Redox and acid reactivity of wolframyl centers on oxide carriers:: Bronsted, Lewis and redox sites

Catalysts prepared by impregnating tungsten oxide on alumina, titania and zirconia and their mixed oxides have been characterized by skeletal IR, IR of adsorbed ammonia, Raman and UV-VIS-NIR spectroscopies and by temperature programmed reduction. In all cases catalysts with W loading well below the monolayer have been taken into consideration. Surface mono-ore wolframyl species with similar low coordination structure have been found to largely predominate in all the supports. However, the W=O bond length, the Lewis acidity, the charge transfer transition energies and the reducibility of the WO, species strongly depend on the support nature. In particular, the wolframyls on alumina are most acidic, have higher charge transfer transition energies and are less easily reducible than those on titania. The wolframyls on zirconia show intermediate properties. Evidence is given for the different behavior of wolframyl centers in spite of their similar geometric "molecular" structure. The different properties of wolframyl centers on the supports used here explain the different behavior of these materials for hydrocarbon conversion, in the selective catalytic reduction of NO by ammonia and as precursors of hydrodesulphurization catalysts. (C) 2001 Elsevier Science B.V. All rights reserved.