Insights into heterogeneous catalytic activation of peroxymonosulfate by Pd/g-C3N4_ The role of superoxide radical and singlet oxygen

Pd nanoparticles anchored on graphitic nitride carbon (Pd/g-C3N4) hybrid catalytic system was prepared by a simple KBH4 reduction method. Pd/g-C3N4 was very active in the degradation of bisphenol A (BPA) by efficiently activating peroxymonosulfate (PMS). Important influencing factors, such as catalyst dosage, PMS concentration, pH and reaction temperature for the BPA removal were investigated. Pd/g-C3N4-PMS demonstrated a wide effective pH range, namely 3.0–9.0. Radical quenching tests and electron spin response (ESR) analyses revealed that superoxide radical (O2%) and singlet oxygen (O2) instead of sulfate radical (SO4%) and hydroxyl radicals (%OH) were the major oxidized species for the degradation of BPA.

Generally, SO 4 % − and/or %OH generated from PMS/PDS activation are responsible for the degradation of organics. However, it was found that the inhibitory effect of ethanol/methanol (scavenger for both %OH and SO 4 % − ) on the degradation of organics in heterogeneous activation of PMS/PDS was insignificant [14][15][16][17]. In these reports, SO 4 % − and/or %OH were not assigned to be the major oxidized species for the degradation of organics. Instead, non-radical oxidation mechanisms were proposed. For example, Zhang et al. [17] suggested that the outer-sphere interaction between the CuO and PDS caused the electron rearrangement of PDS molecular, which subsequently increased the oxidizing power of PDS. Similar phenomena were also observed in the PDS activation by carbon nanotubes (CNTs) [16] and graphitized nanodiamond [18], in which process reactive complexes were formed due to the interplay of PDS and CNTs, where immediately decomposed upon reaction with the organic compounds. Ahn et al. [19] found that noble metals, such as Pt, Pd, Au and Ag also possess catalytic activity for the activation of PMS. The latter was believed to act as an oxidant that directly accepts electrons from the organic compound with the help of noble metal, and no radical is formed during reaction. Feng et al. [20] claimed that degradation of 1,4-dioxane is due to the surface-bound sulfate radical, which is generated from the activation of PMS on alumina-supported palladium (Pd/Al 2 O 3 ). Therefore, more efforts still need to be made for further understanding the intrinsic catalytic mechanism of PMS activation by Pd.
Graphitic carbon nitride (g-C 3 N 4 ) with characteristic N-coordinating cavities has attracted increasing interest in the field of material and catalytic science [21]. It might be facilely produced at a large scale with low cost. In this work, g-C 3 N 4 was prepared via one-step polymerization of urea, where Pd nanoparticles were anchored into the cavities of g-C 3 N 4 to form Pd nanoparticles modified g-C 3 N 4 hybrid system (Pd/g-C 3 N 4 ). The latter was used as heterogeneous catalyst for the activation of PMS. Bisphenol A (BPA), an endocrine disrupting compound, is released from landfill leachates, paper and plastics plants [22] selected as a target pollutant to evaluate the catalytic activity of Pd/g-C 3 N 4 . Compared with the current technologies (e.g., photo-Fenton [23], electro-Fenton [24] and photocatalysis [25]) for the degradation of BPA, Pd/g-C 3 N 4 -PMS system requires no energy input and leads to no secondary pollution. Important influencing factors for the BPA removal, such as PMS concentration, catalyst dosage, pH value and reaction temperature were investigated. A plausible mechanism was proposed based on X-ray photoelectron spectroscopy (XPS) and electron spin response (ESR) studies and radical quenching tests.

Catalyst preparation
The g-C 3 N 4 support material was prepared by heating urea at 600°C with a ramp rate of 5°C/min for 2 h in a closed alumina crucible placed in a muffle furnace. 2.5 g prepared g-C 3 N 4 were dispersed in 250 mL deionized water and magnetically stirred in an ultrasonic bath for 1 h. Subsequently, 11.8 mL of a solution of PdCl 2 (0.01 M) and NaCl (0.04 M) were added, and the resulting suspension was further stirred in the ultrasonic bath for 1 h. Then, freshly prepared aqueous solution of KBH 4 (3.5 mL, 0.5 M) was added to this slurry and stirred for 24 h. The obtained Pd/g-C 3 N 4 catalyst was separated by vacuum filtration, washed with deionized water, and vacuum dried overnight at 50°C.

Catalyst characterization
Detailed information of chemicals and reagents, catalyst characterization studies, catalytic activity tests and analytical methods are provided in Supporting Information.

Characterization of Pd/g-C 3 N 4
Pd/g-C 3 N 4 shows a similar X-ray diffraction (XRD) pattern with the pristine g-C 3 N 4 (Fig. S1), suggesting that the crystal structure of g-C 3 N 4 is not changed after the introduction of Pd nanoparticles. No diffraction peaks of Pd nanoparticles are observed in the XRD pattern of Pd/g-C 3 N 4 , which might be the result of Pd content below the detection limit of XRD and/or the peak broadening from the small size of Pd nanoparticles formed. Scanning electron microscope (SEM) images of g-C 3 N 4 and Pd/g-C 3 N 4 are shown in Fig. S2. It can be seen that Pd/g-C 3 N 4 exhibits a layered structure and no obvious changes are observed compared with pristine g-C 3 N 4 . The bright regions in the scanning transmission electron microscope (STEM) image of Pd/g-C 3 N 4 (Fig. 1a) point out the presence of Pd nanoparticles, which are highly dispersed on g-C 3 N 4 with estimated size in the 2-4 nm range (inset in Fig. 1a).
The N 2 adsorption-desorption isotherms of g-C 3 N 4 and Pd/g-C 3 N 4 reveal that all the samples exhibit typical IV isotherms with H3-type hysteresis loop in the relative pressure range of 0.8-1.0 (Fig. S3). This indicates the existence of mesoporous structure originated from the stacking of g-C 3 N 4 nanosheets. The BET specific surface area of g-C 3 N 4 and Pd/g-C 3 N 4 are 65 and 59 m 2 ·g − 1 , respectively, indicating that the existence of Pd nanoparticles only little might affect the structure of g-C 3 N 4 . The pore size distribution curves present two peaks at about 2 and 30 nm, which might be beneficial to mass transfer effects during the catalytic process.
The surface composition and chemical oxidation states of Pd/g-C 3 N 4 were determined by XPS (Fig. S4). The elements of C, N, O and Pd are observed in the survey spectrum. The N 1s spectrum was fitted into four peaks at 397.5, 398.3, 399.9 and 403.4 eV, which can be assigned to the sp 2 -hybridized nitrogen (CeN]C), tertiary nitrogen (Ne(C) 3 ), amino functional groups (CeNeH) and the charging effects, respectively [26]. The XPS spectra of Pd 3d before and after reaction are presented in Fig. 1b, in which the peaks at 342.6 and 337.2 eV are assigned to Pd II , while the peaks at 340.6 and 335.3 eV to Pd 0 [27]. After the degradation reaction, the Pd 0 /Pd II ratio decreases from 2.02 to 1.19, illustrating that part of Pd 0 changed to Pd II during the PMS activation catalytic process.

The catalytic performance of Pd/g-C 3 N 4
The adsorption ability of Pd/g-C 3 N 4 toward BPA was investigated first and the removal of BPA by adsorption was found to be insignificant (Fig. 2a). Meanwhile, the degradation of BPA by PMS was negligible due to the low oxidative capacity of PMS. The g-C 3 N 4 presented poor catalytic activity for activation of PMS, which is consistent with the previous report [28]. However, degradation efficiency of BPA was achieved using the Pd/g-C 3 N 4 -PMS catalytic system. It was found that 91% of BPA (20 mg·L − 1 ) can be removed in 60 min with 0.1 g·L − 1 of catalyst loading and 1.0 mM of PMS, which indicates the good catalytic performance of Pd/g-C 3 N 4 for the activation of PMS.
To comprehensively evaluate the catalytic performance of Pd/g-C 3 N 4 -PMS, the influence of PMS concentration, catalyst loading, pH value and reaction temperature were systematically investigated. As shown in Fig.  S5a, the removal efficiency of BPA increases from 74% to 91% in 60 min when the PMS concentration increases from 0.2 to 1.0 mM. However, only 5% enhancement is achieved when PMS concentration further increased to 1.5 and 2.0 mM. It is inferred that the active sites were almost occupied by PMS (1.0 mM). Fig. S5b indicates that catalyst loading has a great influence on the BPA removal rate. The degradation rate constant of BPA increases from 0.038 to 0.463 min − 1 with catalyst loading increasing from 0.1 to 0.4 g·L − 1 . One reasonable explanation might be that more active sites are present for the activation of PMS at higher catalyst loadings. To investigate the influence of pH on the BPA removal rate, the pH of the suspension was maintained at a certain pH value ( ± 0.2) after adjustment with 0.1 M H 2 SO 4 and 0.1 M NaOH at all times. As illustrated in Fig. S5c, the catalytic ability of Pd/g-C 3 N 4 is greatly influenced by the pH of solution. With the pH increasing from 3.0 to 9.0, the degradation rate of BPA increases from 0.034 to 0.114 min − 1 . However, when the pH is further increased to 11.0, the degradation rate of BPA decreases to 0.023 min − 1 . The pKa value of PMS is 9.4 [29], thus the SO 5 2 − is the dominant species when the pH of solution is higher than 9.4. As reported elsewhere [30], the redox potential of SO 5 Fig. 2b. The degradation rate constants of BPA were estimated and found to be 0.0367, 0.0629 and 0.0969 min − 1 at 25, 35 and 45°C, respectively. According to the Van't Hoff's equation, the negative slope (inset in Fig. 2b) indicates that the catalytic reaction process is endothermic and the activation energy (E a ) for degradation of BPA was estimated to be 38.6 kJ·mol − 1 (R 2 = 0.989).

Radical quenching tests
To identify the reactive oxygen species (ROS) generated in the Pd/g-C 3 N 4 -PMS system, radical quenching tests were conducted at 0.1 g·L − 1 catalyst loading and 1 mM PMS concentration with different scavengers added at room temperature. Methanol (MeOH) can scavenge both %OH (k MeOH-%OH = 9.7 × 10 8 M − 1 ·s − 1 ) and SO 4 % − (k MeOH-SO4%− = 2.5 × 10 7 M − 1 ·s − 1 ), whereas tert-butyl alcohol (TBA) only scavenge %OH since it reacts with SO 4 % − over 1000-fold slower than with %OH (k TBA-%OH = 3.8-7.6 × 10 8 M − 1 ·s − 1 ; k TBA-SO4%− = 4.0-9.1 × 10 5 M − 1 ·s − 1 ). To guarantee complete quenching of free radicals, the degradation experiments of BPA with different molar ratios between scavenger and PMS were carried out (Fig. S6). The degradation of BPA is not significantly inhibited by TBA/MeOH even using 1000:1 molar ratio of TBA/MeOH:PMS (Fig. S7). Compared with the control experiment, the degradation rate constant of BPA decreased by 3% and 10% with TBA and MeOH as scavengers, respectively (Fig. 3). Therefore, it can be concluded that %OH and SO 4 % − are not the major oxidizing species for the degradation of BPA. Other ROS might be involved in the case of Pd/g-C 3 N 4 -PMS system. As shown in Fig. 3, the degradation rate of BPA is greatly reduced in the presence of p-benzoquinone (p-BQ), which is a scavenger of superoxide radical (O 2 % − ) with a rate constant of 0.9-1.0 × 10 9 M − 1 ·s − 1 [32]. The BPA degradation is almost completely quenched with the addition of L-histidine, which is a scavenger of singlet oxygen ( 1 O 2 ) [18]. These results demonstrate that O 2 % − and 1 O 2 play a key role for the degradation of BPA in the Pd/g-C 3 N 4 -PMS system. It can be concluded that O 2 % − and 1 O 2 instead of % OH and SO 4 % − are the major oxidizing species in the Pd/g-C 3 N 4 -PMS system and the experimental conditions applied.

Conclusions
Pd/g-C 3 N 4 was prepared via a simple KBH 4 reduction method and Pd nanoparticles (d~2-4 nm) were highly dispersed on g-C 3 N 4 . Pd/g-C 3 N 4 exhibited high heterogeneous catalytic activity for the activation of PMS. At the conditions of catalyst loading of 0.1 g·L − 1 and PMS concentration of 1.0 mM, 91% BPA with initial concentration of 20 mg·L − 1 can be degraded in 60 min. The activation energy for BPA oxidation in Pd/g-C 3 N 4 -PMS was found to be 38.6 kJ·mol − 1 . The ROSs involved in Pd/g-C 3 N 4 -PMS system were identified and the results showed that O 2 % − and 1 O 2 instead of SO 4 % − and %OH were the major oxidative species for the degradation of BPA.