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FTIR, extended X-ray absorption fine structure (EXAFS) and X- ray absorption near edge spectroscopy (XANES) and ESR were used to identify the active sites and the adsorption complexes present in a variety of Fe/MFI samples prepared in different ways and displaying vastly different activities and selectivities in the reduction of NOx to N-2 with hydrocarbons. Iron oxide particles, charged ferric oxide nano-clusters, isolated iron ions and oxygen-bridged binuclear iron ions have been identified with various degrees of reliability. In contact with appropriate gases, NO+ ions, mono- and dinitrosyl groups,nitro groups, nitrate ions and superoxide ions have been identified. Peroxide ions, though not detectable with the methods;used here, were postulated by other authors on the basis of density functional calculations. The binuclear iron ore-ions are more abundant in the best catalysts with high Fe/Al ratio that were prepared by sublimation. They are the most probable sites for NO oxidation to NO2 and its further conversion to adsorbed nitro groups and nitrate ions, steps that are crucial for NOx reduction. Superoxide and/or peroxide ions are Likely involved in the NO oxidation to NO2. This process is fast when Fe/MFI is exposed to a mixture of NO + O- 2, but much slower if NO is chemisorbed first, before exposure to O-2. |
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