Efficient regeneration of Pd-SSZ-13 catalysts deactivated by alkali metals for passive NO _x adsorption

Pd-loaded CHA zeolites (e.g., Pd-SSZ-13) have been identified as a promising candidate for passive NO _x adsorption (PNA) in low-temperature environments. The inclusion of lubricant oil additives or biofuels in automotive exhaust after-treatment systems subjects Pd-SSZ-13 to chemical poisoning from contaminants such as alkali metals (e.g., Na and K). In this study, we present a straightforward yet highly effective approach to regenerate Pd-SSZ-13 that has been poisoned by alkali metals, achieved through washing with a dilute NH₄Cl solution. The underlying mechanisms of both poisoning and regeneration were thoroughly explored, revealing that the deactivation due to alkali-metal poisoning is primarily attributed to the transformation of the active Pd²⁺ ion into PdO _x and a concomitant reduction in Brønsted acid sites. The NH₄Cl washing not only removes alkali metal contaminants, thereby restoring the Brønsted acid sites previously occupied by them, but also converts the inert PdO _x back into their active states (i.e., isolated Pd²⁺). This investigation unveils a simplified yet highly efficient NH₄Cl washing technique for the regeneration of the deactivated Pd-SSZ-13, offering profound implications for extending the catalyst's lifespan and enhancing its performance in cold-start exhaust after-treatment applications.