Design And Preparation Of Transition Metal Catalysts And Their Performances In Electrocatalytic Reduction Of CO₂

Carbon dioxide(CO2)is a major industrial waste gas,and it is important to capture and reuse it.Electrocatalytic CO2 reduction reaction(CO2RR)is an important reaction,which can reduce CO2 to a series of useful chemicals.The process uses renewable clean energy as the source of electrical energy to drive a CO2 electrolysis cell to convert CO2 into various carbon-containing products,among which production of carbon monoxide(CO)is one of the most promising reduction products for CO2RR due to its high technical and economic feasibility.However,because CO2 is difficult to be activated at room temperature and limited by thermodynamic and kinetic factors,this system urgently needs to design efficient catalysts to improve the reaction activity.In addition,the selectivity of the prepared catalyst materials should also be considered,considering that the hydrogen precipitation reaction(HER)is the main competing reaction in the CO2RR system where aqueous solution is the electrolyte.Therefore,the suppression of HER while enhancing the catalytic activity of CO2RR is an important strategy to improve the selectivity of CO products.Transition metal-based catalysts have been widely investigated for their efficient catalytic performance.In this thesis,a series of catalysts designed around transition metals were prepared and used in the reaction of electrocatalytic CO2 reduction,and an attempt was made to explain the catalytic mechanism of action by systematically investigating the conformational relationships of the catalysts.The main findings include:(1)Ni-loaded precursors of Cl-doped metal triazole salt skeletal catalysts were prepared,and Ni/(Cl-N-C)-2 catalysts were produced by pyrolysis for the electrocatalytic reduction of CO2 in 0.1 mol/L KCl electrolyte.The characterization results showed that the metal Ni and heteroatomic chloride were uniformly dispersed on the NC-based carrier,and the formation of covalent C-Cl bonds induced the redistribution of electrons in this bond and regulates the electronegativity of the active site of the catalyst.As a result,the adsorption and activation capacity of CO2 was greatly enhanced,exhibiting excellent CO2RR activity.The Cl-modified Ni-based catalyst,i.e.,Ni/(Cl-NC)-2,achieves a high Faraday efficiency of CO(FEco)of 98%at-0.9 V,with a Faraday efficiency much higher than that of the catalyst Ni/(NC)without Cl doping.The high FEco of 90%can be maintained over a wide voltage range(-0.7～-1.1 V(vs.RHE)),and stability tests up to 20 h show no significant decay of the FEco,demonstrating its good stability.(2)Zn-doped chalcogenide Sr2Ti0.95Zn0.05On was synthesized using the solgel route,and the catalyst was able to synthesize a certain percentage of syngas.The catalyst characterization results showed that a small amount of Zn doping did not affect the crystal structure of the chalcogenide,and the doping of Zn was the main active site for the activation of CO2 to generate CO from the chalcogenide.In addition,the doping of moderate amount of Zn increased the concentration of O2-/O-species and oxygen vacancies on the catalyst surface and improved the charge carrier transport ability,which was beneficial to improve the electrocatalytic performance of the catalyst.Among them,when the doping amount of Zn reaches x=0.05,the total Faraday efficiency of CO and H2 is high up to 96%,and the total FE is maintained above 90%in the voltage range of-0.8 to-1.2 V(vs.RHE).The catalyst can maintain a high stability over a long period of time.(3)Precursors were prepared by loading divalent copper acetate coordinated 1,10-phenanthroline molecules onto nitrogen-doped carbon ZIF-8 MOF substrates with abundant carbon defects,and the copper/nitrogen-doped carbon catalysts(Cu(o-P)/NC)were obtained after pyrolysis and applied to the electrocatalytic reduction of CO2 reaction.The maximum FECO of the Cu(oP)/NC catalyst reached 98%at-0.3 V(vs.RHE).The stability results showed that the catalyst maintained more than 90%Faraday efficiency after 22 h of reaction.The catalyst exhibited excellent electrocatalytic CO2 activity and stability at low potentials.The results of structural characterization and electrochemical performance tests showed that the high catalytic performance of Cu(o-P)/NC was mainly attributed to the following three aspects:firstly,the 1,10-phenanthroline molecules with divalent copper acetate coordination avoided Cu agglomeration,and the immobilized 1,10-phenanthroline copper molecules were able to catalyze CO2 reduction as effective active sites;secondly,the direct charge transfer between the 1,10-phenanthroline copper molecules and the nitrogen-doped carbon substrate was promoted,and the direct charge transfer between the 1,10-phenanthroline copper molecules and the nitrogen-doped carbon substrate was improved.This study provides a new approach for the preparation of highly dispersive catalysts and the application of limiting currents in the electrocatalytic reduction of CO2.