| Ammonia(NH3)synthesis is of great importance for people’s production and life.Currently,the production of ammonia mainly depends on the Haber Bosch process under high temperatures and pressures.This technology requires high energy consumption and releases large amounts of CO2,which goes against the sustainable development of society.Therefore,searching for economical and environmental ammonia synthesis alternative strategies has important practical significance.Electrochemical nitrogen reduction reaction(eNRR)which directly converts N2 and renewable electricity to NH3 under ambient conditions has been widely investigated.It’s a hot topic in the field of ammonia synthesis since of the relatively low energy consumption and equipment cost,meeting the requirement of carbon neutralization.Developing efficient catalysts is critical for the development of eNRR for ammonia synthesis.However,the commercial application of the widely researched noble metalbased catalysts is hampered owing to their high cost,low abundance,unsatisfactory activity,and low selectivity.Therefore,it is of great significance to develop highly effective eNRR catalysts with high stability,good activity,and high selectivity.Recently,single-atom catalysts(SACs)show great potential in eNRR due to their high atom utilization,tunable coordination,special electronic structure,and outstanding catalytic activity.With the continuous development of calculation methods and calculation power,theoretical calculations have become an important method to explain experimental data,design and screen catalysts,and investigate the catalytic mechanism.In this paper,based on the first principles calculation,we designed a series of SACs with good stability,excellent catalytic performance,and experimental feasibility for eNRR,and further comprehensively studied the eNRR activity trends,catalytic mechanism,and influence of regulating the coordination environment.The main contents and results of the research are as follows:1.Theoretical design and coordination modulation of graphene-supported transition metal(TM)based SACs for electrochemical reduction of nitrogen to ammonia.This chapter consists of three works.In the first part,26 sulfur coordinated TM SACs are screened,and nine of them(TM=Sc,Ti,V,Cr,Mn,Nb,Mo,W,and Re)exhibit high performance and selectively toward eNRR.Among them,Mo SACs show the best performance with the lowest limiting potential-0.425 V and 100%theoretical Fradic effect.The eNRR activity of SACs presents a volcano relationship with the adsorption energy of NH2*.Particularly,the adsorption energy of NH2*linearly increases with the charge states of the metal atom and an electronic descriptor φ defined based on the valence electron occupancy and electronegativity of transitional metal,sulfur,and nitrogen atoms.In the second part,the eNRR performance of S and N coordinated 25 TM SACs were compared,it can be found that 18 TM-based SACs coordinated by sulfur exhibit better eNRR activity than those coordinated with nitrogen atoms.The eNRR activity of different types of graphene-supported SACs presents a universal volcano relationship with the adsorption energy of NH2*,but the appropriate adsorption energy of NH2*for optimal NRR activity is different.And the adsorption energy of NH2*on different types of SACs is linearly increased with the charge states of TM atom,as well as electronic descriptor φ.In the third part,the eNRR performance of graphene-supported a single Mo atom coordinated by different nonmetal atoms X(X=B,C,N,O,Si,P,and S).It can be found that the Mo atom will donate more electrons and exhibit stronger interaction with N2 with the increase of the electronegativity of X.The smaller the energy level difference between the d orbital of Mo and the 2π*orbital of N2 under different coordination,the adsorbed by N2 will get more electrons.Sulfur-coordinated Mo SACs present the optimal eNRR activity,and the free energy of the first and last hydrogenation reaction is both linearly related to the charge states of Mo.These works suggest the great potential of sulfur-coordinated SACs for eNRR and find universal descriptors for screening catalysts.Moreover,the regulation of the coordination environment on charge state,orbital energy level position,and N2 activation and reduction was thoroughly studied,which provided theoretical guidance for the development of SACs for eNRR.2.The theoretical study of alkaline earth metal(AEM)based SACs for electrochemical reduction of nitrogen to ammonia.This chapter consists of two works.In the first part,4 stable alkaline earth metals(AEM)based single-atom catalysts(SACs)are designed at the edge of nitrogen-doped graphene nanoribbons.Among them,the Ca atom achieves the highest activity with an optimal limiting potential of-0.716 V.The atomic Ca efficiently activates N2 via the special electron "acceptance-donation"mechanism(s→2π*,2π→d),which is associated with the synergistic effect of partially occupied s and empty d orbitals of AEM.Particularly,the NRR activity of atomic AEM exhibits a linear relationship with their charge states.Meanwhile,the construction of AEM atoms with low coordination can effectively increase their remaining s electrons and enhance eNRR activity.In the second part,four kinds of AEM SACs embedded in a covalent organic framework(COF)are designed for eNRR.The results show that inert N2 can be activated more efficiently on multi-AEM active centers,and Sr-COF exhibits the best activity with an optimal eNRR limiting potential of-0.331 V.The activity of the COF-based AEM S ACs is linearly related to the average oxidation state of the AEM center,as well as the obtaining electrons and the spin magnetic moment of adsorbed N2.These researches not only confirm the potential of AEM-based SACs for eNRR but also enhance our understanding of the underlying mechanism of atomic AEM interacting with N2.Meanwhile,it also reveals the mechanism of promoting the activity of atomic AEM with low coordination and multi-center,which provides a new direction for the application of AEM in ammonia synthesis.3.Eighteen kinds of metal-free SACs were designed with two-dimensional transition metal sulfide(TMD)as substrate and a single C atom as the active center,and their eNRR activity was studied.The results show that these C-based SACs can effectively reduce inert N2,and the related eNRR limiting potential is about-0.2～1.2V,C in NbS2-2H has the lowest potential of-0.26 V.The N2 adsorption energy on the C atom is linearly correlated with the electron filling degree of the C pz orbital,as well as the energy level difference(△Ediff)between the C pz orbital and 2π*orbital of N2.The electron filling degree and △Ediff of the C pz orbital are linearly correlated with the C-TM bond angle(θ).There is a volcano relationship between the eNRR activity and the electron filling degree of the C pz orbital,and the C atom gets the optimal eNRR activity when the filling degree of the C pz orbital approaches 0.42.This work reveals the structure-activity relationship between the orbital orientation,electron filling degree,energy level position of C single atom,and the eNRR activity of metal-free SACs,which provides theoretical guidance for ammonia synthesis through metal-free catalysts. |
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