Theoretical Investigations On The Mechanisms Of The Electrosynthesis Of Ammonia On Single Transition Metal Atoms Supported By Two-Dimensional Materials

The synthesis of ammonia from nitrogen and nitrate by electroreduction processes（NRR and NO₃RR）under mild conditions has received much attention because of its advantages such as energy saving and environmental protection.The design and synthesis of electrocatalysts with low overpotential and high Faraday efficiency are particularly important for the synthesis of ammonia by electroreduction.In recent years,single-atom catalysts,especially two-dimensional material-loaded single-atom catalysts,have been widely used in the field of electrocatalytic ammonia synthesis.In this thesis,the study of the mechanism of electrocatalytic ammonia synthesis on twodimensional material supported single-atom catalysts（SACs）and the theoretical design of SACs are carried out based on density functional theory（DFT）calculations.The main research results are as follows:Based on DFT calculations,we systematically investigate the stability and the potential electrocatalytic nitrogen reduction reaction performance of single transition metal atom anchored at the Al vacancy of aluminum arsenide（AIAs）monolayer（TM@AlAs,TM=V～Pt）.It is found that Nb@AlAs 和 W@AlAs are potential electrocatalysts for NRR with limiting potential of-0.20 and-0.27 V,respectively via enzymatic pathway,and the formation of*NH₂-*NH₂ is the potential-determining step.By considering the competition of hydrogen evolution reaction（HER）,it is found that high ammonia selectivity can be reached on Nb@AlAs 和 WAlAs.Therefore,Nb@AlAs and W@AlAs can be used as potential NRR catalysts with highly activity and selectivity.Further,we carry out a theoretical study to investigate the reduction mechanism of electroreduction of nitrate on single metalatoms（from Ti to Au）supported by g-C₂N monolayer.By considering the stability of designed SACs and their potential NO₃RR catalytic performance,Zr/g-C₂N and Hf/g-C₂N are identified as potential NO₃RR catalysts with high activity and ammonia selectivity.A detailed study on the reaction mechanism of NO₃RR show that*NO→*NOH is the limiting potential step and the limiting potentials are calculated as-0.28 and-0.27 V on Zr/g-C₂N and Hf/g-C₂N,respectively.A volcano curve relationship between the limiting-potential and the adsorption free energy of NO₃ is found With the help of machine learning,a descriptor of the adsorption free enrgy of nitrate is revelved and new SACs supported on g-CN are predicted according the descriptor.This work provides theoretical insights for the design of new NO₃RR catalysts.