Palladium exchanged zeolites have been identified as passive NOx adsorbers (PNA) to store NOx at low temperature and release it at higher temperature to a downstream reduction catalyst which converts NOx to N2. PNA occurs with complex exhaust feed compositions and the observed NOx uptake is a strong function of the reaction conditions and Pd loading. Pd can be present in both monomeric and dimeric forms depending on the metal loading and silicon to aluminum ratio. 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 at temperature &gt;100 °C. 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 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, Pd changes its formal oxidation state from +2 to +1 and serves as a storage site for NO. The strong adsorption strength of NO to hydrated Pd+ dictates the NOx release at high temperature. Closing the catalytic cycle requires reoxidation of the palladium by NO2 via HONO formation at high temperature. The NOx storage, oxidation, and release mechanism is further modified in the presence of CO. As the stronger reductant, CO is able to reduce Pd to its metallic state coupled with the release of CO2. The higher degree of Pd reduction leads to an enhanced NO uptake at low temperature but Pd loses coordination with the framework and mobile Pd complexes may agglomerate at elevated temperature causing deactivation. Our proposed mechanisms are consistent with experimental observations and unravel the effect of CO on active site transformation. Water, one of the major constituents of engine exhaust, was found to have a strong effect on oxidation reactions because of the solvated nature of metal cations. Our findings show that the presence of water caused significant changes in the binding preference of adsorbates which opens up an interesting avenue for catalyst performance improvement.

Reductant induced redox behavior of atomically-dispersed palladium in Pd-CHA

ACS Spring Meeting 2021

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