SPECTROSCOPIC INVESTIGATION OF THE STRUCTURE OF SILICA-SUPPORTED VANADIUM-OXIDE CATALYSTS AT SUBMONOLAYER COVERAGES

Surface vanadia species on a high-surface-area silica support have been investigated by UV/visible diffuse reflectance, infrared and laser Raman spectroscopies. The catalysts were prepared and characterized under water-free conditions, as the hydration state of the surface strongly influences the structure of the dispersed vanadia. Two types of surface species have been identified. Monomeric and oligomeric vanadia species, with a tetrahedral coordination geometry of oxygen ligands around the central vanadium ion, give rise to a narrow V = O stretching absorption at 1040 cm-1. The second species is characterized by a Raman band at 920 cm-1 and a charge-transfer absorption at 28 000 cm-1, and corresponds to vanadia ribbons of limited lateral extent, with the vanadium ion situated in the centre of a square pyramid. Vibrational data from aqueous vanadate ions are inadequate as a reference for describing the vanadia species on a dehydrated surface. For vanadia supported on titania and on titania/silica mixed oxides, extended patches of a two-dimensional vanadia monolayer are known to be formed, as characterized by a charge-transfer band at 25 000 cm-1 and Raman bands at 1015, 700, 485 and 265 cm-1. In contrast, no such layer structures are generated on silica owing to the weak interaction of the immobilized species with the support. Instead, rapid formation of crystalline V2O5 is observed even at low loadings. To describe these differences, the concept of wetting is discussed. Adsorption of water onto Lewis-acidic vanadia sites creates new Bronsted centres that are more acidic than the surface silanol groups. A comparison of H2O and D2O adsorption experiments shows that water interacts with the surface vanadyl groups; the V = O double bond is transformed into a geminal V(OH)2 diol in this process. This behaviour is different from that of vanadia supported on TiO2 or TiO2/SiO2 mixed oxides, where water is physisorbed to the vanadyl group with a concomitant decrease in V = O bond order.