Structural Interconversion between Agglomerated Palladium Domains and Mononuclear Pd(II) Cations in Chabazite Zeolites

Abstract

Palladium-exchanged zeolites are candidate materials for passive NO x adsorption in automotive exhaust aftertreatment, where mononuclear Pd cations behave as precursors to the purported NO x adsorption sites. Yet, the structures of zeolite lattice binding sites capable of stabilizing mononuclear Pd 2+ ions, and the mechanisms that interconvert agglomerated PdO and Pd domains into mononuclear Pd 2+ ions during Pd redispersion treatments, remain incompletely understood. Here, we use a suite of spectroscopic methods and quantitative site titration techniques to characterize mononuclear and agglomerated Pd species on zeolites with varying material properties and treatment history. Aqueous-phase methods to introduce Pd onto NH 4 -form zeolites initially form mononuclear [Pd(NH 3 ) 4 ] 2+ complexes, but subsequent thermal treatments (573−723 K; air) lead to in situ formation of H 2 that first reduces Pd 2+ to metallic Pd domains, which are then oxidized by air to PdO domains. Progressive treatment of Pd-zeolites in air to higher temperatures (723−1023 K) converts larger fractions of agglomerated PdO to mononuclear Pd 2+ , as quantified by H 2 temperature programmed reduction, because higher temperature treatments facilitate Pd redispersion toward deeper locations within chabazite (CHA) crystallites, which is corroborated by complementary titrimetric and spectroscopic data. Pd-CHA zeolites synthesized with similar bulk Pd and framework Al content, but varying framework Al arrangement, provide evidence that six-membered rings (6-MR) hosting paired Al sites (Al−O−(Si−O) x −Al, x = 1, 2) stabilize Pd 2+ ions and that otherwise isolated Al sites can stabilize [PdOH]+ species, identifiable by an IR OH stretch at 3660 cm −1 . These findings clarify the underlying chemical processes and gas environments that cause Pd agglomeration in zeolites and their subsequent redispersion to mononuclear Pd 2+ ions, which prefer binding at 6-MR paired Al sites in CHA, and indicate that higher temperature air treatments lead to more uniform Pd spatial distributions throughout zeolite crystallites.

Uniform synthesis of palladium species confined in a small-pore zeolite via full ion-exchange investigated by cryogenic electron microscopy

“…Hence, it can be deduced that the slow ramping rate is required to ensure sufficient time for the Pd 2+ ions to be stabilized at Al-sites thereby suppressing the agglomeration of the Pd 2+ ions. We also note that the redispersion of Pd species during the calcination can contribute to generating isolated Pd 2+ ions, 34 but for the case of the Pd-exchanged SSZ-13 sample prepared with Pd(NH 3 ) 4 Cl 2 and NH 4 -SSZ-13, the redispersion of surface Pd species into the internal zeolite was less noticeable since the Pd species had already been well-distributed through the SSZ-13 (Fig. 1b and Table S3 †).…”

Investigation of CO Deactivation of Passive NOx Adsorption on La Promoted Pd/BEA

“…XPS results also showed a clear shift for Pd for the reacted Pd/BEA sample toward lower binding energies which is most likely due to reduction of ionic Pd to metallic Pd. Metallic Pd can thereafter form PdO x clusters in the form of agglomerated particles [22]. For Pd-La/BEA on the other hand, a lower Pd oxidation state was found for the degreened sample, which increased after the aging, also suggesting more PdO x formation.…”

“…It should be noted that all the used samples had undergone a degreening step at 750 °C with 400 ppm NO, 8% O 2 , and 5% H 2 O. According to Lardionois et al [22], high temperature treatment in air (i.e., high O 2 concentrations) results in more uniform Pd distributions. Each monolith was coated stepwise and between each step dried with a heating gun at around 90 °C for 2 min.…”

Elucidating the role of CO in NO storage mechanism on Pd/SSZ-13 with in situ DRIFTS

“…In fact, a recent study demonstrated that high temperature treatment without water is also effective in redispersing PdO into Pd 2+ ions in SSZ-13 zeolites. 30 Interestingly, our recent study observed that high temperature treatment could not only disperse PdO but also affect the distribution of Pd 2+ ions in FER zeolites. 18 Dispersed Pd 2+ ions anchored at negatively charged AlO4sites within the zeolite frameworks are known to effectively adsorb NO under dry conditions.…”

“…PdO particles are readily reduced at lower temperatures compared to charge compensating Pd 2+ ions. 30,[47][48] When dehydrated Pd/SSZ-13 is exposed to CO at 15°C, only slight CO2 formation was observed probably due to PdO reduction (Figure 7b). The main reduction peak is found around 270°C, which is much higher than the temperature where NOx uptake is conducted during PNA.…”