105HIGHLIGHTS 2020

What makes Fe-modified MgAl2O4 an active catalyst support? Insight from X-ray Raman Scattering, A. Longo (a,b,c), S.A. Theofanidis (a,d), C. Cavallari (b), N.V. Srinath (a), J. Hu (a), H. Poelman (a), M.K. Sabbe (a), C.J. Sahle (b), G.B. Marin (a)

and V.V. Galvita (a), ACS Catal. 10, 6613 6622 (2020); https://doi.org/10.1021/ acscatal.0c00759. (a) Laboratory for Chemical Technology LCT, Ghent University (Belgium) (b) ESRF

(c) Istituto per lo Studio dei Materiali Nanostrutturati (ISMN)-CNR, UOS Palermo (Italy) (d) Department of Chemical Engineering, Aristotle University of Thessaloniki (Greece)

[1] J. Guo et al., Appl. Catal. A 273, 75-82 (2004). [2] S.A. Theofanidis et al., ACS Catal. 8, 5983-5995 (2018).

UNVEILING THE ELECTRONIC STATES AND STRUCTURE OF FUEL CELL CATHODES

Mixed conducting oxides are key functional materials for clean-energy technologies, and their chemical and electronic properties need to be tuned for maximum performance. The information from X-ray absorption fine structure (XAFS) and X-ray Raman scattering (XRS) can be used to derive a comprehensive structure and bonding model in doped barium ferrites.

PRINCIPAL PUBLICATION AND AUTHORS

REFERENCES

50% of Fe gets reduced to Fe2+ (Figure 86d). The indications of distortion in the O and Al signals, together with the partial reduction of Fe, point towards restructuring of the

reduced MgFe0.13Al1.87O4 lattice. Such phase rearrangements facilitate the mobility of oxygen in the Fe-modified MgFe0.13Al1.87O4, whence its reducibility and activity for syngas production.

Solid oxide cells (SOC) are used in high- temperature clean-energy applications, converting hydrogen or other fuel to electricity, or storing excess power as hydrogen through water electrolysis. In a protonic SOC, the electrolyte is a proton conductor and the cathode is a mixed conductor of holes, oxygen vacancies and protons. Achieving both electronic and protonic conduction in an oxide has proven especially hard for materials scientists, and it requires very fine tuning of the chemical compositions and oxidation states. For such a case, a deep understanding of the structure property relations is required to improve the performance of the materials and of the whole device.

Barium ferrites with the perovskite structure, doped with lanthanum or strontium at the A-site and with an oversized transition metal (zinc or yttrium) at the B-site, have recently shown promising proton concentrations [1]. Understanding the origin of the high proton uptake in these oxides will provide the tools for a rational material design. In the present experiments, the atomic and electronic structure of barium ferrites was unveiled using a combination of X-ray absorption fine structure (XAFS) and X-ray Raman scattering (XRS). The structural and oxidation state of iron in a large number of doped ferrite compositions, nominally containing Fe3+ or Fe4+, was investigated with Fe K-edge XAFS at beamline BM26A.

Fig. 87: Iron K-edge spectra of (Ba,Sr,La)(Fe,Zn,Y): (green) reduced, (red) oxidised. The Fe-O-Fe bond geometries are depicted below.