Hydrophobic Modification of a Mesoporous Silica Surface Using a Fluorine-Containing Silylation Agent and Its Application as an Advantageous Host Material for the TiO2 Photocatalyst

Hexagonal mesoporous silica (HMS) material, which is commonly used in catalysis and the nanostructural design field as an adsorbent or host material, was hydrophobically modified via a simple silylation process using a fluorine-containing silylation agent, triethoxyfluorosilane [TEFS or SiF(OEt)(3)]. Structural analyses performed by XRD and N-2 adsorption-desorption suggested that the mesoporous structure and large surface area of the HMS were retained even after the modification. Surface chemical information obtained from XPS indicated that the TEFS reagent was covalently anchored to the surface hydroxyl sites and fluorine atoms existed in a Si-F bonding state. From the H2O adsorption isotherm on the modified HMS, it was clearly shown that the modified HMS was more hydrophobic compared with the unmodified HMS. Moreover, the hydrophobically modified HMS was used as support for the TiO2 photocatalyst. From EXAFS analysis, as well as UV-vis spectroscopy and XRD, it was confirmed that the highly crystallized TiO2 particles were formed in the hydrophobic HMS pores, whereas the titanium species on the original HMS showed physicochemical features similar to those of isolated titanium oxide. With the aim of evaluating its availability as an efficient support, photocatalytic degradation of 2-propanol and phenol diluted in water was performed. Both the adsorption property and the TiO2 photocatalytic activity toward these organic compounds were dramatically improved by increasing the content of the TEFS reagent because the hydrophobically modified HMS worked as an efficient adsorbent to condense organic molecules instead of H2O molecules. By evaluating the obtained physicochemical parameters and degradation rate constant of organic molecules, we clarified the advantages of surface modification by TEFS and the participation of the hydrophobic support in the photocatalytic reactions.