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

Palladium exhibits a complex chemistry in the PNA process, especially in the presence of gas mixtures containing CO and NOx. In this work the mechanism of NOx 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 NOx adsorption was investigated. Insight into the chemisorption mechanism was gained by detailed monitoring of the evolution of the NOx and COx concentrations. Oxidized palladium is required for achieving NOx adsorption. Clustered zerovalent palladium reduced with H2 did not adsorb NOx 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.

Automated summary

This study investigates the mechanism of NOx adsorption on a Pd/FER zeolite in gas mixtures containing CO, O2, and H2O. It is found that oxidized palladium is required for achieving NOx adsorption, while clustered zerovalent palladium reduced with H2 cannot adsorb NOx in the presence of O2. The study provides evidence for a reaction mechanism involving oxygen-bridged PdII-O-PdII moieties of PdO nanoclusters, and the NO adsorption site on palladium is obtained by reducing one of the two PdII atoms to the zerovalent state, which is more readily achieved with CO compared to NO, explaining the beneficial role of CO.