WO x/ZrO 2 catalysts : Part 1. Preparation, bulk and surface characterization

Abstract

Zirconia supported tungsten oxide catalysts (tungsten content up to 33.7 wt.%) were prepared by equilibrium adsorption and impregnation in acid (pH 2.7) and basic (pH 10) conditions using hydrous zirconium oxide. After thermal treatment at 1073 K in air, the catalysts were characterized by surface area determination, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), diffuse optical reflectance, Raman spectroscopy and chemical analysis. The results show that tungsten is present as a dispersed phase and as monoclinic WO 3 crystallites. The density of tungsten in a dispersed form was estimated as about 5 atoms nm −2. Leaching treatments with ammonia revealed that only a fraction of the dispersed tungsten interacts directly with the support surface. The interacting species, in the form of small clusters, as revealed by EXAFS and Raman spectroscopy, control the ZrO 2 surface area and are responsible for the structural changes in the support. Subsequent thermal treatment up to 1073 K in air leaves them unchanged.