Palladium Speciation in Beta and Chabazite Zeolites for Passive NOx Adsorption

Trevor  Lardinois, Davidson School of Chemical Engineering, Purdue University

Trevor M. Lardinois¹, Jason S. Bates¹, Kinga A. Unocic², Jae-Soon Choi², Jeffrey T. Miller¹, Rajamani Gounder¹

¹Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907

²Oak Ridge National Laboratory, Oak Ridge, TN 34830

Lean-burn engines are challenged to meet emission standards during initial cold-start periods and low-load operation when exhaust gas temperatures are below 150 ^oC. Palladium-exchanged zeolites are emerging as effective passive NOx adsorbent technologies because of their NOx storage and release capabilities and resistance to sulfur poisoning and hydrothermal deactivation. However, it remains unclear what structures of extraframework Pd are able to be stabilized onto zeolite supports, the specific intrazeolite locations of such Pd species, and how the framework Al arrangement affects the nature of ion-exchanged Pd species. Two Pd-exchanged zeolites (*BEA, CHA) were prepared via incipient wetness impregnation then treated in air at 550 ^oC for 4 hours. X-ray absorption spectroscopy (XAS) on an ambient and dehydrated Pd-*BEA sample revealed the Pd exists predominantly in the +2 oxidation state. Ex-situ scanning transmission electron microscopy showed Pd particle formation on both Pd-*BEA and Pd-CHA. Two different sized Pd particles were present on the CHA sample: larger 5-20 nm particles and smaller 2 nm particles. Hydrogen temperature programmed reduction (H₂-TPR) was used to differentiate between palladium oxide domains and ion-exchanged Pd. H₂-TPR quantification suggested that all of the Pd was present in the +2 oxidation state, consistent with XAS results on Pd-*BEA. We will also discuss how the Al arrangement in *BEA and CHA can be controlled during synthesis, in order to influence Pd speciation.