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 o f hydrous titanium oxide hydroxide. The influences on the catalyst structur e of tungsten loading tin the range of 0-30 wt % WO3 supported on TiO2), ca lcination temperature (varied from 473 to 973 K), and the form of the appli ed tungstate precursor (ammonium metatungstate or ammonium monotungstate) w ere investigated by surface area measurements, X-ray diffraction, thermal a nalysis, temperature-programmed reduction, vibrational and UV/vis spectrosc opy, 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 t he 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 te mperature of 923 K. Two tungstate species are characterized by two W=O band s in the vibrational spectra. One tungstate species shows a strong dependen ce of its domain size and degree of condensation on calcination temperature and tungsten loading, but the other does not. The first is attributed to a ccessible 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 tungs tate overlayer retards the sintering of the TiO2 support and its phase tran sformation from anatase to rutile. With increasing tungsten loading, the su rface area-increases and the TiO2 particle sizes and pore diameters decreas e. When the tungsten loading exceeds 20 wt % WO3 and the calcination temper ature exceeds 923 K, WO3 is formed. These results are supposed to help to e xplain the properties of these materials including acidity, reactivity in r eduction, and isotope exchange.