Effect of SiO2 on co-impregnated V2O5/WO3/TiO2 catalysts for the selective catalytic reduction of NO with NH3

Co-impregnation of vanadia and tungsta on titania produced highly active catalysts for the reduction of nitrogen oxides with ammonia but showed aging tendencies at temperatures around 600 °C. In order to preserve their low temperature activity after hydrothermal aging up to 650 °C, the catalysts were enriched with Si during the co-impregnation process using different SiO2 precursors and amounts. The optimized catalyst composition comprised ca. 4 wt% SiO2, was stable after aging at 650 °C, could preserve the low temperature activity and was more active than the catalysts prepared from commercially available silica-stabilized TiO2 supports. The origin of the effect of SiO2 was investigated by XRD, BET, TEM, STEM and 29Si MAS NMR. SiO2 remained amorphous at all temperatures and 2–4 wt% SiO2 hampered the anatase crystallite growth at 600 °C and 650 °C. In contrast to WO3, SiO2 was unevenly distributed over the TiO2 particles and ca. 60–70% of Si was present as bulky amorphous SiO2 agglomerates after aging at 650 °C. Small and extended SiO2 polymeric entities were identified to be essential for delaying the offset of the catalyst sintering. With the addition of a small amount of SiO2 in the co-impregnation process, the low temperature activity of these catalysts could be preserved up to an aging temperature of 650 °C.