Thermal decomposition of hydrotalcite with chromate, molybdate or sulphate in the interlayer

Article abstract of DOI:10.1016/j.tca.2005.03.014

The thermal decomposition of hydrotalcites with chromate, molybdate and sulphate in the interlayer has been studied using thermogravimetric analysis coupled to a mass spectrometer measuring the gas evolution. X-ray diffraction shows the hydrotalcites have a d(0 0 3) spacing of 7.98 ? with very small differences in the d-spacing between the three hydrotalcites. XRD was also used to determine the products of the thermal decomposition. For the sulphate-hydrotalcite decomposition the products were MgO and a spinel MgAl ₂O₄, for the chromate interlayered hydrotalcite MgO, Cr₂O₃ and spinel. For the molybdate interlayered hydrotalcite the products were MgO, spinel and MgMoO₄. EDX analyses enabled the formula of the hydrotalcites to be determined. Two processes are observed in the thermal decomposition namely dehydration and dehydroxylation and for the case of the sulphate interlayered hydrotalcite, a third process is the loss of sulphate. Both the dehydration and dehydroxylation take place in three steps each for each of the hydrotalcites.

Full text of DOI:10.1016/j.tca.2005.03.014

Thermochimica Acta 429 (2005) 179–187
Thermal decomposition of hydrotalcite with chromate,
molybdate or sulphate in the interlayer
Ray L. Frost∗, Anthony W. Musumeci, Thor Bostrom, Moses O. Adebajo,
Matt L. Weier, Wayde Martens
Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology,
GPO Box2434, Brisbane Queensland 4001, Australia
Received 24 January 2005; received in revised form 15 March 2005; accepted 19 March 2005
Available online 14 April 2005
Abstract
The thermal decomposition of hydrotalcites with chromate, molybdate and sulphate in the interlayer has been studied using thermogravi-
metric analysis coupled to a mass spectrometer measuring the gas evolution. X-ray diffraction shows the hydrotalcites have a d(0 0 3) spacing
˚
of 7.98 A with very small differences in the d-spacing between the three hydrotalcites. XRD was also used to determine the products of
the thermal decomposition. For the sulphate-hydrotalcite decomposition the products were MgO and a spinel MgAl2O4, for the chromate
interlayered hydrotalcite MgO, Cr2O3 and spinel. For the molybdate interlayered hydrotalcite the products were MgO, spinel and MgMoO4.
EDX analyses enabled the formula of the hydrotalcites to be determined. Two processes are observed in the thermal decomposition namely
dehydration and dehydroxylation and for the case of the sulphate interlayered hydrotalcite, a third process is the loss of sulphate. Both the
dehydration and dehydroxylation take place in three steps each for each of the hydrotalcites.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Hydrotalcite; Takovite; Pyroaurite; Chromate; Molybdate; Thermal analysis; Thermogravimetry
[M2+
M
x(OH)2][An−
]
x/n·yH2O, whereM2+ andM3+ are
3+
1. Introduction
1−x
the di- and trivalent cations in the octahedral positions within
the hydroxide layers with x normally between 0.17 and 0.33.
An− is an exchangeable interlayer anion [3]. In the hydrotal-
cites 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 synthesizing hydrotalcites
any anion may be used. Normally, the hydrotalcite struc-
ture based upon takovite (Ni,Al) and hydrotalcite (Mg,Al)
Hydrotalcites or layered double hydroxides (LDHs) are
fundamentally anionic clays, and are less well-known than
cationic clays like smectites. The structure of hydrotalcite
can be derived from a brucite structure (Mg(OH)2) in which,
e.g., Al3+ or Fe3+ (pyroaurite–sjogrenite) substitutes a part
¨
of the Mg2+. Further, mixtures of these mineral phases
with multiple anions in the interlayer are observed. When
LDH’s are synthesized any appropriate anion can be placed
in the interlayer. This substitution creates a positive layer
charge on the hydroxide layers, which is compensated by
interlayer anions or anionic complexes [1,2]. The hydro-
talcite may be considered as a gigantic cation, which is
counterbalanced by anions in the interlayer. In hydrotal-
cites, a broad range of compositions are possible of the type
˚
has basal spacings of ∼8.0 A where the interlayer anion is
carbonate.
Thermal analysis using thermogravimetric techniques
(TG) enables the mass loss steps, the temperature of the mass
loss steps and the mechanism for the mass loss to be deter-
mined. Thermoanalytical methods provide a measure of the
thermal stability of the hydrotalcite.
∗
The reason for the potential application of hydrotalcites
as catalysts rests with the ability to make mixed metal oxides
Corresponding author. Tel.: +61 7 38642407; fax: +61 7 38641804.
E-mail address: r.frost@qut.edu.au (R.L. Frost).
0040-6031/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2005.03.014

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R.L. Frost et al. / Thermochimica Acta 429 (2005) 179–187
at the atomic level, rather than at a particle level. One would
expect that the potential application of hydrotalcites as cata-
lysts would rest on reactions occurring on their surfaces. The
significance of the formation of the mixed metal oxides is
their importance as a transition material in the synthesis of
catalysts. In this work, we report the thermogravimetric anal-
ysis of hydrotalcite with sulphate, chromate or molybdate in
the interlayer.
SEMusinganEDAXmicroanalyser, andmicroanalysisofthe
clusters of fine crystals was carried out using a full standards
quantitative procedure on the JEOL 840 SEM using a Moran
Scientific microanalysis system. Chromite was used as a stan-
dard for Cr, molybdate for Mo, anhydrite for S. Almandine
garnet and pyrope garnet were also used in the calibration
of the EDX analyses. Oxygen was not measured directly but
was calculated using assumed stoichiometries to the other
elements analysed.
2. Experimental
2.3.1. Thermal analysis
Thermal decompositions of the hydrotalcites were car-
ried out in a TA® Instruments incorporated high-resolution
thermogravimetric analyzer (series Q500) in a flowing ni-
trogen atmosphere (80 cm3/min). Approximately 50 mg of
sample was heated in an open platinum crucible at a rate of
2.0 ◦C/min up to 1000 ◦C. The TGA instrument was coupled
to a Balzers (Pfeiffer) mass spectrometer for gas analysis.
The following gases were analyzed: CO, CO2, SO2, SO3,
and H2O. Mass/charge ratios are measured for example O2
is 32/1 and 32/2.
Band component analysis of the DTG curves was under-
taken using the Jandel ‘Peakfit’ software package, which en-
abled the type of fitting function to be selected and allows
specific parameters to be fixed or varied accordingly. Band
fitting was done using a Gauss-Lorentz cross-product func-
tion with the minimum number of component bands used for
the fitting process. The Gauss-Lorentz ratio was maintained
at values greater than 0.7 and fitting was undertaken until
reproducible results were obtained with squared correlations
of r2 greater than 0.995.
2.1. Synthesis of hydrotalcite compounds
A mixed solution of aluminium and magnesium nitrates
([Al3+] = 0.25 M and [Mg2+] = 0.75 M; 1 M = 1 mol/dm3) 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 wa-
ter). The cationic solution was added to the anions via a peri-
staltic pump at 40 mL/min and the pH maintained above 9.
The mixture was then aged at 75 ◦C for 18 h under a N2 atmo-
sphere. 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 EDAX analyses.
2.2. X-ray diffraction
X-ray diffraction patterns were collected using a Philips
X’pert wide angle X-Ray diffractometer, operating in step
scan mode, with Cu K␣ radiation (1.54052 A). Patterns were
3. Results and discussion
˚
collected in the range 3–90◦ 2θ 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. The Profile
Fitting option of the software uses a model that employs 12
intrinsic parameters to describe the profile, the instrumen-
tal aberration and wavelength dependent contributions to the
profile.
3.1. X-ray diffraction
The X-ray diffraction patterns of the hydrotalcite of
formula (Mg6Al2(OH)16(XO4)·4H2O) where X is S, Cr or
Mo is shown in Fig. 1. The figure clearly shows the X-ray
pattern for hydrotalcite with no peaks due to other phases.
The d(0 0 3) spacing for the sulphate, chromate and molyb-
˚
date interlayered hydrotalcites are 7.99, 7.98 and 7.97 A,
respectively. Such values are close to the d-spacing values re-
ported for the natural hydrotalcite with sulphate in the inter-
layer [4].
2.3. SEM and X-ray microanalysis
Hydrotalcite samples were coated with a thin layer of
evaporated carbon and secondary electron images were ob-
tainedusinganFEIQuanta200scanningelectronmicroscope
(SEM). For X-ray microanalysis (EDX), three samples were
embedded in Araldite resin and polished with diamond paste
on Lamplan 450 polishing cloth, using water as a lubricant.
The samples were coated with a thin layer of evaporated car-
bon for conduction and examined in a JEOL 840A analytical
SEM at 25 kV accelerating voltage. Preliminary analyses of
the hydrotalcite samples were carried out on the FEI Quanta
The XRD of the products of the thermal decomposition
of the chromate interlayered hydrotalcite shows that MgO
(JCPD file 45-0946), Cr2O3 (01-1294) and spinel (75-1798)
are formed (Fig. 2). The products of the thermal decompo-
sition of the molybdate-hydrotalcite were MgO, MgMoO4
(21-0961) and MgAl2O4. The products of the sulphate-
hydrotalcite were a mixture of the oxides of Mg and Al.
These types of products are in agreement with published data
[5].

R.L. Frost et al. / Thermochimica Acta 429 (2005) 179–187
181
ance of the negative charge must be due to hydroxyl ions
in the interlayer. Vibrational spectroscopy supports this con-
clusion as no carbonate or nitrate bands were found in the
Raman spectra of the hydrotalcite. Thus, the formula of
the synthetic hydrotalcites interlayered with chromate may
be given by Mg6Al2(OH)16(CrO4,OH)·4H2O and similarly
for the hydrotalcite interlayered with molybdate and sul-
phate.
3.3. Thermogravimetry and mass spectrometric analysis
of the chromate-hydrotalcite
High resolution thermogravimetry can measure to six dec-
imals of mass enabling subtle mass loss steps to be obtained.
This enables changes in stoichiometries to be made. Phase
changes would need to be identified using XRD. When the
TG is converted to a DTG curve then additional informa-
tion often differentiating between closely overlapping mass
loss steps can be obtained. The use of evolved gas mass spec-
trometry when coupled to the TG allows definitive identifica-
tion of the evolved gases. Normally, the DTG curve matches
the evolved gas mass spectrum curve. Such techniques have
been used to study quite complex mineralogical systems
[6–10].
The thermogravimetric analysis of hydrotalcite with chro-
mate as the counterbalancing ion is shown in Fig. 4. The ion
current graphs reporting the mass of evolved gases are shown
in Fig. 5. Five mass loss steps are observed at 99, 192, 354,
427 and 452 ◦C, which correspond to percentage mass losses
of 7.4, 4.8, 10.1, 10.4 and 3.8. An additional mass loss of
2.6% occurs over a temperature range centred on 498 ◦C. The
ion current curves for masses 18, 17 and 16 show that water
is evolved at around 109, 191, 349 and 446 ◦C. This means
that water and/or OH units are being lost at these tempera-
tures. The mass loss of 44 (CO2) proves that some carbon-
ate is present which decomposes to CO2 at 354 and 414 ◦C.
Evolved oxygen is observed at 463, 490 and 618 ◦C but ap-
pears to be lost continuously over an extended temperature
range.
The first two mass loss steps are attributed to the removal
of water from the hydrotalcite interlayer. The mass loss steps
at 354, 427 and 452 ◦C are attributed to the dehydroxyla-
tion of the chromate interlayered hydrotalcite. The mass loss
changes at 498 and the long-range temperature decrease in
mass are assigned to a loss of oxygen. The theoretical mass
loss based upon the formula Mg6Al2(OH)16(CrO4)·4H2O
for the loss of water is 10.9%, and for the OH units is
21.8%. The experimental mass loss for water if the first two
steps are assumed to be the mass losses due to dehydra-
tion, is 12.2%. This value is slightly higher than the theo-
retical value and may be due to adsorbed water. Therefore,
the compound may be hydrated with more than four water
units in the formula. The DTG profile for temperature range
from 240 to 440 ◦C shows three dehydroxylation steps. The
relative areas of the three DTG peaks are 10.2, 10.47 and
4.55%, which is approximately a ratio of 10 and 10–5. Thus,
Fig. 1. XRD patterns of the hydrotalcite interlayered with sulphate, molyb-
date or chromate.
3.2. EDX analyses
The EDX analyses are illustrated by Fig. 3a–c, being
the analyses for the hydrotalcite interlayered with chro-
mate, molybdate and sulphate. The results of nine analy-
ses are reported in Table 1. In all three analyses the ratio
of Mg to Al is slightly less than the theoretical value of
3:1. For the chromate interlayered hydrotalcite the value is
2.771, for the molybdate interlayered hydrotalcite the value
is 2.89 and for the sulphate interlayered hydrotalcite the
value is 2.75. The surface of the brucite structure should
have no charge per Mg atom. When a trivalent anion is
substituted for the Mg such as Al, a charge of 1 is intro-
duced. For two Al atoms the charge is 2. Thus, this posi-
tive charge will be counterbalanced by the anions chromate,
molybdate or sulphate. The ratio of Al to anion is theoret-
ically 2:1. For the chromate interlayered hydrotalcite the
ratio of Al:Cr is 6.20, for the molybdate interlayered hy-
drotalcite the value is 4.70 and for the sulphate interlay-
ered hydrotalcite the value is 5.06. Thus, the ratio of the
moles of the anions is low compared with the theoretical
value. The EDX analyses clearly show no impurities. No
C for carbonate or N for nitrate is present within the lim-
its of the experiment. Thus, it is concluded that the bal-

182
R.L. Frost et al. / Thermochimica Acta 429 (2005) 179–187
Fig. 2. XRD patterns of the thermal decomposition products of the hydrotalcite interlayered with chromate, molybdate and sulphate together with the patterns
of the reference materials.

R.L. Frost et al. / Thermochimica Acta 429 (2005) 179–187
183
Fig. 3. EDX analyses of the hydrotalcite interlayered with (a) chromate, (b) molybdate and (c) sulphate.
Table 1
EDX analyses of the synthetic hydrotalcites interlayered with chromate, molybdate and sulphate
Element
Scan 1
Scan 2
Scan 3
Average
Mole %
Weight %
Mole %
Weight %
Mole %
Weight %
Mole %
Molybdate
Mg
Al
O
22.32
8.62
62.32
6.75
17.64
6.14
74.87
1.35
22.87
8.7
61.29
7.14
18.2
6.24
74.13
1.44
22.58
8.66
62.95
5.81
17.71
6.12
75.01
1.15
17.85
6.17
74.67
1.31
Mo
Sulphate
Mg
Al
O
24.49
9.79
62.99
2.72
18.68
6.73
73.01
1.57
24.83
10.09
63.15
1.92
18.9
6.92
73.06
1.11
23.33
9.41
65.02
2.24
17.63
6.41
74.68
1.29
18.40
6.69
73.58
1.32
S
Chromate
Mg
Al
O
23.65
9.38
64.58
2.38
18.01
6.43
74.71
0.85
23.9
9.6
63.52
2.98
18.32
6.63
73.98
1.07
24.98
10.09
61.28
3.65
19.38
7.05
72.25
1.32
18.57
6.70
73.65
1.08
Cr

184
R.L. Frost et al. / Thermochimica Acta 429 (2005) 179–187
Step 1 at 99 ◦C: This step includes the loss of adsorbed
water.
Mg6Al2(OH)16(CrO4) · 4H2O
→ Mg6Al2(OH)16(CrO4) · 2H2O + 2H2O
This step represents the first dehydration step and shows
2 mol of water are lost at this temperature.
Step 2 at 192 ◦C:
Mg6Al2(OH)16(CrO4) · 2H2O
→ Mg6Al2(OH)16(CrO4) + 2H2O
This step represents the second dehydration step and
shows 2 mol of water are lost at this temperature.
Step 3 at 354 ◦C:
Fig. 4. TG and DTG patterns of chromate interlayered hydrotalcite.
the ratio of moles of OH units lost in these three steps is
2:2:1.
Mg6Al2(OH)16(CrO4)
→ Mg6Al2O3(OH)10(CrO4) + 3H2O
3.4. Mechanism for the decomposition of hydrotalcite
with chromate in the interlayer
This step represents the first dehydroxylation step and 6
OH units are lost at this temperature. The oxygen is taken
up by the cations present and is shown representatively as
Al2O3.
The following steps describe the thermal decomposition
of the chromate hydrotalcite.
Step 4 at 416 ◦C:
Mg6Al2O3(OH)10(CrO4)
→ Mg3(3MgO)Al2O3(OH)4(CrO4) + 3H2O
This step represents the second dehydroxylation step and
6 OH units are lost at this temperature.
Step 5 at 452 ◦C:
Mg3(3MgO)Al2O3(OH)4(CrO4)
→ (Mg5MgO)Al2O3(CrO4) + 2H2O
This step represents the third dehydroxylation step and 4
OH units are lost at this temperature.
Step 6 at 498 ◦C:
2[(Mg5MgO)Al2O3(CrO4)]
→ 10MgO + 2MgAl2O4 + Cr2O3 + 1.5O2
This step shows that oxygen is lost and the final products of
the thermal decomposition step are the oxides of Cr and Mg
and a magnesium aluminate (spinel).
3.5. Thermogravimetry and mass spectrometric analysis
of the molybdate-hydrotalcite
The TG and DTG of molybdate-hydrotalcite are shown in
Fig. 6 and the ion current curves of the evolved gases in Fig. 7.
The TG analysis shows five mass loss steps at 94, 189, 415,
433 and 481 ◦C with mass losses of 8.3, 7.0, 12.0, 9.1 and
3.6%. The ion current curves show that water is evolved in
each of these steps except the final step. The MS = 44 ion cur-
rent curve showsthat some CO2 islost indicatingthe presence
of some carbonate in the interlayer as well as the molybdate
Fig. 5. Thermal evolved gas analysis (ion current curves) of chromate inter-
layered hydrotalcite for selected gases.

R.L. Frost et al. / Thermochimica Acta 429 (2005) 179–187
185
3.6. Mechanism for the decomposition of hydrotalcite
with molybdate in the interlayer
The following steps describe the thermal decomposition
of the molybdate hydrotalcite.
Step 1 at 94 ◦C: This step includes the loss of adsorbed
water.
Mg6Al2(OH)16(MoO4) · 4H2O
→ Mg6Al2(OH)16(MoO4) · H2O + 3H2O
This step represents the first dehydration step and shows
3 mol of water are lost at this temperature.
Step 2 at 189 ◦C:
Fig. 6. TG and DTG patterns of molybdate interlayered hydrotalcite.
Mg6Al2(OH)16(MoO4) · H2O
→ Mg6Al2(OH)16(MoO4) + H2O
This step represents the second dehydration step and
shows 1 mol of water is lost at this temperature.
Step 3 at 404 ◦C:
ion. Raman spectroscopy shows that the amount of carbonate
is at very low concentrations. This simply shows the difficulty
of keeping CO2 out of the preparation route for the synthe-
sis of the molybdate-hydrotalcite. The total mass loss of the
thermal decomposition is 40.3%. From the mass of the final
products it is calculated that there were 7 mol of water in the
starting material. Thus, the molybdate interlayered hydrotal-
cite is similar to a hydrohonnesite in formulation.
Mg6Al2(OH)16(MoO4)
→ Mg6Al2O4.5(OH)7(MoO4) + 4.5H2O
The DTG curve for the 330–450 ◦C may be curve resolved
with two peaks at 404 and 433 ◦C with relative areas of 12.0
and 9.5%. If these mass loss steps are ascribed to the loss
of the OH units then it is suggested that 9 OH units are lost
at 404 ◦C and 7 at 433 ◦C. This step at 404 ◦C represents
the first dehydroxylation step and 9 OH units are lost at this
temperature. The oxygen is taken up by the cations present
and is shown representatively as Al2O3.
Step 4 at 433 ◦C:
Mg6Al2O4.5(OH)7(MoO4)
→ Mg6Al2O8(MoO4) + 3.5H2O
This step represents the second dehydroxylation step and
7 OH units are lost at this temperature.
Step 5 at 481 ◦C:
Mg6Al2O8(MoO4)
→ 4MgO + MgMoO4 + MgAl2O4 + O2
This step shows the final thermal decomposition step.
3.7. Thermogravimetry and mass spectrometric analysis
of the sulphate-hydrotalcite
The TG and DTG curves for sulphate-hydrotalcite are
showninFig. 8andtheioncurrentcurvesforthegasevolution
in Fig. 9. Three mass loss steps are observed at 56, 110 and
198 ◦C, which are attributed to dehydration. The total mass
loss for these three steps is 13.9%. The theoretical mass loss
based on 4 water units in the structure is 11.12% which is in
agreement with the experimental value. This value is slightly
Fig. 7. Thermal evolved gas analysis (ion current curves) of molybdate in-
terlayered hydrotalcite for selected gases.

186
R.L. Frost et al. / Thermochimica Acta 429 (2005) 179–187
carbonate in the interlayer may account for this lower than
expected value. This would increase the mass loss over the
450–500 ◦C and decrease the mass loss at 928 ◦C. It is very
difficult to synthesise hydrotalcites with no carbonate in the
interlayer [11].
3.8. Mechanism for the decomposition of hydrotalcite
with sulphate in the interlayer
The following steps describe the thermal decomposition
of the sulphate interlayered hydrotalcite.
Step 1 at 110 ◦C: This step includes the loss of adsorbed
water.
Fig. 8. TG and DTG patterns of sulphate interlayered hydrotalcite.
Mg6Al2(OH)16(SO4) · 4H2O
→ Mg6Al2(OH)16(SO4) · H2O + 3H2O
larger as it includes some adsorbed water. Three mass loss
steps are observed at 363, 437 and 476 ◦C with mass loss
steps of 5.2, 15.4 and 4.6%. The first two steps are assigned
to dehydroxylation and the last step to a loss of oxygen. The
theoretical mass loss for 16 OH units is 22.57% which is
slightly higher than the experimental value of 20.6%. It is
noted some oxygen is lost at 484 ◦C which would increase the
experimental mass loss. A higher mass loss step is observed
at 928 ◦C which is accounted for by the loss of sulphate. The
theoretical mass loss of sulphate is 10%. Trace amounts of
This step represents the first dehydration step and shows
3 mol of water are lost at this temperature. The relative areas
of the DTG profile at 110 and 198 ◦C are 10.5–3.03%. This
means the loss of water at 110 ◦C is 3 mol with a further loss
of water at 198 ◦C of 1 mol.
Step 2 at 198 ◦C:
Mg6Al2(OH)16(SO4) · H2O
→ Mg6Al2(OH)16(SO4) + H2O
This step represents the second dehydration step and
shows 1 mol of water is lost at this temperature.
Step 3 at 363 ◦C:
Mg6Al2(OH)16(SO4) → Mg6Al2O2(OH)12(SO4) + 2H2O
This step represents the first dehydroxylation step and 4
OH units are lost at this temperature. The oxygen is taken
up by the cations present and is shown representatively as
Al2O3.
Step 4 at 437 ◦C:
Mg6Al2O3(OH)12(SO4) → (MgO)6Al2O3(SO4) + 6H2O
This step represents the second dehydroxylation step and
12 OH units are lost at this temperature.
Step 5 at 498 ◦C:
(6MgO)Al2O3(SO4) → Mg6Al2SO11 + O2
Oxygen is lost at this step.
Step 6 at 928 ◦C:
Mg6Al2SO11 → 5MgO + MgAl2O4 + SO2
This step shows that sulphur dioxide is lost and the final
products of the thermal decomposition step are the oxide of
Mg and a magnesium aluminate (spinel).
The upper limit of the TG experiment is 1000 ◦C and it
is possible that some sulphate is retained to a higher temper-
ature. Thus, the expected mass loss of 10% becomes in the
experiment 5.2%.
Fig. 9. Thermal evolved gas analysis (ion current curves) of sulphate inter-
layered hydrotalcite for selected gases.

R.L. Frost et al. / Thermochimica Acta 429 (2005) 179–187
187
4. Conclusions
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The thermal decomposition of hydrotalcites based upon a
Mg/Al ratio of 3/1 with chromate, sulphate and molybdate in
the interlayer has been studied using thermal analysis tech-
niques complimented with X-ray diffraction. The products
of the thermal decomposition depend upon the particular in-
terlayer anion. Two processes are observed in the thermal
decomposition firstly dehydration and secondly dehydroxy-
lation. Each of these processes takes place in several steps.
Mechanisms were proposed for each of the steps in the ther-
mal decomposition.
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Acknowledgements
The financial and infra-structure support of the Queens-
land University of Technology Inorganic Materials Research
Program of the School of Physical and Chemical Sciences is
gratefully acknowledged. The Australian Research Council
(ARC) is thanked for funding the thermal analysis facility.
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