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

Abstract

V2O5-TiO2 catalysts with V2O5 contents ranging from 6 to 18wt% were obtained by calcination at 773K of xerogels prepared by a nonhydrolytic sol–gel route from VOCl3, TiCl4, and iPr2O. These materials were characterized by X-ray diffraction, micro-FT-Raman spectroscopy, static 51V NMR, SEM, N2 physisorption, temperature-programmed desorption of ammonia (NH3-TPD), and temperature-programmed reduction by H2 (H2-TPR). In all cases mesoporous solids were obtained with specific surface areas up to 87m2g−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 12wt%), 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 473K (mole of NO converted per mole of V per hour) went through a maximum value (16.6h−1) at a V loading of 8wt%, corresponding to an estimated surface vanadia loading of about 10μmolm−2. The N2O selectivity increased upon V loading as expected. A compromise between activity and high N2 selectivity (98% at 623K) could be found with the material containing 6wt% V2O5.