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

Abstract

To rationalize structure-reactivity relationships for mixed-metal oxide catalysts, well-defined systems are required. Studies involving the deposition of nanoparticles and clusters of VO_x, CeO_x and WO_x 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 VO or WO groups present in VO_x/TiO₂(110), VO_x/CeO₂(111) and WO_x/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/CeO_x/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 MO_x/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.