Palladium-exchanged zeolites have recently attracted attention for their potential to trap NOx emissions produced during vehicle cold starts, and release NOx at temperatures above those required for downstream NOx control catalysts (e.g. urea SCR) to become active. The performance of Pd/zeolite systems for NOx storage has been shown to depend on the extend of Pd exchange, the framework type, the silicon-aluminum distribution, and feedgas composition. These variables ultimately control the oxidation state of palladium, which in turn governs NOx storage and release. Here we employ probe reactions, electronic spectroscopies (XAS and DRUVS), and computational quantum chemistry tools to examine the oxidation state of palladium under varying conditions, and the redox mechanisms that link these oxidation states. In particular, we show from molecular orbital principles why the unusual Pd(I) state is stabilized in zeolites, why this species is important for NOx storage, and how this species can form dynamically under NOx storage conditions through one-electron reduction steps mediated by HONO as a molecular shuttle. We also show a mechanism by which CO and H<sub>2</sub>O combine to efficiently reduce exchanged Pd<sup>2+</sup> to metallic Pd, resulting in degradation of NOx storage performance. The latter result represents a fundamental challenge that must be overcome in order to enable practical implementation of Pd/zeolite-based NOx storage technologies.

NO- and CO-mediated redox chemistry of palladium exchanged zeolite passive NOx adsorbers

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.