Oxidation of Small Silver Clusters: A Density Functional Theory Study

The oxidation of small silver clusters (Ag n, n ≤ 9) was investigated through electronic structure calculations based on density functional theory. The adsorption energies of molecular and dissociated adsorption show a pronounced odd/even alternation, with lower energies calculated for even-sized clusters. Molecular adsorption is favored for n ≤ 5, whereas dissociation is preferred for the larger sizes. Molecular oxygen is adsorbed in atop (Ag, Ag 2, Ag 6, Ag 8 ) or bridge (Ag 3, Ag 4, Ag 5, Ag 7, Ag 9 ) configurations, and atomic oxygen is preferably adsorbed in 3-fold hollow positions. Results for stoichiometric (Ag 2 n O n ) clusters were compared to O 2 adsorption on Ag(111), and ab initio thermodynamics was used to estimate the temperature for the oxide-to-metal phase transition. The barrier for O 2 dissociation on Ag 8 was calculated to be higher than the corresponding barrier on Ag(111), which indicates a slower oxidation process. Adsorption of NO x onto the oxidized clusters was found to proceed through a formal reduction of the clusters; that is, NO x is adsorbed as NO x +1 with x = 1, 2.