Thermal decomposition of hydrotalcite with hexacyanoferrate(II) and hexacyanoferrate(III) anions in the interlayer: A controlled rate thermal analysis study

Article abstract of DOI:10.1007/s10973-008-9448-6

The mechanism for the decomposition of hydrotalcite remains unsolved. Controlled rate thermal analysis enables this decomposition pathway to be explored. The thermal decomposition of hydrotalcites with hexacyanoferrate(II) and hexacyanoferrate(III) in the

Full text of DOI:10.1007/s10973-008-9448-6

Journal of Thermal Analysis and Calorimetry, Vol. 96 (2009) 2, 449–454
THERMAL DECOMPOSITION OF HYDROTALCITE WITH
HEXACYANOFERRATE(II) AND HEXACYANOFERRATE(III) ANIONS
IN THE INTERLAYER
A controlled rate thermal analysis study
Sara J. Palmer1, J. Kristóf2, Veronika Vágvölgyi2, Erzsébet Horváth3 and R. L. Frost1,*
1Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology,
2 George Street, GPO Box 2434, Brisbane, Queensland 4001, Australia
2Department of Analytical Chemistry, University of Pannonia, 8201 Veszprém, PO Box 158, Hungary
3Department of Environmental Engineering and Chemical Technology, University of Pannonia, 8201 Veszprém, PO Box 158,
Hungary
The mechanism for the decomposition of hydrotalcite remains unsolved. Controlled rate thermal analysis enables this
decomposition pathway to be explored. The thermal decomposition of hydrotalcites with hexacyanoferrate(II) and
hexacyanoferrate(III) in the interlayer has been studied using controlled rate thermal analysis technology. X-ray diffraction shows
the hydrotalcites have a d(003) spacing of 10.9 and 11.1  which compares with a d-spacing of 7.9 and 7.98  for the hydrotalcite
with carbonate or sulphate in the interlayer.
Calculations show dehydration with a total loss of 7 moles of water proving the formula of hexacyanoferrate(II) intercalated
hydrotalcite is Mg6Al2(OH)16[Fe(CN)6]0.5·7H2O and 9.0 moles for the hexacyanoferrate(III) intercalated hydrotalcite with the for-
mula of Mg6Al2(OH)16[Fe(CN)6]0.66·9H2O. CRTA technology indicates the partial collapse of the dehydrated mineral.
Dehydroxylation combined with CN unit loss occurs in two isothermal stages at 377 and 390°C for the hexacyanoferrate(III) and in
a single isothermal process at 374°C for the hexacyanoferrate(III) hydrotalcite.
Keywords: controlled rate thermal analysis (CRTA), hexacyanoferrate(II), hexacyanoferrate(III), hydrotalcite
M2+ and M3+ are the di- and trivalent cations in the
Introduction
octahedral positions within the hydroxide layers with
x normally between 0.17 and 0.33. An– is an exchangeable
Hydrotalcites, or layered double hydroxides (LDH’s)
are fundamentally anionic clays, and are less
well-known than cationic clays like smectites [1, 2].
The structure of hydrotalcite can be derived from a
brucite structure (Mg(OH)2) in which e.g. Al3+ or Fe3+
(pyroaurite-sjögrenite) substitutes for some of the
Mg2+ cations [3–14]. This substitution creates a
positive layer charge on the hydroxide layers, which
is compensated by interlayer anions or anionic complexes
[15, 16]. Further mixtures of these mineral phases
with multiple anions in the interlayer are observed.
When LDHs are synthesised any appropriate anion
can be placed in the interlayer. These anions may be
any anion with a suitable negative charge including
the hexacyanoferrate(II) and hexacyanoferrate(III)
ions [17–19]. The incorporation of these ions has
implications in electrochemistry [19–23]. The hydrotalcite
may be considered as a gigantic cation which is
counterbalanced by anions in the interlayer. In
interlayer anion [24]. In the hydrotalcites reevesite
and pyroaurite, the divalent cations are Ni2+ and Mg2+
respectively with the trivalent cation being Fe3+. In
these cases, the carbonate anion is the major
interlayer counter anion. Of course when synthesising
hydrotalcites any anion may be used [7, 14, 25–27].
Reevesite and pyroaurite are based upon the
incorporation of carbonate into the interlayer with
d(003) spacings of around 8  [28, 29]. Normally the
hydrotalcite structure based upon takovite (Ni,Al) and
hydrotalcite (Mg,Al) has basal spacings of ~8.0 
where the interlayer anion is carbonate.
Thermal analysis using thermogravimetric techniques
enables the mass loss steps, the temperature of the
mass loss steps and the mechanism for the mass loss
to be determined [6, 11, 30–34]. Thermoanalytical
methods can provide a measure of the thermal
stability of the hydrotalcite. Controlled rate thermal
analysis (CRTA) has proven extremely worthwhile in
the study of the stability and thermal decomposition
hydrotalcites a broad range of compositions are possible
of the type [M M3x+ (OH)2 ][An –
2+
1- x
]
x/ n •yH2O, where
*
Author for correspondence: r.frost@qut.edu.au
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2009 Akadémiai Kiadó, Budapest

PALMER et al.
pathways of minerals and modified minerals such as
Approximately 50 mg of sample was heated in an
open platinum crucible at a rate of 2.0°C min–1 up to
500°C. The TG instrument was coupled to a Balzers
(Pfeiffer) mass spectrometer for gas analysis. Only
selected gases were analysed.
mechanochemically activated kaolinite and intercalated
kaolinites [34–41]. The application of CRTA technology
to the study of the thermal stability of hydrotalcites
has to the best of our knowledge never been reported.
In this work we report the thermal analysis using
CRTA technology of hydrotalcite with hexacyanoferrate(II)
and hexacyanoferrate(III) anions in the interlayer.
Controlled rate thermal analysis experiment
Thermal decomposition of the intercalated hydrotalcite
was carried out in the Derivatograph under static air
at a pre-set, constant decomposition rate of 0.1
mg min–1. (Below this threshold value the samples
were heated under dynamic conditions at a uniform
rate of 1.0°C min–1.) The samples were heated in an
open ceramic crucible at a rate of 1.0°C min–1 up to
300°C. With the quasi-isothermal, quasi-isobaric heating
program of the instrument the furnace temperature
was regulated precisely to provide a uniform rate of
decomposition in the main decomposition stage.
Experimental
Synthesis of hydrotalcite samples
Hydrotalcites are able to be synthesised in the laboratory
using analytical grade chemicals. The reason for using
synthetic compounds as opposed to the natural minerals
is that difficulties associated with multiple anions in
the interlayer can be minimised, and allow for trends
and characteristics to be more readily determined.
The hydrotalcites reported below were synthesised by
the co-precipitation method.
X-ray diffraction
A mixed solution of aluminium and magnesium
nitrates ([Al3+]=0.25 M and [Mg2+]=0.75 M;
1 M=1 mol dm–3) and a mixed solution of sodium
hydroxide ([OH–]=2 M) and the desired anion, at the
appropriate concentration, were placed in two separate
vessels and purged with nitrogen for 20 min (all
compounds were dissolved in freshly decarbonated
water). The cationic solution was added to the anions
via a peristaltic pump at 40 mL min–1 and the pH
maintained above 9. The mixture was then aged at
75°C for 18 h under a N2 atmosphere. The resulting
precipitate was then filtered thoroughly, with room
temperature decarbonated water to remove nitrates
and left to dry in a vacuum desiccator for several
days. In this way hydrotalcites with different anions in
the interlayer were synthesised. The phase composition
was checked by X-ray diffraction and the chemical
composition by EDXA analyses.
X-ray diffraction patterns were collected using a Philips
X’pert wide angle X-ray diffractometer, operating in
step scan mode, with CuKa radiation (1.54052 ).
Patterns were collected in the range 3 to 90° 2q with a
step size of 0.02° and a rate of 30 s per step. Samples
were prepared as a finely pressed powder into
aluminium sample holders.
Results and discussion
X-ray diffraction
The X-ray diffraction patterns for the hexacyanoferrate(II)
and hexacyanoferrate(III) interlayered hydrotalcites
are shown in Fig. 1. For comparison the XRD patterns
of the sulphate and carbonate intercalated hydrotalcite
are shown. The XRD patterns clearly show the
formation of the hydrotalcites with the different anions
in the interlayer. The XRD patterns also show no
impurities in the synthesised hydrotalcites. Hydrotalcite
normally has a d(003) spacing of 7.9 . The sulphate
intercalated hydrotalcite has a spacing of 7.99 . The
hexacyanoferrate(II) complex has a spacing of 10.9 
and the hexacyanoferrate(III) hydrotalcite 11.1 .
The increased interlayer spacing is due to the size of
the anion between the brucite-like layers.
Thermal analysis
Conventional thermal analysis experiment
Thermal decomposition of the hydrotalcite was carried
out in a Derivatograph PC type thermoanalytical
equipment (Hungarian Optical Works, Hungary)
capable of recording the thermogravimetric (TG),
derivative thermogravimetric (DTG) and differential
thermal analysis (DTA) curves simultaneously. The
sample was heated in a ceramic crucible in static air
atmosphere at a rate of 5°C min–1.
Thermal analysis
Dynamic thermal analysis of hexacyanoferrate(II)
intercalated hydrotalcite
Thermal decompositions of the hydrotalcites
were also carried out in a TA® Instruments incorporated
high-resolution thermogravimetric analyser (series Q500)
in a flowing nitrogen atmosphere (80 cm3 min–1).
The dynamic thermal analysis of 157.95 mg of the
hexacyanoferrate(II) intercalated hydrotalcite is shown
450
J. Therm. Anal. Cal., 96, 2009

THERMAL DECOMPOSITION OF HYDROTALCITE
first step evolution of differently bound water occurs.
Over 500°C a slow mass loss step is observed which
is due to the degradation of the decomposed mineral.
The following steps describe the thermal
decomposition of the
hexacyanoferrate(II)-interlayered hydrotalcite.
Step 1 up to 280°C
Mg6Al2(OH)16(Fe(CN)6)0.5·7H2O®
® Mg6Al2(OH)16(Fe(CN)6)0.5+7H2O
This step includes the loss of hydration water.
Based on the mass loss data, the mineral contains
seven moles of water, although the presence of some
adsorbed water cannot be excluded.
Step 2 between 280 and 410°C
Mg6Al2(OH)16(Fe(CN)6)0.5®
® Mg6Al2O2(OH)13(Fe(CN)4)0.5+HCN+H2O
Mg6Al2O2(OH)13(Fe(CN)4)0.5®
® Mg6Al2O3(OH)11(Fe(CN)4)0.5+H2O
This step represents the simultaneous evolution
of dehydroxylation water and the cyanide ligand in
the form of HCN.
Fig. 1 X-ray diffraction patterns of hydrotalcite with
carbonate, sulphate, hexacyanoferrate(II) and
hexacyanoferrate(III) anions in the interlayer
Step 3 between 410 and 600°C
4Mg6Al2O3(OH)11(Fe(CN)4)0.5+0.5O2®
® 22MgO+2Al2O3+2MgAl2O4+Fe2O3+8HCN+18H2O
In this step the decomposition is complete leading
to the formation of the oxides of Mg and Al as well as
the formation of a magnesium aluminate (spinel).
Dynamic thermal analysis of hexacyanoferrate(III)
intercalated hydrotalcite
The dynamic thermal analysis of the hexacyanoferrate(III)
intercalated hydrotalcite is shown in Fig. 3. Comparing
the curves the following conclusions can be drawn. In
the temperature range from ambient to about 280°C
adsorbed and hydrated water are lost (the wide
temperature range indicates differently bound water).
Dissimilarly to the hexacyanoferrate(II)-type hydrotalcite
the decomposition of the dehydrated mineral takes
place in a more homogeneous process between 280
and 410°C (with a characteristic DTG peak at 372°C
with shoulders only at 360 and 390°C). Between 410
and 600°C a slow process can be observed, as well.
Again, in the 410 to 600°C range dehydroxylation
and the loss of the CN units take place. The proposed
decomposition pathway is as follows:
Fig. 2 The dynamic thermogravimetric analysis of
hexacyanoferrate(II) interlayered hydrotalcite
in Fig. 2. Comparing the curves the following conclusions
can be drawn. In the temperature range from ambient
to about 600°C three main decomposition stages can
be observed in the DTG curve: (a) from ambient to
280°C, (b) from 280 to 410°C and (c) between 410
and 600°C. The first mass loss step is associated with
dehydration, the second with DTG peaks at 361, 374
and 387°C with dehydroxylation and the partial loss
of the cyanide ligand, while the third mass loss step
with loss of the CN units and also the loss of the
additional OH units. It can be supposed that in the
J. Therm. Anal. Cal., 96, 2009
451

PALMER et al.
particle shall be heated under identical conditions.
With the slow and constant decomposition rate of
0.10 mg min–1 the decomposition is carried out under
quasi-isothermal, quasi-equilibrium conditions. This
is achieved by the control of the furnace temperature
regulated by the DTG signal through the computer.
Controlled rate thermal analysis of
hexacyanoferrate(II) intercalated hydrotalcite
Table 1 summarises the mass loss in mg and the %
mass loss over a specific temperature range.
The CRTA curves of 156.22 mg sample are shown
in Fig. 4. In the ambient to 267°C range dehydration
occurs in a single step. The non-isothermal nature of
dehydration process is rather unusual and needs to be
explained. If a decomposition process of constant gas
evolution rate is non-isothermal, it means that hidden
processes slower than the heat transport have a role.
We believe that the layers are partially collapsing
upon dehydration (decrease in the d(003) spacing),
therefore higher energy (i.e. higher temperature) is
needed to maintain the preset, constant rate of
decomposition (i.e. to drive out water vapor through a
reduced space). The small peak between 267 and
310°C is probably due to the loss of more strongly
Fig. 3 The dynamic thermogravimetric analysis of
hexacyanoferrate(III) interlayered hydrotalcite
Step 1 from ambient to 280°C
Mg6Al2(OH)16(Fe(CN)6)0.66·9H2O®
® Mg6Al2(OH)16(Fe(CN)6)0.66+9H2O
This step includes the loss of hydration water.
Based on the mass loss data, the mineral contains nine
moles of water.
Step 2 between 280 and 600°C
Mg6Al2(OH)16(Fe(CN)6)0.66
®
® Mg6Al2O2(OH)12(Fe(CN)6)0.66+2H2O
3Mg6Al2O2(OH)12(Fe(CN)6)0.66
®
® 18MgO+3Al2O3+Fe2O3+12HCN+12H2O
This step represents a dehydroxylation step
accompanied with cyanide loss.
In order to better resolve the decomposition
processes, controlled rate thermal analysis (CRTA)
experiments were carried out as well in the same
equipment using the CRTA control facility. In this
case the decomposition of the mineral was carried out
at a preset, constant, slow rate to provide enough time
for the slow heat and mass transfer processes to occur.
The essence of the technique lies in that each sample
Fig. 4 The controlled rate thermal analysis of
hexacyanoferrate(II) interlayered hydrotalcite
Table 1 Decomposition stages under CRTA conditions
Hydrotalcite with hexacyanoferrate(II)
Hydrotalcite with hexacyanoferrate(III)
(sample mass: 143.76 mg)
(sample mass: 156.22 mg)
Decomposition process
Mass loss
Temp.
Mass loss
Temp.
range/°C
mg
range/°C
%
mg
%
Dehydration
31–267
29.5
18.9
22–226
26.4
4.3
18.4
3.0
226–270
Dehydration
267–310
310–367
367–383
383–428
428–693
1.3
6.1
0.8
3.9
9.7
8.3
6.5
270–315
315–366
366–390
390–427
427–694
0.9
5.7
0.6
4.0
Dehydroxylation/CN loss
Dehydroxylation/CN loss
Dehydroxylation/CN loss
Oxygen loss
15.2
13.0
10.1
21.8
4.9
15.2
3.4
7.4
5.1
452
J. Therm. Anal. Cal., 96, 2009

THERMAL DECOMPOSITION OF HYDROTALCITE
bonded water, although the possibility of early
loss occurs in two main stages corresponding to
dehydration and to the decomposition of the
water-free mineral.
dehydroxylation cannot be excluded.
In the temperature range between 310 and 428°C
dehydroxylation and the decomposition of the cyanide
ligand take place simultaneously. Interestingly, two iso-
thermal processes at 377 and 390°C can be distin-
guished (in harmony with the split DTG pattern) of the
dynamic experiment. The separate decomposition process
between 428 and 500°C correspond to the decomposi-
tion step between 410 and 600°C in the dynamic experi-
ment. With the CRTA method a better resolution of the
closely overlapping reactions can be made.
CRTA offers a better resolution and a more de-
tailed interpretation of the decomposition processes via
approaching equilibrium conditions of decomposition
through the elimination of the slow transfer of heat to
the sample as a controlling parameter on the process of
decomposition. Constant-rate decomposition processes of
non-isothermal nature reveal partial collapse of the lay-
ers, since in these cases a higher energy (higher tempera-
ture) is needed to drive out gaseous decomposition products
through a decreasing space at a constant, pre-set rate.
Controlled rate thermal analysis of
hexacyanoferrate(III) interlayered hydrotalcite
Appendix
The CRTA curves of 143.76 mg sample are shown in
Fig. 5. In the ambient to 226°C range the non-isothermal
process of dehydration takes place. This process can
be interpreted as in the case of the
Calculation of water content for hydrotalcite with
hexacyanoferrate(II)
Composition: Mg6Al2(OH)16[Fe(CN)6]0.5·xH2O
Removing water up to 280°C: 29.50 mg that is 1.637 mmol
Remaining dehydrated mineral up to 2807°C: 137.32 mg that
is 0.238 mmol
hexacyanoferrate(II) complex. In this case, however,
two separate dehydration steps can be distinguished
between 226 and 270 as well as between 270 and
315°C. This separate processes indicate the presence
of differently bound water in the interlayer space.
Dissimilarly to the hexacyanoferrate(II) complex,
only one isothermal process can be seen in the main
decomposition stage at 374°C. This is in harmony
with the dynamic experiment where the DTG peak
shows a more homogeneous decomposition pattern.
Again, the separate peak between 427 and 500°C
indicates the complexity of the decomposition process.
Molar mass of dehydrated mineral: 577.97 g mol–1
Calculation of x:
1 mol dehydrated mineral – x mol H2O
0.238 mol dehydrated mineral – 1.637 mol H2O
x=6.88~7 mol
Formula: Mg6Al2(OH)16[Fe(CN)6]0.5·7H2O
Calculation of water content for hydrotalcite with
hexacyanoferrate(III)
Composition: Mg6Al2(OH)16[Fe(CN)6]0.66·xH2O
Removing water up to 280°C: 30.70 mg that is 1.704 mmol
Remaining dehydrated mineral up to 280°C: 116.06 mg that is
0.190 mmol
Molar mass of dehydrated mineral: 611.88 g mol–1
Calculation of x:
1 mol dehydrated mineral – x mol H2O
0.190 mol dehydrated mineral – 1.704 mol H2O
x=8.97~9 mol
Fig. 5 The controlled rate thermal analysis of
Formula: Mg6Al2(OH)16[Fe(CN)6]0.66·9H2O
hexacyanoferrate(III) interlayered hydrotalcite
Conclusions
Acknowledgements
The thermal decomposition of hydrotalcites based
upon a Mg/Al ratio of 6/2 with hexacyanoferrate(II)
and (III) in the interlayer has been studied using
dynamic and controlled rate thermal analysis techniques
complemented with X-ray diffraction. Both the
dynamic and the CRTA experiments show that mass
This research was supported by the Hungarian Scientific
Research Fund (OTKA) under grant No. K62175. The financial
and infra-structure support of the Queensland University of
Technology Inorganic Materials Research Program is
gratefully acknowledged. One of the authors (SJP) is grateful
to RioTintoAlcan for a PhD scholarship.
J. Therm. Anal. Cal., 96, 2009
453

PALMER et al.
23 K. Yao, M. Taniguchi, M. Nakata and M. Takahashi,
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DOI: 10.1007/s10973-008-9448-6
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