Selective catalytic reduction of NO with NH3 over mesoporous V2O5-TiO2-SiO2 catalysts

Mixed vanadia-titania-silica catalysts (3 or 6 wt% V2O5, and 16-34 wt% TiO2) were one-pot prepared by sol-gel and hydrothermal methods in the presence of surfactants. Sodium silicate (25.5-28.5% silica) or tetraethylorthosilicate was used as a precursor for silica; tetraisopropylorthotitanate or titanyl acetylacetonate, for titania; and vanadyl sulfate or vanadium acetylacetonate, for vanadia. Cetyltrimethylammonium bromide, octadecyltrimethylammonium bromide, or dodecylamine was used as a surfactant. The catalysts were characterized by adsorption and desorption curves of N-2 at 77 K, NH3-DRIFTS, H-2-TPR, XRD, in situ Raman spectroscopy, XPS, and TEM. The catalysts were tested in NO reduction with ammonia using a total flow rate of 100 ml/min and a feed composition of nitric oxide 0.1 vol%, ammonia 0.1 vol%, oxygen 3 vol%, in helium. Vanadia was found to be entrapped in these catalysts as V(V) species in which the population of V=O monomeric bonds strongly depended on the dispersion. Titanium also existed in a very oxidated state, and for high dispersions it adopted a tetrahedral coordination. These structures led to surfaces on which mainly Lewis acid sites are effective under reaction conditions. Under such conditions, the dominant route followed an Eley-Rideal mechanism, yielding in such a way very high activity and selectivity. A comparison with a conventional V2O5-TiO2 catalyst led to the conclusion that the intrinsic activity of one-pot prepared polymeric sol-gel catalysts is higher. (C) 2003 Elsevier Science (USA). All rights reserved.