Energetics of H2O dissociation and COads+OHads reaction on a series of Pt–M mixed metal clusters: a relativistic density-functional study

A relativistic density-functional study of CO adsorption, the energetics of H2O dehydrogenation, and the COads+OHads reaction has been carried out on a series of Pt–M mixed metal clusters. The metal surface–vacuum interface simulation provides insight into the mechanism of COads oxidation on Pt-based bi-functional catalysts. The secondary metals (M) examined are Ru, Sn, Mo, W, Re, Os, Rh, Ir, Cu, Zn, Ge, Pb, and Zr. Cluster models of PtnM10−n were used to simulate the catalyst surfaces. The COads(Pt) adsorption energies on Pt, Pt–C and C–O bond lengths, force constants, stretching frequencies in mixed Pt–M surfaces are calculated. On the basis of the calculated adsorption energies of H2O, OH, and H, the reaction energies and activation barriers for H2Oads(M) dissociation on the M site are estimated. For most of the mixed Pt–M metal surfaces, the presence of M weakens the Pt–C bond and lowers the C–O stretching frequency. The COads(Pt) adsorption energy is decreased dramatically by the presence of Mo, W, Os, and Re. These metals also show much higher activity as bi-functional catalysts toward H2Oads(M) dissociation and formation of OHads(M) than does pure Pt. However, the oxidative removal of COads(Pt) by OHads(M) is not as favorable on bi-metallic Pt–Mo, Pt–W, Pt–Os, and Pt–Re as on pure Pt, because these alloying metals adsorb OH too strongly. On the basis of the energetics of both H2Oads(M) dissociation and the COads(Pt)+OHads(M) combination reaction, the best alloying metals for CO oxidation are predicted to be Mo, W, and Os, with Ru following closely.