Mechanism and Kinetics of Direct N2O Decomposition over Fe−MFI Zeolites with Different Iron Speciation from Temporal Analysis of Products

Abstract

The mechanism of direct N(2)O decomposition over Fe-ZSM-5 and Fe-silicate was studied in the temporal analysis of products (TAP) reactor in the temperature range of 773-848 K at a peak N(2)O pressure of ca. 10 Pa. Several kinetic models based on elementary reaction steps were evaluated to describe the transient responses of the reactant and products. Classical models considering oxygen formation via recombination of two adsorbed monoatomic oxygen species (*-O + *-O --> O(2) + 2*) or via reaction of N(2)O with adsorbed monoatomic oxygen species (N(2)O + *-O --> O(2) + N(2) + *) failed to describe the experimental data. The best description was obtained considering the reaction scheme proposed by Heyden et al. (J. Phys. Chem. B 2005, 109, 1857) on the basis of DFT calculations. N(2)O decomposes over free iron sites (*) as well as over iron sites with adsorbed monoatomic oxygen species (*-O). The latter reaction originates adsorbed biatomic oxygen species followed by its transformation to another biatomic oxygen species, which ultimately desorbs as gas-phase O(2). In line with previous works, our results confirm that the direct N(2)O decomposition is controlled by pathways leading to O(2). Our kinetic model excellently described transient data over Fe-silicalite and Fe-ZSM-5 zeolites possessing markedly different iron species. This finding strongly suggests that the reaction mechanism is not influenced by the iron constitution. The TAP-derived model was extrapolated to a wide range of N(2)O partial pressures (0.01-15 kPa) and temperatures (473-873 K) to evaluate its predictive potential of steady-state performance. Our model correctly predicts the relative activities of two Fe-FMI catalysts, but it overestimates the absolute catalytic activity for N(2)O decomposition.