Study of thermal decomposition of silver acetate

Article abstract of DOI:10.1007/s10973-006-7883-9

Thermal decomposition of silver acetate was studied (TG, DSC, mass-spectrometry, X-ray analysis, electron microscopy). Non-isothermal thermogravimetric data (obtained at two different rates of linear heating) were used for kinetic studies. Kinetic paramet

Full text of DOI:10.1007/s10973-006-7883-9

Journal of Thermal Analysis and Calorimetry, Vol. 90 (2007) 3, 813–816
STUDY OF THERMAL DECOMPOSITION OF SILVER ACETATE
V. Logvinenko1*, O. Polunina2, Yu. Mikhailov2, K. Mikhailov2 and B. Bokhonov2
1Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, Ac. Lavrentyev Ave. 3
Novosibirsk 90, 630090, Russia
2Siberian University of Consumer’s Cooperation, K. Marx Ave. 26, Novosibirsk 630087, Russia
Thermal decomposition of silver acetate was studied (TG, DSC, mass-spectrometry, X-ray analysis, electron microscopy).
Non-isothermal thermogravimetric data (obtained at two different rates of linear heating) were used for kinetic studies. Kinetic
parameters were calculated only for the chosen decomposition step.
Keywords: ‘model free’ kinetics, silver acetate, silver nanoparticles, silver salts, thermal decomposition
Introduction
by Sigma Scan Pro (version 3.0, Jandel Scientific
Software, 1966).
Mass-spectrum were recorded by a mass-spec-
Thermal decomposition of metal carboxylates is now
widely used for the preparation of highly dispersed
metals and metals oxides; synthesized metals and
metal oxides can be used as catalysts [1–3].
Nanoparticles of metal can be obtained in such
processes because of rather low temperature of this
intramolecular reduction of metals.
trometer
with
high
resolution
(R=2000)
Funnigan MAT 8200; ionizing electrons energy
70 eV, accelerating voltage 3000 V. Gaseous
products (from the special heating system) were
injected into the ionic source through the leak (with
molecular leaking regime), with ionic source temper-
ature from 10 till 340°C.
The non-isothermal thermogravimetric data were
processed using the computer program ‘NETZSCH
Thermokinetics’ (version 2001.9d). Since the thermal
decompositions were multi-step, the convenient variant
of calculation was selected. Special program module,
‘Model free’ allows processing several thermo-
gravimetric curves, obtained with different heating
rates, without the information about the kinetic
topochemical equations. Programs ‘Ozawa–Flynn–Wall
Analysis’ and ‘Friedman Analysis’ [5] allow calculating
both the activation energies for the every experimental
point of fractional conversion (in the interval
0.02<a<0.98, conjointly from two curves). The same
set of experimental data was used further for searching
the topochemical equation [selecting from 16 equations
(chemical reaction on the interface, nucleation, and dif-
fusion)]. This calculation is made by the improved dif-
ferential method of Borchardt– Daniels with linear re-
gression [6]. F-test is used for the search of the best ki-
netic description [7]. If the calculations result in two or
three kinetics equations with near values of correlation
coefficients (or F-test), but with noticeably different val-
ues of kinetics parameters, it is rationally to choose the
equation with parameters values near to data of ‘Model
free’ module programs.
Experimental
The salt CH3COOAg was synthesized by the reaction of
silver carbonate with glacial acetic acid at 45–60°C; the
precipitate was filtered after cooling [4].
The study of thermal decomposition was
performed by means of Derivatograph Q-1500-D
(MOM, Hungary). Thermogravimetric curves were
obtained for kinetic studies (plate-like sample holder,
sample mass about 100 mg, heating rate 2.5 and
10°C min–1; helium flow 60 cm3 min–1). DSC curves
were obtained on Mettler DSC-822e/700 (m=
13.5 mg, argon flow 25 cm3 min–1). For the study of
the structural and morphological change in the silver
acetate crystals, the sample (m=1 g) was heated
consecutively at 180±1°C, 210±1°C and 350±1°C
(during 15, 25 and 25 min, accordingly).
SEM and TEM studies of intermediate products
and residue were made by means of
a
JEM-2000 FX II microscope (the resolution 200 ),
equipped by ASID-20 gear, with the accelerating
voltage 200 kV. The dimensions of product decom-
position were evaluated by the examination of
approximately 300 particles on TEM images, formed
*
Author for correspondence: val@che.nsk.su
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2007 Akadémiai Kiadó, Budapest

LOGVINENKO et al.
Results and discussion
The decomposition of the salt (under the conditions of
linear heating) proceeds at 170–280°C, the mass loss
(30%) corresponds to the formation of metallic silver
(Fig. 1).
The mass-spectrum of gaseous products points
to the evolving of CO, CO2, H2O, CH3COOH and
fragments with mass numbers 107, 216, 275.
DSC curve (Fig. 2) confirms that the decom-
position process itself is complex: the heat-evolution
at 260–286°C itself is multistep, the long flat
exothermic peak at 286–331°C is not connected with
any mass loss and can be referred to the fine particles
enlargement (recrystallization).
Fig. 3 SEM images of initial crystals of silver acetate
(l=70–100, d=5–10 mm)
structural and morphological changes) are lower than
appropriate transformation temperatures under linear
heating, which is understandable.
The initial crystals of silver acetate are long
needles: length is 70–100 mm, and thickness is
5–10 mm (Fig. 3). The fine silver particles appearance
is observed at the decomposition beginning (X-ray
analysis detects the metal silver after 15 min heating
at 180°); the initial size of Ag-particles is 40–50 nm
(Fig. 4); they intergrow and form the porous structure
at higher temperatures (Fig. 5). It is worth to note, that
all consecutive temperatures of the long-term iso-
thermal treatment of samples (for the study of the
The kinetic analysis by Ozawa–Flynn–Wall and
Friedman methods shows the complex course of the
decomposition: the relative constancy of the
activation energy realizes only at a=0.45–0.90
(Fig. 6), Ea=330–376 kJ mol–1. The search of the best
kinetic equation in this a-region results in two
equations: C1B and Bna with the catalysis by the
product (Praut–Tompkins equation and n-th order
with autocatalysis, Table 1).
Kinetic parameter (Ea=330–376 kJ mol–1) is
relatively constant only in the region of trans-
formation a=0.45–0.90; the decomposition process is
very complex (the mass-spectrum of the evolved gas
consist CO, CO2, H2O, CH3COOH and small quantity
Fig. 1 Thermal decomposition curves of CH3COOAg;
m=97.3 mg, plate-like sample holder, helium flow
(60 cm3 min–1), heating rate 10°C min–1
Fig. 4 TEM images of Ag-particles (40–50 nm), formed at
210°C (isothermal heating)
Fig. 2 DSC curve for the decomposition of CH3COOAg;
m=13.9 mg, standard aluminum-sample holder (the lid
with holes), heating rate 10 K min–1, argon flow
(25 cm3 min–1)
Fig. 5 The porous structure of silver residue composed of
accreted particles (1–2 mm) at 350°C (isothermal
heating)
814
J. Therm. Anal. Cal., 90, 2007

THERMAL DECOMPOSITION OF SILVER ACETATE
Table 1 Silver acetate AgCH3COO. Data of F-test on fit-quality (for the search of the best kinetic description, a=0.45–0.90)
No.
Code
Type
Fact
Fexp
F
crit (0.95)
Corr. coeff.
1
2
3
4
S
S
S
S
CnB
Bna
D3
5
4
6
6
1.00
1.29
5.10
6.32
4.42
4.42
0.998360
0.998359
0.988083
0.687516
5.96
A2
132.80
ration of highly dispersed silver metal. Rather low
temperature of this intramolecular reduction of metal
(210°C under isothermal heating, 260–286°C from
DSC experiment) results in the formation of non-
equilibrium and metastable nanoparticles of silver;
this nanoparticle phase is kinetically hindered: the
nanoparticles enlargement, occurring with the big
exothermic effect (and the constant rate) at
290–330°C, confirms their non–equilibrium state in
the moment of formation.
The thermal decomposition of salts and complex
compounds of varied metals (lanthanides and tran-
sition metals, in general) with carboxylic and other
organic ligands is widely studied nowadays [8–13].
Most of decomposition processes were conducted in
the air; really the studied process (terminologically)
was not the substance ‘thermal decomposition’
(intramolecular reduction of the metal by organic
ligand), but the substance ‘thermooxidative degra-
dation’, i.e. oxidation by air oxygen under heating
(intermolecular reaction); so the final residues were
metal oxy-salts, oxides or sulfides (nanoparticles
sometimes [11, 14]). Only the noble metals can be
obtained directly during such thermal decomposition
in the presence of air [15].
The silver nanoparticles formation (with the
reduction of silver ions in aqueous solution by
hydroquinone) was studied by the method of titration
microcalorimetry. It is interesting to note that there
are three consecutive steps leading to the formation of
silver nanoparticles: 1) nucleation step (exothermic),
2) growth step (endothermic) and 3) aggregation step
(exothermic) [16]. This reduction reaction is inter-
molecular {Ag++C6H4(OH)2}.
Fig. 6 The kinetic analysis by Friedman method of TG curves
(5 and 10°C min–1)
of radicals, included Ag). For the equations CnB or
Bna (reactions with autocatalysis) Ea=358 kJ mol–1,
logA=33. Both the high overheating (exothermic
reaction) and autocatalysis by the formed metal
particles complicates the decomposition still more. So
the calculated formal kinetic parameters are
evaluative and conditional (which is well seen from
the very high pre-exponential factor).
The overall exothermic thermal effect of decom-
position process (silver reduction at 270–286°C with
corresponding mass loss 30%) is 13 kJ mol–1. The flat
peak (290–330°C, being not connected with mass
loss, can be attributed to the particles enlargement
(Figs 4 and 5). The thermal effect of this process is
33 kJ mol–1. These values are evaluative ones (simple
average from two measurements), but it is obvious
that the formation of the silver nanoparticles
(non-equilibrium and metastable) takes place at low
temperatures, their enlargement occurs at 290–330°C,
and the heat of the enlargement is distinctly more than
the heat of reduction reaction.
We think that the intramolecular reaction of
silver reduction in solid state, considered above, does
not give a chance for clear separation of all these steps
in thermoanalytical experiments, especially with
linear heating. But the existence of nucleation and
aggregation steps is evident.
For checking that the heat of chemical reaction
of reduction is really smaller than the particles en-
largement (aggregation) we synthesized new sample
of CH3COOAg and repeated the DSC study, the heat
ratio is the same.
Acknowledgements
Conclusions
The authors are grateful to Dr. W.-D. Emmerich for providing
the new version of computer program ‘NETZSCH Thermo-
kinetics’.
Thermal decomposition of silver acetate in inert
atmosphere is a favorable technique for the prepa-
J. Therm. Anal. Cal., 90, 2007
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LOGVINENKO et al.
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DOI: 10.1007/s10973-006-7883-9
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