Synthesis Of Copper-based Catalysts And Research On Electrocatalytic CO₂ Reduction

The rapid combustion of fossil fuels and increased CO₂ emissions have caused serious environmental problems,such as global warming and rising sea levels.Electrocatalytic reduction is an effective way to convert carbon dioxide to hydrocarbon fuels and chemical products with high value,as well as solve environmental pollution problems.It has pointed out a new direction for mankind to solve energy and environmental problems,and has become a new energy sustainable development strategy.In recent years,copper-based catalysts have received widespread attention due to their low cost,wide distribution on the earth,and the ability to reduce CO₂ to a variety of hydrocarbon products and alcohols.However,the low Faraday efficiency of CO₂ conversion,poor product selectivity,high overpotential,and catalyst deactivation are the main obtacles that limit the application of copper-based catalysts.In this thesis,we designed and synthesized several copper-nitrogen-carbon materials in order to solve the problems above mentioned.1、Synthesis of Cu-L（L=N,O）coordination polymer for electrocatalytic CO₂reductionLow selectivity is one of the greatest challenges on electrocatalytic CO₂ reduction reaction using copper-based catalyst in water solution.Herein,we report a polymeric Cu-L（L=pyridinic N and carbonyl group）complex core-shell microsphere catalyst（p-Cu L）synthesized by a facile soild-phase reaction of urea and Cu-urea complex.FE-SEM and TEM graphs disclose that inner porous microsphere core attached by a surface thin layer.UV-Vis spectrum presented strong ultraviolet absorption stemming from metal-ligand charge transfer and well-separated unsymmetric d-d transition bands in visible light region due to copper ions binding on sites with low symmetry.Surface and in-depth XPS analyse and Auger electron spectrum suggest copper-ligand coordiantion with mixed oxidation state previeled on surface shell layer,while Cu（I）-pyridine dominated in core with porous framework.The p-Cu L core-shell microspheres show maximal Farady efficient（～74%）on selective reduction of CO₂ to acetate in 0.5M KHCO₃ electrolyte at low applied potential（-0.37V vs.RHE）,without any detectable gas products besides H₂.Ex-situ XAS spectra probe that Cu（II）-carbonyl coordination is relatively stable in electrocatalytic CO₂RR and Cu（I）-N coordination moities are probably the mian active sites to reduce CO₂ to acetate,as well as indirectly prove that the unique porous core-shell structure is favorable for*CO adsorption,dimerization and further reduction to acetate due to distinctly increased O K-edge peaks intensity of C=O and-OH groups on catalyst after CO₂RR.2、Preparation of Cu/CN catalyst and its performance on electrocatalytic CO₂ reduction.The Cu/CN composite catalyst material was successfully prepared by the one-step high-temperature calcination method.FE-SEM,TEM and XPS results showed that the resultant Cu/CN with fluffy porous structure consisting of Cu（0）and CN heterocycles,moreover,the content of pyridinic N was decreased with the enlongated reaction time.At CO₂ saturated 0.5M KHCO₃ solution,the Cu/CN showed large cathodic current and excellent activity on CO₂ reduction,at applied potential of-0.57V vs RHE,CO₂ was highly selective reduction into ethanol and the Faraday efficiency reached 91.5%without other detected products from CO₂ reduction.In addition,it was found that the Faraday efficiency of ethanol gradually decreased with the reduction of the content of pyridinic N in Cu/CN catalyst,which indicated that pyridinic N may be the important site of generating CO*intermediate,and the synergy of pyridinic N and copper active sites resulted in two-point mechanism to highly selective CO₂ reduction to ethanol.3、Preparation of Cu₂O@ppy composite nanowires and application in electrocatalytic reduction of CO₂.A composite Cu₂O and polypyrrole（Cu₂O@ppy）was prepared by one-step hydrothermal method with pyrrole and copper acetate.FE-SEM with EDS and TEM results indicated that polypyrrole acted as morphology controlling agent and palyed an important role in inducing the the formation Cu₂O nanowires through the interaction of pyrrolic N and copper ions.At CO₂ saturated 0.5M KHCO₃ solution,the Cu₂O@ppy showed large cathode current,excellent activity and high selectivity on CO₂ reduction.At applied potential of-0.47V vs RHE,the Faraday efficiency of ethanol reached 88%.During the 9-hour electrolysis process,the cathodic current and Faraday efficiency of Cu₂O@ppy did not significantly decreased,showing good electrocatalytic stability.