Fig. 6.

5D chemical evolution of the Ni-containing species in the catalyst bed. a Phase distribution volumes of NiO, NiAl₂O₄ and Ni as obtained from the Rietveld analysis of the 3D-XRD-CT data collected at the four different operating conditions. The values in the colorbar axes have been chosen to achieve the best possible contrast. b Solid-state evolution of the NiO and NiAl₂O₄ phases at the bottom and top of the catalyst bed during the redox experiment (ie, XRD-CT scan 1 and 30). The results presented in this figure correspond to the Rietveld analysis of ca. 1.2 × 10⁶ diffraction patterns