Tungsten deposits facilitate oxidation of the NiAl(110) surface

The alumina film formed by oxidation of NiAl(110) has gained enormous attention as a surface-science compatible model system for a crystalline and atomically flat oxide surface. A main disadvantage is its small thickness of only 0.5 nm that limits possible uses in catalytic studies at elevated temperature and pressure. This work demonstrates how small amounts of tungsten pre-deposited onto the NiAl surface modify the oxidation characteristic. Oxidizing the surface at 850 K in the presence of W particles increases the film thickness to 2.5 nm, a value that rises to more than 5 nm at 1050 K oxidation temperature. Thickness, stoichiometry, and morphology of the thickened alumina films are analyzed in detail by means of X-ray photoelectron spectroscopy, electron-diffraction, and scanning tunneling microscopy. The promoted oxide formation is ascribed to the unique redox properties of tungsten that stabilizes O₂ molecules from the gas phase and produces active oxygen species for further reaction.