A First-principles Study On Doping-controlled Nitrogen Reduction Reaction

Ammonia（NH₃）as an essential raw material for fertilizers,medicines and other products,plays an important role in human production and life.The traditional industrial method of ammonia（Haber-Bosch Process）is powered by consuming a large amount of non-renewable energy such as petroleum and coal,which accelerates the emission of greenhouse gas（CO₂）,which leads to the greenhouse effect,melting of glaciers and other problems,seriously threatens the ecological environment.As the pressure brought by the ever increasing population and industrial expansion,China has proposed a strategy of carbon neutrality and carbon peaking.The development of green and sustainable ammonia production methods can achieve this goal.At present,electrocatalytic nitrogen reduction reaction（NRR）driven by clean energy such as wind energy and water energy is regarded as a potential means to replace the traditional H-B method.In the reaction process,high-efficiency catalyst is necessary because it can accelerate the reaction rate,lower the reaction energy barrier,and improve the yield.The efficiency of the catalyst depends on the intrinsic properties of active site.Two-dimensional materials can change their electronic structure and other property by doping atoms,thereby improving the catalytic ability of active sites,thus playing the role of supporting active sites of single-metal catalysts.Therefore,in this work,using density functional theory（DFT）calculations,by changing the electronic structure of the two-dimensional supports by doping other atoms,an unsymmetric structure of TM-S₁N₃ and the alkyne ring of pyrazine graphyne（TM-N₂）were designed as active sites for the single-atom catalyst of NRR.The research contents are divided into the following two parts:（1）Transition metal（TM）atoms coordinating with N atoms（TM-N_x）as the catalytically active center for nitrogen reduction reaction（NRR）has greatly promoted the design and development of catalyst materials for NRR,among which the TM-N₄with asymmetric structure has been widely studied.Recently,researches have confirmed that the unsymmetric active center has better catalytic performance.Therefore,an unsymmetric coordinated with S and N atoms is designed in this paper.With the help of DFT calculations,the catalytic performance of a series of single atoms supported on graphene coordinated with S₁N₃（TM-S₁N₃）was systematically investigated.Among them,W-S₁N₃ exhibits the most excellent NRR performance following the distal pathway,only requiring a low overpotential of-0.29 V,and can effectively avoid competitive hydrogen evolution reaction（HER）.Compared with the symmetrical W-N₄ and single W atom supported on graphene,W-S₁N₃ is more effective in adsorbing and activating N₂ and lowering the reaction barrier.The excellent performance is derived from the unique structure,which makes the adsorbed N₂ accept more electrons and effectively weaken the N≡N triple bond.This work demonstrates that tuning the coordination environment around the active site is a viable strategy for the rational design of NRR electrocatalysts.（2）The doping of pyridine N and its doping amount will affect the reductive ability of carbon-based catalysts.In order to optimize the catalyst performance,the N atoms are demarcated at the diagonal positions of the benzene ring in graphyne to form carbon nitrides graphyne（N-GY）with a rhombohedral structure.With the aid of DFT calculations,we discuss suitable sites for loading single metal atoms and their stability.After extensive calculations,the Mo@N-GY can be a potential nitrogen reduction catalyst with the lowest confinement potential（U_L=-0.41 V）and excellent selectivity,and the mechanism of the excellent activity is investigated through electronic structure characterization.