Structure of WOx/TiO2 catalysts prepared from hydrous titanium oxide hydroxide:: Influence of preparation parameters

Tungstated titania catalysts (WOx/TiO2) were prepared by wet impregnation of hydrous titanium oxide hydroxide. The influences on the catalyst structure of tungsten loading tin the range of 0-30 wt % WO3 supported on TiO2), 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 TiO2 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 % WO3/TiO2) 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 TiO2 support. A three-dimensional structure is formed even at low tungsten coverages. This tungstate overlayer retards the sintering of the TiO2 support and its phase transformation from anatase to rutile. With increasing tungsten loading, the surface area-increases and the TiO2 particle sizes and pore diameters decrease. When the tungsten loading exceeds 20 wt % WO3 and the calcination temperature exceeds 923 K, WO3 is formed. These results are supposed to help to explain the properties of these materials including acidity, reactivity in reduction, and isotope exchange.