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

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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.