack high activity. Meanwhile, the non-uniform structure in these catalysts hinders their identification of active sites, mechanistic study, and further rational optimization. In short, there is still a lack of cost-effective electrocatalysts with high catalytic performance for H2O2 synthesis in acidic media. In recent years, single-atom catalysts (SACs) with well-defined active centers have drawn great attention for their particularly high activity and selectivity in various chemical reactions. 21–23 In principle, to increase the selectivity of H2O2 production through ORR, O–O bond breaking needs to be minimized. Benefiting from the desirable features of SACs, in which the active sites are atomically isolated, the adsorption of O2 on SACs is usually of the end-on type, rather than μ-peroxo coordination, which therefore could reduce the possibility of O–O bond splitting. 18 , 24 , 25 This implies that SACs would be suitable for H2O2 generation via ORR. Previous studies of metal-nitrogen-carbon materials mainly focus on the electrocatalytic activity toward four-electron ORR to H2O for fuel cells applications, 26–30 whereas unfavorable two-electron ORR to H2O2 is rarely studied in detail. Although there are few studies of their electrocatalytic activities toward two-electron ORR for H2O2 production, 19 , 20 the fundamental aspects such as active center and reaction mechanism as well as practical electrolytic cell device aspects remain poorly understood. Here, by combining theoretical and experimental methods, the relation between the structure of transition metal (Mn, Fe, Co, Ni, and Cu) SACs anchored in nitrogen-doped graphene and the catalytic performance of H2O2 synthesis via ORR was systematically studied. Both theoretically predicted activity-volcano relation and experimental results show that the Co SAC possesses optimal d-band center and can function as a highly active and selective catalyst for H2O2 synthesis via ORR and even slightly outperforms state-of-the-art noble-metal-based electrocatalysts in acidic media.Inspired by previous work, 16 we first investigated the ORR process on various transition metal SACs anchored in nitrogen-doped graphene for producing H2O2 or H2O along a 2 e − or 4 e − pathway, respectively, by DFT calculations using a computational hydrogen electrode model (details are given in Experimental Procedures). The two-electron (2 e − ) pathway to H2O2 via ORR comprises two proton-coupled electron transfer steps with only one intermediate (*OOH): (Equation 1) ∗ + O 2 + H + + e − → *OOH (Equation 2) ∗ OOH + H + + e − → H 2 O 2 + ∗ where the asterisk (*) denotes the active site of the catalyst. In contrast, for the 4 e − ORR pathway, four proton-coupled electron transfer steps are included, in which O2 is reduced to *OOH, *O, *OH, and H2O in sequence, as displayed in Figure 1 A. Theoretically, an ideal catalyst for H2O2 synthesis should minimize the kinetic barriers for Equations 1 and 2 to provide high activity. Meanwhile, the catalyst needs to maximize the barrier for *OOH dissociation or reduction to *O and *OH (the intermediates of the 4 e − ORR pathway to H2O) to achieve high selectivity. 16 Here, density functional theory (DFT) calculations revealed that *OOH, *O, and *OH were all energetically favored to adsorb on the top site of the metal (M) atom (the most stable configurations are shown in Figure S2). Therefore, the activity of ORR is mainly dependent on the electronic interaction of the intermediates with the M atom rather than the geometrical effects. Figure 1B shows that the binding energies of *OOH, *O, and *OH are generally scaled with the number of valence electrons in the M atom from manganese to copper. The larger the number of valence electrons in M, the weaker the binding of these intermediates to the M atom, which is because the d-band center of M atom shifts down in energy relative to the Fermi level from Mn to Cu (Figure 1B). 31 In detail, the anti-bonding states derived from the coupling between d-orbitals of M atom and 2p-orbitals of bonded O atom of intermediates are shifted down in energy and thus are more filled, which weakens the M–O bonding from Mn to Cu. Then, to compare the ORR activities of these SACs, we calculated the free energy diagrams (Figure S3) and constructed the activity-volcano plots for both the 2 e − and 4 e − pathways by using ΔG*OH as a descriptor, as shown in Figure 1C. For an ideal 2 e − ORR catalyst, the adsorption of *OOH should be thermoneutral at the equilibrium potential (U = 0.7 V versus reversible hydrogen electrode [RHE]), corresponding to ΔG*OOH = ∼3.5 ± 0.2 eV. 16 However, in striking contrast to the 2 e − ORR, even for the optimal catalyst, an overpotential of ∼0.4 V was required to drive the 4 e − reduction of O2 to H2O, because of the scaling relation between *OOH and *OH (Figure S3F), i.e., ΔG*OOH = 0.747 ΔG*OH + 3.32 eV, and similar results have also been found in other models. 32–34 From Figure 1C, it can be seen that the ORR on the Ni and Cu SACs prefers the 2 e − pathway but with a large overpotential because of the large *OOH reduction barrier (Figures S3D and S3E) and high O2 activation energy (Figure 1D), implying that these two catalysts would exhibit low activity but high selectivity for H2O2 production.By contrast, the binding of O2 on the Mn and Fe SACs is so strong (Figure 1D) that it becomes more downhill in free energy for *OOH reduction to *O (Figures S3A and S3B). Therefore, the 4 e − pathway dominates over the 2 e − pathway on the Mn and Fe SACs, thus causing much lower selectivity for H2O2. In addition, the Fe SAC should possess the highest ORR activity via the 4 e − pathway among the five SACs because of its optimized adsorption energy of oxygenated intermediates (Figures 1B and 1C). Considering the relatively stronger binding energies of Fe and Mn SACs toward ORR species, it is possible that the backside of these two catalysts are covered by *OH or *O. Supplemental calculation results show that the backsides are possibly covered by *OH under ORR working potentials (Figure S4). However, this does not obviously change the 4 e − ORR activity (Figure S5), but instead could slightly enhance the 2 e − ORR activity (Figure S6) for H2O2 production. Significantly, the Co SAC with optimal d-band center shows ΔG*OOH = 3.54 eV at U = 0.7 V versus RHE (Figure 1D) for the 2 e − pathway, neither too strong nor too weak, being positioned nearly at the vertex of the activity-volcano map (Figure 1C), suggesting that the Co SAC would be highly active for the 2 e − pathway. In addition, the higher barrier for *OOH reduction to the *O intermediate on the Co SAC (Figure S3C) compared with those on the Mn and Fe SACs (Figures S3A and S3B) would enhance the selectivity of the Co SAC to H2O2. Combined with the predicted high activity, it can be anticipated that the yield of H2O2 on the Co SAC would be the highest among the five SACs.Subsequently, we synthesized five different transition metal SACs anchored in nitrogen-doped carbon (NC) (Mn–NC, Fe–NC, Co–NC, Ni–NC, and Cu–NC) via pyrolysis of melamine, L-alanine, and the corresponding metal acetate mixture (details are given in Experimental Procedures). NC was also prepared by the same method without adding a metal salt for comparison. This synthesis method can be easily scaled up and Figure S8 shows a digital photograph of the SACs obtained by batch synthesis. Figures 2A and S9 show representative scanning electron microscopy (SEM) images of the as-synthesized SACs (Figures 2A and S9 for Co–NC, and Figures S9B–S9F for the rest), which display aggregated two-dimensional (2D) platelets. No obvious Co particles can be observed in the transmission electron microscopy (TEM) images of Co–NC (Figures 2B and S10A), similar to the absence of the corresponding metal particles in the other SACs (Figures S10B–S10F), suggesting that the metal species are highly dispersed in the carbon matrix. The Brunauer-Emmett-Teller (BET) surface areas of the six samples (Mn–NC, Fe–NC, Co–NC, Ni–NC, Cu–NC, and NC) obtained from N2 adsorption isotherms (Figure S11A) are in the range of 360–670 m2/g with total pore volumes of 1.5–2.2 cm3/g (Table S4). All samples have similar pore size distribution. The pore sizes show a wide distribution from several to a few tens of nanometers (Figure S11B), and the pores themselves are mainly formed from the folds or holes in the carbon matrix. The atomic-scale dispersion of the metals was confirmed by aberration-corrected high-angle annular dark field scanning TEM (HAADF-STEM). The bright spots with diameter of ∼0.2 nm in Figure 2C are atomically dispersed Co species in Co–NC. Very similar HAADF-STEM images of the other transition metal catalysts are displayed in Figure S12. The X-ray diffraction (XRD) patterns (Figure 2D) show that all five of the transition metal SACs, together with NC, exhibit a single, similar, broad characteristic diffraction peak of the carbon (002) at 25.8°, suggesting a low degree of crystallization. No other diffraction peaks of metal, metal nitride, or metal oxide are discernible, agreeing well with the TEM and HAADF-STEM results. The Raman spectra (Figure 2E) of the six samples also show very similar patterns with two vibrational bands: the d-band at 1,350 cm−1 is the characteristic peak of vacancies or defects in graphene 35 , 36 and the G band at 1,580 cm−1 is the characteristic peak of graphitic layers, which corresponds to the in-plane vibration of sp2 chains associated with the E2g symmetry. 35 , 37 The relative intensities of D to G band for the six catalysts are nearly identical, suggesting that they have similarly disordered or defective carbon structures. To further examine the structure of the catalysts, we measured extended X-ray absorption fine structure (EXAFS) spectra. Figure 2F shows the Fourier transformation (FT) of the EXAFS spectra of five transition metal catalysts, exhibit only one strong peak at an interatomic distance of ∼1.3 Å (without phase correction, the same below), which is typical for metal-N bonds. 26 , 38 , 39 In addition, the EXAFS spectra of some commercial metal phthalocyanines (M-PC, M=Fe, Co, Ni, and Cu) were measured and Fourier transformation of the EXAFS spectra are shown in Figure S13A for reference. All of them show quite similar peaks at interatomic distances of ∼1.3–1.5 Å, suggesting that the transition metals are mainly coordinated with nitrogen, as was the case in the synthesized SACs. No strong metal-metal bonds (which have interatomic distances of ∼2.1 Å in metal foils as shown in Figure S13B) can be observed in Figure 1F, moreover, the EXAFS data can be fitted well with the model proposed in above DFT calculation section (Figure S14 and Table S5), further proving that the metal species are mainly atomically dispersed, consistent with the HAADF-STEM results (Figures 2C and S12). In addition, the valence states of the metals in the catalysts were also examined through measurement of their K-edge X-ray absorption near-edge structure (XANES) spectra and comparison with those of metallic foils and metal phthalocyanines (Figure S15). Comparison of the first derivative XANES for M–NC catalysts with references indicates that the metals in M–NC are all in positive oxidation states. 39 The composition of the catalysts was analyzed by X-ray photoelectron spectroscopy (XPS) (Figure S16). The carbon, nitrogen, and oxygen contents of all the catalysts display very similar spectra and valance states, and the estimated atomic percentages of N in the transition metal SACs are ∼6–7 atom % (Table S6), which are higher than that in metal-free NC (3.5 atom %). Moreover, the much-enhanced relative intensities of N species coordinated with metal (∼ 398.8 eV) in M–NC compared with bare NC (Figure S17), suggesting that the transition metal and N can stabilize each other in carbon materials by formation of M–N bonds. The metal contents in the catalysts determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) are in the range of 1.0–1.6 wt % (Table S6). By comparison with the typical binding energies of metallic and oxidic states of the corresponding elements (Figure S18 and Table S7), it is deduced that all five metals in the catalysts show positive valance states, agreeing with the XANES spectra (Figure S15) and previous reports. 38 , 39 To examine the catalytic performance, we conducted electrochemical ORR tests in 0.1 M HClO4 on a rotating ring disk electrode (RRDE) at room temperature (24 ± 1°C). Figure S19 shows the cyclic voltammetry (CV) curves of the six catalysts acquired in O2-saturated and N2-saturated 0.1 M HClO4. All the catalysts exhibit similar curves in N2 atmosphere, suggesting their comparable double layer capacitances. Under O2 atmosphere, the reduction peak of oxygen occurs at 0.1–0.2 V versus RHE for Mn–NC, Ni–NC, Cu–NC, and NC, whereas Fe–NC and Co–NC display reduction peaks of O2 at higher potentials (∼0.45 V versus RHE). Subsequently, we conducted linear sweep voltammetry (LSV) measurements on the RRDE and the results are shown in Figure 3 A. As expected, Co–NC and Fe–NC show much higher activity for ORR and had onset potentials at around 0.7 V versus RHE. The ring current of Co–NC, which corresponds to the oxidation of H2O2 also starts at around 0.7 V and the calculated faraday efficiency for H2O2 (Figure 3B) shows that Co–NC is highly selective for H2O2 production in the entire potential range of 0–0.7 V versus RHE. The kinetic current of H2O2 production over Co–NC reached 1 mA / cm disk 2 (corresponding to a mass-normalized current density of 40 A/gcatalyst.) at 0.6 V versus RHE with H2O2 faraday efficiency >90%. In contrast, Fe–NC shows a much lower selectivity for H2O2, consistent with the above DFT predictions and previous reports. 27 , 28 , 40 Although the other catalysts, Mn–NC, Ni–NC, Cu–NC, and NC, are also highly selective for H2O2 (Figure 3B), their catalytic activities are much poorer. The electron transfer number is calculated according to reported method 19 and shown in Figure S20A. The number over Co–NC is close to 2, agreeing with its selectivity for H2O2. The turnover frequency (TOF) values of the catalysts for H2O2 production at different potentials were calculated (Figure S20B) and the highest TOF value of 2.5 s −1 was obtained for Co–NC at a potential of 0.5 V versus RHE. The above experimental results are consistent with our DFT calculations (Figure 1), which show that Co–NC is at the top of the activity-volcano map for H2O2 production reaction. In addition, we further examined the electrochemical characteristics of the transition metal SACs in N2-saturated 0.1 M HClO4 containing 0.1 M H2O2 to monitor their catalytic activity for H2O2 oxidation (H2O2 → O2 + 2H+ + 2e − , E0 = 0.695 V) and reduction (H2O2 + 2H+ + 2e − → 2H2O, E0 = 1.763 V), which are closely related to their selectivities for H2O2. It can be observed from the LSV curves (Figure S21) that the oxidation of H2O2 over Co–NC starts at 0.75 V versus RHE, very close to the onset potential (0.7 V versus RHE, Figure 3A) of its reverse reaction, namely reduction of O2 to H2O2 (O2 + 2H+ + 2e − → H2O2, E0 = 0.695 V). Therefore, the reduction of O2 to H2O2 over Co–NC is nearly reversible, and this is consistent with the high activity of Co–NC for O2 reduction to H2O2 and our DFT calculations. Figure S21 further shows that Fe–NC can efficiently reduce H2O2 to H2O, thus leading to its much lower selectivity for H2O2 production (Figure 3B). Moreover, as deduced from the DFT calculation, Mn–NC should also possess low selectivity for H2O2, because of its strong adsorption of oxygen intermediates. However, Mn–NC in fact shows relatively high selectivity for H2O2, close to that of NC in the ORR (Figure 3B), possibly because the adsorption of oxygenated species is so strong that it blocks the Mn active sites, making Mn–NC behave similarly to bare NC. Meanwhile, Cu–NC and Ni–NC are less active for H2O2 reduction and oxidation because of their too weak adsorption of oxygenated intermediates. Figures 3C and S22 (mass-normalized activity) compares the performances of state-of-the-art catalysts for H2O2 production through ORR. It can be seen that Co–NC is the most effective catalyst for H2O2 synthesis (Table S8), which even slightly outperforms the best previously reported catalyst in acidic media, a Pd-Hg alloy. The effect of the catalyst loading amount on the ORR performance for Co–NC and other SACs was further optimized (Figure S23). Reducing the loading of Co–NC slightly improves the selectivity for H2O2. As shown in Figure S21, H2O2 as an intermediate can be further reduced to H2O over Co–NC. Thus, decreasing the catalyst loading amount reduces the residence time of H2O2 on the catalyst surface, so that less H2O2 is reduced to H2O, therefore increasing the H2O2 selectivity. The rotating speed of the RRDE was found to have little influence on the selectivity for H2O2 (Figure S24). Additionally, NC loaded with Co nanoparticles (CoNPs/NC) was also prepared for comparison (Figure S25), but its ORR activity was much lower than Co–NC (Figure S26). Considering that cobalt porphyrins and phthalocyanines are known for their high selectivities for H2O2 generation by ORR despite their rapidly decaying activities in acidic conditions, 18 , 27 , 41 we prepared tetra-amino-cobalt(II) phthalocyanine (Co-TAPC) loaded on carbon nanotubes (Co-TAPC/commercial carbon nanotubes [CNT]) and tested its ORR performance (Figure S27). However, although the H2O2 selectivity of Co-TAPC/CNT was high, the activity was much lower than Co–NC. The catalytic ORR performances of all six catalysts together with CNT were also tested in alkaline conditions (0.1 M KOH). All of the catalysts exhibited higher activity for ORR in alkaline conditions (Figure S28) than in acidic media and had markedly increased current density at the same potential versus RHE. Among the studied catalysts, NC, CNT, and even blank glassy carbon electrode (GCE) exhibited high selectivity for H2O2 production in alkaline media (Figures S28B and S28D). However, these carbon-based materials show very poor ORR performance in acidic conditions, as shown in Figures 3A and S26. This trend is consistent with previous reports, 5–7 , 10 and might be attributed to the different affinity of protons and hydroxyls toward the various functional groups present on the surface of the catalysts at different pH. 42 Besides activity, stability is another important consideration for a catalyst in practical use. The stability of Co–NC was studied at 0.5 V versus RHE in 0.1 M HClO4 both under static and rotating conditions. The currents of both the ring and the disk electrode remained stable for 10 h without obvious decay (Figure 3D); the slightly increased current of the ring electrode can be attributed to the gradually accumulated H2O2 in the electrolyte. The selectivity for H2O2 determined by titration method (details are given in Experimental Procedures) remained as high as ∼88% throughout the entire process. Figure S31 compares the LSV curves of Co–NC before and after the stability test. A change can be observed in the kinetic range, which might stem from partial detachment of the catalyst from the electrode or some deactivation occurred. The stability of Co–NC was further confirmed by CV cycling for 5,000 cycles (Figure S32). Half-cell experiment at fixed potential of 0.5 and 0.4 V versus RHE chronoamperometry test was further conducted to imitate real case H2O2 production. An average H2O2 production rate of 80 and 275 mmol H 2 O 2 g catalyst  − 1 h − 1 is obtained at 0.5 V and 0.4 V versus RHE, respectively (Figure S33).The HAADF-STEM images of the used catalysts show that the Co species are still atomically dispersed (Figure S34), demonstrating the good catalytic stability of Co–NC in the H2O2 synthesis process.From the above-mentioned DFT calculations, we found that the first step (* + O2 + H+ + e− →*OOH) was the thermodynamic potential-determining step for ORR on Co–NC. To shed more light on the reaction mechanism, we performed kinetic analysis to experimentally probe the rate-determining step. First, the kinetic current of ORR on Co–NC was obtained by Koutecky–Levich analysis from the LSV curves acquired at different rotating speeds (Figures S35 and S36). The reaction order of O2 was determined by performing ORR at different O2 partial pressures. Figure 4 A shows the kinetic current as a function of overpotential at different O2 partial pressures. Then, the logarithm of the kinetic current versus the logarithm of the O2 partial pressure was plotted as shown in Figure 4B, from which, we can deduce that the reaction order of O2 (slope of the line) varied from 0.53 to 0.90 as the overpotential increased from 0 to 250 mV. Simultaneously, the Tafel slope increased from ∼110 mV dec−1 to ∼140 mV dec−1, and finally to ∼240 mV dec−1 as the overpotential increased from 0 to 250 mV (Figure 4A). Figures 4C and 4D show the effect of pH (H+ concentration) on the activity of ORR over Co–NC. By the same analysis as performed for Figures 4A and 4B, the reaction order of H+ in the rate-determining step is −0.05 ∼ −0.07, very close to zero, as shown in Figure 4D. Thus, it is suggested that H+ is not involved in the rate-limiting step, namely, the protonation process is fast. By combining Figures 4A–4D and Table S9, it is deduced that the rate-determining step of H2O2 synthesis over Co–NC is as follows: * + O2 + e − → *O2 −, which is covered in the DFT calculation predicted thermodynamic potential-determining step (* + O2 + H+ + e − → *OOH). 43 In detail, at relatively low overpotential (<50 mV), it is mainly controlled by the electron transfer step of adsorbed O2. (*O2 + e − → *O2 −). The electron transfer step becomes faster by increasing the overpotential. Then the overall reaction rate is more limited by the O2 adsorption process, which agrees well with the observed gradually increasing Tafel slope and reaction order of O2. To monitor the change of Co electronic state and coordination environment, operando X-ray absorption spectroscopy (XAS) was conducted to probe the change of the Co K-edge under H2O2 synthesis conditions in 0.1 M HClO4. The EXAFS spectrum of the Co–NC in air is nearly the same as that of Co–NC immersed in electrolyte saturated with N2, and the Co–N distance is 1.25 Å in both cases (Figure S37). After switching N2 to O2, a dramatic enhancement of Fourier transformed intensity and slight increase of Co–N distance to 1.35 Å are observed (Figure S38), indicating the adsorption of O2 onto Co atoms, which pulls the Co atoms out of plane. This result agrees with the DFT calculations, which show that oxygen species are energetically favored to adsorb on the top site of Co atoms. At the potential of 0.6 V versus RHE, part of the adsorbed O2 is transformed to H2O2 via ORR, and the Co–N distance decreases to 1.32 Å (Figure 4E). At lower potentials, the transformation of adsorbed O2 to H2O2 via ORR becomes more rapid, and the surface coverage of O2 becomes much lower because of the limited rate of O2 adsorption, which explains the further decrease of the Co–N distance to 1.29 Å at 0.3 V versus RHE. Additionally, the Co–N bond distance recovers to its initial value (1.35 Å) after the potential returns to that of open-circuit conditions and the EXAFS spectra in k-space also show similar trend (Figure S37). The operando XANES of Co–NC were also collected (Figure S37). Comparison of the spectra in N2- and O2- (Figure S38B) atmosphere shows an increase in intensity of XANES in O2, suggesting molecular O2 adsorption on the cobalt centers in Co–NC, which agrees with previous report. 29 This trend matches very well with the kinetic analysis. Figure 4F displays a schematic illustration of the ORR steps taking place on Co–NC for H2O2 production, in which step 2 is the rate-limiting process at higher potential whereas step 1 becomes rate-limiting at lower potential. After *OOH is formed, further reduction to H2O2 is rapid, and the active sites are vacated by the desorption of H2O2 to complete the catalytic cycle.In summary, DFT calculations predicted that a cobalt-based SACs anchored in a NC matrix would outperform other transition metal SACs for H2O2 production through ORR. Then, those transition metal SACs were successfully synthesized. Just as predicted by the DFT calculations, Co–NC experimentally behaved as a highly active and selective electrocatalyst for H2O2 synthesis via oxygen reduction in acidic media. A kinetic current density of 1 mA/cm2 was reached on Co–NC at a potential of 0.6 V versus RHE with H2O2 selectivity >90%, and these performance measures could be sustained for 10 h continuous operation without decay. The optimized adsorption energy of oxygenated intermediates on Co–NC with optimal d-band center as compared with other transition metal (Mn, Fe, Ni, and Cu) SACs is chiefly responsible for its high activity and selectivity toward H2O2 production. A kinetic analysis and operando X-ray absorption study combined with DFT calculations demonstrated that nitrogen-coordinated Co single atom was the active site for H2O2 synthesis through ORR and the reaction was rate-limited in the first proton-coupled electron transfer step. This work combines the advantages of both homogeneous catalysts of cobalt macrocycles (well-defined active sites) and heterogeneous catalysts (high catalytic performance) together, moreover, operando XAS combined with kinetics analysis in this work tracked the dynamic change of nitrogen-coordinated cobalt active center under reaction condition, which together lead to higher catalytic performance with enhanced understanding of the reaction process.Chemicals: melamine (C3H6N6, 99%), L-alanine (C3H7NO2, 98%), cobalt(II) acetate tetrahydrate (Co(CH3CO2)2·4H2O, 99%), manganese(II) acetate (Mn(CH3CO2)2, 98%), iron(II) acetate (Fe(CH3CO2)2, 99.99%), nickel(II) acetate tetrahydrate (Ni(CH3CO2)2·4H2O, 98%), nickel(II) phthalocyanine (C32H16N8Ni, 85%), copper(II) acetate (Cu(CH3CO2)2, 98%), copper(II) phthalocyanine (C32H16N8Cu, sublimed grade, 99 %), hydrochloric acid (HCl, 37%), nitric acid (HNO3, 70%), hydrogen peroxide solution (H2O2, 30 wt % in water), and cerium(IV) sulfate (Ce(SO4)2, 98%) were purchased from Sigma-Aldrich and absolute ethanol was bought from Merck. Cobalt(II) phthalocyanine (C32H16N8Co, 95%, product code: 41496) and Iron(II) phthalocyanine (C32H16N8Fe, 96%, product code: 39262) were purchased from Alfa Aesar. Multi-walled carbon nanotubes (CNT, 10–20 nm diameter, 5–15 μm length) were bought from TCI chemical company. All chemicals were used directly without further purification. De-ionized water was obtained from Millipore Q water purification system. Transition metal SACs were synthesized according to our previous method 38 with slight modification. In a typical synthesis, 12 g of melamine, 2 g of L-alanine, and 50 mg of transition metal acetate were homogeneously mixed by ball milling for 1 h. Then, 15 mL of ethanol mixed with 3 mL of hydrochloric acid was added and the slurry was put in a mortar. The mixture was milled in a fume hood until all ethanol was evaporated. The resultant solid was dried in an oven at 60°C overnight and ball milled again for 1 h. The thus obtained powder was pyrolyzed under flowing N2 atmosphere in a tube furnace with the following ramping program: from room temperature to 600°C at a ramping rate of 2.5°C/min, then hold at 600°C for 120 min, ramp to 900°C at 5°C/min and hold for 90 min, finally the furnace was naturally cooled down to room temperature. The obtained black solid materials were grinded and then washed by 2 M HCl aqueous solution at 80°C for 24 h under stirring to remove metal particles. For copper-based material, 1 M HNO3 was used to remove copper metal particles. The acid-washed materials were dried and then annealed again in N2 at 800°C for 1 h at a heating rate of 10 °C/min to recover the crystallinity. The thus obtained SACs were marked as Mn–NC, Fe–NC, Co–NC, Ni–NC, and Cu–NC, respectively, according to the metal acetate used. NC was synthesized by the same method without adding metal acetate and undergoing acid washing. Co nanoparticle supported on NC with Co content of 2 wt % was prepared by impregnation method: 200 mg of NC was dispersed in 20 mL mixed solution of water and ethanol (volume ratio 1:1) containing 18 mg of cobalt(II) acetate tetrahydrate. The mixture was heated to 80°C in an oil bath under stirring to evaporate the solution. The obtained solid was then calcined at 400°C in N2 atmosphere for 2 h. The obtained catalyst was marked as CoNPs/NC. In addition, tetra amino cobalt(II) phthalocyanine (Co-TAPC) was synthesized and loaded on CNT according to the method reported in literature, 44 , 45 and the catalyst was marked as Co-TAPC/CNT.Powder XRD was performed on a Bruker D2 Phaser using Cu Kα radiation with a LYNXEYE detector at 30 kV and 10 mA. The morphological information was examined with field-emission SEM (FESEM, JEOL JSM-6700F). Sub angstrom-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) characterization was conducted on a JEOL JEMARM200F STEM and TEM with a guaranteed resolution of 0.08 nm. The metal content in the catalysts was quantified by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, PerkinElmer). N2 adsorption-desorption was performed on an Autosorb-6 (Quantachrome) at 77 K. Before analysis, the samples were degassed at 200°C for 5 h. BET surface area was calculated in the P/Po range of 0.05–0.2. Pore size destitution was obtained by Barrett-Joyner-Halenda (BJH) method by using the adsorption branch. Pore volume was calculated by the adsorption amount at P/Po = 0.985. Raman spectra were recorded on a Renishaw INVIA Reflex Raman spectrometer using 514 nm laser as the excitation source. XPS measurements were carried out on a Thermofisher ESCALAB 250Xi photoelectron spectrometer (Thermofisher Scientific) using a monochromatic Al Kα X-ray beam (1,486.6 eV). XAS including both XANES and EXAFS at Mn, Fe, Co, Ni, and Cu K-edge were collected in total-fluorescence-yield mode at ambient air in BL-01C1 at the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The spectra were obtained by subtracting the baseline of pre-edge and normalizing to the post-edge. EXAFS analysis was conducted using Fourier transform on k3-weighted EXAFS oscillations to evaluate the contribution of each bond pair to Fourier transform peak. Operando measurement in a typical three-electrode setup was performed in a specially designed Teflon container with a window sealed by Kapton tape with continuous gas bubbling. X-ray was allowed to transmit through the tape and electrolyte, so that the signal of XAS could be collected in total-fluorescence-yield mode in BL-01C1 at NSRRC, Taiwan.The electrochemical performance of various catalysts was evaluated in a three-electrode configuration with carbon rod as the counter electrode and Ag-AgCl electrode with saturated KCl salt bridge as the reference electrode on a RRDE setup (AFE6R1PT model; disk OD = 5.0 mm; ring OD = 7.50 mm; ring ID = 6.50 mm; Pine Research Instrumentation, USA) and a CHI (760E) potentiostat. 0.1 M HClO4 was prepared by diluting perchloric acid (70%, 99.999% trace metals basis, Sigma) with Millipore Q water. 0.1 M KOH was prepared by dissolving KOH pellets (semiconductor grade, 99.99% trace metals basis, Sigma) in Millipore Q water (15 MΩ). A RHE was made with two Pt plates as working and counter electrodes to calibrate the Ag-AgCl electrode and H2 was bubbled over the working electrode. Potentials reported here are referenced to the RHE scale as follows: E RHE = E Ag/AgCl + 0.197 V + 0.059 V × PH or standard hydrogen electrode (SHE) scale: E SHE = E Ag/AgCl + 0.197 V. To prepare the working electrode, the catalyst ink that was prepared by ultrasonically mixing 5 mg of the catalyst, 0.98 mL of Millipore Q H2O (15 MΩ), 0.98 mL of isopropyl alcohol, and 40 μL of 5 wt % D520 Nafion dispersion solution was drop-casted on freshly polished RRDE. In a typical measurement, 2 μL of catalyst ink was used, which corresponds a catalyst loading amount of 25 μg / cm disk 2 . Before collecting the electrochemical data, the electrolyte was bubbled with purified O2 or N2 for 30 min. Subsequently the working electrode together with Pt ring were cycled for ten cycles between 0 and 1.0 V versus RHE at a scan rate of 500 mV s−1 to achieve a stable performance. CV curves were recorded in the potential range of 0−1.1 V versus RHE at 500 mV s−1 under static condition with saturated N2 or O2. LSV curves were recorded at a scan rate of 5 mV s−1 with 100% solution ohmic drop correction under 1,600 rpm or other indicated rotation speed; the potential of the Pt ring in the working electrode was set at 1.2 V versus RHE. In kinetic analysis, according to the Henry's law, the O2 concentration is proportional to its partial pressure in the gas phase, thus partial pressure is used to control the concentration of O2 in electrolyte. In detail, the partial pressure of O2 was adjusted by diluting O2 flow with Argon at controlled flow rate. For example, O2 partial pressure of 25 kPa was obtained by mixing gas of O2 (50 mL/min) and argon (150 mL/min). For different pH (1.5−3.0), KOH tablet was added to 0.1 M H3PO4 solution to tune the pH and KNO3 was added to adjust the ionic strength. 46 Considering that O2 partial pressure and concentration of H+ can influence the equilibrium potential of the reaction (O2 + 2e − + 2H+ → H2O2) according to the Nernst equation: E O 2 / H 2 O 2 = E O 2 / H 2 O 2 0 − R T z F ln a O 2 a H + 2 a H 2 O 2 , where E O 2 / H 2 O 2 0  = 0.695 V (versus standard hydrogen electrode) is the standard potential of the reaction at O2 partial pressure of 1 atm, the activity of H+ (a H+) and H2O2 (a H2O2) equal 1 mol/L. Here, due to the unknown value of a H2O2 in the electrolyte solution, we just assume it as 1 mol/L to get a pseudo-equilibrium potential. Then, the change of O2 partial pressure and activity of H+ would reach a new pseudo-equilibrium potential E O 2 / H 2 O 2 . The overpotential is defined as η = E applied − E O 2 / H 2 O 2 , E applied is the potential (versus SHE) applied to the working electrode, E O 2 / H 2 O 2 is the pseudo-equilibrium potential at designated O2 pressure and H+ activity. It should be noted that the overpotential defined here is not a strict one due to the unknown true value of equilibrium potential, but it still can be used as the driving force of the reaction for kinetic analysis.H2O2 selectivity was calculated on rotating ring disk electrode based on the currents of both disk and ring electrode according to:H2O2 selectivity or Faraday efficiency: H2O2 (%) = 200 I R / N I D + I R / N where I R is the ring current, I D is the disk current and N is the collection efficiency of the RRDE (0.25), which is calibrated by the redox of potassium ferricyanide (Figure S29).During stability test, H-type electrochemical cell with working and counter electrode separated by Nafion film was used to avoid further oxidation of H2O2 on the anode. The formation rate of H2O2 in half-cell experiment was conducted in O2 saturated 0.1 M HClO4 in a H-type cell. The working electrode is prepared with carbon paper (1 × 1 cm) by coating catalysts with loading amount of 100 μg/cm2.The concentration of H2O2 in electrolyte during stability test was determined by cerium sulfate titration (2Ce4+ + H2O2 → 2Ce3+ + O2 + 2H+) as detailed in literature. 7 The concentration of Ce4+ was measured by ultraviolet -visible spectrometer (JINGHUA Instruments, Model: 754PC) at 316 nm and the calibration curve is shown in Figure S30.Spin-polarized DFT calculations were performed using the generalized gradient approximation (GGA) in the form of Perdew–Burke–Ernzerhof (PBE) for the exchange-correlation potentials, 47 , 48 the projector augmented wave (PAW) pseudopotential for the core electrons, 49 and a 480 eV cutoff energy for the valence electrons as implemented in the Vienna ab initio simulation package (VASP). 50 , 51 Transition metals are likely trapped in the vacancies of graphene at various levels of nitrogen doping. The porphyrinic moieties containing 3D transition metal such as Fe and Co are reported to be good ORR catalyst. 30 Herein, we investigated ORR of 2 e − and 4 e − pathways on the nitrogen and transition metal atom (TM = Mn, Fe, Co, Ni, and Cu) co-doped graphene. The single transition metal atom dispersed catalysts are simulated using a cluster model (formula of C40H16N4M), in which the M atom is bound with four pyrrolic nitrogen atoms. Here, we choose cluster model other than periodic one based on two considerations: first, previous studies reported that the porphyrins containing 3d transition metals show promising activity toward production of H2O2. 30 , 52 , 53 If periodic model is constructed using the skeleton of the porphyrin molecule embedded with a metal atom as the basic unit, eight-membered carbon rings inevitably exist, which is significantly different from the honeycomb structure of graphene. Second, previous experimental and DFT studies proposed that the periodic model with MN4 group (M = transition metal, N = nitrogen) compactly embedded in graphene cannot correctly predict the ORR and CO2 reduction activities of these SACs. 30 , 54 Therefore, the cluster model is selected here. The cluster is placed in a box in the size of 30 × 30 × 13 Å, with vacuum layers of ∼13 Å along vertical and lateral directions to decouple the interaction between neighboring images. The energies of gas-phase H2 and H2O molecules were calculated in a cubic supercell with length of 20 Å. All atoms are free to relax until the net force per atom is less than 0.02 eV/Å. We consider the possible spin state of transition metal atom with and without adsorbates and confirm the most stable adsorption configurations of *OOH, *O, and *OH. The corresponding binding energies and spin states are listed in Tables S1 and S2. The stability of SACs is evaluated via calculating the formation energy, which is defined as: (a) E f o r m = E T M / g r a − E g r a − E T M − b u l k where E T M / g r a , E g r a , and E T M − b u l k represent the energies of TM and N co-doped graphene, N-doped graphene, and TM atom in the bulk phase.According to this definition, a more negative formation energy is, the more stable of the SAC will be. The calculated formation energies of these SACs ( E f o r m ) are all negative and are shown in Figure S1, indicating that such single atom dispersed structures are thermodynamically stable. The d-band center of the transition metal atom in SAC is defined as: (b) d center = ∫ − ∞ + ∞ ( ε − E F ) n ( x ) d ε ∫ − ∞ + ∞ n ( x ) d ε where n ( x ) and E F are the projected density of states of the d-orbitals of M atom in C40H16N4M and the corresponding fermi level of C40H16N4M, respectively.Model of pyridinic N coordinated Co SAC was also used for calculation and given in Figure S7 for reference.The ORR after 2 e − and 4 e − mechanisms produces H2O and H2O2, respectively. The associative 4 e − reaction is composed of elementary steps (c, d, e, and f): (c) ∗ + O 2 ( g ) + H + + e − → O O H ∗ (d) O O H ∗ + H + + e − → O ∗ + H 2 O ( l ) (e) O ∗ + H + + e − → O H ∗ (f) O H ∗ + H + + e − → H 2 O l + ∗ The ORR of 2 e − mechanism comprises of elementary steps (g and h): (g) ∗ + O 2 ( g ) + H + + e − → O O H ∗ (h) O O H ∗ + H + + e − → H 2 O 2 l + ∗ The asterisk (*) denotes the active site of the catalyst.The free energy for each reaction intermediate is defined as: (i) G = E D F T + E Z P E − T S + E s o l E D F T is the electronic energy calculated by DFT, E Z P E denotes the zero point energy estimated within the harmonic approximation, and T S is the entropy at 298.15 K (T = 298.15 K). The   E Z P E and T S of gas-phase molecules and reaction intermediates are listed in Table S3. For the concerted proton-electron transfer, the free energy of a pair of proton and electron ( H + + e − ) was calculated as a function of applied potential relative to RHE (U versus RHE), i.e., μ ( H + ) + μ ( e − ) = 1 2 μ ( H 2 ) − eU , according to the computational hydrogen electrode (CHE) model proposed by Nørskov. 55 In addition, the solvent effect is reported to play an important role in the ORR. In our calculations, the solvent corrections ( E s o l ) for *OOH and *OH are 0.45 eV in accordance with previous studies. 56 , 57 We used the energies of H2O and H2 molecules calculated by DFT together with experimental formation energy of H2O (4.92 eV) to construct the free energy diagram. The free energies of O2, *OOH, *O, and *OH at a given potential U relative to RHE are defined as: (j) Δ G ( O 2 ) = 4.92 − 4 eU (k) Δ G ( OOH ) = G ( OOH * ) + 3 G ( H 2 ) 2 − G ( * ) − 2 G ( H 2 O ) − 3 eU (l) Δ G ( O ) = G ( O * ) + G ( H 2 ) − G ( * ) − G ( H 2 O ) − 2 eU (m) Δ G ( OH ) = G ( OH * ) + G ( H 2 ) 2 − G ( * ) − G ( H 2 O ) − eU We would like to acknowledge funding support from the National Key R&D Program of China (2016YFA0202804), Singapore Ministry of Education Academic Research Fund (AcRF) Tier 1: RG10/16 and RG111/15, Tier 2: MOE2016-T2-2-004, and the financial support from Jiangsu Specially-Appointed Professor program.J.G., H.Y., and B.L. conceived and designed the project. X.H. carried out the theoretical calculations. S.H. and H.M.C. performed the X-ray absorption experiments. W.C., C.J., S.M., X.Y., and Y.H. contributed to the structure characterizations. J.G. and B.L. prepared the manuscript. All authors contributed and reviewed the manuscript.The authors declare no competing interests.Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.2019.12.008. Document S1. Figures S1–S38, Tables S1–S9, and Supplemental References Document S2. Article plus Supplemental Information

The electrochemical oxygen reduction reaction in acidic media offers an attractive route for direct hydrogen peroxide (H2O2) generation and on-site applications. Unfortunately there is still a lack of cost-effective electrocatalysts with high catalytic performance. Here, we theoretically designed and experimentally demonstrated that a cobalt single-atom catalyst (Co SAC) anchored in nitrogen-doped graphene, with optimized adsorption energy of the *OOH intermediate, exhibited a high H2O2 production rate, which even slightly outperformed the state-of-the-art noble-metal-based electrocatalysts. The kinetic current of H2O2 production over Co SAC could reach 1 mA / cm disk 2 at 0.6 V versus reversible hydrogen electrode in 0.1 M HClO4 with H2O2 faraday efficiency > 90%, and these performance measures could be sustained for 10 h without decay. Further kinetic analysis and operando X-ray absorption study combined with density functional theory (DFT) calculation demonstrated that the nitrogen-coordinated single Co atom was the active site and the reaction was rate-limited by the first electron transfer step.

Lignin is a complex three-dimensional natural polymer that is obtained from lignocellulose along with cellulose and hemicellulose [1–3] and contains the monomer units of p-coumaryl (H units), coniferyl (G units), and sinapyl alcohols (S units) [4–7]. Unlike the formation of aromatic compounds from methane and acetylene [8], the production of phenolics through lignin depolymerization does not involve the complex condensation of small molecules and is relatively straightforward, thus attracting much attention from researchers. In view of the high stability of the lignin structure, the corresponding depolymerization typically requires the use of catalysts and large amounts of suitable solvents at high temperatures/pressures (e.g., supercritical or near-supercritical solvents), which, together with the poor understanding of the related catalytic process, hinder industrial applications [1].Because of the poor solubility of lignin, several solvents including methanol [9,10], ethanol [10–12], 2-propanol [9], methylcylohexane [9], aqueous methanol [13–15], aqueous ethanol [10,11,13,16,17], tetrahydrofuran [10], amines [18], and others [19] were used for its dissolution. Lignin depolymerization is promoted by these organic solvents [9–13,18] and additives [14], and can be enhanced by improving the mobility of solvent-mixed lignin or providing a hydrogen supply through transfer hydrogenation. Although high-dilution conditions favor the formation of lignin oil and lignin-derived monomers, the associated need to handle large solvent quantities complicates the purification process and reduces economic feasibility. Typically, depolymerization is performed at lignin concentrations (typically 0.01–0.10 g/mL in a suitable solvent [9–12,15,17,20–30]) that are too low for industrial applications and may result in large operation costs associated with purification and solvent removal. Thus, the economic feasibility of lignin depolymerization can be enhanced through the use of higher lignin concentrations, which favor the production of phenolic monomers but lead to char formation and poor processability. Lignin depolymerization is promoted by acids [15,17] and certain metals, as exemplified by the reductive depolymerization of lignin catalyzed by noble and transition metals (Ru, Pd, Ni, Co, Re, Mo, and their mixtures) supported on carbons, zeolites, alumina, and other materials [1,20–25]. Among these catalysts, Ru/H-zeolite β (Hβ) was reported to exhibit high lignin depolymerization activity in our previous studies [14,15,17]. In view of the above, a deep understanding of the roles of metals in lignin depolymerization is essential for the development of optimal depolymerization catalysts.The present study probes the effects of the lignin concentration on the outcome of catalytic lignin depolymerization, elucidates the roles of metal and acid sites in the related catalysts, and provides valuable mechanistic insights, thus paving the way to the efficient industrial depolymerization of lignin. Among the solvents suggested in the literature, aqueous methanol was selected for lignin dissolution based on the results of our previous studies [14,15]. Because of the possible methanol enhancement of the depolymerization reaction [31,32], lignin depolymerization by methanol was also performed without heterogeneous catalysts to determine the catalyst contributions to the lignin depolymerization reaction.All chemicals were used as received without further purification unless otherwise noted. Kraft lignin (KL), tetraamminepalladium(II) nitrate ([Pd(NH3)4](NO3)2, anhydrous pyridine (99.8%), acetic anhydride (99%), fumed silica powder (SiO2, 0.007 µm), and titania (TiO2, nanopowder, 21 nm) were purchased from Sigma-Aldrich (St. Louis, Missouri, USA). Oak-extracted organosolv lignin (OL) was purchased from Sugaren (Yongin, Gyeonggi-do, Korea). Sulfuric acid was purchased from J. T. Baker (Phillipsburg, New Jersey, USA). THF for gel permeation chromatography (GPC) was purchased from Honeywell B&J (Morris Plains, New Jersey, USA). Zeolite β (ammonium form, Si/Al2 = 25 mol/mol) was purchased from Zeolyst (Conshohocken, Pennsylvania, USA). Zirconia (ZrO2) and alumina (γ-form, Al2O3) were purchased from Alfa Aesar (Havenhill, Massachusetts, USA). 1-Butanol (99%) was purchased from Junsei (Tokyo, Japan). Hydrogen gas (H2, 99.999%), nitrogen gas (N2, 99.9%), H2/Ar (5% v/v), O2/N2 (0.5% v/v), and CO/He (10% v/v) were purchased from Shinyang Medicine (Anseong, Gyeonggi-do, Korea).The content (wt%) of acid-insoluble lignin was determined using a two-step hydrolysis process as reported elsewhere [33,34]. A dry sample (0.5 g) was treated with 72 wt% aqueous sulfuric acid at 30 °C (water bath) for 1 h upon stirring with a glass rod at 10 min intervals. The mixture was supplemented with deionized water (84 mL; 18.2 MΩ·cm), heated to 121 °C for 2 h, slowly cooled to room temperature, and filtered, and the residue (acid-insoluble lignin) was dried and weighed. All processes were performed using pressure tubes (8648–30 ACE glass) and screw-on Teflon caps with O-ring seals.H-zeolite β (Ηβ) was prepared by calcining the ammonium form of zeolite β at 550 °C for 3 h. The support powders, including Hβ, Al2O3, TiO2, ZrO2, and SiO2, were sieved to obtain particles smaller than 150 µm and impregnated with 1 wt% Pd using a wet method [14,15]. The impregnated catalysts were calcined in air at 400 °C for 1 h, reduced in a flow of H2/Ar (5% v/v) at 400 °C for 1 h, passivated in a flow of O2/N2 (0.5% v/v) at room temperature for 30 min, and stored under ambient conditions before use. For manipulating the metal dispersion, the reduction time and temperature were adjusted as follows: 400 °C, 1 h for Pd/SiO2-A; 400 °C, 3 h for Pd/SiO2-B; 600 °C, 1 h for Pd/SiO2-C; 600 °C, 3 h for Pd/SiO2-D; and 800 °C, 1 h for Pd/SiO2-E.The catalyst acidity was evaluated by the temperature-programmed desorption of ammonia (NH3 TPD) using a BELCAT-B instrument (MicrotracBel, Osaka, Japan) equipped with a thermal conductivity detector (TCD) and interfaced with a BELMass quadrupole mass spectrometer (MicrotracBel, Osaka, Japan). The catalyst powder (approximately 50 mg) was degassed in a flow of He (50 mL/min) at 500 °C for 60 min and passivated in NH3/He (5% v/v) at 100 °C for 30 min. The TCD was stabilized at 100 °C for 60 min in a flow of He (30 mL/min), and the amount of NH3 desorbed upon heating from 100 °C to 700 °C at 5 °C/min in a flow of He (30 mL/min) was recorded using the TCD and the mass spectrometer (MS). CO chemisorption measurements were performed using a BELCAT-M instrument (MicrotracBel, Osaka, Japan) equipped with a TCD. The catalyst powder (50 mg) was placed in a quartz reactor and heated to 400 °C at 10 °C/min in a flow of He (50 mL/min), oxidized in a flow of O2/He (5% v/v, 50 mL/min), and reduced in a flow of H2/Ar (5% v/v, 50 mL/min) at 400 °C for 15 min. He gas was used to remove O2 and H2 before measurements. The treated catalyst powder was cooled to 50 °C in a flow of He (50 mL/min), and pulses of CO/He (10% v/v) were injected to quantify the surface metal sites. The Brunauer–Emmett–Teller (BET) surface areas, pore volumes, and Barrett–Joyner–Halenda (BJH) pore size distributions of the catalysts were measured using N2 physisorption at 77 K (ASAP 2020, Micromeritics, Norcross, Georgia, USA). High-angle annular dark field (HAADF)–scanning transmission electron microscopy (STEM) images were acquired using a transmission electron microscope (Talos F200X, FEI, Hillsboro, Oregon, USA). Elemental compositions of the catalysts were measured by inductively coupled plasma–optical emission spectrometry (ICP–OES, iCAP 6000 Series, Thermo Fisher Scientific, Waltham, Massachusetts, USA). The crystal structures of the catalysts were probed by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer (Billerica, Massachusetts, USA) operated at 40 kV and 40 mA (Cu Kα1 radiation, λ = 1.54059 Å).The catalytic depolymerization of lignin was performed in a 100 mL stainless steel batch autoclave reactor. In a typical process ( Fig. 1), dry lignin (1, 2, 6, or 12 g) was supplemented with aqueous methanol (30 mL, 65% v/v) and catalyst powder (0.2 g). The reactor was purged three times with H2 at room temperature and pressurized to 50 bar. The reaction mixture was heated to 280 °C for 2 h upon agitation at 500 rpm and cooled to room temperature. The liquid product (LP) was collected and analyzed by gas chromatography-mass spectrometry (GC-MS) and gas chromatography with flame ionization detector (GC-FID) using 1-butanol as an internal standard. The method of effective carbon numbers was used to quantify the lignin-derived monomers. The LP was concentrated using a rotary evaporator and further dried at 55 °C in a vacuum furnace for 16 h to obtain the mass of the depolymerized lignin (DL). The product yield (%) was calculated as [mass of product (g)]/[mass of lignin (g)] × 100, and the selectivity for component i (%) was calculated as [mass of component i (g)]/[mass of LP (g)] × 100. The dispersion remaining after LP isolation was centrifuged to collect the spent catalyst and precipitate solid products, which were further dried at 55 °C in a vacuum for 16 h to obtain the solid residue (SR) mass.The phenolic monomers in the LP were identified using a GC-MS instrument (Agilent 7890 A, 5975 C inert MS XLD) equipped with an HP-5MS capillary column (60 m × 0.25 mm × 250 µm) and quantified using a GC-FID instrument (YoungLin 6500) equipped with an HP-5MS capillary column (60 m × 0.25 mm × 250 µm). Prior to injection into the GC instrument, the LP was filtered using a 0.45 µm Whatman syringe filter.The DL was dissolved in a mixture of acetic anhydride and pyridine (10 mL, 1:1 v/v), and the solution was stirred for 24 h under ambient conditions, supplemented with ethanol (20 mL), and further stirred for 30 min. The solvent was removed in vacuo at 60 °C using a rotary evaporator, and the residue was dried in a vacuum oven at 55 °C for 16 h to afford acetylated lignin. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) of the OL, KL, and DL were determined using a GPC device (Agilent 1200 HPLC) equipped with two Shodex LF-804 columns and a UV detector (λ = 270 nm). The flow rate of the THF eluent equaled 1 mL/min, and the column was calibrated using a polystyrene standard ReadyCal set (Sigma-Aldrich, 250–2,000,000 g/mol). Acetylated lignins were dissolved in THF at a concentration of 1 g/L and filtered using a 0.45-μm Whatman syringe filter prior to injection into the GPC system. The utilized lignin acetylation procedure was described in our previous studies [15,17].The FT-IR spectra of OL and its depolymerization products were recorded on a Nicolet iS10 spectrometer (Thermo Scientific, Waltham, MA, USA) equipped with a Smart Miracle accessory in the transmission mode at UNIST Catalysis Research Center (Ulsan, Korea).Prior to depolymerization, the compositions of KL and OL were measured as reported elsewhere [33,34]. The contents of acid-insoluble lignin in KL and OL were determined as 93.9 wt% and 94.2 wt%, respectively ( Table 1). The moisture contained in KL (3.6 wt%) and OL (4.5 wt%) was removed by drying at 105 °C for 2 h before further processing.The reductive depolymerization of lignin was performed over Pd catalysts deposited on several supports, and the effects of the lignin concentration (expressed as the lignin/solvent or lignin/catalyst ratio) on the depolymerization activity were examined. The solvent and catalyst amounts were fixed while the lignin amount was varied. The distillable phenolic compounds in the LP were characterized by GC-MS and GC-FID ( Fig. 2 and Tables S1, S2). OL depolymerization afforded derivatives of G units (selectivity: 27–53%) and S units (selectivity: 43–67%) as well as in small quantities of heavier products detected at higher retention times. In contrast, KL depolymerization produced G units (selectivity: 63–90%), 4-ethylphenol (H units), and heavier products, whereas S units were not detected. The observed differences in product distributions can be attributed to the biomass origin [35]. The lignin extracted from softwood mainly contained G units (90–95%), and the lignin extracted from hardwood contained G (25–50%) and S (50–75%) units. Grass usually contains G, H, and S units. The depolymerization of OL extracted from oak formed both G and S units in this study, confirming that OL was prepared from hardwood oak. Although the origin of KL was not specified by the manufacturer, the depolymerization products of KL (G units) indicate that it was likely obtained from softwood.Compared to the heterogeneous reaction over supported Pd catalysts, the non-catalytic reaction afforded phenolic monomers in smaller yields of 4.8–16.9 wt% (OL) and 4.1–7.1 wt% (KL) (lignin/solvent = 0.033–0.4 g/mL for both lignin reactants), possibly via solvothermal depolymerization [28–30,36]. In the case of OL, a larger yield improvement was observed at low lignin/solvent or lignin/catalyst ratios, which was ascribed to the better solubility of OL (extracted using aqueous alcohol) in aqueous methanol. With increasing lignin concentration, the yield of phenolics decreased for both OL and KL. The effects of lignin concentration on catalytic activity are further discussed in Section 3.5.In the case of OL, the combined selectivities for 4-propylguaiacol and 4-propylsyringol equaled 23–31% at a lignin/solvent ratio of 0.067 g/mL or a lignin/catalyst ratio of 10 w/w (Table S1). Whereas the detailed distributions of phenolic monomers in the LP did not significantly depend on the catalyst type, high selectivities for guaiacol (7–20%) and syringol (7–18%) and low selectivities for 4-propylguaiacol (4–8%) and 4-propylsyringol (5–13%) were observed at all lignin/solvent ratios in the absence of a catalyst (Fig. 2 and Table S1). In addition, under the conditions of high dilution (lignin/solvent = 0.033 g/mL or lignin/catalyst = 5 w/w), the selectivities for 4-propylguaiacol and 4-propylsyringol increased to 10–18% and 26–33%, respectively. In the case of KL, the selectivity for 4-propylguaiacol decreased from 19–29% to 15–18% with increasing lignin/catalyst ratio (Table S2). The selectivity for heavy molecules (retention time > 33 min) reached 21–36% for all catalytic reactions except for the case of Pd/Al2O3 at lignin/catalyst ratios of 10 and 30 w/w.The compositions of the solid- and liquid-phase products were analyzed further. Liquid-phase products correspond to lignin-derived monomers, dimers, oligomers, and the lignin polymer dissolved in aqueous methanol, while the SR products correspond to insoluble lignin polymer. Solvent removal from the LP afforded DL as a dry powder. The yield of DL equaled 53–62% at a lignin/solvent ratio of 0.033 g/mL and decreased to 22–30% when this ratio increased to 0.4 g/mL ( Fig. 3). In the case of KL, the DL yield was lower than that obtained for OL, equaling 39–52% and 16–21% at lignin/solvent ratios of 0.033 and 0.4 g/mL, respectively. The yield of SR increased with increasing lignin concentration (i.e., with increasing lignin/solvent or lignin/catalyst ratio), equaling 58–64% and 69–75% for OL and KL, respectively. In the case of OL, the DL yield obtained under catalyst-free conditions was lower than those obtained for catalytic reactions. The DL yield was highest for Pd/Al2O3, Pd/ Hβ, and Pd/ZrO2 at lignin/solvent ratios of 0.033 and 0.1 g/mL, exceeding that obtained under catalyst-free conditions by 9 wt%. In the case of KL, the DL yield was highest for Pd/SiO2 at a lignin/solvent ratio of 0.067 g/mL, exceeding that obtained under non-catalytic conditions by 15 wt% and those obtained for Pd/Al2O3, Pd/Hβ, Pd/TiO2, and Pd/ZrO2 by 7–19 wt%. Figs. S1, S2, and Table 2 present the GPC results of acetylated DL, revealing that depolymerization decreased the Mw of OL and KL by 67–83 and 75–84%, respectively. The PDI decreased from 3.5 to 1.8–2.7 for OL and from 4.4 to 1.7–2.2 for KL because of the significant loss of high-molecular-weight compounds and the formation of low-molecular-weight compounds (Figs. S1 and S2). Under non-catalytic conditions, the largest Mw decrease was observed at the highest lignin concentration (lignin/solvnet = 0.4 g/mL) for both OL and KL. As the highest SR yield and the lowest DL yield were also observed at the highest lignin concentration (lignin/solvent = 0.4 g/mL), this behavior was attributed to the significant condensation and precipitation of heavier lignin polymers. For OL, the lowest decrease of Mw and the highest PDI (2.5–2.7) were observed at a lignin/catalyst ratio of 10–30 w/w (lignin/solvent = 0.067–0.2 g/mL), which was indicative of non-catalytic re-condensation during depolymerization. For KL, no such significant re-condensation was observed.The Mw values of DLs obtained under catalytic conditions were not significantly different from those obtained in the absence of catalysts through the lignin concentrations except for the highest lignin reactant concentration (lignin/solvent = 0.4 g/mL) for both OL and KL. These observations indicate that the catalysts suppressed the re-polymerization of heavier lignin polymers to increase the yields of DL and phenolic monomers. The smallest Mw values were observed for Pd/TiO2 in the case of OL and for Pd/ZrO2 and Pd/Hβ in the case of KL, although the effects of the catalysts on the results of GPC analysis were not significant.The analysis of OL, KL, and DL by FT-IR spectroscopy revealed the presence of typical lignin functionalities and confirmed the formation of phenolics (Figs. S3 and S4). The results obtained for KL resembled those obtained for the corresponding DL. As DL is composed of phenolic monomers, these observations indicate that the functionalities of lignin polymers were not significantly different from those of the monomeric products in DL. The bands of C–O bonds in syringyl and guaiacyl rings at 1330 and 1270 cm−1, respectively, were observed only for OL and the corresponding DL. Additionally, C–H (3000–2800 cm−1), CO (1700 cm−1), CC (1600 cm−1), aromatic C–C (1500 cm−1), C–H (1420–1440 cm−1), and C–C (1200 cm−1) stretches were observed for all lignin reactants and products in this study. The C–O peaks of aliphatic ethers at 1150 and 1110 cm−1 were more pronounced for OL than for KL.The surface areas of Pd particles and catalyst pore structures were probed by N2 physisorption, CO chemisorption, and NH3 TPD measurements ( Table 3, Figs. S5–S7). Pd dispersion, expressed as [CO]/[Pd] (mol/mol), decreased in the order of Pd/SiO2 > Pd/ZrO2 ≥ Pd/Al2O3 > Pd/TiO2 > Pd/Hβ, while the quantity of surface acid sites per catalyst mass decreased in the order of Pd/Hβ > Pd/ZrO2 > Pd/TiO2 > Pd/Al2O3 > Pd/SiO2. Notably, the latter parameter was highest for Pd/Hβ, which contained less dispersed Pd, and was lowest for Pd/SiO2, which contained highly dispersed Pd. Interestingly, for a low concentration of OL (0.033 g/mL), which did not contain the catalyst-poisoning sulfur, the smallest monomer yield of 15.8 wt% (Fig. 1 and Table S1) was observed in the case of the Pd/Hβ catalyst with abundant acid sites. This yield was smaller than that obtained under non-catalytic conditions. In the case of OL, high monomer yields were obtained for the Pd/Al2O3, Pd/SiO2, and Pd/TiO2 catalysts with low amounts of acid sites. These observations indicate that the lignin depolymerization activity is determined not only by the quantity of acid sites but also by the properties of the surface metal sites [15,17].The XRD results of all catalysts exhibited no Pd peaks because of the low metal loading (Fig. S8). Moreover, Pd deposition did not markedly affect the XRD results of supports, i.e., the support structure did not change during catalyst preparation. The result of Pd/Al2O3 featured the peaks of boehmite (AlO(OH)) and γ-Al2O3, while that of Pd/SiO2 featured a broad peak of amorphous silica at 2θ = 22°. Although the catalysts were annealed at 400 °C, the formation of crystalline silica (e.g., quartz) was not observed. The result of Pd/Hβ exhibited the broad peaks of zeolite β, indicating the presence of zeolite nanoparticles, while the result of Pd/TiO2 revealed the presence of both rutile and anatase phases. Finally, the result of Pd/ZrO2 featured the broad peaks of monoclinic ZrO2, thus indicating the presence of ZrO2 nanocrystals.The influence of the lignin concentration on the depolymerization activity was probed to shed light on the roles of acid and metal sites. The yield of phenolic monomers decreased with increasing lignin concentration for both OL and KL ( Fig. 4(a, b) and Table 4; based on Fig. 2). Under dilute conditions (lignin/solvent = 0.033 g/mL), higher yields were observed for OL than for KL, whereas no marked difference between OL and KL was observed at high lignin concentrations (lignin/solvent = 0.067–0.4 g/mL; yield = 4.8–12.6% (OL) and 5.6–13.4% (KL)). Notably, higher yields of phenolic monomers were observed in the presence of catalysts than in their absence. At high lignin concentrations, the largest increase in the phenolic monomer yield for both OL and KL was observed in the case of Pd/SiO2, while Pd/Al2O3 also achieved the highest yield of phenolic monomers for OL. Lignin concentrations above 0.4 g/mL were not investigated, as the agitation of the corresponding slurries was difficult.The results of depolymerization were described in terms of the phenolic monomer amount produced per catalyst weight (Fig. 4(c, d)), which increased with increasing lignin concentration up to 0.4 g/mL. These observations indicate that high lignin concentrations (up to 0.4 g/mL) did not significantly deactive the reaction on the catalyst surface, although the pore structures and BET surface areas of the catalysts significantly differred depending on the support type (Table 3). The results also indicated that the production of lignin-derived phenolic monomers can be enhanced by increasing the lignin concentration to up to 0.4 g/mL. Notably, reactions at lignin concentrations above 0.4 g/mL were difficult to perform because of the insufficient wetting of lignin by the solvent.The quantity of phenolic monomers produced per acid site was positively correlated with [CO]/[Pd] (Fig. 4(e)), which indicated that high amounts of Pd atoms on the surface facilitate depolymerization, possibly by favoring hydrogen adsorption. At lower [CO]/[Pd] values, the amount of phenolic monomers produced per surface Pd atom significantly decreased, which indicated that lignin depolymerization can be significantly suppressed by the use of lower [CO]/[Pd] values, lower amounts of surface Pd atoms, or acid sites without Pd atoms. These observations indicate that Pd is required for the catalytic depolymerization of lignin, although acid sites are also required, as demonstrated in our previous studies [15,17].The roles of metals were further studied using the correlation between the weight of the phenolic monomers produced per surface Pd site and acid site quantity (Fig. 4(f)). This correlation was positive, which indicated that acid sites facilitate lignin depolymerization. Interestingly, a certain catalytic activity per Pd atom was observed when the quantity of acid sites approached zero, which indicated that Pd atoms could depolymerize lignin even in the absence of acid sites.Based on the above observations, we further probed the effects of Pd on catalytic depolymerization. In view of the fact that acid-induced depolymerization was observed in our previous studies [15,17], Pd/SiO2 without significant acidity was selected to hinder this depolymerization. The properties of the Pd metal dispersions ([CO]/[Pd]) of Pd/SiO2-A, B, C, D, E are described in Table 5. While the BET surface areas and BJH pore size distributions of these species were not significantly different (Figs. S9 and S10), their [CO]/[Pd] values varied between 0.129 and 0.602 mol/mol. HAADF-STEM imaging confirmed the results of particle size measurements obtained using CO chemisorption ( Fig. 5).In the case of OL depolymerization promoted by Pd/SiO2, higher yields of phenolic monomers or larger quantities of phenolic monomers per surface Pd were observed at lower [CO]/[Pd] values ( Fig. 6), with the largest yield of 16.4% observed at the lowest [CO]/[Pd] value of 0.129 mol/mol. Notably, catalysts containing well-dispersed Pd ([CO]/[Pd] = 0.602 mol/mol) achieved monomer yields of only 5–6 wt%, which were not significantly different from those obtained under non-catalytic conditions and slightly exceeded that obtained using a metal-free SiO2 support (3.4 wt%, data not shown).The analysis of the quantity of produced phenolic monomers per surface Pd also demonstrated that the depolymerization activity increased with decreasing [CO]/[Pd]. Notably, a 14-fold activity difference was observed between catalysts with the highest and lowest [CO]/[Pd] values: Pd/SiO2 with [CO]/[Pd] = 0.602 mol/mol achieved a yield of 94 g monomer/g surface Pd, while Pd/SiO2 with [CO]/[Pd] = 0.129 mol/mol achieved a yield of 1324 g monomer/g surface Pd.The above observations indicated that catalytic depolymerization on the surface of bulk Pd occurs more easily than on the surface of small Pd particles ( Fig. 7), which was ascribed to the better decomposition of multidentate lignin that securely adsorbed on the multiple sites of the bulk Pd surface. The results of GPC analysis indicated that Pd/SiO2-E, exhibiting the lowest [CO]/[Pd], produced DL with the smallest Mw and thus exhibited the highest depolymerization activity (Fig. S11 and Table S3).Herein, we examined the catalytic depolymerization of concentrated lignin and elucidated the roles of Pd metal in this process. Although acid sites have previously been reported to promote lignin depolymerization, efficient lignin depolymerization required the presence of Pd, as indicated by the correlation between the lignin depolymerization performance and reaction conditions. The Pd atoms exhibited a certain depolymerization activity even at a negligible content of acid sites, while depolymerization on acid sites required a certain content of surface Pd atoms. Regarding the effects of Pd dispersion, bulk Pd atoms featured a 14 times higher catalytic activity than highly dispersed ones, which indicated that efficient depolymerization required the multidentate adsorption of the bulky lignin polymer on the Pd surface and suggested the superiority of bulk Pd atoms as catalysts. Thus, the presented results pave the way to the efficient utilization of lignin for the production of phenolic monomers. Aliaksandr Karnitski: Investigation, Writing – original draft. Jae-Wook Choi: Methodology, Visualization. Dong Jin Suh: Conceptualization. Chun-Jae Yoo: Formal analysis. Hyunjoo Lee: Supervision, Validation. Kwang Ho Kim: Validation. Chang Soo Kim: Formal analysis. Kyeongsu Kim: Validation. Jeong-Myeong Ha: Writing – review & editing, Conceptualization, Methodology.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was supported by the Institutional Program of Korea Institute of Science and Technology (KIST) of Republic of Korea (2E31853) and the Technology Development Program to Solve Climate Change of the National Research Foundation (NRF) funded by the Ministry of Science and ICT of Republic of Korea (NRF-2020M1A2A2079798).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.cattod.2022.07.012. Supplementary material .

Lignin is a natural polymer contained in lignocellulose and is a potential feedstock for the production of phenolics or small aromatic molecules. However, lignin depolymerization is typically performed using low lignin concentrations and insufficiently active catalysts, which hinders industrial applications. To address this issue, we herein investigated the depolymerization of concentrated kraft lignin and oak-extracted organosolv lignin promoted by supported bifunctional metal (palladium)-acid catalysts and elucidated the roles of acid and metal sites. In addition to acid sites, which are known to be critical for lignin degradation, metal sites were found to be required to initiate lignin depolymerization. Palladium atoms promoted depolymerization even in the absence of acid sites, while acid site–promoted depolymerization required the presence of surface palladium atoms. Catalysts with bulk palladium particles were much more active than those with highly dispersed palladium particles, which suggested that efficient depolymerization required the bulky lignin polymer to be adsorbed on the palladium surface in a multidentate fashion. Thus, our work provides valuable insights into the mechanism of catalytic lignin depolymerization and paves the way to the industrial-scale application of this process.

Alternatives to traditional oil-derived compounds, like biorenewable compounds, are necessary due to their environmental benefits and due to limitations on fossil fuel resources [1,2]. Several processes have been developed for biodiesel production because it is a renewable, biodegradable, eco-friendly, and nontoxic fuel [3]. Biodiesel consists of fatty acid methyl esters (FAMEs) derived from the transesterification of vegetable oils or animal fats [2], but biodiesel has some problems, such as a lack of stability, limited use under cold conditions, and a slight increase in NOx emissions [4]. Green fuels, resulting from the hydroprocessing of carboxylic esters or triglycerides, are promising alternatives to conventional fuels [5]. The conversion process is called the hydrotreatment of vegetable oils of fats (HVO). Green fuel can also be produced through Fischer-Tropsch synthesis, and the process is known as gas to liquids (GTL). All these processes produce linear hydrocarbons whose properties have to be improved (particularly, their high cloud points need to be reduced) for them to be valid as hydrocarbon-based fuel [6]. For this reason, the hydroisomerization of linear alkanes has been previously studied. Thus, during hydroisomerization, the properties of the feedstock are improved by transforming normal hydrocarbons to branched hydrocarbons containing an identical number of carbon atoms [7].The isomerization process, known as hydroisomerization, usually takes place in the presence of hydrogen, and it transforms a molecule into an isomer with a different chemical structure. The hydroisomerization of hydrocarbons is usually divided into different fractions, such as C4-7, C7-15, and more than C15 [8]. Long-chain hydrocarbons have attracted much attention recently because of their increased availability. These long-chain hydrocarbons or n-paraffins (>C15) comprise up to 80% of the material known as wax [7,8]. The hydroisomerization reaction is always accompanied by a hydrocracking reaction that lowers yield. First, isomerization occurs, and then cracking, which is an undesirable and competitive process that favors multibranched alkanes.Bifunctional catalysts containing metallic sites, including Pt, Pd, or Ni, for hydrogenation/dehydrogenation, and acidic sites for skeletal isomerization via carbenium ions are usually involved in isomerization reactions [8]. First, a linear olefin is formed by the dehydrogenation of an n-alkane; then, this compound is isomerized and finally hydrogenated, giving the isoalkane. Metallic sites also activate and dissociate H2, which plays an important role in decreasing cracking.Several studies are published in the literature about hydroisomerization reaction of alkanes using bifunctional catalysts, specially supported on zirconia. It has been established that the selectivity of paraffin isomerization depends primarily on a proper balance between metal and acid functions. WO3 and Pt are commonly used as acidic function and metallic site respectively. Although numerous published studies have reported the physicochemical properties and activity of WO3/ZrO2 catalysts, the catalytic activity of Pt/WOx/ZrO2 and Pt/WOx/Al2O3 has not been carefully studied for the n-dodecane hydroisomerization reaction. No publications comparing two different preparation methods, two different supports and different W loading at the same time were found for this reaction. This work provides insights contributing to the knowledge of catalyst preparation in the sense that no other articles were found carring out this procedure. The reason why the several consecutive impregnation method was carried out is because acidity plays a main role in the hydroisomerization reaction. Acidity favors the stabilization of carbocationic reaction intermediates and thus, the reaction. Since in our catalysts, we introduce the acidity through the incorporation of tungsten oxide, it could make important differences in the catalysts the different way of introducing tungsten oxide on the supports. Once this phase is introduced, it remains as tungsten oxide on the catalysts which would increase or decrease the acidity depending on the WOx particle size and interaction with the supports surface. This produces significantly changes in the conversion results.Considering the results published in the literature for long chain paraffin hydroisomerization and our previous study on the influence of the reduction temperature and nature of the support on hydroisomerization [6], this work aims to study the influence of W loading, preparation methothithi, and nature of the support during the hydroisomerization of n-dodecane in order to improve the quality of linear alkane fuels obtained by HVO or Fischer-Tropsch synthesis, producing a suitable fuel.Thus, in this study, the hydroisomerization of n-dodecane is mainly discussed with respect to the use of bifunctional catalysts with different W loadings (3–18 wt% W). Pt supported (0.3 wt% Pt) on W-modified alumina or zirconia were used as the catalysts. Correlations between activity and characterization results were studied to establish the properties that predict the best W loading, support type, and preparation method, contributing to the progress in this field.The following salts were used as W and Pt precursors: ammonium metatungstate hydrate ((NH4)6(H2W12O40)·xH2O) (99%) was purchased from Honeywell, and tetraammineplatinum (II) hydroxide hydrate (Pt 58%) (H14N4O2Pt·xH2O) was purchased from Alfa Aesar (Thermo Fisher Scientific, Waltham, USA). Concerning the supports, alumina (γ-Al2O3) was purchased from Saint Gobain-NORPRO (Stow, USA) (1.1 × 2 mm trilobes, SA6975), and the zirconium oxide (2x5 mm pellets, Lot Number X28A052) was purchased from Alfa Aesar (Thermo Fisher Scientific, Waltham, USA).The catalysts were prepared by sequentially impregnating (sequential wetness impregnation method) W and Pt on the supports. Before any impregnation, all supports were dried overnight at 120 °C to remove excess moisture. The catalysts were prepared by the following two procedures: 1) W loading (containing 3, 6, 9, 12, 15, and 18 wt% W) was incorporated in one step. W loading (containing 3, 6, 9, 12, 15, and 18 wt% W) was incorporated in one step.The incorporation of W into ZrO2 and γ-Al2O3 pellets by wet impregnation was achieved following this procedure: the dried pellets were placed in a round flask in contact with an aqueous solution (10 mL for 6 g of ZrO2 and 20 mL for 6 g of Al2O3) of ammonium metatungstate hydrate ((NH4)6(H2W12O40)·xH2O) under stirring for 1 h in a rotary evaporator. Then, the solvent was evaporated under vacuum at 65 °C in a rotary evaporator for 20 min, and finally, the recovered solid was calcined under static conditions at 500 °C for 2 h.The incorporation of Pt by wet impregnation was conducted using this protocol: dried pellets with the corresponding W loading were placed in a round flask in contact with an aqueous solution (10 mL for 6 g ZrO2 and 20 mL for 6 g Al2O3) of tetraammineplatinum (II) hydroxide hydrate (H14N4O2Pt·xH2O) under stirring for 1 h in a rotary evaporator. Then, the solvent was evaporated under vacuum in a rotary evaporator at 65 °C for 20 min, and finally, the recovered solid was calcined in air at 450 °C for 3 h [9]. 2) W loading (containing 6, 9, 12, and 15 wt% W) was incorporated in several consecutive steps. At every step, 3 wt% W was incorporated until reaching the desired loading following the same overall procedure previously described for W and Pt incorporation in a single step. W loading (containing 6, 9, 12, and 15 wt% W) was incorporated in several consecutive steps. At every step, 3 wt% W was incorporated until reaching the desired loading following the same overall procedure previously described for W and Pt incorporation in a single step.Following these procedures, the catalysts (Pt/WO3/ZrO2 or Pt/WO3/Al2O3) were prepared from their corresponding supports (ZrO2 or Al2O3) and denoted as PtW/Zr-[wt.%] or PtW/Al-[wt.%] where [wt.%] indicates the wt.% of W (mainly as WO3 oxide) on the catalysts. Catalysts prepared by successive impregnations are denoted with SI acronyms at the end. The Pt loading was 0.3 wt% for all the catalysts prepared using a single step or several consecutive steps. The catalysts activation or reduction process (350 °C in hydrogen at atmospheric pressure) was performed previously to the reaction in the reaction system.Textural properties of the catalysts were determined from the adsorption–desorption isotherms of nitrogen, recorded at −196 °C with a Micromeritics ASAP 2420 (Norcross, USA). The specific area was calculated by applying the BET method in the relative pressure (P/P0) range of the isotherms between 0.03 and 0.3 and taking a value of 0.162 nm2 for the cross-section of an adsorbed nitrogen molecule at −196 °C. Pore size distributions were computed by applying the BJH model to the desorption curve of the nitrogen isotherms.X-ray diffraction profiles of the samples were recorded with an X’Pert Pro PANalytical diffractometer (Almelo, Netherlands) equipped with a CuKα radiation source (λ = 0.15418 nm) and an X’Celerator detector based on RTMS (real-time multiple strip). The samples were ground and placed on a stainless-steel plate. The diffraction patterns were recorded in steps over a range of Bragg angles (2θ) between 4 and 90° at a scanning rate of 0.04° per step and an accumulation time of 20 s. Diffractograms were analyzed with a X’Pert HighScore Plus software. The mean domain size was then estimated from X-ray linewidth broadening using the Scherrer equation. Width (t) was taken as the full width at half maximum intensity of the most intense and least overlapped peak.Raman spectra of the samples were recorded in air under ambient conditions with a Renishaw in Via Raman Microscope spectrometer (Gloucester, United Kingdom) equipped with a single monochromator, a laser beam emitting 785 nm and 300 mW output power, a thermoelectrically cooled CCD detector and a holographic super-Notch filter. The photons scattered by the sample were dispersed by a 1200 lines/mm grating monochromator and simultaneously collected on a CCD camera. The collection optic was set at 20x and 50x objectives. These measurement conditions gave a Raman shift within an accuracy lower than 0.1/cm.Metal dispersions were determined by CO pulse chemisorption. CO uptake was measured using a Micromeritics Autochem II 2920 apparatus (Norcross, USA). A 100–200 mg portion of the reduced and passivated sample was loaded in the reactor and reduced under a H2 flow (50 mL·min−1) at 250 °C for 1 h (ramp, 10 °C ·min−1). Afterward, the sample was cooled to 40 °C while it was flushed with a He flow (50 mL·min−1). When the TCD signal was stable, pulses of CO (75 μL) were passed through the samples until the areas of consecutive pulses were constant. The total CO uptake was then calculated. D i s p e r s i o n % = C O u p t a k e mmol g M e t a l l o a d i n g mmol g The acidity of the catalysts was measured by the temperature-programmed desorption of ammonia (NH3-TPD). NH3-TPD was carried out using a Micromeritics Autochem II 2920 apparatus (Norcross, USA). A 110–170 mg sample was reduced with a H2 flow (50 mL·min−1) at 350 °C and then cooled to room temperature. Next, an NH3 (5%)/He flow (15 mL·min−1) was passed through the sample for 30 min at 100 °C. In the next step, at 100 °C, the sample was swept with a He flow (25 mL·min−1) for 30 min to remove the physically adsorbed NH3. Afterward, NH3-TPD was performed in a He flow (25 mL·min-1) at a heating rate of 15 °C·min−1 from 100 to 700 °C. The desorbed NH3 was detected by a TCD.Surface acidity (Brønsted and Lewis acid sites) was characterized by in situ FTIR spectroscopy with chemisorbed pyridine in the diffuse reflectance infrared Fourier transform (DRIFT) mode. The spectra were collected with a Nicolet 5700 spectrometer (Waltham, USA) equipped with a Hg–Cd–Te cryodetector of high sensitivity, working in the spectral range of 4000–650 cm−1. A Praying Mantis apparatus (Harrick Scientific Co, Pleasantville, United States) was used as the optical mirror accessory. The samples were placed in a reaction camera equipped with a temperature controller that allows in situ thermal treatments (Harrick Scientific Products, NY). Pyridine (Py) was used as a probe molecule to evaluate surface acidity. The samples were heated from room temperature to 350 °C at a heating rate of 10 °C ·min−1 under a flow of H2 (10 mLN·min−1) and Ar (50 mLN·min−1). The samples were kept at these temperatures for 1 h to reduce them and clean the surface, and then an infrared spectrum of the solid was recorded. After that, the temperature was cooled to 120 °C, the H2 flow was turned off, and Ar was bubbled through the liquid Py for a sufficient time to saturate the sample (10 min). Subsequently, Ar flow was switched on (bypassing the bubbler) at 120 °C for 1 h to remove the physically adsorbed Py. The DRIFT spectra of chemisorbed molecules over the surface sites were then recorded. In all cases, the spectra were recorded with 128 scan accumulations and a resolution of 4 cm−1. The net infrared spectra were obtained after subtraction of the background spectrum of the solid.XPS measurements were recorded using a SPECS GmbH electron spectroscopy system (Berlin, Germany) with a UHV system (pressure approx. 10-10 mbar) with a PHOIBOS 150 9MCD energy analyzer, a multichannel detector (9 channels), and a monochromatic X-ray source (with double anode Mg). The area of the peaks was estimated by calculating the integral of each peak after smoothing and subtracting the S-shaped background and fitting the experimental curve to a mixture of Gaussian and Lorentzian lines of variable proportions. Binding energies (BEs) were referenced to the Zr 3d signal at 182.2 eV for the PtW/Zr-3–18 catalysts and to the Al 2p signal at 74.5 eV for the PtW/Al-3–18 catalysts [6] to correct for charging effects. Quantification of the atomic fractions on the sample surface was obtained by integration of the peaks with appropriate corrections for sensitivity factors [10].The desired metal loadings were confirmed by elemental analysis via inductively coupled plasma optical emission spectrophotometry (ICP–OES) and X-ray Fluorescence (TXRF). Qualitative and quantitative TXRF analyses were performed with a benchtop S2 PicoFox TXRF spectrometer from Bruker (Billerica, USA) equipped with a Mo X-ray source working at 50 kV and 600 μA, a multilayer monochromator with 80% reflectivity at 17.5 keV (Mo Kα), a Bruker XFlash SDD detector (Billerica, USA) with an effective area of 30 mm2 and an energy resolution better than 150 eV for 5.9 keV (Mn Kα), and the methodology for the experimental procedure, called DSA-TXRF (Direct Solid Analysis), can be found in the following paper [11] (p.79) and was developed in the SIDI (Servicio Interdepartamental de Investigación) of the UAM (Universidad Autónoma de Madrid).ICP–OES was performed as follows. First, digestion of the solids was carried out using a Multiwave 3000 model Anton Paar equipment (Graz, Austria), a high-pressure microwave. This digestion consisted of wet mineralization using an acid solution. Decomposition was carried out in closed Teflon containers to avoid losing the volatile components. In all cases, 20 mg of the samples were introduced to the containers containing a mixture of acids, 6HNO3:3HF (ml). The microwave operated at 800 W and 60 bar. Once the sample was dissolved, it was measured by argon nebulization using the inductively coupled plasma optical emission spectrophotometer PQ9000 of Analytik Jena (Jena, Germany).The catalysts were tested for the hydroisomerization of n-dodecane. The reactor operated in a trickle-bed mode in parallel flow and at high pressure, ensuring that the three phases, gas–liquid-solid, were in close contact. The calcined catalysts in the pellets (1 g) were diluted in 8 g of the inert alumina or zirconia supports, placed in the reactor, and reduced at 350 °C and atmospheric pressure. Then, the reduction temperature (350 °C) was maintained, and the reactor was pressurized. The reaction conditions were: Tr = 350 °C, P = 2.0 MPa (20 bar), liquid flow = 0.1 mL·min−1 and H2 flow = 340 mLN·min−1. The gaseous-phase products were analyzed by an online Inficon 3000 micro-GC (Bad Ragaz, Switzerland) equipped with 4 channels, two 5A molecular sieves, a Poraplot Q, and a Stawilwax. The liquid products were collected and analyzed offline by gas chromatography with an Agilent (Palo Alto, USA) 6850A GC and an FID detector.Under the reaction conditions employed and the trickle-bed operation regimen, the external transport limitations are excluded. In consequence the inter-particle heat and mass transport limitations are minimized. The reaction conditions employed in the experiments reach the laboratory reactor compliance criteria for a trickle-bed reactor described in the bibliography [12]. The reactor is filled with glass beads on the catalysts bed assure a good liquid dispersion in the catalyst bed. L/dp is 200 clearly higher than the required criterium of 100. D/dp is 25 higher than the required criterium of 10. Based on these figures, the reaction conditions used clearly reach the compliance of a laboratory scale trickle-bed reactor.The textural properties of the supports and catalysts were determined by N2 adsorption–desorption. The BET surface area, mean pore volume and mean pore diameter of the alumina and zirconia supports and catalysts are reported in Table 1 . In general, when the active phase is impregnated onto the support, the surface areas decrease due to the incorporation of tungsten oxide, which partially covers the support surface, partly blocking the support pores and reducing nitrogen accessibility [13]. This decrease is more evident in the alumina catalysts. Notably, the alumina catalysts show a significantly higher area (196–223 m2/g) than the zirconia catalysts (73–90 m2/g). Alumina support has a high surface area which makes that as the same % W loading is incorporated in the two different procedures, the support surface coverage is similar and BET areas do not significantly differ.To characterize the porosity of these samples, N2 adsorption–desorption isotherms of the supports and catalysts were obtained. All the isotherms belong to type IV (a) according to the IUPAC 2015 classification or type IV according to the BDDT (Brunauer, Deming, Deming, and Teller) classification, given by mesoporous materials [14,15]. According to the IUPAC system, the hysteresis loops obtained in the isotherms (Fig. 1S) can be classified as the H3 type for PtW/Zr and PtW/Al catalysts. In addition, pore types can be deduced from the isotherm hysteresis loop. The zirconia and alumina isotherm hysteresis loops are close to slit-shaped pores (H3 hysteresis type) [15–17]. Comparing the support and catalyst isotherms in all cases, the isotherms are visibly very similar, which indicates a high dispersion of the tungsten and platinum active phases on the supports. All the supports and catalysts show pore size distributions (Fig. 2S) in the mesoporous range from 2 to 50 nm.The crystalline phases present in the samples were studied by X-ray diffraction. In general, the X-ray diffractograms of the catalysts do not show different lines compared with the supports. As an example, X-ray patterns obtained for PtW/Zr-15, PtW/Zr-15-SI, PtW/Al-15, PtW/Al-15-SI, and their supports are shown in Fig. 1 .For the PtW/Zr-15 catalyst (PDF card 00–003-0515), the most intense diffraction line is located at 28.1, associated with the (111) plane of monoclinic ZrO2, and for the PtW/Zr-15-SI (PDF card 00–013-0307) catalyst, the most intense diffraction line is located at 28.3°, associated with the (−111) plane of monoclinic ZrO2 (Fig. 1A). The diffraction lines at 2θ = 17.5, 24.0, 28.1, 34.1, 34.3, 35.0, 38.6, 40.6, 44.8, 49.2, 50.4, 54.2, 55.4, 60.1, 61.5, 62.7, 65.3, 71.2, and 75.2° were attributed to the monoclinic phase of the ZrO2 support. Some minor peaks associated with the tetragonal phase of ZrO2 (PDF card 00-024-1164) are also present. Most WO3 peaks are overlapped with monoclinic ZrO2 peaks suggesting either a homogeneous dispersion of WO3 on the support surface [18] or very small crystallite sizes for the WO3 [15]. The most intense diffraction line for WO3 is the one at 28.1° for PtW/Zr-15 and PtW/Zr-15–SI corresponding to the (200) plane of WO3, but they overlap with the diffraction lines of the support. The absence of platinum diffraction peaks also revealed the high dispersion of this metal on the catalyst, which was expected due to the low Pt loading.For the PtW/Al-15 catalyst (Fig. 1B), the XRD profiles present three main peaks at 36.9, 47.6, and 67.4°, corresponding to the (111), (006), and (215) planes of γ-Al2O3, respectively (PDF card 00-035-0121). For the alumina support and the PtW/Al-15-SI catalyst (Fig. 1B), the XRD spectra present three main peaks at 37.1, 46.0, and 66.8°, corresponding to the (110), (111) and (211) planes of γ-Al2O3, respectively (PDF card 00-001-1303). In both catalysts, few obvious diffraction peaks were found for WO3 (PDF card 00-033-1387) or W3O8 (PDF card 01-081-2262), indicating a high dispersion of tungsten oxide in the support. The lower intensity of the diffraction lines for the catalysts is explained by the lower quantity of sample placed in the sample holder.The Scherrer equation was applied to the most intense and nonoverlapping diffraction lines to determine the average crystalline domains. The average crystalline domain sizes of the corresponding zirconia crystallites are similar for each support and catalyst, showing sizes of approximately 10 nm. In the case of alumina and alumina-supported catalysts, these properties have not been calculated since the alumina phase (gamma) is poorly crystallized (pseudocrystalline state) and pseudocrystals form its structure.The different tungsten oxide structures were studied by Raman spectroscopy (Fig. 2 ). Fig. 2A shows the Raman spectra of the ZrO2 support and the PtW/Zr-3 to 18 catalyst series with profiles corresponding to the monoclinic zirconia phase. In the PtW/Zr catalysts, these bands appear at approximately 180 (with two components), 330, 380, 476, 547 (with two components more visible for the PtW/Zr-18 catalyst) and 635 cm−1 [19,20].All catalysts except PtW/Zr-3 show bands at ∼800 and ∼700 cm−1, which are characteristic of the stretching and bending vibrations of W-O-W, respectively. These bands are also characteristic of crystalline WO3 [13], but no evidence for WO3 crystals is shown in the XRD diagrams. The band at approximately 270 cm−1 is assigned to the W-O-W deformation mode [19]. In addition, a broad band with two components at 955 and 995 cm−1 is attributed to the asymmetric and symmetric vibrations of W = O, respectively, corresponding to highly dispersed WOx species [18], in agreement with XRD results. It can be deduced from the spectrum that a 3% W loading is not sufficient for the formation of bands due to W oxide, but these bands are formed at 6 wt% W loading and above. Fig. 2B shows the Raman spectra of the ZrO2 support and the PtW/Zr-6-SI to 15-SI catalysts with a profile corresponding to the monoclinic zirconia phase. In the PtW/Zr-SI catalysts, bands appear at approximately 180 (with two components), 335, 382, 476, and 547 cm−1. The band at 630 cm−1 of the monoclinic zirconia phase is not clearly visible in the PtW/Zr-12-SI and PtW/Zr-15-SI catalysts.PtW/Zr-6-SI only shows a weak and broad band at approximately 1000 cm−1, assigned to symmetric W = O stretching, which is characteristic of tetrahedrally coordinated surface tungsten oxide species [20]. In the PtW/Zr-9-SI catalyst, the band at approximately 1000 cm−1 is more intense, and a broad band at 800 cm−1 due to the stretching vibration of W-O-W appears. For PtW/Zr-12-SI and PtW/Zr-15-SI, W-O-W bands appear at higher W loadings. These catalysts show bands at approximately 800 and 700 cm−1 due to the stretching and bending vibrations of W-O-W, respectively, at approximately 260 cm−1 due to the W-O-W deformation mode, and a band at approximately 1000 cm−1 due to the symmetric vibrations of W = O [20,21]. These bands are more intense in the PtW/Zr-15-SI catalyst. This means that below monolayer coverage, such as in PtW/Al-6-SI, only isolated tungsten oxide species exist on ZrO2 due to the unique interactions between WO3 and ZrO2. In PtW/Al-9–15-SI, the W = O vibrations are due to polytungstate, meaning two-dimensional oxotungsten species interact with the support [20,21]. Fig. 2C shows the Raman spectra of the γ-Al2O3 support and the PtW/Al-3 to 18 catalysts. The known γ-Al2O3 support has no Raman active modes [21], and the PtW/Al-3 to 18 catalysts only exhibit one broad band at approximately 1000 cm−1 (varying only from 1001 to 1005 cm−1) [22,23], indicating the pseudoamorphous alumina state, in agreement with the XRD profiles obtained for this support and derived catalysts. The band at 1000 cm−1 can be attributed to the W-O stretching mode of the Al2(WO4)3 phase, but it is not very well defined. Considering monolayer coverages of tungsten oxide on alumina-supported catalysts with less than 25–30 wt% WO3, it suggests that this oxide is in a highly dispersed and amorphous state on the alumina surface at low calcination temperatures (500–800 °C) [23]. Fig. 2C reveals that very similar Raman spectra are obtained by varying the tungsten oxide loading in single-step prepared catalysts. Fig. 2D shows the Raman spectra of the γ-Al2O3 support and the PtW/Al-6-SI to 15-SI catalysts. The Raman band at approximately 1000 cm−1, assigned to the symmetric stretching vibration mode of W = O bonding, tends to increase in intensity with W loading. This weak and broad band centered at 989 cm−1 in PtW/Al-6-SI is due to the stretching mode of mono-oxo W = O, corresponding to the highly dispersed WOx species. This band is shifted gradually toward higher wavenumbers, from 989 to 1010 cm−1, in PtW/Al-SI. This shift might be explained by the interaction between platinum and surface tungsten oxides, which affects the distortion of the oxo-tungsten species and transfers electrons from metallic Pt to WOx at the Pt-WOx interface, making the W = O bond stronger [18,21]. The shift in W = O also suggests that, under ambient conditions, different two-dimensional tungsten oxide species may be present in the PtW/Al-SI catalysts: tetrahedrally coordinated tungsten oxide species at low loading (6 wt%) and octahedrally coordinated tungsten oxide species at moderate and high loadings (12 and 15 wt%) [20,21].The dispersion of supported Pt particles in the catalysts with 15% W loading, determined by CO pulse chemisorption, is reported in Table 2 . Catalysts were previously reduced at 250 °C to avoid the partial reduction of tungsten oxide that could result at a temperature of 300 °C and higher. In the zirconia catalysts, WO3 is reduced at approximately 400 °C, and the dispersion could not be measured at the same reduction temperature used in the reaction (350 °C) because the Pt dispersion values would be overestimated since the CO probe molecule can also adsorb to reduced WOx [24,25].The obtained results indicate that the highest dispersions are achieved in zirconia-based catalysts. Zirconia catalysts show a higher dispersion than alumina catalysts because zirconia due to its negative charge on the surface favours tetraammineplatinum (II) cation (Pt(NH4)4 2+) dispersion whereas alumina due to its positive charge on the surface does not favour (Pt(NH4)4 2+) cation dispersion. Comparing catalysts prepared using a single step method to catalysts prepared using several consecutive steps the platinum dispersion is higher in the catalysts prepared using a single step. The dispersion measurements follow the order PtW/Zr-15 > PtW/Zr-15-SI > PtW/Al-15 > PtW/Al-15-SI.The acid properties of the catalysts were investigated by NH3-TPD. This technique measures the total acidity and the strength of acid sites present on the catalyst surface [26]. The NH3 desorption profiles for the four series of catalysts are depicted in Fig. 3 . Fig. 3 shows the NH3-TPD profiles for all the catalysts, with three different desorption peaks that correspond to the following three acid strengths: low acidity sites (desorption peaks less than 250 °C), medium acidity sites (desorption peaks between 250 and 400 °C), and strongly acidic sites (desorption peaks > 400 °C) [27,28]. All the desorption profiles of the catalysts show three main peaks, in agreement with the previous description. For both alumina catalysts, the strong acidic site peaks follow the order PtW/Al-3 > PtW/Al-6 > PtW/Al-9 > PtW/Al-12 > PtW/Al-15 > PtW/Al-18, which is the reverse of the W loading trend. This is due to the formation of tungsten aluminate, which neutralizes the strongly acidic sites. In other words, more W loading implies less strong acid sites in alumina catalysts because they are neutralized by the tungsten aluminate formed. However, in general, the weak acidic site peaks grow in accordance with the W loading: PtW/Al-3 < PtW/Al-6 < PtW/Al-9 < PtW/Al-12 < PtW/Al-15 < PtW/Al-18. These results indicate that with increasing levels of WO3, weak acidic sites increase and strong acidic sites decrease [28].The proportion of strongly acidic sites on the alumina-based catalysts is higher than the same sites on the zirconia-based catalysts due to the higher surface area of the alumina support (Table 1) and the higher dispersion of WOx in these catalysts observed by Raman spectra. As reported in Table 3 , the total acidity value was measured. The PtW/Zr catalysts show more total acidity than the PtW/Zr-SI catalysts. For the alumina catalysts, the total acidity does not show major differences between the PtW/Al and PtW/Al-SI catalysts. Despite this, the proportion of low strength acid sites in alumina catalysts is higher in the catalyst prepared by sequential impregnation while the proportion of high strength acid sites is higher in the catalyst prepared by the one-step method. This fact is explained by the greater formation of tungsten aluminate in the catalyst prepared by sequential impregnation, since each calcination carried out after each impregnation favours the formation of this aluminate phase, decreasing the proportion of strong acid sites.The adsorption of pyridine as a base on the surface of solid acids is used to evaluate the nature of surface acidity. The use of IR spectroscopy of adsorbed pyridine facilitates the identification of distinct acidic sites [26]. The pyridine FTIR (DRIFT) absorption spectra (Fig. 4 ) of the PtW/Zr-15, PtW/Zr-15-SI (Fig. 4A), PtW/Al-15, and PtW/Al-15-SI (Fig. 4B) catalysts were measured at room temperature and were previously reduced at 350 °C. The pyridine FTIR (DRIFT) absorption spectra of the PtW/Zr-3 to 18 catalysts, ZrO2 support, PtW/Al-3 to 18 catalysts, and Al2O3 support are shown in the supporting information (Fig. 3S).The FTIR spectra show the presence of adsorption bands centered at 1607–1616, 1575, and 1447–1450 cm−1, which are assigned to the vibrational modes of pyridine molecules adsorbed on Lewis (L) acid sites. A broad band at 1540 cm−1 is assigned to the pyridinium ions absorbed on Brønsted acid (B) sites. The absorption band at 1488–1490 cm−1 is a combined band that originated from both Lewis and Brønsted acid sites [18,26,29,30]. The presence of Lewis acid sites in the zirconia-based catalysts can be attributed to the presence of coordinately unsaturated Zr4+ cations, while the Brønsted acid sites are likely hydroxyl groups (W–O–W–OH or Zr–O–W–OH) associated with W6+ and W5+ atoms [29]. In alumina, the Lewis acid sites correspond to three possible Al3+ coordination configurations: five-, four-, and three-fold coordinated [31].Surface chemical analyses of the catalysts were carried out by XPS. The XPS spectra for the PtW/Zr-3–18 and PtW/Al-3–18 catalysts are shown in Fig. 5 , and the binding energies (eV) (Al 2p, Zr 3d, and W 4f7/2 core levels) and surface atomic ratios are listed in Table 4 .The evolution of the W 4f and Zr 4p orbitals in the PtW/Zr-3 to 18 (Fig. 5A) catalysts and the W 4f orbitals in the PtW/Al-3 to 18 catalyst (Fig. 5B) peaks is shown in Fig. 5 as a function of W loading. The W 4f signal presents a typical doublet corresponding to spin-orbital splitting. The PtW/Zr-3 to 18 catalysts show a single component with the BE for W4f7/2 at approximately 35.5 eV characteristic of W(VI) attributed to WO3 [32] (nanocrystalline) species. The PtW/Al-3 to 18 catalysts present two components, one attributed to the WO3 species at approximately 35 eV [6,13] and a second attributed to aluminum tungstate, Al2(WO4)3, at approximately 36 eV [6,33]. WO3 was also identified in the Raman bands observed in the spectra (Fig. 2C).The surface atomic ratio W/(Al or Zr) tends to increase with W loading, as observed in Table 4 and Fig. 5C and D. This increase is linear in agreement with the theoretical density of isolated tungsten oxide species and polytungstated oxide species [32] identified by Raman spectroscopy. The PtW/Al-3 to 18 catalysts show lower values for W/Al, which can be explained by the presence of aluminum tungstate species. Due to the formation of this kind of species, surface tungsten migrates to the interior of the alumina, reducing the W signal observed by XPS [6].In summary, with the characterization techniques used (N2 adsorption–desorption isotherms at −196 °C, X-ray diffraction, Raman spectroscopy, FTIR, NH3-TPD, CO pulse chemisorption, and XPS), we can conclude that the alumina catalysts have a higher surface acidity than the zirconia catalysts due to the higher surface area of the alumina support. Catalysts prepared by successive impregnations show more isolated WO3 in the Raman spectra than the single-step prepared catalysts which can produce lower acidity in the SI catalysts. The absence of WO3 diffraction peaks in the XRD analysis revealed the homogeneous dispersion of this oxide on the catalytic supports.The prepared catalysts were studied for the hydroisomerization of n-dodecane. The activation or reduction process (350 °C in hydrogen at atmospheric pressure) and reaction conditions (Tr = 350 °C, P = 2.0 MPa, liquid flow = 0.1 mL·min−1 and H2 flow = 340 mLN·min−1) in a trickle bed-mode reactor were selected based on previous studies [6].Catalysts showed stability in reactions of 24 h. We checked the possible formation of coke analyzing the sample PtW/Al-15 after reaction, obtaining a C/Al atomic ratio similar to the ratio obtained for PtW/Al-15 before reaction, which means that a minimal quantity of coke was formed. In general, all carbon-balance is higher than 90%. Fig. 6 shows the conversion and selectivity results for all the catalysts studied. Catalysts with a low loading of W showed low conversion, especially on the zirconia catalysts, and this effect was also observed for other similar reactions [9]. The conversion of n-dodecane tends to increase with W loading, which is more obvious in the single-step prepared catalysts. Similar studies of varied W loading have been previously reported [9,34]. The relationship between the tungsten loading and the hydrogen uptake capacity of the tungstated zirconia catalysts has been studied by Iglesia et al. They concluded that hydrogen uptake increases with tungsten loading due to the stabilization of hydrogen by polytungstate and crystalline WO3 species on the surface ZrO2 [34]. Conversions reach a maximum value at a WO3 concentration of 15 wt% W, this optimum W percentage occurs because the PtW/Zr-18 and PtW/Al-18 catalysts prepared by a single step showed lower conversions than the catalysts with PtW/Zr-15 and PtW/Al-15. This result is similar to that reported by Iglesia et al. for o-xylene isomerization rates on WOx–ZrO2 samples with various WOx concentrations, reaching a maximum value at a WOx concentration of 12 wt% W [9]. Similarly, Triwahyono et al. reported that the isomerization of n-butane reached a maximum using WOx–ZrO2 with 13 wt% W [34]. The maximum reached for the PtW/Zr-15 and PtW/Al-15 catalysts may have been due to their high concentration of protonic acid sites or their hydrogen uptake capacity [34]. The reason why the PtW/Zr-18 and PtW/Al-18 catalysts are less active than the PtW/Zr-15 and PtW/Al-15 catalysts is probably related to the larger size of the WOx particles formed, presumably because the loading of W, 18 wt%, is too high to adequately disperse over the support and the WOx particles agglomerate. Thus, in the presence of H2, the activities of PtW/Zr, PtW/Zr-SI, PtW/Al, and PtW/Al-SI catalysts for n-dodecane isomerization were strongly determined by the WO3 loading.Comparing both supports, it is clear that the alumina-supported catalysts are more active than the zirconia-supported catalysts. The higher catalytic activity of the alumina-supported catalysts can be related to their higher surface area and acidity, especially in terms of the strongly acidic sites shown by TPD-NH3, which occur at a higher concentration than those on the zirconia-supported catalysts [13]. The FTIR spectra also show that there are more acidic sites in the alumina catalysts than in the zirconia catalysts. In addition, a larger ratio of Lewis acid sites in alumina catalysts is shown, and this ratio is also larger in the most active catalysts prepared by a single step compared with the SI catalysts. The presence of strong Lewis acid sites (monolayer dispersed WO3 particles) facilitates the formation of protonic sites from H2 in the gaseous phase [34], which are the active sites for n-dodecane hydroisomerization. In addition, we checked that the pH of the ((NH4)6(H2W12O40)·xH2O) aqueous solution was between 6 and 7, the isoelectric point (IEP) of Al2O3 is approximately 8 and the IEP of ZrO2 is approximately 5 [35]. This creates a net positive charge on the alumina surface and a net negative charge on the zirconia surface and favors W incorporation on the alumina surface because W is in the anion of the precursor and has a negative charge. However, the incorporation of W to the alumina supports yields a partial formation of tungsten aluminate, which usually migrates to deeper layers resulting in the lower surface atomic ratios (W/Al) in the PtW/Al-3-18 catalysts determined by XPS (Table 4), this lower surface atomic ratio seems to have a positive effect on their catalytic activity because these catalysts showed higher conversions.By comparing the two preparation methods, we can conclude that the single-step impregnation method works better than several successive impregnations for adding the same wt.% of W due to the higher dispersion of WO3 on the catalyst surface prepared using the single-step process. This produces more accessible acidic sites, improving the progress of the reaction and the conversion results. This fact is explained by the greater loss of surface WO3 in the catalysts prepared by SI, since each calcination, after the successive impregnations, entails a solid-state reaction between this oxide and the support. Another reason is that, as observed in the Raman spectra, more isolated WO3 clusters are produced in the successively impregnated catalysts than in the single-step-prepared catalysts, which results in a lower acidity in the SI catalysts and less activity.Selectivity for isomers (Fig. 7 ) varies considerably with W loading, especially when comparing low loadings with high loadings. In general, branched C12 hydrocarbons are the main products obtained, showing selectivity for C12 (>80%) for catalysts with high W loadings (>9 wt% W). The selectivity to C11 varies from 7.4 to 13.9% in these catalysts, and low selectivity values for C6-10 were obtained. Catalysts with 3 and 6 wt% W (independent of the preparation method for 6 wt%) showed high selectivity for C6-10 hydrocarbons and low selectivity for C12 hydrocarbons, but these catalysts showed a very low catalytic activity for the hydroisomerization of n-dodecane.A comparison of our results with other systems used in the bibliography is compiled in Table 5 . The results are selected because a similar catalyst is used or because a similar or the same isomerization reaction is studied. The catalytic activity and isomer yield obtained in this work are better than the values previously, especially for the Pt/WOx/Al2O3 series with certain W loading.Four series of catalysts based on Pt-WOx-alumina and Pt-WOx-zirconia have been successfully prepared by a wet impregnation method via a single step or several consecutive steps (sequential impregnation, SI).The characterization of the catalysts indicates that the alumina catalysts achieve higher surface acidity than the zirconia catalysts, which is explained by the higher dispersion of tungsten oxide on alumina due to the larger surface area of alumina and the IEPs of alumina and zirconia, which influence the dispersion of WOx. The single-step preparation method for the catalysts is a better preparation method because fewer isolated WOx species are formed, as it was revealed in the Raman spectra, which improves the activity results. The high dispersion of WO3 on the catalysts agrees across the XRD, Raman, and nitrogen adsorption/desorption isotherm results.In general, the conversion of n-C12 grows with %W loading for both supports and methods, and branched C12 hydrocarbons are the main products obtained (>80%).We can conclude that the PtW/Zr-15 and PtW/Al-15 catalysts are found to be the most active for the hydroisomerization of n-dodecane, meaning that the catalysts prepared via a single step are more active than catalysts prepared via several consecutive steps. The catalytic activity results clearly show that 15 wt% W is the best W loading and alumina is the best support (67% of n-C12 conversion and 87% of i-C12 selectivity). The different isomerization activities shown by the alumina and zirconia-based catalysts can be related to differences in their physicochemical properties previously mentioned. D. García-Pérez: Investigation, Formal analysis, Writing – original draft. G. Blanco-Brieva: Investigation, Formal analysis, Writing – original draft. M.C. Alvarez-Galvan: Supervision, Writing – review & editing, Funding acquisition. J.M. Campos-Martin: Conceptualization, Supervision, Writing – review & editing, Project administration, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.The Agencia Estatal de Investigación (AEI) (Spain) supported this work through the project ENE2016-74889-C4-3-R. DGP acknowledges her contract (BES-2017-079679) to AEI (Spain). We acknowledge the support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2022.123704.The following are the Supplementary data to this article: Supplementary Data 1

Catalysts based on zirconia and alumina-supported tungsten oxides (3–18 wt% W) with a small loading of platinum (0.3 wt% Pt) were selected to study the influence of W loading, support type, and preparation methods on the hydroisomerization of n-dodecane. The alumina- and zirconia-supported catalysts show different catalytic properties that play an important role in n-dodecane conversion. Specific surface area and acidity of the alumina-supported catalysts are clearly higher than those of zirconia-supported counterparts. The 15 wt% W loaded catalysts were found to exhibit the best catalytic performance for the hydroisomerization of n-dodecane for both zirconia and alumina (67% of n-C12 conversion and 87% of i-C12 selectivity) based catalysts. Catalysts prepared by a single-step wet impregnation method were more active in the hydroisomerization of n-dodecane than catalysts prepared via several consecutive steps.

Fluid catalytic cracking (FCC) is an important process for the conversion of crude oil into valuable products including fuels, lubricants, and precursors for making other products. This importance is evidenced by the fact that there are more than 430 FCC units worldwide today [1–3]. First used commercially in 1942 in Baton Rouge, Louisiana (USA), the FCC process cracks high molecular weight hydrocarbon chains into lighter hydrocarbons using high temperatures (525−575 °C) and a heterogeneous catalyst [1,4–7]. Until the development of the FCC process, refineries were inefficient at making valuable products such as gasoline and liquefied petroleum gas (LPG); however, the parallel development of the FCC process and a catalyst capable of fluidization and cracking chemistry enabled refineries to upgrade less valuable fractions of crude oil into high-value diesel, gasoline, and LPG products. The FCC catalyst is primarily composed of zeolite-Y (in the form of ultra-stable zeolite Y or USY), which has a high surface area, but also features a matrix used for cracking reactions. The catalytic system can also include additional features such as nickel and/or vanadium passivation technologies and additives to tune product yields or to control emissions.The catalyst facilitates beta scission reactions and is relatively robust – a necessity for enduring high temperatures and physical stress during operation. Additionally, FCC catalysts are often exposed to metal contaminants, which are typically introduced into the unit via the FCC feed. A common feed contaminant is nickel, which is often introduced with the feed as a nickel (II) porphyrin structure [8,9]. The concentration of nickel in feed varies widely and can be as high as 100 ppm in extreme cases, although values lower than 25 ppm are more typical [10]. A well-known dehydrogenation catalyst, nickel deposits on FCC catalyst in concentrations ranging up to 19,000 ppm. The deposited nickel induces unwanted dehydrogenation reactions, which lead to an increase in hydrogen and coke yields [11–14]. Excessive amounts of both hydrogen and coke can be problematic for refiners as they push the FCC unit closer to operating limits. For example, increased hydrogen can constrain the FCC unit's downstream compressor, and increased coke can increase the regenerator temperature towards its maximum limit. However, the nickel contaminant becomes less active as it spends more time in the FCC unit. The oxidative environment in the FCC regenerator oxidizes nickel to nickel oxides. This chemical transformation immobilizes the nickel and greatly reduces its dehydrogenation tendency. Interaction of nickel with alumina phases in the catalyst (such as the low surface area crystalline aluminas used for trapping of contaminant nickel in catalysts designed for processing heavier, residue-containing feedstocks) result in various forms of nickel aluminate [15–19]. Once nickel oxides and aluminates are formed, it is important to keep nickel in those states and inhibit its reduction to metallic nickel in the FCC environment.There is precedence in literature that chloride ions can both mobilize and reactivate nickel oxides. Earlier work shows that NiO on activated carbon reacts with hydrochloric acid (HCl) to form NiCl2, a mobile compound, which can then be further reduced by H2 to metallic nickel, a more active dehydrogenation catalyst than NiO [16]. Another study shows the same phenomena using platinum, a metal from the same family as nickel, on zeolite that is exposed to HCl and subsequently reduced with H2. Additionally, a further study observed a redistribution of platinum on the support following the HCl and H2 reactions [20,21]. Another contribution showed that deactivated Ni-erionite catalyst regained its dehydrogenation activity when treated with solutions of HCl or NH4Cl [22]. These examples demonstrate that relatively inert NiO can be reactivated for dehydrogenation chemistry and mobilized by exposure to chloride-containing compounds and set a precedence that this might be possible in the FCC environment. Indeed, industrial reports have noted a correlation between increased chloride content and an increase in unwanted hydrogen production, among other issues. While little literature exists concerning chloride interactions with nickel aluminates, nickel in nickel aluminate exists in a +2 oxidation state (as it does in NiO) and has been shown to be difficult to reduce and less active for dehydrogenation chemistry than metallic nickel [18,19]. Thus, it is hypothesized that chloride could lead to a similar reactivation of nickel aluminate for dehydrogenation chemistry.Interactions of nickel with chloride ions are relevant to the FCC environment as chloride ion sources can enter the FCC both with feed, sometimes a result of insufficient desalting operations, and with fresh catalyst as part of an alumina-based binder used in incorporated catalysts or from the use of chloride-containing precursors/chemicals in catalyst manufacturing [14,23]. The use of the alumina-based binders for incorporated catalysts is needed for particle integrity in order to control the attrition of the final product. Alumina-based binders often contain chloride as a byproduct in its manufacturing. Chloride content in fresh FCC catalyst can be as high as 1.2 wt.%. Chloride sources coming from the feed vary widely. In heavily contaminated feeds, chloride can be as high as 15 ppm. Chloride sources in the feed can react with steam in the FCC to form HCl, while most of the binder-based chloride is released and converted to HCl in the high-temperature, steam partial pressure environment of the FCC regenerator [24–26]. While these chloride contaminants are well known to lead to deposits in the downstream fractionator, fouling in equipment, and having a negative impact on metallurgy, their effect on nickel contaminants in an FCC has never been formally investigated [27].The work described herein constitutes the first exploration of the effect of chloride ions on nickel contaminants deposited on actual FCC catalysts using simulated FCC conditions. While circulating in an FCC unit, a fraction of catalyst is continuously added and withdrawn. The continual addition and withdrawal of catalyst introduces an age-distribution of catalyst particles in periodically withdrawn samples that are tested and tracked to monitor performance. This age-distributed catalyst sample is commonly called equilibrium catalyst (Ecat). Ecat samples taken from two different industrial FCC units were selected for this study. These samples were selected due to their differing nickel levels, marked as "high" and "low". It is important to note that the Ecat samples chosen for this study originate from catalysts manufactured by the “in-situ” manufacturing route. This route differs from conventional catalyst production process in that zeolite is grown in the microsphere after the spray drying step. The zeolite itself acts as the catalyst binder, thus in-situ catalysts do not use chloride-containing binders. As a result, there are no chlorides present in fresh catalyst. In addition, the refineries from which these Ecat samples originate did not report any chlorides coming from the feed. Therefore, this study represents the first time these samples are introduced to chlorides. Table 1 shows the total surface area (TSA), zeolite surface area (ZSA), matrix surface area (MSA) and average particle size (APS) of the Ecat samples studied. It is noted that there are slight differences in surface areas and rare earth oxide content between the low and high nickel containing Ecat samples. For the purpose of this study, we note that these would not have a significant effect on the expected outcome based on the experimental design. Because the Ecat samples that fit the desired criteria (similar technology, similar manufacturing route, no additive usage, no previous chloride exposure) are limited and are based on refineries operating around the world at the moment of this experimental design, these samples represent the best compromise between using industrial Ecats and laboratory generated (deactivated) samples. Ecat samples were chosen over lab-deactivated catalyst as these samples provide the best representation of nickel age distribution in the unit, since it is known that the introduction of nickel contaminants in a laboratory can lead to a distribution of nickel which does not mimic what is seen in an actual FCC unit [1]. To this point, extensive research is focused on the attempt to develop methods to minimize these testing artifacts [28]. Thus, performing such a study on Ecat samples provides results most relevant to industrial application.The Ecat samples chosen for this study were exposed to chloride ions via introduction of gaseous HCl generated by reaction of aqueous HCl with sulfuric acid [29,30]. This procedure is well established in literature for generating HCl. While literature describes the introduction of HCl via liquid solutions as well, such a method was not included in this study, since FCC catalysts are not normally exposed to such liquid media during FCC operation [22]. As a result, they are not designed to withstand this type of liquid interaction; consequently, FCC structural integrity and catalytic performance can be drastically altered by exposure to liquids. The objective of exposure to HCl is to monitor any conversion of oxidized nickel on Ecat into NiCl2 species. A control experiment was also run exposing Ecat to gaseous N2. Following each introduction of chloride ions or control treatment, each catalyst sample was then exposed to H2 to mimic the reducing environment of an FCC riser and reduce any nickel chloride species formed to metallic nickel. The effect of each treatment was then studied by evaluating the physical, chemical, structural, and catalytic changes of the catalysts using particle size measurement, surface area measurement, X-Ray Fluorescence (XRF), Scanning Electron Microscopy (SEM), Advanced Cracking Evaluation (ACE), and CO Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) analyses. The results are presented and discussed below.A round-bottom flask was charged with catalyst (40 g). HCl( g ) was produced by dropwise addition of HCl( aq ) (12.1 M, ca. 1 mL/min) into a Schlenk flask containing a stirred solution of H2SO4( aq ) (100 mL, 18.0 M). The generated HCl( g ) flowed into the catalyst-containing flask via a gas dispersion tube with stirring for 1 h at room temperature and then exhausted into a KOH base trap. The mixture was stirred for 1 h at room temperature. The catalyst was dried using a temperature furnace at 100 °C overnight [29,30].In a control experiment, the same procedure was repeated with a gentle stream of N2( g ) (ca. 1 L/min) replacing the generated HCl( g ). A round-bottom flask was charged with catalyst (40 g). The N2( g ) flowed into the catalyst-containing flask via a gas dispersion tube with stirring for 1 h at room temperature and then exhausted into the atmosphere. The mixture was stirred for 1 h at room temperature. The catalyst was dried in a 100 °C furnace overnight.The reduction process was adapted from a literature procedure [16]. A Fisher-Porter bottle was charged with treated catalyst (40 g). The system was filled with H2( g ) and evacuated five times before being pressurized with 75 psig of H2( g ). The system was heated to 375 °C, kept at that temperature for 30 min, and subsequently cooled (total time elapsed ca. 2 h).Particle size is measured according to ASTM D4464-10. Particle size distribution in the range of 2.8–176 micrometers were measured using a Beckman Coulter LS13320 with Universal Liquid Module and Ultrasonic unit. Material is dispersed in water, exposed to a beam of light, and the diffraction pattern of the light is used to determine the distribution of particle size.Surface area was measured using the Brunauer–Emmett–Teller (BET) method on a Micromeritics TriStar II according to ASTM methods D 3663 and D 4365. BET uses adsorption isotherms to determine material surface area. The sample was pulverized, and outgassing was performed at 250 °C for 4 h. Surface area was measured by N2 adsorption and desorption.X-Ray Fluorescence Spectroscopy analyses were performed using a wavelength-dispersive PANalytical PW2400 spectrometer, calibrated by linear regression to data from standards. All samples were prepared by fusion, using a lithium metaborate/lithium tetraborate flux.The catalyst samples were mounted in epoxy and polished to an ultra-flat surface and carbon coated using a Denton DV-502A Vacuum Evaporation System. The BEI analysis was conducted on a Hitachi 3400S Environmental Microscope at 15−25 kV. EDX (Energy Dispersive X-Ray Spectroscopy) results were collected at 25 kV on a Bruker Quantax EDS system with Dual 30 mm2 Silicon Drift Detectors (SDD).ImageJ was used to calculate the circularity of all particles in each sample. A circularity index was calculated per the following equation: C i r c u l a r i t y = 4 π * A r e a P e r i m e t e r 2 A circularity of 1 equals a perfect circle while a circularity of 0 equals a straight line.Advanced Cracking Evaluation (ACE) is a laboratory-scale FCC testing unit which evaluates the activity and selectivity of FCC catalysts in a fixed-fluidized bed reactor [31,32]. As testing is carried out under fluidized conditions, it is commonly used for evaluating FCC catalysts. Ecat treated by various methods were analyzed on an ACE testing apparatus with the following conditions: Reactor temperature: 532 °C, injector height, 2.125", standard vacuum gasoil feed, variable time on stream method, 1.2 g/min feed rate, 9 g catalyst loading, 575 s catalyst strip time, liquid strip multiplier of 12, 110 °C feed temperature, 116 °C and 177 °C temperature of first and second feedline heater (respectively), and catalyst to oil ratios of 9, 7, 5, and 3. Coke on catalyst is obtained at the end of a run on a LECO unit.A recently developed, 3-temperature (3-T) pretreatment CO DRIFTS method was used to characterize the nickel on samples treated with N2 and HCl [33]. This method, as opposed to traditional CO DRIFTS, is needed due to the presence of other impurities, which can lead to ambiguous CO band assignments. The samples were ground into fine powders and pretreated with 2.4 % H2/Ar at 200, 400, and 600 °C sequentially for 1 h at each temperature. The samples were cooled to 30 °C then underwent a 30 min exposure to 1% CO/Ar for adsorption and a 30 min desorption in Ar while FTIR data were collected. FTIR characterization was performed on a Thermo Fisher Nicolet iS50 FTIR spectrometer equipped with an MCT detector and a Pike Technology high-temperature environmental chamber with a KBr window. Spectrum collection was performed under diffuse reflection mode. Bands were assigned based on CO interaction with metals of different oxidation states and the change in these band intensities with temperature was recorded, which allows the characterization of nickel reducibility upon different treatments.Ecat samples were treated with HCl or N2, reduced by exposure to H2, and then analyzed by several techniques. XRF, surface area, and particle size distribution of each sample were measured and compared to untreated Ecat as a means of evaluating the effect of each treatment on chemical composition and physical integrity of the catalyst. SEM images were also obtained in order to evaluate changes in catalyst particle shape and nickel distribution across different catalyst particles. An image processing method employing ImageJ was used to quantify differences seen between each SEM image. Changes in the dehydrogenation activity of nickel on Ecat following each treatment method was evaluated using ACE analysis. A standard feed was cracked over a range of catalyst to oil (C/O) ratios with each Ecat. The properties of this standard feed are given in the table below (Table 2 ).Hydrogen and coke yields of the Ecat sample are used as a measure for dehydrogenation activity of contaminant nickel. Finally, the oxidation state of nickel, which is hypothesized to be altered by interaction with chloride ions and subsequent reduction, was evaluated by a CO DRIFTS [33]. The results are described herein.Following the treatments described previously, the resulting surface area and chemical composition of the catalysts were analyzed and compared to untreated samples to evaluate how each treatment method influenced the chemical and physical properties of the catalysts. XRF results are shown in Table 3 . Aluminum, lanthanum, iron, nickel and vanadium are reported as oxides. These elements (via their respective oxides) are all of interest. Aluminum is present in both the matrix and zeolite phases of the catalyst and plays the key role in cracking in both the zeolitic domain (especially to provide selectivity towards valuable hydrocarbons such as gasoline and LPG) as well as in the matrix domain (especially to crack large molecules in the feed). Alumina phases can also be used for trapping contaminant metals such as nickel. Lanthanum is a rare earth element which stabilizes active cracking sites. Iron is both a contaminant and found in the structural framework of the catalyst. As a contaminant, iron can act as a dehydrogenation catalyst to generate coke and hydrogen but is considered significantly less active than nickel (ca. one-tenth the activity). Vanadium is also a feed contaminant which contributes to coke and hydrogen yields, but it is also considered to be less active in generating coke and hydrogen compared to nickel (ca. one quarter of the dehydrogenation activity). As a result, it is important to track these elements/oxides before and after treatment methods to assess how any change in their amount might affect the reactivity or selectivity of catalyst samples. It is important to note that no other catalyst contaminants known to increase coke and H2 were present on the catalyst in significant quantities (>50 ppm). For simplicity, they are not included.Following treatment by either HCl or N2 and reduction with H2, both Ecat samples contained amounts of Al2O3, La2O3, Fe2O3, NiO, and V2O5 that were within instrumental error of the untreated Ecat samples. This indicates that loss of nickel, vanadium, aluminum, iron or lanthanum does not occur during treatment (as expected) and will not influence coke and hydrogen yields in ACE analyses. It is also worth noting that there are different amounts of iron and vanadium between the high and low nickel samples; however, these differences are small (ca. 1000 pm Fe and 500 ppm V) compared to the difference in nickel (ca. 4000 ppm Ni) between samples. Furthermore, taking into account the much lower dehydrogenation activities of iron (ca. 1/10th of Ni) and vanadium (ca. 1/4th of Ni), these differences can be “normalized” to much lower levels. When comparing to a 4000 ppm difference in Ni, these small contributions from Fe and V are considered insignificant in this study. Table 1 shows the surface area and average particle size (APS) of the Ecat samples before and after each treatment method. These physical parameters are important to monitor, since any change in the structural integrity of the catalyst could influence coke and hydrogen yields, thus clouding any change in nickel reactivity. Neither treatment method resulted in a change in surface area or particle size that was outside of the instrumental error of the original Ecat. This indicates that HCl and N2 treatment methods do not significantly alter the catalyst structure.Scanning electron microscopy was performed on catalyst samples before and after treatment to understand both the change in nickel distribution and the structural integrity of the catalysts before and after exposure to chloride ions. SEM studies focused on high nickel Ecat, since the low nickel Ecat samples did not contain enough nickel for detection in SEM-EDX (Energy Dispersive X-Ray Spectroscopy). Fig. 1 shows the SEM back-scattering results for treated and untreated Ecat samples. No fragmentation of particles was observed, and the structural integrity of the catalyst particles was maintained. ImageJ was used to calculate the circularity of each particle. A “circularity index” of 0 to 1 was calculated with 1 indicating a perfect circle and 0 indicating a line. The values were averaged for each treatment method and the results are shown in Table 4 . Each catalyst sample had the same circularity index of 0.86, thus confirming that no treatment method was destructive to catalyst integrity and that these treatment methods are an effective way to introduce chloride into the catalyst without influencing the structural integrity of the catalyst particle.The SEM-EDX images of nickel on catalyst particles were also examined before and after treatment. Fig. 2 shows the SEM images for nickel and aluminum overlaid for high nickel Ecat untreated and treated by N2 and HCl.A redistribution of nickel is not apparent from these images; however, this is not surprising given the design of the experiment. The catalysts were not treated in a fluidized environment nor at the high temperatures experienced in an industrial FCC regenerator. As a result, while nickel chlorides can still form, the temperature and lack of fluidization would not be amenable to nickel mobility. A further study of nickel mobility in the presence of chloride ions at conditions closer to that of an FCC unit will be investigated later.Changes in the catalytic behavior of nickel-contaminated Ecat following exposure to N2 or HCl then reduced by H2 were evaluated using ACE analyses. A standard FCC feed was cracked over a fluidized bed of Ecat at different catalyst-to-oil ratios. Since nickel is a known contaminant that produces hydrogen and coke when present on FCC catalyst, the changes in coke yield and H2/CH4 yield ratios during ACE evaluations were compared as a means of assessing nickel activity following different treatment methods. Fig. 3 shows the coke vs. conversion results from an ACE analysis of the Ecat sample containing high amounts of nickel. The results showed treatment with HCl prior to the reduction step gave roughly a 1 wt.% increase in coke yield at a given conversion level. This result highlights that the introduction of chloride ions leads to increased coke yield. Since coke is a known product of dehydrogenation from nickel contamination and it is hypothesized that chloride ions facilitate activation of nickel contaminants on FCC catalyst, a higher coke yield following HCl exposure suggests the reactivation of nickel by exposure to chloride ions. Fig. 3 also shows H2/CH4 yield ratios as a function of conversion for the high nickel Ecat treated by N2 and HCl. As with the coke yield, the H2/CH4 yield ratio was higher for Ecat exposed to HCl than Ecat exposed to N2 (∼0.08 wt.%/wt.%). The increase in H2/CH4 yield ratio with HCl treatment also suggests that there are interactions of chloride ions with nickel which increase the dehydrogenation activity of the nickel contaminants on catalyst.The average yields at 72.5 % conversion are reported in Table 5 . There is a 0.06 wt.%/wt.% and 1.1 wt.% increase in H2/CH4 ratios and coke yields, respectively, when the catalyst is treated with HCl as opposed to with N2. These experiments confirm an average relative increase of 13 % in H2/CH4 ratios and 18 % in coke yields for samples exposed to HCl. These increases in H2/CH4 and coke are both significant, as the average values of 0.52 and 7.4 for H2/CH4 and coke yields following HCl treatment are not within the standard deviation of the H2/CH4 and coke values of the samples treated with N2. Additionally, increases in 13 % and 18 % in H2/CH4 and coke would be considered significant by industry standards as well. These increases in H2/CH4 and coke seen in Table 5 agree with the trends seen in Fig. 3, thus confirming the increased dehydrogenation which occurs when chloride ions are introduced to the system.The combination of increased coke and hydrogen yields following exposure to HCl indicates that nickel contaminant on the catalyst is more active, and that chloride ions play a role in reactivating nickel on the catalyst.Low nickel Ecat samples exposed to N2 and HCl were also analyzed via ACE. The coke yield vs. conversion plots are shown in Fig. 4 . There was a ∼0.5 wt.% increase in coke yield following exposure to HCl. However, this increase in coke is not as large as the increase seen (∼1 wt.%) following treatment of the high nickel Ecat sample with HCl. This is expected as there is significantly less nickel present on the Ecat, thus less nickel available for potential reactivation. Fig. 4 also shows the H2/CH4 yield ratio as a function of conversion for the Ecat samples with lower amounts of nickel. As in the high nickel case, there is an increase in H2/CH4 following exposure of the catalyst to HCl (+0.05 wt.%/wt.%). However, as was seen with coke yields, this increase in H2/CH4 is not as pronounced as seen in the case of catalyst containing high amounts of nickel.Multiple ACE experiments were run with the low nickel Ecat. The coke yields and H2/CH4 yield ratios at constant conversion were averaged and are reported in Table 5. The H2/CH4 yield ratios at constant conversion agreed with the trend seen in Fig. 4. There is a 0.06 wt.%/wt.% increase in H2/CH4 when HCl is introduced.The coke yield at constant conversion agreed with the trend shown in Fig. 4. Exposure to HCl leads to a 0.3 wt.% increase in coke compared to exposure to N2. However, it should be noted, that the increase in coke due to exposure to chlorides is almost within standard deviation of each experimental trial. This is not surprising considering the relatively low amount of nickel present on this catalyst.CO DRIFTS experiments were performed on Ecat samples treated with N2 or HCl then reduced by H2. The goal of DRIFTS experiments is to determine the reducibility of the nickel contaminant. Since the Ecat samples contain 0.8–1.0% of iron, the CO adsorption on iron would show overlapped peaks in DRIFTS with the CO adsorbed on Ni. In the literature, CO adsorbed on the bivalent or single valent state of nickel is assigned in the range of 2100−2200 cm−1, CO adsorbed on top of Ni(0) is assigned in 2000−2100 cm−1, and CO adsorbed on Ni(0) can also be found at 1813−2000 cm−1 for single-fold or multi-fold bridged adsorption on larger particles [34–37]. CO adsorbed on iron (Fe2+, Fe0) is reported with similar peak positions [38–40].In order to differentiate the CO adsorbed on iron and the CO adsorbed on nickel, a sequential CO DRIFTS experiment at 3 temperatures is designed under the pretreatment of hydrogen reduction following the protocol reported in detail elsewhere [33]. In these CO DRIFTS experiments, samples were first treated with H2 at 200 °C before introducing CO, which allows a partial reduction of iron or nickel to a different degree. The CO was then introduced and adsorbed on samples to reach equilibrium. After CO introduction, the CO was allowed to desorb in argon, and CO DRIFTS data were collected during both CO adsorption and desorption time using FTIR. The FTIR spectra presented in this paper were collected at 30 s of CO desorption, which retain the adsorbed CO on solid and remove all the gas phase CO signals. This process was then repeated at 400 and 600 °C. At each of these temperatures, the reduction degree of nickel and iron is examined by the adsorbed CO FTIR signals. As two different metal oxide materials, nickel oxide and iron oxide are expected to have different reducibilities [41–44]. The 3-temperature trend analysis of reduction allows the separation of nickel and iron when their reducibilities are different. The single-beam FTIR spectrum at 30-second-desorption was processed using the IR background spectrum collected before CO was introduced, which allows the comparison of adsorbed CO bond vibration signals at the different reduction temperatures. From this spectrum, information on the oxidation states of metals on the catalyst were determined based upon CO interaction with these sites [45]. With that, the reducibility of nickel can be isolated from the influence of iron, and the effect of N2 or HCl treatment on the Ecat samples can be clearly examined.The CO absorbance spectra of high nickel Ecat treated by gaseous N2 and HCl are shown in Fig. 5 . The CO absorbance spectra of low nickel Ecat samples treated with N2 and HCl are shown in Fig. 6 .The bands between 1940 cm –1 and 2060 cm –1 result from CO bound to Ni(0) and Fe(0) species. The 2090 cm –1 band is a result of CO bound to Ni(0). The 2120 cm –1 band is CO bound to Fe(II) species. The 2140 cm –1 and 2160 cm –1 bands result from Ni(II) species. Bands were deconvoluted and their areas were integrated at different reduction temperatures in order to determine differences in nickel oxidation state between treatments. The 2090 cm –1 and 2120 cm –1, Ni(0) and Fe(II) bands, respectively, have very small areas and it was difficult to infer meaningful information from them in any sample. Thus, the analysis focused on changes in 2140 cm -1 and 2160 cm –1 band areas (Ni(II)) and the areas of the bands in the 1900-2070 cm−1 region (Ni(0) and Fe(0)). Fig. 7 shows the sum of the integrated areas of the nickel(II) derived 2140 cm –1 and 2160 cm –1 bands for the high nickel Ecat samples as a function of temperature. The CO adsorption at these two bands is much smaller than the bands in the 1900 – 2070 cm−1 region, which supports that the sample contains mostly metallic forms of nickel/iron after reduction. The form of Ni(II) may include NiO or nickel aluminate in the Ecat samples, and possibly NiCl2 in the HCl-treated samples. The contribution to the peaks at 2140 cm –1 and 2160 cm –1 is believed to come from NiO or nickel aluminate rather than NiCl2 and are indicative of the amount of NiO/nickel aluminate compound (in short, referred to as Ni-O in discussion below) present on the catalyst. These proposed peak assignments can be supported by the observation that the catalyst treated with HCl showed significantly lower band area and, therefore, less Ni-O containing compounds, than the sample treated with N2. This could indicate that during treatment with HCl, chloride ions reacted with Ni-O, forming NiCl2, which could then be reduced to metallic nickel during the reduction step. Additionally, the N2 treated sample showed a higher band area at 200 °C and a decrease in this band area with increasing reduction temperature, while the chloride ion treated sample was essentially unchanged, indicating Ni-O remaining on Ecat treated with N2 is reduced at higher temperatures, while Ecat sample treated with HCl has much less Ni-O remaining. This result suggests different amounts of Ni-O species remain in the Ecat samples under N2 and HCl treatments, with a higher amount of Ni-O in the N2-treated Ecat.The combined band area between 1900–2070 cm –1 was also examined. These bands are indicative of both Ni(0) and Fe(0) species; as a result, these combined band areas are discussed for both the high nickel and low nickel catalyst samples to understand whether changes in band area are influenced by changes in Fe(0) or Ni(0) compounds, as the iron levels between the high and low nickel catalysts were similar (within 1200 ppm). Fig. 8 shows the combined band area of all bands in the 1900–2070 cm –1 region for the Ecat samples. It is expected that iron would be reduced before nickel is reduced when exposed to H2. The formation of Ni(0) will become more obvious as the reduction temperature increases, thus the Ni(0) can be separated from Fe(0) in the CO DRIFTS.For the low nickel Ecat, as the reduction temperature increases to 400 °C, the bands grow for each sample indicating more Ni(0) and Fe(0) are formed. At 600 °C, important observations can be made. For both low nickel samples, the band area does not increase, which could be an indication that all iron and nickel have been completely reduced to the zero-oxidation state at 400 °C. This result has implications for the analysis of the high nickel sample. The high and low nickel catalyst samples have comparable levels of iron. Thus, a complete reduction of iron in the low nickel Ecat at 400 °C indicates that all iron will be reduced to Fe(0) in the high nickel Ecat at 400 °C as well. Consequently, any changes in band area at 600 °C for the high nickel samples can be attributed to a change in the amount of Ni(0).The combined area of all bands in the 1900–2070 cm –1 region for the high nickel catalyst samples can also be seen in Fig. 8. These band areas should reflect the amount of Fe(0) and Ni(0) present. At 200 °C, the area is similar to the low nickel samples, which is an indication that the primary species being observed here is Fe(0) since the nickel levels are very different between the two catalysts. As the temperature increases to 400 °C, the band of the HCl treated sample grows much more rapidly than the N2 sample. At 600 °C, both high nickel samples show an increase in band area, with the HCl treated sample showing a significantly higher increase than the N2 treated sample. Having established that the reduction of iron to Fe(0) is completed by 400 °C, this would indicate that the increase in band area at 600 °C is due to a change in Ni(0). This difference in the change in band area would then indicate that the HCl treated sample contains more readily reducible nickel than the N2 treated sample.This large increase in Ni(0) formation vs. temperature for Ecat treated with HCl must be reconciled with the fact that its CO adsorbed on Ni-O band at 2140 and 2160 cm –1 does not change with a reduction temperature (Fig. 7). One would expect a large increase in Ni(0) to correspond to a drop in Ni(II). An explanation could be that a nickel species not detected in CO DRIFTS is being reduced to Ni(0) at 600 °C in the Ecat treated with HCl. Given the HCl treatment applied, it could be that NiCl2 is present on the Ecat treated by HCl and is not distinguishable in the FTIR spectra studied. As this NiCl2 is exposed to H2 at 600 °C, it is reduced to Ni(0) which is then visible in the analysis. While the exact mechanism is uncertain, the results show clearly that exposure of Ecat to HCl results in significant differences in the reducibility of nickel compared to exposure to N2. This further supports the conclusion that differences in ACE yields are a result of a change in the reducibility of nickel, and that chloride ions are playing a major role in this transformation.This work attempts to demonstrate and characterize the physiochemical and catalytic effects of chloride ions on contaminant nickel in the FCC environment for the first time. Additionally, by performing the study on catalyst samples from actual FCC units, the age-distribution of nickel on the catalyst studied is representative of what can be expected in actual operation. It is acknowledged that uncertainties are introduced by using actual FCC Ecat, but the method development work performed in this study has laid the groundwork to perform future studies in more carefully controlled laboratory conditions. Uncertainties which will be addressed in future work include aspects such as the use of catalyst with the same properties and non-Ni contaminants, examination of the effect of different Ni passivators, the effect of Cl contamination on catalyst activity through in depth studies, and using additional techniques to characterize the state of nickel on Ecat.By studying the change in physicochemical characteristics and catalytic selectivity of FCC catalysts, as well as the reducibility of the nickel on FCC catalyst, clear differences can be seen when catalyst contaminated with nickel is exposed to HCl and then reduced. Catalyst exposed to HCl showed increased coke and H2 yields and contained less Ni-O bonds. These results bridge the gap between existing literature and the FCC environment by showing that chloride ions can interact with nickel contaminant on FCC catalyst. The interaction results in changes in the electronic environment of nickel, which makes it easier to be reduced in the FCC riser. This reduced nickel poses a significant problem to refineries since it is an active dehydrogenation catalyst which produces undesirable coke and H2. This increased coke and H2 brings the FCC unit closer to its operational constraints and inhibits the refinery from reaching the full potential of this important unit operation. The results from this study enable catalyst manufacturers and refiners to further optimize catalyst design and selection as well as operational strategies to limit H2 and coke from nickel contaminants.The authors report no declarations of competing interest. Corbett Senter: Conceptualization, Methodology, Data curation, Visualization, Writing - original draft, Writing - review & editing, Project administration. Melissa Clough Mastry: Conceptualization, Methodology, Data curation, Visualization, Writing - review & editing. Claire C. Zhang: Data curation, Visualization, Investigation, Writing - review & editing. William J. Maximuck: Data curation, Visualization, Investigation, Writing - review & editing. John A. Gladysz: Data curation, Visualization, Investigation, Writing - review & editing. Bilge Yilmaz: Conceptualization, Methodology, Supervision, Funding acquisition, Writing - original draft, Writing - review & editing.

Nickel, a common contaminant in crude oil, deposits on Fluid Catalytic Cracking (FCC) catalysts and induces unwanted dehydrogenation reactions. These lead to an increase in hydrogen and coke which inhibits the FCC unit from reaching its optimal operation. Modern catalyst technologies can include nickel passivation strategies to minimize such detrimental effects, and, over time, aging of the nickel on catalyst also diminishes its deleterious activity to some extent; however, reactivation of nickel due to chemical interactions within the FCC unit can retard aging and further penalize the catalytic performance. For the first time, we attempt to demonstrate and characterize the physiochemical and catalytic effects of chloride ions on contaminant nickel in the FCC environment. Equilibrium catalyst (Ecat) samples obtained from industrial FCC units are exposed to chloride ions, and changes in physicochemical characteristics, catalytic selectivity, and the reducibility of nickel are analyzed. These changes indicate the reactivation of nickel and an increase in unwanted dehydrogenation reactions following exposure to chloride ions. Spectroscopic analyses show that the interaction with chloride ions alters the electronic environment of nickel, which makes it easier to be reduced in the FCC riser, and Advanced Cracking Evaluation (ACE) studies show equilibrium catalysts that were exposed to chloride ions gave higher coke and H2 yields. These results bridge the gap between existing literature and the FCC environment by demonstrating that chloride ions can interact and reactivate nickel contaminant on FCC catalysts.

In the last few decades, titanium dioxide (TiO2) has been used as an alternative catalyst for the sterilization process of several pathogenic bacteria. The photo-catalyst performance of TiO2 is proven to be more effective in degrading several types of organic matter contaminants than that of the conventional method of chlorination [1]. Chlorination is an in efficient process as it can cause environmental problems that require further treatment [2]. When organic matter contaminants decompose in water containing TiO2, the photo-catalyst surface becomes much more effective after irradiation with ultra-bandgap light with UV radiation (λ ≥ 385 nm) [3, 4, 5, 6]. TiO2 photo-catalyst can decompose organic materials such as dyes, peptides and microbes through a series of oxidation processes initiated by the formation of holes (h+) in the valence band and hydroxyl radicals (·OH), while in the conduction band they form radicals (·O2) in oxidized water.TiO2 photo-catalyst activity is determined by several parameters including crystal structure, surface area, size distribution, porosity and hydroxide density [6, 7]. This performance will affect the electron hole recombination time (e-h+) and the adsorption of organic matter contaminants on the surface of the TiO2 photo-catalyst [8]. TiO2 has three crystal structures namely brookite, rutile, and anatase. The last two crystal structures are thermodynamically more stable [9] with the band gap energy (Eg) of ±3.0–3.2 eV. When compared to the rutile structure, the anatase TiO2 phase structure has more excellent photo-catalytic properties including electron transfer rate that is 89 times greater, chemically and biologically inert, mechanical toughness, low cost and non-toxic [10, 11, 12, 13, 14]. The photocatalytic process requires photon energy. The anatase phase structure (Eg: 3.2 eV) requires UV light energy with λ ≥ 385 nm, which is the energy required to produce illumination energy in the anatase TiO2 photo-catalyst process. The photo-catalytic process is strongly influenced by the electron-hole recombination time (e-h+), whereas the recombination can be extended if the doping process is carried out using transition metal ions on the TiO2 surface [15].Synthesis of TiO2 has been carried out via different methods such as sol-gel process [16], non-hydrolytic sol-gel route [17], ultrasonic technique [18], chemical vapor deposition [19], microemulsion or reverse micelles and hydrothermal process [20]. High calcination temperatures above 450 °C are usually required to form its crystal structures. Up to the present time, no method has been reported without calcination to produce anatase TiO2 particles [21]. The nano-sized TiO2 particles doped with metal alloys are of great interest for further development because they can increase the photo-catalytic activity of TiO2 [22, 23]. It is important to note that powder obtained synthetically by sol-gel has several advantages including low temperature, simplicity, microstructure morphology with different phase compositions can be obtained by varying parameters such as temperature, pressure, process duration, chemical species concentration, solution concentration and pH [24, 25, 26].Sterilization is the rate required at cell suspension to inactivate broad-spectrum microbes such as Escherichia coli, Staphylococcus aureus and Bacillus subtilis. The application of TiO2 photo-catalysts to inactivate microbes has been widely reported. Some examples are explained in the following. Using a doped 1% Pd+3 ion on TiO2, it was found that E. coli was inhibited by ± 98% after 2 h of UV radiation [27]. Floating TiO2 photo-catalyst has been used for inactivation of E. coli [28], S. typhimurium [29], and inactivation and inhibition of P. aeruginosa virulence factor expression [30]. TiO2 coated with polystyrene foam has been used for inactivation of E. coli bacteria [31], and antibacterial against A. baumannii [32]. Pt powder doped TiO2 has been used to inactivate L. acidophilus, S. cerevisiae and E. coli, and it was found that TiO2 can be used to replace conventional disinfectant compounds such as chlorination, ozone, and chloride oxides [33, 34, 35, 36].This paper reports a synthesis of FeCuNi nano-alloy doped TiO2 via the sol-gel method. As has been mentioned previously, dopants from the transition metal group have several advantages because they are the catalysts, low energy levels so that they are easy to capture electrons and hence they can inhibit electron hole recombination [23]. Photo-catalyst activity needs photon energy, one of which from UV irradiation, which has the same energy as the energy gap from TiO2 anatase [37]. Based on this consideration, the FeCuNi doped alloy on TiO2 has antibacterial activity with a higher sterilization rate when synergized with UV irradiation. To study the mechanism in which TiO2 can inactivate microbes is based oxidation process and lipid peroxidation [38, 39], which is usually based on the analysis of refractive index, peroxide value (PV) through the formation of malondialdehyde (MDA) compounds as indicators [40].The synthesis of FeCuNi doped into TiO2 Nanoparticles consists of several phases, initially by preparation of titania sol using titanium isopropoxide (TIP) as the basic element, then mixed in the isopropanol solvent. Diethanolamine (DEA) was used as the additive with a ratio of 1:2 TIP to DEA. TIP addition was done by using nitrogen gas flow. The sol was homogenized for ±15 min and then acetate salt was added from Fe, Cu and Ni metals with different composition ratios. The total concentration number was 4 % mol to TIP matrix. Sol solution was then homo-genized for ±2 h at room temperature. Then, the sol was oven-heated at 100 °C–110 °C for ±15 h to allow dry gel formation. To obtain FeCuNi–TiO2 powder, dry gel was burned in the furnace at 400 °C–600 °C under nitrogen gas flow of 100 psi for ± 2–3 h to prevent oxidation of metal Fe, Cu and Ni. FeCuNi–TiO2 was characterized using XRD (X’ Port PAN Analytical, Rigaku RINT–2400), SEM-EDX (JEOL JSM 6360 LA), TEM (Philips CM 12 Analysis Docuversion 3.2 image) and TG-DTA (Quantachrome, Serial 1089111903. Model: AS-68).In this experiment, E. coli (Gram −), S. aureus and B. subtilis (Gram +) were used as models. Nutrient Broth (NB) was used as a media for bacterial culture stock preparation. Pure bacterial culture of dense slant gelatin Nutrient Agar (NA) of 24–48 h was inoculated into the NB as sterile liquid medium. Aerobic base was incubated inside a rotary shaker for 24 h, at 37 °C and 120 rpm speed. Then, cell production was continued in a medium in the same condition. Cells were harvested after 8 h of incubation process and then centrifuged at 8000 rpm, for 15 min. Cell sedimentation was rinsed with sterile aquadest twice repeatedly and then centrifuged again at 8000 rpm for 15 min. The sedimentation was given cell suspension by adding phosphate buffer (pH: 7.0) at 1:10 ratio. Cell suspension preparation for photo-catalyst reaction samples was made by dilution treatment using sterile phosphate buffer up to 103–106 cell/mL cell concentration.To determine the inhibition power of FeCuNi–TiO2 against the growth of bacteria, a diffusion medium which consisted of NA media for bacteria was prepared. ± 15 mL NA was poured into a Petri dish after the medium was frozen, then the surface of the medium was lubricated evenly by the following: bacterial cell suspension with 105 cell/mL cell concentration, at volume of 0.1 mL. A stainless steel cup was used for the addition of 15 mg of FeCuNi–TiO2. Incubation was performed inside a chamber with a vertical lamp radiating UV (λ ≤ 385 nm) at temperature of 37 °C for 24 h. The intensity of UV radiation was monitored by a detector (Blue Light Safety Detector UV) with intensity set at 3.25 mW/cm2. The results of the inhibition zone diameter were measured in millimeters. The processing was done in an aerobic manner, duplo and aseptic. Controlling was done without addition of FeCuNi–TiO2. Using the diffusion method, inhibition efficiency was determined as the optimum sensitivity boundary of FeCuNi–TiO2 to bacteria cells by adding 0–3.5 g/L of FeCuNi–TiO2 powder. Additionally, the inhibition zone was used as a preparation sample to examine the physical injury of microbes with the use of SEM. Sample preparation was done by applying the freeze drying method. Inhibition zones resulting from FeCuNi–TiO2 in bacteria were called the outer inhibition zone and inner inhibition zone, whereas the growing zone is used as a control. These parts were cut into 5 × 5 mm size, and were steamed with 2 % osmium tetroxide (OsO4). The sample was then dipped into liquid nitrogen steam at −210 °C and placed into the freeze dryer (Emitech K 750) for ±10 h. The sample was then coated in gold plating to the size of 5–10 mm and then monitored by SEM.A total of 0.1 mL bacteria cell supernatant was transferred into NA media inside a petri-dish which was then lubricated evenly (spread plate) on the surface of the media, and incubated at 37 °C for 24 h. After 24 h of incubation, the growth of the colony was examined and counted with a colony counter equipment. The count was then converted using the following Eq. (1) which results in a percentage value reported. (1) % Inhibition = the number of colony control − the number of sample colony the number of colony control × 100 % Bacteria cell sample from photo-catalyst was used to determine the malondialdehyde (MDA). A 2 mL sample was transferred into a test tube, added with 4 mL of 10% trichloroacetic acid (TCA), then homogenized and centrifuged at a speed of 11,000 rpm for 45 min. 6 mL of 0.67% 2-Thiobarbituric acid (TBA) was added into supernatant, incubated for 30 min in a hot water bath, then cooled down in an iced cup for 30 min. Next, it was re-centrifuged at speed of 11,000 rpm for 45 min. Supernatant was used in absorbance measurement using spectrophotometric at λ ≤ 400 nm.The preparation of photo-catalyst media were transferred into 1 mL a beaker glass, with the initial bacteria cell suspension of 103–106 cell/mL, and into 9 ml of sterile NB media and 15 mg of FeCuNi–TiO2 powder. The variations in conditions were: Varied radiation system of UV λ ≤ 365 nm and without UV radiation, incubation time of 30–210 min, and 0–3.5 g/l of FeCuNi–TiO2 concentration. As a control, another experiment was performed without added FeCuNi–TiO2 powder. Then, the mixture of photo-catalyst reaction was stirred in a magnetic stirrer or sonicator (50 kHz ultrasonic wave frequency). Radiation intensity was vertically controlled by putting a beaker glass surface in a 30 cm distance from the radiation source. The intensity was monitored by using a detector (Blue Light Safety Detector UV) of 3.25 mW/cm2. The process was done in an aerobic duplo and aseptic manner. The inhibition percentage of bacteria was quantitatively determined by applying 2 measurement methods namely plate count agar (PCA) that is based on the calculation of the number of colonies and TBARs that is based on the number of MDA product formation as a result of peroxide lipid.The XRD patterns of FeCuNi–TiO2 powder synthesized using varying calcination temperatures (400 °C, 500 °C, and 600 °C) are presented in Figure 1 . All the XRD patterns are indexed according to an anatase TiO2 standard diffraction pattern with the tetragonal I41/amd space group (ICSD-154604) and rutile TiO2 with the tetragonal P42/mnm space group (ICSD-97277). All three XRD patterns matched well with the standard XRD of the TiO2 phase without any additional peaks, confirming the formation of single-phase products. No peaks corresponding to oxides of each Fe, Cu, and Ni metals we reobserved in the doped TiO2 samples, which thus demonstrates that the substitution of all metals in TiO2 host lattice was successful.At calcination temperatures of 400 °C and 500 °C, the observed XRD peaks at 2θ = 24.8°, 37.3°, 47.4°, 53.6°, 54.7°, 62.1° corresponding to reflection planes (101), (004), (200), (105), (211), (204) confirmed the formation of single anatase phase of TiO2. When the calcination temperature was raised to 600 °C, the major peaks characteristic of the rutile phase peaks shown at 2θ: 27.4°, 35.7°, and 40.9° appeared, as highlighted in Figure 1. This suggests that a phase transition from anatase to rutile initially occurred around 600 °C, which agrees with previous reports. It was revealed that the calcination process at 600 °C initially led to the structural transformation from anatase to rutile (A → R). Higher temperatures can cause all crystal position turns to defect crystal wherein the cutting-off in M-TiO2 atoms occurs. Consequently, the FeCuNi–TiO2 structure experienced restructuring and a transformation occurred on the structure. Also, the structure of anatase becomes unstable thermodynamically at high temperatures which causes the anatase particles to stick together to form larger particles and the interface of the anatase particles will become the rutile phase nucleation, resulting in the transformation from anatase to rutile phase [41, 42]. The formation of both anatase and rutile phases was further confirmed by the refinement analysis discussed below.It was also noticeable that the XRD peaks became sharper as the calcination temperature was increased, indicating an increase in crystallinity. Using the full width at half maximum value (FWHM), the crystallite size of the particles was estimated using the Debye–Scherrer’s equation [43]. The average crystallite size was approximately 13.9 nm, 16.8 nm, and 20.2 nm, which increased with increasing calcination temperature. It is expected that the calcination temperature plays a crucial role to accelerate the crystal growth, leading to an increase in larger crystallite size and rising intensity of the anatase phase.XRD data were then refined using the Le Bail refinement technique using Rietica software [27] to determine the phase formations and crystal structure in detail. The initial refinements considered the structural parameters of anatase TiO2 with a tetragonal I41/amd space group (a = b = 3.7862 Å, c = 9.4951 Å; α = β = γ = 90°) (ICSD-154604). All structural parameters were then automatically refined to obtain the best fits between the refinement patterns and optimize the value of reliability factors (R p , R wp , and χ 2). Figure 2 shows the Le Bail fits of the XRD patterns of FeCuNi–TiO2 samples. For sample calcined at 400 °C and 500 °C, the refinement was done with a single-parameter system, since the XRD peaks only show the presence of a single anatase phase. The profile plots in Figure 2a and b show good fits between experimental and calculated patterns for both samples and all peaks matched well with the Bragg reflection of the anatase phases, indicating the existence of both phases. The refinement results confirm that the synthesized FeCuNi–TiO2 samples at 400 °C and 500 °C were a single phase of anatase TiO2 without any formation from rutile or brookite phases, which adopts a tetragonal symmetry with a I41/amd space group.Considering the formation of mixed anatase and rutile phase in the sample with a calcination temperature of 600 °C (as highlighted in Figure 2), we therefore refined the XRD data using the multiphase refinement system. Refinement was done accordingly using the parameter of anatase TiO2 phase as the major phase and added parameter of rutile TiO2 with a tetragonal P42/mnm space group (a = b = 4.6257 Å, c = 2.9806 Å; α = β = γ = 90°) as the secondary phase. As a result, the profile of refinement plots displayed in Figure 2c shows a good fit of all XRD patterns and provides clear evidence for the formation of mixed-phase of anatase and rutile TiO2, according to the Bragg reflection of each phase. The phase fractions obtained from the refinement data were approximately 82.3% for the anatase phase and 17.7% for the rutile phase.The refined lattice parameters and unit cell volumes are shown in Table 1 . All lattice parameters essentially increased as the calcining temperature increased, leading to an increase in cell volume. As expected, the increase in crystal volume can be correlated to the increased crystallite size occurring due to varying calcination temperatures. The appropriate value of reliability factors (R p , R wp , and χ 2) justified the accuracy of refinement results. Since the focus of this study was on the stable anatase TiO2 for higher photo-catalytic activity than that of rutile or mixed phases, the sample calcined at a temperature of 500 °C exhibiting pure anatase phase and higher crystallinity was chosen for subsequent analysis.Thermal analysis of nanoalloys FeCuNi–TiO2 under nitrogen atmosphere was performed to study the effect of mass reduction with the increasing temperature. The mass reduction from the TG analysis and the DTA pattern displayed the effects of temperature on the change of the nano structural phase of TiO2 [44]. The reduction in mass at certain temperatures resulted in a change in the phase structure of FeCuNi–TiO2 into two structural phases, namely the anatase phase and the rutile phase. This can be understood since, thermodynamically, the rutile phase is formed at temperatures above 500 °C and the rutile structure is more stable than the anatase phase [45]. Figure 3 shows the TG-DTA FeCuNi–TiO2 pattern, where there are four exothermic patterns that fluctuate in the temperature range of 200 °C to 500 °C. The first pattern in the temperature range of 25 °C–200 °C indicates a reduction in mass of FeCuNi–TiO2 due to the release of water or organic solvents from the precursor and additive mixtures used in the synthesis process. The second stage at a temperature of 300 °C, in which in this condition the mass reduction is greater, indicates degradation of the organic residue. The exothermic pattern at a temperature of 300 °C–400 °C shows crystal growth and a transformation of the FeCuNi–TiO2, phase in the anatase phase structure. At temperatures ≥500 °C, there is a transformation of the anatase structure pattern to the rutile phase structure with greater weight loss and stability at high temperatures [46]. The SEM pattern of FeCuNi–TiO2 resulting from calcinations at 500 °C is shown in Figure 4 a. Each produced a rough surface like that of a piece of rocky stone where ion dopant particles were distributed evenly and homogeneously on the surface of FeCuNi–TiO2 with different sizes. FeCuNi–TiO2 powder surface shown by SEM indicated similarities. However, using EDX measurements, their different chemical compositions were identified: FeCuNi–TiO2 1:2:1, 97.06 % at 4.5 keV (Figure 4b).The TEM pattern from the FeCuNi–TiO2 powder is shown in Figure 5 a. It is observed that the highest photo-catalytic activity occurs in FeCuNi composition with a 1:2:1 ratio. This FeCuNi–TiO2 nanoparticles form three-dimensional crystals that are regularly structured in a spherical shape. The result of TEM measurement shows that most FeCuNi–TiO2 particle sizes are of 10–15.7 nm in size (Figure 5b). The particles are distributed evenly as much as 45%. The results of the TEM measurement on particle size leads to the correlation with the particle size in Debye-Scherrer formula (Eq. (1)).The free radical ·OH attack from the process of electron-hole photo-generation of FeCuNi TiO2 powder at the bacteria cell partition can be indicated as Malondialdehyde compound (MDA) formation. MDA is the final product from the result of an oxide saturated process in the cell membrane. Figure 10 shows the numbers of MDA product which were formed from a series of ·OH radical attacking process in the bacteria cell partition that was determined by applying the TBARs method [47].When TiO2 nano particles exist in a medium containing moisture or water and then receive UV radiation at appropriate wavelength with energy as needed by TiO2 semiconductor, electron-hole photo-generation will occur that produces free hydroxyl radicals ·OH. The ·OH radical is extremely effective as a toxic compound which kills microorganisms. When the ·OH radical interacts with a microbe cell wall, the DNA chromosome of the microbe will develop a thymine dimer that allows knots among thymine base inside the similar DNA strand. This thymine dimer will obstruct the formation of the double helix and disturbs the normal replication of DNA. Cell growth is obstructed and eventually leads to cell death. The inhibition of FeCuNi–TiO2 photo-catalyst at microbe can be explained by the attack from O2 radicals and ·OH of photo-generated electron holes on the catalyst surface. Among the three types of species, ·OH radical is the most reactive because it has very effective oxidation capabilities for various kinds of organic compounds, such as microbe cells [47].The doping process can increase photo-catalytic activity of FeCuNi–TiO2. Doping can stimulate free radical formation with a high hydroxyl series density through a redox reaction on the FeCuNi–TiO2 surface. Doping of transition metal ions which have been multiplied by three can significantly increase the photo-biocatalytic activity in inhibiting microbes. Based on this consideration, the FeCuNi doped alloy on TiO2 has antibacterial activity with a higher sterilization rate when synergized with UV irradiation. The mechanism of TiO2 inactivation against microbes can be studied from oxidation process and lipid peroxidation. Lipid oxidation is usually based on the analysis of refractive index, peroxide value (PV) through the formation of malondialdehyde (MDA) compounds and 2-thiobarbituric acid reactive substances (TBARS) as indicators, as shown in Figure 6 [38, 39, 40].FeCuNi doped by multiple of three is more effective against bacteria E. coli (+++), S. aureus (++) and B. subtilis (+). E. coli, S. aureus, B. subtilis microbes that were chosen as models for the examination of microbe inhibition of FeCuNi–TiO2. These are pathogens in nature when they interact with humans directly or indirectly. E. coli is a negative gram bacterium, having a thinner layer of peptidoglycan cell wall when compared to positive gram bacteria such as S. aureus, and B. Subtilis [46]. Figure 7 b shows the inhibition power of FeCuNi–TiO2 at E. coli greater than S. aureus greater than B. Subtilis.Microbe cells which were given inhibition treatment with FeCuNi–TiO2 powder that have a wide inhibition zone was used to examine the effect of FeCuNi–TiO2, whereas the zone of microbe growth without the FeCuNi–TiO2 treatment was used as a control (Figure 7b). The reaction effect of photo-biocatalyst was observed based on the interaction between bacteria cells and FeCuNi–TiO2 that were blended continuously and each bacterium was given vertical UV radiation as a function of time. As a result of photo-biocatalytic reaction, a number of bacteria cells died off, thus causing the reduction in the number of initial cells by 104 to 105 cell/mL for each bacterium. This germicide action was examined by Upreti, et al (2018), to inhibit the E. coli bacteria inside Luria Bertani culture which was radiated by UV and nanoparticle composite Nd+3 doped TiO2. A longer time of UV radiation caused a reduction in concentration of E. coli bacteria [48].The effectiveness of FeCuNi–TiO2 powder as an antimicrobial is determined by the performance of FeCuNi–TiO2 photo-catalyst. In these pictures (Figure 7), it can be seen that the inhibition response of FeCuNi–TiO2 to the three bacteria species as a function of time can reduce the number of bacteria colonies following the extension of inhibition time. Using Eq. (1), inhibition percentage was counted based on the reduction of the number of initial colonies at 0 h and at the end of photo-catalyst reaction time between bacteria cells and FeCuNi–TiO2 powder. Figure 7b indicates the zone inhibition of E. coli bacteria cells and the control. Species as the function of time can reduce the number of bacteria colonies following the extension of inhibition time. Using Eq. (1), the inhibition percentage was counted based on the reduction of the number of initial colonies at 0 h and at the end of the photo-catalyst reaction time between bacteria cell and FeCuNi–TiO2 powder.The inhibition percentage of bacteria cells by UV radiation was analyzed based on measurements of cell turbidity during UV radiation at 120 min Figures 8 and 9 display physical changes of microbe cells as a result of FeCuNi–TiO2 powder and UV radiation as observed by SEM to ensure the effect of these treatments on bacterial cell damage. Figure 8a, b and c shows the physical appearance pattern of SEM: E. coli; S. aureus; B. subtilis before inhibition by FeCuNi–TiO2 powder, and Figure 8d and e shows the physical appearance pattern of SEM: E. coli; S. aureus; B. subtilis before inhibition by FeCuNi–TiO2 powder. E. coli is 61.6% in Figure 9a, S. aureus is 52.8%, in Figure 9b and B. subtilis is 34.3% in Figure 9c. Without the addition of FeCuNi–TiO2 and UV radiation, the inhibition percentage for cells are as follows: E. coli is 12.2 %, S. aureus is 8.8 % and B. subtilis is 6.5%. Potential synergy was found when FeCuNi–TiO2 was combined with UV radiation in the application of stirring system, where by an increase in photo-biocatalyst activity occurred for each bacterium: E. coli 96.4%, S. aureus 92.8% and B. subtilis 78.3%.When mechanism treatment was applied by means of ultrasonic wave from sonicator at 50 kHz frequency, there occurred an increase in inhibition efficiency by 6% for 120 min application time. The optimum inhibition efficiency of FeCuNi–TiO2 in each bacterium was noted as follows: E. coli 1.0 g/L, S. aureus 1.5 g/L and B. subtilis 1.5 g/L. This inhibition efficiency was determined by applying Diffusion Method (spread plate) based on the calculation of colony.The free radical ·OH attack from the process of electron-hole photo-generation of FeCuNi–TiO2 powder at the bacteria cell partition can be indicated as Malondialdehyde compound (MDA) formation. MDA is the final product from the result of an oxide saturated process in the cell membrane. Figure 10 shows the numbers of MDA product which were formed from a series of ·OH radical attacking process in the bacteria cell partition that was determined by applying the TBARs method [49].In this work, the performance of TiO2 as an antimicrobial agent has been increased via structural modification and particle size using dopant FeCuNi with a ratio of 1:2:1. The performance of FeCuNi–TiO2 is related to the inhibition efficiency improvement against bacteria in the concentration range of E. coli: 1.5 g/L, S. aureus: 1.5 g/L, B. subtilis: 2.0 g/L. The FeCuNi–TiO2 with particle size of 16.8 nm and surface area of 70.98 m2/g provided more effective inhibition activity based on the measurement of the inhibition zone using diffusion method. The inhibition activity from the highest to the lowest is for E. coli followed by S. aureus and B. Subtilis. The photo-catalytic activity of FeCuNi–TiO2 powder as the antimicrobial agent was more effective when it was irradiated using UV with λ = 365 nm, which provided an inhibition percentage in the range of 78.2%–96.4%. The final product from the series of chemical inhibition processes indicated the formation of MDA product from a series of ·OH free radical attacking process in the bacteria cell partition that was determined by TBARs method.Yetria Rilda: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.Syukri Arief: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.Anthoni Agustien: Performed the experimentsEti Yerizel, Nofrijon Sofyan: analyzed and interpreted the data, analysis tools or data.Hilfi Pardi: Performed the experiments; Analyzed and interpreted the data; Wrote the paper.This work was supported by Andalas University (T/9/UN.16.17/PT.01.03/IS-RPBQ/2022, 7 Juli 2022).Data included in article/supplementary material/referenced in article.The authors declare no conflict of interest.No additional information is available for this paper.

This study reports the application of FeCuNi nano-alloy doped TiO2 synthesized via the sol-gel method as an antibacterial with a sterilization rate greater than 95% under ultra-violet (UV) irradiation. The performance was characterized using X-ray diffraction (XRD), thermal analysis (TG-DTA), scanning electron microscope (SEM-EDX), and transmission electron microscope (TEM). The results showed that the sterilization process of FeCuNi–TiO2 in cell suspension of Escherichia coli, Staphylococcus aureus and Bacillus subtilis increased the effectiveness of UV irradiation at wavelength (λ) ≥ 385 nm after 120 min. The optimum growth inhibition of FeCuNi–TiO2 was observed in the concentrations 1.5 g/L of E. coli, 1.5 g/L of S. aureus and 2.0 g/L of B. subtilis. The highest antimicrobial efficiency of FeCuNi–TiO2 powder was provided by a particle size of 16.8 nm, surface area of 70.98 m2/g. The increased antimicrobial activity in multiplied-three doped ions was related to the increase of illumination energy of UV absorption in the photo-catalyst process. The inhibition mechanism reaction of the three species of bacteria cell affects the lipid peroxidation process at the microbe cell’s wall. This was indicated by the formation of malondialdehyde (MDA). Lipid oxidation was based on the reaction of 2-thiobarbituric acid (TBARS) as an indicator of primary and secondary oxidation.

The hydrogenation of 2,4-dinitrotoluene (DNT) is an important industrial process to produce 2,4-toluelendiamine (TDA). TDA is an intermediate in the production of toluene diisocyanate (TDI), which is one of the main components in the manufacture of polyurethane (PU) [1]. In the catalytic hydrogenation of DNT, carbon-, silica- and alumina-supported transition metals (Pd, Pt, Ni, etc.) or transition metal oxides are the most commonly used catalysts [2–8]. Many intermediates can be formed during the process, such as 4-(hydroxyamino)-2-nitrotoluene (4HA2NT), 2-(hydroxyamino)-4-nitrotoluene (2HA4NT), 4-amino-2-nitrotoluene (4A2NT) and 2-amino-4-nitrotoluene (2A4NT) [9–11]. The best catalysts have high catalytic activity, it is easy to use them, and their recovery is excellent. However, it is difficult to separate powder catalysts (such as supported-activated carbon) from the reaction medium due to their stable dispersion forming ability. The use of magnetic nanoparticles might be a solution to this problem, because the small, mobile particles can be easily removed and isolated by a magnetic field. This simplifies the catalyst recovery and recyclability [12], and by avoiding conventional filtration and centrifugation methods, the associated catalyst loss is also eliminated. Therefore, the use of magnetic nano-catalysts is a promising alternative, especially in heterogeneous catalysis. One of the most widely used magnetic material is maghemite [13,14]. Various methods have been applied to synthesize maghemite nanoparticles, such as sol-gel synthesis [15,16], microemulsion [17,18], coprecipitation [19,20], hydrothermal [21], flow injection [22], and combustion methods [23]. However, the overall process usually includes several steps, and post-treatment to activate the catalysts is also required. Therefore, attempts to simplify the catalyst production process were made and a new method has been developed in our research group [24,25]. By using this method, an active catalyst has been developed during the impregnation step [24,25]. The essence of the method is the exposure of the liquid medium to intense ultrasonic effects, where the induced sound waves create cycles of high and low pressure. Thus, the vapor pressure of the solvent is decreasing momentarily, which results in the formation of bubbles of a few micrometers in size in the mixture. These bubbles are pulsating and growing until they reach a higher pressure range in the liquid, where they collapse as the pressure increases [26]. At this point (“hot spot”), a large amount of energy is released, causing the medium to act as reducing agent in the reaction and to initiate the formation of metal, metal oxide, or metal hydroxide solid particles [27–32]. By using our recently developed sonochemical method, in this work palladium and platinum nanoparticles were deposited on the surface of maghemite and tested in the catalytic DNT hydrogenation reaction.Iron(III) citrate hydrate (FeC6H5O7 x H2O, PanReac AppliChem Ltd) and polyethylene glycol with 400 g/mol molar mass (PEG400, Sigma Aldrich Ltd) were used for the production of maghemite nanopowder. Palladium(II) nitrate dihydrate (Pd(NO3)2 x 2H2O, Merck Ltd), hydrogen hexachloroplatinate (H2PtCl6, Reanal Ltd), hydrazine hydrate (N2H4 x H2O, Sigma Aldrich Ltd) and patosolv (a mixture of 90 vol% ethanol and 10 vol% isopropanol, Molar Chem. Ltd) were used to provide the palladium and/or platinum content of the metal supported maghemite catalysts.2,4-dinitrotoluene (DNT, C7H6N2O4) was used as reactant, and 2,4-diaminotolune (C7H10N2), 2-methyl-5-nitroaniline, 2-methyl-3-nitroaniline, 4-methyl-3-nitroaniline, and 4-methyl-2-nitroaniline (C7H8N2O2) were used as standards (Sigma Aldrich Ltd) for the GC–MS measurements during the catalytic tests. Methanol (CH3OH) was used as solvent (Merck Ltd) during these measurements.The maghemite nanoparticles and the palladium decorated maghemite were examined by high-resolution transmission electron microscopy (HRTEM, FEI Technai G2 electron microscope, 200 kV). During the preparation step, drops of the aqueous suspension of samples were placed onto copper grids (Ted Pella Inc., only carbon, 300 mesh). The size of the nanoparticles was estimated based on the HRTEM images and the original scale bar by using the ImageJ software. Powder X-ray diffraction (XRD) measurements were conducted by using a Rigaku Miniflex II diffractometer with CuKα radiation source (30 kV, 15 mA). To identify the functional groups on the surface of the maghemite nanopowder, a Bruker Vertex 70 Fourier transform infrared spectrometer (FTIR) was used. The prepared maghemite (2 mg) was added to 250 mg spectroscopic potassium bromide, and after homogenization a pellet was formed which was used in the measurements.Specific surface area (SSA, m2/g) of the catalysts was determined by nitrogen adsorption-desorption measurements at 77 K using a Micromeritics ASAP 2020 sorptometer and based on the Brauner-Emmett-Teller (BET) method. The carbon content of the maghemite was measured by Vario Macro CHNS element analyser equipment, and phenanthrene was used as standard (C: 93.538%, H: 5.629%, N:0.179%, S: 0.453%; Carlo Erba Inc). The carrier gas used was He (99.9990%), while O2 (99.995%) was used for oxidation, and the samples were loaded onto tin foils.A recently developed combined method [24,25] which includes combustion and sonochemical steps was applied to synthesize maghemite nanoparticles (Fig. S1, first two steps). In the first step, 7.0 g iron(III) citrate hydrate was dispersed in 40.0 g polyethylene glycol (PEG 400) by using a Hielscher UIP1000Hdt ultrasound tip homogenizer for 5 min (20 kHz). The colour of the PEG 400-based dispersion changed to red, which indicated that iron oxyhydroxide (ferrihydrite and goethite) colloid has formed. In the second step, the pegylated iron oxyhydroxide dispersion was heated up and the organic compound was burned. Thus, magnetic nanopowder (mainly maghemite) was formed.Palladium nitrate (0.25 g) was solved in 50 mL patosolv, and 2.0 g maghemite was added to the solution to synthesize 5.0 wt% Pd/maghemite catalyst. In the case of 5.0 wt% Pt- containing catalyst preparation, 0.21 g H2PtCl6 was added to 2.0 g maghemite, and 1 mL hydrazine hydrate was also used. The alcoholic dispersion of the precious metal and maghemite was sonicated for 2 min by using the tip homogenizer (20 kHz, 78 W). Pd or Pt was deposited onto the magnetic nanopowder solid. The catalysts were then removed from the cleared and transparent alcoholic phase with a neodymium magnet, washed with patosolv, and dried at 105 °C overnight. A bimetallic catalyst (Pd-Pt/maghemite) with 4.5 wt% Pd and 0.5 wt% Pt was also prepared as described above.The catalytic hydrogenation of 2,4-dinitrotoluene (DNT) was carried out in a Büchi Uster Picoclave reactor (200 mL stainless steel vessel with heating jacket). The pressure of H2 was kept at 20 bar, and the reaction mixture was kept at 303, 313, 323 and 333 K. Sampling was carried out after 5, 10, 15, 20, 30, 40, 60, 120, 180, and 240 min on reaction stream. The initial concentration of DNT was 0.05 mol L−1 in methanol, and 150 mL DNT solution and 0.1 g catalyst were applied during the tests. The formed by-products and reaction intermediates were identified by using a JMS-T200GC AccuTOF GCx-plus chromatograph and a JEOL JMS-T200GC mass spectrometer. For the GC measurements, ZB-1MS column (30 m × 0.25 mm, 0.25 μm) was used. The collected data were analysed and detected molecular species were assigned by using “NIST library search”, “Molecular ion search”, “Exact Mass Analysis of Molecular Ion”, “Isotopic Pattern Analysis” and “EI Fragment Ion Analysis”. TDA formation was followed by using an Agilent 7890A gas chromatograph coupled with Agilent 5975C Mass Selective detector. To determine the formed products, analytical standards (2,4-dinitrotoluene, 2,4-diaminotolune, DNT, 2-methyl-5-nitroaniline, 2-methyl-3-nitroaniline, 4-methyl-3-nitroaniline, 4-methyl-2-nitroaniline, Sigma Aldrich Ltd.) have been used.The activity and selectivity (towards TDA) of the catalysts were determined by calculating the conversion (X, %) of DNT and the TDA yield (Y, %) based on the following Eqs. (1) and (2), respectively: (1) X % = consumed n DNT initial n DNT ∙ 100 (2) Y % = n formed TDA n theoretical TDA ∙ 100 Kinetic measuremetns on the studied catalytic reaction in the batch reactor system used were conducted based on initial rates estimation aiming to determine the reaction orders with respect to DNT and H2. Towards this goal, the initial concentration of DNT (cDNT,0) was varied (25, 30 and 40 mmol/L) at constant PH2 of 20 bar, while the total pressure of H2 (PH2) was varied (10, 20, 30 and 40 bar) at constant cDNT,0 = 50 mmol/L for the Pd/maghemite catalytic system. The DNT concentration decay after the first 2 min was used to estimate the initial rate of reaction (v 0) by the following Eq. (3): (3) v 0 = − d DNT dt The initial rate (mmol/L s−1) of reaction can be expressed by Eq. (4): (4) v 0 = k eff DNT α H 2 β The apparent reaction orders with respect to DNT (α) and H2 (β) were determined by the linear fit of the lgc DNT, 0 vs. lgvo relationship at P = 20 bar and lgc DNT, 0 vs. lgvo at cDNT,0 = 50 mmol/L using the logarithmic form of Eq. (4): (5) lg v 0 = lg k eff + αlg c DNT , 0 + βlg p H 2 where, keff is the effective rate constant (T = 303 K).The magnetic catalyst support was examined by HRTEM, and the γ-Fe2O3 nanoparticles are clearly visible (Fig. S2, A). The nanopowder is highly dispersed given the average particle size of 22.0 ± 6.6 nm (Fig. S2, B). The FTIR results indicates that the nanopowder contains carbon as well (Fig. S2, C). The presence of carbon was confirmed by the appearance of the symmetric and asymmetric vibrational bands of the CH stretching at 2892 and 2835 cm−1. Another IR band at 1631 cm−1 also shows the presence of carbon as it can be assigned to the stretching of the CC bonds. Carbon remained in the sample as a product of the combustion of polyethylene glycol. The exact carbon content was measured by CHNS elemental analysis, and it was found that the sample contained 2.83 wt% carbon. The magnetic nanopowder contains hematite as well (6.8 wt%) next to the main maghemite phase (Fig. S2, D).Powder XRD measurements were carried out to clarify the phase composition of the magnetic support. After the sonochemical treatment, the PEG-based dispersion was filtered and washed with distilled water and dried at room temperature in vacuum overnight. Based on the diffractogram of this sample, goethite (α-FeO(OH), 10.7 wt%), ferrihydrite (Fe3+ 10O14(OH)2, 22.1 wt%), and PEG (67.2 wt%) were identified (Fig. S3). Based on this, it can be concluded that during the ultrasonication the iron(III) citrate reacted with polyol forming iron oxyhydroxide species which transformed through dehydration/dehydroxylation processes to maghemite and hematite during combustion.BET surface area analysis was carried out for the maghemite-supported catalysts and the support alone. The Pt/maghemite system had the smallest surface area, ca. 10.1 m2/g, followed by the bimetallic system, ca. 19.2 m2/g. The Pd/maghemite catalyst had a surface area of 23.1 m2/g which is more than double compared to the Pt/maghemite catalyst. The specific surface area of the catalyst support alone is 41.7 m2/g.The morphology of the solid particles of catalysts has been studied by using HRTEM (Fig. S4, A, C and E). The individual maghemite nanoparticles formed aggregates (ca. 100–150 nm). The Pd and Pt nanoparticles were indistinguishable from the support particles on the HRTEM images. However, the powder XRD results confirmed that reduction of platinum and palladium ions was efficient since the samples contain pure metallic phases. On the X-ray diffractogram of the Pd/maghemite system, Pd(111) and Pd(200) lines can be identified at 40.3° and 46.4° 2theta, respectively (Fig. S4, B and Fig. S5). In the case of Pt catalyst, the Pt(111), Pt(200), and Pt(220) lines were detected, which indicates the presence of elemental platinum (Fig. S4, D and Fig. S6). The presence of the precious metals was detectable in the bimetallic Pd-Pt/maghemite catalyst also (Fig. S4, F and Fig. S7). The average size of the Pd and Pt nanoparticles was 4.6 and 7.4 nm, respectively, based on the XRD results and using the Scherrer equation.The maghemite support alone showed a DNT conversion as high as 77.6% at 333 K, while the TDA yield was 30.5% after 240 min of reaction (Fig. S8). However, due to the low TDA yield, adding a precious metal to the system is essential. The apparent reaction order with respect to each of the reactants and the rate equation of reaction was determined experimentally. The reaction order with respect to DNT (α) and H2 (β) were determined according to the linear fitting shown in Fig. 1 .A fractional reaction order of ~1.4 is obtained, while the reaction order parameter with respect to hydrogen can be considered as unity (0.97 ± 0.06), meaning a first order kineticsat the studied experimental conditions. The derived effective rate constant (Eq. (4)) of the Pd/maghemite system was found to be in the range of 6.4 × 10−5 and 3.3 × 10−5 (mmol-0.4 L-0.4 bar−1 s−1). The apparent reaction order with respect to H2 (β) is significantly lower in the case of supported Pt (0.55 ± 0.05, Fig. S9) than Pd catalyst (0.97 ± 0.06) which might suggest some strong interactions of H2 with the catalyst surface, thus leading to high chemical adsorption rates [33].In the case of Pd/maghemite catalyst, TDA yield was not changed significantly with the reaction temperature, where 30 K difference in temperature resulted only in ~8% improvement in the yield. The maximum TDA yield was 82.6% and it was achieved by using the Pd catalyst at 333 K and 20 bar hydrogen pressure (Fig. 2 ). The supported platinum catalyst provided only 62.0% yield. However, both catalysts can be easily separated from the reaction medium by magnet (Fig. 2).A higher TDA yield was achieved by the Pd/maghemite catalyst, and thus, this was applied in the reuse tests (Fig. 3 ). The DNT conversion was stable and did not decreased significantly even after four cycles (Fig. 3). The corresponding TDA yields remained above 80% during the tests (Fig. 3).Two intermediates were identified during the reaction, 4-amino-2-nitrotoluene (4A2NT) and 2-amino-4-nitrotoluene (2A4NT), which are the semi‑hydrogenated species of DNT. The less active Pt/maghemite sample was not able to convert completely the intermediates to TDA (Fig. S10, A, B), while the Pd containing catalyst was successfully achieved this within a reasonable time (Fig. S10, C, D).Various by-products have been formed during the hydrogenation, and these were identified by using isotopic pattern analysis. Condensed derivatives of DNT and TDA such as p-tolualdehyde-2,4-dinitro-phenylhydrazone (C14H12N4O4) and 2-methyl-1-[(2-methyl-4-nitrophenyl)-NNO-azoxy]-4-nitrobenzene (C14H12N4O5) and others have been formed (Table S1).Although both precious metals deposited on maghemite support were highly active, a bimetallic catalyst was also prepared (4.5 wt% Pd and 0.5 wt% Pt) and tested under the same reaction conditions as the monometallic ones. The rates of hydrogenation at 303 and 313 K were similar (Fig. 4 A), and the reaction rate constant (k) values estimated were 5.4 × 10−3 s−1 and 5.7 × 10−3 mmol-0.4 L-0.4 bar−1 s−1, respectively. The reaction rate constants were very similar (1.8 × 10−3 s−1 and 1.9 × 10−3 mmol-0.4 L-0.4 bar−1 s−1) at 333 K when the monometallic Pd and Pt catalysts were used, but a significant increase of k value (5.7 × 10−3 ± 4.0 ∙ 10−5 mmol-0.4 L-0.4 bar−1 s−1) was achieved (~ 3 times) with the bimetallic catalyst (Fig. 4 B). After 10 min of hydrogenation, the DNT conversion was 90.4% at 333 K compared to 65.0% in the case of Pd/maghemite. The TDA yield reached 86.8% at 333 K with the Pd-Pt/maghemite catalyst, which is slightly higher compared to the Pd/maghemite system (82.6% at 333 K).Nanosized maghemite powder was synthesized by using a recently developed combined combustion and sonochemical methods. By ultrasonic treatment, iron oxyhydroxide species formed were transformed through dehydration/dehydroxylation processes to maghemite and hematite phases during the combustion step. The prepared magnetic nanopowder was used as catalyst support. Monometallic palladium and platinum maghemite-supported catalysts and their bimetallic PdPt counterpart were successfully synthesized after using a fast, relatively easy, and efficient catalyst preparation method, which does not include post-treatments. Their catalytic activity for the 2,4-toluenediamine (TDA) synthesis was tested, and in each case full conversion of 2,4-dinitrotoluene (DNT) was achieved after 60 min. However, the TDA yield was higher when Pd/maghemite (82.6% at 333 K) catalyst was used compared to the Pt/maghemite case (62.0% at 333 K). By combining the two precious metals, a more active bimetallic catalyst was developed and 90.4% of DNT conversion was reached after 10 min. Furthermore, after 30 min of reaction, full conversion of DNT was obtained over the bimetallic catalyst, while the monometallic catalysts exhibited lower conversion rates at the same reaction time of 30 min. In the present work, three maghemite-supported magnetic catalysts were successfully produced in an easy and fast synthetic route, and their catalytic activity was remarkable. In addition, these maghemite-based catalysts are easily separable from the reaction medium due to their magnetic behaviour.On behalf of all authors, the corresponding author states that there is no conflict of interest.This research was supported by the European Union and the Hungarian State, co-financed by the European Regional Development Fund in the framework of the GINOP-2.3.4-15-2016-00004 project aimed to promote the cooperation between higher education and the industry. We also thank Angelos M. Efstathiou for his insightful comments and corrections, which significantly improved the quality of the manuscript. Supplementary material Image 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2021.106342.

Maghemite particles were synthesized by using a combined combustion method and sonochemical step. Maghemite was used as carrier to prepare supported Pt and Pd catalysts after deposition via a sonochemical step. The catalysts were immediately ready to be used (metals were catalytically active) for the 2,4-dinitrotoluene hydrogenation to produce 2,4-toluenediamine. The most active catalysts were the Pd/maghemite and bimetallic Pd-Pt/maghemite. The catalysts were easily separable after reaction due to their magnetic properties.

Methanol is an essential commodity to produce key chemicals and intermediates, and an appealing energy carrier [1–3]. Its thermocatalytic production (CO2 + 3H2 → CH3OH + H2O) from captured CO2 and renewable H2 or CO2-rich feeds obtained via biomass gasification is an attractive approach to effectively combat global warming [1,3–7]. In fact, the absolute sustainability of a process for methanol synthesis using captured CO2 was shown by a system engineering analysis based on planetary boundaries [4]. Still, heterogeneous catalysts possessing high methanol productivity and durability and efficiently utilizing costly renewable H2 are needed to meet requirements for industrialization. First uncovered in 2016, indium oxide (In2O3) is regarded as a breakthrough system due to its very high methanol selectivity [8–10]. In-depth kinetic and mechanistic studies revealed that suppression of the competitive reverse water-gas shift reaction (RWGS: CO2 + H2 → CO + H2O) is mostly determined by the heterolytic splitting of H2 on the oxygen vacancies acting as catalytic centers [8,9,11–13]. Among many carriers explored, monoclinic zirconia (m-ZrO2) stood out as a unique material, providing activity improvements beyond conventional dispersion effects and ensuring stability for over 1000 h on stream [8,14–18]. These observations were attributed to its ability to assist CO2 activation and to induce the formation of epitaxially-grown In2O3 or solid In2O3-ZrO2 solutions, featuring additional and/or superior oxygen vacancies [15,16]. Since H2 splitting is rate-limiting on bulk In2O3, several hydrogenation metals (i.e., palladium [19–23], gold [24], rhodium [25], platinum [26], cobalt [27,28], and nickel [19]) were lately evaluated as promoters. While nickel is most suited among low-priced promoters, palladium in the form of low-nuclearity clusters embedded in In2O3 unlocked an unprecedented sustained productivity of ca. 1 gMeOH h−1 gcat −1 aiding H2 activation without displaying the intrinsic RWGS activity of palladium particles [20].In general, catalytic performance in carbon dioxide hydrogenation has so far been determined upon single-pass conversion using purified CO2 and H2, but looking towards practical implementation requires considering that CO2 streams attained from distinct carbon capture technologies or gasification processes commonly contain CO in the range of 0.03−20 vol.% (Fig. 1 ) [29–34]. Additionally, since recycling of unreacted gases is needed to achieve competitive methanol yields and efficiently utilize H2, also CO produced by the RWGS will be recycled back into the reactors in view of the excessive cost for its separation from CO2 [35–38].At present, fundamental understanding of the effect of CO on the performance of In2O3-based systems for CO2 hydrogenation remains elusive, being limited to few studies on the bulk oxide [8,11]. CO co-feeding was shown to significantly boost methanol productivity (CO/CO2 = 4; p CO =0.19 MPa) due to the in situ creation of additional oxygen vacancies [11], or deplete it (CO/CO2 = 1; p CO =0.09 MPa) due to In2O3 over-reduction [8]. Feeding only CO in H2 led to a total activity loss, with In2O3 being fully transformed into metallic indium [8].Herein, we thoroughly examined the behavior of In2O3 in bulk form, and when supported on m-ZrO2 and other carriers, or promoted by palladium and nickel in CO2-based methanol synthesis using hybrid CO2-CO feeds containing practically-relevant concentrations of CO. By applying cycle experiments, in which CO2 is step-wise replaced by CO and vice versa, CO-induced deactivation was observed for all catalysts except for In2O3/m-ZrO2, which was activated. To access structural and electronic alterations underpinning these effects, in-depth characterization was conducted, identifying a main positive contribution by CO and distinct interconnected (de)activation mechanisms. The more attractive In2O3/m-ZrO2 and Pd-In2O3 catalysts were investigated through further dedicated testing protocols to define operation strategies maximizing their methanol productivity. Overall, this study highlights catalytic assessment with CO-containing feeds as an essential element to broaden fundamental understanding and thus guide upcoming design of industrially-amenable systems for CO2-based methanol synthesis.Bulk In2O3 was prepared through controlled calcination of In(OH)3 precipitated according to a reported method [20]. Briefly, In(NO3)3·7H2O (15.5 g, Alfa Aesar, 99.999 %) and Na2CO3 (20 g, Merck, >99 %) were separately dissolved in deionized water (235 and 200 cm3, respectively). Thereafter, the sodium carbonate solution was added to the indium-containing solution under magnetic stirring at ambient temperature until reaching pH 9.2 (ca. 158 cm3, 9 cm3 min−1) and the resulting slurry was aged for 1 h. The precipitate was recovered by high-pressure filtration, washed three times with deionized water (1 L each time), dried in a vacuum oven (2 kPa, 323 K, 12 h), and calcined at 573 K (heating rate =2 K min−1) for 3 h in static air to yield In2O3.Supported catalysts with 5 wt.% In2O3 were attained by wet impregnation (WI) using self-prepared tetragonal zirconia (t-ZrO2) [15] and commercial monoclinic zirconia (Saint-Gobain NorPro, 95 %) and alumina (Sigma Aldrich, 99 %). To produce t-ZrO2, 20 g of a ZrO(NO3)2 solution (Sigma-Aldrich, 35 wt.% in diluted HNO3, >99 % trace metals basis) were diluted with deionized water (100 cm3). Ethylenediamine (Sigma-Aldrich, >98 %) was added dropwise (ca. 3 cm3 min−1) to this solution until reaching pH 10 and the resulting slurry was stirred at 353 K for 3 h. The precipitate was recovered by high-pressure filtration, washed three times with deionized water (250 cm3 each time), dried in a vacuum oven (2 kPa, 323 K, 12 h) and calcined at 973 K (3 K min−1) for 3 h in static air. WI encompassed suspending 2 g of carrier in a mixture of deionized water (54 cm3) and ethanol (70 cm3, Fisher Chemicals, 99.8 %). The resulting slurry was magnetically stirred (500 rpm) for 12 h at room temperature. Thereafter, the solvent was removed using a rotary evaporator (Büchi Rotavap R-114) at 323 K, keeping the slurry constantly at boiling point by lowering the pressure from 180 to 40 mbar. The solid was then dried in a vacuum oven (2 kPa, 323 K, 12 h) and calcined at 773 K (2 K min−1) for 3 h in static air.In2O3 catalysts promoted with 0.75 wt.% Pd or 1 wt.% Ni were produced by coprecipitation (CP, catalysts noted as Pd-In2O3 and Ni-In2O3) and dry impregnation (DI, catalysts coded as Pd/In2O3 and Ni/In2O3) [20]. For CP, In(NO3)3·7H2O (3.5 g) and Pd(NO3)2.5H2O (0.025 g, Sigma-Aldrich) or Ni(NO3)2·6H2O (0.056 g, Sigma-Aldrich, >97 %) were dissolved in deionized water (50 cm3). A solution of Na2CO3 (10 g) in deionized water (100 cm3) was added under magnetic stirring (500 rpm) until pH 9.2. The resulting slurry was aged for 1 h. Thereafter, it was diluted with additional deionized water (50 cm3) and the precipitate was recovered by high-pressure filtration, washed three times with deionized water (1.25 L each time), dried in a vacuum oven (2 kPa, 323 K, 12 h), and calcined at 573 K (2 K min−1) for 3 h in static air. Upon DI, Pd(NO3)2.5H2O (0.095 g) or Ni(NO3)2·6H2O (0.21 g) dissolved in deionized water (0.4 cm3) were added to In2O3 (4 g, calcined at 573 or 773 K in the case of palladium or nickel promotion, respectively) upon mechanical stirring. After drying in a vacuum oven (2 kPa, 323 K, 12 h), calcination was conducted at 573 K (2 K min−1) for palladium-containing samples and at 623 K for those comprising nickel for 3 h in static air.Inductively coupled plasma optical emission spectrometry (ICP-OES) was conducted using a Horiba Ultra 2 instrument equipped with a photomultiplier tube detector. Samples were dissolved in aqua regia during 12 h prior to the analysis, except for zirconia-containing materials. These were digested with the aid of microwave irradiation using a mixture of HCl (Alfa Aesar, 36 wt.%), H2SO4 (Alfa Aesar, 95 wt.%), and HF (Sigma Aldrich, 48 wt.%) with a volume ratio of 2:1:0.5, followed by neutralization with a saturated solution of boric acid (Fluka, 99 %). Nitrogen sorption at 77 K was carried out using a Micromeritics TriStar II analyzer. Prior to the measurement, samples were degassed at 473 K under vacuum for 12 h. The total surface area (S BET) was determined using the Brunauer-Emmet-Teller (BET) method. X-ray diffraction (XRD) was measured in a PANalytical X’Pert PRO-MPD diffractometer operated in the Bragg-Brentano geometry using Ni-filtered Cu Kα radiation (λ=0.1541 nm). Data was acquired in the 10 − 70° 2θ range with an angular step size of 0.025° and a counting time of 12 s per step. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) coupled to energy-dispersive X-ray spectroscopy (EDX) was measured using a Talos F200X instrument operated at 200 kV and equipped with a FEI SuperX detector. Temperature-programmed reduction with CO (CO-TPR) was conducted using a Micromeritics AutoChem HP II at ambient pressure. Samples were loaded in a stainless-steel tube, dried at 423 K in He for 1 h, and cooled down to 273 K (20 K min−1) using dry ice in ethanol. The temperature-programmed reduction was then carried out using 1 mol% CO/He and increasing the temperature to 1073 K (5 K min−1). X-ray photoelectron spectroscopy (XPS) was performed in a Physical Electronics (PHI) Quantum 2000 X-ray photoelectron spectrometer using monochromatic Al Kα radiation generated from an electron beam operated at 15 kV and 32.3 W and a hemispherical capacitor electron-energy analyzer, equipped with a channel plate and a position-sensitive detector. Samples were firmly pressed onto aluminum foil patches, which were then mounted onto a sample platen and introduced into the spectrometer. Analyses were conducted under ultra-high vacuum conditions (residual pressure = 5 × 10−8 Pa) with an electron take-off angle of 45°, operating the analyzer in the constant pass energy mode.The gas-phase hydrogenation of CO x (CO x = CO + CO2) to methanol was performed in a PID Eng&Tech high-pressure continuous-flow setup comprising four parallel fixed-bed reactors (internal diameter =4 mm) (Figure S1 in the Supplementary Material). The undiluted catalyst (mass (w cat) =0.1 g, particle size = 0.2−0.3 mm) was loaded in each reactor, held in place by quartz wool bed set on a quartz frit, and purged with a He (Pangas, purity 4.6) flow of 40 cm3 STP min−1 for 30 min at ambient pressure. Under the same flow, the pressure was increased to 5.5 MPa for a leak test. The reaction was carried out by feeding a mixture of H2, CO (Messer, purity 5.0), and CO2 (20 vol.% in H2, Messer, purity 4.5), with a molar H2/CO x ratio of 4 at 553 K, 5 MPa, and weight hourly space velocity (WHSV) of 24,000 cm3 STP h−1gcat −1, unless stated otherwise. The impact of CO on the performance of the catalysts was assessed by means of CO2-CO cycle experiments, which are summarized in Fig. 2 . During full cycles (FC, Fig. 2a), CO2 in the feed was progressively replaced by CO from R ratio (R = CO/CO x , with CO x = CO + CO2) equal to 0 to 0.5 in steps of 0.1 and then replenished by replacing CO in an analogous manner, while maintaining a constant molar H2/CO x ratio of 4. Half-cycles (HC, Fig. 2b) were conducted starting at R = 0 and increasing the CO concentration to reach R = 0.5, while reverse half-cycles (rHC, Fig. 2c) started at R = 0.5 and ended at R = 0. In both cases, the same step size of 0.1 was kept as in full cycles. The effect of temperature (543−583 K) and pressure (3–5.5 MPa) at R = 0.2 was investigated while keeping all other conditions constant.Additional experiments were specifically designed to further assess In2O3/m-ZrO2 and Pd-In2O3. For the former, CO x hydrogenation was carried out at R = 0, 0.2, or 0.5 after a pre-treatment in syngas (H2/CO = 2) of 4 h at 553 K and 5 MPa (Fig. 2d) and an extended full cycle was conducted, equivalent to the regular full cycles but reaching R = 1. For the latter, threshold limit cycles (Fig. 2e) were applied, alike to the regular full cycles but attaining R = 0.1, 0.2, or 0.3. Long-term cycle experiments (Fig. 2f) were conducted on both systems starting at R = 0, moving to R = 0.2, and then restoring R = 0, keeping each condition for 40 h.Response factors (Fi ) for each compound i in the effluent stream, respective to the internal standard (20 vol.% C2H6 in He, Messer, purity 3.5), in gas chromatographic analysis were determined by Eq. 1, where Ai is the integrated area determined for the peak of compound i and n i in is the corresponding known molar flow at the reactor inlet. (1) F i = A i / n i in A C 2 H 6 / n C 2 H 6 in Their values were calculated using the average of 5 points around the expected concentration of the respective analyte. The unknown effluent molar flow and methanol production rate (r MeOH) were determined using Eqs. 2 and 3, respectively. (2) n i out = A i × F i A C 2 H 6 × n C 2 H 6 out (3) r MeOH = n MeOH out w cat ,     mo l MeOH h − 1 g cat − 1 The methanol space time yield (STY) was determined as the product of r MeOH and the molar weight of methanol. Data reported corresponds to the average of 4 measurements preceding a specific time on stream. The carbon balance (ε) was determined for each experiment according to Eq. 4. (4) ε c = 1 − n C O 2 out + n MeOH out + n CO out n C O 2 in + n CO in × 100 , % The equilibrium composition of the carbon-based species in the reaction mixture upon single-pass conversion was estimated in the temperature range of 423−723 K at 5 MPa and H2/CO2 = 4 using the AspenPlus v10 software as described in Fig. S2. In the calculations, the Gibbs energy was minimized by using the RGibbs model reactor, and thermodynamic constants from Eqs. 5–7 as well as Soave-Redlich-Kwong equations of state were applied. (5) CO + 2H2 ↔ CH3OH, ΔH 298 K = –90.8 kJ mol−1 (6) CO2 + 3H2 ↔ CH3OH + H2O, ΔH 298 K = –49.2 kJ mol−1 (7) CO2 + H2 ↔ CO + H2O, ΔH 298 K = 42.1 kJ mol−1 The per-pass and overall conversion, and R value of the feed with recycling of outlet gases, at 553 K, 5 MPa, 24,000 cm3 STP h−1 gcat −1 and keeping H2/CO x at 4, were calculated using Eqs. 8–10 and results from simulations (Table S1) based on a simplified process flowsheet built in AspenPlus v10 and shown in Fig. S2b. (8) per-pass conversion = C O 2,   reactor   inlet − C O 2,   reactor   outlet C O 2,   reactor   inlet × 100 ,     % (9) overall conversion = C O 2,   fresh   feed − C O 2,     product + C O 2,     purge C O 2,   fresh   feed × 100 ,     % (10) R = CO C O x = C O   reactor   inlet C O   reactor   inlet + C O 2,   reactor   inlet The effect of CO on CO2-based methanol synthesis was explored on a broad set of In2O3 catalysts. Most prominent systems comprised bulk In2O3, In2O3 supported on m-ZrO2, and In2O3 promoted by palladium through co-precipitation and by nickel via dry impregnation. In2O3 carried on tetragonal zirconia and alumina as well as In2O3 promoted by nickel through co-precipitation and palladium via dry impregnation were additionally considered for comparative purposes. These materials were prepared according to reported synthesis methods [15,20]. Compositional analysis of the fresh catalysts (Table S2) revealed that the nominal loadings of indium oxide (5 wt.%) and transition metals (0.75 wt.% for Pd and 1 wt.% for Ni) were closely matched in supported and promoted samples, respectively. The BET surface areas and XRD patterns of all fresh solids were also in line with expectations and previous evidence (Table 1 and Figure S3a,b) [11,15,20].To assess their sensitivity to CO, the catalysts were tested in a four parallel continuous-flow fixed-bed reactor setup by means of CO2-CO cycles. Initially, in so-called full cycles, the CO2 in the feed was progressively replaced by CO, i.e., from R = CO/CO x = 0 to 0.5, and then step-wise restored, i.e., from R = 0.5 to 0. Pressure and H2/CO x ratio were kept at commonly applied values for CO2-to-methanol (5 MPa and 4, respectively), while temperature and WHSV were set at 553 K and 24,000 cm3 STP h−1gcat −1, respectively, as a compromise for the distinct expected reactivity of the catalysts. The results are expressed in terms of methanol STY, since CO2 conversion and methanol selectivity cannot be accurately determined without the use of isotopically-labelled compounds. Indeed, CO shall react with lattice oxygen from In2O3 or carriers forming CO2 (vide infra) especially at higher R values, and with the water formed by CO2 hydrogenation to methanol again to CO2. Moreover, CO is produced to a variable extent by the competitive RWGS reaction and may be hydrogenated to methanol over some of the catalysts. These remarks highlight the relevance of isotopic labelling experiments in future research to deepen knowledge of beneficial and detrimental effects of CO co-feeding through mechanistic understanding.The methanol STY exhibits quite distinct trends in the full cycles (Fig. 3 a). Along the forward branches, it barely increased (1%, Fig. 3b) for bulk In2O3, it was boosted by 8% on In2O3/m-ZrO2 while it decreased by 40 and 55 % for In2O3 carried on t-ZrO2 and Al2O3, respectively, and it dropped almost linearly by 20–40 % for the metal-promoted systems. In the latter case, the dry impregnated palladium-containing catalyst deactivated in a slightly more pronounced manner than the co-precipitated analog (26 vs.23 %), while the reverse holds for nickel-promoted materials (40 vs. 37 %).The backward branches of the cycles followed quite closely their corresponding forward branches only for In2O3/m-ZrO2 and bulk In2O3, while more pronounced hysteresis behavior in the methanol STY was detected for all other catalysts (Fig. 3b). For In2O3/t-ZrO2 and In2O3/Al2O3, the methanol productivity regained 7 and 13 % from the value reached at R = 0.5. Similar evidence was observed for Pd/In2O3 and Pd-In2O3, recovering 10 and 15 % from the same point in the cycle, respectively. In the case of Ni-promoted systems, the methanol STY further decreased by 1–5 %.Overall variations of the methanol productivity upon completion of a full cycle revealed mild (i.e., −8 to −9%) to strong (−15 to −40 %) deactivation of the catalysts. It is worth noting that, in spite of a CO negative impact on methanol production, the global loss in methanol STY for Pd-In2O3 across the full cycle is only −9 %, which is equivalent to that of In2O3/m-ZrO2 (i.e., −8 %). Generally, although methanol production is expected to drop to some extent when lowering the CO2 concentration if the admitted CO is not converted to methanol or does it poorly, the extent of alterations in methanol STY points to concomitant structural reasons. While the beneficial effect of CO is expected to relate to the creation of additional oxygen vacancies in In2O3, the negative impacts appear diverse and of reversible and irreversible nature. Strong CO adsorption on palladium may account for the first type, whereas over-reduction and sintering of the oxide and of the metal promoters would explain the second. Still, sintering shall also be modulated by the variable amount of water dictated by the extent of (R)WGS at different R values.To unravel structural and electronic alterations of the catalysts in detail and detangle individual contributions to the observed changes, in-depth characterization was conducted applying sensible methods. Porous and structural modifications were initially addressed by N2 sorption and XRD (Table 1). The S BET and pore volume of bulk In2O3 dropped by more than 4-fold (from 129 to 28 m2 g−1) and about 2-fold (from 0.41 to 0.22 cm3 g−1) after the forward branch of the full cycle and increased by a very slight extent after the subsequent backward branch. The particle size (d XRD) augmented from 7.9–18.9 nm going from R = 0 to 0.5 and decreased to 17.3 nm upon returning to R = 0, indicating irreversible sintering of the oxide. These modifications explain the overall loss of methanol productivity at the end of cycle, and hint that a positive change counteracts sintering in the forward branch, which displayed a rather constant trend (Fig. 3a). It is worth mentioning that sintering occurs to a significant extent already under pure CO2 hydrogenation conditions, as reported previously [9], with S BET decreasing from 123 to 47 cm3 g−1 and d XRD increasing from 8 to 18 nm, indicating a major role of water in driving this phenomenon. In2O3/m-ZrO2 underwent a minor loss in surface area and pore volume in the forward branch, which was restored upon returning to CO2 hydrogenation conditions. This indicates that the activation of this catalyst upon introducing CO is mainly related to modifications of its surface properties. In2O3/t-ZrO2 experienced progressive slight sintering along the full cycle in agreement with the irreversible depletion of its methanol STY. Palladium- and nickel-promoted In2O3 systems underwent strong modifications, with their surface areas, pore volumes, and particles sizes levelling to equivalent values to bulk In2O3 in the different stages of the full cycle. This is in line with the drop in their methanol productivity. However, the partial restoration of methanol STY over Pd-In2O3 in the backward branch of the full cycle indicates additional and at least partially reversible deactivation mechanisms acting on this specific material. A close analysis of the XRD patterns revealed a weak reflection at 34° 2θ due to metallic indium in bulk and metal-promoted In2O3 after the forward branch and the full cycle and In2O3/m-ZrO2 after the forward branch only (Fig. S3a,b) [39]. Since metallic indium alone was reported to be unable to catalyze CO2 hydrogenation, this might contribute to explaining the decreased methanol productivity of bulk In2O3 and, possibly, of the other systems as well [8,11]. Another weak reflection around 40° 2θ characteristic for metallic palladium might be recognized in the patterns of Pd/In2O3 and Pd-In2O3 after the half- and the full cycle (Figure S3b) [20,23].To gain insights into the distribution and structure of In2O3 and metal promoters in supported and promoted catalysts, fresh and used samples were imaged by HAADF-STEM and chemical element maps were acquired by EDX. The results reveal an effective dispersion of indium on both m-ZrO2 and t-ZrO2 (Fig. 4 a,c) and of the metal promoters (Fig. 4b,d) on In2O3 in all fresh samples. The catalyst architecture remains unaltered for In2O3/m-ZrO2 after the half-cycle (Fig. 4a), while defined aggregates of In2O3 were visualized on In2O3/t-ZrO2 (Fig. 4c). As for the metal-promoted catalysts, palladium remained well dispersed on In2O3 after the same test (Fig. 4b), whereas Ni agglomerated to form nanoparticles (Fig. 4d). In2O3 and palladium stay practically unaltered in In2O3/m-ZrO2 and Pd-In2O3, respectively, even after full cycle experiments, highlighting their robustness against sintering in these materials.Since CO is a strong reducing agent, the reducibility of all fresh catalysts was studied by CO-TPR (Fig. 5 a,b). The curve obtained for bulk In2O3 is composed of a small feature with maximum at ca. 370 K, followed by a pronounced multicomponent peak between 480−720 K, and a further steadily increasing signal at higher temperatures. The profiles of the catalysts carried on the two zirconia polymorphs also show three main signals, which based on the results produced by the supports alone, are considered as surface reduction of zirconia and highly dispersed In2O3, followed by reduction of the two bulk phases in two temperature regions (Fig. 5a). In the second signal, In2O3 contributes more than the carrier for In2O3/m-ZrO2 and the third feature is mostly due to the support for In2O3/t-ZrO2. It is worth noting that the second In2O3 reduction peak starts at a temperature comparable to the reaction conditions (ca. 553 K) for In2O3/m-ZrO2, while at much higher temperatures for In2O3/Al2O3 and In2O3/t-ZrO2 (773 and 800 K, respectively) (Fig. 5a). Based on the XRD and microscopy results (Figs. S3a and 4 c, respectively), this could be due to a larger In2O3 particle size and a strong interaction of In2O3 with the carrier for the alumina- and the tetragonal zirconia-based systems. Furthermore, the peak feature at 553 K for In2O3/m-ZrO2 is also associated with a higher CO consumption, indicating that oxygen is extracted more readily from this system, whereby the carrier may also contribute to this phenomenon due to the tensile force exerted. [15] The curves of the metal-promoted catalysts resemble that of the pure oxide (Fig. 5b). For palladium-containing materials, the first peak was observed at a slightly inferior temperature than for In2O3 (i.e., 340–360 vs. 370 K), suggesting that the promoter facilitates reduction by CO. Interestingly, additional sharp peaks between 400−573 K are present in the curves of Ni/In2O3 and Pd/In2O3, some of which could be related to the reduction of segregated NiO and PdO particles or to the formation of NiIn intermetallic phases (Fig. 5b). STEM-EDX of Ni/In2O3 (Fig. 4d) revealed that nickel sinters into nanoparticles upon reaction at R = 0.5, which are likely reduced to the metallic state by the gaseous environment. Since it is well-documented that metallic nickel is a very active methanation phase, the absence of methane product at any stage of the reaction over Ni/In2O3 suggests that the catalytic properties of nickel were modified by In2O3, whereby the formation of a NiIn intermetallic phase is a plausible option. Indeed, alloying of nickel with indium and other metals such as iron and tin has been reported and shown to foster the production of methanol or CO over methane in CO2 hydrogenation [40–42]. Regarding the Pd-In2O3 catalyst, the in situ generation of intermetallics is unlikely. Indeed, STEM-EDX of this sample (Fig. 4b) indicates that palladium remains well dispersed after half- and full cycle experiments and that irreversible activity loss is mostly due to indium oxide sintering (see Table 1). Moreover, Pd-In intermetallic compounds have been evidenced to lead to permanent depletion in methanol productivity [23,43,44]. The findings from CO-TPR also confirm that CO can generate CO2 while reducing the catalysts during reaction, with the above-discussed implications on the determination of CO2 conversion, especially considering that the partial pressure of CO is much greater in the reactor than in CO-TPR experiments. Indeed, conducting the CO-TPR analysis at 5 MPa showed a broad signal with unresolved components starting at lower temperature (not shown). It is worth noting that reduction of In2O3, carriers, and promoters by CO generally occurs at lower temperatures than with H2 (ca. 400−500 K), highlighting its stronger reducing power and role in facilitating the formation of oxygen vacancies [8,11,14,15,20]. However, it is difficult to define a clear correlation between CO content and catalyst reduction degree upon hydrogenation of hybrid CO−CO2 feeds because the CO and H2 are present simultaneously and CO takes part in the reaction equilibria, being possibly used as a carbon source to produce methanol, as discussed above.To shed light on the catalyst surface composition and electronic properties, bulk In2O3, In2O3/m-ZrO2, and Pd-In2O3 were examined by XPS in fresh form and after the half- and reverse half-cycles. All samples exclusively contained indium in oxidized form and no significant change in its surface content was detected after the full cycles (Table S3 and Fig. 5c) [16,23,45]. Remarkably, all In 3d core-level spectra of In2O3/m-ZrO2 show a shift of ca. 0.5 eV to higher binding energy compared to those of the other catalysts (ca.444.3 eV), indicating charge transfer from metal to support and a more oxidic character of the indium species carried. This corroborates previous evidence that indium cations are directly bound to the zirconia lattice, either through epitaxial growth or in a solid solution [15,16]. For Pd-In2O3, Pd2+ species in the fresh catalyst were predominantly reduced to metallic palladium upon use (Fig. S4a), with residual Pd2+ species still present being likely due to the short exposure of the samples to air for the ex situ analysis. The surface palladium content almost doubled in all samples retrieved after the full cycles (Table S3). A similar observation was reported for an equivalent material exposed to pure CO2 hydrogenation conditions and was associated with the characteristic removal of lattice oxygen upon catalyst reduction [20,23]. Concerning Ni/In2O3, no reduction of Ni2+ species to metallic nickel was evidenced, which is expected since this metal is more easily oxidized upon exposure to air (Fig. S4b) [46,47]. Interestingly, In2O3 coverage by nickel significantly decreased (i.e., 4.1 to 1.4 at.%) in catalysts retrieved after the full cycle (Table S3), which is in line with STEM-HAADF-EDX results (Fig. 5d) showing major sintering of the nickel phase into large particles.The O 1s core-level spectra (Figs. 5d–f and S4c) were analyzed to access information about the amount of oxygen vacancies, determining the relative contribution of the distinct oxygen species (Table S3). The signals related to oxygen atoms next to a defect (Odefect) were ascribed to the presence of vacancies, as previously reported [8,15,20,48]. While bulk In2O3 (Fig. 5d), Pd-In2O3 (Fig. 5f), and Ni/In2O3 (Fig. S4c) are associated with a slight modification in the content of Odefect, a significant alteration was evidenced for In2O3/m-ZrO2 (from 14 to 22 % and 20 % after the half- and reverse half-cycles, respectively, Fig. 5e). Although the relative contribution of Odefect is quite similar for all catalysts retrieved after cycle experiments, a recent report showed that the increased density of oxygen vacancies for In2O3/m-ZrO2 relates to the formation of additional In-Vo-Zr centers rather than Vo sites typically generated on bulk In2O3, highlighting that the former better activates CO2 and H2 to produce methanol [48]. This claim is supported by the identification of indium cations directly bound to the m-ZrO2 lattice in this study, and the possible superiority of such vacancies was also postulated for In2O3 epitaxially-grown on m-ZrO2 [15,16]. These findings indicate that CO has a moderate beneficial effect on promoted and unpromoted indium oxide upon moving from R = 0 to 0.5, fully obscured by detrimental phenomena, and a greater positive role on In2O3/m-ZrO2, which is not counterbalanced by negative structural changes.The main knowledge gathered by characterization is graphically combined in Fig. 6 to provide a comprehensive rationalization of the (de)activation mechanisms acting on most relevant In2O3-based catalysts in methanol synthesis from hybrid feeds. Notably, In2O3/m-ZrO2 showcases an increased density of distinctive and superior oxygen vacancies that along with its resistance to sintering explain the improved performance in CO-rich feeds. Unlike activation, deactivation mechanisms are vaster and intricate. Sintering of In2O3 induced by water and/ or over-reduction by CO is a common denominator for all deactivated systems, but can be associated with other factors resulting in greater depletion in methanol productivity. This is clearly evidenced for the metal-promoted systems. Ni/In2O3 irreversibly deactivates also due to sintering of the nickel phase. For Pd-In2O3, CO inhibition of H2 splitting over the promoter due to strong adsorption is put forward as another factor contributing to the drop in methanol STY. Nonetheless, such kinetic effect is reversible and can be modulated by tuning the CO concentration in the feed.It is worth mentioning that state-of-the-art commercial and lab-prepared Cu-ZnO/Al2O3 systems for CO-based methanol synthesis have also been tested in the hydrogenation of hybrid CO2−CO feeds in steady-state under a fixed feed composition and in cycles where the CO/CO2 ratio was varied, providing significantly diverse results. Similar to In2O3-based systems, some catalysts experienced a boost in methanol productivity, while others displayed diminished performance [49–51]. These divergent behaviors are traced back to distinct catalyst compositions and preparation method as well as the presence of additives in commercial samples and indicate that, unlike In2O3-based catalysts, there is no consensus on a top-performer applied to CO2-containing feeds.Based on the findings of the full cycles, In2O3, In2O3/m-ZrO2, Pd-In2O3, and Ni/In2O3 were selected for further testing. To gain insights into the catalytic performance at R = 0.5 without prior exposure to any other environment, reverse CO2−CO half-cycles (rHC), i.e., from R = 0.5 to 0, were carried out (Fig. 3c). While similar trends to the backward branches of the full cycles developed for all systems with a decline of methanol STY for In2O3, In2O3/m-ZrO2, and Ni/In2O3 and an improvement of the same for Pd-In2O3 (Fig. 3a), differences in the absolute methanol productivity values were observed at the start of these experiments. For both bulk In2O3 and In2O3/m-ZrO2, methanol STY at R = 0.5 was higher than under the same condition in the full cycles, suggesting that water-induced sintering in the CO2 hydrogenation environment counteracts the positive effect of CO (Fig. 3a). For Ni/In2O3 and, especially, Pd-In2O3, starting the reaction at R = 0.5 was detrimental to the methanol STY. For all catalysts, the methanol productivity at R = 0 was comparable to that determined at the same ratio at the end of the full cycles, except for the palladium-promoted sample, pointing to a plausible greater role of CO than water on structural modifications of the latter. In line with these findings, characterization by N2 sorption, XRD, and XPS of the catalysts after the reverse half-cycle provided equivalent results to those described for the same materials after the whole full cycle (Tables 1 and S3, and Figs. S3a,b, S4a–c, and 5 c–f).To further examine the impact of CO, the effect of temperature and pressure was studied at R = 0.2 since this is the ratio expected in the reactor at equilibrium when recycling CO (vide infra) and corresponds to the upper concentration boundary of CO present as a feed impurity. The methanol productivity increased as a function of pressure within 3–5.5 MPa and temperature between 543−583 K for all systems (Fig. S5), which is in good agreement with their performance trends under CO-free CO2 hydrogenation conditions. This hints that these operational parameters do not have a critical influence on either beneficial or detrimental CO effects in the practically-relevant ranges investigated.Since In2O3/m-ZrO2 displayed an improved performance upon the introduction of CO in the feed and Pd-In2O3 still produced a significant amount of methanol even when experiencing deactivation by CO, additional experiments were devised to assess how their methanol productivity could be individually optimized. To predict the most representative R ratio to which the catalysts shall be exposed upon per-pass operation, calculations were conducted using the Aspen Plus software. Firstly, the thermodynamic equilibrium composition (Fig. S2a) of carbon‐based species during methanol synthesis was determined under typically applied reaction conditions (T = 533–593 K, P = 5 MPa, H2/CO2 = 4, and WHSV = 24,000 cm3 h−1 gcat −1), which shows that R equals 0.20 at 553 K in a single-pass conversion process. To quantify the benefit of recycling a gas mixture with this composition on methanol productivity at the latter temperature, per-pass and overall conversion levels were estimated based on a simplified flowsheet comprising the methanol synthesis reactor, a flash separator to separate water and methanol from the gases, and a recycle loop for the latter (Fig. S2b) using molar flows of relevant reaction streams as detailed in Table S1. Values of 21 and 67 % were attained for the two parameters, resp6ctively, highlighting that per-pass operation would triplicate conversion in CO2-based methanol synthesis. A similar R value to that obtained for single-pass conversion (R = 0.17 vs. 0.20, Fig. S2a) was determined upon recycling of the outlet gases. Accordingly, 0.2 was applied in the testing of In2O3/m-ZrO2 and Pd-In2O3.Concerning In2O3/m-ZrO2, a pre-treatment in syngas (H2/CO = 2) was carried out for 4 h at 553 K prior to using the catalyst in CO x hydrogenation at R = 0, 0.2, and 0.5 expecting that such activation protocol would lead to a more defective and thus better performing material. Fig. 7 a shows an enhanced methanol productivity, which is sustained when applying feeds containing CO (R = 0.2 and 0.5) but not using a stream with only CO2 and H2 (R = 0), emphasizing that the catalyst surface dynamically responds to the reducing potential of the feed mixture. Although methanol STY at R = 0.5 is similar to that recorded at R = 0.2, the actual yield with respect to CO2 shall be effectively higher under the former conditions, which shall trace back to the hindering of the RWGS reaction and a better preservation of oxygen vacancies upon having more CO in the feed. No variations in the surface area were recorded (Table S4), indicating that the syngas treatment did not significantly or irreversibly alter the catalyst structure. O 1s core-level XPS spectra indicate that the amount of surface oxygen vacancies present on In2O3/m-ZrO2 and pure m-ZrO2 after activation with syngas does not change during the reactions (Fig. S6). Some depletion would be expected for the exposure to R = 0, but is it likely masked by the large contribution of zirconia to the oxygen signal. The catalytic data in Fig. 7a also reveal that In2O3/m-ZrO2 can hydrogenate CO to methanol in pure syngas conditions, in contrast to bulk In2O3, which fully reduces and melts [8,11]. In line with this, an extended full-cycle experiment spanning from pure CO2 (R = 0) to pure CO (R = 1) hydrogenation (Fig. S7) displayed that CO is converted to methanol over In2O3/m-ZrO2 at H2/CO = 4. This points to a possible utilization of CO as a carbon source for methanol along with CO2 in hybrid feeds. These findings are in line with m-ZrO2 showing high CO adsorption capacity (i.e., 5- to 10-fold higher than for t-ZrO2) and CO activation ability [52–55]. The extended cycle evidenced that increasing CO concentration in the feed to R > 0.5 is irrelevant or detrimental to methanol productivity and that the catalyst performance is readily recovered upon restoring CO2 in the feed. Thus, over-reduction of In2O3 to metallic indium is less likely to be the reason for deactivation in this supported system. It is plausible that decreased methanol productivity at R > 0.5 traces back to the lower methanol formation rate upon CO hydrogenation (r = 2.9 mmolMeOH h−1 gcat −1) as compared to CO2 hydrogenation (r = 7.5 mmolMeOH h−1 gcat −1).To evaluate stability under variable conditions, In2O3/m-ZrO2 was subjected to a long-term cycling test comprising three stages and a total time on stream of 120 h (Fig. 7b). During an initial 40 h at R = 0, the system exhibits a stable performance, which is improved with no induction time upon changing feed composition to 0.2 and sustained for an additional 40 h. The latter regime mimics the situation in which the outlet gas stream purified from condensable species is fed back into the reactor upon per-pass operation. The final switch back to pure CO2 hydrogenation occurs sharply as well, and the catalytic behavior is equivalent to the first phase. These findings emphasize that In2O3/m-ZrO2 can effectively undergo reversible cycling and is amenable for practically-relevant operation.Regarding Pd-In2O3, the first set of tests encompassed three full cycles reaching lower R values than in the case described above, to unravel if the methanol productivity could be better recovered when returning to CO2 hydrogenation from hybrid feeds poorer in CO. In such threshold experiments the highest R was set at 0.1, 0.2 or 0.3. As expected, Pd-In2O3 deactivated almost linearly upon introduction of CO in all cases (Fig. 8 a). The recovery in methanol productivity was moderately higher than in the full cycle reaching R = 0.5 (ca. 90 vs. 85 %) and similar regardless of the R value reached in the forward branch of the cycles (vide the backward branches in Fig. 8a).The second experiment comprised the same long-term cycling test as described for In2O3/m-ZrO2. At R = 0 during 40 h, Pd-In2O3 displayed an equilibration phase after which the methanol STY levelled off (Fig. 8b), in line with a previous report [20]. A drop in methanol productivity (from 0.65 to 0.56 gMeOH h−1 gcat −1) was then observed at R = 0.2, which was partially recovered upon returning to the initial condition of R = 0 but with an induction time. Indeed, the same catalyst would be expected to display stable behavior after 93 h on stream under pure CO2 hydrogenation conditions [20]. These findings indicate that the irreversible restructuring of the catalyst described above occurs to a consistent extent also in feeds less rich in CO. Indeed, the sample retrieved after the threshold experiment at R = 0.1 featured a strongly decreased surface area (i.e., 133 to 37 m2 g−1, Table S4) indicating substantial In2O3 sintering. However, since the surface area of bulk In2O3 was reported to decrease from 113 to 38 m2 g−1 upon reaction with CO2 hydrogenation for a comparable time on stream [8], such structural change is most likely due to the water byproduct rather than only to the CO co-fed. Overall, the results indicate that Pd-In2O3 could not be regenerated in swing operation. However, they hint that this catalyst could show steady, although reduced, performance in a per-pass process for a longer time than probed in this test.The impact of CO on CO2-based methanol synthesis over bulk, supported, and promoted In2O3 catalysts was systematically assessed by a set of purposely devised cycle experiments. This approach revealed diverse catalytic responses despite all catalytic systems share the same active phase. Upon CO addition until reaching an equimolar CO−CO2 ratio, In2O3/m-ZrO2 experienced an increase in methanol productivity, bulk In2O3 remained almost unaffected, and moderate-to-substantial deactivation was determined for nickel- or palladium-promoted In2O3, In2O3/t-ZrO2, and In2O3/Al2O3. Detailed characterization uncovered that a variable interplay between formation of surplus oxygen vacancies, sintering of In2O3 or metal promoters induced by CO and/or water, and strong adsorption of CO on palladium underpins the observed behaviors. Focusing on the more relevant In2O3/m-ZrO2 and Pd-In2O3, operational regimes were probed to identify conditions to maximize their performance in a practical process. The methanol productivity of In2O3/m-ZrO2 in hybrid feeds was boosted by pre-activation of the catalyst in syngas and exposure of Pd-In2O3 to CO-poorer feeds (R < 0.3) enabled a better recovery of methanol STY upon restoring pure CO2 hydrogenation conditions. Moreover, the two catalysts could adapt rapidly to a switch from CO2 hydrogenation to processing of a feed with a CO/CO2 ratio as dictated by thermodynamics upon a per-pass regime and sustain for a longer period their activated and moderately depleted states, respectively. Overall, this study uncovered the positive and negative impacts of hybrid CO2-CO feeds on CO2-based methanol synthesis on a relevant class of catalysts, offering a novel approach to assess performance under practically-relevant conditions, which shall serve as a stepping-stone to accelerate the development of industrially-viable catalytic technologies for this reaction and other CO2-based conversions. Thaylan P. Araújo: Investigation, Data curation, Methodology, Visualization, Writing - original draft. Arjun Shah: Investigation, Software. Cecilia Mondelli: Supervision, Visualization, Writing - review & editing. Joseph A. Stewart: Project administration, Writing - review & editing. Daniel Curulla Ferré: Project administration. Javier Pérez-Ramírez: Conceptualization, Funding acquisition, Writing - review & editing.The authors declare that they have no competing interests.Dr. S. Mitchell is acknowledged for the electron microscopy measurements, and the Scientific Center for Optical and Electron Microscopy (ScopeM) at the ETH Zurich for the use of their facilities. We are grateful to S. Büchele for performing the XPS analyses and to J. Lüthi for designing the 3D models of the catalysts.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2021.119878.The following is Supplementary data to this article:

Catalysts for CO2-to-methanol are typically evaluated in a single-pass regime using pure CO2streams. In a practical process however, CO shall be present as a feed impurity or as a recycled byproduct. Herein, the sensitivity to CO was evaluated on In2O3 catalysts in bulk, supported, or metal-promoted forms, using cycle experiments with variable CO2 and CO contents at H2/(CO + CO2) = 4. The methanol productivity was decreased (−20-−40 %) on all catalysts except In2O3/monoclinic-ZrO2, the activity of which was boosted by 10 %. In-depth characterization of the catalysts uncovered controlled formation of oxygen vacancies and resistance to sintering as the main reasons for the activation of the latter and an interplay of CO/H2O-induced sintering and CO inhibition as the origin of performance loss. Focusing on the most representative systems, operation protocols were explored to maximize their methanol yield. We emphasize that assessment with hybrid CO2-CO feeds is key for the design of industrially-viable catalysts for sustainable methanol production.

With the increasing demand for energy supply and the ever-worsening climate change, the development of sustainable technologies, such as the solar-driving catalysis process, to generate clean energy and chemicals has been recognized as the most promising way to address this pressing global issue. Photocatalysis has attracted significant attention due to its sustainable characteristics and limited environmental impacts (Lei et al., 2021; Zhu and Zhou, 2019). To date, many semiconductor materials, such as ZnO (Liu et al., 2019b; Shekofteh-Gohari et al., 2018), TiO2 (Gao et al., 2020b; Guo et al., 2019), and C3N4 (Patnaik et al., 2021; Zhu et al., 2021), have been demonstrated with photocatalytic activity. However, as pristine semiconductor materials, their photocatalytic performance is still below the common expectation due to their unsatisfactory photocatalytic processes (i.e., limited light-harvesting capability, severe recombination of photo-induced charge carriers, and poor catalytic selectivity and activity) (Gao et al., 2019a; Zhou et al., 2018). Therefore, it is of great importance to developing novel photocatalysts with an efficient photocatalytic process to perform high-efficient solar-driven chemicals/fuel production.Recently, single-atom catalysts (SACs), with the active metal species existing as isolated single-atoms (SAs) and stabilized by bonding with the substrate materials or by alloying with another metal (Lai et al., 2019; Zang et al., 2020), have attracted significant attention in the field of photocatalysis due to the unique advantages of SAs, such as maximal metal dispersion and tuneable local coordination environments, exhibiting excellent reaction activity. For the supported metal nanoparticles, it has been widely recognized that their catalytic performance (product selectivity, reaction activity, and the stability of the catalysts, etc.) is strongly influenced by the metal particle size and the local coordination environment between the metal particle and the supporting materials (Campbell et al., 2002; Liu, 2017; Rao et al., 2008; Subramanian et al., 2004). With the particle size decreasing, the surface atoms ratio to the total atoms will sharply increase and result in more unsaturated coordinated surficial metal atoms (Liu, 2017), which could act as the active sites for the reactant molecules' adsorption. When decreased to the small nano-size (2–10 nm) (Halperin, 1986; Li et al., 2021a), the electron energy level of the metal species will split into discrete energy levels (Yang et al., 2015; Yang et al., 2013), which will directly influence the orbital hybridization and charge transfer at the metal/reactant interface (Wang and Lu, 2020).Generally, SACs possess five main compelling advantages: (1) maximized metal dispersion (Zhang et al., 2021b): as a result, with the same metal loading amount, SACs possess more reactive sites; (2) better product selectivity: all the single-atom sites possess the similar composition and coordination structure, resulting in a uniform catalytic environment (Chen et al., 2021); (3) bridged homogeneous and heterogeneous catalysts (Chen et al., 2018a; Gao et al., 2018): SACs have atomically dispersed metal sites on solid supports and consist of well-defined mononuclear active centers, expected to combine homogeneous and heterogeneous catalysts; (4) unsaturated coordination structure or valence state: the unsaturated coordinated structure or valence state will make the SAs act as the reaction active center and directly participate in the reaction (Chen et al., 2018b); (5) size effect (Hu et al., 2014): the surface free energy of the SAs obviously increases compared with the metal nanoparticles, making them highly active in chemical interactions for reactant molecules (Yang et al., 2013).SACs have been at the forefront of catalysis research due to their maximized atom utilization, unique structures, and properties. Among many applications, electrochemical energy conversion is one of the most promising areas, including oxygen evolution (OER), CO2 reduction (CRR), etc (Daiyan et al., 2020a). Compared with traditional catalysts, SACs expose more active sites, leading to enhanced electrocatalytic activity. In addition, SACs are also demonstrated with improved selectivity toward the target products. Taking CRR as an example, it was demonstrated that the coordination structure of the SACs was beneficial for reducing the formation energy barrier of the CRR intermediates (e.g., ∗COOH), thereby improving the product selectivity (Daiyan et al., 2020b; Leverett et al., 2021; Leverett et al., 2022). In addition, SACs have also been applied and studied in the photocatalysis fields. It is found that by constructing single-atom photocatalysts (SAPs) with rationally introduced SAs, the overall photocatalysis process could be significantly impacted (Figure 1 ) (Xia et al., 2021). For instance, it is demonstrated that the introduction of SAs can efficiently alter the electronic structure of the supporting semiconductors, resulting in tuneable optical response behavior (Gao et al., 2020a), and the enhanced charge transfer kinetics of the SAPs can be ascribed to the unique band bending effect (Su et al., 2018). Moreover, due to the tuneable coordination structure between the SAs and the supporting materials, the surface adsorption and activation of the reactant molecules can be boosted (Yang et al., 2020). In this regard, SAPs provide a great platform to regulate the overall photocatalytic reaction process. However, it is also worth to be pointed out that the practical application of SAPs is limited due to the following reasons: (1) the agglomeration of SAs; (2) the structure-performance relationship between the SAPs and the surface photocatalytic reaction is unclear, resulting in vague SAPs-based reaction mechanisms. Therefore, it is of great importance to systematically summarize the current research progress of SAPs for the guidance of future studies on SAPs’ synthesis.In this review, the unique phenomenon of SAPs brought to the overall photocatalysis processes (i.e., optical response, the separation, and surface reaction process) will be systematically overviewed. As such, the local coordination environment between SAs and the substrates materials will be deeply discussed, and the sequent impact on products' selectivity will be explained. At last, the current challenge and potential research points of SAPs are pointed out. We believe this review will provide guidance knowledge for the future development of novel SAPs catalysts for photocatalytic reaction.Photocatalysis is regarded as one sustainable method for solar-driven chemicals/fuels generation. Therefore, within a typical photocatalytic reaction process, it contains three steps to obtain the final redox products, including, (1) the photoexcitation of the photocatalysts and the generation of photo-induced carriers; (2) the spatial separation of photo-induced electrons and holes; (3) the redox of surface-adsorbed reactant molecules. As such, the photocatalytic performance of the catalysts is closely related to the aforementioned three steps: (a) for some wide bandgap semiconductors, such as TiO2, ZnO, etc., the narrow optical response performance leads to relatively poor light-harvesting capability and limited amounts of photo-induced carriers; (2) during the spatial separation process, severe carrier recombination processes (both surface recombination and volume recombination) are commonly accompanied, resulting in extra carrier recombination loss; (3) the uncontrollable surface adsorption and activation of the reactant molecule lead to poor product selectivity (Figure 2 ). Therefore, rationally regulating the aforementioned three steps is of great importance to obtain high-performance photocatalytic processes.In addition, from the perspective of reaction types, the photocatalysis techniques can be applied to perform various types of solar-driven chemicals/fuels production reactions, including (Wang et al., 2021d), CO2 reduction (Tang et al., 2022), N2 reduction (Guan et al., 2021), NOx reduction (Zhang et al., 2021d), biomass oxidation reforming (Wang et al., 2021c), and pollutant oxidation degradation (Bui et al., 2021). By virtue of photocatalysis, value-added chemicals and green fuels can be generated in a sustainable and environmental-friendly way. As such, it requires sound development of novel catalysts with favorable adsorption and dissociation modes toward the specific molecules. For instance, in the past decades, noble metal/semiconductor catalysts have been reported as one of the most efficient catalysts for the selective oxidation of aromatics (Chen et al., 2017a). However, the high cost of the catalysts severely limit the scale-up application of such kind of catalysts. In this regard, it makes the development of SAPs of great practical significance. Due to the insufficient understanding of the influence caused by the introduction of SAs on the photocatalysis process, the study on the application of SAPs for the solar-driven chemicals/fuels production is still in its infancy, calling for urgent investigation.When the particle size is reduced to atom level, the metal single atoms with ultra-high surface energy tend to aggregate and form nanoclusters during the synthesis process (Zhao et al., 2020; Zhu et al., 2020). Currently, the reports on SAPs are still limited. Fortunately, the widely reported SACs for other catalysis reactions, such as thermal catalysis, electrocatalysis, etc., can bring us some insights. To date, the synthesis methods of SACs are plentiful, such as impregnation, co-precipitation, chemical vapor deposition (CVD), ion exchange, galvanic replacement, thermochemical method (flame spray pyrolysis [FSP], and pyrolysis of organic materials [metal-organic framework, covalent organic framework, etc.]), atomic layer deposition (ALD), atom trapping, and photochemical reduction. Taking the impregnation method as an example, the prepared substrate material is put into a solution containing SAs precursor. The metal ions are adsorbed on the surface of the substrate material, which are then reduced to produce SACs (Wang et al., 2018). The mass loading of metal SAs for the SACs prepared by the impregnation method is very low, but the procedure is simple (Xia et al., 2022). In addition, the co-precipitation method is also widely used for the preparation of SACs. At least use two cations to form a homogeneous phase in the solution (Qiao et al., 2011). For SACs prepared by co-precipitation, the mass loading must be kept below 1% to prevent agglomeration of the metal particles during calcination or reduction (Xia et al., 2022).Besides the method mentioned earlier, the following four strategies will be further discussed and compared, including ALD, atom trapping, thermochemical method, and photochemical reduction methods, which are proven to be effective in preventing metal SAs sintering (Figure 3 ).Atomic layer deposition (ALD) is demonstrated to be a precise method that can deposit the SAs on the surface of the supporting materials by alternately exposing the supports to pulsed vapors of various precursors. As the method is based on chemisorption, deposition occurs only in areas with reactive surface sites (Fonseca and Lu, 2021). Generally, the ALD method includes four steps: (1) exposure to the first precursor; (2) purge of the reaction chamber; (3) exposure to the second reactant precursor; and (4) a further purge of the reaction chamber (Cheng et al., 2016). The morphology, size, density, and loading of the deposited material on the carrier can be precisely controlled by simply tuning the ALD cycle (Cheng et al., 2019). For instance, Cao et al. successfully applied ALD technology and synthesized Co-based catalysts (Co1/PCN), which were used in the photocatalytic hydrogen evolution (Figure 4A) (Cao et al., 2017). Specifically, cyclopentadienyl cobalt (Co(Cp)2) was used as the cobalt-precursor for the ALD, which was then treated with O3 to remove the cyclopentadienyl ligand, resulting in the Co1-N4 structured SACs. However, nucleation delay and island growth are considered the key issues that need to be optimized (Cheng and Sun, 2017), resulting from the limited functional groups on the surface of supporting materials. Besides, the surface energy difference of metal and support will also bring a challenge to the deposition process. When the surface energy of the support is lower than the free energy of deposited metal, the support cannot be wetted by the deposited metal, which will lead to insufficient adsorption sites and result in an island growth mode finally (Cheng and Sun, 2017).The atom trapping method is another effective strategy to synthesize the SACs with stably anchored SAs (Wang et al., 2018). Specifically, to plant SAs in the commonly used semiconductors materials, constructing surface defect sites (i.e., C-defect (Chen et al., 2020c; Liu et al., 2021a), O-defect (Cai et al., 2020; Wang et al., 2021e), N-defect (Qin et al., 2021; Zhang et al., 2022a), S-defect (Wang et al., 2020b; Zhang et al., 2021e), metal-defect (Wu et al., 2021; Zhang et al., 2018), etc.) can efficiently capture the SAs. It means that, through defect-engineering strategies, defect sites can be created on the surface of the supporting materials, leading unsaturated coordination environments to the adjacent atoms (Liu et al., 2021a), which can be used as the anchors to trap and stabilize SAs. The necessary synthesis condition of atom trapping requires the mobile metal species and trapping sites on the supports (Qu et al., 2018). Thus, by adjusting the concentration of the surface defects, the SAs loading amount and the catalytic performance of the SACs can be easily regulated (Wang et al., 2019a). For instance, Jones et al. demonstrated that the Pt SAs could be captured and anchored in CeO2, forming atomically dispersed Pt1/CeO2 SACs (Figure 4B) (Jones et al., 2016). In this process, Pt nanoparticles supported on Al2O3 were aged in the air at 800°C, and the PtO2 was released and captured by CeO2 in a high-temperature environment, which exhibited quite good thermal stability.Pyrolysis and flame spray pyrolysis (FPS) are two of the current main thermochemical methods to prepare SACs. The major difference between these two strategies lies in the treatment atmosphere, as the general pyrolysis process calls for inert atmosphere annealing, whereas the FPS process can be processed in air condition (Wang et al., 2012). For pyrolysis, a common method is to adsorb the metal complexes with N-ligand onto the porous support, followed by a pyrolysis step of the metal-organic framework, covalent organic framework (Liu et al., 2019a). For example, it is widely reported that the Co and Fe atoms can coordinate with the N from the support materials to form the Co-N and Fe-N atomic dispersion structure (Wang et al., 2019d; Wu et al., 2019a; Zhang et al., 2021c; Zhu et al., 2017). To synthesize such SACs, generally, the nitrogen/metal precursors and the carbon supports will be pyrolyzed and carbonized under inert atmospheres at a quite high temperature. In addition, Wei et al. also demonstrated that the nanoparticles of noble metals (e.g., Pd, Pt, Au) can be converted to thermal-stable SAs with an annealing temperature of 900°C in the inert atmosphere (Figure 4C) (Wei et al., 2018). With the continuous study on the pyrolysis method, it is confirmed that controlling the temperature change during the pyrolysis process is crucial for the evolution of metal SAs (Wang et al., 2020a).Unlike pyrolysis, the flame spray pyrolysis method allows the SACs’ synthesis to be carried out in the air conditions, making it a flexible way to produce catalysts with controllable morphology and particle size by adjusting the synthesis conditions (Ding et al., 2021). During the combustion process, the metal salt solution and the solvent of the supporting materials are sprayed into the high-temperature flame simultaneously. The solvent will be vaporized and the metal salt solution will be burned or hydrolyzed in the high-temperature flame, allowing the SAs to be anchored on the supporting materials in one step (Michalow-Mauke et al., 2015; Pongthawornsakun et al., 2015; Thybo et al., 2004). As shown in Figure 4D, Ding et al. reported that, by virtue of the flame spray pyrolysis method, the Pt SAs can be introduced to a series of oxide supports with excellent stability (Ding et al., 2021). Compared with other methods, the flame spray pyrolysis strategy shows unique advantages for the catalysts’ preparation: the flame spray pyrolysis method can synthesize the catalyst quickly and is suitable for scaling up; the SACs produced by the flame spray pyrolysis strategy can exhibit a quite small particle size and well-dispersion (Gavrilović et al., 2018).Photochemical reduction method is a widely used postpreparation strategy to synthesize SACs. This method generally requires that the support materials and the metal salt precursor should be mixed within a reducing agent solution at first. Then the reducing agent will release free radicals under the irradiation of the UV light, reducing and anchoring the SAs to the supporting materials (Li et al., 2018). This method is easy to tailor the SAs loading amount. For instance, by the photochemical strategy, Liu et al. successfully planted the Pd SAs into ultra-thin TiO2 nanosheets (Pd1/TiO2) (Liu et al., 2016), as shown in Figure 4E. The TiO2 powder was first dispersed in ethylene glycol (EG) by ultrasonic. Then, the H2PdCl4 was added as the metal precursor. Under mild UV conditions, the ethylene glycol free radicals can be formed on the surface of the TiO2 nanosheets, which promoted the Cl− releasing in the Pd precursor solution and the Pd-O bond formation, resulting in the formation of Pd1/TiO2. The as-synthesized Pd-TiO2 catalysts exhibited excellent catalytic activity for the hydrogenation of C=C bonds and C=O bonds, which was nine times higher than that of commercial Pd catalysts. To further prevent the agglomeration of SAs, Wei et al. demonstrated a novel ice-photochemical reduction method to synthesize Pt SAs (Figure 4F), which were confined and dispersed in the crystal lattice of the supporting materials (Wei et al., 2017). The Pt SAs were obtained by exposing the frozen H2PtCl6 solution under UV radiation. Then, the molten frozen solution containing Pt atoms was physically mixed with various supports (e.g., TiO2) to synthesize Pt-based SACs. Due to the low-temperature feature of the iced-photochemical reduction strategy, it can further avoid the agglomeration of the SAs, which is unavoidable in the room temperature photochemical reduction processes (Lu et al., 2020; Wei et al., 2019). Due to the reduced diffusion rate caused by the low-temperature condition, a hindrance to the agglomeration of SAs thus will be caused, leading to better atomic dispersion of SAs (Lu et al., 2020).Current research on the SAPs have demonstrated that the introduction of SAs will influence the overall three-step photocatalysis process (i.e., photo-excitation, carrier transfer, and surface reaction). Therefore, the related work will be comprehensively overviewed and compared in this section.The optical response capability of the semiconductor catalyst could directly influence the photocatalytic performance, as narrow light-harvesting behaviors will severely limit the incident light utilization efficiency of the catalysts. Previous work demonstrated that the introduction of SAs is effective to enlarge the optical response performance of the pristine catalysts. Some work claimed that the introduction of SAs will cause impurity energy level, thereby broadening the optical response range of the pristine catalysts (Figures 5A–5B) (Yang et al., 2016). In addition, some work also claimed that the introduction of SAs could directly reduce the bandgap of the semiconductor catalysts, rather than introducing impurity energy level, leading to enhanced light absorption capability of the SAPs (Figure 5C) (Yang et al., 2021).For instance, Jin et al. proposed SAPs with Fe SAs implanted into the surface of Bi4O5I2 (Bi4O5I2-Fe30) (Jin et al., 2021). As such, Fe SAs were considered as the dopant to replace the Biatoms in Bi4O5I2, which generated impurity energy levels within the bandgap of Bi4O5I2, leading to a broadened light-harvesting range toward the Bi4O5I2-Fe30 SAPs. From the ultraviolet-visible (UV-Vis) diffuse reflectance spectra, a red-shifting light absorption trend was demonstrated when the Fe SAs were introduced to the Bi4O5I2 (Figure 6A). The Tauc plot diagram in Figure 6B also confirmed that the bandgap values reduced from 2.17 eV of Bi4O5I2 to 1.56 eV of Bi4O5I2-Fe30. The aforementioned evidence indicated that the light absorption and electronic structure of pristine semiconductors can be modulated by incorporating SAs. By virtue of both experimental (Figures 6C and 6D) and theoretical calculation (Figures 6E and 6F), the energy band structure of Bi4O5I2 and Bi4O5I2-Fe30 were further determined. It was confirmed that, after the introduction of Fe SAs, impurity energy levels appeared near the conduction band of Bi4O5I2, therefore enlarging the optical response range of Bi4O5I2. Li et al. also demonstrated Ru SAs doped monolayered TiO2 nanosheets (Ru1/TiNS) and confirmed that after Ru1 SAs inducing, an isolated impurity energy level was formed, broadening the optical absorption range up of the Ru1/TiNS to 700 nm (Li et al., 2020b).Moreover, it was reported that the introduction of SAs could directly reduce the bandgap of the catalysts, leading to an enlarged light absorption range. For instance, Jiang et al. developed a novel Ag SAs incorporated carbon nitride photocatalyst (Ag-N2C2/CN) with the Ag-N2C2 configuration (Jiang et al., 2020). As shown in Figure 6G, with the Ag-N2C2 and Ag-N4 coordination structure, the optical response range of the obtained catalysts can be efficiently expanded. The inset in Figure 6G showed a reduced narrowed bandgap after incorporating Ag SAs. The DFT calculation also confirmed the introduction of Ag SAs would lead to the directly reduced bandgap but no impurity energy level (Figures 6H and 6I).The efficient spatial separation and transfer of photogenerated carriers are important for performing high-efficient photocatalytic reactions. But for pristine semiconductor catalysts, severe carrier recombination is unavoidable (Liu et al., 2020). This unavoidable energy loss before the surface reaction would result in relatively low photocatalytic activity. The introduction of SAs is also demonstrated to contribute to efficient carrier separation (Dong et al., 2021b; Xiao et al., 2020). According to the previous studies, generally, when the metal nanoparticle is in contact with the semiconductor, a Schottky barrier is generated on behalf of the energy level matching of the metal and the semiconductor. After the electronic equilibrium is established at the interface, the photogenerated electrons in the semiconductor will pass through the Schottky barrier and transfer to the metal particles (Gao et al., 2020a). This charge transfer behavior across the metal-semiconductor interface can be inherited by the SAPs (Gao et al., 2020a; Gopalakrishnan et al., 2021). It was demonstrated that, when the metal SAs were contacted with the semiconductor substrate, efficient charge transfer behaviors could be expected due to the interfacial barrier (Meng et al., 2019; Wang et al., 2019a). In addition, it was considered that, by introducing SAs, the charger transfer distance between the light-harvesting units and the photocatalytic sites could be shortened (Gao et al., 2020a).For instance, Li et al. demonstrated that, by incorporating Pt SAs into ultra-thin covalent triazine framework nanosheets (Pt-SACs/CTF) with unique Pt-N3 structure, the carrier migration in the Pt-SACs/CTF catalysts can be efficiently enhanced (Li et al., 2020a). It was demonstrated that the electrons captured by the Pt SAs were subsequently utilized for the nitrogen fixation reaction. In sharp contrast with the ultra-thin covalent triazine framework nanosheets (CTF-PDDA-TPDH), the Pt-SACs/CTF catalysts showed a weaker PL intensity (Figure 7A), indicating the suppressed carrier recombination in Pt-SACs/CTF. The photoresponse of the Pt-SACs/CTF and CTF-PDDA-TPDH catalysts were further studied by the chronoamperometry curves under chopped optical illumination (Figure 7B). The CTF-PDDA-TPDH nanosheet catalysts showed lower photocurrent density, whereas the photocurrent density increased obviously after incorporating Pt SAs. These results elucidated the enhanced carrier separation efficiency of the Pt-SACs/CTF catalyst. The proposed carrier transfer mode was shown in Figure 7C. Under visible light irradiation, the CTF-PDDA-TPDH nanosheets were excited with the electrons on the CB of the CTF-PDDA-TPDH nanosheets transferred to the Pt SAs, bringing about the enhanced carrier separation. The electrons captured by the Pt SAs were then consumed to reduce the N2 molecule into NH3.Similarly, Dong et al. demonstrated that the Pt SAs could be anchored on the covalent organic framework (COF) catalysts, linked by β-ketoenamine (Pt1@TpPa-1) (Dong et al., 2021a). As-synthesized photocatalysts showed high activity (99.86 mmol gpt −1 h−1) and selectivity (100%) for H2 evolution, which were attributed to the successful anchoring of Pt SAs to facilitate the transfer efficiency of photogenerated electrons. The photoluminescence (PL) spectra exhibited two fitting peaks centered at 625 and 710 nm in Figure 7D. And the PL peak at 625 and 710 nm was attributed to the bandgap radiative recombination and the π-π interaction between the COF and β-ketoenamine layers, respectively. Moreover, these two emissions peaks of Pt1@TpPa-1 were quenched significantly compared with TpPa-1, due to the interfacial charge transfer from TpPa-1 to Pt SAs. The charge transfer behavior was further probed through the time-resolved PL (TRPL) decay spectra (Figure 7E). It was shown that the anchoring of Pt SAs (3% Pt1@TpPa-1) led to a shorter lifetime (0.27 ns) compared with TpPa-1 (0.50 ns), which was attributed to the addition of Pt SAs in the COF. The possible charge transfer behavior and reaction routes are illustrated in Figure 7F. As a result, the Pt1@TpPa-1 provided more photocarriers for the subsequent surface photocatalysis reactions, thus improving the photocatalytic performance. The protons (H+) produced by the dissociation of H2O were then reduced to the transitional state (H∗) and finally evolved into H2. The aforementioned result implied that the Pt SAs could facilitate the efficient migration of photoelectrons, thus improving the efficiency of the photocatalytic reactions.Besides affecting the photocatalysts' light-harvesting and charge transfer capability, it is demonstrated that the introduced SAs could also act as the reaction active sites and directly participate in the reaction (Liu et al., 2021a; Wang et al., 2021b). Compared with general metal nanoparticle/semiconductor catalysts, the SAPs, maximizing the atomic utilization, exhibit boosted reaction active sites and consequent excellent photocatalytic performance (Figures 8A and 8B) (Jiao et al., 2021; Wang et al., 2019e; Zhang et al., 2022a). It is generally considered that the surface reaction process is strongly influenced by the geometric effect and the electronic structure of the catalysts (Gao et al., 2020a). Constructing SAPs provides an effective method to manipulate the local coordination environments of the SAs, offering an easy way to regulate the adsorption/activation mode of the reactant as the surface of the catalysts. In addition, as the SAs offered a large number of unsaturated coordination centers, numerous reaction active sites could be provided for the surface reaction process (Wang et al., 2019b).Currently, metal oxides (TiO2, ZnO, etc.) and metal sulfides (CdS, MoS2, etc.) are widely studied as the photocatalysts (Kumar et al., 2021; Kusiak-Nejman et al., 2021; Premaratne et al., 2004). As discussed in section atomic layer deposition method, by creating defect sites (i.e., O-defect, S-defect), the metal-oxides- and metal-sulfides-based SAPs can be easily synthesized by the atom trapping methods. Due to the synergistic effect of the SAs and the adjacent defect sites, it is reported that enormous reaction active sites could be provided.For instance, Xing et al. successfully loaded various SAs (i.e., Pt, Pd, Ru, Rh, etc.) onto the TiO2 substrate for photocatalytic hydrogen evolution reactions (HER) (Xing et al., 2014). The turnover frequencies (TOFs) of 0.2Pt/TiO2 photocatalytic hydrogen production was about 24 and 6 times higher than that of 2Pt/TiO2 and 1Pt/TiO2 (PD, denoted nanoparticles), respectively (Figure 8C). It was found that the same phenomenon also existed in some other SAPs (Pd, Ru, and Rh), confirming the introduction of SAs could provide extra reaction active sites compared with bulk catalysts. In addition, Wu et al. successfully introduced various concentrations of Pt SAs onto the TiO2 nanotubes in different concentrations of HPtCl4 solution (2–0.0005 mM) and applied them for the photocatalytic HER (Wu et al., 2021). It was found that compared with Pt nanoparticles, the Pt SAPs also exhibited better HER performance.For metal-sulfides-based SAPs, similar results were evidenced. For instance, Zhao et al. reported Co SAs/N-doped graphene-modified CdS (Co-NG/CdS) (Zhao et al., 2017), exhibiting efficient photocatalytic HER performance. The 0.25 wt% of Co-NG/CdS showed an H2 evolution rate of 1077 μmol h−1, which was 1.3 times higher than that of the Pt-NPs/CdS photocatalyst (1382 μmol h−1), confirming the contribution of SAs on the reaction active sites (Figures 8D–8E). Moreover, the turnover numbers (TONs) were calculated to be 58.2 and 474.764 for the CdS and Co-NG/CdS, respectively. For the 0.25 wt% Co-NG/CdS photocatalyst, the TOF was approximately 8.8 s−1. These TON and TOF values show that, under the same reaction conditions, the SAPs showed better reaction activity than the metal nanoparticle/semiconductor catalysts.As the coordination environment of the active atoms in SAPs can be flexibly regulated, it affords us an effective effort to regulate the reactant adsorption/activation modes on the catalysts’ surface, thereby altering the reaction pathway and finally achieving high product selectivity in the photocatalytic process (Gao et al., 2020a; Wang et al., 2021g). As discussed in section principle of photocatalysis, photocatalysis techniques have been widely applied to various solar-driven chemicals/fuels generation, including HER (Alarawi et al., 2019; Zhang and Guan, 2020), CRR (Gao et al., 2018; Wang et al., 2019c), nitrogen reduction reaction (NRR) (Huang et al., 2018; Li et al., 2020a), etc. Table 1 overviewed some recent research on SAPs for the inorganic photocatalytic reactions.For photocatalytic CO2 reductions, it was demonstrated that the presence of some SAs can enhance the adsorption of CO2 molecules, stabilize the photocatalytic CO2 reduction intermediates, and accelerate the CO desorption, thereby achieving better CO selectivity in the photocatalytic process (Zhang et al., 2020). For instance, Di et al. demonstrated that, by replacing Bi3+ with Co SAs, the CoBi3O4Br atomic shell could be negatively charged, which facilitated the adsorption of CO2, as shown in Figure 9A (Di et al., 2019). By virtue of in-situ Fourier transform infrared spectroscopy (FTIR), it allowed insight into the reaction intermediates of photocatalytic CO2RR (Figure 9B). The, peaks at 1256, 1337, and 1508 cm−1 were attributed to the CO2 −, symmetrical O-C-O extended bidentate carbonate (b-CO3 2−) and monodentate carbonate (m-CO3 2−) groups, respectively. The increasing peak intensity at 1567 cm−1 was attributed to the COOH∗ intermediate, which was an important intermediate for the formation of CO. Finally, the CO desorption was also considered to be an important factor in determining the comprehensive photocatalytic efficiency. The temperature-programmed desorption (CO-TPD) curves in Figure 9C demonstrated that Co-Bi3O4Br-1 possessed a lower initial CO desorption temperature, indicating that as-formed CO∗ could be easily removed from Co-Bi3O4Br-1 surface and higher CO yield rates (Figure 9D). Therefore, the introduction of Co SAs could promote the adsorption of CO2 molecules and reduce the activation energy barrier of CO2 by stabilizing the COOH∗ intermediate and adjusting the rate-limiting step to CO∗ desorption (Figures 9E–9F), thus exhibiting excellent photocatalytic activity and selectivity.The SAPs have also been applied for photocatalytic NRR. For instance, Li et al. successfully anchored Pt SAs to the ultra-thin CTF nanosheet (Pt-SAC/CTF) (Li et al., 2020a). The high-angle annular dark-field scanning transmission electron microscopy (HADDF-STEM) images of the obtained catalysts were shown to confirm the appearance and atomic diameter distribution of the Pt-SAC/CTF catalyst (Figures 9G and 9H). The even dispersion of Pt, C, and N atoms corresponding to the EDS mapping image could be observed in Figure 9I. The electronic state of the Pt species in the Pt-SAC/CTF catalyst was explored by the X-ray absorption near-edge structure analysis (XANES) (Figure 9J). The white line intensity of the Pt-SAC/CTF was lower than that of PtO2, but higher than that of Pt foil, indicating that the unoccupied density lay between PtO2 and Pt foil. The extended X-ray absorption fine structure (EXAFS) spectrum confirmed the same coordination structure of the Pt-N3 site in the ultra-thin CTF-PDDA-TPDH nanosheets. The Fourier transform EXAFS (FT-EXAFS) shown in Figure 9K showed a main peak at 2.34 Å, which corresponded to the metal Pt bond of the standard Pt foil. The sharp peak at 1.57 Å for the Pt-SAC/CTF catalyst indicated that the Pt presented as SAs in the Pt-SAC/CTF catalyst. Then under the visible light radiation, the photocatalytic N2 immobilization experiment was carried out. The average NH3 production rate of the Pt-SAC/CTF catalyst was 171.40 μmol g−1 h−1 (Figure 9L), which was 6 times and 1.5 times higher than that of the CTF-PDDA-TPDH and Pt-NPs/CTF catalysts, respectively.Besides the inorganic photocatalytic reaction, SAPs can also be applied in some organic-related photocatalytic reactions, such as biomass reforming, organic synthesis, and pollutant degradation, which were summarized and listed in Table 2 . For instance, da Silva et al. mixed Na-PHI and FeCl3 together to introduce Fe3+ into the poly(heptazine imides) (PHI) matrix, thereby obtaining the target catalyst Fe-PHI (Figure 10A) (da Silva et al., 2022). In Figure 10B, the Fe-PHI XRD peaks were mainly significant differences between 25° and 30°. This apparent difference was assigned to the statistical positioning of Fe ions in the mainframe of the crystal. In Figure 10C, Fe SAs could be well distinguished, confirming the successful synthesis of the Fe-PHI catalyst. EXAFS, Fourier transforms (FTs), and wavelet transforms (WTs) spectra were shown in Figures 10D–10F. Detailed analysis showed that once Fe3+ was introduced into the PHI structure, Fe3+ would combine with the N atom of the heptazine ring. The DFT calculations showed that Fe3+ ions were located between the PHI layers, with each Fe3+ ion coordinating with four N atoms and two in each PHI layer (Figure 10G). Moreover, earlier, it was claimed that the C-H bond had high dissociation energy and the C-H bonds were easier to be over-oxidized and lead to the side reactions (Liu et al., 2017). However, it was found that the geometric structure enabled the Fe-PHI catalyst to promote the selective oxidation of C-H bonds. As a result, the Fe-PHI SAP was applied for the catalytic oxidation of the ethylbenzene. It was demonstrated that the Fe-PHI (0.1%) exhibited the superior oxidation activity (Figure 10H), with the 99.6% ethylbenzene conversion rate and 98.4% acetophenone selectivity.In addition to the aforementioned biomass refining reaction, the researchers put another focus on organic synthesis to promote the C-C coupling reaction (Toe et al., 2021). Zhou et al. successfully synthesized Pt SAs-loaded TiO2 (PtSA-TiO2) and applied it for the production of 2,5-hexanedione (HDN), an important chemical in biofuels and medicinal chemistry, from low-cost acetone dehydrogenation (Zhou et al., 2020a). It was the first application of the in-situ icing-assisted photocatalytic reduction method to anchor Pt SAs on TiO2. The HAADF-STEM image shown in Figure 11A confirmed the presence of Pt SAs on TiO2. The coordination structure of Pt SAs on TiO2 was analyzed by X-ray absorption near edge structure (XANES) spectra in Figure 11B, which indicated that the absorption edge of Pd SAs was higher than that of Pt nanoparticles-loaded TiO2 (PtNP-TiO2) and Pt foil (Figure 11B). The EXAFS spectrum of the PtSA-TiO2 showed a main peak at 1.61 Å in the R space and a maximum at 5.61 Å−1 in the k space (Figure 11C), both of which were assigned to the Pt-O bond (Figures 11D and 11E). Subsequently, the photocatalytic acetone conversion was carried out under the irradiation of a 300 W xenon lamp at 25°C. The results showed that the HDN production rate of PtSA-TiO2 was 3.87 mmol g−1 h−1, which was 6 times higher than that of PtNP-TiO2, confirming the excellent reaction activity achieved by the Pt SA catalysts. The gas chromatography-mass spectrometry (GC-MS) in Figure 11E confirmed that the photocatalytic product contained HDN and H2. In addition, the HDN-production activity of PtSA-TiO2 can maintain four cycles in 16 h (Figure 11F). To further explore the catalysis mechanism, the attenuated total reflection infrared ((ATR)-IR) spectrum discovered that the two IR peaks at 2921 and 2852 cm−1 were ascribed to the C-H bond in the methyl group of acetones. As for the PtSA-TiO2, these two peaks revealed a sharp decrease (Figure 11G), which implied the activation of methyl and acetone tended to be dehydrogenated at the surface of PtSA-TiO2. In Figure 11H, the electron spin resonance (ESR) spectrum exhibited the CH3COCH2 • radical on PtSA-TiO2, which was an important intermediate for the production of HDN by C-C coupling. This result showed that Pt SAs exhibited a significant influence on the acetone dehydrogenation reactions. All of the aforementioned results suggested that the PtSA-TiO2 possessed a relatively low reaction barrier for acetone dehydrogenation reaction, which was also proved in Figure 11I. Similarly, Wang et al. successfully synthesized Pt/gC3N4 SAPs using a photo-deposition method, combining the oxidation of benzaldehydes with simultaneous proton reduction (Wang et al., 2021c). The benzaldehyde conversion rate of Pt/gC3N4 reached 49.5 mmol/gPt, and the hydrogen evolution rate of Pt/gC3N4 was 24 mmol/gPt. Pt/gC3N4 SAPs exhibited nearly 100% efficiency per atom in the production of benzoic acid and clean H2 fuel.SAPs usually consist of two parts, the supporting substrate and the SAs. By decorating the substrate materials with SAs, enhanced photocatalytic performance could always be obtained. Therefore, in the following section, the current research progress of both the substrate and SAs will be comprehensively overviewed.To synthesize SAPs, the applied supporting substrates are usually semiconductor materials. Meanwhile, acting as the substrates of SAPs, the applied semiconductors should be able to anchor the SAs and prevent the aggregation of the SAs. In this section, the supporting substrate catalysts will be categorized as organic, inorganic, and carbon-based materials.Currently, organic materials have been widely used as SAPs substrates, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). For the MOF substrates, the selection criteria are mainly based on the following three aspects (Jiao and Jiang, 2019; Li et al., 2016): (1) possessing large specific surface area, which is conducive to the adsorption of reactants; (2) exhibiting optical activity, which can generate photo-induced carriers to participate in the photocatalytic reaction; and (3) providing pore confinement, which can prevent the aggregation of metal with a relatively high metal loading. For MOF itself, its unsaturated coordination sites, defects, and the porous structure can be utilized to anchor metal SAs, making it an ideal substrate for anchoring the SAs (Jiao and Jiang, 2019). For example, the Pt1/SnO2/UiO-66-NH2 catalysts were successfully synthesized by Sui et al., applying for the visible-light-driven HER (Sui et al., 2021). The obtained Pt1/SnO2/UiO-66-NH2 SAPs showed a superior H2 evolution rate of 2167 μmol g−1 h−1. Further, Li et al. synthesized MOF-808-EDTA with implanted Pt SAs (Li et al., 2019), which exhibited an excellent photocatalytic H2 evolution rate (68.33 mmol g−1 h−1) under visible light irradiation (Figure 12A).As for the COFs, SAs can be confined within the COF through the coordination interaction between the metal atom and the binding groups in COF (Wei et al., 2020). Moreover, COFs possess heteroatom-rich pore walls that can facilitate reactant adsorption and charge transfer, resulting in more efficient photocatalytic reactions (Zeng and Xue, 2021). Therefore, the utilization of COFs as the substrates to capture SAs is expected to bring new opportunities for the development of SAPs. For example, Dong et al. reported a two-dimensional β-ketoenamine-linked COF supporting Pt SAs (Pt1@TpPa-1) for photocatalytic HER (Dong et al., 2021a). TpPa-1-COF showed special holes and dispersed unsaturated coordinating nitrogen atoms, which made the Pt SAs highly dispersed. The optimal 3% Pt1@TpPa-1 showed the best H2 evolution rate of 99.86 mmol gPt −1 h−1 (Figure 12B). Besides Pt, the active Mo SAs were also impregnated in the TPBPY-type COF to get Mo-COF, realizing an efficient photocatalytic reduction of CO2 to produce C2H4 (3.57 μmol g−1 h−1) (Kou et al., 2021).To date, metal oxides are the most used inorganic substrate for the synthesis of SAPs, as the SAs can be anchored on metal oxides through the metal-oxygen bonds or be stabilized through oxygen vacancies, contributing to the enhanced stability of the SAPs. For instance, Hu et al. demonstrated that the Pt SAs could incorporate defective TiO2 nanosheets (Pt SA/Def-s-TiO2) for photocatalytic water splitting (Figure 12C) (Hu et al., 2021b). The surface oxygen vacancies could efficiently stabilize the Pt SAs by forming a three-center Ti-Pt-Ti structure, which also contributed to the enhanced charge transfer processes. As a result, greatly enhanced photocatalytic HER was evidenced. Notably, the Pt SA/Def-s-TiO2 SAPs exhibited an enhanced H2 evolution performance (13460.7 μmol h−1 g−1), which was 29.0 times higher than that of TiO2 nanosheets.Similar to metal oxides, the unsaturated coordinated sulfur atoms in metal sulfides could also bond with metal SAs to form SAPs. For instance, Li et al. synthesized CdS-Pd SAPs through the photoreduction method (Li et al., 2022). It was demonstrated that the CdS-Pd SAPs exhibited considerable structural stability and photocatalytic HER performance due to the synergistic interaction between CdS and Pd, achieving an efficient charge transfer process to the catalysts’ surface. The obtained H2 evolution rate (947.93 μmol g−1 h−1) was about 110 times higher than that of pure CdS NPs (8.64 μmol g−1 h−1).Besides the metal oxides and sulfides, recently, halide perovskites materials are also demonstrated to be a potential substrate material to synthesize the SAPs. Halide perovskites possess fascinating properties such as broad light absorption, long charge carrier migration lengths, etc. (Fu and Draxl, 2019). Currently, the halide perovskites are demonstrated with excellent photocatalytic performance, besides being applied in the photovoltaic fields. In this regard, synthesizing halide perovskites-based SAPs is promising to obtain extraordinary catalytic performance. The presence of SAs is expected to effectively enhance the interaction between the halide perovskite and the reactant molecules (Fu and Draxl, 2019). For instance, Wu et al. successfully anchored Pt SAs onto FAPbBr3-xIx (Pt/FAPbBr3-xIx) with high dispersibility and stability (Figure 12E) (Wu et al., 2022). The obtained Pt/FAPbBr3-xIx SAPs showed enhanced photocatalytic hydrogen production activity, reaching 682.6 μmol h−1 (100 mg). In addition, Hu et al. demonstrated that the Pt SAs could be deposited onto the CsPbBr3 NCs (Pt-SA/CsPbBr3) through the formation of Pt-O and Pt-Br bonds (Hu et al., 2021a). Compared with pristine CsPbBr3 NCs, the trap levels exhibited in the Pt-SA/CsPbBr3 were ascribed to the deposition of Pt SAs, leading to an enhanced separation capability of the photogenerated carriers. Because of the fast carrier transfer from CsPbBr3 to Pt SAs, the Pt-SA/CsPbBr3 exhibited a superior activity toward the photocatalytic propyne semi-hydrogenation (TOF = 122.0 h−1).Because of the excellent conductivity of graphene, carbon-based substrates have been widely used to anchor metal SAs for not only electrocatalysis (Su et al., 2021a; Su et al., 2021b; Tian et al., 2021; Wang et al., 2021f) but also the photocatalysis field (Zhuo et al., 2020). Similar to organic and inorganic substrates, structurally modified graphene can bind with SAs through the coordination interactions with oxygen- or nitrogen-containing functional groups. For instance, Gao et al. used oxidized graphene nanosheets as the substrates to immobilize the isolated Co SAs (Co1-G). Under this circumstance, the graphene acted as a bridge to connect the Ru(bpy)3 photosensitizer and the Co SAs, thereby realizing effective charge transfer and CO2 reduction (Gao et al., 2018). It was demonstrated that the Co SAs were coordinated with the carbon and residue oxygen on the graphene surface and exhibited outstanding TON (678) and TOF (3.77 min−1) toward photocatalytic CRR. In addition, N-doped carbon substrates are also widely applied in photocatalysis, which provide rich coordination sites for the anchoring of the SAs (Liu et al., 2021b). For instance, Zhao et al. demonstrated that the Ni SAs-decorated N-graphene/CdS (Ni-NG/CdS) could be efficient SAPs for photocatalytic HER (Zhao et al., 2018). In this work, Ni SAs were anchored on the vacancies in nitrogen-doped graphene (Ni-NG). In the obtained catalysts, the Ni-NG acted as the electron storage medium to suppress the carrier recombination and the active site for the reduction reaction. As a result, the Ni-NG/CdS received an outstanding photocatalytic HER performance with a quantum efficiency of 48.2% at 420 nm.As discussed earlier, to regulate the charge carriers’ generation/transfer and surface reaction process, loading metal nanoparticles to modify the pristine semiconductor catalysts is a generally applied strategy. However, due to the high expense and scarce reserves of noble metal, increasing the utilization efficiency of metal atoms is of great importance, which is also applicable to nonnoble metal species (Li et al., 2021b). In the following section, the currently studied SAs species are systematically summarized.Currently, various noble metals, such as Pt, Pd, Ir, Au, Ag, Rh, Ru, etc., have been applied to synthesize SAPs due to their excellent catalytic activities. For example, Liu et al. applied g-C3N4 with carbon vacancies (Cv-CN) to anchor Pd SAs (Pd-Cv-CN), applying for the photocatalytic NO reduction reaction (Figure 13A) (Liu et al., 2021a). The results showed that the Pd SAs could be successfully anchored to the carbon vacancies and uniformly dispersed on the Cv-CN surface, thereby forming isolated Pd-N3 sites. In the case of photocatalytic NO conversion, Pd-Cv-CN not only exhibited higher conversion efficiency of 56.3% but also higher selectivity and stability toward NO3 − generation compared with Cv-CN. Similarly, Li et al. prepared Pd/TiO2 SAPs by the liquid-phase reduction method and applied it in the photocatalytic CRR (Li et al., 2017). The results showed that the Pd SAs could be uniformly dispersed on the surface of TiO2, leading to improved CRR activity. The enhanced CRR efficiency was attributed to the synergistic effect of Pd SAs and TiO2, as the Pd SAs could act as the electron trap center to capture photogenerated electrons and inhibit the recombination of photo-induced electrons and holes.The nonnoble metal-based SAPs are focused on the transition metals such as Fe, Co, Cu, Ni, etc. The transition metals have vacant orbitals that can accept electrons as electron traps and avoid the recombination of photogenerated electron-hole pairs (Abdullah et al., 2017). Ma et al. dispersed Co SAs on g-C3N4 nanosheets with ultra-high density of Co-N2C active sites and applied the obtained SAPs for the photocatalytic CRR (Figure 13B) (Ma et al., 2022). They demonstrated that the Co-N2C sites served not only as the electron aggregation center but also as the CO2 adsorption/activation sites, which subsequently promoted the photocatalytic methanol generation performance. As a result, the methanol formation rate for 4 h was 941.9 μmol g−1 over Co/g-C3N4-0.2, which was 13.4 times of g-C3N4 (17.7 μmol g−1). Moreover, Zhang et al. dispersed Co SAs into MOFs (MOF-525-Co) for the CO2 photoreduction (Zhang et al., 2016). They proved that the Co SAs could act as the CO2 adsorption sites. Simultaneously, the photogenerated electrons could transfer from the MOFs to the Co active sites feasibly, thereby improving the photocatalytic efficiency.With empty and occupied orbitals, the atomic structures of some nonmetal elements (e.g., B, Si, etc.) are similar to that of the transition metals (Zhao et al., 2022). Compared with metal-based SAPs, metal-free-based SAPs have also been extensively studied due to their low cost and environmental friendliness. Although metal-free-based SAPs show weaker catalytic activity compared with metal-based SAPs, they yet have good stability and strong resistance to poisoning and deactivation. Ling et al. designed a boron-atom-decorated graphitic-carbon nitride (B/g-C3N4) for the photocatalytic NRR (Figure 13C) (Ling et al., 2018). By analyzing the extranuclear electronic structure of boron atoms, they found that the sp3-hybridized boron atoms were similar to transition metals with empty and occupied orbitals, which could be used as the reaction active center for the NRR. Furthermore, the modification of B SAs can significantly enhance the visible light absorption capability of g-C3N4, thus promising to realize the visible-light-driven NRR. Lv et al. also reported that B SAs could be applied to reduce dinitrogen to ammonia spontaneously (Lv et al., 2019).Generally, for SAPs, the metal/nonmetal species are dispersed on the supporting substrates in the form of SAs, acting as the active sites for the photocatalytic reaction. Therefore, it is crucial to clarify the geometric structure, electronic structures, and the spatial distribution of SAs for the deep study of SAPs. Advanced electron microscopy analysis techniques, such as scanning tunneling microscope (STM) and high-angle annular dark-field-scanning transmission electron microscope (HAADF-STEM), can provide advanced understandings of the structure of SAPs, making it possible to identify the SAs at the magnitude at c.a. ∼ 0.1 nm; spectroscopy techniques, such as X-ray absorption fine structure (XAFS) spectroscopy and infrared (IR) spectroscopy, can also be applied to identify the existence of SAs and clarify the electronic structure and chemical state of the obtained SAPs.The typical electron microscopes techniques, scanning electron microscope (SEM) and transmission electron microscope (TEM), can toughly identify the SAs at the atomic level. In this regard, HAADF-STEM is applied to observe SAs due to the improved spatial resolution of its sub-Angstrom probe (Gao et al., 2019b; Peng et al., 2004). This technique has been chosen for heavy elements on light substrates for the strong correlation between atomic number and imaged intensity, called Z-contrast (LeBeau et al., 2008; Nellist et al., 2010). Under dark-field conditions, different atoms have different Z-contrasts, making the atoms distinguishable by observing their brightness in the HAADF-STEM images (Chung et al., 2019). In Figure 14A, the spherical-aberration-corrected HAADF-STEM images of O/La-CN SAPs showed the bright dots, which corresponded to the even dispersed La SAs on CN substrate due to the different Z contrasts of La, C, and N atoms (Chen et al., 2020a).STM is a characterization technique applied to probe the surfaces and absorb substances at the atomic level with ultra-high resolution of 0.1 nm laterally and 0.01 nm in depth. The SAs can be imaged and manipulated with the conductive tips (Gao et al., 2020a). For example, Deng et al. used STM to reveal the existence of Fe SAs with FeN4 center in the graphene matrix as shown in Figure 14B (Deng et al., 2015). The iron center was shown as a bright spot, whereas adjacent atoms (C and N) exhibited a higher apparent height than other C atoms in the graphene matrix. In the simulated STM images, the FeN4 centers embedded in the graphene lattice were consistent with the STM images, which better revealed the iron centers significantly alter the density of states of adjacent atoms (N and C) (Figure 14C).In addition to the aforementioned microscopy techniques, XAFS spectroscopy, including XANES and EXAFS, is another effective way for the characterization of SAPs, which is used to analyze the coordination environment and electronic structure in the material structure. According to the characteristics of peaks and shoulders in the XANES spectrum, the electronic structure and chemical valence state of SAs can be obtained. In the EXAFS spectra, SAs can be identified through morphological imaging characterization and corresponding spectral information sensitive to atomic structure, so as to obtain the coordination number, coordination form, and coordination distance of the planted SAs to the adjacent atoms in the SAPs. For instance, Sui et al. performed XAS to determine the coordination environment and chemical state of Pt species in Pt1/SnO2/UiO-66-NH2 SAPs (Sui et al., 2022). From the L3-edge image, it could be seen that the peak intensity of Pt1/SnO2/UiO-66-NH2 was closer to that of PtO2, implying the presence of a highly oxidized Pt state (Figure 14D). The Fourier transform expansion X-ray absorption fine structure spectra (FT-EXAFS) of the Pt1/SnO2/UiO-66-NH2 gave only a dominant peak at about 1.63 Å, which can be attributed to the first shell of the Pt-O bond, rather than the Pt-Cl bond and Pt-Pt bond, suggesting the existence of atomically dispersed Pt sites in Pt1/SnO2/UiO-66-NH2 (Figure 14E).IR spectroscopy can also be utilized to identify the presence of SAs and to quantify the percentage of metal SAs to some extent (Chen et al., 2017b; Liu, 2017). The principle is as follows, IR is used to detect the interaction between the substrate and the adsorbed molecule by utilizing probe molecules (e.g., CO, NH3, pyridine, etc.) (Gallenkamp et al., 2021). For instance, Ding et al. applied the IR spectra to confirm the Pt SAs in the Pt/HZSM-5 catalysts by analyzing the CO adsorption mode (Ding et al., 2015). As shown in Figure 14F, the peak at 2115 cm−1 was attributed to CO molecules adsorbed on Pt SAs. Meanwhile, the peak at 2090 cm−1 was ascribed to CO molecules linearly adsorbed on Pt nanoparticles. It could be inferred that Pt existed as SAs on Pt/HZSM-5 with a low Pt loading at 0.5 wt % by studying the changes in peak intensity for the four catalysts. And the Pt atoms tended to agglomerate to form Pt nanoparticles after increasing the Pt loading from 0.5 wt% to 2.6 wt%. In addition, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) offers another technique to gain insight into the local information of the SAPs, as it is usually applied to in-situ collect information of the surface reactive species and intermediates during the reaction (Yang et al., 2019). Typically, CO is used as the probe molecule in DRIFTS studies because of its advantages in characterizing the exposed noble metal sites on loaded catalysts (Liang et al., 2022). For instance, Fang et al. collected the in-situ DRIFTS spectra of the CO adsorption behavior over the Al-TCPP-0.1Pt SAPs (Fang et al., 2018). After purging with Ar to remove gaseous CO, the peak centered at 2090 cm−1 corresponded to the CO chemisorbed on Pt SAs (Figure 14G). In the ranges of 2080–2030 cm−1 and 1920–1950 cm−1, no bands that could linearly and bridge CO adsorption on Pt clusters and nanoparticles appeared, implying that all Pt species were atomically dispersed.To date, SAPs have been widely studied in solar-driven chemicals/fuels generation, with various SAPs synthesis strategies being established. The pioneer works demonstrated that the introduction of SAs to the commonly used semiconductor catalysts can directly influence the overall photocatalysis process: (1) by modulating the band structure with impurity level or directly reducing the bandgap, SAPs, therefore, exhibits greatly enlarged optical absorption range; (2) due to the unique band bending effect at the metal SAs/semiconductor interfaces, the spatial separation and transfer of the photogenerated carriers can be significantly promoted; (3) with the tuneable coordination environment of the SAs, SAPs are equipped with boosted reaction active sites and better product selectivity. Moreover, rationally choosing the supporting substrate materials and the loaded SAs species is expected to regulate the surface reaction process efficiently. However, due to the insufficient understanding of the structure-catalytic performance relationship based on SAPs, in the future, the study of SAPs still faces some crucial issues: (1) Due to the strong influence of the local electronic structure of the material, the rational design of SAPs with high loading rates of the SAs is still a significant challenge. The knowledge to prepare stable and efficient SAPs with high SAs loading is still in its infancy. The loading amounts of SAs can reach 23 wt% now for nitrogen-doped carbon and polymeric carbon nitride (Lu et al., 2021). However, for other supports, the loading rates are still less satisfactory. Consequently, it will be promising to achieve higher SAs loading rates by ligands protection or through the ambient multistep method to regulate the removal of the ligands from the metal precursors and enhance the associated interactions between SAs and substrates. (2) To date, there is still lack of efficient methods to monitor the reaction dynamics of the catalytic process. Currently, the characterizations of the ligand environment and associated charge transfer processes mainly rely on theoretical calculations. Direct monitoring of the reaction dynamics is still challenging. In the future study, in-situ characterization techniques, such as in-situ electron microscopy or in-situ synchrotron radiation technique, need to be applied to monitor and probe the photocatalytic reaction process. Combined with theoretical calculations, the relationship between their structure and catalytic performance can be better explained. (3) The advanced understanding of the photocatalytic mechanism over the SAPs is insufficient, making this technique challenging to practical application. To date, only a few studies tried to uncover the effect of the SAs on the reaction mechanism. To illustrate the impact of SAs on the reaction pathway, it will be promising to apply homogenized substrates for the SAs loading to investigate the accurate active sites of SAPs. This will benefit the design of some specific SAPs fitted for the targeted reactions on an atomic scale. Abbreviations SACs Single-atom catalysts SAs Isolated single-atoms SAPs Single-atom photocatalysts CVD Chemical vapor deposition FSP Flame spray pyrolysis ALD Atomic layer deposition CB Conduction band VB Valence band COFs Covalent organic frameworks PL Photoluminescence TRPL Time-resolved PL TOFs Turnover frequencies TONs Turnover numbers FTIR Fourier transform infrared spectroscopy TPD Temperature-programmed desorption PHI Poly(heptazine imides) EXAFS Extended X-ray absorption fine structures FTs Fourier transforms WTs Wavelet transforms HDN 2,5-hexanedione XANES X-ray absorption near edge structure GC-MS Gas chromatography-mass spectrometry ESR Electron spin resonance HADDF-STEM High-angle annular dark-field scanning transmission electron microscopy FT-EXAFS Fourier transform EXAFS HER Hydrogen evolution reaction CRR Carbon dioxide reduction reaction NRR Nitrogen reduction reaction DRIFTS Diffuse reflectance infrared Fourier transform spectroscopy STM scanning tunneling microscope XAFs X-ray absorption fine structure IR Infrared (ATR)-IR Attenuated total reflection infrared MOFs Metal-organic frameworks Due to the strong influence of the local electronic structure of the material, the rational design of SAPs with high loading rates of the SAs is still a significant challenge. The knowledge to prepare stable and efficient SAPs with high SAs loading is still in its infancy. The loading amounts of SAs can reach 23 wt% now for nitrogen-doped carbon and polymeric carbon nitride (Lu et al., 2021). However, for other supports, the loading rates are still less satisfactory. Consequently, it will be promising to achieve higher SAs loading rates by ligands protection or through the ambient multistep method to regulate the removal of the ligands from the metal precursors and enhance the associated interactions between SAs and substrates.To date, there is still lack of efficient methods to monitor the reaction dynamics of the catalytic process. Currently, the characterizations of the ligand environment and associated charge transfer processes mainly rely on theoretical calculations. Direct monitoring of the reaction dynamics is still challenging. In the future study, in-situ characterization techniques, such as in-situ electron microscopy or in-situ synchrotron radiation technique, need to be applied to monitor and probe the photocatalytic reaction process. Combined with theoretical calculations, the relationship between their structure and catalytic performance can be better explained.The advanced understanding of the photocatalytic mechanism over the SAPs is insufficient, making this technique challenging to practical application. To date, only a few studies tried to uncover the effect of the SAs on the reaction mechanism. To illustrate the impact of SAs on the reaction pathway, it will be promising to apply homogenized substrates for the SAs loading to investigate the accurate active sites of SAPs. This will benefit the design of some specific SAPs fitted for the targeted reactions on an atomic scale.SACs Single-atom catalystsSAs Isolated single-atomsSAPs Single-atom photocatalystsCVD Chemical vapor depositionFSP Flame spray pyrolysisALD Atomic layer depositionCB Conduction bandVB Valence bandCOFs Covalent organic frameworksPL PhotoluminescenceTRPL Time-resolved PLTOFs Turnover frequenciesTONs Turnover numbersFTIR Fourier transform infrared spectroscopyTPD Temperature-programmed desorptionPHI Poly(heptazine imides)EXAFS Extended X-ray absorption fine structuresFTs Fourier transformsWTs Wavelet transformsHDN 2,5-hexanedioneXANES X-ray absorption near edge structureGC-MS Gas chromatography-mass spectrometryESR Electron spin resonanceHADDF-STEM High-angle annular dark-field scanning transmission electron microscopyFT-EXAFS Fourier transform EXAFSHER Hydrogen evolution reactionCRR Carbon dioxide reduction reactionNRR Nitrogen reduction reactionDRIFTS Diffuse reflectance infrared Fourier transform spectroscopySTM scanning tunneling microscopeXAFs X-ray absorption fine structureIR Infrared(ATR)-IR Attenuated total reflection infraredMOFs Metal-organic frameworksThe authors acknowledge the financial support from the Sydney Nano Grand Challenge, at the University of Sydney and Australia Research Council Linkage Project (LP200200615). H.S. is grateful to Lizhuo Wang for his help in the discussions of the characterization section. Dedication: This work is dedicated to Professor Jianzhong Chen on the occasion of his 70th birthday.Conceptualization: J.H.; Writing - Original Draft: H.S.; Writing-Review & Editing: R.T., J.H.; Supervision: J.H.The authors declare no competing interests.

With the ever-increased greenhouse effect and energy crisis, developing novel photocatalysts to realize high-efficient solar-driven chemicals/fuel production is of great scientific and practical significance. Recently, single-atom photocatalysts (SAPs) are promising catalysts with maximized metal dispersion and tuneable coordination environments. SAPs exhibit boosted photocatalytic performance by enhancing optical response, facilitating charge carrier transfer behaviors or directly manipulating surface reaction processes. In this regard, this article systematically reviews the state-of-the-art progress in the development and application of SAPs, especially the mechanism and performance of SAPs on various reaction processes. Some future challenges and potential research directions over SAPs are outlined at the final stage.

Since Al 3 Ni 2 intermetallic compound (IMC) is utilized in the preparation of Raney-type Ni catalysts, it is considered as an industrially important material and has been highlighted in Intermetallic Materials Processing in Relation to Earth and Space Solidification (IMPRESS) project  [1–13]. Another important application of Al-rich IMCs is in the use of metal surface coatings due to the easier tendency to form Al 2 O 3 thin films [14]. Despite the significant application potentials of Al-rich Al 3 Ni 2 IMC, there have been less research about this IMC phase compared to its Ni-rich counterparts  [7,15]. Industrial applications such as the preparation procedure of Raney-type Ni catalysts by Al-leaching from Al 3 Ni 2 IMC, and development of metallic surface coatings need a better understanding of interdiffusion effects and IMC growth at the interface of LIQUID and face centered cubic (FCC) phases [16]. At temperatures equal to or greater than 1123.15 K, Al 3 Ni 2 intermetallic compound is formed at the interface between Al-rich LIQUID and Ni-rich FCC phases. Designing experiments at such high temperature is quite complicated. Because of the opaque nature of Al–Ni alloy, it is quite difficult to study the evolution of the interfacial IMC phases during the reactive wetting solely from experiments [17]. In this context, multiscale computational models are essential tools to obtain a better understanding of the mechanisms of IMC growth at the interface. A major challenge in simulating the growth behavior of Al 3 Ni 2 grains between the LIQUID and FCC phases is the uncertainty regarding the values of interfacial energies σ i / j between two adjacent phases i and j. While some information is available on the interface properties for Ni-rich IMC phases [18,19], there is little information regarding the experimental or theoretical values of the interfacial energies ( σ f c c / i m c , σ f c c / l i q , σ i m c / i m c , σ i m c / l i q ) for the interfaces involving Al-rich IMC phases in Al–Ni system. As the Al-rich IMCs are formed during reactive wetting at higher temperatures, it is difficult to determine the interfacial energies using conventional experiments. Alternatively, the current state-of-the-art of quantitative phase field models makes it possible to determine properties such as liquid diffusion coefficient and interface energies, provided the results can be compared with experimental observations directly related to the effects of these material parameters [20]. However, when considering IMC growth at material interfaces, too many different types of interfaces are involved to be able to determine the individual properties of the interfaces based on phase-field simulations. Therefore, in this work, interfacial energies are calculated using molecular dynamics (MD). As it is quite challenging to upscale MD simulations to reach the length scales of macroscale experiments, the mesoscale phase field model can be used to scale bridge results from MD computations and the continuum scale. To be specific, the interfacial energies computed from MD simulations are utilized in multi-phase field simulations for the study of the growth of Al 3 Ni 2 IMC grains at the LIQUID/FCC interface.Numerous studies have been devoted to molecular dynamics (MD) simulations that can measure the interfacial energies between different materials  [21–24]. For instance, Hu et al. [21] performed MD simulations to measure the solid/liquid interfacial energy of uranium. Critical nucleation method (CNM) and capillary fluctuation method (CFM) were used to measure interfacial energy. Results show good agreement between these methods (CNM and CFM). Benedek et al.  [22] performed atomistic simulations in heterogeneous interfaces with due consideration to the role of misfit. The influence of inter-diffusion and reaction layer was not considered, hence not promising for reactive systems at high temperatures. The interfacial energy also can be represented by calculating the work needed to create two free surfaces, which is the work of separation. This method has been utilized in the work by Yang et al. [23] and Gong et al. [24].Bhaskar [25] has studied the precipitate coarsening in ternary Ni–Al–Mo alloys using phase field method. The model is based on constant interfacial energy (CIE) formulation i.e. a single constant value of interfacial energy ( σ ) is employed for the determination of model parameters of a given phase field simulation. For a total of 6 phase field simulations performed with 6 different values of interfacial energy, it has been revealed that the coarsening rate is increased with the increase in the value of σ . This illustrates that interfaces with different physical structure will evolve differently and cannot be represented by a single value of interfacial energy. Thus, in a multi-phase field system characterized with distinct interfaces, it is very important to account for the effect of the role of heterogeneity caused by the differing magnitude of the interfacial energies. The influence of interfacial energies in the pattern formation during thin film growth has been assessed using phase field method in work of Khanna and Choudhury  [26]. However, in their work, the interfacial energy has been utilized directly as a model parameter in the gradient term of the phase field equation.The present study is designed on the framework of multiscale simulations. The interfacial energies of the FCC/IMC, FCC/LIQUID, IMC/IMC and IMC/LIQUID interfaces are first computed at T = 1173.15 K using MD simulations. Subsequently, atomistically informed multi-phase field simulations of the growth of Al 3 Ni 2 IMC grains at the interface of Al-rich LIQUID and Ni-rich FCC phases are performed at T = 1173.15 K. The MD computed interfacial energies are supplied into the finite element method (FEM) based phase field simulations through the appropriate model parameter choice. At T = 1173.15 K, Al 3 Ni 2 is the only stable IMC phase between LIQUID and FCC. Therefore, from now onwards the term IMC will refer to AL3NI2 phase.Interfacial energy σ i / j between phases i and j is a physical quantity of interest for materials scientists and engineers. However, we did not find any experimental work measuring the σ i / j at such an elevated temperature as the temperature of interest for our study, namely around T = 1173 K . MD simulation is considered to be the most relevant approach to compute the interfacial properties at the nanoscale, and is therefore chosen in this study for the calculation of the interfacial energies. The Embedded Atom Method (EAM) potential developed by Zhou et al. [27,28] has been employed for all the MD simulations in this work to describe the interactions between the Al and Ni atoms.In order to set up the MD model, slabs of FCC Al, FCC Ni and Al 3 Ni 2 IMC are defined with an identical geometry (length = 70 Å, breadth = 70 Å, and height = 90 Å) at room temperature (298.15 K). The Ni Slab consists of 42,865 Ni atoms only, the IMC slab is composed of 20,376 Al + 13,090 Ni atoms, whereas the Al slab is defined with 27,562 Al atoms only. Then these slabs were heated to the elevated temperature of 1173.15 K at a linear heating rate of 0.01 K/fs  [29], and after reaching this temperature each slab is relaxed for 1 ns. Then this is followed by the procedure of uniting two slabs. When two slabs of different materials are joined as shown in Fig. 1, an interface will be created between the materials. Fig. 1 illustrates the procedure for the construction of a solid/solid Ni/Al 3 Ni 2 interface. When two slabs of a same material are united to produce a slab, as shown in Fig. 2, a union slab of single phase is created. In Fig. 2, both Ni and Al 3 Ni 2 are in solid state at T = 1173.15 K. The illustrated methodology can thus be applied to compute the surface energies of these solids. We ignored possible variations in interface energy with orientation of the interface plan to restrict the computational work. Al is in liquid state at this temperature. For liquid, surface energy and surface tension are equal. Therefore, for Al, instead of preparing the union of two heated Al slabs, it is opted to apply a mechanical approach [30] to compute the surface tension based on simulations for a single heated slab.At 1173.15 K, only Ni retains its FCC crystal structure, whereas Al transforms into liquid. Since the properties are computed at this temperature, the subscripts fcc, liq and imc appearing with mathematical quantities nowonwards refer to FCC (pure Ni or Ni-rich), LIQUID (pure Al or Al-rich) and Al 3 Ni 2 IMC phases. The energy needed to create an interface between two different phases (i and j) at a given temperature is called the interfacial energy ( σ i / j ) and is calculated by the following equation  [31]. (1) σ i / j = E i / j − E i − E j 2 S i n t where, S i n t is the interface area in the simulated system. It is equal to 70 Å × 70 Å. For FCC/IMC interface, the interfacial energy is denoted as σ f c c / i m c . E f c c / i m c refers to the time-averaged total energy obtained for a union material as depicted in Fig. 1, whereas E f c c and E i m c are the time-averaged total energy of the single phase union materials as shown in Fig. 2. For the computation of σ l i q / i m c , the interface energy of IMC/LIQ interface, E l i q / i m c and E i m c are obtained in the same way as just discussed for the FCC/IMC interface, while the time-averaged total energy E l i q is computed using the mechanical approach for surface tension determination. While the interfacial energy corresponding to i and j phases is denoted as σ i / j , the surface energy for solid phase i or surface tension of a liquid phase is denoted as σ i .Following the mechanical approach, the surface tension ( σ A l ) of liquid Al slab can be expressed by the following equation: (2) σ A l = 1 2 ∫ − ∞ ∞ [ P N ( z ) − P T ( z ) ] d z In Eq. (2), P N ( z ) and P T ( z ) are, respectively, the normal and tangential components of pressure to the interfaces. The z -axis is defined as the normal to the base area (x–y plane) of the slab. The total height of the slab in this z-dimension is divided into layers, each of width dz. In a 3D cartesian co-ordinate system, the normal pressure component P N ( z ) can be alternatively expressed as P z z ( z ) whereas the average tangential pressure component is defined by 1 2 ( P x x ( z ) + P y y ( z ) ) .The MD simulation is performed in LAMMPS software  [32]. The Open Visualization Tool (OVITO)  [33] has been utilized for the visualization of the atoms and their positions. Periodic boundary conditions were applied in x and y directions whereas non-periodic boundary conditions were applied in the z -direction. Canonical ensemble (NVT) was used to control the temperature of the systems. The velocity-Verlet algorithm [34] has been implemented to solve Newton ′ s equation of motion at each time step. A time step of 1 fs was taken.To validate the MD calculated values, experimental data for surface tension of liquid Al ( σ l i q or σ A l ) and surface energy of FCC Ni ( σ N i ) were collected. Pendant drop method [35] is one of the most commonly utilized approaches for surface tension measurement. The exact details of the experimental steps required to determine the surface tension of pure Al using this method at different temperatures in ultra-high vacuum condition (7 × 10−9-9 × 10−9 Pa) has been outlined in Sun et al. [36]. For the present work, the average surface tension values obtained at two temperature, namely, 1023.15 K and 1123.15 K are considered. Moreover, experimental values of surface energy of pure Ni reported in the literature [37–40] were used for comparison with the calculated data. The MD computed values of surface energy of FCC Ni ( σ f c c ) for T = 1023.15 K and 1123.15 K, are presented along with the available experimental values in Fig. 3. The uppermost value of surface energy among the experimental data is 3.7 J/m 2 corresponding to a temperature T = 298.15 K  [37]. The lowest measured value is 1.78 J/m 2 corresponding to a temperature T = 1573 K  [40]. Our simulation results are within these two extreme values and are thus considered acceptable. Fig. 4 shows surface tension data against temperature for liquid Al. At T = 1023.15 K, the surface tension of Al as obtained from experiments is 0.903 N/m [36], while the MD simulation yields σ l i q = 0.909 N/m at T = 1000 K. The experimental σ l i q at T = 1123.15 K is 0.883 N/m [36] and the computed σ l i q at T = 1200 K is 0.883 N/m. At 1023.15 K, the percentage error of the numerically computed σ l i q values with respect to the experimental value is obtained as 0.38% whereas at a higher temperature of 1123.15 K, the numerical model deviates from the experimental value by an error of 1.16%. The deviations between the experimental and computational values is very low, indicating that the MD computed data, using the considered interatomic potential and for the considered system, are most probably reliable even from a quantitative point of view. The above benchmark studies thus confirm the strength and acceptability of Zhou interatomic potential for describing the surface and interface properties in Al–Ni materials systems.The computed results of time-averaged total energy (E i for single phase or E i / j for two phase slabs) at T = 1173.15 K are presented in Table 1. The values of interfacial energies σ i / j obtained from Eq. (1) are listed in Table 2.The MD calculation has demonstrated that the various interfaces in the Al–Ni system at T = 1173.15 K, have different interfacial energies. Hence, for the accurate understanding of the spatio-temporal dynamics of the AL3NI2 IMC grains at the interface of LIQUID (Al-rich) and FCC (Ni-rich) phases, it is essential to develop a mesoscale phase field model that has the capability to address the differences in interfacial energies. Fig. 5 shows a initial condition (IC) of a 2D computational model consisting of LIQUID phase (upper part), FCC phase (lower part) and 16 square AL3NI2 IMC grains at the LIQUID/FCC interface at T = 1173.15 K. In this study, the width of the computational domain is 1200 nm and height is 700 nm. The length of the squares representing the IC of IMC grains is maintained within a range of 60–80 nm. The total area of IMC phase (sum of area of individual grains) at t = 0 is equal to 7.91E+04 nm 2 . The initial areas of FCC and LIQUID phase are 3.015E+05 nm 2 and 4.59E+05 nm 2 respectively. The phases of the computational domain are represented by a set of non-conserved order parameters ( η i ) in such a way that η i = 1 inside a phase i and 0 outside it. The composition of a single component is sufficient to numerically represent the composition information in a binary Al–Ni system. It is chosen to represent the composition by the mole-fraction of Al, represented by c , and which is a conserved variable. We use c to represent the global composition at each position in the computational domain. Besides, c f c c , c i m c and c l i q are introduced to represent the mole-fraction of Al in FCC, IMC and LIQUID phases, respectively. In a multi-phase system with possibly more than one grain per phase, the material properties at the interface region between two different phases are interpolated in a thermodynamically consistent manner using the following function  [41]: i AL3NI2 (IMC) phase having q grains per phase (3) h i = ∑ j = 1 q η i , j 2 ∑ k = 1 N η k 2 ii FCC and LIQUID phases having 1 grain per phase (4) h i = η i 2 ∑ k = 1 N η k 2 AL3NI2 (IMC) phase having q grains per phase (3) h i = ∑ j = 1 q η i , j 2 ∑ k = 1 N η k 2 FCC and LIQUID phases having 1 grain per phase (4) h i = η i 2 ∑ k = 1 N η k 2 where i represents the phase and η i , j are the order parameters representing the IMC grains and η i are the order parameters representing FCC phase or LIQUID phase. In the current model consisting of 3 phases and represented as in Fig. 5, the FCC phase and LIQUID phase are each represented by a single grain. Whereas the AL3NI2 IMC phase is assumed to initially consist of 16 grains and for this phase q = 16. Thus, the total number of order parameters required in the model, denoted by N in Eq. (4), is 18. The computational model is thus for a 3-phase 18-order parameter system.The driving force for microstructural evolution in the computational model is derived from the free energy of the system. The total free energy of the thermodynamic system F t o t a l can be described as the sum of bulk free energy ( F b u l k ) and interfacial free energy ( F i n t ) as following: (5) F t o t a l = F b u l k + F i n t = ∫ V ( f b u l k + f i n t ) d V The bulk free energy per unit molar volume, f b u l k , is expressed as following. (6) f b u l k = ∑ i h i f c h e m i c a l i = ∑ i h i G m i V m i In Eq. (6), V m i is the molar volume of phase i [42]. In the present study, the molar volume of FCC, IMC and LIQUID phases are taken equal and are assigned the value of 11.4 cm 3 mol−1. The molar Gibbs free energy of each phase, denoted by G m i in the equation, is considered to have a parabolic composition dependence and is represented by the following expression [43]: (7) G m i = A i 2 ( c i − c e q i ) 2 + B i ( c i − c e q i ) + C i . The fitted parabolic Gibbs free energy curves of the phases are depicted in Fig. 6(a) for T = 1173.15 K. Table 3 presents the coefficients A i , B i and C i for the FCC, AL3NI2 (IMC) and LIQUID phases. In Fig. 6(b), a sketch is shown to illustrate how the projected values for the equilibrium compositions of co-existing phases are obtained using the common tangent construction. The composition ( c f c c , e q , i m c f c c ) at which FCC phase is in equilibrium with AL3NI2 IMC phase is 0.16245, and the composition ( c i m c , e q , f c c i m c ) of the IMC in equilibrium with FCC is 0.58269. Similarly, the equilibrium compositions of IMC and LIQUID when in equilibrium with each other are c i m c , e q , l i q i m c = 0 . 6275 and c l i q , e q , i m c i m c = 0 . 8967 , respectively. The coefficients of the parabolic Gibbs energy functions were chosen so that the equilibrium compositions agree well with those obtained from the phase diagram calculated with Thermo-Calc software using TCNI8 database.The interfacial free energy density (f i n t ) of Eq. (5) can be mathematically decomposed into the following parts [44–46] (8) f i n t = f o + f g r a d where, (9) f o = m ( ∑ i ( ( η i ) 4 4 − ( η i ) 2 2 ) + ∑ i ∑ j ≠ i γ i , j η i 2 η j 2 + 1 4 ) and, (10) f g r a d = κ i , j 2 ∑ i ( ∇ η i ) 2 Thus, the quantity f0 contains two model parameters, namely, m and γ i , j . The gradient free energy density, denoted as f g r a d in Eq. (8) consists of the model parameters κ i , j .It is important to note that diffuse interface width ( δ ) and interfacial energy ( σ i / j ) are both related to model parameters m , κ i , j and γ i , j . When the interface energy is the same for all the interfaces, κ i , j = κ , ∀ i , j and γ i , j = γ , ∀ i , j , and only three parameters m , κ and γ have to be defined for a given value of δ and σ  [43]. However, as outlined by the atomistic calculation in Section 2, the interfacial energy is different for different interfaces. When considering different interfacial energies, the model parameters need to be calculated following the procedure outlined in Moelans et al. [44]. More details of the calculation procedure of the parameters m , κ i , j and γ i , j is presented in Section 3.2.The evolution of the conserved variable (c) is expressed by the following equation: (11) ∂ c ∂ t = ∇ . ∑ i h i M i ∇ ( ∂ f b u l k ∂ c i ) where i goes over the three phases and wherein the Cahn–Hilliard mobility M i is related to phase diffusivity (D i ), thermodynamic factor (A i = ∂ 2 f b u l k ∂ ( c i ) 2 ) and grain-boundary mobility (M g b ) as following : (12) M i = D i A i + ∑ j ≠ i h j M g b The grain boundary mobility (M g b ) of Eq. (12) is defined as following (13) M g b = 3 δ g b D g b δ ( h i A i + h j A j ) with D g b being the grain-boundary diffusivity and δ g b = 0.5 nm is the width of the grain boundary channel. For a binary system, there is only one interdiffusion coefficient for each phase, describing the intermixing of both elements.The phase evolution is characterized by the following equation representing the spatio-temporal evolution of non-conserved order parameters : (14) ∂ η i ∂ t = − L ( η 1 , … , η N ) ( ∂ f b u l k ∂ η i + ∂ f o ∂ η i − ∇ . ∂ f g r a d ∂ ∇ η i ) In Eq. (14), L ( η 1 , … , η N ) is mathematically defined by the following expression (15) L = ∑ i = 1 N ∑ j > 1 N ( L i , j η i 2 η j 2 ) ∑ i = 1 N ∑ j > 1 N η i 2 η j 2 The kinetic coefficients L i , j of Eq. (15) are determined as described in Section 3.2.For a system with varying interfacial energies (VIE) at different interfaces, the following expressions relate the different interfacial energy ( σ i / j ) and diffuse interface width ( δ ) to the phase field coefficients κ i , j , m and γ i , j .  [44,45]: (16) σ i / j = g ( γ i , j ) κ i , j m (17) δ = κ i , j m f 0 , m a x ( γ i , j ) The two functions of γ i , j namely, g ( γ i , j ) and f 0 , m a x ( γ i , j ) , present in Eqs. (16) and (17) are evaluated following the procedure defined in [44]. The interfacial mobilities of the different interfaces are defined using the following formulae: i For grain boundaries between two grains of the same phase (18) L i , j = μ i / j g ( γ i , j ) δ f 0 , m a x ( γ i , j ) with μ i / j the mobility of the grain boundary between grains i and j . ii For phase interfaces (19) L i , j = 4 m κ i , j λ i , j where, 1 λ i , j = M i + M j 2 ( c i , e q , j i − c j , e q , i j ) ( c i , e q , k i − c k , e q , i k ) and i and j refer to the two phases in the neighboring grains. For grain boundaries between two grains of the same phase (18) L i , j = μ i / j g ( γ i , j ) δ f 0 , m a x ( γ i , j ) with μ i / j the mobility of the grain boundary between grains i and j .For phase interfaces (19) L i , j = 4 m κ i , j λ i , j where, 1 λ i , j = M i + M j 2 ( c i , e q , j i − c j , e q , i j ) ( c i , e q , k i − c k , e q , i k ) and i and j refer to the two phases in the neighboring grains.The quantity λ i , j corresponding to the interface of the phase i and phase j is a function of the equilibrium composition and diffusion mobility in phases i and j [47]. With δ = 25.0 nm, and m = 2.4 × 108 J/m 3 , the functional values of g ( γ i , j ) and f 0 , m a x ( γ i , j ) are outlined. Then the model parameters κ i , j , γ i , j and L i , j (for both grain boundaries and phase boundaries) are estimated corresponding to the different values of interfacial energies σ i / j using the procedure outlined in  [44]. The magnitudes of these model parameters are provided in Table 4. In order to set up the phase field model with a correct assignment of the model parameters κ i , j , γ i , j and L i , j at the corresponding interfaces, the following expression is defined in the model: (20) ψ k = ∑ i = 1 N ∑ j > 1 N ( ψ i , j η i 2 η j 2 ) ∑ i = 1 N ∑ j > 1 N η i 2 η j 2 where, ψ i , j represents κ i , j , γ i , j or L i , j . The phase dependent interfacial mobility has already been mathematically expressed in Eq. (15).The values of interfacial energies at different types of interfaces, and their corresponding model parameters are discussed in the preceding section. In addition to these model parameters, the other parameters and material properties required as input in the phase field simulations are listed in Table 5. The values for phase diffusivities used in the simulations are presented in the table and are obtained from [48–51]. As mentioned earlier, the interfacial energy ( σ i / j ) listed in the table are obtained from the molecular dynamics calculation of the present work. The energies of the boundaries between the IMC grains were not computed with the MD method, as it would have required a huge amount of compute time. Instead, we used the MD computed IMC surface energy as a first estimate for the grain boundary energies of the IMC phase (i.e. we took σ i m c / i m c = σ i m c ) in the phase-field model. The model parameters κ i , j , γ i , j , and L i , j will vary in the simulation domain to represent the different values of interfacial energies, and their values are mentioned in Table 4.The non-conserved order parameters are initially described as a function of co-ordinates (x,y) to define the initial geometrical conditions of the phases represented in Fig. 5. At t = 0, the mole fraction of Al in FCC, AL3NI2 and LIQUID phases are set as 0.15, 0.55 and 0.89, respectively. No flux boundary condition is applied for both c and η variables at the top and bottom boundaries of the computation domain. At the right and left sides of the domain, periodic boundary conditions are enforced for both types of variables.The partial differential Eq. (11) and  (14) are solved using finite element method (FEM) in Multiphysics Object Oriented Simulation Environment (MOOSE) Framework  [52,53]. The GeneratedMesh object of MOOSE has been utilized to construct 2D mesh with QUAD4 elements for the initial geometry described in Fig.  5. In the phase field simulation, the following assumptions have been made in accordance to Kim–Kim–Suzuki (KKS) model [54] for the binary Al–Ni system consisting of three phases (FCC, IMC and LIQUID). i Chemical potential between any two adjacent phases is equal. ii The expression c = ∑ i h i c i relates the global composition of Al with its local phase compositions ( i goes over the three phases). Chemical potential between any two adjacent phases is equal.The expression c = ∑ i h i c i relates the global composition of Al with its local phase compositions ( i goes over the three phases). With the information of interfacial energies obtained from molecular dynamics simulation and having outlined the corresponding values of model parameters for the mesoscale phase field models, we can now present the results of a 2D grain growth simulation for Al 3 Ni 2 IMC grains at Al/Ni interface for the VIE model. Fig. 7 presents the evolution of the grains showing the structure at t = (i) 0.05, (ii) 0.1, (iii) 0.14 and (iv) 0.19 s. The interfacial Al 3 Ni 2 IMC grains with a composition c ≈ 0.6, appear in red color according the color scale bar. The LIQUID (Al-rich) and FCC (Ni-rich) regions are depicted respectively by yellow and cyan colors. It is evident from the figure that as the time passes the IMC phase (total sum of IMC grains) area expands at the expense of LIQUID and FCC phases. At t = 0.05 s (Fig. 7(i)), all the IMC grains (regardless of their initial sizes) have nearly equal exposure to the LIQUID phase at phase boundaries. But as the time passes the Ostwald ripening phenomenon influences the relative growth of grains with a different size. While all the grains tend to grow longitudinally at the expense of LIQUID and FCC phase, they are marked by differentiated lateral growths at the LIQ/IMC and FCC/IMC interfaces. At the LIQUID/IMC interface, the locally larger grains tend to expand laterally whereas the locally smaller grains show lateral constriction. As revealed clearly in the image at t = 0.19 s (Fig. 7(iv)), the initially larger grains are facilitated to enlarge their contact with the liquid whereas the initially smaller grains lose their contact with the LIQUID phase. At FCC/IMC interfaces, on the contrary, the lateral dimension of the IMC grains hardly varies, regardless of their initial size. A clearly different morphology is thus observed at these two interfaces.In order to understand whether the choice of interfacial properties affects the growth pattern of phases in the Al/Ni system, it is necessary to compare the result of varying interface energy (VIE) model with those obtained using constant interfacial energy (CIE) model. The simulations in CIE formulation start with the same initial geometry as the VIE model, and use the same bulk material properties. Only the interfacial properties are taken differently. The interfacial properties of the CIE model are taken as given in Table 6. With the interfacial energy for all the interfaces (denoted as σ c ) being assigned a value of 1.0 J/m 2 and the interface width taken as δ = 25 nm, the model parameter m = 2.4E+08 J/m 3 in the CIE model, and this is equal to that of VIE model. However, in contrast to VIE model where { κ i , j } and { γ i , j } have different values at various interfaces, the CIE model has a constant κ and a constant γ at all of the interfaces. As mentioned in the table, κ = 1.875E−08 J/m and γ = 1.5 at all of the interfaces of the CIE model. It is to be noted that the kinetic coefficients L i , j defined by Eqs.  (18) and  (19) are influenced by diffusion mobilities of grains/phases besides the interfacial energy values. The ratio of diffusivity (refer Table 5) to thermodynamic factor A i (refer Tables 3) for LIQUID phase is 8.35 × 10−21 m 5 /(J s) whereas this ratio for FCC phase is 6.14 × 10−24 m 5 /(J s) for FCC phase. In context of model parameters shown in Tables 4 and  6, all elements of { κ i , j } vector are within the range (1.69–2.25) × 10−8 J/m and the components of { γ i , j } vector vary just between 1.32 and 1.98. Thus for the Al–Ni system at 1173.15 K it can be inferred that the L i , j for interface i/j are solely dominated by the magnitude of diffusion mobilities of phase i and phase j, and are generally independent of the variation in interfacial energies.The morphology of a grain (G1) neighboring two smaller grains at its either sides is compared between the two models for t = 0.1375 s in Fig. 8. The image (a) represents the simulation result of CIE model whereas the image (b) is of VIE model. It is interesting to note that the FCC/IMC interface moves relatively faster in the CIE model, and thus G1 grows more towards the bottom in the CIE model. Moreover, the IMC/LIQUID phase boundary length of the G1 is larger for the VIE model. Though not so distinct, the curvatures of the phase boundaries are definitely different for the two models. Fig. 9 presents a comparison of the evolution of phase areas for the two models. The AL3NI2 IMC grows at the expense of LIQUID and FCC phases in both cases. As shown in Fig. 9(a), the area of FCC phase decreases at a faster rate for the CIE model compared to the VIE model. At t = 0.4 s, the area of FCC in CIE model is reduced to a value of 1.48E+05 nm 2 whereas FCC phase in VIE model attains an area of 1.84E+05 nm 2 . In contrary to this, the phase area of LIQUID phase for VIE model decreases at a faster rate than that of CIE model (Fig. 9(b)). The LIQUID area in CIE model is lowered to a value of 2.64E+05 nm 2 at t = 0.41 s whereas the LIQUID phase in VIE model already shrinks to a value of 2.56E+05 nm 2 within the same time. This dynamics is consistent with the fact that the interfacial energy at FCC/IMC interface of VIE model is larger than σ c (1.0 J/m 2 ) whereas that for LIQUID/IMC interface of VIE model is smaller than σ c . A larger interface energy results in a larger curvature driving force which counteracts the growth, since the center of curvature of the FCC/IMC and LIQUID/IMC interfaces is inside the IMC. Finally, as revealed by Fig. 9(c), the total IMC phase area is observed to increase at a faster rate in the CIE model than in the VIE model. At t = 0.4 s, the corresponding IMC phase areas of CIE and VIE models are respectively 4.18E+05 and 3.97E+05 nm 2 . Since the deviation between CIE and VIE model is larger for FCC phase than for LIQUID, the IMC phase in CIE model grows slightly faster than in VIE model. From these observations, it is clear that the selection of interfacial properties influences the growth kinetics of the phases in the simulations. Hence it is important to determine the interface properties accurately and use a phase-field model that can account for different interfacial energies. This highlights the need to formulate a phase-field model that can take into account different interface energies (VIE) for a realistic description of IMC morphology in multi-phase materials systems. Moreover, it is needed to determine interface properties accurately for all different types of interfaces. The following conclusions can be derived from the study: i This work employs the mesoscale multi-phase field method for mathematical description of the spatio-temporal dynamics of AL3NI2 IMC phase at the interface of LIQUID (Al-rich) and FCC (Ni-rich) phases. Sixteen square Al 3 Ni 2 IMC grains were initially introduced at the interface of LIQUID and FCC phases and the phase field simulation is performed at 1173.15 K to understand the structural evolution of these interfacial IMC grains. ii The interfacial energies required to determine the model parameters of the VIE formulation based mesoscale phase field method, have been supplied through computations performed using nanoscale Molecular Dynamics. The MD computation yielded the interfacial energies of 1.2, 1.02 and 0.9 J/m2 at FCC/IMC, FCC/LIQUID and IMC/LIQUID interfaces respectively. For the IMC/IMC grain boundaries, a value of 0.957 J/m2 was used in the phase-field model. This value was estimated, taking it equal to the MD calculated surface energy for IMC, since computing the different grain boundary energies from MD would become highly complex. Additionally, a CIE formulation based phase field model is also designed corresponding to δ and σ c of 25 nm and 1.0 J/m 2 . iii As compared to the VIE model, phase area reduction for FCC phase is faster in the CIE model whereas the decrease in area of LIQUID phase is slower in the CIE model. Overall there is a net faster growth rate of IMC area in the CIE model. iv The choice of interfacial properties can thus ardently influence the size of bulk phases. The development of microstructure models with varying interface energies and calculation of interface energies from MD is thus important to allow for accurate microstructure prediction. v The presentation of methodologies, namely, the utilization of EAM interatomic potential for the computation of surface as well as interface properties of Al–Ni system at nanoscale, upscaling of these interfacial properties and subsequently combining them compatibly with other thermodynamic properties of bulk phases at mesoscale to simulate the structural evolution of Al 3 Ni 2 grains at the interface of Al-rich LIQUID and Ni-rich FCC phases, in this work, outline the basic cornerstone of virtual experiments of materials with reactive interfaces undergoing high temperature applications. This work employs the mesoscale multi-phase field method for mathematical description of the spatio-temporal dynamics of AL3NI2 IMC phase at the interface of LIQUID (Al-rich) and FCC (Ni-rich) phases. Sixteen square Al 3 Ni 2 IMC grains were initially introduced at the interface of LIQUID and FCC phases and the phase field simulation is performed at 1173.15 K to understand the structural evolution of these interfacial IMC grains.The interfacial energies required to determine the model parameters of the VIE formulation based mesoscale phase field method, have been supplied through computations performed using nanoscale Molecular Dynamics. The MD computation yielded the interfacial energies of 1.2, 1.02 and 0.9 J/m2 at FCC/IMC, FCC/LIQUID and IMC/LIQUID interfaces respectively. For the IMC/IMC grain boundaries, a value of 0.957 J/m2 was used in the phase-field model. This value was estimated, taking it equal to the MD calculated surface energy for IMC, since computing the different grain boundary energies from MD would become highly complex. Additionally, a CIE formulation based phase field model is also designed corresponding to δ and σ c of 25 nm and 1.0 J/m 2 .As compared to the VIE model, phase area reduction for FCC phase is faster in the CIE model whereas the decrease in area of LIQUID phase is slower in the CIE model. Overall there is a net faster growth rate of IMC area in the CIE model.The choice of interfacial properties can thus ardently influence the size of bulk phases. The development of microstructure models with varying interface energies and calculation of interface energies from MD is thus important to allow for accurate microstructure prediction.The presentation of methodologies, namely, the utilization of EAM interatomic potential for the computation of surface as well as interface properties of Al–Ni system at nanoscale, upscaling of these interfacial properties and subsequently combining them compatibly with other thermodynamic properties of bulk phases at mesoscale to simulate the structural evolution of Al 3 Ni 2 grains at the interface of Al-rich LIQUID and Ni-rich FCC phases, in this work, outline the basic cornerstone of virtual experiments of materials with reactive interfaces undergoing high temperature applications. Anil Kunwar: Conceptualization, Methodology, Software, Investigation, Visualization, Validation, Formal analysis, Writing – original draft. Ensieh Yousefi: Software, Methodology, Investigation, Validation, Visualization, Formal analysis, Writing – review & editing. Xiaojing Zuo: Software, Validation, Writing – review & editing. Youqing Sun: Investigation, Validation, Writing – review & editing. David Seveno: Methodology, Validation, Supervision, Writing – review & editing, Resources, Funding acquisition. Muxing Guo: Validation, Supervision, Writing – review & editing, Resources, Funding acquisition. Nele Moelans: Conceptualization, Methodology, Validation, Supervision, Writing – review & editing, Resources, Funding acquisition.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by the KU Leuven Research Fund (C14/17/075). The computational resources and services used in this work were provided by the VSC (Flemish Supercomputer Center), funded by the Research Foundation - Flanders (FWO) and the Flemish Government - department EWI. Open access funding was provided by Silesian University of Technology.

Considering its application in developing Raney-type Ni catalysts and in metal surface coatings, the study on the growth behavior of Al 3 Ni 2 intermetallic compound (IMC) at the Al/Ni material interface is of utmost importance. The present work integrates nanoscale molecular dynamics (MD) calculation with mesoscale phase field model for studying the interfacial phenomena associated with Al 3 Ni 2 growth in Al/Ni interface at 1173.15 K. The interfacial energies computed from MD are in the range 0.9–1.2 J/m 2 with FCC/IMC featuring as the interface with the largest value and IMC/LIQUID as the one with the lowest value. Phase field model parameters characterizing a varying interface energy formulation are established to simulate the 2D growth of interfacial IMC grains. With the help of an atomistically informed phase field model, it has been revealed that the phase areas and morphology are obviously sensitive to the interfacial properties. The methodologies and results of these multiscale simulations for IMC interfaced between Al and Ni microstructures offer the complementary and accelerated design route of in-silico studies for materials systems experimented at high temperature.

Heterocyclic compounds have gained great attention in organic synthesis. Five-membered heterocyclic compounds containing one heteroatom such as pyrrole, furan and thiophene are the most prominent scaffolds in organic chemistry having wide-spread biological activities [1]. Especially, pyrroles have an essential role in pharmaceuticals [2], agrochemicals [3], optoelectronic materials [4] and biologically active natural products [5]. Pyrrole shows notable pharmaceutical properties such as anti-oxidative [6], anti-fungal [7], anti-bacterial [8], anti-inflammatory [9] and ionotropic nature [10]. Moreover, pyrroles are the important building blocks for functional materials due to their occurrence, which exhibits inevitable properties [11]. Knorr [12], Paal-Knorr [13], and Hantzsch [14] reactions are the well-established classical synthetic strategies for pyrroles. However, new interesting and attractive methodologies have been introduced to synthesize substituted pyrroles. Over the past decades, plenty of methods have been developed for the construction of N-substituted pyrroles through modified Paal-Knorr or Clauson-Kaas reaction using acid catalysts [15] and metal catalysts such as Mg [16], Fe [17], Cu [18], Zr [19], Bi [20] and Ce [21]. In this context, we focused on the sustainable and environmentally benign protocol for synthetic reactions [22]. We extended our efforts on organic synthesis catalyzed by transition metals such as Mn [23], Fe [24], Co [25], Ni [26], Cu [27], Zn [28], etc. Very recently our group introduced a solvent-free manganese-catalyzed microwave-assisted version of Clauson-Kaas reaction (Scheme 1 ) [29]. To the best of our knowledge, no solvent-free zinc-catalyzed synthesis of N-aryl pyrroles through Clauson-Kaas reaction are reported yet. On comparison with other metals, zinc is the low-cost and less-toxic 3d series transition metal, which has a vital role in synthetic chemistry due to its relatively great abundance in earth and high concentration in ores. In addition, zinc has an attractive biological relevance and is an essential trace element in the human body [30]. Due to the relevance of zinc, in 2017, Tran and co-workers reported an efficient, simple and green protocol for the synthesis of substituted pyrroles through Paal-Knorr reaction utilizing the deep eutectic solvent ([CholineCl][ZnCl2]3) under solvent-free sonication [31]. We continued our efforts towards zinc-catalyzed C-N bond formation and due to the ecofriendly and less-toxic nature of zinc, we herein disclose our findings on the zinc-catalyzed protocol for the synthesis of N-aryl pyrroles through Clauson-Kaas reaction under neat condition.At the beginning of our studies, we have chosen aniline (1a, 1 mmol) and 2,5-dimethoxytetrahydrofuran (2, 1.2 mmol) as the model substrates for the reaction, which along with 10 mol% of Zn(OTf)2 catalyst, and heating at 60 °C for 2 h under neat condition resulted in the formation of N-phenyl pyrrole (3a) in 56% yield (Scheme 2 ).Encouraged by this result, we screened various zinc catalysts such as Zn(OTf)2, Et2Zn, ZnI2, Zn powder, anhydrous ZnCl2, Zn(NO3)2·6H2O, ZnSO4·H2O, Zn(OAc)2·2H2O and ZnO (Table 1 , entry 1–9). Catalytic screening was started with Zn(OTf)2, which furnished 56% yield of the product within 2 h. Subsequently, we used Et2Zn, Zn powder and Zn(OAc)2·2H2O, which did not afford the product. When we used ZnI2, anhydrous ZnCl2 or Zn(NO3)2·6H2O, the N-substituted pyrrole product was obtained in 18%, 15% and 31% respectively while ZnSO4·H2O and ZnO when used as the catalyst, only trace amount of the product could be obtained. From these catalyst screening, it was revealed that Zn(OTf)2 was the best catalyst.After the catalyst screening, we tested the effect of temperature and time. The reaction carried out at room temperature provided only trace amount of the product (Table 2 , entry 1). On increasing the reaction temperature to 60 °C, we obtained the desired product in 56% yield (Table 2, entry 2). When we increased the temperature to 80 °C, the reaction did not take place due to the solidification of the reaction mixture within 25 min (Table 2, entry 3). We presume that the solidification is due to polymerization or other side reaction. The solidified reaction mixture was found to be insoluble in most of the solvents. Then we carried out the reaction at 60 °C and increased the reaction time to 4 h, 6 h and 8 h, we observed a gradual increase in the yield of product (Table 2, entry 4–6). Then we increased the reaction temperature to 70 °C for 8 h, which provided 94% yield of the product (Table 2, entry 7). No change in the product yield was observed when the reaction was carried out at 70 °C by increasing the reaction time to 10 h (Table 2, entry 8). Later, we increased the reaction time to 12 h and 24 h, and noticed a decrease in the yield of the product (Table 2, entry 9–10). From these observations, we concluded that the optimum temperature and time for the reaction were 70 °C and 8 h respectively.Finally, we investigated the effect of the amount of catalyst loading in this reaction. We tried the reaction using 10 mol% catalyst, which afforded the product in 90% yield (Table 3 , entry 1). Then, we increased the amount of catalyst to 15 mol% and found a decrease in the product yield to 80% (Table 3, entry 2). Then the reaction was carried out without any catalyst, which did not offer any product (Table 3, entry 3). When we performed the reaction using 5 mol% catalyst, the product was formed in 94% yield (Table 3, entry 4). The reaction when carried out with 2.5 mol% catalyst furnished the product in 59% of yield (Table 3, entry 5). Finally, we concluded that the suitable reaction condition for this reaction as 5 mol% Zn(OTf)2 as the catalyst at 70 °C for 8 h when aniline and 2,5-dimethoxytetrahydrofuran were used as the substrates.With the optimized reaction condition in hand, we conducted substrate scope studies using a wide variety of anilines substituted with either electron-donating or electron-withdrawing groups (Scheme 3 ). From these studies, it is revealed that anilines with electron-donating groups such as –Me, –OMe, etc. provided higher yield than those with electron-withdrawing groups such as –COMe, –NO2, etc. and the anilines bearing substituents at -ortho position with electron-donating groups –Me, –OMe, etc. furnished good to excellent yield than those bearing at -meta and -para positions. 2,6-Dimethyl substituted aniline (3e) provided a relatively higher yield than -ortho, -meta and -para toluidine (3b-3d). Ortho bromoaniline (3i) afforded the product in excellent yield when compared to the para bromoaniline (3j) whereas chloro substitutions at -ortho, -meta and -para furnished the desired product in satisfactory yields (3k-3m). But, iodo substitutions at -ortho, -para position and –OH at -para position did not give the expected excellent yield due to the clogging of the reaction mixture at 70 °C (3n-3p). The presence of electron-withdrawing groups such as –NO2, –COMe, etc are resulted the desired products in lower yields (3q-3r). We also tried the reaction with aliphatic amines such as benzylamine and cyclohexylamine, which unfortunately did not undergo this reaction.In summary, we have disclosed a green zinc-catalyzed protocol for the synthesis of N-substituted pyrroles through a modified Clauson-Kaas reaction using 2,5-dimethoxytetrahydrofuran and various anilines without using any solvent, base or ligand. A wide range of anilines reacted with 2,5-dimethoxytetrahydrofuran under the optimized reaction conditions to provide the N-substituted pyrroles in moderate to excellent yields.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.CMA and KRR thank the Council of Scientific and Industrial Research (CSIR, New Delhi) and the University Grants Commission (UGC, New Delhi) for the award of junior research fellowships respectively.Supplementary data to this article can be found online at https://doi.org/10.1016/j.rechem.2022.100350.The following are the Supplementary data to this article: Supplementary data 1

The first zinc-catalyzed simple and convenient protocol for the synthesis of N-substituted pyrroles through a modified Clauson-Kaas reaction without co-catalyst, ligand, base and solvent has been described. N-substituted pyrroles were prepared from various aniline derivatives and 2,5-dimethoxytetrahydrofuran under green condition by utilizing low cost, eco-friendly, non-toxic and easily accessible Zn(OTf)2 as catalyst. A wide variety of N-substituted pyrroles were afforded in moderate to excellent yields from easily available starting materials at 70 °C.

With the rapid development of society, the excessive consumption of fossil fuels and the dramatically increasing of energy demand have caused serious environmental pollution and energy crisis. The development of renewable energy and energy conversion technologies is considered to be an effective way to solve these two issues. 1–6 Especially, electrocatalytic conversion devices composed of electrocatalysts, membrane and electrolytes have attracted extensive attention from researchers because of their high efficiency and cleanliness. 7 However, their large-scale application is greatly limited by the slow kinetics of the cathodic reaction. Therefore, the development of highly active and stable electrocatalysts is necessary to improve energy conversion efficiency of emerging devices and propel their commercialization.Traditional heterogeneous catalysts (e.g. Pt-based catalysts and RuO2) usually contain a mixture of metal particles with a wide size distribution. However, only a small fraction of metal particles with appropriate size distribution can be used as catalytically active species, while others are either inert or probable to trigger undesired side reactions, which results in low metal utilization efficiency and poor selectivity of the reaction. 8–10 Homogeneous catalysts have well-structured active sites and tunable ligand environments, which exhibit excellent activity and high selectivity for the target reaction. However, the catalysts still suffer from limitations such as poor stability and unsatisfactory recoverability. 11 Single atom catalysts (SACs) combine the advantages of “isolated sites” from homogeneous catalysts and the structural stability as well as convenience of separation from heterogeneous catalysts, which serve as a bridge between homogeneous and heterogeneous catalysts. 12–14 Since Zhang and coworkers reported single Pt atom on iron oxide for CO oxidation in 2011, 15 single atom catalysts have attracted a lot of attention in various aspects, such as photocatalysis, 16–20 electrocatalysis 21–25 and thermal catalysis 26–28 because of their maximal atom utilization (nearly 100%) and high catalytic activity, stability and selectivity. The size reduction of nanoparticles to the sub-nanometer level leads to an increasing in number of low-coordinated metal atoms that can be used as catalytically active sites, which will be maximized at the single-atom level when individual metal atoms are accessible and catalytically active. 29 However, these low coordination single atoms have very high surface energy and tend to aggregate during the synthesis process. 27 Therefore, it is of great importance to select suitable supports to interact with metal single atoms to prevent their aggregation for the synthesis of single-atom catalysts. 30 Common support materials include metal oxides, 15 metal sulfides, 31 metal nitrides, 32 metal surfaces, 33 MOFs, 34 porous carbon materials, 35 , 36 g-C3N4 37 , 38 and graphene. 39 , 40 Among them, nitrogen-doped porous carbon is considered as an ideal single-atom support due to its hierarchical pore structure, high mechanical strength and special defect effect. 14 Metal-nitrogen-carbon catalysts, in which dispersive metal atoms are coordinated to nitrogen atoms doped in carbon nanomaterials, have been used as effective catalysts for lots of electrochemical reactions, such as carbon dioxide reduction reaction (CO2RR), 41–44 nitrogen reduction reaction (NRR), 45 , 46 oxygen reduction reaction (ORR), 47–49 oxygen evolution reaction (OER), 50 hydrogen oxidation reaction (HOR) 51 and hydrogen evolution reaction (HER). 52 Precious metal catalysts show excellent activity for these reactions, but their high cost and low natural abundance have forced the search for suitable metal substitutes. In contrast, iron is well-known to be abundant in nature and inexpensive, and also exhibits high durability, tunability in both acidic and alkaline medium, and methanol tolerance, which has led to many iron-based catalysts being explored for numerous reactions, especially in the reduction of C, N and O.In this review, as shown in Fig. 1 , we firstly conclude several essential methods for the synthesis of Fe–N–C. Then, we present the property of Fe–N–C catalysts, including spin-related interaction of electronic structures as well as the orbital coupling between the electronic structure of Fe–N x and the electronic orbital of C, N and O. Furthermore, we summarize the electrocatalytic applications about the chemical conversion of C, N and O with Fe–N–C. Lastly, we discuss some challenges in structural characterization, mechanistic investigation and industrial application of Fe–N–C, and propose some possible solutions.Since 2008, MOFs have been widely recognized as ideal sacrificial templates for preparing of highly stable and conductive nanostructured carbon. 53 By charring the MOF precursors with the target metals, it was found that the single atoms (SAs) were firmly embedded into the carbon carriers through strong metal heteroatom (S/N/O) coordination bonds, successfully providing single-atom catalysts with excellent catalytic performance. 54 The iron source is first anchored into the networks of the metal nodes and organic linkages, as well as in the cavities of the entire MOF crystal to form iron-containing MOF precursors, which are then converted into Fe SACs in one-step pyrolysis treatment. So far, the most commonly used MOF precursor in this method is a zinc imidazole skeleton doped with a small amount of iron because of its good structural tailorability, high nitrogen content and the pore confinement effect. Under solvent or solvent-free conditions, Fe2+/Fe3+ is spatially separated by Zn2+ and 2-methylimidazole ligands, which are atomically diluted throughout the MOF crystals. During pyrolysis at high temperatures, Zn metal readily evaporates out of the system due to its low boiling point, leaving the Fe SAs distributed on the MOF-derived porous carbon framework (Fig. 2 a). 55 It is worth mentioning that the nitrogen atoms from the organic linkers will firmly anchor and stabilize these Fe single atoms, thus effectively avoiding their migration and aggregation.To improve the activity of Fe–N–C, Zhang et al. replaced ZIF-8 with MIL-101 to act as a precursor to introduce abundant mesopores (Fig. 2b). 56 However, the iron-containing MIL-101 has high iron content and insufficient nitrogen content, so amino groups are further introduced in preparing of the precursor to reduce the formation of Fe nanoparticles in the catalyst. Firstly, NH2-MIL-101(Al) was pyrolyzed to obtain nitrogen-doped carbon materials (NC-MIL101-T) at a temperature range of 800–1100 °C under N2. After acid etching, Fe(phen)3 2+ and NC-MIL101-T support were thoroughly mixed. FeSAC-MIL101-T catalysts were finally obtained after freeze-drying and the second pyrolysis at 800 °C. Later, Yang et al. proposed to replace Zn by Cd as a sacrificial metal for the synthesis of Fe–N–C, which could reduce the pyrolysis temperature of the precursor from 1000 °C to 750 °C and thus help to preserve the individual iron atom active sites. Meanwhile, as a comparison, the ZIF-8/Fe catalyst was prepared by pyrolysis at the same temperature. In contrast to the ZIF-8/Fe catalyst, the Fe–N–C retained a smaller amount of sacrificial metal and formed a higher density of single-atom structures. 59 In addition, Jiang and co-workers used a hybrid ligand strategy to prepare high-content (1.76 wt%) single-atom iron-implanted nitrogen-doped porous carbon (FeSA–N–C) by pyrolysis of porphyrinic MOFs (Fe–PCN-222). The mixed porphyrin ligands, Fe-TCPP and H2-TCPP made the Fe(Ⅲ) ions form a long spatial distance in the MOF skeleton, which was conducive to the formation of single iron atoms by pyrolysis. 60 Later, Jiang et al. developed a similar strategy to obtain FeSA–N–C with high Fe loading (3.46%) by the pyrolysis of SiO2@MOF composite (Fig. 2c). 57 The pre-synthesized PCN-222(Fe) has a one-dimensional pore structure of ∼3.2 nm, which ensures that tetraethylorthosilicate (TEOS) can be fully permeable in the internal space. Under hydrochloric acid vapor treatment, TEOS was hydrolyzed and condensed to silica, forming the SiO2@MOF composite with well-retained MOF crystallinity. Silica interacts with iron atoms to increase the energy barrier for iron atoms migration and thus prevent their aggregation. Moreover, the removal of silica increases the porosity and specific surface area of the material, which facilitates the exposure of active sites and mass transfer. In contrast, Fe particles could be detected from the catalyst obtained by the direct pyrolysis of PCN-222(Fe) without SiO2, which proved that the presence of SiO2 inhibited the migration of Fe atoms under pyrolysis.Moreover, heteroatom doping is an important means to increase the density of active centers and improve the electrocatalytic activity of catalysts. The doping of heteroatoms can change the coordination environment and electronic properties of iron centers as well as change the density of active centers through long-range or short-range interactions. Wang et al. designed a Zn/Fe bimetallic mixed-ligand metal-triazolate (MET) as the precursor and 4,5-dichloroimidazole as the source of Cl to obtain FeN4Cl/NC (Fig. 2d). The higher electronegativity of chlorine could change the d-band delocalization and electronic structure of Fe atoms to ensure the presence of FeN4Cl coordination configuration. 58 Irregular metal-containing complexes and polymers can replace MOFs as precursors to prepare atomically dispersed Fe–N–C catalysts with special structures via pyrolysis. Tang et al. designed atomically dispersed iron atoms anchored on N-doped carbon nanosheets (Fe–N–C HNSs) with well-defined FeN4 structures and unique spherical hollow architecture via SiO2-templated strategy. Histidine (His) served as the source of N and C due to high content of heteroatoms and natural abundance. During the process of preparation, the pre-synthesized SiO2 nanospheres could absorb Fe3+ ions through electrostatic attractions after surface modification with negative charges. Subsequently, the His molecules bound with Fe3+ ions to form SiO2@Fe-His nanospheres, which would be pyrolyzed at high temperature and acid leached. The 3D hollow spherical structure prevented aggregation of iron atoms, offered shorten pathway for mass diffusion and exposed more active sites. 61 Li et al. synthesized Fe–N–C catalysts that possess special atomically dispersed Fe-N x structure via changing the ratio of acrylic acid (AA) and maleic acid (MA). AA could be polymerized into PAA and chelated with Fe3+ to form a cross-linked hydrogel, while MA could be co-polymerized with AA to increase the carboxylic content of the copolymer (P(AA-MA)). These polymers and cyanamide were treated at high-temperature to obtain PAA–Fe–N and P(AA-MA)–Fe–N. Structural characterization results showed that the introduce of MA elongate the bond strength of Fe–N and create the exclusive Fe–N4/C moiety of P(AA-MA)-Fe-N. 62 Generally, high temperature pyrolysis is the most mature method for preparing FeSA–N–C. By adjusting the size, coordination number and composition of the MOF precursor, a more precise tuning of the catalyst can be achieved. In addition, pyrolysis of irregular metal-containing complexes and polymers enable more structurally diverse FeSA–N–C catalysts with more abundant iron sources, carbon sources and nitrogen sources. Remarkably, during the process of high temperature pyrolysis, the introduction of template can further regulate the microstructure and pore size of the FeSA–N–C catalysts, thus improving their catalytic performance and stability.Due to low metal loading in high temperature pyrolysis methods, the chemical vapor deposition (CVD) method was designed to synthesize Fe–N–C catalysts (Fig. 3 a). 63–67 One of the most important advantages of the CVD method is the ability to select different templates and adjust deposition conditions to precisely control the morphology and pore structure of the resulting catalysts. 68 Wu et al. prepared atomically dispersed Fe–N–C catalyst with increased Fe loading via CVD method compared to wet-chemistry synthesis. As shown in Fig. 3b, with the presence of argon gas and heating, gaseous 2-MeIm was deposited onto Fe–ZnO nanosheets and underwent a gas-solid reaction to form Fe–Zn(MeIm)2 intermediate and its crystalline structure gradually evolved from zif towards kat phase. The formation of CVD/Fe-kat increased the number of active sites, which was attributed to the fact that the formed narrower pores could promote the coordination of single Fe sites with N and thus slow down the diffusion and agglomeration of Fe atoms. Finally, CVD/Fe–N–C-kat was obtained under 1000 °C heating. According to the analysis of in-situ electrochemical measurements through the nitrite absorption followed by reductive stripping, the FeN4 active site density of CVD/Fe–N–C-kat is about 26 mmol g−1, which is higher than most Fe–N–C catalysts synthesized by other methods. 63 Similarly, Peng et al. chose ferrocene-doped calcium oxide as a template and pyridine as carbon and nitrogen sources to synthesize atomic Fe–N–C catalyst by pyrolysis at 700 °C under an argon atmosphere via CVD method (Fig. 3c). 66 Later, they used iron(III) acetylacetonate dissolved in pyridine as a precursor, magnesium hydroxide as a substrate for CVD pyrolysis, and finally acid etching to obtain single iron atoms anchored on porous N-doped carbon (Fe-N-PC) (Fig. 3e). 64 Jia and co-workers creatively synthesized Fe–N–C with high active sites by flowing ferric chloride vapor over Zn–N–C at 750 °C via CVD method (Fig. 3d). Zn–N–C material was obtained by pyrolysis of ZIF-8 under argon atmosphere at 1050 °C, which contained 2.16% Zn and abundant microporous structures. In the presence of FeCl3, the Zn was removed and Fe took the place of Zn under high temperature (>650 °C), which contributed to the formation of Fe–N4 sites according to the reaction mechanism: FeCl3(g) ​+ ​Zn–N4 ​+ ​X → Fe–N4 ​+ ​ZnCl2(g) ​+ ​XCl (X refers to H or Cl). The obtained FeNC-CVD-750 catalyst had an activity site density of 1.92 ​× ​1020 sites per gram with 100% site utilization. 67 In short, although the CVD method can appropriately increase the loading of iron atoms, its complex synthesis steps and high temperature condition force a preference for simpler methods to synthesize single-atom catalysts.Traditional ball milling method involves mixing metal salts, nitrogen-containing compounds and supports by ball milling and then thermal reducing the mixture to form single atom catalysts. The purpose of ball milling is to improve metal dispersion before pyrolysis. Dai et al. reported a universal domino reaction strategy to produce M-SA/NC catalysts including Fe, Co, Ni, Mn, Mo, Pd and arbitrary combinations SA/NC catalysts (Fig. 4 a). Polyaniline (PANI), appropriate metal salt, NaCl and NaNO3 were mixed and ball-milled with the aim of making PANI chains doped with metal ions and wrapped around salt particles. NaNO3 was decomposed and released gases to cause PANI blew up and carbonized with the formation of porous carbon nanosheets by pyrolysis at 1000 °C. The role of the gas is to etch the nanosheets for obtaining a microporous structure and also to anchor the metal atoms to the carbon framework and prevent them from aggregating into metal particles. In addition, the structure and pore size of M-SA/NC could be regulated by changing the content of NaNO3. 69 The other means of synthesis is to directly high-energy ball milling coordination precursors (such as iron phthalocyanine) and carbon supports to achieve mixing in molecular-level and provide precursors for the subsequent pyrolysis. 70 , 71 Deng et al. used the ball milling approach to synthesize a highly dispersed single FeN4 center with coordinatively unsaturated iron sites confined in a graphene matrix at a large quantity with iron phthalocyanine (FePc) and graphene nanosheets as precursors (Fig. 4b). 70 During the ball milling process, on one side, the outside macrocyclic structure of FePc is disrupted to produce a fragmented structure possessing FeN4. On the other side, graphene produces defective sites and interacts with isolated FeN4 centers.Under the influence of the above two ball milling methods, Baek et al. discovered an eco-friendly top-down strategy, namely mechanochemical abrasion method, which means the direct atomization of bulk metals on different supports by means of abrasion, without any solvent and the generation of by-products and waste in the process (Fig. 4c). 32 In the experiment, iron balls, N2 gas and graphite were loaded into a steel container and the ball-milling was conducted at a constant rotation speed for 30 h. Iron balls are not only the source of iron single atoms, but also transfer kinetic energy to drive reactions. N2 gas is dissociated on the surface of the iron balls and enters the graphite framework to anchor the iron atoms. The graphite is served as an active matrix to accommodate the nitrogen and atomized metal. An additional advantage of the abrasion method is that the amount of catalyst can be expanded in equal proportions by increasing the volume of the ball-milling container.In general, Table 1 lists the advantages and disadvantages of these three preparation methods for Fe–N–C single atom catalysts. Compared to other synthesis methods, ball milling method simplifies the synthesis process and is prone to be applicable for the large-scale preparation of Fe–N–C. However, during the ball milling process, some of iron atoms may be encapsulated inside the catalysts, which in turn results in a low atomic utilization and uneven structures.There are many factors affecting the properties of Fe–N–C electrocatalysts, such as the electronic structure of the central metal, the metal coordination environment, and the metal-support interactions. But little attention has been paid to the role of electron spin of Fe. As the following, we will discuss spin-related properties in Fe–N–C catalysts.In general, FeN x moieties serve as active sites of most Fe–N–C catalysts, while the number of coordination x usually depends on the synthesis conditions. The coordination environment and the valence state of iron determine the spin state of single Fe atom. Fig. 5 shows the electronic configuration and spin state of the common FeN4 species. When oxidation state of Fe is +1 or +4, the 3d electron configuration of FeN4 is d x y 2 d y z 2 d x z 2 d z 2 1 or d x y 2 d y z 1 d x z 1 , respectively, belonging to low spin states. When oxidation state of Fe is +2, the 3d electron configuration of Fe(Ⅱ)N4 can simply be classified into three forms, including d x y 2 d y z 2 d x z 2 , d x y 2 d y z 2 d x z 1 d z 2 1 and d x y 2 d y z 1 d x z 1 d z 2 1 d x 2 − y 2 1 , corresponding to low spin t2g6eg0, medium spin t2g5eg1 and high spin t2g4eg2, respectively. When oxidation state of Fe is +3, the 3d electron configuration of Fe(Ⅲ)N4 can simply be classified into three forms, including d x y 2 d y z 2 d x z 1 , d x y 2 d y z 1 d x z 1 d z 2 1 and d x y 1 d y z 1 d x z 1 d z 2 1 d x 2 − y 2 1 , corresponding to low spin t2g5eg0, medium spin t2g4eg1 and high spin t2g3eg2, respectively. Most of the reports demonstrate that the preparation method and preparation conditions have important effects on the spin of iron in Fe–N–C catalysts, including the type and content of iron, pyrolysis temperature, pyrolysis duration, atmosphere, etc. Additionally, it must be mentioned that heteroatom doping and the modulation of supports could also change the iron electron polarization as well as the electron spin state and complete mutual transformation of different spin states.Commonly used detection tools for measuring the spin state of Fe include electron paramagnetic resonance (EPR), L-edge X-ray absorption near edge structure (XANES) and Mössbauer spectroscopy. When there are unpaired electrons in the outer electron orbitals of iron, under an applied magnetic field and an applied electromagnetic wave, the electrons at the low spin energy level absorb energy and transfer to the high spin energy level to produce paramagnetic resonance absorption, which is the reason why EPR can measure the spin state of iron. However, there is a serious limitation of EPR that it can only provide information on iron with half-integer spin multiplicity and cannot measure the valence state of all iron species. Fe L-edge XANES is employed to analysis the structure of Fe, because the valence and spin states of 3d tradition metals significantly affect the L-edge spectra. The L-edge of 3d tradition metals is generated by electronic transitions between the 2p level and the mostly unoccupied 3d electronic states. The L3 edge (706–712 eV) involves transitions from 2p3/2 to 3d states, while the L2 edge (718–726 eV) comes from transition 2p1/2 to 3d states. Additionally, it has been found that there is a relationship between the area ratio of L3/L2 and the spin state. A higher ratio indicates that high spins dominate, and vice versa. 72–74 However, the sensitivity of XANES to the spin state of the fine structure containing iron is not as good as that of the Mössbauer spectrum.The 57Fe Mössbauer spectroscopy, involving the resonant and recoil-free emission and absorption of γ-rays by atomic nuclei, is used to study the valence, spin polarization and coordination environment of iron in single-atom catalysts based on the values of isomer shift (IS) and quadrupole splitting (QS). 74–76 Each type of iron with a defined coordination structure and spin state corresponds to a specific value of IS and QS, and the shift of values implies the change of iron species. In addition, with the development of operando spectroscopy, operando Mössbauer spectroscopy could be used to characterize the spin state of iron-based materials during reactions. As a result, the Mössbauer spectrum has become the most reliable means to characterize the spin state of iron-based materials.Recently, non-resonant X-ray emission spectroscopy (XES) was demonstrated to quantify the average, ex situ spin state of a series of Fe–N–C catalysts. Herranz et al. used two-component fitting to analyze the Kβ main lines based on a linear relation between the relative area of the Kβ’ spectral peak and the spin state of several reference compounds, and in turn established a potential-induced spin change in the catalysts prepared by pyrolysis of an Fe-porphyrin. 77 Excitingly, this method has the potential to be extended to measure the spin state of other transition metal materials. However, it always reflects an average spin state information and cannot help us to determine the influence of the spin state on the reaction mechanism at a deeper level.Numerous experimental evidences show that the spin of Fe affects the occurrence of the reaction as well as the rate and selectivity of the reaction. Next, we will further analyze the effect of spin on the reduction of C, N and O from the perspective of orbital coupling.The activation of CO2 plays an important role in the process of CO2→CO. The HOMO of CO2 localize on the O atom, while the LUMO localize on the C atom and performed as a C–O σ∗ orbital. Taking Fe(Ⅰ) L.S. as an example, the electron-rich iron center conducts nucleophilic attacks on the electrophilic C-center of CO2, which means that occupied d z 2 , d y z and d x z orbitals of iron could offer electrons to populate these empty σ∗ and π∗ orbitals, which is beneficial to the activation of CO2 (Fig. 6 a, d). When CO2 gets an electron, a new splitting of the CO2 orbitals occurs, but the coupling tendency with Fe will not change greatly. 78 Taking Fe(Ⅲ) M.S. as an example, the iron centers have empty d x 2 − y 2 orbitals and the half-filled d z 2 orbitals (Fig. 6b). Upon the N2 side-on adsorption, N2-σ electrons will be transferred to higher-energy empty spin-down d-orbitals ( d x 2 − y 2 ). The empty spin-down orbitals ( d z 2 ) at the higher energy interact with the antibonding orbitals of N2 to weaken the N–N triple bond and lower the N2 adsorption energy (Fig. 6e), which can explain why Fe(Ⅲ) has excellent NRR performance. 79 The low spin Fe(Ⅱ)N4 lack of unpaired d electrons and the high spin Fe(Ⅱ)N4 lack of empty d orbitals showed poorly activity of N2 reduction reaction. Similarly, the high spin Fe(Ⅲ)N4 exhibits worse activity than middle spin Fe(Ⅲ)N4 and low spin Fe(Ⅲ)N4 due to the shortage of empty d orbitals. 80 Fig. 6c illustrates the major orbital interactions between O2 and high spin Fe(Ⅱ) during the O2 adsorption process. The antibonding orbital (π∗) of O2 could couple with the half-filled d z 2 , d y z and d x z orbitals to form four new low-to-high orbitals, corresponding to d x z − d y z − π ∗ BD , d z 2 − π ∗ BD , d x z − d y z − π ∗ BD ∗ and d z 2 − π ∗ BD ∗ , which increases the d orbital splitting and form a more stable system (LS Fe(Ⅱ)-O2). From the electronic point of view, it can be roughly considered that the d electrons of d z 2 , d y z and d x z orbitals leap to the π∗ orbital of O2, resulting in electrons rearrangement that occur to reduce the energy of the system (Fig. 6f). It is also worth mentioning that the interaction between iron species and O2 should not be too strong. Low spin Fe(Ⅲ) without eg filling has empty σ∗ antibonding orbital of FeN4, and leads to a very strong Fe(Ⅲ)/O2 interaction and a quite stable Fe4+–O2 2− bond. Thus, it is difficult for the occurrence of Fe(Ⅳ)-O2/Fe(Ⅲ)-OOH transition. 81 , 82 Electrocatalytic carbon dioxide reduction reaction is a significant strategy to solve the problem of energy shortage and environmental pollution. Carbon dioxide could be reduced to value-added chemical products powered by electricity generated from renewable energy sources. 83 Many studies have shown that Fe–N–C exhibits remarkable performance concerning the reduction of CO2 to CO and other simple chemicals (Table 2 ).Hu et al. reported Fe–N–C catalyst with atomically dispersed iron sites, which produced CO at the overpotential of −0.08 V in the CO2-saturated 0.5 M KHCO3 catholyte. When the cathode potential decreased to −0.45 V (vs. RHE), particle current density of CO could reach 94 mA cm−2 with FECO higher than 90%. Fe 2p3/2 XPS spectrum and Fe K-edge XANES spectrum results indicated that the iron oxidation state of Fe–N–C was +3. Compared to Fe–N–C, the current density of Zn–N–C synthesized under the same conditions could be neglected at −0.1 ​V to −0.6 ​V (vs. RHE), indicating that Fe sites was the origin of Fe3+–N–C during CO2RR. Operando XANES displayed that the Fe K-edge of Fe3+–N–C showed no obvious shift at −0.1 ​V to −0.4 ​V (vs. RHE), indicating the Fe species remained in +3 oxidation state; while Fe K-edge shifted to lower energies at −0.4 ​V to −0.5 ​V (vs. RHE), the same as the potential of the deactivation of Fe3+–N–C, indicating the reduction of Fe3+ to Fe2+ (Fig. 7 a). Furthermore, it could be observed that Fe3+ was reduced to Fe2+ in the as-prepared Fe3+–N–C at −0.1 V to −0.2 V (vs. RHE) (Fig. 7b). These phenomena proved that Fe3+ sites were more active for generating CO. According to the kinetic and mechanistic analysis (Fig. 7e), CO2 adsorption is the rate-limiting-step for Fe2+–N–C, while the protonation of the adsorbed CO2 − to form an adsorbed COOH intermediate is the rate-limiting-step for Fe3+–N–C. Additionally, the CO2RR rate would be also limited by CO desorption for Fe2+–N–C but not limited for Fe3+–N–C. As a result, the superior activity of Fe3+ could be proven to derive from faster CO2 adsorption and weaker CO adsorption compared to Fe2+ sites. 84 Liu et al. identified low-spin Fe(Ⅰ)N4 is the reactive center for the conversion of CO2 to CO. Operando 57Fe Mössbauer results showed that three doublets were detected in the Fe–NC–S at OCV, corresponding to LS Fe(Ⅱ)N4, MS Fe(Ⅱ)N4 and HS Fe(Ⅱ)N4, respectively. When polarized at −0.3 V (vs. RHE), a new doublet was observed and was assigned to LS Fe(Ⅰ)N4. When the potential was gradually decreased to −0.9 V (vs. RHE), the relative content of LS Fe(Ⅰ)N4 increased accompanied by the decreasing of relative content of LS Fe(Ⅱ)N4, which reflected that LS Fe(Ⅱ)N4 was reduced to LS Fe(Ⅰ)N4. Additionally, the new doublet disappeared when removing the potential, further proving that Fe(Ⅰ)N4 transited from LS Fe(Ⅱ)N4 was the real active center during CO2RR (Fig. 7c). DFT calculations indicated that the CO2 molecule is activated on the Fe(Ⅰ) site and then forms the ∗COOH intermediate after the hydrogenation step. During the process, the singly occupied d z 2 orbital of Fe(Ⅰ) coupled with the singly occupied π1∗ orbital of COOH to generate one fully occupied bonding ( d z 2 − π 1 ∗ BD ) orbital and one empty antibonding ( d z 2 − π 1 ∗ BD ∗ ) orbital (Fig. 7d). Next, ∗CO and H2O are generated in the presence of electrons and protons. Finally, the adsorbed CO desorbs from the Fe(Ⅰ) site to complete the catalytic cycle. 75 To further understand the effect of spin state of single-atom FeN4, Chen et al. conducted a more detailed analysis on the electroreduction of CO2 to CO/HCOOH. Combined with the Fe2+ radius, energy order and corresponding HOMO–LUMO gap of calculated Fe(II)N4 in the different spin states, the order of catalyzing activity is inferred to Fe(II)N4(MS) > Fe(II)N4(LS) > Fe(II)N4(HS). Moreover, the calculation of CO2 absorption energy indicated the adsorption strength decreases with the increase of the spin states for Fe(II)N4. Furthermore, it is clear that the middle spin Fe(II)N4 has the lowest energy barrier for the first-step reduction of CO2 (0.52 eV) compared to other two spin states. As a result, the middle spin Fe(II)N4 have the highest selectivity and best activity from the perspective of mechanism. The same approach proves that the middle spin Fe(III)N4C favors the conversion process of ∗CO2 to ∗COOH as compared with the other two spin states. 89 The industrial production of ammonia is mainly dependent on the Haber–Bosch process under harsh conditions, which accounts for 1%–2% of the earth's energy supply. 97 , 98 Moreover, this process can only obtain relatively low conversions due to the constraints of chemical equilibrium. Therefore, it is of great significance to develop efficient nitrogen fixation routes under mild conditions. Inspired by the fact that bacteria can electrochemically reduce nitrogen in the presence of enzyme nitrogenase, electrochemical N2 reduction reaction via N 2 ( g ) + 6 H + + 6 e − → 2 NH 3 ( g ) received a lot of attention. 99 Unfortunately, hydrogen evolution reaction (HER) is more likely to occur at similar potentials at most of the metal active sites due to the yield of a large amount of electrons and protons. From the kinetic point of view, the activation of N2 is the rate determining step of nitrogen reduction reaction. 100 Fe–N–C is thought to be ideal electrocatalysts for lowering the free barrier of N2, weakening hydrogen absorption, and improving ammonia selectivity due to the dispersion of active sites and the positive charge of the metal (Table 3 ). 101 Thanks to high stability and high carrier mobility, graphene is expected to facilitate charge transfer in catalytic reactions. As early as 2016, Luo et al. proposed a new catalyst, FeN3-embedded graphene, for activating N2 and converting it into NH3 at room temperature from first-principles calculations. From the perspective of chemical coordination, the FeN3 center is strongly spin-polarized with a localized magnetic moment, which greatly facilitates the adsorption of N2 and activates the inert NN bond. The synergistic interaction between graphene and FeN3 gives the system novel properties to catalyze the conversion of activated N2 to NH3 via a six-proton and six-electron process at room temperature following three possible reaction paths. 102 On this basis, Zheng et al. designed and synthesized Fe–N/N–CNTs with built-in Fe–N3 sites by pyrolysis of Fe-doped ZIF–CNTs templates. The NRR performance indicated that Fe–N/N–CNTs possessed the highest NH3 average yield of 34.83 μg h−1 mg−1 cat., and corresponding FE of 9.28% at −0.2 V (vs. RHE). 90 Later, more and more studies have found Fe–N4 structure possess higher intrinsic activity and obvious ability to inhibit hydrogen evolution. Liu et al. reported a Fe single-atom catalyst with well-defined Fe–N4 active sites and in neutral media, achieving high Faradaic efficiency (18.6 ± 0.8%) and NH3 yield rate (62.9 ± 2.7 μg h−1 mg−1 cat.) at −0.4 V (vs. RHE) at room temperature. 91 Hu et al. developed a method to prepare iron-nitrogen-carbon materials for electrocatalysis N2 reduction reaction by loading iron phthalocyanine (FePc) on nano/microporous carbon at a molecular level. It delivered a high selectivity and activity with a NH3 yield rate of 137.95 μg h−1 mg−1 FePc at the potential of −0.3 V (vs. RHE) in 0.1 M Na2SO4 aqueous solution. On the basis of systematic electrochemical analyses, poisoning experiments and theoretical calculations, it suggested that Fe center in FeN4 was the most active site for NRR rather than N or C sites. 92 Yan et al. demonstrated a single atomic iron catalyst on nitrogen-doped carbon (FeSA–N–C), which promoted NRR process with a Faradaic efficiency of 56.55% at an onset potential of 0.193 V and a desirable ammonia yield rate of 7.48 μg h−1 mg−1 at 0 V in alkaline solution. Molecular dynamics simulation unveiled that N2 molecules tended to accumulate at approximately 0.45 nm from the Fe site, leading to high local concentrations, which would promote the following adsorption (Fig. 8 a). DFT calculations proved that the energy barrier of N2 absorption is much lower than that of water dissociation (Fig. 8b) and the alternating pathway was prone to achieve NRR (Fig. 8c). These results illustrated that FeSA–N–C was more favorable for nitrogen adsorption than hydrogen adsorption with a small energy barrier. 46 In order to understand the relationship between the electronic state of FeN4 and NRR, Zhang et al. fabricated Fe and Mo co-coordinated polyphthalocyanine electrocatalyst (FeMoPPc) by a low-temperature melt polyphthalocyanine. The zero-field cooling temperature-dependent magnetic susceptibility measurements and 57Fe Mössbauer spectra revealed that Fe(Ⅱ) changed from high spin to middle spin after adding Mo, which weakened the NN bond and promoted the first hydrogenation of N2. 80 In addition, Feng et al. found that F surface modification could induce Fe(Ⅲ) in the high spin state, which facilitated π-backdonation process, promoted the activation of N2 and reduced the limiting potential of NRR. 103 The misuse of nitrogen fertilizers and the consumption of fossil fuels have made nitrate ions one of the most spread water contaminants, posing a serious threat to the ecology and human health. 104 , 105 Nitrate reduction reaction converts NO3 − to N2 or NH3 under a wild temperature and pressure, playing a vital role in promoting the earth's nitrogen cycle and solving water pollution problems. In recent years, a series of metal catalysts have been used to convert nitrate to nitrogen, including Ru, Rh, Ir and Cu. 106 , 107 However, there have been few studies on the reduction of nitrate wastes to value-added ammonia. As a vital competition, NO3 − reduction to N2 involves an N–N coupling step, which needs two neighboring active sites. Therefore, selecting suitable single atom catalyst can improve the selectivity towards ammonia. So far, Fe–N–C was reported to perform excellent activity of NitRR, which could help us understand the complex pathways and mechanism of the 8 protons and 8 electrons transfer process (Table 3). 93–95 Wang et al. synthesized a single atomic Fe–N–C catalyst by a TM-assisted carbonization method with highly mesoporous structures. At a potential of −0.66 V (vs. RHE), Faradaic efficiency of the ammonia increased to a maximal of ∼75%. A large NH3 partial current density of ∼100 mA cm−2 and a yield rate of ∼20,000 μg h−1 mg cat−1 (0.46 mmol h−1 cm−2) were obtained at the potential of −0.85 V (vs. RHE). The catalyst still exhibited excellent stability with a high NH3 yield rate and FE after 20 consecutive electrolysis cycles under the best NH3 selectivity reaction condition. DFT calculations reveal the minimum energy pathway for NO3 − reduction to NH3 on Fe single atom site (Fig. 9 a). Furthermore, Fig. 9b exhibited NO∗ is a key intermediate and a limiting potential of U = −0.30 V is needed to make all steps downhill in free energy. These results proved the high NH3 yield rate and activity of Fe SAC contributed to intrinsic high-efficiency active Fe–N4 centers that exhibiting lower thermodynamic barriers and optimized electrocatalytic conditions. 93 Later, Yu et al. demonstrated a polymer-hydrogel strategy for preparing single atom Fe catalysts anchored on N-doped porous carbon (Fe-PPy SACs). When the cathode potential varied from −0.3 V to −0.7 V (vs. RHE), The catalyst displayed a maximum ammonia yield rate of 2.75 mgNH3 h−1 cm−2 with nearly 100% Faradaic efficiency. Besides, The Fe–PPy SACs delivered a twelve times higher turnover frequency than Fe nanoparticles. The NitRR mechanism illustrated that the single active Fe-N x site experienced a nitrate-preoccupied transition center and efficiently eliminated the competing water adsorption. 94 Later, Liu et al. found that doping of S could significantly enhance the activity of NitRR. 95 The catalyst (Fe–CNS) has many folds and defects according to the electrochemical active surface areas and pore size distributions, which suggested that S-doping could create more defect sites and shift the previously balanced electrons, leading to well electrical conductivity. Compared with Fe–CN, Fe–CNS showed higher NH4 +–FE at all potentials. Moreover, the potential of maximum NH4 +–FE (78%) with Fe–CNS was −0.57 V, higher than that of Fe–CN (−0.67 V). DFT calculations reflected that the energy of the basic reaction ( NO 3 - → NO 2 ∗ ) changed from 2.02 ​eV to 2.37 ​eV after the doping of sulfur, which was consistent with the better catalytic performance of nitrate removal of Fe–CNS. And the energy changes for the basic reactions from N∗ to N2∗ and from N∗ to NH∗ on Fe–CNS are −0.14 ​eV and 2.78 ​eV, respectively, which indicates that the ammonia path is thermodynamically favored. Unfortunately, no study has yet shown that the spin state of the iron center can affect the activity of nitrate reduction.Proton exchange membrane fuel cells have broad application prospects due to their high efficiency and zero emission. 108 However, the slow cathodic reaction kinetics severely limits their development. Current electrocatalysts for this reaction are usually expensive, low storage capacity commercial Pt-based catalysts. 109 Therefore, it has become a top priority to search for efficient and stable ORR catalysts that could replace Pt. So far, among many single-atom transition metal catalysts, Fe–N–C with atomically dispersed iron sites has shown the best ORR catalytic performance (Table 4 ). 110 , 111 Oxygen reduction reactions are normally divided into a two-electron transfer process and a four-electron transfer process. The two-electron process reduces O2 to hydrogen peroxide (H2O2), and the four-electron process directly reduces O2 to H2O under acidic conditions or hydroxide (OH−) under alkaline conditions. 112 , 113 Hydrogen peroxide can react with iron sites in the Fenton reaction to produce reactive oxygen species, which in turn continuously leads to catalyst deactivation and degradation, and damage to the proton exchange membrane. 114 Therefore, we hope to further find Fe–N–C catalysts with high selectivity towards H2O and well-defined mechanism.Xu et al. proposed a defined explain about the influence of local Fe(Ⅱ) spin configuration on ORR. The higher spin state of iron in FeN4 with bond contraction can create a wider spin-related channel in FeN4, promoting the charge transport during ORR. Moreover, the oxygen molecule can be more easily captured by FeN4 with Fe–N bond contraction because of higher bond order resulted from the spin–orbital interactions between iron and O2, which should be the intrinsic factor dominating the DFT calculated trend of O2 adsorption. 117 Liu et al. firstly developed Operando 57Fe Mössbauer to identify the exact structures and spin state of active atomically dispersed iron moieties during ORR. When polarized at 0.9 V (vs. RHE), it indicated that O2 adsorbed on the HS Fe(Ⅱ)N4 sites along with the generation of O2–Fe(Ⅱ)N5 intermediate and the spin state of Fe2+ transited from HS to LS with the central Fe2+ moving to the N4-plane. When polarized at 0.7 V (vs. RHE), O2 adsorbed on the LS Fe(Ⅱ)N4 sites along with the generation of O2–Fe(Ⅱ)N4 intermediate and the spin state of Fe2+ transited from LS to HS with Fe2+ moving out of the N4-plane (Fig. 10 a and b). As shown in Fig. 10c, the spin crossover of Fe2+ significantly reduces the energy barrier for the dynamic cycle. Quantum chemical studies provide the structural and dynamic evolutions of Fe(Ⅱ)N4 and spin-crossover-involved mechanism for ORR (Fig. 10d). Due to the exchange stabilization, the interaction between O2 and HS Fe2+ increases the d-orbital splitting, resulting in the conversion of the spin state of iron (Fig. 10e and f). 74 Later, Zhai and co-workers reported that the incorporation of S in the second sphere of Fe–NC could enhance catalytic activity of oxygen electroreduction reaction via inducing the transition of spin polarization configuration. Mössbauer spectroscopy showed that there are three different doublets (D1−D3) existing in three FeNSC catalysts, corresponding to low spin (LS) Fe3+ (D1: X–Fe3+N–Y), high spin (HS) Fe2+ (D2: Fe2+N4) and high spin (HS) Fe2+ (D3: X–Fe2+N–Y), respectively (X and Y refer to S and C). Among them, Fe1-NS1.3C possessed more D1 moiety which had been proven to be active for ORR (Fig. 11 b). In order to clarify the active sites of Fe1–NS1.3C for ORR, in-situ Mössbauer spectroscopy was performed in O2-saturated 0.1 M KOH at room temperature (Fig. 11a). As shown in Fig. 11c, the D1 content decreased as the D3 content increased at the potential of 0.85 V (vs. RHE), indicating conversion of spin state from LS Fe3+ to HS Fe2+. At the potential of 0.65 V (vs. RHE), the D1 content decreased as the D2 content increased, indicating conversion of spin state from LS Fe3+ to HS Fe2+. While at the potential of 0.45 V (vs. RHE), both the D1 and D3 content decreased as the D2 content increased, implying LS Fe3+ and HS Fe2+ both acted as active sites. Combined with the results that the LS Fe3+ still converted into HS Fe2+ at the same potential without the existence of O2, it is proved that D1 active site is sensitive to O2 molecules and the LS Fe3+ of C–FeN4–S moiety could be the active site for Fe1-NS1.3C catalyst in alkaline ORR. DFT indicated that the doping of S impacted the spin polarization and adjusted the spin state of Fe center, resulting in the decrease of the adsorption free energy of ∗OH, which is further enhanced the activity of ORR (Fig. 11d–f). 115 Additionally, Zhang et al. designed dual-metal atomically dispersed Fe,Mn/N–C catalyst and revealed that the reduction of oxygen occurred preferentially on Fe(Ⅲ) in the intermediate spin state. 122 The analysis of Mössbauer spectroscopy and DFT calculation proved that the implant of Mn–N moieties led to Fe(Ⅲ) 3d electron delocalization and caused the spin state of Fe(Ⅲ) transition from low spin (t2g5eg0) to intermediate spin (t2g4eg1), which easily penetrated the antibonding π-orbitals of oxygen. Similarly, Zhai et al. found that the introduce of Se could also tune charge redistribution and the spin-state of Fe active sites to improve the electrochemical performance for ORR. 116 Guo et al. introduced Ti3C2T x as the support of iron phthalocyanine (FePc) and achieved a significant enhancement of ORR activity. Temperature-dependent magnetic susceptibility measurement results unveiled that the introduction of Ti3C2T x weakened the paramagnetic state of FeN4 moieties and increased the number of unpaired d electron of Fe(Ⅱ) ions, such that more occupied 3d electrons were easily transferred to antibonding π-orbital of oxygen. Compared to the pristine FePc, an additional D1 doublet appeared in the Mössbauer spectrum of the Ti3C2T x -supported FePc, belonging to high spin Fe(Ⅱ). These evidence suggested that van Der Waals forces or hydrogen bonding between FeN4 moieties and Ti3C2T x induced electron density redistribution and spin-state transition and electron configuration transition from d x y 2 d y z 1 d x z 1 d z 2 1 d x 2 − y 2 1 to d x y 2 d y z 2 d x z 1 d z 2 1 was thought to be responsible for the enhanced ORR activity through yielding an easier dioxygen adsorption and reduction. 117 Hydrogen peroxide is a versatile chemical, however, its industrial synthesis involves an energy intensive and tedious anthraquinone process. 123 Therefore, the synthesis of hydrogen peroxide by ORR offers a simpler and more sustainable approach to tackle this challenge. The production of H2O2 via 2e− ORR involves two proton-coupled electron transfer steps: (1) ∗  +  O 2 +  H + + e − →  ∗OOH  (2) ∗OOH + H + + e − → H 2 O 2 + ∗ Liu et al. proved that Fe–N–C adsorbed OOH∗ so strongly that the selectivity of H2O2 is low. 124 Although the Fe–N–C has poor selectivity for hydrogen peroxide, the selectivity can be improved after doping and other treatments due to the modification of metal center. Xue et al. prepared a cheap sodium ferric EDTA-derived material (EDTAFeNa-KB-HT1), an iron and nitrogen co-doped Fe–N–C catalyst with micro-mesoporous structure and large surface area via one-step pyrolysis. The catalyst exhibited high selectivity (80%–100%) towards H2O2 with a large current density. 125 Choi et al. used H2O2 to introduce oxygen functional groups to the carbon surface of Fe–N–C, which changed the reaction path of ORR and led to a 30% increase towards H2O2. 126 Yagi et al. synthesized Cu-, Fe-, and N-doped carbon nanotubes, (Cu, Fe)–N-CNT as an ORR catalyst. It showed a high selectivity of 99% towards H2O2. Kinetic analysis revealed that the rate constant for the reduction of O2 to H2O2 is two orders of magnitude higher than that for the reduction of O2 to H2O. 127 Besides, H2O2 could be trapped in the micropores of powders and further reduced to H2O. Therefore, constructing a freestanding SACs electrode could prevent the further reduction of H2O2 and improve the selectivity.In summary, we concluded common methods for the preparation of Fe–N–C single-atom catalysts, including high-temperature pyrolysis, chemical vapor decomposition (CVD) and ball milling. Then, the relationship between the electronic structure of Fe single atoms and spin configurations is outlined, and based on this, the common methods of spin regulation are briefly mentioned, and the electronic structure of Fe and the orbital coupling of C, N and O are summarized. Finally, the application of Fe–N–C in electrocatalytic C, N and O conversion is introduced, including CO2RR, NRR, NitRR and ORR. Here, we would like to present the challenges and outlook of Fe–N–C, which may provide some opportunities for the future development of SACs. (1) The loadings of catalysts synthesized with the existing methods are still not high, so it becomes an important task to find more reasonable and effective means to synthesize catalysts with high site densities. It should not be overlooked that the increase in loading can cause inferior mass transfer, thus the structural design of the catalyst is particularly important. (2) A reasonable Fe–N bond length can effectively prevent the aggregation of iron atoms and increase the stability of the active sites. However, there is a lack of study in such topic. More efforts shall be devoted to exploring the relationship between catalyst structure and reaction performance from a smaller scale. (3) The structures of SACs do not maintain unchanged during the reactions. Therefore, it is important to develop new techniques to achieve in situ characterization of catalysts to disclose the underlying mechanism and discover better catalysts. (4) The spin regulation of Fe–N–C by external experimental conditions is seldom studied at present. For example, the modification of catalytic properties by applying a magnetic field in the electrocatalytic process has not received enough attention. (5) The spin regulation of Fe–N–C by changing prepared conditions remains to be further developed. Although many reports expounded temperature, heteroatom doping, and other factors can change the spin state of the iron center, the mechanism is not clearly explained. In addition, it is unknown whether some conditions such as the applied magnetic field will exert an effect on the spin state. (6) DFT calculations were reasonably used to predict the spin electron transfer trends on different reactions of Fe–N–C prepared under different conditions. Proper utilization of the DFT calculations can lead to suitable thermodynamic data, 128 , 129 such as adsorption energy, dissociation energy and splitting energy, which may provide new possibilities for the design of Fe–N–C catalysts and mechanistic explanations. The loadings of catalysts synthesized with the existing methods are still not high, so it becomes an important task to find more reasonable and effective means to synthesize catalysts with high site densities. It should not be overlooked that the increase in loading can cause inferior mass transfer, thus the structural design of the catalyst is particularly important.A reasonable Fe–N bond length can effectively prevent the aggregation of iron atoms and increase the stability of the active sites. However, there is a lack of study in such topic. More efforts shall be devoted to exploring the relationship between catalyst structure and reaction performance from a smaller scale.The structures of SACs do not maintain unchanged during the reactions. Therefore, it is important to develop new techniques to achieve in situ characterization of catalysts to disclose the underlying mechanism and discover better catalysts.The spin regulation of Fe–N–C by external experimental conditions is seldom studied at present. For example, the modification of catalytic properties by applying a magnetic field in the electrocatalytic process has not received enough attention.The spin regulation of Fe–N–C by changing prepared conditions remains to be further developed. Although many reports expounded temperature, heteroatom doping, and other factors can change the spin state of the iron center, the mechanism is not clearly explained. In addition, it is unknown whether some conditions such as the applied magnetic field will exert an effect on the spin state.DFT calculations were reasonably used to predict the spin electron transfer trends on different reactions of Fe–N–C prepared under different conditions. Proper utilization of the DFT calculations can lead to suitable thermodynamic data, 128 , 129 such as adsorption energy, dissociation energy and splitting energy, which may provide new possibilities for the design of Fe–N–C catalysts and mechanistic explanations.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.We are grateful for the financial support from National Natural Science Foundation of China (No. 21974103) and the start-up funds of Wuhan University.

Single atom catalysts (SACs) are constituted by isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and amorphous carbon. The thermal stability, electronic properties, and catalytic activities of the metal center can be controlled via manipulating the neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomical dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased. Furthermore, new possibilities are offered to easily control the selectivity of a given transformation process as well as to improve turnover frequencies and turnover numbers of target reactions. Among them, Fe–N–C single atom catalysts own special electronic structure, and have been widely used in many fields of electrocatalysis. This review aims to summarize the synthesis of Fe–N–C based on anchoring individual iron atoms on carbon/graphene. The spin-related properties of Fe–N–C catalysts are described, including the relation between spin and electron structure of Fe–N x as well as the coupling between electronic structure of Fe–N x and electronic (orbit) of CO2, N2 and O2. Next, mechanistic investigations conducted to understand the specific behavior of Fe–N–C catalysts are highlighted, including C, N, O electro-reduction. Finally, some issues related to the future developments of Fe–N–C are put forward and corresponding feasible solutions are offered.

The increase in atmospheric CO2 level has created a foremost environmental concern, as it is known to participate in the global climate change [1]. Sustainable solutions for the decreasing of atmospheric CO2 requires the use of fossil-free energy sources as well the development of new chemical technologies that can convert CO2 to useful chemicals, preferably with economic value [2–8]. For example, the catalytic coupling of CO2 with epoxides to form cyclic carbonates (Scheme 1 ) has attracted considerable interest, especially due to its 100% atom economy. As a result a wide range of such catalytic systems have been developed [9].The active homogeneous systems include various metal-organic catalysts [10–19] as well as some organocatalysts [20–26]. The catalytic coupling reaction with metal-based catalysts follows several steps: (i) the activation of the epoxide by Lewis acidic metal upon coordination through the oxygen atom, (ii) epoxide ring opening by a nucleophile and concomitant formation of a M-OR bond, (iii) the insertion of CO2 into the metal-alkoxide bond and formation of the organic carbonate by cyclization, which is finally followed by (iv) the release of the cycloaddition products. In general, the catalytic performance of coupling catalysts is based on the cooperation of a Lewis acid centre with a nucleophile, e.g. a halide ion. For example, Miceli et al. have used vanadium(V) aminotriphenolate complexes with an Bu4NI co-catalyst as archetypal examples on highly active catalyst system for the coupling of terminal and internal epoxides with CO2 [10]. Several two-component organocatalysts based on phenols and Bu4NX (X = Br, I) have also been shown to couple CO2 and epoxides efficiently [27–29]. Recently, Hong et al. synthesized an amine bisphenol carrying a quaternary ammonium/iodide ion pair in a pendant arm (Scheme 2 ), and used it as a single-component organocatalyst for the coupling reaction of propylene oxide with CO2 [26].We have previously used amine bisphenol ligands [30] to prepare high oxidation state metal complexes as bio-inspired model compounds and catalysts e.g. for epoxidation of alkenes [31–35]. Here we report the use of ammonium-functionalised amine bisphenol to prepare oxovanadium(V), dioxomolybdenum(VI) and dioxouranium(VI) complexes, which carry a cationic group in the ligand pendant arm with the aim of preparing active single-component catalysts that combine Lewis acidic metal centres and a nucleophile part necessary for the coupling reaction. The vanadium complex was studied as a catalyst for the coupling of CO2 with styrene oxide.The ligand precursor [H2L]I was prepared by the reaction between a tripodal amine bisphenol and iodomethane in acetonitrile applying a known procedure for corresponding compounds [26]. The reactions of [H2L]I and metal precursors VO(O-i-Pr)3, MoO2(acac)2 and UO2(OAc)2·H2O in methanol lead to the precipitation of the oxometal species [VO(OMe)(L)]I·2MeOH (1), [MoO2(L)(H2O)]·2MeOH (2) and [UO2(L)(OAc)]·[H2L]I·4MeOH (3), respectively (Scheme 3 ). Vanadium complex 1 crystallized as dark brown needles in a 63% yield. The solid compound is moderately stable under dry air, however, the crystals slowly deteriorate if kept in open atmosphere due to the loss of the solvate molecules. 1 is stable in dry organic solvents, but degrades gradually if wet solvents are used. The 1H NMR spectrum in CDCl3 indicates the presence of few isomers or conformations, as typical for pentacoordinated oxovanadium(V) aminophenolates in non-coordinating solvents [32,36]. However, in MeOH-d4, one major component (> 99%) is present in the 1H and 51V spectra. The 1H and 13C NMR spectra show anticipated chemical shifts for the deprotonated tridentate ligand. Interestingly, the methoxide ligand is not visible in the spectrum, probably due to the rapid interchange by deuterated solvent. Principally, phenols may act as redox-active, non-innocent ligands through the formation of phenyl radicals upon coordination. For 1, the 51V chemical shift, -470 ppm, is within the expected range for an oxidation state V(v), which indicates a redox-inactive behaviour [37]. The V=O stretch in the IR spectrum is seen at 948 cm−1, as characteristic for oxovanadium(V) aminophenolates [38].Complex 2 precipitated from the reaction mixtures as yellow plates, contaminated with some amount of solid impurities. The crystals as well as the contamination were moderately soluble in DMSO but practically insoluble in any other common solvents and therefore 2 could not be purified by washing or recrystallization. The IR spectrum show the ν(MoO2)s and ν(MoO2)a for cis-MoO2 as strong peaks at 914 and 900 cm−1 [39]. The 1H and 13C NMR spectra of the crude product comprise typical chemical shifts for the tridentate amine bisphenolate ligand, e.g. benzylic methylene protons are seen as two two-proton doublets at 4.33 and 3.43 ppm, respectively. Similarly, 3 crystallised as brown crystals with some solid impurities and could not be further purified. The NMR spectra in DMSO-d6 show for coordinated amine bisphenolate ligand show chemical shifts ascribed for the coordinated ligand, e.g. doublets for the benzylic methylene protons at 5.02 and 4.00 ppm, as well as the for the free amine bisphenol molecule. In the IR spectrum, a strong peak was observed at 865 cm−1 due to ν(UO2), as typical for the presence of a linear O=U=O group [40]. In addition, compounds 2 and 3 were successfully characterized by single-crystal XRD determination of selected crystals.In the solid state, 1 is formed of ion pairs, which crystallize with two molecules of solvate methanol in the asymmetric unit. In the complex part, the central V(V) ion is coordinated to the tridentate dianionic amine bisphenolate, one oxide anion and one methoxide to form a trigonal bipyramidal coordination sphere. Two phenolate oxygen atoms and an oxo ligand occupy the basal coordination sites while the nitrogen atom in the ligand backbone and a monodentate methoxide group occupy apical positions. The vanadium ion is located slightly above the plane formed by the O atoms. The V–Ophenolate distances are 1.829(2) and 1.825(2) Å, respectively, whereas the V–Omethoxide distance, 1.788(2) Å, is noticeably shorter. The V–N bond in a trans position to the methoxide ligand is rather long, 2.303(3) Å. The terminal V=O bond length is 1.585(2) Å. In general, the structure and the coordination sphere around the metal centre are typical for the pentacoordinated amine bisphenolate V complexes [36,38,41,42]. The positive charge of the complex unit is located in the pendant ammonium cation, whereas the iodide anion and the solvent molecules are positioned in the cavities of the crystal lattice.In complex 2, the amine bisphenolate is coordinated to the dioxomolybdenum(VI) ion as a tridentate ligand through two phenolate oxygen atoms and the nitrogen donor in the ligand backbone. A water molecule is coordinated trans to the oxo group to complete the distorted octahedral coordination sphere. Two terminal oxides, two monoanionic phenolate groups and two neutral donors form a typical cis-oxo,trans-X,cis-L configuration around the metal centre [43]. The O=Mo=O angle, 103.8(2)°, the Ophenolate–Mo–Ophenolate angle, 152.8°, and the Mo=O distances, 1.687(5) and 1.715(4)  Å, respectively, are typical for cis-dioxomolybdenum(VI) complexes with tridentate amine bisphenolate ligands [44,45]. The Mo-Ophenolate distances are 1.940(4) and 1.930(4) Å, whereas the Mo-N and Mo-Owater distances, i.e. the bond lengths to the neutral donors, are 2.493(5) and 2.275(5) Å, correspondingly. Again, the positive charge of the complex is located in the pendant-arm ammonium cation while the iodide anion and two solvent molecules reside in the crystal lattice.Compound 3 is a zwitterion where the anionic charge of the complex unit, formally the uranate anion, is balanced with the cationic charge in the pendant ammonium group. It crystallizes together with one ion pair of [H2L]I and four methanol molecules. The acetate anion is coordinated as the bidentate ligand and amine bisphenolate in a tridentate manner to the linear dioxouranium(VI) ion, which generates a distorted pentagonal bipyramidal geometry around the metal centre. The O=U=O angle is 179.2° and the U=O bonds are 1.792(3) and 1.790(3) Å, respectively. The U–Ophenolate distances are 2.208(4) and 2.196(4) Å, while the U–Oacetate distances are noticeably longer, 2.464(4) and 2.483(4) Å, respectively. The U–N distance is also rather long, namely 2.642(4) Å. The overall structure and the geometrical parameters of the complex unit resemble those found previously for UO2(HL’)(NO3)]·2CH3CN (H2L’ = (N,N-bis(2-hydroxy-3,5-dialkylbenzyl)-N′,N′-dimethylethylenediamine; alkyl = Me or tBu) [46].In all complexes, the tridentate ligand coordinates as an O,N,O donor to form two six-membered chelate rings, though the ligand conformation varies in different compounds. In pentacoordinated 1, the chelate rings have adopted half-chair conformations. The O1-V1-N8 and O2-V1-N8 bite angles are 79.4 and 79.8°, respectively, whereas the bite distances O1···N8 and O2···N8 are 2.665(4) and 2.673(4) Å. In the six-membered rings, the dihedral angle between the facing bonds O1-C1 and C7-N8 is 33.5°, whereas the dihedral angle between O2–C15 and N8-C12 is -28.1°. The related bite angles in hexacoordinated 2 are of same magnitude, 81.0 and 79.2° as in 1, while the bite distances are longer, 2.910(6) and 2.853(7) Å, as a result of the longer metal-donor distances. The dihedral angles O1-C1···C7-N8 and O2-C15···N8-C9 are remarkably larger than in 1, i.e. 52.2 and -51.3°, showing more puckered rings. The structures of the chelate rings bear resemblance to boat conformation. In heptacoordinated 3, the bite angles, 70.4 and 75.1°, are remarkably smaller than in 1 and 2 due to the larger central atom and longer metal-donor bonds. The bite distances, 2.818(6) and 2.968(5) Å, are shorter compared to those in 2. Similarly to 1, the conformation of the chelate rings can be described as a half-chair.As vanadium complex 1 carries a Lewis acid centre and a nucleophilic iodide in a single, isolated compound, it presents as a potential catalyst for the coupling of CO2 with styrene oxide. In our reaction setup, 0.01 mmol of catalyst sample was mixed with 7 mmol of styrene oxide in an open vial and the reaction mixture was kept in an autoclave at 80 °C for five hours under a CO2 pressure of 10 bar. The reaction mixtures were subsequently analysed by 1H NMR (Table 4). Along with 1, the ammonium iodide proligand [H2L]I as well as the known oxovanadium(V) complex [VO(OMe)(L’)] (H2L’ is N,N′-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N′,N′-dimethylethylene-1,2-diamine), a neutral analogue of 1 [47], were tested as references. A stoichiometric mixture of precursors [H2L]I and VO(OPr)3 was also tested as an in situ prepared analogue for 1. Under applied reaction conditions, compound 1 gave styrene carbonate in a 26% yield, the turn-over number (TON) being 182. Interestingly, the free proligand [H2L]I, isolated vanadium compound 1 and the VO(OPr)3/[H2L]I mixture gave all styrene carbonate in practically similar yields of which the in situ prepared catalyst displayed highest activity. On the contrary, [VO(OMe)(L’)] did not show any catalytic activity in the absence of an additional nucleophile, but it could be activated by a Bu4NI co-catalyst. On the other hand, it is noteworthy that [VO(OMe)(L’)] is hexacoordinated in the solid state, so the activation may require the dissociation of the pendant side-arm donor.We may suppose that the reactions catalysed by different catalyst systems apply different reaction mechanisms, as well. Specifically, the reaction involving [H2L]I as a catalyst most likely follows the organocatalytic mechanism proposed by Hong and co-workers [26]. In this reaction mechanism, a phenolic hydrogen bond donor activates the epoxide at the alpha carbon, then iodide nucleophile attacks leading to the formation of the ring-opened alkoxide intermediate. This intermediate subsequently reacts by a CO2 insertion leading finally to the formation of a cyclic product [26]. Conversely, complex 1 as well as the two-component systems VO(OPr)3/[H2L]I and [VO(OMe)(L’)]/Bu4NI follow apparently the mechanism proposed by Licini and co-workers for the oxovanadium(v)aminotrisphenolate/ammoniumiodide system. In the suggested mechanism, the coordination of the epoxide to the metal through the oxygen donor is followed by either an internal (by the alkoxide/phenoxide oxygen) or an external (by the halide) nucleophilic attack, a CO2 insertion and a final cyclisation [10]. As [VO(OMe)(L’)] did not show any activity without an external nucleophile, the internal nucleophiles, i.e. phenolate oxygens or coordinated nitrogen donor seem not to participate the reaction.In conclusion, an ammonium iodide -functionalised amine bisphenol reacts with V, Mo and U precursors as a tridentate O,N,O donor to form mononuclear oxovanadium(V), dioxomolybdenum(VI) and dioxouranium(VI) species, respectively. In the oxovanadium(V) and dioxomolybdenum(VI) complexes, the cationic charge in the pendant arm is balanced by iodide counter ion. By contrast, uranyl cation forms a zwitterionic complex, in which the anionic charge of uranate complex unit is compensated by the cationic pendant arm of the ligand. The oxovanadium(V) complex combines a Lewis acid metal centre and Lewis basic iodide moiety, which makes it the catalyst for the coupling of CO2 with styrene oxide. Study of the catalytic activity of the vanadium complex provided evidence on the importance of the ammonium moiety of the ligand since it serves the role of carrying the iodide nucleophile in the reaction.All syntheses and manipulations were carried out under ambient atmosphere. The solvents and chemicals purchased from commercial suppliers were used without further purifications. The IR spectra were measured with Bruker Optics, Vertex 70 device with a diamond ATR setup, whereas the NMR spectra were recorded with Bruker Avance 500 NMR (1H: 500 MHz, 51V: 132 MHz, 13C: 125 MHz) NMR spectrometer at 25 °C (298 K). The spectrometer was equipped with a broad-band observe probe (Bruker BBO-5 mm-Zgrad). The 0 ppm vanadium reference frequency was calculated from the TMS 1H frequency using the unified chemical shift scale by IUPAC (Ξ(51V, VOCl3) = 26.302948) [48]. Complex [VO(OMe)(L’)] was prepared as previously reported [47]. The NMR spectra are given inThe ligand precursor was made applying a known procedure for corresponding compounds [26] 2.1 g (4 mmol) of N,N′-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N′,N′-dimethylethylene-1,2-diamine [47] and 1.4 g of MeI (10 mmol) were mixed in 20 ml of acetonitrile and heated to the reflux temperature for three hours. The reaction mixture was allowed to cool to room temperature and 2.1 g (78%) of [H2L]I was isolated by filtration, washed with cold acetonitrile and dried in vacuum. 1H NMR (CDCl3): δ 7.43 (s, 2H, ArOH), 7.26 (d, J = 2.2 Hz, 2H, ArH), 6.99 (d, J = 2.1 Hz, 2H, ArH.), 3.98 (t, J = 6.3 Hz, 2H, CH2 NMe3), 3.83 (s, 4H, ArCH2 ), 3.22 (s, 9H, NMe3 ), 3.00 (t, J = 6.2 Hz, 2H, NCH2 ), 1.41 (s, 18H, t-Bu), 1.29 (s, 18H, t-Bu).134 mg (0.20 mmol) of [H2L]I was dissolved in 4 ml of MeOH and 50 µl (0.20 mmol) of VO(OPr)3 was added. The dark solution was kept at +4 °C for three days to obtain 105 mg (63%) of 1 as dark brown needles. A sample was kept in a vacuum desiccator for two days prior to the elemental and spectral analyses to remove the possible non-stoichiometric amount of the solvent of crystallisation. Found: C: 56.54; H: 7.81; N: 3.68. Calcd. for C36H60IN2O4V: C: 56.69; H: 7.93; N: 3.67. IR (cm−1) 2954w, 1438w, 1236m, 1168w, 1054s, 989w, 948m (V=O), 914m, 852m, 806w, 757m, 595s, 549w, 476w, 368w. ESI(+)-MS: m/z 635.4062 ([VO(OMe)(L)]+ calcd. m/z 635.3993). UV-Vis: 285 nm (ε = 25 500 M−1cm−1), 360 nm (ε = 10 500 M−1cm−1). 1H NMR (MeOH-d4): δ 7.39 (d, J = 2.2 Hz, 2H, ArH), 7.30 (d, J = 2.2 Hz, 2H, ArH), 4.29 (d, J = 12.7 Hz, 2H, ArCH2 ), 3.64 (m, 2H, NCH2 ), 3.54 (d, J = 12.7 Hz, 2H, ArCH2 ), 3.12 (m, 2H, NCH2 ), 2.84 (s, 9H, NMe3 ), 1.55 (s, 18H, t-Bu), 1.34 (s, 18H, t-Bu). 51V NMR (MeOH-d4): -470 (major component, > 90%), -532 (minor), -553 (minor). 13C NMR (MeOH-d4): 165.4, 143.1, 137.2, 125.2, 125.1, 123.8, 60.4, 58.0, 54.2, 45.3, 36.3, 35.4, 32.3, 31.4.134 mg (0.20 mmol) of [H2L]I was dissolved in 4 ml of MeOH and 65 mg (0.20 mmol) of MoO2(acac)2 was added. The solution was kept at room temperature for three days to obtain 120 mg of 2 as yellow crystals together with a small amount colourless precipitate. The crystalline material is poorly soluble in common solvents and was not further purified. IR: 3421w(br), 3280w, 2958m, 2904m, 2869w, 1475vs, 1443m, 1413s, 1388m, 1360m, 1303m, 1254s, 1236s, 1203s, 1169s, 1128s, 1099w, 1022m, 1016s, 991w, 970w, 940s, 914s, 900vs, 843vs, 808w, 783w, 754s, 653w, 600w, 569s, 555s, 499m, 478m cm−1. 1H NMR (DMSO-d6,): δ 7.46 (d, J = 2.2, 2H, ArH), 7.24 (d, J = 2.2 Hz, 2H, ArH), 4.33 (d, 2H, J = 11.5 Hz, ArCH2 ), 4.09 (q, J = 5.5 Hz, 2H, CH3 OH), 3.64 (m, 2H, CH2 NMe3), 3.43 (d, 2H, J = 11.5 Hz, ArCH2 ), 3.18 (d, J = 5Hz, 6H, CH3 OH), 2.91 (m, 2H, NCH2 ), 2.78, (s, 9H, NMe3 ), 1.39 (s, 18H, t-Bu), 1.29 (s, 18H, t-Bu). 13C NMR (DMSO-d6): 159.7, 141.7, 135.8, 125.3, 123.4, 123.1, 55.0, 52.8, 48.6, 34.65, 34.2, 31.6, 30.1.67 mg (0.10 mmol) of [H2L]I was dissolved in 3 ml of MeOH and 42 mg (0.10 mmol) of UO2(CH3COO)2·2H2O was added. The solution was kept at room temperature for three days to obtain 60 mg of 3 as brown crystals. The product contained a small amount of slightly coloured microcrystals, whereas attempts to purify the sample by washing or recrystallization failed. IR: 3344w(br), 2953m, 2906m, 2862m, 1543m, 1477vs, 1445vs, 1414s, 1387m, 1360m, 1308s, 1284m, 1271s, 1238s, 1205s, 1167m, 1130m, 1109m, 1049w, 989w, 969w, 914m, 865vs, 837s, 806m, 785m, 770m, 744m, 673m, 648m, 619s, 602s, 526s cm−1. 1H NMR (DMSO-d6): δ 9.00 (s, 2H, ArOH), 7.36 (s, 2H, ArH), 7.34 (s, 2H, ArH), 7.15 (s, 2H, ArH), 7.04 (2H, ArH), 5.01 (d, 2H, J = 12.3 Hz, ArCH2 ), 4.00 (d, 2H, J = 12.3 Hz, ArCH2 ), 3.75 (s, 4H, ArCH2 ), 3.53 (t, J = 6.3 Hz, 2H, CH2 NMe3), 3.44 (m, 2H, CH2 NMe3), 3.25 (m, 2H, NCH2 ), 2.92 (s, 9H, NMe 3), 2.85 (t, J = 6.3 Hz, 2H, NCH2 ), 2.53 (s, 9H, NMe 3), 2.33 (s, 3H, OAc), 1.68 (s, 18H, t-Bu), 1.36 (s, 18H, t-Bu), 1.32 (s, 18H, t-Bu), 1.24 (s, 18H, t-Bu). 13C NMR (DMSO-d6): 184.3, 166.2, 152.4, 141.0, 137.4, 136.8, 136.2, 125.0, 124.9, 124.8, 123.0, 122.6, 122.5, 61.2, 61.0, 60.4, 54.5, 52.4, 52.3, 45.0, 42.6, 34.8, 34.6, 33.9, 33.6, 32.0, 31.4, 30.3, 29.6.In each experiment, the 0.01 mmol sample of the catalyst (1, [H2L]I, [H2L]I + VO(OPr)3, [VO(OMe)(L’)] or [VO(OMe)(L’)] + Bu4NI) was mixed in 0.8 ml (7 mmol) of styrene oxide and the reaction mixture was put in a stainless steel autoclave. The blank reaction was run without any catalyst. The reactor was then pressurized with CO2 to 10 bar at 80°C for five hours, whereas the reaction mixtures were subsequently analysed by 1H NMR by comparing the integrated intensities of aliphatic hydrogens in styrene oxide at 5.70, 4.83 and 4.37 ppm, respectively, to those chemical shifts of styrene carbonate at 3.88, 3.17 and 2.84 ppm.Data were collected on a Bruker-Nonius KappaCCD diffractometer with Apex II detector using Mo Kα radiation and the crystals kept at 170 K during data collection. For data collection, processing, and absorption correction the software packages COLLECT [49], DENZO-SMN [50] and SADABS [51] were, respectively used. The structure solving (direct methods) and refinement on F 2 by full-matrix least-squares techniques were done within Olex2 [52] environment using SHELXS [53] and SHELXL [54] software packages, respectively. All non-hydrogen atoms were refined anisotropically whereas hydrogen atoms were refined using isotropic displacement parameters. O–H hydrogen atoms were located from the difference density map when possible (all phenol groups and water molecules as well as some of the methanol solvent molecules) and refined using O–H distance restraints. The remaining MeOH O–H and all C–H hydrogen atoms were refined with a riding atom model. The final refinement of structure of 2 was carried out using a HKLF5 file consisting of two domains in a ca. 6:4 ratio, which resulted in significant improvement of the refinement (Figs. 1–3 , Tables 1–3 , Table 5 ). Anssi Peuronen: Investigation, Writing – review & editing. Esko Salojärvi: . Pasi Salonen: . Ari Lehtonen: Conceptualization, Supervision, Investigation, Writing – original draft.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Anssi Peuronen reports financial support was provided by Academy of Finland.We greatly acknowledge Mr. Qingan Wang for the number of catalyst tests. This work was supported by Academy of Finland (project no. 315911, for A.P.).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2022.132827. Image, application 1 Image, application 2

An amine bisphenol ligand with an ammonium iodide group in the pendant arm (H2L) reacts with V, Mo and U oxometal precursors to form oxovanadium(V), dioxomolybdenum(VI) and dioxouranium(VI) species, respectively. In methanol solutions, vanadium(V) and molybdenum(VI) form 1:1 complexes [VO(OMe)(L)]I·2MeOH and [MoO2(L)(H2O)]I·2MeOH, where the cationic charge in the pendant arm is counterbalanced by an iodide anion. Uranium(VI) forms a complex in which the anionic charge of uranate complex unit is compensated by the cationic pendant arm. The complex crystallises as a co-crystal containing a neutral ligand precursor, namely [UO2(L)(OAc)]·[H2L]I·4MeOH. The oxovanadium(V) complex combines a Lewis acid, i.e. a pentacoordinated metal centre with a Lewis basic iodide moiety, which makes it a suitable catalyst for the coupling of CO2 with styrene oxide. The role of the ammonium moiety of the ligand is to carry the iodide nucleophile in the reaction.

Energy crisis and environmental pollution hinder the sustainable development of society. People have realized the significance of clean energy. Hydrogen energy is widely concerned and researched because of its high combustion efficiency, rich resources, clean and recyclable characteristics. Hydrogen energy is regarded as the most potential energy in the 21st century [1–5].Water electrolysis is an effective method to acquire hydrogen with the aid of electrocatalyst [6,7]. This method is simple, reliable, high conversion efficiency, which is able to achieve large-scale production. Platinum and palladium are the most common catalysts for hydrogen evolution reaction, which show good catalytic activity due to their low overpotential [8]. However, platinum and palladium are precious metals, which have low reserves on the earth and are expensive, which is not conducive to industrial production [9]. People began to study new materials to replace the precious metals in the hydrogen evolution reaction. It is found that Mo, W, Fe, Ni and Pd can be used as catalysts for hydrogen evolution reaction [10,11]. In the process people gradually realized that the binary alloy catalysts could not only reduce the cost, but also significantly reduce the overpotential and improve the stability of the material, which make the research on binary catalysts is attractive [12].For example, Paolo Salvi et al prepared porous Ni-Fe, Ni-Mo, Ni-Ti, Ni-Cr Alloys by chemical doping method [13]. The properties of Ni alloy doped with Mo did not significantly reduce after several hours of cathodic polarization, and its conductivity and electrode stability were improved.On the basis of binary catalyst, it is found that the surface roughness of the catalyst increases after adding the third element, which makes the specific surface area of the catalyst increase and further improves the activity and stability of the catalyst [14–16]. For example, Shervedani et al prepared Ni-Mo-P electrode and studied its cathodic polarization and alternative current (AC) impedance curve in 1 M NaOH, the test results show that the increase in electrode activity was due to increases in (i) surface roughness and (ii) intrinsic activity [17].Herein, PtAuFe/C ternary composite catalyst and Pt/C catalyst were prepared by direct reduction of sodium borohydride. PtAuFe/C and Pt/C catalysts were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The performance of PtAuFe/C composite catalyst was tested using the electrochemical workstation, and the mechanism of hydrogen evolution was clarified.All chemical reagents used in this experiment were of analytical grade and used without further purification, including carbon black (VulcanXC-72), chloroplatinic acid (H2PtCl6), chlorogold acid (HAuCl4·4H2O), ferrous chloride (FeCl2), sodium borohydride (NaBH4), Nafion solution, ethanol (CH3CH2OH), isopropanol, alumina polishing powder, sulfuric acid.A certain amount of chloroplatinic acid solution (3.7 mg•mL−1), chloroauric acid solution (4.78 mg•mL−1) and ferrous chloride were mixed with 0.5 mol•L−1 sodium borohydride solution. The acquired solution appeared black precipitation and bubbles. After standing for several hours, the solution was washed with anhydrous alcohol for three times and centrifuged using ultrasonic dispersion to remove impurity ions. Then PtAuFe powder with a mass ratio of 1:1:1 was obtained by drying in a 333 K vacuum oven for 6 h. Finally, PtAuFe/C (PtAuFe:C = 1:1) composite catalyst was prepared by ultrasonic dispersing of PtAuFe powder and carbon powder in isopropanol solution.Taking a certain amount of chloroplatinic acid solution (the theoretical loading is 50%) to prepare Pt/C catalyst. The preparation method is the same as above. Sung Mook Choi’s group characterized the prepared Pt/C by TEM characterization, test results prove that uniformly-dispersed Pt nanoparticles are found on the carbon support [18].The catalysts were characterized by XRD, SEM and EDS. The crystal structure and composition of the catalyst were measured by XRD-6100. The working voltage is 40 kV, the working current is 30 mA, the scanning range is 20°–80°, and the scanning rate is 8˚/min. The surface morphology and composition of the catalyst were analyzed by SEM and EDS.All the electrochemical tests were carried out on the electrochemical workstation (CHI760e) in a three electrode system. The glass carbon electrode (GCE, the diameter is 3 mm) with catalyst drop (0.0757 mg/cm2), platinum wire and the saturated calomel electrode (SCE) are used as working electrode, counter electrode and reference electrode respectively. Taking 0.5 mg of prepared catalyst powder, 0.5 mg of carbon powder and 1.5 ml of isopropanol and mix them using ultrasonic dispersing for 30 min. 8 μl of prepared catalyst solution was dropped onto the working electrode surface, and then a drop of 0.5% Nafion solution was added onto the electrode to protect the catalyst.The XRD patterns of Pt/C and PtAuFe/C catalysts are shown in Fig. 1 a. Fig. 1a shows that the diffraction peaks of (1 1 1), (2 0 0) and (2 2 0) crystal planes of Pt are corresponding to the 2θ of 40.02°, 46.46° and 67.76° respectively. The diffraction peaks of (1 1 1), (2 0 0), (2 2 0) and (3 1 1) crystal planes of Au are corresponding to the 2θ of 38.32°, 47.02°, 64.82° and 77.64°. The diffraction peak of Fe appears at 2θ of 44.62°, which corresponds to the (1 1 0) crystal plane of Fe. The results show that PtAuFe/C catalyst has been successfully synthesized. Fig. 1b & c are the SEM images of Pt/C catalyst and PtAuFe/C composite catalyst. In Fig. 1b, Pt/C is aggregated in granular form and the aggregation was more serious. From Fig. 1c, we can find that adding Au and Fe changes the surface morphology. Compared with Pt/C catalyst, the agglomeration of PtAuFe/C composite catalyst is obviously reduced and the dispersion of catalyst is increased. Fig. 1d&e are the energy dispersive spectrum of Pt/C and PtAuFe/C composite catalysts respectively. Fig. 1d shows that the sample is mainly composed of Pt and carbon. Fig. 1e demonstrates that the composite sample is mainly composed of Pt, Au, Fe and carbon. The results are in accordance with those of XRD. Fig. 2 a shows the hydrogen evolution reaction (HER) catalytic performance of PtAuFe/C composite catalyst and Pt/C catalyst at 298 K. The HER performance of electrocatalysts were tested by LSV in 0.5 M H2SO4. It can be seen that the initial hydrogen evolution potential of PtAuFe/C composite catalyst shifted positively about 10 mV compared with that of Pt/C catalyst, and the current density is 1.4 times of that of Pt/C catalyst at −0.4 V. It indicates that PtAuFe/C composite catalyst is of more positive initial hydrogen evolution potential and higher current density. The results show that the addition of Au and Fe improves the electrocatalytic hydrogen evolution performance of the catalyst.To understand the hydrogen evolution mechanism of the catalysts, Tafel curves of the prepared catalysts were calculated. Fig. 2b shows the Tafel curves of PtAuFe/C composite catalyst and Pt/C catalyst, and the corresponding electrochemical kinetic parameters are shown in Table 1 . It can be seen from Fig. 2b that the kinetic characteristics of HER for the catalysts are in accordance with Tafel relationship. Table 1 shows that the Tafel slope b of Pt/C catalyst is 36 mV•dec−1 demonstrating that the reaction mechanism of Pt/C catalyst is Volmer Heyrovsky reaction. The Tafel slope b of PtAuFe/C composite catalyst is 30 mV•dec−1, and the reaction mechanism of PtAuFe/C composite catalyst is Volmer Tafel reaction. Compared with Pt/C catalyst, PtAuFe/C composite catalyst has a smaller Tafel slope b, which means that the current density of PtAuFe/C composite catalyst increases faster with the increase of overpotential and the faster hydrogen evolution rate was achieved. Fig. 2c is the CV curves of PtAuFe/C and Pt/C. PtAuFe/C has a higher current density and a stronger redox peak than Pt/C under the same conditions. The electrochemical surface area (ECSA) of the samples are calculated according to the CV curves. It is found that the ECSA of Pt/C is 45.21 m2/g and that of PtAuFe/C is 57.96 m2/g. The higher the value of ECSA is, the higher the catalytic activity is. PtAuFe/C has a larger ECSA than Pt/C, indicating that PtAuFe/C has better catalytic activity, which is consistent with the results of LSV.Besides the activity, the stability of the catalysts is also an important factor to evaluate their performance. Fig. 2d&e show the LSV curves of Pt/C catalyst and PtAuFe/C composite catalyst. The stability of these two catalysts were judged by comparing the polarization current density after different scanning cycles. The current density corresponding to the LSV curve of Pt/C catalyst scanned by 1000 cycles of cyclic voltammetry (CV) at −0.4 V is 15% lower than that of the 1st cycle, while the current density corresponding to the LSV curve of PtAuFe/C composite catalyst scanned by 1000 circles of CV at −0.4 V is 14% higher than that of 1st cycle. This may be due to the emergence of new active sites in the catalyst after multi-cycle scanning. Therefore, PtAuFe/C composite catalyst has better stability than that of Pt/C. Fig. 3 a is the LSV curves of PtAuFe/C composite catalyst at 298 K in different concentration of H2SO4 solutions. With the increase of the electrolyte concentration, the initial potential of hydrogen evolution has a significant positive shift and the current density has a significant increase. The initial potentials in 0.1 M, 0.5 M and 1 M of H2SO4 were 0.32 V, 0.29 V and 0.27 V respectively. The initial potentials in 0.5 M of H2SO4 were about 30 mV higher than that in 0.1 M of H2SO4, and the current density in 1 M of H2SO4 was 8.8 times higher than that in 0.1 M of H2SO4. It indicates that the concentration of H2SO4 has an obvious effect on the hydrogen evolution performance of PtAuFe/C. There are two ways of electrode polarization. One is the electrochemical polarization, which is caused by the slow reaction rate of a certain step in the process of electrolytic product precipitation (such as ion discharge, atom binding to molecules, bubble formation, etc.). The other is concentration polarization, which is caused by the difference between the concentration near the electrode and the concentration in the middle part of the solution due to the slow diffusion rate of the ions. By increasing the concentration of H2SO4, the ion concentration in solution will increase, which lead the increase of ion diffusion rate and the reduction of concentration polarization. For PtAuFe/C composite catalyst, the results show that the over potential of hydrogen evolution is reduced, the initial potential of hydrogen evolution is shifted positively, and the hydrogen evolution performance of the catalyst is improved. Fig. 3b is the LSV curves of PtAuFe/C composite catalyst at different temperatures in the concentration of 0.5 M H2SO4 solution. Table 2 shows the current density at −0.3 V for PtAuFe/C composite catalyst at different temperatures. It can be seen that temperature has an obvious effect on the hydrogen evolution reaction. As the temperature increase, the initial potential of hydrogen evolution moves forward and the overpotential of hydrogen evolution decreases. At the same potential, the current density increased gradually and the reaction rate of hydrogen evolution increased. The reason is that the temperature has an effect on the concentration polarization. When the temperature increases, the concentration polarization is reduced, so the hydrogen evolution overpotential is reduced and the hydrogen evolution performance of the catalyst is improved. In addition, with the increase of temperature, the chemical kinetic constant increases, the reaction speed is accelerated, and the hydrogen evolution performance of the catalyst is also improved.In this paper, Pt/C catalyst and PtAuFe/C composite catalyst were successfully prepared by direct reduction of sodium borohydride. XRD, SEM and EDS were used to characterize the crystal structure, morphology and composition. The hydrogen evolution mechanism of the two catalysts in H2SO4 and the effect of temperature and electrolyte concentration on the performance of PtAuFe/C composite catalyst were studied.In summary, Pt/C and PtAuFe/C composite catalysts have crystal structure. The prepared catalysts are relatively pure. Pt/C catalyst agglomerated seriously. With the addition of Au and Fe, the morphologies of the catalyst were changed and increased the specific surface area of the catalyst. PtAuFe/C composite catalyst is of more positive initial hydrogen evolution potential and higher current density. The reaction mechanism of Pt/C catalyst is probably Volmer Heyrovsky reaction, while PtAuFe/C composite catalyst is Volmer Tafel reaction. In addition, the PtAuFe/C composite catalyst has better stability. With the increase of the concentration of H2SO4, the initial potential of hydrogen evolution of PtAuFe/C composite catalyst shifted positively, the current density increased, and the hydrogen evolution performance is significantly improved. With the increase of temperature, the initial potential of PtAuFe/C composite catalyst for hydrogen evolution reaction gradually moves positively, the over potential of hydrogen evolution gradually decreases, the current density gradually increases at the same potential, and the activity of hydrogen evolution continuously increases.PtAuFe/C composite catalyst and Pt/C catalyst are prepared by sodium borohydride direct reduction method. The PtAuFe/C composite catalyst has larger specific surface area, smaller overpotential, higher polarization current density and better activity as well as stability than that of the Pt/C catalyst. The addition of non-precious metal Fe improves the hydrogen evolution property of PtAuFe/C composite catalyst. M. Nie: Writing - review & editing, Project administration. H. Sun: Data curation, Formal analysis. X.H. Tian: Supervision, Validation. J.M. Liao: Data curation. Z.H. Xue: Investigation, Methodology. Z.Z. Zhao: Software, Conceptualization. F. Xia: Validation, Formal analysis. J. Luo: Formal analysis, Visualization.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This work was supported by Chongqing Key Laboratory for Advanced Materials & Technologies of Clean Energies (JJNY202002), Fundamental Research Funds for the Central Universities (XDJK2020B004) and Chongqing Graduate Research Innovation Project (CYS19106).

In this paper, PtAuFe/C composite catalyst and Pt/C catalyst are prepared by sodium borohydride direct reduction method. The physical properties of the catalysts are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The electrocatalytic hydrogen evolution performance of the prepared catalysts is tested by linear scanning voltammetry (LSV) in H2SO4 solution. The results show that the PtAuFe/C composite catalyst has larger specific surface area, smaller overpotential, higher polarization current density and better stability than that of the Pt/C catalyst. The hydrogen evolution mechanism of PtAuFe/C composite catalyst is determined by Tafel curve, and the results show that PtAuFe/C composite catalyst is of composite desorption mechanism. In addition, the effect of temperature and electrolyte concentration on the performance of PtAuFe/C composite catalyst is also studied.

Hydrogen is a perfect way to store, transport, and convert energy for a comprehensive and clean energy concept. One of the main challenges that need to be overcome, before hydrogen economy is implemented worldwide, is safe and efficient hydrogen storage. Metal hydrides are promising materials to fulfil the target for hydrogen storage applications. To ensure the best hydrogen absorption/desorption properties, metal hydrides are often catalysed by carbon materials. It has been proven that pure carbon materials such as carbon nanotubes, graphite, porous carbon, and activated carbon can store hydrogen and the amount of physisorbed hydrogen is proportional to the specific surface area of carbon material [1]. Among recently studied metal hydride-based composites containing carbon structures one can find: Mg [2–56], Mg–Ni elemental mixture [35,57], Mg–Ni alloy [58–68], Ti [15,63,69,70], Ti–Ni alloy [71–74], TiFe [63], LaNi5 [75], V [63], Ca [8,9], Na [8,9], NaAlH4 [76–78], NaMgH3 perovskite hydride [79], Li [8], LiBH4 [80–82], LiBH4–MgH2 composite [83], Co–Cu–Si elemental mixture [84], C15 type Laves phase alloy [85] and body-centered-cubic (BCC) solid solution [2,86,87].It is clearly visible from the number of published papers that the influence of carbon addition on the hydrogen storage properties has been especially studied for Mg and Mg-based systems and the other groups of materials have not been studied in detail (see also Fig. 1 A). For instance, BCC alloys that are promising hydrogen storage materials and are known to absorb up to two atoms of hydrogen per atom of metal have been studied only in three papers. In these works, Yu et al. have shown that carbon catalysts can improve the activation properties and hydrogen storage capacity of BCC alloy in solid-gas and electrochemical reaction, respectively [86,87]. Moreover, Ranjbar et al. have indicated that carbon nanotubes can significantly lower the hydrogen absorption and desorption temperature of MgH2-BCC composite [2].Recently, Balcerzak has shown that Ti–V BCC alloy absorbs up to 3.67 wt % of hydrogen at room temperature [88]. However, the application of this hydrogen storage material is limited mainly by the high temperature of hydride decomposition. Therefore, in this paper, we present our studies on the carbon materials' catalytic function on BCC alloys. Furthermore, these research findings are preceded and enriched by a short review of the carbon catalysts that have been used in the past to promote hydrogenation reaction of various metal-based systems.Several authors have shown that ball milling of graphite with metal hydrides (Mg, Mg2Ni, Ti-based composite) resulted in particle size refinement and an increase in the specific surface area of the composite [31,62,69]. Graphite acts here as a process control agent that prevents cold-welding and favours powder crushing process, which is essential for effective activation process and fast hydrogenation kinetics [63,89]. For example, Milanese et al. have shown that addition of 5–15 wt % of graphite can highly reduce the number of activation cycles (from 15 to only 3) of Mg–Ni elemental mixture [89]. Furthermore, graphite is known to give origin to carbon-based radicals that react with oxygen-containing species during milling, thus, preventing hydrogen storage material from oxidation [6,11] and hindering oxygen back diffusion from the bulk to the surface [64]. In another study, Borchers et al. have shown that carbon atoms occupy the near-surface layer in the ball-milled composites, which leads to a radical decrease in the effective activation energy of the hydrogenation process [69].In other studies it has been shown that graphite is also an excellent addition to (i) promote Mg hydride formation during ball milling under reactive hydrogen atmosphere; (ii) reduce hydrogenation/dehydrogenation hysteresis; (iii) accelerate the hydrogen absorption and desorption kinetics [8,9,12,17]. The complementary studies revealed that the improvement is not related to any change in the hydrogen storage material structure or morphology and should be directly connected to the catalytic function of graphite [17]. It is noteworthy that the kinetics of the dehydrogenation process is one order of magnitude faster for graphite-containing composite than for commercial Mg. A similar performance has been also presented by Furster et al., who have shown that Mg can reach the maximum capacity in a twice shorter time compared to pure magnesium [31]. However, it is important to note that Lototskyy et al. have shown that the excess (more than 1 wt %) of expandable and thermally-expanded graphite in the Mg-based composite causes the deterioration of the hydrogenation kinetics. The authors have observed an incubation time during hydrogenation which as believed was required to delaminate the graphene layers from the bulk graphite [20]. The same group has observed that even a relatively modest addition of graphite (5 wt %) can greatly affect the cyclic stability during repeated hydrogen absorption/desorption (up to a hundred cycles) of Mg and Mg–Ti hydrides at elevated temperatures (623 K) [15].To mitigate the disadvantages of the magnesium hydride, Jang et al. have studied the influence of graphene on reactively milled Mg. The composite with 5 wt % of graphene showed the maximum capacity of 3.7, 5.1, and 5.7 wt % at 423, 523, and 623 K, respectively. Moreover, the hydrogen uptake at 423 K can be increased to 5.1 wt %, when the content of graphene is increased to 10 wt %. Graphene was found to play the role of absorbent to capture hydrogen, as well as a catalyst [53]. In another work, Liu et al. have shown results for ball-milled MgH2 with highly crumpled graphene that was obtained by a thermal exfoliation method. The synthesized material exhibited improved hydrogen absorption capacity and kinetics allowing to capture 6.6 wt % of H2 within 1 min at 573 K [39].As stated by Cai et al., carbon nanotubes (CNTs) can destabilize the metal-hydrogen bonding and, therefore, reduce the energy barrier for H2 nucleation [37]. It is probably related to the electron affinity of their curved surface. Therefore, to alter the charge distribution in the hydrides and weaken the interaction between hydride forming material and H atoms the contact between CNTs and metal hydride particles has to be ensured.Single-walled carbon nanotubes (SWCNTs) were used together with metallic catalysts to promote the hydrogenation properties of mechanically milled magnesium. Wu et al. have shown that Mg-based composite with purified nanotubes absorbs 4.2 wt % and 6.0 wt % within an hour at 373 and 423 K (under an initial hydrogen pressure of 2 MPa), respectively [56]. Moreover, the MgH2-SWCNTs composite exhibits faster hydrogen absorption/desorption kinetics and decreased hydrogen desorption activation energy compared to pure magnesium hydride. Wu et al. have proven that SWCNTs mechanically milled with MgH2 reduce the hydrogen absorption time by five times and hydrogen desorption temperature by 70 K compared to pure Mg [7,45,56]. SWCNTs have, however, only a small beneficial effect on the desorption temperature of LiBH4 and NaAlH4 – the dehydrogenation temperature is only 20 K lower compared to the unmodified material [80,81].Extraordinary hydrogenation kinetics have been observed for Mg ball milled with SWCNTs and 5 wt % of ZrO2 or FeTi [50,51]. The composite with SWCNTs and ZrO2 can absorb 6.75 wt % at 423 K in the first 100 s of the process and 4.0 wt % at room temperature within 700 s, while the composite with SWCNTs and FeTi captured 6.6 wt % in 60 s at 423 K [50,51]. Moreover, the study of Chen et al. have revealed that ball milling of Mg with this carbon additive resulted in the formation of 50 nm magnesium grains [51].The research on metal hydrides revealed significant improvement of hydrogenation kinetics after the addition of multi-walled carbon nanotubes (MWCNTs). For example, a Mg-based material with 5 wt % of MWCNTs absorbs 90% of its total hydrogen storage capacity within 150 s at 573 K, while the pure Mg requires hours or days to reach the same storage capacity under the same conditions [2]. In another study, Aminorroaya et al. have shown that the full hydrogen uptake can be reached in 2 min at 643 K under 2 MPa of H2 [35].The study of Laves phase alloy modified by MWCNTs has shown a decrease in the slope of hydrogenation plateau, which is extremely important for its applicability [85]. The versatility of this carbon additive has been also presented for Mg and Mg2Ni alloy with MWCNTs that were co-catalysed by TiF3, Pd, Al, and K2NbF7 according to their solid-gas or electrochemical hydrogen storage properties [34,43,44,57,61]. A study of Chen et al. has revealed that 5 wt % of MWCNTs allowed keeping the maximum hydrogen capacity under cyclic hydrogen absorption/desorption of Mg hydride [33]. Moreover, Pukazhselvan et al. who have studied the influence of MWCNTs on hydrogen storage properties of NaAlH4 for the first time, have shown good rehydrogenation characteristics with reversible capacity at 4.2 wt % level for this composite [77]. It is important to note that in some cases the improvement may be less pronounced for MWCNTs than for SWCNTs. Chuang et al. have explained it by the various Young's modulus of these two carbon materials. SWCNTs have a higher modulus than MWCNTs but smaller strain and higher hardness and this can effectively affect the reduction of Mg crystallite size and thus hydrogen storage properties [48].Other studies prove that MWCNTs affect also the hydrogen desorption process. For example, Mg2Ni mixed with this carbon catalyst shows a 90% increase in hydrogen desorption rate in comparison to pure Mg2Ni [60]. Ranjbar et al. have shown that 5 wt % of MWCNTs added to the Mg can reduce the MgH2 decomposition temperature by 125 K [2].MWCNTs were also co-mixed with graphene oxide to enhance the hydrogen storage properties of NaMgH3. In effect, the composite material was characterized by significantly reduced activation energy for both dehydrogenation steps. The synergetic effect of both carbon materials resulted in better dehydrogenation kinetics and lower dehydrogenation temperature [79].There are various proposed reasons for the positive impact of carbon nanotubes on hydrogen storage properties. One of them describes CNTs as channels for hydrogen diffusion to and from the interior of hydrogen storage material grains [2,44,60,66,86]. Their presence results in faster hydride formation and decomposition kinetics. Moreover, Yahya et al. have stated that carbon nanotubes should also be considered as thermal bridges that facilitate the heat exchange process during the hydrogen absorption [44]. Cai et al. have proposed that CNTs act as a three-dimensional framework that prevents the metal/metal hydride particles from sintering during hydriding/dehydriding cycles [40]. The sintering process is considered as one of the reasons for the deterioration of hydrogen storage properties.The influence of carbon nanofibres/graphite nanofibres (CNFs) on hydrogen storage properties of metal hydrides has not been studied so thoroughly as in the case of graphite or carbon nanotubes. However, the helical graphitic nanofibres were used to enhance the properties of NaAlH4 [78]. The authors have found that this carbon material possesses superior catalytic activity in improving the desorption kinetics and decreasing the hydrogen desorption temperature. Moreover, the NaAlH4 can be easily re-hydrogenated at moderate conditions, at 393 K, and under 9.12 MPa. It is worth mentioning that CNFs can greatly accelerate the kinetics of Mg hydrolysis. This composite releases 95% of the theoretical hydrogen generation yield within 4 min [16].As in the case of the above-mentioned carbon catalysts, also ultrafine diamonds have been used to promote the hydrogenation in Mg. The published studies showed that the addition of diamond powder can substantially increase the hydrogen absorption rate, strongly decrease the hydrogen decomposition temperature (by 100 K compared to the unmodified system) and significantly increase the storage capacity (by 25% compared to pure magnesium) for MgH2 [18,24].Activated carbon has also been used to catalyse hydrogenation reactions in different laboratories. Its co-milling with MgH2 decreased the onset and peak temperature of the hydrogen desorption process (compared to 693 K for as received Mg). The temperature depends on the content of carbon catalyst in the composite – for 1 wt % and 10 wt % of activated carbon is 622 K and 589 K, respectively. Moreover, these composites are able to absorb 6.5 wt % of H2 within 7 min at 573 K, 6.7 wt % of H2 within 2 h at 473 K and release 6.5 wt % of H2 within 30 min at 603 K [54]. The significant improvement of hydrogenation/dehydrogenation reactions (also kinetics) at even low activated carbon concentration has been also observed by Lototskyy et al. and Wu et al. [20,45]. Concluding, carbon acts as a carrier of the ‘activated’ hydrogen by a mechanism of spill-over.In a different study, Mandzhukova et al. have studied the influence of activated carbon and 3d-metal-containing compounds on the hydrogenation properties of magnesium. The carbon-containing composites preserve their hydrogen storage capacity during prolonged cycling (at least up to 80 cycles, at 573 K, 1 MPa of H2). After 80 cycles the hydrogen absorption capacity of the composite was 6.9 wt %, which is close to the theoretical 7.2 wt % [90]. Scanning electron microscopy (SEM) images of the cycled composite showed very fine powder which favours the hydrogenation/dehydrogenation performance due to serious shortening of the hydrogen diffusion distance [55].So far, fullerene has been used to catalyse the hydrogenation reaction of Mg and Mg2Ni alloy. Alsabawi et al. have systematically studied the catalytic effect of C60 addition to MgH2 with a concentration of fullerenes up to 10 wt %. The ball milling of MgH2 with 10 wt % of fullerenes for 10 h resulted in the reduction of hydrogen desorption temperature by nearly 35 K [49]. Moreover, the desorption of hydrogen from C60 containing composite is incomparably faster than from pure MgH2 (at 623 K) [45]. Fullerenes have been also used to create nanocrystalline Mg2Ni alloys with the increased surface area. In the work of Bouaricha et al., the initially ball milled Mg2Ni/C60 composite was poured in toluene to dissolute the C60 and form an alloy with a highly reduced size of crystals and enlarged specific surface area [62]. As a result, C60 can at least twice accelerate the hydrogen desorption rate of Mg2Ni based composite [62].Only in a few studies, carbon black has been considered as a catalyst to improve the properties of metal hydrides [45,46]. The addition of carbon black to the Mg by ball milling considerably affects the kinetics of hydrogenation (at 423, 473, 573 K) and dehydrogenation (at 623 K) [45]. The fully hydrogenated composite with carbon black can desorb nearly 6 wt % within 15 min at 623 K, while the unmodified MgH2 needs an hour to release 4 wt % only.As demonstrated by Rud et al., the addition of amorphous carbon can effectively improve the kinetics of the hydrogen absorption process and substantially increase the hydrogen storage capacity [18]. The composite material obtained via reactive ball milling (under initial pressure of 0.5–0.6 MPa of hydrogen) absorbs around 4.6 wt % of H2 after 25 h of milling, while the pure Mg reaches only 4.1 wt % of H2 in 100 h of milling [18]. In another study, Spassov et al. have studied the hydrogen storage properties of Mg/amorphous carbon soot composite [24].The demineralized anthracite coal was used as a support in milling the Mg particles down to the nanoscale without cold welding. The studies on the influence of this carbon catalyst on hydrogen storage properties of MgH2 have shown that it improves the hydride decomposition kinetics, decreases the hydrogen desorption onset temperature, reduces the enthalpy change, and diminishes the dehydrogenation activation energy compared to bulk MgH2 [25–27].Although most of the mentioned research papers are focused on solid-gas reactions between metal and hydrogen, there are, however, some studies that deal with the influence of carbon addition on the performance of negative electrode in Ni-MHx secondary batteries [59,62,65,71–73,75,84,87]. For example, CNTs are beneficial to charge-transfer reactions improving the overall discharge capacity, strengthening high-rate dischargeability, reducing charge-transfer resistance, plateau voltage, influencing the hydrogen diffusion in the bulk of the electrode, and improving the cycle life of the Ni-MHx batteries [71,72,75]. The enhancement of Ni-MHx batteries performance is related not only to the electro-catalytic function of carbon nanotubes but also to carbon-induced changes in microstructure and morphology of electrode material. In particular, the reduction of particle size and simultaneous rise of the specific surface area provides a larger electrochemically accessible area and rapid channel for hydrogen transportation [84]. However, the improvement of properties is observed only when the addition of carbon material is relatively small – too high content of MWCNTs can cause a decrease in hydrogen storage properties due to the decrease in the active material density [75]. In another study, Guo et al. have also observed a negative impact of carbon structures on the electrochemical properties of Mg–Ni alloy. In this case, the carbon addition resulted in the decrease in initial discharge capacity, which was apparently caused by the fact that carbon atoms blocked the active sites for hydrogen storage [59].Despite that the above discussion shows rather unambiguously the desired effect of carbon materials on the hydrogen storage properties of metal hydrides, the properties of a specific composite strongly depend on the metal hydride and carbon catalyst used. Therefore, carbon catalyst impact studies should be undertaken each time a new composite is designed and synthesized. In our study, we pay attention to six different carbon catalysts. Two of them were the most studied carbon forms (taken as reference) – graphite and carbon nanotubes (see also Fig. 1B). We supplemented this group with four relatively novel materials that were less tested in terms of hydrogenation catalysis – mesoporous carbon, carbon nanofibres, ultrafine diamonds, and fullerenes. In this paper, we extensively studied the effect of carbon catalysts on the hydrogen absorption/desorption properties of BCC alloy. We believe that these results can be used to design new BCC-based composites and a novel undeveloped group of materials – high-entropy alloys (HEA)-based composites for hydrogen storage.Ti1 . · 5V0.5 alloy was prepared from Ti and V elemental powders (Alfa Aesar, −325 mesh, 99.5% purity). This alloy was chosen, as it is one of the recently studied alloys from the Ti–V system, that is characterized by high gravimetric hydrogen storage capacity but high hydrogen desorption temperature [88]. The alloy has been synthesized using the ball milling method under an argon atmosphere. The detailed synthesis procedure has been already published elsewhere [88].A series of carbon allotropes were used to prepare Ti1 . · 5V0.5-based composites: multi-walled carbon nanotubes – CNT (Sigma Aldrich, 6–13 nm outer diameter, 2–6 nm inner diameter, 2.5–20 μm long, 7–13 graphene layers, >98% purity), mesoporous carbon nanopowder - CM (Sigma Aldrich, 99.95% purity), carbon nanofibers - CNF (Sigma Aldrich, 100 nm diameter, 20–200 μm long, ≥98% purity), diamond powder - UFD (Alfa Aesar, <1 μm, 99.9% purity), graphite - G (Alfa Aesar, ∼45 μm, 99.9995% purity), fullerene – C60 (Alfa Aesar, 99% purity). The pre-synthesized alloy was mixed with 5 wt % of carbon structures using SPEX 8000 M shaker mill under an inert argon atmosphere. As several groups reported the destructive nature of ball milling on carbon structures we decided to mill alloy with carbon structures without milling balls – to reduce the energy during composite preparation [63]. This process lasted 30 min in each case. Additionally, various amounts of fullerenes (1, 5, 10 wt %) were milled with an alloy for 30 min and the obtained materials were tested to study the effect of amount of addition on hydrogen storage properties. Moreover, 5 wt % of fullerene was milled with an alloy for 10, 30 and 60 min, and the influence of milling time on the hydrogenation performance was determined. The nomenclature of the studied materials together with their Brunauer–Emmett–Teller (BET) specific surface areas (provided by suppliers) are listed in Table 1 . All the material handling was performed in a high-purity Ar atmosphere M-Braun glovebox (H2O and O2 levels below 1 ppm).The synthesized alloy and composite materials were characterized by powder X-ray diffraction (XRD) using a Panalytical Empyrean X-ray diffractometer with a Cu Kα radiation. The surface morphology was observed by SEM, using TESCAN MIRA 3 operating at 10 kV in secondary electrons (SE) and backscattered electrons (BSE) image modes. The microstructural characterization of carbon materials, alloy, and synthesized composites was acquired by employing a Hitachi Scanning Electron Microscope S3000 N (operated at 100 kV) and FEI TEM Titan3 G2 (operated at 80 kV). The Raman spectra of Ti1 . 5V0.5-based composites were recorded at room temperature to investigate the milling effect on the structure of carbon allotropes using Renishaw inVia Raman microscope equipped with a thermoelectrically (TE) - cooled CCD detector and an argon-ion laser working at 514.5 nm wavelengths. The Raman spectra were recorded in the spectral range 100–3200 cm−1 with a spectral resolution better than 2 cm−1. The exposure time of CCD detector was 300 s. To avoid sample overheating, the power of the laser beam was kept below 0.1 mW. The position of Raman peaks was calibrated before collecting the data using a crystalline silicon sample as an internal standard. The spectral parameters of bands were determined using the fitting package of Wire 3.4 software. The dehydrogenation behaviour was studied by differential scanning calorimetry (DSC) isochronal experiments that were performed utilizing the Netzsch DSC 404 apparatus with various heating rates q (5–40 K/min) and for temperatures from 323 to 1023 K. After initial equilibration, the continuous heating curves were measured in an argon flow of 100 ml/min. The effective activation energies E a (of the dehydrogenation process) were calculated using the Kissinger formula from the ln (T p 2/q) vs. 1/T p plots [91].Hydrogen absorption properties were determined using an automated Particulate Systems HPVA-200 Sievert's volumetric apparatus. Prior to the measurements, all of the as-prepared materials (alloy and composites) were evacuated at 303 K for 1 h under vacuum and activated with 3 MPa of H2 at room temperature. Around 1 g of material was placed in a stainless steel reactor for each measurement. The absorption kinetic measurements were performed at 303 K with an initial pressure of 3 MPa of H2.As written in the experimental section, a wide range of carbon allotropes was selected to improve the hydrogen storage properties of Ti1 . 5V0.5 BCC alloy. Table 1 compares the results of physical absorption of N2 measurements (BET specific surface areas) that were performed by suppliers. It is obvious that carbon allotropes highly differ in terms of porosity, ranging from non-porous to highly porous structures (with BET up to 500 m2/g). The X-ray diffraction patterns and representative TEM/HRTEM micrographs of various carbon materials used as the catalyst for the present study are shown in Fig. 2 . All the materials are characterized by the well-defined crystal structures. The HRTEM image of a CNT shows the number of walls and large central hollow that provides an easy channel for the transport of hydrogen [2,3,60,61]. The CNTs present an inner diameter of about 5 nm, an external diameter of about 12 nm. Some of them were closed by a fullerene-shaped cap, while others were open at the ends. The removal of these caps provides an easy pathway for the diffusion of hydrogen through nanotubes’ inner channels. The HRTEM images of some of the studied carbon materials show microporosity (pointed by arrows). It has been assumed in the past that the microporosity has an influence on the adsorption properties [1]. Fig. 3 shows the results of the detailed investigation of morphology and microstructure of Ti1 . 5V0.5 and studied composites. The SE SEM micrographs present particles characterized by irregular shapes and varying from a few to hundreds of micrometers in size. The micrograph obtained for Ti1 . 5V0.5_UFD_5_30 shows that the ultra-fine diamond particles form a tight layer on the alloy surface. Furthermore, the large carbon nanofibres are clearly visible on the alloy surface after milling. The BSE SEM observations were performed to evaluate the dispersion of carbon allotropes (black regions) on the Ti1 . 5V0.5 particles (bright regions). Ti1.5V0.5 alloy, which has been used as a reference, shows no presence of carbon structures. In contrast to the BCC alloy BSE SEM micrographs prove the dispersion of carbon structures on surfaces. However, the degree of dispersion varies with the type of carbon structure used. UFD and C60 are well distributed on the alloy particles. The other allotropes (G, CNT, CNF, CM) tend to concentrate in some material's areas, while the other areas remain completely devoid of them. The insets show BSE SEM micrographs of some of the materials obtained at higher magnification to show the undamaged carbon structures.TEM and HRTEM images of Ti1 . 5V0.5 show that the alloy consists of micrometer-sized particles (some nanoparticles were also observed) characterized by nanocrystalline and multicrystalline structure. TEM observation proved that carbon structures retained their structures after the synthesis of composites. However, it can be seen that the length of some CNTs is reduced from the submicrometer scale (Fig. 2) to dozens or hundreds of nanometers (Fig. 3). Some of the carbon nanofibres were also damaged or destroyed. For CNF-, G- and CNT-containing samples the connection of alloy nanoparticles with undamaged carbon structure was confirmed. Moreover, the TEM observation proved that UFDs are well attached to the alloy surface. It should also be mentioned that Lillo-Rodenas et al. have stated that incorporation of carbon allotropes resulted in a reduction of metal-hydrides particles and stabilized the size of powder particles [4]. Notwithstanding, such a conclusion cannot be made based on the micrographs in Fig. 3.The XRD patterns obtained from as-prepared alloy and composites are shown in Fig. 4 A. Ti1 . 5V0.5 alloy crystalized in BCC phase. A residual V-based minority BCC phase was also detected. According to the recent paper of Balcerzak [88], the fraction of the Ti–V main BCC phase to the V-based BCC phase is 92.6%–7.4% (as obtained from Rietveld refinement). The lattice parameter of the main phase equals 3.239 Å. The crystal structure of an alloy was not tacked after milling with carbon allotropes. This is contrary to the results obtained for ball-milled Mg2Ni-based composites for which a reduction of peaks intensity has been noticed after milling [62]. However, in these studies, balls were not used during milling and therefore much lower energies were generated during milling. It led to much lower deformations and strain formations in alloy material. Moreover, peaks related to specific carbon crystal structures are visible on some of the patterns. Ti1.5V0.5_G_5_30 pattern contains two peaks related to the graphite structure. The most intense one, at around 26.6 deg., corresponds to the interlayer distance between graphitic layers of 0.335 nm. The most intense peaks of fullerenes and UFD are also visible. The other carbon structures were not detected due to their low concentration or low crystallinity.Structural characteristics of carbon allotropes attached to the surface of particles can be inferred from Raman spectra, as presented in Fig. 4B. For a better understanding of the milling effect on the carbon structures, the composites spectra are compared with spectra of original carbon allotropes structures. As the composites were prepared by milling without balls, no high impact mixing occurred. Therefore, the breakdown of carbon allotropes to form amorphous carbon was not allowed in such conditions and the characteristic Raman peaks were not altered. It confirms that the crystal structures of these carbon allotropes attached to the surface of alloy particles remained essentially unchanged. In carbon allotropes containing a mixture of sp2 and sp3 type carbon-carbon bonding, there are two important characteristic Raman bands at 1580 cm−1 and at 1350 cm−1, which were assigned to the in-plane vibrations of the C–C bonds (G bands, stretching of all sp2 bonds, both in rings and chains) and vibration mode originated from the distorted hexagonal lattice of graphitic sp2 network near the crystal boundary (D bands), respectively. A closer look at the G peak shows in some cases (for composites with CNF and CNT) an asymmetry in the line shape that appeared at 1620 cm−1, which can be assigned to structural defects (distorted stacking order in c-axis direction) introduced to the structure with milling (D’ bands). The analysis of Raman bands in the 1000-2000 cm−1 range is presented in detail in Fig. S1. The insignificant peak at around 2460 cm−1 (D + D″) is related to a combination of a D phonon and an acoustic longitudinal phonon D″. At higher wavenumber for some of the carbon allotropes, additional G′ (2650 cm−1) and D + G (2920 cm−1) peaks were observed. The structure of UFD and C60 differs from the above-described ones. Unmilled UFD exhibits one main band at 1332 cm−1 (T2g). However, for UFD-containing composite additional amorphous bands were observed (D and G), which stem from the milling of UFD with alloy material. Their presence in the spectrum indicates partial amorphisation of UFD. The Raman spectra of C60 and its composite show the sharp peak at 1460 cm−1 as indicative of C60 structure.It has been shown that the relative intensity ratio (ID/IG) of the D to the G band is an indicator of the perfection of the graphite layer surface [46]. The increase in the ID/IG ratio indicates an increase in the number of defects of carbon species and a simultaneous decrease in the degree of carbon graphitization in the composite. Table 2 shows the comparison of ID/IG ratio for CNT, CNF, G, and CM containing composites before and after milling (due to the different hybridization of carbon atoms a similar analysis is not possible for UFD and C60). It is clear that the level of graphitization is the lowest for Ti1 . 5V0.5_G_5_30, intermediate for Ti1 . 5V0.5_CNT_5_30, while the highest for Ti1 . 5V0.5_CNF_5_30 and Ti1 . 5V0.5_CM_5_30. Moreover, most of these carbon structures became more defected after milling (with an exception for mesoporous carbon). The ID/IG ratio can also be used to estimate a planar domain size La (also named as the size of graphene stacks) in carbon allotropes [92]. The calculated La values have been listed in Table 2. Fig. S2 shows that the dependence of the degree of graphitization (ID/IG) on the domain/crystallite size of tested carbon materials and its composites has a linear character. We can conclude here that the XRD, Raman spectroscopy, and microscopic observations prove that the carbon allotropes remained their structures after their synthesis process.The activated alloy and composites were hydrogenated at 303 K under initial H2 pressure of 3 MPa. The results of these studies were presented and summarized in Fig. 4C and Table 3 . All of the studied materials absorb hydrogen without any incubation time. The Ti1 . 5V0.5 alloy absorbs nearly 95% of maximum hydrogen storage capacity within 48 min. In all the cases, the carbon-containing materials perform much better than the reference BCC alloy. The hydrogenation time was significantly reduced for carbon-containing composites. The fastest H2 uptake kinetic, with an uptake time of 12 min, was observed for Ti1 . 5V0.5_C60_5_30. Regarding this, it is interesting that studies on Mg2Ni-carbon structures composites have not shown any improvement of hydrogen absorption kinetics after milling with carbon structures (C60, graphite, Vulcan) [62]. One of the possible reasons for the different effects of carbon materials on the kinetics of hydrogen sorption is the use of ball milling [62], which led to the deformation of the crystalline structure of the carbon catalyst, but did not take place in this study. The hydrogen uptake of Ti1 . 5V0.5 alloy (3.68 wt %) was decreased to the level of 3.46–3.59 wt % for composite materials. The loss of the uptake is related to the addition of 5 wt % of carbon allotropes. Since carbon materials do not participate in hydrogen absorption dilution effect is observed, as the mass of active material is decreased and therefore the hydrogen uptake for composites decreases.The decomposition process of hydrogenated samples was measured by DSC. The most important outcomes from these studies can be seen in Fig. 4D and Table 3. It is clearly visible that the hydrogen desorption process of Ti1 . 5V0.5 alloy can be divided into three steps. The first one at Tp = 750.3 K is related to the decomposition of dihydride to monohydride. The second one (814.1 K) corresponds to phase transformation of monohydride to hydrogenated BCC phase, while the last one (890.9 K) to desorption of hydrogen from hydrogenated BCC solid solution to the dehydrogenated BCC phase. The presence of carbon allotropes affects the temperature of the first decomposition process, while the position of the other two steps remains basically unchanged. The temperature of decomposition of dihydride phase is the most reduced for Ti1 . 5V0.5_C60_5_30, reaching 662.7 K. Comparing this temperature with the one observed for Ti1 . 5V0.5 alloy, the desorption temperature was decreased by 87.6 K. The reduction of hydride decomposition temperature was also observed for carbon-modified magnesium hydride [4]. DSC curves were measured for each studied sample with different heating rates (Fig. S3-Fig. S9). The obtained results were used to calculate the energy of activation (Ea) of the dehydrogenation process (utilizing Kissinger plots). Kissinger analysis was performed for the first endothermic effect, which is related to the decomposition of dihydride, as stated before. Nevertheless, one has to bear in mind that this is not a solitary process (overlapping peaks in heat flow curves) and can be affected to some degree by succeeding reactions. Table 3 shows the Ea values calculated for all of the studied materials. In the best case, in terms of hydrogenation kinetics, the Ea is twice reduced compared to the Ti1 . 5V0.5 alloy. However, due to the measurement uncertainties, it is impossible to identify the composite with the lowest Ea value. Nevertheless, it is obvious that a fullerene-containing composite is characterized by a much lower activation energy of hydrogen desorption. The reduction of dehydrogenation Ea indicates that the energy barrier of hydrogen release from BCC alloy is reduced. As Ti1 . 5V0.5_C60_5_30 showed the best hydrogenation/dehydrogenation properties among all studied materials, further studies were focused on this fullerene-containing composite only. Surprisingly, in the previously published study on MgH2-type materials, the addition of fullerene negatively affected their absorption kinetics [49].As we did not observe any change in particle size after synthesis of composites (Fig. 3) we exclude the influence of the carbon materials addition on the specific surface area of the BCC alloy and thus the improvement of hydrogen storage properties is with high probability connected to the catalytic function of each carbon allotrope. Moreover, we did not find any clear tendency of the BET area of carbon catalysts (provided by suppliers) on any specific hydrogenation/dehydrogenation property. Similarly, Lillo-Rodenas et al. have shown that the addition of selected carbon materials considerably improves the hydride decomposition kinetics of MgH2 [4]. As in the previous case mentioned authors did not observe any relationship between the porosity and surface area of the carbon materials and hydride decomposition kinetics/temperature. Further detailed research is required to understand the mechanisms that led to improved properties in each studied composite.The hydrogen absorption and desorption measurements performed on the studied materials clearly showed that all carbon allotropes catalyse both hydrogenation and dehydrogenation reactions in BCC alloy. Since the best performance has been observed for composite with fullerene the further studies have been focused on this carbon-containing composite.To study the influence of milling time on hydrogenation/dehydrogenation properties of the fullerene-containing composite, 5 wt % of C60 was milled for 10, 30, and 60 min under the same milling conditions. Fig. 5 shows that the increase of the milling time did not affect the structure of alloy, but caused a gradual decrease of crystallinity of carbon structure (see the inset of Fig. 5A). It has also been previously observed for MgNi2-based composite, that the diffraction peaks of C60 become broader and the intensity decreases with increased milling time, indicating grain refinement and introduction of strain into the structure [62]. However, for composites studied in this paper, the Raman spectra did not show any significant changes in the C60 structure (Fig. 5B). We infer the presence of a small amount of C60 and alloy particles that get mechanically damaged in a contact with the jar wall during milling. The microstructure of the sample milled for 60 min (Ti1 . 5V0.5_C60_5_60) was studied by BSE SEM. A representative micrograph can be seen in Fig. 5C. Even 60 min of milling was insufficient to disperse the carbon material over the entire surface area of the alloy and many uncovered Ti1 . 5V0.5 alloy areas can be easily found in Fig. 5C (white regions). Hydrogen absorption measurements showed that extended milling time was detrimental to hydrogen storage properties. First of all, the hydrogen uptake is significantly reduced when the milling time increases: from around 3.5 wt % for samples milled for 10 and 30 min to only 2.8 wt % after 60 min. Secondly, the hydrogenation kinetics is lowered when the carbon allotrope is milled with an alloy material for a longer time (see Table 3.). As shown by Alsabawi et al. and Wu et al., there is an optimal milling time that favours the hydrogen storage properties of MgH2 co-milled with C60 or SWCNTs [7,49]. As it has been shown in Ref. [49], further milling leads to a serious degradation on these properties of the composite which are related to the destruction of carbon catalysts (proven by Raman spectra). However, in the present study Raman spectra did not prove any structural changes after milling. The only proof of carbon catalyst degradation has been observed by XRD.Most importantly, the Ti1 . 5V0.5_C60_5_10 composite can absorb nearly 3.3 wt % of hydrogen within a minute (at room temperature). This superfast absorption is almost fifty times faster than for Ti1 . 5V0.5 alloy. The DSC studies of hydrogenated fullerene-containing composites showed that dihydride decomposition temperature (648–663 K) and Ea of dehydrogenation process (148–157 kJ/mol) displayed comparable values and are independent of milling time (Figs. S9–11). It is important to note, that even relatively low energy milling (without milling balls) can significantly affect the functionality of C60.Furthermore, the influence of various amounts of fullerene addition was also studied. For this reason 1, 5, and 10 wt % of C60 were milled with Ti1 . 5V0.5 for 30 min. The XRD patterns obtained for these composites exhibit no influence of the amount of carbon addition on the crystal structure of Ti1 . 5V0.5 alloy (Fig. 6 A). The inset of Fig. 6A shows the angular region of the (311) C60 peak. The peak was not detected for the sample with only 1 wt % of fullerene, while for the other two composites it was clearly visible. The absence of the (311) peak in the Ti1 . 5V0.5_C60_1_30 pattern is probably related to the very low content of C60 in this particular sample, which is below the sensitivity threshold of XRD method. Especially, since Raman spectroscopy which is more sensitive to carbon materials showed no major changes in the structure after milling of samples with 1, 5, and 10 wt % of C60 (Fig. 6B). The sample with the greatest content of fullerene was tested by BSE SEM (Fig. 6C). Surprisingly, even a relatively large content of carbon allotrope does not guarantee the formation of a uniform C60 layer on the surface of alloy particles. As the size and morphology of particles remained the same changes in hydrogen storage properties should be connected with catalytic properties of carbon additive. Each of the samples was studied by Sieverts type apparatus to characterize its hydrogen storage properties. As can be seen in Table 3 the decrease of H2 uptake is proportional to the amount of carbon allotrope in the composite structure. The more C60 that does not participate in hydrogenation reaction is used the less hydrogen storage capacity is. The same tendency has been observed by Milanese et al. They have reported that the overall hydrogen uptake of Mg–Ni elemental mixture decreased with increased graphite content in the composite (due to the inability of C to absorb H2) [89].We did not observe any clear relation between the amount of C60 and the kinetic properties of the alloy. It seems that 5 wt % of fullerene is an optimal concentration, as Ti1 . 5V0.5_C60_5_30 composite shows the fastest hydrogen absorption among materials with various amounts of C60. Furthermore, worse hydrogenation kinetic properties were observed for the Ti1 . 5V0.5_C60_10_30 composite. A very similar tendency has been observed in the study of Alsabawi et al. They have found that for MgH2-based composite there exists an optimal amount of fullerene addition. When the C60 concentration exceeds 2 wt % in composite, the carbon addition harms the hydrogen storage properties [49], However, it should also be mentioned that in another work Dal Toe et al. have not observed any significant improvement of hydrogenation kinetics when a larger amount of graphite was added to MgH2 [10].DSC studies of hydrogenated samples showed that the dihydride decomposition temperature is decreased, while the amount of C60 used in the composite increases (670.2 K for 1 wt %, 662.7 K for 5 wt %, and 651 K for 10 wt %). The same tendency was also observed for Ea (see also Figs. S9, S12, S13). Ti1.5V0.5_C60_10_30 is the composite with the lowest dehydrogenation activation energy among all materials studied in this work (136 ± 5 kJ/mol).The presented results clearly show the beneficial effect of carbon allotropes on the hydrogen storage properties of BCC alloy. We believe that this complex study can help in the design of other BCC alloys and composites for hydrogen storage. They can be especially useful concerning the growing hydrogen community interest in the BCC HEA. The very recent studies on HEA have shown that these alloys can be considered as materials capable to reach the H/M ratio over 2, which is the upper limit for conventional BCC alloys [93]. Moreover, groups of Zepon and Zlotea considered HEAs as possible light-weight alloys for hydrogen storage [94,95]. The interest in HEAs for hydrogen storage has just begun and there are still many undeveloped areas in this topic. What is meaningful, there are no reports on the HEA-based composites synthesized with carbon catalysts. Therefore, we hope that this paper can serve as a guide for research on a new group of materials for hydrogen storage.Based on the presented and discussed studies, the following conclusions can be drawn: • Milling of BCC alloy with carbon additives without milling balls is an effective way to disperse carbon catalysts on the hydrogen storage material surface without significant changes in both the alloy and the catalyst structures. • All of the proposed catalysts promote hydrogenation and dehydrogenation in studied BCC alloys. • The activation energy of the hydride decomposition process is at least twice reduced for composites with ultrafine diamonds, carbon mesoporous, or fullerenes in comparison to BCC alloy. • The temperature of the hydride decomposition process was decreased by nearly 100 K for fullerene-containing composites. • The extremely fast hydrogen absorption was observed for a composite containing 5 wt % of fullerene co-milled with BCC alloy for 10 min. The alloy absorbs 3.3 wt % of H2 within a minute at room temperature. The absorption is nearly fifty times faster compared to pure BCC alloy. • Prolonged milling of BCC alloy with C60 leads to serious degradation of hydrogen storage properties of a composite. • In terms of hydrogenation/dehydrogenation properties, the optimal concentration of fullerenes has been set at 5 wt %. • Since milling with carbon allotropes did not cause any major microstructural and structural changes in the BCC alloy, the improvement of hydrogen storage properties is related to the catalytic function of added carbon materials. Milling of BCC alloy with carbon additives without milling balls is an effective way to disperse carbon catalysts on the hydrogen storage material surface without significant changes in both the alloy and the catalyst structures.All of the proposed catalysts promote hydrogenation and dehydrogenation in studied BCC alloys.The activation energy of the hydride decomposition process is at least twice reduced for composites with ultrafine diamonds, carbon mesoporous, or fullerenes in comparison to BCC alloy.The temperature of the hydride decomposition process was decreased by nearly 100 K for fullerene-containing composites.The extremely fast hydrogen absorption was observed for a composite containing 5 wt % of fullerene co-milled with BCC alloy for 10 min. The alloy absorbs 3.3 wt % of H2 within a minute at room temperature. The absorption is nearly fifty times faster compared to pure BCC alloy.Prolonged milling of BCC alloy with C60 leads to serious degradation of hydrogen storage properties of a composite.In terms of hydrogenation/dehydrogenation properties, the optimal concentration of fullerenes has been set at 5 wt %.Since milling with carbon allotropes did not cause any major microstructural and structural changes in the BCC alloy, the improvement of hydrogen storage properties is related to the catalytic function of added carbon materials. Mateusz Balcerzak: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Visualization, Funding acquisition. Tomasz Runka: Formal analysis, Investigation, Writing – original draft. Zbigniew Śniadecki: Formal analysis, Investigation, Writing – original draft.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Financial assistance from National Science Centre, Poland (no. 2015/17/N/ST8/00271).The following is the Supplementary data to this article: Multimedia component 1 Multimedia component 1 Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2021.06.030.

Body-centered cubic (BCC) alloys are considered as promising materials for hydrogen storage with high theoretical storage capacity (H/M ratio of 2). Nonetheless, they often suffer from sluggish kinetics of hydrogen absorption and high hydrogen desorption temperature. Carbon materials are efficient hydrogenation catalysts, however, their influence on the hydrogen storage properties of BCC alloy has not been comprehensively studied. Therefore, in this paper, composites obtained by milling of carbon catalysts (carbon nanotubes, mesoporous carbon, carbon nanofibers, diamond powder, graphite, fullerene) and BCC alloy (Ti1.5V0.5) were extensively studied in the non-hydrogenated and hydrogenated state. The structure and microstructure of the obtained materials were studied by scanning and transmission electron microscopes, X-ray diffraction (XRD), and Raman spectroscopy. XRD and Raman measurements showed that BCC alloy and carbon structures were in most cases intact after the composite synthesis. The hydrogenation/dehydrogenation studies showed that all of the used carbon catalysts significantly improve the hydrogenation kinetics, reduce the activation energy of the dehydrogenation process and decrease the dehydrogenation temperature (by nearly 100 K). The superior kinetic properties were measured for the composite with 5 wt % of fullerene that absorbs 3.3 wt % of hydrogen within 1 min at room temperature.

activated carboncarbon blackcarbon felt/graphite feltcarbon nanofibercarbon nanotubeelectrochemically exfoliated graphenehierarchical porous carbonsmesoporous carbonmultiwall carbon nanotubeordered mesoporous carbonreduced graphene oxidereticulated vitreous carbonreed straw activated carbonspectrographically pure graphitesingle-wall carbon nanotubebioelectrochemical systemdensity functional theoryelectrochemical advanced oxidation processhydrogen evolution reactionliquid crystal displaymagic angle spinningmicrobial electrolysis cellmicrobial fuel cellnuclear magnetic resonanceoxygen evolution reactionoxygen reduction reactionperoxi-coagulationpersistent organic pollutionrotating disk electroderotating ring-disk electrodestatistical Raman spectroscopyanthraquinonedisulphonateN-butyl-3-methylpyridinium dicyanamideCo-polypyrrolecobalt tetra-methoxy-phenyl porphyrindihydroxynaphthalene1-Ethyl-3-methylimidazolium dicyanamidehexaminepolyanilinepolyethyleniminePolypyrrolepolytetrafluoroethylenetert-butyl-anthraquinoneTriethylenetetramineHydrogen peroxide (H2O2) is a universal oxidizing agent which can be utilized either alone or in combination with other reagents for various purposes, such as pulp and textile bleaching [1], chemical synthesis [2], and wastewater treatment [3]. In the environmental field, H2O2 is used in many advanced oxidation processes (AOPs), such as H2O2/UV, H2O2/Fe2+, and H2O2/O3 [4]. In these H2O2-based AOPs, strong oxidant ·OH radicals (E°(·OH/H2O) = 2.80 VSHE) are generated in situ. Afterwards, this free radical can non-selectively oxidize various contaminants at relatively high rate constants in the order of 106-1010 M−1 s−1 [5]. Moreover, the self-quenching of ·OH radicals makes their lifetime in water as short as a few nanoseconds [5]. AOPs have been widely applied for the degradation of various pollutants such as antibiotics, herbicides, insecticides, endocrine-disrupting chemicals, pharmaceutical and personal care products (PPCPs), and effluent organic matter [6], showing their potential for water and wastewater treatment in the future [7,8].Currently, over 95% of commercially produced H2O2 worldwide is derived from the anthraquinone oxidation (AO) process (the alternate name is auto-oxidation process) [9]. Hydrogen, anthraquinone, and air are employed as raw materials in the AO process. The alkylanthraquinone precursor dissolved in an admixture of organic solvents is catalytically hydrogenated and then oxidized to obtain a diluted solution of H2O2 at 0.9–1.8% (wt). The following liquid-liquid extraction and distillation processes produce concentrated H2O2 solution at 35–50% (wt) (Fig. 1 a). The major drawbacks of the AO process are (1) the use of large quantities of hazardous organic solvents, (2) highly concentrated H2O2 is explosive, which brings potential risks during transport and storage, (3) about 0.1% (wt) H2O2 at most is needed in the wastewater treatment process, which makes the concentration-dilution process a waste of cost and energy. These disadvantages and the decentralized requirements of users motivated the academic community and industry to develop other H2O2 synthesis methods and go beyond the AO process.An alternative method is the direct synthesis of H2O2 from H2 and O2. In this straightforward batch process [10], gaseous H2 and O2 are introduced into the liquid medium with catalysts. The proposed catalytic mechanism for direct synthesis is sequential hydrogenation of molecular oxygen. Firstly, the H2 molecule is dissociated into H atoms on the surface of the catalyst. Afterwards, an O2 molecule adsorbs onto the surface of the catalyst, followed by reacting with the H atom and thus forming the HOO∗ intermediate. H2O2 is finally obtained by hydrogenating the HOO∗ intermediate (Fig. 1b) [11]. Although the first patent was granted in 1914, there is still no industrial process based on the direct synthesis for over 100 years because of the following three critical disadvantages [12,13]: (1) Safety - the direct synthesis avoids transportation of H2O2 to the site, but H2 is more explosive relative to H2O2. The H2 and O2 mixture gas has to be diluted by other “inert” gases (N2 or CO2) to operate below the lowest explosive limit, which also limits the productivity of H2O2; (2) Competing side reactions - the hydrogenation of O2 towards H2O2 (ΔH = −135.9 kJ mol−1) is along with the direct formation of H2O (ΔH = −241.6 kJ mol−1), the further reduction to H2O (ΔH = −105.8 kJ mol−1) and H2O2 decomposition reaction (ΔH = −211.5 kJ mol−1) (Fig. 1b), which are all thermodynamically more favored than the desired main synthesis reaction; (3) Cost of catalyst - Although noble metal catalysts, such as Pd, Pd–Au and Pd–Sn, are proven to be effective, the rareness and high price of these materials make the direct synthesis hard for large-scale applications.Photo-catalysis through proton-coupled electron transfer is another alternative to generate H2O2. Briefly, in the heterogeneous photocatalytic process, an optical semiconductor is activated by irradiation of an appropriate light source to form photo-generated electron/hole (e−/h+, Fig. 1c) pairs, which under certain conditions induces the reduction of O2 to produce H2O2 [14]. Currently, it is accepted that H2O2 can be produced via either a one-step two-electron direct reduction or a two-step one-electron indirect reduction route [15]. Photo-catalysis has emerged as a promising alternative since it only requires an optical semiconductor, water, oxygen, as well as sufficient and renewable light as the driving force. As a hot topic in recent research, multiple photo-catalysts have been investigated and employed, including TiO2, graphite carbon nitride (g-C3N4), metal-organic compounds and their modification materials. However, photocatalytic H2O2 generation is still in its initial stage with several problems that need to be solved, such as poor selectivity toward 2 e− O2 reduction, relatively low response to sunlight, and high recombination rate of photo-generated species [15]. Moreover, the H2O2 can be further reduced with e− or decomposed by the irradiated UV light (λ < 400 nm), which causes the H2O2 production rate to be lower than the above two methods [14,16].The electrosynthesis of H2O2 via two-electron oxygen reduction reaction (ORR) is attracting growing attention. The electrosynthesis of H2O2 was first reported by Berl et al., in 1939 by applying activated carbon as a cathode to achieve a 90% current efficiency [17]. Based on the two-electron ORR pathway, the Huron-Dow process was developed in the 1980s (by Dow and Huron Technologies, Inc.) to produce dilute alkaline H2O2 onsite (Eq. (1)) for pulp and paper bleaching (Fig. 1d) [9]. Although it was successfully commercialized in 1991, the inherent disadvantages includes corrosion of the electrodes from the highly alkaline environment, carbonate formation from CO2 and the high Ohmic resistance in the system limiting its further development [12]. As a variant of the Huron-Dow process, the electro-Fenton (EF) process was first investigated and developed by Brilla's research group and Oturan's group in the 1990s [18,19]. The EF technology is based on the continuous H2O2 generation on a cathode (Eq. (2)) in an acidic electrolyte. ·OH radicals are generated via Fenton's reaction (Eq. (3)) with the addition of a sufficient amount of Fe2+ ions. The homogeneous regeneration of Fe2+ on the cathode (Eq. (4)) makes the persistent organic pollution (POPs) continuously degraded in EF. Until now, EF has become the most popular electrochemical technology to degrade a variety of POPs, including pesticides [19], dyestuffs [20], PPCPs [21,22], and industrial pollutants [18]. Moreover, novel electrochemical advanced oxidation processes (EAOPs) based on the cathodic generation of H2O2 were developed for remediation of wastewater, such as photoelectro-Fenton [23], sono-electro-Fenton [24], peroxi-coagulation [25], and electro-peroxone processes [26]. (1) H2O + O2 + 2 e− → HO2 − + OH− (2) O2 + 2H+ + 2 e− → H2O2 (E◦ = 0.68 VSHE) (3) Fe2+ + H2O2 → Fe3+ + ·OH + OH− (4) Fe3+ + e− → Fe2+ (E◦ = 0.77 VSHE) Highly efficient cathode and the optimized system are two crucial prerequisites for the development of these EAOPs. Recently, substantial research has been devoted to the prediction and design of catalysts for electrochemical two-electron ORR and some of the progress has been previously presented in several reviews [12,27–35]. However, systematic and comprehensive reviews on carbonaceous two-electron ORR catalysts from the angle of mechanism and catalyst design to electrode fabrication have not been reported. As an emerging research field, it is necessary and indispensable to review and summarize the latest work on the development of these areas. Furthermore, from our point of view, the rapid development of electrode fabrication technology requires solid theoretical research in material science and catalytic science, as well as continuous optimization in engineering. In this review, we discuss the recently discovered mechanistic understanding of carbon materials catalysis and present important developments in carbon-based catalysts for two-electron ORR. Currently there are only a few comprehensive studies on two-electron ORR materials and the close correlation between two-electron ORR and four-electron ORR reactions. In the following sections we present some enlightening mechanisms and research results on four-electron ORR for the first time, including the predictive design by density functional theory (DFT) calculations and controllable doping/functionalization configurations as well as the construction of porous structures and defects to guide two-electron ORR design (Chapter 4). The remaining uncertainty on the real active sites for two-electron ORR are illustrated and discussed. Depending on the raw carbonaceous material and the operation mode, we also systematically summarize the preparation and modification methods for the formed carbon-based electrodes (Chapters 6, 7, and 8). Finally, we provide a detailed perspective on the challenges and opportunities in this rapidly developing field. We attempt to take full advantage of carbon-based materials in constructing highly efficient two-electron ORR catalysts and provide a thought for the amplification and application of electrocatalytic synthesis H2O2 with high efficiency and low cost. Additionally, based on the knowledge amassed from the references and our former work experience, we hope to provide guidance and suggestions for future research by summarizing the inconsistent or divergent experimental and computational methods. We encourage future studies to use more unified experimental methods and expressions, while avoiding the oversights from previous studies.The mechanism of electrochemical ORR is outlined in Fig. 2 a. Generally, the ORR involves either a four-electron transfer pathway, which reduces O2 to H2O (Eq. (5)) and is attractive for fuel cells, or a two-electron pathway to produce H2O2 (Eq. (2)), which is desirable for environmental remediation [36]. Overall, the direct four-electron ORR involves multiple steps and intermediates (HOO∗, HO∗, O∗), which can be divided into the dissociative and associative way, depending on the oxygen dissociation barrier on the catalyst surface (Eqs. (6)–(13)) [37,38]. (5) O2 + 4H+ + 4 e− → 2H2O Dissociative pathway: the O–O bond breaks into two O∗, which could be reduced to H2O as the final product. (6) O2 + 2 ∗ → 2 O∗ (7) 2 O∗ + 2H+ + 2 e− → 2 HO∗ (8) 2 HO∗ + 2H+ + 2 e− → 2H2O + 2 ∗ Associative pathway: the activated O2 molecule firstly couples the proton & electron to produce HOO∗, and then the O–O bond of HOO∗ is cleaved and reduced to H2O. (9) O2 + ∗ + H+ + e− → HOO∗ (10) HOO∗ + H+ + e− → O∗ + H2O (11) O∗ + H + e− → HO∗ (12) HO∗ + H+ + e− → H2O + ∗ (13) HOO∗ + ∗ → O∗ + HO∗ Where ∗ denotes an unoccupied active site, and HOO∗, HO∗, O∗ represent the single adsorbed intermediates on the catalyst surface [36].Conversely, the two-electron pathway is comprised of two coupled electron & proton transfers together with one intermediate (HOO∗) (Eqs. (14) and (15)) [39]. (14) O2 + ∗ + H+ + e− → HOO∗ (15) HOO∗ + H+ + e− → H2O2 + ∗ From the above reactions and the schematic diagram shown in Fig. 2a, it is observed that breaking the O–O bond (Eqs. (6) and (10)) is a necessary step in both dissociative and associative four-electron pathways. Therefore, preventing the O–O bond from dissociation is critical in the selective catalysis for H2O2 synthesis [30]. Moreover, the obtained H2O2 could be further reduced to H2O, making the process an indirect four-electron ORR to reduce the H2O2 production (Eqs. (16)–(19)). Consequently, shortening the H2O2 residence time on the catalyst surface is also critical to maintaining the stability of H2O2 during electro-synthesis. In summary, the ideal electrocatalyst with high activity and high selectivity toward two-electron ORR should have the property of minimizing the kinetic barriers for Eqs. 14 and 15 and maximizing the barrier for H2O2 reduction and HOO∗ dissociation to HO∗ and O∗ [36]. (16) H2O2 + ∗ + H + e− → HO∗ + H2O (17) HO∗ + H + e− → H2O + ∗ (18) H2O2 + 2 ∗ → 2 HO∗ (19) 2 HO∗ + 2H+ + 2 e− → 2H2O + 2 ∗ The performance of the electrocatalyst depends on the binding energy of the reaction intermediates to the catalyst surface [36]. However, because of the existence of scaling relationships Eq. (20) between these intermediates, the activity is governed by a single parameter ΔG OH∗ [40,41]. Benefiting from the development of DFT calculations on numerous close-packed metal surfaces, a volcano framework has established that the theoretical overpotential relates to the free energy of HO∗ for the ORR activity (Fig. 2b) [42,43]. In brief, for the materials that bind HO∗ too weakly (i.e., to the right side of the peak), the ORR is limited by the activation of O2. The weak interaction with O∗ and HO∗ increases the selectivity toward two-electron ORR, but simultaneously lowers the ORR activity. For the materials that bind HO∗ too strongly (i.e., to the left side of the peak), the limiting step for the H2O2 and H2O production is associated with removing HOO∗ and HO∗, respectively. Considering the two-electron pathway is determined by only one intermediate, it is feasible to find an electrocatalyst with zero theoretical overpotential that has an optimal ΔG HOO∗, binding HOO∗ neither too strongly nor weakly [43]. (20) ΔG HOO∗ = ΔG HO∗ + 3.2 ± 0.2 eV The electrocatalysts for two-electron ORR covers a wide range from noble metals (Pb, Au, etc.) and metal alloys to carbonaceous materials. The metal alloys, such as Pd-Au [44,45] and Pt–Hg [36], were verified to have high selectivity toward H2O2 (70.8–92.5% under 0.1–0.3 V vs. RHE at pH 1). However, the wide application of these noble-metal catalysts is impacted by their scarcity and high cost [46]. Accordingly, metal-free and non-noble-metal catalysts are developed as sustainable alternatives. Carbon materials are a promising alternative for H2O2 electrosynthesis because of their high abundance, conductivity, activity, and lower price. Most importantly, with a variety of allotropes, the carbon materials have multiple morphologies and highly tunable electronic structures. This unique feature makes them an ideal platform for designing electrocatalysts at the atomic level [47].The characterization of electrocatalyst performance via the rotating ring disk electrode (RRDE) technique in a three-electrode system is necessary for evaluating ORR activity and H2O2 selectivity in the material design. RRDE is a convective electrode system containing a disk electrode, a coaxial Pt ring electrode together with a rotating shaft (Fig. 3 ) [48]. In the RRDE, the ORR takes place on the disk electrode to generate both H2O2 and H2O. Thereafter, H2O2 is radially transferred to the coaxial Pt ring electrode by the forced convection resulting from the rotation of the electrode. Subsequently, H2O2 is oxidized back to O2 (Eq. (21)) at the ring electrode. The overall ORR activity and H2O2 selectivity could be quantified by analyzing the corresponding reduction disk current (i D ) and oxidation ring currents (i R ), respectively [49]. The selectivity of H2O2 is quantified by the Faradaic efficiency (λ Faradaic ) and the average transferred electrons number (n). (21) H2O2 → O2 + 2H+ + 2e− (22) λ F a r a d a i c = i R N i D × 100 % (23) n = 4 | i D | | i D | + i R N where N represents the collection efficiency of the RRDE, which means the fraction of product from the disk to the ring, based on the geometries of the ring and disk electrodes [30].Rotating disk electrode (RDE) is another tool for assessing the catalyst ORR activity. Compared with RRDE, the coaxial ring electrode is removed in the RDE system (Fig. 3). The hydrodynamic and electrochemical properties of RDE are related to the Koutecky-Levich (K-L) equation [50]. (24) 1 J = 1 J K + 1 J L = 1 n F k C 0 + 1 0.62 n F C 0 D 0 3 2 v − 1 6 w 1 2 where j K and j L are the kinetic-limiting current density and diffusion-limiting current density (mA cm−2), n stands for the electron-transfer number, F is the Faraday constant (C mol−1), C 0 and D 0 are the bulk concentration (mol cm−3) and O2 diffusion coefficient (cm2 s−1) in the electrolyte, v is the kinematic viscosity of the electrolyte (cm2 s−1), ω represents the angular velocity (rad s−1), and k is the electron-transfer rate constant. n and k can be obtained from the slope and intercept of K-L equation, respectively.Typically the onset potential or overpotential (η) in (R)RDE tests are used to compare the activities among different electrocatalysts. And the selectivity of newly designed electrocatalysts toward H2O2 is mainly quantified in terms of n and λ. In Tables 1 and 2 , the performance of newly developed electrocatalysts on (R)RDE are systematically summarized.Before discussing the carbon catalysts, it is vital to discuss the different types of obtained carbon materials. There are three allotropes as dictated by the carbon precursor [51]: graphite (sp 2 bonding), diamond (sp 3 ), and amorphous carbon (disordered mixture of sp 2 and sp 3 ). Due to the different combinations of carbon atom hybridization, carbon allotropes with different structures and properties are obtained. Until now, most of the two-electron ORR catalysts are graphite carbon and amorphous carbon.Graphite is formed by multilayered two-dimensional (2D) sheets of sp 2 hybridized carbon atoms with hexagonal lattice in the basal plane. The edges of its planes have terminations with carbon atoms arranged with zigzag or armchair configurations, and a van der Waals interaction along the transverse direction between the layers that is relatively weak. Apart from naturally existing graphite, the discovery of graphene and carbon nanotubes (CNTs) expanded the categories of sp 2 carbon materials. Graphene (a single layer form of graphite) is a 2D sheet honeycomb structure composed of sp 2 carbon atoms. Due to the remarkable electrical, thermal, physical, optical, and mechanical properties together with high specific surface area, it has received increasing attention [28,52]. Graphene can be prepared by a variety of processes, including mechanical cleavage [53], chemical vapor deposition growth [54], epitaxial growth [55], electrochemical exfoliation of graphite [56], and thermal/chemical/electrochemical reduction of graphene oxide (GO) [57]. Among those, the mechanical cleavage method results in high-quality graphene sheets but low yield, which cannot meet the large demand for graphene. The GO reduction method is considered to be one of the most promising routes for large-scale graphene production, which restores the essential properties. However, the production of graphene with minimum defects remains challenging. CNTs are tubular cylinders of carbon atoms that can be conceptually viewed as one or up to dozen graphene sheet(s) that are rolled up into a single-wall carbon nanotube (SWCNT) or a multiwall carbon nanotube (MWCNT) [51]. CNTs have been at the forefront of materials research in the last decade due to their high electrical conductivity (∼5000 S cm−1) [58], high surface area (∼2630 m2 g−1) [59], high charge mobility (∼100,000 cm2 V−1 s−1) [60], as well as chemical stability, and significant mechanical strength. Currently, SWCNTs and MWCNTs are mainly produced by three techniques: arc-discharge [61], laser-ablation [62], and catalytic growth [63]. Among these methods, catalytic growth of nanotubes by the chemical vapor deposition (CVD) is the dominant mode of high-volume CNT production. In 2013, bulk purified MWCNTs were sold for less than 100 $ kg−1 [64]. The decrease in CNTs price increases their potential for application in various technological areas such as the chemical, medical, aerospace, energy, and automotive industries.Carbon black (CB) is an amorphous particle of nearly pure elemental carbon, consisting of grape-like aggregates of spherical primary particles, with the aggregates clustered into larger-sized agglomerates [65]. CB has relatively low quantities of extractable organic compounds and total inorganics (usually <1%) [66]. As a manufactured commercial product for over a century, it has plentiful applications as well as a variety of different trade names and physicochemical properties. A variety of CB grades with different properties (surface area, structure, aggregate size, abrasion resistance, etc.) are manufactured by controlling the conditions of the oil furnace production process. The most widely used CB material is Vulcan XC-72(R) (produced by Cabot Corporation, US) which is used in 80% of electrocatalysts [67], due to the large surface area (∼250 m2 g−1), high mesopore and macropore percentage (54%), and good electric conductivity (4.0 S cm−1 at the packing fraction of 0.3 and 7.4 S cm−1 at 0.4) [68,69]. In addition to Vulcan XC-72(R), there are some other commercial CB materials such as Printex L6, Black Pearls 2000, Acetylene Black, and Macsorb [70].Ordered mesoporous carbon (OMC) is a type of carbon material with regular arrays of uniform mesopores. The OMCs with different compositions vary from pure organic/inorganic frameworks to organic-inorganic hybrid frameworks have been widely investigated and reported in the past two decades. Normally, OMCs can be prepared by two different methods, the hard templating process (nano-casting strategy) by using mesoporous silicas and a soft templating process (direct synthesis) via self-assembly of block copolymers/surfactants and carbon precursors [71]. Except for high surface areas, OMCs also have outstanding special properties, including tunable pore sizes, alternative pore shapes, periodically arranged monodispersed mesopore space, and uniform nano-sized frameworks [72].Because of the above outstanding physical and chemical properties, tremendous investigations have been conducted to reveal the ORR catalytic characteristics of graphene, CNTs, CB, and OMCs. However, the electroactivity of pristine carbon catalysts still lags behind that of their metal counterparts because pristine carbon materials are inert to the adsorption and activation of O2 and ORR intermediates (Fig. 2c) [36,44,73]. They always show high overpotentials and thus unpromising catalytic property toward ORR [74]. Therefore, the catalytic properties of carbon materials need to be improved by doping, functionalization, and structural regulation.Heteroatom doping refers to replacing part of carbon atoms in the carbon skeleton by other heteroatoms, including N, O, B, P, S, halogens, etc. (Fig. 4 a) Because of the differences in atomic size, electronegativity, and binding states, heteroatom doping can regulate the spin and charge distribution, tune the absorption and activation of the ORR intermediates, and further change the catalytic performance of carbon [74,75]. For instance, B and P dopants tend to give electrons to carbon due to their lower electronegativity (B: 2.04; P: 2.19) compared with carbon (2.55), which creates a partial positive charge on the dopant atoms [76]. In contrast, nitrogen with higher electronegativity (N: 3.04) tends to rob electrons from carbon to generate a partial positive charge on the carbon atoms. Typically, the formation of partial positive/negative charges can both promote the interaction between the catalyst and O2, and the adsorption of O2 on the carbon materials. Though S doping does not disrupt the charge uniformity of carbon materials because of its similar electronegativity (2.58), larger size, and greater polarizability of the S atom enhance the spin density and charge delocalization on the neighboring carbon atoms to promote the ORR [77,78].As a convenient method to tune the electrochemical catalytic properties of carbon materials, doping has been utilized widely in developing ORR catalyst materials. Here, carbon materials with multiple heteroatoms doping for H2O2 production are reviewed.N-doping is the most extensively and promising form to modify sp 2 carbon for ORR for two reasons: one, N has a similar atomic radius as C, allowing it to easily replace C atoms without lattice mismatch; and two, the higher electron affinity of N makes the N dopant easy to change the atomic structures and electron arrangements of the carbon skeleton [46,74,78]. First synthesized by Gong et al. [75], N-doped carbon materials for four-electron ORR have attracted much attention. Typically, there are two strategies for fabricating N-doped carbon catalysts. The first one is post-doping of porous carbons in the presence of nitrogen-containing precursors, which can effectually control the structures of the catalyst. However, the N-doping efficiency of post-doping is low and diverse. The other method is direct (in-situ) doping during the synthesis of catalysts to enhance the N-doping content and vary the N-doping structure. Nevertheless, the pore size and porosity of the catalyst are difficult to be precisely controlled in direct doping.Resulting from different synthetic precursors, catalysts, conditions, and procedures, there are various forms of N dopant that exist in the carbon skeleton, such as pyridinic N, pyrrolic N, graphitic N (or quaternary N), and pyridine-N-oxide (Fig. 4b). However, not all N dopant-related dopants on the carbon materials constitute highly catalytic active species. There is debate on the real active sites in N-doped carbon for ORR. In general, planar pyridinic-N can enhance the electron-donating capability and weaken the O–O bond due to its lone electron pair. As a result, pyridinic-N is thought to be the active site for four-electron ORR [79]. Guo et al. [80]. Reported that carbon atoms adjacent to pyridinic-N are the real active sites for ORR in acidic media. Others have suggested that pyrrolic-N plays a key role in reducing O2 to H2O2, and pyridinic-N is the site for reducing H2O2 to H2O [81,82]. Conflictingly, Geng, et al. [83] believed that graphitic-N instead of pyridinic-N might be responsible for the two-electron ORR. Kabir et al. [84] claimed graphitic-N contributes significantly to peroxide generation in 0.5 M H2SO4. The co-generation and co-existence of different types of N-doping species in the carbon materials are inevitable, making it difficult to distinguish their contributions. Moreover, the inter-transition of the different doping types induced by temperature makes the situation even more complex.In the recent 7–8 years, the N-doped carbon two-electron ORR catalysts have been systematically investigated to obtain different results on two-electron ORR active pores. N-doped OMC with a mean pore diameter of 13.2 nm was obtained from the ionic liquid N-butyl-3-methylpyridinium dicyanamide (BMP-dca) at 800 °C by hard-templating strategy using silica nanoparticles [85]. H2O2 production rate reached 0.17 g gcat −1 h−1 with a current efficiency of 65.2% at 0.1 V (vs. RHE). The RDE verified that the resultant material was highly active for the selective H2O2 generation. However, a comparable material synthesized at 1000 °C was favorable to the four-electron ORR [86]. The authors assumed the lower degree of delocalization, higher N content, and exposure of pyrrolic-N sites may favor the two-electron process.Sun et al. [87] fabricated a series of N-doped OMC materials by pyrolysing the mixture of 1-Ethyl-3-methylimidazolium dicyanamide (EMIM-dca) and CMK-3 at different temperatures. Compared with six potentially suitable two-electron ORR carbon materials (Ketjen black EC 300J, Ketjen black EC 600JD, Black pearls 2000, Vulcan XC 72R, Graphene nano-plates, and CMK-3), the structural, compositional, and other physical properties were correlated with their catalytic performance. For six pristine carbon materials, large BET surface areas, positive zeta potentials, and high defect sites were all beneficial for H2O2 generation. Dissimilarly, the H2O2 selectivity of N-doped carbon catalyst is governed more by the N doping effect. The selectivity of the optimal N-doped material reached 95–98% in the potential range of 0.1–0.3 V (vs. RHE). In order to get a better understanding of the potentially related mechanistic roles of the different N species during the two-electron ORR, a novel N-doped OMC was prepared by annealing the mixture of CMK-3 and polyethylenimine (PEI) in the N2 atmosphere; achieving the highest H2O2 selectivity of 95–98.5% with the potential range of 0.1–0.4 V [88]. Analyzing chemical state trajectories of N species in the catalysts suggested pyridinic-N played a key role as an active site in acidic solution, while graphitic-N groups seemed to be active catalytic moieties in neutral and alkaline conditions.Other N-doped porous carbons were prepared from paraformaldehyde cross-linked collagen by sintering at various temperatures (400–800 °C) [89]. Higher carbonization temperature brought more porous and sheet-like structures into the materials, and led to the formation of graphitic-N structure, the removal of oxygen-containing functional groups, and the decrease of N content, thus enhancing graphitic crystallinity. According to the electrochemical tests, N-doped carbon prepared at 400 °C showed excellent two-electron ORR with a selectivity of 93% over a wide potential range from 0.17 to 0.6 V (vs. RHE) due to the combination of pyridinic-N, pyrrolic-N, and the surface oxygen-containing functional groups.Since ORR generally proceeds on the surface of the catalysts, the unexposed active sites hidden in the catalyst body contribute little to the catalytic activity [74]. Therefore, the appropriate doping location and catalyst micro-configuration are more crucial than the gross doping content.To obtain a special carbon material with N-doping mainly at the surface, to provide active sites and high graphitic carbonaceous core to provide high electrical conductivity, N-doped graphitic carbon materials were fabricated by sequential pyrolysis of aniline and dihydroxynaphthalene (DHN) inside the SBA-15 hard silica template (Fig. 5 a) [90]. To cover the template surface with monolayered aniline, the amount of aniline employed was determined based on the precise calculation of the molecular cross-sectional area of precursor and total pore volume of the template. The resultant materials displayed an ordered, hexagonal array of carbon rods, with a narrow pore size distribution centred at 4.3 nm, and specific surface area of 877 m2 g−1 (Fig. 5b). The novel N-doped OMC exhibited outstanding performance with a transfer number of 2.1, and a H2O2 selectivity of 95% (Fig. 5c). Which attributed to the high surface area, regular mesopores structure, a graphitic character, high content graphitic-N andpyridinic-N configurations.Except for the N species, other influence factors were also investigated. Park et al. [91] studied the effect of the mesopores to discover that N-doped carbon materials with 3.4–4 nm well-ordered mesopores had high activity and selectivity (>90%) for H2O2 synthesis. In comparison, micropore-dominant N-doped activated carbon showed a higher onset potential than N-doped OMC, but a lower selectivity (56–60%). The excellent mass transfer of mesoporous structure enhanced the release of H2O2 within a relatively short contact time, which resulted in high selectivity toward H2O2 synthesis. Hasché et al. [92] proposed that the electrochemical formation mechanisms for peroxide are dependent on the pH and the species of electrolyte, as well as the respective change of the peroxide species from H2O2 to hydroperoxide. In order to obtain a kinetic understanding of N-doped carbon catalysts in acidic media, a porous N-doped carbon with a surface area of 992 m2 g−1 was obtained by carbonization of polyimide nanoparticles through a two-step pyrolysis [93]. RRDE revealed that lower catalyst loading on the disk suppresses the further reduction of H2O2 in the catalyst matrix layer. When the catalyst loading density decreased to 30 μg cm−2, the H2O2 selectivity was much higher than 80%. This study provided quantitative insight into the ORR mechanism over an N-doped carbon catalyst.Although N-doped carbon materials have been investigated extensively, carbon materials doped with B, P, S, and halogens have also been explored recently for their potential applications for electrocatalysis of ORR. However, almost all of the B/P/S doped carbon materials demonstrated an affinity for four-electron ORR instead of two-electron ORR for H2O2 generation [94–97].Zhang et al. [98] studied the formation energy, electronic structures, transition states, and energy barriers of S-doped graphene clusters by DFT calculation to predict ORR activity of four types of S-doped graphene clusters (Fig. 4c); including S atoms adsorbed on the graphene cluster surface (Type 1), S atom replacement at the graphene cluster armchair edge or zigzag (Type 2), SO2 substitution at the graphene edge (zigzag and armchair. Type 3), and two graphene clusters connected by an S ring (Type 4). Carbon atoms with high spin density or positive charge density are the active catalytic sites, which are often located at the zigzag edges or close to the SO2 doping structure. Two-electron ORR proceeds on the substitutional S atom with a high charge density, while four-electron ORR occurs simultaneously on the carbon atoms with a high positive spin or charge density.Considering the high electronegativity of Fluorine (3.98), the carbon electronic structure can be adjusted significantly by F atom doping. In addition, F-doping regulated the electron transfer properties by inducing polarization and changing the Fermi level [99,100]. Recently, a F-doped hierarchically porous carbon catalyst was developed from an aluminum-based metal-organic framework (MOF, MIL-53) precursor [101,102]. The selectivity of the ORR pathway strongly depended on the F doping species configurations. The covalent CF2 and CF3 facilitate the two-electron pathway because of the strong adsorption of O2 and the weak binding energy of the HOO∗ intermediate. Hence, the fabricated F-doped catalysts exhibited a high H2O2 yield of 113–793 mmol h−1 gcat −1, and selectivity reached 97.5–83% in the potential range of −0.1 V to −0.6 vs. RHE (pH 1).In 2021, Xia et al. [103] systematically studied the effects of different dopants (B, N, P, S) in carbon material on its performance in 2e− ORR performance. Among all these dopants, B-doped carbon shows the highest activity and selectivity, with an onset potential of 0.773 V (vs. RHE) while maintaining over 85% selectivity across a broad potential window in 0.1 M KOH. BET, XPS, XAS, and Raman results excluded the possible morphological, structural, and electronic side effects on 2e− ORR. DFT calculations revealed the B-doped at single vacancy has nearly-zero overpotential, while molecule dynamics at constant potential indicates that the energy barrier for the 2e− pathway is lower than its 4e− counterpart.It has been recently proven that the co-doping of multiple types of heteroatom into carbon materials would increase the density of electrocatalytic active sites for two-electron or four-electron ORR processes [78]. N & S, N & B, and N & P co-doped carbon materials have been investigated for catalyzing two-electron ORR. The comparison of N-doped, S-doped, and N & S co-doped mesoporous carbons showed that a higher N content enhanced the catalytic activity while the effect of sulfur was opposite [104]. Though the N & S co-doped carbon showed a lower activity, the selectivity toward H2O2 (75%) was higher than N-doped samples (67–69%). In similar work, mesoporous carbons doped with either N, S, or both, were obtained by a one-pot molecular precursor auto-assembly followed by hydrothermal carbonization [105]. The dopant molecule was found to govern the ensuing structure and resulted in different average mesopore sizes (3.5 nm, 8.2 nm, 32 nm, and 34 nm corresponding to un-doped, N-doped, S-doped, and N & S co-doped carbons). The RDE test demonstrated that no beneficial effect was achieved by the co-doping of S & N. The best performance for two-electron ORR was achieved by N-doped catalyst with 4% (wt) N content and about 80% pore volume in the mesopore range. Dissimilarly, Zhu et al. introduced N, S atoms into a carbon-based cathode [106]. Results showed the optimized N & S co-doped cathode presented over 42% improvement of H2O2 yield, which was higher than single N/S doping. Mechanism studies show that “End-on” O2 adsorption was achieved by adjusting electronic nature via N doping, while HOO∗ binding capability was tuned by spin density variation via S doping.Hybrid boron-carbon-nitrogen (BCN) materials have been tested for several catalytic applications [107,108]. To increase the selectivity toward H2O2 production, B & N co-doped carbons were prepared. BET surface area, together with the total content of B and N dopants were modulated by controlling the initial co-monomer precursor ratios [109]. Compared to solely N-doped carbon, the final loading of N by co-incorporating B with N increased significantly due to the formed isolated patches of h-BN, which provided higher activity and selectivity for the two-electron ORR. Moreover, systematic DFT calculations were performed to study the structures of different size h-BN domains doped into graphene, and different size C domains doped into an h-BN lattice [110,111]. The relationship between stimulated limiting potential and HO∗ adsorption energy is shown in Fig. 5d. The results predicted 13% h-BN to have the best two-electron ORR performance.Li et al. confirmed that N & P co-doping increases the two-electron ORR activity of cotton-stalk-derived activated carbon fibers significantly [112]. Co-doping N & P in the carbon lattice slightly changed the pore structures. Remarkably, (NH4)3PO4 treatment could not only embed N and P into the carbon skeleton but also introduced additional mesopores on the catalyst.Chemical functionalization is another powerful “regulation screw” to tailor the electron density and/or electron density distribution in the materials by introducing specific electrophilic/nucleophilic, ionic, or chiral sites. Oxygen functional groups (OFGs) are the most popular species modified onto carbon-based materials.Surface OFGs are often introduced into carbon materials by oxidation treatment. OFGs break the electrical neutrality of sp 2 carbon lattice to enhance the ORR activity. Zhong et al. [113] discovered that the carboxyl group (OC–OH) could weaken the CNF–O bond more easily and exhibit the highest four-electron ORR activity. Moreover, all the OFGs on CNFs were found to be easily bonded with H2O2 to furtherly reduce H2O2 to H2O, thus making n of the resultant materials close to 4. Until now, this was the challenge to further pinpoint the active site to a specific group still remains.Recently, it was found reactivity of OFGs will change in different environments [114]. Kim et al. [115] prepared a mildly reduced graphene oxide (mrGO) by heating purified GO at 100 °C flowing N2 overnight. The mrGO, which kept parts of the OFGs, showed stable peroxide formation activity together with highly selective at low overpotentials (about 0.01 V) in 0.1 M KOH solution. The experiments proved that carbonaceous catalysts with epoxy or ring ether groups situated either at plane edges or on their basal planes, exhibited remarkable two-electron reactivity, which was able to produce HO2 − with nearly 100% selectivity and high stability (15 h at 0.45 V vs. RHE) in alkaline conditions. In other research with N-doped rGO [116], sp 2 carbon sites located next to oxide regions were identified as dominating the ORR activity by experimental and DFT calculation, which underlined the importance of OFGs rather than nitrogen functional groups (NFGs). These references suggested that the enhancement effect of OFGs on H2O2 production activity requires a synergistic contribution of the carbon lattice environments. Based on this assumption, Sun, et al. [117] provided a novel idea into the coupling role of carbon cluster size and OFGs in H2O2 production. An activated coke electrocatalyst with size-tailored amorphous carbon clusters modified by OFGs yielded high activity (onset potential 0.83 V vs. RHE), high H2O2 selectivity (∼90%), and long-term stability. Based on this result and a series of control experiments, it was concluded that the size-reduced amorphous carbon lattices with abundant edges contributed to the high activity, while the basal carbon atoms in ether-modified small-size carbon planes are the most active sites towards H2O2 selectivity.Lu et al. [118] demonstrated a facile approach to oxidize the raw CNTs by HNO3 to obtain O-CNTs. The O-CNTs drastically lowered the needed overpotential by ∼130 mV at 0.2 mA compared with raw CNTs and increased the selectivity from ∼60 to ∼90%. Based on DFT calculations, ester groups (C–O–C) in the basal plane of the graphene and OC–OH in the armchair edge were proved active and selective for H2O2 production.Zhang et al. [119] introduced OFGs onto the Vulcan XC-72 CB by a simple calcination method at 200–600 °C exposed to air. Characterization results showed both structural defects and OFGs content increased with the calcination temperature. Furthermore, many types of OFGs, such as C–O–C, C–OH, CO, and OC–OH, were successfully introduced onto the CB surface. With calcination at 600 °C, the RRDE onset potential increased from −0.27 V to −0.14 V (vs. Ag/AgCl) and the H2O2 selectivity increased slightly from 47.0-56.2% to 52.6–56.1% at −0.35 to −0.6 V (vs. Ag/AgCl).In order to reveal the nature and quantity of two-electron ORR active sites in the alkaline media, Lu, et al. [120] synthesized various oxidized carbon black (OCB) with adjustable surface OFGs (CO, OC–OH, -C-OH) by HNO3 treatment at 30–120 °C. The OCB-120 °C had the most stable ring current and λ of ∼60% at 0.26–0.36 V (vs. RHE). It was also observed that the intrinsic activity of OC–OH is much higher than that of CO.To investigate the synergistic influence of different N doping species and OFGs in carbon materials on the H2O2 production, N & O co-doped OMC was fabricated from HMT (hexamine), Pluronic F127, and resorcinol by a one-step hydrothermal method at 600–900 °C [121]. The 700 °C carbonization sample had 443 m2 g−1 BET surface area, and COOH, CO, total N, graphitic N content together with the highest zeta potential and pyridinic N, C–O–C content (Fig. 6 ). DFT calculations on the account of the adsorption energy of HOO∗ were applied to study the interactive effects between N species and OFGs (Fig. 6i). Compared with the pure graphitic carbon (−0.608 eV), pyridinic N (−0.289 eV), graphitic N (−0.494 eV), COOH (−0.362 eV), C–O–C (−0.175 eV) doped carbon possessed a lower HOO∗ adsorption energy, which positively affects the production of H2O2. Among these, pyridinic N & C–O–C co-doped carbonaceous catalyst exhibited the lowest HOO∗ adsorption energy (−0.092 eV), which accelerated the HOO∗ protonation toward H2O2. Combined with the ideal dispersed performance, the 700 °C carbonization sample had the highest activity and selectivity (∼95%) at 0.4 V vs. RHE.OFGs can also be in-situ introduced onto the carbon-based electrode by physical/chemical/electrochemical methods to promote H2O2 yield. These will be described in detail in Chapters 6 and 7 based on the electrode type and the method.Non-noble metal (NPM)-based materials have been investigated as four-electron ORR electrocatalysts for more than a few decades. Recently, some researchers tried to load NPMs onto the carbon to test their catalytic performance for two-electron ORR. In 2011, series transition metal-carbon composite catalysts (M = V, Fe, Co, Ni, Cu, Zn, Sn, Ba, Ce) were obtained by heating the mixture of Vulcan XC-72 CB and metal nitrate salts at a high temperature of 900 °C in N2 [122]. As shown in Fig. 7 a, it is clear that Co-activated samples have outstanding performance for the electrosynthesis of H2O2 than other samples in an acid medium. An optimized catalyst with 4% (wt) Co showed a high H2O2 selectivity of 80–90% at 0.1–0.4 V (vs. RHE). The selectivity of ORR is also related to the geometric arrangement of atoms on the surface of the catalyst [123]. HOO∗ normally binds onto atop sites, whereas O∗ binds onto hollow sites. Eliminating hollow sites will specifically destabilize O∗ without necessarily changing the activity. Therefore, Siahrostami, et al. [36] predicted that catalysts such as Co-porphyrins that lack hollow sites might have high selectivity toward H2O2. Zhang et al. [124] developed a Co-based catalyst with a negligible amount of onset overpotential and nearly 100% selectivity by modulating the oxygen functional groups near the atomically dispersed cobalt sites. It was revealed that the presence of epoxy groups near the Co–N4 centers exceptionally enhanced H2O2 generation.Because of the incremental improvement witnessed in NPM electrocatalysts, various novel efficient NPM-based catalysts were developed. Among these NPM-based electrocatalysts, metal carbonitrides, including non-pyrolyzed transition metal macrocycles and pyrolyzed NPM-N-doped carbon (M-N-C) (M = Fe, Ni, Co, etc.) catalysts, have shown the most promising potential because of their efficient activity toward ORR.The investigation of non-pyrolyzed transition metal macrocycles on ORR dates back to the 1960s since Jasinski [125] first discovered the promoted ORR performance by cobalt phthalocyanine with a metal-N4 center. The electronic configuration of the metal centers is beneficial to bond with the O2 molecule and subsequent reduction of O2 [125]. Subsequently, multiple M − N4 macrocycles, such as porphyrins, phthalocyanines, and tetraazaannulenes, have been widely investigated [126]. It was found that these catalysts are prone to catalyze two-electron reductions if they are adsorbed onto the surface of the electrode. The surface electrochemical behavior of adsorbed Co tetra-methoxy-phenyl porphyrin (CoTMPP) was investigated at different pH values [127]. The Co center one-electron redox process and the N4-ring two-electron redox process were recognized during the ORR. The adsorbed CoTMPP displayed strong activity for both O2 reduction and peroxide reduction. O2 can only be reduced to the stage of H2O2 in acidic conditions. In contrast, in neutral or alkaline solutions, the ORR was observed through a two-electron pathway in the low potential polarization range (0.13 to −0.5 V vs. SCE) and the overall four-electron pathway to H2O in the high potential polarization range (about −0.5 to −1.5 V vs. SCE). However, the major problem of these non-pyrolyzed transition metal macrocycles is demetallation from the active sites resulting from the collapse of the macrocyclic structure caused by peroxide and superoxide intermediates during ORR [128,129].Enlightened by the high ORR performances of the transition metal macrocyclic complexes, pyrolyzed NPM-N-doped carbon (M-N-C), prepared via the thermal treatment of either metal N4-macrocyclic complexes or the mixture of metal salts, carbon and nitrogen precursors, have been extensively investigated. Until now, the role of the transition metals in the M-N-C catalysts is still controversial, and numerous types of active sites were inferred to be responsible for four-electron ORR activity [126,128,130], while the investigation on two-electron ORR was limited.Jaouen and Dodelet [131] have confirmed that Fe or Co, together with N, followed by a pyrolysis treatment, resulted in catalytic sites highly active for two-electron or four-electron ORR. Fe-activated carbons are active for H2O production, and Co-activated carbons are reported to be responsible for reducing O2 toward H2O2. However, according to the review by Bezerra et al., 2008, both Fe–N–C and Co–N–C materials catalyze the ORR mainly through a four-electron process instead of a two-electron process [132]. Campos et al. [133] reported that the electrocatalytic performance of Co catalysts obtained from nitrogen-ligands is greatly affected by heat treatment. Once the heat temperature exceeded 500 °C, a drop in H2O2 selectivity resulted from the progressive formation of metallic cobalt particles. H2O2 reduction was almost invisible without cobalt or when the cobalt is in the form of a complex. Olson et al. [134] have studied the ORR mechanism of Co-polypyrrole-C (CoPPy/C) in alkaline media through structure-to-property analyze. Initially, two-electron ORR occurred on a Co-Nx type site to form HO2 −. The HO2 − species further reacted either to form OH− via electrochemical reduction or to form OH− and O2 by chemical decomposition. It was speculated that decorating CoxOy/Co nanoparticles appears to be the site of HO2 − destruction.In summary, the M-N-C catalysts (where M = Fe or Co) were thought to exhibit high activity towards the four-electron ORR following the peroxide formation-reduction pathway (O2 – H2O2 – H2O) in acidic media [135]. The active sites for H2O2 generation and reduction all exist in the catalysts. To obtain efficient M-N-C catalysts for H2O2 formation, the suppression of H2O2 further reduction is pivotal. Recently, researchers have tried to obtain highly active and selective M-N-C two-electron ORR catalysts by introducing other functional groups. Byeon et al. [136] demonstrated that co-doping of MnO nanoparticles together with Mn-Nx moieties into carbon are efficient for peroxide production λ of 74% at 0.2 V (vs. RHE). The favored two-electron ORR resulted from the increasing number of Mn-Nx sites inside the mesoporous N-doped carbon. Moreover, strong evidence showed that a further reduction of H2O2 was remarkably suppressed by adjacent MnO species. Li et al. [137] pointed out that the atomic Co-Nx-C sites improve the ORR activity but lack the selectivity for H2O2 generation, while OFGs promote the selectivity for the two-electron ORR but exhibit limited kinetics for ORR. Therefore, a rational combination of Co-Nx-C sites and OFGs into 3D interconnected conductive hosts was prepared by heating the predesigned precursor prior to HNO3 treatment (noted as Co-POC-O). The Co-POC-O exhibited excellent catalytic performance in KOH (0.1 M) with a high onset potential (0.84 V vs. RHE) and selectivity of over 80%. Moreover, the synergy effect of atomic Co-Nx-C reactive sites and OFGs was identified by the control samples with only atomic Co-Nx-C reactive sites (Co-POC-R) or only OFGs (POC–O) (Fig. 7b–f).Multiple metal oxides, especially group IV and V metals, are proven to be catalyst supports to replace carbon materials due to their abundant surface hydroxyl groups and chemical stability in acidic electrolytes [130,138]. However, their bulk form exhibit extremely low ORR activity resulting from the poor electrical conductivity and reduced reactive sites for oxygen species adsorption. Recently, alloying, forming highly dispersed nanoparticles, and reducing the crystalline sizes have been reported as an effective way to enhance the catalytic activities of metal oxides by increasing their exposed reactive sites, surface available defects, and electrical conductivity. Different carbon varieties were modified by various metal alloys or metal oxides with nanostructure to improve the two-electron ORR of carbonaceous electrocatalysts. Typically, the synthesized or purchased metal composites were supported onto the carbon by a modified polymeric precursor method [139] or sol-gel method [140]. Vulcan XC-72(R) with n = 3.1–2.5 and λ = 41–73% was used frequently in Santos' team as the support to study the catalytic capacity of two-electron ORR. After being modified with V2O5, SnNi, WO2.72, MnO2, or W@Au, the n of metallic nanostructure modified CB decreased to 2–2.6, while λ increased to 68–96% [141–146]. Among them, a core-shell type W@Au nanostructures (1% W@Au/CB) presented the highest selectivity toward H2O2 with n of ∼2 [147].Printex L6 CB (BET surface area of ∼250 m2 g−1, primary particle size of 18 nm and density of 1.8 g cm−3) with a λ of 65.3–68% and n of 2.6–2.7 was another important carrier to develop metal compounds nanoparticles modified carbonaceous electrocatalysts for two-electron ORR [148,149]. After preparation optimization, 4% CeO2/CB specimen showed a λ = 88% and n = 2.2 at −0.4 V vs. Hg/HgO, while Ta2O5/CB (5% (w/w) Ta/C) exhibited a λ = 83.2% and n = 2.3 at −0.3 to −0.5 V (vs. Ag/AgCl). A λ of 1% Pd/CB was over 80% at about 0 V (vs. Ag/AgCl) [150]. In another work, the synthesized rGO with mean particle size of 5.7 ± 0.8 nm showed λ = 73.7% and n = 2.52. With the Nb2O5 loading, Nb2O5/rGO composite (Nb/GO = 15% w/w) exhibited λ = 85.3% and n = 2.28 (−0.20 to −0.40 V vs. Ag/AgCl) [151]. In contrast, after loading 5% Fe3O4 nanoparticles, the λ of Fe3O4/rGO was only 62% [152].Based on the above research, the conclusions were that the decoration of metallic nanostructures would bring more acidic oxygen species or special morphology to the surface of the carbons, resulting in a more acidic and hydrophilic surface, and thus improving H2O2 generation by enhancing oxygen adsorption or oxygen diffusion.Some researchers claimed the doped heteroatoms are the real ORR active sites in the carbon structures [81,82,84,87], however, another group of research recently proved that defects created by heteroatoms might be the actual active sites [153,154]. Some novel carbon-based electrocatalysts were developed with the guidance of this newly established defect-driven catalysis mechanism. In this section, we will not discuss this mechanism debate. Instead, we will provide a new insight on the mechanism for ORR and give specific examples on promoting two-electron ORR by creating different defective carbon materials.Perfect and defective graphene clusters are summarized and plotted in Fig. 8 a. The Graphene with G585 divacancy defects (consisting of two pentagons and one octagonal) facilitated the O2 adsorption and lowered the following reaction energy barriers. DFT calculations showed that the point and line defects in graphene could tailor the local electronic structures and the distributions of nearby carbon atoms [155]. A pentagon ring located at the zigzag edge, the odd number of octagon ring, and fused pentagon ring line at the edge of the defective graphene are all proposed to be ORR active sites (Fig. 8). Hu and co-workers proposed that pentagon and zigzag edge defects are more reactive in four-electron ORR [156]. Moreover, defective graphene fabricated by N doping following a removal approach was a trifunctional catalyst for the four-electron ORR, hydrogen evolution reaction, and oxygen evolution reaction [157]. DFT models predicted N-doped into the graphene is beneficial to lower the adsorption energy of O2 but unfavorable for the reduction.These results proved the versatility of the defect-driven catalysis for electrocatalysis. However, until now, insufficient effort has been placed on understanding what defective active sites selectively promote the two-electron ORR. Only a few research applied the defect-driven catalysis mechanism to design and explore two-electron ORR materials.Carbonization of MOF-5 under H2 will transform sp 2 -C bonds to sp 3 -C bonds [158]. The harvested hierarchical porous carbons (HPC) exhibited H2O2 selectivity of 80.9–95% in acid solution (pH 1 and 4), with both defects and sp 3 -C acting as active sites of two-electron ORR. This research certified the un-doped and un-functionalized defective carbon also has potential for the two-electron mechanism. However, later researchers presented opposing views about the active sites. For example, Tao, et al. [159] confirmed that the defect sp 3 carbon atoms served as main active sites for four-electron ORR instead of two-electron ORR. Chen et al. [160], Kim, et al. [161] all stated the active sites of two-electron ORR are from sp 2 carbons.Chen et al. [160] have experimentally and theoretically investigated the defect and pore size effect to the electrochemical H2O2 synthesis. Two porous carbon catalysts (predominantly microporous/mesoporous carbon, Micro C, and Meso C) were synthesized from similar precursors but different synthetic procedures. Characterizations showed the two materials had similar chemical identity and content of defects but different pore structures (surface area, pore size, and pore volume) (Fig. 8b–e). Electrochemical tests showed both carbons exhibit high activity with an onset potential of about 0.7 V (vs. RHE) and selectivity of >70% toward H2O2 (Fig. 8f–i). The better performance of the Meso C was attributed to the easier mass transfer in mesoporous structures. Spectroscopic analyses revealed that microporous/mesoporous carbon (Micro C and Meso C) contain sp 2 -type defects that might be the reactive sites for the two-electron ORR. DFT calculations indicated that the pentagon edge, single vacancies (SV), and 585 double vacancies (DV) in 2D graphene sheets are too reactive to contribute to ORR. While some of the defect configurations (555-6-777, 555–777 line defect, and 555–777 point defects) were identified as having comparable activity with PtHg4 (Fig. 8j and k) (the ideal catalyst until now shown in Fig. 2b and c) for the two-electron ORR. Kim et al. [161] developed two N-rGO materials with different defect compositions. Based on the nuclear magnetic resonance technique and other X-ray-based tools, sp 2 carbon defects associated with epoxy or ether groups were identified to play a more critical role in promoting H2O2 formation than other functionalities, such as N defects or carboxylic acid edge sites.In recent research [162], the functionalized graphene sample with the largest electrochemical active surface area and the highest in-plane carbon defect density did not show the most efficient ORR activity and H2O2 selectivity, which was due to excess in-plane carbon defects that would lead to a conductivity decrease. In comparison, the graphene sample with the lowest in-plane carbon defect density had the highest H2O2 selectivity. These results emphasized that the optimization of graphene precursors defect site density is pivotal for adjusting the catalysts’ catalytic activity and reaction pathway. Defect modulation is regarded as a burgeoning strategy to regulate the electronic structure of carbon-based materials.The Electrode is the key component of the electrochemical cell because it contacts the electrolyte and provides the reaction sites for the reactants. Ideal carbon-based electrodes must possess a large surface area, suitable porosity, internal channels, and low electronic resistance for high electrochemical activity. The following chapters mainly introduce the development of formed carbon-based electrodes, including general electrode preparation methods, various physical and chemical modification methods, and their applications. There are two kinds of modification methods for enhancing two-electron ORR activity of the electrodes: (1) modify the pristine electrodes (graphite plate & rod, graphite felt, reticulated vitreous carbon, and activated carbon fibers, Fig. 9 a) by physical/chemical methods to tune the surface properties or to load effective two-electron ORR functional groups (N, O) onto the electrode surface; (2) use the pristine carbon electrodes as support and introduce other highly active & selective carbon materials, such as CNTs, CB, and graphene (Fig. 10 a). For example, carbon felt and carbon paper are often used as soft current collectors for in situ construction of electroactive nano-carbon structures due to their simple handling and excellent conductivity.Various electrochemical testing techniques can be applied to investigate the electrochemical activity and interface properties of formed electrodes, including linear sweeping voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry (CA), and scanning electrochemical microscopy (SECM) in the three-electrode system [163]. The above test results can be used to support the demonstration of electrode performance and mechanism analysis. In our estimation, the most critical parameter is the H2O2 yield and the current efficiency of the electrodes. In order to contrast the performance of electrodes clearly and intuitively, the expression of H2O2 yield in this paper was uniformly transformed to mg h−1 unless specially noted. The current efficiency (CE, also known as Faradaic efficiency) of the cathodes was calculated from Eq. (25): (25) CE = n F C H 2 O 2 V I t × 100 % where n is the number of the transferred electrons from O2 to H2O2 (n = 2), F represents the Faraday constant (96,486 C mol−1), CH2 O2 represents the concentration of H2O2 (mol L−1), V stands for the bulk volume (L), I is the current (A), and t is the time (s). The performances of all mentioned cathodes are summarized in Table 3 , including the electrolyte conditions, operation mode, H2O2 yield, and CE.Graphite has excellent chemical stability and electrical conductivity. Furthermore, it is relatively easy to get at a low price. Early graphite-based electrodes were usually made of graphite rods or plates (Fig. 9a) [164,165]. In 2010, a spectrographically pure graphite (SPG) rod was applied as a cathode in a microbial fuel cell (MFC) without any energy input to synthesize H2O2 with a yield of 0.46 mg h−1 and the CE of 69.5% [166]. 3D graphite particle electrodes (GPE) prepared via extrusion-spheronization method from graphite-polytetrafluoroethylene (PTFE) were also used in MFCs to generate H2O2 [167]. Although anodic COD removal, electricity generation, and cathodic H2O2 production were realized in one single system, H2O2 yield was relatively low as 0.13 mg h−1. Acidic pretreatment on raw graphite powder promoted the surface area and the OFGs content of GPE to increase the H2O2 yield of MFC by 41% (0.19 mg h−1) [168]. Whereas the yield of H2O2 in the MFCs was still sparse due to the relatively low current density provided by the bio-anode.In the abiotic electrochemical system, the H2O2 yield mostly depends on the performance of the cathode. Thus research efforts aimed to improve H2O2 yield by modifying graphite electrodes.Polymer-modified electrocatalysts are very promising materials for ORR [169]. Consider quinonoid compounds were often used to modify electrodes because then can stop the ORR at the peroxide stage [170,171], Polypyrrole/anthraquinonedisulphonate (PPy/AQDS) composite film modified graphite produced H2O2 with the yield of 4.2 mg h−1 and CE of 64–73% at −0.65 V (vs. SCE) [172]. Quinonoid compounds were also employed to modify other kinds of carbon-based cathodes, and they will be discussed in the following chapters. In 2011, commercial MWCNTs (surface area 233 m2 g−1, outer diameters 8–15 nm, inside diameters 3–5 nm) were immobilized onto graphite surface by Khataee and co-workers [173]. 1.4 mg h−1 of H2O2 was yielded while a CE of about 2.1% was achieved at 100 mA. Conducting polymers, such as Ppy and polyaniline (PANI), are widely used as catalysts or catalyst supports for ORR due to their stability, ease of electro-polymerization, and high conductivity [174]. Rabl et al. [175] presented results that Ppy and PANI coating on carbon electrodes considerably improved the H2O2 selectivity by preventing undesired side or further reactions of H2O2 to H2O. Based on the above properties, Ppy/MWCNT and PANI/MWCNT nanocomposites were fabricated and electro-polymerized onto the graphite electrode or stainless steel [176,177]. ORR activity of different concentrations of MWCNT was investigated. The PANI/MWCNT nanocomposite modified stainless steel cathode with MWCNT content of 2% (wt) generated 1.1 mg h−1 of H2O2 with a CE of about 42% at −0.6 V (vs. SCE), 2.5 times higher than raw stainless steel, which was due to the superior electrocatalytic activity of MWCNT. Meanwhile, 2.5% w/w Ppy/MWCNT had the highest ORR electrocatalytic activity and H2O2 yield was increased by 70% to 3.4 mg h−1 at −0.55 V (vs. SCE). Recently, Chu, et al. [178] fabricated multi-layer super-hydrophobic cathode by mixing graphite powder with CNT and PTFE. The hydrophobic property of carbon powder and heat treatment induced strong aerophily of cathodes, by which the cathode could adsorb more air bubbles under the air aeration than the hydrophilic cathode, and it exhibited an ideal performance for H2O2 generation at 37.6 mg h−1 with an observed CE of 40% with 60 min of electrolysis.H2O2 yield on graphite-based cathodes is not usually satisfactory because of the small surface area of graphite rods, plates, or particles. The graphite cathodes could only be used under very low current density in the early prototype design. Although modification led to a significant improvement in the performance of ORR, the H2O2 yield and CE of graphite-based electrodes were still relatively low for application. Recently, 3D porous electrodes are becoming increasingly popular to counteract the low yield limitations of 2D electrodes in the electrochemical cells [179].Graphite felt (GF) or carbon felt (CF) are the most frequently used commercially available carbon materials. (GF is obtained from the graphitisation of the CF). As a typical 3D electrode (Fig. 9b), GF has excellent features such as good electrical conductivity (370.4 S m−1) [180], high volumetric surface area (22,100-22,700 m2 m−3), good mechanical integrity, high chemical resistance and stability, it is easy to fabricate and scale-up, and has a low cost [181]. It has been extensively used for H2O2 production in the field of EAOPs for wastewater treatment.Raw CFs (provided by Carbon-Lorraine Company) were applied as electrodes in EAOPs to electrogenerate H2O2 and fabricate an electro-Fenton system to successfully degrade 2, 4-dichlorophenoxyacetic acid, p-Nitrophenol, pentachlorophenol, methyl parathion, malachite green, and phenol [182–187]. In early studies of EAOPs, pollutant degradation pathway and intermediates, optimal technical and economical degradation conditions, the kinetic mechanism of degradation were the major concern, rather than the performance of H2O2 production. Nevertheless, as demand continues to increase, researchers are demanding higher performance of GF/CF based cathode, which gave rise to surface modification or coating techniques.OFGs were verified to increase the hydrophilicity of carbon materials and thus promote the transfer of both electrons and dissolved oxygen [188]. Furthermore, OFGs or defects associated with OFGs are identified as two-electron reactive sites of carbon materials [115,118,161]. Therefore, increasing the number of OFGs on the electrode surface becomes the primary choice for electrode modification.Zhou et al. [189,190] used hydrazine hydrate to increase O and N functional species content on the GF microfilaments surface. With the stronger hydrophilicity and faster electron transportation of modified GF, the H2O2 yield improved 160% to 11.5 mg h−1 with CE of 82% at −0.65 V (vs. SCE). Ou et al. [191] successfully loaded O, N, and S-containing functional groups onto the GF by modification through the concentrated H2SO4, KMnO4, and NH3 activation. With better hydrophilicity and conductivity, the modified GF realized 47.9 mg h−1 of H2O2 yield with CE of 11.8% at 640 mA, which was 73% higher than the raw GF. Wang et al. [192] activated GF with KOH at a high temperature (900 °C) to harvest electrodes with higher surface area, higher hydrophilicity, and more OFGs for higher a H2O2 yield. The H2O2 yield of activated GF reached 40 mg L−1 h−1 (volume unknown) at −0.7 V (vs. SCE). After activating the GF cathode, the apparent rate constant of dimethyl phthalate degradation in the electro-Fenton system increased from 0.02 min−1 to 0.20 min−1. However, large amounts of energy were required due to the high temperature in this method. In 2020, Lai et al. [193] demonstrated GF cathodes treated with NaOH at a lower temperature (400 °C) facilitated the OFGs loading and enhanced hydrophilicity. As a result, the modified GF realized about 30 mg h−1 of H2O2 yield with CE of 94.6% at 50 mA. In order to develop simple methods to achieve large-scale modification, Jiang, et al. [194] investigated the HNO3 and KOH reagents treatment under a much milder temperature (70 °C). Though both methods could increase the OFGs content, surface area, hydrophilicity, and HNO3 treated GF exhibited better performance to synthesis; resulting in 742.5 mg h−1 of H2O2 at 20 V applied voltage, 8% and 69% higher than the KOH modified GF and unmodified GF, respectively. Acid pretreatment was also applied to activate GF via a low-cost and simple gaseous acetic acid activation [195]. The H2O2 yield of activated GF is enhanced by 6 fold–10.3 mg h−1 with the CE of 75% at −0.7 V (vs. SCE) due to higher contents of macropores, micropores, sp 3 carbon bonds, defects, and OFGs.Anodization was applied on GFs to increase the H2O2 yield of electrochemical modified GFs by 170% due to the generation of carbonyl, carboxyl, quinone, and ester groups [196]. However, CE of anodized GF decreased from 87% to 79%, indicating the anodizing modification encouraged both two-electron ORR and four-electron ORR. Electrode polarity reversal was also applied for situ anodically modification of GF to improve the hydrophilicity of the electrode surface and O2 mass transfer [197]. With high contents of carbonyl and hydroxyl groups, the H2O2 yield of GFs increased by 2.9 times, reaching 8.1 mg h−1 at 100 mA.Comparing three oxidation modifications, Wang, et al. [198] discovered that the H2O2 yield of electrochemically oxidized carbon fiber generated was 9 mg h−1, 11.6 times that of raw CFs, and 16–98% higher than H2O2 oxidized CFs and Fenton (·OH) oxidized CFs. However, after 10-rounds of continuous runs, the H2O2 production of electrochemical modified GFs decreased by 42–61% due to the loss of OC–OH species, and the destruction of the electrode structure. In the most recent research, the activity and selectivity of GF electrodes were improved for H2O2 electrogeneration by integrating chemical oxidation, electrochemical oxidation, and thermal treatment [199]. It was reported that HNO3 oxidation facilitated OFGs and defects formation, while electrochemical oxidation favored carboxyl removal and carbonyl groups formation. Moreover, the following thermal treatment engendered the rebounded hydrophilicity and thus enhanced the activity. The modified electrode (GF–HNO3–EC-N2) benefited from the above treatments, and exhibited a 5-fold higher H2O2 yield, 9 mg h−1 with CE of 86% at 0 V (vs. RHE) than the pristine samples.Highly active and selective materials were also applied onto GFs to improve the H2O2 yield. For example, ZIF-8 was carbonizated under a N2 atmosphere to load N-doped porous carbon onto the GF [200]. The existing graphitic-N and sp 2 carbon promoted the electron transfer between catalyst surface and O2 molecules, as well as accelerating the ORR. With optimal condition, H2O2 yield increased 10 times and reached 6 mg h−1 with CE of about 9% at 100 mA. Another N-doped carbon modified GF prepared by electro-deposition of PANI, carbonization, and activation was applied in electro-Fenton to generate and activate H2O2 to remove 85% of phenol in 180 min with a residual H2O2 accumulation of 4.6 mg h−1 [201].Vulcan XC-72R CB was deposited on GF to increase the surface area as well as the pore volume of the electrode. This modification improved the H2O2 yield by about 10.7 times to reach H2O2 47.3 mg h−1 with CE of 74.6% at 100 mA [202]. This was a milestone in developing a highly efficient modified GF electrode, because H2O2 yield was improved to tens of mg h−1 at 100 mA with CE far more than 10%. Vertical-flow electro-Fenton reactor, peroxide-coagulation system, and flow-through electro-peroxone systems were developed based on this CB modified GF electrode to realize different wastewater treatment functions [203–205]. Most recently, an MWCNT & CB co-modified GF was fabricated [206]. Although the hydrophobicity was slightly increased (contact angle increased from 145.2° to 154.9°), higher BET surface, pore volume, and OFGs content caused by modification still improved the ORR activity. As a result, a comparable H2O2 yield of 48 mg h−1 with CE of 63% was obtained at 120 mA.In the past 7–8 years, graphene has been regarded as a promising material for H2O2 electrogeneration. In 2016, Mousset et al. [207] tested three commercial pristine graphene materials (2D graphene monolayer, 2D graphene multilayer, and 3D graphene foam) as electrodes for H2O2 generation. Although 3D graphene foam exhibited the least hydrophilicity, it could surprisingly achieve the highest H2O2 generation,0.6 mg cmcat −3 at −0.61 V (vs. Ag/AgCl), due to the contribution of higher surface area as well as superior electrical conductivity. However, compared with other carbon materials, the H2O2 electrogeneration from graphene was not satisfactory, indicating that pristine graphene itself was not the preferred electrode for two-electron ORR. Graphene was mostly used for coating various substrates to improve the surface area and the conductivity of raw electrodes and thus increase the catalytic performance.Le et al. [208] loaded homogeneous dispersion of GO onto the CF and investigated the effect of electrochemical, chemical, and thermal reduction of GO on the electrodes performance. The reduction of GO was beneficial to H2O2 production because it enhanced hydrophilic characteristics and conductivity, as well as created more active sites. Though thermal reduction exhibited the highest electrochemical properties, electrochemical reduction had both high performance as well as low cost, which is regarded as the best modification method [209]. Analogously, GO was drop-casted onto a substrate disposed of liquid crystal display (LCD) glass, and then electrochemically reduced to form an ErGO-LCD electrode [210], which generated H2O2 with the yield of 2.3 mg h−1 at −1.5 V (vs. Ag/AgCl).Encouraged by the above results, electrochemically exfoliated graphene (EEGr) was utilized as the functional coating material to decorate carbon cloth (CC) [211] and carbon-fiber brush [212]. After the optimization of EEGr and Nafion concentration, the EEGr decorated cathodes increased by 40–100% in H2O2 yield, and 26.3–106% in phenol degradation rate in EF processes.Quinone-functionalized electrochemically exfoliated graphene (QEEGr) was coated on the CC electrode to generate 5 mg h−1 H2O2, which was 9 times higher than the unmodified CC [213]. The presence of the quinone group was thought to facilitate two-electron ORR, thus initiating H2O2 generation without compromising the electrode electrical property. Moreover, QEEG-Fe3O4 coated CC composite electrode could continuously generate reactive oxygen species for complete degradation of Bisphenol A.EEGr and Vulcan XC-72R CB co-modified GF cathode were developed to generate 38.5 mg h−1 of H2O2 at −0.9 V (vs. SCE) in a neutral solution, which was 2 times that of the unmodified cathode [214]. N-doped graphene (N-EEGr) was derived by mixing the EEGr with ammonium nitrate followed by calcination under a N2 atmosphere to activate H2O2 to ·OH for organics degradation, rather than increase the electrochemical generation of H2O2 [215]. A significantly different result was obtained with another N-EEGr modified electrode by loading the mixture of CB, Nx-EEGr, and PTFE on the GF [216]. The Nx-EEGr was prepared by annealing of melamine and graphene mixture under a N2 atmosphere, where x represented the mass ratio of melamine to graphene. The optimized N3-EEGr-CB-GF cathode improved H2O2 yield to 86 mg h−1 due to the generated active graphite N and pyridinic N species and CC. Moreover, the presence of pyridinic N was able to catalyze H2O2 to produce ·OH, which is beneficial to overcoming the effect of the initial pH on EF [201,217].RVC is a disordered glassy porous carbonaceous material with a solid foam network structure (Fig. 9c). RVC has an exceptionally high surface area, high void volume, rigid structure, and low resistance to fluid flow [218]. These properties encouraged the applications of RVC in diverse areas such as sensors and monitors, chemical catalyst supports, and energy conversion [219]. Over the last 10 years, the corresponding research showed that the performance of RVC as cathodes for H2O2 synthesis are comparable or even better relative to the GF electrode.Coria et al. [220] investigated the mass transport of GF, RVC and boron-doped diamond (BDD) cathodes during two-electron ORR in a filter-press electrolyzer to discover that the performances of porous GF and RVC with higher limiting currents were obviously better than the non-porous BDD cathode. Petrucci et al. [221] confirmed that electrogeneration of H2O2 on a RVC electrode was 210% and 60% higher than that of graphite and CF, respectively, under 5 mA cm−2. This observation was due to the better oxygen diffusion and larger reactive surface from a porous 3D structure.Recently a RVC electrode modified by anodic polarization was developed for drinking water disinfection. The H2O2 yield was 6.4 mg h−1 with a CE of 43% at 24 mA, which was about 4 times of the unmodified RVC cathode [222]. The modification and application of RVC electrodes has gradually gained attention [223].ACFs are considered as a group of advanced porous materials with a fiber shape and a well-defined porous structure (Fig. 9d) [224]. Except for the extremely large surface area (2000–2500 m2 g−1), the micropores of the ACFs are directly exposed to the surface, which reduces the mass transfer resistance and enhances the adsorption of various compounds. ACFs and their modification composites were extensively utilized in environmental remediation, such as the adsorption of organic and inorganic pollutants in water/air, capacitive deionization, the degradation of organic pollutants, and microbial decontamination [225]. However, only a few studies have focused on the catalytic production of H2O2 from ACF.Commercial ACF felt was utilized in an electro-Fenton system to degrade Acid Red 14 and levofloxacin [226,227]. Although almost 100% of Acid Red 14 or levofloxacin and 61–70% TOC were removed, the H2O2 yield property of the ACF cathode was only about 3.6 mg h−1 at 500 mA with poor CE of 1.1% in the absence of Fe2+ during 180 min of electrolysis, which means most of the electricity was wasted. Similarly, a commercial ACF cathode was utilized in an electro-Fenton system for cationic red X-GRL degradation with a maximum H2O2 yield of 4.8 mg h−1 and CE of about 1% [228]. Zhang et al. [229] compared the electrocatalytic properties of two ACFs, which shared similar pore volumes as well as pore diameters but varied BET surface areas. Results showed that ACF with a higher surface area was correlated to faster H2O2 and ·OH accumulation. However, the better ACF only had a H2O2 yield of 4.1 mg h−1 and CE of about 1.8%.In 2018, Zhou et al. [230] proposed an activated carbon-stainless steel mesh composite cathode (ACSS), which integrated H2O2 electrogeneration and activation together with pollutants adsorption. Although H2O2 yield was only 1.9 mg h−1 at 100 mA with a CE of 3%, the ACSS enabled the iron-free electro-Fenton feasible under neutral pH to remove 61.5% of reactive blue after 90 min. Ren et al. [231] successfully prepared a novel multilayer ACF-based composite cathode with rGO as the conducting layer and OMC as the oxygen diffusion channel. The electroactive surface area, oxygen diffusion rate and electron transport rate were all increased, and the H2O2 yield of ACF@rGO@OMC electrode reached 2.8 mg h−1 with CE of 40.4% at −0.7 V (vs. SCE).In summary, on a few activated carbon-based cathodes have been employed for H2O2 production and until its catalytic selectivity is substantially improved the potential for further applications will be limited [232].Generally, cathodes mentioned in the last chapter are immersed in the electrolyte. The gaseous O2 is dissolved into the electrolyte by aeration, and then the dissolved O2 diffuses with the electrolyte into the internal pores of the cathodes and reacts at active sites (Fig. 10b). However, the immersed cathodes are usually unable to maintain high H2O2 yield and CE at large current densities (usually >10 mA cm−2) because of the low solubility and inferior O2 mass transfer [164]. Lower CE of immersed cathodes results in the waste of the electricity and brings hidden safety trouble due to higher parasitic HER [233]. The birth of gas diffusion electrode (GDE) solved these challenges. As a kind of film electrode, GDE often consists of a reactive catalyst layer (CL), a gas diffusion layer (GDL), and an optional current collector. Applied for the electrogeneration of H2O2, the hydrophilic CL, which faces to the electrolyte, provides the reactive sites for the ORR, while the hydrophobic GDL facing to reactant gas provides a stable gas diffusion channel for the oxygen towards the catalyst layer and prevents electrolyte leakage. CLs are usually fabricated from the mixture of carbon-based catalyst powder and the binder, followed by being coated/rolled/painted/sprayed onto the gas diffusion layer. Based on the species of the main catalyst in the CL, the GDE can be sorted into carbon black-based-, carbon nanotube-based- and hybrid carbon GDE.The most widely utilized carbon materials for GL preparation are the Vulcan XC-72(R) and Printex L6 CB. GDE has been developed by E-TEK by painting Vulcan XC-72 CB and PTFE wet paste mixture uniformly onto a face of the carbon cloth. The H2O2 yield of Vulcan XC-72 CB-based GDE was 82.7 mg h−1 with CE of about 29% at 450 mA. Based on this electrode, several different EAOP systems were developed, and numerous target POPs were successfully degraded [18,234–240]. Although the advent of GDE substantially altered the mass transfer of O2, there is still much room for CE further improvement by promoting the CL catalytic activity and selectivity. Developing from Vulcan XC-72R CB and PTFE via a rolling method, the harvested GDE produced 158 mg h−1 H2O2 at 520 mA with CE of 48%. Unlike traditional GDE, the CL of the electrode simultaneously acted as a GDL [241]. The GDE was also prepared by pressing and sintering a series of metal oxide modified Vulcan XC-72R CB mentioned in Chapter 4.3.2. With 0.2 bar pressurized pure O2 supply, H2O2 yield of W@Au/CB GDE, CeO2/CB GDE and MnO2/CB GDE were 21, 51, and 68 mg h−1 at −1.1 V (vs. Ag/AgCl) [141,145,147]. Meanwhile, 102 mg h−1 of H2O2 was generated from WO2.72/CB GDE at −1.3 V [144], and 102 mg h−1 of H2O2 was generated from V2O5/CB GDE at −1.5 V [142]. Nevertheless, CE was not calculated or mentioned in those studies, which makes it hard to compare those GDE with others’ intuitively.Compared to Vulcan XC-72 CB, Printex L6 CB was demonstrated to be a better choice for H2O2 generation due to more oxygenated acid species content and higher hydrophilicity [242–244]. When the aforementioned 5% Ta2O5/CB material was made into GDE, the H2O2 yield was 11 mg h−1 at −1.0 V (vs. Ag/AgCl) [148]. As quinones have been investigated as efficient catalysts for the improvement of two-electron ORR [245], GDEs were developed by modifying CB with different amounts of tert-butyl-anthraquinone (TBAQ) [246]. According to RRDE results, 1% TBAQ/CB showed the highest selectivity (89.6% with 2.2 electrons exchanged). The obtained GDE realized a H2O2 yield of 80 mg h−1 at −1 V (vs. SCE). Rocha et al. [171] investigated the electro-activity of various quinone compounds (acenaphthoquinone (APQ; acenaphthylene-1,2-dione), menadione (MDA; 2-Methyl-1,4-naphthoquinone), and Alizarin Red S (ALZ; 1,2- dihydroxyanthraquinone)) modified CB for H2O2 production in an acid medium. The results showed that 1% of 1,2-dihydroxyanthraquinone was efficient for two-electron ORR and the resultant GDE realized a H2O2 yield of 77.1 mg h−1 at 1900 mA (CE = 6.4%), which was 116% higher than the unmodified CB GDE.In 2009, Zarei et al. [247] developed CNT-based GDE by bonding the ointment mixture of commercial MWCNT and PTFE to 50% PTFE-loaded carbon papers and calcined at 350 °C under N2 atmosphere. H2O2 yield was 24 mg h−1 with CE of about 38% at 100 mA. The prepared GDE was utilized for C.I. Basic Yellow 2 removal via peroxi-coagulation, as well as C.I. Basic Red 46 degradation through the oxalate catalyzed photo-electro-Fenton [248,249].During the synthesis of modified GDE, the carbon materials also serve as a template or a platform that will disperse or adsorb the modifying reagents. The subsequent heat/hydrothermal treatment caused a uniform distribution of active sites associated with modified moieties. Therefore, the impact of carbon supports was different. Considering the contribution of quinone compounds toward two-electron ORR, Lu, et al. [250] investigated the ORR activity of different TBAQ modified carbon materials (carbon aerogel, CNT, CB, graphene doped CB) to discover the TBAQ modified CNT exhibited the highest H2O2 yield (30.1 mg h−1 at 50 mA), which was 27% higher than the unmodified CNT GDE and 9–56.4% higher than other TBAQ modified carbon GDEs. The characterization results showed more C–C sp 3 carbon, and OFGs content together with the mesoporous structure resulting in the outstanding performance of the TBAQ modified CNT GDEs.According to the former results in Chapter 4, Co-based catalysts in the form of Co oxides, Co chalcogenides, or Co nanoparticles are the most efficient electrocatalysts for enhancing two-electron ORR in the acidic medium [133,251–253]. The spraying of CoS2-MWCNTs was employed to manufacture GDE [254]. In the galvanostatic test, the H2O2 yield of CoS2-MWCNTs GDE reached 95 mg h−1 with CE of about 50%. CoS2 particles were proven to play a significant role in enhancing the two-electron ORR as well as preventing H2O2 from further reduction to H2O to some extent. Enlightened by the above research, (Co, S, P)-decorated MWCNTs were prepared through a hydrothermal route [255]. The electrocatalyst was mixed with 2-propanol and Nafion. The mixture ink was sprayed onto a carbon cloth several times together with a carbon microporous layer to form the GDE. Compared with undecorated MWCNT GDE, (Co, S, P)-decorated MWCNTs GDE enhance the electrocatalytic H2O2 production to about 225 mg h−1 with CE of 51–53% at 800 mA.Recently, GDE was fabricated by rolling, pressing, and calcining the mixture of P-doped CNTs and PTFE [256]. The successful doping of P increased the activity of CNT, which exhibited about 0.2 V more positive onset potential and 100% higher current density at −0.8 V (vs. SCE). However, P-doping decreased the selectivity of two-electron ORR with n changing from 2.6 (CNTs) to 3.06 (P-CNTs). Although P-CNTs tended to four-electron pathway, the P-CNTs GDE still had excellent performance with a H2O2 yield of 207 mg h−1 and CE of 88.5%, which was obviously higher than CNTs-GDE (67 mg h−1 with CE of 64.7%). This result demonstrated the difference between the RRDE calculated selectivity and electrolysis calculated CE, which in fact was the difference between the theoretic selectivity of the material and the actual selectivity of the electrode in operation.Recently, researchers began to take advantage of the structural and physical properties of different carbon materials and started to investigate the mixture of different carbon materials as a catalyst. Carbon composite materials containing at least two kinds of carbons were reviewed in this section.Xu et al. [257] investigated the performance of graphite-PTFE GDE and rGO & graphite-PTFE GDE to discover H2O2 yield of rGO & graphite-PTFE GDE (12.5 mg h−1 with CE of about 40% at 24 mA) was nearly four times higher relative to graphite-PTFE GDE, due to the addition of rGO improved electrochemical conductivity and mesopores contents [258].Typically, rGO aggregation is an overlooked issue during the fabrication of electrodes. Because of the strong hydrophobic interactions between nano-sheets, rGO can aggregate easily in solution or in the drying process [115,259], which substantially reduces the accessibility of the reactants toward rGO basal planes on the electrodes [118]. CNTs stacked between rGO nano-sheets prevent the rGO from restacking to increase basal space and bridge the defects to enhance the electrical conductivity [260]. Liu et al. [261] fabricated a novel N-rGO & CNT-PTFE GDE by doping N atoms in the rGO & CNT composite. RRDE results showed that the onset potential of N-rGO & CNT shifted positively in the range of 146–363 mV with reference to bare rGO, CNT, and graphite. Moreover, N-rGO & CNT-PTFE GDE generated 1 mg h−1 H2O2 at a relatively positive potential (−0.2 V vs. SCE), which was 2–10 times higher than the reference GDEs.Chen et al. [262] reported that trace MWCNTs could construct “electron-bridges” interconnecting the CB particles and thus increase the conductivity and porosity of CB. Therefore H2O2 yield of CB & MWCNT-PTFE GDE was 41.3 mg h−1 with a high CE of 65.1% at 100 mA. However, once poorly conductive AC embedding into the hybrid material, the CB particles would be huddled to some scattered aggregates to destroy the bridges, resulting in poor porous structure and conductivity. Furthermore, H2O2 is prone to be further reduced to H2O with the presence of AC.Zhu et al. [263] mixed graphite powder with g-C3N4 to fabricated g-C3N4@GDE via the rolling method. Characterization results showed that hydrophilicity could be increased by a g-C3N4 modification, which could induce fast electrolyte penetration to the cathode surface. With moderate g-C3N4 mixing, g-C3N4@GDE generated the highest H2O2 (457.5 μM) compared with pure graphite GDE (328.2 μM) and pure g-C3N4 GDE (302.2 μM).In summary, multiple GDEs based on newly designed high-performance catalysts were developed, which could realize maximum H2O2 yield in the case of the excellent three-phase interface provided by GDE. Nevertheless, some technical issues remain, which require special attention, such as flooding issues caused by improper water management [264]. When the porous GDE was supersaturated, the electrolyte hindered the ability of oxygen to diffuse towards the active sites and thus destroyed the three-phase interfaces (TPIs) equilibrium and decreased the electrode performance. Characterizing, measuring, and solving flooding issues are still challenges for both two-electron and four-electron reactions [265,266].Compared with the immersed electrode, GDE greatly improved the utilization rate of oxygen during H2O2 production relative to the CE of the electrode [164]. However, the aforementioned GDE always needs pressurized air or pure oxygen gas, which increases the construction cost, reactor complexity, and safety. In order to reduce costs, while ensuring the satisfied cathodic performance in engineering applications, a novel sandwich-like air self-diffusion cathode manufactured via rolling method was developed by our group in 2015 [267], which was a deformation and expansion of our formerly developed AC-PTFE air cathodes for four-electron ORR [268–272]. Instead of expensive and multistep-prepared CNTs, OMC or graphene, commercial carbon powder Vulcan XC-72R CB and graphite were used to fabricate CL. The mixture of carbon powders and PTFE was rolled onto a stainless steel mesh (SSM), while the CB-PTFE breathable waterproof GDL was rolled onto the other side. This air self-diffusion cathode is a practical design where oxygen in the air can actively diffuse through GDL to the internal interface of the CL, which needs no aeration or pressurized gas to generate H2O2. In order to distinguish our air cathode from GDE, the former is named “air-breathing cathode” (ABC).During the exploration of different proportions of CB and graphite in CL, it was found the pore area and volume of pure CB-PTFE electrode were 11.6 and 4 times of those of pure graphite-PTFE electrode, but the pore diameter of the former was only 31.6% of the latter. Both electrodes exhibited poor performance in electrogenerating H2O2 due to the indirect four-electron reaction or lack of active sites [267]. When two kinds of carbon materials were mixed together, the hybrid carbon-PTFE CL had moderate pore diameter, area, and volume. With the optimal mass ratio of CB to graphite (1:5), H2O2 yield reached 50 mg h−1 (with CE of 92%) in an electrolysis cell at 86 mA. Aimed at improving the two-electron ORR activity for efficient H2O2 generation, Zhao et al. tuned catalyst mesostructure and hydrophilicity/hydrophobicity by adjusting PTFE content in CB & graphite-PTFE CL and avoiding calcination under atmospheric conditions [273,274]. It was found that the electroactive area was more relevant to the specific surface area of the 3–10 nm mesopores rather than the total BET surface area, and the electroactive area decreased from 41 cm2 gcat −1 to 19 cm2 gcat −1 with PTFE increased from 0.57 g to 4.56 g. Higher PTFE content led to an excessive supply of H+ and induced the H2O2 decomposition and decreased the hydrophobicity to limit the amount of O2 diffused to catalytic sites. The ABC PTFE0.57 with the lowest PTFE content exhibited super-hydrophobic, highest H2O2 yield of 74.6 mg h−1, and highest CE of 84% at 140 mA.These researchers emphasized the balance among the pore diameter, specific area, and volume, as well as the balance between hydrophilicity and hydrophobicity. In former studies, people devoted themselves to increasing the surface area together with the hydrophilicity of the electrode by numerous treatments [192,197,208,214,275]. However, conflicting results were obtained in our research. Compared with the coating or sparging method, the rolling method was an advanced method for catalyst layer fabrication because it could load more active sites onto the unit area [233]. On the one hand, higher active sites density improves the activity of the cathode. On the other hand, excessive active sites would further reduce H2O2 in the porous rolling cathode. Dissimilar to the immersed electrodes, our rolling ABC mainly utilized the O2 from the air rather than the dissolved O2 in the electrolyte by aeration. Consider the two-electron ORR need electron, proton, and O2 as reactants, and each of those is from solid, liquid, air, respectively, the solid-liquid-air three-phase equilibrium inside the porous catalyst layer is critical to the high performance of the electrode. Excessive pursuit of hydrophilicity in the cathode would induce the three-phase interfaces out of balance and engender the decrease of H2O2 production. Recently, the hydrophobicity property of the electrode has attracted a growing number of research, and H2O2 yield, as well as stability of the newly developed hydrophobic/super-hydrophobic/super-aerophilic electrode, have been significantly improved [276–278]. Some scholars also preferred to use more hydrophobic materials as substrates for GDE [124].Based on our optimized highly efficient ABC, multiple electrochemical systems have been built for various wastewater (cyanobacterial boom water [279], formaldehyde-containing wastewater [280], phenolic wastewater [163], antiviral drug-containing wastewater [281]) treatments as well as sterilization and disinfection [274]. In conclusion, our series of research demonstrated highly efficient production of H2O2 via two-electron ORR can be realized by rolling hybrid carbon cathodes without using noble metals or other complex chemical promoters. The cheap commercial raw carbon materials combined with an easy manufacturing process makes the rolling ABC potential for large-scale application of in situ electrosynthesis of H2O2.In recent years, the active gas diffusion electrode has attracted growing attention, and multiple electrodes were developed with various names. rGO@graphite-based air diffusion cathode (rGO@GADC) was applied in an electro-UV/H2O2 system for penicillin sodium degradation to remove 91.1% of penicillin sodium with 56.8% of mineralization current efficiency [282]. A novel “floating cathode” with half submerged inside the electrolyte synergistic utilized gaseous O2 from ambient air and the dissolved O2 in the electrolyte. With the formation of TPIs, the optimized floating GF electrode realized a H2O2 yield of 13.3 mg h−1 with CE of 21% at 100 mA, which was 4.3 times of the submerged GF electrode [283]. A super-hydrophobic natural air diffusion electrode (NADE) was developed by coating CB onto CF followed by calcination. After the catalyst loading optimization, H2O2 yield reached 518.5 mg h−1 with CE of 66.8% at 1200 mA [277,284]. In the catalysis materials fabrication field, researchers are also beginning to use self-diffusing substrates for material modifications to reduce electrode complexity [285,286].The electrosynthesis of H2O2 via two-electron ORR provides an alternative to the mature AO process or the emerging direct synthesis and photo-catalysis. The increased usage and decreased cost of renewable electricity will transform the chemicals industry. However, current developments of the cathode are still limited by at least four major challenges, including designing catalyst materials with high activity and selectivity, establishing theoretical calculation models closer to the actual experiments, fabricating materials and electrodes by simple and low-cost methods, maintaining stability over the long-term operation. 1) Designing catalyst materials with high activity and selectivity Designing catalyst materials with high activity and selectivityTwo major challenges, namely improving activity and selectivity, need to be addressed before carbon-based electrocatalysts can compete with the current state-of-the-art. There is plenty of scope for the improvement of pristine catalyst materials via changing the substrate composition, architecture & defects, and surface property. However, the catalysis mechanism and critical active sites for ORR on carbonaceous electrocatalyst are still confusing and controversial [138,287]. There are remaining debates and discrepancies on the intrinsic properties of the catalytic sites, the effect of heteroatom/functional groups/defect, micro/mesoporous, hydrophilic/hydrophobic nature of the materials to the selectivity to the two-electron ORR process. This ambiguity in determining the true nature of two-electron ORR active sites in those carbon-based catalysts hinders the development of efficient catalysts. The current cutting-edge research is to identify the influence of a single factor on ORR catalytic activity and selectivity. Further development is still primarily based on trial-and-error approaches until now [288], and it is still difficult to realize the controllable synthesis of various defective or doping carbon materials. Meanwhile, new mechanisms are emerging continuously: In contrast to the ordinary ad-O2 mechanism, Chai, et al. [39] found O2 adsorption is not required in the new mechanism. Instead, the H atom on carbon catalyst is abstracted by O2 molecule to generate either a HO2 − ion or a HOO· radical and thus generate H2O2. 2) Establish a theoretical calculation model closer to the actual experiments Establish a theoretical calculation model closer to the actual experimentsIn most studies, researchers have reported to successfully develop ideal two-electron ORR catalysts based on the RRDE or RDE results when the electron transfer numbers of catalysts were close to 2. However, there is a significant leap from pure material testing to realistic electrode operation that is needed. The (R)RDE measurements are in O2 saturated, high electrolyte content solution together with almost unlimited mass transfer. These idealized systems only provide an upper boundary to the CE to H2O2 at most, while practical equipment tends to underperform. On the one hand, the internal channels and pore structures of the final fabricated electrode are relatively different from those of a pure carbon material or testing modes on the (R)RDE. On the other hand, even with pure O2 aeration and magnetic stirring, the O2 supply and mass transfer in 100–1000 mL level real reactors cannot compare with the conditions of RRDE. At this point, the reference value of the DFT model and (R)RDE results decrease. Such phenomena require modeling experiments in order to predict or represent the real equipment conditions more accurately. 3) Fabricate materials & electrodes by simple and low-cost methods Fabricate materials & electrodes by simple and low-cost methodsIt should be noted that catalytic activity and the selectivity of novel catalysts have only exhibited a marginal increase while the preparation methods become more intricate, making the materials deviate from the target of more cost effective approaches. For many of the aforementioned high-performance catalysts, it is reported that a lower catalysts loading engenders the active sites sparse distribution, which decreases the probability of H2O2 further reduction. The selectivity toward H2O2 increases while decreasing the loading amount [87,93]. Typically, catalyst loading density on the RRDE was controlled at one hundred μg cm−2 level. Considering the mass transfer limitations in practical devices, catalyst loading density on the real electrode should be lower to keep high selectivity. This calls for the highly precise electrode fabrication method.In the future development, a simple fabrication method with low costs should be another critical criterion in designing catalysts and fabricating electrodes. 4) Maintaining stability over the long-term operation Maintaining stability over the long-term operationExcept for catalytic activity and selectivity, the practical value of any material or electrode also relies on its long-term stability. Wang et al. [198] found the performance of the OFGs modified cathode was reduced after 10-times continuous runs, which was ascribed to the cathode structure destruction and OFGs content decrease due to the H2O2 oxidation. This illustrated the poor stability of the cathode in the long-term operation [289]. Meanwhile, the stability tests in most previous research are far from enough. Presently, researchers often document the stability of new fabricated electrodes by showing negligible changes in current response or H2O2 yield after 5–20 h of operation [87,101,102,118,196]. These are not convincing results to obtain a stable performance conclusion. Active materials should not be proven stable until they are subjected to more rigorous electrosynthesis trials conducted over hundreds or even thousands of hours. In the most recent research, Cao, et al. [290] presented a highly hydrophobic architecture GDE consisting of densely distributed N-doped carbon nano-polyhedra, thus enabling the 200 h durable electrolysis at 100 mA cm−2. Li et al. [291] evaluated the feasibility of electrochemical H2O2 production with CB-PTFE GDE. The results showed that the GDE could maintain high CE (>85%) as well as low energy consumption (<10 kWh per kg H2O2) for about 1000 h. This research suggests that electrochemical H2O2 production with GDE holds great promise for the development of compact treatment technologies. In the following exploratory research, the long-term stability and decay mechanism of materials, as well as electrodes in the EAOP systems, also need special attention. In our recent research [163], although the rolling ABC showed good stability under the condition of generating H2O2 (H2O2 yield decreased 17.8% after 200 h of operation), the electrodes decayed obviously when operated in the EAOP systems. It was found salt precipitation occurred due to the local alkalinization and enrichment of Na+, which would cause the block of the active sites and mass transfer channels. Meanwhile, the ·OH generated in the EAOP system would cause damage to the carbonaceous electrode by adding defects and oxygen-containing functional groups onto the electrode during the non-selective oxidation. Four electrode performance decay factors were illustrated during the synthetic phenol wastewater degradation. For actual wastewater treatment, the operating life of the cathode in such conditions will only be shorter because the components of sewage are more complex and diverse. Thus, lifetime experiments under different conditions with longer testing times are suggested to analyze the correlations of physicochemical properties and catalyst/electrode performance decay. When the decay mechanism of the electrode performance in long-term operation is clear and definite, it can further guide the development and upgrading of the new long-life electrodes [292,293].The development of carbon-based materials should be combined with theoretical studies, regarded as a requisite aid for catalysts designed or electrode modification to tune ORR selectivity to H2O2. In particular, the models which can better reflect the performance of catalysts in realistic devices will be more popular. To achieve these ends, further studies of the fundamental principles is needed to fully understand the origin of activity enhancement.Although multiple theoretical simulations and physicochemical techniques have been applied to reveal the catalytic mechanism of various carbonaceous materials, it's still hard to distinguish the nature of active sites of two-electron & four-electron ORR at the current technology state. More advanced characterization techniques and sophisticated experimental design are needed to distinguish the active sites for two-electron and four-electron pathways. Pinpointing such sites or chemical motifs would have guiding significance for both two-electron and four-electron carbon-based catalyst designs in the future. Once the two-electron active sites are determined, these sites could be purposefully increased to enable more efficient two-electron ORR or be eliminated from catalysts where four-electron ORR is required.The ORR needs O2, protons and electrons with mole ratio of 1:4:4 (four-electron pathway) or 1:2:2 (two-electron pathway) as reactants, from liquid, gas, and electrode, respectively. The active sites that stay in liquid-gas-solid TPIs could efficiently catalyze the ORR [233,294,295]. The accessibility of the active sites to O2 molecules is critical, but usually deficient due to the low O2 solubility in the realistic aqueous solution. Therefore, series electrodes styles, such as 3D particle electrodes [167], “floating” electrodes [283,296], and GDE, were invented to optimize the catalytic interface. Recently, scholars tried to accelerate the gas diffusion or tune water distribution to create more adequate TPIs inside the porous electrodes by porosity control and microarchitecture engineering. Therefore, superaerophilic CNT-array electrodes [294], superwetting electrodes [297], and breathing-mimicking electrodes [298] were invented to improve four-electron ORR activity. It could be observed from Eqs. (26) and (27) that the O2 demand of a two-electron pathway is twice that for a four-electron reaction at the same Faraday electron flux and proton supply, which means O2 supply is more vital. More simple surface/structure engineering techniques need to be developed in the future to enhance overall catalytic performances in H2O2 production. (26) 4H+ + 4 e− + 1 O2 → 2H2O (27) 4H+ + 4 e− + 2 O2 → 2H2O2 Process optimization and cost-efficiency are at the core of a suitable treatment strategy [299]. From the engineering perspective, reducing the cost to an acceptable level is a prerequisite for the application of this technology. According to Yang et al.‘s calculation [12], the total costs per mole of H2O2 (C total) via two-electron ORR can be calculated and expressed as the sum of two parameters, electricity costs (C electricity) and cathode costs (C cathode) (Eq. (28)) C total = C electricity + C cathode = p electricity UIt + p cathode S (28) = p electricity UnF/λ FE  + p cathode nF/jtλ FE Where, p electricity represents the cost per unit energy of electricity ($ J−1), U stands for the cell potential (V), n = 2, which represents the electrons transfer number for generating H2O2, F stands for Faraday constant (96486 C mol−1), and λ FE is the Faradaic current efficiency. Meanwhile, p cathode is the capital cost per unit cathode area ($ cm−2), j represents the current density (A cm−2), and t is the total operating time of the cathode over its lifetime (s). As λ FE stands for the denominator for electricity costs as well as cathode costs, it is clear that high CE plays a significant role in the performance of the electrode as well as the economic efficacy of the process. Meanwhile, in the cathode costs part, decreasing the numerator p cathode and increasing cathode lifetime t can also reduce the overall cost of the H2O2 synthesis, which highlights the relevance of costs of raw materials and preparation process together with electrode stability.Except for stability [163], longer-term goals for H2O2 electrosynthesis should focus on scalability: moving from benchtop experiments to syntheses on pilot or even industrially relevant scales [29]. Rapid H2O2 accumulation to desired concentration in large volume aqueous solution is the prerequisite of up-scaling [233]. Therefore, a more realistic aim would be to increase the current density on the electrodes in the H2O2 electrosynthesis without lowering selectivity. This would improve the synthesis efficiency and decrease the costs (Eq. (26)). Traditional carbon electrodes such as graphite rod/plate/particle, GF, and ACF are inefficient for H2O2 production in bi-dimensional configurations due to the limited dissolved O2 mass transfer in water [300]. In the presently available literature, most studies focus on selectivity on the premise of neglecting the current density. For those electrodes that can only withstand low current density, they have to be fabricated bigger and bigger to increase H2O2 production at higher current flux, which makes them impractical for applications. Recently, multiple GDEs have been reported for faster H2O2 productions with high current efficiencies. These GDEs could maintain the current efficiency over 70% at current densities >25 mA cm−2 [301–304]. Based on these electrodes, numerous contaminations were degraded in EAOP systems at pilot/pre-pilot plant scale (2.5–100 L). Developing electrodes with efficient H2O2 production at large current flux is the inevitable trend in the future. Until now, the largest air cathode so far came from Zhang et al. [305]. A 707 cm2 air cathode was utilized in a 3 L EF reactor for efficient Rhodamine B degradation. The shape and operating conditions of film air cathode determine that it cannot be enlarged without limit. As a result, new electrode and auxiliary equipment structures need to be developed to accommodate modular assembly. Reactors applied for EAOP treatments of wastewater are also needed to be well developed to fulfill the optimum properties of the electrodes [306]. More effort should be dedicated to other aspects such as equipment, scale-up, engineering, and economic issues in applying EAOP technologies to real wastewaters at an industrial scale [300].The various applications of aqueous H2O2 will require a certain H2O2 concentration and may only tolerate a certain pH range [29]. According to RRDE & RDE measurements shown in Tables 1 and 2, all the catalysts showed higher O2 reduction activity in alkaline media than in acid media. However, they were also less selective toward H2O2. In the practical application of H2O2, wastewater treated by AOPs is often in acid media, while pulp and paper bleaching is usually in an alkaline environment. Here it is recommended that researchers could first focus their efforts on H2O2 electrogenerating under acid conditions: AOP treatments only need 0.1% (wt) content of H2O2, which is two orders of magnitude below the demand for bleaching, making the application relatively easier. Moreover, Proton conducting polymeric membranes are much more technologically mature and cheaper than hydroxide conducting counterparts [307].In our review, there are multiple differences in systems, method, expression and, calculations, which make comparisons among the different research difficult. We suggest the following studies can use more unified experimental methods and expressions while avoiding mistakes described below. 1) System: in the electrogeneration of H2O2 experiments or (R)RDE tests, except for pH value, different types and concentrations of electrolyte were used. Until now, the most commonly used electrolyte in the electrolysis is 50 mM Na2SO4 with a pH of 3 or 7, while O2 saturated 100 mM KOH or 500 mM H2SO4 electrolyte is often employed in the (R)RDE tests. 2) Method: in general, the cathodic electrogeneration of H2O2 was conducted through galvanostatic mode in a two-electrode system connecting to a DC power or via chronoamperometry at constant potentials in a three-electrode system powered by a potentiostat. We found that a slight change in the distance between the reference electrode and the working electrode would dramatically change in H2O2 yield. Therefore, it is not easy to transversely compare the H2O2 yield at chronoamperometry because the researchers did not specify the distance between the electrodes. Moreover, as shown in Table 3, CE of electrodes operated in chronoamperometry mode are usually unknown and even cannot be calculated by us due to the unpublished current flux. Considering the three-electrode system has many disadvantages, including the high cost of the equipment and the fuzzy parameters in the practical water treatment processes, the electrogeneration of H2O2 through galvanostatic mode in a two-electrode system is recommended in future investigations. As summarized in Tables 1 and 2, in the RDE and RRDE tests, some parameters like onset potential, the definition of onset potential, and the potential range of calculated n were not given by authors. This would also bring troubles for readers, which should be avoided in the future. 3) Expression: various expressions were utilized to describe the H2O2 yield in different studies, including mg L−1 h−1, mg h−1 cm−2, μM h−1, mmol h−1 gcat −1. The disunity of units makes direct comparison different. Although we convert the most H2O2 yield into mg h−1 in this review, there are still several results that cannot be normalized due to the incomplete system parameters (Table 3). Furthermore, in some research, the newly fabricated or modified electrodes were directly utilized in the EAOP systems. The target pollutant removal efficiency/TOC removal efficiency was the only indicators to evaluate the electrodes, and the H2O2 yield of the cathodes were not mentioned. In our point of view, H2O2 yield expressed in the form of “mg h−1” could visualize the performance, and it would not be affected by solution volume and electrode area. Furthermore, the CE at a certain current is the most significant indicator of the cathode performance. We highly recommend the publishing of H2O2 yield together with the CE and current in the future papers. Meanwhile, with the development of materials, electrodes, and the continuous improvement of material requirements, more advanced and rational parameters or criteria are welcomed to be proposed in the future. 4) Calculation: there is also divergence in the reactive area of the immersed cathodes. Some calculated the current density (mA cm−2) or H2O2 yield (in the form of mg h−1 cm−2) via dividing the projection area of the electrode. However, another group of researchers [193,200,201], including us [279], all believe the effective area should be at least twice the projected area because each side of the electrode is in contact with the electrolyte. This error in the arithmetic will double the calculation result and mislead the readers, which should be avoided in the future. System: in the electrogeneration of H2O2 experiments or (R)RDE tests, except for pH value, different types and concentrations of electrolyte were used. Until now, the most commonly used electrolyte in the electrolysis is 50 mM Na2SO4 with a pH of 3 or 7, while O2 saturated 100 mM KOH or 500 mM H2SO4 electrolyte is often employed in the (R)RDE tests.Method: in general, the cathodic electrogeneration of H2O2 was conducted through galvanostatic mode in a two-electrode system connecting to a DC power or via chronoamperometry at constant potentials in a three-electrode system powered by a potentiostat. We found that a slight change in the distance between the reference electrode and the working electrode would dramatically change in H2O2 yield. Therefore, it is not easy to transversely compare the H2O2 yield at chronoamperometry because the researchers did not specify the distance between the electrodes. Moreover, as shown in Table 3, CE of electrodes operated in chronoamperometry mode are usually unknown and even cannot be calculated by us due to the unpublished current flux. Considering the three-electrode system has many disadvantages, including the high cost of the equipment and the fuzzy parameters in the practical water treatment processes, the electrogeneration of H2O2 through galvanostatic mode in a two-electrode system is recommended in future investigations. As summarized in Tables 1 and 2, in the RDE and RRDE tests, some parameters like onset potential, the definition of onset potential, and the potential range of calculated n were not given by authors. This would also bring troubles for readers, which should be avoided in the future.Expression: various expressions were utilized to describe the H2O2 yield in different studies, including mg L−1 h−1, mg h−1 cm−2, μM h−1, mmol h−1 gcat −1. The disunity of units makes direct comparison different. Although we convert the most H2O2 yield into mg h−1 in this review, there are still several results that cannot be normalized due to the incomplete system parameters (Table 3). Furthermore, in some research, the newly fabricated or modified electrodes were directly utilized in the EAOP systems. The target pollutant removal efficiency/TOC removal efficiency was the only indicators to evaluate the electrodes, and the H2O2 yield of the cathodes were not mentioned. In our point of view, H2O2 yield expressed in the form of “mg h−1” could visualize the performance, and it would not be affected by solution volume and electrode area. Furthermore, the CE at a certain current is the most significant indicator of the cathode performance. We highly recommend the publishing of H2O2 yield together with the CE and current in the future papers. Meanwhile, with the development of materials, electrodes, and the continuous improvement of material requirements, more advanced and rational parameters or criteria are welcomed to be proposed in the future.Calculation: there is also divergence in the reactive area of the immersed cathodes. Some calculated the current density (mA cm−2) or H2O2 yield (in the form of mg h−1 cm−2) via dividing the projection area of the electrode. However, another group of researchers [193,200,201], including us [279], all believe the effective area should be at least twice the projected area because each side of the electrode is in contact with the electrolyte. This error in the arithmetic will double the calculation result and mislead the readers, which should be avoided in the future.In summary, with the comprehensive and critical review, we hope to attract scholars from different research fields and use their knowledge to push electrosynthesis of H2O2 to pilot or even industrially relevant scales.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This research was financially supported by the National Natural Science Foundation of China (No. 52070140), the Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. HC202151) and the Postdoctoral Science Foundation of China (2021M702439). Jingkun An also thank the scholarship from the Shanghai Tongji Gao Tingyao Environmental Science & Technology Development Foundation (STGEF).

Hydrogen peroxide (H2O2) is an efficient oxidant with multiple uses ranging from chemical synthesis to wastewater treatment. The in-situ H2O2 production via a two-electron oxygen reduction reaction (ORR) will bring H2O2 beyond its current applications. The development of carbon materials offers the hope for obtaining inexpensive and high-performance alternatives to substitute noble-metal catalysts in order to provide a full and comprehensive picture of the current state of the art treatments and inspire new research in this area. Herein, the most up-to-date findings in theoretical predictions, synthetic methodologies, and experimental investigations of carbon-based catalysts are systematically summarized. Various electrode fabrication and modification methods were also introduced and compared, along with our original research on the air-breathing cathode and three-phase interface theory inside a porous electrode. In addition, our current understanding of the challenges, future directions, and suggestions on the carbon-based catalyst designs and electrode fabrication are highlighted.