Studies on reduction dynamics of nickel atoms incorporated in MCM-41 by XANES with two-dimensional correlation spectroscopy approaches

The reduction kinetics of Ni cations incorporated in MCM-41 was investigated by in situ XANES measurements performed at a dispersive beamline. The detailed analysis of XANES spectra was conducted by perturbation-correlation moving-window 2D correlation spectroscopy (PCMW2D) and generalized 2D correlation spectroscopy (2DCORA) formalisms. PCMW2D results showed important changes on white-line intensities associated with water desorption and dehydroxilation during the heating process. PCMW2D successfully located the onset of nickel reduction during H-2-TPR and characterized the pattern of changes on white-line and preedge intensities from rapid to gradual. The contributive character of progressive 2D correlation analysis calculations induced neglecting of variations on rate of spectral feature changes. To circumvent this limitation, we employed an alternative approach, calculating contributions to the 2D correlation function using a subset of spectra to take into account the temporal complexity structure of in situ collected data. The results show that the Ni reduction is a dynamic process in which the rate of spectral variations changes as the reaction progresses.