Construction Of Graphene Aerogel-supported Modified Carbon Nitride Composite Photocatalyst And Study Of CO₂ Reduction Performance

With the development of industry,human production activities use a large amount of non-renewable energy,which seriously causes excessive carbon dioxide（CO₂）gas emissions.CO₂ is one of the main gases which cause the greenhouse effect.Photocatalytic CO₂reduction can alleviate the energy crisis and reduce the greenhouse effect,which has been widely concerned.The core problem of photocatalytic CO₂reduction technology is to design efficient and stable photocatalysts.As an n-type semiconductor material,carbon nitride has the characteristics of high stability and visible light response,and is one of the focuses of current scholars’research.The pure phase of carbon nitride has the disadvantages of low separation efficiency of electron-hole pair and narrow light absorption range,which is difficult to directly save as a material for photocatalytic CO₂ reduction.At the same time,Graphene aerogel（GA）is a 3D network structure,consisting of carbon atoms in a two-dimensional layer,which has the characteristics of high specific surface area,high porosity,high electrical conductivity and good thermal conductivity,making it an ideal porous electrode material.Functional groups or nanoparticles are grafted to form functional graphene aerogels,which have good application prospects in electronics,catalysis,energy and other industries.Therefore,by modifying carbon nitride and supporting it on graphene aerogel could construct heterogeneous junction to speed up the effective separation of electron-hole pair and broaden their absorption range of visible light,which is one of the ideal strategies to promote the efficiency of photocatalytic CO₂ reduction.The purpose of this paper is to design a series of graphene aerogel（GA）supported modified carbon nitride composite material,to characterize their physical properties,and to explore possible catalytic mechanisms.It consists of the following three parts:Part one,C₃N₄ nanomaterials were synthesized by a simple thermal condensation method,and then C₃N₄-Co Se₂ nanoparticles were synthesized by hydrothermal method,and then anchored on the surface of graphene aerogel（GA）to prepare a honeycomb shape（C₃N₄-Co Se₂）/GA photocatalyst.At the same time,using lamp as a simulated visible light source,without any precious metals and sacrificial agents,the photocatalytic activity of the prepared nanomaterials is evaluated by reducing the photocatalytic CO₂ to CO.The results show that Co Se₂ as a co-catalyst can improve the photocatalytic performance of carbon nitride.Introduce Co Se₂ and GA on the surface of C₃N₄nano frame and prepare a cellular structure（C₃N₄-Co Se₂）/GA catalyst,The coupling of GA,Co Se₂ and C₃N₄ can significantly improve the light absorption density and extend light response range which show a lower fluorescence intensity and a larger electron transfer rate.Under the same kind of photocatalysis,（C₃N₄-Co Se₂）/GA has the highest catalytic efficiency for CO₂ reduction,with CO yield reached 5.75μmol·g^-1·h^-1,and the repeated use performance is good.Part two,（C₃N₄-Bi₂O₃）/GA is prepared by calcination,solvothermal method and freeze-drying technology.The porous rose-shaped Bi₂O₃ and C₃N₄ form an interface interaction and matching energy band structure,found that the electron-hole pair recombination rate of（C₃N₄-Bi₂O₃）/GA material is reduced and the charge separation efficiency is high.By changing the mass ratio of Bi₂O₃,different proportions（C₃N₄-Bi₂O₃）/GA were prepared.Using the same test conditions,the results show5wt%（C₃N₄-Bi₂O₃）/GA is the optimal CO₂ reduction performance,and the production rate is 11.36μmol·g^-1·h^-1,which is 32 times and 71 times faster than the Bi₂O₃ and C₃N₄generation rates.Through 4 tests,the final rate is 10.08μmol·g^-1·h^-1,which can be seen that is no obvious change in the rate of CO production,and proves that the material has good repeatability.Part three,the（C₃N₄-ZnO）/GA composite material was successfully synthesized through calcination,hydrothermal method and freeze-drying technology,Hexagonal crystal ZnO and C₃N₄ form heterogeneous junctions,reducing the band gap and widening the absorption range of ultraviolet visible light.（C₃N₄-ZnO）/GA was analyzed through a series of characterizations such as photocurrent,impedance,and fluorescence spectroscopy.And by changing the mass ratio of C₃N₄,different ratios of（C₃N₄-ZnO）/GA were prepared.Using test conditions consistent with Parts 1 and 2,the results of photocatalytic CO₂ show that 7wt%（C₃N₄-ZnO）/GA exhibits optimal performance,with a production rate of 33.87μmol·g^-1·h^-1,which is 25.6 times and 21.2times the production rate of ZnO and C₃N₄.After 5 cycles of testing,the final CO production rate is 31.16μmol·g^-1·h^-1,which can still achieve a photocatalytic efficiency of 91.99%,indicating that the（C₃N₄-ZnO）/GA catalyst has good cycle stability.