Transition Metal Oxide Monolithic Catalysts On Three-Dimensional Macroporous Ni Foam For Soot Combustion

Nowadays,diesel engines dominate in the heavy-duty vehicles due to their low operation costs,good durability,and high fuel efficiency.However,as one of the hazardous air pollutants,soot particulates in diesel engine emissions cause a serious threat to environment and human health.Diesel particulate filters(DPF)can collect more than 90%of soot particulates.The most challenging task in this regard is to eliminate the trapped soot particulates by catalytic oxidation within the temperature range of diesel engine exhaust in order to achieve in-situ regeneration of DPF.However,the classical-powder washcoated catalysts have undesirable catalytic performance for soot combustion,due to uneven distribution of active components and low material utilization efficiency.In order to improve the utilization efficiency of catalysts,this work aims to design and synthesize a series of nanostructured transition metal oxide monolithic catalysts in situ on DPF.Moreover,the structure-activity relationship and reaction mechanism of the monolithic catalysts were detailedly studied based on the activity evaluation and characterization results.The macroporous Ce O₂/Mn₂O₃ monolithic catalysts(Ce/Mn-NF)were successfully prepared via the tandem hydrothermal and impregnation methods.The macroporous nano-structures improve the soot-catalyst contact efficiency on the external surface of catalysts.The superficial Mn-Ce interaction can produce Mn⁴⁺and oxygen vacancies on the Ce/Mn-NF catalysts through the redox process of Ce⁴⁺+Mn³⁺?Mn⁴⁺+Ce³⁺,simultaneously generating surface active oxygen species.For the Ce/Mn-NF-2,the surface atomic ratio of Ce and Mn is close to 1:1,which will create more Mn-Ce interaction sites to generate active oxygen species compared with other catalysts.Accordingly,the Ce/Mn-NF-2 exhibits the remarkable catalytic activity with the T₁₀ and T₅₀,as low as 324 and 385oC,respectively,for soot oxidation in presence of NO.The tactful design of the Ce/Mn-NF catalysts successfully overcomes the problems for the application of Mn O_x-Ce O₂ catalysts in the reaction of catalytic soot oxidation.In view of the gas-solid-solid reaction characteristics of soot combustion,the hierarchically meso-macroporous Co₃O₄/Ce O₂ nanosheet monolithic catalysts(Co/Ce-NS)were successfully constructed in situ on DPF substrate.The hierarchically porous nano-structures increase the soot-catalysts contact chances,and promote mass transfer of gaseous reactants and products in catalytic soot combustion.The introduction of cobalt enhances the redox ability of the catalysts,and the Co-Ce interaction results in the generation of the superficial Co²⁺-?-Ce³⁺species,inducing the formation of surface active oxygen species on the Co/Ce-NS catalysts.The adsorbed oxygen species on the Co-Ce interfaces are more active than these on surface Ce O₂ or Co₃O₄.Accordingly,the 0.6Co/Ce-NS with more Co-Ce interaction sites exhibits the higher intrinsic activity and better catalytic activity than other catalysts for soot combustion.In order to further improve the oxidation ability of the catalysts,the Ag/Ce O₂monolithic catalysts(Ag/Ce-A and Ag/Ce-H-O)were successfully constructed in situ on DPF substrate.For the Ag/Ce-A,the nano-structures created by nanowires can promote the contact between soot and active sites on the catalyst.However,the Ag/Ce-H-O with the morphology of nanoparticles exhibits better catalytic activity than the Ag/Ce-A for soot oxidation.The Ag⁰ on the Ag/Ce-A can adsorb and activate O₂ to form surface adsorbed oxygen species,while the Ag/Ce-H-O generates the superficial Ag⁺induced by Ag-Ce interaction besides the superficial Ag⁰.Hydrogen reduction promotes the interaction of Ce⁴⁺+Ag⁰?Ag⁺+Ce³⁺on Ag/Ce-H-O,and the surface adsorbed oxygen species induced by the Ag-Ce interaction are more active than that formed by adsorbing and activating O₂ of Ag⁰.The K/Ce-NW monolithic catalysts grown on DPF substrate were successfully designed and synthesized by the hydrothermal and impregnation methods.The K/Ce-NW catalysts exhibit the excellent catalytic activity and stability for soot oxidation.The nano-structures of nanowires promote the high dispersion of potassium species and enhance the contact efficiency between soot and active sites on the catalysts.The introduction of potassium species provides the new active sites for catalytic soot oxidation.Moreover,the interaction between potassium species and Ce O₂ can effectively stabilize the potassium species on the catalyst surface,and promote the formation of surface active oxygen species.