Thermal investigation of strontium acetate hemihydrate in nitrogen gas

Article abstract of DOI:10.1007/s10973-007-8711-6

The thermal decomposition of strontium acetate hemihydrate has been studied by TG-DTA/DSC and TG coupled with Fourier transform infrared spectroscopy (FTIR) under non-isothermal conditions in nitrogen gas from ambient temperature to 600°C. The TG-DTA/DSC experiments indicate the decomposition goes mainly through two steps: the dehydration and the subsequent decomposition of anhydrous strontium acetate into strontium carbonate. TG-FTIR analysis of the evolved products from the non-oxidative thermal degradation indicates mainly the release of water, acetone and carbon dioxide. The model-free isoconversional methods are employed to calculate the E _a of both steps at different conversion α from 0.1 to 0.9 with increment of 0.05. The relative constant apparent E _a values during dehydration (0.5<α<0.9) of strontium acetate hemihydrate and decomposition of anhydrous strontium acetate (0.5<α<0.9) suggest that the simplex reactions involved in the corresponding thermal events. The most probable kinetic models during dehydration and decomposition have been estimated by means of the master plots method.

Full text of DOI:10.1007/s10973-007-8711-6

Journal of Thermal Analysis and Calorimetry, Vol. 94 (2008) 1, 169–174
THERMAL INVESTIGATION OF STRONTIUM ACETATE HEMIHYDRATE
IN NITROGEN GAS
Y. Duan1, J. Li1, X. Yang1, X.-M. Cao1, L. Hu1, Z.-Y. Wang1, Y.-W. Liu1,2* and C.-X. Wang1
1College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China
2College of Life Sciences, Wuhan University, Wuhan 430072, China
The thermal decomposition of strontium acetate hemihydrate has been studied by TG-DTA/DSC and TG coupled with Fourier
transform infrared spectroscopy (FTIR) under non-isothermal conditions in nitrogen gas from ambient temperature to 600°C.
The TG-DTA/DSC experiments indicate the decomposition goes mainly through two steps: the dehydration and the subsequent de-
composition of anhydrous strontium acetate into strontium carbonate. TG-FTIR analysis of the evolved products from the non-oxi-
dative thermal degradation indicates mainly the release of water, acetone and carbon dioxide. The model-free isoconversional meth-
ods are employed to calculate the Ea of both steps at different conversion a from 0.1 to 0.9 with increment of 0.05. The relative
constant apparent Ea values during dehydration (0.5<a< 0.9) of strontium acetate hemihydrate and decomposition of anhydrous
strontium acetate (0.5<a<0.9) suggest that the simplex reactions involved in the corresponding thermal events. The most probable
kinetic models during dehydration and decomposition have been estimated by means of the master plots method.
Keywords: master plots method, model-free isoconversional method, strontium acetate hemihydrate, TG-FTIR, thermal decomposition
Introduction
determine the ‘kinetic triplets’ of the decomposition
processes. Vyazovkin and Wight pointed out that the
use of the model-free approach is recommended as a
trustworthy way of obtaining reliable and consistent
kinetic information from both non-isothermal and
isothermal data [13–16].
The thermal decomposition of metal acetates are of
great importance for their frequent application in the
preparation of nano or complex material precursor.
Their products which are obtained through precisely
controlled calcination temperature and time in spe-
cific atmosphere, can be used as prospective materials
for superior technical ceramics, materials for mag-
netic recording, many electronic components, sensor,
catalysts as well as the high-temperature supercon-
ductors [1]. d-electron metal salts contain organic an-
ions such as oxalates, acetates, formates etc. are com-
mon precursor systems for the controlled synthesis of
d-metal oxides or alloys under mild condition [2].
A lot of studies have been performed for the
thermal decomposition behavior of metal acetates by
a variety of measurements such as TG/DTG, DTA,
DSC and TG coupled with FTIR or MS (mass spec-
troscopy), etc. [3–10]. Recently thermal analysis cou-
pled with EGA has been developed rapidly and are
frequently used for the quantification of each single
gaseous evolution process as the result of an increas-
ing thermal ramp or a defined isothermal temperature.
Compared with conventional methods, the coupled
technique offers an invaluable interpretation to reveal
the possible degradation mechanism in the thermal
events. The thermal analysis coupled with EGA
reviews can be seen in literatures [11, 12].
In present work, an investigation of thermal de-
composition of strontium acetate hemihydrate was
carried out under non-isothermal condition from am-
bient to 600°C in N2 atmosphere via TG-DTA/DSC,
DSC, TG-FTIR and XRD techniques. Simultaneous
techniques of coupling TG and FTIR were utilized to
monitor the gas released from the decomposition pro-
cess. The kinetics of the non-isothermal dehydration
and decomposition of strontium acetate was also stud-
ied. The kinetic parameters were obtained by the
model-free isoconversional method and the master
plots method [17].
Theoretical
For a reaction under non-isothermal condition, its
kinetic function can be expressed as the following
form [18]:
AE
a
g(a )=
P(u)
(1)
bR
where a is an extent of conversion, b the heating rate,
Ea the apparent activation energy, R the gas constant,
A pre-exponential factor, g(a) the integral expression
of kinetic model function, and
The model-fitting method and the model-free
method are the two primary computational methods to
*
Author for correspondence: ipc@whu.edu.cn
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2008 Akadémiai Kiadó, Budapest

DUAN et al.
u
u
(France). Instrument calibration was performed with
æ
P(u)=ò-ç
ö
÷du
e
2
ç
è
÷
standard indium, tin, lead, zinc, silver and gold sam-
ple of known melting temperature. All standards were
of purity³99.99%. For the kinetics measurement
about 5 mg sample was weighted into an open alu-
mina crucible. The furnace temperature was pro-
grammed to rise from ambient temperature to 600°C
at the rates of 5, 10, 15 and 20 K min–1. Nitrogen gas
of high purity (purity³99.999%) was used as carrier
gas at flow rate of 50 mL min–1.
u
ø
¥
where u=Ea/RT.
Because the exponential integral, P(u), has no
analytical solution. An approximation formula of
high accuracy [19] was used.
-lnP(u)=0.37773896+
1.89466100lnu+1.00145033u
(2)
Inserting Eq. (2) in Eq. (1) , one can obtain:
Simultaneous TG coupled with FTIR was done
on the Setaram Setsys 16 TG-DTA/DSC Instrument
and a Thermo Nicolet Nexus 670 Fourier Transform
Infrared Spectrometer. About 10 mg sample was
loaded into an open alumina crucible. The heating
rate of TG furnace was 20 K min–1 and nitrogen gas of
high purity (purity³99.999%) with a flow rate of
100 mL min–1 was used as carrier gas. The sample
was heated from ambient temperature to 600°C. The
transfer line used to connect TG and FTIR was a 1 m
long stainless steel tube with an internal diameter of
2 mm, of which the temperature is maintained at
200°C. The TG accessory of the IR Spectrometer was
used, which has a 45 mL gas cell with a 200 mm path
length. It was also heated at the constant temperature
of 200°C. The IR spectra were collected at 8 cm–1 res-
olution, co-adding 8 scans per spectrum. This resulted
in a temporal resolution of 4.32 s. Lag time that the
gas products went from furnace to gas cell was about
7 s. The FTIR spectra have been identified based on
the FTIR reference spectra available on the World
Wide Web in the libraries of NIST [20].
ì
ù
= ln
í
ú
AE
b
é
ù
é
a
ln
+3.63504095-
ê
ë
ú
û
1.89466100
ê
T
Rg(a )
ë
û
î
E
a
1.89466100lnE -1.00145033
}
a
(3)
RT
The first term at the right side of Eq. (3) is a con-
stant corresponding to a given value of a. So for a se-
ries of experiments at different heating rates, the plot
of ln(b/T1.89466100) vs. 1/T with the same conversional
ratio should be
a
line with the slope of
–1.00145033Ea/R. Then, the apparent activation en-
ergy Ea can be calculated from the slope.
Inserting a=0.5 into Eq. (1), one can get
AE
a
g(0.5)=
P(u )
(4)
0.5
bR
where u0.5=Ea/RT0.5, T0.5 is the temperature when a
equals to 0.5. When Eq. (1) is divided by Eq. (4), the
following equation is obtained:
g(a )/g(0.5)=P(u)/P(u )
(5)
0.5
The differential scanning calorimetry (DSC)
measurements were performed on a Mettler-Toledo
calorimeter, model DSC 822e. About 6 mg sample
was added in the aluminium sample pan. The refer-
ence pan was empty. The DSC experiments were run
at the heating rate and cooling rate of 10 K min–1.
X-ray diffraction (XRD) patterns were recorded
on Bruker D8 (Germany) model X-ray diffractometer
with a graphite monochromator and CuKa and scan
rate of 4° min–1.
By plotting g(a)/g(0.5) vs. a corresponds to dif-
ferent theoretical model functions, the theoretical
master plots can be obtained for different kinetic
mechanism. With Ea calculated from Eq. (3), the ex-
perimental master plots of P(u)/P(u0.5) vs. a could be
drawn from the experimental data obtained under dif-
ferent heating rates. Equation (5) indicates that, for
arbitrary a, the experimental value P(u)/P(u0.5) and
theoretically calculated values of g(a)/g(0.5) are
equivalent when an appropriate kinetic model is used.
So this integral master plots method can be used to
determine the most probable mechanism function.
Then, the pre-exponential factor A can be ob-
tained from the slope of the plot of g(a) vs.
EaP(u)/bR.
Results and discussion
TG/DTG-DSC and DSC analysis
The results of TG experiments carried out in N2 under
600°C at the heating rates of 5, 10, 15 and 20 K min–1
are shown in Fig. 1. It can be observed that the mass
losses of each step are almost identical at different
heating rates. The TG curves and DTG peaks were
gradually shifting to the right accompanied with in-
crease of heating rates. The TG/DTG-DSC curves ob-
tained at heating rate of 10 K min–1 is presented in
Fig. 2, which displayed Sr(AC)2·0.5H2O thermally
Experimental
Strontium acetate hemihydrate powder (analytical
grade) was purchased from Beijing Reagent Factory
without further purification.
TG experiments were carried out on
a
Setaram Setsys 16 TG-DTA/DSC thermal analyzer
170
J. Therm. Anal. Cal., 94, 2008

STRONTIUM ACETATE HEMIHYDRATE IN NITROGEN GAS
Fig. 2 TG-DTG-DSC curves for the decomposition of
Sr(AC)2·0.5H2O at 10 K min–1, N2 flow rate
50 mL min–1
Fig. 1 a – TG and b – DTG curves for the decomposition of
Sr(AC)2·0.5H2O at different heating rate, N2 flow rate
50 mL min–1
Fig. 3 X-ray powder diffraction patterns of final decomposi-
tion product of Sr(AC)2·0.5H2O in N2 at ambient tem-
perature
decomposed mainly through two-step mass losses.
The first step of mass loss 4.11% agreed with the the-
oretical value 4.16% between 180 to 260°C was at-
tributed to the dehydration of Sr(AC)2·0.5H2O. Two
overlapped endothermic peaks with maximum values
213.1 and 231.9°C of the dehydration process can be
seen in the DSC curves. This implies that the dehy-
dration occurs in at least two stages, which is verified
by the DTG curves at different heating rates presented
in Fig. 1b. The second step of mass loss 26.92%
agreed with theoretical value 27.10% between 400 to
480°C can be assigned to the decomposition of anhy-
drous Sr(AC)2 to SrCO3. A sharp endothermic peak at
430.6°C and a broad exothermic peak between 440
and 500°C in the DSC curve superimposed upon the
decomposition process. The XRD experiment was
carried out for the final decomposition product of
Sr(AC)2·0.5H2O at ambient temperature. The result
(shown in Fig. 3) conformed very closely to the stan-
dard XRD patterns of SrCO3 (JCPDS No. 87-1778).
Figure 2 shows that there were two sharp endo-
thermic peaks with maximum values of 314.3 and
331.6°C in DSC curve with no corresponding mass
loss in TG curve suggests two-phase change pro-
cesses occurred in this temperature range. In order to
identify nature of the two sharp DSC peaks, DSC ex-
periments (shown in Fig. 4) were carried out at the
heating rate and cooling rate of 10 K min–1. In Fig. 4a,
the DSC experiment was firstly heated from 30 to
Fig. 4 DSC curves of Sr(AC)2·0.5H2O a – heated from 30 to
323°C and cooled to 30°C, b – rescan a heated from 30
to 360°C, c – heated from 30 to 360°C and cooled to
30°C, d – rescan c from 30 from 360°C
323°C which just exceeded the first phase change and
the second phase change was not taken place, then
cooled down to ambient temperature. Figure 4b
shows the rescan of the sample in Fig. 4a. The sharp
DSC peak at about 314°C appeared again in the
rescan DSC curve indicated that the broad peak ap-
peared during the cooling process between
250–290°C in Fig. 4a can be assigned to the solidifi-
cation or recrystallization and the former phase
change peak was the endothermal melting process. In
J. Therm. Anal. Cal., 94, 2008
171

DUAN et al.
Fig. 4c, the similar DSC run was done as Fig. 4a ex-
cept for the higher terminated temperature 360°C
which just exceeded the second phase change, there
was no endotherm or exotherm peak during the cool-
ing process. Figure 4d shows the rescan of the sample
in Fig. 4c, there presented a broad exothermal peak
between 230 to 270°C and followed an endothermal
peak between 270 to 300°C which may due to the
crystallization peak and the melting of the new crys-
talline of anhydrous Sr(AC)2, respectively. McAdie
[21] have reported that the endothermal fusion of the
anhydrous strontium acetate occurs at 321±1°C (peak
temperature), with no other enthalpic or mass changes
being observed between dehydration and fusion. The
dissimilar observations may lie in the different resolu-
tion of the apparatus being used.
Fig. 6 The compared FTIR spectra of evolved gases from
Sr(AC)2·0.5H2O in N2 measured at 442.1°C
TG-FTIR analysis
By utilization of TG coupled with evolved gas analy-
sis (EGA), the nature of volatile products released by
a substance subjected to a controlled temperature pro-
gram are on-line determined, with the possibility to
proven a supposed reaction, either under isothermal
or under heating conditions [7]. The evolved gaseous
species are analyzed by FTIR. The 3D plot FTIR gas
spectra from the thermal decomposition process of
Sr(AC)2·0.5H2O is presented in the Fig. 5. The char-
acteristic bands at 1700–1500 and 3740–3500 cm–1
can be assigned to the bending vibration and stretch-
ing vibration of O–H which corresponding to the de-
hydration of Sr(AC)2·1/2H2O in the range of
180–260°C. Carbon dioxide was easily identified by
the characteristic peak at 2358 cm–1 during
400–480°C. The FTIR spectra observed at the Gram
Schmidt peak temperature (442.1°C) is shown in
Fig. 6. By comparing with NIST database, the gases
evolved at 442.1°C were identified as a mixture of
gaseous acetone and minor carbon dioxide. The same
result was also obtained by Borger et al. [22].
Fig. 7 The curves of TG, DTG and the total FTIR absorbance
intensity of evolved gases, gotten during the
Sr(AC)2·0.5H2O decomposition process by TG-FTIR,
heating rate 20 K min–1, N2 flow rate 100 mL min–1
hemihydrate at the heating rate 20 K min–1 and N2
flow rate 100 mL min–1. With consideration of lag
time from furnace to gas cell, the DTG peaks of mass
loss is well corresponding to peaks of the IR intensity
during both the dehydration and decomposition of
strontium acetate.
The plots of IR absorbance at several character-
istic wavenumbers vs. temperature are shown in Fig. 8
which indicated the changing of the evolution rates
with temperature for each gaseous product. The peak
temperatures which represent the temperatures of the
maximum evolution rates show that the water is the
gaseous product of the first stage mass loss, while mi-
nor carbon dioxide and acetone are the gaseous prod-
ucts of the second stage mass loss.
In Fig. 7, it shows that the plot of TG/DTG/total
IR intensity of decomposition of strontium acetate
In addition, the corresponding decomposition
processes can be summarized in the following steps:
Sr(CH3COO)2·0.5H2O(s)®
Sr(CH3COO)2(s)+0.5H2O(g)
structural change
fusion
Sr(CH3COO)2(s)¾¾®Sr(CH3COO)2(l)¾¾¾¾®
Sr(CH3COO)2(l)®SrCO3(s)+CH3COCH3(g)+CO2(g)
Model-free isoconversional method to obtain the
activation energy Ea
Fig. 5 The 3D surface graph for the FTIR spectra of the
evolved gases produced by Sr(AC)2·0.5H2O decompo-
sition at heating rate 20 K min–1 and N2 flow rate
100 mL min–1
With Eq. (3), by plotting ln(b/T1.89466100) vs. 1/T at dif-
ferent heating rates and a, values of the apparent acti-
172
J. Therm. Anal. Cal., 94, 2008

STRONTIUM ACETATE HEMIHYDRATE IN NITROGEN GAS
tion of liquid anhydrous Sr(AC)2 indicate a high prob-
ability of a single-step mechanism.
Determination of the kinetic models of dehydration
and decomposition process
The data analysis was carried out by means of master
plot methods [23]. The conventional model functions
[24] are well known equations.
It is worth noting that the two-phase changes
have taken place prior to the decomposition of anhy-
drous Sr(AC)2. So the decomposition occurred in the
form of liquid state Sr(AC)2 whose process can be as-
signed to the homogeneous reaction. It is more rea-
sonable to descript this reaction by the simple order
reaction model.
Fig. 8 IR absorbance vs. temperature curves of identified
evolved gaseous species evolved from Sr(AC)2·0.5H2O
decomposed in N2, measured by online-coupled
TG-FTIR system (heating rate 20 K min–1, N2 flow rate
100 mL min–1)
According to Eqs (4) and (5), plotting
g(a)/g(0.5) vs. a corresponds to theoretical master
plots of various g(a) functions. To draw the experi-
mental master plots of P(u)/P(u0.5) vs. a from experi-
mental data obtained under four heating rates, the
knowledge of temperature as a function of a and the
value of Ea for the process should be known in ad-
vance, and P(u) can be calculated directly using nu-
merical Simpson’s procedure or some approximate
formulas (Eq. (2)). Both of the experimental and the
theoretical master plots are shown in Fig. 10. For a
vation energy Ea are obtained and shown in Fig. 9. In
the dehydration process, the Ea decreased from 198.0
to 129.5 kJ mol–1 when conversion 0.1<a<0.5 and re-
mained constant with values of 133.4±6.6 kJ mol–1 in
the range 0.5<a<0.9. The evidence of an inflection
point on the mass loss curves supports the concept of
a two-stage type of water loss. At the same time, the
doublet DSC endothermal peaks during dehydration
may represent separation of two modes of bonding of
water molecules with the strontium acetate lattice.
Unfortunately, lattice structures of Sr(AC)2·0.5H2O
required for further interpretation have not been
found. Similar to the dehydration process, the Ea dur-
ing decomposition when conversion 0.1<a< 0.5 pre-
sented a decreased from 254.7 to 215.5 kJ mol–1 and
kept constant with values of 210.6±2.9 kJ mol–1 in the
range 0.5<a< 0.9. Though the single DTG peak exists
during the decomposition, the two superimposed
DSC peaks and the variation of Ea suggest the occur-
rence of multiplex reaction. McAdie [21] considered
that the liberation of acetone occurs with endothermal
process initially, however thermal degradation of the
liberated organic fragment gives rise to considerable
exothermal activity superimposed upon the decompo-
sition process.
The obtained relative constant values of Ea dur-
ing dehydration of Sr(AC)2·0.5H2O and decomposi-
Fig. 10 The theoretical master plots (—) and the experimental
master plots at heating rates of q – 5, p – 10, ˜ – 15
and ¢ – 20 K min–1 for a – dehydration(0.5<a<0.9)
and b – decomposition (0.5<a<0.9) in flowing N2
Fig. 9 The Ea of dehydration and decomposition process deter-
mined by model-free isoconversional method
J. Therm. Anal. Cal., 94, 2008
173

DUAN et al.
Table 1 Kinetic triplets during the dehydration (0.5<a< 0.9) and decomposition process (0.5<a< 0.9)
Reaction
E/kJ mol–1
133.4±6.6
lnA/s–1
n
model function/g(a)
[–ln(1–a)]1/3
dehydration
27.50±0.11
3
1-(1-a)1- 0.62
1-0.62
decomposition
210.6±2.9
13.18±0.04
0.62
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Acknowledgements
Received: November 17, 2007
Accepted: April 23, 2008
This work was financially supported by the National Nature
Sciences Foundation of China (Grant No. 20373050,
No. 30600116), Nature Sciences Foundation of Hubei and
China Postdoctoral Science Foundation.
OnlineFirst: August 15, 2008
DOI: 10.1007/s10973-007-8711-6
174
J. Therm. Anal. Cal., 94, 2008