Visible-Light-Induced Photocatalytic CO₂ Reduction Over Ti-Based Metal-Covalent Organic Frameworks

The utilization of CO₂ could not only solve the environmental problems caused by the greenhouse effect,but also relieve the resources shortage.Photocatalytic CO₂ reduction into hydrocarbons is a green carbon fixation technology,and the development of new visible-light-induced photocatalysts is considered as a key topic to the efficient utilization of CO₂ by photocatalytic reduction technology.Ti-based metal-covalent organic framework materials which combine the advantages of metal-organic frameworks and covalent-organic frameworks are a new class of porous materials synthesized through the coordination between metal clusters and organic covalent ligands.Due to their unique large?conjugated structures and the nitrogen-containing functional groups that are favorable for CO₂ adsorption,Ti-based MOF materials are considered as suitable candidates for CO₂ reduction with visible light response and high redox activity.In this thesis,based on the structures of MOF-901 and MOF-902,three new Ti-based MOF materials were synthesized via the regulation of the organic units,and systematically characterized by several techniques such as XRD,XPS,UV-vis DRS,photoluminescence,photocurrent and so on.Then,their photocatalytic performance for CO₂ reduction was studied to elucidate how the composition and the structure of these Ti-based MOF materials can influence their photocatalytic activity.Furthermore,the mechanism of the photocatalytic reduction of CO₂ over Ti-based MOF materials has been also proposed.The main results are summarized as follows:1.MOF-901?PDA?with similar structure to MOF-901?BDA?reported in the literature was successfully prepared by regulating the positions of amino and aldehyde functional groups of organic units.Compared with MOF-901?BDA?,MOF-901?PDA?exhibits stronger visible light response,stability,and better photocatalytic performance for the reduction of CO₂ with triethanolamine?TEOA?as sacrificial agent.2.Another two Ti-based metal-covalent organic framework materials,MOF-901?PyDA?and MOF-902?BPyDA?,were successfully synthesized by substituting pyridine ring or bipyridyl ring for the benzene ring on the amino ligands.The introduction of pyridine rings is conducive to enhancing the visible light response,increasing the adsorption of CO₂,and thus exhibiting a higher activity of photocatalytic CO₂ reduction.In our photocatalytic CO₂ reduction,MOF-901?PyDA?showed the best photocatalytic activity with 745.9?mol g_cat^-11 of the produced formic acid under 10 h of visible light irradiation.3.In combination with a series of characterization techniques such as photoluminescence spectroscopy and photoelectrochemical testing,it is clarified that the microstructure regulation of organic units in Ti-based metal-covalent organic frameworks can influence of the energy band structure and photo-generated charge separation.And it initially revealed the relationship between material composition-structure catalytic activity.Novelty of the thesis research:the strategy of the microstructure regulation of organic units by combining metal clusters and covalent organic ligands is used to synthesis Ti-based metal-covalent organic framework materials,and for the first time applied in the photocatalytic CO₂ reduction to expound the relationship between the composition,structure and photocatalytic performance.These results can provide a new avenue for development of novel visible-light-driven photocatalysts and also broaden the application of MOFs and enrich the theory of the photocatalytic CO₂ fixation.