Interface Regulations For Constructing Copper Based Catalysts And Their Carbon Dioxide Electrochemical Reduction Study

Since the industrial revolution,the emission of a large amount of carbon dioxide（CO₂）caused by human activities has led to a continuous increase in the concentration of CO₂in the atmosphere,which has caused global warming and a series of ecological and environmental problems.This situation has reached a point where it cannot be ignored.The utilization of CO₂can not only alleviate the rising carbon dioxide concentration,but also convert CO₂into valuable fuels and chemicals.The electrochemical reduction of CO₂using renewable electric energy is such a way to simultaneously achieve efficient utilization and storage of renewable energy,efficient resource utilization of CO₂and provision of important fuels and chemicals.The CO₂electrochemical reduction reaction（CO₂RR）has many advantages,such as mild reaction condition,simple process,controllable reaction performance,and the use of renewable electric energy.Therefore,CO₂RR process has attracted great attentions.In order to improve the catalytic performance of CO₂RR and promote the application of this technology,researchers around the world have carried out a lot of investigations on CO₂RR.The development of high-efficiency catalysts is the key to CO₂electrocatalytic reduction reaction（CO₂RR）and is the hotspot of current researches.At present,non-precious metal copper（Cu）based catalysts are the only catalysts that can catalyze CO₂into multi-carbon(C₂₊)products including ethylene,ethanol,n-propanol,etc.,and have received extensive attentions and researches.However,the catalytic performance of the existing Cu-based catalysts used for CO₂RR is still unsatisfactory,mainly due to high overpotential,low activity and selectivity,and poor catalytic stability.In addition,the reaction mechanism of CO₂RR on Cu-based catalysts is still unclear and the key factors affecting the performance of Cu-based catalysts are still unclear.These problems hinder the development of high-efficiency Cu-based catalysts for CO₂RR.Aiming at the above problems,in this thesis,we try to design and synthesize high-performance Cu-based catalysts for CO₂RR.In the process of CO₂RR,catalytic reaction occurs at the interface between catalyst and electrolyte.Therefore,this thesis focused on the regulation of catalytic interface through the regulations of heteroatom doping,surface oxophilicity,and carbon support confinement for Cu-base catalysts,aiming to constructing high-performance Cu-based catalysts for CO₂RR.Through the study of structure and surface properties of catalysts,we explore the structure-activity relationship between the copper-based catalysts and the catalytic performance of CO₂RR.We try to explore the reaction pathways of some reduction products through experimental results and theoretical calculations and deepen the understanding of the reaction mechanism of CO₂RR.In addition,in this thesis,we also pay attention to the catalytic stability of Cu-based catalysts in CO₂RR and propose strategies to improve the catalytic stability of CO₂RR.In this paper,a traditional H-type cell is used as the electrolyzer for CO₂RR（Chapter 2）.The reagents,equipment and testing methods are summarized,as well as the preparation and comparison of working electrode and the reaction condition.The gas phase and liquid phase reduction products of CO₂RR were analyzed by gas chromatograph and magnetic resonance spectrometer,and the calculation and evaluation methods of catalytic reaction performance are presented in detail.Aiming the problem of unclear active sites in carbon materials supported Cu-based catalysts,we utilized graphitic carbon nitride（g-C₃N₄）with high nitrogen content and definite metal embedding sites to support Cu.We adopted a simple in-situ thermal polymerization method and synthesized a series of Cu-doped g-C₃N₄materials.The Cu sites confined in g-C₃N₄and the influence of these Cu sites on the catalytic performance of CO₂RR were studied.High methane activity and selectivity were achieved in this thesis（Chapter 3）.When the Cu content in the catalyst is low,isolated Cu atoms are mainly confined to the nitrogen vacancies of g-C₃N₄,while with the increasing Cu content in the catalyst,in addition to the confined Cu sites in the nitrogen vacancies,a second Cu sites are formed and some Cu atom pairs and sub-nano clusters are formed.The CO₂RR tests showed that the Cu content in the catalysts and the Cu sites restricted by g-C₃N₄scaffold have a great impact on the activity and selectivity of CO₂RR.The Cu-CN-20 catalyst showed the highest CH₄Faraday efficiency of 49.04%and the largest CH₄current density of 9.78 m A/cm².It also exhibited good catalytic stability in the stability test of 10 hours.The Faraday efficiency ratio of CH₄/C₂H₄reached the maximum of 35.03 on Cu-CN-100 catalyst.With the increasing Cu content in the catalysts,the Faraday efficiency of CH₄decreased while the Faraday efficiency of H₂increased.We propose that the Cu single site embedded in the g-C₃N₄nitrogen pots is an efficient active site for CH₄production.The regulation of the confined Cu sites in carbon materials is an effective means to improve the selectivity of Cu-based catalysts for CO₂RR.In order to construct favorable Cu active sites for the formation of C₂₊products,we have proposed a strategy to induce and stabilize Cu active sites by residual chlorine（Cl）and obtained a highly selective and stable Cu-based catalyst for CO₂RR（Chapter 4）.We designed and synthesized a novel porous Cl-doped Cu-based catalyst,e-Cu OHFCl,derived from the precursors containing halogens.The e-Cu OHFCl catalyst exhibits a high Faraday efficiency of 53.8%for C₂₊products at-1.00 V_RHEand the current density of the C₂₊product reached 15 m A/cm²at-1.05 V_RHE.The results showed that the Cu⁰-Cu⁺sites induced by residual Cl and the porous nanosheet structure of e-Cu OHFCl catalyst promoted the production of C₂₊products.In addition,the e-Cu OHFCl catalyst exhibited an excellent CO₂RR catalytic stability in the long-term reaction up to 240 hours.It is found that the stable Cu⁰-Cu⁺site induced by residual chlorine and the robust structure of the catalyst are essential for the Cu-based catalyst to maintain high C₂₊Faraday efficiency during the long-term operation of CO₂RR.This study provides new insights into the construction of highly efficient Cu active sites and provides valuable strategies for improving the catalytic stability of Cu-based catalysts for CO₂RR.Ethanol is the main liquid C₂₊product in CO₂RR.However,the Faraday efficiency of ethanol is usually low on Cu-based catalysts.To address this problem,in this thesis,we have proposed a surface oxophilicity regulation strategy to improve the selectivity of ethanol in CO₂RR by stabilizing oxygen-containing intermediates（Chapter 5）.A bimetallic strategy was adopted to regulate the surface oxophilicity of the catalysts and tin（Sn）with higher oxophilicity than Cu was selected.We have synthesized a series of Cu Sn_xbimetallic catalysts with dendritic structure the surface-enriched Sn that highly dispersed in the Cu matrix.The highest CO Faraday efficiency reached 96.36%at-0.8 V_RHEon Cu Sn catalyst.While at high overpotential,high ethanol activity and selectivity were obtained and the ethanol/ethylene Faraday efficiency ratio increased with the increase of Sn content in Cu Sn bimetallic catalysts.We have investigated the change of the surface oxophilicity of Cu Sn bimetallic catalysts and studied the stability of the selectivity-determining intermediate between ethanol and ethylene by theoretical calculation method.The results showed that the introduction of Sn enhanced the surface oxophilicity of Cu-based catalysts and promoted ethanol pathway as well as ethanol/ethylene Faraday efficiency ratio by stabilizing the common selectivity-determining intermediate between ethanol and ethylene.We propose that the precise regulation of surface oxophilicity of Cu-based catalysts is an effective strategy to improve CO₂RR to oxygen-containing compounds and provides a new idea for the design of catalysts.At last,we summarize all the chapters of the thesis,illustrate the innovations and shortcomings of this thesis.We also propose the future perspectives for the research direction.