Design Of MOFs Based Heterojunction Composite Photocatalyst And Its Photocatalytic Performance For CO₂ Reduction

In the energy crisis and environmental pollution increasingly serious at present,the development of new energy and technology is urgent.The photocatalytic CO2 reduction technology,which converts the inexhaustible solar energy into chemical bond energy for preservation by simulating plant photosynthesis,can not only effectively delay the energy crisis,but also improve the greenhouse effect,which has aroused the interest of scholars.Metal-organic Frameworks(MOFs)have been proved to be excellent materials for photocatalytic reduction of CO2 due to their excellent crystallinity,high porosity,controllable band structure and morphology.However,in the actual use of MOFs and its derivatives,due to the presence of C=O in CO2,a large energy(750 kJ/mol)is needed to interrupt it,so that the photocatalytic yield is severely limited.Moreover,the electron-hole pair recombination of single MOFs materials is fast,and the light response range is small,which also affects the photocatalytic activity.Therefore,we design MOFs derivative composite materials using MOFs as a template.Using the structural advantages of MOFs,we obtain better catalytic conversion performance under ultraviolet light and the addition of sacrificial agent.Considering the ideal mode of natural photosynthetic reaction,the system of gas-solid photoreduction of CO2 was designed,and the CdS/Ni-MOF three-dimensional composite photocatalyst was used to achieve the selective photoreduction of CO2 to produce CO.Considering the enhancement of photocatalytic activity by plasma effect,a Cu2O/Ni-MOF composite photocatalyst with plasma effect was further designed to enhance electron migration ability and achieve high selective catalytic conversion of CO2 in gas-solid reaction system simulating photosynthesis.Carbon-doped In2O3(C-In2O3)hollow tubes were synthesized by calcination using MIL-68(In)as sacrificial template.C-In2O3/g-C3N4 heterojunction was obtained under hydrothermal condition with C-In2O3 and g-C3N4.The unique hollow tube structure of C-In2O3/g-C3N4,carbon doping and Ⅱ type heterojunction expand the light absorption capacity and promote the separation of photogenerated electron hole pairs,thus significantly improving the activity of photocatalytic reduction of CO2.Among them,C-In2O3/g-C3N4 had the highest CO2 reduction activity for CO,and the yield of CO and CH4 reached 153.42 μmol/g/h and 110.31 μmol/g/h,respectively,under the addition of sacrificial agent and UV light.Considering the simulation of natural photosynthesis,we further constructed the gas-solid photocatalytic reaction system,designed the layered hierarchical CdS/NiMOF heterostructure photocatalyst,and realized the selective photoreduction of CO2 to CO in the constructed gas-solid reactor under the simulated sunlight conditions.Among the prepared photocatalysts,the CO yield of 20%-CdS/Ni-MOF is the highest,reaching 1.9μmol/g/h,which is 16 times and 7 times of that of Ni-MOF and CdS,respectively.By controlling the morphology of the catalyst,we found that the synergistic effect of heterostructure in CdS/Ni-MOF and its unique layered structure can improve the charge transfer efficiency and provide abundant active sites.Three dimensional(3D)flower spherical Ni-MOF was prepared by hydrothermal method,and Cu2O nanoparticles were uniformly dispersed as heterogeneous nucleation sites.The hierarchical Cu2O/Ni-MOF composite photocatalyst with plasma effect was obtained.The catalyst was tested in gas-solid photocatalytic system for 4 h.It was found that 30%-Cu2O/Ni-MOF had the highest photocatalytic activity,and the CO yield reached 21.7 μmol/g,which was 4.1 and 11.1 times of Cu2O(5.32(μmol/g)and Ni-MOF(1.95 μmol/g),respectively.The improved catalytic performance can be attributed to the hierarchical structure,heterogeneous structure and the LSPR effect of Cu on the surface of Cu2O nanoparticles.