Density functional theory study of the initial oxidation of the Pt(111) surface

We used density functional theory calculations to examine the initial stages of oxidation of the Pt(111) surface. Consistent with prior studies, our calculations predict that oxygen atoms adsorb on fcc sites and form p(2x2) and p(2x1) structures at coverages of 0.25 and 0.50 ML, respectively. In addition to various surface configurations of oxygen on fcc sites, we examined subsurface oxygen and clustering of oxygen atoms on the surface. We find that subsurface oxygen is not the precursor to the oxidation of the Pt(111) surface. Instead, we predict a strong preference for the formation and growth of one-dimensional Pt oxide chains within the p(2x1) structure. In particular, at coverages above 0.50 ML, additional oxygen atoms prefer to aggregate between the close-packed oxygen rows formed by the p(2x1) structure and induce large buckling (similar to 1.8 A) and modification of the charge of the surface Pt atoms. The result is an oxide compound with threefold and fourfold Pt-O coordination that grows as a one-dimensional chain running parallel to the oxygen rows of the p(2x1) structure. Furthermore, half of the oxygen atoms in the Pt oxide chains reside near hcp sites, contrary to some reports that oxygen atoms reside only on the fcc sites on Pt(111). Our results agree well with a recent scanning tunneling microscopy study and suggest a precursor mechanism to the oxidation of metal surfaces involving Pt oxide chain formation and growth on terraces at moderate oxygen coverages. Our results should have important implications to current models of NO and CO oxidation on Pt(111) and potentially on studies of the initial oxidation of other transition-metal and bimetallic surfaces.