Nonhydrolytic vanadia-titania xerogels:: Synthesis, characterization, and behavior in the selective catalytic reduction of NO by NH3

V2O5-TiO2 catalysts With V2O5 contents ranging from 6 to 18 wt% were obtained by calcination at 773 K of xerogels prepared by a nonhydrolytic sol-gel route from VOCl3, TiCl4, and (Pr2O)-Pr-i. These materials were characterized by X-ray diffraction, micro-FT-Raman spectroscopy, static V-51 NMR, SEM, N-2 physisorption, temperature-programmed desorption of ammonia (NH3-TPD), and temperature-programmed reduction by H-2 (H-2-TPR). In all cases mesoporous solids were obtained with specific surface areas up to 87 m(2) g(-1). Raman spectroscopy indicated the presence of the same vanadia species as in conventional vanadia-titania catalysts. Up to a critical V2O5 loading (between 10 and 12 wt%), all the vanadium was highly dispersed in the form of monomeric vanadyl and polymeric vanadate species, whereas for higher loadings V2O5 crystallites also formed. Most of the vanadium species (80-90%) were accessible to reduction by H2. The catalytic properties of these materials were evaluated in the selective catalytic reduction of NO by NH3. The estimated turnover frequency 'TOF at 473 K (mole of NO converted per mole of V per hour) went through a maximum value (16.6 h(-1)) at a V loading of 8 wt%, corresponding to an estimated surface vanadia loading of about 10 mu mol m(-2). The N2O selectivity increased upon V loading as expected. A compromise between activity and high N-2 selectivity (98% at 623 K) could be found with the material containing 6 wt% V2O5. (c) 2006 Elsevier B.V. All rights reserved.