ransactions of the Japan Institute of Metals, Vol. 24, No. 11 (1983), pp. 754 to 758

Study on Thermal Decomposition of MnO2

and Mn2O, by Thermal Analysis

By Kiyoshi Terayama* and Masao Ikeda*

The present paper is concerned with the kinetics of thermal decomposition of manganese

oxides and nonstoichiometric cnmpounds formed in these reaction processes, studied by means of the

thermal analyses (TG, DTA and DSC) and gas chromatography (GC).

The decomposition of MnO2 to Mn2O3 proceeds at about 756K, and the heat of reaction is

176±8kJ/mol of O2. For the decomposition of Mn2O, to Mn3O4, the activation energy of about

163kJ/mol of O2 is obtained. The nonstoichiometric compounds, MnO1 .61 and MnO1.42, seem

to be formed in the thermal decomposition processes of MnO2 and Mn2O3, respectively.

(Received April 2, 1983)

Keyrwords: thermal decomposition, manganese oxide, thermal analysis, gas chromatograph,

nonstoichiometric compound

Ⅰ. Introduction

In the Mn-O system, MnO2, Mn2O3, Mn3O4

and MnO are known to exist as stable crystal-

line phases, and MnO2 is continuously con-

verted into lower oxides under a suitable

condition of temperatures and oxygen partial

pressures. Thermal decomposition of MnO2 to

Mn2O3 and that of Mn2O3 to Mn3O4 take

place by heating, and Mn3O4 is easily formed

upto about 1273K. Further reduction of

Mn3O4 to MnO can be achieved by CO or

carbon.

In spite of many studies dealing with various

aspects of the chemistry of manganese oxides,

only a few studies (1)-(5) have been reported

about the nonstoichiometry in the carbon

thermic reduction of these oxides.

The present paper is concerned with the ki-

netics of thermal decomposition of MnO2 and

Mn2O3 and nonstoichiometric compounds

formed in these reaction processes, studied by

the simultaneous thermal analyses (TG, DTA

and DSC) and gas chromatography (GC).

Ⅱ. Experimental Procedure

1. Specimen

MnO2 was prepared from a reagent of

recrystallized manganese nitrate by heating at

473K in air. Then it was heated at 873K in an

electrical resistance furnace under an Ar

atmosphere for 43.2ks. It was known by the

X-ray diffraction method, that the oxide sam-

ples obtained had chemical formulas of MnO2

and Mn2O3.

2. Experimental apparatus

The decomposition process was studied with

a thermal analytical apparatus (TG, DTA and

DSC: Type 9500, Rigaku Denki Co., Japan) in

a He gas flow using Al2O3 as a reference

material.

For the thermal decomposition of MnO2, the

heat of reaction was calculated from a DSC

curve by means of the computing system (DSC:

Type 9500+HP-85F).

The oxygen content in a fixed volume (2×

10-6m3) of gas evolved during the decomposi-

tion of MnO2 and Mn2O3 was determined

by a gas chromatograph (GC: Type GS-5000,

Gaschrokogyo Co., Japan+Type 163, Hitachi

Seisakusho Co., Japan) at intervals of 30s.

The effluent gas analysis (EGA) technique (6)

was also used, and the application of this EGA

to the reduction process of manganese oxides

was described in the previous papers (4) (7) in

some detail.

The sample quenched at a chosen condition

was used for identifying phases present by

means of the X-ray diffraction method using

Fe-Kα radiation and the chemical analysis.

* Department of Metallurgy, Faculty of Engineer-

ing, Toyama University, Takaoka 933, Japan.

Study on Thermal Decomposition of MnO2 and Mn2O, by Thermal Analysis 755

Ⅲ. Results and Discussion

1. Simultaneous TG, DTA, DSC and GC

For the thermal decomposition of MnO2,

results obtained by simultaneous TG, DTA,

DSC and GC under the same experimental

conditions are presented in Fig. 1. A good

agreement is found among results obtained by

different methods: The oxygen evolution deter-

mined by GC and the weight loss of the sample

are recognized, when the heat change is ob-

served in DTA curve. The weight loss in TG

curve occurs gradually at about 703K. The

weight loss, an endothermic peak in DTA

curve, a clear peak in DSC curve and the oxy-

gen evolution in GC curve occur at the same

temperature of about 756K. The maximum

peak of the reaction is found at about 860K.

This reaction corresponds to the thermal de-

composition of MnO2 expressed by eq. (1):

4MnO2=2Mn2O3+O2. (1)

The heat of reaction, 176±8kJ/mol of O2,

calculated from the DSC curve agrees well with

the value based on the thermochemical data (8),

165kJ/mol.

The heat of reaction (1) in the present study

Fig. 1 Simultaneous depiction of TG, DTA, DSCand GC curves for the thermal decomposition ofMnO2 to Mn2O3.

is in reasonable agreement with those of Wickset al. (9) and Kapustinsky et al. (10), (164.0 and132kJ/mol of O2) obtained by equilibrium andcalorimetric measurements, respectively, whilethe result of the present study slightly differsfrom those obtained by the differential thermalanalysis, 108 (11) and 102.2 (12)kJ/mol of O2,and that obtained by equilibrium measurement,241.8 (13)kJ/mol of O2.It is known that the heat of reaction can be

quantitatively determined by DSC more ac-curately than by DTA, and the equilibriummeasurement is thought to be a standardmethod by which it takes a considerable longtime.For the thermal decomposition of Mn2O3 to

Mn3O4 expressed by eq. (2), TG, DTA and GCcurves are shown in Fig. 2.

6Mn2O3=4Mn3O4+O2. (2)

The weight loss in TG curve, the endothermicreaction in DTA curve and the oxygen evolu-tion in GC curve occur at about 923K, and the

peak temperature of the thermal decomposi-tion is about 1072K. In the present study,DSC technique can not be applicable becauseof the limitation of DSC apparatus (from roomtemperature to 1073K).The weight loss obtained by TG curve

exactly equals that calculated for the reactionexpressed by eq. (2).

2. Isothermal decomposition process

Typical effects of temperature on the thermal

Fig. 2 Simultaneous depiction of TG, DTA and GCcurves for the thermal decomposition of Mn2O, toMn3O4.

756 Kiyoshi Terayama and Masao Ikeda

Fig. 3 Effect of temperature on the thermal decom-

position of MnO2.

decomposition of MnO2 to Mn2O3 measured

by EGA technique are shown in Fig. 3. At an

earlier stage, the decomposition does not

quickly proceed for some time and the sig-

moidal curves are observed. The decomposition

rate increases as the temperature increases

from 805 to 880K.

In the present study, a He flow rate of 1.17×

10-6m3/s controlled by a gas flow equipment

is sufficient for neglecting the effect of the

transport of the product gas from the outer

surface of the sample to the bulk gas.

It has been mentioned in the previous paper (4)

that the rate determining step of the reaction

expressed by eq. (1) is an interfacial reaction

and the rate equation in the form (14) (15) of

1-(1-α)1/3=kt (3)

is applied, k being an apparent velocity con-

stant. In the following discussion, α1 and α2

are fractions of the thermal decomposition in

converting MnO2 to Mn2O3 and Mn2O3 to

Mn3O4, respectively.

The plots of [1-(1-α1)1/3] vs t are shown in

Fig. 4. It can be seen that they are linear in the

α1 range from about 0.2 to 0.8, but this rela-

tionship holds no longer at the later steps,

where mixed kinetics of interfacial reaction and

diffusion are to be considered.

The activation energy, about 163kJ/mol of

O2, is determined from an Arrhenius-type plot

shown in Fig. 5, and this value is in good

agreement with the values reported in the

literatures (4) (15).

The effects of temperature on the thermal

decomposition of Mn2O3 to Mn3O4, meas-

Fig. 4 Relation between time and 1-(1-儒1)1/3 for

the thermal decomposition of MnO2.

Fig. 5 Arrhenius plots for the thermal decomposi-tion of MnO2 and Mn2O3.

ured by EGA technique, are shown in Fig. 6.

As shown here, the decomposition rate in-

creases at higher temperatures and approaches

unity in the first period of about 0.7ks. The

relation between [1-(1-α2)1/3] and t is shown

in Fig. 7. From the slope of these straight lines

at earlier stages, the activation energy of this

reaction expressed by eq. (2) is estimated to be

about 226kJ/mol of O2, nearly equal to those

of Siemonsen (16) and Hahn et al. (17) (210.0,

Study on Thermal Decomposition of MnO2 and Mn2O3 by Thermal Analysis 757

Fig. 6 Effect of temperature on the thermal decom-

position of Mn2O3.

Fig. 7 Relation between time and 1-(1-α2)1/3 for

the thermal decomposition of Mn2O3.

193±13.8kJ/mol of O2, respectively). The

interfacial reaction is thought to be a rate

determining step for the thermal decomposition

of Mn2O3 to Mn3O4.

These activation energies obtained in the

present study are thought to be accurate, be-

cause of the adoption of simultaneous measure-

ments of thermal analyses (TG, DTA and

DSC) and gas analyses (GC, EGA). The gas

analysis method, which is sensitive and trouble-

less compared with other methods in thermal

analysis (6), can be applicable to studying the

carbon reductions of oxides having high

vapour pressures, such as manganese ox-

ides (4) (7).

3. Formation of nonstoichiometric

compound

During the thermal decomposition of MnO2

and Mn2O3, the reaction rates increase at 833

and 1053K in the DTA curves illustrated in

Fig. 1 and Fig. 2, respectively. The decomposi-

tion rate (dα/dt), α and x are calculated from the

EGA curves at temperature from 823 to 843K

for MnO2 and from 1043 to 1063K for Mn2O3,

where the decimal notation, x, in MnOx is used

to specify a certain composition of manganese

oxide.

A procedure similar to that described in the

previous paper (4) for determining the nonstoichi-

ometric compound can be applicable to the ther-

mal decomposition of MnO2. The relation be-

tween dα1/dt, α1 and x is illustrated in Fig. 8. The

value of α1, at which the decomposition rate

becomes slower, is obtained by calculating the

intersection of the lines tangent to the dα1/dt-

α1 curve (4) at 833K illustrated graphically in

Fig. 8. The calculated value of al is 0.78 which

corresponds to the nonstoichiometric com-

pound, MnO 1.61 (5), and this compoulld has

also been presented by Amirova et al. (1) based

on the oxidation of MnO in an oxygen atmo-

sphere.

A similar procedure of plotting dα2/dt-α2, x

can be applicable to the thermal decomposition

of Mn2O3, as shown in Fig. 9. The value of α2,

Fig. 8 Relation between dα1/dt and α1 obtained from

EGA curve for the thermal decomposition of MnO2.

758 Kiyoshi Terayama and Masao Ikeda

Fig. 9 Relation between dα2/dt and α2 obtained from

EGA curve for the thermal decomposition of

Mn2O3.

0.48, which corresponds to the nonstoichiomet-

ric compound, MnO1 .42, is obtained at 1053K,

and this compound is also formed in the reduc-

tioll process of Mn2O3(4).

These nonstoichiometric compounds ob-

tained in the present study are in good agree-

ment with those determined by a study for the

Mn-O system in CO2-H2 mixtures (5).

Ⅳ. Summary

The thermal decomposition of MnO2 and

Mn2O3 is studied in order to clarify the

kinetics of these reactions as well as the

nonstoichiometric compound by the simultane -

ous application of TG, DTA , DSC and GC.

The results may be summarized as follows

(1) The decomposition of MnO2 to Mn2O3

proceeds at about 756K. The heat of reaction,

176±8kJ/mol of O2, is obtained .

(2) For the decomposition of Mn2O3 to

Mn3O4 which occurs at about 923K, the

activation energy, about 163kJ/mol of O2, is

obtained.

(3) The nonstoichiometric compounds ofMnO1 .61 and MnO1.42 seem to be formed in

the thermal decomposition processes of MnO2

and Mn2O3 at 833 and 1053K , respectively.

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