Mechanistic Study Of Carbon Dioxide Reduction By Two-Dimensional Carbon Supported Single-Atom Catalysts

Renewable electrochemical carbon dioxide reduction reaction（CO₂RR）has gained significant attention as a promising technology for CO₂ conversion.The CO₂RR process not only helps to mitigate CO₂ emissions but also facilitates the production of vital raw materials and value-added chemicals such as carbon monoxide（CO）,formic acid（HCOOH）,methane（CH₄）,and ethylene（C₂H₄）for the chemical industry.To achieve stable and efficient CO₂RR,catalysts play a crucial role,and the study of catalytic mechanisms provides theoretical support and guidance for designing efficient catalysts.Therefore,it is imperative to explore the catalytic mechanisms of CO₂RR to facilitate the long-term goal of sustainable utilization of carbon resources.Single-atom catalysts（SACs）offer numerous advantages such as high atom utilization,precise active site structure,uniform and adjustable structure,and high catalytic activity,making them highly promising for electrocatalytic CO₂RR systems.However,the microscopic mechanism of CO₂RR on SACs,particularly the structureactivity relationship of catalysts under complex electrochemical conditions,remains incompletely understood.In light of these considerations,this paper employs twodimensional carbon material-supported single-atom catalyst as a model to investigate the CO₂RR mechanism via theoretical calculations,exploring the impact of material structure and microscopic environment on the reaction process.The thesis structure comprises an introduction providing background on CO₂RR,an overview of the theoretical simulation method,three to five chapters detailing the main research contents,and a conclusion.The primary research content and progress are as follows:Building on experimental data,the catalytic reduction of CO₂ to CH₄ was studied over graphidiyne（GDY）-supported single-atom copper catalysts（Cu SA/R-GDY,R=-F,-H,-OMe）with different chemical modifications.Using the experimental structure characterization as a reference,a unique reaction mechanism for the catalytic reduction of CO₂ by double-layer Cu SA/F-GDY was proposed.The corresponding elementary reaction process was calculated and simulated using density functional theory（DFT）.The DFT calculations revealed that the bilayer’s two adjacent Cu atoms can adsorb CO₂ and H2O,respectively,stabilize the adsorbed CO₂ via hydrogen bonds,and form adsorbed COOH through proton-electron co-transfer,supported by the Cu-OH hydrogen bond to stabilize CO₂.Subsequently,CH₄ was formed through multi-step proton-electron co-transfer and rearrangement reactions,leaving O adsorbed on the Cu site,and H2O was formed and removed via hydrogenation reduction,completing the catalytic cycle from CO₂ to CH₄.Comparing the reaction free energy changes confirmed that Cu SA/F-GDY was more favorable for reducing CO₂ to CH₄,which supports the experimental findings.This work is presented in the third chapter of the thesis.The electrochemical interface’s chemical environment is believed to significantly affect the electrocatalytic CO₂RR.However,there are limited studies on how interface factors,such as electric double layers,impact the reaction process for heterogeneous single-atom catalyst systems.This study focuses on the impact of alkali metal cations,commonly used in the CO₂RR system,on the reaction.The catalyst model used is the graphene nitride-supported copper single-atom catalyst（CuN4/G）,with sodium trihydrate（Na⁺·3H₂O）used as the base in the solution.An electric double layer model was constructed,including surface charges,solvent water,and polarized environments such as cations.DFT calculations of the two-electron reduction process of CO₂ to CO revealed that Na⁺·3H₂O significantly impacts the electrosorption,activation,and reduction process of CO₂.With the help of Na⁺·3H₂O,CO₂ can chemisorb and activate on Cu or N atoms on CuN4/G.The free energy change data indicated that the step of generating COOH became the potential-limiting step of CO₂RR.However,in the double CuN4 site environment,Na⁺·3H₂O promotes the preferential adsorption of CO₂ on N,which transfers to Cu in the first hydrogenation reduction step and finally forms CO.This study demonstrates the critical role of alkali metal cations in the early stage of CO₂RR,providing new insights into tuning copper-nitrogen-carbon（Cu-N-C）single-atom catalysts for better CO₂ conversion performance.The fourth chapter of this thesis covers this topic.The electric double layer model,which does not account for changes in the total number of electrons in the actual electrochemical reaction system,cannot disclose the true nature of charge transfer step,making it difficult to explain the elementary reaction mechanism.Building upon the findings of the previous chapter,I employed the constant potential method,which closely resembles actual electrochemical conditions,to elucidate the mechanism of CO₂RR formation of CO on Cu-N-C single-atom catalysts.Grand canonical density functional theory（GC-DFT）calculations reveal that under sufficiently large negative bias conditions（U=-1.2V vs SHE）,CO₂ electrosorption aided by Na+3H2O is not only an electrochemical process,but also a potential-limited step.This finding contradicts the conclusion that the formation of COOH is classified as a potential-limited step under electrically neutral conditions and is consistent with the conclusions of recent theoretical and experimental studies on the formation of CO by CO₂RR.Furthermore,under constant potential conditions,the formation of COOH is also an electrochemical step,and the formed COOH groups carry a significant negative charge.Analysis of the electronic structure of the system suggests that the monovalent Cu with fully filled 3d orbitals may be the determining factor for the substantial negative charge of the adsorbed COOH.Additionally,Na+ 3H2O facilitates the stable chemisorption of all intermediates,confirming the crucial role of hydrated Na ions demonstrated by simulations under electrically neutral conditions.This chapter highlights the significance of simulating real electrochemical conditions for the theoretical modeling of electrocatalytic reaction mechanisms.This section is presented in the fifth chapter of the thesis.