Preparation Of COFs-based Composite Materials And Its Photocatalytic Performance Of The Green Energy Source

Through the photosynthesis of plants,people proposed to use artificial photosynthesis system to solve the energy shortage and environmental problems caused by excessive consumption of fossil fuels.Although the use of artificial semiconductor materials for artificial photosynthesis is an ideal and potential way to obtain solar energy,it is still challenging to build an efficient photosynthesis system that can produce solar fuels at a scale that can compete with fossil energy.Therefore,the use of high-efficiency photocatalysts is essential to promote the photocatalytic process.Covalent organic frameworks（COFs）are emerging crystalline porous materials connected by covalent bonds,with flexible molecular design and synthesis strategies,large surface area and porosity.These advantages make COFs show broad prospects in the fields of photocatalytic decomposition of water to produce hydrogen,reduction of CO₂,nitrogen fixation,and photocatalytic production of H₂O₂.In this thesis,a series of COFs materials were mainly prepared and their photocatalytic properties were studied.The first is based on the bipyridyl COF（Tp Bpy）using a top-down method to introduce monoatomic MoN₂ active sites to prepare Mo-COF materials,through XRD,TEM,SEM,XPS,HAAD-STEM and AFM,etc.Characterization means verify the successful preparation of COFs and the existence of MoN₂ active sites.The performance of reducing CO₂ under visible light is studied,and the reaction mechanism of photocatalytic reduction of CO₂ is explored through Fourier in-situ infrared and transition state theoretical calculations.The reaction results showed that the reduction products of Mo-COF under visible light after the introduction of MoN₂ active sites newly appeared CH₄ and C₂H₄.This was the first time that C₂H₄appeared in the process of COFs material photocatalytic reduction of CO₂.And the system can produce C₂H₄ at a rate of 3.57μmol g-1h-1 with a selectivity of 32.92%.FT-IR and DFT results show that the introduction of Mo single atom promotes the COFs’adsorption of CO₂ and CO and the subsequent hydrogenation coupling process,and also reduces the reaction energy barrier for intermediate products,and realizes the photocatalytic reduction of CO₂ to COFs.The second is to prepare COFs of different structures by adjusting the monomers used in synthetic materials,and to explore the performance and reaction mechanism of H₂O₂ produced by photocatalysis under visible light.First,Tp Bpy-COF with pyridine structure and DAAQ-COF without pyridine structure were synthesized by solvothermal method,and the structure of the material was determined by XRD,IR,and NMR.Under the same experimental conditions and visible light（400～700 nm）,the performance of the two COFs photocatalysis to generate H₂O₂ was studied.The results showed that when the solution concentration was 0.5 mg/ml and 40min of light,the yield of Tp Bpy reached 821.74μM,which was 18.5 times and 69.6times higher than DAAQ（44.47μM）and g-C₃N₄（11.80μM）,respectively.By changing a series of reaction conditions,it is proved that light,H₂O and O₂ play an important role in the photocatalytic production of H₂O₂.ESR detected the production of superoxide and hydroxyl during the reaction,and explored the mechanism of this photocatalytic reaction based on free radical capture experiments and Fourier in-situ infrared.Tp Bpy with a pyridine structure directly produces H₂O₂ through two-electron oxidation/reduction.In contrast,DAAQ produces H₂O₂ through a single-electron oxidation/reduction process.