Palladium exchanged zeolites have been extensively studied as passive NOx adsorbers (PNA) to store NOx at low temperature and release it at higher temperature to a downstream selective catalytic reduction (SCR) catalyst which converts NOx to N2. PNA occurs with multicomponent exhaust feed compositions and the complex interplay between the various reductants, oxidants and water results in rich features observable in typical temperature programmed desorption (TPD) experiments. Herein, we use periodic, van der Waals corrected density functional theory (DFT) along with flow reactor experiments to assign a role to the various gases present in the diesel exhaust on the storage and oxidation behavior of Pd/SSZ-13. Water dynamically hydrates isolated Pd sites and upon the favorable adsorption of NO to [Pd(H2O)4]2+ in the absence of CO the key intermediate HONO is formed after a water activation step. Water activation is more easily accomplished when CO is present by forming a carboxyl intermediate on the path to CO2. Re-oxidation of the reduced Pd site may occur with water or oxygen. The pathway mediated by water leads to metallic Pd, which agglomerates and leads to long-term catalyst deactivation, whereas the active Pd site can be restored with oxygen. We find that the active site modification by oxygen, not its reduction by CO, leads to enhanced low temperature NO uptake in the presence of CO. These findings are corroborated by TPD experiments performed on Pd(1%)/SSZ-13 with various feed compositions to elucidate the intertwined roles of the individual exhaust components. Our combined computational and experimental investigation suggests that water plays a critical role in dynamic active site transformations and oxygen prevents deactivation by CO during PNA. Both findings are vital for catalyst design and further catalyst performance improvement.

Influence of redox environment on the mechanism of passive NOx adsorption on Pd/SSZ-13

ACS Fall 2021

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