Theoretical Design And Performance Investigation Of Two-Dimensional Materials Supported Single-Atom Catalysts For Electrochemical Synthesis Of Ammonia

Ammonia（NH₃）is an important substance widely used in chemical,agricultural and medical fields.However,the traditional NH₃ synthesis method——the Haber-Bosch process——suffers from serious problems such as harsh reaction conditions,high energy consumption and substantial CO₂ emissions.Therefore,the use of electrochemical technology to convert nitrogen sources in the atmosphere and water into ammonia through renewable energy has gradually become an environmentally friendly and economically valuable NH₃ synthesis method.However,this method is still at the research stage,and effective solutions still need to be found to address the problems of low activity and poor ammonia selectivity in the electrochemical N₂reduction reaction（NRR）and NO reduction（NORR）processes.Among them,one of the most key problems is the development of stable and efficient electrocatalysts.In this paper,based on density functional theory calculations,a series of two-dimensional material supported single-atom catalysts（SACs）were designed by defect construction,atom doping,coordination regulation to promote efficient catalysis of NRR and NORR.Through electronic structure analysis,the origin of catalytic activity was revealed,and the intrinsic relationship between the electronic structure and catalytic activity of the catalyst was explored,providing new ideas and theoretical basis for the optimal design of the catalysts of electrochemical synthesis ammonia.The research contents and main results of this thesis are as follows:1.Nitrogen-doped porous graphene and graphitic carbon nitride（g-C₂N）supported transition metal single-atom catalysts for electrochemical NRR to ammoniaNitrogen-doped porous graphene（NPG）is a kind of promising support of single-atom catalysts.In this study,based on the graphitic carbon nitride g-C₂N structure with N6 cavity,a six-fold N6-cavity NPG was designed as a support for single transition metal atoms（TM@N6-G）,and the stability and NRR catalytic performance of TM@N6-G SACs were systematically investigated.Six promising catalysts（Nb@N6-G,Mo@N6-G,Tc@N6-G,Ta@N6-G,W@N6-G,and Re@N6-G）were selected from14 candidates by screening the free energy changes of the first and last electric chemical steps.Detailed NRR mechanism studies show that six SACs have superior electrocatalytic NRR performance,especially W@N6-G,reaching low limiting potentials of-0.23 V.Further calculations reveal that except for Re@g-C₂N,the other five SACs using g-C₂N（TM@g-C₂N）as a support exhibit good stability,high NRR catalytic activity and selectivity.Electronic structure analysis indicates thatΔG_*NNH,d _N-N,and ICOHP can be used as descriptors to understand the origin of NRR catalytic activity of these two kinds of catalysts.The results of this study show that the coordination environment of the active center atoms has a great influence on the electrocatalytic performance of single-atom catalysts,which provides an important reference for the design of efficient NPG supported NRR electrocatalysts.2.Two-dimensional boron phosphide（BP）supported transition metal single-atom catalysts for electrochemical NORR to ammoniaBoron phosphide（BP）is a novel two-dimensional material with high thermal conductivity,chemical stability and charge mobility.In this study,BP was used as a support and B atom defect was introduced to enhance the stability of transition metal single-atom catalysts（TM@BP,TM=Ti～Zn,Zr～Ag and Hf～Au）.By calculating the stability of the catalyst and the free energy changes of the first（*NO→*NOH or*NO→*NHO）and the last hydrogenation steps（*OH→*H₂O or*NH₂→*NH₃）of NORR,7 TM@BP（TM=Ti,V,Cu,Rh,Ag,Ir,and Au）were selected from 23 candidate SACs.Further,all possible reaction pathways of NORR to produce NH₃ and byproducts（N₂O,N₂ and H₂）were systematically investigated.The results indicate that Ti@BP,V@BP,Cu@BP,and Au@BP exhibit excellent catalytic activity with a limiting potential（U_L=-0.60～0 V）close to Cu based catalysts,and high NORR selectivity toward NH₃.Therefore,these SACs can serve as potential NORR electrocatalysts.This study provides useful guidance for the future design of boron phosphide based highly efficient NORR electrocatalysts.3.Two-dimensional C₃N supported main-group metals（Al,Ga and In）single-atom catalysts for electrochemical NORR to ammoniaHow to adjust the selectivity of the catalyst for NH₃ products in the NORR is the main challenge of electrocatalytic NORR.Different from transition metals,the main group metal atoms（MGM）have weak binding ability with H atom,which can play a role in inhibiting hydrogen evolution reaction.In this study,the main group metal atoms（Al,Ga and In）were used as the active centers of SACs,and four supports with different coordination environments were constructed on the two-dimensional C₃N surface:single defect（C and N defects）and double defect（C-C and C-N defects）.Based on DFT calculations,the NORR pathways of 12 SACs（MGM-C₂N,MGM-C₃,MGM-C₂N₂ and MGM-C₄）were systematically investigated.The calculated results show that7 SACs exhibit excellent NORR activity and high NH₃ selectivity.Through further stability calculations,5 potential NORR electrocatalysts were finally screened,namely Al-C₂N₂,Ga-C₂N₂,Ga-C₄,In-C₂N₂ and In-C₄.Meanwhile,electronic structure analysis shows that there is a good linear relationship between the Bader charge of the main group metal atoms(Q_MGM)and the?G_max of the element steps of NORR,which promotes the understanding of the relationship between coordination environment and catalytic activity.This study not only provides valuable theoretical reference and guidance for the future exploration of monolayer C₃N supported main group metal SACs,but also contributes to the further understanding of the effect of coordination environment on the catalytic activity of SACs.