Highly enhanced photocatalytic degradation of tetramethylammonium on the hybrid catalyst of titania and MCM-41 obtained from rice husk silica

A hybrid photocatalyst consisting of TiO2 and the mesoporous support was prepared by loading TiO2 nanoparticles on MCM-41 which was synthesized from silica obtained from rice husk (abundant agricultural waste in Thailand). The catalysts with varying content of TiO2 were characterized by X-ray diffraction (XRD), nitrogen adsorption–desorption, high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray (EDX) analysis and UV–vis diffuse reflectance spectroscopy. The MCM-41 structure was retained after the loading of TiO2 but its surface area decreased as a result of the partial pore blocking. The TiO2 morphology and band gap were not affected by the dispersion on the support but the presence of the support prevented the TiO2 nanoparticles from agglomerating in the neutral pH range. When the TiO2 loading on the support was higher than 10 wt.%, the activity of the hybrid catalyst was saturated. The activity of TiO2/MCM-41 for the photocatalytic degradation of tetramethylammonium (TMA) in aqueous slurry was significantly higher than that of the unsupported TiO2. The optimal TiO2 loading on the support was 10 wt.% at which the complete conversion of TMA was achieved in 90 min irradiation at pH 7. The unsupported TiO2 photocatalyst could convert only 20% of TMA in the same irradiation time and condition. The photocatalytic degradation of TMA proceeded by generating demethylated intermediates such as tri-, di- and monomethylammonium ions which were finally converted to ammonium ions.