Mechanism and kinetics of thermal decomposition of nickel(II) sulfate(VI) hexahydrate

Article abstract of DOI:10.1023/B:JTAN.0000033184.32714.7f

This work presents results of research on thermal decomposition of nickel(II) sulfate(VI) hexahydrate in air and in helium atmosphere. On the base of TO and XRD results a mechanism of thermal decomposition of NiSO₄ hydrate was established. For calculations of kinetic parameters of the Arrhenius equation, the Coats-Redfern approximation was applied, Choice of g(α) function and thus of a mechanism best describing given stage of decomposition was performed by testing 12 kinetic models.

Full text of DOI:10.1023/B:JTAN.0000033184.32714.7f

Journal of Thermal Analysis and Calorimetry, Vol. 77 (2004) 25–31
MECHANISM AND KINETICS OF THERMAL
DECOMPOSITION OF NICKEL(II) SULFATE(VI)
HEXAHYDRATE
E. Tomaszewicz* and M. Kotfica
Technical University of Szczecin, Institute of Chemistry and Environment Protection,
Al. Piastów 42, 71-065 Szczecin, Poland
Abstract
This work presents results of research on thermal decomposition of nickel(II) sulfate(VI) hexahydrate in
air and in helium atmosphere. On the base of TG and XRD results a mechanism of thermal decomposi-
tion of NiSO4 hydrate was established. For calculations of kinetic parameters of the Arrhenius equation,
the Coats–Redfern approximation was applied. Choice of g(α) function and thus of a mechanism best
describing given stage of decomposition was performed by testing 12 kinetic models.
Keywords: Coats–Redfern’s approximation, kinetics of thermal decomposition,
nickel(II) sulfate(VI) hydrates
Introduction
Heterogeneous catalysts for dimerization and oligomerization of olefins, consisting
mainly of nickel compounds supported on oxides, have been known for many years. In
many papers [1–4], it has been shown that NiO–TiO2, NiO–ZrO2 and NiO–γ-Al2O3
modified with sulfate or tungstate ions are very active for ethylene dimerization. High
catalytic activities in reactions are related to acidic properties of modified catalysts,
which originated from the inductive effect of S=O or W=O bonds of the complex
formed by the interaction of oxides with sulfate or tungstate ions.
Thermal decomposition of d-electron metal sulfates has also been used as an im-
portant reaction in chemical and metallurgical industries. The reaction has also been
under recent investigation for many potential applications, e.g. possible storage of
solar energy or nuclear heat. One of suggested applications is hydrogen production
by a thermochemical water-splitting process [5].
The basic kinetic equation describing the rate of thermal dissociation of solids
(Arrhenius equation) can be written in a form:
dα
dT
A
E



=
exp −
f (α)
(1)

α
RT


*
Author for correspondence: E-mail: tomela@ps.pl
1388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
© 2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

26
TOMASZEWICZ, KOTFICA: NICKEL(II) SULFATE(VI) HYDRATES
where T – is the current temperature of sample, A – pre-exponential factor, E – activation
energy, R – universal gas constant, α – conversion degree.
By integrating the Eq. (1) a solution can be obtained in the form:
α
AT
E


g(α) =
exp −
dT
(2)


∫
β
RT


T
0
This integral has no analytical solution and therefore in the kinetics theory of non-
isothermal decomposition of solids some approximate dependence formulas are used [6].
One of convenient forms of them is the Coats–Redfern approximation [6] written as:


2RTm
g(α)
T 2
 AR 
E







ln
=ln
1−
−
(3)







βE
E
RT








where Tm – is the temperature corresponding to maximum rate of conversion for a given
stage, β – the rate of heating a sample.
The dependence 3 is a basic equation making it possible, on the base of experi-
mental data, to establish the reaction mechanism by determining the g(α) function as
well as to find the values of parameters in the Arrhenius equation.
Thermal decomposition of hydrates of many d-electron metal sulfates (also
NiSO4⋅6H2O) was studied by many investigators [7–10]. Unfortunately, many publica-
tions on a decomposition of nickel(II) sulfate(VI) hexahydrate do not contain kinetic data
for an inert atmosphere. In this work, thermal decomposition of NiSO4⋅6H2O was studied
in two different atmospheres (air and helium). For calculations of kinetic parameters of
the Arrhenius equation, the Coats–Redfern approximation was applied.
Experimental
A reagent used for research was nickel(II) sulfate(VI) hydrate NiSO4⋅6H2O obtained by a
slow crystallisation from a saturated solution of nickel(II) sulfate(VI) (pure, product of
POCh Gliwice, Poland). The obtained crystals of hydrate were dried in acetone and
ground in an agate mortar. DTA/TG investigations were carried out by means of a
SDT 2960 apparatus, product of TA Instruments. The measurements were performed in
corundum crucibles in the atmosphere of air or helium (gas flow 110 mL min–1), at heat-
ing rates of 2, 4, 6, 8 and 10 K min–1. The mass of NiSO4⋅6H2O portions were within the
16–40 mg range. The final and intermediate solid products of NiSO4⋅6H2O decomposi-
tion in air and in helium atmosphere were identified by XRD. Investigations by this
method were conducted with the use of an X-ray diffractometer DRON-3, the radiation
CoKα (λ=0.179021 nm). Solid products of reaction were identified by comparing the
X-ray powder diffraction patterns of the investigated samples with the data contained in
the PDF cards of the JCPDF database [11].
Results and discussion
The investigations were begun with trying to establish the number of water mole-
cules contained by the obtained nickel(II) (VI) hydrate. For this purpose the prepara-
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TOMASZEWICZ, KOTFICA: NICKEL(II) SULFATE(VI) HYDRATES
27
tion obtained by crystallisation was subjsulphateected to DTA/TG and XRD exami-
nation. On the base of the mass change and the XRD analysis it was found that the
hydrate of NiSO4 contained 6 water molecules.
Figure 1 presents the DTA/TG curves recorded while heating the samples of
NiSO4⋅6H2O in the atmosphere of air and of helium, at a heating rate of 10 K min–1.
In each DTA curve four endothermic effects were recorded. In order to determine the
characteristic of these effects, the samples of NiSO4⋅6H2O were heated for 4 h, in a
flow furnace, in the following temperatures: 383, 423, 473, 533, 773 and 1173 K.
These temperatures were established on the base of DTA curves. After the required
heating time the samples were rapidly cooled to ambient temperature and subjected
to XRD investigations. The results of such investigations for ‘frozen’ samples are
showed in Table 1.
Fig. 1 DTA/TG curves of NiSO4 hydrate (air and helium atmosphere, β=10 K min–1)
On the ground of the mass changes recorded in the TG curves, the results of XRD
analysis and literature information [12–20], it was established that the mechanism of
thermal decomposition of NiSO4⋅6H2O, in air and helium atmosphere, can be described
by the following equations:
I
NiSO4⋅6H2O(s)=NiSO4⋅2H2O(s)+4H2O(g)
NiSO4⋅2H2O(s)=NiSO4⋅H2O(s)+H2O(g)
NiSO4⋅H2O(s)=NiSO4(s)+H2O(g)
(4)
(5)
(6)
(7)
II
III
IV
NiSO4(s)=NiO(s)+SO2(g)+1/2O2(g)
J. Therm. Anal. Cal., 77, 2004

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TOMASZEWICZ, KOTFICA: NICKEL(II) SULFATE(VI) HYDRATES
Table 1 Conditions of constant-temperature treatment and XRD results for NiSO4⋅6H2O samples
heated in a flow furnace
Identified phases
(air atmosphere)
Identified phases
(helium atmosphere)
Measurement No.
‘Freezing’ temp./K
NiSO4⋅6H2O
(tetragonal)
NiSO4⋅6H2O
(tetragonal)
1
2
3
ambient temperature
383
423
NiSO4⋅2H2O
NiSO4⋅2H2O
NiSO4⋅H2O
NiSO4⋅2H2O
NiSO4⋅H2O
4
5
473
533
NiSO4⋅H2O
NiSO4⋅H2O
NiSO4⋅H2O
NiSO4⋅H2O
Tables 2 and 3 list individual stages of thermal treatment conducted in air and in
helium, applied heating rates, temperature ranges in which the individual stages of de-
composition occur (∆T) and maximum temperatures of peaks recorded in DTA curves
(Tm). The calculated values of the Arrhenius equation parameters (A and E) and the val-
ues of correlation coefficient r as well as the kinetic model best describing given stage
are also presented there. The kinetic model for given stage was established on the base
of the best values of the correlation coefficient and identical courses of the k(T) de-
pendence for five applied heating rates. This last criterion determined the selection of
kinetic model. The course of the dependence of rate constant for NiSO4⋅6H2O thermal
decomposition on the temperature, concerning the IV stage of this reaction, at the ap-
plied heating rates, both in air and in helium atmosphere, is presented in Fig. 2.
Fig. 2 Dependence of reaction rate constant k on temperature for IV stage of
NiSO4⋅6H2O thermal decomposition (air and helium atmosphere)
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TOMASZEWICZ, KOTFICA: NICKEL(II) SULFATE(VI) HYDRATES
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TOMASZEWICZ, KOTFICA: NICKEL(II) SULFATE(VI) HYDRATES
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TOMASZEWICZ, KOTFICA: NICKEL(II) SULFATE(VI) HYDRATES
31
The results of calculations presented in Tables 2 and 3 indicate that both in air and
in helium atmosphere the I stage of decomposition is best described by the kinetic
model F2 (second-order reaction g(α)=[(1–α)–1–1]), the II stage of decomposition in
air atmosphere – the model F3 (third-order reaction g(α)=[(1–α)–2]), and in helium at-
mosphere – the model F2. On the other hand, the stages III and IV, regardless of the
kind of gas atmosphere, are best described by the models F2 and A2 (random nucle-
ation – I Avrami equation g(α)=[–ln(1–α)]1/2), respectively.
Conclusions
It has been established on the ground of DTA/TG and XRD investigations that the pro-
cess of thermal decomposition of nickel(II) sulfate(VI) hydrate both in air and in helium
atmosphere occurs in four endothermic stages. First three stages are associated with the
salt dehydration, while the last one – with decomposition of anhydrous NiSO4.
For each stage of decomposition and for each heating rate the dependence of con-
version degree on temperature has been determined. By applying the Coats–Redfern ap-
proximation the respective kinetic model expressed in terms of the g(α) function has
been established and the Arrhenius equation parameters A and E have been calculated.
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