Oxygen dissociation at close-packed Pt terraces, Pt steps, and Ag-covered Pt steps studied with density functional theory

Using density functional theory we have characterized O2 dissociation on flat and stepped Pt(1 1 1) surfaces. The reactivity of the steps is significantly higher than that of the flat terraces. Inclusion of Ag monoatomic chains along the Pt steps modifies the reactivity of the Pt steps towards that of the flat Pt terraces. Our investigations reveal the reaction energetics and the geometries for the molecular precursor states (MPS), transition states (TS) and final states of the dissociating oxygen. Both the MPS and TS geometries on stepped Pt involve oxygen species at the top of the step with no oxygen atoms diffusing onto the lower terrace. We further find that the Ag chains mainly modify the adsorption and reaction bond strengths while they leave the MPS and TS geometries essentially unchanged. The high reactivity of the Pt steps is explained in terms of the coordinative unsaturation of the Pt step atoms which cause a highlying valence 5d-electron system.