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 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 87 m2 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 μmol m−2. The N2O selectivity increased upon V loading as expected. A compromise between activity and high N2 selectivity (98% at 623 K) could be found with the material containing 6 wt% V2O5.
Journal: Applied Catalysis B: Environmental - Volume 69, Issues 1–2, 1 December 2006, Pages 49–57