VANADIA, VANADIA TITANIA, AND VANADIA TITANIA SILICA-GELS - STRUCTURAL GENESIS AND CATALYTIC BEHAVIOR IN THE REDUCTION OF NITRIC-OXIDE WITH AMMONIA

Metal oxide xerogels of pure vanadia, vanadia-titania, and vanadia-titania-silica were prepared by the hydrolysis of vanadium, titanium, and silicon alkoxides. The structural and chemical transformations of the gels occurring during thermal treatment under oxidizing and inert gas atmosphere were studied using thermoanalytical methods (TG, DTA) combined with mass spectroscopy, X-ray diffraction, and electron microscopy. Crystallization of the amorphous vanadia was found to be considerably retarded in the mixed gels compared to the pure vanadia gel. The mixed gels remain amorphous at 520 K, whereas crystallization already occurs in the pure vanadia gel at this temperature. All mixed gel phases separate after stabilization at 870 K, with the formation of V2O5 platelets and globular TiO2 (anatase) particles. No other phases were detected by X-ray diffraction at this temperature. Addition of silica sol increases the overall surface area of the gel catalysts. This, however, does not increase the dispersion of vanadia in the amorphous matrix or stabilize it sufficiently to prevent crystallization under oxidizing conditions. Heat treatment of the gels at temperatures close to the melting point of vanadia results in rapid transformation of anatase to rutile. The stability and catalytic activity of the gels for the selective catalytic reduction of NO with NH3 (SCR) were studied following thermal treatments under in situ and oxidizing (7% O2) atmosphere. Low temperature SCR testing (T ≤ 520 K) showed that the same kinetic behavior typical of titania-supported vanadia exists in all gels, with activity behavior being a function of gel composition and dispersion. © 1992.