The Study On Nitrogen Fixation Performance Of Two–dimensional Materials And The Underlying Mechanism Of The Energy Conversion

Ammonia,as the basic chemical substance for the production of various synthetic chemicals,plays an indispensable role in industry and agriculture.Besides,due to the outstanding features of high hydrogen content,high energy density,facile storage/transportation,and zero–carbon emission,amomonia is identified as a promising green energy to replace the fossil fuel.Currently,the ammonia is mainly from the Haber–Bosch process,which operates under extremely harsh conditions,leading to the huge energy consumption and the massive emission of carbon dioxide.To overcome the mentioned shortages of the Haber–Bosch method,the electrocatalysis of the N₂–to–NH₃ conversion paves a green and sustainable avenue for the ammonia synthesis under the mild reaction,wherein the performance is significantly depended on the efficiency of the catalysts as used.Despite tremendous efforts in recent decades,electrocatalysts that selectively and efficiently produces ammonia from nitrogen reduction remain elusive.Therefore,the development of the highly–active materials for the ammonia synthesis has attracted great attentions.This thesis aims to investigate the nitrogen reduction performances of single–transition metal atom modified bismuthene,single–transition metal atom functionalized graphene as well as single–transition metal atom and single–non–metal atom codoped functionalized graphene.By screening 3d/4d/5d transition metal element,our gaol is to identify the potential candidates by means of element screening for the experimental preparation and to uncover the catalytic mechanism by means of electronic structure analysis.The specific focuses on the following:1.Firstly,the nitrogen reduction activity of the transition metal decorated bismuthene TM@Bis is fully investigated by means of density functional theory calculations in Chapter3.The results demonstrate that W@Bis delivers the best efficiency,wherein the potential–determining step is located at the last protonation step via the distal mechanism with the limiting potential U_L of 0.26 V.Furthermore,the dopants of Re and Os are also promising candidates for experimental synthesis due to its good selectivity,in despite of the slightly higher U_L of NRR with the value of 0.55 V.However,the candidates of Ti,V,Nb and Mo delivered the relative lower U_L of 0.35,0.37,0.41 and 0.43 V might be suffered from the undeniable hydrogen evolution reaction.More interestingly,a volcano curve is established between U_L and valence electrons of metal elements wherein W with 4 electrons in d band located at the summit.Such phenomenon originates from the underlying acceptance–back donation mechanism.Therefore,our work provides a fundament understanding for the material design for nitrogen reduction electrocatalysis.2.Secondly,the nitrogen electro–reduction toward ammonia on the functional graphene with the six–coordinated transition metal site has been systematically investigated by the density functional theory calculations in Chapter 4.The results demonstrate that Mo N₃C₃ and Zr N₆ can effectively promote nitrogen fixation process under the limiting potentials of 0.51 V,respectively.However,in despite of low limiting potentials of 0.44 and0.16 V,VN₆ and Nb N₆ may suffer from low efficiency of ammonia production caused by the competition of the undesirable hydrogen evolution reaction.Furthermore,Mo N₃C₃shows lower activity in comparison with Mo N₃ and Mo N₄ and Zr N₆ performs best compared to Zr N₃ and Zr N₄.Therefore,Zr N₆ is attractive for experimental assessment.Meanwhile,for the macroporous model,the pre–transition group metals have better nitrogen reduction performance than the post–transition group metalsthe.Therefore,our work provides some possible efficient catalyst of nitrogen electro–reduction to ammonia synthesis.3.Finally,the application potential of the combination of metal and non–metal atoms as NRR electrocatalysts for NH₃ synthesis are systematically investigated via density functional theory calculations in Chapter 5.Herein,according to the previous works,the transition metal elements of Fe,Nb,Mo,Ru and W and the non–metal elements of B,P,S are taken into consideration.Fully considering the structural stability,NRR activity and selectivity,the results identify Mo S delivers an optimal NRR performance with a relative low limiting potential of 0.47 V.Furthermore,the overpotential of hydrogen evolution catalyzed by Mo S is 0.51 V,which is higher than those of Mo B and Mo P.The introduction of S secondary dopant enhances the hydrogen adsorption and increase the thermodynamic barrier of hydrogen evolution.It improves the NRR selectivity of Mo S in comparison with Mo B and Mo P.In addition,according to the first–principle molecular dynamic calculation,the Mo S configuration is preserved under the temperature of 300 K without any bond breakage.As well–known,the controllable synthesis of the desired material with the identified active is extreme–challenge duringthe experimental preparation.It means different configurations would be merged in the samples,which would raise different performance.Therefore,our work validates the influence of secondary non–metal atom influence of active center and offers a basic understanding for the material optimization during the experimental synthesis.