Preparation of vanadia-containing aerogels by rapid supercritical extraction for applications in catalysis

We have developed a variety of vanadia-containing aerogels using a rapid supercritical extraction method that employs a contained mold held within a hydraulic hot press. Sol–gels were prepared with silica, silica–titania, titania and alumina backbones using a variety of precursor chemicals: tetramethyl orthosilicate, tetraethyl orthosilicate, vanadyl acetylacetonate, titanium (IV) butoxide and titanium (IV) isopropoxide. Total fabrication time (including processing) was ca. 9 h for sol–gels prepared without solvent exchange. When sol–gel aging and solvent exchange steps are employed, the overall fabrication time was on the order of several days. The aerogels were characterized by physical appearance (color), bulk density, Brunauer–Emmett–Teller surface area, Barrett–Joyner–Halenda pore distribution, scanning electron microscopy and Fourier transform infrared spectroscopy. All of the resulting aerogels have average bulk densities below 0.12 g/cm3 and surface areas ranging from 500 to 770 m2/g, with the exception of the vanadia–titania materials, which have surface areas of 140 m2/g. Variation in speciation and complexation of vanadia in the resulting aerogels is evidenced by significant differences in color observed for various precursor recipes and processing conditions. Preliminary catalytic testing under conditions that mimic automotive exhaust gas indicates that the aerogels are catalytically active for the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides.