Preparation Of Polyoxometalate Composite Photocatalysts And Study On Their Environmental Catalytic Activity

As the industry develops to become more refined,more and more new chemicals enter the water environment,which may lead to paralysis of biological treatment systems.Traditional water treatment technologies with biological methods as the mainstream have been unable to meet actual needs,and is urgently looking for new technologies.Photocatalytic technology represented by TiO2 can reduce heavy metal ions while degrading organic pollutants under ultraviolet l,ight irradiation.It is a clean and effective pollution control technology.However,there are problems in that photogenerated charges are easily recombined,quantum efficiency is low,and solar energy utilization rate is low.Polyoxometalates(POMs)consist of polyatomic(Mo,W,V,etc.)and heteroatoms(P,Si,Fe,etc.)coordinated by oxygen atoms.POMs are a very ordered and structurally complete molecular ion.It is usually one to several nanometers in size and can be regarded as a quantum-level semiconductor material.The excited state of POMs can act as a storage tank for electrons,and its structure remains unchanged even if it accepts multiple electrons.Compared with TiO2,the structural composition of POMs is more abundant,and the oxidation/reduction potential and LUMO position(similar to the conduction band)are easier to control,which should be more conducive to the development of highly active photocatalysts.Based on POMs,several novel,high-efficiency and stable composite photocatalysts were prepared by complex Ag3VO4,Ag3PO4,metal organic framework(MOFs)and Ag plasma resonance effect(SPR).Then it is applied to the treatment of organic wastewater containing Cr(?).Based on POMs,Ag@ Ag3 V O4/AgPMo photocatalysts were prepared by simple hydrothermal and in situ UV reduction methods.XPS,XRD,SEM and UV-vis DRS results show that Ag3VO4 and AgPMo are successfully combined,and Ag atoms with plasmon resonance(SPR)effect are formed on the surface,and visible light absorption increases.Further,the photocatalytic activity of Cr(?)-containing wastewater was investigated.The results show that the reduction rate of Cr(?)by Ag@Ag3VO4/AgPMo is about 16 times that of AgPMo,and the catalyst can be reused many times.Further,Ag@Ag3PO4/AgPMo was prepared by hydrothermal and ultraviolet reduction methods.Compared with Ag@Ag3VO4/AgPMo,it has the advantages of reduced band gap,increased specific surface area,and reduced impedance.Under the same conditions,the reduction efficiency of Cr(?)by Ag@Ag3PO4/AgPMo is about 1.8 times that of Ag@Ag3VO4/AgPMo,which can achieve the simultaneous reduction of Cr(?)and organic matter.Through the free radical detection test,it was found that the signal intensity of hydroxyl radical(HO)and superoxide radical(O2·-)was enhanced,and the oxidation performance of the catalyst was also improved.Considering the high cost of precious metal Ag,and in order to simplify the preparation process,a variety of iron-based POMs(FeSiW,FePMo,FePW)and heterojunction FeSiW/Fe-MOF were prepared by simple hydrothermal method.It is found that after Fe-MOF is loaded with FeSiW,the absorption of visible light is enhanced,the band gap is reduced,and the visible light catalytic activity is significantly improved.It is applied to the treatment of Cr(VI)-containing wastewater with good effect and stability.Under the condition that the dosage is 1/10 of Ag@Ag3VO4/AgPMo and other conditions are the same,the reduction effect of FeSiW/Fe-MOF on Cr(VI)is still dominant.