What Makes Fe-Modified MgAl2O4an Active Catalyst Support? Insight from X-ray Raman Scattering

Fe-modified MgAl2O4 makes a surprisingly active catalyst support, likely linked to a structural effect of the Fe incorporation. Two catalyst supports, MgAl2O4 and MgFeAlO4, have been studied in fresh and reduced state to determine the effect of high-temperature H2 reduction upon ion distribution in the lattices. To this end, an X-ray Raman scattering study has been performed, focusing on the oxygen K edge and magnesium and aluminum L2,3 and iron M2,3 soft edges. MgAl2O4 shows a random cation distribution and only small changes occur at the Mg L2,3 and Al L2,3 edges upon reduction at 1073 K. The main oxygen signal does lose intensity and its simulation points to a lower O covalency and more confined state after reduction. Introducing 8.9 wt % Fe into the spinel pushes Mg towards mostly tetrahedral position in the MgFeAlO4 lattice, whereas Fe and Al share the octahedral sites. Concomitant lattice distortion is observable in the O signal. Reduction of MgFeAlO4 leads to enhanced distortion visible in the O and Al signals and the presence of 50% Fe2+. Both disorder and reduction lead to partial segregation of MgFeOx from the MgFeAlO4 lattice. This combination of distortion and phase restructuring in the Fe-modified MgFeAlO4 material facilitates the lattice oxygen mobility and hence its catalytic activity.