Comparison of the activation energies for the thermal decomposition of copper and nickel compounds obtained by various thermogravimetric methods

Article abstract of DOI:10.1016/s0040-6031(99)00277-4

The thermal decomposition of representative transition metal compounds has been examined using both standard and high resolution thermogravimetry. Using two approaches for plotting variable heating rate data, it has been demonstrated that comparable value

Full text of DOI:10.1016/s0040-6031(99)00277-4

Thermochimica Acta 340±341 (1999) 311±314
Comparison of the activation energies for the thermal
decomposition of copper and nickel compounds
obtained by various thermogravimetric methods
*
B.A. Howell , B.B.S. Sastry
Department of Chemistry, Center for Applications in Polymer Science, Central Michigan University, Mt. Pleasant, MI 48859, USA
Accepted 6 August 1999
Abstract
The thermal decomposition of representative transition metal compounds has been examined using both standard and high
resolution thermogravimetry. Using two approaches for plotting variable heating rate data, it has been demonstrated that
comparable values for activation energies may be obtained by either method. Thus high resolution thermogravimetry is
reliable method for the determination of decomposition activation energies. # 1999 Elsevier Science B.V. All rights reserved.
Keywords: Thermogravimetry; Activation energy; Thermal decomposition
1
. Introduction
However, this procedure requires multiple determina-
tions at several temperatures to permit the construc-
tion of a reliable Arrhenius plot and the extraction of
an activation energy. For this reason, several dynamic
methods have been developed for the estimation of the
activation energy for decomposition processes to
which the material of interest may be subject [6].
The most popular of these is the Flynn±Wall constant
heating rate method which requires three or more
determinations at different linear heating rates [7].
For ®rst order processes, a plot of the natural logarithm
of the heating rate (ln ꢀ) vs the reciprocal of the
absolute temperature (1/T) affords a slope which
may be used to calculate the activation energy (Ea)
[6]. In the constant reaction rate approach the heating
rate is adjusted as required to maintain a selected rate
of weight loss [8,9]. Assuming a reaction order of one,
the activation energy may be determined directly by
plotting ln(1/1 � a) vs 1/T where ꢁ represents the
Thermogravimetry (TG) has found great utility in
the analysis of polymeric materials, particularly in the
elucidation of degradation pathways available for the
thermal decomposition of these materials [1]. Less
prominent, perhaps, but nonetheless, important has
been the in¯uence of the thermogravimetric analysis
of inorganic and organometallic compounds [2±5]. In
this case information about both decomposition mode
and elemental composition may be obtained [2]. In
either case kinetic information which re¯ects the rate
of degradation (and thermal stability) may be
obtained. Still the most reliable method for determin-
ing kinetic perimeters is carefully-controlled isother-
mal decomposition in which weight loss at a constant
temperature is monitored as a function of time [1].
*
Corresponding author.
0040-6031/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 9 9 ) 0 0 2 7 7 - 4

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12
B.A. Howell, B.B.S. Sastry / Thermochimica Acta 340±341 (1999) 311±314
fraction of decomposition and T the absolute tempera-
ture [6]. With the advent of high resolution thermo-
gravimetry a dynamic heating rate approach has
become available [10±12]. In this approach both the
heating rate and rate of weight loss continuously vary
during decomposition. As the rate of weight loss
increases, the heating rate decreases. Thus the heating
rate is high when no weight loss is occurring but low
when weight loss is high, i.e., when a thermal event is
occurring. Consequently, the advantages of both high
heating rate (rapid throughput) and very low heating
rate (good resolution) may be at hand in the same
experiment. The heating rate at maximum decompo-
sition is a function of the initial heating rate and an
instrumental resolution parameter. Using this
approach and assuming a reaction order of one, the
2.3. Synthesis
2.3.1. bis(Diethyl dithiophosphato)nickel(II)
A solution of 2.01 g (0.00845 mole) of nickel chlor-
ide hexahydrate in 30 ml of methanol was treated with
a solution of 3.11 g (0.016 mole) of sodium diethyl
dithiophosphate in 30 ml of methanol. The product, a
violet solid, which formed immediately was collected
by ®ltration at reduced pressure, washed with three
20 ml of portions of methanol, and allowed to dry at
room temperature and atmospheric pressure: mass
‡
� 1
spectrum, M (m/e) 428; ir (cm , KBr) 2880 (C-
H), 650 (asymmetric P-S stretch), 539 (symmetric
P-S stretch).
2.3.2. bis(Diethyl dithiophosphato)copper(II)
activation E for decomposition may be obtained from
a
bis(Diethyldithiophosphato)copper(II) wasobtained
as a blue solid in manner directly analogous to that
used for the preparation of the corresponding nickel
2
a plot of ln(H /T ) vs 1/T, where H is the heating rate
r
r
at maximum weight loss and T is the absolute tem-
perature at maximum weight loss [5].
‡
� 1
compound: mass spectrum, M (m/e) 433; ir (cm
;
Examination of the thermal decomposition for four
representative transition metal compounds using both
standard and high resolution TG and these variable
rate methods was undertaken to explore the reliability
of the methods and the applicability of the high
resolution approach for the determination of kinetic
parameters.
KBr) 2980 (C-H), 630 (asymmetric P-S stretch), 520
(symmetric P-S stretch).
2.3.3. Dichloro(1,10-phenanthrolein)copper(II)
A solution of 2.50 g (0.014 mole) of 1,10-phenan-
throlein in 30 ml of ethanol was added dropwise to a
stirred solution of 2.50 g (0.015 mole) of copper(II)
chloride dihydrate in 25 ml of water. The blue solid
which formed was collected by ®ltration at reduced
pressure, washed with three 20 ml portions of ethanol,
and dried at 408C and 20 torr for 48 h: mass spectrum,
2
. Experimental
‡
� 1
2
.1. Materials
M (m/e) 313; ir (cm , KBr) 1560 ꢀC - N†.
0
Diethyl dithiophosphate, 1,10-phenanthrolein, and
0
2.3.4. Dichloro(2,2 -bipyridine)copper(II)
0
2
,2 -bipyridine were obtained from the Aldrich Che-
Dichloro(2,2 -bipyridine)copper(II) was obtained
as a blue solid in a manner directly analogous to that
used for the preparation the 1,10-phenanthrolein com-
pound: mass spectrum, M (m/e) 289; ir (cm ) 1508
C - N†.
mical Company. Nickel chloride hexaydrate, copper
II) chloride dihydrate, and common solvents were
obtained from Fisher Scienti®c Company.
(
‡
� 1
ꢀ
2
.2. Characterization
2
.4. Thermogravimetry
Infrared spectra were recorded using approximately
% solid solutions in anhydrous potassium bromide
as discs) and a Perkin-Elmer model 1600 FT-IR
spectrophotometer. Mass spectra were obtained at
an ionizing potential of 70 eVusing a Hewlett-Packard
1
(
The thermal degradation of bis(diethyl dithiopho-
sphato)nickel(II), bis(diethyl dithiophosphato)cop-
per(II), dichloro(1,10-phenanthrolein)copper(II), and
dichloro(2,2 -bipyridine)copper(II) was examined
under a variety of conditions using a TA Instruments
model 2950 TGA unit interfaced with the TA Instru-
0
5995 gas chromatograph/mass spectrometer equipped
with a direct insertion probe.

B.A. Howell, B.B.S. Sastry / Thermochimica Acta 340±341 (1999) 311±314
313
ments Thermal Analyst 2100 control unit. Decay
plots, weight loss vs temperature, were generated
by feeding the analyzer output (TA Instruments soft-
ware was used for all data manipulation) to a model
In this case complete loss of both ligands occurs in a
single step. The decomposition begins at 1418C and is
accompanied by a loss of 81% of the initial sample
weight corresponding to the mass of both ligands. The
degradation is complete at 2428C and a residue of 19%
of the initial sample weight, corresponding to the mass
of metal present in the compound, remains. The
analagous copper compound displays a similar decom-
position pattern. In contrast, the decomposition of
7
440 Hewlett-Packard plotter. The TGA cell was
swept with nitrogen at 50 ml/min during degradation
runs and the sample, approximately 10 mg, was con-
tained in a platinum sample pan. Kinetic plots were
‡
generated using statView 512 software and an Apple
Macintosh IIci computer.
0
dichloro(2,2 -bipyridine)copper(II) and dichloro-
(
1,10-phenanthrolein)copper(II) begins at approxi-
mately 1008C higher temperature and occurs in two
steps with the initial loss corresponding to the organic
ligand followed by loss of the chloride ligands. In all
cases the residue remaining after decomposition cor-
responded to the metal component of the compound.
The activation energy for the initial decomposition
step for each compound was determined by two
methods using both standard and high resolution
techniques. The results are displayed in Table 1.
While the activation energies for the decomposition
of the dithiophosphate compounds are considerably
smaller than those for the decomposition of the phe-
3
. Results and discussion
The thermal decomposition of four representative
transition metal compounds, bis(diethyl dithiophospha-
to)nickel (II) [Ni(dtp) ], bis(diethyl dithiophosphato)-
2
copper (II) [Cu(dtp) ], dichloro(1,10-phenanthrolein)-
2
0
copper (II) [Cu(phen) Cl ], and dichloro(2,2 -bipyridi-
2
2
ne)copper (II) [Cu(bpy) Cl ] has been examined by
2
2
thermogravimetry. The thermogram for the decomposi-
tion of bis(diethyl dithiophosphato)nickel (II) is dis-
played in Fig. 1.
Fig. 1. Thermogram for the decomposition of bis(diethyl dithiophosphato)nickel (II).

3
14
B.A. Howell, B.B.S. Sastry / Thermochimica Acta 340±341 (1999) 311±314
Table 1
Activation energies (kcal/Mole) for the thermal decomposition of selected transition metal compounds
Compounds
Methods
2
/T ) vs 1/T
ln ꢀ vs 1/T
ln(H
r
a
b
a
b
Standard
Hi-Res
Standard
Hi-Res
Ni(dtp)
Cu(dtp)
Cu(bpy)Cl
2
16.33
18.21
40.25
42.44
12.92
17.71
42.68
43.05
14.45
17.45
38.01
40.00
14.63
16.27
40.48
40.48
2
2
Cu(phen)Cl
2
a
Determined at heating rates of 58, 108 and 208C/min without the high resolution feature of TA Instruments.
Determined with an initial heating rate of 208C/min with Hi-Res control at resolution parameters of 3, 4 and 6. For Cu(bpy)Cl this
2
b
corresponded to heating rates of 3.10, 1.11 and 0.758C/min with temperatures of maximum degradation rate at 311.58, 295.38 and 289.28C,
respectively.
nanthrolein and bipyridine compounds, it is clear that
the variation in values obtained using the various
methods is rather small, 2±3 kcal/mole. In general,
the values obtained using a plot of ln ꢀ vs 1/Tare larger
nique may be used with con®dence for the determina-
tion of decomposition activation energies.
Acknowledgements
by about 2 kcal/mole than the corresponding values
2
obtained from a plot of ln(H /T ) vs 1/T. More impor-
r
Stipend support (BBSS) and funding for the estab-
lishment of the Center for Applications in Polymer
Science, which permitted the purchase of the thermal
analysis equipment used in this work, from the Michi-
gan Research Excellence Fund is gratefully acknowl-
edged.
tantly the variation between values obtained using
standard vs high resolution techniques is smaller,
usually 1±2 kcal/mole. It is clear that satisfactory
values for activation energies for the decomposition
of metal compounds may be obtained using the high
resolution feature of the TA instrument. As has been
noted previously for the degradation of polymeric
systems using Hi-Res TG [12], somewhat better
results were obtained by maintaining a constant initial
heating rate and varying the resolution parameter to
obtain a variable heating rate at the temperature of
maximum decomposition rate rather than by selecting
a particular resolution value and varying the heating
rate.
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5
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