Vanadia grafted on TiO_2-SiO_2 aerogels Structural properties and catalytic behaviour in selective reduction of NO by NH_3

摘要

Vanadia has been grafted on high surface area titania-silica, titania and silica aerogels prepared using the sol-gel method and ensuing drying by extraction with supercritical CO_2. 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 NH_3 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. V_2O_5-SiO_2 as well as V_2O_5-TiO_2-SiO_2 catalysts showed significant deactivation at temperatures ca. >500 K. Deactivation of vanadia deposited on TiO_2-SiO_2 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.