Microkinetic modeling of nitrous oxide decomposition on dinuclear oxygen bridged iron sites in Fe-ZSM-5

The decomposition of N2O on dinuclear oxygen-bridged iron sites in Fe-ZSM-5 was simulated under steady-state conditions considering the reaction mechanism and the rate parameters proposed by Hansen et al. [J. Phys. Chem. C 111 (2007) 2092] on the basis of DFT calculations. The presence of low concentrations of water vapor in the feed stream (ppb to ppm levels) affects the calculated values for the apparent activation energy and the pre-exponential factor and thus can explain the wide variation in experimental values for these quantities, as well as the appearance of an apparent compensation effect. The activity of the dinuclear oxygen-bridged site was compared with that of the mononuclear iron site proposed earlier by Heyden et al. [J. Phys. Chem. B 109 (2005) 1857]; the latter was found to be slightly more active. Microkinetic models for both mononuclear and dinuclear iron sites were used to reproduce temperature-programmed reaction experiments reported for Fe-ZSM-5 samples with low and high iron content. This analysis leads to the conclusion that at very low Fe/Al ratios, mononuclear iron sites prevail, whereas at higher Fe/Al ratios, both mononuclear and dinuclear iron sites are likely to be present simultaneously.