VANADIA SUPPORTED ON TITANIA AEROGELS - MORPHOLOGICAL PROPERTIES AND CATALYTIC BEHAVIOR IN THE SELECTIVE REDUCTION OF NITRIC-OXIDE BY AMMONIA

Vanadia on titania-aerogel catalysts with a wide range of surface areas (S-BET:70-190 m(2) g(-1)) and vanadia loadings (2.5-17 mu mol m(-2)) have been prepared and tested for the selective catalytic reduction of nitric oxide with ammonia (SCR). The high surface area of the titania aerogels allows the immobilisation of larger amounts of active vanadia species per gram of support, which results in higher overall SCR activities, compared to catalysts based on conventionally prepared titania. Transmission electron microscopy, X-ray diffraction measurements and the SCR activities indicate total dispersion of the vanadia in the VOx/TiO2-aerogel samples up to a theoretical monolayer of V2O5. Raman spectroscopy showed that monomeric vanadyl species and two-dimensional polyvanadate layers were present on the titania surface. Increase of the vanadia loading favoured the formation of polyvanadate species. The intrinsic SCR activity depends on the vanadia (surface) concentration, whereas the morphology and acidity of the titania (anatase) support seem to have little influence. Temperature-programmed desorption after exposure to SCR feed gas (SCR-TPD) of both the titania supports and the VOx/TiO2 catalysts, indicated that the acidity of the titania aerogel supports depends on the crystallite size, as smaller crystallites (d(c) < 10 nm) result in increased acidity. Upon deposition of less than 6 mu mol m(-2) (V) on the TiO2-aerogels, the adsorption capacity of the samples decreases and centres are formed that bind ammonia more strongly. These catalysts show low SCR activity. Further vanadia immobilisation restores the total amount of adsorbed species and leads to the formation of adsorption centres with moderate acidity, which results in up to ten times higher SCR activity. The occurrence of oxidation products of adsorbed ammonia with high vanadia loadings reflects the enhanced oxidation capability of these catalysts at elevated temperatures.