Thermal deamination kinetics of tris(ethylenediamine)nickel(II) sulphate in the solid-state

Article abstract of DOI:10.1007/s10973-007-8923-9

Thermogravimetric techniques have been used to study the kinetics of thermal deamination of tris(ethylenediamine)nickel(II) sulphate. The complex was synthesized and characterized by various chemical and spectral techniques. Thermal decomposition studies were carried at different heating rates (5, 10, 15 and 20°C min^-1) in dynamic air. The complex undergoes a four-stage decomposition pattern. The stages are not well resolved. Decomposition path can be interpreted as a two-stage deamination, and a two-stage decomposition. Reaction products at each stage were separated and identified by means of IR and XRD. The morphology of the complex and the residue were studied by means of SEM. Final residue of the decomposition was found to be crystalline NiO. The deamination kinetics was studied using model-free isoconversional methods viz., Friedman, Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) methods. It is observed that the activation energy varies with the extent of conversion; indicating the complex nature of the deamination reaction.

Full text of DOI:10.1007/s10973-007-8923-9

Journal of Thermal Analysis and Calorimetry, Vol. 93 (2008) 1, 213–217
THERMAL DEAMINATION KINETICS OF TRIS(ETHYLENE-
DIAMINE)NICKEL(II) SULPHATE IN THE SOLID-STATE
*
K. S. Rejitha and S. Mathew
School of Chemical Sciences, Mahatma Gandhi University, P.D. Hills, Kottayam 686 560, Kerala, India
Thermogravimetric techniques have been used to study the kinetics of thermal deamination of tris(ethylenediamine)nickel(II) sul-
phate. The complex was synthesized and characterized by various chemical and spectral techniques. Thermal decomposition studies
were carried at different heating rates (5, 10, 15 and 20°C min–1) in dynamic air. The complex undergoes a four-stage decomposition
pattern. The stages are not well resolved. Decomposition path can be interpreted as a two-stage deamination, and a two-stage de-
composition. Reaction products at each stage were separated and identified by means of IR and XRD. The morphology of the com-
plex and the residue were studied by means of SEM. Final residue of the decomposition was found to be crystalline NiO.
The deamination kinetics was studied using model-free isoconversional methods viz., Friedman, Flynn–Wall–Ozawa (FWO)
and Kissinger–Akahira–Sunose (KAS) methods. It is observed that the activation energy varies with the extent of conversion; indi-
cating the complex nature of the deamination reaction.
Keywords: deamination, kinetics, nickel oxide, thermal decomposition
Introduction
The kinetic analyses of the deamination stages
were done using model-free isoconversional methods
viz., Friedman [12], Flynn–Wall–Ozawa (FWO)
[13, 14], Kissinger–Akahira–Sunose (KAS) [15, 16]
methods. A series of activation energy (E) were deter-
mined for the deamination stage.
Thermal decomposition studies of transition metal am-
mine complexes especially those of Cu, Ni and Co
have been extensively studied over years. However,
thermal analyses of certain complexes have received
renewed interest because of the formation of certain in-
termediates, which are otherwise difficult to synthesize
[1–6]. Transition metal ammine complexes can be used
as template for zeolite synthesis as they carry high pos-
itive charge density for interaction with the anionic sil-
icate species and offer novel architecture which is not
common with classic quaternary ammonium templates
[7]. Obviously, thermal decomposition studies have
immense application in materials synthesis where the
knowledge of the decomposition temperature of the
precursor is a pre-requisite for the synthesis of
nanosized metal oxide and metals under appropriate
conditions. In fact, now thermal decomposition tech-
nique is an important route for the synthesis of nano-
materials [8–10]. In this context it is interesting to in-
vestigate the thermal decomposition behaviour of
tris(ethylenediamine)nickel(II) sulphate in the solid-
state. The thermolysis of nickel compounds always
lead to the formation of NiO as the final product.
Li et al. synthesized nano-sized NiO using pyrolysis of
nickeldimethylglyoximate rods [11]. Present work
aims to explore the thermal behaviour of tris(ethylene-
diamine)nickel(II) sulphate and to evaluate the kinetic
scheme of thermal deamination reaction.
Experimental
Preparation of the complex
The complex, Ni(en)3SO4 was synthesized as per the
procedure in [17]. Stoichiometric amount of ethylene-
diamine was added to nickel sulphate solution with
stirring and cooled in ice bath. The complex was pre-
cipitated out by adding ethanol. The isolated complex
was washed with ethanol and then with ether and dried
over vacuum. The resulting complex was characterized
by various spectral and chemical means. The nickel
content in the complex was determined by gravimetry
[18]. Nickel is precipitated by the addition of ethanolic
solution of dimethylglyoxime to a hot faintly acidic so-
lution of the nickel complex, and then adding a slight
excess of aqueous ammonia solution. The precipitate is
washed with cold water and then weighed as nickel
dimethylglyoximate after drying at 110–120°C.
Instrumentation
Thermogravimetric analyses were carried out using
Shimadzu DTG-60 instrument connected to TA60 on-
*
Author for correspondence: sureshmathews@sancharnet.in
1388–6150/$20.00
© 2008 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands

REJITHA, MATHEW
line analyser. The samples were loaded in a platinum
where da/dt is the rate of conversion, and f(a) is the
mechanism function.
crucible and the mass was kept constant around
12±0.6 mg. The experiment was carried out in dynamic
air. The heating rates employed were 5, 10, 15,
Flynn–Wall–Ozawa equation is as follows
AE
E
20°C min–1. IR spectra were recorded on
a
lnf =ln
-lng(a )-5.3305-1.052
(6)
R
RT
Shimadzu 2600 FTIR instrument using KBr pellet tech-
nique. CHNS analyses were carried out on a Vario
Elementar III analyzer. The X-ray powder patterns were
recorded using Bruker D8 Advance diffractometer us-
ing CuKa radiation (l=1.542 ). The particle size can
be calculated using Scherrer equation [19],
where f is the heating rate, a is the degree of conver-
sion, g(a) is the mechanism function, E is the activa-
tion energy, A is the pre-exponential factor, and R is
the gas constant.
Kissinger–Akahira–Sunose (KAS) equation is
f
A
Ea
t=0.9l/bcosq
(1)
æ
ç
è
ö
÷
ø
ln
=ln
-
Eg(a ) RT
(7)
2
T
where t is the thickness of the particle, l is the wave-
length, b is full width at half maximum (FWHM), q is
the corresponding angle.
From the above Eqs (5)–(7) it is seen that the
2
plots of lnf vs. 1/T, ln(da/dt) vs. 1/T and ln(f/T ) vs.
SEM studies were carried out on a JEOL
JSM 6390 instrument. The samples were uniformly
spread on a carbon tape and coated with platinum.
1/T would give straight lines with slopes –1.052E/R,
–E/R and –E/R, respectively for the FWO, Friedman
and KAS equations. The slope of the straight lines is
directly proportional to the activation energy.
Kinetic studies
Solid-state reaction often follows the basic kinetic equa-
tion
Results and discussion
The results of the elemental analysis are summarized
in Table 1. The decomposition patterns at different
heating rates are given in Fig. 1. The various stages of
decomposition are as follows
da
dt
–E
= Aexp
f (a )
(2)
RT
where A is the pre-exponential factor, E is the activa-
tion energy, R is the gas constant and T is the tempera-
ture, f(a) is the kinetic model function. For a non-iso-
thermal reaction Eq. (2) can be written as
Ni(en)3SO4®Ni(en)SO4+2en
Ni(en)SO4®NiSO4+en
da
A –E
= exp dT
f (a ) f RT
(3)
NiSO4®mixture NiO and NiSO4
mixture NiO and NiSO4®NiO+SO3
where f is the heating rate employed and
The shapes of the TG curves remain unchanged
while there is a noticeable increase in the temperature
of inception (Ti), final temperature (Tf) and the DTG
peak temperature (Ts) with increase in the heating
dT
f =
(4)
dt
Isoconversional method has the advantage that it
allows the estimation of kinetic parameters without
choosing a reaction model. Friedman, FWO and KAS
equations represent isoconversional methods, and are
frequently employed to study the kinetics. These
methods yield activation energy (E) as a function of
extent of conversion (a). From the dependence of E
on the extent of conversion, we can infer the
multi-step nature of solid-state reactions. Besides, it
reveals some mechanistic conclusions.
In order to study the kinetics using the model-
free methods, several TG measurements were carried
out at different heating rates.
Friedman equation is
da
dt
E
ln
=ln[Af (a )]-
(5)
RT
Fig. 1 TG plots at different heating rates
214
J. Therm. Anal. Cal., 93, 2008

THERMAL DEAMINATION KINETICS
Table 1 Elemental analysis data
Ni/%
C/%
H/%
N/%
S/%
obsd.
obsd.
obsd.
obsd.
obsd.
(calcd.)
(calcd.)
(calcd.)
(calcd.)
(calcd.)
17.52
21.51
7.22
25.08
9.57
(17.49)
(21.52)
(7.16)
(24.98)
(9.55)
rates. The decomposition starts at low temperature for
a slow heating rate and ends at low temperature. It
was observed that the reaction interval (Tf–Ti) is in-
creased with heating rate [20]. TG/DTA plot for the
thermal decomposition at a heating rate of 10°C min–1
is shown in Fig. 2.
Fig. 2 TG/DTA plot at heating rate 10°C min–1
The phenomenological data for the thermal de-
composition at 10°C min–1 are shown in Table 2.
From the TG curve at 10°C min–1, it is seen that the
complex starts to loss mass at 300°C, indicating the
thermal stability of the complex. This stability is due
to the bidentate nature of the ligand coordinating with
the metal, resulting in the formation of complex with
octahedral geometry. The first stage of the decompo-
sition of tris(ethylenediamine)nickel(II) sulphate cor-
responds to the loss of two molecules of ethylene-
diamine (36% loss) to give mono ethylenediamine
complex. The mono(ethylenediamine)nickel(II) sul-
phate is very unstable and suddenly decomposes at
360°C. The subsequent stages are overlapping and
hence not distinguishable.
decomposition carried at a single heating rate cont-
rary to our investigation. The obtained decomposition
profile differs from those of reported earlier.
Wendlandt and George [1] reported a single stage
decomposition pattern for the complex. The DTA
contained three endotherms. While Mitra et al. [21]
reported two stage decomposition pattern for
tris(ethylenediamine)nickel(II) sulphate. The DTA
pattern contained two endotherms and exotherms.
Independent pyrolysis was carried out in a muffle
furnace under non-isothermal conditions. Results are
given in Table 2, which are in good agreement with
the thermogravimetric results. The complex and the
intermediates were separated and identified by means
of XRD and IR. Figure 3 shows the IR spectra of the
complex, intermediate at 500°C and the final residue.
IR spectrum of the intermediate isolated at 500°C
contains the peaks corresponding toSO24– groups. The
peaks at 1110, 980 and 610 cm–1 are characteristics of
SO24– group [22].
Intermediate formed at 500°C was separated and
identified. This stage corresponds to the partial disso-
ciation of NiSO4 to a mixture of NiSO4 and NiO. This
was further confirmed by IR and XRD analyses. Mass
loss of 77.5% at 800°C corresponds to the formation
of NiO. During the temperature range (500-800°C) an
almost continuous process was observed for which
well defined DTA could not be obtained.
XRD powder patterns of the complex, intermedi-
ate at 500°C and the residue at 800°C are shown in
Fig. 4. Structural elucidation by single crystal X-ray
investigation revealed that tris(ethylenediami-
ne)nickel(II) sulphate crystallizes in trigonal space
group p31c with unit cell dimensions a=b=8.946 and
Thermal decomposition studies of tris(ethylene-
diamine)nickel(II) sulphate have been reported in the
literature [1, 21]. However both studies pertain to the
Table 2 Phenomenological data for the thermal decomposition of tris(ethylenediamine)nickel(II) sulphate (f 10°C min–1)
Independent pyrolysis
TG results
Mass loss/%
mass loss/%
Decomposition
Stages
Ti/°C Tf /°C Ts/°C
Theor.
Obs.
36.3
Theor.
36.5
Obs.
36
Residue
Loss of two
Ni(en) SO4
NiSO4
1
2
3
300
360
466
360
466
500
350
410
490
36.5
18
molecules of en
Loss of one
17.89
18
17.85
molecule of en
Decomposition
of NiSO4
Mixture of NiSO4
and NiO
Decomposition
of mixture of
696
786
77.7
77.5
77.7
78
NiO
4
NiSO4 and NiO
J. Therm. Anal. Cal., 93, 2008
215

REJITHA, MATHEW
Fig. 5 SEM pictures of a – Ni(en)3SO4 and b – NiO
Fig. 3 IR spectra of a – Ni(en)3SO4, b – mixture of NiSO4 and
NiO, c – NiO
Fig. 6 Variation of activation energy with conversion (a)
appearance and NiO (residue) is found to be as hollow
structure and got agglomerated.
A plot of activation energy vs. conversion func-
tion is given in Fig. 6. The kinetic parameters obtained
from FWO and KAS equations are almost equal while
higher values are obtained for Friedman method.
The activation energy varies with the extent of
conversion. Activation energy was found to be de-
creasing with the extent of conversion in the range
0.05 to 0.7 and then found to be increasing. The de-
creasing dependence of activation energy on the con-
version function can be concluded as a kinetic scheme
of endothermic reversible reaction followed by an ir-
reversible one. Increasing dependence of activation
energy with conversion indicates that reaction in-
volves parallel reaction [26–28].
Fig. 4 XRD pattern of a – Ni(en)3SO4, b – mixture of NiSO4
and NiO, c – NiO
c=9.634  [23]. The XRD peaks obtained are
matching with that of JCPDS file of the complex [24].
The XRD pattern of the intermediate isolated at 500°C
has (Fig. 4b) peaks of NiSO4 and NiO. However, the
majority peaks are those of NiO phase. The elemental
analysis of the intermediate separated confirmed the
presence of sulphur. The XRD analysis revealed that
the final residue is NiO [25]. NiO has got well defined
peaks with cubic symmetry. From the peak broadening
value particle size of NiO was calculated using
Scherrer equation. Average particle size was found to
be 17.47 nm. The SEM pictures of the complex
tris(ethylenediamine)nickel(II) sulphate and the
residue are shown in Fig. 5. The complex has a rod like
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