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 “fcc-like” over “hcp-like” sites by $\sim 0.4\mathrm{eV}$. 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 three-fold, fcc hollow site behind a B step, while twofold edge bridging is favored for A steps.