Density functional theory (DFT) calculations are used to describe the adsorption of H-2, CO and H2O on the close packed surfaces of Pt, Ru, Pt3Sn and PtRu. The calculations show substantial differences in the adsorption energies from one system to the next, even in cases where the local adsorption geometry is the same. This ligand effect is ascribed to changes in the electronic structure of the surface atoms due to interactions with the surroundings, and the physical origin of the effect is discussed. The calculated adsorption energetics is used as input into a simple kinetic model, which is developed to describe the electro-oxidation of CO and H-2 at conditions relevant to the anode of a PEM fuel cell. The model is very simple and allows us to express the kinetics of a promoted anode surface relative to the activity of pure Pt directly from the calculated adsorption energy differences. In agreement with experiments, the model shows Ru, PtRu and Pt3Sn to start oxidizing CO at lower potentials than Pt. In addition, the model shows PtRu to have a considerably lower overpotential for H-2 oxidation in the presence of CO than Pt. Pt3Sn is even better than PtRu, while Ru is found to be considerably poorer than Pt. Within the model we can analyze the origin of the promoting effect of alloying. We find that for the CO oxidation reaction the lower overpotentials for Ru, PtRu and Pt3Sn are due to the fact that Ru and Sn are much better than Pt at dissociating H2O. The promoting effect of Ru and Sn on H-2 oxidation reaction in the presence of CO, on the other hand, can be explained by the fact that Ru and Sn affects Pt in the surface to bond CO weaker, thus decreasing the CO coverage under working conditions of the electrode.
|Publication status||Published - 2003|