Low-Temperature Pd/Zeolite Passive NOx Adsorbers: Structure, Performance and Adsorption Chemistry

Janos  Szanyi, Pacific Northwest National Laboratory

Pd/zeolite passive NOx adsorber (PNA) materials are prepared with solution ion-exchange between NH₄/zeolites (Beta, ZSM-5 and SSZ-13) and PdCl₂ solutions. The nature of Pd (dispersion, distribution and oxidation states) in these materials is characterized with Na⁺ ion-exchange, TEM imaging, CO titration with FTIR and in situ XPS. The NOx trapping and release properties were tested using feeds with different compositions. It is concluded that multiple Pd species coexist in these materials: some Pd is atomically dispersed in the cationic sites of zeolites, and some stays as PdO₂ and PdO particles on the external surfaces. While Pd is largely atomically dispersed in ZSM-5, the small pore opening for SSZ-13 inhibits Pd diffusion such that the majority of Pd stays as external surface PdO₂ clusters. NOx trapping and release are not simple chemisorption and desorption events, but involve rather complex chemical reactions. In the absence of CO in the feed, cationic Pd(II) sites with oxygen ligands and PdO₂ clusters are reduced by NO to Pd(I) and PdO clusters. These reduced sites are the primary NO adsorption sites. In the presence of H₂O, the as-formed NO₂ desorb immediately. In the presence of CO in the feed, metallic Pd and “naked” Pd²⁺ and Pd⁺ sites are responsible for NO adsorption. For Pd sites with the same oxidation states but in different zeolite  frameworks NO binding energies are not expected to vary greatly. However, NO release temperatures do vary substantially with different zeolite structures. This indicates that NO transport within these materials play an important role in determining release temperatures. Finally, some  rational design principles on efficient PNA materials are suggested.

Download oral presentation: