Structure and reactivity of titania-supported molybdenum and tungsten oxides

Abstract

TiO 2 supported MoO 3 and WO 3 have been prepared by aqueous impregnation of low-area anatase (∼10 3m 2g −1) and Degussa P-25 (55 m 2g −1) with aqueous solutions of molybdate and tungstate salts, followed by calcination: products have been characterised by temperature-programmed reduction (TPR), Laser Raman spectroscopy (LRS) and X-ray photoelectron spectroscopy (XPS). Reaction of Ti-OH groups on these supports with MoOCl 4, and the form of the variation of Mo/Ti and W/Ti XPS intensity ratios, lead to estimates of Mo and W contents corresponding to monolayers of MoO x and WO x adsorbed species, the surface Mo:Ti and W:Ti ratios being a little more than 0.5. Possible structures are suggested. The techniques employed provide evidence that with MoO 3 contents between one and four monolayer equivalents a “disordered MoO x” phase is formed. A corresponding phase in the case of WO x has not been detected, but in both cases “paracrystalline” oxide phases are seen at higher loadings: these cover 2.5–20% of the monolayer surface and differ in their reducibility from the corresponding bulk oxides. A simple model is presented to account quantitatibely for the variation in XPS intensity ratios with oxide content.