Preparation Of Copper-modified MOF Materials And Their Application To Photoelectrocatalytic CO₂ Reduction

Since the first industrial revolution in the 18^th century,human society has relied on fossil fuels such as coal,oil,and natural gas.The overuse on these non-renewable resources has caused worldwide energy shortage and a large number of CO₂ emissions,leading to uncontrollable climate change and extreme weather events.That’s why we want to look for clean,renewable energy to replace fossil fuels.Reducing CO₂ to chemical fuels can not only solve the problem of energy shortage,but also effectively reduce the CO₂ concentration in the atmosphere and alleviate the greenhouse effect.Among the proposed methods and reaction schemes,photocatalytic and electrocatalytic CO₂ reduction are the most promising means of reducing CO₂ due to their potential scalability and environmental friendliness,while photoelectrocatalytic CO₂ reduction combines the advantages of photocatalysis and electrocatalysis,and is an effective way to achieve CO₂ reduction to chemical fuels.MOFs have the advantages of high porosity and high specific surface area,and can capture CO₂ and catalytic CO₂ reduction.In this thesis,the regular octahedral UiO-66-NH₂ and the regular dodecahedral ZIF-8/67 materials are selected as the basis,by loading copper and doping copper on MOFs to solve the problem of the poor conductivity and high photogenerated carrier composite efficiency with MOFs to improve catalytic performance of CO₂ reduction。（1）UiO-66-NH₂/Cu-x photoelectrocathode materials were prepared and applied to the photoelectrocatalytic reduction of CO₂.First,the regular octahedron UiO-66-NH₂sample was prepared by solvothermal method method,then exchanged with copper ion by impregnation,and finally calcined to obtain UiO-66-NH₂/Cu-x（x used to distinguish different Cu contents）.The morphology and valence composition of the catalyst were characterized by SEM,TEM,XRD and XPS,and we obtained the information of valence state（0 valence）and the size（2 nm）of the copper nanoparticles.The optical-electrical properties were evaluated by UV-vis,PL,LSV,EIS and transient photocurrent responses.The characteristic absorption peaks in UV-vis can be ascribed to the surface plasmon resonance（SPR）effect of Cu nanoparticles,thereby promoting the photogenerated charge separation efficiency.Meanwhile,loaded of the metallic element Cu significantly reducing the starting potential of the reaction.Finally,under the Xenon lamp irradiation and the external bias voltage,the photoelectrocatalytic CO₂ reduction experiment was carried out in the three-electrode system.Experimental results show that,the UiO-66-NH₂/Cu-2 photoelectrocathode exhibits the best catalytic activity,under-0.8 V（vs.SCE）,the products of CO₂ reduction were converted from methanol to C₂₊products,and the apparent faraday efficiency is 4.6 times higher than that of the UiO-66-NH₂.（2）X-Zn O/Co₃O₄@CuO_x photoelectrocathode materials were prepared and applied to the photoelectrocatalytic reduction of CO₂.First,Cu（OH）₂nanowires were grown on the surface of copper foam by chemical method,and based on this,ZIF-8/67of different sizes were in situ grown with Cu（OH）₂ nanowires as a template,and then calcined at 400°C to obtain dendritic X-Zn O/Co₃O₄@CuO_x heterojunction catalytic materials（X be written as S,M and L stands for small,middle,and large respectively,CuO_xrepresenting oxide of copper,0.53O₄@CuO_x improved the stability relative to the CuO_x@Cu photoelectrocathode,and improved the optical response ability and the separation efficiency of the photogenerated carriers relative to the Zn O/Co₃O₄@Cu photelectrocathode.Photoelectrocatalytic CO₂ reduction experiments in the three-electrode system also exhibited that L-Zn O/Co₃O₄@CuO_xphotoelectrocathode materials have the best performance in CO₂ reduction,and the production rate reached 2 times than that of CuO_x@Cu,and the electron transfer rate reached 268μMh^-1cm^-1 under a bias of-0.8 V（vs.SCE）.