Theoretical Design Of Metal-embedded Carbon Nanotubes Single-atom Catalyst For Nitrogen Reduction

Electrochemical ammonia synthesis is a green and efficient means of ammonia synthesis,which is expected to replace the traditional Haber-Bosch process as the main way to produce ammonia.At the present stage,the electrocatalytic catalyst for ammonia synthesis is difficult to be used widely due to the low ammonia production rate and the difficulty in suppressing hydrogen evolution reaction.With the wide application of nanomaterials and single-atom catalysts electrocatalysis,the electrocatalytic synthesis of ammonia technology has also been further developed.Carbon nanotubes are one-dimensional nano-scale hollow tubular materials formed by curling graphene,which are widely used in catalysis.In this paper,based on density functional theory,the selectivity and catalytic activity of the nitrogen reduction reaction of different metals embedded in carbon nanotubes were studied.At the same time,the influence of the curvature of carbon nanotubes and nitrogen atom doping on the catalytic activity was investigated.Research indicates:(1)Among the 16 transition metals including Sc,Ti,V,Cr,Mn,Fe,Co,Ni Cu,Zn,Mo,Ru,Rh,Ir,Pt,and Pd,15 metals can be stably embedded on carbon nanotubes except for Pb,14 metals can be stably embedded on nitrogen-doped carbon nanotubes without migration and agglomeration except for Mo and Ru.At the same time,the curvature will not have a decisive influence on the relative magnitude of the binding energy and cohesive energy,so that it will not affect its stability.(2)Among the 15 stable metal-embedded carbon nanotubes,8 of them(Sc,Ti,V,Cr,Mn,Fe,Co,and Mo)can inhibit the side reaction of the hydrogen evolution reaction effectively.However,only V@CNT(3,3)and Ti@CNT(3,3)have certain nitrogen reduction catalytic ability,which potential control steps are the first step of hydrogenation and the limiting potentials are0.78 and 0.98 e V,respectively Through charge analysis,nitrogen molecules adsorbed on titanium-embedded carbon nanotubes obtained 0.22 e~-.This negative charge is mainly concentrated on the anti-bonding orbitals of nitrogen molecules and mainly comes from the contribution of Py electronic state.(3)Among the 14 stable metal-embedded nitrogen-doped carbon nanotubes,6 of them(Sc,Ti,V,Cr,Fe,and Co)are more prone to nitrogen reduction reactions.At the same time,the doping of nitrogen atoms significantly improves the catalytic activity of the catalyst for nitrogen reduction.Among these catalysts,the limiting potentials of Ti N4@CNT(3,3)and Ti N4@CNT(5,5)are both 0.51 V,which is the smallest of all metals embedded in nitrogen-doped carbon nanotubes.The potential control steps are the first hydrogenation and the generation of the second adsorbed ammonia molecule,respectively.Through charge analysis,0.47 e~-was obtained after nitrogen molecules were adsorbed on the titanium-embedded nitrogen-doped carbon nanotubes,which was 0.25 e~-more than that on titanium-embedded carbon nanotubes.More negative charges are concentrated on the anti-bonding orbitals of nitrogen molecules,which makes it easier to form feedbackπbonds andσorbitals.Thus,This is extremely conducive to the hydrogenation process of nitrogen atoms,thereby greatly reducing the energy barrier of the nitrogen reduction reaction.