Abstract
Palladium-exchanged zeolites are candidate materials for passive NOx adsorption in automotive exhaust aftertreatment, where mononuclear Pd cations behave as precursors to the purported NOx adsorption sites. Yet, the structures of zeolite lattice binding sites capable of stabilizing mononuclear Pd2+ ions, and the mechanisms that interconvert agglomerated PdO and Pd domains into mononuclear Pd2+ 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 NH4-form zeolites initially form mononuclear [Pd(NH3)4]2+ complexes, but subsequent thermal treatments (573–723 K; air) lead to in situ formation of H2 that first reduces Pd2+ 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 Pd2+, as quantified by H2 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 Pd2+ 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 Pd2+ 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.
