Photooxidation of dichloroacetic acid controlled by pH-stat technique using TiO2/layer silicate nanocomposites

Nanocomposites containing anatase nanoparticles were prepared by heterocoagulation, using Na-montmorillonite and titanium dioxide obtained by hydrothermal sol–gel method. Heterocoagulation was carried out at pH 1 and 4. Based on X-ray diffraction measurements, an average particle size of 3.8–4.0 nm was calculated by the Scherrer equation for the particles intercalated between the silicate lamellae. Nitrogen adsorption studies revealed that the specific surface area of nanocomposites prepared at pH 1 varies in the range of 157–284 m2/g, depending on the TiO2 content. After preparation at pH 4, the specific surface area of the samples is lower (123–248 m2/g). UV–vis analyses of the nanocomposites showed that as TiO2 content is increased, band gap energies relative to TiO2 decrease and gradually approach the value obtained for the pure sol–gel TiO2 sample (Eg = 3.12 eV). The nanocomposites obtained were tested in photocatalytic degradation of dichloroacetic acid (DCA) in a suspension photoreactor. The reaction was quantitatively monitored during the entire irradiation time using the pH-stat technique. We found that higher catalytic efficiencies could be achieved when increasing sample TiO2 content. The photocatalytic efficiency of composites prepared at pH 1 was well below that of the samples prepared at pH 4, which was attributed to structural changes in the support brought about by the highly acidic medium. When photocatalytic degradation data were normalized to pure TiO2, composite samples containing 47% and 57% TiO2 were found to be the most efficient as compared to the 100% TiO2 sample prepared by the sol–gel method.