An ultraviolet-visible-near infrared study of the electronic structure of oxide-supported vanadia-tungsta and vanadia-molybdena

The ultraviolet-visible-near IR spectra of a series of catalysts have been recorded and discussed. In particular bulk and alumina-, titania- and silic a-supported V, W, and Mo oxides have been considered. Additionally mixed V- W and V-Mo-supported oxides have been investigated. The data show that, in agreement with vibrational data and other literature data, the oxides suppo rted on silica are similar to the corresponding bulk oxides. In contrast, t he spectra of the oxides supported on alumina and titania correspond to sur face oxide species where the metal stays in a lower overall coordination wi th respect to the bulk oxides. Finally, the spectra show that only in the c ase of titania-supported oxides an electronic interaction between the suppo rted metal oxide centers through the support conduction band is possible. T his allows to justify on electronic bases the activating effect of titania for vanadia, vanadia-tungsta and vanadia-molybdena catalysts used in the hy drocarbon selective oxidation catalysis and for the selective catalytic red uction of nitrogen oxide by ammonia. (C) 2001 Elsevier Science B.V. All rig hts reserved.