Effect of anion doping on the thermal decomposition of potassium bromate

Article abstract of DOI:10.1023/A:1010194508160

Structural defects were introduced into the potassium bromate (PB) lattice in the form of SO₄² and Cl^- ions in the process of crystal growth. It was assumed that these doped crystals PB(Cl^-) and PB(SO₄^2-) are composed of a two phase system, one being the perfect PB lattice and the other distorted regions due to induced defects. Isothermal decomposition of doped and normal PB samples was carried out gasometrically between the temperature range 653-663 K. The α-t plots reveal that the process occurs though initial gas evolution, acceleratory and decay stages. It also confirmed that doping enhances the rate of the reaction, the effect being more pronounced in the case of PB(SO₄²). The data are found to be well fitted to the Prout-Tompkins and Avrami-Erofe'ev mechanisms.

Full text of DOI:10.1023/A:1010194508160

Journal of Thermal Analysis and Calorimetry, Vol. 61 (2000) 879–883
EFFECT OF ANION DOPING ON THE THERMAL
DECOMPOSITION OF POTASSIUM BROMATE
B. C. Das and D. Patnaik*
P. G. Department of Chemistry, Ravenshaw College, Cuttack-753003, Orissa, India
(Received August 8, 1999)
Abstract
Structural defects were introduced into the potassium bromate (PB) lattice in the form of SO24– and
Cl– ions in the process of crystal growth. It was assumed that these doped crystals PB(Cl–) and
PB(SO24− ) are composed of a two phase system, one being the perfect PB lattice and the other dis-
torted regions due to induced defects. Isothermal decomposition of doped and normal PB samples
was carried out gasometrically between the temperature range 653–663 K. The α-t plots reveal that
the process occurs through initial gas evolution, acceleratory and decay stages. It also confirmed that
doping enhances the rate of the reaction, the effect being more pronounced in the case of PB(SO24– ).
The data are found to be well fitted to the Prout-Tompkins and Avrami-Erofe’ev mechanisms.
Keywords: anion doping, defects, isothermal decomposition
Introduction
Extensive work has been done on the thermal behaviour of alkali metal halates [1–9]
which are characterised by their oxygen generating capacity on decomposition. This
is of importance as chemical transformation of solids play a significant role in mod-
ern technology [9]. Several investigations have been carried out on the effect of addi-
tives [1, 5] irradiation [4, 6, 7] and dopants [2–4, 7, 8] on the thermal decomposition
of halogen oxosalts. It has been reported that irradiation increases the rate of decom-
position by lowering the energy of activation [2] which may be attributed to the lat-
tice defects generated, chemical damage fragments and products of decomposition.
Lattice defects introduced during crystal growth, even at low density, influences the
most characteristic features of the host crystals [9]. Diffusion which depends on the
presence and movement of defects, plays a vital role in solid state reactions. Observa-
tions reveal that thermal properties of doped inorganic solids change especially in the
vicinity of phase transition temperature [10]. The thermal decomposition of PB is
characterised by the fact that it proceeds through a molten/semi-molten state due to
the formation of an eutectic below the melting point of the substance [5, 8]. It was
*
Author to whom all correspondence should be addressed.
1418–2874/2000/ $ 5.00
Akadémiai Kiadó, Budapest
© 2000 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

880
DAS, PATNAIK: THERMAL DECOMPOSITION
therefore of interest to study the effects of the induced defects as doped anions on the
thermal decomposition of PB. The present work is on the study of the comparative ef-
fect of doping by anions (Cl– andSO42– ) having different size and valency which act as
potential sites for nucleation in the solid phase and as catalysts in the mol-
ten/semi-molten phase of the decomposition as no topochemical aspects are involved
at this stage [4a].
Experimental
Analar grade potassium chloride, potassium sulphate and PB were used for the prepa-
ration of samples. Crystals of potassium chloride and potassium sulphate (5·10–2 mo-
lar ratio) were incorporated into the lattice of PB by slow crystallisation of solutions
containing calculated amount of the dopants [3, 10] and PB. The samples were dried
at 430 K and were stored over P2O5 in desiccator. The purity and homogeneity of the
samples were tested by chemical and X-ray diffraction methods. Isothermal decom-
position studies of the samples were carried out (653–663 K) gasometrically in vac-
uum [11, 12] apparatus. The fraction decomposed α(=pt/pf) values were obtained
from the pressure of the evolved oxygen at different time intervals (pt) and at the end
of decomposition (pf) process.
Results and discussion
The isothermal decomposition of PB proceeds with evolution of oxygen according to
the reaction (1) through initial gas evolution, acceleratory and decay stages.
KBrO3=KBr+1.5O2
(1)
The initial gas evolution which may be due to the occluded air at the crystal sur-
face is enhanced by increase in temperature and upon doping; the effect being more
pronounced with PB(SO42– ) (Figs 1 and 2). α3min values for PB, PB(Cl–) and PB(SO24– )
were found to be 0.012, 0.03 and 0.06 respectively at 658 K. The absence of induction
Fig. 1 Effect of doping on the isothermal decomposition of PB
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DAS, PATNAIK: THERMAL DECOMPOSITION
881
Fig. 2 Effect of temperature on the isothermal decomposition of doped samples of PB
period and linear stage (Figs 1 and 2) is indicative of the fact that instantaneous nucle-
ation takes place in the surface of normal and doped samples of PB [5, 8]. This corre-
sponds to the applicability of kinetic models like Prout-Tompkins [13] and
Avrami-Erofe’ev [14]. (Figs 3 b and a, Eqs (2) and (3) respectively.)
log(α/1–α)=kA,D+C1
(2)
(3)
[–log(1–α)]1/n=kA′, D′+C2 (n=2)
kA,A and kD,D represent rate constants for acceleratory and decay stages respec-
′
′
tively, C1 and C2 being constants. The ranges of applicability of the above equations
over the sigmoidal region being 0.01<α<0.86 PB, 0.05<α<0.92 PB(Cl–) and
0.05<α<0.96 PB(SO24– ).
Fig. 3 Avrami-Erofe’ev (a) and Prout-Tompkins (b) analysis of data on different sam-
ples of PB
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The acceleratory and decay stages are shortened upon increase of temperature
and it is found that at 663 K, the reaction is almost complete after 50 min of the com-
mencement of the decomposition for PB(SO24– ) whereas and same stage is reached af-
ter 70 and 80 min for PB(Cl–) and PB respectively (Figs 1 and 2). This may be attrib-
uted to the fact that the reacting mass becomes homogeneous during the acceleratory
stages of the decomposition of PB due to the formation of an eutectic and DTA show
that the melting point is lowered from 730 to 670 K where the onset of decomposition
take place [5, 15, 16]. In this molten/semimolten stage, doping favours the decompo-
sition to a large extent (Figs 1 and 2). This may be attributed to the catalytic activity
of the product bromide ions and that of the induced defects in the crystal lattice [4a].
By displacing an ion from the regular lattice site, an induced defect introduces local
strain which results in the alteration of the frequency of the phonon vibration. These
changes bring about a decrease in the symmetry of the host bromate ions [17, 18] and
weaken the chemical bonds in the vicinity of the doped ions leading to a marked in-
crease in the reactivity. Table 1 indicates that the rate constants of both acceleratory
and decay stages are enhanced upon doping, the effect being more in the case of
PB(SO24− ). The applicability of Eqs (2) and (3) also points to the possibility that each
product nucleus catalyzes the formation of neighbouring nuclei which is more effec-
tive in the molten state than in the solid state thus indicating that nucleation occurs in
the chain branching manner and there is two dimensional growth of nuclei [7, 8].
Table 1 Effect of doping on rate constants (kA, kD and kA’ and kD’ in 102 min–1)
Prout-Tompkins
PB(Cl–)
Avrami-Erofe’ev
PB(Cl–) PB(SO24− )
Temp./
K
PB
PB(SO24− )
PB
kA
kD
kA
kD
kA
6.2
kD
kA′
1.3
1.7
2.2
kD′
0.7
1.0
1.2
kA′
1.5
2.5
2.7
kD′
0.8
1.6
1.2
kA′
1.7
3.0
4.0
kD′
1.0
1.7
1.2
653
658
663
3.9
5.8
6.2
1.4
2.2
2.5
4.3
6.5
8.7
1.8
2.8
3.5
2.5
3.0
3.7
8.3
11.2
It was of interest to observe that at all temperatures (Table 1) kA>kD and kA′ >kD′
for all the samples which may be attributed to the slowing down of the decomposition
process due to the presence of excess bromide ions which facilitates the resolidi-
fication of the melt and restricts the escape of oxygen [5, 8]. The enhanced rate of de-
composition due to the presence of Cl– and SO24– can also be explained on the basis of
the electron donating character of both the anions to an oxygen atom of an
undecomposed bromate ion thereby promoting the splitting of the latter [15]. De-
pending on their valency, the incorporation of foreign ions changes the number of
cation and anion vacancies and these changes induce modification in the rate of de-
composition. Since anion vacancies mean a +ve charge in the crystal lattice, the
higher charge on the SO24– ion has a greater effect on the electron transfer process and
hence enhanced rate of reaction results. The introduction of the bivalent ion forces
two bromate ions to leave the lattice to maintain the charge neutrality condition of the
lattice, [(+)(–)=constant at a constant temperature for an ionic solid where (+) and (–)
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DAS, PATNAIK: THERMAL DECOMPOSITION
883
represent the cation and anion vacancy concentrations respectively] which contrib-
utes to the higher rate of decomposition [19–21].
* * *
This work is dedicated to the memory of Late Professor S. R. Mohanty. The authors thank the De-
partment of Science, Technology and Environment, Government of Orissa, for financial support. B.
C. Das gratefully acknowledges the award of a Junior Research Fellowship by the University Grants
Commission, New Delhi, which made the work possible.
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