Theoretical Study And Design Of Atom-loaded Two-dimensional Nitrogen Reduction Electrocatalysts

With the rapid growth of the world’s population and the continuous depletion of fossil fuels,new demands are being placed on the development of green and clean energy.Ammonia（NH₃）is not only a high value-added fuel,but also a key raw material for fertilizer,medicine and plastics.At present,the industrial synthesis of NH₃ mainly relies on the Haber-Bosch process,but it has the disadvantages of harsh reaction conditions,high energy consumption and high environmental pollutability.It is imperative to find a green and sustainable ammonia synthesis method.The non-polluting and low energy consumption electrocatalytic nitrogen reduction reaction（NRR）can reduce N₂ to NH₃ at ambient temperature and pressure using renewable electricity.However,the NH₃ yield and Faraday efficiency（FE）of NRR ammonia synthesis are still far from the industrial production standard.Therefore,it is urgent to design electrocatalysts with excellent catalytic performance,high stability and high selectivity.Single atom catalysts（SACs）have become one of the research hotspots in the field of electrocatalysis due to their 100%atomic utilization and low cost.However,the design of SACs also slove many challenges,such as how to avoid the interaction of d-d orbitals between metal atoms to achieve high dispersive stability;how to precisely regulate the size of catalyst pore size to break the drawbacks of multiphase catalytic systems;how to solve the problem of low loading of single-atom catalysts;how to accurately regulate the coordination environment to impove the catalytic performance and how to break the scaling relationship of single-atom catalysts.Therefore,to address these scientific problems,the dissertation investigated and designed the two-dimensional novel and highly efficient electrocatalysts based on the density functional theory（DFT）method using the strategies of pore size regulation,metal atom loading regulation,ligand environment regulation and heteronuclear metal atom regulation,and investigates catalytic mechanism in detail,which mainly includes the following contents:1.Based on pore-size modulation strategies,a series of two-dimensional conducting metal-organic frameworks（MOFs）TM₃（HHTT）₂（TM=Sc,Ti,V,Cr,Mo,W,Mn,Fe,Co,Ni,Cu,and Zn）NRR catalysts with mesoporous structures were designed.DFT calculated results show that the pore size of TM₃（HHTT）₂ is in the range of 2.30～2.40nm,which can effectively solve the mass transport and gas diffusion problems of the NRR process.Using theΔG values of*N₂,*N₂H,and*NH₃ intermediates as activity descriptors,Mo₃（HHTT）₂ monolayer was screened from the 12 structures to efficiently reduce N₂ to NH₃ via the distal pathway at a low Limiting Potential（U_L）of-0.60 V.Through molecular dynamics simulations,formation energy and dissolution potential(E_f=-2.96 e V and U_diss=1.28 V)calculations,it is found that the Mo₃（HHTT）₂ monolayer exhibits excellent structural stability,and it can effectively inhibit the occurrence of hydrogen evolution reaction（HER）.2.Based on the design strategy of high metal atom loading,a total of 29 two-dimensionalπ-d conjugated TM₂B₃N₃S₆structures with TM-selected 3d～5d transition metals were constructed,and their performance as NRR catalysts was investigated.The DFT results showed that TM₂B₃N₃S₆（TM=Mo,Ti,and W）possessed excellent NRR electrocatalytic properties（U_L=-0.38,-0.53,and-0.68 V）.By studying the limiting potential difference between the NRR and HER process,it was demonstrated that the TM₂B₃N₃S₆（TM=Mo,Ti and W）monolayers could effectively inhibit the HER competition reaction.Meanwhile,the effect of aqueous solution on catalyst performance and stability was evaluated.It was demonstrated by the implicit solvent method that the aqueous solution could promote the NRR process,which reduced theΔG value of the potential determination step（PDS）of the Mo₂B₃N₃S₆ monolayer by 0.10 e V.In addition,TM₂B₃N₃S₆（TM=Mo,Ti,and W）also shows good stability in the aqueous phase,which demonstrated by calculating the Pourbaix diagram.Moreover,the multilevel descriptors of the NRR proess(ΔG_*N2H,ICOHP,andε_d)are proposed in the aspects of basic features,electronic properties,and chemical properties.This work provides a new approach for the development and utilisation of NRR catalysts with high atomic loadings.3.Based on the coordination environment regulation,four types ofπ-d conjugated TM_xB₃N₃S₆（x=2,3,TM=Ti,V,Cr,Mn,Fe,Zr,Nb,Mo,Tc,Ru,Hf,Ta,W,Re,and Os）monolayers with 3-and 4-coordination environments have been designed to study the effect of the coordination environment on the catalytic performance of SACs.The structural stability screening results showed that I-TM₂B₃N₃S₆ and III-TM₃B₃N₃S₆ are 3-and 4-coordination structures with high thermal stability,respectively.Then,the structural configurations,electronic properties and catalytic properties of a total of 30 3-coordinated I-TM₂B₃N₃S₆ and 4-coordinated III-TM₃B₃N₃S₆ with 3d～4d transition metal atoms were systematically investigated.The results show that the B₃N₃S₆ ligands and TM atom are ideal carriers for TM atoms due to the presence of strong TM-S bonds.3-coordinated I-V₂B₃N₃S₆ has the best catalytic performance（U_L=-0.01 V）,excellent stability(E_f=-0.32 e V,U_diss=0.02 V)and good selectivity.This work not only provides the idea of designing novelπ-d conjugated SACs,but also gives a theoretical basis for the effect of coordination environment on the catalytic performance.4.Based on the heteronuclear metal atom modulation,21 novelπ-d conjugated M_IM_IIOH₂BDC₂(M_I,M_II=Cr,Mo,W,Fe,Ru and Os)double-metal atom catalysts have been designed,and their potentials as NRR catalysts have been investigated.In addition,the improvement mechanisms of the heteronuclear metal atoms on the catalytic activities of SACs have been studied.21 double-metal atom catalysts were screened according to the"five-step"screening criteria(E_f<0 e V;ΔG_*N2<0 e V;ΔG*N2→*N2H<0.55 e V;ΔG*NH2→*NH3<0.55 e V;ΔG_*N2<ΔG_*H).The results show that the Mo Ru OH₂BDC₂ and Mo Os OH₂BDC₂ monolayers have high stability,high catalytic activity（U_L=-0.53 and-0.55 V）and high NRR selectivity(ΔG_*N2<ΔG_*H).At the same time,the Mo atom was found to be the main catalytic active site of Mo Ru OH₂BDC₂ and Mo Os OH₂BDC₂.Finally,the catalytic mechanism of Mo Ru and Mo Os double atom sites was investigated.The synergistic interaction of Mo atom with Ru and Os heteronuclear metal atoms resulted in the better NRR catalytic performance of Mo metal sites.