Controllable Synthesis Of Low-Dimensional Cu-based Catalysts And Their Application In Carbon Dioxide Photo/Electroreduction

Since the Industrial Revolution in the 19th century,the increasing CO2 concentration in the atmosphere has caused increasingly severe environmental problems and energy crisis.It is urgent to find a sustainable development road for substitution.Photo/electro catalytic CO2 reduction into high value-added industrial chemicals with renewable energy is one of the most promising ways to close the anthropogenic carbon cycle.In recent years,Cu-based catalysts have attracted extensive interest due to their unique catalytic performance in CO2 reduction.Unfortunately,due to the lack of a thorough understanding on the structure of active sites and the mechanism of the CO2 reaction,the CO2 reduction activity and selectivity of Cu-based catalysts are still far from industrial expectation.Therefore,in this paper,the electronic structure of the active centers and their chemical environment were regulated by a variety of means.Besides,in-situ evolution process of the catalytic sites and their effects on CO2 reduction were discussed by combining in-situ characterization and theoretical calculations,which would provide the direction for the design of Cubased highly active sites for CO2 reduction.The main research content of this paper is as follows:1.Cu-MOF nanosheets with reversible CuⅡ/CuⅠ sites were constructed:In order to achieve high activity and selectivity of photocatalytic CO2 reduction of CO,Cu-MOF nanosheets with a large number of well-defined monodisperse Cu2+ sites were designed and synthesized.Quasi-in-situ synchrotron X-ray photoelectron spectroscopy revealed that reversible valence conversion between CuⅡ/CuⅠ occured in Cu-MOF nanosheets during CO2 photoreduction,where Cu2+ underwent the process of receiving electrons to form Cu+,transferring electrons to CO2,and finally restoring the original Cu2+.In-situ Fourier transform infrared spectroscopy and Gibbs free energy calculations confirmed that in-situ Cu+sites promoted charge transfer during CO2 photocatalysis and greatly reduced the energy barrier of CO2 activation forming CO2δ-,which was the rate-determoine step.As a result,Cu-MOF nanosheets exhibited remarkable photocatalytic performances.Without any photosensitizer and sacrificant,the CO selectivity on the MOF nanosheets was up to nearly 100%,and the CO yield reached as high as 860 μmol g-1 h-1,which was significantly better than the state-of-art performance of CO2 photocatalysts.In addition,the CO formation rate of Cu-MOF nanosheets also reached 830 μmol g-1 h-1 within the solar radiation,showing great potential in practical application.2.A surface modification strategy based on anion-exchange ionomer(AEI)was proposed to achieve industrial-current-density CO2-to-C2+ electroreduction:In order to achieve efficient CO2 electroreduction to multicarbonproducts,a catalyst surface modification strategy based on anion-exchange ionomer was developed to construct a local microenvironment conducive to the generation of multicarbon products.Using oxide-derived copper nanosheets as model catalysts,the results of in-situ spectroscopy characterization and theoretical calculation confirmed that the modification of anion exchange ionomer obviouslty improved the local pH values near the catalyst surface,and greatly reduced the kinetic energy barrier of the rate-determine step in C2+formation,which was*COCO to*COCOH.In addition,by optimizing swelling properties of the ionomer,the catalyst layer could achieve the reasonable water content of 3.5%,which helped to establish the gas-liquid equilibrium on the catalyst surface,inhibiting the hydrogen evolution reaction and improving the overall activity of multicarbon products by facilitating proton coupled electron transfer steps.In a customized local microenvironment,oxide-derived copper nanosheets achieved Faraday efficiencies of more than 85%for multi-carbon products at 800 mA cm-2,with ethylene reaching 65%and half cell energy conversion efficiencies of more than 50%.This work revealed the regulation effect of ionomer on local environment and had important significance in rational design of local environment.3.CeO2 containing Cuδ+ sites under CO2 reduction conditions was constructed to achieve selective generation of hydrocarbon products:In order to achieve selective CO2 electroreduction to CH4 under the industrial current density,the strong metal-support interaction was used to construct the Cuδ+ sites under electroreduction conditions.Taking Cu/CeO2 as an example,the exposed crystal surface of CeO2 with different morphologies and the monodisperse Cu2+sites on the surface of CeO2 were confirmed by high Angle dark field scanning transmission electron microscopy and synchrotron radiation X-ray absorption fine structure spectroscopy.It was found that different crystal faces of CeO2 exhibited different O defect formation energies,leading to different metal-support interaction with Cu,which would affect its electronic structure.The CeO2(111)was beneficial to improve the anti-reducing property of Cuδ+.In-situ Raman spectroscopy revealed CuOx/(OH)x species on CeO2(111)during electrocatalysis,confirming the existence of stable Cuδ+ sites.In-situ Raman spectroscopy and in-situ attenuated total reflection surface enhanced infrared absorption spectroscopy observed the key intermediates in CH4 formation such as CO*,CHO*,CH2O*,etc.Theoritical calculation results confirmed that the electron-deficient Cuδ+ sites contributed to reducing the activation energy barrier from CO*to CHO*,which accelerated the formation of CH4.Finally,the monodisperse Cuδ+ sites on CeO2(111)achieved the CH4 Faraday efficiency of up to 77.2%and the industrial current density of 450 mA cm-2,which was significantly higher than the performance of the atomic Cu catalyst supported on CeO2(100).This work revealed the substantial effects of metal-support interaction on the performance of CO2 reduction,which was of great significance for the design of high performance supported catalysts for CO2 reduction.