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-0 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-0 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. (C) 2002 Elsevier Science B.V. All rights reserved.