Preparation And Photocatalytic Performance Of Polyoxometalates-based Metal Organic Framework Composite Photocatalysts

The continuous consumption of fossil fuels has brought about serious energy problems and environmental issues,and photocatalysis is considered an important technology because solar energy is a renewable energy source and is used for photocatalysis to reduce CO₂ emissions.The effective conversion of CO₂ into carbon-based fuels by photocatalysis is an effective strategy to solve the above problems.Metal-organic frameworks（MOFs）are a new class of crystalline inorganic-organic hybrid materials with tunable semiconductor behavior.MOFs-based photocatalysts have received widespread attention for their unique morphology,excellent photocatalytic performance,good chemical stability,and high porosity,and are frequently applied as carriers in photocatalytic CO₂ reduction.In contrast,polyoxometalates（POMs）have a unique structure and strong redox ability,but the poor recyclability and small specific surface area of POMs limit their application in photocatalytic CO₂ reduction.By using MOFs as carriers to prepare polyoxometalates-based metal-organic frameworks（POM@MOFs）,the stability of the materials can be enhanced by combining the advantages of both,and the performance of the materials for photocatalytic CO₂ reduction can be improved.Based on the above considerations,the main work carried out in this paper is to design and synthesize polyoxometrate-based mental organic frameworks and their composite materials and study the performance of its application in photocatalytic CO₂reduction.The main work is as follows:1.NENU-5 was prepared simply and quickly by grinding method in this paper,and its performance of photocatalytic CO₂ reduction in gas-solid system was tested.However,the photocatalytic effect of NENU-5 was not satisfactory,so a series of Ag/NENU-5 composites were prepared by introducing silver nanoparticles（Ag NPs）into NENU-5 by photodeposition.The photocatalytic CO₂ reduction test results showed that the introduction of Ag NPs resulted in a maximum CO formation rate of 22.28μmol g^-1 h^-1.The average particle size of Ag NPs in the Ag/NENU-5 composite photocatalysts was investigated,and the results showed that the catalytic performance was optimal when the particle size of Ag NPs was 6-7 nm.2.Considering the relatively high economic cost of precious metals.Therefore,in the second work,NENU-3,NENU-5 and NENU-9 were prepared by loading three POMs,respectively,to modulate the photocatalytic performance of the photocatalysts according to the difference of light absorption properties between different POMs.The performance of photocatalytic CO₂ reduction was also tested in the gas-solid system,and the test results showed that NENU-9 had the best photocatalytic CO₂ reduction ability,which was better than the best performance of Ag/NENU-5,and the CO formation rate reached 27.2μmol g^-1 h^-1.The results indicated that the best photocatalytic effect of NENU-9 composed of loaded H₅PMo₁₀V₂O₄₀.3.Tp Pa-1,a covalent organic framework（COF）with excellent light-absorbing properties,was selected as a photosensitizer in this work,and combined with POM@MOF to construct composites for photocatalytic CO₂ reduction.PMo₁₂@ZIF-8was firstly prepared,and by functionalizing it with 4-pyridinecarboxaldehyde,PMo₁₂@ZIF-8/Tp Pa with heterogeneous structure could be synthesized by further in situ reaction.the covalent bond between the two components provided a bridge for the effective transfer of photogenerated charges with good stability and reproducibility.The test results showed that this catalyst can efficiently convert CO₂ to CO with the best photocatalytic CO formation rate of 53.72μmol g^-1 h^-1 in the gas-solid system.