The generality of surface vanadium oxide phases in mixed oxide catalysts

The nature of VO(x) sites in mixed oxides of supported VO(x) (on both pure oxide and mixed oxide supports), molecular sieves, zeolites, clays, hydrotalcites, stochiometric bulk oxides and bulk solid solutions were investigated. For supported metal oxides, zeolites and molecular sieves, the VO(x) species are exclusively present as surface VO(x) phases below monolayer coverage or the maximum dispersion limits. For layered clays and hydrotalcites, the VO(x) is present in the hydroxide layers at modest temperatures and react with the clays and hydrotalcites at higher temperatures (>350 degrees C) when their layered structures decompose. Surface VO(x) species are always also present for bulk oxides and bulk solid solutions. The rapid diffusion kinetics of VO(x), due to its low Tammann temperature, coupled with the lower surface free-energy of vanadium oxide are responsible for the universal presence of surface VO(x) sites on all mixed oxide materials. Furthermore, surface reactivity studies demonstrate that the surface VO(x) sites are the catalytic active sites for all V-containing mixed oxide catalytic materials. (C) 2010 Elsevier B.V. All rights reserved.