Construction Of Two-Dimensional Donor-Acceptor Covalent Organic Framework Materials And Studies On Their Photoelectrocatalytic Performance

With the promotion of the national"double carbon"strategy,the development of clean energy and environmental pollution have received more and more attention.Electric energy and light energy are clean energy.Fuel cells and metal-air cells play an important role in the process of energy generation and storage,and their positive oxygen reduction reaction（ORR）restricts the development of high-efficiency fuel cells and metal-air cells.Therefore,it is of great significance to develop efficient and stable electrocatalysts for ORR.Hexavalent chromium（Cr（Ⅵ））is a toxic heavy metal ion with high carcinogenicity and acute toxicity to most organisms,which is widely used in industrial production.Cr（Ⅵ）in wastewater is harmful to human health and environmental protection.Reducing highly toxic Cr（Ⅵ）light to low toxic Cr（III）under visible light is a low-cost and sustainable green method to reduce the harmful effects of Cr（Ⅵ）.However,so far,it is still challenging to prepare efficient and reusable photocatalysts for Cr（Ⅵ）reduction.Covalent organic frame materials（COFs）are emerging porous crystalline materials in recent years.They have periodic regular structure,high specific surface area,excellent stability,structural designability and semiconductor properties,and have attracted great attention in many fields.Aiming at energy and environmental problems,this thesis designed COFs materials as electrocatalysts and photocatalysts for ORR reaction and Cr（Ⅵ）photoreduction:Ⅰ:Four new two-dimensional porphyrin donor–acceptor COFs（M-TP-COF,M=H₂,Co,Ni and Mn）with different catalytic activity centers were synthesized by carrying out Schiff-base condensations between different 5,10,15,20-tetrakis（paraaminophenyl）porphyrins（M-TAPP,M=H₂,Co,Ni and Mn）and 1,1,2,2-tetrakis（4-formyl-（1,10-biphenyl））ethane（PTPE）,and the relationships between their chemical structures and ORR catalytic activities were investigated.The structure and catalytic active sites of M-TP-COFs can be regulated at the atomic level to study the structure-activity relationship.Our experimental results showed that the ORR catalytic activity of M-TP-COF can be tuned by changing the M atom in the center of the porphyrin structure with the order of catalytic activity being Co-TP-COF>Mn-TP-COF>Ni-TP-COF>H₂-TP-COF.The results of density functional theory（DFT）calculations based on the O₂-O₂*-OOH*-O*-OH*-OH^-catalytic route were in accordance with our experimental results.This research disclosed an effective way to tune the ORR performance of COFs as well as to design COFs showing improved ORR catalytic activities.Ⅱ:We combine the single-atom substitution strategy with the construction of donor–acceptor COFs（D-A COFs）to investigate the structure and photocatalytic activity relationship.By exploiting 1,3,6,8-tetrakis（4-formylphenyl）pyrene（TFPPY）as a donor and4,4’-（benzoselenadiazole-4,7-diyl）dibenzaldehyde（SEZ）and4,4’-（benzothiadiazole-4,7-diyl）dibenzaldehyde（BTZ）as acceptors,Se and S doped D–A COFs,named PYE-COF and PYS-COF.In comparison with PYS-COF,PYE-COF exhibited enhanced Cr（Ⅵ）reduction performance with higher reduction efficiency at 100%within 60 min under visible light.After 5 continuous photocatalytic cycles,the Cr（Ⅵ）reduction ratio by PYE-COF was still over 95%.Further photophysical and electrochemical investigations demonstrated that the enhanced photocatalytic performance of PYE-COF resulted from the high charge separation and transfer efficiency.The current study provides new insights into the design and preparation of efficient photocatalysts with atomic precision for environmental remediation.