Progresses on Soot Combustion Perovskite Catalysts

Abstract

The literature available related to the utilization of perovskites as soot combustion catalysts has been reviewed in this chapter. The general formula of perovskites is ABO3, and soot combustion perovskite catalysts with many different cations have been reported for both A and B positions. The most studied formulations include transition metals that are able to accomplish redox cycles, such as Mn, Ni, Cu, Co, and Fe among others. The partial substitution of one of the cations usually has a positive effect on the perovskite activity, and alkali metals seem to be the most studied dopants. However, an excess of dopant or an inappropriate synthesis method can lead to the segregation of phases and this could decrease the activity.The catalytic activity for soot combustion of perovskite catalysts has been reported to be related to the creation of active oxygen species that are suitable for the direct oxidation of soot and/or for the catalytic oxidation of NO to NO2. The introduction of dopants on the parent perovskite structure favors the formation of these active oxygen species and promotes the formation of oxygen vacancies that improve oxygen mobility into the oxide framework. It has also been proposed that the presence of dopants affects the chemical behavior of the main cations.One of the properties that limits the activity of perovskite catalysts for soot combustion is their very low surface area, and well-ordered perovskite nanocrystals have not been reported. The positive consequence of the formation of large perovskite crystals is the high thermal stability usually attributed to soot combustion perovskite catalysts. Three-dimensionally ordered macroporous (3DOM) perovskites have been investigated as soot combustion catalysts, and this approach could be a solution for the low surface area of perovskites.In spite of the successful utilization of perovskite catalysts by some authors in real diesel exhausts, some basic research seems still necessary to understand the effect of SO2, H2O, and CO2 in the catalytic combustion of soot, and advanced perovskite synthesis methods should also be developed in order to prepare high-surface-area perovskite nanocrystals.