Investigation Of Performance Over Nd Modified MnO_x-CeO₂ Soot Combustion Catalyst

Diesel engines are extensively used in cars because of its low CO₂ emissions,good dynamic property and reasonable price.However,diesels emit particulate matter with high soot content,which pose a threat to the environment and human health.Combining diesel particulate filter and oxidation catalysts is a direct and efficient technique for soot remove.Despite the noble metal catalysts have excellent activity,but it is expensive and rare,which limits its wide use.Instead of precious metal catalyst,developing cheap and high performance soot combustion catalyst become the research hot spot.MnMnO_x with the strong redox ability are cheap,efficient,environmentally friendly.CeO₂ exhibit the super oxygen storage capacity.MnMnO_x-CeO₂ composite oxidate have been extensively research in the field of catalytic oxidation.This paper focuses on the preparation and characterization of MnMnO_x-CeO₂ and Nd-doped MnMnO_x-CeO₂ soot combustion catalysts.In this work,firstly,trying to prepare PMMA microspheres with different diameter by change the dosage of precursor MMA and mechanical stirring speed.Secondly,investigating the effect of the preparation methods,Mn content and doping Nd on the performance of MnMnO_x-CeO₂ catalysts.The catalysts were characterized by XRD,BET,Raman,XPS,H2-TPR,O₂-TPD and FESEM,and the soot combustion activities over catalysts were evaluated.In order to detect the thermal aging performance MnMnO_x-CeO₂ and Nd-doped MnMnO_x-CeO₂ catalysts,the aged catalysts were obtained by thermal aging treatment at 800°C for 12 h and denoted with a prefix of “A-”.The aged catalysts were characterized by XRD,BET and H2-TPR,and the activities over these catalysts were tested.The SEM images revealed that the catalysts have the long-range ordered macroporous structure,the macroporous have the regular shape and are interconnected through small open windows,the average pore size and wall thickness are uniform.According to the SEM images,the average pore size of the macroporous is about 160 nm,the diameters of the small window are approximately 60 nm and the wall thickness of pore is around 20 nm.The introduction of P123 make the mesoporous dispersed on the wall thickness of macroporous.Compared with the nonporous partical,the edges of mesoporous and macroporous as well as thin wall of macroporous potentially expose more electron donor centers which provide the electron to the adsorbed oxygen,therefore enhancing soot combustion activity.Because the average size of soot particles is approximately 25 nm,the small window is conducive to the penetration of soot particles into the inner pore of catalyst,therefore increasing the contact probability of the active site and soot and further promoting soot combustion.The researches on MnMnO_x-CeO₂ or Nd-doped MnMnO_x-CeO₂ catalysts with different Mn content show that when Mn content are low to 0.4 and 0.5,respectively,3DOM and tronditional catalysts form the solid solution,but continuing to increase Mn content,MnMnO_x appear in the catalysts.The specific surface area,oxygen vacancy concentration,surface adsorbed oxygen and Mn4+ content all first increase and then decrease with the increasing of Mn content,thus catalytic activities also first enhance and then weaken.The catalysts obtained by template method have more oxygen vacancy and adsorption oxygen in the surface of the catalyst,stronger redox ability and oxygen desorption capacity compared with the catalysts prepared by coprecipitation method,therefore the former has better catalytic activity than the latter.The researches on MnMnO_x-CeO₂ and Nd-doped MnMnO_x-CeO₂ catalysts show that the Nd-doped Mn-O-Ce solid solutions have more oxygen vacancy and adsorption oxygen and Mn4+ in the surface of the catalyst and stronger redox ability than the Mn-O-Ce,which is the reason that Nd-doped Mn-O-Ce catalysts have the better catalytic activities.Doping Nd into MnMnO_x-CeO₂ is also found to enhance the thermal aged performance of the catalyst.The incorporation of Nd into MnMnO_x-CeO₂ prevents to some degree the solid solutions from severe sintering and phase separation,and maintains the partial low-temperature redox capacities after aging,impeding the catalytic activity serious inactivation.The catalytic activity and thermal aging performance of 3DOM Mn0.3Ce0.6Nd0.1O₂ catalyst are best in the as-synthesized catalysts for soot combustion.