Theoretical Studies On Single Atom Or Single Cluster Electrocatalysts For Ammonia Synthesis

Ammonia(NH3)is an important raw material for many chemicals and an important carbonfree energy carrier.Currently,the synthesis of NH3 mainly relies on the Habor-Bosch method that is energy-intensive,highly polluting,and inefficient.Thus,there is an urgent demand to develop a green process for NH3 production.Electrocatalytic synthesis of ammonia with small molecules(N2 and NO)is considered as an ideal alternative.On the one hand,sustainable energy(wind,solar,etc.)can be used as the driving force,herein the whole process is carbon-free.On the other hand,the catalytic reaction takes place at ambient temperature and pressure with low equipment and production costs.However,designing efficient electrocatalysts for ammonia synthesis is still a"trial and error" process that relies on experience and chemical intuition with long cycle times,low efficiency,and high cost.To improve the development of electrocatalysts for ammonia synthesis and promote the application of electrocatalytic ammonia synthesis technology,we takes Single Atom Catalysts(SACs)and Single Cluster Catalysts(SCCs)with unique geometric and electronic structures as starting point in this thesis.A series of SACs or SCCs were theoretically designed to reveal the structure-activity relationship of SACs or SACCs on reaction mechanism of electrocatalytic ammonia synthesis.Two available reactions are considered,including Electrochemical Nitrogen Reduction Reaction(NRR))and Electrochemical Nitric Oxide Reduction Reaction(ENORR)).The prediction of efficient ENRR/ENORR electrocatalysts provides an important theoretical understanding and guidance for experiments.The main research contents of this thesis are as follows:(1)To expand the research map of SACs in ENRR and reveal the structure-activity relationship of Single Atom Alloy Catalysts(SAACs),Au-based SAACs(TM@Au(111))were studied theoretically.The numerical correlations between various descriptors and the energy change of key step of the reaction(*N2+e-+H+→*NNH)are constructed to bulid up the relationship between the electronic structure of the doped single-atom and the adsorption strength of the key reaction intermediates.Furthermore,combing with the Random Forest(RF)Algorithm in machine learning,the weight relations between the intrinsic physical quantities of single-atom centers and the energy change of potential determining step(*N2+e-+ H+→*NNH)were comprehensively explored.Based on experimental results,this work designs multi-dimensional screening criteria to screen promising SAACs,including activity,selectivity,feasibility,and theoretical thermal/kinetic stability.Four SAACs(i.e.Mo,Ru,Ta,and W@Au(111))were screened out as promising electrocatalysts for ENRR.Among them,Mo and W@Au(111)possess the highest theoretical activity.The corresponding limiting potentials(UL)are both-0.30 V,which outperforms the vast majority of reported SACs.(2)Given the problems of a single reaction site(only top-site)and expanding the candidate material library,this work theoretically designs a series of Dual Atom Single Cluster Catalysts(DSCCs)where dual atom single clusters anchored on nitrogen-doped carbon(M1M2@N6,M1,M2=Mn,Fe,Co,Ni,and Mo).A new approach was proposed to evaluate the stability of DSCCs.Through exploring the ENRR mechanism of M1M2@N6,the top-site and bridge-site were both identified as the possible reaction sites.To understand the synergistic effect of metal atoms in DSCCs,the various reaction mechanisms and electronic structures were compared between two groups.One group contains MnMo@N6 and CoMo@N6 with the same activity but different potential determining steps.The other group is composed of CoMo@N6 and the corresponding SACs(i.e.,Co@N4 and Mo@N4).Combined with molecular dynamics simulations and electrochemical stability analysis,CoMo@N6 was suggested as an ideal ENRR electrocatalyst with UL of-0.52 V.(3)Through the research of DSCCs,the bimetallic cluster can only be in a linear configuration embedded in the nitrogen-doped defective carbon material,resulting in the same coordination environment of the two metal atoms.Without asymmetric active sites,the candidate library is limited.By increasing the number of metal atoms and constructing SCCs with rich spatial configurations,it is expected to expand the candidate library and enable the search for catalytic systems with higher activity.Based on the existing experimental results,a series of homonuclear Triple Atom Single-Cluster Catalysts systems were designed,marked as TM3@N4.First,the most stable configuration among five possible configurations of triatomic clusters was identified,and the corresponding ENRR reaction mechanism was explored.Benefiting from the unique configuration,TSCCs composed of uncommonly used Mn,Fe,and Co atoms exhibit high activity,with corresponding ULs of-0.53,-0.56,and-0.41 V,respectively.The activity is close to that of Mo3@N4(-0.51 V).Finally,selectivity and thermal stability were analyzed to confirm that Mn3@N4,Fe3@N4,Co3@N4,and Mo3@N4 are ideal electrocatalysts for ENRR.(4)Compared with ENRR,activating polar nitroxide bonds in ENORR is more feasible experimentally.Inspired by the research of carbon-based SACs and SCCs,the potential application of carbon-based SACs was further investigated for ENORR.C558 with metallic properties was selected as the support to construct carbon-based SACs(TM@C558).There are abundant single-atom loading sites in C558,leading to different single atoms in various coordination environments.At first,the most stable adsorption sites of single atoms on C558 were located,then the NO adsorption strength was studied as the starting point to explore the ENORR mechanism on TM@C558.The volcano-type relationship between NO adsorption strength and UL was constructed,confirming that moderate NO adsorption is related to the high ENORR activity.Cu-and Au@C 558 were screened out as the possible ENORR catalytic systems by evaluating activity,selectivity,experimental feasibility,and thermodynamic stability.Au single atom is predicted to show high ENORR catalytic activity for the first time.A series of SACs and SCCs were designed theoretically to accelerate the development of electrocatalysts for ammonia synthesis,reduce the research cost and change the research mode of"trial and error".To reveal the structure-activity relationship,the reaction mechanisms of ammonia synthesis(including ENRR and ENORR)of SACs and SCCs were simulated.In this thesis,we predicted a variety of potential electrocatalysts for ammonia synthesis and provided theoretical guidance for experiments.Moreover,the methodology and procedure for the investigations on complex ENRR and ENORR provide implications for carrying out the Materials Genetic Engineering in the field of catalysis.