Selective catalytic reduction of nitric oxide with ammonia: II. Monolayers of Vanadia Immobilized on Titania—Silica Mixed Gels

Abstract

Monolayers of vanadia were immobilized on pure titania and silica, and on mixed gel carriers of titania and silica containing 1, 10, 20 and 50 mol% of titania using the selective reaction of vanadyl triisobutoxide with surface hydroxyl groups of the carriers. The catalysts were investigated with regard to their structural properties and their activity in selective catalytic reduction (SCR) of nitric oxide with ammonia. The textural properties of both pure and impregnated carriers depended strongly on their chemical composition. Low titania content (1 mol%) led to a marked increase of the BET surface area and the pore volume, whereas with a higher titania content (>10 mol%) these properties decreased drastically. X-ray diffraction and high-resolution electron microscopy indicated that silica was present as an amorphous phase in all carriers, whereas crystalline domains of titania (anatase) were found in carriers containing 10 mol% and more of titania. On all carriers the immobilized vanadia species were well dispersed and disordered. Temperature programmed reduction showed for all samples only a single peak for the reduction of the immobilized vanadia layer. The temperature of maximum reduction rate which reflects the ease of reduction of the supported vanadia layer decreased with increasing titania content of the carrier. It was highest for vanadia supported on pure silica (790 K) and lowest for vanadia supported on titania (700 K). This behavior was attributed to the markedly stronger support interaction of titania compared with silica. The highest activity for SCR was found for vanadia supported on mixed gels containing 20 and 50 mol% titania. As a result of the weak support interaction, the vanadia species supported on pure silica tended to agglomerate when exposed to higher temperatures under SCR conditions. This agglomeration was suppressed when titania was added to the silica matrix of the carrier. Of all the catalyst preparations only the vanadia layer supported on equimolar titania-silica and on pure titania maintained stable activity for SCR when exposed to sulfur dioxide-containing feed.