| Since the Industrial Revolution,the rapid development of the global economy has also consumed a large number of fossil fuels such as coal,oil and natural gas,which human beings rely on for survival.As a result,a large number of greenhouse gases such as carbon dioxide and methane were discharged into the atmosphere,which triggered global warming,sea level rise,extreme weather and other environmental disasters,seriously threatening human survival.At present,there are two strategies to solve this problem:one is to reduce carbon emissions from the source by vigorously developing solar energy,wind energy,geothermal energy,hydrogen energy and other renewable energy and promoting the transformation of energy structure from fossil fuels to clean energy;the other is to vigorously develop carbon recovery,carbon conversion and carbon utilization technologies to convert greenhouse gases into high-value-added chemicals.Electrocatalytic carbon dioxide reduction(CO2RR)and carbon monoxide reduction(CORR)are the promising carbon utilization technologies,which can convert carbon dioxide and carbon monoxide into high value-added carbon-based fuels at room temperature and pressure without causing additional carbon emissions.However,due to the high structural stability and chemical inertness of CO2 and CO molecules,and their low solubility and diffusivity in aqueous solution,CO2 and CO supply near the catalytic sites in CO2RR and CORR processes is usually insufficient.This brings a series of problems such as low reaction rate,poor product selectivity and operation stability,seriously limiting the industrial application of CO2RR and CORR.The reaction rate and product selectivity can be significantly improved by enriching CO2 and CO around the catalytic sites through the regulation of catalysts.However,how to achieve the enrichment of reactants and intermediates through the structural regulation of catalysts at the nanoscale is still a core scientific problem to be solved.In this thesis,a series of Cu-based catalysts were reasonably designed and controllably synthesized to achieve the enrichment of CO2 and CO reactants and carbon-based intermediates at the nanometer scale,and the catalytic performance and reaction mechanism of the catalyst were also deeply studied.In view of the poor activity and selectivity of Cu-based catalysts,a series of highly efficient Cu-based electrocatalysts were designed and created under the guidance of theoretical calculation and simulation.At the same time,the catalytic reaction mechanism was further explored by means of high-energy X-ray spectroscopy,in situ Raman and in situ ATR-SEIRAS.The main research results obtained in this thesis are as follows:1.A multihollow cuprous oxide(Cu2O)catalyst with confinement effect for efficient conversion of CO2RR to multicarbon products was developed.A series of cuprous oxide catalysts with different morphologies(multihollow,solid and fragment)were designed and synthesized.The multihollow Cu2O could confine the CO2 and C1 intermediates during the CO2RR process,thus significantly increasing the local concentration of CO2 and C1 intermediates.Moreover,the confined C1 intermediates could adsorb on the surface of the Cu2O cavities,which retarded the reduction of Cu+during CO2RR process,eventually leading to the efficient conversion of CO2 to multicarbon(C2+)products.The experimental results showed that in a flow cell,the multihollow Cu2O exhibited the maximum C2+Faraday efficiency of 75.2%at-0.61V vs.RHE in 2M KOH,with the C2+current density of 267 mA cm-2.The catalyst also exhibited a large C2+-to-C1 ratio of 7.2,which was about 9-fold and 8-fold higher than that of solid and fragmental Cu2O,respectively.The finite element simulation showed that the selectivity ratio of C2/C1 of multihollow Cu2O was 6.4,which was 8-fold and 7-fold higher than that of solid and fragmental Cu2O,respectively,in good agreement with the experimental results.In situ Raman and synchrotron radiation X-ray absorption showed the calculated Cu+species in multihollow Cu2O was up to 32.1%even after CO2RR for 20 min at-0.61 V vs.RHE in a flow cell,while the solid and fragmental Cu2O were completely transformed into metallic Cu only after CO2RR for 1min.This work provides a useful reference for regulating catalyst morphologies to achieve the enrichment of reactants and intermediates to improve the catalytic activity and selectivity.2.A dua-phase Cu catalyst with enriching chlorine ability was developed for neutral CO2RR to C2+products.A series of Cu-based catalysts with different crystallographic form were designed and prepared,including amorphous Cu oxide catalyst,a dual-phase Cu catalyst comprised of amorphous Cu oxide matrix and Cu crystalline particles,and a crystalline Cu particle catalyst.Electron energy loss spectroscopy and theoretical calculation showed that Cu particles would transfer electrons to amorphous Cu oxide matrix and thus form Cu+ at the interface.The strain analysis showed that Cu particles was subjected to compressive strain at the interface due to lattice mismatch with amorphous copper oxides.The compressive strain could restrict lattice oxygen loss and thus stabilize Cu+during CO2RR process.In situ Raman and theoretical calculation showed that Cu+could combine with Cl-in the electrolyte and thus enrich Cl-at the interface.The presence of Cl-could enhance the adsorption of*CO intermediates on Cu surface and reduce the energy barrier of C-C coupling,thus promoting the conversion of CO2 to C2+products.The CO2RR test in a flow cell showed that dual-phase Cu catalyst could deliver a maximum C2+Faraday efficiency of 81%with the partial current density of 322 mA cm-2 under the total current density of 400 mA cm-2 in 3 M KCl,of which the Faraday efficiency was 10.8 times and 2.0 times higher than that of amorphous Cu oxide matrix and crystalline Cu particles,respectively.Moreover,at the total current density of 600 mA cm-2,the C2+current density of dualphase Cu could reach 400 mA cm-2,which was 29.7 times and 2.2 times higher than that of amorphous Cu oxide matrix and crystalline Cu particles,respectively.The strategy of enriching electrolyte ions by regulating the crystallographic form of catalyst provides a new idea for developing highly active catalysts.3.Cuprous oxide polyhedral crystals with selective growth of gold particles was developed for efficient conversion of carbon monoxide to acetic acid.The Cu2O decahedral catalysts exposed(111)and(100)crystal planes were successfully prepared,and the Au particles were successfully deposited on the exposed(111)and(100)crystal planes,respectively,thus obtaining three kind of Cu2O catalyst precursors.In the CORR process,the Cu2O decahedron crystals could be converted in situ into pure Cu decahedron crystals with exposed Cu(100)and Cu(111),and the Au particles still remained deposited on the Cu(100)and Cu(111),respectively.In situ Raman and theoretical calculations showed that the growth of Au on the Cu(100)would prevent the adsorption of CO on the Cu(100)crystal plane and promote the enrichment of CO on Cu(111).As Cu(111)crystal surface is conducive to*CObridge adsorption,*CObridge and*COatop could coexist on the surface of Cu(111),leading to the asymmetric coupling of COatop and CObrigde.Such asymmetric C-C coupling could reduce the energy barrier of the key intermediate ketene*CH2CO formation,thereby promoting the large conversion of CO to acetic acid.The CORR test in a flow cell showed that for the catalyst of Au deposited on the Cu(100),the Faraday efficiency of acetic acid could reach 60.5%at the total current density of 300 mA cm-2,which was 1.4 times and 2.3 times higher than that of pure Cu and Au deposited on the Cu(111),respectively.This strategy of selective enrichment of carbon-based intermediates by modifying the crystal surface of catalyst opens up a new idea for developing highly selective catalysts. |
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