(99e) Reductant Induced Redox Behavior of Atomically-Dispersed Palladium in Pd-CHA

Palladium exchanged zeolites have been identified as passive NO_x adsorbers (PNA) to store NO_x at low temperature and release it at higher temperature to a downstream reduction 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 loading. Notably, the presence of water leads to solvated and mobile Pd complexes that behave differently from the cationic species anchored to the zeolite framework under dry conditions. Here, we used density functional theory (DFT) and bench-flow reactor experiments to simulate the treatment of diesel exhaust feed during warmup, with the objective of elucidating the complete reaction mechanism for NO storage and oxidation on dynamically hydrated and isolated Pd within chabazite (CHA) zeolite. We also study the effect of and propose a mechanism for CO on active site transformation and deactivation.

Upon binding, NO facilitates the activation of one of the water molecules coordinated to Pd, forming the key intermediate HONO, which subsequently disproportionates to NO, NO₂, and H₂O as shown in Fig.1(a). The reduced form of Pd serves as a strong storage site for NO and the mechanism is favorable under reaction conditions as shown in Fig.1(b).

We find that active site transformation in the presence of CO is facilitated by both oxygen and water forming Pd-hydroperoxyl, which leads to enhanced NO uptake at low temperature. Consistent with our DFT results, the experiments shown in Fig.1(c) also confirm that CO does not change the NO_x uptake under dry condition whereas NO_x uptake increases when both water and oxygen are present as shown in Fig.1(d). Finally, our results suggest that the presence of both water and CO is required to deactivate the catalyst.