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(e) ESRF (f) CNRS, Univ. Bordeaux, Institut de Chimie de la Matière Condensée de Bordeaux, Pessac (France) (g) Department of Chemistry, University of California Berkeley (USA) (h) Department of Chemical and Biomolecular Engineering, University of California Berkeley (USA)
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WHAT MAKES Fe-MODIFIED MgAl2O4 AN ACTIVE CATALYST SUPPORT? INSIGHT FROM X-RAY RAMAN SCATTERING
Fe-modified MgAl2O4 makes a surprisingly active support for heterogeneous catalysts, strongly linked to structural effects due to Fe incorporation. MgAl2O4 and MgFe0.13Al1.87O4 were studied in fresh and reduced states using X-ray Raman scattering (XRS) to determine the effect of H2 reduction upon the lattice cation distribution.
PRINCIPAL PUBLICATION AND AUTHORS
Additionally, the combined analysis of high- field EPR and magnetisation data evidences a marked enhancement of the magnetic anisotropy when compared to the [ReIVF6]2- and trans-[ReIVCl4(CN)2]2- anions [3,5]. The anisotropy energy gap, ∆/kB, between the two lowest Kramers doublets, is estimated at 160 K (∆/kB = 2√(D2 + 3E2)/kB) with D/kB = −69.5 K and |E|/kB = 22.6 K using the following spin- Hamiltonian: ^H = D(^Sz2
1 3S(S + 1) + E( ^Sx2
^Sy2)), being one of the largest values measured for a 5d transition metal complex. This remarkable
result suggests that the prevalence of an Orbach relaxation mechanism in [ReIVF6]2- and trans- [ReIVCl4(CN)2]2- is most likely due to the presence of smaller energy gaps.
In conclusion, this work provides an original synthetic approach to tune the ligand field and, consequently, the magnetic anisotropy of existing heavy transition metal complexes and building blocks, which paves the way for the preparation of high-performance molecule- based magnets.
Magnesium aluminate spinel (MgAl2O4) is a material with a wide range of technological applications due to its high melting point, strength and resistance to chemical attack. Moreover, its high surface area makes it ideal as catalyst support . Two types of MgAl2O4 can be distinguished in nature: normal (all Al3+ in octahedral and Mg2+ in tetrahedral sites) and inverse (Al3+ spread over both sites, Mg2+ in octahedral sites). Aside from these natural spinels, phases with intermediate cation distribution can equally occur, characterised by an inversion parameter.
Recent work has suggested the use of a Fe-modified MgAl2O4 as catalyst support for syngas production . The improved
performance of such Fe-doped magnesium aluminate (MgFexAl2-xO4) was attributed to the stabilisation of Fe within the spinel lattice, resulting in less sintering and highly active and carbon-resistant catalysts for syngas production. To link the MgFexAl2-xO4 structure to its improved performance (Figure 85), an X-ray Raman scattering (XRS) study was performed by exploiting the large-solid-angle spectrometer of beamline ID20.
The detailed investigation of the spinels in fresh and reduced state by XRS yields insight in the rearrangements that take place upon Fe incorporation and high temperature reduction. Interestingly, at the O K-edge, no difference between the fresh samples with and without