ELECTRONIC STRUCTURE, MAGNETISM AND DYNAMICS
Fe can be detected, indicating that the average local geometry around oxygen is not influenced by the small amount of iron used to modify the MgAl2O4 spinel (Figure 86a).
Upon reduction of the undoped spinel, the Mg L2,3 and Al L2,3 edges show slight changes in peak ratio (Figure 86b), which, however, do not lead to a significant change in the spinel inversion parameter. The latter remains at ~0.3, i.e., keeping the Mg and Al cations randomly distributed over the octahedral and tetrahedral sites. The O K-edge signal for MgAl2O4 loses intensity upon reduction, while displaying a sharp pre-edge peak. Simulation of the O K-edge (Figure 86c) indicates that in reduced state, O presents a more confined electronic structure and a lower covalency. The introduction of Fe into the lattice (MgFexAl2-xO4, x = 0.13) gives rise to a different site distribution for Mg and Al, according to the lower inversion parameter of ~0.1. The latter indicates that Mg now occupies mainly tetrahedral sites, while Fe takes up octahedral positions, mostly as Fe3+, together with Al. This entails more lattice distortion, which is reflected in the O K-edge spectrum by the presence of a pre-edge feature, even in fresh state. Upon reduction of MgFe0.13Al1.87O4, up to
Fig. 85: Consumption rate of CO2 and CH4 for reduced MgAl2O4 and MgFeAlO4 after 30 min methane dry reforming (1023 K, 111.3 kPa and
CH4/CO2~1). Secondary axis: H2/CO product ratio.
Fig. 86: a) Oxygen K-edge for fresh and reduced MgAl2O4 and MgFe0.13Al1.87O4. The arrows indicate the main features of the edge, which change upon reduction treatment. Inset: zoom of the peaks c and d. b) Top: Fe M2,3, Mg L2,3, Al L2,3 and Fe M1-edges for
undoped and Fe-doped samples before and after reduction. Left: zoom of Mg L2,3. Right: zoom of Al L2,3-edges. c) Experimental and calculated spectrum at the O K-edge (540 eV) for fresh (left) and reduced (right) MgAl2O4. Green dashed curve: adsorbed molecular
oxygen component added to the calculation. d) Left: experimental data for the Fe M2,3-edge of MgFe0.13Al1.87O4 (a: fresh, b: reduced). Right: simulation for fresh (solid) and reduced (dashed) MgFe0.13Al1.87O4 together with the normalised experimental data.