Thiolate Ligands as a Double-Edged Sword for CO Oxidation on CeO2 Supported Au25(SCH2CH2Ph)18 Nanoclusters
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Abstract
The effect of thiolate ligands was explored on the catalysis of CeO2 rod supported Au25(SR)18 (SR = -SCH2CH2Ph) by using CO oxidation as a probe reaction. Reaction kinetic tests, in situ IR and X-ray absorption spectroscopy, and density functional theory (DFT) were employed to understand how the thiolate ligands affect the nature of active sites, activation of CO and O2, and reaction mechanism and kinetics. The intact Au25(SR)18 on the CeO2 rod is found not able to adsorb CO. Only when the thiolate ligands are partially removed, starting from the interface between Au25(SR)18 and CeO2 at temperatures of 423 K and above, can the adsorption of CO be observed by IR. DFT calculations suggest that CO adsorbs favorably on the exposed gold atoms. Accordingly, the CO oxidation light-off temperature shifts to lower temperature. Several types of Au sites are probed by IR of CO adsorption during the ligand removal process. The cationic Au sites (charged between 0 and +1) are found to play the major role for low-temperature CO oxidation. Similar activation energies and reaction rates are found for CO oxidation on differently treated Au25(SR)18/CeO2 rod catalysts, suggesting a simple site-blocking effect of the thiolate ligands in Au nanocluster catalysis. Isotopic labeling experiments clearly indicate that CO oxidation on the Au25(SR)18/CeO2 rod catalyst proceeds predominantly via the redox mechanism where CeO2 activates O2 while CO is activated on the dethiolated gold sites. These results point to a double-edged sword role played by the thiolate ligands on Au25 nanoclusters for CO oxidation.
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