Heterogeneous mechanism of NOx-assisted soot oxidation in the passive regeneration of a bench-scale diesel particulate filter catalyzed with nanostructured equimolar ceria-praseodymia

Abstract

An experimental study of passive regeneration of bench-scale diesel particulate filters (DPF) catalyzed by morphologically varied ceria and ceria-praseodymia (equimolar mixture) washcoats was carried out. The microstructured washcoat was prepared in the DPF via in situ solution combustion synthesis, whereas the nanostructured counterpart was deposited in the DPF via wetness impregnation with its suspension. Soot model was loaded via impregnation until the weight ratio of soot to catalytic washcoat was 1–20. The experiments revealed that after 4 h of isothermal regeneration at 400 °C, in the presence of 10% O2 and 1000 ppm NO, the reaction over nanostructured ceria-praseodymia washcoat resulted in the highest soot conversion (82%). Moreover, this catalyst achieved the highest NO2 exploitation efficiency in soot conversion, estimated as the ratio of the converted soot C atoms per mole of consumed NO2 (formed by NO oxidation), resulting in a C-to-NO2 ratio of 3.43. The catalyst reducibility analysis via Soot- and NO-TPR showed that nanostructured ceria-praseodymia had the highest catalytic activity toward soot oxidation in the absence of bulk-phase oxygen (i.e. via Mars-van Krevelen mechanism), as well as the ability to adsorb NOx at lower temperatures. Additional experiments with NO2-assisted soot oxidation were conducted to characterize the participation of NO2 in the catalytic reaction. They revealed that, in the absence of bulk-phase oxygen, the oxidation by NO2 over nanostructured ceria-praseodymia was more intense at low temperatures than that without catalyst. It was concluded that the participation of NO2 during the passive DPF regeneration was not only in the bulk phase, where NO2 directly oxidizes soot, but also on the catalyst surface, where adsorbed NO2 species and possibly reactive oxygen species, generated through an interaction between NO2 and the surface, attack soot at soot–catalyst interface, i.e. via a heterogeneous mechanism. As a matter of fact, the heterogeneous pathway of NO2-assisted soot oxidation was not observed to the same extent for the microstructured ceria-praseodymia catalyst, leading to the observation of different structure-sensitive mechanisms of soot oxidation by NO2. Finally, in the upscaled reaction system, nanostructured ceria-praseodymia remained the most performing catalyst despite the shorter residence time and the weaker soot–catalyst contact.