A Reaction Mechanism for the Nitrous Oxide Decomposition on Binuclear Oxygen Bridged Iron Sites in Fe-ZSM-5

The reaction mechanism for the decomposition of nitrous oxide (N2O) on hydroxylated and dehydroxylated binuclear oxygen bridged extraframework iron sites in Fe-ZSM-5 has been studied using density functional theory. The results show that if two charge exchange sites in the zeolite are in close proximity, two isolated dihydroxylated iron sites readily form an oxygen bridged iron site while releasing water. Different mechanisms for N2O decomposition were examined on these hydroxylated binuclear iron sites. The activity for the N2O decomposition on these sites is low. At elevated temperatures, water desorbs from hydroxylated sites and N2O is readily decomposed on dehydroxylated iron sites. The rate-limiting step in the reaction path is N2O dissociation. The overall activity of the N2O decomposition on binuclear iron sites is qualitatively the same as that on isolated, single iron sites reported previously by Heyden et al. (J. Phys. Chem. B 2005, 109, 1857). More important than the nuclearity of the iron site (mono- or binuclear) for the activity of Fe-ZSM-5 in N2O decomposition is the influence of small amounts of water in the reaction system on the nature of the iron site. As in the case of isolated, single iron sites, only the dehydroxylated binuclear iron sites show a significant activity for N2O dissociation, while the hydroxylated sites are virtually inactive.