Theoretical analysis of reactivity on Pt(1 1 1) and Pt-Pd(1 1 1) alloys

The reactivity of Pt-Pd alloy surfaces towards the oxygen reduction reaction is studied as a function of the alloy overall composition and surface atomic distribution and compared to that on pure Pt surfaces. The systems include Pd and Pt monolayers on various substrates and Pt3Pd, PtPd and PtPd3 surfaces of ordered alloys. Reactivity is evaluated on the basis of the adsorption strength of oxygenated compounds which are intermediate species of the four-electron oxygen reduction reaction, separating the effect of the first electron-proton transfer from that of the three last electron-proton transfer steps. None of the alloys are found to provide better sites than those of pure Pt both for O2 dissociation and for the reduction of O and OH to water; with the skin surfaces being the closest to pure Pt. The results are discussed in relation to those found in 10-atom clusters of similar compositions and to experiments.