EXAFS characterization of the local structure of Fe in Fe-ZSM-5: an experimental and theoretical study

The local structure of Fe in Fe-ZSM-5 prepared by solid-state exchange was investigated with EXAFS. Fe K-edge spectra taken at liquid nitrogen temperature of He- and CO-pretreated sample show two main peaks, one at 1.6 {angstrom} and the other at 2.5 {angstrom}. To interpret the origin of these peaks, RSFs were simulated for a number of mono- and di-iron structures obtained from quantum chemical calculations. The peak at 1.6 {angstrom} is clearly identified with scattering from O atoms coordinated to a Fe atom. The peak at 2.5 {angstrom} has been used to argue for the presence of di-iron oxo species. However, the origin of this peak and its interpretation remains an open question. The imaginary part of the Fourier-transformed data for the peak at 2.5 {angstrom} has the same characteristics as that generated theoretically for Fe-Al scattering. This evidence strongly suggests that iron in Fe-ZSM-5 is present as isolated cations in an aluminum framework. Further evidence for such a structure is the absence of any change in the magnitude of the peak at 2.5 {angstrom} with sample treatment.