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.

• Received 16 July 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.4481