Thermal decomposition of Cd(CH3COO)2· 2H 2O studied by a coupled TG-DTA-MS method

Article abstract of DOI:10.1023/B:JTAN.0000046117.25037.5a

The thermal decomposition of cadmium acetate dihydrate in helium and in air atmosphere has been investigated by means of a coupled TG-DTA-MS method combined with X-ray diffraction analysis. Dehydration of Cd(CH ₃COO)₂·2H₂O is a two-stage process with Cd(CH₃COO)₂H₂O as intermediate. The way of Cd(CH₃COO)₂ decomposition strongly depends on the surrounding gas atmosphere and the rate of heating. CdO, acetone and CO ₂ are the primary products of decomposition in air. In helium decomposition goes by two parallel and consecutive reactions in which intermediates, Cd and CdCO₃, are formed. Metallic cadmium oxidizes and cadmium carbonate decomposes giving CdO. Some of the metallic cadmium, depending on the heating rate and the concentration of oxygen, evaporates. Acetone is partially oxidized in secondary reactions with oxygen.

Full text of DOI:10.1023/B:JTAN.0000046117.25037.5a

Journal of Thermal Analysis and Calorimetry, Vol. 78 (2004) 535–544
THERMAL DECOMPOSITION OF Cd(CH3COO)2 2H2O
STUDIED BY A COUPLED TG-DTA-MS METHOD
Barbara MaÓecka*
AGH University of Science and Technology, Faculty of Materials Science and Ceramics,
30 Mickiewicz Av., 30-059 Cracow, Poland
Abstract
The thermal decomposition of cadmium acetate dihydrate in helium and in air atmosphere has been
investigated by means of a coupled TG-DTA-MS method combined with X-ray diffraction analysis.
Dehydration of Cd(CH3COO)2·2H2O is a two-stage process with Cd(CH3COO)2·H2O as intermedi-
ate. The way of Cd(CH3COO)2 decomposition strongly depends on the surrounding gas atmosphere
and the rate of heating. CdO, acetone and CO2 are the primary products of decomposition in air. In
helium decomposition goes by two parallel and consecutive reactions in which intermediates, Cd
and CdCO3, are formed. Metallic cadmium oxidizes and cadmium carbonate decomposes giving
CdO. Some of the metallic cadmium, depending on the heating rate and the concentration of oxy-
gen, evaporates. Acetone is partially oxidized in secondary reactions with oxygen.
Keywords: cadmium acetate, dehydration, evolved gas analysis, thermal decomposition
Introduction
d-electron metal salts containing organic anions such as oxalates, acetates, formates
etc. are common precursor systems for the controlled synthesis of d-metal oxides or al-
loys under mild conditions. The mechanism of thermal decomposition of anhydrous or
hydrated metal salts is important in determining the properties of the solid product.
A few studies have been published for different cations with the acetate an-
ion [1–5]. The hydrates of d-electron metal acetates decompose in two main consecu-
tive steps: dehydration and decomposition of the anhydrous salt. After dehydration,
however, basic salts are sometimes formed [6]. The decomposition of acetates
strongly depends on the surrounding atmosphere. Depending on the conditions the
solid product can be the metal or/and the metal oxide. On decomposition acetates of-
ten yield acetone and acetic acid. In addition some other volatile products have been
reported like CO2, CO, methane, isobutene and acetaldehyde [7].
The decomposition of cadmium acetate has not been investigated previously ex-
cept for a kinetic study [8]. At 130°C the compound becomes anhydrous. The anhy-
drous cadmium acetate melts at about 250°C and between 250–280°C decomposes [9].
*
Author for correspondence: E-mail: bmalecka@uci.agh.edu.pl
1388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
© 2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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MA˜ECKA: THERMAL DECOMPOSITION OF Cd(CH3COO)2⋅2H2O
Cadmium acetate dihydrate is a complex compound. The cadmium interacts with seven
oxygens (Cd–O distances range from 229.4 to 259.7 pm), two from the waters of
hydration and five from the acetate groups [10]. The co-ordination polyhedron of oxy-
gen atoms around the central metal atom is best described as a distorted square base-
trigonal capped polyhedron (idealized CS symmetry). Both of the acetate groups are
bidentate, but an oxygen atom in one of them is also in a bridging position forming a
continuous cadmium-oxygen spiral around the two-fold screw axis parallel to c. Both
acetate groups are planar. The oxygen atoms of each acetate ion can be regarded as
three-co-ordinate. The compound is extensively hydrogen-bonded. All oxygen atoms
and water hydrogen atoms are involved.
In the presented work the thermal decomposition of cadmium acetate dihydrate
under non-isothermal conditions was studied to reveal the mechanism of the process.
Experimental
Commercial Cd(CH3COO)2·2H2O of analytical grade (Polskie Odczynniki Chem-
iczne S.A.) was used. The experiments were carried out on TA Instruments SDT 2900 si-
multaneous thermal analyzer connected with Balzers QMD 300 Thermostar quadrupole
mass spectrometer for detection of gaseous products of reaction. The samples were heated
in a standard platinum crucible. The measurements were carried out under dynamic condi-
tions in inert and oxidizing atmospheres (helium or synthetic air). The flow
of 100 cm3 min–1 was controlled using a mass-flow controller. The heating rates used
were 2 and 5°C min–1. Some experiments were performed at 20°C min–1.Mass of the sam-
ples ranged between 8 and 11 mg.
The composition of the solid intermediates and the final products of
Cd(CH3COO)2·2H2O decomposition were analyzed by means of an X-ray diffracto-
meter using CuKα radiation (Seifert XRD-7).
Results and discussion
All the results from TG, DTA and MS analysis are normalized. TG-DTA signals ob-
tained during the decomposition of Cd(CH3COO)2·2H2O in helium and in air at the
heating rate 5°C min–1 are presented in Fig. 1.
Dehydration of Cd(CH3COO)2·2H2O
The TG curves for experiments carried out in air are quite different from those for ex-
periments performed in helium. The TG curve obtained during heating
Cd(CH3COO)2·2H2O in air exhibits three steps. The first two steps can be attributed to
the dehydration of the dihydrate. The only MS signals detected for this process were of
m/z=18, 17 and 16 corresponding to the mass spectrum of water. In each of the steps
the mass loss corresponds to the evolution of one molecule of H2O for each molecule
of cadmium acetate. The steps on the TG curve are accompanied by separate endo-
therms on the DTA curve. In helium atmosphere the dehydration proceeds in the same
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MA˜ECKA: THERMAL DECOMPOSITION OF Cd(CH3COO)2⋅2H2O
537
Fig. 1 TG and DTA curves for the decomposition of Cd(CH3COO)2·2H2O in helium and
air atmospheres at a heating rate of 5°C min–1. The inset presents the results of de-
hydration at the heating rate of 20°C min–1
way as in air however the separation of two dehydration steps on the TG curve is no-
ticed at higher heating rates. The stepwise loss of two water molecules is in accordance
with the crystal structure of Cd(CH3COO)2·2H2O, where the two water molecules are
bound in non-equivalent positions around the metal [10]. Dehydration is complete at
about 180°C. XRD analysis confirmed the presence of anhydrous Cd(CH3COO)2 in
samples cooled down from 180 and 200°C.
The exotherm at about 120°C observed in all experiments with inseparable de-
hydration steps can be attributed to the recrystallization of amorphous cadmium ace-
tate which occurs after dehydration connected with melting of the sample. No signifi-
cant change in the TG curve at this temperature is noticed as well as no mass
spectrometric signal.
The continuous mass loss in the TG curve between 100 and 240°C (more noticeable
for the measurements in helium) indicates the evolution of some volatile substance.
Before the beginning of the decomposition of Cd(CH3COO)2 the DTA curves
show endotherms at 200 and 220°C, for measurements in helium, and at 220°C for
measurements in air, which are not connected with changes in the TG curve. These
endotherms could result from some phase transitions of anhydrous cadmium acetate,
however very thorough X-ray diffraction analysis of the sample performed in a high
temperature camera did not confirm the presence of any different phases. DSC scans
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MA˜ECKA: THERMAL DECOMPOSITION OF Cd(CH3COO)2⋅2H2O
for anhydrous cadmium acetate in the range 120–230°C made during heating and
then during cooling showed that these effects are irreversible.
Decomposition of Cd(CH3COO)2 in air
The decomposition of anhydrous cadmium acetate is preceded by melting of the sam-
ple accompanied by an endotherm with a maximum at about 250°C (according to
NIST data, the melting temperature of anhydrous Cd(CH3COO)2 in air is 256°C). For
the decomposition carried out in air the total loss of mass of 51% is in good agree-
ment with the 51.8% mass loss calculated for CdO as the final solid product. XRD
analysis confirmed this result. At about 380°C decomposition is complete. On the
DTA curve, except for the endotherm corresponding to the melting of sample, two
exotherms are present. The endothermic effect of the decomposition seems to be su-
perimposed on the exothermic effect of some process and this results in DTA curve
having a horizontal segment in the temperature range of decomposition.
The mass spectrum of the gas over the decomposing cadmium acetate contained
the following m/z signals: 12, 14, 15, 16, 17, 18, 25, 26, 27, 28, 29, 30, 31, 32, 37, 38,
39, 40, 41, 42, 43, 44, 45, 53, 55, 56, 57, 58, 59, 60, 83 and 95. The main gases identi-
fied are: acetone CH3–CO–CH3 (mass spectrum: 14, 15, 25, 26, 27, 28, 29, 31, 37, 38,
39, 40, 41, 42, 43 – 100%, 53, 55, 57, 58, 59), CO2 (mass spectrum: 12, 16, 28,
44 – 100%), H2O (mass spectrum: 16, 17, 18 – 100%) and vapors of acetic acid (mass
spectrum: 15, 29, 42, 43 – 100%, 45, 60).
Figure 2 shows the signals m/z=18, 43 and 44 characteristic of water, acetone
and carbon dioxide, respectively. The composition of the gaseous phase over the de-
composing cadmium acetate depends strongly on the carrier gas. However, both in
helium and in air acetone is formed mainly in the beginning of the decomposition.
In order to connect the volatile products with the stage of the decomposition the
degree of evolution of different gases was determined from the intensities of the ion
currents using the procedure described elsewhere [11] and the degree of conversion
was determined from the TG curve. Figures 3 and 4 show the calculated degrees of
evolution of CH3COCH3, CO2, H2O and CH3COOH and the degree of conversion.
For decomposition in air atmosphere the degree of conversion corresponds with
the degree of evolution of acetone, which indicates that the main mass loss is related
to the formation and evolution of acetone. Thus during decomposition in air the fol-
lowing reaction occurs:
Cd(CH3COO)2(s)→CdO(s)+CH3COCH3(g)+CO2(g)
(1)
The degree of evolution of CO2 should be the same as the degree of evolution of
acetone, however, it is lower during the whole course of decomposition. Additionally
the maximum intensity of CO2 occurs 20°C above the temperature of the first maxi-
mum of the acetone intensity (Fig. 2). These effects as well as the presence of water
and acetic acid in the gaseous phase (Fig. 5 shows signal of m/z=60 which is charac-
teristic for acetic acid) indicate that the following secondary reactions can take place:
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MA˜ECKA: THERMAL DECOMPOSITION OF Cd(CH3COO)2⋅2H2O
539
CH3COCH3+O2→CO2+H2O
(2)
and
CH3COCH3+2O2→CH3COOH+CO2+H2O
(3)
These are catalytic reactions occurring probably on the surface of cadmium ox-
ide or on the surface of the platinum crucible and can explain the exotherms in the
DTA curve. Thus CO2 results not only from the primary reaction of Cd(CH3COO)2
Fig. 2 Selected MS signals for the decomposition of Cd(CH3COO)2 in helium and air at-
mospheres at a heating rate of 5°C min–1
Fig. 3 The degree of evolution of volatile products and the degree of conversion for the
decomposition of Cd(CH3COO)2 in air
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MA˜ECKA: THERMAL DECOMPOSITION OF Cd(CH3COO)2⋅2H2O
Fig. 4 The degree of evolution of volatile products and the degree of conversion for the
decomposition of Cd(CH3COO)2 in helium
Fig. 5 The MS signal for m/z=60 characteristic for acetic acid detected during decompo-
sition of Cd(CH3COO)2 in helium and air atmospheres at a heating rate of 5°C
decomposition, but also from the secondary reactions of the oxidation of acetone
which are delayed in relation to the primary reaction. The total amount of CO2 is
greater than that resulting from Cd(CH3COO)2 decomposition. This explains the dif-
ferences between the degrees of evolution of CO2 and of acetone.
Decomposition of Cd(CH3COO)2 in helium
Decomposition of Cd(CH3COO)2 in helium is quite different. Decomposition begins
at about 250°C. The TG curve shows several overlapping steps with corresponding
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MA˜ECKA: THERMAL DECOMPOSITION OF Cd(CH3COO)2⋅2H2O
541
endotherms on the DTA curve. All the mass changes are completed by about 520°C,
almost 200°C higher than in the experiments carried out in air. The total mass change
of 72% is much higher than the mass loss of 57.8% calculated for the formation of
metallic Cd (Fig. 1). This suggests that some part of the cadmium vaporizes. XRD
analysis showed that the solid residue from decomposition at 550°C is CdO.
The MS analysis showed the presence of the same signals as for decomposition in
air. The main amount of acetone is evolved until 280°C and the main amount of CO2 is
produced when acetone disappears (Fig. 2). The evolution of acetone and acetic acid
ends at about 350°C, while CO2 and H2O are detectable in the gas phase up to 520°C.
When the decomposition of Cd(CH3COO)2 proceeds in helium, the degree of
conversion is much lower than the degree of evolution of acetone during the whole
course of reaction (Fig. 4). The large mass loss is thus not only a consequence of ace-
tone evolution, but also results from the evolution of some volatile substance far later
than evolution of acetone occurs.
The solid residue sampled at several temperatures in the region 280–350°C is
composed mainly of CdO and of traces of metallic cadmium (Fig. 6) which was ob-
served only in few diffractograms. The formation of metallic cadmium during de-
composition of cadmium acetate in helium was confirmed in another experiment,
where 5 g sample of Cd(CH3COO)2 was placed in quartz tube and was heated in the
flow of helium. On the cold ends of the tube some black powder condensed which
was found to be metallic Cd.
Metallic Cd thus forms one of the solid products of Cd(CH3COO)2 decomposi-
tion in helium according to the following equation:
Cd(CH3COO)2(s)→Cd(s)+0.5O2(g)+CH3COCH3(g)+CO2(g)
(4)
The oxygen which forms during this reaction was observed in experiments per-
formed at high heating rates (Fig. 7).
The metallic cadmium formed during decomposition is very active and reacts
instantaneously with the oxygen formed in the same reaction, or with traces of oxy-
gen present in the inert atmosphere:
Cd(s)+0.5O2(g)→CdO(s)
(5)
The temperature of melting of cadmium is 321°C [12]. The cadmium vapor
pressure close to this temperature is relatively high and rises as the temperature in-
creases. The extra mass loss observed thus results from the vaporization of the part of
metallic cadmium formed during decomposition. The rate of vaporization depends
on the rate of heating.
The MS signal m/z=44 corresponding to CO2 reveals four peaks. All these peaks
can be ascribed to four endothermic effects on the DTA curve having extrema at the
same temperatures. The peaks with maxima at 249, 282 and 316°C correspond with
the peaks on the MS signal for acetone (Fig. 1). However, the degree of evolution of
CO2 is significantly lower than the degree of evolution of acetone during the whole
course of decomposition. In addition evolution of CO2 is complete by 520°C while
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MA˜ECKA: THERMAL DECOMPOSITION OF Cd(CH3COO)2⋅2H2O
Fig. 6 The XRD pattern of the solid residue taken at 350°C during the decomposition
Cd(CH3COO)2 in helium
acetone is not detectable above 350°C. All these observations indicate that there must
be another compound which decomposes giving CO2.
The most probable explanation is the formation of CdCO3 in a parallel reaction:
Cd(CH3COO)2(s)→CdCO3(s)+CH3COCH3(g)
(6)
The cadmium carbonate formed is assumed to be amorphous because XRD anal-
ysis performed in situ in a high-temperature camera did not reveal any other phases
than CdO and Cd. The grains of CdCO3 can decompose on the surface up to 350°C:
CdCO3→CdO+CO2
(7)
The remaining CdCO3 decomposes between 450–500°C that is confirmed by a
small peak on the CO2 signal, the endothermic effect on the DTA curve and the mass
Fig. 7 The MS signals for oxygen, acetone and carbon dioxide observed during the de-
composition of Cd(CH3COO)2 in helium at a heating rate of 20°C min–1
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MA˜ECKA: THERMAL DECOMPOSITION OF Cd(CH3COO)2⋅2H2O
543
loss on the TG curve. The total amount of CdO at the end of reaction at 520°C results
from the oxidation of metallic Cd and decomposition of CdCO3.
The presence of small amounts of water and acetic acid in the gas over the react-
ing cadmium acetate results from oxidation of acetone as proposed for the decompo-
sition in air – Eqs (2) and (3). This explains the decrease in the intensity of m/z=32
signal in the temperature range of decomposition in the experiments carried out with
low heating rates.
Some comments should be made on the evolution of acetic acid. From Fig. 4 we
see that evolution of acetic acid begins before the start of decomposition. These small
amounts of acetic acid could arise at a result of hydrolysis of cadmium acetate in the
presence of the crystallization water. When the temperature rises the acetic acid
evaporates but some of it can be trapped in the occlusions and be liberated at higher
temperatures.
No reliable explanation for the weak mass spectrometric signals of m/z=56, 83
and 95 can be given. The presence of ions of that high mass is evidence that some
secondary processes between gaseous products occur, but in a very small scale.
Conclusions
This study of the thermal decomposition of Cd(CH3COO)2·2H2O in helium and in air at-
mospheres has revealed the complexity of this process. The dehydration of the dihydrate
proceeds by two stages with the formation of Cd(CH3COO)2·H2O as an intermediate.
The path of decomposition of Cd(CH3COO)2 depends strongly on the surround-
ing atmosphere, as well as on the heating rate. The decomposition of Cd(CH3COO)2
in air leads to the formation of CdO as a solid product and acetone and carbon diox-
ide as the gaseous products. The decomposition of Cd(CH3COO)2 in helium occurs
by two parallel reactions. In the first reaction the solid product is metallic cadmium
and the gaseous products are acetone, carbon dioxide and oxygen. In the second reac-
tion cadmium carbonate and acetone are formed. CdCO3 then decomposes and metal-
lic Cd oxidizes. The final product of cadmium acetate decomposition in helium atmo-
sphere is CdO. Some of the metallic cadmium evaporates and its amount depends on
the heating rate and on the rate of oxidation of cadmium.
In air, and to a lesser extent in helium, secondary reactions between the gaseous
products occur, mainly oxidation of acetone.
* * *
The assistance of Miss Agnieszka ˜·cz, M.Sc., in TG-MS measurements is highly appreciated. The
author thanks the Polish State Committee for Scientific Research for financial support of this work
under project No. 4 T08A 020 24.
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