EXAFS and kinetic study of MnOx/γ-alumina in gas phase catalytic oxidation of toluene by ozone

Abstract

This work addresses characterization of Mn2O3 materials by EXAFS technique and the mechanism of toluene oxidation by ozone over MnOx/γ-alumina (10%) catalyst. Bond Valence Model was used to model EXAFS signal of bulk and nano-particles of Mn2O3. The Bond Valence Model relates formal oxidation state of Mn atoms to the coordination number of oxygen atoms at the first Mn shell, adding restraints to the EXAFS theoretical models. Use of the Bond Valence Model resulted in distinguishing closely located oxygen atoms around each Mn site without any simplifying assumption on the structure of Mn2O3. Kinetic study of catalytic oxidation of toluene by ozone using MnOx/γ-alumina (10%) was performed at constant toluene or ozone partial pressures by varying the reactor space time in the range of 0–1212kgcat.moltol.−1s. Differential method of analysis was used to estimate the initial reaction rates. Apparent activation energy of the reaction was determined to be 31kJmol−1, obtained by a power law model. The power law model also determined the reaction orders with respect to toluene and ozone as −1 and 2, respectively. To explain the initial reaction rates, two Langmuir–Hinshelwood mechanisms based on different toluene activation steps were examined. The first mechanism was based on activation of adsorbed toluene molecule by atomic oxygen while the second model was based on activation of toluene molecule via abstraction of hydrogen from the methyl group. The second Langmuir–Hinshelwood mechanism led to the same rate equation as the power law model, explaining the effect of toluene and ozone partial pressures on toluene oxidation rate.