Energetics of Manganese Oxides
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n manganese oxides the cations exist in three oxidation states (Mn 2 +, Mn 3 +, Mn +) and the oxides crystallize in several different structures. The oxides MnO, 4
Mn304, Mn203 and MnO2 can be converted from one to the other by suitable adjustments of temperature and 02 partial pressure. The present work is a systematic study, using high temperature reaction calorimetry, of well characterized stable phases in the Mn-0 system. The heat contents, the enthalpies of solution, and
the enthalpies of oxidation have been determined. We studied well defined oxides : MnO, Mn304, Mn203 and MnO2, which reach a known final state at the calorimetric temperature. The new values for enthalpies of oxidation combined with the reliable standard entropies have been used to plot a phase diagram for the Mn-0 system. Development of such methods for oxides containing manganese in oxidation states of +2, +3 and +4 would have interesting applications to the thermochemistry of systems such as LaMnO3-based perovskites and metastable manganese oxides and oxyhydroxides, phases encountered in fuel cell, battery,
electronic and environmental applications. The study presented in this paper is a first step in developing and validating calorimetric techniques for other complex manganese-containing compounds. EXPERIMENT All the studied samples were synthetic. High temperature calorimetric experiments were performed using a Tian Calvet twin type calorimeter described in 667 Mat. Res. Soc. Symp. Proc. Vol. 398 01996 Materials Research Society
detail by Navrotsky [1]. Heat contents of manganosite (MnO), hausmannite (Mn304) and bixbyite (Mn203) were obtained using the transposed temperature drop technique under an inert flowing gas atmosphere (argon). Under oxidizing atmosphere, all the samples transform to the bixbyite phase. In this case, the heat effect contains both the heat content and the enthalpy of reaction (oxidation or reduction). Drop solution calorimetry was used to determine the heat of solution of the manganese oxides. RESULTS AND DISCUSSION Heat contents and enthalpies of solution The heat contents (H 9 77 -H 298 ) measured (Table 1) are in excellent agreement with the values in the literature and show the consistency and accuracy of our method. The enthalpies of solution of the manganese oxides, in 2PbO-B203 at 977 K determined in this study are reported in Table 1.
Table 1: Heat content and enthalpy of solution of manganese oxides Manganese oxide
Heat content H977-H298 (kJ/mol)
Enthalpy of solution (kJ/mol)
MnO
34.3 ± 0.3 34.5 [21 34.4 [3]
8.8 ± 0.9 5.8 ± 0.7 [4]
116.5 ± 0.9 116.3 [21 116.1 [31
113.3 ± 7.3
Mn203
82.8 ± 0.9 82.8 [21
63.8 ± 1.6 62.6 ±1.4 [5]
MnO2
47.8 [21
Mn304
82.8 [3]
48.0 [3]
Measured enthalpies of oxidation and reduction : MnO/Mn2Q3, Mn-O4/Mn?_Q3 and MnO?_/MnO3
By dropping pellets of MnO, Mn304 or MnO2 from room temperature into the hot calorimeter with no solvent present, those oxides converted to the stable phase Mn203. The enthalpies of oxidation (at 298 K and 977 K) have been calculated
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