Preparation Of Visible Light Responsive Fe-POMs And Complex Heterojunctions And Study On Their Environmental Catalytic Activity

In recent years,water pollution has become one of the most urgent problems in environmental pollution control.Taking heavy metal chromium（Cr）as an example,chromium mainly exists in the form of trivalent chromium（Cr（Ⅲ））and hexavalent chromium（Cr（Ⅵ））in the waste water containing chromium,among which Cr（Ⅵ）is highly toxic,which is 100 times more than Cr（Ⅲ）.As an emerging environmental purification technology,photocatalytic method represented by Ti O₂ has attracted much attention due to its advantages such as mild reaction conditions and small secondary pollution,and has been widely used in water pollution control.Electron-hole pairs are generated under the excitation of light.Electrons have reducibility and can reduce high-toxic Cr（Ⅵ）to low-toxic Cr（Ⅲ）.Holes can oxidize water or organic matter to realize catalytic cycle.Polyoxometalates（POMs）have been widely used in the field of catalysis due to their high activity,good selectivity and non-toxicity.At the same time,it has the characteristics of reversible redox,obtaining one or more electrons without the change of its own structure,which is also applied in the field of photocatalysis.However,due to the good solubility of POMs in strong polar and non-polar solvents,it is difficult to recover,separate and recycle POMs,which seriously limits its practical application.Therefore,researchers are committed to heterogeneous POMs,generally through loading,ion deposition and other methods.Therefore,this study intends to develop POMs heterogeneous catalysts with excellent catalytic performance and good stability for application in the field of photocatalysis.Firstly,FeSiW powder was obtained by deposition of Keggin POMs silicotungstate acid(H₄Si W₁₂O₄₀)by Fe³⁺under solvothermal conditions.As can be seen from the infrared spectrum（FTIR）,FeSiW maintains a good Keggin structure.In addition,the effect of different initial mole ratios（4:1～4:4）of Fe³⁺and H₄Si W₁₂O₄₀ on visible light catalytic performance was investigated.Through photoelectric chemical characterization,FeSiW has the highest visible light response and the lowest charge transfer resistance when the initial molar ratio is 4:3.The photocatalytic reduction of Cr（Ⅵ）was carried out with Cr（Ⅵ）as the target pollutant.It was found that the FeSiW prepared had the optimal photocatalytic performance when the initial molar ratio of Fe³⁺to H₄Si W₁₂O₄₀ was 4:3.The optimal reduction rate of Cr（Ⅵ）of FeSiW reached 99%when the additive amount was 1 g/L and pH was 3 under visible light irradiation for 90 min.In addition,cyclic dynamics experiments show that FeSiW has good stability.Secondly,Fe³⁺and Keggin type POMs(H₃PW₁₂O₄₀,H₄Si W₁₂O₄₀ and H₃PMo₁₂O₄₀)were used to prepare Fe-POMs with visible light activity under solvothermal conditions,which are denoted as ferric phosphotungstate（FePW）,FeSiW and ferric molybdate（FePMo）.It was found that among the three Fe-POMs,FePMo had the optimal photocatalytic reduction performance,and showed the highest visible light absorption rate,photocurrent response and electron transport efficiency in the photoelectric chemical characterization.Characterized by XRD,FTIR,etc.,it is found that the Keggin structure is maintained well.By changing the preparation conditions,it was found that FePMo prepared at the solvent heat of 160℃for 4 h had the optimal photocatalytic reduction activity.After 50 min of visible light irradiation,the reduction rate of Cr（Ⅵ）by FePMo with the dosage of 1 g/L reached 88%.The fitted first-order kinetic rate constant was 0.042 min^-1,which was 2.5 times and 1.8 times of FePW and FeSiW,respectively.After 5 cycles of photocatalytic reduction of Cr（Ⅵ）at pH=2,it is found that FePMo has good stability in photocatalytic reduction of Cr（Ⅵ）and XRD structure.Finally,the FePMo/FeVO₄ heterojunction material was prepared by solvent-heat method in one pot.Compared with the monomer FePMo and FeVO₄,the heterojunction composite presented higher photocurrent response and faster electron transfer rate.Among them,when the molar ratio of FePMo and FeVO₄ was 1:1,the reduction rate of Cr（Ⅵ）reached 99%after the0.1g/L FePMo/FeVO₄ heterostructure material was prepared after pH=3 and visible light irradiation for 50 min.The fitting rate constant of first order dynamics of FePMo/FeVO₄ was0.061 min^-1,which was 1.6 and 4.3 times that of FePMo and FeVO₄,respectively.After 5 cycles,the restore performance is stable.At the same time,the FePMo/FeVO₄ heterojunction material was loaded on the nickel network by solvent thermal method to realize simple recovery.After70 min of visible light irradiation,the reduction rate of Cr（Ⅵ）is over 99% with good stability.