Towards advanced structural analysis of iron oxide clusters on the surface of γ-Al2O3 using EXAFS

• Published in: Applied surface science 2016-11, Vol.386, p.234-246
• Main Authors: Boubnov, Alexey, Roppertz, Andreas, Kundrat, Matthew D., Mangold, Stefan, Reznik, Boris, Jacob, Christoph R., Kureti, Sven, Grunwaldt, Jan-Dierk
• Format: Article
• Language: English
• Subjects: Absorption; Catalyst; Catalysts; CO oxidation; EXAFS; Iron; Iron oxide; Iron oxides; Lattice sites; Molecular model; Surface chemistry; Transmission electron microscopy; X-rays; γ-Al2O3
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2016.05.148

[Display omitted] •Analysis of isolated and oligomeric FeOx (x=4, 5) on Al2O3 by XANES and EXAFS.•Iron is trivalent and is mainly located at octahedral lattice sites of Al2O3.•Low Fe loading (0.1%) guarantees high dispersion of catalytically active iron sites.•Surface Fe-cluster on Al2O3 and DFT-optimised Fe-doped Al2O3 as input models for EXAFS.•Interactions of iron with support are well-accounted for using realistic models. Iron oxide centres are structurally investigated in 0.1% Fe/γ-Al2O3, which is known as highly active catalyst, for instance in the oxidation of CO. The sample was characterised by using X-ray absorption spectroscopy (XAS) in terms of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), Mössbauer spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). These analyses evidenced high dispersion of the iron oxide entities without significant presence of bulk-like aggregates associated with the low Fe content of the catalyst. A library of structural models of Al2O3-supported surface Fe was created as input for EXAFS fitting. Additionally, several model structures of Fe substituting Al ions in bulk γ-Al2O3 were created with optimised geometry based on density-functional theory (DFT) calculations. From EXAFS refinement of the best 8 out of 24 models, it was found that the trivalent Fe ions are coordinated by 4–5 oxygen atoms and are located on octahedral lattice sites of the exposed surfaces of γ-Al2O3. These iron oxide species exist mainly as a mixture of monomeric and binuclear species and due to the low concentration represent suitable model systems as alternative to single crystal systems for structure-function relationships.