What Makes Fe-Modified MgAl2O4 an 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, MgAl(2)O(4 )and MgAl2O4, have been studied in fresh and reduced state to determine the effect of high-temperature H-2 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 L-2,L-3 and iron M(2,3 )soft edges. MgAl(2)O(4 )shows a random cation distribution and only small changes occur at the Mg L-2,L-3 and Al L-2,L-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 MgAl2O4 lattice, whereas Fe and Al share the octahedral sites. Concomitant lattice distortion is observable in the O signal. Reduction of MgAl2O4 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 MgAl2O4 lattice. This combination of distortion and phase restructuring in the Fe-modified MgAl2O4 material facilitates the lattice oxygen mobility and hence its catalytic activity.