CLEERS Teleconference: Dr. Raj Gounder, Purdue University

Structural Interconversion of Agglomerated Pd Domains and Mononuclear Pd Sites in CHA Zeolites for Passive NOx Adsorption

Dr. Raj Gounder
Purdue University

Abstract:

Pd-exchanged chabazite (CHA) zeolites have received interest as passive NOx adsorbers (PNA) that mitigate vehicular NOx emissions during cold-start conditions. Mononuclear Pd cations are proposed precursors to low-temperature NO adsorption sites. In this talk, we will discuss how the external environment conditions (e.g., temperature, gas-phase pressure and composition, presence or absence of water) and different reaction pathways and mechanisms influence the structural interconversion of aggregated Pd domains (Pd, PdO) into mononuclear Pd cation site types. We interrogate model Pd-zeolite powders of well-defined Pd and zeolite domain sizes and compositions, characterized using a range of spectroscopic, reaction, titration and microscopic imaging techniques. These data reveal the strong influences of CHA zeolite material properties (e.g., crystallite size, framework Al content and distribution) and of the sizes and locations (e.g., extracrystalline or intracrystalline) of agglomerated Pd domains that are present prior to high-temperature Pd re-dispersion treatments, providing insights into both reversible and irreversible changes that occur to Pd site structures as they evolve with treatment and sample history.