【作者】 赵刚；

【导师】 江俊；

【作者基本信息】 中国科学技术大学 ， 物理化学， 2023， 硕士

【摘要】 可再生电化学二氧化碳还原反应（CO₂RR）已经成为最受关注的CO₂转化技术之一。CO₂RR技术不仅能帮助减少CO₂排放,而且可为化学工业提供一氧化碳（CO）、甲酸（HCOOH）、甲烷（CH₄）、乙烯（C₂H₄）等重要的生产原料和有附加值的化学品。催化剂是实现稳定高效的CO₂RR的必要条件,催化机理的研究,则为催化剂设计提供理论支持和指导。因此,CO₂RR催化机理的研究,对助力催化剂设计朝向可持续地利用碳资源这一长远目标不可或缺。单原子催化剂（SACs）具有原子利用率高、催化活性高、活性位点结构精准,均一可调等许多优点,在电催化CO₂RR这个体系上也有鼓舞人心的效果。同时,人们对单原子催化剂上的CO₂RR微观机理了解并不充分,特别是复杂的电化学条件下催化剂的构效关系如何,尚待探索。基于这些思考,本论文以前景广阔的二维碳材料负载单原子催化剂为模型,从理论计算的角度研究CO₂RR机理,探讨了材料结构和微观环境对反应过程的调控。本论文的结构是,第一章为绪论,介绍CO₂RR的背景,第二章为理论模拟的方法介绍,第三至第五章为主要研究内容,第六章为总结。主要研究内容及进展如下:（1）与实验合作,研究了有不同化学修饰的石墨双炔（GDY）负载的单原子铜催化剂（Cu SA/R-GDY,R=-F,-H,-OMe）上CO₂还原为CH₄的反应过程。本人以实验结构表征为参考,提出了一种独特的双层Cu SA/F-GDY催化还原CO₂的反应机理,并用密度泛函理论（DFT）计算模拟了相应的基元反应过程。DFT计算发现,双层两个邻近Cu原子可分别吸附CO₂与H2O,以氢键稳定吸附态CO₂,通过质子-电子共转移形成吸附态COOH,并辅以Cu-OH与之形成的氢键稳定,然后以多步质子-电子共转移步和重排反应形成了 CH₄,留下O吸附在Cu位点,再通过加氢还原形成H2O并脱去,完成一次从CO₂到CH₄的催化循环。通过反应自由能变化的比较,证实了 Cu SA/F-GDY更有利于CO₂还原为CH₄,支持了实验发现。这部分内容在论文第三章。（2）人们普遍相信电化学界面的化学环境对电催化CO₂RR有重要影响。但对异相单原子催化剂体系,界面因素如双电层如何影响反应过程的研究较少。本人聚焦于探讨CO₂RR体系广泛使用的碱金属阳离子对反应的影响,以氮化石墨烯负载铜单原子催化剂（CuN4/G）为催化剂模型,以三水合钠离子（Na⁺·3H₂O）为溶液中碱金属阳离子模型,构造了包含表面电荷,溶剂水和阳离子等极化环境的双电层模型。对CO₂还原到CO这一两电子还原过程的DFT计算发现,Na⁺·3H₂O对CO₂的电吸附、活化和还原过程产生显著的影响。在Na⁺·3H₂O的帮助下,CO₂可以在CuN4/G上的Cu或N原子上发生化学吸附并活化。自由能变化数据表明,生成COOH步骤成为CO₂RR的电位限制步骤。而在双CuN4位点环境中,Na⁺·3H₂O促成CO₂优先吸附在N上,再在第一个加氢还原步转移到Cu上,最终形成CO。我们的研究证明了碱金属阳离子在CO₂RR早期的关键作用,为调节铜氮碳（Cu-N-C）单原子催化剂获得更好的CO₂转化性能提供了新的见解。这部分内容在论文第四章。（3）由于电中性条件的双电层模型未考虑真实电化学反应过程系统的总电子数变化,未能明确归属电荷转移步的属性,而给基元反应的机理解释造成了困难。本人在上一章工作基础之上,应用更接近真实电化学条件近似的恒电势法,梳理了 Cu-N-C单原子催化剂上CO₂RR形成CO的机理。巨正则密度泛函理论（GC-DFT）计算表明,在足够大的负偏压条件下（U=-1.2Vvs SHE）,Na⁺·3H₂O辅助下的CO₂电吸附不仅是一个电化学过程,而且是电位限制步骤,修正了电中性条件下将COOH的形成划为电位限制步骤的结论,并且与近期的CO₂RR形成CO的理论和实验研究结论一致。而且,在恒电势条件下,COOH的形成也是电化学步骤,所形成的COOH基团带有相当的负电荷。对体系的电子结构分析表明,具有全充满3d轨道的一价Cu可能是促成吸附态COOH显著带负电的决定性因素。同时,Na⁺·3H₂O促成了所有中间体的稳定化学吸附,确认了电中性条件模拟揭示的水合钠离子的重要作用。本章工作表明了模拟真实电化学条件对理论模拟电催化反应机理的重要性。这部分内容在论文第五章。更多还原

【Abstract】 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.更多还原