Equilibrium Pressures of Oxygen over MnO[sub 2]-Mn[sub 2]O[sub 3] at Various Temperatures

Abstract

Oxygen pressures of the pyrolusite form of MnO2 have been determined in a temperature range of 441°–642°C. The dissociation reaction 4MnO2false(normalcfalse)⇄2Mn2O3false(normalcfalse)+O2false(normalgfalse) has been found to reach a steady state in the forward direction; reaction in the reverse direction was exceedingly slow and did not reach a steady state. normalΔH° and normalΔS° for the reaction at 813°K are 37.87 kcal and 51.28 cal/deg, respectively. With little error these values can be used in the equation −4.5756logpO2 false(normalatmfalse)=37,870/T−51.28 to calculate the pressure for any temperature in the above mentioned range. Conversion to 298.15°K resulted in normalΔH°298=38.8 normalkcal and normalΔS°298=52.8 normalcal/normaldeg for the dissociation. The limits of uncertainties are not given here because equilibria were not attained by recombination.

Assessment of the Mn-O system

“…87 Zordan and Hepler [108] 59. 75 Todd and Bonnickson [86] 59.02 this work Hausmannite (Mn 3 O 4 ) 155. 75 Pankratz [107] 166.…”

“…75 Todd and Bonnickson [86] 59.02 this work Hausmannite (Mn 3 O 4 ) 155. 75 Pankratz [107] 166. 6 O'Neill and Pownceby [79] 149.…”

“…The scatter of the measured decomposition temperature of MnO 2 is very large. In air the measured temperatures range from 697 K [75] to 943 K [5] in 1 atm O 2 from 742 K [75] to 1238 K, [69] with the other measurements [82,83] lying in between. The measurements of Klingsberg and Roy [73] are not considered as they are clearly wrong.…”

“…This might be due to some metastable product formed before true equilibrium is reached [58] or the fact that the reaction proceeds sluggishly, as the temperatures involved are low. The only measurements that represent true equilibrium values seem to be the ones by Otto, [75] who equilibrated the samples for very long times.…”

“…12, Table 9) was used; [75] for the reaction ␣-Mn 3 O 4 + 1 ⁄2O 2 (g) ‫ס‬ 1.5Mn 2 O 3 the oxygen potential measurements by Shenouda and Aziz, [70] Ingraham, [71] Hahn and Muan, [29] Hochgeschwender and Ingraham, [58] and Charette and Flengas [67] (Fig. 11, Table 8) were used.…”