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Chapter 5
Thermal Decomposition of Hexaamminenickel(II)
nitrate and Tris(ethylenediamine)nickel(II) nitrate
5.1 Introduction
In chapter 5, results of the investigations on the thermal decomposition of
hexaamminenickel(II) nitrate and tris(ethylenediamine) nickel(II) nitrate
are discussed. Thermal decomposition studies of nitrate containing
transition metal amine complexes are of significance because of its
exothermic decomposition and can be used in propellants, explosives and
pyrotechniques.1-4
The explosive decomposition of these complexes is
attributed to the simultaneous presence of both oxidising (nitrate) and
reducing (en or ammonia) groups in the same compound.1
At present TG-MS and TR-XRD studies have extensively been used to
follow the intermediates and various species formed during heating.5-6
Present investigation focus on the TG-MS and TR-XRD studies of
these complexes. The kinetics and mechanism were also evaluated for
the different decomposition reactions of hexaamminenickel(II) nitrate
and tris(ethylenediamine) nickel(II) nitrate. Model free isoconversional
methods viz., Flynn-Wall-Ozawa (FWO),7-8
Friedman,9 Kissinger-
Akahira-Sunose (KAS)10-11
in combination with non-mechanistic and
mechanism based equations were employed for the kinetic analysis.
148 Chapter 5
5.2 Experimental
5.2.1 Preparation of the complexes
The complexes were synthesized as per the standard procedure reported in
the literature.12
Hexaamminenickel(II) nitrate and tris(ethylenediamine)
nickel(II) nitrate were prepared by adding stoichiometric amount of
ammonia and ethylenediamine respectively to nickel(II) nitrate solution.
Ethyl alcohol was then added to precipitate the complex. The crystals were
separated and washed with water and dried in a vacuum desiccator. Nickel
content in the complexes was determined by gravimetry.13
The complexes
were further characterized by other spectral and chemical analysis.
5.2.2 Instrumentations
TG/DTA-MS studies were carried out in a thermogravimetric apparatus
(TG; Rigaku, TG-8120) combined with mass spectroscopy (Anelva, M-
QA200TS) under high-purity He (99.9999%) at a flow rate of 200 ml min-1
.
The heating rates employed were 5, 10, 15 and 20ºC min-1
.
DSC studies were carried out using a Shimadzu DSC-60 instrument at a
heating rate of 10ºC min-1
under flowing nitrogen (50 ml min-1
).
The elemental analyses were carried out using Vario El III instrument and
the analysis details are given in Table 3.1. X-ray powder patterns were
recorded on a Bruker D8 Advance diffractometer attached with a
programmable temperature device (TTK 450) from Anton Paar, (using Cu
Kα radiation, λ = 1.542 Å). 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 Scherrer equation,
Thermal Decomposition of Hexaamminenickel(II) nitrate … 149
t = 0.9 λ / β cos θ,
where t is the thickness of the particle, λ is the wave length, β is the line
broadening (FWHM) and cos θ is the corresponding angle.
5.3 Mathematical Treatment of Data
The kinetic and mechanistic aspects of the thermal decomposition of these
nickel amine nitrate complexes were investigated by means of
isoconversional methods, mechanism non-invoking and mechanism based
equations. The details are given in Chapter 4.
5.4 Results and Discussion
5.4.1 Thermal decomposition studies
5.4.1.1 Hexaamminenickel(II) nitrate
From thermogravimetry (TG) it can be seen that hexaamminenickel(II)
nitrate undergoes a three stage thermal decomposition. The different
stages of thermal decomposition are as follows:
[Ni(NH3)6](NO3)2 → Ni(NH3)5(NO3)2 + NH3 (Stage I)
Ni(NH3)5(NO3)2 → Ni(NH3)4(NO3)2 + NH3 (Stage II)
Ni(NH3)4(NO3)2 → NiO + 4NH3 + NO3 + NO2 (Stage III)
Simultaneous TG/DTA coupled online with mass spectral plot of
hexaamminenickel(II) nitrate is shown in Fig. 5.1.
150 Chapter 5
Fig. 5.1. TG/DTA-MS plot of [Ni(NH3)6](NO3)2 at 10ºC min-1
The complex starts to lose mass at 78ºC with the liberation of one
molecule of ammonia. The second stage at 116ºC is also a deamination
stage in which one molecule of ammonia is liberated to give an
intermediate tetraammine complex. Third stage involves (164-300ºC) the
simultaneous deamination and decomposition of the tetraammine complex
to give NiO as the final residue. The phenomenological details regarding
the decomposition viz., temperature of inception Ti, final temperature Tf,
temperature of summit Ts and the mass loss data are shown in Table 5.1.
Temperature / ºC
200 400 600 800
-100
-80
-60
-40
-20
0
Inte
nsi
ty /
a.
u.
ma
ss l
oss
/%
m/z 14m/z 15m /z 2
m/z 44
m/z 30
m/z 28
m/z 18
m/z 16
m/z 17
e
nd
o
∆T
ex
o
Thermal Decomposition of Hexaamminenickel(II) nitrate … 151
Table 5.1
Phenomenological data for the thermal decomposition of nickel
amine nitrate complexes at = 10ºC min-1
in He atmosphere
TG results Percentage weight loss
Stages Ti
(°C)
Tf
(°C)
Ts
(°C) Theoretical Observed Residue
[Ni(NH3)6](NO3)2 Step
wise Cumulative Stepwise Cumulative
I 78 116 87 5.97 5.97 5.7 5.7 Ni(NH3)5(NO3)2
II 116 164 136 5.97 11.9 5.8 11.5 Ni(NH3)4(NO3)2
III 164 300 266 61.9 73.8 61.3 72.8 NiO
[(Ni(en)3] (NO3)2
220 300 262 79.4 78 NiO
The DTA and DSC profiles are shown in Fig. 5.1 and Fig. 5.2
respectively. Both the DTA and DSC traces show three endotherms and
one intense exotherm.
Fig. 5.2. DSC plot of [Ni(NH3)6](NO3)2 at 10ºC min-1
0 50 100 150 200 250 300
-5
0
5
10
15
he
at f
low
/mw
flo
w/m
w
o Temperature / C
152 Chapter 5
The temperature of inception (Ti), temperature of completion (Tf) and the
peak temperature (Tp) from DTA and DSC are given in Table 5.2.
Table 5.2
DTA and DSC peak temperatures for the thermal decomposition of
hexaamminenickel(II) nitrate
Stages Ti (°C) Tf (°C) Tp (°C)
I
DTA
87 116 107
II 116 162 157
III 162 230.8 211.5
IV 230.8 295 278
I
DSC
94.5 121.1 113.5
II 125.2 158.5 142.2
III 204.1 223 208.7
IV 268.9 294.5 281.3
The first two endotherms correspond to the liberation of one molecule of
ammonia each to form tetraammine complex. The third small endotherm
is immediately followed by a sharp exotherm and is due to the initial
decomposition of the tetraammine complex.
The sharp exothermic peak can be explained on the basis of the
decomposition of Ni(NO3)2, overlapping the deamination reaction. Nickel
nitrate on decomposition gives rise to oxides of nitrogen and oxygen as
detected by TG-MS (vide infra) (scheme 1) which in turn would oxidize
the liberated ammonia.1 In TG curve, simultaneous deamination and
decomposition appear as a single stage whereas in DTA and DSC traces
the initial deamination appear as an endotherm.
Thermal Decomposition of Hexaamminenickel(II) nitrate … 153
TG-MS plot reveals the presence of ion peak with mass number 17 in the
temperature range 78-164ºC, indicating the evolution of ammonia. Along
with the peaks of ammonia, ion peaks with mass numbers 15 and 16 were
also observed during this temperature range. This could be due to the
presence of NH and NH2 ion species. These fragments were formed by the
thermal fragmentation of the evolved ammonia. The peaks corresponding to
m/z values 14, 16, 28, 30, 32 and 44 indicate the presence of ions like N, O,
N2, NO, O2 and N2O formed by the fragmentation of NO3 group. The
possible fragmentation pattern for NO3 is shown below.
Scheme 1. Fragmentation of NO3
The oxidation of ammonia also generates gaseous species like NO and N2O.
The presence of ion peak with m/z value 18 can be attributed to the formation
of water. The formation and evolution of water has been reported during the
thermal decomposition of Co(NH3)6(NO3)2, Pd(NH3)2(NO2)2 and
Pt(NH3)2(NO2)2 complexes.14-15
The formation of water due to the reaction
between the evolved gaseous products is shown below.15
2 NH3 + 3 O2 + N2 → 2 NO + N2O + 3 H2O
or 2 NH3 + 2.5 O2 → 2 NO + 3 H2O
Scheme 2. Formation of water from gaseous products
2 NO3
N2 (28) or 2N (14) + 3 O2 (32) N2O (44) + 5O (16)
I
II
2 NO (30) + 2O2 (32)
III
154 Chapter 5
5.4.1.2 Tris(ethylenediamine)nickel(II) nitrate
Fig. 5.3 shows the simultaneous TG/DTA coupled online with MS for
tris(ethylenediamine)nickel(II) nitrate. The complex starts to decompose
at 220ºC with an initial mass loss of ~ 7% followed by a large mass loss
of 65.3% to give nickel oxide as the residue. In general, the
decomposition of the complex is as follows
[Ni(en)3](NO3)2 NiO + gaseous products
The gaseous products include the oxides of nitrogen and ethylenediamine.
It is seen from the mass spectra (scheme 1) that the possible gaseous
products like NO3 and NO2 are fragmented to different species like N, O,
N2, NO and N2O at elevated temperature. The fragmentation possibility of
nitrate group and ethylenediamine are shown in scheme 1 and 3
respectively.
Fig. 5.3. TG/DTA-MS plot of [(Ni(en)3] (NO3)2 at 10ºC min-1
e
nd
o
∆T
ex
o
200 400 600 800
-100
-80
-60
-40
-20
0
m/z 18
m/z 2
m/z 30
m/z 44m/z 17
m/z 16
m/z 14
m/z 28
In
ten
sity
/a.
u.
mas
s lo
ss/
%
Temperature / ºC
Thermal Decomposition of Hexaamminenickel(II) nitrate … 155
0 50 100 150 200 250 300
-10
0
10
20
30
40
Hea
t fl
ow
/mw
Temperature / o
C
The thermal stability of tris(ethylenediamine)nickel(II) complex is high
compared to that of nickel hexaammine complex and is due to the
chelating effect.1
The phenomenological data regarding the decomposition
of tris(ethylenediamine)nickel(II) nitrate are given in Table 5.1.
The DTA and DSC curves are shown in Fig. 5.3 and Fig. 5.4 respectively.
Both the DTA and DSC curves show one small endotherm followed by a
sharp exothermic peak.
Fig. 5.4. DSC plot of [Ni(en)3](NO3)2 at 10ºC min-1
The small endotherm is due to the partial initial deamination (loss of
ethylenediamine) in the tris(ethylenediamine) complex and the exotherm
is due to the insitu oxidation of the liberated ethylenediamine.
Thermoanalytical data from DTA and DSC for tris(ethylenediamine)
complex are given in Table 5.3.
156 Chapter 5
Table 5.3
DTA and DSC peak temperatures for the thermal decomposition of
tris(ethylenediamine)nickel(II) nitrate
Stages Ti (°C) Tf (°C) Tp (°C)
I
II
DTA 219.9 245.7 228
245.7 272 262
I
II
DSC
213.8 228.9 218
236.6 276.9 253.7
In the mass spectra (Fig. 5.3), the ion peaks with mass numbers 2, 14, 16,
17, 18, 28, 30 and 44 were observed during the thermal decomposition of
this complex. These peaks appear in the temperature range 260-290ºC and
correspond to the formation of ion species like H2, N, NH2, NH3, H2O,
N2/C2H4, NO and N2O. The detection of oxides of nitrogen formed from
the fragmentation of NO3- group in the temperature range 260-290ºC,
surmise the presence of Ni(NO3)2 phase during the thermal decomposition
of the tris(ethylenediamine) complex. The formation of Ni(NO3)2 as an
intermediate phase has been further confirmed by TR-XRD analysis (vide
infra). Among the ion peaks H2, N, NH3 and N2 are formed by insitu
fragmentation of the evolved ethylenediamine as shown below.
3 NH2-CH2-CH2-NH2
Scheme 3. Fragmentation of ethylenediamine
4 NH3 (17) + 3 CH2 = CH2 (28) + N2 (28) or 2 N
2 N2 (28) or 4 N + 6 H2 (2)
Thermal Decomposition of Hexaamminenickel(II) nitrate … 157
20 40 60
*
oo
##
##
#
#
##
#
##
#
##
***
**
**
*
**
*
*
*
*
*
280oC
260oC
240oC
220oC
180oC
160oC
140oC
120oC
100oC
80oC
60oC
o NiO
# Ni(NO3
)2
* [Ni(NH3
)]6
(NO3
)2
(200)
***(222)*
*
two theta/ degrees
Inte
nsi
ty/
a.u
40oC
300oC
*
(111)
200oC
(111) (220)(311) (420)
(511)
(222) (400)
(400)
Mass numbers 16, 30 and 44 correspond to the presence of O, NO and
N2O formed by the fragmentation of NO3- group
as shown in scheme 1.
Detection of water (m/z 18) possibly formed by the reaction between the
gaseous products (scheme 2) was also observed in the mass spectral
analysis of tris(ethylenediamine) complex.
5.4.2 Temperature resolved X-ray diffraction studies
5.4.2.1 Hexaamminenickel(II) nitrate
In order to complement the TG-MS results, insitu temperature resolved X-
ray diffraction patterns (TR-XRD) were recorded to identify the
structure/stability of the phases formed during the deamination stages of
the hexaamminenickel(II) nitrate. The typical non isothermal diffraction
series with a step wise heating of 20ºC per patterns from 40-300ºC for
[Ni(NH3)6](NO3)2 are shown in Fig. 5.5.
Fig. 5.5. TR-XRD pattern of [Ni(NH3)6](NO3)2
158 Chapter 5
The temperature resolved X-ray diffraction patterns in the range 40-80ºC
contain peaks corresponding to (111), (220), (222), (311), (400), (420)
and (511) planes and can be indexed to the cubic lattice of the complex
(JCPDS No. 45-0027). The temperature resolved XRD patterns in the
temperature range 100-140ºC show the structural changes occurred due to
the deamination resulting in the formation of nickel nitrate at 160ºC. The
peak corresponding to (420) plane at 2θ 37.2º of hexaammine complex
appears with very low intensity in the temperature range (100-140ºC),
indicating that this plane forms part of the lattice plane of the intermediate
complex structure formed due to the deamination. The intermediate
complex decomposes to give Ni(NO3)2 at 160ºC and is stable up to 280ºC
and the peaks at 42.68 and 49.92º 2θ are indexed as (222) and (400)
planes of Ni(NO3)2 (JCPDS no. 74-2261). At 300ºC, NiO is formed as the
residue by the decomposition of Ni(NO3)2 and the peaks at 37.39 and
43.44º 2θ correspond to the (111) and (200) planes of NiO (JCPDS no.
47-1049). Average crystallite size of NiO calculated from the peak
broadening value using Sherrer equation is 25.5 nm.
5.4.2.2 Tris(ethylenediamine)nickel(II) nitrate
The temperature resolved X-ray diffractograms for the thermal
decomposition of [Ni(en)3](NO3)2 are shown in Fig. 5.6.
At 40ºC the pattern corresponds to the tris(ethylenediamine)nickel(II)
nitrate and the peaks at 11.74, 15.76, 20.39 and 26.24º 2θ indicate the
different planes of the complex. In the temperature range 100-200ºC the
complex undergoes thermal decomposition resulting in the intensity
reduction of these peaks. At 220ºC the deamination (release of 3
ethylenediamine molecules) process was completed resulting in the
Thermal Decomposition of Hexaamminenickel(II) nitrate … 159
30 60
o
##
##
##
#
o
****
*
*
*
*
*
**
***
o
280oC
260oC
240oC
220oC
180oC
160oC
140oC
120oC
100oC
80oC
60oC
two theta/ degrees
#
* * *
(222)
Inte
nsi
ty/
a.u
*
NiO
# Ni(NO3
)2
* [Ni(en)3
](NO3
)2
(400)
(111)(200)
40oC
200oC
300oC
formation of Ni(NO3)2 as intermediate in the temperature range 220-
280ºC. The peaks at 42.91 and 49.97º 2θ correspond to (222) and (400)
planes of Ni(NO3)2 (JCPDS no. 74-2261).
Fig. 5.6. TR-XRD pattern of [(Ni(en)3] (NO3)2
The intermediate Ni(NO3)2 phase is stable till 280ºC and on further
heating decomposes to NiO as the residue. The peaks at 37.29 and 43.43º
2θ correspond to the (111) and (200) planes of NiO. Average crystallite
size of NiO calculated from the peak broadening value using Sherrer
equation is 23 nm.
5.4.3 SEM analysis
5.4.3.1 Hexaamminenickel(II) nitrate
The SEM images of hexaamminenickel(II) nitrate and the NiO residue
are shown in Figs. 5.7 a-b. NiO formed (Fig. 5.7 b) by the decomposition
of hexaamminenickel(II) nitrate is in the nano range (20nm).
160 Chapter 5
Fig. 5.7. SEM pictures of (a) [Ni(NH3)6](NO3)2 (d) NiO
5.4.3.2 Tris(ethylenediamine)nickel(II) nitrate
The SEM images of tris(ethylenediamine)nickel(II) nitrate and the NiO
residue are given in Figs. 5.8 a-b. The SEM image shows that nano NiO
(20 nm) (Fig. 5.8 b) formed is highly porous in nature. The porosity of the
residue may be due to the escape of gaseous products during pyrolysis.
Fig. 5.8. SEM pictures of (a) [(Ni(en)3] (NO3)2 (b) NiO
Morphologies of the nano NiO residues formed by the thermal
decomposition of these two amine complexes are different, as they are
formed from different precursors. These findings are of significance since
the morphology of nickel oxide nanoparticles can be tailored by merely
changing the precursors.
(a) (b)
(a) (b)
Thermal Decomposition of Hexaamminenickel(II) nitrate … 161
0.0 0.2 0.4 0.6 0.8 1.0
110
115
120
125
130
135
140
145
150
155
160
FWO
KAS
Freidman
E/
kJ
mo
l-1
alpha
5.4.4 Kinetics and mechanism
The first two clear cut and non overlapping deamination reactions of
hexaamminenickel(II) nitrate involving the loss of one ammonia molecule
in each stages are subjected to kinetic analysis.
5.4.4.1 Kinetic analysis using isoconversional approach
Hexaamminenickel(II) nitrate
By employing isoconversional methods, a series of activation energies were
obtained as a function of conversion for the two deamination stages of the
hexaammine complex. Variations of activation energy with conversion for
the first and second deamination stages for the hexaamminenickel(II)
nitrate complex are shown in Figs. 5.9-5.10 and the corresponding values
are given in Table 5.4. The activation energy calculated by FWO and KAS
equations for the deamination reaction from hexaamminenickel(II) nitrate
to pentaamminenickel(II)nitrate (Fig. 5.9) exhibited a decreasing
dependence of activation energy with conversion. This decreasing
dependence of activation energy with conversion is attributed to the
reversible nature of the reaction.16
Fig. 5.9. Variation of activation energy with conversion (α) for the
deamination reaction of [Ni(NH3)6](NO3)2 to Ni(NH3)5(NO3)2
162 Chapter 5
0.0 0.2 0.4 0.6 0.8 1.0
130
140
150
160
alpha
E /
kJ
mo
l-1
FWO
KAS
Friedman
For the deamination reaction i.e pentaamminenickel(II) nitrate to
tetraamminenickel(II) nitrate, activation energy also shows a decreasing
dependence with conversion (α). The decreasing dependence shows that
the reaction is reversible, which is very common to many solid state
reactions of the type solid ↔ solid + gas.16
Fig. 5.10. Variation of activation energy with conversion (α) for the
deamination reaction of Ni(NH3)5(NO3)2 to Ni(NH3)4(NO3)2
The shapes of the plots showing the dependence of activation energy on
the extent of conversion are similar for the three equations viz., FWO,
KAS and Friedman. However, for the Friedman method the activation
energy values are comparatively higher as this method is very sensitive to
experimental noise.17
Thermal Decomposition of Hexaamminenickel(II) nitrate … 163
Table 5.4
Activation energy and conversion () values for deamination reactions of
hexaamminenickel(II) nitrate
Stage 1 Stage II
5.4.4.2 Kinetic analysis using non-mechanistic approach
Hexaamminenickel(II) nitrate
The kinetic parameters evaluated for the first two deamination stages of
the hexaamminenickel(II) nitrate decomposition using the non-mechanism
based equations are given in Table 5.5.
alpha E (kJ mol-1
)
FWO KAS Friedman
0.05 138.6 139.6 157.7
0.1 139.1 140.1 158.9
0.2 135.9 132.1 150.8
0.3 130.6 132.8 147.9
0.4 128.1 127.1 146
0.5 127.2 126.2 145.5
0.6 127.1 125 139.1
0.7 125.1 124 134.8
0.8 124.2 122 133
0.9 123 121 131
1 122 120 134.4
alpha E (kJ mol-1
)
FWO KAS Friedman
0.05 157.5 153.4 155.8
0.1 155.9 152.4 157.9
0.2 151.2 147.8 155
0.3 147.9 147.9 153.6
0.4 139.7 141.4 157.1
0.5 138.4 140.6 154.5
0.6 137.4 139.7 158.7
0.7 135.5 133.6 152.3
0.8 136.4 136.8 145.8
0.9 130.9 135.9 140.6
1 131.4 135.9 144.3
164 Chapter 5
Table 5.5
Kinetic parameters for the two deamination stages of hexaamminenickel(II)
nitrate decomposition from non-mechanistic equations
Stage1 Stage II
n 1.47 1.49
E (kJ mol-1)
CR 125.80 101.37
MKN 125.82 100.65
HM 161.25 105.46
MT 124.60 99.62
A (s-1)
CR 1.23×1014 5.95×1010
MKN 1.34×1014 5.01×1010
HM 2.83×1020 1.83×1011
MT 8.23×1016 3.40×1010
∆S≠ (JK-1 mol-1)
CR -2.34×101 -4.14×101
MKN -1.61×101 -4.28×101
HM -1.44×102 -3.21×101
MT -7.65×101 -4.61×101
r
CR 0.9982 0.9976
MKN 0.9982 0.9976
HM 0.9982 0.9964
MT 0.9984 0.9979
CR, Coats–Redfern; MKN, Madhusudanan-Krishnan-Ninan; HM, Horowitz-Metzger;
MT, MacCallum-Tanner.
Kinetic parameters computed from the four non-mechanistic equations are
comparable. The correlation coefficient values in the table show good
linear fit. The comparatively higher values of kinetic parameters obtained
using Horowitz-Metzger equation are due to the inherent error involved in
the approximation employed in the Horowitz-Metzger equation. It can be
Thermal Decomposition of Hexaamminenickel(II) nitrate … 165
seen from the Table that the order parameters are fractional numbers. It is
known from the literature18
that the order parameters can have decimal
number. The entropy of activation for the two deamination stages shows
negative values. The negative values of entropy indicate that the activated
complex has an ordered structure than the reactant and the reaction in this
case is said to be slower than the normal.19
The kinetic parameters (E and
A) calculated for the first deamination stage (i.e. hexaamminenickel(II)
nitrate to pentaamminenickel(II) nitrate) show higher values compared to
the second stage of deamination (pentaamminenickel(II) nitrate to
tetraamminenickel(II) nitrate).
5.4.4.3 Deduction of reaction mechanism
Hexaamminenickel(II) nitrate
The kinetic parameters (viz., E, A) and the correlation coefficient obtained
for the two deamination stages of hexaamminenickel(II) nitrate
decomposition evaluated using the mechanism based equations are given in
Table 5.6. Among the two deamination reactions of hexaamminenickel(II)
nitrate, the best linear fits (r = 0.9947 and 0.9913) are obtained for the
Mampel equation (eqn. no. 5) and therefore the rate controlling process for
these two deamination stages are assigned as random nucleation with the
formation of one nucleus on each particle.
166 Chapter 5
Table 5.6
Kinetic parameters for the two deamination stages of
hexaamminenickel(II) nitrate from mechanistic equations
Mechanistic eqns no. Stage1 Stage II
1
E
A
r
152.1
6.95×1016
0.9706
116.3
7.79×1011
0.9509
2
E
A
r
167.9
6.11×1018
0.9792
129.55
2.54×1013
0.9649
3
E
A
r
189.9
1.73×1021
0.9889
148.6
2.17×1015
0.9813
4
E
A
r
175
1.38×1019
0.9829
135.6
3.85×1013
0.9711
5
E
A
r
104.1
1.03×1011
0.9947
81.8
1.27×108
0.9913
6
E
A
r
48.8
3.21×103
0.9939
374
2.24×102
0.9893
7
E
A
r
30.3
8.08×100
0.9928
22.6
2.13×100
0.9867
8
E
A
r
86.3
1.45×108
0.9836
66.1
4.34×105
0.9714
9
E
A
r
91.7
5.79×108
0.9879
70.8
1.30×106
0.9792
E - kJ mol-1
, A - s-1
5.5 Conclusions
Investigations on the thermal behaviour of hexaamminenickel(II) nitrate and
tris(ethylenediamine)nickel(II) nitrate have been carried out using simultaneous
Thermal Decomposition of Hexaamminenickel(II) nitrate … 167
TG/DTA coupled online with mass spectroscopy (MS) and temperature
resolved X-ray diffraction (TR-XRD) techniques. Hexaamminenickel(II)
nitrate undergoes a three stage thermal decomposition and tris(ethylenediamine)
nickel(II) nitrate undergoes two stage thermal decomposition. Decomposition
reactions involve highly exothermic oxidation reactions. Evolved gas
analysis by MS studies shows that along with ammonia, ion species like
NH2 and NH are formed during the deamination process. These species
could be formed due to the fragmentation of the evolved ammonia during
pyrolysis. The decomposition of nickel nitrate generates gaseous products
like N, N2, NO, O2 and N2O. Oxidation of NH3 evolves N2O, NO and
water and these species are detected during pyrolysis. Thermal
fragmentation of ethylenediamine produces various ion fragments like
NH3, CH2 = CH2, N2, N and H2. The formation of the intermediates was
monitored by insitu TR-XRD. The final residues during the thermal
decomposition of both these amine complexes are highly crystalline NiO
nanoparticles. Kinetic analysis of both stages of deamination of
[Ni(NH3)6](NO3)2 using isoconversional methods show that the activation
energies vary with the extent of conversion, indicating the multi step
nature of solid state reactions. Using the mechanism based equations it is
deduced that the rate controlling processes for the thermal deamination
reactions of nickel hexaammine complex are random nucleation with the
formation of one nucleus on each particle.
168 Chapter 5
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