Multivariate curve resolution analysis applied to time-resolved synchrotron X-ray Absorption Spectroscopy monitoring of the activation of copper alumina catalyst

• Copper catalyst supported on hierarchical porous alumina with macro-mesoporosity.
• Fast in situ acquisitions of the copper catalysts using a Quick-XAS monochromator.
• Two-step reduction of Cu(II) to Cu(0) involving a Cu(I) species at 250 °C.
• MCR-ALS method applied to time-resolved XAS data of mixtures of components.

The pure X-ray Absorption (XAS) spectral constituents and related concentration profiles characterizing the reduction of a copper-based catalyst supported on hierarchically porous alumina were disentangled from a set of time-resolved Quick-XAS using a Multivariate Curve Resolution-methodology. First, the number of components involved in the data set and the range of existence of intermediate species were determined by Principal Component Analysis (PCA). Then, a Multivariate Curve Resolution with Alternating Least Squares fitting of the set of time-resolved Quick-XANES was performed using the constraints of non-negativity of both absorption and concentration and of closure of concentration. Consideration about the strategy of use of the chemometric methods and robustness regarding the reliability of results for time-resolved XAS data is discussed. The intermediate copper species determined by MCR-ALS is at the monovalent oxidation state and characterized by a local order of 2 oxygen atoms at 1.915 Å. Upon isothermal treatment under 60% H2/He at 250 °C for 30 min, the intermediate species appears quite resistant to complete copper reduction. Copper speciation obtained using MCR-ALS method leads to very different results than those obtained using bulk CuO, Cu2O and metallic Cu as constituents, MCR-ALS concentration profile so-determined being fully consistent with the reducibility of the catalyst measured by TPR.

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