Elucidating the role of CO in NO storage mechanism on Pd/SSZ-13 with in situ DRIFTS

Abstract

Pd ion exchanged zeolites emerged as promising materials for the adsorption and oxidation of air pollutants. For low-temperature vehicle exhaust, dispersed Pd ions are able to adsorb NOx even in H2O-rich exhaust in the presence of carbon monoxide. In order to understand this phenomenon, changes in 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 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 alternative pathway through Pd2+(NO)(CO) complex with lower energy barrier for accelerating NO adsorption on hydrated Pd2+ ion, which is kinetically limited in the absence of CO. 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 a high temperature of 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.

Cold-Start NOx Mitigation by Passive Adsorption Using Pd-Exchanged Zeolites: From Material Design to Mechanism Understanding and System Integration

“…20,41,83,86 The vibrational features below ∼2100 cm −1 are related to metallic Pd derived from the reduction of small PdO x clusters by CO. 58,86 Besides, DRIFTS could also provide solid evidence for the formation of a bidentate complex Pd 2+ (NO)(CO) by coadsorption of CO and NO on Pd 2+ ions. 65,87 Considering the high sensitivity of CO vibrations to the environment, researchers have employed DRIFTS to determine the siting locations of ionic Pd species in/on the framework structure of a zeolite. 43,58,64 The strong Lewis basicity of NH 3 makes it readily bonded to various acid sites (e.g., protons and Pd ions) of Pd-zeolites to form NH 4 + and Pd(NH 3 ) x 2+ species.…”

“…30,43 The change in NO x storage/release induced by CO was supposed to be associated with the formation of Pd 2+ (NO)(CO) species. 30,65,87 As shown in Figure 9c, exposure of NO-saturated Pd-SSZ-13 to CO led to a decrease of band intensity at 1865 cm −1 for the ν(NO) vibrations of Pd 2+ -NO species and an increase of band intensities at 2150 cm −1 for ν(CO) vibrations and at 1800 cm −1 for ν(NO) vibrations, corresponding to a selective conversion of Pd 2+ -NO species into a stable mixed carbonyl−nitrosyl complex, i.e., Pd 2+ (NO)(CO), as evidenced by DFT calculations. 30,65 This process of selective transformation is summarized in eq 10, based on a stronger binding energy of Pd 2+ -NO than that of Pd 2+ -CO as proven by subsequent vacuum desorption studies.…”