Modulation Of Interfacial Sites Over Ceria-Based Catalysts Towards Enhanced CO Oxidation Reaction Performance

The efficient removal of CO from automobile exhaust is related to the national health and control of air pollution.The advanced combustion engines with high fuel efficiency will generate higher levels of CO and hydrocarbons,while discharging less NO_x.Thus,the exhaust catalysts are required to remove CO more efficiently.Currently,the ceria-based catalysts are the most effective catalysts towards CO oxidation due to their excellent oxygen storage capacity and unique electronic properties.During the oxidation reaction,the interfacial sites between the active metal oxide and the support are the place for the electron transfer.In order to further improve the CO oxidation performance of ceria-based catalysts,the modulation of interfacial sites is the principal way to control the electron transfer and redox properties of catalysts.In this dissertation,aiming at the highly-efficient removal of CO,the innovative research was carried out in three aspects,including the controllable preparation of ceria-based catalysts,modulation of interfacial sites and improved catalysis.The interfacial evolution over ceria-based catalysts induced by alkali cations was systematically studied.The structure-activity relationship between the transformed interfacial sites and the CO oxidation activity was disclosed.The“non-chemical”way of strengthening the micro-mixing of reactants was used to adjust the particle size,morphology and defect structure of catalysts.The alkali cations served as the electronic modifier to adjust the interfacial electron transfer of Pt/CeO₂and CuO/CeO₂catalysts,and the mechanism for the effect of modified interfacial sites on the CO oxidation activity and resistance to hydrocarbons inhibition was clarified.The main research contents of this dissertation are as follows:1.“Non-chemical”way to modulate the morphology of Pt/CeO₂catalysts and interfacial site characterization.The scientific basis for regulating the microstructure of catalysts by the rotating packed bed（RPB）reactor was studied.Compared with the traditional method,the morphology,particle size and oxygen vacancy defects of CeO₂synthesized by the RPB reactor were significantly changed.Meanwhile,the RPB reactor was also used to synthesize the VPO catalyst to verify the feasibility of this“non-chemical”way controlling the morphology and size of nanoparticles.The CeO₂with different morphologies was employed as support to load Pt,and the Pt/CeO₂catalysts prepared by the RPB reactor showed better catalytic activity.However,due to the limitation of the over-stabilized Pt-O-Ce interface of the Pt/CeO₂system itself,the catalytic performance was not significantly improved in comparison with the high-level catalysts reported in literatures.2.Alkali cation modulates the interfacial site of Pt/CeO₂catalyst and improves the CO oxidation activity.Since the Pt-O-Ce interface with high Pt-O coordination number in the Pt/CeO₂system over-stabilized the Pt sites,the further improvement of CO oxidation performance was not facile.An interface remodeling strategy was proposed to transform the Pt-O-Ce interface by the interaction of Pt with another component.The alkali cation Na⁺was used as the electronic modifier of Pt/CeO₂catalyst.The effect of Na⁺on the evolution of Pt sites was investigated based on the in-situ DRIFTS.It was confirmed that the Pt-Na⁺mutual interaction weakened the Pt-O-Ce interface,promoting the electron transfer from Ce³⁺-O_vsites to Pt to form the Na⁺stabilized metallic Pt cluster sites,researched by H₂-TPR,XPS,and EXAFS.These sites performed excellent CO oxidation performance and high resistance to hydrocarbon poisoning,which was higher than the optimal performance reported in recent literatures.Additionally,to verify the interface remodeling strategy,the Pt-FeO_xinteraction over the Pt/FeO_x/CeO₂catalyst was also investigated to reconstruct the Pt-O-Ce interface and improve CO oxidation activity.3.“Non-chemical”way to regulate oxygen vacancies of CuO/CeO₂catalyst.Noble metal（Pt,Pd）catalysts are the most effective catalysts for CO oxidation.However,due to their high price,using non-noble metal catalysts to replace noble metal catalysts will be a popular tendency.Oxygen vacancies serve as sites for oxygen adsorption,and play an important role in CO oxidation as a medium for electron transfer from the active metal oxide to the carrier.A“non-chemical”strategy was proposed to improve the oxygen vacancy content of CuO/CeO₂catalyst by strengthening the micro-mixing of reactants.The CuO/CeO₂catalyst was synthesized by a RPB reactor.The excellent micro-mixing efficiency of the reactants was beneficial for Cu²⁺to enter the CeO₂lattice to form the Cu-O-Ce solid solution structure.This caused the fluctuation of Ce-O bond due to the difference of coordination numbers of Ce-O bond and Cu-O bond,resulting in more oxygen vacancies in the samples synthesized by the RPB reactor.Furthermore,the Co₃O₄/CeO₂and Fe₂O₃/CeO₂with tremendous oxygen vacancies were also synthesized by the RPB reactor to verify the feasibility of this“non-chemical”strategy to improve the oxygen vacancy concentration.4.Alkali cation enhances the interfacial electron transfer and CO oxidation performance of CuO/CeO₂catalyst.The further improvement of CO oxidation performance of CuO/CeO₂catalyst depends on the change of the redox properties to increase the Cu⁺content.The above study shows that alkali cation can be used as electron modifiers for transition metal catalysts.Therefore,in this work,alkali cation K⁺was used to modify the CuO/CeO₂catalyst,and the effect of K⁺modification on the CuO_xdispersion and redox properties was studied.The result showed that the K⁺shifted the redox equilibrium of Cu²⁺+Ce³⁺（?）Cu⁺+Ce⁴⁺to be right toward the increase of Cu⁺,which greatly improved the CO oxidation performance and the ability to resist hydrocarbon inhibition.The K⁺-modified CuO/CeO₂catalyst continuously worked for 48 hours with high activity.The CO oxidation activity of the K⁺-modified CuO/CeO₂catalyst was in the same order of magnitude as the high-level noble metal catalysts reported in literatures.In addition,K⁺deteriorated the dispersion of CuO_xspecies on the surface of the CuO/CeO₂catalyst.Combined with the Na⁺-modified Pt/CeO₂system,the single-atom Pt agglomerated to form metallic Pt clusters during the reduction pretreatment.It was concluded that the alkali action could induce the agglomeration of transition metal components supported by CeO₂under the reductive atmosphere.