(229b) Mechanistic Role of Water in the Storage and Oxidation of NO on Pd/CHA
- Conference: AIChE Annual Meeting
- Year: 2021
- Proceeding: 2021 Annual Meeting
- Group: Catalysis and Reaction Engineering Division
- Time: Monday, November 8, 2021 - 2:35pm-2:55pm
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.