Theoretical Study On The Electrochemical Synthesis Of Ammonia Over Mo₂C With N Doping Or Vacancy

The Haber-Bosch method using N2 as the raw material is the main method for industrial ammonia synthesis.It requires high temperature and pressure,consumes a lot of fossil fuels and causes a lot of greenhouse gas emissions.The electrochemical nitrogen reduction reaction(NRR)can use renewable electricity to convert water and nitrogen into ammonia under mild conditions,which conforms to the concept of green and sustainable development,and is therefore widely studied.N2 adsorption is closely related to the electronic structure of the catalyst,because only atoms that contain both occupied and unoccupied orbitals can accept the lone pair of N2 electrons and provide electrons to the N2 molecule,thereby weakening the N≡N bond.The Mo atom in Mo2C,which is often used as a catalyst for the hydrogen evolution reaction,has both occupied and unoccupied d orbitals.Therefore,Mo2C has the potential to activate the N≡N bond of N2.However,there are few studies to explore the potential of HER catalysts(like Mo2C)as a NRR catalysts.The reaction mechanism of Mo2C for NRR is still unclear.Therefore,we choose Mo2C as the electrocatalyst for NRR to study the thermodynamic mechanism and explore its potential as a NRR electrocatalyst.Also,the effects of N doping and vacancies on the performance of Mo2C for eNRR are explored.The main research contents and results are as follows:(1)By using density functional theory,the electronic structure before and after N2 adsorption,the free energy,the reaction mechanism and the effect of N doping on the selectivity of NRR over the pristine Mo2C(121)surface and the Mo2C(121)surface with N-doping are calculated and compared.The results show that with the increase of N-doping ratio,the valence electrons number of surface Mo atom gradually decreases.And at the same time,the d-band center of Mo atom moves away from the Fermi level,which makes the N2 adsorption strength gradually weaken from strong adsorption to moderate adsorption,which is conducive to the synthesis of ammonia reaction.On the pristine Mo2C(121)surface and the Mo2C(121)surface with N-doping,the potential determining step(PDS)ΔGPDS of the enzymatic is significantly lower than that of the alternating and distal,which indicates that enzymatic is the domainant pathway.The ΔGPDS gradually decreases when the N doping ratio increase.Among diffierent Mo2C(121)surface,6N@Mo2C(121)surface has the lowestΔGPDS,which is only 0.32eV,which highlights that 6N@Mo2C(121)has excellent catalysis for the synthesis of ammonia.ability.With the increase of the N atom doping ratio,the reaction selectivity of NRR gradually increased,and 6N@Mo2C(121)had the highest selectivity(2)The surface of pristine Mo2C(121)with vacancies and 6N@Mo2C(121)with defect are constructed and screened.The electronic structure before and after N2 adsorption,the free energy,the reaction mechanism and selectivity are studied.And the results are compared with the Mo2C(121)without defecets.It indicates that Mo vacancy are easier to form than C vacancy.The introduction of vacancy further reduces the valence electrons of Mo atoms on the surface,which further weakens the adsorption of N2 on the defect surface,and is favorable for the reaction to occur.The defect surface along the distal,alternating and enzymatic pathways to catalyze the ammonia synthesis reaction are studyed.It is found that the predominant pathway of the reaction is the enzymatic route.Vacancy further reduce the ΔGPDS of the surface for NRR.In particular,the lowest ΔGPDS(only 0.24 eV)is found in 6N@Mo2C(121).Vacancy further improve the catalytic activity of the ammonia synthesis reaction.The selectivity of surface with vacancy is higher than that of the perfect surface,which indicates that the introduction of vacancy can improve the selectivity of the Mo2C(121)for NRR.This work not only explores the potential of Mo2C(which is a HER catalysts)for NRR,but also explores how the vacancy and N doping effect the performance of the Mo2C.It provides instruction for the screening of synthetic ammonia catalysts,and at the same time provides useful theoretical information for the design and preparation of synthetic ammonia catalysts containing vacancy and doping.