Lower emissions requirements challenge lean-burn engines and catalytic conversion systems to limit NOx output during initial cold-start periods and low-load operation when exhaust gas temperatures are below 423 K. The need for effective NOx storage (&lt;423 K) and release (&gt;423 K) during transient operation, coupled with requirements for hydrothermal stability and sulfur poisoning resistance, make Pd-exchanged zeolites attractive materials for passive NOx adsorption applications. Additional investigation is needed, however, to understand the dynamic structures that extraframework Pd can adopt on zeolite supports, the specific intrazeolite locations of such Pd species, and the influence of framework Al arrangement on the structure of ion-exchanged Pd. Pd-exchanged zeolites (Beta, Si/Al=11; CHA, Si/Al=14) were prepared via incipient wetness impregnation of Pd(NH3)4(NO3)2 precursors on the NH4-form zeolite (Pd/Al&lt;0.1), then treated in flowing air at 823 K (0.033 K s-1) for 4 hours. X-ray absorption spectroscopy (XAS) on a Pd-Beta sample at ambient conditions and after treatment in flowing 10% O2 to 473 K indicated the Pd exists predominantly in the +2 oxidation state. Ex-situ scanning transmission electron microscopy detected extra-crystalline PdO particles (&gt;1 nm) on both Pd-Beta and Pd-CHA. On Pd-CHA, Pd particles were dispersed with a broad range of sizes, including larger 5-20 nm Pd particles and smaller 2 nm particles. Temperature-programmed reduction in flowing 5% H2/Ar (H2-TPR) was used to identify and quantify palladium oxide (PdO) domains and ion-exchanged Pd according to their reduction temperatures and H2 consumption, respectively. Quantifying H2 consumption on all Pd-zeolites (1 H2/Pd) indicated that all Pd was present in the +2 oxidation state, consistent with XAS results on Pd-Beta. A substantial fraction of Pd was present as PdO (0.2–0.7 PdO/Pdtotal) on samples synthesized by incipient wetness impregnation, which was corroborated by quantifying ionic Pd species in solution that were replaced by exchange with NH4+. Varying the Pd loading on Beta from 0.01–0.09 Pd/Al resulted in values of 0.2–0.4 PdO/Pdtotal that did not correlate with the Pd density, suggesting that incipient wetness impregnation is ineffective for stabilizing solely ion-exchanged Pd after treatments in flowing air at 823 K. Approaches to achieve cationic Pd via ion-exchange and post-synthetic treatments, and the effects of Al arrangements will be discussed.

Palladium speciation in beta and chabazite zeolites for passive NOx adsorption

2019 Spring National Meeting

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