AN IR SPECTROSCOPY STUDY OF THE STATE AND LOCALIZATION OF VANADIUM OXO SPECIES ADSORBED ON TIO2 (ANATASE)

The state and localization of vanadium-oxo species adsorbed on anatase have been studied by IR spectroscopy of probe molecules, UV-vis spectroscopy, XPS, and chemical analysis. The adsorption was carried out from acidic solutions of ammonium vanadate. Two samples were used as adsorbents: (i) pure anatase, characterized by the presence of two kinds (strong and weak) of Lewis acid sites and two kinds of surface hydroxyl groups, and (ii) anatase treated with hydrogen peroxide where physically and chemisorbed peroxides exist. It was established that the active sites for adsorption were, in the first case, the surface hydroxyl groups and the strong Lewis sites only. The catalyst thus obtained (VT1) was characterized by a Lewis acidity, mainly due to the weak acid sites of the support, as well as by a small amount of Brønsted acid sites representing surface VOH groups. When the adsorption proceeded on the peroxide-treated sample, the vanadium coverage was higher, since all Lewis acid sites of anatase participated in the process. A protonic acidity was also observed on this sample (VT2), but, contrary to VT1, the Lewis acidity was associated with the existence of V5+. On the basis of the results obtained, structures of the surface species are proposed. Some possibilities of using the samples as catalysts are discussed. © 1992.