Theoretical Design And Mechanism Study Of Electrocatalysts For Nitrogen Reduction Reaction On Two-Dimensional Materials

Ammonia（NH₃）is an important chemical raw material that is widely used in pharmaceutical industry,agriculture and industry fields.At present,Haber-Bosch method is commonly used in industry to synthesize ammonia,but this method not only has high energy consumption and low efficiency,but also causes excessive carbon emission and environmental pollution,which is not conducive to the sustainable development of human society.As an emerging method of ammonia production,electrocatalytic nitrogen reduction reaction（NRR）can convert nitrogen（N₂）into NH₃under mild conditions by using electric energy.It has the advantages of low energy consumption,no pollution and zero"carbon emission",and is one of the current research hotspots in the field of nitrogen fixation.However,electrocatalytic NRR technology is faced with severe challenges such as difficulties in nitrogen activation,low reaction efficiency,and strong competition in hydrogen evolution reaction,and is still far from the standard of industrial application.Therefore,the search for an efficient electrocatalyst with excellent stability,selectivity and catalytic activity is one of the key tasks in this field.In recent years,single atom catalysts（SACs）supported by two-dimensional materials have attracted wide attention in the field of electrochemical nitrogen reduction due to their excellent catalytic performance.With the development of experimental synthesis and characterization methods,some single atom catalysts have been used in the study of NRR.At the same time,the rational design and performance prediction of electrocatalytic catalysts based on first principles have become the necessary methods and means for the research of electrocatalytic nitrogen reduction,which is of great significance for promoting the research of electrochemical nitrogen fixation at normal temperature and pressure.In this paper,a series of single atom catalysts based on two-dimensional transition metal carbide（MXene）and covalent organic framework（COF）were designed by density functional theory（DFT）calculation to discuss the structural stability of the catalysts and determine the mechanism of nitrogen reduction reaction.Through calculation of electronic structure properties,the intrinsic nature of efficient nitrogen fixation performance of catalyst was revealed.The theoretical design work of NRR electrocatalysts in this paper can provide the basic theoretical basis and guidance for the related experimental studies.The main conclusions of the paper are as follows:（1）2D materials MXene（carbides and nitrides）are widely used in the field of catalysis due to their excellent chemical stability and electrical conductivity.Except for the SACs with transition metal atoms as the active center,the strong acceptor-donor interaction between the electron-deficient non-metallic B atoms and N₂ molecule is conducive to the activation of N₂,thus improving the NRR performance of the catalyst.Nine NRR single atom catalysts B@M₃C₂O_2-x（M=Ti,V,Cr,Zr,Nb,Mo,Hf,Ta,W）have been designed using transition metal carbide M₃C₂O_2-x with terminal O vacancies as support material.DFT calculation shows that the B@M₃C₂O_2-x（M=V,Cr,Nb,Mo,W）catalysts not only have thermodynamic and electrochemical stability,but also exhibit excellent NRR activity（corresponding limiting potential in the range of-0.32V～-0.48 V）and good selectivity（inhibition of hydrogen evolution reaction）.Therefore,they are promising NRR electrocatalysts.This work can provide theoretical guidance for the experimental study of non-metallic doping MXene catalysts and help to promote the potential application of MXene in NRR and other electrocatalytic reactions.（2）Because of its high stability and abundant active sites,COF materials have broad application prospects in the field of catalysis.Changing metal ion center of COF is an effective means and strategy to control the performance of catalyst.In this paper,based on the structure of the synthesized Co phthalocyanine COF linked by polyimide,six metal COF（TM-COF,M=Nb,Mo,Co,Hf,Ta,W）were designed for electrocatalysis of NRR by changing the metal ion center.Three kinds of TM-COF（TM=Nb,Mo,Ta）were selected by comparing by comparing the competitive adsorption of N₂ and proton-electron pairs in solution.The free energy diagram shows that NRR is carried out in consecutive pathway,distal pathway and mixed pathway on the three catalysts,respectively.The results show that Nb-,Mo-and Ta-COF have excellent NRR activity and NH₃ selectivity,especially Nb-COF has the best performance with a limiting potential of-0.02 V.Ab initio molecular dynamics simulation,dissolution potential and phonon spectrum calculation prove that they have good thermodynamic,electrochemical and dynamic stability.This study not only predicts the potential of 2D TM-COFs as electrocatalysts for NRR,but also provides a theoretical basis for the experimental design and synthesis of COFs-based catalysts.