Investigation On The Preparation Of Composite CuO_x-CeO₂ Nanorod Catalyst And The Catalytic Performance For Preferential Oxidation Of CO

The large-scale extraction and utilization of fossil fuels has caused increasingly severe problem of environmental pollution and energy shortage,and the development of clean and high-efficient new energy source is of great significance to the sustainable development of human society.Hydrogen energy has the potential to become an alternative to fossil fuels due to its advantages of high energy density,extensive availability and non-pollution.Proton exchange membrane fuel cell（PEMFC）is one of the most effective ways for hydrogen energy utilization,and is also the optimal power source for the future fuel cell vehicles.While high purification of the hydrogen produced from hydrocarbon reforming through preferential oxidation of CO（CO-PROX）,to avoid the poisoning of Pt anode by trace CO,is an important part of the PEMFC application technology.CuO_x-CeO₂ based catalysts are the most extensively investigated catalysts for CO-PROX by virtue of the compromise between catalytic performance and cost.Nevertheless,the operation temperature window of the present CuO_x-CeO₂ catalysts for CO-PROX is not yet able to completely meet the demand of PEMFC application,and CO₂ and H₂O existed in realistic reaction system can also lead to further catalytic performance degradation,thus developing CuO_x-CeO₂ catalysts with superior catalytic performance remains the target for researchers.The high catalytic performance of CuO_x-CeO₂ catalysts mainly originates from the strong Cu-Ce synergistic effect by electron transfer,and modulating the structure of catalysts via appropriate methods,especially the surface/interface structure,can optimize the distribution status and redox properties of the reactive Cu species,and thereby promote the catalytic performance.Investigating the Cu-Ce interaction regime,clarifying the existing form of reactive Cu species as well as establishing the structure-performance correlation are key factors for the design and application of high-performance CuO_x-CeO₂ catalysts.In the present dissertation,a method involving high temperature thermal treatment of composite CuO_x-CeO₂ nanorod precursor to induce outward migration of Cu species is put forward,and high-performance CuO_x-CeO₂ catalysts with large amounts of surface active Cu species,abundant Cu-Ce interfaces and specific crystal planes exposed are constructed.By employing multiple techniques including high resolution transmission electron microscope（HRTEM）,X-ray diffraction（XRD）and Rietveld refinement,N₂ adsorption-desorption isotherm,X-ray photoelectron spectroscopy（XPS）,UV-Raman spectroscopy,temperature-programmed reduction by H₂/CO（H₂/CO-TPR）,temperature-programmed desorption of H₂/CO₂（H₂/CO₂-TPD）and in situ diffuse reflectance infrared Fourier transform spectroscopy（DRIFTs）,the impacts of thermal treatment temperature,Cu content and CeO₂ coating on the existence form and distribution status of Cu species in the catalysts are systematically investigated,and the correlation between Cu-Ce interaction,properties of reactive Cu species and catalytic performance is in-depth discussed.Moreover,Co₃O₄ is introduced to the CuO_x-CeO₂ catalysts,and the possibility to eliminate CO by combined CO-PROX and CO methanation reaction is suggested.The main insights and conclusions are as follows:（1）Composite CuO_x-CeO₂ nanorod precursors with uniform structure are synthesized via one-pot coprecipitation method at ambient temperature,and high-performance CuO_x-CeO₂ catalysts with the advantages of large amounts of surface active Cu species,abundant Cu-Ce interfaces and specific crystal planes exposed are constructed by high temperature thermal treatment induced migration of Cu species.For instance,the 7Cu Ce-R-700 catalyst exhibits the T_50%value of merely 62℃ and the operation temperature window of 85-140℃ for CO-PROX reaction.It is indicated that in the CuO_x-CeO₂ catalysts,Cu⁺is the main active sites for CO adsorption and oxidation,while Cu⁰ is responsible for the side reaction of H₂ oxidation.The catalytic performance of CuO_x-CeO₂ catalysts for CO-PROX is closely related with the amount and redox properties of surface highly dispersed CuO_x species and strongly bonded Cu-[O_x]-Ce species.The amount of surface highly dispersed CuO_x species and its difficulty to be reduced from Cu²⁺to Cu⁺decides the low temperature catalytic performance,while the amount of strongly bonded Cu-[O_x]-Ce species and its difficulty to be reduced from Cu⁺to Cu⁰ determines the high temperature catalytic performance.Through in-depth discussion of the correlation between Cu-Ce interaction,properties of reactive Cu species and catalytic performance for CO-PROX,it is clarified that Cu-Ce interaction impacted the catalytic performance mainly by affecting the amount and redox properties of the reactive Cu species,especially the strongly bonded Cu-[O_x]-Ce species.Strengthening Cu-Ce interaction can facilitate the formation of large amounts of Cu-[O_x]-Ce species,which are easy to be reoxidized in the reduction-oxidation cycles,and inhibit the adsorption and dissociation of H₂ even after being reduced to Cu⁰,thus promotes the O₂ selectivity toward CO oxidation and improves the high temperature catalytic performance.In addition,since CO₂ and H₂O in the reaction stream impacts the catalytic performance mainly by competitive adsorption,it is easy for surface highly dispersed CuO_x species to be occupied.Therefore,the presence of large amounts of strongly bonded Cu-[O_x]-Ce species also contributes to the durability to CO₂ and H₂O for the catalysts.（2）Thermal treatment at 400℃ leads to the formation of Cu-Ce solid solution in the composite CuO_x-CeO₂ nanorod catalysts,while higher thermal treatment temperature results in the migration of Cu species from bulk to the surface and subsurface of CuO_x-CeO₂ nanorods,and enrich in the surface lattice relaxation layer of CeO₂.On one hand,the thermal induced Cu migration phenomenon increases the amount of surface highly dispersed CuO_x species,but slightly decreases the amount of strongly bonded Cu-[O_x]-Ce species.On the other hand,it reduces the thickness of CeO₂ surface lattice relaxation layer but increases its tightness,thus strengthens the Cu-Ce interaction.Therefore,the raising of thermal treatment temperature from 400 to 700^oC promotes the low temperature catalytic performance of the CuO_x-CeO₂ catalysts,but slightly degrades the high temperature catalytic performance.Moderate increment in Cu content（<5%）increases the amount of surface highly dispersed CuO_x species,and strengthens the interaction between this CuO_x species and CeO₂ as well,which increases the concentration of oxygen vacancies and facilitates the electron transfer between the reversible Cu⁺/Cu²⁺and Ce³⁺/Ce⁴⁺redox couples,thus promotes the reducibility of CuO_x species.The improvement in the amount and reducibility of surface highly dispersed CuO_x species increases the low temperature catalytic performance,but the improvement in the reducibility of strongly bonded Cu-[O_x]-Ce species decreases the high temperature catalytic performance.Further increment in Cu content causes the agglomeration of CuO_x species,which weakens the interaction between CuO_x species and CeO₂ as well as reduces its reducibility,but increases the amount of strongly bonded Cu-[O_x]-Ce species and thereby improves the high temperature catalytic performance to some extent.（3）Coating composite CuO_x-CeO₂ nanoparticles（short nanorods）with CeO₂ can inhibit the excessive outward migration of CuO_x species and the agglomeration of crystalline CuO in the high temperature thermal treatment process.Meanwhile,thermal induced Cu migration supplements the surface highly dispersed CuO_x species and strongly bonded Cu-[O_x]-Ce species for the CeO₂ coated CuO_x-CeO₂ catalysts,thereby improves the low temperature catalytic performance as well as maintains the high temperature operation window.Before the high temperature thermal treatment,on one hand,the coating of CeO₂ covers the surface highly dispersed CuO_x species and decreases the content of surface Cu⁺of the catalysts,thus degrades the low temperature catalytic performance.On the other hand,it facilitates the incorporation of more Cu ions into CeO₂ lattice,which strengthens the Cu-Ce interaction as well as increases the amount of strongly bonded Cu-[O_x]-Ce species,thus delays the reduction of Cu⁺to Cu⁰in the reaction process and promotes the high temperature catalytic performance.（4）Trace CO in H₂-rich stream can be eliminated by CO-PROX at low reaction temperature and CO methanation at high reaction temperature.Appropriate amount of Co deposition can significantly broaden the operation temperature window of CuO_x-CeO₂ catalysts.For instance,the 1Co-7Cu Ce-P-500 catalyst presents the operation temperature window of 85-over 240℃.Reactions occurred on the Co deposited CuO_x-CeO₂ catalysts are complicated.At relatively low reaction temperature（below 120℃）,CO-PROX reaction is predominant and CH₄ is not formed.At over 120℃,the O₂selectivity toward CO oxidation gradually decreases and the side reaction of H₂oxidation intensifies,CO methanation reaction takes place after O₂ is totally consumed,and gradually becomes the predominance.At over 230℃,CO-PROX reaction is negligible,the CO elimination is achieved entirely by CO methanation,and O₂ is totally consumed by reacting with H₂.At over 400℃,reverse water-gas shift reaction（RWGS）occurs,and the CO concentration starts to increase again.For the Co deposited CuO_x-CeO₂ catalysts,Co³⁺is also the active sites for CO oxidation,while Co⁰ is responsible for CO methanation.The introduction of small amount of Co to CuO_x-CeO₂ catalysts（Co/（Cu+Ce）molar ratio below 1/2）slightly decreases the low temperature catalytic performance for CO-PROX,but significantly broadens the high temperature operation window by CO methanation.Further increment in Co deposition amount induces the transferring of Cu species from CeO₂ to Co₃O₄,which strengthens the Cu-Co interaction and stabilizes the surface Cu⁺and Co³⁺,thus promotes the low temperature catalytic performance for CO-PROX as well as maintains the high temperature operation window.