The generality of surface vanadium oxide phases in mixed oxide catalysts

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

The nature of VOx sites in mixed oxides of supported VOx, pure oxides and mixed oxides, molecular sieves, zeolites, clays, hydrotalcites, stochiometric bulk oxides and bulk solid solutions were investigated. For supported metal oxides, zeolites and molecular sieves, the VOx species are exclusively present as surface VOx phases below monolayer coverage or the maximum dispersion limits. For layered clays and hydrotalcites, the VOx is present in the hydroxide layers at modest temperatures and react with the clays and hydrotalcites at higher temperatures (>350 °C) when their layered structures decompose. Surface VOx species are always also present for bulk oxides and bulk solid solutions. The rapid diffusion kinetics of VOx, due to its low Tammann temperature, coupled with the lower surface free-energy of vanadium oxide are responsible for the universal presence of surface VOx sites on all mixed oxide materials. Furthermore, surface reactivity studies demonstrate that the surface VOx sites are the catalytic active sites for all mixed oxide catalytic materials.Figure optionsDownload high-quality image (138 K)Download as PowerPoint slideResearch highlights▶ Surface VOx species are present for supported metal oxides, zeolites, and molecular sieves contain VOx species at or below monolayer coverage. ▶ VOx is present in the hydroxide layers of clays and hydrotalcites. ▶ Surface VOx is always present for bulk oxides and bulk solid solutions. ▶ Surface VOx sites are the catalytic active sites for all V-containing mixed oxide catalytic materials.