The Electrochemical Properties Of The Keggin-type Polyoxometalates And Its Photocatalytic Activity,

Polyoxometalates, a category of compounds that have similar properties to semiconductor metal oxide, have a vast advantages involving photocatalytic and electrocatalytic performance. Since the polyoxometalate anions could be reduced by electrochemical means as well as by photochemical method, it is significant to establish the relationship between their photochemical and electrochemical behaviors and to explore the mechanism of H₂ evolution by correlation with these dual behaviors.In this work, we investigate the photocatalytic H₂ evolution and electrochemical behaviors of a series of Keggin-type polyoxometalates. It is found that PW₁₂O₄₀^3-, GeW₁₂O₄₀^4- and SiW₁₂O₄₀^4- show steady photocatalytic activity for H₂ evolution under light illumination. Their activities increase in the order of PW₁₂O₄₀^3- < GeW₁₂O₄₀^4- < SiW₁₂O₄₀^4- and increase with the concentration of H₂SO₄. Based on electrochemical results, the two-electron reduced species of them are recognized to be active for the reduction of H⁺. It is concluded that the more negative the potential of XW₁₂O₄₀^{(n+2)-/(n+1)-} is, the higher the activity for hydrogen evolution is, which is due to the larger thermodynamic driving force for H⁺ reduction.XW₁₂O₄₀^n- (X= B, Fe, Co, H₂) are investigated as catalysts for the photocatalytic hydrogen production from water in the solution of 0.1 mol/L H₂SO₄-0.9 mol/L Na₂SO₄ with methanol as sacrificial reagent. By correlating photochemical and electrochemical behaviors of polyoxotungstate anions, the one-electron reduced species of BW₁₂O₄₀^5-, the two-electron reduced species of H₂W₁₂O₄₀^6-, FeW₁₂O₄₀^5- and the four-electron reduced species of CoW₁₂O₄₀^5- are active for H₂ evolution. And their mechanism of photocatalytic H₂ evolution is proposed.Cyclic voltammetric pattern of PMo₁₂O₄₀^3- shows that the potentials of four reduced species are more positive than φ^θH⁺/H₂, which is in good agreement with the previously reported result that PMo₁₂O₄₀^3- was unable to reduce H⁺ to H₂. In addition, two new compounds, Ba(DMF)₆·HPMo₁₂O₄₀·2H₂O and {[Zn(DMF)₅]₂PMo₁₂O₄₀}·H₂PMo₁₂O₄₀· 4DMF, were synthesized and structurally characterized by IR, TG, CV and X-ray diffraction.