(582f) Zeolite Caging Effect on Particle Agglomeration of Pd Ion-Exchanged Model Passive NOx Adsorbers

Cold start emission control is challenging due to the limited activity of exhaust after-treatment catalysts at temperatures below 200 °C. Pd/zeolite based passive NOx adsorbers (PNA) circumvent this failure by trapping NOx at low temperatures and releasing NOx to the NOx reduction catalyst when the exhaust temperature is above 200 °C. However, as prototypes of commercial catalysts are developed, catalyst degradation induced by other emission components such as CO becomes a major concern. Previous literature shows that PNA degradation induced by CO involves a two-step process: the reduction of Pd²⁺ cations and the agglomeration of Pd particles. Further investigation suggests that the zeolite cage opening size significantly influences this two-step process and a smaller zeolite cage opening size could potentially stop the degradation. This presentation focuses on the effect of zeolite framework on the reducibility of Pd²⁺ cations and the zeolite caging effect on the rate of Pd particle agglomeration after reduction. In-situ extended X-ray absorption fine structure spectroscopy (EXAFS), CO temperature-programmed reduction (CO-TPR), and NOx temperature programmed desorption (NOx-TPD) experiments suggest that Pd cations are readily reduced to metallic Pd at temperatures below 300 °C, whereas Pd particle agglomeration requires both O₂ and high temperature (>500°C) to irreversibly form PdO particles on the external surface of the zeolites. Comparison of the Pd particle agglomeration rates between Pd/SSZ-13 and Pd/BEA indicates that this is an activated process, of which the energy required is inversely proportional to the zeolite cage opening size.