TiO_x And Graphene Quantum Dots In MOF And Their Photocatalytic Reduction Of CO₂

Photocatalytic CO₂reduction has become a promising solar energy conversion and utilization strategy.However,the thermodynamic stability of CO₂,the complexity of the reaction,and the high dissociation energy of the C=O bond hinder its effective activation on the catalyst and improve the energy barrier of the catalytic reaction.The design of semiconductor photocatalysts to achieve efficient photocatalysis for the reduction of carbon dioxide is currently the main problem to be solved in this field.Metal-organic framework compounds（MOFs）as CO₂photocatalysts have attracted extensive attention from researchers due to their unique molecular structure,excellent porous structure,and adjustable light absorption.However,due to the serious carrier recombination,poor light absorption,or lack of corresponding catalytic sites in most MOF materials,the further improvement of their photocatalytic performance is limited.In this thesis,based on the MOF material MIL-101（Cr）,three kinds of composite photocatalysts were designed and prepared with TiO_xmolecular clusters rich in surface oxygen vacancies and graphene quantum dots（GQD）with infrared up-conversion optical properties as pore fillers.The research content is mainly divided into the following three parts:（1）Based on MIL-101（Cr）with porous and strong water molecular adsorption capacity,TiO_x/MIL-101（Cr）composite photocatalyst was prepared by using atomic layer deposition technology to accurately fill its pores with TiO_xmolecular clusters.The molecular compartment formed by the MIL-101（Cr）channel inhibits the carrier recombination on the surface of TiO_xand promotes the photoelectron transfer from TiO_xto MIL-101（Cr）.Photoelectric testing,impedance testing,and fluorescence testing all indicate that TiO_x/MIL-101（Cr）enhances the separation of photogenerated carriers.The secondary building unit（SBU）containing Cr metal in MOF acts as the center of CO₂adsorption and catalytic reaction.Among them,TiO_x-50/MIL-101（Cr）exhibits the best catalytic performance in photocatalytic CO₂reduction reactions.Under the irradiation of ultraviolet light(100 m W cm^-2),the rate of CO generation reaches 50μmol g_cat.^-1h^-1,CH₄generation rate reached 2.3μmol g_cat.^-1h^-1;Under the irradiation of AM 1.5 G(100 m W cm^-2),the CO generation rate reached 24.6μmol g_cat.^-1h^-1,CH₄generation rate reached 2.0μmol g_cat.^-1h^-1.（2）Based on MIL-101（Cr）,the pore filling of rGQDs was realized through a self-assembly process,and MIL-101（Cr）/rGQDs composite photocatalyst was prepared.The high dispersion of rGQDs in the channel of MIL-101（Cr）promotes the effective separation and transfer of photogenerated carriers between them and significantly improves the photocatalytic reduction efficiency and the light absorption range of CO₂.Among them,MIL-101（Cr）/rGQDs-3.2%showed the best photocatalytic performance in the photocatalytic CO₂reduction reaction,with the CO rate could reach 9.8μmol g_cat.^-1h^-1under infrared light;CO rate reaches 25.5μmol g_cat.^-1h^-1under AM1.5 G light,which is 14 times of MIL-101（Cr）.（3）Using TiO_x-50/MIL-101（Cr）material（TM）obtained from work（1）,load rGQD into the MIL-101（Cr）channel through self-assembly.TM/rGQDs-2.6%showed the best photocatalytic performance.CO rate reaches 13.8μmol g_cat.^-1h^-1under infrared light;CO rate is 27.3μmol g_cat.^-1h^-1under AM1.5 G（simulated sunlight）,and showed good stability in the light reaction test within 30 h.