Vanadia–silica aerogels. Structure and catalytic properties in selective reductionof NO by NH 3

Abstract

Vanadia-silica aerogels, containing 10 to 30 wt% V 2O 5, and a xerogel were prepared from vanadium(V) oxide triisopropoxide and vanadium (III) acetylacetonate (V(III) acac) precursors using the solution-sol-gel method and different drying processes, including conventional evaporative and high-temperature and low-temperature supercritical drying. The behavior of these mixed oxides in the selective catalytic reduction of NO by NH 3 was tested and compared to that of other vanadia-silica and vanadia-titania catalysts. The structural and catalytic properties of the sol-gel derived vanadia-silica mixed oxides were found to be mainly influenced by the drying method, the vanadia content and the vanadia precursor used. For a particular vanadia content (10 wt%), low-temperature supercritical drying and evaporative drying resulted in significantly higher vanadia dispersion than high-temperature supercritical drying, which led to crystalline V 2O 5. Turnover frequencies for SCR at temperatures T < 475K were highest for low-temperature aerogels containing well-dispersed vanadium oxide species. Exposing these catalysts to higher temperatures under SCR conditions resulted in agglomeration/redispersion phenomena and at temperatures T > 550K best catalytic behavior was observed with vanadia-silica mixed oxides for which Raman spectroscopy indicated the presence of crystalline V 2O 5, as was the case for aerogels obtained by high-temperature supercritical drying and the low-temperature aerogel with the highest vanadia content (30 wt%).