Transition metal surfaces under oxygen-rich conditions (abstract only)

The catalytic oxidation activity of transition metal particles in automobile catalysts is influenced by the superficial oxide phases which form under oxygen-rich reaction conditions. Here we study the thermodynamic stability of ultrathin oxide films on low-index surfaces of transition metals by means of first-principles atomistic thermodynamics calculations based on density functional theory. On Pd(111) a surface oxide with Pd(5)O(4) stoichiometry is stable, which does not correspond to any bulk structure. At the same time, a variety of metastable structures form. On Pd(100) a PdO(101) trilayer is the stable surface oxide. All structures display two-oxygen-coordinated and four-oxygen-coordinated Pd atoms, the difference lying in the spatial arrangement of these basic structural elements. On rhodium surfaces a Rh oxide trilayer forms that corresponds to a bulk oxide structure. On the Pt(111) surface the most stable superficial oxide phase is found to be a thin layer of α-PtO(2), which appears not to be reactive to either methane dissociation or carbon monoxide adsorption. On Pt(100) an α-PtO(2) layer is thermodynamically the most stable, even if Pt(3)O(4) films could grow in a coherent and stress-free manner. Bulk Pt(3)O(4) is found to be thermodynamically stable in a region around 900 K at atmospheric pressure. The enthalpy barrier for the adsorption of CO molecules on oxygen atoms of this surface is as low as 0.34 eV, and desorption of CO(2) is observed in molecular dynamics simulations to occur without any appreciable energy barrier.