Pd/FER vs Pd/SSZ-13 Passive NOx Adsorbers: Adsorbate-controlled Location of Atomically Dispersed Pd(II) in FER Determines High Activity and Stability

Pd-loaded FER and SSZ-13 zeolites as low-temperature passive NOx adsorbers (PNA) are compared under practically relevant conditions. Vehicle cold-start exposes the material to CO under a range of concentrations, necessitating a systematic exploration of the effect of CO on the performance of isolated Pd ions for PNA. NO release temperature of both adsorbers decreases gradually with the increase of CO concentration from a few hundred to a few thousand ppm. This beneficial effect results from local nano-“hot spots” formation during CO oxidation. Dissimilar to Pd/SSZ-13, increasing the CO concentration above ~1,000 ppm improves the NOx storage significantly for Pd/FER, attributed to the presence of a Pd ions in FER γ-site that is shielded from NOx. CO mobilizes this Pd atom to the NOx accessible position where it becomes active for PNA. This behavior explains the very high resistance of Pd/FER to hydrothermal aging: Pd/FER materials survive hydrothermal aging at 800⁰C in 10% H2O vapor for 16 hours with no deterioration in NOx uptake/release behavior. Thus, by allocating Pd ions to the specific microporous pockets in FER, we have produced very hydrothermally stable and active PNA materials with immediate practical applications.