Complete Mechanism for NOx Adsorption and Release on Atomically Dispersed Palladium in Pd-CHA

Palladium exchanged zeolites are an established class of passive NOx adsorber (PNA) that effectively stores and catalytically oxidizes NO at low temperature and releases NOx (x = 1, 2) at higher temperature, enabling its downstream selective catalytic reduction to N2. Notably, the presence of water leads to solvated and mobile Pd complexes that behave drastically different from the cationic species anchored to the zeolite framework under dry conditions. Herein, we use theoretical and experimental techniques to develop a complete reaction mechanism for NO adsorption and release on dynamically hydrated, isolated Pd ions within chabazite zeolites (Pd-CHA). van der Waals corrected density functional theory (DFT) calculations, subsequently verified with HSE06 single-point calculations, manifest that NO preferentially binds to the hydrated PdII site and facilitates the activation of water leading to the transient formation of a Brønsted acid site and hydrated palladium hydroxide. Adsorbed NO reacts with hydroxide to form HONO and Pd changes its formal oxidation state from +2 to +1 upon HONO desorption. HONO subsequently disproportionates to form NO, NO2, and H2O. The strong binding of NO to hydrated PdI leads to NO release at temperatures in excess of 200 °C with concomitant reoxidation by NO2 or O2. The complete NOx storage, oxidation, and release mechanisms identified by DFT are consistent with NOx uptake experiments on Pd-CHA. A consistent catalytic cycle for NO oxidation by water on isolated palladium active sites is described, and a mechanistic framework for further improvement of PNA performance is established.