Thermogravimetric analysis of hydrotalcites based on the takovite formula NixZn6-xAl2(OH)16(CO 3)?4H2O

Article abstract of DOI:10.1007/s10973-005-0749-8

High resolution thermogravimetric analysis (HRTG) coupled to a gas evolution mass spectrometer has been used to study the thermal decomposition of zinc-modified takovites of formulae Ni_xZn_6-xAl ₂(CO₃)(OH)₁₆·4H₂O) where x varies from 0 to 6. X-ray diffraction data indicate an increase in the crystallinity and decrease in the interlayer spacing of the materials with increase in the zinc composition. HRTG and mass spectrometry results indicate that water is lost in two major steps, with carbon dioxide coming off simultaneously at the second step. Further loss of CO₂ occurred at higher temperatures due to decomposition of carbonate chemically bound to layer Zn²⁺ and Al³⁺ cations. The variations of the temperatures of dehydration and dehydroxylation/decarbonation with the zinc composition are discussed in terms of the effects of stabilisation of the layer structure, the electronegativity of the layer metals the charge-to-size ratio of the layer cations and the layer-interlayer hydrogen bonding interactions.

Full text of DOI:10.1007/s10973-005-0749-8

Journal of Thermal Analysis and Calorimetry, Vol. 81 (2005) 83–89
THERMOGRAVIMETRIC ANALYSIS OF HYDROTALCITES BASED ON
THE TAKOVITE FORMULA Ni Zn Al (OH) (CO )·4H O
X
6-X
2
16
3
2
*
Yu-Hsiang Lin, Moses O. Adebajo , Ray L. Frost and J. Theo Kloprogge
Inorganic Materials Research Group, School of Physical and Chemical Sciences, Queensland University of Technology, GPO
Box 2434, Brisbane, Qld 4001, Australia
High resolution thermogravimetric analysis (HRTG) coupled to a gas evolution mass spectrometer has been used to study the ther-
x 2 3 2
mal decomposition of zinc-modified takovites of formulae (Ni Zn6-xAl (CO )(OH)16·4H O) where x varies from 0 to 6. X-ray dif-
fraction data indicate an increase in the crystallinity and decrease in the interlayer spacing of the materials with increase in the zinc
composition. HRTG and mass spectrometry results indicate that water is lost in two major steps, with carbon dioxide coming off si-
2
multaneously at the second step. Further loss of CO occurred at higher temperatures due to decomposition of carbonate chemically
2
+
3+
bound to layer Zn and Al cations. The variations of the temperatures of dehydration and dehydroxylation/decarbonation with the
zinc composition are discussed in terms of the effects of stabilisation of the layer structure, the electronegativity of the layer metals,
the charge-to-size ratio of the layer cations and the layer-interlayer hydrogen bonding interactions.
Keywords: decomposition reactions, high-resolution thermogravimetric analysis, mass spectrometric analysis,takovite,
zinc-substituted hydrotalcites
Introduction
a broad range of compositions are possible of the type
2
+
3+
n-
2+
3+
[
are the di- and trivalent cations in the octahedral posi-
M M (OH) ][A ]x/n.yH O, where M and M
1-x x 2 2
Interest in the study of hydrotalcites results from their
potential use as catalysts [1–5]. The reason for the po-
tential application of hydrotalcites as catalysts rests
with the ability to make mixed metal oxides at the
atomic level, rather than at a particle level. There are
many other uses of hydrotalcites. Such mixed metal
oxides are formed through the thermal decomposition
of the hydrotalcite [6, 7]. Hydrotalcites may also be
used as components in new nano-materials such as
nano-composites [8]. Hydrotalcites are important in
the removal of environmental hazards in acid mine
drainage[9, 10] . Hydrotalcite formation also offers a
mechanism for the disposal of radioactive wastes
tions within the hydroxide layers with x normally be-
n–
tween 0.17 and 0.33. A is an exchangeable interlayer
anion [20].
The use of thermal analysis techniques for the
study of the thermal decomposition of hydrotalcites is
not uncommon [21]. Heating sjögrenite or pyroaurite
at <200°C caused the reversible loss of H O. At
2
200–250°C on static heating, or 200–350°C on dy-
namic heating, very little H O or CO were lost, but
2
2
changes in the infrared spectrum and DTA effects
were observed . To date, the number of thermal analy-
sis studies of these minerals is very limited. Thermal
analysis has been applied to the study of complex
mineral systems [22–28] . No studies of the thermal
decomposition of zinc-modified takovites have been
forthcoming. In this study, the thermal decomposition
of synthesised takovite and zinc-substituted takovites
was investigated using high resolution thermo-
gravimetric analysis and mass spectrometry of the
evolved gases. Such investigation will be quite useful
for the development of the materials for potential ap-
plications as catalysts and catalyst precursors and as
components in new nano-composites.
[
11]. Hydrotalcite formation may also serve as a
means of heavy metal removal from heavy metal con-
taminated waters [12]. These hydrotalcites are readily
synthesised by a co-precipitation method [13–15].
Hydrotalcites, or layered double hydroxides
(
LDH) are fundamentally anionic clays, and are less
well-known and more diffuse in nature than cationic
clays like smectites [16, 17]. The structure of
hydrotalcite can be derived from a brucite structure
3
+
3+
(
(
Mg(OH) ) in which, for examples, Al or Fe
2
pyroaurite- sjögrenite) substitutes a part of the Mg .
2
+
This substitution creates a positive layer charge on the
hydroxide layers, which is compensated by interlayer
anions or anionic complexes[18, 19]. In hydrotalcites,
*
Author for correspondence:m.adebajo@qut.edu.au
1
388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
©
2005 Akadémiai Kiadó, Budapest

YU-HSIANG LIN et al.
Experimental
Synthesis of hydrotalcites
The hydrotalcites were synthesised by the co-precipita-
tion method. Hydrotalcites with a composition of
Ni Zn Al (OH) (CO ).4H O where x varied from 6
x
6-x
2
16
3
2
to 0, were synthesised. Two solutions were prepared,
namely solution 1 containing 2M NaOH and 0.125M
2
+
Na CO and solution 2 containing 0.75M Ni
2
3
2+
(Ni(NO ) .6H O) and 0.75M Zn (Zn(NO ) .6H O) to-
gether with 0.25M Al (as Al(NO ) .9H O). Solution 2
3 2 2 3 2 2
3+
3
3
2
in the appropriate ratio was added to solution 1 using a
3
–1
peristaltic pump at a rate of 40 cm min , under vigor-
ous stirring, maintaining a pH of 10. To prepare hydro-
talcites with different molecular formulae, the ratio of
2
+
2+
[
Ni ]/[Zn ] was varied according to the required for-
mula. In this way, seven hydrotalcites with Ni:Zn ratios
of 6:0, 5:1, 4:2, 3:3, 2:4, 1:5 and 0:6 were prepared. The
precipitated minerals are washed at ambient tempera-
tures thoroughly with water to remove any residual ni-
trate. The composition of the hydrotalcites was checked
by ICP and ICP-AES analysis. The phase composition
was checked by X-ray diffraction.
Thermogravimetry
Differential thermal and thermogravimetric analyses
Fig. 1 Powder X-ray diffraction [001] peak of Ni-Zn/Al
hydrotalcites
®
of the hydrotalcites were obtained using a TA instru-
ments Inc. Q500 high resolution TGA operating at
–1
high resolution ramp 10°C min resolution 6.0°C from
room temperature to 1000°C in a high purity flowing
for Zn makes the OH groups to be more strongly bound
to Zn, resulting in the stabilization of the layered struc-
ture. There is also an apparent linear relationship be-
tween the moles of zinc and the d(003) spacing. Increas-
ing the moles of Zn substituted for Ni in the takovite
formula results in the decrease of the d(003) spacing.
The linear relationship is found to be y=-0.0374x+7.827
3
–1
nitrogen atmosphere (80 cm min ). Approximately
0 mg of sample was heated in an open platinum cruci-
5
ble. No preparation was required other than grinding
the sample up finely. The TGA instrument was cou-
pled to a Balzers (Pfeiffer) mass spectrometer for gas
analysis. Only selected gases evolved, namely water
vapour, carbon dioxide and oxygen, were measured.
2
with R value of 0.9847, where y is the d(003) spacing
and x is the moles of zinc. The significance of this result
is that not only are the interlayer spacings of
hydrotalcites anion dependent but they are also cation
dependent, as reported recently [30]. The lowering of
the basal spacing with increase in the moles of zinc may
Results and discussion
X-ray diffraction
2+
be related to the bigger ionic size of Zn (0.74 Å) com-
2+
pared with that of Ni (0.69 Å). The bigger ionic size of
It is important to ensure that the synthesized hydro-
talcites are layered structures. This was achieved using
X-ray diffraction (XRD). The XRD patterns obtained
are shown in Fig. 1. As illustrated in Fig. 1, the XRD
d(003) peak becomes sharper with increase in the moles
of Zn, indicating an increase in the crystallinity. Con-
versely, a decrease in the moles of Zn leads to the broad-
ening of the peak, indicating a more disordered structure
or a decrease in crystallinity. The stabilization of the
layered structure with increase in the moles of Zn may
be attributed to the lower electronegativity of Zn (1.65)
than that of Ni (1.91). The lower electronegativity value
2+
Zn makes the layer thickness to increase leading to the
ultimate reduction in the interlayer spacing. The
interlayer spacing is thus progressively reduced as the
moles of Zn is increased.
High-resolution thermogravimetric analysis
The thermogravimetry (TG) and derivative thermo-
gravimetry (DTG) curves of zinc modified takovites
of formula Ni Zn Al (OH) (CO )·4H O are shown
x
6-x
2
16
3
2
84
J. Therm. Anal. Cal., 81, 2005

THERMOGRAVIMETRIC ANALYSIS OF HYDROTALCITES
Fig. 2 Plot of d(001) spacing against the moles of zinc for the
Ni–Zn/Al hydrotalcites. The broken line is the curve
passing through the experimental points while the solid
line is the linear trendline through the points
in Figs 3a to 3g. The results of the thermogravimetric
analyses are reported in Table 1. For takovite,
(
Ni Al (OH) (CO )·4H O), two mass loss steps are
6 2 16 3 2
observed at 91.06 and 284.65°C with mass losses of
4.35 and 17.70%. The theoretical mass loss for the
1
first step is 8.89%. This mass loss is attributed to de-
hydration. However, the higher experimental value
than the theoretical figure for this mass loss suggests
that some dehydroxylation may also be possible at
this step. The second mass loss step occurs at
Fig. 3c, 3d TG and DTG curves for c – 4:2 Ni-Zn/Al
hydrotalcite; d – 3:3 Ni–Zn/Al hydrotalcite
2
84.65°C with a mass loss of 17.70%. This may be
compared with a theoretical mass loss of 23.21%. The
theoretical mass loss for steps 1 and 2 is 32.1%. The
Fig. 3e, 3f TG and DTG curves for e – 2:4 Ni-Zn/Al
hydrotalcite; f – 1:5 Ni–Zn/Al hydrotalcite
Fig. 3a, 3b TG and DTG curves for a – Ni/Al hydrotalcite
(takovite); b – 5:1 Ni–Zn/Al hydrotalcite
J. Therm. Anal. Cal., 81, 2005
85

YU-HSIANG LIN et al.
Fig. 3g TG and DTG curves for Zn/Al hydrotalcite
experimental mass loss for steps 1 and 2 is 32.05%, in
agreement with the theoretical value of 32.1%. The
second mass loss step is attributed to the loss of the
hydroxyl units and interlayer carbonate. These two
units appear to be lost simultaneously.
For the other hydrotalcites of formula
Ni Zn6-xAl (OH)16(CO )·4H O, two mass loss steps
x 2 3 2
are also identified. The first, as in the case of takovite,
is attributed to dehydration although the results above
suggest that some dehydroxylation may also take
place at the lower temperatures. The second step for
the Zn-substituted takovites is also attributed to the
combination of dehydroxylation and decarbonation.
The data in Table 1 suggests that there is a relation-
ship between the temperature of dehydration and the
moles of Zn. Such a relationship is shown in Fig. 4a.
The temperature of dehydration increases with the
moles of Zn substituted. The data may be fitted to a
Fig. 4 Plots of the temperature of a – dehydraion/dehydroxy-
lation and b – dehydroxylation/decarbonation against
the moles of zinc for the Ni–Zn/Al hydrotalcites. The
broken lines are the curves passing through the experi-
mental points while the solid lines are the linear
trendlines through the points
even with layer hydroxyl groups. In contrast, how-
ever, the temperature of dehydroxylation/ decarbona-
tion was found to decrease linearly with increase in
the moles of zinc substituted into the takovite formula
as shown in Fig. 4b. The data is fitted to a linear func-
2
linear function y=8.7251x+92.216 with an R value of
0
.9578, where y is the temperature of dehydration and
x is the moles of Zn. This observation may be related
to the lowering of the interlayer spacing with increase
in moles of zinc. The reduction in the interlayer spac-
ing on Zn incorporation would favour stronger hydro-
gen bond interactions between the interlayer water
molecules and stronger hydrogen bonding of the wa-
ter molecules with the interlayer carbonate ions or
2
tion y= –21.484x+321.34 with an R value of 0.966,
where again y is the temperature of dehydroxylation/
decarbonation and x is the moles of zinc. On going
from the zinc-substituted hydrotalcite formula
(Ni
to (Zn
5
Zn
1
Al
Al
2
(OH)16(CO
(OH)16(CO
3
)·4H O) with one mole of zinc
)·4H O), the temperature of the
2
2
6
2
3
Table 1 Results of weight losses of the TG and DTG for Ni/Zn/Al hydrotalcites
Step 1: Dehydration & Dehydroxylation
Step 2: Dehydroxylation & Decarbonation
Weight loss %
Temp./ºC
Weight loss %
Temp./ºC
found
theor.
found
theor.
Ni
Ni
Ni
Ni
Ni
Ni
Ni
6
Zn
5
Zn
4
Zn
3
Zn
2
Zn
1
Zn
0
Zn
0
1
2
3
4
5
6
14.35
13.64
13.16
13.87
14.03
13.85
11.48
8.89
8.81
8.74
8.67
8.60
8.53
8.47
91.06
99.34
17.70
17.14
16.96
16.66
15.47
13
23.21
23.02
22.82
22.64
22.46
22.29
22.12
284.65
295.78
277.33
277.08
250.09
209.13
187.67
138.13
140.32
135.37
134.27
140.64
8.65
86
J. Therm. Anal. Cal., 81, 2005

THERMOGRAVIMETRIC ANALYSIS OF HYDROTALCITES
Fig. 5a, 5b DTG and MS curves for a – Ni/Al hydrotalcite
takovite); b – 5:1 Ni–Zn/Al hydrotalcite
Fig. 5c, 5d DTG and MS curves for c – 4:2 Ni–Zn/Al
hydrotalcite; d – 3:3 Ni–Zn/Al hydrotalcite
(
dehydroxylation/decarbonation is observed to de-
crease significantly from 295.78 to 187.67°C. How-
ever, the dehydroxylation/decarbonation temperature
increases from 284.65 to 295.78°C on going from
vailing effects of the stabilization of layer structure
and hydrogen bonding interactions resulting from
zinc incorporation. On the other hand, the effects of
stabilization of the layered structure and stronger hy-
drogen bonding interactions by zinc incorporation
predominate in the process of dehydration such that
the zinc-substituted hydrotalcites are more stable than
the takovite.
Ni
Al (OH)16(CO )·4H O to Ni Zn Al (OH)16(CO )·
6 2 3 2 5 1 2 3
4
the takovite formula. The incorporation of Zn with
H O by incorporation of only one mole of zinc into
2
2
+
2
+
lower charge-to-size ratio than Ni would facilitate
the process of decarbonation due to the weakening of
the interaction between the layers and the interlayer
carbonate. This effect therefore appears to compete
strongly with the effects of stabilization of the layered
structure and stronger hydrogen bonding interactions
that lead to increase in the temperature of dehydration
in zinc-substituted takovites. These results thus pro-
vide a measure of the stability of takovite compared
with zinc-substituted takovites. It appears that the ef-
fect of stronger interaction of the interlayer carbonate
with the layer cations predominates in the takovite
during the process of dehydroxylation/decarbonation.
Thus, the takovite is more stable than the zinc-substi-
tuted hydrotalcites with respect to dehydroxy-
lation/decarbonation. However, the 5:1 Ni–Zn
hydrotalcite has a higher temperature of dehydroxy-
lation/decarbonation and therefore slightly more sta-
ble than takovite containing no zinc due to the pre-
In addition to the two steps of dehydration and
dehydroxylation/decarbonation, three higher temper-
ature mass losses are also observed at about 300°C,
550–560°C and 735–765°C in the Zn-substituted
takovites containing higher moles of Zn, i.e. Ni
Ni Zn and Ni Zn (Figs 3e to 3g). These secondary
Zn ,
2 4
1
5
0
6
higher temperature losses are ascribed to further loss
of interlayer carbonate. This is confirmed later in sec-
tion 3.3 by mass spectrometric (MS) analysis of
evolved gases that shows that CO gas is evolved at
2
these temperatures. It is suggested that during the
dehydroxylation and decarbonation of these Zn-sub-
stituted takovites, some of the carbonate ions are
forced to bond chemically to the layer cations to form
molecules such as NiCO
300°C peak overlaps with the dehydroxylation step in
, ZnCO , and Al (CO ) . The
3 2 3 3
3
Ni
Ni
Zn
Zn
takovite (Fig. 3e), and is virtually absent in
and Ni Zn . On the basis of this observation
0 6
2
1
4
5
J. Therm. Anal. Cal., 81, 2005
87

YU-HSIANG LIN et al.
Fig. 5g DTG and MS curves for g – Zn/Al hydrotalcite
that water is lost in two major steps at 90–150°C and
1
2
60–300°C while carbon dioxide comes off at about
00–300°C, in agreement with the TG/DTG data. The
MS data therefore confirms that the first step is the
dehydration step and second step the dehydroxy-
lation/decarbonation step. Again, as observed in the
TG/DTG results, the MS curves for the Zn-substituted
takovites containing 4 to 6 moles of Zn show further
higher temperatures loss of CO
and 735–765°C thereby confirming the loss of car-
at about 550–560°C
2
2
+
3+
bonate chemically bound to layer Zn and Al cat-
ions, respectively. The following chemical reactions
are therefore proposed for the thermal decomposition
of the synthesized takovite or Zn–Al hydrotalcite:
Fig. 5e, 5f DTG and MS curves for e – 2:4 Ni–Zn/Al
hydrotalcite; f – 1:5 Ni–Zn/Al hydrotalcite
and the fact that the electronegativity of the metals re-
duces in the order Ni>Zn>Al, the higher temperature
mass losses at 300°C, 550–560°C and 735–765°C are
Decomposition Step 1 at 91.06 (Ni
40.64°C (Ni Zn )
Zn ) or
6 0
1
0
6
2
+
2+
M Al (OH) (CO ).4H O→
assigned to loss of carbonate bound to Ni , Zn and
6
2
16
3
2
3
+
Al cations, respectively. The gradual decrease in the
mass loss in step 2 due to dehydroxylation/ de-
carbonation on increasing the moles of Zn from 0 to 6
M Al (OH) (CO ) +4H O
6
2
16
3
2
Decomposition Step 2 at 284.65 (Ni Zn ) or
6
0
1
84.67 (Ni Zn )°C
6
(
Table 1 and Figs 3, 4), may therefore be attributed to
0
this higher temperature loss of carbonate ions bound
to the layer cations. The experimental total mass loss
for steps 1 and 2 for Ni Zn (26.85%) and Ni Zn
M Al (OH) (CO )→M Al O CO +8H O
6
2
16
3
6
2
8
3
2
M
Al
O
CO
→6MO+Al
O
+CO
2
1
5
0
6
6
2
8
3
2
3
(
3
20.13%) are lower than the theoretical values of
0.82% and 30.59%, respectively due to this higher
where M=Ni or Zn.
For the Ni–Zn–Al hydrotalcites, the proposed
chemical reactions can be represented as:
temperature secondary loss of carbonate ions bound
to layer cations. In the case of the other Zn-substituted
takovites, the experimental total mass losses for steps
Decomposition Step 1 at 99–134°C
1
and 2 compare well with the theoretical values.
Ni Zn Al (OH) (CO )·4H O→
6-x
x
2
16
3
2
Νι Zn Al (OH) (CO )+4H O
6-x
x
2
16
3
2
Mass spectrometric analysis
Decomposition Step 2 at 209–295°C
The DTG of the takovites and MS curves of CO
(
2
Ni Zn Al (OH) (CO )→Ni Zn O CO +8H O
6–x
x
2
16
3
6-x
x
8
3
2
MS=44), H O (MS=18) and OH (MS=17) are shown
2
Ni Zn Al O CO →(6–x)NiO+xZnO+Al O +CO
in Figs 5a to 5g. It is a fundamental principle that the
mass spectrometric curves follow the DTG curves.
Figs 5a to 5g show that the patterns of the DTG and
MS curves are identical. The MS results also show
6–x
x
2
8
3
2
3
2
88
J. Therm. Anal. Cal., 81, 2005

THERMOGRAVIMETRIC ANALYSIS OF HYDROTALCITES
Conclusions
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thermogravimetry and mass spectrometric analysis of
evolved gases. Powder X-ray diffraction was also
used to determine the crystallinity and interlayer
spacing of the materials. The results of the investiga-
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The X-ray diffraction data show that the
crystallinity of the materials increases and the interlayer
spacing decreases linearly with increase in the moles of
zinc incorporated into the formula. The lower electro-
negativity value for Zn than that of Ni makes the OH
groups to be more strongly bound to Zn, resulting in the
stabilization of the layered structure. Also, the bigger
ionic size of Zn compared with Ni causes the layer
thickness to increase such that there is ultimate reduc-
tion in the interlayer spacing.
TG/DTG and mass spectrometric analysis of
evolved gases indicate that that water is lost in two
major steps at 90–150 and 160–300°C, while carbon
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2
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2
0 H. C. B. Hansen and C. B. Koch, Appl. Clay Science,
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1
0 (1995) 5.
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Acknowledgments
406 (2003) 221.
8 R. L. Frost, M. L. Weier, M. E. Clissold, P. A. Williams,
and J. T. Kloprogge, Thermochim. Acta, 407 (2003) 1.
9 J. T. Kloprogge, D. Wharton, L. Hickey, and R. L. Frost,
Am. Miner., 87 (2002) 623.
The financial and infra-structure support of the Queensland Uni-
versity of Technology Inorganic Materials Research Program is
gratefully acknowledged. The Australian Research Council
(ARC) is thanked for funding the thermal analysis facility.
0 R. L. Frost, Z. Ding, W. N. Martens, and T. E. Johnson,
Thermochim. Acta, 398 (2003) 167.
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