Migration and oxidation of tungsten species at the origin of acidity and catalytic activity on WO3-ZrO2 catalysts

A WO3-ZrO2 catalyst, prepared by a precipitation method, was annealed at 560, 700 and 800 degrees C with a view to examine the nature of WO, species using infrared and Raman spectroscopy, X-ray photoelectron spectroscopies (XPS), high resolution and energy filtered transmission electron microscopy (HRTEM and EIFTEM). When calcining at 560 degrees C, a large amount of W atoms remain in W5+ oxidation state, in strong interaction with the ZrO2, forming Zr-O-W bonds. Only a little fraction of the W atoms was expelled to the surface as isolated WOx species, which are responsible for the Bronsted acid sites. After calcining at 700 degrees C, all W atoms migrated to the surface, remaining in full W6+, oxidation state, forming ca. 0.5 nm nanoclusters. In these nanoclusters, all W atoms are exposed on the surface and promote acid sites, mainly Bronsted acidity. When the sample was calcined at 800 degrees C, the tungsten nanoclusters grew to partially reduced ca. 3.0 nm WOx domains. These WOx domains showed very low acid sites density and strength, however, they showed the highest catalytic activity in n-hexane isomerization reaction; supporting the hypothesis that redox properties of WO, domains are responsible for this reaction. (c) 2006 Elsevier B.V. All fights reserved.