Theoretical evidence of the observed kinetic order dependence on temperature during the N2O decomposition over Fe-ZSM-5

Abstract

The characterization of Fe/ZSM5 zeolite materials, the nature of Fe-sites active in N₂O direct decomposition, as well as the rate limiting step are still a matter of debate. The mechanism of N₂O decomposition on the binuclear oxo-hydroxo bridged extraframework iron core site [Fe^II(μ-O)(μ-OH)Fe^II]⁺ inside the ZSM-5 zeolite has been studied by combining theoretical and experimental approaches. The overall calculated path of N₂O decomposition involves the oxidation of binuclear Fe^II core sites by N₂O (atomic α-oxygen formation) and the recombination of two surface α-oxygen atoms leading to the formation of molecular oxygen. Rate parameters computed using standard statistical mechanics and transition state theory reveal that elementary catalytic steps involved into N₂O decomposition are strongly dependent on the temperature. This theoretical result was compared to the experimentally observed steady state kinetics of the N₂O decomposition and temperature-programmed desorption (TPD) experiments. A switch of the reaction order with respect to N₂O pressure from zero to one occurs at around 800 K suggesting a change of the rate determining step from the α-oxygen recombination to α-oxygen formation. The TPD results on the molecular oxygen desorption confirmed the mechanism proposed.