Visible-light-induced Photocatalytic CO₂ Reduction Over NH₂-UIO-66?Zr?modified With Different Organic Groups

The reduction of CO₂ into high value-added chemicals and fuels by photocatalytic technology can relieve energy shortage and the environmental problems caused by greenhouse effect.And the key to the realization of photocatalytic carbon fixation technology is to develop highly efficient photocatalysts with visible-light response.Metal-organic frameworks?MOFs?have been widely concerned as emerging materials because of the following advantages:?1?the compositions and structures are easy to design,modify and modulate;?2?the metal-oxo clusters on the framework are similar to the discrete semiconductor quantum dots,which are not easy to agglomerate during the reaction process and are conducive to maintaining the stability of the catalyst;?3?the charge transition from ligand to metal can be adjusted by changing the type of metal ion and organic ligand to get visible-light response;?4?the high specific surface area,some of organic ligands have nitrogen-containing functional groups,which are in favor for CO₂ adsorption.However,as photocatalytic materials,MOFs are still faced with the disadvantages of poor quantum efficiency and poor separation of photogenerated charges.In this thesis,the effects of the modified NH₂-UiO-66?Zr?with different organic functional groups on photocatalytic CO₂ reduction under visible light irradiation were mainly studied.The amino functional groups on NH₂-UiO-66?Zr?were post-synthetic modified with pyridine-2-carbaldehyde,salicylaldehyde,benzaldehyde and trifluoroacetic acid to study the influence of different organic functional groups on the visible-light response,photoelectric performance and other physico-chemical properties of this type of materials and clarify the relationship between the structure and composition of MOF materials and the performance of photocatalytic CO₂ reduction.The main results and conclusions are as follows:1.The covalent modification of NH₂-UiO-66?Zr?by pyridine-2-formaldehyde,benzaldehyde,salicylaldehyde and trifluoroacetic acid has been successfully carried out through amine aldehyde condensation and amide reaction.The C=N bond formed by the condensation of amine aldehyde is conducive to the absorption of visible light,while the peptide bond formed by the amide reaction has no effect on the absorption of visible light.2.Modification of pyridine-2-formaldehyde,benzaldehyde and salicylaldehyde increased the CO₂ adsorption capacity of NH₂-Ui O-66?Zr?to a certain extent;in the photocatalytic system using triethanolamine as sacrificial agent,the activity of CO₂reduction over NH₂-Ui O-66?Zr?increases with the introduction of such functional groups.And pyridine-2-carbaldehyde-modified MOF material showed the highest activity with 92.6?mol g^-11 h^-1.However,the activity of NH₂-Ui O-66?Zr?modified by trifluoroacetic acid was obviously inhibited.3.Through further analysis of the fluorescence spectrum and photoelectrochemical test results,the larger conjugated structures of the ligands on NH₂-Ui O-66?Zr?were modified by the aldehyde groups?such as pyridine-2-carbaldehyde?to suppress the photogenerated electron-hole pair,and thus improve the performance of photocatalytic CO₂ reduction;with the introduction of the trifluoroamide structure,the ligands on the MOF material have greatly reduced the ability of donating electrons due to the strong electronegativity of the fluorine atom,which reduces the photocatalytic activity.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.