Constrcting Photocatalytic Composites Based On Electronic Regulation Of Polyoxometalates For Removal Of Typical Pollutants From Water Performance And Mechanisms

Photocatalysis technology is considered to be a new technology with the most potential to solve the energy crisis and environmental pollution.Therefore,finding,designing and developing efficient and stable photocatalysts has become the key to the application of this technology.At present,the single photocatalyst has many shortcomings,such as narrow photoresponse range,low photoquantum efficiency of electron/hole composite,and poor photostability.Semiconductor material compounding has proved to be an effective way to solve the above problems.Polyoxometalates（POMs）have similar properties to semiconductors,which are good electronic storage devices including excellent photoelectric performance and adjustable redox potential.However,most POMs are difficult to excite under visible light and easily soluble in polar solvents,so their wide application is limited.Therefore,in the paper,the insoluble POMs with UV photoactivity are combined with narrow bandgap semiconductor materials to synthesize a composite photocatalyst with wide response.At the same time,POMs is expected to act as an electronic trap in the composite material to promote the separation of carriers of composite photocatalysis and improve the catalytic activity.Based on the above considerations,this article combines CdS and AgBr with cesium salt of heteropoly acid to construct a series of catalysts and characterize their systems;the photocatalytic performance is evaluated by simulating the sunlight in the laboratory and discuss the mechanism of catalytic removal of pollutants in water.The main contents are as follows:（1）CdS-sensitized nanocomposite CdS/Cs₃PW₁₂O₄₀was synthesized by electrostatic self-assembly-hydrothermal post-treatment method.The crystal structure of the sample was studied by XRD.FT-IR spectroscopy showed that the Keggin-type polyacid structure in the composite catalyst remained intact.Potential titration determined that the zero charge point of the composite catalyst was about p H 5.1.Further,the specific surface area and pore structure of the sample were measured by BET,the morphology of the sample was analyzed by SEM/TEM and EDX,the valence state of the element was characterized by XPS,the light response range of the catalyst was analyzed by UV-vis DRS.In addition,the charge separation performance was characterized by PL.The study found that the composite catalyst with EDTA to reduce Cr（VI）had good catalytic activity under the irradiation of UV and visible light.We further studied the effect of catalyst hydrothermal treatment temperature,CdS loading,solution p H,catalyst dosage,Cr（VI）and EDTA concentration on photocatalytic reduction of Cr（VI）.Finally,the mechanism of reducing hexavalent chromium under visible light was discussed.Although the activity of the composite catalyst had decreased in the cycle experiment,the activity could basically be restored after corresponding treatment.Therefore,CdS/Cs₃PW₁₂O₄₀was a stable and excellent UV and visible light driven catalyst.（2）A simple precipitation method was used to synthesize the composite materials of Keggin series POMs(Cs_2.5PW₁₂,Cs_3.5Si W₁₂,Cs_2.5PMo₁₂)and silver bromide.The samples were characterized by XRD,FT-IR,SEM,UV-vis DRS and XPS,and the charge transfer and flat band potential were studied by electrochemical method.The performance of the catalyst was evaluated by degrading AR18 under visible light.The results showed that the catalytic activity of AgBr combined with Cs_2.5PW₁₂and Cs_3.5Si W₁₂was significantly improved,but the catalytic activity of AgBr combined with Cs_2.5PMo₁₂was not significantly improved.AgBr/Cs_2.5PW₁₂and AgBr/Cs_3.5Si W₁₂have similar activity in visible light catalytic degradation of AR18.The experimental results of selective inhibition on active species showed that·O₂^-,·OH and h⁺were all involved in the degradation of AR18 in the catalytic degradation system of Cs_2.5PW₁₂and Cs_3.5Si W₁₂,but the main active species was·O₂^-.In the catalytic degradation of AR18 system by Cs_2.5PMo₁₂,the main active substances were·O₂^-and h⁺,·OH has little effect.Different POMs and AgBr would affect the performance,the optimum recombination ratio and the optimum p H condition of the final composite catalyst.Finally,according to the different band positions of POMs/AgBr composite systems and the different performance of catalytic degradation of AR18,the mechanism was reasonably speculated.