Kinetics of thermal decomposition of copper(II) acetate monohydrate

Article abstract of DOI:10.1023/A:1010179532267

The thermal decomposition of copper(II) acetate monohydrate was studied in air and nitrogen atmospheres by means of DTA-TG and SEM measurements. The kinetics of the thermal decomposition steps in air was studied by using isothermal and non-isothermal thermogravimetric techniques. The results are discussed in terms of various reaction interface models and different techniques of computational analysis of non-isothermal data. The activation parameters, calculated by using a composite method of integral analysis of non-isothermal data, revealed not only their independence from the heating rate and fractional reaction, but also a better correlation and agreement with the results obtained under isothermal conditions.

Full text of DOI:10.1023/A:1010179532267

Journal of Thermal Analysis and Calorimetry, Vol. 61 (2000) 985–994
KINETICS OF THERMAL DECOMPOSITION OF
COPPER(II) ACETATE MONOHYDRATE
A. Y. Obaid, A. O. Alyoubi, A. A. Samarkandy, S. A. Al-Thabaiti,
S. S. Al-Juaid, A. A. El-Bellihi* and El- H. M. Deifallah
Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, P.O. Box 9028
Saudi Arabia
(Received January 20, 1999; in revised form September 5, 1999)
Abstract
The thermal decomposition of copper(II) acetate monohydrate was studied in air and nitrogen atmo-
spheres by means of DTA-TG and SEM measurements. The kinetics of the thermal decomposition
steps in air was studied by using isothermal and non-isothermal thermogravimetric techniques. The
results are discussed in terms of various reaction interface models and different techniques of com-
putational analysis of non-isothermal data. The activation parameters, calculated by using a compos-
ite method of integral analysis of non-isothermal data, revealed not only their independence from the
heating rate and fractional reaction, but also a better correlation and agreement with the results ob-
tained under isothermal conditions.
Keywords: copper acetate, decomposition, ESM, DTA, TG
Introduction
Several studies have been performed on the thermal decomposition behaviour of metal
acetates [1, 5], but less information is available on the kinetics of the reactions [4, 5]. On
decomposition, metal acetates often yield acetone and acetic acid, and there may be in-
complete melting and some hydrolysis of hydrated salts [1]. Leicester and Redman [2]
studied the thermal decompositions of nickel and cobalt acetates and found that they pro-
ceeded via an initial cleavage to metal oxide and acetic anhydride, and thence to various
pyrolysis products of the anhydride. Clough et al. [3] suggested that the initial step in the
thermal decomposition of uranyl acetate could be the fission of the UO2–O bond, produc-
ing acetate radicals which react to give various pyrolysis products.
In the thermal decomposition of Cu(II) carboxylates, Cu+ salts have been identified
as intermediates, and the reduction of Cu(II) proceeds in two steps (Cu2+→Cu+→Cu0),
with a possibly appreciable overlap of the consecutive rate processes [6].
*
To whom all correspondence should be addressed.
1418–2874/2000/ $ 5.00
Akadémiai Kiadó, Budapest
© 2000 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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OBAID et al.: COPPER(II) ACETATE MONOHYDRATE
All methods of kinetic analysis of the thermal decomposition of solids yield results
of a relative character, depending markedly on procedural variables [7], including the
sample mass, the heating rate, the particle size and the partial pressure of the ambient at-
mosphere. There are a variety of computational methods and the analysis of kinetic data
may not allow the choice of a single model to explain the results, which makes the estab-
lishment of a dependable correlation more difficult. However, applicable reaction models
identify the chemical change occurring at the reaction interfaces, which progressively ad-
vance into the unchanged reactant [6]. The importance of characterizing the chemistry of
the interface reactions and of elucidating the chemical steps through which the reactants
are converted into products has been emphasized.
In the present investigation, the thermal decomposition behaviour of copper(II)
acetate monohydrate has been studied by using DTA-TG and SEM techniques. The
kinetics of the isothermal and non-isothermal dehydration and decomposition of cop-
per(II) acetate monohydrate was studied in air. The composite method of kinetic inte-
gral analysis of non-isothermal data was applied to identify the mechanistic equation
describing the geometry of advance of the reaction interface and to calculate the acti-
vation parameters. The results were compared with those obtained by using the iso-
thermal technique and other integral methods of analysis of non-isothermal data.
Experimental
The copper(II) acetate monohydrate crystalline powder, a BDH product of reagent
grade, was used without further purification.
Simultaneous DTA-TG experiments were performed with a Shimadzu DT 30
thermal analyser. The experiments were performed under isothermal conditions or at
different specified heating rates in air at a flow rate of 3.0 l h–1. The sample mass in
the Pt cell of the thermal analyser was kept at about 8 mg in all experiments, in order
to ensure a linear heating rate and accurate temperature measurements.
Samples of copper acetate monohydrate were inserted into an electric oven at room
temperature in air and the temperature was raised to the desired value, which was main-
tained for 30 min before the sample was removed and cooled to room temperature in air
in a desiccator. The changes taking place in morphology and texture during the thermal
decomposition of the salt were investigated by using a JEOL T 300 scanning electron mi-
croscope. Samples of the parent compound and the products formed in the different
stages during the decomposition were examined.
Kinetic consideration
In general, the rate constant, k of a solid-state reaction is given by the formal kinetic
equation:
dα
=kf (α )=Ae–E/RT f (α )
(1)
dt
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OBAID et al.: COPPER(II) ACETATE MONOHYDRATE
987
where α is the fractional reaction; t, is time; A is the pre-exponential factor; E is the
activation energy; R is the gas constant; T is temperature in Kelvin, and f(α) is the ki-
netic function, which takes different forms depending on the particular reaction rate
equation [7–9]. In isothermal kinetic studies, the rate equation used to calculate the
rate constant has the form
g(α)=kt
(2)
α
dα
where g(α )=
is the integrated form of f(α) [7–9]. However, non-isothermal
∫
f (α )
0
methods are becoming more widely used because they are more convenient than the
classical isothermal methods [10].
In non-isothermal kinetics, the time dependence on the left side of Eq. (1) is
eliminated by using a constant heating rate β=dT/dt, so that T=T0+β, where T0 is the
starting temperature and t is the time of heating. With the use of integral methods of
analysis, Eq. (1) may be written as
A T
∫
β T
0
g(α )= e–E/RTdt
(3)
(4)
(5)
The reaction rate is negligible at low temperatures. Hence:
AT
g(α )= e–E/RTdt
∫
β
0
Integration of this equation leads to Doyle’s equation [11]:
∞
AE e–x e–u
AE
g(α )=
−
du =
P(x)
∫
Rβ
x
u
Rβ
0
where u=E/RT and x is the corresponding value of u at which a fraction of material
has decomposed. This equation has been reformulated [12] as
AE
lng(α )−lnP(x)=ln
=B
(6)
Rβ
where B is a constant for a particular reaction at a constant heating rate β. The integral
function P(x) is not definite; it may be written in an expanded form and estimated by
using a procedure of trial-and-error type involving iteration [13].
In the present study, analysis of non-isothermal data was performed by using the
approximate computational approaches due to Coats and Redfern [14], Ozawa [15],
Mahusudanan [16] and Diefallah [17]. Integral methods are often more reliable and
generally preferred than differential methods of kinetic analysis.
In the Coats-Redfern method [14], the function g(α) is approximated to the form
ART 2 2RT
g(α )=
1−
e–E/RT
(7)
βE
E
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988
OBAID et al.: COPPER(II) ACETATE MONOHYDRATE
The equation has been written in the form:
g(α )
T 2
AR
2RT
E
−ln
=−ln
1−
+
(8)
βE
E
RT
The quantity ln[(AR)/(βE)](1–2RT/E) is reasonably constant for most values of
E and in the temperature range over which most reactions occur. However, both E
and A could vary with the experimental heating rate.
In the Ozawa method [15], a master curve has been derived from the TG data ob-
tained at different heating rates (β) by using Doyle’s equation and assuming that
[AE/βR]p(E/RT) is a constant for a given function of material decomposed. The func-
tion p(E/RT) was approximated by the equation:
E
E
log p
=−2.315−0.4567
(9)
RT
RT
so that
E
−logβ=0.4567
+constant
(10)
RT
Hence, the activation energy is calculated from the thermogravimetric data ob-
tained at different heating rates. The frequency factor is calculated from the equation
E
E
logA=logg(α )−log
p
(11)
βR RT
Although the calculated activation energy is independent of the reaction model,
and the frequency factor depends on the determined form of g(α), both E and A could
vary considerably with the fractional reaction [7].
In the Madhusudanan method [16], the equation used has the form
g(α )
T 1.921503
AR
E
−ln
=−ln
+3.7720501−1.921503lnE−
(12)
βE
RT
where the symbols have their usual significance. The activation energy E and the fre-
quency factor A are obtained from the slope and intercept of the linear fit of the plot of
–ln[g(α)/T1.921503] vs. 1/T.
In the composite method of analysis of dynamic data [7], the results obtained
(not only at different heating rates, but also at different α values) are superimposed on
one master curve. This was achieved by rewriting the approximate integral equations
due to different workers in a form such that the kinetic function g(α) and the linear
heating rate β lie on one side of the equation and 1/T on the other side. When use is
made of the modified Coats-Redfern equation [14], then in order to allow the com-
posite analysis the equation is written in form
βg(α )
T 2
AR
E
E
ln
=ln
−
(13)
RT
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OBAID et al.: COPPER(II) ACETATE MONOHYDRATE
989
Hence, the dependence of ln[βg(α)/T2] calculated for different α values at their re-
spective β values on 1/T must give rise to a single master straight line for the correct form
of g(α ), and a single activation energy and frequency factor can readily be calculated.
When Doyle’s approximate equation [11] is used, the equation for composite
analysis has the form
AE
R
E
log[βg(α )]= log
−2315 −0.4567
(14)
RT
Use may also be made of the approximate equation [16] of Madhusudanan, re-
written as
βg(α )
T 1.921503
AR
E
E
−ln
=−ln
+3.7720501−1.9211503lnE−
(15)
RT
Again, the dependence of the left side of Eqs (14) and (15) on 1/T should give
rise to a single master straight line for the correct form of g(α), and hence the activa-
tion energy and the frequency factor can readily be calculated.
In general, the use of the different approximate integral equations for the kinetic
analysis of non-isothermal decomposition kinetic data according to the composite
method of analysis gave rise, within experimental error, to identical values of the ac-
tivation parameters and the correct form of g(α). A computer program has been writ-
ten to perform the data analysis [17].
Fig. 1 DTA-TG curves for copper(II) acetate monohydrate in A – air and B – nitrogen
at a heating rate of 15°C min–1
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OBAID et al.: COPPER(II) ACETATE MONOHYDRATE
Results and discussion
Figure 1 shows the DTA-TG curves obtained for Cu(CH3COO)2 H2O in air (A) and nitro-
gen (B), respectively. The TG curve contained two steps due to dehydration and decom-
position of the anhydrous salt, in general agreement with previous studies [4]. The first
step was endothermic and took place in the temperature range 110–150°C, involving the
loss of one molecule of water with a mass loss of about 10%. The second step started at
about 200°C and was complete by about 300°C; it was endothermic in nitrogen, and exo-
thermic in air, due to oxidation. It corresponded to a mass loss of about 58%, due to de-
composition of the anhydrous salt to copper metal. In air, above 300°C, a slight mass in-
crease was observed in the TG curve and a corresponding broad exothermic peak in the
DTA curve, due to the partial oxidation of Cu to CuO.
SEM micrographs indicating the changes in morphology and texture that ac-
company the salt decomposition are shown in Fig. 2. Each sample was calcined for
Fig. 2 Scanning electron micrographs showing the changes in texture and morphology
that accompany the thermal decomposition of copper(II) acetate monohydrate in
air: a – parent compound at room temperature, displaying relatively large crys-
tals of different sizes; b – parent compound after partial dehydration at 120°C;
c – sample calcined at 200°C after complete dehydration of the parent com-
pound to the anhydrous salt; d – sample calcined at 300°C, showing partial de-
composition and fissioning of the anhydrous salt; e – sample calcined at 500°C,
showing complete decomposition of the anhydrous salt into small crystallites
with a cubic crystal habit and some aggregation of particles; f – sample calcined
at 800°C, showing coalescence of the small crystallites into large aggregates ac-
companying the oxidation of Cu to CuO; (scale bar: a, b and c – 100 µm; d, e
and f – 10 µm)
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OBAID et al.: COPPER(II) ACETATE MONOHYDRATE
991
about 30 min at the specified temperature. The results reveal that the particle shape
and size change throughout the decomposition. The regular shape of the parent reac-
tant crystallites (Fig. 2(a)), underwent some superficial roughening, after partial de-
hydration at 120°C, (Fig. 2(b)), whereas complete dehydration at 200°C resulted in
more roughening and cracking of the crystallite surface (Fig. 2(c)). At 300°C, the an-
Table 1 Activation parameters of the thermal dehydration and decomposition of Cu(CH3COO)2
H2O, calculated according to the R2 model
Dehydration step
lgA/min–1
Decomposition step
E/kJ mol–1
Method of analysis
E/kJ mol–1
lgA/min–1
Isothermal
107.8±4.3
13.1±0.5
71.2±3.7
6.0±0.3
Non-isothermal
β
2
180.3
129.3
23.0
16.0
92.7
95.0
8.2
8.4
5
10
a) Coats-Redfern
118.5
14.6
95.9
8.6
15
118.8
13.7
106.0
9.7
20
87.8
10.7
117.7
10.3
9.0±0.8
Average
β
125.5±23.4
15.6±3.1
100.3±6.9
2
180.6
134.8
23.0
16.8
93.1
95.3
8.2
8.5
5
b) Madhusudanan
10
118.8
14.6
96.2
8.6
15
112.0
13.7
106.3
9.7
20
88.0
10.6
112.0
10.3
9.1±0.8
Average
1–α
0.1
126.8±24.7
15.7±3.3
100.6±6.9
182.7
145.8
23.8
18.5
92.3
93.7
8.2
8.3
0.2
0.3
129.7
16.2
98.7
8.9
0.4
123.2
15.3
107.9
9.9
c) Ozawa
0.5
115.7
14.3
117.2
10.9
0.6
112.0
13.7
124.5
11.7
0.7
105.7
12.9
129.6
12.2
0.8
99.2
12.0
142.0
13.5
0.9
88.9
10.6
148.0
14.2
Average
122.5±20.2
117.7±4.2
15.3±2.8
14.5±1.3
117.1±16.8
100.2±1.9
10.9±1.8
9.1±0.5
d) Composite
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OBAID et al.: COPPER(II) ACETATE MONOHYDRATE
hydrous salt, after partial decomposition, had undergone extensive degradation with a
reduction in particle size from more than about 100 µm to less than about 4 µm. At
500°C, after complete decomposition of the anhydrous salt to Cu° metal, the particle
size was further reduced to less than about 2 µm, with a cubic crystal habit. At 800°C,
the small crystallites coalesced into aggregates of irregular shape and size, accompa-
nying the oxidation of copper to Cu(II) oxide.
Figure 3 shows plots of isothermal fractional reaction vs. time curves, whereas
Fig. 4 depicts the fractional reaction remaining as a function of temperature, for the
dehydration and decomposition steps. The curves in Fig. 3 exhibited a deceleratory
Fig. 3 Isothermal α/t curves for the decomposition of copper(II) acetate monohydrate
in air: a – dehydration step; b – decomposition step
Fig. 4 Dynamic measurements for the thermal decomposition of copper(II) acetate
monohydrate in air: a – dehydration step; b – decomposition step. Heating rate:
curve A – 2°C min–1; curve B – 5°C min–1; curve C – 10°C min–1;
curve D – 15°C min–1 and curve E – 20°C min–1
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OBAID et al.: COPPER(II) ACETATE MONOHYDRATE
993
Fig. 5 Composite analysis of the dynamic TG data based on the modified
Coats-Redfern equation (A) and Doyle’s equation (B), assuming the contracting
surface (R2) model. 1 – dehydration step; 2 – decomposition step
shape, and kinetic analyses of the two steps were performed with reference to the dif-
ferent models of heterogeneous solid-state reactions [7–9]. The method of least
square regression analysis was performed for each of the tested functions, and it was
found that the two-dimensional phase boundary and the first-order reaction models
gave the best fits for both the dehydration and the decomposition steps. Analyses of
the non-isothermal data were performed according to the integral methods of
Coats-Redfern, Ozawa, Madhusudanan and Diefallah, and the activation parameters
were calculated. They are listed in Table 1, in comparison with those obtained from
the analysis of the isothermal data.
Figure 5 shows typical composite plots for the dehydration and decomposition
steps according to the contracting surface (R2) equation, which gives better regression
factors and standard deviations than those for the other models.
The results in Table 1 demonstrate that the integral composite analysis of the dy-
namic data gave better agreement with the isothermal results, with a higher correla-
tion and less deviation in the calculated experimental parameters. The activation en-
ergy for the dehydration step is in agreement with that reported by Mu and Perlmutter
[5] and indicates that the water of hydration is co-ordinated water. The activation en-
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994
OBAID et al.: COPPER(II) ACETATE MONOHYDRATE
ergy for the decomposition of the anhydrous salt is lower than the values of many
thermal decomposition reactions, which may indicate the catalytic action of the
fine-grained copper metal particles formed in the early stages of the decomposition
step. This supports the view that charge transfer is the rate-controlling step in the
mechanism of carboxylate decompositions [1].
* * *
The authors thank Mr. M. Abdel Fattah (Chem. Dept., KAU) for the DTA-TG analysis, and Mr. M.
Sief (Production Eng. Dept., KAU) for performing the SEM experiments.
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