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

[Display omitted] •In Pd-CHA PNAs, Pd redox state is the key parameter controlling NO storage/release.•CO and NO are able to reduce PdIIOH to PdI, which is the primary NO storage site.•CO enhances NO storage due to its strong reducing power.•H2O enhances NO storage inhibiting the oxidation of Pd species by O2.•Strong oxidizing mixtures favour the release of the stored NO at lower temperatures. 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 (O2, CO, H2O) with the aim to clarify the mechanism determining the capability of Pd-CHA to trap and desorb NO. PdI was identified as the main NO storage site, both in mixtures containing a strong reductant like CO and in the presence of O2 only. When the oxidizing capability of the environment was enhanced, the Pd utilization, i.e. NO stored/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.