Visible-Light-Driven Photocatalytic Activities Of Vitamin B₁₂-based Nanocomposites Under Aerobic Conditions

Vitamin B₁₂ and its derivatives,which are naturally green and non-polluting,exhibited high photocatalytic activity for dehalogenation reaction.But so far,most of the research based on B₁₂ catalyst was carried out under nitrogen atmosphere,which seriously limited their practical applications.In this thesis,three nanocomposites N3-TiO₂-B₁₂,N3-ZrO₂-B₁₂2 and RhB-TiO₂-B₁₂2 were systhesized through co-immobilizing the photosensitizer and the vitamin B₁₂ derivative on the surface of P25 TiO₂ or ZrO₂ nanoparticles.The visible-light-driven photocatalytic activities of these nanocomposites for benzotrichloride dechlorination were investigated systematically under aerobic conditions.The main contents of the thesis are summarized as follows:Firstly,the composite RhB-TiO₂-B₁₂ was prepared by immobilizing Rhodamine B（RhB）as photosensitizer and the B₁₂ derivative[（CN）（H₂O）Cob（III）7COOH]Cl as catalyst onto the surface of TiO₂ nanoparticles simultaneously.Its structure was characterized by UV-vis diffuse reflectance spectroscopy,XRD and Zeta potential measurement et al.The content of the B₁₂ derivative was 1.2×10^-5 mol/g and the ratio of RhB and B₁₂ content in the composite was about 1.3:1,which was determined by the change of the UV-vis absorption spectroscopy.The visible-light-driven photocatalytic activity of RhB-TiO₂-B₁₂ under aerobic conditions was investigated using benzotrichloride as a substrate in methanol.The trichlorotoluene was almost completely converted into methyl benzoate after 2.5 h irradiation.When using B₁₂-TiO₂ as catalyst,the productive rate was only 39%.The significantly enhanced photocatalytic activity of the composite RhB-TiO₂-B₁₂ should be attributed to the effective utilization of visible light by using RhB as photosensitizer.In addition,this composite catalyst can be easily reused without obvious loss of catalytic efficiency.In order to improve the utilization of visible light and construct more efficient photocatalytic system,N3 dye was used instead of RhB.Two novel composites N3-TiO₂-B₁₂ and N3-ZrO₂-B₁₂ were synthesized by co-immobilizing N3 dye as photosensitizer and the B₁₂ derivative[（CN）（H₂O）Cob（III）7COOH]Cl as catalyst onto the surface of TiO₂ and ZrO₂ nanoparticles respectively.Their structures were characterized by UV-vis diffuse reflectance spectroscopy,XRD and Zeta potential measurement et al.The modification with N3 dye and the B₁₂ derivative didn’t change the crystal type of TiO₂ and ZrO₂.The content of the B₁₂ derivative was4.9×10^-5 mol/g and the ratio of N3 dye and B₁₂ content in the two composites was all about 2:1,which was determined by the change of the UV-vis absorption spectroscopy.The composite N3-TiO₂-B₁₂2 exhibited high photocatalytic activity for benzotrichloride dechlorination driven by visible light under aerobic conditions.Benzotrichloride was investigated using as a substrate in methanol.The trichlorotoluene was almost completely converted into methyl benzoate after 1 h irradiation.When using N3-ZrO₂-B₁₂2 as catalyst,the yield of methyl benzoate was only 35%.These results indicated that TiO₂ as catalyst support was far superior to ZrO₂ in such a hybrid catalytic system,attributing to its appropriate energy band structure between the Ru（II）photosensitizer and the B₁₂ catalyst.The utilization of P25 TiO₂ as catalyst support not only accelerated the electron transfer from the Ru（II）photosensitizer to the Co center of the catalyst,but also effectively suppressing the back electron-transfer process.Furthermore,the composite catalyst N3-TiO₂-B₁₂could also be easily recycled for use and maintained high catalytic activity.