Characterization of Fe/ZSM-5 by isotopic exchange with 18O2

Abstract

Exchange of ¹⁸O₂ with O atoms in two groups of Fe containing ZSM-5 catalysts was studied. One class was prepared by sublimation of FeCl₃ onto H-ZSM-5 with various Si/Al ratios. These materials are highly active and selective catalysts in the reduction of NO_x with alkanes. A second group, further called Fe₂O₃/ZSM-5, was prepared by impregnation. Significant differences in the exchange behavior were found between both groups; the exchange rate is higher for Fe₂O₃/ZSM-5 than for Fe/ZSM-5, in accordance with the higher selectivity for alkane combustion of Fe₂O₃/ZSM-5 and its low selectivity for NO_x reduction. The data suggest that negligible amounts of iron oxide particles exist in the Fe/ZSM-5 catalyst. For Fe/ZSM-5 catalysts the exchange is not limited to the oxygen atoms directly attached to iron; some zeolite oxygen is also exchanged, although this does not occur for the metal-free zeolite. Kinetic analysis shows that with Fe₂O₃/ZSM-5 the R² exchange mechanism prevails in which ¹⁸O₂ is exchanged in one step against two ¹⁶O atoms. For the selective Fe/ZSM-5 catalyst R¹ exchange prevails at low temperature, i.e., ¹⁸O₂ molecules exchange only one O atom against an atom of the solid. The qualitative difference between both types of catalyst indicates that Fe/ZSM-5 catalysts owe their selectivity in NO_x reduction to Fe oxo-ions.