(42f) Effect of Water and CO on the NOx Operating Cycle in Pd/CHA Passive NOx Adsorbers

H₂O and CO affect the NO_x adsorption/desorption characteristics of Pd/CHA passive NO_x adsorbers. However, the presence of various Pd sites (Pd²⁺, Pd¹⁺, [PdOH]⁺, and [Pd-O-Pd]²⁺) makes it challenging to decipher their role on the NO_x adsorption-desorption cycle. DFT calculations are performed in this work 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 on a [PdOH]⁺ site with pre-adsorbed H₂O strengthens NO binding. Even though the presence of H₂O facilitates the reduction of [PdOH]⁺ and [Pd-O-Pd]²⁺ by NO, it negatively impacts the desorption of NO₂ (formed by NO oxidation) from the Pd¹⁺ sites. 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 Pd^II species under an oxidizing atmosphere, such that the NO desorption is facilitated. However, in the presence of H₂O, NO oxidation is favored over Pd¹⁺ reoxidation, thus resulting in the formation of monodentate nitrates (Figure 1(a)). The monodentate nitrates transform to the more stable bidentate nitrates upon water desorption. This finding, along with the higher NO₂ desorption energy in the presence of H₂O can explain the increase in the NO_x desorption temperature in the presence of H₂O. The proposed effect of H₂O on the NO_x operating cycle is depicted in Figure 1(b). In the full paper, the inferences obtained on the effect of H₂O and CO will be used to explain the reported experimental data.