Condition-dependent NO_x adsorption/desorption over Pd/BEA: A combined microreactor and in situ DRIFTS study

Pd/BEA is chosen as a model passive NO_x adsorber (PNA) to elucidate the effect of the feed gas composition on the NO_x adsorption/desorption behavior. The Brønsted acid and the partially hydrolyzed framework Al (P-HAl(OH)) sites in HBEA adsorb NO and NO₂ under dry conditions. Moreover, the performance of HBEA is not affected by CO, while CO inhibits nitrate formation and promotes NO adsorption via the Pd(NO)(CO) complexes formation over Pd/BEA. H₂O inhibits NO adsorption over the Brønsted acid and P-HAl(OH) sites, and ionic Pd is the only active site for NO_x adsorption under wet conditions. Furthermore, NO adsorption over hydrated Pd (Pd²⁺(OH)(NO)(H₂O)₃) is weaker than NO adsorption over bare ionic Pd (Z₂[Pd²⁺(NO)], Z[Pd²⁺(OH)(NO)]). Dehydration of Pd²⁺(OH)(NO)(H₂O)₃ forms more stable Z[Pd²⁺(OH)(NO)] during desorption. The NO adsorption capacity of Pd/BEA improves in the presence of CO under both dry and wet conditions by forming a stable carbonyl–nitrosyl complex.