Theoretical evidence of the observed kinetic order dependence on temperature during the N2O decomposition over Fe-ZSM-5
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 [FeII(m-O)(m-OH)FeII]+ 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 FeII core sites by N2O (atomic a-oxygen formation) and the recombination of two surface a-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 a-oxygen recombination to a-oxygen formation. The TPD results on the molecular oxygen desorption confirmed the mechanism proposed.