Diphosphine-Protected Au22 Nanoclusters on Oxide Supports Are Active for Gas-Phase Catalysis without Ligand Removal

Abstract

Investigation of monodispersed and atomically-precise Au nanoclusters provides a route to understand the roles of coordination, size, and ligand effects in Au catalysis. We have explored the catalytic behavior of a newly-synthesized Au22(L8)6 nanocluster (L = 1,8-bis(diphenylphosphino) octane) with in situ uncoordinated Au sites supported on TiO2, CeO2 and Al2O3. Stability of the supported Au22 nanoclusters was probed structurally by EXAFS and HAADF-STEM, and their adsorption and reactivity for CO oxidation were investigated by IR absorption spectroscopy and temperature programed flow reaction. Low temperature CO oxidation activity was observed for the supported pristine Au22(L8)6 nanoclusters without ligand removal. Isotopically labeled O2 was used to demonstrate that the reaction pathway occurs through a redox mechanism, consistent with the observed support-dependent activity trend: CeO2 > TiO2 > Al2O3. Substantiated by density functional theory (DFT) calculations, we conclude that the uncoordinated Au sites in the intact Au22(L8)6 nanoclusters are capable of adsorbing CO, activating O2 and promoting CO oxidation reaction. Thanks to the presence of the in situ coordination unsaturated Au atoms, this work is the first clear demonstration of a ligand-protected Au nanocluster that are active for gas phase catalysis without the need of ligand removal.