The Synthesis Of Transition Metal Single Atom Catalyst By ZnO Template Strategy And Its Application For Electrochemical CO₂ Reduction

Electrochemical CO2 Reduction(ECR)technology can achieve CO2 recovery and high-value utilization by using the unstable electrical energy such as wind energy and solar energy,which should be worthy of in-depth investigations and of great application value.The preparation methods of electrochemical catalysts with high efficiency,high selectivity,and high stability are one of the key technologies of ECR.Among them,metal single-atom catalysts have shown good catalytic ability in ECR applications in recent years.However,catalyst preparation still suffers from harsh conditions,complex processes and high cost,and furthermore it is difficult to achieve large-scale industrial applications.In this thesis,a preparation strategy based on ZnO template was developed,which is easier to realize the controllable synthesis of carbon-based transition metal single-atom catalysts.The obtained metal single-atom catalysts show good ECR catalytic performance and industrial application prospects.The main research results of this thesis are as follows:Firstly,transition metals such as Fe,Co,Ni,Cu were doped into ZnO(hereinafter generally referred to as doped-ZnO)prepared by a solution-sol-gel synthesis method,and the formation mechanism of doped-ZnO was investigated.It was demonstrated that the cross-linking reaction of citric acid with metal ions promoted the highly uniform dispersion of transition metals and immobilized them in the ZnO lattice through the subsequent heat treatment reaction,which was of great importance for the successful preparation of single-atom catalysts.Then,the doped metal was introduced into ZIF-8 by the template effect of doped-ZnO by using the reaction of ZnO and 2-methylimidazole at high temperature,and the catalyst series MxNC-900 was synthesized.It was observed that the well-dispersed metal supported on the carbon material.emerged as single atoms by using aberration-corrected high angle annular dark field scanning transmission electron microscope(HAADF-STEM),and furthermore the existence of Fe-N4 coordination was proved by the synchrotron X-ray absorption results of Fe1NC-900.The catalyst has high selectivity to CO,in which the Faradaic efficiency of Fe1NC-900 can reach 94.8%at-0.5 V,and its CO current density can reach 4.49 mA cm-1,which is a high level of similar materials.It was shown that with the increase of voltage,the reaction rate-determinig step gradually changed from CO2 adsorption to*COOH protonation by in-situ infrared test.Theoretical calculations showed that the Fe-N4 active site could significantly reduce the reaction energy barrier for the formation and protonation of*COOH intermediates,thereby improving CO selectivity.Furthermore,the graphite-N modified Fe-N4 structure was prepared,which improved the intrinsic activity of the catalyst.ZnO1010 with exposed 1010 crystal planes was firstly prepared by hydrothermal synthesis,and the growth law of ZnO1010 in the hydrothermal process was studied.Then the transition metal single-atom supported ZnO was obtained by the impregnation method,the crystal plane(1010)was proved to be favorable for single-atom adsorption by thermodynamic simulation,and then the adsorption kinetics were studied to optimize the adsorption conditions.Finally the transition metal single-atom supported ZnO1010 was selected as the template to synthesize the catalyst series MxNC-900@ZnO1010.It was proved that the method can effectively obtain dispersed transition metal single atoms by aberration-corrected HAADF-STEM,and the results of synchrotron X-ray absorption proved that iron single atoms have a similar coordination structure to Fe1NC-900.Fe1NC-900@ZnO1010 can achieve a Faradaic efficiency of 90.8%at-0.5 V,while the CO current density per unit load is increased by about 3.5 times.The modification of graphite-N at the Fe-N4 meta-position was indicated by in-situ infrared and theoretical calculations the key species to enhance the intrinsic activity.In addition,the hydrothermal-impregnation method greatly simplifies the preparation process,reduces the cost,and makes the catalyst more industrially practical.In order to make up for the above-mentioned defect of low catalyst loading and further improve the material performance and application value,ZnO1010 was used as a template to in situ generate ZIF-8 to capture ferrocene by exposing ZnO1010 in a mixed atmosphere of 2-methylimidazole and ferrocene and the catalyst Fe3FcNC-900@ZnO1010 was prepared.The results of inductively coupled plasma mass spectrometry(ICP-MS)and X-ray photoelectron spectroscopy(XPS)showed that Fe loading increased by about 10 times.Highly dispersed iron single atoms were observed by aberration-corrected HAADF-STEM,and in situ infrared characterization results proved that the catalyst had a similar catalytic mechanism to Fe1NC-900@ZnO1010.The high loading of the catalyst significantly increases the CO partial current density and enables the material to obtain a higher Faradaic efficiency(94.7%,-0.5 V).The more concise preparation process and the results of the stability test and the application of zinc-CO2 batteries both showed that the improved material had industrial application value.