Selective Catalytic Reduction of NO by NH₃ over Aerogels of Titania, Silica, and Vanadia

Abstract

Aerogels of titania, titania-silica, and titania-vanadia were prepared with titanium butoxide, silicon ethoxide, and vanadium triisopropoxide as precursors. The titania and titania-silica aerogels were then used as supports for vanadia, introduced by the incipient wetness impregnation of vanadium triisopropoxide and the subsequent calcination at 773 K. The structures of vanadia and titania in these samples were characterized by X-ray diffraction and Raman spectroscopy, and their catalytic properties by the selective catalytic reduction (SCR) of NO with NH 3 . With H 2 O and SO 2 in the feed stream, the SCR activity decreased with increasing vanadia loading. Samples of high activity all contained anatase TiO 2 , but an active vanadia species only needed to be in close proximity and interacting with, and not necessarily deposited on the surface of crystalline titania. Co-gelling was thus an effective way to prepare an active sample in a single step, as demonstrated by the SCR data of the titania-vanadia aerogel. The addition of niobia, up to 10 weight %, did not appreciably change the surface area, structure, or SCR activity of the titania aerogel supported vanadia.Titania-supported vanadia catalysts have been widely used in the selective catalytic reduction (SCR) of nitric oxide by ammonia (1, 2). In an attempt to improve the catalytic performance, many researchers in recent years have used different preparation methods to examine the structure-activity relationship in this system. For example, Ozkan et al (3) used different temperatureprogrammed methods to obtain vanadia particles exposing different crystal planes to study the effect of crystal morphology. Nickl et al (4) deposited vanadia on titania by the vapor deposition of vanadyl alkoxide instead of the conventional impregnation technique. Other workers have focused on the synthesis of titania by alternative methods in attempts to increase the surface area or improve its porosity. Ciambelli et al. (5) used laser-activated pyrolysis to produce non-porous titania powders in the anatase phase with high specific surface area and uniform particle size. Solar et al. have stabilized titania by depositing it onto silica (6). In fact, the new SCR catalyst developed by W. R. Grace & Co.-Conn., SYNOX™, is based on a titania/silica support (7).