The Photocatalytic Activity Enhancement And Mechanism Research For Organic Degradation Over TiO₂and Bi₂WO

Photocatalytic degradation of organic contaminant is an attractive method to solve energy crisis and environment problem, because it can realize mineralization of organic contaminant under O2, water and sunlight. In most catalysts, TiO2possesses the highest value for scientific research as it is chemical stable, low-cost and eco-friendly. However, there are still some challenges for practical application. On the one hand, TiO2only absorbs UV light at wavelength shorter than400nm, which accounts for a small part of sunlight reaching the Earth surface. The photocatalytic activity of TiO2is very low under sunlight irradiation. On the other hand, under UV irradiation, the recombination rates of photogenerated carriers of TiO2are very high, which is disadvantage for organic contaminant mineralization.According to the two defects of TiO2, in this work, we try to improve the charge carrier separation efficiency of TiO2by choosing proper promoter and investigate the detailly mechanism. In addition, we researched the visible light photocatalytic activity of Bi2WO6and enhanced the activity by adding Bi2O3. The main contents and results are as follows:In the first part, sky-blue and pure CuB2O4was synthesized by high temperature solid-phase reaction and was used as a cocatalyst for TiO2. The mixture of CuB2O4/TiO2was prepared by adding CuB2O4particle in TiO2aqueous suspension. The samples were characterized by X-Ray Powder Diffraction (XRD), N2adsorption-desorption curves, X-ray photoelectron spectroscopy (XPS) and UV-vis diffuse reflection spectrum (DRS). The results showed that the mixture of CuB2O4and TiO2has no physical changes. The photocatalytic activity of catalysts was assessed by phenol degradation and potassium dichromate reduction. The results show that CuB2O4/TiO2is more active than TiO2and/or CuB2O4, the best activity of CuB2O4/TiO2is about2.7times that of TiO2. We hypothesize that the photocatalytic activity enhancement was ascribed to high charge separation efficiency through excluding pH, catalyst separation and light adsorption. The charge transfer between CuB2O4and TiO2has been demonstrated by photoelectrochemical experiment, fluorescence spectra and phenol degradation under N2atmosphere. The charge transfer promotes the carrier separation efficiency.In the second part, flower-like Bi2WO6was prepared by modified hydrothermal method, and then sintered in air at350℃for3h. the photocatalytic acitivity for phenol degradation of is very low. Therefore, series of Bi2WO6containing Bi2O3catalysts were prepared through a simple mixing method. Phenol oxidation in water under UV light was used as a model reaction. As the amount of Bi2O3in the mixture increased, the photocatalytic activity of the mixture increased, and then decreased. A maximum activity was observed with the catalyst containing12.5%(w) of Bi2O3, which was about4times that of Bi2WO6. Solid characterization revealed that the composite was a mixture of β-Bi2O3and Bi2WO6. Furthermore, the photocurrent of water oxidation over β-Bi2O3/Bi2WO6thin film electrode was much larger than the sum of the phootocurrents of Bi2O3and Bi2WO6thin film electrodes. It is proposed that there is a valence hole transfer from Bi2WO6to β-Bi2O3, improving the efficiency of charge separation, and consequently increasing the rate of phenol degradation.