Design Of Metal-nitrogen-carbon Catalyst And Theoretical Study On The Performance Of Electrocatalytic Ammonia Synthesis

Ammonia（NH₃）is an important raw material in agriculture and industry.However,the current ammonia synthesis still depends on industrial Haber-Bosch process,which not only requires harsh equipment condition but also results in great energy consumption and serious environmental pollution.Electrocatalytic ammonia synthesis have the advantages of mild conditions and low energy consumption.However,due to the limitation of catalytic activity of electrocatalyst and the influence of competitive hydrogen evolution reaction,the yield and Faraday efficiency of electrocatalytic ammonia synthesis are low,which is difficult to meet the needs of practical application.Therefore,it is urgent to develop high activity and high selectivity electrocatalysts for ammonia synthesis.By means of density functional theory（DFT）,this paper studies the performance of a series of metal-nitrogen-carbon catalysts for electrocatalytic ammonia synthesis.The key factors to improve the catalytic performance was revealed,and the activity descriptor was developed.The relationship between structure and activity were also determined.These findings provide a theoretical basis and prediction tool for subsequent experimental research.For investigating the electrocatalytic ammonia synthesis performance of adjacent single metal active sites,28 bimetallic metal-nitrogen-carbon catalysts（MN₄/M’N₄-C）were designed.DFT calculation was used to predict the stability of the catalyst,the activity and selectivity of electrocatalytic ammonia synthesis,and the mechanism of the active sites of adjacent single metal atoms on improving ammonia synthesis activity was clarified.Theoretical calculations show that all designed catalysts have high thermodynamic stability.Among them,the limiting potential of ammonia synthesis for Cr N₄/M’N₄-C（M’=Cr,Mn,Fe,Cu,Zn）catalysts is about-0.60 V,which is higher than that of single metal-nitrogen-carbon catalysts reported in previous literatures.In addition,they can effectively inhibit the side reaction of hydrogen evolution and show good catalytic activity and selectivity.It is found that Cr is the active center for adsorbing and activating N₂.Another metal atom regulates the charge distribution of the system through electronic modulation effect,assisting the Cr active center in ammonia synthesis reaction,which improves the activity of the catalyst.In order to further enhance the interaction between metal atoms,a series of M-M’/N₆-C（M=Fe,M’=Mn,Fe,Co,Ni,Cu,Zn,Mo）catalysts with bonded bimetallic atoms as the active center was studied.The thermodynamic stability,electrocatalytic ammonia synthesis activity and hydrogen evolution activity of the catalysts were studied by DFT calculation.The selectivity of ammonia synthesis reaction and hydrogen evolution reaction was determined,and the mechanism of electronic structure change to improve the activity of ammonia synthesis was discussed.The calculation results show that Fe-Mn/N₆-C can effectively adsorb and activate N₂ molecule,showing excellent activity and selectivity for ammonia synthesis with the limiting potential of-0.37 V.It is much better than the single Fe metal-nitrogen-carbon catalyst with limiting potential of-0.85 V,and the above adjacent bimetallic catalysts with limiting potential of-0.60 V.The limiting potential of hydrogen evolution reaction is-0.62 V,indicating that Fe-Mn/N₆-C catalyst has high selectivity for ammonia synthesis.Comparing with Fe-N₃C,the energy levels of Fe-Mn/N₆-C and antibonding orbital of nitrogen match each other.The bonded Fe and Mn atoms simultaneously participate in the acceptance-donation process through synergistic effect,which can not only donate the electrons of d orbital to the antibonding empty orbital of N₂ molecule,but also have empty orbital to accept the lone pair electrons of N₂.It will promote the activation of N₂ molecule and improve the performance of Fe-Mn/N₆-C for ammonia synthesis.To further investigate the influence of the number and type of metal atoms in the active center on the electronic structure of the catalyst and the activity of ammonia synthesis,the ammonia synthesis properties of 24 metal-nitrogen-carbon catalysts（M₃/N₆-C）were studied.The active center of M₃/N₆-C is homonuclear triatomic clusters composed of fourth,fifth and sixth periodic transition metals.Theoretical calculation shows that Ni₃/N₆-C,Ru₃/N₆-C and Ir₃/N₆-C catalysts have high thermodynamic stability,excellent ammonia synthesis activity and selectivity with the limiting potential of-0.30 V,which is better than most of the widely used catalysts reported in the literature.The mechanism study shows that the three-dimensional structure of the cluster active center weakens the steric effect for the N₂ adsorption,making it adsorb on the catalyst surface as a side-on configuration.This will enhance the multiple"push-pull"effect between metal atoms and N₂molecule,which will be a great benefit to weaken the strength of N≡N triple bond and accelerate the progress of ammonia synthesis.Based on the relationship of structure and activity obtained from the above calculation,an activity descriptor including the first ionization energy of metal/coordination atom and the number of d/valence orbital electrons of the active center is proposed.It can provide a reasonable prediction tool for the design and development of electrocatalytic ammonia synthesis catalysts.