Nitrous Oxide Decomposition on the Binuclear <Fe(μ−O)(μ−OH)Fe>\left<Fe\left(\mu -O \right)\left(\mu -OH \right)Fe \right> Center in Fe-ZSM-5 Zeolite

The reaction mechanism for nitrous oxide (${N}_{2}O$) direct decomposition into molecular nitrogen and oxygen was studied on binuclear iron sites in Fe-ZSM-5 zeolite using the density functional theory (DFT). Starting from the hydroxylated bi-iron complex {\left}^{+}, a reductive dehydroxylation pathway was proposed to justify the formation of the active site {\left}^{+}. The latter contains two FeII ions linked via oxo and hydroxo bridges, {{Z}^{-}\left}^{+}, and for the first time was considered to catalyze the ${N}_{2}O$ ecomposition. The DFT results show the activity of {\left}^{+} complex for the ${N}_{2}O$ decomposition. The first step of the catalytic reaction corresponds to a spontaneous adsorption of ${N}_{2}O$ over FeII sites, followed by the surface atomic oxygen loading and the release of molecular nitrogen. The formation of molecular ${O}_{2}$ occurs through the migration of the atomic oxygen from one iron site to another one followed by the recombination of two oxygen atoms and the desorption of molecular oxygen. The computed reactivity over the binuclear iron core complex {\left}^{+} is consistent with experimental data reported in the literature. Although the dissociation steps of the ${N}_{2}O$ molecules, calculated with respect to adsorbed ${N}_{2}O$ intermediates, are highly energetic, the energy barrier associated with the atomic oxygen migration is the highest one. Up to 700 K, the oxygen migration step has the highest free energy barrier, suggesting that it is the te-limiting step of the overall kinetics. This result explains the absence of ${O}_{2}$ formation in experimental study of ${N}_{2}O$ decomposition at temperatures below 623 K

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