First-Principles Study of Carbon Monoxide Oxidation on Ag(111) in Presence of Subsurface Oxygen and Stepped Ag(221)

The role of subsurface oxygen in Ag(111) and the step edge of vicinal Ag(221) on CO oxidation were studied by using density functional theory calculations. For high oxygen coverage, the formation of subsurface oxygen is not only energetically favorable but also kinetically likely. CO oxidation with on-surface atomic oxygen is facile with a barrier of about 0.16 eV, whereas the reaction with subsurface oxygen is hindered because of the significant barrier for diffusion of the subsurface oxygen to the surface from the subsurface region. It is found that the adsorption of molecules (CO, atomic O and O-2) is stabilized in the presence of subsurface oxygen, and surface reactivity is enhanced. An energetically favorable reaction pathway for CO oxidation with O-2 is identified, with a reaction barrier of 0.23 eV via a unique four-center O-2 center dot center dot center dot CO intermediate. A catalytic cycle for CO oxidation on Ag(111) in the presence of subsurface oxygen is proposed. On Ag(221) surfaces, our calculations show that both the adsorption of the reactants and 02 dissociation, with a calculated barrier of 0.42 eV compared to 0.95 eV for the clean Ag(111), are promoted significantly at the step edge, and the reactivity for CO oxidation is improved accordingly.