Abstract
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 Mossbauer 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 Mossbauer spectroscopy allowed determining the fraction of Fe2+ in situ. The distribution of Fe2+ and Fe3+ in the catalysts depends on the Fe concentration and the conditions of the thermal treatment. It is possible to stabilize isolated Fe2+ 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 NH3 (NH3-SCR), reflecting the equilibrium for NO oxidation.
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