Catalysis and the nature of mixed-metal oxides at the nanometer level: special properties of MOx/TiO2(110) {M= V, W, Ce} surfaces

To rationalize structure-reactivity relationships for mixed-metal oxide catalysts, well-defined systems are required. Studies involving the deposition of nanoparticles and clusters of VOx, CeOx and WOx on TiO2(110) and other well-defined oxide surfaces have shown novel structures that have special chemical properties. Dimers of vanadia and ceria have been found on TiO2(110), monomers of vanadia on CeO2(111), and (WO3)(3) clusters on TiO2(110). The V=O or W=O groups present in VOx/TiO2(110), VOx/CeO2(111) and WOx/TiO2(110) surfaces dislay a very high activity for the selective oxidation of alkanes and the dehydrogenation of alcohols. The non-typical coordination modes imposed by TiO2(110) on ceria nanoparticles make possible the direct participation of this oxide in catalytic reactions and enhance the dispersion of metals on the titania substrate. Au/CeOx/TiO2(110) surfaces display an extremely high catalytic activity for CO oxidation and the water-gas shift reaction. In general, the chemical behavior of the MOx/TiO2(110) {M = V, Ce or W} surfaces reflects their unique structure at the nanometer level. These simple models can provide a conceptual framework for modifying or controlling the chemical properties of mixed-metal oxides and for engineering industrial catalysts.