Interconversion of Agglomerated Pd Domains and Ion-Exchanged Pd2+ in CHA Zeolite Materials for Passive NOx Adsorption

Pd-exchanged chabazite (CHA) zeolites offer promise as passive NO_x adsorbers (PNA) that mitigate vehicular NO_x emissions during cold-start conditions when exhaust temperatures are below (<450 K) the operating temperature window of currently used catalytic converters. Site-isolated, ion-exchanged Pd species in CHA zeolites are proposed precursors to the NO adsorption sites [1]. Yet their formation, structure (e.g., Pd²⁺, [PdOH]⁺, Pd⁺), and the zeolite lattice sites and framework Al configurations that bind them remain unclear. Here, we study how the material properties and gaseous treatments of Pd-CHA influence Pd speciation and NO_x adsorption.

A suite of CHA zeolites was synthesized with different bulk Al densities (Si/Al = 5–30) and framework Al arrangements (i.e., number of 6 membered-rings, 6-MR, containing 1Al or 2Al sites, as quantified by Co²⁺ [2]), onto which Pd-amine complexes were deposited. As-made Pd-CHA materials contained predominantly isolated [Pd(NH₃)₄]²⁺, evident in DRUV-Visible spectra that showed d-d transition bands at ~300 nm. Thermal desorption of amine ligands (>500 K) resulted in the reduction and agglomeration of Pd, forming metallic Pd in inert atmosphere or PdO in air. PdO particles formed during synthesis converted to ion-exchanged Pd in progressively higher amounts as the air treatment temperature increased (500–1023 K), as quantified by H₂ TPR. Pd-CHA samples prepared to contain predominantly isolated framework Al (i.e., no 6-MR containing 2Al sites) were found to host ion-exchanged Pd²⁺, demonstrating that ion-exchanged Pd²⁺ may be stabilized on Pd-CHA at framework Al configurations that are distinct from those that stabilize Co²⁺. Common methods to characterize Pd speciation (H₂ TPR, cation back-exchange) alter the sample, but a proposed Δ-EXAFS characterization method can quantify the fraction of ion-exchanged Pd²⁺ without altering the sample.

References

1. Chen et al., Catal. Lett. (2016) 146, 1706-1711
2. Di Iorio et al., J. Am. Chem. Soc. (2020) 142, 4807-4819