A first-principles study of bulk oxide formation on Pd(100).

The catalytic oxidation activity of palladium is influenced by the oxidation state of the metal. Under technologically relevant conditions, bulk and surface oxides may form and decompose. By employing first-principles calculations based on density functional theory, we have investigated the transition from the surface oxide to the bulk oxide on Pd(100). We show that the most stable orientation of the oxide film is PdO(101)@Pd(100) at any film thickness. The monolayer has unique electronic, chemical, and thermodynamic properties in comparison to thicker oxide films. In particular, carbon monoxide adsorbs by approximately 0.3 eV more strongly on thicker oxides than on the surface oxide, a fact that should influence the catalytical activity. Finally, we show that a simple model employing density functional theory energies predicts a Stranski-Krastanov growth mode for the oxide film, with a critical thickness of 1 ML. Our results give a framework for the interpretation of experiments of Pd oxide growth.