Preparation And Photocatalytic Properties Of Silver Salts Of Transition Metal-Substituted Phosphomolybdate And Vanadium-Substituted Phosphotungstate Composite Materials

Water pollution caused by the discharge of various pollutants?such as organic and inorganic pollutants?from industrial wastewater and municipal wastewater is one of the most concerned issues in the past few decades.Photocatalytic technology has been used as an important technology in environmental remediation because of its mild reaction conditions and advantages of pollution-free.In most cases,photocatalytic technology can degrade and mineralize organic contaminants into final products such as carbon dioxide,water and other small inorganic molecules.Polyoxometalates?POMs?have a similar chemical composition and electronic structure to semiconductor materials?such as TiO₂?and are a semiconductor-like material.Therefore,the application of POMs in photocatalysis has received more and more attention in environmental control and clean energy.However,there are two problems in the application of POMs.First,most of the POMs are water-soluble,and difficult to recycle.Second,most of POMs have photocatalytic activity only under UV light and low absorption of visible light.The above two points limit the application of POMs in photocatalysis.Therefore,it is very important to design a POM-based heterogeneous catalyst which is insoluble and has high activity.In this paper,the transition metal is used to modify the Keggin-type phosphomolybdatetoadjustitssolubilityand catalyticactivity;the vanadium-substituted phosphotungstates are supported on the mesoporous semiconductor material mTiO₂,and the Ag nanoparticles are further deposited to promote the photocatalytic performance and reusability of the prepared POM-based heterogeneous catalyst composite.The main works are as follows:1.Using the large cations Cs⁺and Ag⁺as the counter cations of monosubstituted PMo₁₁M?H₂O?O₃₉^5-?M=V,Co,Zn?,five kinds of solid insoluble salts of Cs₅[PMo₁₁Co?H₂O?O₃₉],Ag₃Cs₂[PMo₁₁Co?H₂O?O₃₉],Ag₅[PMo₁₁Co?H₂O?O₃₉],Ag₅[PMo₁₁Zn?H₂O?O₃₉]and Ag₄PMo₁₁VO₄₀ were synthesized,and used as heterogeneous catalysts.The photocatalytic activity of as-prepared materials was studied by using methyl orange?MO?as a degradation dye.The results show that the photocatalytic activity gradually increased with the increasing proportion of Ag⁺.When the cation of the POMs salt is completely replaced by Ag⁺,the highest photocatalytic activity achieved.The degradation order of dyes by monosubstituted Ag_nPMo₁₁M?H₂O?O₃₉ is Ag₅PMo₁₁Zn>Ag₅PMo₁₁Co>Ag₄PMo₁₁V>Ag₃PMo₁₂.Ag₅PMo₁₁Zn has the highest catalytic activity and can degrade MO completely under simulated light irradiation for 160 min.Experiments show that the Ag₅PMo₁₁Zn sample has the lowest fluorescence intensity among these samples,indicating that the combination of holes and electrons in Ag₅PMo₁₁Zn is effectively retarded,and thus has high photocatalytic activity.After five repeated experiments,the catalytic activity of the catalyst remained stable.2.In order to obtain a photocatalyst with high activity,a series of mTiO₂/POMs/Ag(POMs=H₃PW₁₂O₄₀,H₄PW₁₁VO₄₀0 and H₅PW₁₀V₂O₄₀)composite materials with pore structure and different amount of POMs were prepared by sol-gel method combined with hydrothermal method using P123 as template.It was found that the highest photocatalytic activity was obtained when the loading of POM was about 20%and the deposition of Ag was about 1%.Their catalytic performance as photocatalyst for the degradation of MO was studied.The order of catalytic activity is as follows:mTiO₂/PW₁₀V₂/Ag>mTiO₂/PW₁₂/Ag>mTiO₂/PW₁₁V/Ag>mTiO₂/Ag.Among them,mTiO₂/PW₁₀V₂/Ag has the highest catalytic activity,and MO can be completely degraded under simulated light irradiation for 15 min.Fluorescence spectroscopy experiments show that the recombination of electrons and holes can be effectively suppressed in mTiO₂/PW₁₀V₂/Ag sample due to the surface plasmon resonance effect?SPR?of Ag NPs.At the same time,mTiO₂/PW₁₀V₂/Ag has a low band gap value?3.02eV?.This composite material exhibited excellent stability and maintained a consistent level of photoactivity after five catalytic cycles.