Chemisorbed Oxygen on the Au(321) Surface Alloyed with Silver: A First-Principles Investigation

The adsorption of oxygen on kinked Au(321) slabs is investigated theoretically on the basis of density functional theory. On-surface, subsurface, and surface-oxide forms of O are analyzed and compared on pure gold and on the surfaces containing silver atoms. At low O coverage (0.1 ML) subsurface O species are shown to be unstable both thermodynamically and kinetically due to a low barrier for conversion to stronger bound on-surface chemisorbed oxygen. The presence of Ag in the near-surface region was shown to increase the binding strength of on-surface as well as subsurface O, but the activation barrier for releasing subsurface O to the surface remains essentially unaffected by the presence of Ag. At oxygen coverage 0.2 ML or higher, the most stable surface arrangements of Oatoms are chain-like structures consisting of linear -O-Au-O- fragments. Subsurface O atoms being a part of such chains are significantly stabilized. We examine phase transitions between the clean surface and possible stable oxidized surface structures as a function of temperature and O-2 partial pressure. Ag atoms replacing Au on the Au(321) surface are shown to stabilize the O-covered surface with respect to the clean surface. Pre-existent chemisorbed atomic oxygen is predicted to facilitate the dissociation of molecular oxygen on the pure and alloyed gold surfaces.