UV-Raman characterization of iron peroxo adsorbates on Fe/MFI catalyst with high activity for NO_x reduction

The interaction of O₂ with Fe supported on zeolite MFI has been investigated by the in situ UV Raman technique. Previously, the formation of adsorbed superoxide ions, O₂^- on the Fe/MFI prepared by sublimation of FeCl₃ vapor onto HMFI, was identified by ESR at 77 K. In situ Raman data indicate that adsorbed peroxideions, O₂^2-, are formed on the same catalysts with Fe/Al = 1 even at 300 K. A band at 730 cm^-1, which was found to be sensitive to the partial pressure of O₂, is tentatively assigned to the stretching vibration of peroxide ions. With the ¹⁸O₂ isotope, a red shift of 32 cm^-1 was observed for the 730 cm^-1 band. Raman features of other catalyst samples show that the binuclear iron sites in the Fe/MFI are essential for the formation of peroxide ions. As proposed before, the same bridging sites between two Fe ions, that are occupied by O^2- in the calcined catalyst, are also able to adsorb di-oxygen complexes.