Effect of the morphological and surface properties of CeO2-based catalysts on the soot oxidation activity

Abstract

In diesel particulate filters, a solid (soot)–solid (catalyst) reaction occurs and, in these cases, the catalyst morphology plays a relevant role, being the number of contact points a critical feature for the resulting reaction rate.For this reason, two different ceria-based catalyst morphologies have been investigated in this work, for the soot oxidation reaction: first, a ceria catalyst was prepared by solution combustion synthesis (SCS), which lead to a specific surface area of 29m2/g. On the other hand, three-dimensional self-assembled (SA) stars ceria, so-called due to their branched morphology starting from a central nucleus, exhibited a higher specific surface area (124m2/g), a high availability of contact points between soot particles and the catalyst itself, due to the concavities of the obtained morphology, and interesting surface features at the nanoscale. The synthesis method was tuned in order to resort to a desired and well-defined morphology in terms of shape and size.Aged stars were also taken into account, in order to fully evaluate the advantages and drawbacks of this new material against classic SCS catalysts.Compared to the SCS ceria catalyst, the SA stars have a much higher microporous volume, being of about 0.04cm3/g, and a finer crystallite size. SA stars showed a better activity towards soot oxidation: the peak oxidation temperature dropped from 614°C (in the non-catalytic case) to 403°C in tight contact conditions, and to 552°C in loose ones. XPS analysis elucidated that this can be due to a higher surface oxygen availability as compared to SCS ceria, while HRTEM revealed that surface crystallites show oxygen defective structures, almost absent at the SCS catalyst surface, which also might involve a higher redox capability.In loose contact conditions, being the most realistic testing condition because soot deposits on the catalytic layer of the diesel particulate filter without any force exertion, the SA stars exhibited the lowest onset temperature, even after aging, thus proving their higher intrinsic activity. Moreover, aged stars, which are less active than fresh SCS powders in tight contact conditions, regain good results in the loose ones, thanks to intrinsic capability to catch soot particles, peculiar of the SA star morphology.In the perspective of applying such engineered morphologies in diesel particulate filters, the three-dimensional shape of SA stars may involve an enhancement of the number of contact points between the cake layer and the catalyst one, thus fostering the final activity.