Theoretical Study On Electrocatalytic Reduction Of CO₂ And Synthesis Of Urea Over Metal Nitrogen Carbon Single-atom（Bimetallic Single-atom） Molecular Catalyst

Electrocatalytic CO₂ reduction（CO₂RR）is an effective means of CO₂ resource utilization,which provides a potential way to achieve the goal of carbon neutralization.In addition,the growing population increases the need for food,which involves the use of fertilizers.Nature benefits humans through“crops brought by thunderstorms”.Urea is also one of the most important nitrogen fertilizers with the highest nitrogen,so that the development of the urea industry is of great significance.This thesis mainly focuses on carbon electrochemical reduction and nitrogen electrochemical reduction to synthesize raw materials for basic chemical production-hydrocarbons and hydrocarbon derivatives.The main research contents are as follows:1.Due to the excellent performance of metal nitrogen-doped carbon catalysts（M-N-C catalysts）,in this article,we choose M-N-C catalysts（M=a series of transition metals）to conduct a series of studies.We used the PBE method of density functional theory（DFT）to conduct a detailed study on the changes of the electroreduction of CO₂ on M-N-C（M=Fe/Ni/Cu/Zn/Ru/Rh/Pd/Ag/Cd/Os/Ir/Pt/Au）catalysts,compare the catalytic performance of the selected nitrogen-doped metal carbon catalyst,and focus on simulating the mechanism of various CO₂ electrochemical reduction reactions.The research points out that single-atom molecular catalyst M-N-C（M=Ir）catalysts have good performance in catalyzing the reduction of carbon dioxide into carbon products with economic value,especially methane and methanol,and multicarbon products such as ethanol.For the electroreduction of CO₂ to methane and methanol,the path of CO₂（g）→COOH*→CO*+H₂O→HCO*→HCOH*→CH₂OH*is the most favorable path.For the electroreduction of CO₂ to ethanol,compared with other coupling methods,it is more inclined to couple HCO-CO to HCOCO,then proton coupled electron transfer to HCOCOH,and finally electroreduction to CH₃CH₂OH.The potential determination step（PDS）is CO*→HCO*step(ΔG_max=+0.63 e V).2.In this article,the focus is to combine carbon reduction and nitrogen reduction to produce urea.A series of catalysts supported on the substituted corrole substrates in the form of a double transition metal are investigated by DFT calculations.The best catalyst V2N6C was selected,and the carbon reduction and nitrogen reduction were combined to further explore the catalytic performance of urea synthesis.In the mechanism analysis,the potential barriers of the speed determination steps of each path were compared.It was found that 0.59 e V<0.63 e V<0.75 e V,and the optimal path was found,that was,*→*N2→*NN→*NCO2N→*NCOOHN→*NCON→*NCONH→*HNCONH→*HNCONH₂→*H₂NCONH₂.Based on this study,it was found that the synergistic catalytic strategy of double active sites had broad prospects in urea synthesis.The selection of homonuclear bimetallic single-atom molecular catalysts could also provide new development strategies for the design of other efficient molecular catalysts.3.Due to the synergistic effects of different metal sites,the double atom or multiple atomic catalysts are very promising in electrocatalysis.This article focuses on the synthesis of urea under the synergistic effects of bimetallics.The heteronuclear bimetallic single-atom molecular catalysts were designed and screened,and the details and energy information of elementary reactions were provided from atomic level through theoretical simulation,which provided a new understanding for the study of reaction mechanism.It was found that Nb VN₆C catalyst was the best catalyst for electrosynthesis of urea（0.37 e V<0.44 e V<0.67 e V<0.74 e V<0.84 e V）,and the optimal path was*→*N₂→*NN→*NNH→*NNH₂→*HNNH₂→*H₂NNH₂→*H₂NNH₂-CO₂→*H₂NNH₂-COOH→*H₂NCONH₂.At the same time,compared with homonuclear bimetallic single-atom catalyst V₂N₆C,heteronuclear bimetallic single-atom molecular catalyst Nb VN₆C had a smaller limiting potential（-0.37 V and-0.59 V）,which demonstrated the usefulness of this study.In this study,from mono-metal atom molecular catalysts to homonuclear bimetallic atomic molecular catalysts,and finally to heteronuclear bimetallic atomic molecular catalysts,whether it was to study the electrocatalytic reduction of carbon dioxide to synthesize C₁ products such as methane and methanol,C₂ products such as ethanol,or the combination of carbon dioxide reduction and nitrogen reduction to synthesize urea,it was found that there were a large number of active sites on the bimetallic molecular catalyst for unsaturated coordination,which could promote the activity of the catalyst,and the synergistic effect of heteronuclear bimetallic atoms was beneficial to the catalytic synthesis of urea.This study provided a valuable reference for the accurate design and construction of single-atom（bimetallic single-atom）molecular catalysts.