Thermochemistry of manganese oxides in reactive gas atmospheres:: Probing redox compositions in the decomposition course MnO2→MnO

The thermal decomposition course MnO2 --> MnO was examined in various gas atmospheres (O-2, air, N-2 and H-2) by temperature-programmed studies employing thermogravimetry and differential thermal analysis. Weight-invariant thermal events encountered were subjected to non-isothermal and isothermal kinetic analysis. Product analysis was carried out using infrared spectroscopy and X-ray diffractometry. Cyclic TG experiments carried out in air have revealed that, of the intermediate decomposition products characterized, viz. Mn5O8, Mn2O3 Mn3O4, the mixed-valence Mn3O4 (=Mn(II)Mn-2(III)O-4) can tolerate reversible oxygenation-deoxygenation processes at (500-1050 degrees C). Moreover, the presence of Mn(II) in the mixed-valence Mn5O8 (=Mn-2(II)Mn-3(IV)O-8) is seen to sustain a synproportionation of Mn(II)/Mn(IV) during the oxide deoxygenation, giving rise to Mn(III) species (=Mn2O3). The electron-mobile environment thus established in such mixed-valence oxides is seen to promise a catalytic potential in oxidation/reduction reactions. (C) 1997 Elsevier Science B.V.