Abstract
Exchange of 18O 2 with O atoms in two groups of Fe containing ZSM-5 catalysts was studied. One class was prepared by sublimation of FeCl 3 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 Fe 2O 3/ZSM-5, was prepared by impregnation. Significant differences in the exchange behavior were found between both groups; the exchange rate is higher for Fe 2O 3/ZSM-5 than for Fe/ZSM-5, in accordance with the higher selectivity for alkane combustion of Fe 2O 3/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 Fe 2O 3/ZSM-5 the R 2 exchange mechanism prevails in which 18O 2 is exchanged in one step against two 16O atoms. For the selective Fe/ZSM-5 catalyst R 1 exchange prevails at low temperature, i.e., 18O 2 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.
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