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

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


Gu, Y. - Presenter, University of Houston
Marino, S., University of Virginia
Epling, W., University of Virginia
Mandal, K., University of Virginia
Pihl, J. A., Oak Ridge National Laboratory
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.