Controlled Synthesis Of Novel Two-dimensional Materials For Photocatalytic And Electrocatalytic Ammonia Synthesis

Ammonia is important in human production and life.The Haber-Bosch method is the most widely used nitrogen fixation method in the modern chemical industry.However,this method has intrinsic disadvantages such as high energy consumption,extreme reaction conditions of 300-500? and 15-25 MPa,and severe emissions of CO2,leading to environmental issues.Photocatalytic and electrocatalytic nitrogen fixation can be carried out at normal temperature and pressure,which has the advantages of reducing energy consumption and pollution.Therefore,photocatalytic nitrogen fixation and electrochemical nitrogen fixation are one of the most promising ammonia synthesis methods.This thesis focuses on photocatalytic and electrocatalytic N2 reduction to ammonia by two-dimensional materials under normal temperature and pressure.Existing semiconductor photocatalytic materials have large forbidden band widths,which makes it difficult to effectively use visible light and near-infrared spectral responses,resulting in a decrease in photocatalytic performance during application.The semiconductor forbidden band width can be reduced by controlling the crystal plane,constructing a heterojunction structure.In view of the serious problem of the side reaction of hydrogen evolution in the electrocatalytic synthesis of ammonia,the hydrophobic modification of the electrode surface is designed to reduce the local proton concentration around the electrode,which effectively reduces the occurrence of hydrogen evolution side reactions,and improves the selectivity of the electrocatalytic ammonia synthesis reaction.The above catalyst design strategy improves the ammonia yield rate and reaction selectivity of photocatalytic and electrocatalytic processes.The above catalyst design strategies have improved the energy conversion efficiency in photocatalytic and electrocatalytic processes,and provide a crucial catalyst design and theoretical basis for the practical application of artificial nitrogen fixation technology at room temperature and pressure.This research is divided into the following three research subjects:1.The new antimony nanosheets with different thicknesses were obtained by ultrasonic liquid phase exfoliation method and fractional centrifugation with an average thickness of 2 nm and a lateral size of up to 100 nm.The ammonia yield rate could reach 297.5 ?g h-1 gcat.-1 under visible light,and 422.3 ?g h-1 gcat.-1 under full light irradiation,which is 4.6 times that of the body antimony.The visible light absorption range was extended to 800 nm,which greatly improved the light absorption capacity.The surface oxidation increased as the thickness of antimony nanosheets decreased,and the ammonia yield rate increased.Calcination in a reducing atmosphere could enhance the catalytic active site.The optimal calcination time under H2 was 1 h,and the calcination temperature 300?.The ammonia yield rate of the defective antimony nanosheets could be greatly increased under visible light.The antimony nanosheets had the highest reactivity under a neutral reaction medium.The photocatalytic nitrogen fixation performance got better as the solvent polarity of the reaction medium increased.The antimony nanosheets had good stability,and the crystal structure and band structure of the catalyst did not change significantly before and after the reaction.Finally,the efficient photocatalytic nitrogen fixation activity of antimony nanosheets was mainly attributed to the ultra-thin layer structure,which exposed a large number of active surface defect sites,promoting the migration and separation of charge carriers and enhancing the N2 adsorption capacity.2.A new heterojunction composite catalyst composed of new antimony nanosheets and titanium dioxide with wide spectrum absorption was successfully prepared by ultrasonic method.Heterojunction composite catalysts had high carrier density,low impedance,and stronger photocurrent response capabilities.The antimony/titanium dioxide heterojunction composite maintained a good ammonia yield rate of 547.4 and 353.9 ?g h-1 gcat.-1 under full spectrum and visible light irradiation,which was 2 times and 16 times higher than that of antimony nanosheets and amorphous titanium dioxide.Tight interfacial contact in the antimony and titanium dioxide heterojunctions greatly enhanced the photogenerated carrier migration and separation,and enhanced the photocatalytic nitrogen fixation activity through the synergistic effect of the two components.This result provided new research for the development of new and efficient photocatalytic nitrogen fixation catalysts and ideas.3.The metal phase TaTe2 nanosheets were prepared by simple ultrasonic liquid phase exfoliation method.TaTe2 nanosheets were used for the first time in the electrocatalytic nitrogen fixation reaction.The ammonia yield rate was 6.3 ?gNH3 h-1 mgcat.-1,2.8 times higher than that of bulk TaTe2.At-0.17 V,the Faraday efficiency of TaTe2 nanosheets was 7.3 times higher than that of bulk TaTe2.The surface of the catalyst was hydrophobically modified to suppress the hydrogen evolution reaction,and improve the selectivity of the electrocatalytic nitrogen fixation reaction.The TaTe2 nanosheet catalyst had good stability.After 12 cycles of use experiments,the ammonia yield rate and Faraday efficiency of the catalyst remained stable.After 100 hours of electrolysis,the current value remained stable,and the catalyst activity and stability were maintained after the end of electrolysis.It provided a new catalyst for two-dimensional materials in electrocatalytic nitrogen fixation,and also provided a new idea for reducing the hydrogen evolution reaction and improving the selectivity of nitrogen fixation.