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 N2 at 77 K, NH3-DRIFTS, H2-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.