Reactivity and stability of platinum and platinum alloy catalysts toward the oxygen reduction reaction

Density functional theory (DFT) is used to study the reactivity of Pt and Pt-M (M: Pd, Co, Ni, V, and Rh) alloy catalysts towards the oxygen reduction reaction (ORR) as a function of the alloy overall composition and surface atomic distribution and compared to that on pure Pt surfaces. 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. It is found that most homogeneous distribution PtxM catalysts thermodynamically favor the dissociation of adsorbed OOH in comparison with pure Platinum and adsorb strongly O and OH due to the strong oxyphilicity of the M elements. On the other hand, in all cases skin Platinum surfaces catalysts do not favor the dissociation of adsorbed OOH and do favor the reduction of M-O and M-OH with respect to Platinum. Considering the overall pathway of the reactions to catalyze the ORR most of the skin Platinum monolayer catalysts provide more negative free energy changes and should behave at least in a similar way than Platinum in following order: Pt3V (skin Pt) > Pt3Co (skin Pt) > Pt3Ni (skin Pt) > Pt > PtPd (skin) > Pt4Rh (skin Pt) > PtPd3 (skin ). In all cases, the reactivity is shown to be not only sensitive to the overall composition of the catalyst, but most importantly to the surface atomic distribution. Proposed electrochemical dissolution reactions of the catalyst atoms are also analyzed for the ORR catalysts, by computing the free energy changes of Platinum and bimetallic Pt-X (X: Co, Pd, Ni, and Rh) catalysts. It is found that Platinum is thermodynamically more stable than Pt-alloys in Pt3Co, Pt3Pd, Pt3Ni and Pt4Rh.