First-principles Study On Two-dimensional Nitride-based Materials In The Catalytic Reaction Of Electrochemical Synthesis Of Ammonia

Ammonia（NH₃）,as the most important basic raw material for the preparation of fertilizers,occupies an important position in the global economy.At the same time,NH₃ is not only one of the largest chemical products in the world,but also widely used in chemical,agricultural,pharmaceutical and energy fields.Currently,the industrial production of NH₃relies on the traditional Haber-Bosch process,accounting for about90%of the annual output.Due to the harsh condition required to break the strong N≡N,the reaction process takes place under high temperature（300-500°C）and pressure（150-300 atm）with the Ru-based catalysts.This process is highly fossil fuels consuming and a mass of greenhouse gases will be produced.In recent years,the electrochemical nitrogen reduction reaction（NRR）has attracted extensive attention,which can convert N₂ to NH₃ by simply changing the operating potential and reaction environment under mild temperature and pressure conditions.Unfortunately,electrocatalytic NRR is still seriously limited by low ammonia yield and reaction efficiency,hence the development of high activity and selectivity NRR catalysts is desirable.Up to now,2D nitride-based materials have been widely used as substrate materials for NRR catalysts due to their natural N ligand atoms which can stably anchor metal active sites.A large number of theoretical and experimental results have demonstrated that transition metal（TM）atoms anchored on 2D nitride-based materials with N4/N3 coordination environment can exhibit excellent catalytic performance.However,two-dimensional nitride-based single-atom catalysts（SACs）have problems such as low stability and single active site,which limit the further research of this type of catalyst in NRR.In order to solve this problem,this paper introduced single metal atom or bimetallic pair into the active site of two-dimensional boron nitride（BN）substrate and two-dimensional C₃N substrate with 6N coordination cavity to construct NRR catalysts.Then,the synergistic effect between bimetals was used to explore its performance in electrochemical synthesis of ammonia.Compared with single-atom catalysts,dual-atom catalysts not only increase the loading of active sites,but also provide more bimetallic adsorption modes to exhibit different catalytic performances.The main research contents and results of this paper are as follows:（1）The two-dimensional BN with B defects（V_B-BN）as a substrate was applied to build a series of DACs for the NRR（16TM₁-TM₂@V_B-BN catalysts,TM=Ti,Fe,Mo and Ru）.The density functional theory calculations were performed to investigate the stabilities of the constructed structures.Then we studied the N₂adsorption ability,the Gibbs free energy of intermediates and the catalysts selectivity of TM₁-TM₂@V_B-BN.Through a“three-step”strategy,several catalysts are highlighted as the best catalysts for NRR.Finally,we analyzed the electronic structure to uncover the NRR catalytic activity and the synergistic effect of the bimetallic active sites during the catalytic process.Particularly,the calculation results showed that Ru-Mo/Ru/Ti@V_B-BN are candidates meeting the requirements,and Ru-Ti@V_B-BN is the most active NRR catalyst.Under the enzymatic mechanism,the limiting potential of Ru-Ti@V_B-BN can reach a favorable value of-0.40 V.The reason for its good activity can be attributed to the fact that the Ru-Ti dimer has the largest magnetic moment,which significantly activate the N₂molecule and thus break the N≡N triple bond.Furthermore,the second metal atom in the heteronuclear DACs（Ru-Ti/Mo）can effectively tune the d-band center of the Ru active site compared with the homonuclear double atom catalyst（DACs）（Ru-Ru）.Therefore,the rate-determining step（RDS）value of the first hydrogenation（^*N₂+H⁺+e^-→^*N₂H）process decreases because the d band center of Ru-Ti is closer to the Fermi level.（2）The potential of C₃N with hexanitrogen cavity as a support material for single metal atom and homonuclear bimetallic pairs（TM=Sc～Zn）has been methodically investigated for potential NRR catalysts.Based on density functional theory calculations,the adsorption energy and cohesive energy were calculated to evaluate the stability of the catalyst structure.Then,the adsorption and activation abilities of TM/TM₂@C₃N for N₂ molecules were estimated by adsorption energy based on the stable structure.In addition,the promising RDS of NRR:first hydrogenation（^*N₂→^*N₂H）and the final step of hydrogenation（^*NH₂→^*NH₃）,are deemed as screening criteria to initially assess the performance of catalyst.Finally,exploring the relationship of reaction intermediate is meaningful to select appropriate descriptor.The calculation results show that most single/double atom catalysts seem to be able to activate N₂ effectively.In terms of catalytic activity and selectivity,Mn₂/Fe₂@C₃N exhibit outstanding catalytic performance which possesses ultra-low limiting potentials of-0.33 V and-0.47 V through the distal mechanism.Furthermore,the volcano plots suggest thatΔG_NH2 is an effective descriptor of NRR catalytic activity for DACs.This theoretical study provides both a new research idea and candidate materials for designing and supporting experimental exploration,which will also guide the design of two-dimensional high-performance nitride-based catalysts for NRR and other fields of electrocatalysis.