Structural Evolution of Copper-Oxo Sites in Zeolites upon the Reaction with Methane Investigated by Means of Cu K-edge X-ray Absorption Spectroscopy

Abstract

The structure of copper sites formed under an oxidative environment and their evolution in the course of the reaction with methane at elevated temperature was investigated by means of Cu K-edge X-ray absorption spectroscopy for a series of copper-containing MFI, MOR, and FAU zeolites. The pretreatment in oxygen at 723 K leads to the formation of copper(II)-oxo sites, whose nature depends on the framework type. Dimeric species are formed in CuMFI material, dimeric and monomeric sites coexist in CuMOR, and agglomerated copper-oxo nanoclusters are found in large-pore copper-containing faujasite (CuFAU). For all studied materials, the reaction with methane resulted in the exclusive formation of copper(I) species; no formation of metallic copper was detected even at 748 K. The nature of formed copper(I) species is governed by the structure of corresponding copper(II) centers. In particular, monomeric and dimeric copper(II)-oxo sites hosted in CuMOR and CuMFI are transformed into isolated copper(I) cations coordinated to ion-exchange positions of the zeolite. Contrarily, copper(II)-oxo clusters present in CuFAU undergo restructuring with only a partial loss of extra-framework oxygen and form aggregated species with a structure similar to that of bulk copper(I) oxide.

Understanding C–H activation in light alkanes over Cu-MOR zeolites by coupling advanced spectroscopy and temperature-programmed reduction experiments

“…Its spectroscopic signature is fully consistent with bare Cu I ions coordinated to the MOR framework. 34,70 Overall, the XAS results obtained under CH 4 -TPR well align with the recent report by Artsiusheuski et al, 92 conrming for Cu-MOR the absence of either metallic or oxidic aggregates at the end of the TPR step. Nonetheless, during both C 2 H 6 -and CO-TPR, additional Cu I species were found.…”

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper‐Containing Mordenite: Insights into Reaction Mechanism and Product Protection

“…The spectra of the oxygen-activated CuMOR (dark blue lines) contain a weak peak at 8977 eV due to 1s!3d quadrupole transition and a shoulder at 8986 corresponding to 1s!4p transition typical of copper(II) species in CuMOR. [28,62] Upon reaction of CuMOR with alkanes, the intensity of the aforementioned signals decreases and, simultaneously, a signal at 8983 eV appears and develops. The signal is characteristic of 1s!4p transition in copper(I)-containing zeolites [19,28] indicating the reduction of copper (II) sites.…”

“…[28,62] Upon reaction of CuMOR with alkanes, the intensity of the aforementioned signals decreases and, simultaneously, a signal at 8983 eV appears and develops. The signal is characteristic of 1s!4p transition in copper(I)-containing zeolites [19,28] indicating the reduction of copper (II) sites. Figure 4D illustrates the evolution of the Cu II fraction versus temperature from the MCR-ALS analysis (Section 2.7 of the Supporting Information).…”

“…[23][24][25][26] Simultaneously, copper(II)-oxo sites are reduced to copper(I) species. [20,27,28] Finally, the products of partial oxidation are converted to methanol by hydrolysis. [29] While the activation of methane over copper-containing zeolites has been studied in depth, much less attention has been paid to CÀ H bond activation in other hydrocarbons.…”

Oxidizing Role of Cu Cocatalysts in Unassisted Photocatalytic CO2 Reduction Using p-GaN/Al2O3/Au/Cu Heterostructures

“…Cu K-edge X-ray absorption spectroscopy is an ideal technique to probe the composition of the Cu cocatalyst. − The elemental specificity of X-ray absorption allows for selective probing of Cu in the multicomponent p-GaN/Al 2 O 3 /Au/Cu system. The X-ray absorption near-edge structure (XANES) region of the Cu K-edge absorption spectrum is highly sensitive to both the oxidation state and coordination environment of Cu. , The method can also be applied under operating conditions, for example, under CO 2 and H 2 O vapor atmosphere and under visible light illumination, which is relevant here when characterizing the Cu cocatalyst composition in p-GaN/Al 2 O 3 /Au/Cu.…”