First Principles Study Of Two-dimensional Materials Supported Transition Metal Single Atom Catalysts

The increasingly severe energy shortage and greenhouse effect have seriously threatened the future survival of human beings and the sustainable development of today’s society.Electrochemical energy conversion technology,including N₂ reduction to NH₃,H₂O electrolysis to H₂,H₂O electrolysis to O₂ and O₂reduction,has developed into one of the current research hotspots in the field of new energy technology.Two-dimensional materials,as a rising star in the field of electrocatalysis,have been widely employed as an ideal substrate for transition metal catalysts due to their large specific surface area,high chemical stability and easy adjustment of electronic structures.However,the developments of novel electrocatalysts with high activity,high selectivity,good stability and low price still face significant challenges.Therefore,in this paper,a series of transition metal single-atom catalysts supported on two-dimensional materials were designed,and their electrocatalytic activities for nitrogen reduction reaction（NRR）,hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）were systematically explored by first-principles calculations.Meanwhile,the origin of catalytic activity was revealed from the electronic point of view.The main research contents and conclusions are as follows:The catalytic performances of a series of single transition metal atom supported on WS₂ monolayer（TM@WS₂）for electrocatalytic N₂ reduction were systematically explored.Descriptor closely related to the intrinsic characteristics of transition metal atom(φ=θ_d×E_TM,whereθ_d and E_TM represent the valence electron number and the electronegativity of transition metal atom,respectively)was proposed,which shows a volcano relationship with N₂ reduction catalytic activity.The results show that TM@WS₂（TM=Cr,Tc and Os）exhibit excellent catalytic activity and selectivity,all of which tend to catalyze N₂ reduction via distal mechanism,with the limiting potentials of-0.37,-0.27and-0.33 V,respectively,which are significantly better than Ru（0001）catalysts（-0.98 V）.In addition,the adsorption energy of*N-NH intermediate also shows a good volcano relationship with the N₂ reduction catalytic activity,corresponding to the ideal adsorption energy of*N-NH intermediate of-1.50 e V.The density of states and the difference charge density of TM@WS₂（TM=Cr,Tc and Os）with N₂ adsorption were explored,showing that N₂ activation mainly comes from the“acceptance-donation”process between d orbitals of Cr,Tc and Os atoms and the 2π*orbital of N₂ molecule.Single-atom catalysts with TMN_x active center usually exhibit high N₂ reduction activity.To this end,the potentials of a series of single transition metal atom embedded on the N-doped defective h-BP（TMN₃-BP）as N₂ reduction electrocatalysts were comprehensively investigated.Structural descriptor correlating the local environment of TMN_x active center and the NRR catalytic activity was constructed(φ=θ_d×(x_TM+nx_N)/（（n+1）x_N）,whereθ_d,x_TM,x_N and n represent the number of valence electrons and electronegativity for transition metal atom,the electronegativity of N atom and the number of N atoms,respectively),and it was found that transition metal atoms located in the middle of the same period exhibit stronger ability to adsorb and activate N₂molecule.Through a four-step strategy of“high stability,stable chemisorption of N₂molecules,high activity and high selectivity”,Mo N₃-BP was finally screened from 25candidates and can effectively impede the competing hydrogen evolution side reactions,with a theoretical limiting potential of-0.25 V,which has the optimal catalytic activity for N₂ reduction.Further analysis of electronic properties revealed that the excellent N₂reduction catalytic activity of Mo N₃-BP is mainly due to its good electrical conductivity,the“acceptance-donation”process between the d-orbital of Mo atom and the molecular orbital of*N₂,as well as the electron transfer of Mo N₃ active center to N_xH_y intermediate during N₂ reduction.The potentials of a series of individual transition metal atom anchored on g-CN monolayers（TM@CN）as multifunctional electrocatalysts for HER/OER/ORR were explored.It was shown that g-CN modified by individual transition metal atom could effectively improve the electrical conductivity and the charge transfer efficiency,and significantly enhance the catalytic performance.Meanwhile,the catalytic activities for hydrogen evolution,oxygen evolution and oxygen reduction reactions exhibit the volcano relationships with the Gibbs free energy of H*adsorption(ΔG_H*),the difference of Gibbs free energy of O*and OH*adsorption(ΔG_O*-ΔG_OH*)as well as the Gibbs free energy of OH*adsorption(ΔG_OH*),respectively,which shows that Fe@CN,Rh@CN and Au@CN exhibit optimal catalytic activity for hydrogen evolution,oxygen evolution and oxygen reduction reactions,with the overpotentials of 0.01,0.33 and 0.46 V,respectively,which are comparable to or even better than the commercial Pt and Ir O₂ catalysts.In addition,the d-band center of transition metal atoms can be used as an effective descriptor to measure the adsorption strength of the oxygenated intermediates on TM@CN surface.More importantly,Rh@CN and Pd@CN can not only serve as efficient bifunctional catalysts for HER/OER and OER/ORR,but also exhibit excellent catalytic activity in catalyzing HER/OER/ORR.