Adsorption and photocatalytic properties of TiO₂/mesoporous silica composites from two silica sources (acid-leached kaolinite and Si-alkoxide)

Two different mesoporous silicas (MPS) were synthesized by hydrothermal treatment in NaOH solution of two SiO₂ sources. These were microporous silica (MicroPS) derived from selectively acid leached metakaolinite and tetraethylorthosilicate (TEOS). The hydrothermal syntheses of the MPSs were performed at a ratio of SiO₂/cetyltrimethyl- ammonium bromide (CTABr)/NaOH/ H₂O = 1/0.1/0.3/150. The specific surface areas (SBET) of the MPSs from MicroPS (MPS(M)) and TEOS (MPS(T)) were 1070 and 1020 m ²/g, respectively. Composites of MPS (75 mass%) with TiO₂ (25 mass%) were prepared using both SiO₂ and two commercial TiO ₂ powders, P25 and ST-01. The adsorption-desorption behavior of methylene blue (MB) by the four resulting composites and the two MPSs alone was unique in showing partially reversible behavior. The maximum MB adsorption, observed in the composite of ST-01 with MPS(M), designated (S/M), was 0.034 mmol/g. The rates of MB adsorption in the dark and photodecomposition under UV illumination were considerably different for the four composites and two TiO₂ powers, and followed the order ST-01\S/T\P25\P/ T ≈ P/M ≪ S/M. The removal rate of MB by the composite S/M by adsorption and photodecomposition was further enhanced by heating at 700C. Direct photodecomposition of MB without adsorption in the dark was greatly enhanced in the composites, especially in that composed of MPS(M) and ST-01.