Theoretical study of O2 interaction with subnanometer-sized Ag clusters supported on defective SiO2 surface

Abstract

The effect of the nature of paramagnetic adsorption centers of the dehydroxylated silica surface - nonbridging oxygens (NBO) or silicon atoms with dangling bond (E′) - on the oxidation of adsorbed subnanometer-sized silver clusters Agn (n = 3, 4, 7) has been clarified by density functional theory using embedded cluster models. The interaction with NBO centers results in the formation of positively charged Ag moieties, while at E′ centers metal species remain rather neutral. At both surface defects, the electronic and structural properties of Ag species resemble the features of free Agn+ clusters. As a result, O2 molecular adsorption on the supported Ag clusters follows the trends established for cationic species. O2 is weakly adsorbed in a terminal mode on the Ag trimers and heptamers and is strongly bound to the Ag tetramers in a bridge mode. The stability of O2 molecular and dissociative adsorbed forms at the supported Ag3 and Ag7 species is similar, whereas on Ag4 cluster O2 dissociation is preferred. The heights of the O2 dissociation barriers are determined by the initial activation of molecularly adsorbed O2 and the deformation of supported metal clusters along the reaction pathway. The high activation energies make O2 dissociation unlikely and manifest that small Ag clusters trapped by silica paramagnetic defects highly resistant to the oxidation.