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. Here, 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 resonance 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 (CuxOy) complexes, which are interpreted alongside Cu(I) oxidation kinetics extracted from in situ X-ray absorption spectroscopy (XAS). Raman spectra of several plausible CuxOy species simulated using density functional theory suggest that experimental spectra (λex = 532 nm) capture the formation of mono(μ-oxo)dicopper species (ZCuOCuZ). Transient experiments show that the timescales required to form CuxOy structures that no longer change in Ra-man 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 timescales for the complete conversion of the initial Cu(I) intermediates to their Cu(II) states (<0.3 h, reflected in XANES spectra), which demonstrates that CuxOy complexes continue to evolve structurally following rapid oxidation. The dependence of ZCuOCuZ formation rates on O2 pressure, H2O pressure, and temperature are consistent with a mechanism in which ZCuOH reduce to form ZCu+ sites that bind molecular oxygen and form ZCu-O2 intermediates. Subsequent reaction with proximate ZCu+ form 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 unrec-ognized complexities in the processes by which Cu ions in zeolites activate O2 to form active CuxOy complexes, which under-score the insight afforded by judicious combinations of experimental and theoretical techniques.