Kinetics of the thermal decomposition of CuC2O4-ZnC2O4 mixture in air

Article abstract of DOI:10.1016/S0040-6031(02)00616-0

The thermal decomposition processes taking place in the solid state oxalate mixture of CuC₂O₄-ZnC₂O₄· 2H₂O (1:1 mole ratios) have been studied in air using DTA-TG, XRD and SEM techniques. DTA-TG curve

Full text of DOI:10.1016/S0040-6031(02)00616-0

Thermochimica Acta 402 (2003) 199–208
Kinetics of the thermal decomposition of
CuC2O4–ZnC2O4 mixture in air
M.A. Gabal
Chemistry Department, Faculty of Science, Benha University, Benha, Egypt
Received 15 August 2002; received in revised form 3 December 2002; accepted 4 December 2002
Abstract
The thermal decomposition processes taking place in the solid state oxalate mixture of CuC2O4–ZnC2O4·2H2O (1:1 mole
ratios) have been studied in air using DTA–TG, XRD and SEM techniques. DTA–TG curves showed that, the decom-
position proceeds through three well-defined steps with DTA peaks closely correspond to the weight loss obtained. The
decomposition behaviour of the oxalate mixture is in good agreement with that of the individual oxalates. XRD showed
an increase in the intensity of the diffraction lines on rising the calcinations temperature from 400 to 800 ◦C attributed
to the grain growth of the decomposition products. SEM experiments confirmed this result as the very small crystallites
obtained at 400 ◦C are re-textured and coalesced into aggregates of large crystals with sharp edges and angles when cal-
cined at 800 ◦C. Kinetic analysis of the oxalate decomposition steps was performed under non-isothermal conditions us-
ing different integral methods of analysis. The dynamic TG curves obeyed the Avrami–Erofeev equation characteristic of
solid-state nucleation-growth mechanism and consistent with the textural changes accompanying the decomposition revealed
by SEM experiments. The activation parameters were calculated and discussed in view of the electronegativity of the metal
ions.
© 2003 Elsevier Science B.V. All rights reserved.
Keywords: Copper oxalate; Zinc oxalate; DTA–TG; SEM; Kinetic
1. Introduction
gas sensor with layered structure of CuO/ZnO was in-
troduced to enhance the sensitivity and selectivity to
CO gas [2]. CuO supported on ZnO prepared by the
impregnation of basic zinc carbonate with a water so-
lution of copper nitrate followed by heat treatment at
different temperatures was found to possess surface
and catalytic properties depend upon CuO content and
the calcination temperature [3].
The thermal decomposition of copper oxalate dif-
fers substantially from that of the other transition metal
oxalates since it exhibits a strongly exothermic reac-
tion with the formation of copper metal when heated in
nitrogen and this reaction is described as autocatalytic.
In oxygen, the overall reaction is strongly exothermic,
Investigation of chemical, physical and structural
properties of mixed metal oxides have found very im-
portant applications in new technologies. They can be
used as prospective materials for superior technical ce-
ramics, materials for magnetic recording, many elec-
tronic components, sensors, catalysts as well as the
high-temperature superconductors.
CuO/ZnO catalyst was used for the catalytic gener-
ation of hydrogen from the reaction of methanol with
oxygen in the presence of steam [1]. Heterocontact
E-mail address: mgabalabdo@yahoo.com (M.A. Gabal).
0040-6031/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0040-6031(02)00616-0

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M.A. Gabal / Thermochimica Acta 402 (2003) 199–208
owing to oxidation of the evolved CO to CO2 beside
the oxidation of metal to metal oxide [4].
Dollimore and coworkers [4,5] studied the thermal
decomposition of ZnC2O4·2H2O in air and nitrogen
and found that the end product in both atmospheres is
ZnO.
2. Experimental
2.1. Materials
Individual metal oxalates were prepared by pre-
cipitation from aqueous solutions containing the
calculated amounts of AR metal chloride salt with
equimolar quantity of AR oxalic acid. The fine pre-
cipitate obtained were filtered, washed with distilled
water until free of chloride ions and dried.
Physical mixture of CuC2O4–ZnC2O4·2H2O (1:1
mole ratios) was prepared using the impregnation tech-
nique [11]. Bidistilled water was added in drops to
the appropriate mole ratios of the two metal oxalates
with vigorous stirring to assure complete homogene-
ity. The wetted oxalates mixture was then dried in a
thermostatic oven at 100 ◦C for about 2 h.
Many researchers have paid their attention to the
different synthesis methods of the mixed metal oxides
and their characterization. Mixed cerium–gadolinium
oxalates [6] have been used as precursors for the prepa-
ration of important materials exhibiting high ionic con-
ductivity. The thermal behaviour of the coprecipitated
mixed metal oxalates [7]; MCu(C2O4)2·xH2O where
M: Fe, Co or Ni examined in air and nitrogen atmo-
spheres showed that, their thermal behavior is differed
from that of the individual metal oxalates.
Thermal techniques have been used for studying
the kinetics of the thermal decomposition reactions
of solids. In analyzing the kinetic data it is true that,
the conventional isothermal method is important for
estimating the kinetic model but the dynamic method
has some advantages over it in several respects
[8].
The kinetics of copper oxalate [9] decomposition
reaction showed that the decomposition obeyed the
Avrami–Erofeev (A2) mechanism when studied by
isothermal and rising temperature experiments in both
nitrogen and air atmospheres.
Yankwich and Zavitsanos [10] have been reported
the activation energies for zinc oxalate decomposition
reaction, based on the observed pressure of the gaseous
products during the decomposition, assuming different
reaction interface models.
2.2. Experimental apparatus
Simultaneous DTA–TG experiments were per-
formed using a Shimadzu DT-40 thermal analyzer.
Experiments were carried out in air at flow rate of
30 l h−1 against ␣-alumina as a reference, at heating
rates of 1, 2, 3 and 5 ◦C min−1. The sample mass in
the Pt cell of the thermal analyzer was kept at about
8 mg in all experiments, in order to ensure a linear
heating rate and accurate temperature measurements.
In accordance with DTA–TG results, samples of the
oxalates mixture were thermally heated in an electrical
oven at different temperatures for specified durations
then removed and cooled in a desiccators to room
temperature.
XRD patterns for the different calcined samples
were recorded at room temperature using a Philips PW
1710 X-ray diffraction unit using a Cu target and Ni
filter.
The changes in morphology and texture taking place
during the thermal decomposition of the mixture were
investigated using a Jeol T 300 scanning electron mi-
croscope.
In the present study, the thermal decomposition of
physical oxalate mixture of CuC2O4–ZnC2O4·2H2O
(1:1 mole ratios) in air has been studied using
DTA–TG techniques. The parent mixtures and mix-
tures calcined at different temperatures were char-
acterized using XRD and SEM techniques to gave
some information about the decomposition products.
Kinetic analysis of TG data were performed and
considered with reference to the various models and
computational methods of solid state reactions. The
results of various techniques used to examine the
chemical phases, morphology and texture changes
that occur during the thermal decomposition and the
kinetics of the decomposition reactions are compared
and discussed.
3. Results and discussion
3.1. DTA–TG
Fig. 1 shows the DTA–TG curves in air at heat-
ing rate of 5 ◦C min−1 for CuC2O4–ZnC2O4·2H2O

M.A. Gabal / Thermochimica Acta 402 (2003) 199–208
201
Fig. 1. DTA–TG curves of CuC2O4–ZnC2O4·2H2O (1:1 mole ratios) mixture in air at heating rate of 5 ◦C min−1
.
mixture (1:1 mole ratios). The DTA peaks closely cor-
respond to the weight changes observed on the TG
curves. The DTA–TG curves show that, the thermal
decomposition of the mixture at temperatures below
400 ◦C occurs in three well-defined steps. The first
step starts at about 112 ◦C, characterized by a broad
endothermic DTA peak at 132 ◦C and is accompa-
nied by a weight loss of 11% at 170 ◦C in accordance
with calculated weight loss of 10.6% attributed for the
dehydration of ZnC2O4·2H2O and the formation of
anhydrous oxalate mixture. The mixture is stable up
to about 260 ◦C, and is then decomposed in the sec-
ond step. This step shows an exothermic process with
broad DTA peak at 300 ◦C, indicating a weight loss of
22% in accordance with calculated weight loss 21.1%
due to the decomposition of CuC2O4 and the forma-
tion of CuO–ZnC2O4 mixture. The third decomposi-
tion step is also exothermic with DTA peak at about
360 ◦C, accompanied by a weight loss of 21.5% com-
pared with theoretical weight loss of 21.1% attributed
to the decomposition of ZnC2O4 in the mixture and
the formation of CuO–ZnO mixture at 390 ◦C.
copper oxalate with iron(II) oxalate, cobalt oxalate or
nickel oxalate indicate that the dehydration, the de-
composition in nitrogen and the reactions in oxygen
are the same as those of the individual oxalates. The
obtained dehydration temperature of ZnC2O4·2H2O
suggests that the water molecules are present as crys-
talline water [12]. The exothermic character of the
DTA peaks accompanying the oxalate decomposition
steps is due to the air oxidation of CO to CO2. The
enthalpy change associated with this reaction has to
be higher than the endothermic heat required to drive
off CO [5].
The obtained data also indicates the temperatures
equivalent to the theoretical weight loss which can be
selected as the minimum temperatures for the calci-
nation processes. Thus, samples of CuC2O4–ZnC2O4·
2H2O mixture were calcined at 320 ◦C for 30 min, at
400 ◦C for 1 h or at 800 ◦C for 2 h.
XRD and SEM experiments were carried out for the
parent mixture and mixtures calcined at different tem-
peratures to characterize the decomposition products
at different stages.
From the above results it is clear that the DTA–TG
behavior of the metal oxalates in their physical mix-
ture are in general agreement with those reported for
the individual metal oxalates [4,5,7]. In other words,
the thermal decomposition behaviors of the metal ox-
alates are not much affected by their presence in the
mixture and each one behaves as it is present alone. A
similar behavior was found by Coetzee et al. [7] where
TG and DSC results on ground physical mixtures of
3.2. X-ray diffraction
The starting mixture gave XRD pattern in general
agreement with the results reported in the ASTM
data cards for CuC2O4 and ZnC2O4·2H2O. In accor-
dance with DTA–TG results, the mixture calcined at
320 ◦C shows the XRD lines characteristic of CuO
and the disappearance of any XRD lines characteristic

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M.A. Gabal / Thermochimica Acta 402 (2003) 199–208
features. Picture quality was often poor due to charge
retention arising from the low electrical conductivity
of the porous aggregates that constituted the particle
assemblages. Mohamed and Galwey [14] showed a
similar behavior for copper(II) oxalate on the studying
kinetic and mechanistic studies of its thermal decom-
position.
The SEM micrographs showing the changes in
morphology and texture that accompany the thermal
decomposition of CuC2O4–ZnC2O4·2H2O mixture
in air are shown in Fig. 3. The results show that the
particle shape and size change throughout the decom-
position process. Micrograph of the mixture calcined
at 320 ◦C (Fig. 3a) shows two types of crystals. The
first type was attributed to the decomposition of cop-
per oxalate and breaking into small and fine granules
with roughening of the crystal surfaces. The other
type showing relatively large crystals of different size
and shape, assigned to cadmium oxalate. Micrograph
of the mixture calcined at 400 ◦C (Fig. 3b) shows
the complete breaking into very small and fine gran-
ules of the crystal surface due to the decomposition
of cadmium oxalate crystals in the mixture. Micro-
graph of the mixture calcined at 800 ◦C (Fig. 3c)
shows a re-texturing and coalescence into aggregates
of hexagonal large crystals of different sizes with
sharp edges and angles. The results of SEM exper-
iments are thus consistent with the results of XRD
analysis.
Fig. 2. Characteristic parts of XRD patterns of CuC2O4–ZnC2O4·
2H2O (1:1 mole ratios) mixture calcined at 400 ◦C (a) and 800 ◦C
(b). Phases: (᭝) CuO and (᭺) ZnO.
of CuC2O4. Fig. 2 shows the characteristic parts of
the X-ray powder diffraction patterns for the mixture
calcined at 400 and 800 ◦C. The XRD pattern of the
mixture calcined at 400 ◦C (Fig. 2a) gave the char-
acteristic lines of metal oxide mixture, CuO–ZnO.
The small broadening of the diffraction lines ob-
tained indicate the poorly crystalline structure at this
decomposition stage. For the mixture calcined at
800 ◦C (Fig. 2b), the increase in the intensities of
XRD lines, without changing their position, which
was accompanied by a change in the color of the
mixtures compared with those obtained at 400 ◦C can
be attributed to the grain growth and the formation of
well-crystallized oxide mixture. A similar behavior
was found by Osvaldo et al. [13] for (ZnO)x(CdO)1−x
thin films prepared with different compositions by
spray pyrolysis on glass substrates and exposed to
different annealing treatment, where the crystallite
sizes were found to increase with annealing.
3.4. Kinetic studies
The kinetics of the oxalate decomposition steps (viz.
the last two decomposition steps) have been studied
under dynamic conditions using heating rates of 1, 2,
3 and 5 ◦C min−1. Kinetic analysis of the dynamic TG
curves were carried out using three integral methods:
the Ozawa method [15], the Coats–Redfern method
[16] and Diefallah’s composite method [12] which is
based either on the modified Coats–Redfern equation
[16]; (composite method I) or on the Doyle’s equation
[17]; (composite method II).
With dynamic techniques, the temperature rate is
set to a constant value β and the function g(α) is given
by Doyle’s equation [17]:
3.3. Scanning electron microscope
SEM of the parent mixture are not produced here
because the crystallite of the reactants were too small
(<0.5 ␮m) to permit satisfactory resolution of textural
ꢁ
ꢂ
ꢀ
∞
A
β
E
AE
g(α) =
exp −
dT =
P(χ)
(1)
RT
βR
0

M.A. Gabal / Thermochimica Acta 402 (2003) 199–208
203
In the Coats–Redfern method [16], the function g(α)
is approximated to the form:
ꢃ
ꢄ
ART2
2RT
g(α) =
1 −
e−E/RT
(3)
βE
E
This equation has been written in the form:
ꢃ
ꢄ
ꢃ
ꢄ
g(α)
T 2
AR
2RT
E
−ln
= −ln
1 −
+
(4)
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.
In the Ozawa method [15], a master curve has been
derived from the TG data obtained at different heating
rates (β) using Doyle’s equation [17] and assuming
that [(AE/Rβ)P(E/RT)] is a constant for a given frac-
tion of material decomposed. The function P(E/RT)
was approximated by the equation:
ꢁ
ꢂ
ꢁ
ꢂ
E
E
log P
= −2.315 − 0.4567
(5)
RT
RT
So that:
−log β = 0.4567
ꢁ
ꢂ
E
+ constant
(6)
RT
Hence, the activation energy is calculated from the
thermogravimetric data obtained at different heating
rates. The frequency factor is calculated from the equa-
tion:
ꢃ
ꢁ
ꢂꢄ
E
E
log A = log g(α)
P
(7)
βR
RT
Thus, it is obvious that the calculation of E is inde-
pendent of the reaction model used to describe the re-
action, whereas the frequency factor depends on the
determined form of g(α).
Fig. 3. Scanning electron micrograph showing the changes in tex-
ture and morphology that accompany the thermal decomposition
of CuC2O4–ZnC2O4·2H2O (1:1 mole ratios) mixture in air. Mix-
ture calcined at: (a) 320 ◦C; (b) 400 ◦C and (c) 800 ◦C (scale bar
10 ␮m).
In the composite method of dynamic data analy-
sis, the results obtained not only at different heating
rates but also with different fractional reaction values
are superimposed on one master curve [11,12,18–21].
The method employs multiple sets of non-isothermal
data and uses all (α, T, βi) values obtained at the dif-
ferent heating rates, βi. The use of single-heating rate
methods should be very limited and the application of
multi-set methods is expected to enrich kinetics with a
deeper insight into the multi-step nature of solid-state
reactions [22–24].
The function P(χ) has been defined as follows:
ꢀ
e−χ
∞ e−u
P (χ) =
−
du
(2)
χ
u
0
where u = E/RT and χ is the corresponding value of
u at which a fraction α of material has decomposed.

204
M.A. Gabal / Thermochimica Acta 402 (2003) 199–208
Fig. 4. Dynamic measurements for the thermal decomposition of CuC2O4–ZnC2O4·2H2O (1:1 mole ratios) mixture in air. Decomposition
of: (a) CuC2O4 to CuO and (b) ZnC2O4 to ZnO. Heating rate: curve A, 1 ◦C min−1; curve B, 2 ◦C min−1; curve C, 3 ◦C min−1 and curve
D, 5 ◦C min−1
.
The integral form of the equation showing the tem-
perature dependence of the kinetic model function
g(α) is rewritten in a form that provides a plot of all
non-isothermal data on a single master straight line for
the correct reaction model. For example, in the com-
posite method I, the modified Coats–Redfern equation
[16] was rewritten in the form:
values obtained at the different heating rate experi-
ments. Deviation from a straight line relationship are
interpreted in terms of multi-step reaction mechanism
[12,15,26].
Fig. 4 shows representative weight changes as a
function of temperature obtained from the dynamic
measurements of the oxalate decomposition steps.
A computer program [19] was used for the kinetic
analysis of non-isothermal data according to the two
composite methods using the different models of het-
erogeneous solid state reactions [12] listed in Table 1.
The data can be entered manually or from a data
file and the results can be printed out, plotted and
saved. The results of calculation allow us to choose
the kinetic mechanism which best fits the data and
gives the highest correlation coefficient and the low-
est standard deviation. The program also calculates
the activation energy E and the frequency factor A
from the slope and intercept of the linear fit line. The
results always show that both the composite methods
of analysis gave equivalent curves and nearly iden-
tical values for the activation parameters. The values
of the activation parameters of the non-isothermal
decomposition of CuC2O4 and ZnC2O4 in their mix-
ture, calculated according to the composite method
II, assuming different kinetic models are reported in
Table 2. From the table, it is clear that the kinetics of
the reaction are best described by the Avrami–Erofeev
random nucleation model (A2), in which the reac-
tion is controlled by initial random nucleation fol-
lowed by overlapping growth in two dimensions. The
ꢃ
ꢄ
ꢁ
ꢂ
βg(α)
T 2
AR
E
ln
= ln
−
(8)
E
RT
Hence, the dependence of ln[βg(α)/T2] calculated for
different α values at their respective 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 be readily calculated. In a similar
way, in the composite method II, Doyle’s equation
[17] has been rewritten in the form:
ꢃ
ꢄ
AE
E
log g(α)β = log
− 2.315 − 0.4567
(9)
R
RT
Again, the dependence of log g(α)β, calculated for
the different α values at their respective β values on
1/T must give rise to a single master straight line for
the correct form of g(α). Vyazovkin and Wight [25]
pointed out that the use of the model-free approach is
a trustworthy way of obtaining a reliable result from
both non-isothermal and isothermal data. Although,
the composite method involves a model-fitting kinetic
approach, however it does not assume a particular re-
action model, but it allow us to choose the model func-
tion that gives the best representation of all (α, T, βi)

M.A. Gabal / Thermochimica Acta 402 (2003) 199–208
205
Table 1
Kinetic equations examined in this work
Reaction model
g(α)
Symbol
One-dimensional diffusion
Two-dimensional diffusion
α2
D1
D2
D3
D4
R2
R3
F1
A2
A3
E1
α + (1 − α) ln(1 − α)
2
Jander equation, three-dimensional diffusion
Ginsling–Braunshtein equation, three-dimensional diffusion
Two-dimensional phase boundary reaction
Three-dimensional phase boundary reaction
First order kinetic, Mampel unimolecular law
Random nucleation: Avrami equation
Random nucleation: Erofeev equation
Exponential law
[1 − (1 − α)1/3
]
(1 − 2/3α) − (1 − α)2/3
[1 − (1 − α)1/2
[1 − (1 − α)1/3
[−ln(1 − α)]
[−ln(1 − α)]1/2
[−ln(1 − α)]1/3
ln α
]
]
fine-grained decomposition products formed in the
first oxalate decomposition step catalyze the reaction
as it proceeds. This result is in consistent with the
formation of hexagonal crystals (as revealed by SEM
experiments) and their subsequent growth in C-axis
direction. The exponential low and other heteroge-
neous reaction kinetic models gave a less satisfactory
fit to the experimental results. Fig. 5 shows typical
composite plots of the non-isothermal data based on
Doyle’s equation according to (A2) and (E1) models.
From the figure it is obvious that single master straight
line for all (α, T, Bi) values was obtained at different
heating rate experiments. Fig. 6 shows a plot of the
activation energies estimated by the Ozawa method
for the oxalate decomposition steps versus α with
0.1α step. The figure shows approximately constant
E with α. So, from the above results it is likely that
the rate-limiting step is a single reaction step [11].
Analysis of the dynamic data was also carried out
using Coats–Redfern and Ozawa methods, assuming
A2 model, and the results were compared with those
obtained using the composite method of analysis.
Table 3 shows the results of the activation parameters
of the non-isothermal study of the oxalate decom-
position steps calculated according to the A2 model
using different computational methods. The values
of the activation parameters listed in the case of the
Coats–Redfern and the Ozawa methods are the aver-
age ones for the data calculated at different heating
rates (β) and fractional reaction (α) values, respec-
tively. The integral composite analysis showed the less
standard deviation in the calculated experimental pa-
rameters. In general, there is a good agreement within
experimental errors between the activation parameters
calculated according to the composite method and
those according to Coats–Redfern method however,
Table 2
Activation parameters of the non-isothermal decomposition in air of CuC2O4 and ZnC2O4 in their mixture, calculated according to the
composite method II, assuming different kinetic models
Model
Decomposition of CuC2O4 (step II)
E (kJ mol−1 log A (min−1
Decomposition of ZnC2O4 (step III)
E (kJ mol−1 log A (min−1
)
)
r∗
)
)
r∗
D1
D2
D3
D4
R2
R3
F1
A2
A3
E1
348
369
397
379
208
215
229
131
98
20
22
25
23
7
8
10
4
31.4
33.2
33.2
33.4
18.2
18.7
20.7
11.4
8.2
4.3
4.8
5.4
5.0
1.6
1.7
2.1
0.8
1.1
5.8
0.896
0.860
0.848
0.856
0.946
0.939
0.923
0.983
0.883
0.406
491
525
568
539
294
304
328
184
137
193
27
29
33
31
9
9
11
3
14.6
14.6
14.6
14.6
23.3
24.0
26.6
14.5
10.4
17.2
5.2
5.7
6.4
5.9
1.7
1.8
2.2
0.7
1.3
7.0
0.892
0.889
0.884
0.888
0.965
0.962
0.953
0.986
0.913
0.507
5
27
7
36
118
12.0

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M.A. Gabal / Thermochimica Acta 402 (2003) 199–208
Fig. 5. Composite analysis of TG data for the non-isothermal decomposition in air of (a) CuC2O4 and (b) ZnC2O4 in their mixture based
on Doyle’s equation assuming: (1) A2 model and (2) E1 model.
Fig. 6. Plots of the activation energies vs. α calculated by Ozawa method for the thermal decomposition of CuC2O4 and ZnC2O4 in their
mixture.

M.A. Gabal / Thermochimica Acta 402 (2003) 199–208
207
Table 3
Activation parameters of the non-isothermal decomposition in air of CuC2O4 and ZnC2O4 in their mixture calculated according to A2 model
Method of analysis
Decomposition of CuC2O4 (step II)
E ( kJ mol−1 log A (min−1
Decomposition of ZnC2O4 (step III)
E ( kJ mol−1 log A (min−1
)
)
)
)
Composite II
Coats–Redfern
Ozawa
131
126
162
4
7
4
11.4
10.9
14.0
0.8
0.6
1.4
184
178
216
3
9
6
14.5
13.9
17.2
0.7
0.8
0.9
the values calculated by the Ozawa method are higher
than that calculated by the other two methods which is
possibly due to the neglect in the Ozawa method of the
reaction mechanism in calculating energies of activa-
tion. In many cases, the activation energies estimated
by the composite method are, within experimental er-
rors, in good agreement with the values estimated us-
ing the model-free method of Ozawa [19–21,27].
Mohamed and Galwey [14] have provided evidence
that the decomposition of copper oxalate proceeds via
a Cu(I) intermediate with some overlap of stages. The
activation energy of the first stage is 140 7 and
180 7 kJ mol−1 for the second. Dollimore et al. [28]
have been investigated the kinetics and thermal stabil-
ity of copper oxalate by isothermal and rising tempera-
ture experiments in air. The decomposition was found
to obey the Avrami–Erofeev (A2) mechanism with ac-
tivation energy of 182 10 kJ mol−1 in the α range
from 0.1 to 0.9. Yankwich and Zavitsanos [10] have re-
ported an activation energy of about 202 13 kJ mol−1
for the thermal decomposition of zinc oxalate assum-
ing Avrami–Erofeev equation.
From Table 3 and assuming the composite method
II, the activation energy of 184 3 kJ mol−1 obtained
for the thermal decomposition of zinc oxalate is com-
parable within experimental errors to that reported by
Yankwich and Zavitsanos [10], however, the value of
activation energy for the thermal decomposition of
copper oxalate is lower than that reported by Dol-
limore et al. [28]. The decomposition of copper ox-
alate in nitrogen gives copper metal, and in air the
metal powder formed in the decomposition is oxi-
dized to the metal oxide; on the other hand, zinc
oxalate gives ZnO on decomposition in both atmo-
spheres [4]. For the oxalates of bivalent metals [5]
ature at which the M–OI link is ruptured and will de-
pend critically on the size and charge of metal ion,
whereas in those decompositions where the oxide is
produced in nitrogen the decomposition temperature
represents the energy required to break the C–OI bond
and then will depend less critically on the nature of
cation. Also, the extent to which the M–OI bond is co-
valent depends on the electronegativity of the metal.
The electronegativity of copper (1.9) is more than that
of zinc (1.6) [29] and consequently, the addition of
zinc ions to CuC2O4 will results in an increased pos-
itive charge on the copper ion, and a less covalent
type of bond thus occurs in the mixed oxalate than in
pure CuC2O4. This results in weakening of the Cu–O
bond and a lowering in activation energy of CuC2O4
decomposition.
Acknowledgements
The author would like to thank Prof. Dr. El-H.M.
Diefallah for the useful discussion. Thanks are also
due to Dr. A.A. El-Bellihi for doing SEM experiments.
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