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 K-edge of Ag 16 @TiO 2 is more prominent in air than in carbon monoxide environment, but no variation was found between the spectra of Ag 4 @TiO 2 in air and in carbon monoxide environments. Density functional theory calculations show a preference for molecular oxygen adsorption for Ag 4 @TiO 2 and that for a dissociative one on Ag 16 @TiO 2, while carbon monoxide reactions with adsorbed oxygen reduced the Ag 16 @TiO 2 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 K-edge 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.