Photocatalytic Degradation Of Methylene Blue Using Bi₂WO₆ Catalyst Under Visible Light Irradiation

In recent years, a large number of investigations have focused on the semiconductor photocatalyst for its wide applications in solar energy conversion and environmental purification since Fujishima and Honda discovered the photocatalytic splitting of water on the TiO₂ electrodes in 1972．Among various oxide photocatalyst, Bi₂WO₆ was reported that it had a suitable valence band and possessed interesting physical properties, such as ferroelectric piezoelectricity, pyroelectricity, a nonlinear dielectric susceptibility and catalytic behavior. It has been recognized as a photocatalst for water splitting and pollutant degradation under visible light irradiation.The Bi₂WO₆ photocatalyst were prepared by simple co-precipiation process and solid-state reaction method, metal particles were loaded on the Bi₂WO₆ catalyst by deposition method. The photocatalytic activity of Bi₂WO₆ was evaluated by methylene blue under visible light irradiation (λ>400 nm).The MB degradation was examined to assess the pH value, initial concentration of MB , illumination intension, air flux, illumination times, the dosage of catalyst and metal particles loaded. The results demonstrated that the activity of catalyst which were prepared by co-precipiation process much better than prepared by solid-state reaction method. In the former MB/Bi₂WO₆ system, MB was effectively degraded by about 80% within 4 h, when the dosage of catalyst was 0．2 g/L, the initial concentration of MB was 10 mg /L and pH was 9．30．Results of kinetic study show that the degradation of MB using Bi₂WO₆ followed the first-order reaction kinetic model.In the present study the adsorptive properties of Bi₂WO₆ to MB were studied. The results include the following: the maximum MB adsorption capacity was 16．46 mg/g and the adsorption accord with Langmuir adsorption isotherm preferable; the adsorption velocity of menthylene blue executed by B₂WO₆ is comparable high. The adsorption rate could reach adsorption balance within 30 min at 25℃when we added 0．02 g Bi₂WO₆ to 100 mL 10 mg/L menthylene blue solution. To sum up, Bi₂WO₆ could rapidly and effectively adsorb menthylene blue in solution; the adsorption of menthylene blue increases alongside the rise of temperature and augment of the Bi₂WO₆ usage.But the activity of catalyst was decreased when Bi₂WO₆ catalyst loaded metal particles. It maybe because of the variation of crystal structure after metal ions entered. Among the metal-Bi₂WO₆, Ag-loaded photocatalyst had a better effect, and also found that when the content of Ag was 3 wt.%, the best photocatalytic degradation efficiency of MB was obtained. The samples were characterized with XRD, SEM, TG-DSC. It was found that the diffraction peaks of Bi₂WO₆ catalyst appeared at 600℃. TG-DSC analysis indicated that NO₃ had been removed at 450℃, and Bi₂WO₆ had been in steady state after 800℃.