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K. S. Rejitha et al.
residues were determined using JEOL JSM-6390 scanning
electron microscope (SEM). For SEM analyses, the samples
were spread on a carbon tape and made uniform by blowing
air.
context. To the best of our knowledge, TG-MS studies on the
nature of the gaseous species evolved and the TR-XRD
studies on the structural changes happening during the
thermal decomposition of the title complexes have not been
reported. Thermolysis of materials often leads to the for-
mation of metal oxides or metals in the nano range [8, 9].
Mathew et al. [2] have reported the formation of ultrafine
metallic copper wires by the thermal decomposition of
tris(ethylenediamine)copper(II) halides. Syntheses of nano
NiO or Ni by the thermal decomposition of nickel complexes
have also been reported [10, 11]. These findings are of
importance as it opens up a strategy for the synthesis of
ultrafine nano metals and metallic oxides with controlled
morphology in contemporary material research. This aspect
also makes the thermal decomposition studies of amine
complexes quite significant. The kinetics of deamination and
dehalogenation of hexaamminenickel(II) halides have been
studied using model-free isoconversional methods viz.,
Flynn–Wall–Ozawa (FWO) [12, 13], Friedman [14] and
Kissinger–Akahira–Sunose (KAS) [15, 16].
Kinetic studies
Isoconversional methods
Solid state reaction often follows the basic kinetic equation
dðaÞ
dt
ꢁE
¼ A exp
fðaÞ;
ð1Þ
RT
where A is the pre-exponential factor, E is the activation
energy, R is the gas constant and T is the temperature, f(a)
is the kinetic model function. For a non-isothermal
reaction, Eq. 1 can be written as
dðaÞ
fðaÞ
A
ꢁE
dT
dt
¼
exp
dT; where the heating rate / ¼
:
ð2Þ
/
RT
The solid state reactions involve complex reaction steps,
and a single rate equation is unable to explain the com-
plexities of solid state reactions. In this context, isocon-
versional method could be used as an alternative to study
the solid state reactions. Model-free isoconversional
methods are a versatile way to investigate the kinetics of
solid state reactions as these methods possess several
advantages over the conventional methods [19–22]. For a
single step reaction, E is constant over the whole conver-
sion function. For multi-step kinetics, E varies with the
extent of conversion, and this reflects the variation in rel-
ative contributions of single steps to the overall reaction
rate. Friedman, FWO and KAS are isoconversional meth-
ods and are frequently employed to study the kinetics.
These methods yield effective activation energy (E) as a
function of extent of conversion (a).
Experimental
The nickel ammine halide complexes were synthesized as
per the procedure reported in the literature [17]. Nickel
content in the complexes was determined by gravimetry
[18]. The complexes were further characterized by spectral
and chemical analysis. The halide content in the complexes
was determined by Volhard’s method [18].
Instrumentations
TG-MS studies were carried out in a thermogravimetric
apparatus (TG; Rigaku, TG-8120) combined with mass
spectroscopy (Anelva, M-QA200TS) under high-purity He
gas flow (99.9999%). The heating rates employed were 5, 10,
15 and 20 K min-1. For the TG/DTA analyses, the sample
mass used were 10 0.2 mg for all the experiments.
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 Kissinger–Akahira–Sunose (KAS)
methods are based on multiple heating rate experiments.
Flynn–Wall–Ozawa equation is as follows [12, 13]
The elemental analyses were carried out using Vario
Elemental III instrument. X-ray powder patterns were
recorded on a Bruker D8 Advance diffractometer attached
with a programmable temperature device from Anton Paar
AE
R
E
ln / ¼ ln
ꢁ ln gðaÞ ꢁ 5:3305 ꢁ 1:052
ð3Þ
˚
RT
(TTK 450) up to 400 °C (using Cu Ka radiation, k = 1.542 A).
The measurements were performed by placing the sample
on a flat sample holder, while the samples were heated by a
programmable temperature controller. Crystallite size was
calculated using the Scherrer equation,
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.
Friedman equation is [14]
t ¼ 0:9k=b cos h;
wheretisthethicknessofthe particle, kisthe wave length,bis
the line broadening (in radians) and cos h is the corresponding
angle. Morphology of the complexes, intermediates and
da
dt
E
ln
¼ ln½AfðaÞꢂ ꢁ
;
ð4Þ
RT
where da/dt is the rate of conversion and f(a) is the
mechanism function.
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