N2O decomposition over iron-containing zeolites prepared by different methods: a comparison of the reaction mechanism

Iron-containing zeolites were prepared by aqueous ion exchange, sublimation of FeCl3, or hydrothermal synthesis and were modified by steaming at 873 K. The samples contained different kinds of iron species ranging from isolated iron atoms to small clusters that contain a few atoms and somewhat larger oligomers. Yet, the dynamic behavior of all samples in N2O decomposition was qualitatively similar. Pulse experiments showed that a slow and fast pathway of O2 formation, which is the rate-limiting step of the reaction, exists. Both contributed to the overall activity. The fast pathway dominated on isolated iron sites, whereas the contribution of the slow pathway increased on catalysts with high iron loadings. Steaming led to an agglomeration of the iron clusters and increased the activity (with one exception). The steamed samples had a peculiar time-on-stream behavior. They exhibited a very high initial activity and then slowly decayed to steady state. This indicates that steaming did not merely agglomerate the iron clusters, but qualitatively changed the nature of the active sites.