Theoretical Study On Modified Carbon-based Single Atom Catalysts For Electrocatalytic Carbon Dioxide Reduction

With the extensive use of fossil fuels,the circumstance of energy crisis and the greenhouse effect have become increasingly serious.The strategy of reducing CO₂ into fine chemicals or high value-added products could not only alleviate above problem,but also serves as an effective method to achieve the target of carbon neutrality.Since its moderate reaction conditions and adjustable products,electrocatalyzing CO₂ has received more and more attention and become one of favored research filed.The carbon-based single-atom catalyst（M-N₄）formed by four nitrogen atoms anchoring a single metal atom is an important type of CO₂ reduction electrocatalyst.However,there are some problems in carbon-based catalysts,such as poor catalytic activity,high overpotential and low selectivity.Accordingly,this thesis study on the modification of M-N₄ catalytic center to improve their catalytic performance,which involves changing the coordination number of metals（M-NxV4-x）and introducing heteroatoms to replace coordinated nitrogen atoms（M-N₃X）.The main research contents are discussed as follows:Firstly,a series of M-N₃V and M-N₂V₂ catalysts were smartly designed in this work by constructing N vacancies in M-N₄.The calculation results have shown that the adsorption strength of intermediates become stronger as the number of nitrogen vacancies increases.Interestingly,there is a volcano-shaped relationship between the adsorption energy of*CO and the overpotential(η^CO)of CO₂RR.The catalyst of Mn-N₂V₂,Zn-N₃V,Mn-N₃V and Zn-N₂V₂ located at the summit and exhibited the excellent activity in catalyzing CO₂ to CO products,and their η^CO are 0.12,0.15,0.16 and 0.22 V,respectively.The electronic structure analysis shows that the negative Crystal Orbital Hamiltonian Population Integral（-ICOHP）of metal atom and C atom in CO has a good linear relationship with ΔG₃.Secondly,we have regulated the catalytic unit of M-N₄ by replacing one N atom with hetero-elements X（B,C,O,P,and S）to design various of M-N₃X catalysts and attempted to explore their CO₂RR catalytic performance.Here,the calculation results exhibited that the active site of CO₂RR origin from the metal atom for M-N₄,M-N₃C,M-N₃O,M-N₃P and M-N₃S catalysts,while the active site in M-N₃B turn to boron atom since the stronger adsorption ability toward intermediates.We have successfully established a volcano-shaped between the adsorption energy of*COOH intermediates（ΔG1）and the overpotential(η^CO)of CO₂RR,in which the best catalysts could be easily identified.The Mn-N₃S located at the summit and shown the highest catalytic performance with the lowest overpotential(η^CO=0.03 V).The Fe-N₃B,Zn-N₃S,Mn-N₃B,Co-N₃O,Zn-N₃O,Cr-N₃O and Zn-N₃B also shown better catalytic activity than their corresponding M-N₄ catalysts and noble bulk catalysts（e.g.,Au and Ag）.This improvement has been revealed by the analysis of electronic structure that the differences in electronegativity and atomic radius of hetero-elements will redistribute the charge state on catalytic site and further to adjust the adsorption energy of intermediates to reach a ideal state,which promoted the catalytic process of CO₂RR.