理论研究Cu@C_(2)N催化剂表面上水分子对电催化CO_(2)还原反应机理的影响

Mechanistic Insights into Water-Mediated CO_(2) Electrochemical Reduction Reactions on Cu@C_(2)N Catalysts:A Theoretical Study

电催化CO_(2)还原反应(CO_(2)RR)的反应途径涉及多个质子-电子对转移,在水溶剂条件下,质子的来源是水分子,考虑水分子对质子-电子对的转移机制十分必要。本研究提出水辅助氢穿梭模型,与常用的以氢原子作为氢源的直接加氢模型对比,研究水分子在CO_(2)RR中对质子-电子对转移的影响。采用密度泛函理论,系统地研究了铜原子嵌入C_(2)N单层催化剂(Cu@C_(2)N)和石墨烯作为衬底的Cu@C_(2)N/石墨烯复合催化剂(Cu@C_(2)N/G)表面上不同加氢模型的CO_(2)RR反应机理。在水辅助氢穿梭模型中,氢原子与水分子结合形成水合质子,水合质子将自身的氢原子转移到催化剂表面的反应物上形成反应中间体,增强了中间体与催化剂之间的相互作用。此外,在Cu@C_(2)N/G催化剂中,石墨烯将电子转移到表面的Cu@C_(2)N上,提高了催化剂的CO_(2)RR催化活性。进一步,计算了Cu@C_(2)N和Cu@C_(2)N/G催化剂上CO_(2)RR和析氢反应的极限电位,讨论催化剂的活性和选择性。结果表明CO_(2)在低电位下容易生成HCOOH,施加高电位时可以生成CO、CH3OH和CH4并伴随着H2的生成。

CO_(2) molecules can be converted into various fuels and industrial chemicals through electrochemical reduction,effectively addressing the problems of global warming,desertification,ocean acidification,and other adverse environmental changes and energy supply issues such as excessive utilization of nonrenewable fossil fuels.Generally,the pathway of the CO_(2) reduction reaction(CO_(2)RR)involves multiple proton–electron pairs transferred to the reactants,resulting in the production of multiple reduction products.Here,protons are derived from water molecules under aqueous solvent conditions.Therefore,exploring the effect of water molecules on the proton–electron pair transfer process in CO_(2)RRs is essential.In this study,we developed a water-mediated hydrogen shuttle model(H-shuttling)as a hydrogenation model to investigate the effect of water molecules on the proton–electron pair transfer process in CO_(2)RRs and compared it with the widely used water-free direct hydrogenation model(H-transfer),wherein the hydrogen atom is used as a proton.Because copper is a metal electrode material capable of producing hydrocarbons from CO_(2) electroreduction with a high faraday efficiency,and nitrogen-doped graphene(C_(2)N)exhibits excellent catalytic CO_(2) activation,we selected a single copper atom-embedded C_(2)N(Cu@C_(2)N)as the catalyst.Furthermore,to study the effect of graphene on the CO_(2)RR activity of Cu@C_(2)N/G,we selected a graphene-loaded Cu@C_(2)N composite(Cu@C_(2)N/G)as the catalyst because graphene was utilized as a substrate to boost the conductivity of the catalyst.In the two hydrogenation models,we investigated the mechanisms of CO_(2)RRs on Cu@C_(2)N and Cu@C_(2)N/G catalysts through density functional theory calculations.Notably,in the H-shuttling model,the H atom combines with the water molecule to form H3O,which transfers one of its own H atoms to a reactant on the catalyst surface,yielding a reaction intermediate.The H-shuttling model enhances the interaction between the catalyst and intermediate.Graphene,as a substrate,transfers electrons to the Cu@C_(2)N surface of the Cu@C_(2)N/G catalyst,which is demonstrated by calculations of the Bader charge transferred between the reaction intermediate and catalyst,as well as the Gibbs free energy of the CO_(2) reduction elementary reaction.This effectively lowers the Gibbs free energy of the potential-determining step and enhances the CO_(2)RR catalytic activity of Cu@C_(2)N/G.Moreover the limiting potentials of the CO_(2)RR and hydrogen evolution reaction are determined to obtain the activity and selectivity of the Cu@C_(2)N and Cu@C_(2)N/G catalysts.The results indicate that CO_(2) molecules on the Cu@C_(2)N and Cu@C_(2)N/G catalysts generate HCOOH at low applied potentials,and are able to produce CO,CH3OH,CH4,and H2 as the applied potentials increases.