UV−Raman Characterization of Iron Peroxo Adsorbates on Fe/MFI Catalyst with High Activity for NOx Reduction

The interaction of O 2 with Fe supported on zeolite MFI has been investigated by the in situ UV Raman technique. Previously, the formation of adsorbed superoxide ions, O 2 -, on the Fe/MFI prepared by sublimation of FeCl 3 vapor onto HMFI, was identified by ESR at 77 K. In situ Raman data indicate that adsorbed peroxide ions, O 2 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 O 2, is tentatively assigned to the stretching vibration of peroxide ions. With the 18 O 2 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.