Theoretical Study On Catalytic Performance Of Heteronuclear Diatomic Catalysts Supported On Two-dimensional Porous Materials In Electrochemical Ammonia Synthesis

Ammonia synthesis is one of the most important catalytic reactions in chemical production and is of great significance to the sustainable development of society and economy.The traditional Haber-Bosch process is characterized by high energy consumption,fossil fuel consumption and large pollution emission,so scientists are trying to develop an electrocatalytic nitrogen reduction reaction to replace it.Currently,electrochemical ammonia synthesis can not be widely used.The main obstacle is that the electrocatalyst has not met expectations,and there is a lack of efficient and high Faradaic efficiency catalysts.In recent years,the study of Single-atom catalysts has been extended to Diatom catalysts.Compared with Single-atom catalysts,Diatom catalysts supported on suitable substrates have richer and more flexible active sites,as well as the interaction between diatoms,which can not only maximize the advantages of Single-atom catalysts in multi-step reactions,but also provide more opportunities for the selection of catalysts.In particular,heteronuclear Diatom catalysts often bring unexpected surprises through the loading or doping of heteronuclear atoms.In this paper,based on the first-principle DFT calculation,heteronuclear Diatom catalysts with different substrates were designed,and their e NRR catalytic activity and reaction mechanism were studied,and their activity essence was analyzed from the electronic structure.The main research contents and conclusions are as follows:（1）Extended heteronuclear bimetallic phthalocyanine has both isolated monoatomic structure and overall double-site structure,which can provide a variety of flexible adsorption sites for reactants,and is expected to break the linear adsorption relationship between reaction intermediates and further improve the catalyst activity.Therefore,we designed a series of extended heteronuclear bimetallic phthalocyanine electrocatalysts MM’-Pc for electrocatalytic nitrogen reduction reaction,including:Ta W-Pc,Ti Ta-Pc,Ti V-Pc,Ti W-Pc,VTa-Pc,VW-Pc,Mo V-Pc,Mo Ti-Pc and Mo Ta-Pc.Then,the stability,activity and selectivity of MM’-Pc catalyst were comprehensively evaluated by spin polarization DFT method.The results show that Mo Ti-Pc and Ti V-Pc have good e NRR catalytic activity,and the U_Lare-0.29 V and-0.31 V,respectively.Secondly,they have obvious inhibitory effect of HER,especially the HER energy barrier of Mo Ti-Pc is as high as 1.07 e V,which is considered as the most promising catalyst.In order to reveal the essential reason of its excellent activity,we also analyzed the electronic structure changes of Mo Ti-Pc before and after N₂adsorption from p DOS and charge density difference diagram.In addition,we also found that the Bader charge of N atom bonded to the catalyst surface is negative,while the Bader charge of N atom far away from the catalyst surface is positive.This also explains why the N₂adsorbed by end-on mode is extremely difficult in the first protonation step of NRR process.（2）A series of Mo-based heteronuclear Diatom catalysts Mo TM/C₂N（TM=3d transition metal）were designed by tuning the interaction between Mo and TM.Then,the stability,activity and selectivity of Mo TM/C₂N catalyst were comprehensively evaluated by spin polarization DFT method.Among all Mo TM/C₂N,Mo Fe/C₂N is considered as the most promising catalyst because of its lowest U_L（-0.26V）and effective inhibition of HER.Compared with the homonuclear counterparts Mo Mo/C₂N and Fe Fe/C₂N,we found that heteronuclear Mo Fe/C₂N has the advantages of both Mo-based and Fe-based catalysts,which can balance the first protonation step and the sixth protonation step simultaneously.Finally,we also revealed the essence of excellent activity of Mo Fe/C₂N from the electronic structure,which came from the strong interaction between the catalyst and the side-on adsorption of N₂,and effectively activated N₂.（3）A new method for designing heteronuclear DACs by geometric constraints was proposed,and two high-performance e NRR catalysts YRe/C₂N and YMn/C₂N were screened.In this work,we designed a series of heteronuclear Diatom catalysts YTM/C₂N（TM=transition metal）.By using the pore size limitation of C₂N and the large atomic radius of Y element,the active center of bimetallic Y-TM properly protrudes from C₂N plane,which makes the 2D catalyst three-dimensional,thus increasing the stable adsorption of N₂with side-on mode.Then,the stability,activity and selectivity of YTM/C₂N catalyst were comprehensively evaluated by spin polarization DFT method.Finally,it was found that YTM/C₂N had the best e NRR catalytic performance when TM was theⅦB element.Among them,YRe/C₂N has the lowest limiting potential,only-0.12 V,which is the lowest in all theoretical catalysis reports at present,followed by YMn/C₂N（-0.24 V）.This result in turn proves the reliability and practicability of our new method.