Vanadia-Titania Aerogels: III. Influence of Niobia on Structure and Activity for the Selective Catalytic Reduction of No by NH 3

Abstract

Niobia-doped vanadia-titania aerogels with 20 wt% nominal "V 2O 5" and 2-6 wt% nominal "Nb 2O 5" have been prepared by a two-stage sol-gel process with subsequent supercritical drying. A titania gel was synthesized by the addition of an acidic hydrolysant to tetrabutoxytitanium(IV) in methanol. The methanolic, pre-mixed solution of niobium(V) pentaethoxide and vanadium(V) oxide tri- n-propoxide was added after redispersing the titania gel. The influence of niobia loading on the morphological and chemical properties of the aerogels was studied. Similarly a ternary xerogel (20 wt% V 2O 5, 6 wt% Nb 2O 5) was prepared for comparison with the corresponding aerogel. The aero(xero-)gels were characterized by means of N 2 physisorption, X-ray diffraction, thermal analysis coupled with mass spectrometry, scanning electron microscopy, and vibrational spectroscopy. The meso- to macroporous aerogels possess BET surface areas of 176-203 m 2 g −1 calcination at temperatures ≤ 673 K and contain well-developed anatase crystallites of 7-8 nm mean size. For the calcination at 723 K niobia is suggested to retard both crystallization of the vanadia phase and phase transformation of anatase to rutile. The corresponding BET surface area is 114 m 2 g −1. In contrast, the xerogel calcined at 673 K is micro- to mesoporous, possesses a BET surface area of 117 m 2 g −1, and contains anatase as well as monoclinic TiO 2 with mean crystallite sizes of 11 and 15 nm, respectively. Crystalline V 2O 5 was only detected for the xerogel calcined at 773 K. In all other samples the vanadia phase consists of highly dispersed vanadium oxo clusters together with more extended structures, especially at temperatures ≥ 673 K. The catalytic properties of the aero(xero-)gels were tested with the selective catalytic reduction of NO by NH 3. The variation of the niobia loading from 2 to 6 wt% Nb 2O 5 did not affect the activity. An increase of the calcination temperature from 573 to 723 K led to a significant activity rise of the aerogel catalysts. The xerogel calcined at 673 K exhibited the highest activity among all catalysts tested.