The catalytic oxidation activity of platinum particles in automobile catalysts is thought to originate from the presence of highly reactive superficial oxide phases which form under oxygen-rich reaction conditions. Here we study the thermodynamic stability of platinum oxide surfaces and thin films and their reactivities toward oxidation of carbon compounds by means of first-principles atomistic thermodynamics calculations and molecular dynamics simulations based on density functional theory. On the Pt(111) surface the most stable superficial oxide phase is found to be a thin layer of alpha-PtO2, which appears not to be reactive toward either methane dissociation or carbon monoxide oxidation. A PtO-like structure is most stable on the Pt(100) surface at oxygen coverages of one monolayer, while the formation of a coherent and stress-free Pt3O4 film is favored at higher coverages. Bulk Pt3O4 is found to be thermodynamically stable in a region around 900 K at atmospheric pressure. The computed net driving force for the dissociation of methane on the Pt3O4(100) surface is much larger than that on all other metallic and oxide surfaces investigated. Moreover, 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 CO2 is observed to occur without any appreciable energy barrier in molecular dynamics simulations. These results, combined, indicate a high catalytic oxidation activity of Pt3O4 phases that can be relevant in the contexts of Pt-based automobile catalysts and gas sensors.

The thermodynamic equilibrium shape of rhodium and palladium crystals are predicted under conditions from ultrahigh vacuum to high oxygen pressures using the Gibbs-Wulff construction. The analysis is based on data obtained from ab initio calculations for the adsorption of oxygen on the low-index ͑111͒, ͑100͒, and ͑110͒ surfaces, and the stepped ͑311͒, ͑211͒, and ͑331͒ surfaces. While the close-packed ͑111͒ facets dominate the shape in the low-coverage cases, the higher adsorption energies at the more open surfaces lead to a rounding of the crystallite. A linear correlation between the surface energies of the clean surfaces and the respective adsorption energies is found.

Adsorption and dissociation of water on different oxygen- and iron-terminated hematite(0001) surfaces at monolayer coverage have been studied by density-functional theory calculations, including a Hubbard-like+U correction. We considered six possible surface terminations, including four oxygen- and two iron-terminations. Binding energy of water on these terminations can be as large as 1.0 eV. On these terminations the energy barrier for the dissociation of the molecularly adsorbed water is less than 0.3 eV, and in few cases the dissociation is even spontaneous, i.e., without any detectable barrier. Our results thus suggest that water can be adsorbed on the α-Fe2O3(0001) surface dissociatively at room temperature, as previously found by experiment. This study also presents a very first theoretical insight into the adsorption and dissociation of water on all known terminations of the hematite(0001) surface.