Platinum-group and noble metals under oxidizing conditions

Platinum-group metals and noble metals play an important role in catalysis, for total oxidation as well as for partial oxidation reactions. Only in recent years have advances in microscopic, spectroscopic and computer simulation techniques made it possible to investigate the interaction of oxygen with metallic substrates at an atomistic level. We present an overview on the formation of adsorption structures and surface oxides on Rh, Pd, Ag, Cu and Pt surfaces, with particular focus on the phase diagrams calculated from first-principles thermodynamics. The low-index (111), (100) and (110) surfaces as well as selected high-index surfaces have been considered. We predict the stability of novel structures such as the c(4 x 6) on Cu(100) and the alpha-PtO2 trilayer on Pt(100). The knowledge of the Gibbs free surface energies allows us to predict the adsorbate-induced changes in the thermodynamic equilibrium shape of metal nanoparticles. At low oxygen chemical potential, corresponding to clean surfaces, the (111) facets dominate the particle shape, with a significant contribution from (100) facets. But even under these conditions a small fraction of the overall surface corresponds to more open facets. As oxygen adsorption sets in, their contribution becomes larger. At high oxygen partial pressures, surface oxides form on the platinum-group metals. They do not only display different chemical properties than the metal, but also determine the exposed surface orientations of the particles. The latter effect might play an important role for the catalytic activity of transition metal nanoparticles.