CHARACTERIZATION OF V2O5/SIO2-MIXED GEL CATALYSTS AND TIO2/SIO2-MIXED GEL CATALYSTS BY RAMAN-SPECTROSCOPY

The preparation of V2O5/SiO2 mixed gels by the sol-gel process provides a novel class of active catalysts for the selective catalytic reduction (SCR) of nitric oxide [1]. Raman spectroscopy is used to obtain information on the structure of these systems, in which the vanadia species are molecularly dispersed in an amorphous SiO2 matrix. This three-dimensionally linked network exhibits a high stability against thermal aggregation of the vanadia particles. At high vanadia contents small crystalline domains of V6O13 and V3O7 are detected in the Raman spectrum, in accordance with high resolution electron microscopy studies. The controlled grafting of vanadia layers onto the surface of oxidic supports is another attractive route to highly active SCR catalysts. The interaction of the carrier material with the immobilized vanadia layers can be tuned by using TiO2/SiO2 mixed oxide supports. Raman spectroscopy has been used to characterize monolayers of vanadia, which had been immobilized on a 20% TiO2/80% SiO2 mixed oxide from a vanadyl alkoxide precursor. Characteristic vibrations observed in the Raman spectrum, as well as UV-visible reflection data, indicate that the most abundant vanadia species on the surface consists of disordered polymeric arrays: the central vanadium ion is surrounded by one tightly bound vanadyl oxygen, four oxygen ions in the basal plane, and a distant sixth oxygen ligand in bridging position to the support.