TG–MS study on the kinetics and mechanism of thermal decomposition of copper ethylamine chromate, a new precursor for copper chromite catalyst

Article abstract of DOI:10.1007/s10973-015-5207-7

Copper chromite is a well-known burn rate modifier for the combustion of composite solid propellants. In this study, basic copper ethylamine chromate (CEC), a new precursor for copper chromite catalyst, was synthesized by precipitation method. The thermal decomposition of the precursor was followed by thermogravimetry–mass spectroscopy (TG–MS) and X-ray diffraction techniques and compared with that of copper ammonium chromate, a conventional precursor for copper chromite catalyst. TG–MS analysis for the decomposition of CEC revealed that the decomposition starts with the liberation of ethylamine. The change in enthalpy for the decomposition reaction of copper ethylamine chromate was higher than that of copper ammonium chromate due to the oxidation of ethyl group. The reducing atmosphere created by the presence of carbon during the decomposition of CEC produced a mixture of Cu, CuCr₂O₄, CuCrO₂ and CuO, while the oxidizing atmosphere of copper ammonium chromate produced a mixture of CuCr₂O₄ and CuO. Mechanistic study based on Criado and Coats–Redfern methods showed that CEC follows random nucleation (F1) mechanism as the rate-determining step for the thermal decomposition process.

Full text of DOI:10.1007/s10973-015-5207-7

J Therm Anal Calorim
DOI 10.1007/s10973-015-5207-7
TG–MS study on the kinetics and mechanism of thermal
decomposition of copper ethylamine chromate, a new precursor
for copper chromite catalyst
Sanoop Paulose1 Deepthi Thomas1 T. Jayalatha1 R. Rajeev1
Benny K. George1
•
•
•
•
Received: 1 September 2015 / Accepted: 12 December 2015
´
´
Ó Akademiai Kiado, Budapest, Hungary 2015
Abstract Copper chromite is a well-known burn rate
modifier for the combustion of composite solid propellants.
In this study, basic copper ethylamine chromate (CEC), a
new precursor for copper chromite catalyst, was synthe-
sized by precipitation method. The thermal decomposition
of the precursor was followed by thermogravimetry–mass
spectroscopy (TG–MS) and X-ray diffraction techniques
and compared with that of copper ammonium chromate, a
conventional precursor for copper chromite catalyst. TG–
MS analysis for the decomposition of CEC revealed that
the decomposition starts with the liberation of ethylamine.
The change in enthalpy for the decomposition reaction of
copper ethylamine chromate was higher than that of copper
ammonium chromate due to the oxidation of ethyl group.
The reducing atmosphere created by the presence of carbon
during the decomposition of CEC produced a mixture of
Cu, CuCr2O4, CuCrO2 and CuO, while the oxidizing
atmosphere of copper ammonium chromate produced a
mixture of CuCr2O4 and CuO. Mechanistic study based on
Criado and Coats–Redfern methods showed that CEC
follows random nucleation (F1) mechanism as the rate-
determining step for the thermal decomposition process.
Introduction
Composite solid propellants (CSP) are the major source of
chemical energy for the propulsion of space vehicles and
missiles. CSP are heterogeneous mixtures consisting of an
oxidizer, usually ammonium perchlorate (AP), a metallic
fuel like aluminium powder and a fuel-cum-binder, gen-
erally hydroxyl-terminated polybutadiene. In addition, they
contain curing agents, plasticizers, bonding agents and
deoxidizers for improving their mechanical and storage
properties and burn rate modifiers for improving burning
rate. Being complex mixtures, their combustion behaviour
is quite complex. Studies on various kinds of catalysts and
additives for controlling the burn rate of CSP and the
thermal decomposition kinetics of its oxidizer, AP, were
well documented in the literature [1–6].
The most commonly used and effective burn rate
modifiers are transition metal oxides (TMO), especially
copper chromite and iron oxide [7]. The catalytic system of
copper chromite, CuO.CuCr2O4, first reported by ‘Adkins’,
has wide commercial application as catalyst for chemical
reactions such as hydrogenation, dehydrogenation, oxida-
tion and alkylation [8]. In addition to versatile commercial
applications as a catalyst, copper chromite finds application
as a ballistic modifier for composite solid propellants [9].
Copper chromite accelerates the thermal decomposition of
AP, thereby increasing the burn rate of propellant com-
bustion [10, 11]. In AP decomposition, copper chromite
acts as heterogeneous catalyst, facilitating the decomposi-
tion of perchlorate/perchloric acid formed on the catalyst
surface as well as the oxidation of ammonia [12].
Keywords Copper ethylamine chromate Á Copper
chromite Á Burn rate modifier Á Thermal decomposition
& R. Rajeev
arerajeev@gmail.com
Sanoop Paulose
sanoop_ap@vssc.gov.in
The catalytic activity of copper chromite depends on its
chemical nature, particle size and surface area. The chemical
and physical characteristics of copper chromite catalyst vary
according to the method of preparation [13, 14]. The type of
1
Analytical and Spectroscopy Division, Analytical,
Spectroscopy and Ceramics Group Propellants, Polymers,
Chemicals and Materials Entity, Vikram Sarabhai Space
Centre, Trivandrum 695022, India
123

S. Paulose et al.
precipitating agent used for the preparation of precursor to
copper chromite, pH of the precipitating medium, thermal
decomposition temperature, etc. can affect the catalytic
activity of copper chromite.
spectrometer was used for the TG–MS study. The crystal
phase of the precursors and non-volatile decomposition
products were identified by Bruker D8-Discover powder
X-ray diffraction spectrometer using CuKa radiation
(1.5418 A°) at a scan rate of 2.5° min-1. Simultaneous
TG–DSC, TA Instruments Q600, was employed for eval-
uating the thermal decomposition of the precursors. In all
the TG–DSC experiments, a sample mass of 5 1 mg in
platinum sample cup was used and furnace was purged
In this work, we are focusing on the kinetics and mecha-
nism of thermal decomposition of basic copper ethylamine
chromate (CEC), a new precursor for copper chromite cat-
alyst. The synthesis, characterization and application of CEC
were already reported [15]. TG–MS and XRD techniques
were employed to evaluate thermal decomposition and to
identify the composition of volatile and non-volatile prod-
ucts. Kinetic parameters such as activation energy and pre-
exponential factor were derived from the TG data using
multiple heating rate methods, viz. Kissinger–Akahira–
Sunose (KAS) and Flynn–Wall–Ozawa (FWO) [16–21].
Criado method was employed to find out a reaction mecha-
nism for the thermal decomposition of precursor [22, 23].
The results were compared with the thermal decomposition
of copper ammonium chromate (CAC), the precursor for
conventional copper chromite catalyst [24–29].
with ultrapure nitrogen gas at a flow rate of 100 mL min-1
.
For evaluating the kinetic parameters, TG analysis of the
precursor was carried out at variable heating rates (5, 10,
20 and 40 K min-1).
Thermal decomposition kinetics
Kinetic parameters for the thermal decomposition of CEC
and CAC were calculated using two isoconversional mul-
tiple heating rate methods, viz. Kissinger–Akahira–Sunose
(KAS) and Flynn–Wall–Ozawa (FWO) [16–21]. Both KAS
and FWO methods are suitable for the determination of the
kinetic parameters without any prior knowledge of the
reaction mechanisms.
Methodology
Materials
KAS method is based on the following equation
ꢁ
ꢀ
b
AR
Ea
ln ¼ ln
Ta2
À
ð1Þ
Analytical-grade reagents, viz. copper nitrate trihydrate,
ammonium dichromate, ethylamine (70 % aqueous solu-
tion) and ammonia solution (25 %), from SD Fine Chem-
icals, India, and chromium trioxide trihydrate from SRL
Chemical, India, were used for the synthesis of catalysts.
gðaÞEa
RTa
where b is the heating rate, Ta is the temperature in Kelvin
corresponding to a fixed degree of conversion, a, A is the
pre-exponential factor, R is the gas constant, Ea is the
activation energy at a given degree of conversion and g(a)
is the integral form of kinetic model function. Ea for a
given degree of conversion is obtained from the slope of
Preparation of precursors and copper chromite
catalysts
b
the linear fit of the plot ln T2 versus 1/Ta.
a
The precursor, basic copper ethylamine chromate was
synthesized according to a previously reported precipita-
tion method [15]. Different batches of the precursors were
then heated in a muffle furnace at 598, 773, 973 and
1123 K for 1 h under nitrogen atmosphere. Similarly, the
conventional basic copper ammonium chromate was pre-
pared from cupric nitrate (240 g in 250 mL of water) and
ammonium dichromate (126 g in 100 mL of water) using
ammonia as the precipitating agent. Different batches of
CAC were then heated in a muffle furnace for 1 h at 623,
773 and 973 K (23, 24 and 26) to obtain respective sam-
ples. CEC and CAC samples heated at different tempera-
tures are subjected to XRD analysis.
The isoconversional FWO method is represented by the
following equation
ꢀ
ꢁ
AEa
Ea
log b ¼ log
À 2:315 À b
ð2Þ
gðaÞR
RTa
where ‘b’ is an approximation constant [21]. A plot of log (b)
against 1/Ta gives a straight line with slope d (log b)/d (1/Ta).
The Ea of the reaction is calculated using the equation
ꢀ ꢁ
R d log b
ꢂ ꢃ
Ea ¼ À
ð3Þ
1
Ta
b
d
The Ea values obtained from FWO method are
approximate and hence require refining as per the method
given by ASTM E 1641 [21]. The activation energy
values obtained by both methods are used to determine
the corresponding pre-exponential factors, A, using the
equation,
Characterization
Perkin Elmer Pyris 1 TGA thermogravimetric analyzer
clubbed with
a Perkin Elmer Clarus SQ8T mass
123

TG–MS study on the kinetics and mechanism of thermal decomposition of copper ethylamine…
ꢀ
ꢁ
bEaeE =RTa
a
dðaÞ
dT
A
Ea
¼
exp À
fðaÞ
ð7Þ
A ¼
ð4Þ
RTa2
b
RT
To find out a solution for this non-isothermal kinetic
equation, Coats–Redfern used the integral form of the rate
law, which utilized the asymptotic series expansion for
approximating the temperature integral, producing [30]:
Several kinetic equations have been proposed and used
during the last decades for the description of solid-state
reactions such as random nucleation, nucleation and
growth, diffusion and phase boundary controlled reactions
[22]. As given in Table 1, the algebraic expressions that
represent the theoretical mechanisms are separated into
four groups, viz., Dn, Fn, An and Rn, representing nuclei
formation processes for the propagation of thermal degra-
dation, diffusion processes that are related to the heat
transfer capacity along the material structure, random
nucleation and reaction mechanisms controlled by the
phase boundary reactions.
ꢀ
ꢆ
ꢄ
ꢅꢇꢁ
gðaÞ
AR
2RT
Ea
ln
¼ ln
1 À
À
ð8Þ
T2
bEa
Ea
RT
This equation was used to obtain the master curves by
replacing g(a) with corresponding models listed in Table 1.
Plotting ln gðaÞ versus 1/T gives a linear fit with a slope,
T2
from which Ea can be calculated. The model that gives the
best linear fit and comparable Ea value with that obtained
by model free methods is chosen as the preferred one to
explain the reaction mechanism.
All kinetic studies assume that the isothermal rate of
conversion, da/dt, is a linear function of a temperature-
dependent rate constant, k, and a temperature-independent
function of the conversion, a, that is:
Results and discussion
dðaÞ
¼ kðTÞfðaÞ
ð5Þ
dt
First-stage thermal decomposition of CEC and CAC
(423–598 K)
If a reaction is proven to be isokinetic over the range of
temperature studied, Arrhenius equation can be applied to
Eq. (5) to give
The TG–DSC curves for the thermal decomposition of
CEC and CAC are shown in Fig. 1a, b, respectively, and
the phenomenological data corresponding to each stage of
decomposition are given in Table 2. Figure 1a, b shows
that the thermal decomposition behaviour of CEC is dif-
ferent from that of CAC. CEC shows four stages of mass
loss, while CAC shows only three stages. In both cases,
except for the exothermic first-stage decomposition, all the
other stages are endothermic in nature. From Fig. 1a, b, it
ꢀ
ꢁ
dðaÞ
dt
Ea
¼ A exp À
fðaÞ
ð6Þ
RT
where A is frequency factor/pre-exponential factor (s-1), Ea
is activation energy (kJ mol-1), R is universal gas constant
(8.314 J mol-1 K-1) and T is absolute temperature (K).
In non-isothermal condition of constant heating
rate = dT/dt, the conversion degree can be analysed on a
function of temperature and the Eq. (6) becomes
Table 1 Algebraic expressions for the frequently used mechanisms of solid-state decomposition processes
Function g(a)
F(a)
Rate-controlling process
D1
D2
D3
D4
a2
‘(a)
One-dimensional diffusion
(1 - a)ln (1 - a) ? a -ln(1 - a)-1
[1 - (1 - a)1/3 2
Two-dimensional diffusion, cylindrical symmetry
(1 - a)}2/3 Three-dimensional diffusion, spherical symmetry; Jander equation
]
{(3/2)(1 - (1 - a)1/3 -1
(3/2)(1 - (1 - a)1/3 -1
)
(1 - 2/3a)
)
Three-dimensional diffusion, spherical symmetry;
Ginstling–Braunshtein equation
- (1 - a)2/3)
F1
F2
F3
A2
A3
A4
R1
R2
R3
-ln(1 - a)
(1 - a)
(1 - a)2
(1/2)(1 - a)3
2(1 - a) - ln{(1 - a)}1/2
3(1-a) - ln{(1 - a)}2/3
4(1-a) - ln{(1 - a)}3/4
1
2(1 - a)1/2
3(1 - a)2/3
Random nucleation, one nucleus on each particle
Random nucleation, two nuclei on each particle
Random nucleation, three nuclei on each particle
1/(1 - a)
1/(1 - a)2
-ln{(1 - a)}1/2
-ln{(1 - a)}1/3
-ln{(1 - a)}1/4
a
(1 - a)1/2
(1 - a)1/3
Random nucleation, Avrami equation I
Random nucleation, Avrami equation II
Random nucleation, Avrami equation III
Phase boundary reaction(1D movement)
Phase boundary reaction, cylindrical symmetry
Phase boundary reaction, spherical symmetry
123

S. Paulose et al.
0.5
0.4
0.3
0.30
100
90
80
70
60
50
(a)
(b)
100
0.25
0.20
0.15
0.10
0.05
80
60
40
20
0.2
0.1
0.0
0.00
–0.1
1200
–0.05
1200
600
600
800
1000
800
1000
400
400
Temperature/K
Temperature/K
Fig. 1 TG and DSC curves for the thermal decomposition of CEC and CAC
Table 2 Phenomenological data for thermal decomposition of CEC and CAC
Sample
CEC
First stage
Second stage
Third stage
Fourth stage
Ti/K
Ts/K
Tf/K
Ti/K
Ts/°C
717
Tf/K
Ti/K
Ts/K
Tf/K
Ti/K
Ts/K
1088
Tf/K
423
473
598
683
768
849
908
953
1023
Dm = 3.0 %
Endo
Nil
1123
Dm = 24.1 %
Dm = 0.72 %
Dm = 2.2 %
Exo
Endo
Endo
CAC
473
534
598
673
698
773
973
958
9
Dm = 23.1 %
Dm = 1.8 %
Dm = 3.4 %
Endo
Nil
Exo
Endo
Nil
Dm = Mass loss
is evident that the first-stage decomposition of CEC starts
at a lower temperature than that of CAC which may be due
to the instability of the complex containing bulky ethy-
lamine group. DSC measurements of first-stage decompo-
sition of CEC showed an overall DH of 570 J g-1 which
was higher than the DH for the first-stage decomposition of
CAC (DH = 260 J g-1), indicating the higher exother-
micity of the reaction.
diffractogram of the CEC is almost matching with CAC at
some intervals, it shows more number of sharp peaks than
CAC. The presence of peaks at 2h values of 20.9, 28.2,
32.2 and 41.1 may be specific for the structure of CEC
along with the standard peaks at 26.9, 30.1 and 35.1° of
CAC.
The X-ray diffractograms of CEC and CAC subjected to
heating at 598 K for 1 h in a muffle furnace under nitrogen
atmosphere are shown in Fig. 4a, b, respectively. They
show broad and weak peaks, indicating the amorphous
nature of the products with respect to that of precursors.
For CEC heated at 598 K for 1 h (Fig. 4a), all the
diffraction peaks can be readily indexed to Cu (JCPDS No.
00-004-0836), CuCrO2 (JCPDS No.04-010-3330), CuO
(JCPDS No. 00-048-1548) and CuCr2O4 (JCPDS No.
00-005-0657) phases, whereas Fig. 4b shows the presence
of CuCr2O4 and CuO phases only. The volatile reaction
products from the thermal decomposition of CEC and CAC
were identified, and the corresponding mass spectra are
given in Fig. 5a, b, respectively. Ethylamine (C2H5NH2),
ammonia (NH3), water (H2O) and carbon dioxide (CO2)
are the volatile products identified by MS during the first-
stage thermal decomposition of CEC. The absence of
oxygen or oxidizing species like N2O and the presence of
Total ion chromatograms for various volatile products
obtained from TG–MS analysis of CEC and CAC during
the thermal decomposition from 300 to 1173 K are shown
in Fig. 2a, b, respectively. Total ion chromatograms of
CEC (Fig. 2a) indicate that the reaction starts with the
evolution of C2H5NH2 and NH3 which is immediately
followed by the evolution of H2O and CO2, indicating the
oxidation of initial volatile products. However, CAC shows
the presence of H2O, NH3 and nitrous oxide (N2O) during
the first-stage thermal decomposition (Fig. 2b) with H2O
and NH3 as the initial products followed by N2O.
Figure 3 represents the XRD of CEC and CAC as pre-
pared. The peak positions and relative intensities of CAC
agree well with the XRD patterns of copper ammonium
chromate with the formula Cu (OH) (NH4) CrO4
(2h = 26.9, 30.1, 35.1) [26]. Even though X-ray
123

TG–MS study on the kinetics and mechanism of thermal decomposition of copper ethylamine…
(a)
(b)
C H NH
2
5
2
NH
H O
3
2
NH
CO
3
2
N O
2
H O
2
O
2
O
2
400
600
800
1000
1200
400
600
800
1000
1200
Temperature/K
Temperature/K
Fig. 2 Total ion chromatograms obtained from TG–MS of CEC and CAC decomposition
CO2 among the volatile products of first-stage decompo-
sition of CEC as well as the presence of carbon and
metallic copper in the non-volatile residue confirmed that
incomplete oxidation has been taken place during this stage
and the prevailing atmosphere was reducing in nature.
In contrast, the total ion chromatograms and mass
spectrum (Fig. 2b and 5b) of CAC show the presence of
H2O, NH3 and nitrous oxide (N2O) during the first-stage
thermal decomposition. The formation of N2O indicates the
presence of excess oxygen, which oxidizes NH3 to H2O
and N2O. A closer look at the total ion chromatograms
(Fig. 2b) shows that H2O and NH3 are the initial products
which are followed by the appearance of N2O, an oxidation
product at high temperature.
Based on these observations, the possible reactions
taking place during the first-stage decomposition of CEC
are
4Cu(C2H5NH3)(OH)CrO4 ! 2CuO þ 2CuCr2O4
ð9Þ
þ 4C2H5NH2 þ 4H2O þ 3O2
Cu(C2H5NH3)(OH)CrO4 ! CuCrO4 þ C2H5NH2 þ H2O
ð10Þ
2C2H5NH2 þ 3O2 ! CO2 þ 4H2O þ 2NH3 þ 3C
2CuO þ 2CuCr2O4 þ C ! 4CuCrO2 þ CO2
2CuO þ C ! 2Cu þ CO2:
ð11Þ
ð12Þ
ð13Þ
Second-stage thermal decomposition (673–773 K)
A second-stage mass loss is observed in the curves
(Fig. 1a, b) of CEC and CAC from 673 to 773 K. Reac-
tions are endothermic in nature for both cases, but the mass
loss for CEC is almost half (0.7 %) of that in CAC (1.8 %)
(Table 2). The MS analysis (Fig. 6) showed that oxygen is
the only volatile product evolved during this stage, and
XRD of CEC (Fig. 7) heated at 773 K shows the presence
of Cu, CuO, CuCrO2 and CuCr2O4 phases.
It was reported that the second-stage mass loss in CAC
is due to the decomposition of CuCrO4, which was formed
during the first-stage decomposition [26]. Similarly,
CuCrO4 is formed during the decomposition of CEC also
(Eq. 10). However, the reducing environment during CEC
decomposition prevents the formation of more quantities of
CuCrO4 which is basically an oxidized form of Cu and Cr.
This was reflected in the observed lower mass loss for
the second-stage decomposition of CEC than that of CAC.
10
20
30
40
50
60
70
80
90
2θ
Fig. 3 X-ray diffractograms of CEC and CAC
123

S. Paulose et al.
Cu
CuCr2O4
CuO
CuCr2O4
(a)
(b)
1800
1800
CuCrO2
CuO
1500
1200
900
1500
1200
900
70
10
20
80
30
90
50
60
40
70
10
20
80
30
90
50
60
40
2θ
2θ
Fig. 4 X-ray diffractograms of CEC and CAC heated at 598 K in nitrogen atmosphere
m/zvalue
44,30,28,14
18
m/zvalue
18
44
CO2
(a)
44,28,12
45,30
17,16
18
(b)
100
80
60
40
20
0
N2O
100
80
60
40
20
0
C2H5NH2
NH3
H2O
NH3
H2O
17,16
18
44
17
17
30
28
30
28
45
16
14
16
12
14
10
20
30
40
50
60
70
0
10
20
30
40
50
60
70
m/z
m/z
Fig. 5 Mass spectra of first-stage decomposition of CEC and CAC
The second-stage decomposition reaction for both CEC
and CAC can be represented as,
copper (I) in these compounds in pure state took place
only above 1073 K. This indicates the occurrence of a
solid-state reaction between CuO and CuCr2O4 with the
reduction of copper (II) to copper (I) and formation of
CuCrO2 and copper (III) oxide (Cr2O3) along with the
liberation of oxygen during the third-stage decomposition
of CEC.
2CuCrO4 ! CuO þ CuCr2O4 þ 1:5O2:
ð14Þ
Third-stage thermal decomposition (873–973 K)
A third-stage endothermic decomposition was observed
in both CEC and CAC from 873 to 973 K accompanied
by mass loss of around 2.2 and 3.4 %, respectively
(Fig. 1a, b). MS data (Fig. 8) at this stage show the
evolution of oxygen during decomposition. This stage is
attributed to the solid-state reaction between CuO and
CuCr2O4 to form cuprous chromite (CuCrO2). Indepen-
dent experiments conducted by us with CuCr2O4
and CuO showed that the reduction of copper (II) to
X-ray diffractogram of CEC heated at 973 K in inert
atmosphere is shown in Fig. 9, and it shows sharp and
intense peaks, indicating the formation of highly crystalline
materials. Formation of Cu and CuCrO2 phase during the
third-stage decomposition of CEC is confirmed by peaks at
2h values of 32.5, 35.7, 38.9, 48.7, 53.4, 58.2, 61.4, 65.8
and 82.6 and 31.5, 36.4, 41.0, 56.1, 62.5, 65.8, 71.5, 74.6
and 88.7°, respectively. From XRD, it is inferred that small
amount of Cr2O3 (JCPDS No.00-038-1479) was also
123

TG–MS study on the kinetics and mechanism of thermal decomposition of copper ethylamine…
m/z value
32
m/z value
32
100
80
100
80
O2
32,16
O2
32,16
60
40
20
0
60
40
20
0
16
16
0
10
20
30
40
50
60
70
0
10
20
30
40
50
60
70
m/z
m/z
Fig. 6 Mass spectrum of second-stage decomposition of CEC and
CAC
Fig. 8 Mass spectrum of third-stage decomposition of CEC and CAC
8000
4500
4000
3500
3000
CuCrO2
Cr2O3
Cu
7000
Cu
CuO
CuCr2O4
CuCrO2
6000
5000
4000
3000
2000
1000
0
2500
2000
1500
1000
500
10
20
30
40
50
60
70
80
90
2θ
10
20
30
40
50
60
70
80
90
2θ
Fig. 9 X-ray diffractogram of CEC heated at 973 K in nitrogen
atmosphere
Fig. 7 X-ray diffractogram of CEC heated at 723 K in nitrogen
atmosphere
(CO2 at m/z of 44, 28 and 12) during fourth-stage
decomposition (Fig. 10a) reveals that the oxygen produced
during the previous low-temperature stages is not capable
to oxidize the carbon present in the system at that tem-
perature. The reactions taking place during this stage are
represented as,
formed during decomposition of CEC. The third-stage
decomposition reaction of CEC can be represented as,
CuO þ 2CuCr2O4 ! 2CuCrO2 þ Cr2O3 þ Cu þ O2:
ð15Þ
Fourth-stage thermal decomposition of CEC
(1063–1123 K)
2CuCrO2 ! 2Cu þ Cr2O3 þ 0:5O2
C þ O2 ! CO2:
ð16Þ
ð17Þ
CEC alone shows a fourth-stage decomposition from 1063
to 1123 K accompanied by a mass loss of 3 %. Figure 10a,
b shows the MS and XRD spectra of the sample heated at
1123 K in nitrogen atmosphere. MS analysis (Fig. 10a)
showed an interesting observation that two volatile prod-
ucts, O2 and CO2, are liberated at this stage. XRD
(Fig. 10b) showed the presence of Cu and Cr2O3 only
which indicates the reduction of CuCrO2. Evolution of CO2
Kinetic parameters for thermal decomposition
CEC
The first-stage decomposition of CEC is the major step in
the formation of copper chromite catalyst, and hence, it
was subjected to detailed kinetic analysis using KAS and
123

S. Paulose et al.
14000
12000
10000
8000
6000
4000
2000
0
m/z value
44,28,12
32,16
32
Cr2O3
Cu
(a)
(b)
100
80
CO2
O2
44
60
40
20
0
28
16
12
0
10
20
30
40
50
60
70
10
20
30
40
50
60
70
80
90
2θ
m/z
Fig. 10 Mass spectrum and X-ray diffractogram of fourth-stage decomposition of CEC
Table 3 Ea and A values for the thermal decomposition of CEC by
KAS and FWO methods
KAS
FWO
Method
Ea/kJ
mol-1
A/min-1
KAS
124.5
123.9
3.76 9 1013
3.23 9 1013
FWO
Mechanism of decomposition
The Ea average values obtained using the KAS and FWO
methods were used to identify the mechanism of thermal
decomposition of CEC from the different mechanisms
proposed by Criado [22]. Using Eq. 8, proposed by Coats
and Redfern, the activation energy for every g(a) functions
listed in Table 1 was calculated from log g(a)/T2 versus
1/T plots, where a is the degree of conversion at the tem-
perature T for a heating rate of 10 K min-1. The 13 kinetic
plots for CEC are shown in Fig. 12. Ea values obtained
from the kinetics plots are given in Table 4. Analysis of
Fig. 12 and Table 4 shows that, among the 13 kinetics
plots derived for CEC, activation energy corresponding to
F1 type mechanism is in good agreement with that obtained
using KAS/FWO methods. The process was repeated with
thermogravimetric data obtained from experiments with
heating rates 5, 20 and 40 K min-1 and found that the Ea
values obtained from F1 mechanism are 124.5, 125.5 and
122.6 kJ mol-1, respectively, which are in good agreement
with that from KAS/FWO methods.
0
10
20
30
40
50
60
70
80
90
Conversion/%
Fig. 11 Variation of Ea with degree of conversion for CEC using
KAS and FWO methods
FWO methods. The activation energy values obtained by
FWO and KAS methods were drawn against the percentage
of conversion, ranging from to 5 to 90 %, and are shown in
Fig. 11. The average activation energy and pre-exponential
factor for CEC by both the methods are given in Table 3.
From Fig. 11 and Table 3, it can be seen that both KAS
and FWO methods show comparable kinetic parameters
and similar variation with degree of conversion. It can also
be seen that Ea values for CEC increase with degree of
conversion. This is expected as the decomposition of CEC
starts at a lower temperature with the liberation of less
stable ethylamine group, followed by its oxidation. As the
reaction progresses, oxidation becomes prominent, thereby
increasing the overall activation energy. These observa-
tions are confirmed by the total ion chromatogram of
reaction products shown in Fig. 2a.
In the F1 mechanism that corresponds to the random
nucleation with one nucleus on the individual particle, the
decomposition of CEC is initiated from random points that
act as growth centre for the development of the decom-
position reaction. From TG–MS results for the decompo-
sition of CEC, the decomposition starts with the liberation
123

TG–MS study on the kinetics and mechanism of thermal decomposition of copper ethylamine…
by TG–MS and XRD showed that first-stage decomposition
D1
D2
D3
D4
F1
F2
F3
A2
A3
A4
R1
R2
R3
–8
–10
–12
–14
–16
–18
of CEC took place in a reducing atmosphere, while that of
CAC took place in oxidizing atmosphere. The non-volatile
products after the first-stage decomposition of CEC were
Cu, CuCr2O4, CuCrO2 and CuO, while those of CAC are
CuCrO4 and CuO. Second- and third-stage decompositions
were almost similar for both the precursors. These stages
correspond to the reduction of CuCrO4 to CuO and
CuCr2O4 and a solid-state reaction between CuO and
CuCr2O4 to form CuCrO2, respectively. CEC showed an
additional fourth-stage decomposition corresponding to the
reduction of CuCrO2 to Cu, and Cr2O3. The kinetic
parameters for the first-stage thermal decomposition were
derived from thermogravimetric data, and the detailed
kinetic analysis showed that the first-stage decomposition of
CEC followed F1 mechanism. The Ea values calculated by
KAS and FWO isoconversion methods for the first-stage
thermal decomposition of CEC show similar variation with
the percentage conversion.
1.8
1.9
2.0
2.1
2.2
2.3
2.4
1000/T
Fig. 12 Kinetic curves obtained for CEC with mechanistic equation
Table 4 Activation energy calculated for CEC from different
mechanistic equations
Correlation coefficient
Ea/kJ mol-1
D1
D2
D3
D4
F1
F2
F3
A2
A3
A4
R1
R2
R3
0.9430
0.9644
0.9750
0.9716
0.9983
0.7423
0.7929
0.9808
0.9646
0.9793
0.9239
0.9676
0.6363
144.7
162.2
7.1
Acknowledgements The authors thank Director, VSSC, for per-
mission to publish this work and colleagues in Analytical and Spec-
troscopy Division, VSSC, for their support. One of the author (Sanoop
Paulose) thanks ISRO for the fellowship.
169.5
125.3
58.3
135.0
38.4
19.5
36.3
63.2
77.6
8.81
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