Thermal decomposition studies of [Ni(NH3)6]X 2 (X = Cl, Br) in the solid state using TG-MS and TR-XRD

Article abstract of DOI:10.1007/s10973-010-1054-8

Detailed investigation on the thermal behaviour of hexaamminenickel(II) chloride and hexaamminenickel(II) bromide has been carried out by means of simultaneous TG/DTA coupled online with mass spectroscopy (TG-MS) and temperature-resolved X-ray diffraction (TR-XRD). Evolved gas analyses by TG-MS revealed the presence of NH₂, NH, N₂ and H₂ fragments in addition to ammonia during the deamination process. These transient species resulted due to the fragmentation of the evolved ammonia during pyrolysis. The intermediates formed during the thermal deamination stages were monitored by in situ TR-XRD. The final product of the decomposition was found to be nano size metallic nickel in both cases. Morphology of the complexes, intermediates and the residue formed at various decomposition stages was analysed by scanning electron microscope (SEM). Kinetic analyses using isoconversional method for deamination and dehalogenation reaction show that the activation energies vary with the extent of conversion, indicating the multi-step nature of these solid state decomposition reactions.

Full text of DOI:10.1007/s10973-010-1054-8

J Therm Anal Calorim (2011) 103:515–523
DOI 10.1007/s10973-010-1054-8
Thermal decomposition studies of [Ni(NH3)6]X2 (X = Cl, Br)
in the solid state using TG-MS and TR-XRD
•
K. S. Rejitha T. Ichikawa S. Mathew
•
Received: 22 April 2010 / Accepted: 14 September 2010 / Published online: 8 October 2010
´
´
Ó Akademiai Kiado, Budapest, Hungary 2010
Abstract Detailed investigation on the thermal behaviour
of hexaamminenickel(II) chloride and hexaamminenick-
el(II) bromide has been carried out by means of simulta-
neous TG/DTA coupled online with mass spectroscopy
(TG-MS) and temperature-resolved X-ray diffraction
(TR-XRD). Evolved gas analyses by TG-MS revealed the
presence of NH2, NH, N2 and H2 fragments in addition to
ammonia during the deamination process. These transient
species resulted due to the fragmentation of the evolved
ammonia during pyrolysis. The intermediates formed dur-
ing the thermal deamination stages were monitored by in
situ TR-XRD. The final product of the decomposition was
found to be nano size metallic nickel in both cases. Mor-
phology of the complexes, intermediates and the residue
formed at various decomposition stages was analysed by
scanning electron microscope (SEM). Kinetic analyses
using isoconversional method for deamination and de-
halogenation reaction show that the activation energies
vary with the extent of conversion, indicating the multi-
step nature of these solid state decomposition reactions.
Introduction
Thermal decomposition studies of transition metal amine
complexes have extensively been investigated over years
[1–3]. A scan through the literature reveals that various
reports are available regarding the description of the
evolved gaseous species and the transient intermediates
formed during thermal deamination [3, 4]. Albeit these
conflicting reports, mostly all the studies take a simplified
approach in dealing the interpretations of the thermal
fragments during the deamination process, as all these
studies were enabled only with the aid of TG/DTA for
thermal analysis. Therefore, it was felt imperative to have a
detailed investigation on the evolved gaseous products to
ascertain the nature of the fragmented species without any
ambiguity and the structural/phase changes if any occur-
ring during the deamination event. A detailed investigation
of this sort warrants the use of techniques like thermo-
gravimetric-mass spectroscopy (TG-MS) and temperature-
resolved X-ray diffraction (TR-XRD).
Conventional thermoanalytical techniques like TG,
DTA and DSC do not tell the nature of the gaseous prod-
ucts. Evolved gas analysis (EGA) offers a useful tool to
identify the fragments which cannot be detected by other
techniques [5, 6]. TR-XRD is a powerful tool to study the
structural/phase changes occurring during a solid state
reaction as well as in identifying the reaction intermediates
in situ during pyrolysis. This method enables the recording
of a series of pattern, while the samples are heated con-
tinuously, stepwise or isothermally during the reaction.
These series contain information on the lattice of solids and
on the structural changes as a function of temperature [7].
The present investigation aims to explore a detailed thermal
decomposition behaviour of hexaamminenickel(II) chloride
and hexaamminenickel(II) bromide in the aforementioned
Keywords TG-MS ꢀ TR-XRD ꢀ SEM ꢀ Nano nickel ꢀ
Kinetics ꢀ Isoconversional methods
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
T. Ichikawa
Institute for Advanced Materials Research, Hiroshima
University, Higashi-Hiroshima 739-8530, Japan
123

516
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.
123

Thermal decomposition studies of [Ni(NH3)6]X2 (X = Cl, Br)
Kissinger–Akahira–Sunose (KAS) equation is [15, 16]
517
the DTA shows an endotherm with TP 136 °C corre-
sponding to this deamination stage. Broadly, the second
stage of decomposition comprises the release of two
ammonia molecules in the temperature range 140–280 °C.
(However, a very slow mass loss is observed in the TG
depicting the partial release of ammonia in the temperature
regime 140–200 °C. A close perusal of the DTA curve shows
a small endothermic (TP = 185 °C) peak indicating this slight
mass loss of 2%.) The MS peak (m/z 17) with very low
intensity at 184 °C and an intense ion peak (m/z 17) at 266 °C
substantiate the above observation. This second stage of
deamination (–2NH3) resulted in the formation of NiCl2
(mass loss 14.3%) as shown in Table 2.
ꢀ
ꢁ
/
T2
AR
E
ln
¼ ln
ꢁ
:
ð5Þ
EgðaÞ RT
From the above Eqs 3–5 the plots of ln / versus 1/T, ln
da/dt versus 1/T, ln //T2 versus 1/T 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.
Results and discussion
TG-MS studies of hexaamminenickel(II) halide
complexes
m/z 17
m/z 16
The results of elemental analyses are shown in Table 1 and
are in good agreement with those calculated theoretically,
which confirm the formation of the complexes. The phe-
nomenological details like temperature of inception Ti,
final temperature Tf and the temperature of summit Ts for
the thermal decomposition of hexaamminenickel(II) halide
complexes and the mass loss data are given in Table 2. The
plot of simultaneous TG/DTA coupled online with MS for
[Ni(NH3)6]Cl2 is given in Fig. 1. From the TG plot, it is
seen that the first stage of thermal decomposition of
hexaamminenickel(II) chloride is in the temperature range
79–140 °C and this stage corresponds to the release of four
ammonia molecules. Mass spectra show a strong peak with
m/z value 17, signaturing the presence of ammonia mole-
cules at 132 °C because of this deamination reaction. Also
m/z 2
m/z 15
m/z 28
m/z 18
m/z 14
0
–20
–40
–60
TG
DTA
–80
–100
Table 1 Elemental analysis/%
Complexes
Ni obsd/
calcd
H obsd/
calcd
N obsd/
calcd
Halogen
obsd/calcd
200
400
600
800
Temperature/°C
[Ni(NH3)6]Cl2
[Ni(NH3)6]Br2
24.9/25.3
17.8/18.3
8.1/7.8
6/5.7
36.1/36.3
26.6/26.2
31.1/30.6
50.1/49.9
Fig. 1 TG/DTA-MS plot for hexaamminenickel(II) chloride at
heating rate 10 °C min-1
Table 2 Phenomenological data for the thermal decomposition of hexaamminenickel(II) halides// = 10 °C min-1
Complexes
Stages
TG results
Percentage mass loss
Residue
Ti/°C
Tf/°C
Ts/°C
Theoretical
Stepwise
Observed
Theoretical
Cumulative
Observed
[Ni(NH3)6]Cl2
[Ni(NH3)6]Br2
1
2
3
79
140
609
140
132
29.3
14.7
30.6
28.9
14.3
31.8
29.3
44
28.9
43.2
75
Ni(NH3)2Cl2
NiCl2
280
250.7
727.8
743.8
74.6
Ni
1
2
3
90
180
155.5
239.6
716.8
21.2
10.6
49.9
21
10
49
21.2
31.8
81.7
21
31
80
Ni(NH3)2Br2
NiBr2
180
260
580.5
726.7
Ni
123

518
K. S. Rejitha et al.
The decomposition of NiCl2 commences at 609 °C and
decomposition of hexaamminenickel(II) bromide starts at
90 °C with the loss of four molecules of ammonia and this
is evidenced by the MS ion peak at 165 °C. The second
stage of deamination involves the liberation of two
ammonia molecules in the temperature range 180–260 °C,
and the corresponding ion peak with mass number 17 (in
Fig. 2) at 250 °C indicates the evolution of two ammonia
molecules. The observed intensity of the two MS peaks is
consistent with the amount of ammonia released viz., four
molecules of ammonia in the first stage and two molecules
of ammonia in the second stage of thermal decomposition.
The final mass loss of 49% corresponds to the formation of
metallic nickel as the final product of thermal decompo-
sition. The liberation of Br2 or its fragments, during the
final stage of decomposition, is not detected by the mass
spectral analysis. Corresponding thermal decomposition
stages appear as three endothermic peaks in DTA. From the
TG-MS data, the decomposition of hexaamminenickel(II)
bromide is shown below.
the mass loss of 31.8%, corresponding to the release of
chlorine resulted in the formation of metallic nickel at
744 °C as the residue. However, the evolution of Cl2 or its
fragments during the final stage of thermal decomposition
is not detected in the TG-MS. The DTA curve shows an
endotherm at 740 °C corresponding to the dechlorination
reaction. The overall thermal decomposition pattern for
hexaamminenickel(II) chloride can be written as follows:
Â
Ã
NiðNH3Þ6 Cl2 ! NiðNH3Þ2Cl2 þ 4NH3
NiðNH3Þ2Cl2 ! NiCl2 þ 2 NH3:
NiCl2 ! Ni þ Cl2
However, Tanaka et al. have reported that the two
ammonia molecules are released in two successive steps,
and NiCl2.NH3 is formed as an intermediate. According to
this report, the formation of NiCl2.NH3 takes place only at
relatively lower heating rates [23]. Since the TG-MS
analysis in the present study was done at 10 °C min-1, no
distinct and well-separated stages of deamination (–NH3) is
observed.
Â
Ã
NiðNH3Þ6 Br2 ! NiðNH3Þ2Br2 þ 4NH3
NiðNH3Þ2Br2 ! NiBr2 þ 2NH3
NiBr2 ! Ni þ Br2:
TG-MS analysis shows ion peaks with mass numbers 2,
14, 15, 16 and 28. These mass numbers indicate the presence
of fragments like H2, N, NH, NH2 and N2, respectively.
These species can be formed due to the fragmentation of the
liberated ammonia during pyrolysis [24, 25].
MS analysis has also detected peaks with m/z values 2, 18
and 28. The ion peak 18 may be due to the adsorbed
moisture. The other two peaks with mass numbers 2 and 28
may be due to H2 and N2 fragments formed by the
decomposition of ammonia as shown below.
The plot of simultaneous TG/DTA coupled online with
MS for [Ni(NH3)6]Br2 is shown in Fig. 2. The thermal
4NH3 ð17Þ ! 2 N2 ð28Þ þ 6H2 ð2Þ:
ð6Þ
Peaks with mass number 15 and 16 in the mass spectra
indicate the formation of NH and NH2 species formed by
the fragmentation of ammonia.
m/z 17
m/z
2
TR-XRD studies of hexaamminenickel(II) halide
complexes
m/z 15
For complementing the TG-MS analysis, in situ TR-XRD
studies were done for the first time to probe the structure/
stability of the phases formed during the deamination
stages. The non-isothermal X-ray diffraction series
(40–400 °C) is shown in Fig. 3. The TR-XRD pattern in
the range 40–60 °C contains characteristic peaks corre-
sponding to (111), (220), (222), (400) and (422) planes of
this complex (JCPDS no. 24-0803). On heating peaks
corresponding to these planes disappear to form new lattice
planes corresponding to the formation of diammine com-
plex by the loss of four molecules of ammonia. The pattern
at 120 °C is the intermediate structure between parent
hexaammine complex and the diammine species. The
pattern obtained at 140 °C represents the diammine spe-
cies. Peaks with two h values 15.7, 20, 30.2, 35.5 and 40.6
m/z 16
0
–20
–40
–60
–80
–100
TG
DTA
200
400
600
800
Temperature/°C
Fig. 2 TG/DTA-MS plot for hexaamminenickel(II) bromide at
heating rate 10 °C min-1
123

Thermal decomposition studies of [Ni(NH3)6]X2 (X = Cl, Br)
519
o NiBr2
o NiCl2
# Ni(NH3)2Br2
* [Ni(NH3)6]Br2
# Ni(NH3)2Cl2
* [Ni(NH3)6]Cl2
o
o
o
o
O
o
O
O
O
O
O
O
400 °C
400 °C
280 °C
O
O
O
o
260 °C
180 °C
#
#
#
#
#
#
#
#
#
#
#
#
#
#
140 °C
120 °C
120 °C
80 °C
*
*
*
*
*
*
*
*
*
*
*
60 °C
*
*
*
*
*
*
*
40 °C
40 °C
10
20
30
40
50
60
70
10
20
30
40
50
60
70
2 /°
2 /°
Fig. 3 TR-XRD pattern for [Ni(NH3)6]Cl2
Fig. 5 TR-XRD pattern for [Ni(NH3)6]Br2
of the diammine complex are in agreement with reported
refinement data of diamminenickel(II) chloride [26]. The
diammine complex is not stable and suddenly undergoes
decomposition by the release of two ammonia molecules.
At 280 °C, peaks corresponding to (003), (101) and (104)
planes of NiCl2 (JCPDS no. 71-2032) lattice start to appear
by the loss of remaining ammonia molecules. TG-MS data
also show ion peak corresponding to the evolution of
ammonia at this stage. NiCl2 begins to decompose above
400 °C to give Ni as the final residue. This is not recorded
as the temperature limit of the heating attachment of the
sample holder for TR-XRD is up to 400 °C. In order to
identify the final residue, the complex was heated in a
muffle furnace maintaining similar conditions as in a TG
furnace. Figure 4 shows the XRD pattern of the residue,
which shows the characteristic planes of metallic nickel
(JCPDS no. 04-0850). The average crystallite size of the
residue calculated by the Scherrer equation is found to be
*20 nm.
The TR-XRD pattern of [Ni(NH3)6]Br2 is shown in
Fig. 5. The patterns at 40–80 °C contain the peaks corre-
sponding to (111), (220), (222) and (400) planes of
[Ni(NH3)6]Br2 (JCPDS no. 24-0802). As the temperature
increases, all the planes except (111) plane of the hexa-
ammine complex disappear and new peaks start to appear,
this is due to the loss of ammonia from the hexaammine
complex. The TR-XRD pattern at 120 °C contains the
(111) plane of the hexaammine complex with lower
intensity. The pattern at 180 °C is due to the formation of
Ni(NH3)2Br2 by the loss of four molecules of ammonia as
evident from TG analysis. The diammine species also
contain the (111) plane of the hexaammine complex with
lower intensity. TR-XRD results show that (111) planes of
the parent hexammine complex become part of the inter-
mediate as well as the diammine complex lattice as can be
seen from the lowering of intensity of this peak in the
temperature range 100–180 °C. The main peaks obtained
for diamminenickel(II) bromide at 2h values 30.9, 32.6,
34.0, 39.2 and 44.5 match well with the reported values
[26]. The evolution of ammonia is confirmed by the TG-
MS results. Diammine complex is very unstable and starts
to decompose above 180 °C and ends at 260 °C by the
release of two ammonia molecules. TG-MS detected the
liberation of ammonia at this temperature range. At
260 °C, peaks corresponding to NiBr2 appear. The for-
mation of planes corresponding to (003), (101), (104),
(107) and (110) at 260 °C confirm the presence of NiBr2
*
* Ni
*
10
20
30
40
50
60
70
2 /°
Fig. 4 XRD pattern for metallic nickel formed from [Ni(NH3)6]Cl2
123

520
K. S. Rejitha et al.
*
* Ni
*
20
30
40
50
60
70
Fig. 7 SEM images of a [Ni(NH3)6]Cl2, b Ni(NH3)2Cl2, c NiCl2,
d Ni
2 /°
Fig. 6 XRD pattern for metallic nickel formed from [Ni(NH3)6]Br2
(JCPDS no. 73-0331). The final residue was separated by
heating the complex in a muffle furnace and analysed by
XRD. The pattern in Fig. 6 shows the presence of well-
crystalline fcc structure of nickel. The average crystallite
size of the residue calculated from the peak broadening
value is found to be 20 nm.
SEM analyses of the complexes
The SEM pictures of the complexes, intermediates and the
residue formed during thermal decomposition of
[Ni(NH3)6]Cl2 and [Ni(NH3)6]Br2 are shown in Figs. 7 and
8, respectively. All the decomposition products in each
stage were separated by heating the complex under iden-
tical heating rate in a programmable muffle furnace sim-
ulating the conditions of thermogravimetric experiments.
SEM pictures show that both the complexes appear as
octahedral crystals and the surfaces of the octahedra cor-
respond to (111) plane [27], while there is considerable
morphological change occurring for these complexes dur-
ing heating. Even though, the intermediates Ni(NH3)2Cl2
(Fig. 7b) and NiCl2 (Fig. 7c) formed from [Ni(NH3)6]Cl2
retain the octahedral structure, the topology of the surfaces
seems to be not as smooth as the original complex. Liter-
ature reports reveal that the intermediates of hexaam-
minenickel(II) chloride possess polymeric pseudo
octahedral geometry [23]. In Ni(NH3)2X2 (X = halogens),
each nickel ion is considered to be surrounded by four
bridging halogens in a square planar configuration and two
ammonia molecules located in trans position. Nickel ion
keeps the coordination number of six throughout the
decomposition process when ammonia molecules are
replaced stepwise by halogens [23]. The porous structure
can be due to the release of ammonia from the complex.
Fig. 8 SEM images of a [Ni(NH3)6]Br2, b Ni(NH3)2Br2, c NiBr2,
d Ni
The final residue, i.e. nickel metal, appears as flower-
shaped nano structure (Fig. 7d). SEM images show the
porous nature of Ni(NH3)2Br2 (Fig. 8b). NiBr2 is also
showing a porous structure (Fig. 8c), but it looks denser
than its precursor. These pores may be formed due to the
release of ammonia from the surfaces during heating.
Similar porous structures have been reported during the
thermal decomposition of [Ni(2-picolylamine)2(NO3)2] due
to the elimination of NO [28]. The final residue appears as
agglomerated and is shown in Fig. 8d. Morphology dif-
ference arises since these residues are formed from two
different precursors namely NiCl2 and NiBr2.
It is found that the size and morphology of nickel
nanoparticles can be tailored by merely changing the pre-
cursors. In this way, thermal decomposition can be used as
a facile and effective route for the synthesis of nano
materials with controlled morphology and this is quite
significance as the morphology and the size influence their
123

Thermal decomposition studies of [Ni(NH3)6]X2 (X = Cl, Br)
521
physical properties. Nickel nanoparticles have interesting
magnetic properties and have the potential in the usage as
data storage devices and in ferrofluids [29].
The decrease in E from 0.45 till the end of the reaction
may be an indication of the mass transfer-controlled dif-
fusion mechanism [30]. The second deamination stage
also shows a decreasing tendency of E with a. It is
reported that the kinetics of the decomposition of hexa-
ammine complex is limited by the heat and mass transfer
through the porous surfaces which is formed during the
decomposition process [32]. For the dechlorination step
(Fig. 9c),the activation energy increases in the a range
0.1–0.65 and decreases after 0.65. Millan et al. [32] have
reported that the decomposition of the hexaammine
complex is due to the pressure of the desorbed ammonia
gas which leads to the collapse of the structure at point,
line or plane defects.
Kinetic studies of hexaamminenickel(II) halide
complexes
The activation energies of the decomposition were calcu-
lated using isoconversional methods. Isoconversional
methods help to calculate activation energy as a function of
conversion without the prediction of a reaction model.
Existence of the dependence of E on conversion (a) is an
indication on the multi-step solid state reaction. The
effective activation energy of the process is a composite
value determined by the 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
as well as with the temperature. Hence, the variation of
E with a is an indication of multi-step kinetics [30]. From
the dependence of activation energy on the extent of con-
version, the nature of solid state reactions can be predicted.
The results of kinetic analyses calculated using isocon-
versional methods for the complexes are given in Figs. 9a–c
and 10a–c. In the case of hexaamminenickel(II) chloride, for
the first deamination step (Fig. 9a) the activation energy
(calculated by FWO and KAS methods) increases in the a
range 0.1–0.45 and then decreases after 0.45. The increasing
dependence 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 [31].
For hexaamminenickel(II) bromide, first deamination
(Fig. 10c) shows increasing dependence of E in the a range
0.05–0.4, thereafter the value of E is found to be decreasing.
The second deamination step shows a decreasing value of
E with a. The decrease in the activation energy indicates the
mass transfer controlled diffusion mechanism. For the
debromination step (Fig. 10c), activation energy increases in
the a range 0.05–0.85 and then tends to decrease. As men-
tioned above, the increasing dependence of E on a shows that
the reaction is competing or consecutive.
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 val-
ues 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 [33].
Fig. 9 Variation of E with a for
a deamination, b deamination
and c dechlorination for
[Ni(NH3)6]Cl2
350
(a)
(b)
280
FWO
KAS
Friedman
300
FWO
KAS
Friedman
260
250
240
200
150
100
50
220
200
180
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
140
130
120
110
100
90
(c)
FWO
KAS
Friedman
80
70
60
50
40
30
0.0
0.2
0.4
0.6
0.8
1.0
123

522
K. S. Rejitha et al.
Fig. 10 Variation of E with a for
a deamination, b deamination
and c debromination of
[Ni(NH3)6]Br2
300
250
200
150
100
50
(a)125
(b)
120
115
110
105
100
95
FWO
KAS
Friedman
FWO
KAS
Friedman
90
85
80
75
70
65
60
55
50
45
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
400
350
300
250
200
150
100
50
(c)
FWO
KAS
Friedman
0
0.0
0.2
0.4
0.6
0.8
1.0
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