Theoretical Study On Electrocatalytic Nitrogen Reduction By Two-Dimensional Materials MXene

There is an urgent need for exploring alternative resources based on the continuous consumption of non-renewable energy sources.Ammonia（NH₃）is a basic raw material for fertilizer,a carbon-free carrier of hydrogen,and an alternative fuel.Therefore,artificial nitrogen fixation plays an indispensable role in people’s daily life.The conventional industrial Haber-Bosch process for ammonia production requires high energy consumption and produces large amounts of greenhouse gases.Therefore,a more moderate approach to NH₃production is of great interest and green ammonia synthesis methods including electrocatalytic nitrogen reduction reaction（NRR）have been proposed.MXenes are a type of transition metal carbides,nitrides or carbon-nitrogen compounds,which have unique properties such as high electrical conductivity,good thermal stability,adjustable surface terminations and corrosion resistance,making MXenes promising for applications in catalysis.In this thesis,a density functional theory（DFT）approach is used to study the catalytic performance of transition metal doping/defect engineering in the electrocatalytic NRR of MXenes.Chapter 1 introduces the structures and properties of MXenes,the research progress of MXenes-based catalysts and electrocatalytic NRR;Chapter 2 briefly introduces the theoretical calculation methods;Chapters 3 to 5 are the main research contents of this thesis:1.The catalytic reduction of N₂ catalyzed by Nb₂C-doped transition metal monoatoms（TM-Nb₂C,TM=Ti,V,Mn,Fe,Co,Y,Zr,Mo）was systematically investigated by DFT calculations,in which Nb₂C doped with Fe,Mo,and Co show excellent performance in electrocatalytic NRR,and the formation energy and dissolution potential indicate that the three catalysts had good stability.Among the studied catalysts,Fe-Nb₂C exhibits the best catalytic activity with low limiting potential（U_L（NRR）=-0.47 V）and excellent NRR selectivity.Finally,a volcano plot between U_L（NRR）and the ICOHP values of the N-H bond in^*NH₂ was established to determine the optimal ICOHP values that can be used as a simple descriptor for NRR performance of Fe-Nb₂C.2.The electrocatalytic activity of V_C-Mo₂C catalyzed N₂ reduction to NH₃ was investigated by DFT calculations.The results show that V_C-Mo₂C has excellent catalytic activity and the optimal pathway for NRR is the dissociative-associative pathway with a limiting potential of-0.43 V.Its catalytic performance is superior to most of the reported metal catalysts.The presence of coordination-unsaturated Mo atoms in the defect contributes to the catalytic activity compared to Mo₂C.V_C-Mo₂C also has good NRR selectivity and can inhibit HER side reactions.This work provides a new idea for the design of efficient NRR catalysts and guides the experimental and theoretical studies of nitrogen fixation catalysts.3.The activity of Mo₃C₂ electrocatalytic NRR was investigated by DFT calculations.It is shown that Mo₃C₂ can promote the stable adsorption and effective activation of N₂ through the"acceptance-donation"process,and Mo₃C₂ exhibits excellent catalytic activity for NRR along a consecutive path with a limiting potential of 0.02 V.In addition,we determined that Mo₃C₂ has a good suppressed effect on the competitive reaction HER.The work provides valuable information for experimental and theoretical studies to explore the NRR catalytic potential of Mo-based MXenes catalysts.