Vanadia supported on TiO 2SiO 2 mixed oxide gels: Structure of the dispersed phase and activity for the selective catalytic reduction of NO with NH 3

Abstract

Supported vanadia catalysts were prepared by reacting vanadyl triisopropoxide with titania-silica mixed oxide supports of fixed Ti: Si mol ratio (1 : 4 and 1 : 1), but of varying pore structure, surface area, and crystallinity. The latter properties were controlled by varying the sol-gel preparation parameters. Although the selectivity pattern for the selective catalytic reduction (SCR) remained essentially the same over all catalysts, the specific activity, defined on a per V 5+ basis, varied with the support. The vanadia content in all catalysts corresponded to less than one monolayer (defined as 0.10 wt% V 2O 5m 2 or 11 μmol V 5+ /m 2), and structural data indicate that the vanadia is well dispersed. Raman spectroscopy was used to differentiate between vanadia species immobilized on TiO 2 and SiO 2 domains, respectively. These results revealed that the mixed oxide gels can be categorized into two main types exhibiting titania- or silica-like behavior. A weak interaction that results from a two-stage hydrolysis procedure of the alkoxide precursors of the constituents is characteristic for silica-like supports. This contrasts with the titania-like behavior observed for TiO 2SiO 2 mixed oxides prepared by single-stage hydrolysis, where the vanadia-support interaction is strong. No crystalline V 2O 5 phases were detected. TPR profiles show that the reducibility of vanadia, as evidenced by shifts in the temperature of maximum hydrogen consumption, is dependent upon the composition and preparation method of the mixed gel supports. An increase in the value of T max can be correlated with a decrease in the turnover frequency (NO converted per V 5+ site) of the SCR reaction.