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

Abstract

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 peroxide ions, O22-, 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 O2- in the calcined catalyst, are also able to adsorb di-oxygen complexes.