Syntheses Of Covalent Organic Framework-based Photocatalytic Materials For Carbon Dioxide Reduction

The excessive use of fossil fuel has led to a sharp rise in the concentration of CO₂in the atmosphere,causing serious environmental problems such as global warming and rising sea levels.A series of environmental problems brought by solving the CO₂concentration in various ways have become a critical task facing modern human society.CO₂restoration of photocatalytic CO₂is one of the effective ways to achieve carbon neutralization and strategic goals.The design synthesis of photocatalytic materials is still a key scientific issue in the field.Covalent Organic Frameworks（COFs）is the fastest-growing type of porous organic material in recent years.It is usually formed by organic ligands connected through covalent bonds.Owing to the advantages of low density,large specific surface area,high porosity,adjustable pore size and easily modified structure,it has attracted widespread attention and has shown broad application prospects in the field of photocatalytic CO₂.In this paper,different COFs building blocks were selected and a series of COFs based photocatalytic materials were prepared through appropriate synthesis strategies.The effects of the structure,active center types,coordination microenvironment,and inorganic compounds of COFs on the photocatalytic CO₂reduction performance were comprehensively investigated,and the relationship between the structure and performance of COFs was established.The research work in this thesis provides theoretical basis and material support for the design and synthesis of COFs based photocatalytic materials and their applications in photocatalytic CO₂reduction.The specific research content and results are summarized as follows:（1）The relationship between metal active centers in COFs and CO₂photoreduction activity and selectivity was established.Isomorphic COFs（MP-TPE-COF,M=H₂,Ni,Co）with different active centers were synthesized using porphyrin based organic ligands containing different metal ions and tetra-aldehyde phenylethylene.Experimental and theoretical studies have shown that metalloporphyrin units play a crucial role in the selective adsorption,activation,and conversion of CO₂,as well as the separation and electron transfer of charge carriers,allowing flexible adjustment of photocatalytic activity and selectivity.Co P-TPE-COF shows a high CO precipitation rate of 2414μmol g^-1h^-1under visible light,the selectivity for H₂generation is 61%,and the Ni P-TPE-COF provides 525μmol g^-1h^-1CO precipitation rate and 93%selectivity with excellent durability.In addition,the photocatalytic system is feasible for simulated flue gas,with a CO emission rate of 386μmol g^-1h^-1with a selectivity of 77%.（2）A low concentration CO₂photocatalytic reduction system with high catalytic activity and selectivity was constructed.Using the organic ligand of triaminoguanidine hydrochloride as the core module,covalent organic framework nanosheets（CONs）Ni-TP-CON containing bis-chelating Ni coordination sites were synthesized for selective photoreduction of low concentration CO₂（15%）.X-ray absorption and X-ray photoelectron spectroscopy fine structure analysis confirmed the chemical environment of the Ni active site.The experimental results of low concentration CO₂reduction show that Ni-TP-CON exhibits high catalytic activity for CO(8720μmol g^-1h^-1)and selectivity（98%）,the apparent quantum efficiency at 420 nm is 1.2%,and the number of CO cycles in 5 hours is 28.7.Experimental and theoretical results show that compared to the mono-chelating coordination mode,the bis-chelating metal coordination units in CON are more favorable active sites for dilute CO₂photoreduction,effectively reducing energy barriers,accelerating the formation of reactive radicals*COOH,and promotes the transport of photogenerated charges.（3）An artificial photosynthesis system involving only water was designed and synthesized.The irreversible covalent bond linked organic inorganic composite（PCTN）was synthesized using the covalent bond interaction between Ti₃C₂MXene and polyarylether-based COF（COF-318）.The results of photocatalytic experiments show that PCTN-3 exhibits high photocatalytic efficiency in converting CO₂into CO and CH₄under visible light conditions,with a CO yield of 173.5μmol g^-1in 5 hours,CH₄production reaches 24μmol g^-1,while showing very high cycle stability.The study of photocatalytic mechanism found that the obtained PCTN composite exhibited strong covalent coupling,good conductivity,and effective charge transfer.At the same time,the improvement of specific surface area and pore structure endows the PCTN composite with enhanced CO₂adsorption capacity.COF-318 generates photogenerated electrons and holes under the excitation of visible light,through the covalent bond between COF-318 and Ti₃C₂,electrons and holes can be efficiently separated and quickly transferred to Ti₃C₂.At the same time,thanks to the good catalytic effect of Ti₃C₂material,CO₂is converted into CO and CH₄.Simultaneously,a large number of positive holes exist in COF-318,and water oxidation reaction occurs,which finally realizes artificial photosynthesis.