N 2 O decomposition catalyzed by oxidized Fe clusters localized in the micropores of Fe/ZSM-5 has been studied using the DFT approach and a binuclear cluster model of the active site. Three different reaction routes were found, depending on temperature and water pressure. The results show that below 200 °C the binuclear cluster is hydroxylated and is probably inactive. Above this temperature and up to 500 °C the catalytic site has the [HO-Fe-O-Fe-OH] 2+ structure, and above 500 °C the site is predominantly [Fe-O-Fe] 2+ . The reaction paths on the latter two forms of the site are similar. N 2 O dissociates on each of the Fe ions with subsequent oxygen recombination and desorption. Some of the side reactions including NO formation are also considered.
Cluster model quantum chemical calculations have been performed to compare stabilization energies (E
st) of
Zn2+ ions in four-, five-, and six-membered zeolitic rings. E
st was evaluated as energy of the reaction Zn2+/Z
+ H2 ⇒ 2H+/Z + Zn0. It was found that E
st substantially decreases in the series six-, five-, and four-membered
ring, and this trend is essential to the understanding of the comparative adsorption ability and reactivity of
Zn2+ in cationic sites of different zeolites. This conclusion was proved in calculations of the heterolytic
dissociation of ethane. The molecular structure of active sites in ZnHY and ZnHZSM-5 zeolites and the
question of the stability of small intrazeolite zinc oxide species are discussed.
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