Enhanced photocatalytic activity of Ce³⁺-TiO₂ for 2-mercaptobenzothiazole degradation in aqueous suspension for odour control

A series of cerium ion-doped titanium dioxide (Ce³⁺-TiO ₂) catalysts with special 4f electron configuration was prepared by a sol-gel process and characterized by Brunauer-Emmett-Teller method, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), and also photoluminescence (PL) emission spectroscopy. The photocatalytic activity of Ce³⁺-TiO₂ catalysts was evaluated in the 2-mercaptobenzothiazole (MBT) degradation in aqueous suspension under UV or visible light illumination. The experimental results demonstrated that the overall photocatalytic activity of Ce³⁺-TiO₂ catalysts in MBT degradation was signigicantly enhanced due to higher adsorption capacity and better separation of electron-hole pairs. The experimental results verified that both the adsorption equilibrium constant (K_a) and the saturated adsorption amount (Γ_max) increased with the increase of cerium ion content. The results of XPS analysis showed that the Ti ³⁺, Ce³⁺, and Ce⁴⁺ ions reside in the Ce ³⁺-TiO₂ catalysts. The results of DRS analysis indicated that the Ce³⁺-TiO₂ catalysts had significant optical absorption in the visible region between 400 and 500 nm because electrons could be excited from the valence band of TiO₂ or ground state of cerium oxides to Ce 4f level. In the meantime, the dependence of the electron-hole pair separation on cerium ion content was investigated by the PL analysis. It was found that the separation efficiency of electron-hole pairs increased with the increase of cerium ion content at first and then decreased when the cerium ion content exceeded its optimal value. It is proposed that the formation of two sub-energy levels (defect level and Ce 4f level) in Ce³⁺-TiO ₂ might be a critical reason to eliminate the recombination of electron-hole pairs and to enhance the photocatalytic activity.