Influence of calcination treatment on the structure of grafted WOx species on titania

TiO2/WOx supported metal oxide materials were prepared by grafting a tungsten(V)ethoxide precursor onto a titania support. The supported metal oxides were characterized by Raman spectroscopy under ambient conditions in oxygen and inert gas atmosphere. Measured under ambient conditions, the samples are sensitive to a variation in laser power. This effect is a function of the tungsten loading. During calcination of the samples in either inert gas atmosphere (N-2) or oxidizing atmosphere (O-2), this sensitivity is not observed. After cooling the samples to room temperature under inert gas conditions, a sensitivity of the surface structures to a laser power variation is again recorded. The structural changes are due to thermal effects and not to an adsorption of water. The composite band structure in the Raman spectra of the samples leads to the conclusion that octahedrally and tetrahedrally coordinated WOx surface structures as well as polytungstates are co-existing on the surface, and are forming cross linked structures. According to the XPS data, the structure of the metal oxide species depends on the monolayer coverage on the support. (C) 1999 Elsevier Science B.V. All rights reserved.