Preparation Of Transition Metal-based Catalysts And Their Performances In CO₂ Electroreduction

Fossil fuels play an important role in our daily lives.With the development of society,the excessive utilization of fossil fuels leads to an obvious increase in CO₂ concentration in the air,subsequently causing a series of environmental problems.CO₂ conversion and utilization is one of an ideal ways to solve energy crisis and environmental problems.Especially,electrocatalytic CO₂ reduction（CO₂ER）can not only utilize CO₂ resources but also generate value-added chemicals,so it has important theoretical significance and application value.However,CO₂ is a thermodynamically stable and kinetically inert molecule and is difficult to be converted.The development of high-efficient catalysts for the CO₂ER is highly important.In this thesis,a series of transition metal-based（Cu,Fe,Zn,and Mn）catalysts were prepared by constructing bimetals and interfaces,doping heteroatoms（N,P,etc.）,and supporting on two-dimensional materials（GO,MXene,etc.）.These catalysts were applied to catalyze CO₂ electroreduction to the products of ethylene,CO,and syn-gas.The relationship between the catalyst structure and performance was studied by using a variety of characterization techniques,and the reaction mechanisms were proposed according to the results of in-situ Raman spectra and density functional theory（DFT）calculation.The main contents are as follows.（1）A bimetallic ZrO₂/Cu-Cu₂O catalyst was prepared by a co-precipitation method,which showed good catalytic performances in catalyzing the CO₂ER to C₂H₄.The catalyst could offer an ethene Faradic efficiency（FEc2H4）of 62.5%and a current density of 24 mA·cm^-2 at-1.28 V（vs.RHE）in a 0.1 mol/L KCl electrolyte solution.The characterization results of TEM,XRD,XPS,etc.confirmed that the catalyst was composed of Cu₂O,Cu0,and ZrO₂.The results of in-situ Raman spectroscopy confirmed that the reaction involved a*COOH intermediate.The DFT calculations confirmed that the*COOH formed on the Zr site can be transferred to the adjacent Cu⁺species at the Cu/Zr interface.The*COOH is then converted to*CO on Cu⁺ sites,followed by dimerizing of two*CO to*OCCO species,which is further reduced to C₂H₄.（2）The N,P co-doped 3D graphene aerogel supported Mn-Zn bimetallic catalyst((Mn_xZn_1-x)2P2O7N/3D-GO)was prepared by a hydrothermal method,and it showed good CO synthesis efficiency in catalyzing the CO₂ER.The characterization results showed that the catalyst exhibits a sheet-like structure similar to that of 3D-GO.When the potential was-0.917 V（vs.RHE）,the FEco reached 96.6%,and the current density was 12 mA·cm^-2,and the catalyst was stable at least for 20 h.In addition,in the potential range of-0.967～-1.117 V（vs.RHE）,the FEco obtained over the catalyst were all more than 90%,indicating that it has a wide electrochemical window.The excellent catalytic performance of the catalyst originated from the synergistic effect among the MnZn,N,P,and GO.（3）Using FeCl₃·6H₂O,C₃H₆N₆,and Ti₃C₂ MXene as raw materials,a FexC@CNT/N-MXene catalyst with dual active sites was prepared by adopting a high-temperature calcination method.The catalyst showed good performance in the CO₂ER to syn-gas,which is composed of FexC,N-MXene,and carbon nanotubes（CNTs）.In 0.1 mol/L KCl electrolyte,when the potential is-1.1 V（vs.RHE）,the current density reached 26 mA·cm^-2,and the syn-gas with a H₂/CO molar ratio of 0.97 was obtained,which can be directly used in Fischer-Tropsch（FT）synthesis,hydroformylation,and other reactions.When the applied potential was in the range of-0.8～-1.2 V（vs.RHE）,the syn-gas with a H₂/CO molar ratio of 0.22～2.82 could be formed.There was a synergistic effect between the FexC@CNT and N-MXene,and especially the FexC@CNT could improve the electrochemical specific surface area and CO₂ adsorption capacity,and reduce the electrochemical impedance of the catalyst.The in situ Raman spectroscopy results indicated that the reaction also involved the*COOH intermediate,and metal poisoning experiments and DFT calculations revealed that the Fe_xC was the active site of CO₂ ER.（4）A heteroatom N-doped 2D MXene material supported ZnO catalyst（ZnO/N-MXene）was prepared and used in the CO₂ER to CO.The catalyst was composed of MXene nanosheets stacked in a monolayer or a few layers.In 0.5 mol/L KCl solution,when the apllied potential was-0.967 V（vs.RHE）,the catalyst could produce 96.4%of FEco in the reaction,the current density is 7.2 mA·cm^-2 with a stability of at least 6 h.N-MXene can effectively improve the mass transfer capacity of the ZnO catalyst and reduce its electrochemical impedance.The metal poisoning experiment showed that Zn²⁺ is the active site of the catalyst.