Theory for the Potential Shift for OH[sub ads] Formation on the Pt Skin on Pt[sub 3]Cr(111) in Acid

We present results of a periodic spin-density-functional theory study of the electronic structure and the local adsorption properties of the annealed alloy surface in comparison with the Pt(111) surface. Each is modeled as a four-layer slab where the two topmost layers and the adsorbates are allowed to relax. The annealed alloy has Pt segregated to the surface and is modeled by a Pt(111) monolayer covering the face bulk phase alloy. We have calculated OH and structures and adsorption energies at low coverage. The top adsorption sites are predicted to be the most stable for these adsorbates on both surfaces, but adsorption energies for both decrease on the Pt skin, with a larger decrease for OH. An empirical model based on reaction energy calculations in acid is used to estimate the reversible potential of formation from on the Pt skin and on Pt(111) in acid. A positive shift V) is predicted for the surface of the Pt skin relative to Pt(111). This result is in qualitative agreement with the 40-60 mV reduction in overpotential observed experimentally for oxygen reduction on the alloy compared to Pt surfaces, which is attributed to being a surface poison.