M. A. Hasan

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

Abstract The thermal decomposition course MnO2 → MnO was examined in various gas atmospheres (O2, air, N2 and H2) 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)Mn2(III)O4) can tolerate reversible oxygenation-deoxygenation processes at (500–1050°C). Moreover, the presence of Mn(II) in the mixed-valence Mn5O8 (= Mn2(II)Mn3(IV)O8) 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.

Low-temperature Synthesis of Hausmannite Mn3O4

[15, 16] resulted in a major product thatcontained varied amounts of â-MnOOH (Feitknech-tite), depending in whether the oxidant was merelyair or pure oxygen atmosphere and on the exposureperiod to the oxidizing atmosphere [15, 16]. Employ-ing X-ray diffractometry, the major product wassometimes identified as being Mn

Bulk and surface characteristics of pure and alkalized Mn2O3: TG, IR, XRD, XPS, specific adsorption and redox catalytic studies

α-Mn2O3 (containing a minor proportion of Mn5O8) was obtained by calcination of pure MnO2 at 700°C for 2 h. It was alkalized by impregnation of the parent dioxide with potassium and barium nitrate solutions prior to the calcination. K-Mn2O3 (α-Mn2O3+KMn8O16) and Ba-Mn2O3 (α-Mn2O3) thus respectively produced were subjected, together with the unmodified Mn2O3, to the title bulk and surface characterization techniques. It has been implied that the alkalization improves the electron density and the mobility of lattice and surface oxygen species. As a result, the bulk thermochemical stability is reduced on heating in a CO atmosphere, and a capacity towards CO2 uptake is developed. Moreover, the surface catalytic behaviour towards CO oxidation in the gas phase is maintained, and the behaviour towards H2O2 decomposition in the liquid phase is considerably promoted.

Thermochemistry of manganese oxides in reactive gas atmospheres: Probing catalytic MnOx compositions in the atmosphere of CO+O2

Abstract A series of Mn-oxides in the range MnO 2 –MnO were subjected to thermogravimetry on heating up to 1100°C in the reactive gas atmosphere of O 2 , CO and CO+O 2 . Some selected solid residues were analyzed for the MnO x composition by infrared spectroscopy, and for carbon deposits by CHNS-analyzer. The CO+O 2 gas stream was chromatographically analyzed, following heating the gas/solid interface to some selected temperatures. The results indicate that Mn-oxides in the range MnO 2 –Mn 1.3 exhibit catalytic properties towards the CO oxidation in the gas phase. The higher thermal stability (up to ∼1000°C) of compositions in the range MnO 1.5–1.3 , i.e. in the Mn 2 O 3 –Mn 3 O 4 system, may render them particularly interesting in the field of applied catalysis. The experimental approach adopted in the present investigation may help accounting for the interfacial chemistry of solid-state reactions involving the surrounding gas atmosphere.