Investigation On Regulation Of Interface Structure Of CuO-CeO₂ Catalyst And Catalytic Performance For Selective Oxidation Of CO

With the severe problem of environmental pollution and energy shortage,the development and utilization of new energy has become an urgent problem in today 's society.Hydrogen,an emerging secondary energy source,is well suited to complement the renewable energy.As an efficient energy conversion device,proton exchange membrane fuel cell(PEMFC)is the most important application of hydrogen energy.Hydrogen-rich streams produced by on-line reforming of hydrocarbons is primary fuel source of PEMFCs,and CO purification of the H2-rich streams is of vital importance.With excellent catalytic performance and low cost,CuO-CeO2 catalyst is the most ideal catalyst for CO selective oxidation(CO-PROX)in hydrogen-rich gas.However,the present CuO-CeO2 catalysts still cannot fully meet the reaction temperature requirements in PEMFC feed gas purification applications,and its resistance to CO2 and H2O is poor,which proposed higher requirements for the development of efficient CuO-CeO2 catalysts.The fundamental reason for the high activity of CuO-CeO2 catalyst is the strong synergetic interaction between copper and cerium.Therefore,interface structure regulation is of great importance to develop CuO-CeO2 catalyst with higher performance.The cubic fluorite-structured CeO2nanocrystal mainly has three low-index crystal planes:{111},{110}? and {100},whose crystal plane stability and defect formation energy are different.The exposed crystal plane of CeO2 determines the surface composition and surface structure of the crystal,and affects the concentration and structure of oxygen vacancy.Therefore,controlling the morphology of CeO2supports with specific crystal faces to regulate its surface properties and the electronic structure of active site,has drawn more and more attention in the design and application of CuO-CeO2 catalysts for CO-PROX.In the present dissertation,we used X-ray powder diffraction(XRD)and its Rietveld Refinement,N2 adsorption-desorption,X-ray photoelectron spectroscopy(XPS),High resolution transmission electron microscope(HRTEM),UV-Raman spectra,Static/dynamic oxygen storage capacity(OSC),H2 temperature-programmed reduction(H2-TPR),In situ diffuse reflection Fourier transform infrared spectroscopy(In situ DRIFTs)and catalytic activity test techniques to systematically study the effect of the morphology of ceria,the microstructure of ceria nanorods,the copper coverage and the transition metals doping on the interface structure and CO selective oxidation performance of CuO/CeO2 catalysts.In addition,we developed a new precipitation method to synthesize mixed CuxCe-XO2 nanorod catalysts,regulated the interfacial copper-ceria interaction by changing preparation conditions and analyzed the types and migration of the copper species in the catalyst.Some specific conclusions can be drawn as follows1.The morphology of CeO2 determines its exposed crystal plane.CeO2-rod mainly exposes {111} and {200} crystal planes and grows along[110]direction.CeO2-cube exposes{200}crystal plane,and CeO2-plate exposes{111?crystal plane.On one hand,the exposed crystal plane of CeO2 affects the interfacial copper-ceria interaction in CuO/CeO2 catalyst and its catalytic activity in CO selective oxidation reaction.The{111} crystal plane with high concentration of oxygen vacancy,facilitates the strong interaction between the highly dispersed copper oxide and ceria support at the interface,thus promotes the low-temperature activity of CuO/CeO2-rod and CuO/CeO2-plate catalysts in CO-PROX.The {200} crystal plane with lower concentration of oxygen vacancy is against to anchor copper oxide species and generate strong copper-ceria interaction,resulting in poor low-temperature activity of CuO/CeO2-cube catalyst.On the other hand,the exposed crystal plane of CeO2 can change the coordination structure of copper species on the surface of CuO/CeO2 catalyst,which affects the stability of copper species under the reaction atmosphere and the selectivity of O2 for CO oxidation.Copper oxide dispersed on the{200}crystal plane is in a symmetrical eight-coordinate structure,which is beneficial to stabilizing the active Cu+species.Therefore,copper species on CuO/CeO2-cube and CuO/CeO2-rod catalysts with more{200}crystal faces exposed are difficult to be completely reduced to Cu0,which delaying the production of active sites(Cu0)of HZ oxidation,resulting in their higher O2 selectivity for CO oxidation in CO-PROX.2.The microstructure of CeO2 nanorods and the surface copper coverage have an important influence on the interfacial copper-ceria interaction of CuO/CeO2(rod)catalyst and its catalytic performance in CO selective oxidation.On one hand,changing the hydrothermal synthesis time can regulate the microstructure parameters of CeO2(rod)(such as pore structure,twin fault probability,oxygen vacancy concentration,etc.).With the prolongation of hydrothermal synthesis time,the cubic grains of ceria are oriented by overlapping along specific crystal faces,follows with Ostwald ripening to complete the crystal growth process,which leads to the size and length of ceria nanorods simultaneously increased.The prolongation of hydrothermal synthesis time favors the formation of more oxygen vacancies on the surface of CeO2(rod),and promotes the generation of Cu-[OX]-Ce species and the strong interaction between the highly dispersed CuOx species and CeO2(rod)support,which enhances the catalytic performance of CuO/CeO2(rod)catalyst.On the other hand,moderate increase of surface copper coverage results in the increased amount of highly dispersed CuO strongly interacting with CeO2(rod),and greatly promotes the catalytic performance of CuO/CeO2(rod)for CO-PROX.However,the over high copper content would not only lead to the formation of crystalline phase CuO,but also cause the agglomeration of highly dispersed copper oxide species with increased particle size,which weakens the interfacial interaction between copper and ceria and leads to the decline of their catalytic performance for CO-PROX.3.Transition metal dopants remarkably change the microstructure parameters of CeO2 nanorods and significantly affect the interfacial copper-ceria interaction of CuO/CeO2(rod)catalysts.Rietveld refinement analysis results indicate that doping Mn and Ti obviously increases the microstrain and defect probability of the high energy crystal plane in CeM(rod),and promotes the formation of oxygen vacancies.Therefore,Mn and Ti dopants facilitate the high dispersion of copper species on the surface of CeM(rod)and cause more copper ions to enter the surface ceria lattice,thus promote the strong interaction between Cu-Ce-Mn/Ti and the generation of Cu-[Ox]-Ce species in CuO/CeM(rod)catalysts,which greatly enhances the catalytic performance of CuO/CeO2(rod)catalyst in CO-PROX.The doped Zr4+could enter the CeO2 lattice and replace Ce4+to form the uniform Ce-Zr solid solution,which increase the microstrain,growth deformation probability,and bulk oxygen vacancy concentration of CeZr(rod).However,since the substitution of Zr4+for Cer.site in ceria lattice hinders the contact of copper and ceria species,the doping of Zr could not improve the interfacial copper-ceria interaction in CuO/CeZr(rod)catalyst,which basically maintained the same catalytic performance as CuO/CeO2(rod)creduces atalyst.The introduction of Ni2+increases the basic grain size of CeNi(rod)and the microstrain,and leads to the formation of crystalline phase NiO,which is aginst to the dispersion of copper oxide in CuO/CeNi(rod)catalyst and weakens the interfacial copper-ceria interaction and results in its worst catalytic performance in CO-PROX4.The composite CuXCe1-xO2 catalyst with uniform nanorod structure was successfully synthesized by low-temperature coprecipitation method.The existence of copper species in CuCe(rod)catalyst is in the form of highly dispersed CuOx clusters,strongly bound Cu-[Ox]-Ce species,and bulk phase CuO particles.The highly dispersed CuO,species on CuCe(rod)catalyst determines its low-temperature CO oxidation activity,while Cu-[Ox]-Ce species at the copper-ceria interface determines the high-temperature CO oxidation activity of CuCe(rod)catalyst,especially Cu-[Ox]-Ce in the bulk phase.Appropriate copper doping increases the highly dispersed CuOx species on the catalyst surface,and enhances the copper-ceria interaction of CuCe(rod)catalyst,which significantly improve its catalytic performance in CO-PROX.The most optimal doping amount of copper in CuCe(rod)catalyst is 7-11%,whose corresponding T50%values are 66 ? and 63 ? and their temperature windows are 100-130 ? and 90-120?,respectively.With the increase of calcination temperature,copper species in the bulk of CuCe(rod)catalyst gradually migrate to the surface with improved redox property,which significantly enhances the low-temperature CO oxidation activity.Excessive calcination temperature leads to the precipitation of copper species on the surface of the catalyst,weakens the interfacial copper-ceria interaction and has a negative impact on its high-temperature CO oxidation activity of in CO-PROX.