Nanocomposites of crystalline TiO2 particles and mesoporous silica: molecular selective photocatalysis tuned by controlling pore size and structure

Preformed well-crystallized TiO2 particles (P-25, 20-30 nm in diameter) were directly embedded into surfactant-templated mesoporous silica having tuned pore sizes (1.4-5.3 nm). These nanocomposites (containing as much as 60 wt% TiO2) demonstrated molecular-selective photocatalysis toward the decomposition of mixed alkylphenols in water. Even in the presence of highly concentrated phenol, the nanocomposite decomposed dilute nonylphenols (NP) and heptylphenol (HP) completely, which is in contrast to pristine TiO2, highlighting the importance of the nanocomposite structure. The molecular selectivity could be tuned by systematically controlling the pore size: catalysts having large pores (2.7 5.3 nm) decomposed NP and HP at similar fast rates due to their preferential adsorption onto the catalysts. Catalysts having small pores (1.4-1.9 nm and smaller micropores) decomposed HP faster than NP because of their different diffusion rates. Furthermore, it was found that the nanocomposite structures were significantly improved by TiO2 particle surface alkyl-grafting: SEM and TEM images of the nanocomposite prepared from surface alkyl-grafted TiO2 particles revealed that the TiO2 particles were almost completely surrounded by mesoporous silica, whereas in the case of unmodified TiO2 particles, parts of them were left uncovered with the mesoporous silica. The nanocomposite prepared from alkyl-grafted TiO2 particles showed much higher molecular selectivity and activity than that prepared from unmodified TiO2.