Molecular Mechanism of Low-Temperature Passive NO_x Adsorption (PNA) on Palladium-Loaded FER Zeolite

Abstract

Palladium exhibits a complex chemistry in the PNA process, especially in the presence of gas mixtures containing CO and NO x . In this work the mechanism of NO x adsorption in gas mixtures with CO, O2, and H2O on a Pd/FER zeolite is investigated. The redox state of palladium and its clustering during high-temperature pretreatment and NO x adsorption was investigated. Insight into the chemisorption mechanism was gained by detailed monitoring of the evolution of the NO x and CO x concentrations. Oxidized palladium is required for achieving NO x adsorption. Clustered zerovalent palladium reduced with H2 did not adsorb NO x in the presence of O2. Evidence is provided for a reaction mechanism departing from oxygen-bridged PdII–O–PdII moieties of PdO nanoclusters. The NO adsorption site on palladium is obtained by reduction of one of the two PdII atoms to the zerovalent state which occurs more readily with CO as compared to NO explaining the beneficial role of CO.

RHO Zeolites with High Hydrothermal Stability

“…Multiple reports from the literature describe the dispersed Pd as PdO. Such a PdO phase has been reported in CHA, ,,− FER, , BEA ,, and MFI ,,− zeolites. The exact nature of the phase is, however, still a matter of debate.…”

“…Palladium-loaded small-pore zeolites are candidates for application in automotive aftertreatment systems to store NO x produced by an engine at cold start until downstream deNO x systems in the exhaust gas aftertreatment system are sufficiently heated to become active. For that application, in particular, the hydrothermal stability of palladium-loaded RHO is highly relevant. − …”

Investigating Palladium Nanoparticle Morphogenesis on Zeolite for Passive NOx Adsorption via In Situ Transmission Electron Microscopy Analysis under Hydrothermal Conditions

“…In our previous research, through ADF-STEM examination of Pd-loaded ferrierite (FER) zeolites subjected to hydrothermal conditions compared to those applied in this study, we observed the homogeneous dispersion of minute Pd particles primarily situated within the micropores of the zeolite. Subsequent X-ray photoelectron spectroscopy analysis of these Pd/FER zeolites indicated the presence of a substantial quantity of PdO species . The excellent PNA performance of the Pd-loaded RHO substantiated the presence of nicely dispersed Pd species within the zeolite.…”

Noncontact In Situ Multidiagnostic NMR/Dielectric Spectroscopy

“…Characterization of interfacial interactions is, however, extremely tedious, especially in 3D porous materials such as zeolites, mesoporous silicates, metal–organic frameworks (MOFs), and covalent organic frameworks (COFs). Such materials are nevertheless abundantly implemented in chemical applications, − in molecular water research, , and for energy storage applications. − Specifically for zeolites, solvent (e.g., water, alcohol) pore wall interactions impact catalysis and adsorption processes. , …”