Theoretical Study On The Mechanism Of Ammonia Synthesis Catalyzed By MXene-supported Single-atom Catalyst

Reduction of N₂to NH₃plays an extremely important role in agriculture and industry,but the Haber-Bosch process is widely used currently,which requires harsh reaction conditions such as high temperature and pressure,high energy-consumption,and generates a large amount of greenhouse gas CO₂.Therefore,it is very urgent to develop environmentally friendly,low-cost,efficient catalysts that can reduce N₂to NH₃under mild conditions.Single atom catalysts（SACs）have become a new frontier in heterogeneous catalysis because they can achieve the highest atomic utilization and excellent catalytic activity by having fully dispersed single metal atoms on support as the active center,especially SACs based on two-dimensional materials,which have great potential for applications.MXene,a two-dimensional material composed of transition metal carbides,nitrides and carbonitrides,has excellent electrical properties and good chemical stability,especially in MXene with surface defects,which can be embedded into metal single atoms to build MXene-based SACs.In this work,we systematically screen highly stable MXene-based SACs using density functional theory calculations,and further study the catalytic mechanism of reduction of N₂to NH₃.The main research contents are as follows:（1）Density functional theory calculations are carried out to systematically investigate the geometries,stability,electronic properties of SACs with the 3d-transition metal M（Sc,V,Cr,Mn,Fe,Co,Ni,Cu,Zn）atoms embedded in the Ti defect sites of Ti₂CO₂（denoted as M₁@Ti₂CO₂）.A new SAC of the highly stable V₁@Ti₂CO₂has been found to show excellent catalytic performance for N₂reduction reaction to produce NH₃after screening the 3d transition metals.The results show that not only can V₁@Ti₂CO₂strongly adsorb N₂,but also exhibits an excellent NRR catalytic activity with a limiting potential of only-0.20 V and a high ability to suppress the competing hydrogen evolution reaction（HER）.The excellent NRR catalytic activity of V₁@Ti₂CO₂is attributed to the strong covalent metal-support interaction that leads to superb N₂adsorption ability of V atom.Furthermore,the embedded V single atoms facilitate electron transfer,thus improving the catalytic performance for NRR.These results demonstrate that V₁@Ti₂CO₂is potentially a highly promising 2D material for building robust and excellent electrocatalyst for NRR.（2）Based on the electronic controllability of terminal groups on MXene,we further compared and studied the catalytic performance of Ti₂CF₂with-F terminal group for NRR.Using DFT calculations,the most stable V₁@Ti₂CF₂SAC is screened from a series of 3d transition metals embedded in Ti₂CF₂,and the catalytic performance for NRR to NH₃is studied.Comparison with SAC V₁@Ti₂CO₂,the limit potential of V₁@Ti₂CF₂for NRR is-0.67 V,significantly higher than that（-0.20 V）of V₁@Ti₂CO₂,which indicates that different terminal groups can regulate the catalytic activity of Ti₂C MXene towards NRR,and the-O terminal group is more conducive to improving its electrocatalytic nitrogen reduction activity than the-F terminal group.（3）Based on our previous research,here,a preliminary exploration is conducted on the possible thermal catalytic reaction mechanism of V₁@Ti₂CF₂SAC for NRR to NH₃using DFT.Calculations indicate that N₂has strong adsorption and activation in the side-on mode on V₁@Ti₂CF₂.With the occurrence of NRR to NH₃,N₂has a relatively low（0.89 e V）dissociation energy barrier on V₁@Ti₂CF₂,and the rate-determining step is*NH₂→*NH₃with an energy barrier of 1.99 e V.This may be due to the strong adsorption of*NH₂(E_ads=-5.60 e V).In summary,this preliminary research suggests that V₁@Ti₂CF₂may be a potential SAC for the thermocatalytic synthesis of ammonia synthesis.