CeO 2 -FeInterfacial Interaction Of 2 O 3 Catalysts And Their Catalytic Properties For Soot Combustion

Catalytic oxidation of soot is an effective route for reducing particulate emissions from diesel engines.Numerous studies have shown that ceria modified by transition metal oxides are one of the most promising catalysts for soot oxidation and the activity and stability of CeO₂ will be consolidated tremendously by means of doping other metal ions into ceria lattice.Based on this concept,introducing undersized and low valence ions such as Fe3+ into CeO₂ lattice could strongly enhance the activity and stability of the CeO₂ catalyst for catalytic oxidation.An alternative strategy to improve the activity and stability of CeO₂ is to use catalyst supports.Fe₂O₃ is a typical active support in the area of catalytic oxidation.It therefore generates considerable interest in loading CeO₂ nanoparticles on the Fe₂O₃ support to create effective catalysts for catalytic soot oxidation.In the first part of this paper,two series of CeO₂-Fe₂O₃ catalysts?CeO₂-based and Fe₂O₃-supported oxides?with varying composition were synthesized by a hydrothermal method and characterized using various techniques.The comparison on the activity and thermal stability of the two types of catalysts for catalytic oxidation of soot was also performed.The presence of both CeO₂-Iike solid solution and CeO₂-Fe₂O₃ interaction were observed over the two types of catalysts.The role of CeO₂-based solid solution and the physical contact between the isolated oxides in the catalytic process was discussed.The oxygen vacancy in the solid solution is the main active site to facilitating the soot combustion over the CeO₂-based samples(i.e.,Ce_1-xFexO₂-?).Small CeO₂ nanoparticles are well dispersed on the Fe₂O₃-supported catalysts?i.e.,CeO₂/Fe₂O₃?y??,which results in the formation of Fe-O-Ce species?the coupling of the Ce4+-Ce3+ and Fe3+-Fe2+ couples in the CeO₂-Fe₂O₃ interface?due to the strong Ce02-Fe₂O₃ interaction.The Fe-O-Ce species is also indentified as an active species for catalytic soot oxidation.The concentration of oxygen vacancy is closely related to the content of iron in the ceria lattice,while the formation of Fe-O-Ce species strongly relies on the particle size of CeO₂.It is also found that the oxygen vacancy is more active than the Fe-O-Ce species for soot oxidation.However,the solid solution is very easy to decompose after calcination at high temperature,resulting in obvious deactivation of catalysts.By contrast,the Fe-O-Ce species is very stable under high-temperature treatments.For the fresh samples,the Ce0.8Fe0.2O₂-? and CeO₂/Fe₂O₃?3/7?samples show comparable catalytic activity.After long term aging at 800 ?,the loss on the activity over Ce0.8Fe0.2O₂-? catalyst is much higher than that over the CeO₂/Fe₂O₃?3/7?sample.The Fe₂O₃-supported catalysts are more suitable for practical application than the Ce-Fe-O solid solution.The influence of carrier structure on catalytic activity was also investigated.The activity evaluation result demonstrates that the Fe₂O₃ with 3DOM structure is a suitable support.The satisfactory activities of the catalysts with 3DOM structure can be summarized into the following two parts.First of all,the diameter of ordered macropores is 250±20 nm,large enough for soot particles to enter.The soot particles can be catalyzed both in the external and internal surface of catalysts.The second,the diffusional resistance can be decreased by the well-ordered interconnected open windows.We can thus conclude that 3DOM structure contributes to enhance the catalytic activity of soot oxidation.