Strong oxide-oxide interactions in silica-supported magnetite catalysts. 1. X-ray diffraction and Mössbauer spectroscopy evidence for interaction

X-ray diffraction and Mössbauer spectroscopy were used to identify a strong oxide-oxide interaction between Fe₃O₄ and SiO₂ in a silica-supported magnetite catalyst. This interaction results in an iron oxide phase which, when oxidized at temperatures up to ca. 800 K, yields γ-Fe₂O₃ rather than α-Fe₂O₃. Furthermore, the Mössbauer parameters of the octahedrally coordinated iron cations in silica-supported magnetite were significantly different from those of bulk magnetite, while the Mössbauer parameters of the tetrahedrally coordinated iron cations were essentially unchanged by the presence of silica. This can be interpreted in terms of a model in which Si⁴⁺ substitutes for Fe³⁺ in the tetrahedral sites near the surface of magnetite. This displaced Fe³⁺ is transferred into adjacent octahedral sites where it participates in the characteristic electron hopping process of the octahedral sublattice of magnetite.