Copper Pairing in the Mordenite Framework as a Function of the Cu^I/Cu^II Speciation

Abstract

Aseries of gas-phase reactants is used to treat aCuexchanged mordenite zeolite with the aim of studying the influence of the reaction environment on the formation of Cu pairs.The rearrangement of Cu ions to form multimeric sites as af unction of their oxidation state was probed by X-ray absorption spectroscopy( XAS) and also by applying advanced analysis through wavelet transform, am ethod able to specifically locate Cu-Cu interactions also in the presence of overlapping contributions from other scattering paths.T he nature of the Cu-oxo species formed upon oxidation was further crosschecked by DFT-assisted fitting of the EXAFS data and by resonant Raman spectroscopy. Altogether,the Cu I / Cu II speciation clearly correlates with Cu proximity, with metal ion pairs quantitatively forming under an oxidative environment.

Relationships among the Catalytic Performance, Redox Activity, and Structure of Cu-CHA Catalysts for the Direct Oxidation of Methane to Methanol Investigated Using In Situ XAFS and UV–Vis Spectroscopies

“…10,12,16,18,30−34 In fact, the previous study has revealed using a special EXAFS analysis technique, e.g., EXAFS wavelet transform analysis, although it is difficult to evaluate Cu−Cu contributions from the usual EXAFS analysis. 25,29 The flow of wet He applied after this step caused almost no change in the first shell peak but a decrease in the second shell, suggesting that the interaction between Cu 2+ ions and the zeolite framework was partly disrupted because of ligand exchange with H 2 O under the wet gas flow conditions. After the reaction gas flow, the spectrum showed a slight decrease in the first shell peak at 1.5 Å, and the reduction gas flow caused a further decrease in the first shell peak.…”

“…However, note that this result does not deny the presence of Cu−Cu pairs in the Cu-CHA catalysts. 25,29 The Cu-CHA Cu single sites are not active species. 10,12,16,18,30−34 In fact, the previous study has revealed using a special EXAFS analysis technique, e.g., EXAFS wavelet transform analysis, although it is difficult to evaluate Cu−Cu contributions from the usual EXAFS analysis.…”

“…Cu structure provided improved CH 3 OH selectivity but slower Cu 2+ reduction rates and lower catalytic activity for CH 4 oxidation than the Cu-CHA with a high proportion of the ZCuOH structure. According to the literature, Cu 2 O x species formed by two Cu sites are active for CH4 oxidation, but single Cu sites are not.10,12,16,18,30−34 Although the EXAFS curve fitting analysis was performed using the simple model in this study, the EXAFS spectra do not deny the presence of Cu−Cu pairs, and Cu−Cu pairs are considered to be formed in the Cu-CHA catalysts according to previous reports 25,29. In addition, single Cu sites can be intermediate of Cu 2 O x species 30,32.…”

Influence of ion mobility on the redox and catalytic properties of Cu ions in zeolites

“…In a recent study by Deplano et al , the tendency of Cu ions to form ion pairs as a function of their oxidation state was discussed. 81 By performing a detailed wavelet transform analysis, a method enabling the discrimination of contributions overlapping in the R space if visualized by conventional FT-EXAFS, of the XAS data obtained on the same Cu-MOR sample, subjected to different redox pretreatments, the preference toward the formation of Cu neighbouring sites (Cu–Cu distance <3.5 Å) in the presence of Cu 2+ was proved. Cu + ions, instead, do not exhibit a clear mutual interaction fingerprint, testifying their tendency to segregate from each other within the zeolitic framework.…”

Assessing the Influence of Zeolite Composition on Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-CHA DeNOx Catalysts by Machine Learning-Assisted X-ray Absorption Spectroscopy

“…Together with the conventional Fourier transform (FT)-EXAFS representation, we exploited a WT-based analysis 23 , 24 to unambiguously identify possible Cu–Cu scattering contributions possessing a characteristic lobe centered at ca. 7 Å –1 along the k direction 14 , 20 , 25 − 28 (SI, Section 3 for details).…”