Molecular Precursors in the Dissociative Adsorption ofO2on Pt(111)

Ab initio local-spin-density calculations for the adsorption of ${\mathrm{O}}_2$ on Pt(111) are presented. We identify two distinct, but energetically almost degenerate chemisorbed precursors. A superoxolike paramagnetic precursor is formed at the bridge site, with the molecule parallel to the surface. A second peroxolike nonmagnetic precursor is formed in the threefold hollow, with the atom slightly canted in a top-hollow-bridge geometry. The nature of the barrier for dissociation into atoms adsorbed in the hollows is explored.