Thermoanalytical investigations of tris(ethylenediamine)nickel(II) oxalate and sulphate complexes : TG-MS and TR-XRD studies

Article abstract of DOI:10.1007/s10973-010-0823-8

Investigations on the thermal behaviour of [Ni(en)₃]C ₂O₄?2H₂O and [Ni(en)₃]SO ₄ have been carried out in air and helium atmosphere. Simultaneous TG/DTA coupled online with mass spectroscopy (MS) in helium atmosphere detected the presence of H₂, O, CO, N₂/CH₂=CH ₂ and CO₂ fragments during the decomposition of tris(ethylenediamine)nickel(II) oxalate and H₂, O, NH, NH ₂, NH₃ and N₂/CH₂=CH₂ fragments for tris(ethylenediamine)nickel(II) sulphate complex. The thermal events during the decomposition were monitored by temperature-resolved X-ray diffraction. In air, both the complexes give nickel oxide as the final product of the decomposition. In helium atmosphere, tris(ethylenediamine)nickel(II) oxalate gives nickel as the residue, whereas tris(ethylenediamine)nickel(II) sulphate gives a mixture of nickel and nickel sulphide phases as the final residue. Kinetic analyses of these complexes by isoconversional methods are discussed and compared.

Full text of DOI:10.1007/s10973-010-0823-8

J Therm Anal Calorim (2010) 102:931–939
DOI 10.1007/s10973-010-0823-8
Thermoanalytical investigations of tris(ethylenediamine)nickel(II)
oxalate and sulphate complexes
TG–MS and TR–XRD studies
K. S. Rejitha Suresh Mathew
•
Received: 4 February 2010 / Accepted: 19 April 2010 / Published online: 7 May 2010
Ó Akad e´ miai Kiad o´ , Budapest, Hungary 2010
Abstract Investigations on the thermal behaviour of
[
During heating these complexes undergo stepwise decom-
Ni(en) ]C O ꢀ2H O and [Ni(en) ]SO have been carried
position to form various intermediates [1, 2]. Among these
ethylenediamine complexes are well studied and these
complexes are thermally stable due to the chelating effect
[3]. It is well known that experimental conditions have a
telling effect on the thermal decomposition pattern of the
complexes. In air, most of the complexes give metal oxide as
the final residue [3]. However, in inert condition metal is
obtained as the final product [2]. In this study, we have
investigated in detail the thermal decomposition of two
nickel complexes containing ethylenediamine as the ligand
viz., tris(ethylenediamine)nickel(II) oxalate dihydrate and
tris(ethylenediamine)nickel(II) sulphate in dynamic air
and helium. We have detected the evolved gaseous species
and studied the structural changes happening during the
decomposition of amine complexes by TG–MS and tem-
perature-resolved XRD. Despite numerous reports in the
literature with regard to the thermal decomposition studies
on the nickel amine complexes, the aforesaid aspects have
not been much investigated yet [2].
3
2
4
2
3
4
out in air and helium atmosphere. Simultaneous TG/DTA
coupled online with mass spectroscopy (MS) in helium
atmosphere detected the presence of H , O, CO, N /
2
2
CH =CH and CO fragments during the decomposition of
2
2
2
tris(ethylenediamine)nickel(II) oxalate and H , O, NH,
2
NH , NH and N /CH =CH fragments for tris(ethylene-
2
3
2
2
2
diamine)nickel(II) sulphate complex. The thermal events
during the decomposition were monitored by temperature-
resolved X-ray diffraction. In air, both the complexes give
nickel oxide as the final product of the decomposition. In
helium atmosphere, tris(ethylenediamine)nickel(II) oxalate
gives nickel as the residue, whereas tris(ethylenedia-
mine)nickel(II) sulphate gives a mixture of nickel and
nickel sulphide phases as the final residue. Kinetic analyses
of these complexes by isoconversional methods are dis-
cussed and compared.
Keywords Mass spectroscopy ꢀ Temperature resolved
X-ray diffraction ꢀ Kinetic analysis ꢀ Isoconversional
method
We have also studied the thermal decomposition kinet-
ics of tris(ethylenediamine)nickel(II) oxalate dihydrate and
tris(ethylenediamine)nickel(II) sulphate using model-free
isoconversional methods viz., Flynn–Wall–Ozawa (FWO)
[4, 5], Friedman [6] and Kissinger–Akahira–Sunose (KAS)
Introduction
[
7, 8].
Amines like propylenediamine, ethylenediamine and
ammonia are well known for their complexing capacity and
they form well-defined complexes with transition metals.
Experimental
The complexes were synthesized employing reported pro-
cedures [9, 10]. Nickel content in the complexes was
determined by means of gravimetry [11]. The complexes
were further characterized by spectral and chemical
analysis.
K. S. Rejitha ꢀ S. Mathew (&)
School of Chemical Sciences, Mahatma Gandhi University,
P.D. Hills P.O., Kottayam, Kerala 686-560, India
e-mail: sureshmathews@sancharnet.in
123

9
32
K. S. Rejitha, S. Mathew
Instrumentation
alternative to study the solid state reactions. Now a days
model-free isoconversional methods have gained much
attention as a useful approach to investigate the kinetics of
the solid state reactions as these methods possess several
advantages over conventional approaches [12–15]. These
methods are based on the principle that the reaction rate at
a constant extent of conversion (a) is only a function of the
temperature (T). For a single step reaction, the activation
energy (E) is constant over the whole conversion function.
For multi step kinetics, the activation energy (E) vary with
the extent of conversion and this reflects the variation in
relative contributions of single steps to the overall reaction
rate. Friedman, FWO and KAS are isoconversional
methods and are frequently employed to study the
kinetics. These methods yield effective activation energy
(E) as a function of extent of conversion (a) [16].
Thermogravimetric analyses in air were carried out using
Shimadzu DTG-60 instrument connected to a TA-60 online
analyser. The heating rates employed were 5, 10, 15 and
-
1
2
0 °C min . TG–MS studies were carried out using a
Rigaku, TG-8120 thermogravimetric apparatus combined
with mass spectroscopy (Anelva, M-QA200TS) at a heating
-
1
rate of 10 °C min
under high-purity He gas flow
(
99.9999%). For the thermogravimetric analyses, the sam-
ple mass used were 10 ± 0.2 mg for all the experiments.
The elemental analyses were carried out using Vario
Elemental III instrument. X-ray powder patterns were
recorded on a Bruker D8 Advance diffractometer using Cu
˚
Ka radiation, k = 1.542 A attached with a programmable
temperature device from Anton Paar, (TTK 450). The
measurements were performed by placing the sample on a
flat sample holder, while the samples were heated by the
programmable temperature controller. A series of diffrac-
tion patterns were recorded at every 20 °C rise of tempera-
ture. Crystallite size was calculated using Scherrer equation
In order to study the kinetics using the model-free
methods, several TG measurements were carried out at
different heating rates. Friedman, Flynn–Wall–Ozawa
(FWO) as well as KAS methods are based on multiple
heating rate experiments.
Flynn–Wall–Ozawa equation is as follows [4, 5]:
t ¼ 0:9k=bcos h;
where t is the thickness of the particle, k is the wave length,
b is full width at half maximum (FWHM) intensity
AE
E
ln / ¼ ln R ꢁ ln gðaÞ ꢁ 5:3305 ꢁ 1:052
;
ð3Þ
RT
where / is the heating rate, a is the degree of conversion, g
a) is the mechanism function, E is the activation energy, A
is the pre-exponential factor and R is the gas constant.
(
in radians) and cos h is the corresponding angle. Mor-
(
phology of the complexes, intermediates formed at differ-
ent temperatures and the residues were determined using
JEOL JSM-6390 scanning electron microscope. For SEM
analyses, the samples were spread on a carbon tape and
made uniform by blowing air.
Friedman equation is [6]:
da
E
ln ¼ ln½Af ðaÞꢂ ꢁ
;
ð4Þ
dt
RT
where dd at is the rate of conversion and f (a) is the mecha-
nism function.
Kinetic studies
Kissinger–Akahira–Sunose (KAS) equation is [7, 8]:
ꢁ
Isoconversional methods
ꢀ
/
T2
AR
E
ln
¼ ln
ꢁ
:
ð5Þ
Solid state reaction often follows the basic kinetic
equation:
EgðaÞ RT
From the above equations (3–5), plots of ln / versus 1/T,
da
dt
/
2
dðaÞ
dt
ꢁE
ln
versus 1/T, ln T
versus 1/T give straight lines with
¼
A exp
f ðaÞ;
ð1Þ
RT
slopes -1.052E/R, -E/R and -E/R, respectively, for the
FWO, Friedman and KAS equations. The slope of the
straight line is directly proportional to the activation energy.
where A is the pre-exponential factor, E is the activation
energy, R is the gas constant, T is the temperature and f (a)
is the kinetic model function. For a nonisothermal reaction,
Eq. 1 can be written as:
Results and discussion
dðaÞ
fðaÞ
A
ꢁE
dT
¼
exp
dT; where the heating rate / ¼ dt
:
ð2Þ
/
RT
Thermal decomposition studies
The above equation pertains to a single step reaction
kinetics, but quite often the solid state reactions contain
complex reaction steps and a single rate equation is unable
to explain the complexities of solid state reactions. In this
context, isoconversional methods could be used as an
Tris(ethylenediamine)nickel(II) oxalate dihydrate
The details of the elemental analyses are given in Table 1,
which are in agreement with the formulae of the
1
23

Thermoanalytical investigations
933
Table 1 Elemental analysis/%
Complexes
Ni
Obsd./calc.
C
Obsd./calc.
H
Obsd./calc.
N
Obsd./calc.
S
[
Ni(en)
Ni(en)
3
]C
2
O
4
ꢀ2H
2
O
16.2/15.9
17.5/17.5
26.5/26.6
21.5/21.5
7.7/7.2
7.2/7.2
25.1/24.9
25.1/25
[
3
]SO
4
9.6/9.6
complexes. For tris(ethylenediamine)nickel(II) oxalate
dihydrate, the decomposition in air involves three stages.
The decomposition pattern of the complex in air is as
follows:
in a stable complex i.e. tris(ethylenediamine)nickel(II)
oxalate. The second stage commences at 260 °C and
corresponds to the loss of two molecules of ethylenedia-
mine to give mono(ethylenediamine)nickel(II) oxalate.
This monoamine intermediate is not stable and quickly
decomposes at 325 °C. The final stage corresponds to the
decomposition of this monoamine intermediate by the loss
of one molecule of ethylenediamine along with the evolu-
Â
Ã
Â
Ã
NiðenÞ C O ꢀ2H O ! NiðenÞ C O þ 2H O,
3
2
4
2
3
2
4
2
Â
Ã
NiðenÞ C O ! NiðenÞC O þ 2en,
3
2
4
2
4
NiðenÞC2O4 ! NiO þ en þ CO2 þ CO:
tion of CO and CO to form NiO as the residue. The DTA
2
results complement this observation. The DTA pattern
contains two endotherms at 105 and 312 °C and one exo-
therm at 395 °C. The two endotherms correspond to the
dehydration and the deamination reactions while the exo-
therm corresponds to the decomposition of mono(ethy-
lenediamine)nickel(II) oxalate to give NiO in an oxidizing
atmosphere. A shoulder of an exothermic peak at 332 °C is
seen in the DTA curve. This can be due to the overlapping
of two reactions viz., the deamination and the decomposi-
tion reactions in the final stage. Under oxidizing condition
ethylenediamine is oxidized instantaneously to oxides of
nitrogen and carbon and the exotherm at 395 °C in the DTA
can be attributed to the oxidation of ethylenediamine. The
obtained results are in agreement with the reported thermal
decomposition pattern of the complex [9].
TG/DTA plot for the thermal decomposition of the
-
1
complex in air at a heating rate of 10 °C min is shown in
Fig. 1. The values of the temperature of inception (T ), final
temperature (T ) and the temperature of summit (T ) for the
i
f
s
thermal decomposition and the mass loss data are given in
Table 2. From the TG curve, it is seen that the complex
starts to lose mass at 70 °C. The first stage in the thermo-
gram corresponds to the loss of two molecules of water in
the temperature range 70–135 °C. This dehydration resulted
1
8
6
4
2
0
–
000
00
00
00
00
1
00
80
60
40
20
The plot of simultaneous TG/DTA coupled online with
MS spectra carried out under helium atmosphere is shown in
Fig. 2. In helium atmosphere, the dehydrated (dehydration
occurs due to the heating of the sample before MS analysis)
tris(ethylenediamine)nickel(II) oxalate complex undergoes
a two step decomposition pattern as shown below:
200
Â
Ã
NiðenÞ C2O4 ! NienC2O4 þ 2en,
NienC2O4 ! Ni þ en þ CO2 þ CO þ O:
3
0
100
200
300
400
500
600
Temperature/°C
The deamination stage occurs at 245 °C in helium (inert)
atmosphere with the liberation of two ethylenediamine
Fig. 1 TG/DTA plot of tris(ethylenediamine)nickel(II) oxalate dihy-
-
1
drate (in air) at heating rate 10 °C min
-
1
Table 2 Phenomenological data for the thermal decomposition of tris(ethylenediamine)nickel(II) oxalate dihydrate in air// = 10 °C min
Stages
TG results
/8C
Percentage mass loss
Stepwise
Residue
T
i
T
f
/8C
T
s
/8C
Cumulative
Theoretical
Theoretical
Observed
Observed
1
2
3
70
260
325
135
325
400
80
290
385
9.9
33.1
36.4
10.0
32.9
35.3
9.9
43
10.0
42.9
78.2
3 2 4
Ni(en) C O
Ni(en) C
NiO
2 4
O
79.4
123

9
34
K. S. Rejitha, S. Mathew
mass
*
*
Ni
2
1
2
4
6
8
4
0
–
–
–
–
–
20
40
60
80
100
*
TG
DTA
1
0
20
30
40
2θ/°
50
60
70
200
400
600
800
Temperature/°C
Fig. 2 TG/DTA-MS plot of tris(ethylenediamine)nickel(II) oxalate
1
Fig. 3 XRD pattern of metallic nickel
-
(
in helium) at heating rate 10 °C min
Tris(ethylenediamine)nickel(II) sulphate
molecules. The second stage begins at 320 °C and ends at
05 °C. The phenomenological data for the thermal decom-
position of the dehydrated complex in helium is given in
Table 3. The formation of metallic nickel as the final product
is due to the reduction of NiO by CO formed during the
decomposition [17]. The XRD pattern of the metallic nickel
4
Thethermal decompositionoftris(ethylenediamine)nickel(II)
sulphate complex in air involves a four stage decomposition
[
18] as shown below:
NiðenÞ SO4 ! NiðenÞSO4 þ 2en,
3
(
JCPDS no. 04-0850) is shown in Fig. 3. Average crystallite
NiðenÞSO ! NiSO þ en,
4
4
size of metallic Ni calculated from the peak broadening value
using Scherrer equation was found to be 22.5 nm.
NiSO ! mixture of NiO and NiSO ! NiO þ SO :
4
4
3
The DTA profile in Fig. 2 shows two endotherms with Ts
values 298 and 370 °C corresponding to the two decompo-
sition stages of tris(ethylenediamine)nickel(II) oxalate. The
MS spectra of the complex show fragments with m/z values
The phenomenological data for the thermal decomposition
of tris(ethylenediamine)nickel(II) sulphate are given in
Table 4. This complex starts to lose mass only at 300 °C,
indicating the thermal stability of the complex. The stability is
due to the strong coordination of the bidentate ligand (en) with
nickel resulting in the formation of complex with octahedral
geometry. The TG/DTA plotfor the thermal decompositionof
2
, 16, 28 and 44 in the temperature range 356–388 °C. The
reported peaks are due to the evolution of H (2), O/NH (16),
2
2
CO, N , CH =CH (28) and CO (44). The species H , NH ,
2
2
2
2
2
2
N and CH =CH are formed by the fragmentation of the
2
2
2
tris(ethylenediamine)nickel(II) sulphate in air at a heating rate
-1
evolved ethylenediamine as shown below.
of 10 °C min
decomposition of tris(ethylenediamine)nickel(II) sulphate
is shown in Fig. 4. The first stage of
2 2
CH – CH
1
2 2 2
0 NH (16) + 5 CH = CH (28)
corresponds to the loss of two molecules of ethylenediamine
5
NH
2
NH
2
(36% loss) to give mono ethylenediamine complex. The
mono(ethylenediamine)nickel(II) sulphate is very unstable
and quickly decomposes at 360 °C. The subsequent stages are
overlapping and hence not distinguishable. Intermediate
5
N
2
2
(28) + 10 H (2)
5
3
NH (17) + 5 NH (15)
Scheme 1 Fragmentation of ethylenediamine
ꢃ
-1
Table 3 Phenomenological data for the thermal decomposition of tris(ethylenediamine)nickel(II) oxalate in helium=/ ¼ 10 C min
Stages
TG results
Percentage mass loss
Residue
T
i
/8C
f
T /8C
s
T /8C
Stepwise
Cumulative
Theoretical
Theoretical
Observed
Observed
1
2
245
320
320
405
300
375
36.7
45.3
36.5
42
36.7
82
37.5
78.5
Ni(en)C
Ni
2 4
O
1
23

Thermoanalytical investigations
935
ꢃ
-1
Table 4 Phenomenological data for the thermal decomposition of tris(ethylenediamine)nickel(II) sulphate in air=/ ¼ 10 C min
Stages
TG results
Percentage mass loss
Residue
T
i
/8C
T
f
/8C
s
T /8C
Stepwise
Cumulative
Theoretical
Theoretical
Observed
Observed
1
2
3
4
300
360
466
696
360
466
500
786
346
424
487
760
35.8
17.9
36
35.8
53.7
36
Ni(en) SO
4
17.9
53.9
NiSO
4
Mixture of NiSO
NiO
4
and NiO
23.9
23.9
77.6
77.8
6
00
00
100
mass
5
2
1
1
1
2
5
6
7
8
8
0
0
0
0
400
3
2
1
0
–
00
00
00
6
4
2
0
–
–
–
–
20
40
60
80
100
0
100 200 300 400 500 600 700 800 900
Temperature/°C
TG
Fig. 4 TG/DTA plot of tris(ethylenediamine)nickel(II) sulphate
1
in air) at heating rate 10 °C min
DTA
-
(
–100
2
00
400
600
800
Temperature/°C
formed at 500 °C corresponds to the partial dissociation of
Fig. 5 TG/DTA-MS plot of tris(ethylenediamine)nickel(II) sulphate
-
1
(
in helium) at heating rate 10 °C min
NiSO to a mixture of NiSO and NiO phases. The total
4
4
cumulative mass loss of 77.8% at 786 °C corresponds to the
formation of NiO.
The plot of simultaneous TG/DTA coupled online with
MS during the thermal decomposition of tris(ethylenedia-
mine)nickel(II) sulphate in helium atmosphere is shown in
Fig. 5. The TG–MS analysis indicates the following ther-
mal decomposition patterns:
∗
Ni S
3 2
Ni
NiðenÞ SO ! NiðenÞSO þ 2en,
3
4
4
∗
Ni
4
NiðenÞSO4 ! Ni3S2 þ Ni þ 4en þ 8O2 þ 2S:
Tris(ethylenediamine)nickel(II) sulphate undergoes a
∗
∗
Ni
∗
∗
Ni
two stage decomposition in helium atmosphere, resulting in
the formation of a mixture of Ni and Ni S (JCPDS no. 44-
3
2
1
0
20
30
40
50
θ/°
60
70
80
1
418) phases as the residues. Figure 6 shows the XRD
2
pattern of the decomposition products namely, Ni and
Ni S . The mass spectra of tris(ethylenediamine)nickel(II)
3
2
3 2
Fig. 6 XRD pattern of mixture Ni and Ni S
sulphate show peaks with m/z values 2, 15, 16, 17 and 28.
These peaks are due to the presence of H , NH, NH , NH
3
2
2
TR–XRD studies of the complexes
and N /CH =CH . These species are formed due to the
2
2
2
fragmentation of the liberated ethylenediamine molecule as
shown in Scheme 1.
The high temperature XRD patterns of tris(ethylenedia-
mine)nickel(II) oxalate dihydrate complex in the
123

9
36
K. S. Rejitha, S. Mathew
o
o
o NiO
o
#
NiO
o
# NiSO4
[Ni(en) ]C O
400°C
80
3
2
4
o
o
o
o
* [
Ni(en) ]C O .2H O
* Ni(en) SO
3
800°C
3
2
4
2
4
o
o
0
0
0
0
0
10
20
30
30
40
50
50
50
60
60
60
60
60
70
70
70
70
70
90
90
90
90
90
3
20°C
1
0
20
30
40 o 50
60
70
80
90
100
100
o
o
500°C
10#
10*
10
20
40
80
#
#
o
o
#
1
20°C
#
#
#
#
#
1
0
20
30
40
50
60
70
80
90
*
20
30
40
80
*
6
0°C
0°C
*
40°C
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
20
30
40
50
*
80
0
10
20
30
2θ/°
40
50
60
**
*
4
*
*
*
*
*
*
Fig. 8 High temperature XRD patterns of tris(ethylenediamine)
nickel(II) sulphate
10
20
30
40
50
80
2
θ/°
SEM analyses
Fig. 7 High temperature XRD patterns of tris(ethylenedia-
mine)nickel(II) oxalate dihydrate
SEM images of tris(ethylenediamine)nickel(II) oxalate
dihydrate, intermediates isolated at different temperatures
and the residue in air are shown in Fig. 9. It is seen from
SEM images that the complex has an elongated morphol-
ogy. The SEM image shows that the dehydrated amine
complex is having more or less same morphology as that of
temperature range 40–400 °C are shown in Fig. 7. The
patterns recorded at 40–60 °C with peaks having 2h values
1
1.29, 14.20, 16.60, 19.82, 22.98, 24.33, 30.80, 43.19 and
4
9.25 represent the planes of tris(ethylenedia-
mine)nickel(II) oxalate dihydrate. Above 60 °C, the com-
plex begins to decompose by the liberation of two
molecules of water. The XRD pattern at 120 °C corre-
sponds to the dehydrated complex. This complex is stable
up to 240 °C and subsequently decomposes at 260 °C by
releasing two en molecules. The XRD pattern at 320 °C
corresponds to the monoethylenediamine complex. The
monoethylenediamine complex decomposes at 400 °C to
give NiO as the residue.
the original complex [Ni(en) ]C O .2H O. This is due to
3 2 4 2
the structural similarity of the complexes. Both these
complexes possess octahedral geometry (as there is no
change in coordination during the dehydration process)
hence they exhibit similar structure. SEM image reveals
that in the case of mono(ethylenediamine) nickel(II) oxa-
late there is an apparent change in morphology and a
reduction of the size. The final residue NiO appears as very
small rods.
All the XRD patterns are highly crystalline and the final
NiO residue formed contains well-defined peaks with fcc
structure (JCPDS no. 47-1049). Average crystallite size of
NiO was calculated from the peak broadening value using
Scherrer equation and was found to be 23.4 nm.
The SEM pictures of tris(ethylenediamine)nickel(II)
sulphate and the residue show that the complex has a rod
like appearance and NiO (residue) is found to be agglom-
erated (Fig. 10).
The XRD patterns of tris(ethylenediamine)nickel(II)
sulphate, intermediate and the residue are given in Fig. 8.
The XRD pattern at 40 °C contains peaks corresponding to
Kinetic analyses
(
100), (101), (002), (110), (102), (200), (201), (112), (202),
Kinetic analyses of the thermal decomposition of amine
complexes were carried out by means of isoconversional
methods. Isoconversional methods help to evaluate
Arrhenius parameters without the prediction of a reaction
model. The method yields a series of activation energies
(E) as a function of conversion. Often the solid samples are
polycrystalline in nature and hence the rates of solid state
reactions are different for crystals of different size and
occur in multi steps [19]. The effective activation energy of
the process is a composite value determined by the
(
211), (301) and (303) planes of tris(ethylenediamine)
nickel(II) sulphate (JCPDS no. 23-1796). The XRD pattern
of the intermediate at 500 °C has peaks of NiSO and NiO.
4
However, the majority peaks are those of NiO phase. The
XRD analysis revealed that the final residue at 800 °C is NiO
and has well-defined peaks with cubic symmetry. From the
peak broadening value, the crystallite size of NiO was cal-
culated using Scherrer equation. Average crystallite size was
found to be 17.47 nm.
1
23

Thermoanalytical investigations
937
Fig. 9 SEM pictures of
a [Ni(en)
b [Ni(en)
d NiO
3
]C
]C
2
O
4
.2H
2
O,
3
2
O
, c Ni(en)C O ,
4 2 4
Fig. 10 SEM pictures of
a [Ni(en) ]SO , b NiO
3
4
activation energies of elementary steps as well as by the
relative contributions of these steps to the over all reaction
rate. The contributions of the individual steps can change
with the extent of conversion (a) and with the temperature
0.05–0.2 and then increases (a = 0.4). Increasing depen-
dence of E on a shows that the reaction is competing or
consecutive. Solid state reaction of the type solid ?
solid ? gas, undergo such type of reactions. The activation
energy (E) again decreases in the a range 0.45 to 1.
Decreasing dependence shows the reaction is reversible in
nature [20]. These variations of E with a, may be due to the
occurrence of simultaneous deamination (loss of en) and
decomposition (decomposition of nickel oxalate to give
(
T). Hence, the variation of E with a is an indication of
multi step kinetics [16]. From the dependence of activation
energy on the extent of conversion, we can predict the
nature of solid state reactions.
The activation energy versus conversion plot of
tris(ethylenediamine)nickel(II) oxalate dihydrate for all
three stages viz., dehydration, deamination and the
decomposition are shown in Figs. 11a–c. It is observed that
for the dehydration reaction (Fig. 11a), the activation
energy decreases with conversion. The decreasing depen-
dence shows that the reaction is reversible, which is very
common to many solid state reactions of the type
solid $ solid ? gas. This type of behaviour is a typical
characteristic of the reversible dehydration of crystal
hydrates like calcium oxalate monohydrate as well as other
process showing reversible reactions [20]. For the deami-
nation stage (stage II, Fig. 11b) also the activation energy
decreases with conversion. This behaviour indicates that
the deamination is a reversible reaction. For the final stage
of decomposition (Fig. 11c), the activation energy derived
from FWO and KAS equations decreases in the alpha range
NiO, CO and CO) steps in a single stage.
2
The kinetic analyses for the first deamination stage
(i.e. Ni(en) SO ? Ni(en)SO ? 2 en) of tris(ethylenedi-
3
4
4
amine)nickel(II) sulphate was derived using isoconver-
sional methods and the corresponding kinetic plots are
shown in Fig. 12. It is seen that the activation energy
depends on conversion function (a) which indicates the
complex decomposition mechanism. From the activation
energy versus conversion plot using FWO and KAS
equations, it is seen that activation energy was found
to be decreasing with the extent of conversion in the
range 0.05–0.7 and then found to be increasing [17]. As
mentioned above, the decreasing dependence can be
attributed to the reversible reactions and the increasing
dependence can be attributed to the competing or con-
secutive reactions.
123

9
38
K. S. Rejitha, S. Mathew
Fig. 11 Variation of activation
energy with conversion (a) for
a dehydration, b deamination,
c decomposition reactions of
tris(ethylenediamine)nickel(II)
oxalate dihydrate
110
100
300
280
260
240
220
(
a)
(b)
FWO
Friedman
KAS
FWO
Friedman
KAS
90
80
70
60
50
2
00
180
60
140
20
100
1
1
80
60
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
α
α
FWO
Friedman
KAS
(c)
280
260
240
220
200
180
160
0.0
0.2
0.4
0.6
0.8
1.0
α
3
2
2
2
2
2
1
1
1
00
80
60
40
20
00
80
60
40
conditions (air and He). In air, tris(ethylenedia-
mine)nickel(II) oxalate dihydrate decomposes in three
steps and tris(ethylenediamine)nickel(II) sulphate decom-
poses in four steps. In air, both the complexes give nickel
oxide as the final product of decomposition. In helium
atmosphere, the dehydrated tris(ethylenediamine)nickel(II)
oxalate complex undergoes a two step decomposition to
give nickel as the final product. Tris(ethylenedia-
mine)nickel(II) sulphate undergoes a two stage decompo-
sition in helium and gives a mixture of nickel and nickel
sulphide as the final residues. Simultaneous TG/DTA
coupled online with mass spectra (MS) in helium atmo-
sphere revealed the presence of H , O, CO, N and CO
2
FWO
Friedman
KAS
0.0
0.2
0.4
0.6
0.8
1.0
α
2
2
fragments during the decomposition of tris(ethylenedia-
mine)nickel(II) oxalate and H , O, NH, NH , NH and N in
Fig. 12 Variation of activation energy with conversion (a) for the
deamination of [Ni(en) ]SO to Ni(en)SO
2
2
3
2
3
4
4
the case of tris(ethylenediamine)nickel(II) sulphate decom-
position. SEM analyses show that both tris(ethylenedia-
mine)nickel(II) oxalate dihydrate and tris(ethylenediamine)
nickel(II) sulphate complexes have rod like appearances.
Kinetic analyses by isoconversional methods reveal that the
dehydration and deamination stages of tris(ethylenedia-
mine)nickel(II) oxalate dihydrate are reversible in nature.
The decomposition reaction of the tris(ethylenediamine)
nickel(II) oxalate dihydrate and the first deamination stage of
tris(ethylenediamine)nickel(II) sulphate show increasing as
well as decreasing dependence of activation energy on
conversion, probably due to the simultaneous occurrence of
various thermal events in a single stage.
It is seen that the activation energy calculated by FWO
and KAS methods are comparable. However, the activation
energies calculated by Friedman method show higher values
in most cases. This is because the Friedman method is very
sensitive to experimental noise and tends to be numerically
unsound when employing instantaneous rate values [21].
Conclusions
Thermal behaviour of [Ni(en) ]C O ꢀ2H O and [Ni(en) ]
3
2
4
2
3
SO have been carried out in different atmospheric
4
1
23

Thermoanalytical investigations
939
Acknowledgements The authors are grateful to the Sophisticated
Test and Instrumentation Centre (STIC), Cochin, for recording TR–
XRD patterns. The authors are also grateful to Prof. T. Ichikawa,
Institute for Advanced Materials Research, Hiroshima University,
Japan, for the TG–MS analyses.
10. Rochow EG, editor. Inorganic synthesis, vol VI. New York:
McGraw-Hill; 1960. p. 198–200.
11. Vogel AG. Text book of quantitative inorganic analysis. 4th ed.
Longmann: London; 1978.
12. Cai JM, Bi LS. Kinetic analysis of wheat straw pyrolysis using
isoconversional methods. J Therm Anal Calorim. 2009;98:325–
3
0.
1
3. Su TT, Zhai YC, Jiang H, Gong H. Studies on the thermal
decomposition kinetics and mechanism of ammonium niobium
oxalate. J Therm Anal Calorim. 2009;98:449–55.
4. Badrinarayanan P, Zheng W, Simon SL. Isoconversional analysis
of the glass transition. Thermochim Acta. 2008;468:87–93.
5. Jankovic B, Adnadevic B, Javanovic J. Application of model
fitting and model free kinetics to the study of non isothermal
dehydration of equilibrium swollen poly(acrylic acid) hydrogel:
thermogravimetric analysis. Thermochim Acta. 2007;452:106–
References
1
. Singh G, Pandey DK. Studies on energetic compounds: kinetics
and mechanism of thermolysis of bis(ethylenediamine)metal
nitrate and their role in the burning rate of solid propellants.
Propellants Explos Pyrotech. 2003;28:231–9.
1
1
2
3
. Mathew S, Nair CGR, Ninan KN. Thermal decomposition studies
on amine complexes of copper(II) nitrate in solid state. Bull
Chem Soc Jpn. 1991;64:3207–9.
. Mathew S, Nair CGR, Ninan KN. Kinetics and mechanism of
thermal decomposition of bis(ethylenediamine)copper(II) halide
monohydrate. Thermochim Acta. 1991;181:253–68.
1
5.
1
1
6. Vyazovkin S. A unified approach to kinetic processing of non-
isothermal data. Int J Chem Kinet. 1996;28:95–101.
7. Małecka B, Małecki A, Dro z_ d z_ -Cie s´ la E, Tortet L, Llewellyn P,
Rouquerol F. Some aspects of thermal decomposition of
4
5
. Ozawa T. A new method of analyzing thermogravimetric data.
Bull Chem Soc Jpn. 1965;38:1881–6.
. Flynn JH, Wall LA. A quick, direct method for the determination
of activation energy from thermogravimetric data. J Polym Sci B.
NiC
2 4
O ꢀ2H
2
O. Thermochim Acta. 2007;466:57–62.
1
1
8. Rejitha KS, Mathew S. Thermal deamination kinetics of
tris(ethylenediamine)nickel(II) sulphate in the solid state.
J Therm Anal Calorim. 2008;93:213–7.
9. Vyazovkin S. Kinetic concepts of thermally stimulated reactions
in solids: a view from a historical perspective. Int Rev Phys
Chem. 2000;19:45–60.
1
996;4:323–8.
6
7
8
9
. Friedman HL. New methods for evaluating kinetic parameters
from thermal analysis data. J Polym Sci B. 1969;7:41–6.
. Kissinger HE. Reaction kinetics in differential thermal analysis.
Anal Chem. 1957;29:1702–6.
. Akahira T, Sunose T. Research report of Chiba Institute Tech-
nology. Sci Technol. 1971;16:22–31.
2
2
0. Vyazovkin S, Linert W. Kinetic analysis of reversible thermal
decomposition of solids. Int J Chem Kinet. 1995;27:73–84.
1. Vyazovkin S. Modification of the integral isoconversional
method to account for variation in the activation energy. J Com-
put Chem. 2001;22:178–83.
. Haschke JM, Wendlandt WW. The thermal decomposition of
metal ethylenediamine complexes. Anal Chim Acta. 1965;32:
3
86–93.
123