Elucidating the Role of CO in the NO Storage Mechanism on Pd/SSZ-13 with <i>in Situ</i> DRIFTS
Inhak Song, Konstantin Khivantsev, Yong Wang, János Szanyi
Abstract
Pd-ion-exchanged zeolites have emerged as promising materials for the adsorption and oxidation of air pollutants. For low-temperature vehicle exhaust, dispersed Pd ions can adsorb NOx even in H2O-rich exhaust containing carbon monoxide. To understand this phenomenon, changes in the Pd ligand environment have to be monitored in situ. Herein, we directly observe the activation of hydrated Pd ion shielded by H2O into a carbonyl–nitrosyl complex Pd2+(NO)(CO) in SSZ-13 zeolite. The subsequent thermal desorption of ligands on the Pd2+(NO)(CO) complex proceeds to nitrosyl Pd2+ rather than to carbonyl Pd2+ under various conditions. Thus, CO molecules act as additional ligands to provide an alternative pathway with a lower energy barrier in accelerating NO adsorption on hydrated Pd2+ ions through the Pd2+(NO)(CO) complex. We further demonstrate that hydration of Pd ions in the zeolite is a prerequisite for CO-induced reduction of Pd ions to metallic Pd. The reduction of Pd ions by CO is limited under dry conditions even at temperatures as high as 500 °C, while water makes it possible at near RT. However, the primary NO adsorption sites are Pd2+ ions even in gases containing CO and water. These findings clarify additional mechanistic aspects of the passive NOx adsorption (PNA) process and will help extend the NOx adsorption chemistry in zeolite-based adsorbers to practical applications.