Structure of WOx/TiO2 Catalysts Prepared from Hydrous Titanium Oxide Hydroxide: Influence of Preparation Parameters

Tungstated titania catalysts (WO x /TiO 2 ) were prepared by wet impregnation of hydrous titanium oxide hydroxide. The influences on the catalyst structure of tungsten loading (in the range of 0−30 wt % WO 3 supported on TiO 2 ), calcination temperature (varied from 473 to 973 K), and the form of the applied tungstate precursor (ammonium metatungstate or ammonium monotungstate) were investigated by surface area measurements, X-ray diffraction, thermal analysis, temperature-programmed reduction, vibrational and UV/vis spectroscopy, and X-ray absorption spectroscopy. The data show that tungsten loadings giving higher than monolayer coverage of the TiO 2 and the application of a high-surface-area titania precursor lead to new structural properties of the surface tungstate phase. A tungstate overlayer is formed that is stable at loadings up to ca. two monolayers (20 wt % WO 3 /TiO 2 ) at a calcination temperature of 923 K. Two tungstate species are characterized by two W O bands in the vibrational spectra. One tungstate species shows a strong dependence of its domain size and degree of condensation on calcination temperature and tungsten loading, but the other does not. The first is attributed to accessible outer segments of a three-dimensional tungstate structure and the latter to the interface providing the linkage to the TiO 2 support. A three-dimensional structure is formed even at low tungsten coverages. This tungstate overlayer retards the sintering of the TiO 2 support and its phase transformation from anatase to rutile. With increasing tungsten loading, the surface area increases and the TiO 2 particle sizes and pore diameters decrease. When the tungsten loading exceeds 20 wt % WO 3 and the calcination temperature exceeds 923 K, WO 3 is formed. These results are supposed to help to explain the properties of these materials including acidity, reactivity in reduction, and isotope exchange.