Mesoporous TiO2–SiO2 aerogels with hierarchal pore structures

Freestanding blocks of binary oxides, TiO2–SiO2 aerogel containing highly ordered mesophase structures were successfully prepared by a new synthesis method involving partial solvent evaporation followed by supercritical extraction and drying. The new method allows the routine preparation of large, crack-free aerogels of high titanium content (i.e., Ti/Si ⩽ 0.75 or up to 50 wt.% Ti), ordered mesopores (i.e., 2–20 nm), large surface area (i.e., 400–900 m2 g−1) and pore volume (i.e., 0.7–2.6 cm3 g−1). Aerogels with well-ordered mesopores were obtained for Ti/Si atom ratios of 0.04–0.08. The size of ordered mesopore domains decreases with increasing titanium loading, and TS75 aerogels with Ti/Si = 0.75 display discontinuous microdomains of ordered mesoporosity within disordered phases interspersed with crystalline anatase TiO2. The greater permeability of the TS75 pore network resulted in fifteen times better activity for photocatalytic oxidation of airborne trichloroethylene compared to commercial Degussa P25 TiO2 and more than twice that TiO2–SiO2 aerogel (TS100) of similar titanium loading but with disordered and tortuous pore network.