Experimental and Theoretical Studies of Pd Cation Reduction and Oxidation During NO Adsorption on and Desorption from Pd/H–CHA

Passive NO<sub>x</sub> adsorbers (PNAs) have been proposed for trapping NO<sub>x</sub> 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<sub>x</sub> reduction. Pd-exchanged chabazite (Pd/H–CHA) is a good candidate for passive NO<sub>x</sub> adsorption due to its ability to store NO and retain it to high temperatures (>473 K). Previous research suggests that NO adsorbs on both Pd<sup>2+</sup> and Pd<sup>+</sup> cations and that NO desorption from Pd<sup>2+</sup> cations occurs at lower temperatures than from Pd<sup>+</sup> cations. Since experimental evidence shows that Pd exchanges into CHA exclusively as Pd<sup>2+</sup>, it is not clear how these cations are reduced to Pd<sup>+</sup>. In this study we show through experiments and theoretical analysis that Pd<sup>+</sup> 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 + Pd<sup>2+</sup>Z<sup>–</sup>Z<sup>–</sup> + 0.5 (H<sub>2</sub>O)H<sup>+</sup>Z<sup>–</sup> → (NO)Pd<sup>+</sup>Z<sup>–</sup>H<sup>+</sup>Z<sup>–</sup> + 0.5 NO<sub>2</sub> + 0.5 H<sup>+</sup>Z<sup>–</sup>. Experiments confirm that the ratio of the NO2 formed upon NO adsorption to the NO desorbing from Pd<sup>+</sup> at elevated temperatures corresponds to 0.5. Pd<sup>2+</sup> can also be reduced via the reaction 1.5 CO + Pd<sup>2+</sup>Z<sup>–</sup>Z<sup>–</sup> + 0.5 (H<sub>2</sub>O)H<sup>+</sup>Z<sup>–</sup> → (CO)Pd<sup>+</sup>Z<sup>–</sup>H<sup>+</sup>Z<sup>–</sup> + 0.5 CO<sub>2</sub> + 0.5 H<sup>+</sup>Z<sup>–</sup>. Upon subsequent adsorption of NO, NO fully displaces CO from Pd<sup>+</sup> to form (NO)Pd<sup>+</sup>Z<sup>–</sup>H<sup>+</sup>Z<sup>–</sup>. In this case, the amount of CO<sub>2</sub> formed upon CO adsorption is 0.5 of the NO desorbing at elevated temperatures from Pd<sup>+</sup>. 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<sup>+</sup> is not formed in the absence of adsorbed water and is readily reoxidized to Pd<sup>2+</sup> by trace amounts of O<sub>2</sub>.