Performance, Deactivation and Regeneration of SnO2/TiO2 Nanotube Composite Photocatalysts

SnO2/TiO2 nanotube composite photocatalysts were synthesized by microwave-assisted hydrothermal and micro-emulsion methods. The photocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM/EDX), and electrochemical techniques. Toluene was chosen as a model pollutant to evaluate the performance, deactivation, and regeneration behavior of the photocatalysts under ultraviolet (UV) and vacuum ultraviolet (VUV) irradiation. The results show that ternary heterojunctions of SnO2/TiO2 nanotube composite photocatalysts including anatase TiO2 (A-TiO2)/rutile TiO2 (R-TiO2), A-TiO2/SnO2, and R-TiO2/SnO2 were successfully created. They were able to separate photogenerated electron-hole pairs efficiently, and promote photocatalytic activity accordingly. SnO2/TiO2 showed the best photocatalytic performance. Under UV or VUV irradiation, the toluene degradation rate of SnO2/TiO2 was 100%, and the CO2 formation rate (k(2)) of SnO2/TiO2 was approximately 3 times higher than that of P25. Because of the low mineralization rate under UV irradiation, the refractory intermediates generated can occupy active photocatalytic sites on the photocatalyst surface, which hinders the photocatalytic oxidation rate. After 20 h of UV irradiation, the k(2) of SnO2/TiO2 decreased from 138.5 to 76.1 mg.m(-3).h(-1), implying that the photocatalysts can be deactivated quickly. VUV irradiation was employed to regenerate the deactivated SnO2/TiO2 because reactive species such as center dot OH, O-2(-center dot), O(D-1), O(P-3), and O-3 can be generated. These play an important role in the oxidation of refractory intermediates on the photocatalyst surface, and k(2) increased to 143.6 mg.m(-3).h(-1) accordingly. Therefore, UV photodegradation combined with VUV regeneration could be a feasible photocatalytic process because of a synergistic effect between UV and VUV.