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

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.

Complete Mechanism for NOx Adsorption and Release on Atomically Dispersed Palladium in Pd-CHA

“…Both oxidants NO 2 and O 2 bind significantly stronger on Z[Pd I ] compared to the Z 2 [Pd II ], regardless of the extent of hydration. Our binding energy values are in good agreement with previous reports, 25,37,42 and we attribute the strong binding to a significant electron transfer from Z[Pd I ] to the oxidants NO 2 and O 2 . We quantified the charge transfer as the Bader charge difference before and after adsorption of NO 2 and O 2 to Z[Pd I ] and show the results in Figure 1a.…”

“…This is consistent with the recent findings of Kim et al suggesting that water is necessary during the reduction of palladium from +2 to +1. 37 To further support our assertion that oxygen is not needed for NO 2 formation, we conducted additional flow experiments.…”

“…This oxidation is commonly attributed to the reduction of palladium which in turn acts as an active site for high temperature NO release. 11,27,28,37 To accommodate the 2|e − | oxidation of NO, reported mechanisms for NO oxidation over Pd/zeolites involve the formation of dimeric active sites analogous to dimeric Cu sites in Cu/zeolites during SCR of NO. 11,12,26,27,38 The assumption of dimeric sites allows formulating mechanisms in which Pd takes formal oxidation states of +2 and +1 and avoids the complete reduction to metallic Pd.…”

“…Gupta et al and Kim et al have attributed the additional NO uptake to an increase in available sites after H 2 O desorption. 11,37 With a further increase in temperature from 200 to 400 °C, only NO desorbs, while no significant amounts of NO 2 are detected. Finally, in the presence of oxygen and at temperatures above 400 °C, NO is oxidized to NO 2 as evidenced by the blue trace in Figure 3.…”

“…Overall, the comparison between these two experiments supports our mechanism, which does not assign any role to O 2 for low temperature NO oxidation or Pd site transformation. 11 Similarly, Kim et al postulated that NO could be oxidized by water, 37 and Song et al highlighted the favorable reduction of Pd by CO in the presence of water in Pd/SSZ-13. 53 Moreover, Gupta et al reported that ethylene oxidation to acetaldehyde on 1%Pd/SSZ-13 only required H 2 O.…”