Exchange of 18 O with O atoms in two groups of Fe containing ZSM-5 catalysts was studied. One class was prepared by 2 sublimation of FeCl onto H-ZSM-5 with various SirAl ratios. These materials are highly active and selective catalysts in 3 the reduction of NO with alkanes. A second group, further called Fe O rZSM-5, was prepared by impregnation.
x 2 3
Significant differences in the exchange behavior were found between both groups; the exchange rate is higher for Fe O rZSM-5 than for FerZSM-5, in accordance with the higher selectivity for alkane combustion of Fe O rZSM-5 and 2 3 2 3 its low selectivity for NO reduction. The data suggest that negligible amounts of iron oxide particles exist in the FerZSM-5
x catalyst. For FerZSM-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 O rZSM-5 the R 2 exchange mechanism prevails in which 18 O is exchanged in one step against two 16 O atoms. For the 2 3 2 selective FerZSM-5 catalyst R 1 exchange prevails at low temperature, i.e., 18 O molecules exchange only one O atom 2 against an atom of the solid. The qualitative difference between both types of catalyst indicates that FerZSM-5 catalysts owe their selectivity in NO reduction to Fe oxo-ions.