O binding sites on stepped Pt(111) surfaces

First-principles calculations show that O adatoms are attracted to step edges on Pt(111), gaining 0.2 to 0.3 eV per nearest neighbor step-edge Pt atom, and that they favor ''fee-like'' over ''hcp-like'' sites by similar to 0.4 eV. These preferences account for the different appearances, in scanning tunneling micrographs, of O-saturated, A- and B-type island edges on Pt(111). They imply that O binds in a threefold, fee hollow site behind a B step, while twofold edge bridging is favored for A steps.