Studies on the thermal behavior of ammonium permanganate

Article abstract of DOI:10.1016/S0040-6031(99)00212-9

The thermal decomposition of ammonium permanganate was studied by thermogravimetry-mass spectrometry (TG-MS). A Perkin Elmer TGS-2 thermobalance and a Hiden's HAL 3F/PIC mass spectrometer equipped with a fast ion counter was used. Products of thermal decomposition of ammonium permanganate are oxygen, manganese oxides, water and ammonia. There is no gas phase oxidation of ammonia by oxygen. The possible intermediates of low-temperature decomposition reaction are ammonium nitrite or nitrate which transform into N₂, N₂O, or H₂O. The oxidation of adsorbed ammonia by manganese oxides via the superoxide active centers takes place on the surface of manganese oxide.

Full text of DOI:10.1016/S0040-6031(99)00212-9

Thermochimica Acta 338 (1999) 129±133
Studies on the thermal behavior of
ammonium permanganate
a,*
Laszlo Kotai , Piroska Szabo , Agnes Keszler
a
Âb
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aInstitute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences,
H-1525 Budapest, P.O. Box 17, Hungary
bResearch Laboratory of Materials and Environmental Chemistry, Chemical Research Center,
Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary
Received 11 June 1999; accepted 11 July 1999
Abstract
The thermal decomposition of ammonium permanganate was studied by thermogravimetry-mass spectrometry (TG-MS). A
Perkin Elmer TGS-2 thermobalance and a Hiden's HAL 3F/PIC mass spectrometer equipped with a fast ion counter was used.
Products of thermal decomposition of ammonium permanganate are oxygen, manganese oxides, water and ammonia. There
is no gas phase oxidation of ammonia by oxygen. The possible intermediates of low-temperature decomposition reaction are
ammonium nitrite or nitrate which transform into N2, N2O, or H2O. The oxidation of adsorbed ammonia by manganese oxides
via the superoxide active centers takes place on the surface of manganese oxide. # 1999 Elsevier Science B.V. All rights
reserved.
Keywords: Ammonium permanganate; Thermal decomposition; Thermogravimetry-mass spectrometry; Ammonia oxidation
1. Introduction
the composition of solid phase under thermal treat-
ment were also studied [6].
Ammonium permanganate and the products of its
thermal decomposition are widely used as oxidants or
catalysts in material technology [1].
Although all the authors have de®ned the chemical
composition of evolved gases under thermal decom-
position of ammonium permanganate, the relationship
between the composition of the gas phase and the
temperature ranges belonging to the degradation steps
has not been studied yet. The mechanism of formation
of some unusual gaseous reaction products e.g., N2O
has not been determined either.
The thermal decomposition of ammonium perman-
ganate was investigated by many authors. Bircusmhaw
and Taylor have studied the kinetics of decomposition
of this salt under oil [2]. The kinetics and mechanism
of decomposition under vacuum were performed by
Pavlyuchenko et al. [3]. The effect of UV irradiation
[4] and of ultrasonic initiation [5] on the thermal
behavior of ammonium permanganate, as well as
2. Experimental
*Corresponding author.
E-mail address: kotail@cric.chemres.hu (L. Kotai)
Ammonium permanganate was prepared from bar-
ium manganate and ammonium sulphate [7]. The
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0040-6031/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 9 9 ) 0 0 2 1 2 - 9

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L. Kotai et al. / Thermochimica Acta 338 (1999) 129±133
130
product was recrystallized from water and dried in the
dark.
The thermal decomposition of the sample was
studied by thermogravimetry-mass spectrometry
(TG-MS). This computerized atmospheric pressure
thermobalance-mass spectrometer (TG-MS) system
had been built of a Perkin Elmer TGS-2 thermoba-
lance and a Hiden's HAL 3F/PIC mass spectrometer
equipped with a fast ion counter. Small amounts of
samples (0.72 mg) were heated from 208C at a heating
1
rate of 18C min in a platinum sample pan. The
surface of the sample was covered with a layer of
high purity Al2O3 (1 mg) to prevent the explosion-like
degradation of NH4MnO4. The ambient gas was high
purity argon with a ¯ow rate of 140 ml min 1. The
evolved gases were led from the sample pan to the
mass spectrometer through a heated capillary within
approximately 1 s. A small portion of the carrier gas
was introduced into the ion source of mass spectro-
meter through a molecular hole. The ion source was
operated at 70 eV electron energy. Selected mass
spectrometric intensities were measured as functions
of temperature. The mass spectrometric intensities
were normalized by the initial sample mass and by
the sensitivity factors calculated from the observed
Fig. 1. Comparison of TG, DTG and ion intensity curves of H2O
and O2 (m/z 18: -*- and m/z 32: -!, respectively) during the
thermal decomposition of NH4MnO4. (Ion intensity curves are in
arbitrary scales.)
gas phase. The presumed oxidation agents of these
reactions are permanganate ion, oxygen or primary
reaction products having higher oxidation states as the
higher valence manganese oxides, nitrogen oxides or
transition unstable ions (e.g. as superoxide ion). Cat-
alytic effects of reaction products (e.g. manganese
oxides) cannot be excluded in the entire decomposi-
tion processes.
‡
isotope intensity (38Ar ) of the ambient gas. The DTG
curves were calculated by spline smoothing. The
performance of the system was checked by measuring
the three decomposition steps of calcium oxalate
monohydrate [8].
Figs. 1±3 show the gaseous thermal decomposition
products of ammonium permanganate. Table 1 lists
3. Results and discussion
During the thermal decomposition of ammonium
permanganate ammonia releases and the destruction
of anion provides manganese oxides besides oxygen
evolution. The oxidation reactions of ammonia or
ammonium ion resulting in the formation of water
and nitrogen containing oxidation products (e.g. ele-
mentary nitrogen or nitrogen oxides and ammoninum
nitrate) can also be observed [1±6].
All possibilities of fragmentation pathways were
taken into consideration at the same time in order to
distinguish the species with identical masses.
Nitrogen containing oxidation products can be
formed in two ways: via oxidation of ammonium
ion or retained ammonia in solid phase; or in further
oxidation or ignition of previously formed ammonia in
+
Fig. 2. Comparison of TG, DTG and ion intensity curves of NH2
and NH3 (m/z 16: -*- and m/z 17: -!-, respectively). (Ion
intensity curves are in arbitrary scales).

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L. Kotai et al. / Thermochimica Acta 338 (1999) 129±133
131
in this temperature interval, thus above 1158C the
amount of formed oxygen is negligible.
2NH4MnO4 ˆ 2NH3 ‡ H2O ‡ 2MnO2
‡ 1:5O2
(1)
As can be seen from Eq. (1), the water is not an
oxidation product of ammonia.
It is known [3] that the presence of ammonia and
water decreases and the presence of oxidants (e.g.
oxygen) increases the rate of decomposition. Based on
the formation pro®les of ammonia and oxygen we can
state that the evolution of ammonia and oxygen starts
at the same time. The autocatalytic character of this
reaction is complex due to the presence of many active
species as manganese oxides, oxygen, water or ammo-
nia.
If the reaction heat is distributed, the local over-
heating can be avoided and the ammonia desorption
from the solid phase is slower, thus NH3 can be
oxidized in the solid phase. After the total decom-
position of permanganate ion (oxygen evolution is
ended) the oxidation process also seems to be ®nished,
and only ammonia and water can be observed (110±
1408C) as decomposition products. It means that the
active oxidants are the permanganate ion or its decom-
position intermediates.
Fig. 3. Comparison of TG, DTG and ion intensity curves of N2,
NO+ and N2O (m/z 28: -*-, m/z 30: -!- and m/z 44: -^-,
respectively). (Ion intensity curves are in arbitrary scales).
data of the maximum mass spectrometric ion intensity
values and temperature regions belonged to the ion
curves.
3.1. On the decomposition reaction of NH4MnO4
The heat transfer among ammonium permanganate
crystals plays an important role in the rate of decom-
position [2,9], thus the high reaction heat causes a fast
overheating of each site of crystals. As it can be seen
from Table 1 and from Figs. 1 and 2 the decomposition
starts below 708C and becomes very quick at around
858C. The main step is the simple decomposition
reaction with formation of ammonia, oxygen and
water. The permanganate anion totally decomposes
3.2. On the mechanism of thermal decomposition of
NH4MnO4
Although the ignition characteristics of ammonia
gas with oxygen gas and the ¯ame nature under the
thermal decomposition of NH4MnO4 are similar
[3,10], the oxidation of ammonia in gas phase with
Table 1
Assignement and intensity maxima of the mass spectroscopic ions during the thermal decomposition of NH4MnO4 (covered with alumina)
m/z
Assignmentsa
Temperature ranges of single and
double peaks (8C)
Max. intensitiesb of single and
double peaks (arbitrary units)
16
17
18
28
30
32
44
NH2+, O+
NH3, OH+
H2O
70±160
80±180
5.9
7.3
70±130
70±130
145±180
70±115
70±130
130±170
130±180
114
13
54
20
CO, N2
NO+
1.7
O2
7.5
9.1
N2O, CO2
130±180
10.1
a The compounds refer to the probable source of the ions. The `+' charge sign indicates that the corresponding formula represents a mass
spectrometric fragment ion.
b The intensities are normalized by the initial sample mass.

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L. Kotai et al. / Thermochimica Acta 338 (1999) 129±133
132
gaseous oxygen under thermal decomposition of
NH4MnO4 is not probable, since no typical oxidation
products of ammonia (N2, NO, NO2, N2O) were
found. The role of manganese oxides in the low
temperature (around 1008C) oxidation of gaseous
ammonia is also not probable [11].
step the manganese oxides play an important role in
the formation of nitrogen oxides.
At higher temperatures there is a very little ammo-
nia in the solid phase, thus the oxidation process is not
signi®cant.
Not only does the presence of gaseous products
(NH3, H2O or O2,), but the presence of dislocations
(due to formation of thermal decomposition products)
which cause stresses in crystal lattice also plays an
important role [4] in the thermal decomposition pro-
cess of NH4MnO4.
It is known [3] that ammonium permanganate
decomposes slowly at room temperature. However,
the longer storage of this salt at the same temperature
decreases the decomposition rate compared with
unstoraged salt [4]. With increasing number of dis-
locations (formed under storage) the number of active
stress centers in crystal lattice is increased. Under
fairly long storage, the crystal lattice is interrupted due
to the high rate of decomposition, therefore, the role of
stresses ceases.
At higher temperatures (130±1908C) another oxida-
tion process occurs, where H2O, N2O NO, N2 and a
small amount of O2 are formed as the main oxidation
products. Above 1458C NO and H2O are formed in the
oxidation step and are retained. Since the decomposi-
tion reactions were carried out under argon atmo-
sphere, this phenomenon should exclusively be the
result of the reaction between ammonia and manga-
nese oxides originated from permanganate anion.
The formation of N2O in the thermal decomposition
of ammonium salts of oxoanions chromate [12,13],
dichromate [14], chlorate [15], perchlorate [14±20] or
MOn4 metallates (MV, Mo and W) [21] is known.
NH4NO3 is the possible precursor of N2O and water as
well according to the following equation:
In the slow decomposition of NH4MnO4 at room
temperature the hydrogen bonds which are present
between the hydrogen of ammonia and the oxygen of
permanganate also [24] play an important role. These
interactions are the active centers of redox processes
between permanganate ion and ammonia (via forma-
tion of Mn-oxides) resulting in crystal defects and
dislocations.
The presence or absence of NH4NO3 or NH4NO2 as
precursors of N2O or N2 is also important in thermal
decomposition of NH4MnO4. Some authors have
detected ammonium nitrate among the solid decom-
position products of ammonium permanganate (under
vacuum in oil as heat conducting media [2] or under
atmospheric condition [6]). However, in the residue of
the totally decomposed NH4MnO4 in vacuum neither
ammonium nor nitrate ions were found [3] at 1008C.
Under these conditions (vacuum, in the absence of
retarding media e.g. oil) the ammonia is desorbed
from manganese oxides, and further oxidation yield-
ing ammonium nitrate can be excluded. The formation
of ammonium nitrite together with ammonium nitrate
is also detected [9] in the reaction of NH4MnO4 and
NH3, but NH4NO2 decomposes above 708C due to its
thermal instability. The formation of N2 starts at 708C,
see Table 1 or Fig. 1, thus the origin of N2 between 70±
1308C from decomposition reaction of NH4NO2 can-
NH4NOx ‡ D ˆ N2Ox ‡ 2H2O
(2)
2
NO and N2O always form together in the oxidation
process of ammonia [11,19,20]. The presence of metal
oxides, however, as potential catalysts [11,22] accel-
erate the decomposition of NH4NO3 resulting in the
exclusive formation of N2O. The experimental results
led us to assume that during the first decomposition
step of NH4MnO4 as a temporary compound, NH4NO3
can be formed.
There is another possibility for the formation of
oxidation products N2, NO or N2O in the second
decomposition step, too. It is the oxidation of adsorbed
ammonia by manganese oxides. On the basis of some
references [15,23] N2O can also be formed in the
oxidation of ammonia. In this oxidation processes, e.g.
by oxygen, nitrogen and nitrogen oxides (NO or also
N2O) are produced. The amount of N2O formed is
temperature-dependent [23] and so is the increase in
the presence of various catalysts [15] which inhibit the
further decomposition of the previously formed N2O.
Since the catalytic effect of manganese oxides (MnO2
and Mn2O3) which are formed in the thermal decom-
position of ammonium permanganate (the composi-
tion of oxide phase is temperature dependent [6]) in
the ammonia oxidation process at relatively low tem-
peratures [11] is well de®ned, in the second oxidation

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L. Kotai et al. / Thermochimica Acta 338 (1999) 129±133
133
not be excluded. Desorption of ammonia does not
occur under atmospheric pressure (sorptioned ammo-
nia have been detected in solid phase decomposition
products of NH4MnO4 above 1008C [6]), thus the
oxidation of ammonia to ammonium nitrate may take
place. In the presence of ammonium nitrate the
decomposition of NH4MnO4 is slower [3] than in
the case of pure salt, due to the endothermic effect
of the phase transitions of ammonium nitrate [13] at
the temperature where NH4MnO4 decomposes.
The mechanism of catalytic action is based on the
presence of superoxide ion ꢀO2 † on the surface of
manganese oxides [11]. Increasing the partial pressure
of oxygen the formation of ꢀO2 † covered sites on
manganese oxides are increased, but the presence of
oxygen can accelerate the oxidation thermal decom-
position [3] process of ammonium permanganate.
Furthermore, by raising the temperature the ammo-
nia acts as a reducing agent towards MnO2 by the
formation of Mn2O3.
erating effect of oxygen in the thermal decomposition
(including the redox reaction) of ammonium perman-
ganate con®rms the above meaning, because by
increasing partial pressure of oxygen the covering
of the surface of manganese oxides by O2 molecules
(precursors for ꢀO2 † ions) increases as well.
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