Fabrication of titania and titania-silica aerogels using rapid supercritical extraction

Titania (TiO2) and titania-silica (TiSi) aerogels are suitable for photocatalytic oxidation of volatile organic compounds for pollution mitigation; however, methods for fabricating these aerogels can be complex. In this work we describe the use of a rapid supercritical extraction (RSCE) technique to prepare TiO2 and TiSi aerogels in as little as 8 h. The RSCE technique uses a metal mold and a four-step hydraulic hot press procedure to bring the solvents in the sol-gel pores to a supercritical state and control the supercritical fluid release process. Resulting TiO2 aerogels were powdery with BET surface areas of 130-180 m(2)/g, pore volumes similar to 0.5 cm(3)/g and skeletal densities of 3.6 g/mL. Monolithic TiSi aerogels were made using two different methods. An impregnation process, in which titania precursor was added to a silica sol-gel, took 4-8 days to complete with a 7-h RSCE and resulted in translucent aerogels with high surface area (560-650 m(2)/g) and pore volume (2.0-2.6 cm(3)/g), bulk densities ranging from 0.1 to 0.4 g/mL and skeletal densities of 2.3 g/mL. A co-precursor method for preparing TiSi aerogels took 8 h to complete. The precursor chemical mixture was poured directly into the mold and processed in a 7-h RSCE process. The resulting aerogels were opaque, with high surface areas (510-580 m(2)/g), low bulk density (0.03 g/mL), skeletal densities of 2 g/mL and pore volumes of 2.6-3.5 cm(3)/g. Preliminary solar simulator studies show that TiO2 and TiSi aerogels are capable of photocatalytic degradation of methylene blue in aqueous solution.