The Effect of Si/Al Ratio for Pd/BEA and Pd/SSZ-13 Used as Passive NOx Adsorbers

Abstract

The mitigation of cold-start emissions involves the development of passive NO x adsorbers (PNA) systems that store NO x at low temperature, being designed to release the trapped NO x at higher temperatures, where downstream NO x reduction catalysts are efficient. Pd-based zeolites (BEA, SSZ-13) with different SAR (Si-to-Al ratio) were used for PNA investigation, and Pd/Ce/Al 2 O 3 catalyst was used as a reference for comparison. In this study, NO x adsorption is investigated at low temperature (80 °C) and it is released during a temperature ramp (to 400 °C) under various gas feed composition. Moreover, detailed characterization was performed using BET, XRD, XPS, TPO, STEM and ICP-SFMS and the stored NO species was studied using in-situ DRIFTS. The addition of CO to the storage mixture resulted in that for Pd/zeolites with low and medium SAR the binding energy for NO was increased. In addition, NO was stored in larger quantities, especially for the Pd/SSZ-13 samples. However, for Pd/BEA (SAR = 300) no such stable NO species was formed and for Pd/Ce/Al 2 O 3 the CO addition was even negative. Moreover, in-situ NO DRIFTS showed that there was large amount of nitrosyls on ionic palladium for the Pd/zeolites with low and medium SAR, indicating that a significant fraction of the palladium was in ionexchanged positions, while this peak was small for the Pd/BEA (SAR = 300) and non-existing for Pd/Ce/Al 2 O 3. Thus, CO addition is beneficial for Pd species that are in ion-exchanged positions, but this is not the case for Pd particles and this can explain the observations that CO is only beneficial for Pd/zeolites with low and medium SAR. Moreover, experiments with similar SAR [Pd/BEA (SAR = 25) and Pd/SSZ-13 (SAR = 24)], showed that there is larger stability of the stored NO x in the small pore Pd/SSZ-13.

Zeolite Beta Doped with La, Fe, and Pd as a Hydrocarbon Trap

“…These results are consistent with the study on propene adsorption by Yoda et al [17]. In that study, they found that propene adsorption occurs on Brønsted acid sites, which are blocked by the presence is known that there are significant interactions between Pd and water, which has been found in passive NOx adsorption studies, where the NO adsorption on Pd/zeolites is significantly reduced in the presence of water [31]. However, DRIFTS studies have shown that NO can replace some of the water species, causing water release [31].…”

“…The results from the TPO experiments are shown in Figure 1. The results in the figure shows that Pd/BEA has a distinct temperature of oxidation centered at 330 • C, which has been confirmed by other studies of BEA impregnated with Pd [31]. The major peak of oxidation for Fe/BEA was found at 230 • C; however, this peak is considerably lower magnitude than the peak associated with the Pd-containing samples.…”

Stabilization of Super Electrophilic Pd+2 Cations in Small-Pore SSZ-13 Zeolite

“…Its additional charge transfer character is also evident based on the intense pink-purple color of the complex upon CO adsorption ( Fig S5). Previous studies claimed the formation of Pd(IV) cation on various solid supports and zeolites based on (1) the indirect evidence from stoichiometry from XRD refinement 27 (2) DFT-modeling 46 and (3) the presence of the high binding energy feature (relative to PdO with BE of Pd 3d5/2 at 336.5-337.0 eV) in the Pd 3d XPS spectra 24,28,29,47,48 . Interestingly, in some works a feature at 337.7-338.0 eV on ceria supports, for example, 49,50,51 [8][9][10] We also note that upon dehydration either in He or O2, the color of the sample changes from light yellow to light pink (Fig.…”

Stabilization of Super Electrophilic Pd+2 Cations in Small-Pore SSZ-13 Zeolite

“…However, based on the provided evidence, this Pd/SSZ-13 system selectively forms non-classical Pd +2 (CO)2 fragments and not Pd +3 (CO)2 as previously proposed in the literature. 11,12,24,28 The non-classical nature of this complex is further indirectly corroborated by the fact that its infrared signature does not depend on zeolite type to any significant extent and presents a Pd +2 (CO)2 fragment stabilized as ion pairs (with the Al-associated negative oxygen atoms of zeolite acting as a negative part of the ion pair, whereas Pd +2 and Pd +2 (CO)2 are the positive +2-charged part of the ion pair) in a small (~0.85 nm) SSZ-13 cage ( Fig. S10).…”

“…Its additional charge transfer character is also evident based on the intense pink-purple color of the complex upon CO adsorption ( Fig S5). Previous studies claimed the formation of Pd(IV) cation on various solid supports and zeolites based on (1) the indirect evidence from stoichiometry from XRD refinement 27 (2) DFT-modeling 46 and (3) the presence of the high binding energy feature (relative to PdO with BE of Pd 3d5/2 at 336.5-337.0 eV) in the Pd 3d XPS spectra 24,28,29,47,48 . Interestingly, in some works a feature at 337.7-338.0 eV on ceria supports, for example, 49,50,51 was attributed to Pd(IV) ions or PdO2 .…”