Design Of Novel Oxygen Bearing Copper Catalyst For Electrocatalytic Carbon Dioxide Reduction Reaction

Electrocatalytic carbon dioxide reduction reaction(CO2RR)can convert greenhouse gases carbon dioxide into valuable chemicals and fuels through renewable electricity,which is crucial for the storage of renewable energy and mitigation of climate change and has attracted a lot of attention.Particularly,the electrocatalytic process can effectively and selectively form high-energy-density multi-carbon oxygenate and hydrocarbon(C2 or C2+)products,which will improve the energy efficiency of CO2RR.For the electrocatalytic CO2RR to produce C2 or C2+products,a very critical intermediate rate-limiting step is the C-C coupling reaction.This intermediate process requires the catalyst surface to be firmly bound with*CO intermediate,so as to establish sufficient*CO coverage to further perform*CO dimerization.The oxygen-bearing copper structure(Cuδ+)is considered to be the key for*CO dimerization.However,the instability of oxygen-bearing copper in the electrocatalytic reduction system raises doubt about its role.Therefore,it is necessary to accurately identify the structure and function of oxygen-bearing copper,and then reasonably design a Cu-based catalyst with high selectivity,good activity and stability.This is of great significance for promoting the development of electrocatalytic CO2RR.In this thesis,oxygen-bearing copper materials are set to be research object.Firstly,an oxygen-bearing copper structure that can stably exist under an electrocatalytic reduction system is prepared for electrocatalytic CO2RR.Secondly,we systematically investigate the catalyst structure by reasonably building in situ and quasi-in situ characterization devices.Combining density functional theory(DFT)calculations,the "structure-activity relationship" in the electrocatalytic CO2 reduction process has been studied in depth.The specific research contents are mainly as follow:Firstly,we designed novel oxygen-bearing copper nanowires with hierarchical pore and grain-boundary structures.The novel oxygen-containing copper catalysts can stably exist in the electrocatalytic reduction system,and exhibit unique catalytic selectivity towards C2H4 under the best negative bias potential without forming CO or CH4.The detailed contents are as follow:(1)The structural evolution of novel oxygen-bearing copper nanowires was studied.Under in situ electrochemical reduction conditions,a stable and novel oxygen-bearing copper structure was successfully synthesized through utilizing CuO nanowires with hierarchical pore and grain-rich boundary structures as precursor.(2)We designed and constructed a series of in situ and quasi-in situ material characterization systems,including in situ Raman,quasi-in situ X-ray photoelectron spectroscopy(XPS)and in situ high-energy X-ray absorption spectroscopy(XAS).Combining in situ and ex situ characterization methods,this novel oxygen-bearing copper structure was studied in depth,which was finally determined to be a metastable sub-oxide copper Cu4O.(3)Through DFT calculations,we confirmed that the surface of the novel oxygen-bearing copper catalyst surface enhanced the adsorption of*CO and the dimerization reaction,thereby promoting the C-C coupling reaction.Thus,we deeply understood the "structure(novel oxygen-bearing copper structure)-activity(CO2RR)relationship" of the system in the process of electrocatalytic CO2 reduction.Secondly,we found that oxygen-bearing copper nanowires with hierarchical pore and grain-boundary structure displayed good stability following continuous CO2 electroreduction,while it showed poor cycling stability.After repeated oxidation and reduction of oxygen-bearing copper,that is,after the oxygen atoms are repeatedly inserted and removed from the copper crystal,the oxygen-bearing copper nanowires are transformed into nanoparticles,which spread on the copper foam substrate.The disappearance of the nanowire structure leads to the disappearance of the oxygen-bearing copper structure,and finally its ability for electrocatalytic CO2 reduction is greatly weakened.This result indicates that the nanowire with hierarchical pore and grain-boundary structure is the key to maintain the new oxygen-bearing copper structure.Finally,based on the above research,the role and characteristics of the "oxygen-bearing copper" structure are confirmed.In order to take advantage of the special structure,we discovered another method that can prepare a more stable and efficient "oxygen-bearing copper" catalyst.That is,we prepared quasi metal-organic frameworks(MOFs)by a"spray" method at room temperature.After the quasi-MOFs crystals have been electrochemically reconstructed,the electrocatalyst with "oxygen-bearing copper"structure was also successfully acquired.In addition,it showed good CO2 adsorption characteristics and excellent electrocatalytic CO2RR performance,with about 85%Faraday efficiency of the hydrocarbon product.Utilizing our pioneering "spray" method,the transition of the prepared samples from quasi-MOFs to complete MOFs crystals can be observed through controlling the reaction time,thus controllable structure is achieved.The surface groups,morphology and electronic structure of the samples were investigated thoroughly by vibrational spectroscopies,X-ray powder diffraction,and electron microscope analysis,which proved that the MOFs is HKUST-1.Moreover,the material characterization results indicate that the quasi-MOFs(QMOF samples)have incomplete[Cu2(COO)4]metal clusters(nodes)and organic coordinating ligands(linkers).In addition,we used in situ(XAS and Raman)and quasi-in situ material characterization methods to study the structural reorganization of quasi-HKUST-1 and pure HKUST-1 crystals during electrochemical process.Unlike high-valent copper was electrochemically reconstructed from pure HKUST-1 structure,it is oxygen-bearing copper specie(Cuδ+)similar to sub-oxide copper structure that was electrochemically reconstructed from quasi-HKUST-1,which enhances the catalyst C2H4 activity and selectivity.