Reduction of supported iron oxide studied by temperature-programmed reduction combined with mössbauer spectroscopy and X-ray diffraction

Abstract

The reduction of iron oxide supported on alumina, anatase and rutile and the interactions between iron oxides and these supports have been studied using temperature-programmed reduction, Mössbauer spectroscopy and X-ray diffraction. The intermediates formed during reduction depend strongly on the nature of the support, the iron loading and the reduction conditions used. Several iron species such as bulk α-Fe₂O₃, superparamagnetic α-Fe₂O₃ and surface iron–aluminium oxides are found in the alumina-supported iron oxide samples. The iron–aluminium species can be formed either by incorporation of Al³⁺ into the structure of iron oxide in acidic solution during the preparation of the samples or by incorporation of Fe²⁺ into the structure of the alumina support under high-temperature reduction. For the as-prepared Fe/TiO₂ samples, the anatase and rutile have only weak interactions with the iron oxide as only bulk α-Fe₂O₃ species have been detected. However, some intermediates such as (1 –x)FeTiO₃–xFe₂O₃ solid solution, surface iron–titanium oxide and bulk FeTiO₃ phase can be formed during reduction because of the reducibility and mobility of titanium ions on the surface. Surface titanium ions can act as an electron-transfer medium for the reduction of Fe³⁺ to Fe²⁺ and can even be substituted for Fe³⁺ ions in the octahedral sites of magnetite formed during the reduction. In all cases, when the loading of iron oxide is low, Fe³⁺ and Fe²⁺ ions are stable during reduction due to the formation of iron oxide–support intermediates by strong interactions between iron oxides and the supports.