Energetics of H 2O dissociation and CO ads+OH ads reaction on a series of Pt–M mixed metal clusters: a relativistic density-functional study

Abstract

A relativistic density-functional study of CO adsorption, the energetics of H 2O dehydrogenation, and the CO ads+OH ads 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 CO ads 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 Pt n M 10− n were used to simulate the catalyst surfaces. The CO ads(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 H 2O, OH, and H, the reaction energies and activation barriers for H 2O ads(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 CO ads(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 H 2O ads(M) dissociation and formation of OH ads(M) than does pure Pt. However, the oxidative removal of CO ads(Pt) by OH ads(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 H 2O ads(M) dissociation and the CO ads(Pt)+OH ads(M) combination reaction, the best alloying metals for CO oxidation are predicted to be Mo, W, and Os, with Ru following closely.