Abstract

Passive NO_x adsorbers (PNAs) have been proposed for trapping NO_x present in automotive exhaust during the period of cold start during which the three-way convertor is not yet hot enough to be effective for NO_x reduction. Pd-exchanged chabazite (Pd/H–CHA) is a good candidate for passive NO_x adsorption due to its ability to store NO and retain it to high temperatures (>473 K). Previous research suggests that NO adsorbs on both Pd²⁺ and Pd⁺ cations and that NO desorption from Pd²⁺ cations occurs at lower temperatures than from Pd⁺ cations. Since experimental evidence shows that Pd exchanges into CHA exclusively as Pd²⁺, it is not clear how these cations are reduced to Pd+. In this study we show through experiments and theoretical analysis that Pd⁺ cations can form via two processes, each of which involves water adsorbed on Brønsted-acid sites of the zeolite. The first of these processes is 1.5 NO + Pd²⁺Z^–Z^– + 0.5 (H₂O)H⁺Z^– → (NO)Pd⁺Z^–H⁺Z^– + 0.5 NO₂ + 0.5 H⁺Z^–. Experiments confirm that the ratio of the NO₂ formed upon NO adsorption to the NO desorbing from Pd+ at elevated temperatures corresponds to 0.5. Pd²⁺ can also be reduced via the reaction 1.5 CO + Pd²⁺Z^–Z^– + 0.5 (H₂O)H⁺Z^– → (CO)Pd⁺Z^–H⁺Z^– + 0.5 CO₂ + 0.5 H⁺Z^–. Upon subsequent adsorption of NO, NO fully displaces CO from Pd⁺ to form (NO)Pd⁺Z^–H⁺Z^–. In this case, the amount of CO₂ formed upon CO adsorption is 0.5 of the NO desorbing at elevated temperatures from Pd⁺. Gibbs free energy calculations for the above processes at various potential ion-exchange sites in the CHA framework indicate that these reactions are thermodynamically feasible. We also find that Pd⁺ is not formed in the absence of adsorbed water and is readily reoxidized to Pd²⁺ by trace amounts of O₂.

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