Nanocavity effects on N(2)O decomposition on different types of Fe-zeolites (Fe-FER, Fe-BEA, Fe-ZSM-5 and Fe-FAU): A combined theoretical and experimental study

Nitrous oxide decomposition on iron-exchanged zeolites (Fe-FER, Fe-ZSM-5, Fe-BEA, and Fe-FAU) has been studied both theoretically, by using the ONIOM (B3LYP/6-31G(d,p):UFF) method, and experimentally, by temperature programmed reaction, to determine the effect of different zeolitic nanostructured pore networks on the catalytic activity. The ONIOM quantum mechanical calculations show that the nitrous oxide molecule adsorbs with slightly stronger interactions energies on Fe-FER and Fe-ZSM-5 than on the larger pore Fe-BEA and Fe-FAU zeolites. In the transition state leading to the decomposition of nitrous oxide, the smallest pore ferrierite zeolite exerts the strongest van der Waals interactions on the reacting species and, thus, results in the lowest activation energy. Therefore, the predicted intrinsic activity trend is Fe-FERFe-BEAFe-ZSM-5Fe-FAU. On the other hand, the temperature programmed reaction on zeolites containing trace amounts of iron impurities shows an observed activity trend of Fe-FERFe-BEAFe-ZSM-5Fe-FAU. The experimentally observed activity trend can be explained by the intrinsic activity of each zeolite except for Fe-FAU. Nitrous oxide decomposition in Fe-FAU could be limited by the mass transfer process and not governed by the intrinsic activity. It is known that cations are preferentially located on the six-membered ring in the sodalite cage of the faujasite, to which the reactants have a very limited access.