Determining Cu–Speciation in the Cu–CHA Zeolite Catalyst: The Potential of Multivariate Curve Resolution Analysis of In Situ XAS Data

The Cu-CHA zeolite today represents an attractive platform to design catalysts for deNOx applications by NH3-assisted Selective Catalytic Reduction (NH3-SCR) and for the low-temperature selective oxidation of methane to methanol (MTM). Accessing a quantitative understanding of Cu-speciation in this material is a key step to unveil structure-performance relationships for these high-impact processes. Herein, we select Cu-CHA as a case study to demonstrate the potential of chemometric approaches, such as Multivariate Curve Resolution (MCR) applied in combination with Principal Component Analysis (PCA). We employ these methods to assist the interpretation of X-ray Absorption Spectroscopy (XAS) experiments in the near-edge (XANES) region, determining the spectroscopic signatures and concentration profiles of the pure Cu-species formed. We pinpoint the composition impact on the material reducibility and highlight Cu-speciation-productivity relationships for the MTM process. Furthermore, we report novel insights on the formation of O2-derived species in Cu-CHA, obtained from MCR analysis of High Energy Resolution Fluorescence Detected (HERFD) XANES data collected during thermal treatment of Cu-CHA in both He and O2 gas flow. Multivariate analysis, in combination with the superior energy resolution adopted, allows us to identify an additional Cu(II) species. This component, different from the previously characterized Z[Cu(II)OH] moiety, is only formed at significant concentrations in O2 and it is envisaged to play an important role in the MTM conversion.