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 °C with a view to examine the nature of WOx species using infrared and Raman spectroscopy, X-ray photoelectron spectroscopies (XPS), high resolution and energy filtered transmission electron microscopy (HRTEM and EFTEM). When calcining at 560 °C, a large amount of W atoms remain in W5+ oxidation state, in strong interaction with the ZrO2, forming ZrOW bonds. Only a little fraction of the W atoms was expelled to the surface as isolated WOx species, which are responsible for the Brönsted acid sites. After calcining at 700 °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 Brönsted acidity. When the sample was calcined at 800 °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 WOx domains are responsible for this reaction.

Tungsten atoms are initially in strong interaction with the ZrO2 in a reduced W5+ oxidation state. As temperature increases from 560 to 800 °C, W atoms are expelled to the surface of ZrO2 forming WOx nanoclusters of ≤0.5 nm and showing high acid sites density, but poor catalytic activity. When these nanoclusters grow to ca. 3.0 nm WOx domains, acid sites decreased whereas catalytic activity increased. Figure optionsDownload as PowerPoint slide