Theoretical Study Of Graphene-Based Single/Double-Atom Catalysts For Nitrogen Fixation Performance

Ammonia（NH₃）synthesis by the catalytic process is of enormous significance to thedevelopment and progress of modern society,as NH₃ is the raw material for fertilizer and various other important commodity chemicals and industrial products.Moreover,NH₃ is also considered as a promising hydrogen carrier.Electrocatalytic ammonia synthesis technology is a green and sustainable development process.Efficient and low-cost electrocatalysts are the key to electrocatalytic technology.Single-atom catalyst（SAC）has emerged as a research focus for the electrocatalysis field because of its unique properties,which shows excellent catalytic performance in a variety of electrochemical reactions.Recently,as the extension of SAC,double-atom catalyst（DAC）is another nova of the atomic catalyst,which has attracted intensive research attention.For the electrocatalytic N₂ reduction to NH₃,besides the defects on the basal plane,very recently,the one-dimensional edge universally existing in the finite graphene or carbon sheet has gained attentions as the anchoring site for SACs,which may enable unique catalytic mechanism.Herein,using first-principles calculations,the first part of this paper systematically investigated the NRR over SACs of transition-metal（Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Ru,Rh,Pd,Nb,Mo,and W）anchored by the N-modified edge of the graphene armchair nanoribbon（denoted as TM@GNR）.Three criteria were employed to screen the best candidate from all the TM@GNR,including the high stability of TM@GNR,the preferable adsorption of N₂ compared with H,and the lower applied potential for the first protonation of N₂ compared with that of the active site.Accordingly,V（Nb）@GNR were theoretically demonstrated to be promising NRR electrocatalyst towards NH₃ with low limiting potentials of-0.65（-0.52）V,excellent selectivity of～100%（97%）,and good stability.Particularly,NRR on the V@GNR and Nb@GNR precedes through a novel reaction mechanism with three spectator N₂ molecules.Further analysis reveals that the strong capture and activation of N₂ molecules by the edge-anchored V（Nb）atoms derives from their localized spin moment and the charge“acceptance-donation”mechanism.Our studies emphasize the great potential of the edge of carbon materials to synthesize SACs for NRR and other reactions,and further reveal a novel NRR reaction mechanism on SACs.Electrocatalytic reduction of N₂O is another green process for effective production of NH₃.N₂O molecule is listed as the third largest greenhouse gas after carbon dioxide and methane due to its adverse environmental effects.Electrocatalytic reduction of harmful N₂O（N₂ORR）is also critical for environmental remediation.Transition-metal dimer anchored N-doped graphene basal plane may have unique catalytic mechanism as well as excellent catalytic performance.Herein,to screen for efficient catalysts and explore the synergetic effect of metal dimer on N₂ORR,using first-principles calculations,e N₂ORR on transition-metal dimer（TM1/TM2=V,Cr,Mn,Fe,Co,Ni,Cu）anchored by the N-doped graphene（TM₂N₆@G）was thoroughly investigated in the second part of this paper.Based on thermal stability,selectivity,and catalytic activity,the results reveal that Fe₂N₆@G can efficiently adsorb and activate N₂O and prompt the electrocatalytic N₂ORR to produce valuable NH₃ along the eight-electron pathway with a low limiting potential of-0.45 V.Moreover,the synergetic effect for the metal dimer has been studied in terms of energy and electronic structures.The effective adsorption and activation of N₂O can be ascribed to the charge“acceptance-donation”mechanism and its moderate binding due to the occupation of the d-p antibonding orbital.Our work reveals that graphene-based DACs have great potential in N₂ORR process,which provides an efficient strategy for synthesis of valuable ammonia.