Preparation Of G-C₃N₄@MOF Composite And Its Application In Photocatalytic Reduction Of CO₂

The massive burning of fossil fuels and the massive emissions of automobile exhausts are key factors in the greenhouse effect.Excessive CO₂ in the air has seriously threatened human life and has caused people to pay attention to environmental issues.Developing and finding green,energy-efficient,and efficient ways to reduce CO₂ from the air into clean,renewable energy sources can both address energy scarcity and reduce CO₂ emissions in the air,and turn waste into treasure.Photocatalytic technology has the advantages of mild conditions,high green efficiency,no secondary pollution,etc.The use of photocatalytic reduction of CO₂ in air to prepare organic chemical raw materials is an effective means to solve the greenhouse effect.However,many photocatalysts have been reported to limit their application in the field of photocatalysis due to a series of shortcomings such as small light absorption range,wide band gap and unstable decomposition.The newly developed g-C₃N₄ is a two-dimensional nano-semiconductor material with narrow band gap（2.7 eV）,which can effectively utilize visible light,and has the advantages of stable structure,non-toxicity and simple preparation.However,since photogenerated electron-holes are easily recombined,g-C₃N₄ has low catalytic activity,which limits its efficient application in the field of photocatalysis.Therefore,this paper mainly relies on the composite modification of g-C₃N₄ to alleviate the problem that the pure g-C₃N₄ photogenerated electron-holes are easily recombined.（1）The g-C₃N₄ nanosheets were prepared by high temperature calcination decomposition of melamine.The g-C₃N₄/ZIF-67 composite photocatalysts with different ratios were prepared by g-C₃N₄ encapsulation of ZIF-67 by simple stirring and aging.By X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FT-IR）,UV-visible diffuse reflectance spectroscopy（UV-vis DRS）,fluorescence spectroscopy（PL）,X-ray photoelectron spectroscopy（XPS）and N₂ adsorption-desorption isotherm to analyze the structure and physicochemical properties of the catalyst;the morphology of the samples was observed by transmission electron microscopy（TEM）and scanning electron microscopy（SEM）.The characterization results show that the size of the g-C₃N₄ nanosheet is about 1μm,and the ZIF-67 is uniform in size and dispersed.Compared with pure g-C₃N₄,the specific surface area of the catalyst increased from 17 m²·g^-1to 489 m²·g^-1;the band gap decreased from 2.24 eV to1.81 eV;the light absorption range was red-shifted as a whole;The compounding of electrons and holes rate is greatly reduced,and these changes are beneficial to photocatalytic reduction of CO₂ to ethanol.Ethanol production increased by about 3 times,and ethanol production decreased by about 6%after 5 cycles,indicating that the photocatalyst has better cycle stability.Secondly,the photocatalytic mechanism of CO₂ reduction is also analyzed and discussed.The photocatalytic mechanism analysis of CO₂ reduction shows that g-C₃N₄ and ZIF-67 have synergistic effects,prolonging the lifetime of electrons and holes in the composite,and reducing the recombination probability of electrons and holes.（2）Using melamine as the main raw material,polyacrylamide is used as a stripping agent to prepare g-C₃N₄ nanosheets,and CoO and ZnO nanoparticles are grown on g-C₃N₄,and then some metal oxides are dissolved to form ZIF-67 and ZIF-8 polyhedron,respectively.The final successful preparation of g-C₃N₄/CoO/ZIF-67 and g-C₃N₄/ZnO/ZIF-8 composites.The morphology,structure and properties of the catalysts were characterized by XRD,SEM,FT-IR,BET,PL and UV-vis.The results show that the photocatalytic performance of g-C₃N₄/CoO/ZIF-67 under visible light is relatively good in the two ternary composites prepared.Compared with pure g-C₃N₄,the specific surface areas of g-C₃N₄/CoO/ZIF-67 and g-C₃N₄/ZnO/ZIF-8 increased by 17.1 m²·g^-1and 9.26 m²·g^-1,respectively;significantly lower fluorescence intensity;after exposure to visible light for 1h,g-C₃N₄/CoO/ZIF-67 reduced CO₂to produce ethanol up to 1300μmolg^-1cat,while g-C₃N₄/ZnO/ZIF-8 reduced CO₂ to produce ethanol in an amount of 250μmolg^-1cat.After 5 cycles,the yield of ethanol produced by the two composites decreased below 10%,indicating that g-C₃N₄/CoO/ZIF-67 and g-C₃N₄/ZnO/ZIF-8 ternary composite photocatalysts have higher cycle stability.