H2O and CO affect the adsorption/desorption temperature and storage capacity of Pd/CHA passive NOx adsorbers. The presence of various Pd sites (Pd2+, Pd1+, [PdOH]+, and [Pd-O-Pd]2+) makes it challenging to decipher the mechanisms of the adsorption-desorption cycle. Density functional theory (DFT) simulations are performed to understand the impact of H2O and CO on NO interaction with various Pd sites, and explain the mechanisms of NO adsorption, oxidation, and desorption in their presence. Calculations show that while H2O has a negligible effect on the NO adsorption energetics for various Pd sites (Pd2+, Pd1+, and [Pd-O-Pd]2+), it significantly weakens the NO binding on [PdOH]+. However, the presence of CO strengthens NO binding on [PdOH]+ with pre-adsorbed H2O. The presence of H2O facilitates the reduction of [PdOH]+ and [Pd-O-Pd]2+ by NO but negatively impacts the desorption of NO2 (formed by NO oxidation) from the Pd1+ sites (Edes = 198 kJ/mol in the presence of H2O as compared to 123 kJ/mol in its absence). In addition, CO is shown to be a better reductant as compared to NO when both CO and NO are co-adsorbed on [Pd-O-Pd]2+, thus resulting in the formation of Pd1+. The Pd1+ sites can reoxidize to PdII species under an oxidizing atmosphere. However, the presence of H2O favours NO oxidation over the reoxidation of Pd1+. The high NO2 desorption energy along with inhibition of Pd1+ reoxidation in the presence of H2O can explain the experimentally-observed increase in the NOx desorption temperature in the presence of H2O.

Effect of water and CO on the NOx operating cycle in Pd/CHA passive NOx adsorbers

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