Vanadia-Titania Aerogels: I. Preparation, Morphological Properties, and Activity for the Selective Catalytic Reduction of NO by NH 3

Abstract

Highly dispersed vanadia-titania aerogels with high surface area have been synthesized by a two-stage sol-gel process with ensuing high-temperature supercritical drying. A titania gel was prepared by the addition of an acidic hydrolysant to tetrabutoxy-titanium(IV) in methanolic solution. The vanadium alkoxide precursor was added after redispersing the titania gel. The influences of different preparation conditions on the morphological and chemical properties of the aerogels were studied. The aerogels were characterized by means of nitrogen physisorption, X-ray diffraction, transmission electron microscopy, and thermal analysis (thermogravity, differential thermal analysis) coupled with mass spectrometry. The meso- to macroporous vanadia-titania aerogels possess a BET surface area of 140 to 220 m 2 g −1 after calcination up to 673 K and contain well-developed anatase crystallites of ca. 10 nm mean size. Crystalline V 2O 5 was detected only for samples calcined at 723 K. Ah other aerogel catalysts, calcined at lower temperature, showed no indication for long-range ordered vanadia domains. Thermal analysis revealed that even calcination at 723 K was not sufficient for complete removal of organic residues, which were entrapped during supercritical drying. The catalytic properties of the aerogels were tested for the selective catalytic reduction of NO by NH 3. An increase in the vanadia loading from 5 to 30 wt% V 2O 5 resulted in a marked increase in overall as well as specific activity, whereas the apparent activation energy decreased from 66 to 55 kJ mol −1. An increase in the calcination temperature from 573 to 673 K led to a significant rise in the activity of the vanadia-titania aerogel catalyst. The aerogel with 30 wt% V 2O 5 showed a reaction rate referred to the vanadium content (turnover frequency) that is similar to that of multiply grafted well-dispersed vanadia/titania catalysts. Among all catalysts presented in this work, this aerogel exhibited the highest reaction rate per gram of catalyst.