Title: Dioxygen Activation Kinetics over Distinct Cu Site Types in Cu-Chabazite Zeolites

Abstract

Cu-exchanged zeolites activate dioxygen to form active sites for partial methane oxidation (PMO), nitrogen oxide decomposition, and carbon monoxide oxidation. Apparent rates of O₂ activation depend both on the intrinsic kinetics of distinct Cu site types and the distributions of such sites within a given zeolite, which depend on the density and arrangement of the framework Al atoms. In this work, we use hydrothermal synthesis methods to control the arrangement of framework Al sites in chabazite (CHA) zeolites and, in turn, the distinct Cu site types formed. Time-resolved in situ Raman spectroscopy reveals the kinetics of O₂ adsorption and activation within these well-defined Cu-CHA materials and the concomitant structural evolution of copper–oxygen (Cu_xO_y) complexes, which are interpreted alongside Cu(I) oxidation kinetics extracted from in situ X-ray absorption spectroscopy (XAS). Raman spectra of several plausible Cu_xO_y species simulated using density functional theory suggest that experimental spectra (λ_ex = 532 nm) capture the formation of mono(μ-oxo)dicopper species (ZCuOCuZ). Transient experiments show that the timescales required to form Cu_xO_y structures that no longer change in Raman spectra correspond to the durations of oxidative treatments that maximize CH₃OH yields in stoichiometric PMO cycles (approximately 2 h). Yet, these periods extend well beyond the timescalesmore » for the complete conversion of the initial Cu(I) intermediates to their Cu(II) states (<0.3 h, reflected in X-ray absorption near edge spectroscopy spectra), which demonstrates that Cu_xO_y complexes continue to evolve structurally following rapid Cu(I) oxidation. The dependence of ZCuOCuZ formation rates on O₂ pressure, H₂O pressure, and temperature is consistent with a mechanism in which ZCuOH reduces to form ZCu⁺ sites that bind molecular oxygen and form ZCu-O₂ intermediates. Subsequent reaction with proximate ZCu⁺ forms bridging peroxo dicopper complexes that cleave O–O bonds to form ZCuOCuZ in steps facilitated by water. These data and interpretations provide evidence for the chemical processes that link rapid and kinetically irrelevant Cu oxidation steps (frequently probed by XAS and UV–vis spectroscopy) to the relatively slow genesis of reactive Cu complexes that form CH₃OH during PMO. In doing so, we reveal previously unrecognized complexities in the processes by which Cu ions in zeolites activate O₂ to form active Cu_xO_y complexes, which underscore the insight afforded by judicious combinations of experimental and theoretical techniques.« less

Authors:

Bregante, Daniel T.  ^[1];

• Search DOE PAGES for author "Bregante, Daniel T."
• Search DOE PAGES for ORCID "0000-0003-2157-1286"

Wilcox, Laura N. ^[2]; Liu, Changming ^[3]; Paolucci, Christopher  ^[3];

• Search DOE PAGES for author "Paolucci, Christopher"
• Search DOE PAGES for ORCID "0000-0002-4506-9306"

Gounder, Rajamani  ^[2];

• Search DOE PAGES for author "Gounder, Rajamani"
• Search DOE PAGES for ORCID "0000-0003-1347-534X"

Flaherty, David W.  ^[1]

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• Search DOE PAGES for ORCID "0000-0002-0567-8481"

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+ Show Author Affiliations

1. Univ. of Illinois at Urbana-Champaign, IL (United States)
2. Purdue Univ., West Lafayette, IN (United States)
3. Univ. of Virginia, Charlottesville, VA (United States)

Publication Date:

Fri Sep 10 00:00:00 EDT 2021

Research Org.:

Purdue Univ., West Lafayette, IN (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)

OSTI Identifier:

1835588

Grant/Contract Number:  

SC0019026; SC0020224; AC02-06CH11357; CBET-1942015

Resource Type:

Accepted Manuscript

Journal Name:

ACS Catalysis

Additional Journal Information:

Journal Volume: 11; Journal Issue: 19; Journal ID: ISSN 2155-5435

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Cu-SSZ-13; partial methane oxidation; Raman spectroscopy; spectrokinetics; in situ spectroscopy