(628a) Nitrous Oxide Decomposition over Fe-Zsm-5 in the Presence of Nitric Oxide: a Comprehensive Dft Study

Most catalytic systems active for nitrous oxide (N₂O) decomposition are inhibited by nitric oxide (NO). In contrast, for iron zeolite catalysts, it has been reported that nitric oxide significantly enhances the N₂O decomposition rate. In the present work, a novel explanation of this promotional effect of nitric oxide consistent with quantum chemical calculations and experimental findings is given. The NO-assisted N₂O decomposition over mononuclear iron sites in Fe-ZSM-5 was studied on a molecular level using density functional theory (DFT) and harmonic transition state theory. For a reaction network consisting of over 100 elementary reactions, the geometries and energies of potential energy minima, as well as transition states were determined on different spin potential energy surfaces. In the absence of NO and at temperatures below 690 K, most active single iron sites are poisoned by small water impurities in the gas stream, leading to a low overall activity of the catalyst. However, in the presence of nitric oxide, these poisoned iron sites are converted into a novel active iron center. On these iron sites, nitrous oxide dissociates into a surface oxygen atom and a N₂ gas molecule. The surface oxygen atom is removed by a nitric oxide or a nitrogen dioxide molecule. The N₂O dissociation is the rate-limiting step in the reaction mechanism. At higher temperatures, water desorbs from inactive iron sites and the reaction mechanism of the N₂O decomposition becomes independent of nitric oxide, leading to the reaction mechanism previously reported by Heyden et al.^1,2 Altogether, macroscopic reactor simulations based solely on parameters obtained from first principles can reproduce experimental observations such as a 1-2 orders of magnitude increase in N₂O decomposition rate in the presence of low concentrations of nitric oxide (at 600 K) and a maximum in the nitrogen dioxide concentration with temperature at around 660 K as reported by Mul et al.³

[1] A. Heyden, B. Peters, A. T. Bell, F. J. Keil, J. Phys. Chem. B 109, 1857 (2005).

[2] A. Heyden, A. T. Bell, F. J. Keil, J. Catal. 233, 26 (2005).

[3] G. Mul, J. Pérez-Ramírez, F. Kapteijn, J. A. Moulijn, Catal. Lett. 77, 7 (2001).