Phase transformations in the thermoactivated MnO x–Al 2O 3 catalytic system

Abstract

The phenomenon of the sharp increase of activity in reactions of deep oxidation found for the manganese oxide–alumina system calcined at 900–1000 °C was named as thermoactivation effect. This effect was shown to be associated with phase transformations in this system. Peculiarities of phase transformations in the MnO x /Al 2O 3 system after its calcination at 950 °C for 1–10 h were studied with XRD, TEM, XPS methods; the catalysts activity in deep oxidation of n-pentane was measured in a flow-circuit setup. It was found that a set of phase transformations occurred in the system, starting from the formation of the metastable nonstoichiometric manganese–alumina cubic spinel (after 1–2 h calcination) and nonequilibrium solid solution of Mn 3+ ions in the structure of γ-Al 2O 3 which decomposes producing high-temperature aluminas modified by manganese ions and particles of imperfect β-Mn 3O 4 phase doped with aluminium ions. Increase of the calcination time to 10 h and more results in the formation of biphasic system composed of α-Al 2O 3 and nanocrystalline β-Mn 3O 4 modified by Al 3+ ions. These transformations are associated with the maximum activity showed by catalysts calcined for 5–6 h in the reaction of n-pentane combustion.