Soot combustion over Ce1-xFexO2-δ and CeO2/Fe2O3 catalysts: Roles of solid solution and interfacial interactions in the mixed oxides

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 different catalysts for low-temperature soot oxidation was also performed. The presence of both Ce–Fe–O solid solution and CeO2–Fe2O3 interaction were observed over the two types of catalysts. The oxygen vacancy in the solid solution is the crucial active site to facilitating the soot combustion over the CeO2–based samples. Small CeO2 nanoparticles are well dispersed on the Fe2O3–supported catalysts, which results in the formation of Fe–O–Ce species due to the strong CeO2–Fe2O3 interaction. The Fe–O–Ce species could achieve the coupling of the Ce4+–Ce3+ and Fe3+–Fe2+ couples in the CeO2–Fe2O3 interface, which is also identified as an active species for catalytic soot oxidation. The concentration of oxygen vacancy is closely related to the content of iron in 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, but it is very easy to decompose 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 CeO2–based and Fe2O3–supported catalysts showed comparable catalytic activity. After long term aging at 800 °C, the loss on activity over the CeO2–based catalyst (Ce–Fe–O solid solution) is much higher than that over the Fe2O3–supported sample. The Fe2O3–supported catalysts are more suitable for practical application than the Ce–Fe–O solid solution.