Excitonic Au4Ru2(PPh3)2(SC2H4Ph)8 cluster for light-driven dinitrogen fixation†

The surface plasmon resonance of metal nanoparticles has been widely used to improve photochemical transformations by plasmon-induced charge transfer. However, it remains elusive for the molecular-like metal clusters with non-metallic or excitonic behavior to enable light harvesting including electron/hole pair production and separation. Here we report a paradigm for solar energy conversion on an atomically precise Au4Ru2 cluster supported on TiO2 with oxygen vacancies, in which the electron–hole pairs can be directly generated from the excited Au4Ru2 cluster and the TiO2 support, and the photogenerated electrons can transfer to the Ru atoms. Importantly, the Ru atoms in the Au4Ru2 cluster are capable of injecting the electrons into adsorbed N2 to activate N2 molecules. The cooperative effect in the supported Au4Ru2 catalyst efficiently boosts the photocatalytic activity for N2 fixation in comparison with homogold (Aun) clusters.

3 hexane and extraction with CH2Cl2. The crude product dissolved in CH2Cl2 was pipetted on PTLC plate (10 cm × 20 cm), and the separation was conducted in developing tank (developing solvent: CH2Cl2) for 20 min. A knife was used to cut the bands in the PTLC plate, and the green product was extracted by pure CH2Cl2. Green needle-like crystals of Au4Ru2(PPh3)2(SC2H4Ph)8 were crystallized from CH2Cl2/menthol for 3~5 days.

Synthesis of [Au11(PPh3)8Cl2]:
The residue was dissolved in a minimum amount of CH2Cl2 and the product was precipitated with 20 times this volume of pentane. The resulting mixture was centrifuged, and the supernatant was discarded. The residue was stirred with pentane (20 mL) and centrifuged. The final solid was dissolved in EtOH, transferred to a round-bottom flask, and the solvent was evaporated to obtain the crude product. 125 µL 1-cyclohexanethiol was added in the mixture at room temperature. After 30 min, 20 mg NaBH4 of 2 mL ethanol solution was added to the above solution. The mixed solution was stirred for 5 h. Finally, the obtained product washed with methanol and extracted with CH2Cl2.
Synthesis of Au24(PPh3)10(SC2H4Ph)5Cl2: S5 0.23 mmol HAuCl4· 4H2O was first dissolved in 5 mL water, and then 180 mg PPh3 was added into organic phase under vigorous stirring. 26 mg NaBH4 of 5 mL ethanol solution was added to the above solution. After vigorous stirring for 4 h, the reaction product was separated using 10~12 mL CH2Cl2, and 200 μL 2-phenylethylthiol was added to the solution. Then, the mixture was kept at 40 o C for 5~8 h. Then, 1.2 g PPh3 was added in the above mixture, and the reaction was continued for 24 h at 40 o C. Finally, the product washed with n-hexane and extracted with toluene.
Synthesis of Au25(SC2H4Ph)18: S6 0.203 mmol of HAuCl4· 3H2O and 0.235 mmol of TOAB were dissolved in 15 mL tetrahydrofuran in an ice bath. Until the above solution turned a wine red (30min), 140 µL of phenylethanethiol was added into the above solution. The mixture was slowly stirred about 1~3 h, then 5 mL cold water of 78 mg NaBH4 was added.
After stirring for 3 h, the ice water bath was removed. Then, the mixture solution was stirred to proceed overnight. The obtained product was washed with methanol five times.
Synthesis of Au28(TBBT)20: S7 10 mg Au25(SC2H4Ph)18 was added to the mixture of 0.5 mL toluene and 0.5 mL TBBT. The reaction solution was heated at 80 o C oil bath for 2 h.

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The reaction product was washed with methanol three times. Finally, the Au28(TBBT)20 clusters were extracted with CH2Cl2.
Synthesis of Au36(TBBT)24 and Au44(TBBT)28: S7,S8 The Au36(TBBT)24 and Au44(TBBT)28 clusters were synthesized by a two-step size focusing process. In the first step, HAuCl4•3H2O (90 mg) of 2 mL water was added to 15 mL of CH2Cl2 solution containing TOAB (154 mg) in a 50 mL flask. After 30 min, the water phase was removed, then 125 µL TBBT was added to the organic phase. The reactants are stirred vigorously in an ice bath. After 4 h, 50 mg NaBH4 dissolved in 5 mL water was quickly poured into the reaction mixture. The mixture was stirred to proceed overnight in the ice bath. The reaction product was washed with methanol three times to remove redundant TOAB and TBBT. In the second step, the product was divided evenly into two parts. One of parts is added with 0.5 mL toluene and 100 µL TBBT at 80 o C for 22 h, and then the Au36(TBBT)24 clusters were separated from the reaction mixture by washing with methanol and extraction with CH2Cl2. The other part was added 0.5 mL toluene and 100 µL TBBT and kept at 60 o C for 22 h, and then the Au44(TBBT)28 clusters were separated from the reaction mixture by washing with methanol and extraction with CH2Cl2.
Synthesis of Au23(SC6H11)16: S9 0.3 mmol HAuCl4· 3H2O and 0.348 mmol TOAB were dissolved in 15 mL methanol. After vigorously stirring for 15 min, 1.6 mmol 1cyclohexanethiol was added to the mixture at room temperature. After 60~120 min, NaBH4 (3 mmol dissolved in 6 mL cold water) was added to the solution under vigorous stirring.
The solution turned black immediately, which then precipitated out of the methanol 6 solution. The reaction mixture was further allowed to overnight and finally gave rise to pure Au23(SC6H11)16 clusters.
Synthesis of TiO2-Ov: S11 10 mL tetra-butyl titanate was added to a mixture of 1.2 mL HF and 30 mL ethanol, which was stirred for 30 min. The above solution was transferred to a 100 mL hydrothermal kettle and kept at 180 o C for 2 h. After 2 h, the product was filtered and washed with water, and then dried at 60 o C for 24 h.
Preparation of supported Au4Ru2 catalysts: 10 mg Au4Ru2(PPh)3(SC2H4Ph)8 was dissolved in 2 mL CH2Cl2. Then 500 mg TiO2-Ov was added into the above solution under vigorous stirring for 10 h. Finally, the Au4Ru2/TiO2-Ov catalyst was obtained by removing the solvent in flowing nitrogen at room temperature. The actual loadings of Au and Ru on the Au4Tu2/TiO2-Ov catalyst were 0.56 wt% and 0.14 wt% respectively by ICP-AES analysis. The preparation methods of other supported cluster catalysts were the same as the above described method.

Characterization 7
The X-ray crystallography was performed on a Bruker D8 VENTURE with Mo Kα radiation (λ = 0.71073 Å). The crystal structures were resolved by direct methods and refined by full-matrix least-squares methods with SHELXL-2013(Sheldrick, 2013. UV/vis/NIR spectra with a range of 300-1400 nm were recorded on a UV3700 spectrophotometer (SHIMADZU). Diffuse reflectance UV-vis spectra were recorded on a UV-vis spectrometer using SHIMADZU UV-3600. Electrospray ionization (ESI) mass spectra were collected on a Waters Q-TOF mass spectrometer using a Z-spray source. The sample was first dissolved in toluene (0.5 mg/mL) and then diluted (2:1 v/v) with a methanol solution containing 50 mM CsOAc. The morphology and size of catalysts were characterized using a JEOL JEM-2100F field-emission high-resolution transmission electron microscope operated at 200 kV. The XRD patterns were recorded with an X'Pert PRO MPD (PANalytical) diffractometer using Cu K as the radiation source at 40 kV and 40 mA. Electron paramagnetic resonance (EPR) spectra were conducted on a Bruker 500 spectrometer at room temperature. The loading weight of metal was determined via the inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Optima 5300DV).
A home-built wide-field fluorescence microscope based on Olympus IX73 was used with 450 nm CW diode laser for the fluorescence spectra measurements and 450 nm pulse laser from super continuous laser (Fianium SC-400) for the PL lifetime measurement. The fluorescence was collected by a dry objective lens (Olympus LUCPlanFI 40×, NA=0.6) and detected by an EMCCD camera (iXon Ultra 888, Andor). A transmission grating (Newport, 150 lines/mm) was put in front of the camera for the spectra measurement. The 8 PL lifetime measurement was done by using a single photon counting system (TCSPC, Picoharp 300).
Thermal gravimetric analyses (TGA) were performed using a NETZSCH Instruments TGA STA-449C under N2 flow. Samples were run from 20 to 800 °C with a heating rate of 10 °C/min.
In situ diffuse reflectance FTIR spectra were collected using Nicolet iS50FT-IR spectrometer (Thermo, USA) with a designed reaction cell. The substrate (Au4Ru2/TiO2-Ov film) is lying at the center of the reaction cell. Then, the gas in the reaction cell and the gas adsorbed on the surface of the catalyst was extracted by an ultra-high vacuum pump.
A layer of water molecules was necessary to provide protons in our functional model. Therefore, the water vapor in the nitrogen gas is passed through the substrate to form a layer of adsorbed water molecules, after which the molecular N2 is pumped into the cells to obtain a N2 atmosphere. Finally, the Xe lamp was turned on and the in situ FTIR spectra was collected using a MCT detector along with the reaction.
The photoelectrochemical (PEC), electrochemical impedance spectroscopy (EIS) measurement and Mott-Schottky plots were performed on a CHI660B electrochemical workstation in a standard three-electrode system with a platinum foil as a counter electrode, a glassy carbon as working electrode and a saturated Ag/AgCl as a reference electrode. A 300W Xe lamp was utilized as the light source on the PEC and EIS measurement. PEC and ESI measurements were carried out at room temperature in 0.5 mol/L Na2SO4, which had been deoxygenated by bubbling high-purity Ar/N2 for 30 min. The Nyquist plots were recorded from 1 to 100 kHz frequency range at an applied 0.5 V bias voltage. Catalytic reaction: Photocatalytic N2 reduction performance was tested using a quartz glass as the photocatalytic reactor under atmospheric pressure and ambient temperature, and a 300W Xe lamp (CEL-HXF300) was used as a light source. Specific catalytic evaluation scheme was as follows: 100 mg catalyst (0.7 wt% metal from ICP), 20 mL water and a magnetic stir bar were loaded into the quartz glass. The reactor was equipped with a circulating water shell to maintain room temperature. The suspension was stirred constantly in the dark, and a constant high-purity N2 (>99.9%) at a flow rate of 50 mL/min for 30 min was bubbled to obtain a saturated aqueous suspension of N2. The mixture was irradiated afterward with a 300W Xe lamp. 3 mL of the reaction solution was taken out every 30 min by a syringe, and then the catalyst was removed by centrifugation immediately. Concentration of NH4 + was determined using Nessler's reagent method S11 on a SHIMADZU UV-1800 spectrometer and ion chromatography (930 compact IC Flex), which the and standard curves for NH4 + with Nessler's reagent and ion chromatography were shown in Figure S13. The content of N2H4 was determined using colorimetry of para-(dimethylamino) benzaldehyde. S13 No NH4 + was detected when bubbling Ar only, indicating that background NH3 was not produced in the system. In addition, the photocatalytic activity for H2 as byproduct production was investigated and the rate of H2 production was 3.45 μmol/g/h on the Au4Ru2/TiO2-Ov under full spectrum illumination. However, given the established fact that H2 production in N2 photofixation is not a serious issue, NH3 highly dissolves in water and is easily separated from gaseous H2. Therefore, in this study, we focused on the evolution of ammonia.
The detailed preparation method of the standard curve for NH4 + of Nessler's reagent was as follows. In alkaline environment, ammonia can react with mercury iodide and potassium iodide to produce a reddish gelatinous precipitate, which has an absorption peak at 420 nm. The specific method to obtain the standard curve of Nessler's reagent is as follows: 0, 50, 150, 250, 350, 450, and 500 µL of 10 mg/L NH4Cl standard solution was transferred into seven colorimetric tubes respectively. And, the samples tubes were diluted with ultrapure water to 5 mL. Then, 100 µL potassium sodium tartrate was added to the sample tubes. After mixing well, 150 µL of Nessler's reagent was also added to the sample tubes and mixed for ageing 20 min, and then measured by the SHIMADZU UV-3600 spectrometer. The absorbance at 420 nm was measured and plotted as a function of ammonia concentration.

Computational methods
DFT calculations were performed by the Vienna ab initio simulation Package (VASP), S14 using a planewave basis set with energy cutoff of 500 eV, projector augmented wave (PAW) pseudopotentials, S15 and the generalized gradient approximation parameterized by Perdew, Burke, and Ernzerhof (GGA-PBE) for the exchange-correlation functional. S16 The Grimme's DFT-D3 scheme for dispersion correction was adopted for better description of the interactions between the gold clusters and reaction intermediates involved in N2 fixation. S17 The clusters were placed in a cubic supercell with a dimension of 30 Å, and its 11 Brillouin zone was sampled by the Γ point. All of structures were fully optimized for the ionic and electronic degrees of freedom using the convergence criteria of 10 -4 eV for electronic energy and 10 -2 eV/Å for the forces on each atom. The kinetic barriers and transition states for elementary steps of N2 fixation were calculated by the climbing-image nudged elastic band (CI-NEB) method implemented in VASP, S18 with five images to mimic the reaction path. The intermediate images were relaxed until the perpendicular forces were less than 10 -2 eV/Å. To simplify our models and improve the computational efficiency, the -triphenylphosphine (-PPh3) and -phenylethanethiol (-SC2H4Ph) ligands were replaced by -trimethylphosphine -P(CH3)3 and -SCH3, respectively, which, according to our test calculations, does not affect the binding strength of the cluster with N2 molecule.
We used a slab model consisting of the six atomic layers of anatase TiO2 (101)  A single oxygen atom of surface and subsurface was removed to simulate oxygen vacancy on the surface (TiO2-Ov1) and subsurface (TiO2-Ov2) of anatase TiO2(101). For model of ligand-protected Au4Ru2 cluster, the -triphenylphosphine (-PPh3) and -phenylethanethiol (-SC2H4Ph) ligands were replaced by -trimethylphosphine -P(CH3)3 and -SCH3, respectively, which, according to our test calculations, does not affect the binding strength of the cluster with N2 molecule ( Figure S12). We examined the adsorption of a N2 molecule on the cluster with one ligand shed from either Ru or Au atom ( Figure S14 and Table S1).
Our calculations show that the exposed Au atom cannot chemisorb N2 molecule. Only the exposed Ru atom in the cluster has activity for N2 fixation.

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To characterize the interactions between ligand-protected gold clusters and N2 molecule, we defined the adsorption energy as (1) where Etotal and Ecluster are the energies of a gold cluster with and without adsorption of a N2 molecule, respectively; EN2 is the energy of the N2 molecule in the gas phase.     The H, C, O, P, S, Au, Ru and Ti atoms are shown in light blue, gray, red, green, yellow, orange, blackish green and yellowish, respectively. The closest distance between the cluster and substrate is shown as d. respectively. When the cluster is fully covered by ligands (A, C), N2 adsorption leads to severe structure distortion. Table S1. Geometrical parameters including the bond length of N-N (dN-N), Ru-N (dRu-N) and Ru-S (dRu-S) bonds and N2 adsorption energy (ΔE*N2) for a N2 molecule on the ligand-protected Au4Ru2 clusters shown in Figure S14.