Size-Selective Reactivity of Subnanometer Ag4 and Ag16 Clusters on a TiO2 Surface

Size-selected Ag-4 and Ag-16 clusters on a titania surface have been studied for their potential in CO oxidation using theoretical calculations and X-ray absorption near edge spectroscopy. The first peak at the measured Ag Kedge of Ag-16@TiO2 is more prominent in air than in carbon monoxide environment, but no variation was found between the spectra of Ag-4@TiO2 in air and in carbon monoxide environments. Density functional theory calculations show a preference for molecular oxygen adsorption for Ag-4@TiO2 and that for a dissociative one on Ag-16@TiO2, while carbon monoxide reactions with adsorbed oxygen reduced the Ag-16@TiO2 cluster. The dissociated oxygen atoms increased the oxidation state of Ag-16 cluster and resulted in the prominent first peak in Ag K-edge spectrum in quasi-particle theory calculations, with the subsequent carbon monoxide oxidation reversing the character of Ag Kedge spectrum associated with the reduction of the cluster. The results provide insight into the size selectivity of supported subnanometer silver clusters in their interactions with oxygen and carbon monoxide, with implications on the cluster catalytic properties in oxidative reactions.