Structural Interconversion Between Agglomerated Pd Domains and Isolated Pd2+ Cations in Chabazite Zeolites
Trevor Lardinois, Purdue University
Trevor M. Lardinois,1 Jason S. Bates,1 Andrew “Bean” Getsoian,2 Jeffrey T. Miller,1 Rajamani Gounder1,*
1Charles D. Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47906 (USA)
2Ford Motor Company, Dearborn, MI 48121 (USA)
Pd-exchanged chabazite (CHA) zeolites are reported as potential passive NOx adsorbers (PNA) that minimize vehicular NOx emissions during cold-start when exhaust temperatures are below (<450 K) the operating temperature window of downstream catalytic converters. The proposed NO adsorption precursors are cationic, single-site (ion-exchanged) Pd charge compensated by zeolite framework Al sites, but the identity of the precursor site (Pd2+, [PdOH]+, Pd+) and the framework Al configurations to stabilize such species are unclear. Additionally, Pd-exchanged zeolites often contain a distribution of ion-exchanged and agglomerated Pd species where the influences of the zeolite material properties, synthesis procedure, and post-synthetic treatment conditions on the interconversion between ion-exchanged and agglomerated Pd are not investigated fully.
Parent CHA zeolites were synthesized with varied 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+ ), and single-site Pd-amine complexes were deposited via incipient wetness impregnation. DRUV-Vis spectra of as-exchanged Pd-CHA materials contained a single d-d transition band at ~300 nm, consistent with transmission UV-Vis spectra of aqueous [Pd(NH3)4]2+ complexes. [Pd(NH3)4]2+-exchanged CHA zeolites treated to high temperatures (>500 K) resulted in Pd reduction and agglomeration of Pd, forming metallic Pd in inert atmosphere or PdO domains in air. Agglomerated Pd particles formed during synthesis are converted to ion-exchanged Pd in progressively higher amounts as the air treatment temperature increased (500–1023 K), as quantified by H2 TPR. H2 consumption stoichiometries per Pd for all samples treated in flowing air >600 K were unity, suggesting the majority of Pd species are divalent after air treatment. UV-Vis spectra of hydrated, predominantly ion-exchanged Pd-CHA contained a single d-d transition located at 390 nm, consistent with hydrated Pd2+ cations . Parent CHA zeolites prepared to contain predominantly isolated framework Al site (i.e., no 6-MR containing 2Al sites) were observed to stabilize ion-exchanged Pd2+, demonstrating that ion-exchanged Pd2+ may be stabilized at framework Al configurations that are distinct from those that stabilize Co2+, such as [PdOH]+ at single framework Al sites. Common methods to characterize Pd speciation (H2 TPR, cation back-exchange) alter the sample, but a proposed ∆-EXAFS characterization method can quantify the fraction of ion-exchanged Pd2+ without altering the sample.
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