Subsurface oxygen and surface oxide formation at Ag„111...: A density-functional theory investigation

To help provide insight into the remarkable catalytic behavior of the oxygen/silver system for heterogeneous oxidation reactions, purely subsurface oxygen, and structures involving both on-surface and subsurface oxygen, as well as oxidelike structures at the Ag~111! surface have been studied for a wide range of coverages and adsorption sites using density-functional theory. Adsorption on the surface in fcc sites is energetically favorable for low coverages, while for higher coverage a thin surface-oxide structure is energetically favorable. This structure has been proposed to correspond to the experimentally observed (434) phase. With increasing O concentrations, thicker oxidelike structures resembling compressed Ag2O(111) surfaces are energetically favored. Due to the relatively low thermal stability of these structures, and the very low sticking probability of O2 at Ag~111!, their formation and observation may require the use of atomic oxygen ~or ozone, O3) and low temperatures. We also investigate the diffusion of O into the subsurface region at low coverage ~0.11 ML!, and the effect of surface Ag vacancies in the adsorption of atomic oxygen and ozonelike species. The present studies, together with our earlier investigations of on-surface and surface-substitutional adsorption, provide a comprehensive picture of the behavior and chemical nature of the interaction of oxygen and Ag~111!, as well as of the initial stages of oxide formation.