Study On Catalytic Combustion Of Soot Particles Over Iron Elements And Manganese-based Transition Metal Oxides

On account of strong power,low oil consumption rate,preferable thermal efficiency,and stable operation,diesel vehicles are gaining more and more market share in the high-duty vehicle market.However,particulate matters?PM?,nitrogen oxides?NOx?,carbon monoxide?CO?and hydrocarbons?C H?discharged from diesel engines are considered to be important sources of pollution which seriously endanger the atmosphere environment and human health.Among the main pollutants,soot particles can penetrate the human respiratory system and have toxic human health effects leading to lung cancer,allergic diseases,bronchitis,cardiovascular,asthma,etc.Diesel particulate filters?DPFs?are considered as one of the efficient and economical post-treatment technology as well as the development and optimization of oxydic catalysts serve as vital factor.As several important basic materials,iron oxides,cobalt oxides and manganese oxides possess excellent redox property so that they can be widely used in catalysis.In this work,a series of iron oxides,cobalt oxides and manganese oxides were prepared and systematically characterized to study physicochemical property.The soot combustion was used as model reactions.And the correlation between their catalytic performance and physicochemical properties were also investigated.A series of Co-Fe co-doping ceria solid solution catalysts Ce₈₀Co_20-xFe_xO₂?x=0,5,10,15,20?with mesoporous structure were successfully synthesized using the CTAB-assisted co-precipitation method.The doped samples maintain the cubic fluorite structure of CeO₂,but they have large surface area,pore volume and small pore diameter.The synergistic effect of the Co and Fe species can induce additional oxygen vacancies and increase the redox performance of the composite catalysts,providing more active sites for soot oxidation.Co-Fe co-doping accelerates soot oxidation under both tight and loose contact conditions because of its unique mesoporous structure,additional oxygen vacancies,high oxygen ads-species concentration and good low-temperature reducibility.Furthermore,the presence of NO in O₂also promotes soot combustion as in the NO₂-assisted mechanism.A series of highly active amorphous manganese oxide catalysts?MnO x-A-550,MnO x-S-550,MnOx-N-X?X=550,650,750,850??were synthesized using the colloidal solution combustion synthesis?CSCS?method.Manganese precursors?Mn?AC?₂·4H₂O,MnSO₄·H₂O,Mn?NO₃?₂·4H₂O?and calcination temperatures?550oC,650oC,750oC,850oC?affect the crystal structure,surface properties,and redox properties of MnO x.As revealed by XPS,H₂-TPR and Soot-TPR results,the synthesized MnO x samples possess abundant Mn⁴⁺species and more surface active oxygen species.With the calcination temperature increasing from 550 to 850oC,the crystalline structure of MnO x changed from amorphous phase to crystal phase.In general,the amorphous MnO x showed better catalytic activity for soot oxidation than that of the crystal ones,which can be ascribed to the improved low-temperature reducibility,more surface active oxygen species and abundant surface Mn⁴⁺ions.MnO x-N-650 revealed the highest catalytic soot combustion activity with the lowest T₅₀value of 372°C in O₂ and the lowest T₅₀ of 332°C in NO+O₂.For crystal MnOx catalysts,their catalytic activity for soot combustion decreased as calcination temperature rising.The KMnO x sample was successfully prepared by redox precipitation method and revealed Mn₂O₃/K_2-XMn₈O₁₆ crystalline phase with hierarchical porous including micropores,mesopores and macropoes.Microporous and mesoporous materials usually have larger surface area and pore volume,which can provide a large number of active sites.These active sites are conducive to gas adsorption and diffusion.The existences of macropores expand the contact area between catalysts and soots so that they can improve reactive efficiency.The soot combustion with NO+O₂ of the KMnO x prepared in this study showed excellent catalytic activity.Therefore,the enhanced activity of KMnO x can be attributed to a number of favorable properties including macropores,improved redox ability,the presence of abundant Mn³⁺and Mn⁴⁺and promotion effect of K.As a basic metal with low melting point,K of molten state not only promotes the contact between the catalyst and soot,but also makes the soots form carbonate species and adsorb CO₂ to promote the reaction.