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The nature and oxidation state of iron species in Fe-exchanged BEA zeolites treated in synthetic air or nitrogen were determined by a combination of Mössbauer and X-ray absorption spectroscopy. The linear correlation between the edge energy of the X-ray absorption near edge structure (XANES) and the oxidation state determined by Mössbauer spectroscopy allowed determining the fraction of Fe 2+ in situ. The distribution of Fe 2+ and Fe 3+ in the catalysts depends on the Fe concentration and the conditions of the thermal treatment. It is possible to stabilize isolated Fe 2+ cations under ambient atmosphere in the zeolite pores, while FeBEA catalysts show a temperature-dependent oxidation and reduction of the active Fe species during the selective catalytic reduction of nitrogen oxides by NH 3 (NH 3 -SCR), reflecting the equilibrium for NO oxidation.
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Maier et al. (2011) studied this question.
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