X-ray Absorption Fine Structure Characterization of the Local Structure of Fe in Fe−ZSM-5

The local structure of Fe in Fe−ZSM-5 prepared by solid-state exchange was investigated with XAFS. Fe K-edge spectra were taken at liquid nitrogen temperature for samples with Fe/Al ratios of 0.33, 0.66, and 0.80. The radial structure function (RSF) of He- and CO-pretreated Fe−ZSM-5 shows two main peaks, one at 1.6 A and the other at 2.5 A. To interpret the origin of these peaks, RSFs were simulated for a number of mono- and di-iron structures obtained from quantum chemical calculations. By this means, the peak in the RSF at 1.6 A is clearly identified with backscattering from O atoms coordinated to an Fe atom. The peak at 2.5 A has been previously ascribed to Fe−Fe scattering and 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 A has the same characteristics as that generated theoretically for Fe−Al backscattering and is distinctly differe...