Photocatalytic Hydrogen Production Catalyzed By Earth-abundant Element-based Polyoxometalates Containing Cobalt, Iron And Manganese

It is well known that fossil flues are non-renewable energy source, and the issues of global warming caused by releasing of green house gas CO2 generated from the burning of fossil fuels together with the problems of environmental pollution have been increasingly emerging. Exploration of clean, environmentally friendly, sustainable and non-fossil fuel energy sources has attracted much attention. Recently, it is supposed that solar energy will play an important role in the development of new energy sources in the future because it is inexhaustible, clean and especially renewable. Meanwhile, with advantages of high combustion energy and water as its only product without environmental pollution, hydrogen is an ideal candidate for the replacement of fossil fuels. Therefore, it is a promising strategy to produce hydrogen utilizing solar energy.Polyoxometalates (POMs) are well-defined metal-oxygen clusters formed by early transition metal and have unique structural features and versatility. POMs are constructed by d0 electronic structure of transition metal oxygen anion clusters and show similar properties to metal oxide semiconductors (MOSs), so it may be regarded as the most promising class of photocatalytic hydrogen production photocatalyst. In addition, POMs are attractive candidates for catalysis of multielectron processes because of their rich redox chemistry and high stability toward hydrolysis in water. So, it can be used as H2 evolution catalyst, photosensitizer and sacrifical reagent were added to build a high efficient homogeneous photocatalytic hydrogen production system, the addition of photosensitizer successfully completed the hydrogen production reaction under visible light irradiation. POMs combined with the advantages of both heterogeneous catalysts stability and homogeneous molecular catalysts, many POMs with photocatalytic hydrogen production performance have been investigated. However, there are very few reports of the photocatalytic hydrogen evolution system of earth-abundant element-based POMs. Thereby, development and explore the photocatalytic activity of POMs in different photocatalytic hydrogen production system has a lot of space. Here, we took the earth-abundant element-based POMs for the object and detailed investigated the photocatalytic hydrogen evolution activity in different photocatalytic hydrogen production system and the mechanism for hydrogen production. The paper includes the following aspects:(1) Synthesis of earth-abundant element-based POMs containing cobalt (K7[CoⅢCoⅡ(H2O)W1ⅠO39]·15H20), iron(Na27[Fe11(H2O)14(OH)2(W3O10)2(α-SbW9033)6]103H2O), manganese(Na11(NH4)[(MnⅡ (H2O))3(SbW9O33)2] ·45H2O,K18{[Mn(H2O)3]2[Mn(H2O)2][(B-β-SiW9O33(OH))-Mn3(H2O)(B-β-S iW8O30(OH))]2}·16H2O and Na,o[Mn4(H20)2(VW9034)2]·26H2O) and characterization of its structure.(2) The photocatalytic hydrogen production activity for synthesis of these compounds were comprehensive studied in different photocatalytic hydrogen production system under different reaction conditions, at the same time, variables of the photocatalytic reaction conditions, including catalyst concentrations, photosensitizer concentrations, sacrifical reagent concentrations, pHs, co-catalyst concentrations, etc., were systemically optimized to obtained the best hydrogen evolution efficiency. Among of them, cabalt-based POM achieved the highest average rate of hydrogen production (13395 umol h-1 g-1) in the visible light dye sensitized system, which is the hightest among of all the POMs photocatalytic systems reported so far. And the iron-based POM photocatalyst exhibits good stability in the system of UV irridation photocatalytic hydrogen evolution, the rate of hydrogen production reaches as high as 820μmolh-1g-1(3) The stability of catalysts were tested and confirmed with multiple characterization methods (UV-vis, IR, DLS, CV, catalyst recycling and aged experiment) in reaction solution and photo-reaction processes.(4) The mechanism of different photocatalytic hydrogen evolution system were preliminary studied, which combined with the steady-state fluorescence spectroscopy, UV-Vis diffuse reflectance and cyclic voltammetry.