The Study On Electrocatalytic Reductions Of N₂ And CO₂ On Mo₂C-MXenes:A First-Principles Calculations

In the past decade,two-dimensional（2D）layered transition metal carbide（nitride）（MXenes）have begun to grow fast.It has aroused wide interest from researchers in the field of electrocatalysis for their distinctive electronic structure and flexible performance adjustability.Electrocatalytic energy conversion technology has been progressing slowly due to the lack of low-cost and high-efficiency catalysts.MXenes can act as excellent reaction catalysts or promoters to enable efficient and stable catalytic reactions.In order to deal with the energy profligacy and environmental pollution caused by the production of important industrial raws such as NH₃ and urea（CON₂H₄）,and to alleviate the greenhouse effect caused by the excessive concentration of CO₂ in the atmosphere.Based on the density functional theory,this paper investigated the structure and stability of catalysts and the catalytic reaction mechanisms in nitrogen reduction reaction（NRR）,Carbon dioxide reduction reaction（CO₂RR）,and Urea synthesis reaction（CNRR）using the two-dimensional MXenes.The mainly includes were summarized as follows:（1）The structural models of TM-Mo₂CS₂（TM=Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn）were constructed as non-noble metal CO₂ reduction electrocatalystst.Combining First-Principles computation and Ab-initio molecular dynamics（AIMD）,the stability of catalysts and the adsorption and activation of CO₂ on the active sites of catalysts were systematically investigated in this paper.Our results show that Zn-Mo₂CS₂is not steady,while Cu-Mo₂CS₂ have poorly CO₂RR selectivity.As for other TM-Mo₂CS₂,Cr-,Fe-,Co-and Ni-Mo₂CS₂ have higher CO₂RR activity and the maximum reaction energy barriers were only around 0.3 e V.The effects of different surface functional groups on the performance of Fe-Mo₂CS₂ electrocatalytic of CO₂reactions reduction were revealed.And the results showed that,when-S functional groups were replaced by-O and-Se functional groups,the maximum reaction energy barrier of CO₂RR would increase to 1.18 e V and 0.55 e V,showing the activity of CO₂RR was reduced.But the CO₂RR activity of the catalyst was significantly increased after the Mo-S-Fe activity center was constructed on the surface of Fe-Mo₂CO₂,which proved the promotion effect of Mo-S-Fe structure on CO₂RR reaction.（2）Based on DFT,the structure model of nano Fe-Mo₂CS₂ catalyst whose structure was similar to the active center of nitrogenase was constructed.And the effects of surface-supported Fe single atoms on N₂ adsorption,the NRR reaction pathways,and catalytic activity were systematically studied.The results reveal that the Mo-S-Fe structure created after the introduction of Fe atoms is conducive to the adsorption and activation of N₂ molecule,and the maximum reaction energy barrier is lower when N₂ molecule is adsorbed by lying-on mode.Although the theoretical maximum reaction energy barrier is 0.836 e V,somewhat slightly higher than that of the precious metal Pt（1 1 1）（0.78 e V）.This noble metal-free catalyst design method offers a novel approach to the synthesis of NRR catalysts.（3）The reaction mechanism of directly electrocatalytic coupling synthesizing of CO₂ and N₂ into urea on the surface of MXenes was explored.The performances of urea synthesis on MXenes covered with different surface functional groups were also investigated.The results show that the C/N coupling reaction is not conducive to urea synthesis when there is a-S or-O functional group on the surface of MXenes.And the Mo₂C-MXene without any functional group on the surface has great CO₂/N₂ co-adsorption performance,while the thermodynamic activation barrier of CNRR is really low（only 0.69 e V）.But the result of CINEB calculation shows that the transition state barrier of the C/N coupling reaction is very high（about 1.5 e V）,which is not conducive to the smooth progress of the urea synthesis reaction.Based on these researches,the effects of Mo₂C surface-loaded transition metal atoms on its CNRR activity were explored.It is found that the thermodynamic and transition state energy barriers of C/N coupling reaction are significantly reduced while Mo defect loads Ti or Fe single atom,which helped to improve the urea synthesis performance of the catalyst.