Preparation Of Nanorod-shaped La₂O₃/La₂O₂CO₃-supported Noble Metal Catalysts And Their Catalytic Performances For Soot Oxidation

Particulate matter（PM,consisting mainly of soot particles）emitted from diesel engines has caused serious environmental and human health problems.Thus,study on the purification of soot particles in diesel exhaust has an important significance in the environmental protection.The combination of diesel particulate filters（DPF）and highly active oxidation catalysts was considered to be one of the most efficient after-treatment techniques for soot purification,and the purification efficiency for soot particles is strongly related to the catalytic oxidation activity of the catalysts.The essence of catalytic soot oxidation is a complex deep oxidation reaction which occurs at the interfaces of a triple-phase contact boundary among the solid soot particle,the solid catalyst,and the gaseous reactants（O₂,NO）.The adsorption and activation properties of the catalysts for O₂ play a central role in determining the catalytic activity for soot oxidation.Supported noble metal（Au,Pt）nanocatalysts have exhibited good catalytic performances in oxidation reactions because of their excellent ability for the adsorption and activation of gaseous O₂.Therefore,rod-shaped metal oxides are used as carriers to support noble metals（Au,Pt）acting as an active components,and the fabrication of strong interactions between metal and oxide/support is not only beneficial for the effective utilization of noble metals and the sintering resistance of the Au-based nanocatalysts at high temperatures,but also increases the density of active sites located at metal-oxide/support interfaces for enhancing the adsorption-activation properties of O₂.Physicochemical properties of the catalysts were characterized using the following techniques:XRD,Raman,TEM,N₂ adsorption-desorption,ICP-OES,H₂-TPR,XPS,and NO-TPO.The temperature programmed oxidation of soot particles（soot-TPO）tests would be used to evaluate the activities of all catalysts for soot oxidation.The results of catalytic performances for soot oxidation were discussed on the basis of the results（structure and physicochemical properties）of characterization.The catalytic mechanisms of soot oxidation were systematacially studied and proposed.The main contents and results are shown as follows:La₂O₂CO₃ nanorods with lengths of 200-500 nm and diameters of 20-50 nm were successfully prepared by a hydrothermal method,and the Au and La₂O₂CO₃interaction-induced self-assembly of Au@La₂O₃ core-shell nanoparticles（NPs）on the surface of La₂O₂CO₃ nanorods（Au_n@La₂O₃/LOC-R）was achieved by the combined methods of gas bubbling-assisted membrane reduction（GBMR）and subsequent calcination treatment.The La₂O₂CO₃ as a precursor of the La₂O₃ shell plays an important role in the fabrication of self-assembled Au@La₂O₃ core-shell NPs.Our experimental results indicated that Au@La₂O₃ core-shell NPs are highly dispersed on the surface of La₂O₂CO₃ nanorods,and the sizes of Au NPs have a narrow distribution in the range 1-7 nm,while the thicknesses of La₂O₃ shells are 1-2 nm.The strong interaction between Au NPs and La₂O₃ oxides increases the density of active sites（oxygen vacancies）for enhancing the adsorption-activation properties of O₂,which is favorable for increasing the amount of surface active oxygen species.Au_n@La₂O₃/LOC-R catalysts exhibit highly efficient catalytic activity for soot oxidation under the conditions of loose contact between soot and catalyst.For example,the catalytic activities(T₅₀,TOF)of Au₄@La₂O₃/LOC-R catalyst are 375 ℃ metal（Au）-oxide（La₂O₃）interaction will stabilize Au NPs effectively to inhibit sintering at high temperatures,the Au_n@La₂O₃/LOC-R catalysts exhibit excellent stability.In addition,the catalytic mechanism of soot oxidation is discussed in depth,in which the catalytic oxidation of NO to NO₂ over Au@La₂O₃ core-shell NPs is the key step for catalytic soot oxidation;the active sites at the interfaces of the Au core and La₂O₃ shell can effectively promote catalytic NO oxidation.The exposed crystal surface of oxide supports is crucial to further improve the catalytic performances of supported noble metal catalysts.La₂O₃ nanorods（La₂O₃-R）with active{110}crystal facets were successfully prepared via a hydrothermal method and used as substrate to support Pt NPs by a gas bubbling-assisted membrane reduction（GBMR）method.The exposed active{110}crystal facets of La₂O₃-R play a crucial role in the high dispersion of Pt NPs and the fabrication of strong metal（Pt）-support（La₂O₃-R）interaction.The hemispherical Pt NPs with mean particle size of 4.5 nm are highly dispersed on the{110}surfaces of La₂O₃-R support（Pt/La₂O₃-R）.The strong interaction between metal（Pt）and support（La₂O₃-R）is beneficial for enhancing redox ability.Pt/La₂O₃-R catalyst exhibits higher activity(T₅₀=369°C,La₂O₃-supported Pt nanoparticle catalyst（Pt/La₂O₃-P）under conditions of loose soot-catalyst contact.The activity of Pt/La₂O₃-R catalyst for catalytic soot oxidation is dependent on the rod morphology（{110}crystal facets）of La₂O₃-R and the strong interaction of Pt NPs with{110}crystal facets of La₂O₃-R,which is beneficial for the formation of the active oxygen species.Therefore,the activity of Pt/La₂O₃-R catalyst for catalytic soot oxidation can be dramatically improved.In addition,the Pt/La₂O₃-R catalyst exhibits also good stability of activity and nanostructure during catalytic soot oxidation.In conclusion,the work not only enhances the effective utilization of noble metal components and the sintering resistance of Au-based nanomaterials at high temperatures,but also establishes the structure-activity relationship in catalytic reactions,and investigates the mechanism of catalytic oxidation of soot particles in depth,which provide the theoretical guidance and practical foundation for the further development of high-efficiency catalysts for diesel soot purification.