Vanadia grafted on TiO2-SiO2, TiO2 and SiO2 aerogels -: Structural properties and catalytic behaviour in selective reduction of NO by NH3

Vanadia has been grafted on high surface area titania-silica, titania and silica sterogels prepared using the sol-gel method and ensuing drying by extraction with supercritical CO2. The aerogels exhibited favourable textural properties (high surface area combined with dominant mesoporosity) and the capability to stabilise vanadia in a well-dispersed form up to high loadings. Titania aerogel with a BET-surface area of 165 m(2) g(-1) proved to be the most suitable support for vanadia affording excellent activity for the selective catalytic reduction (SCR) of NO by NH3 at low temperature (400 K). With titania-silica aerogels, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy indicated a preferential deposition of vanadia on Ti sites at low vanadia loading, SCR activities were found to depend mainly on the vanadia loading and the support material. High loadings of dispersed vanadia (6-10 mu mol m(-2)) were favourable on all aerogel supports. Vanadia grafted on titania aerogel was superior in activity compared to vanadia deposited on the other aerogels. V2O5-SiO2 as well as V2O5-TiO2-SiO2 catalysts showed significant deactivation at temperatures ca. >500 K. Deactivation of vanadia deposited on TiO2-SiO2 could be avoided at loadings ca. >6 mu mol m-2. Deactivated catalysts showed a decrease in both Bronsted as well as Lewis sites, as revealed by desorption studies after exposure to SCR conditions. (C) 1999 Elsevier Science B.V. All rights reserved.