【作者】 杨晶；

【导师】 陈艳霞；

【作者基本信息】 中国科学技术大学 ， 物理化学， 2018， 硕士

【摘要】 二氧化碳(CO2)电催化还原是制备碳氢燃料、高附加值的碳基原料、实现碳循环的有效手段之一,但其反应机理、反应条件下电极/溶液界面的变化规律以及该变化对反应的影响机制等,目前仍不是十分清晰。水溶液体系中CO2电催化还原主要受到氢析出反应(HER)的竞争,因此研究CO2及其中间体对HER的影响,有助于找出有效抑制析氢并实现CO2还原的最优条件。近期的一些研究认为,吡啶(Pyr)可有效提高C02催化还原生成甲醇与甲酸,而另一些研究者认为,Pyr对C02催化还原根本没有促进作用。针对以上科学问题,本论文以铂(Pt)、铜(Cu)为工作电极,开展了以下两方面的研究:1、不同pH溶液中Pt、Cu电极上CO2还原和HER关系探究:利用旋转圆盘结合循环伏安法、电势阶跃法和红外光谱等技术,研究了不同pH溶液中Pt电极表面CO2还原和HER的关系。在H欠电位沉积(H-UPD)和HER的电势范围内,CO2可在Pt电极表面还原生成COad,其生成速率随着UPD-H的覆盖度而增加,并在HER起始电位达到最大值。CO2吸附、还原所需中间产物Had,在HER的作用下只有有限的吸附位点和停留时间,因此COad随HER逐渐减少。我们发现即使在CO2饱和溶液中,Pt电极上主要反应都是HER,溶液pH的变化主要改变HER的平衡电势与用于HER的质子源的浓度。在铜电极上,CO和C02饱和溶液中还原起始电位差值恰好等于2种体系中界面pH不同引起的电势差,且都在电极表面观察到COad,验证了 CO是CO2还原的中间产物。CO和CO2的反应速率以及产物分布会因Cu膜电极的制备方式以及由此导致的电极表面结构的不同而不同。2、Pt、Cu电极上Pyr与CO2体系中电极反应探究:利用电化学与原位红外光谱技术考察了 Pt(111)单晶和Pt、Cu薄膜上Pyr体系的电化学行为,以及溶液pH和Pyr浓度对其电化学行为的影响。并在此基础上探究了 Pyr与CO2共存体系中的电极反应。我们发现,在Pyr溶液中Pt电极主要的阴极反应是HER,Pyr扮演两种角色,一方面在强缓冲体系中Pyr会在电极表面吸附、毒化活性位点而抑制HER;另一方面,在H+浓度较低的无缓冲溶液中充当缓冲剂,作为质子源还原生成H2。在Pyr与CO2共存的体系,Pyr抑制了 CO生成,可能略微增强了 COOH-的选择性。系统对比三种不同电极上的相关数据发现Pyr存在的溶液体系HER和CO2还原的电化学行为均与电极材料有很强的依赖性,不同电极上可能涉及不同的中间体和反应机理。更多还原

【Abstract】 Electrocatalytic CO2 reduction reaction(CO2RR),to produce hydrocarbon fuel and high value-added carbon-based materials,as one of the most effective means to realize carbon cycle in the ecosystem,has been intensely investigated through both experimental and theoretical studies.However,the reaction mechanism,the variation of the electrochemical interface as well as its influence on the reaction mechanism are all not very clear.Under aqueous conditions,CO2RR and hydrogen evolution reaction(HER)compete with each other and hence decrease the faradaic efficiency for CO2RR.Understanding the effect of CO2 and intermediates during CO2RR process on HER is of fundamental importance to inhibit the HER pathway and realize the optimal conditions for CO2 reduction.Recent studies have demonstrated that pyridine(Pyr)greatly enhance the catalytic efficiency of CO2 reduction to CH3OH and HCOOH,while other researchers argue that pyridine makes no difference to promote CO2RR.To the end,concerning the scientific problems mentioned above,Pt and Cu are employed as working electrodes to carry out investigations.And the main results and conclusions are summarized as follows:1.Investigations on CO2RR and HER on Pt and Cu electrodes with different pH.With rotating disk electrode(RDE),cyclic voltammetry(CV),potential step method and in-situ FTIR,studies in the relationship between CO2RR and HER on Pt electrode with different pH have been carried out.At the potential range of H-UPD and HER,CO2 can be reduced to COad on Pt surface.The yielding rate of COad increased with the coverage of UPD-H,and reached the maximum value at the onset potential of HER.The coverage of COad decreased gradually during HER due to less unoccupied sites and shorter occupied time of Had which was necessary for CO2 adsorption and reduction.Studies have revealed the pH depence of the equilibrium potential in HER and the concentration of H+,and it was found that H2 generation was more dominant even in CO2-saturated solution on Pt electrode.On Cu electrode,difference in the onset potential for CO and CO2 reduction corresponded equally to the potential difference caused by different pH.And COad was always observed at Cu surface,indicating that COad acted as an intermediate in CO2RR.Reaction rate and the main product in the two reactions depended much on the surface structure of the electrodes.2.Investigations on the role of pyridine(Pyr)in CO2RR on Pt and Cu electrodes.Combined electrochemical techniques with in-situ infrared spectroscopy,electrochemical behaviors of pyridine on Pt(111),Pt and Cu thin film electrodes and the effect of pH and the concentration of Pyr were explored.Moreover,the electrode reactions in CO2-saturated solution with pyridine were investigated.It was discovered that the main cathodic reaction in pyridine-containing solution on Pt electrode was still HER.Pyridine,on the one hand,inhibits HER by occupying and poisoning active sites at the electrode surface in the buffer system;on the other hand,acting as buffer in the solution with high pH,generates H2 as the proton source.In the process of CO2 reduction,the presence of pyridine may improve the selectivity of producing COOH-.Finally,comparing data on the three electrodes used,it was found that the electrochemical behaviors of HER and CO2RR in pyridine solution depended strongly on the nature of electrode materials,which may result in different intermediates and reaction mechanism.更多还原