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 z irconia and their mixed oxides have been characterized by skeletal IR, IR o f 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 sp ecies with similar low coordination structure have been found to largely pr edominate in all the supports. However, the W=O bond length, the Lewis acid ity, the charge transfer transition energies and the reducibility of the WO , species strongly depend on the support nature. In particular, the wolfram yls on alumina are most acidic, have higher charge transfer transition ener gies 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 "molecul ar" structure. The different properties of wolframyl centers on the support s used here explain the different behavior of these materials for hydrocarb on 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.