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Abstract: The reaction mechanism of the fast selective catalytic reduction (SCR) of NO x has been investigated by means of a microkinetic model based on DFT calculations. First, potential additional reaction mechanisms are presented that include the intermediate formation of Z−[NO x ]+ (x =1, 2) from the decomposition of N2O y (y =3, 4) on a Brønsted acid. The formed reacts with ammonia to nirosamine or nitramide. The DFT results are applied in a microkinetic model together with prior studies of Brüggemann et al. The simulated conversion of NO x is in agreement with experimental data over a wide range of temperature. The activity of the H-ZSM5 for the fast SCR is based on the reaction sequence via Z−[NO x ]+, the decomposition of nitrosamine and nitramide, and the reaction of nitrous and nitric acid with adsorbed ammonia at low temperatures. Ammonia blocks the active sites at low temperatures while thermodynamic limitations of the surface species N2O y restrict the conversion at high temperatures. Heat of formations and reaction rate constants were adjusted within the accuracy of the applied method for important elementary steps to cope with experimental data. [Copyright &y& Elsevier]

Abstract: A reaction mechanism for the decomposition of N2O on Fe-ZSM-5 has been proposed by Heyden et al. based on a detailed density functional theory analysis of the energetics for alternative pathways. This work suggested that isolated Fe cations bound to a single oxygen atom (viz., Z−[FeO]+) are the active sites for N2O decomposition. It was also shown that the presence of water vapor in the feed stream to the catalyst can deactivate these sites, but this process was demonstrated to be reversible at temperatures above about 690 K. In the present work, the decomposition of N2O was simulated under non-steady-state conditions using the mechanism of Heyden et al. and the rate coefficients calculated by these authors. These simulations closely reproduce the results of temperature-programmed reaction and pulsed N2O decomposition experiments. The dynamics of water desorption from deactivated Fe sites are found to be slow. Thus at 773 K, dehydration of a fully hydrated sample of Fe-ZSM-5 can take up to 10 h. The present study also shows that temperature-programmed desorption of O2 from Fe-ZSM-5 after N2O decomposition arises from the reaction Z−[FeO2]+ ⇄Z−[Fe]+ +O2, but that this process is not a part of the mechanism for N2O decomposition under steady-state conditions. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]