Identification of active sites and adsorption complexes in Fe/MFI catalysts for NO x reduction

Abstract

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 NO x 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 oxo-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 NO 2 and its further conversion to adsorbed nitro groups and nitrate ions, steps that are crucial for NO x reduction. Superoxide and/or peroxide ions are likely involved in the NO oxidation to NO 2. 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.