Mechanistic insight in NO trapping on Pd/Chabazite systems for the low-temperature NOx removal from Diesel exhausts

We have applied transient response methods to study Pd-CHA systems as Passive NOx Adsorbers (PNA) for the low-temperature NOx removal from Diesel exhausts. NO adsorption/TPD experiments were carried out at different temperatures in gaseous mixtures containing the main exhausts components (O-2, CO, H2O) with the aim to clarify the mechanism determining the capability of Pd-CHA to trap and desorb NO. Pd-1 was identified as the main NO storage site, both in mixtures containing a strong reductant like CO and in the presence of O-2 only. When the oxidizing capability of the environment was enhanced, the Pd utilization, i.e. NOstored/Pdtot, dropped significantly: this loss is attributed to the competition between the NO storage on the reduced Pd sites and their re-oxidation. Pd oxidation played an important role also in the desorption phase: TPD experiments performed in gaseous mixtures with greater oxidizing strength enabled NO desorption at lower temperatures. The present data emphasize therefore the impact of the Pd redox state on the NO storage/release performances of Pd-CHA based PNAs.