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

Trevor M. Lardinois; Jason S. Bates; Harrison H. Lippie; Christopher K. Russell; Jeffrey T. Miller; Harry M. Meyer; Kinga A. Unocic; Vitaly Prikhodko; Xinyi Wei; Christine K. Lambert; Andrew Bean Getsoian; Rajamani Gounder

doi:10.1021/acs.chemmater.0c04465

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

Trevor M. Lardinois, Jason S. Bates, Harrison H. Lippie, Christopher K. Russell, Jeffrey T. Miller, Harry M. Meyer, Kinga A. Unocic, Vitaly Prikhodko, Xinyi Wei, Christine K. Lambert, Andrew Bean Getsoian, Rajamani Gounder

Research output: Contribution to journal › Article › peer-review

45 Scopus citations

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.

Original language	English
Pages (from-to)	1698-1713
Number of pages	16
Journal	Chemistry of Materials
Volume	33
Issue number	5
DOIs	https://doi.org/10.1021/acs.chemmater.0c04465
State	Published - Mar 9 2021

Funding

The authors acknowledge the financial support from the Department of Energy, Energy Efficiency and Renewable Energy (DE-EE0008213). The authors thank the technical support of 9-BM and 10-BM staff at the Advanced Photon Source in Argonne National Lab during XAS characterization. The use of the Advanced Photon Source is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract no. DE-AC02-06CH11357. MRCAT operations and beamlines are supported by the Department of Energy and the MRCAT member institutions. The authors thank all members of our collaborative research team from the University of Kentucky, the University of California—Berkeley, Ford Motor Company, the Oak Ridge National Laboratory, and BASF for helpful technical discussions. The authors additionally thank Casey B. Jones (Purdue) for his critical review of this manuscript and Brian Bayer (Purdue) for assistance with CHA zeolite synthesis. The authors also thank Sachem, Inc. for providing the organic structure-directing agent used to synthesize CHA zeolites. Microscopy research was supported by both the Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy and the Department of Energy, Office of Nuclear Energy, Fuel Cycle R&D Program, and the Nuclear Science User Facilities (using instrumentation FEI Talos F200X S/TEM). The authors also thank Shawn K. Reeves for assistance with TEM sample preparation. This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the U.S. Department of Energy (DOE). The U.S. government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ).

Funders	Funder number
Center for Nanophase Materials Sciences
Department of Energy, Energy Efficiency and Renewable Energy	DE-EE0008213
Scientific User Facilities Division	DE-AC05-00OR22725
U.S. Department of Energy
Office of Science
Basic Energy Sciences	DE-AC02-06CH11357

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Lardinois, T. M., Bates, J. S., Lippie, H. H., Russell, C. K., Miller, J. T., Meyer, H. M., Unocic, K. A., Prikhodko, V., Wei, X., Lambert, C. K., Getsoian, A. B., & Gounder, R. (2021). Structural Interconversion between Agglomerated Palladium Domains and Mononuclear Pd(II) Cations in Chabazite Zeolites. Chemistry of Materials, 33(5), 1698-1713. https://doi.org/10.1021/acs.chemmater.0c04465

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

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.",

author = "Lardinois, {Trevor M.} and Bates, {Jason S.} and Lippie, {Harrison H.} and Russell, {Christopher K.} and Miller, {Jeffrey T.} and Meyer, {Harry M.} and Unocic, {Kinga A.} and Vitaly Prikhodko and Xinyi Wei and Lambert, {Christine K.} and Getsoian, {Andrew Bean} and Rajamani Gounder",

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Lardinois, TM, Bates, JS, Lippie, HH, Russell, CK, Miller, JT, Meyer, HM, Unocic, KA, Prikhodko, V, Wei, X, Lambert, CK, Getsoian, AB & Gounder, R 2021, 'Structural Interconversion between Agglomerated Palladium Domains and Mononuclear Pd(II) Cations in Chabazite Zeolites', Chemistry of Materials, vol. 33, no. 5, pp. 1698-1713. https://doi.org/10.1021/acs.chemmater.0c04465

TY - JOUR

T1 - Structural Interconversion between Agglomerated Palladium Domains and Mononuclear Pd(II) Cations in Chabazite Zeolites

AU - Lardinois, Trevor M.

AU - Bates, Jason S.

AU - Lippie, Harrison H.

AU - Russell, Christopher K.

AU - Miller, Jeffrey T.

AU - Meyer, Harry M.

AU - Unocic, Kinga A.

AU - Prikhodko, Vitaly

AU - Wei, Xinyi

AU - Lambert, Christine K.

AU - Getsoian, Andrew Bean

AU - Gounder, Rajamani

PY - 2021/3/9

Y1 - 2021/3/9

N2 - 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.

AB - 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.

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

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