(229b) Mechanistic Role of Water in the Storage and Oxidation of NO on Pd/CHA

Palladium exchanged zeolites have been extensively studied as passive NO_x adsorbers (PNA) to store NO_x at low temperature and release it at higher temperature to a downstream selective catalytic reduction (SCR) catalyst which converts NO_x to N₂. PNA occurs with complex exhaust feed compositions and the observed NO_x uptake is a strong function of the reaction conditions and Pd speciation. 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. As secondary function Pd-zeolites also catalyze the oxidation of NO to NO₂.

Herein, we use periodic, van der Waals corrected density functional theory (DFT) to propose a complete reaction mechanism for NO storage and oxidation on dynamically hydrated isolated Pd within chabazite (CHA) zeolite. We report the effect of water solvation on binding preference of adsorbates on various monomeric Pd active sites, Fig 1(i), and find that NO preferentially binds to the [Pd(H₂O)₄]²⁺ site and facilitates the activation of one of the water molecules coordinated to Pd. The water activation step results in the formation of the key intermediate HONO, which subsequently disproportionates to NO, NO₂, and H₂O. In the process of HONO formation Bader charge analysis indicates a reduction of Pd, which serves as a storage site for NO. NO desorption at high temperature requires re-oxidation of Pd using either NO₂ or O₂ as oxidant.

Our proposed mechanism predicts the role of water in oxidizing NO at low temperature and is consistent with experiments, showing low temperature NO₂ formation even without oxygen in Fig. 1(ii and iii). Our findings suggest that the presence of water plays a major role in active site transformation, thus, opening up an interesting avenue for catalyst performance improvement.