Study On The Modification Of G-C₃N₄ And MXene-based Layered Materials And Their Photocatalytic Nitrogen Fixation Performance

Nitrogen is essential to every life on earth.At present,the commonly used industrial nitrogen fixation is the Haber-Bosch process,but this process must be at high temperature and high pressure to produce NH3,which has a huge demand for fossil energy,and the harm to the environment is not small.Photocatalytic nitrogen fixation uses solar energy and appropriate catalysts to convert nitrogen molecules in the air into required ammonia under normal temperature and pressure,which has great research value.However,due to some common defects(such as some photocatalyst band gap limitation,photogenerated electrons and holes generated in semiconductors under illumination are too fast for recombination charge recombination efficiency,many most catalysts have weak chemisorption energy for nitrogen,etc.),most photocatalysts have problems such as poor absorption of light and the inability to continue the photocatalytic nitrogen fixation reaction.Therefore,the efficiency of photocatalytic nitrogen fixation reported so far has not been greatly improved.The two-dimensional layered material has the characteristics of large specific surface area,high surface atom to overall atom ratio,which makes it have the advantages of high surface activity and fast electron migration speed,and is very suitable as a photocatalyst.Two layered materials,g-C3N4 and MXene,were selected for modification studies to explore the changes in their photocatalytic nitrogen fixation capabilities.The main contents are as follows:(1)g-C3N4 is a planar two-dimensional lamellar polymer semiconductor with suitable band gap and stable chemical properties,and the raw materials are abundant and easy to prepare.It is considered a good photocatalyst.However,due to visible light absorption edge and grain boundary effects,its photocatalytic nitrogen fixation activity is low.Moreover,the weak van der Waals interaction between adjacent conjugate planes of g-C3N4 also limits the electronic coupling between the planes,which has a negative impact on electron migration.In view of the weak electron mobility of g-C3N4,this work chose graphene to compound with it.Graphene has excellent electrical properties and can be used as a channel to transport electrons,helping g-C3N4 to separate and migrate photogenerated carriers.In this work,the graphene-gC3N4 composite material with the best performance was explored after compounding g-C3N4 with several different forms of graphene through hydrothermal reaction.After analyzing the experimental results,it is found that the nitrogen fixation performance of the g-C3N4 and graphene oxide(GO)composite material is improved the most,and the nitrogen fixation yield of CNG(20%)is 95.95μmol g-1 h-1,which is 4.7 times that of pure g-C3N4.Through the structural characterization and photoelectric performance test of the composite material,it is found that GO has good conductivity,improves the separation efficiency of g-C3N4 carriers,and enhances the utilization of visible light,thereby enhancing the photocatalytic activity.(2)MXene is a new type of layered transition metal carbide(Ti3C2).Because of its good hydrophilicity,conductivity and large specific surface area,it has application prospects in the field of catalytic nitrogen fixation.However,Ti3C2-MXene is not a semiconductor,and it does not have the ability of photocatalytic nitrogen fixation.It is necessary to modify Ti3C2-MXene to give full play to its advantages in the field of phococatalytic nitrogen fixation.This work makes full use of the characteristics of the MXene layered compound,selects CTAB as a pillaring agent to expand the layer spacing,and then successfully inserts Sn ions between the Ti3C2-MXene layers by the water bath method.After hightemperature calcination,TiO2 is generated in situ on the surface of the material to form an MXene/TiO2/Sn/C heterojunction.The nitrogen fixation capacity of the prepared composite catalyst is as high as 110.34 μmol g-1 h-1,which is 11.5 times that of the pure Ti3C2-MXene calcined product.Electrochemical tests have proved that the charge separation efficiency and migration ability under light are significantly improved.