Identifying Cu-oxo species in Cu-zeolites by XAS: A theoretical survey by DFT-assisted XANES simulation and EXAFS wavelet transform

Abstract

Abstract Cu-exchanged zeolites are attracting increasing attention as platforms for high-impact selective redox chemistry, including NH3-mediated selective catalytic reduction of harmful nitrogen oxides (NH3-SCR) and selective oxidation of methane to methanol (MTM). X-ray absorption spectroscopy (XAS) today represents a well-established and widespread technique for the characterization of these systems, but open challenges remain to accurately determine nuclearity and chemical identity of Cu-oxo species stabilized under high-temperature oxidative conditions. To systematically assess limitations and potential of XAS in this context, here we consistently explored by DFT a set of monomeric and dimeric CuII species incorporated in the chabazite (CHA) and mordenite (MOR) frameworks, envisaged as key players for both NH3-SCR and MTM. We present a combined DFT-assisted analysis based on both full-potential XANES simulation and Wavelet Transform Analysis (WTA) applied to computed EXAFS signals. The resulting theoretical insights set the bases for an easier and more robust interpretation of the XAS data collected on Cu-zeolite systems. In particular, we evidenced possible fingerprint features in the XANES region, together with the difficulties in discriminating among monomeric and dimeric CuII moieties solely based on their XANES signatures. We then show the potential of Wavelet Transform Analysis as a new tool in the identification and visualization of EXAFS atomic contributions, to possibly enhance the EXAFS sensitivity to the nuclearity of Cu-oxo species in Cu-zeolites.