Understanding the Degradation Mechanism of Pd/zeolite Based Passive NOx Adsorbers: The Interchange Between Particles and Cations
Kevin Gu, University of Virginia
Pd/zeolite based passive NOx adsorbers (PNA) ideally trap NOx at low temperatures and subsequently release NOx to the downstream reduction catalyst when the exhaust temperature is above 200 °C. By doing so, NOx emissions during cold start can be mitigated and PNAs are automatically regenerated for reuse. However, as this concept has developed and model catalysts tested, PNA degradation induced by CO and other reductants from the exhaust has become a major concern. In this presentation, in-situ extended X-ray absorption fine structure spectroscopy (EXAFS), temperature programmed desorption (TPD), CO temperature-programmed reduction (CO-TPR), and steady-state CO oxidation kinetics are used to study the chemistry behind the irreversible loss of NOx storage capacity upon CO exposure. Results show that PNA degradation induced by CO involves a two-step process: the reduction of Pd2+ cations and the agglomeration of PdO particles. Hydrated Pd cations formed under cold start conditions are the most active Pd species for CO oxidation and can easily be reduced by CO at temperatures above 300 °C. The reduced Pd requires both O2 and high temperature to facilitate particle agglomeration and irreversibly form PdO particles on the external surface of the zeolites. However, the regeneration of Pd cations via protonolysis competes against particle agglomeration at high temperature, thus different strategies might be applied to protect PNAs from degrading.