Preparation Of MIL-101 Supported Copper Composite Catalysts And Their Electrochemical CO₂ Reduction Performance

With the aggravation of the energy crisis and global warming,the electrochemical reduction of CO₂ into value-added carbon-based products opens up new possibilities for the utilization of CO₂,which is a promising solution conducive to environmental improvement and sustainable development.Copper and its derivatives are considered to be the most potential catalysts in the field of CO₂ electrochemical reduction（CO₂ER）due to their abundant reserves,low price and good electrical conductivity.Among them,free Cu nanoparticles（Cu NPs）can provide more catalytic active sites,but also have the disadvantage of serious agglomeration tendency,so various coating agents need to be added in the preparation process.Metal-organic frameworks（MOFs）,as a class of materials with high specific surface area and high porosity,are used as carrier templates to limit the growth and agglomeration of Cu nanoparticles with the help of their pore channels.Meanwhile,it is also conducive to the adsorption of CO₂ molecules.On this basis,this paper selects the classic MOFs material—MIL-101-Cr and its aminofunctionalized material as the precursor to support Cu NPs to prepare Cu-based catalysts for the research of CO₂ER.The main contents are as follows:1.Preparation of Cu@MIL-101-Cr catalyst and study on the performance of CO₂ER:The MIL-101-Cr precursor of 80,200,300nm was hydrothermally synthesized by controlling the concentration of reactants,and the specific surface area and pore volume of the precursor increased with the shrinking of the size.Cu@MIL-101-Cr catalyst was prepared by supporting Cu NPs,in which Cu NPs is mainly a mixture of Cu（0）,Cu（Ⅰ）and Cu（Ⅱ）with average size of～3.9 nm.The catalytic performance of CO₂ reduction was tested by CV,LSV,ECSA,EIS,i-t curve and other electrochemical methods.The experimental results show that the main products of catalytic reduction of CO₂ by Cu@MIL-101-Cr/C composite electrode are H2,CO and methanol in the potential range of～1.1 V～-1.5V vs Ag/AgCl.Among them,the Faraday efficiency of methanol reached 29%at the-1.4V vs.Ag/AgCl potential of 100mM materials,and with the increase of the size,the Faraday efficiency of methanol decreased,while the Faraday efficiency of CO increased continuously.Therefore,the control of precursor size will affect the performance of CO₂ER of Cu@MIL-101 materials,and the reduction of precursor size will improve the reduction efficiency of CO₂ to methanol.2.Preparation of Cu@NH₂-MIL-101-Cr catalyst and study on the performance of CO₂ER:Amino functionalized MOF-NH₂-MIL-101-Cr with high specific surface area,～70 nm size and good thermal stability was successfully synthesized using 2-aminoterephthalic acid as ligand.Then the Cu@NH₂-MIL-101-Cr catalyst was prepared by impregnation method,and its CO₂ER performance was tested.The results show that the activity of Cu@NH₂-MIL-101Cr/C composite electrode for CO₂ER is better than that of Cu@MIL-101-Cr.In the range of1.1V vs Ag/AgCl potential to-1.5V vs Ag/AgCl potential,the Cu@NH₂-MIL-101-Cr/C composite electrode decreased the reaction overpotential,and the Faraday efficiency of methanol reached 31%at-1.2V vs.Ag/AgCl potential,slightly higher than the optimum Faraday efficiency of 29%of Cu@MIL-101-Cr electrode,but combined with five potentials of-1.1 V vs Ag/AgCI.The Faraday efficiency of methanol has been significantly improved as a whole.The experimental results show that the introduction of amino groups by direct synthesis can reduce the reaction overpotential and promote the performance of CO₂ER.3.Preparation of Cu@ED-MIL-101-Cr catalyst and study on the performance of CO₂ER:In this chapter,ethylenediamine（ED）was grafted onto MIL-101-Cr（～80 nm）to prepare ED-MIL-101-Cr by post-synthesis modification.ED-MIL101 material was synthesized by controlling the amount of ethylenediamine（25,50,75,100 μ L）,and Cu@ED-MIL-101-Cr catalyst was prepared by loading Cu NPs as precursor.Then the performance of electrocatalytic reduction of CO₂ was tested,and the experimental results showed that the Faraday efficiency of methanol increased with the increase of the amount of ethylenediamine.Among them,the FECH3OH of Cu@75-ED-MIL-101 catalyst at the optimal potential（1.4Vvs.Ag/AgCl）reached 36%,while maintaining a high current density,but when the amount of ethylenediamine reached 100 μL,the Faraday efficiency of methanol decreased at all potentials.To sum up,ethylenediamine modification will affect the performance of Cu@MIL-101 materials for electrocatalytic reduction of CO₂,but excessive ethylenediamine will destroy the structure of MIL-101-Cr and then reduce the activity of CO₂ER.Comparing the Faraday efficiency of three kinds of electrode materials in this paper,the introduction of amino group can improve the catalytic activity of Cu@MIL-101 catalyst and the selectivity of methanol.The FECH3OH of Cu@NH₂-MIL-101-Cr prepared by direct synthesis method is 31%at the optimum potential（-1.2Vvs.Ag/AgCl）,and can react with overpotential at the same time.The Cu@ED-MIL-101-Cr catalyst was prepared by postsynthesis modification,while maintaining a high current density,the FECH3OH reached 36%,the electrocatalytic reduction activity of CO₂ was stronger,and the selectivity of methanol was higher.