High-temperature transformations of Cu-rich hydrotalcites

Article abstract of DOI:10.1016/j.jssc.2003.08.023

CuM(II)Al ternary hydrotalcites (M(II) = Ni, Co and Mg) with a (Cu+M(II))/Al atomic ratio of 3.0 and Cu/M(II) atomic ratio of 5.0 were synthesized by coprecipitation under low supersaturation. Powder X-ray diffraction of the as-synthesized samples showed a pattern characteristic of hydrotalcite-like (HT-like) structure (JCPDS: 41-1428). Thermal analyses of these samples showed four stages of weight loss/heat change when recorded in nitrogen. Analysis of the evolved gases characterized the nature of these transformations. The thermoanalytical effects differed significantly especially for the high-temperature transformations, when the treatment was performed in oxygen. In situ powder X-ray diffraction of the samples was carried out to elucidate the phase evolution of these compounds. Surprisingly formation of CuO was noted at temperatures around 200°C well below the destruction of the layered network. The nature of the resulting phases varied with both the nature of the co-bivalent metal ion and the heating atmosphere. FT-IR spectroscopy confirmed the retention of carbonate ions at higher temperatures (above 700°C), although the concentration of carbonate anion (most likely unidentate) varied with the calcination temperature. The crystallinity of CuO increased significantly above 600°C, probably through dissociation of copper oxycarbonate. Significant differences in the thermal transformation temperatures (for the third and the fourth transformations) of these samples containing different co-bivalent metal ions were not observed. This suggests that an association of the co-bivalent metal ions and/or trivalent metal ion in this phase is unlikely. A plausible thermal evolution scheme of these hydrotalcites is proposed.

Full text of DOI:10.1016/j.jssc.2003.08.023

ARTICLE IN PRESS
Journal of Solid State Chemistry 177 (2004) 319–331
High-temperature transformations of Cu-rich hydrotalcites
S. Kannan,a,ꢀ V. Rives,b and H. Knozingerc
a Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002, India
b
´
´
Departamento de Quımica Inorganica, Universidad de Salamanca, 37008-Salamanca, Spain
c Department Chemie, Physikalische Chemie, Universita¨t Mu¨nchen, Butenandtstrasse 5–13 (Haus E), 81377 Mu¨nchen, Germany
Received 31 May 2003; received in revised form 12 August 2003; accepted 20 August 2003
Abstract
CuM(II)Al ternary hydrotalcites (M(II) = Ni, Co and Mg) with a (Cu+M(II))/Al atomic ratio of 3.0 and Cu/M(II) atomic ratio
of 5.0 were synthesized by coprecipitation under low supersaturation. Powder X-ray diffraction of the as-synthesized samples
showed a pattern characteristic of hydrotalcite-like (HT-like) structure (JCPDS: 41-1428). Thermal analyses of these samples
showed four stages of weight loss/heat change when recorded in nitrogen. Analysis of the evolved gases characterized the nature of
these transformations. The thermoanalytical effects differed significantly especially for the high-temperature transformations, when
the treatment was performed in oxygen. In situ powder X-ray diffraction of the samples was carried out to elucidate the phase
evolution of these compounds. Surprisingly formation of CuO was noted at temperatures around 200ꢁC well below the destruction
of the layered network. The nature of the resulting phases varied with both the nature of the co-bivalent metal ion and the heating
atmosphere. FT-IR spectroscopy confirmed the retention of carbonate ions at higher temperatures (above 700ꢁC), although the
concentration of carbonate anion (most likely unidentate) varied with the calcination temperature. The crystallinity of CuO
increased significantly above 600ꢁC, probably through dissociation of copper oxycarbonate. Significant differences in the thermal
transformation temperatures (for the third and the fourth transformations) of these samples containing different co-bivalent metal
ions were not observed. This suggests that an association of the co-bivalent metal ions and/or trivalent metal ion in this phase is
unlikely. A plausible thermal evolution scheme of these hydrotalcites is proposed.
r 2003 Elsevier Inc. All rights reserved.
Keywords: Cu-hydrotalcite; In situ powder X-ray diffraction; Thermal behavior; FT-IR spectroscopy
1. Introduction
have observed an endothermic high-temperature effect
around 600ꢁC, which is characteristic of copper-contain-
ing hydrotalcites, as also reported by others [10,11].
Velu and Swamy [10] have observed this effect for
CuMnAl hydrotalcite and attributed this feature to a
reaction occurring between brucite sheets and carbonate
anions to form an oxycarbonate of chemical composi-
tion M(II)M(III)xOy(CO3)z. However, Alejandre et al.
[11] have found a similar thermal feature in an attempt
to synthesize pure CuAl hydrotalcites and attributed this
effect to differences in the coordination of carbonate
anions within the brucite-like lattice. Fierro and cow-
orkers [12] reported the thermal decomposition of
CuZnAl ternary hydrotalcite having an (Cu+Zn)/Al
atomic ratio of 3.0 and Cu/Zn atomic ratio of 0.5. They
attributed the high-temperature transformations occur-
ring around 600ꢁC to the removal of carbonate anions
occluded in the solid and demonstrated by FT-IR
spectroscopy that coordination of carbonate anions in
Hydrotalcite-like compounds are two-dimensional
layered materials with alternating positively charged
mixed metal hydroxide sheets and negatively charged
interlayer anions along with water molecules [1]. They
are represented by the general formula [M(II)1ꢂx
nꢂ
M(III)x(OH)2][A ] ꢃ mH2O where M(II) is a bivalent
x/n
metal ion, M(III) is a trivalent metal ion, A is the
interlayer anion, and x can have values between 0.2 and
0.4 [2]. Transition metal containing hydrotalcites,
particularly copper-containing hydrotalcites are receiv-
ing increasing attention owing to their diverse catalytic
applications [3–7]. Understanding the thermal behavior
of these materials is important since they are used as
catalysts after calcination [8]. In our earlier work [9], we
ꢀCorresponding author. Fax: +91-278-2566970.
E-mail address: kanhemad1@sancharnet.in (S. Kannan).
0022-4596/$ - see front matter r 2003 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2003.08.023

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320
the interlayer space changes upon loss of interlayer
water molecules. Although thermal properties of cop-
per-containing hydrotalcites have been investigated in
the past, a detailed account of the high-temperature
processes, particularly at temperatures greater than
600ꢁC, is still lacking. The aim of this paper is to
unravel these high-temperature effects by varying both
the co-bivalent metal ions associated with copper and
the gas atmosphere. The need for studying various co-
bivalent metal ions (both redox as well as non-redox) is
to understand their influence on these high-temperature
transformations.
and heated at 5ꢁC minꢂ1 in steps of 50ꢁC and stabilized
for 5 min before measurements. The operating voltage
and current wereꢁ 40 kV and 40 mA, respectively. The
stepsize was 0.05 with a steptime of 1 s. Identification
of the crystalline phases was made by comparison with
the JCPDS files [13].
FT-IR spectra were recorded on a Perkin-Elmer
FT1730 instrument using KBr pellets; 100 spectra
(recorded with a nominal resolution of 4 cmꢂ1) were
averaged to improve the signal-to-noise ratio. In situ
FT-IR spectroscopy studies were carried out with a
Bruker IFS-66 apparatus at a spectral resolution of
2.0 cmꢂ1 accumulating 128 scans. Self-supporting wafers
(ca. 10 mg cmꢂ2) were prepared from the sample and
heated directly in a purpose-made IR cell. The latter was
connected to a vacuum/sorption apparatus with a
residual pressure less than 10ꢂ3 Pa.
2. Experimental
The samples were prepared by coprecipitation under
low supersaturation. Two solutions, namely, solution
(A) containing the desired amount of metal (Cu, Co, Ni,
Mg and Al) nitrates and solution (B) containing the
precipitating agents (i.e., NaOH and Na2CO3) were
added slowly (B1 mL minꢂ1) and simultaneously into a
beaker containing distilled water, while maintaining the
pH constant around 9–10 under vigorous stirring at
room temperature. The addition of these solutions took
ca. 90 min and the final pH was adjusted to 10. The
samples were aged in the mother liquor at 65ꢁC for 18 h,
filtered off, washed (until total absence of nitrates in the
washing liquids and neutral pH) and dried in an air oven
at 110ꢁC for 12 h. In all cases, the atomic ratio between
the divalent and trivalent cations was maintained
around 3.0, while the Cu:M(II) atomic ratio was 5:1.
The reason for selecting higher Cu-content in our
samples is that the weight loss corresponding to high-
temperature transformations (occurring above 600ꢁC)
is higher when the Cu-content was higher, assisting in
precise analysis of results. The samples are denoted
CuM(II)Al-HT where M(II) = Ni, Co and Mg. Binary
CuAl hydroxycarbonate was also prepared and included
in this study for comparison.
Thermogravimetric (TG) and differential thermal
analyses (DTA) were carried out on TG-7 and DTA-7
instruments from Perkin-Elmer in flowing oxygen or
nitrogen (from L’Air Liquide, Spain, flow 50 mL minꢂ1
)
at a heating rate of 10ꢁC minꢂ1. Alumina (Merck)
calcined at 1200ꢁC was used as a reference for the DTA
measurements. The gases evolved during thermal treat-
ment were simultaneously analyzed by an on-line
quadruple mass spectrometer (Balzers 420) connected
with a Netzsch thermo balance with a focus on m/e =
18 (H2O) and 44 (CO2).
Specific surface area measurement and pore size
analysis was carried out on a Gemini instrument from
Micromeritics. The sample (ca. 80–100 mg) was pre-
viously degassed in flowing nitrogen at 150ꢁC for 2 h in
order to remove physisorbed water in a FlowPrep060
apparatus, also from Micromeritics, and the data were
analyzed using published software [14].
4. Results and discussion
Elemental chemical analysis of the samples (Table 1)
showed a reasonable correspondence within experimen-
tal error limits between the (Cu+M(II))/Al and Cu/
M(II) atomic ratios of the solids and the starting
solutions suggesting completion of precipitation. The
lack of coincidence between the initial ratio of cations in
the solutions and the ratio in the solids isolated, is,
however, rather common in literature, and has been
ascribed to a preferential precipitation of one or another
cation as hydroxide [15]. The molecular formulae of the
samples were calculated and are given in Table 1. In
deriving the molecular formula, it was assumed that the
carbonate is the only charge compensation anion (see
also FT-IR data), while interlayer water was calculated
from TG curves. Further, TPR and UV-vis analysis of
these samples also confirmed the retention of +2
3. Techniques
Elemental chemical analysis for Cu, Mg, Co, Ni, and
Al was carried out by atomic absorption in a Mark 2
ELL-240 apparatus, in Servicio General de Analisis
Quimico Aplicado (University of Salamanca, Spain).
Powder X-ray diffraction (PXRD) patterns were
recorded on a Siemens D-500 instrument, using CuKa
radiation (l=1.54050 A). Data analysis was carried out
with AT Diffract software. In situ powder X-ray
diffraction (PXRD) was carried out on a Philips X’Pert
MPD system connected to an Anton-Paar high-
temperature XRK assembly using CuKa radiation.
The sample was mounted in a high-temperature cell

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Table 1
Elemental composition and formula of the samples synthesized
Sample
(Cu+M(II))/Ala
Cu/M(II)a
Formulab
Solution
Solid
Solution
Solid
CuMgAl-HT
CuNiAl-HT
CuCoAl-HT
3.0
3.0
3.0
3.06
3.04
2.96
5.0
5.0
5.0
5.18
4.40
4.60
[Cu0.63Mg0.12Al0.25(OH)2](CO3)0.13 ꢃ 0.58H2O
[Cu0.61Ni0.14Al0.25(OH)2](CO3)0.13 ꢃ 0.96H2O
[Cu0.62Co0.13Al0.25(OH)2](CO3)0.13 ꢃ 0.62H2O
a Atomic ratio.
b Values rounded to two significant figures.
Table 2
Lattice parameters, surface areas and pore volumes of as-prepared
samples
Sample
a (A)
c (A)
SBET (m2 gꢂ1
)
Vp (cm3 gꢂ1
)
CuMgAl-HT
CuNiAl-HT
CuCoAl-HT
3.081
3.074
3.081
22.78
22.48
22.43
60
89
32
0.48
0.23
0.07
Fig. 1. Powder X-ray diffraction patterns of (a) CuAl-HT, (b)
CuMgAl-HT, (c) CuNiAl-HT, (d) CuCoAl-HT (ꢀ - gerhardite).
oxidation state for these metal ions in the brucite-like
network.
Powder X-ray diffraction patterns of these three
samples along with binary CuAl-HT (Cu/Al atomic
ratio is 3.0) are shown in Fig. 1. All three ternary
hydrotalcites crystallized in HT-like phase without any
discrete crystalline impurity, exhibiting sharp and
symmetric reflections at lower diffraction angles. On
the contrary, binary CuAl-HT consisted of a mixed
phase containing both gerhardite (or malachite) and
hydrotalcite [11, 16–18]. The lack of single-phase
crystallization as HT-like structure of CuAl-HT is well
known [1] and accounted for by the Jahn–Teller
distortion of copper in octahedral coordination. Among
the three bivalent metal ions studied, Mg offered a well-
ordered material followed by nickel and cobalt. Lattice
parameters ‘a’ and ‘c’ were calculated from the first peak
of the doublet observed near 2y = 60ꢁ (diffraction by
(110) planes) and the first basal peak observed near 2y
= 12ꢁ (diffraction by (003) plane), respectively. The
numerical values are summarized in Table 2. It is seen
that the values are very nearly identical, although small
variations in ‘a’ result from the differences in the ionic
radii of the bivalent metal ions [19]. The ‘c’ parameter is
a measure of the Coulomb forces between layer and
Fig. 2. TG and DTG traces of (a) CuNiAl-HT, (b) CuMgAl-HT, (c)
CuCoAl-HT recorded in nitrogen.
interlayer, and their almost identical values suggest a
similar extent of electrostatic interaction irrespective of
the nature of the cobivalent metal ions. BET measure-
ments showed a maximum specific surface area for
CuNiAl-HT and a minimum for CuCoAl-HT, whose
values along with pore volumes are also summarized in
Table 2.
Thermogravimetric and differential thermal analyses
of these samples were carried out both in nitrogen and in
oxygen, in order to detect any oxidation process
occurring during calcination. The TG curves together
with the corresponding derivative TG curves (DTG)
recorded in nitrogen are shown in Fig. 2 and the
parameters are summarized in Table 3. The reasons for
these transformations are summarized below.
Upto four different stages can be envisaged in the
TG curves recorded in nitrogen. According to data

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Table 3
Thermogravimetric parameters of the samples synthesized
Sample
T1 (ꢁC)
T2 (ꢁC)
T3 (ꢁC)
T4 (ꢁC)
W3
(%)
W4
(%)
Net weight
loss (%)
CuMgAl-HT (N)a
CuNiAl-HT (N)a
CuCoAl-HT (N)a
CuMgAl-HT (O)b
CuNiAl-HT (O)b
CuCoAl-HT (O)b
127
114
126
129
114
124
270
261
565
576
582
588
577
584
846
852
840
3.3
3.0
5.1
4.1
3.9
4.5
4.6
4.4
4.6
28.1
32.0
29.9
26.2
25.3
27.4
253
245c
230c
228c
a Heated under nitrogen.
b Heated under oxygen.
c Center of the broad peak.
previously reported [20], the first weight loss, identified
by a rather sharpDTG minimum at 104–121 ꢁC
corresponds to removal of interlayer water molecules.
This weight loss was immediately followed by a second
ꢁ
weight loss, upto ca. 350 C. The corresponding DTG
signal covers a broad temperature window and splits
into two broad overlapping signals, usually attributed to
dehydroxylation of the brucite-like layers and decarbo-
nation. Gas evolution analysis (Fig. 3) showed that the
first DTG signal is only related to dehydration through
removal of the interlayer water molecules (no evolution
of CO2 was noted till 200ꢁC) while the second
transformation comprises both dehydroxylation and
decarbonation. Generally, for MgAl-hydrotalcites, such
a weight loss (due to decarbonation and dehydroxyla-
tion) extends upto ca. 500 ꢁC, and no further weight loss
is usually recorded above this temperature. However, in
the case of transition metal containing hydrotalcites,
particularly with those containing copper, additional
transformations were noted. Two well-defined transfor-
mations were noted in the temperature range between
ca. 590ꢁC and 850ꢁC. However, their position did not
significantly vary with the nature of the bivalent metal
ions (Fig. 2). For the third transformation (occurring
around 590ꢁC), the variation was around 710ꢁC while
for the fourth transformation it was only around 75ꢁC.
These minor variations remain within the temperature
resolution of the instrument and/or are related to
different textural effects of the solids. We therefore
infer that these transformations are only scarcely
influenced by the nature of the co-bivalent metal ion.
In other words, these transformations seem to be
characteristic of copper-containing phases. Further,
these transformations occur in a rather narrow tem-
perature range (more clear from DTG traces). This
suggests that these transformations should involve
structural transformations. A total weight loss of
around 30% was observed for these samples, of which
the first two weight losses account for B21% while the
latter two weight losses contribute around 9%. The net
weight loss of the two high-temperature transformations
(third and fourth put together) was very similar for these
Fig. 3. MS profiles of evolved gases in situ during decomposition of
(a) CuNiAl-HT, (b) CuMgAl-HT, (c) CuCoAl-HT under nitrogen.
materials irrespective of the nature of the bivalent metal
ion. This observation is consistent with the above
inference that the two respective thermal effects are
characterizing the copper-containing phase irrespective
of the co-bivalent and trivalent metal ion.
The TG and DTG curves recorded in oxygen, where
oxidation processes are expected, are plotted in Fig. 4. A
careful examination of these curves, and their compar-
ison with those recorded in nitrogen (Fig. 2), shows that

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tion values for the calcined samples were higher than
one (M/H2 mole ratio) and the values were nearly 1.1
when considering the average oxidation state values
between 2.67 and 3.0, indicating the presence of Co3+
.
The thermal effects recorded between 400ꢁ and 600ꢁC as
well as the changes responsible for the weight loss
recorded above 800ꢁC in nitrogen are clearly related to
transformations involving copper-containing species. In
our earlier work, we have proposed a catalytic role of
the cobivalent metal ion, namely cobalt, since a shift of
the third transformation to lower temperature occurred
with an increase in the concentration of cobalt. We now
propose a different explanation, namely a correlation of
temperature shift with variation in the copper content of
the sample. The DTA curves recorded in nitrogen or in
oxygen (not shown) also provided information comple-
menting that from the TG/DTG analysis; the positions
of the DTA minima approximately correspond to the
positions of the minima in the DTG curves. In the low-
temperature region, the DTA curves are rather similar
to the DTG curves. The curves recorded in nitrogen and
in oxygen are acceptably similar, with two main
differences: (i) the high temperature effect is only
recorded for the Cu-rich samples in nitrogen in
agreement with the TG results, and (ii) the second
endothermic effect is sharper in oxygen than in nitrogen,
i.e., they occur in a narrower temperature range. In
order to understand the nature of the phase evolution
upon calcination, powder X-ray diffraction and infrared
spectroscopy of these samples were carried out under in
situ conditions with increasing temperatures.
Fig. 4. TG and DTG traces of (a) CuNiAl-HT, (b) CuMgAl-HT, (c)
CuCoAl-HT recorded in oxygen.
the TG curves are almost coincident below ca. 550–
600ꢁC, the main difference being the weight loss above
800ꢁC recorded in nitrogen, but absent in oxygen. The
DTG curves provide a clearer description of the
differences between the processes taking place in an
inert or an oxidizing atmosphere. The first sharp DTG
peak corresponding to the first weight loss, is coincident
for all samples when recorded in nitrogen or oxygen,
thus confirming that it corresponds to non-oxidizing
processes, i.e., dehydration via removal of the interlayer
water molecules. However, the second process, which is
recorded as a single or split process in the DTG analysis
and which has been ascribed to dehydroxylation and
simultaneous decarbonation, shows some differences:
while it is relatively sharpor splits into two peaks in
nitrogen, it is broad and ill-defined in oxygen. Such a
difference as observed for sample CuCoAl-HT could
possibly be attributed to the simultaneous oxidation of
Co2+ species [21–23]. It is also known that variations in
the dehydroxylation process occur with the variation in
heating atmosphere and is also influenced by the nature,
the oxidation state and coordination of metal ions
present in the layered lattice. Further, a comparison of
TG traces of CuCoAl-HT recorded in either nitrogen or
oxygen shows that the curves are nearly coincident from
room temperature to 200ꢁC and between 650ꢁ and
800ꢁC. A small difference between the curves in the
range 200–650ꢁC was noted (not observed for the other
samples) with the weight loss being slightly larger when
the sample was heated in nitrogen rather than in oxygen.
This might indicate evidence for the oxidation of Co2+
to Co3+ in oxidizing atmosphere. Assuming the weight
loss difference between the formation of 3 CoO and
Co3O4 and considering the weight percent of cobalt
present in the sample, a theoretical value of 0.63% was
calculated while experimentally the value was found to
be 0.9%, not too far from the theoretical value, but
rather within the experimental error. Further, our earlier
TPR experiments [24] demonstrated that H2 consump-
Interesting information emerged upon investigating
the phase evolution of these materials under in situ
conditions. PXRD spectra of a representative sample
CuMgAl-HT after calcination at various temperatures
in air are given in Fig. 5 (diffractograms of other
samples namely CuCoAl-HT and CuNiAl-HT are
Fig. 5. PXRD patterns of CuMgAl-HT in situ calcined at (a) 50ꢁC, (b)
100ꢁC, (c) 150ꢁC, (d) 200ꢁC, (e) 300ꢁC, (f) 400ꢁC, (g) 500ꢁC, (h) 600ꢁC,
(i) 700ꢁC, (j) 800ꢁC.

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shown in Fig. 6 and 7). As reported earlier, for these
materials [25], the basal reflections (003) and (006)
progressively shift with increasing temperatures. This
could be attributed to the removal of interlayer water
accompanied by a reduction of the interlayer spacing. A
closer inspection of the (003) reflection for these samples
showed its complete absence at 250ꢁC, 200ꢁC and 150ꢁC
for CuMgAl-HT, CuNiAl-HT and CuCoAl-HT, respec-
tively. Consequently, the stability of the layered network
is highest for the Mg-containing sample followed by Ni
and Co, corroborating the results obtained from thermal
analysis. Interestingly, the crystallization of CuO occurs
for these samples at temperatures as low as 150ꢁC (i.e.
well below the first weight loss in TG is completed). This
suggests that the crystallization of CuO occurs even
when the layered network prevails. This may be possible
when dehydroxylation of the layered network starts at
the edges of the sheets. However, the crystallinity of
CuO is poor up to temperatures around 500ꢁC. With a
further increase in the temperature, the intensities of the
CuO reflections (representative diffractograms are
shown in Fig. 8) increase significantly. This may be
correlated to the unusual third transformation seen in
TG analyses, substantiating the fact, that the transfor-
mation mainly involves a copper-containing phase. With
a further increase in temperature, the crystallinity of
copper oxide increases, probably due to sintering. At
higher temperatures, crystallization of a spinel phase,
whose composition is influenced by the nature of the
cobivalent metal ion, also occurs. In addition, the onset
temperature of spinel crystallization varied with the
nature of the cobivalent metal ion, with the crystal-
lization being observed around 750ꢁC for Mg- and
Ni-containing systems, while it occurred already at
500ꢁC for the Co-containing system. This clearly depicts
the unusual behavior of cobalt-containing hydrotalcites
[26], for which the crystallization of a spinel occurs at
lower temperature, because of the facile oxidation of
Co2+ to Co3+. An attempt to identify the nature of
spinels for these systems was difficult from these results,
because of their poor intensities and minimal variations
in the peak positions for various spinels [13, 27, 28].
It is also known that the crystallization of spinel
phase increases with a further increase in calcination
temperature.
CuO
HT
HT
500
HT
a
d
g
In order to complement the structural transforma-
tions, infrared spectra were recorded under in situ
conditions for one of the samples, namely CuMgAl-HT
calcined at different temperatures under vacuum in a
purpose-made cell, in which samples can be heated up to
750ꢁC. The results are given in Fig. 9. The spectrum
recorded at room temperature showed bands corre-
sponding to synthetic hydrotalcite exhibiting broad
bands in the hydroxyl region (due to extensive hydrogen
bonding), a band around 1640 cmꢂ1 assigned to the
j
10
20
30
40
50
60
70
2 Theta, deg
Fig. 6. PXRD patterns of CuCoAl-HT in situ calcined at (a) 50ꢁC,
(b) 100ꢁC, (c) 150ꢁC, (d) 200ꢁC, (e) 300ꢁC, (f) 400ꢁC, (g) 500ꢁC,
(h) 600ꢁC, (i) 700ꢁC, (j) 800ꢁC.
CuO
HT
HT
500
200
a
a
d
b
h
k
c
a
d
10
20
30
40
50
60
70
30
40
50
60
70
2 Theta, deg
2 Theta, deg
Fig. 7. PXRD patterns of CuNiAl-HT in situ calcined at (a) 50ꢁC,
(b) 100ꢁC, (c) 150ꢁC, (d) 200ꢁC, (e) 300ꢁC, (f) 400ꢁC, (g) 500ꢁC,
(h) 600ꢁC, (i) 700ꢁC, (j) 800ꢁC, (k) 850ꢁC.
Fig. 8. PXRD diagrams of CuNiAl-HT calcined at (a) 500ꢁC,
(b) 550ꢁC, (c) 600ꢁC, (d) 650ꢁC.

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elimination still above 300ꢁC (upto 350 ꢁC). This
difference is presumably caused by differences in the
conditions of the experiment. The IR experiment was
done under dynamic vacuum while TG was done in a
flow of nitrogen. Furthermore, the carbonate vibrations
are present even after complete dehydroxylation (second
stage of transformation) corroborating the evolved gas
analysis (EGA) results. Carbonate vibrations (see
spectrum m of Fig. 9) were found even after calcination
of the sample at 750ꢁC (at which EGA did not show any
peaks for CO2 evolution) suggesting the presence of
strongly bonded carbonate species in the mixed metal
oxide network, probably located in the lattice as
oxycarbonates; nevertheless, they should be in a low
concentration or be amorphous, as no indication could
be obtained from the XRD diagrams. Although the
process involving decarbonation of oxycarbonate during
decomposition of hydrotalcites is reported earlier [9, 10],
the presence of carbonate even after treatment at 750ꢁC
under dynamic vacuum for a Cu-containing hydrotalcite
network, reported here for the first time, suggests the
presence of oxycarbonates. We infer, that it may not be
simple carbonate based on the fact, that the intensity of
the carbonate vibration is too small to be present as a
metal carbonate. The splitting of the n3 vibration, which
is at 1425 cmꢂ1 for the free carbonate, was found to be
158 cmꢂ1. It is possible to determine the coordination of
a C2v carbonate (mono- or bidentate) in the interlayer
space from this splitting, which was suggested to be ca.
100 and 300 cmꢂ1 for uni- and bidentate carbonates,
respectively [30]. After rearrangement only one pair of
bands was observed indicating that only one of the two
possible coordinations depicted in Scheme 1 can be
present. The experimentally observed splitting of the n3
carbonate vibration is indicative of a unidentate
coordination of the carbonate.
Fig. 9. In situ FT-IR spectra of CuMgAl-HT calcined at different
temperatures for 30 min (a) 30ꢁC, (b) 100ꢁC, (c) 200ꢁC, (d) 250ꢁC,
(e) 300ꢁC, (f) 350ꢁC, (g) 400ꢁC, (h) 450ꢁC, (i) 500ꢁC, (j) 600ꢁC,
(k) 650ꢁC, (l) 700ꢁC, (m) 750ꢁC.
deformation mode of water molecules present in the
interlayer and a sharpband around 1380 cm ꢂ1, assigned
to the n3 asymmetric stretching vibration of carbonate.
However, a significant change in the n3 vibration of
carbonate was noted when the sample was calcined at
100ꢁC. The band then splits into two well-defined bands
located around 1514 and 1360 cmꢂ1. This may suggest a
rearrangement of the carbonate anions in the interlayer
space, caused by a partial removal of interlayer and/or
physisorbed water molecules from the HT-like network.
In other words, the coordination of carbonates changes
upon removal of water molecules where they will then
coordinate to hydroxyl groups of the layered network.
Fierro and coworkers [12] also reported a similar
carbonate rearrangement, for which a band splitting
was observed after mild calcination of a CuZnAl system.
They concluded that the carbonate coordination may be
either monodentate or bidentate with the band at
1370 cmꢂ1 being assigned to a C–O vibration interacting
with OH groups in the brucite-like network and the
other band at 1530 cmꢂ1 was assigned to C=O
vibration. A band was also observed at relatꢂiv1ely higher
The Anton Paar camera attached to our XRD for
high-temperature studies has a high temperature limit of
900ꢁC. Therefore, to investigate the phase evolution in
both oxygen and nitrogen, PXRD was carried out for
samples CuNiAl-HT and CuCoAl-HT after ex situ
calcination at temperatures of 400, 650 and 1000ꢁC
(temperatures were chosen on the basis of TG traces).
The corresponding diffraction patterns are shown in
Figs. 10 and 11. The diffraction patterns observed for
temperatures (B250ꢁC) around 1050 cm
(Fig. 9
spectrum-d), attributed to the n1 vibration of carbonate,
an IR active band when the symmetry of carbonate is
reduced from D3h to lower symmetry (such as C3v or
C2v) [20, 29]. In addition, there is a continuous decrease
in the intensity of the hydroxyl vibrations and a
complete disappearance of the deformation mode of
water molecules occurs because of the desorption of
physisorbed water molecules with increasing calcination
temperature. At 300ꢁC, a nearly complete disappearance
of the hydroxyl vibrations was noted, suggesting the
absence of OH species in the calcined material. This is in
accordance with TG results (Fig. 2), although the gas
evolution during TG experiments (Fig. 3) showed water
Mn+
Mn+
OII
OI
OI
OI
C
C
OII
OII
Unidentate, Cs
Bidentate, C2v
Scheme 1. Modes of co-ordination of carbonate ion with metal center.

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326
Fig. 10. PXRD diagrams of CuNiAl-HT thermally treated at designated temperatures in nitrogen and oxygen for 1 h (T—Tenorite; B—Bunsenite;
S—Spinel; C—Cuprite).
Fig. 11. PXRD diagrams of CuCoAl-HT thermally treated at designated temperatures in nitrogen and oxygen for 1 h (T—Tenorite; B—Bunsenite;
S—Spinel; C—Cuprite).
samples calcined at 1000ꢁC in either oxygen or nitrogen
will clearly characterize the nature of the phases formed,
as the temperature is well above all the total weight
losses recorded by thermogravimetry. The phases
identified are the following:
formation of CuAl2O4, possibly containing small
amounts of nickel, is most likely.
CuNiAl in oxygen: Main peaks correspond to tenorite
with weak reflections of spinel and NiO. However, no
cuprite phase was detected.
CuNiAl in nitrogen: Mainly cuprite (Cu2O) was
formed along with weak reflections of a spinel phase
(which may be CuAl2O4 or NiAl2O4 or solid solutions
thereof). Pure NiO (bunsenite) was also noted, however,
pure tenorite (CuO) was not detected. Because of the
high concentration of copper present in the sample, the
CuCoAl in nitrogen: Dominant peaks again char-
acterize cuprite. However, additional less intense peaks
characterizing a spinel were also noted. The diffraction
patterns of most first row transition metal ions spinels
are very similar, so that a discrimination between Cu-
and Co-spinel is difficult. A peak search tentatively

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327
suggested that CoO ꢃ Al2O3, i.e. Co2+ spinel is the more
likely structure. This is not unusual, as under non-
oxidizing conditions, the oxidation of Co2+ to Co3+ is
unlikely.
CuCoAl in oxygen: The main peaks correspond to
tenorite like CuNiAl system, with no peaks of cuprite
found. Additional spinel peaks may correspond to
mixed spinels with Cu2+, Co2+, Co3+ and Al3+ being
present.
It is known that thermodynamically favored red
Cu2O is formed by decomposition of CuO at high
temperatures preferably under inert conditions [31]. We
believe, that the final high-temperature transformation,
which was noted in nitrogen but not in oxygen
characterizes the decomposition of CuO to Cu2O. The
quantitative weight loss values can be compared with
the theoretically expected values. Formation of Cu2O
from CuO can be written as
nitrogen no copper was present as CuO as evidenced
from PXRD. Hence, one can conclude that the high-
temperature weight loss occurring (around 850ꢁC) in
nitrogen for these samples corresponds to quantitative
formation of Cu2O.
The samples were also treated at intermediate
temperatures (i.e. after the second and third transforma-
tion in the TG traces) and studied for their phases using
PXRD (also shown in Figs. 10 and 11). PXRD of
CuNiAl-HT thermally treated at 400ꢁC in nitrogen
showed reflections corresponding to tenorite. The
PXRD traces were nearly coincident for samples
calcined either in nitrogen or in oxygen, suggesting that
redox processes do not occur at 400ꢁC. When the
temperature was raised to 650ꢁC, also only tenorite was
detected, but with increased crystallinity, probably due
to aggregation of the particles, which was more favored
in nitrogen than in oxygen. However, when the
temperature was raised to 1000ꢁC development of
Cu2O peaks was noted in nitrogen, but not in oxygen
as discussed above. These results confirm that reduction
of Cu2+ to Cu+ occurs after the fourth stage of
transformation (cf. quantitative TG analysis) for this
sample. To confirm our results, an independent experi-
ment was carried out wherein pure CuO (Merck) was
calcined under both nitrogen and oxygen and cooled
under respective atmospheres and followed both by
PXRD and thermal analysis. As observed for our
samples, a weight loss of around 10% was noted when
CuO was calcined in nitrogen in accordance with the
equilibrium 2CuO 3 Cu2O + 12 O2 and PXRD (Fig. 12)
2CuO-Cu2O þ 1=2O2:
Hence, 2 ꢄ 63.54 g of copper can release 16 g of
oxygen. Considering the weight percentage of copper
present in CuNiAl-HT (35.8%), CuCoAl-HT (34.9%)
and CuMgAl-HT (39.3%), the expected amounts of
oxygen released (for 100 g of solid) are 4.5, 4.4 and 4.9 g.
Experimentally, the high-temperature weight loss in
nitrogen yielded 4.4, 4.6 and 4.6 g for CuNiAl-HT,
CuCoAl-HT and CuMgAl-HT, respectively (Table 3).
These values are sufficiently close to the theoretical
values to support the interpretation of the high-
temperature transformation. We emphasize that under
+Al
T
C
T
C
Al
C
C
Al
T
N
T
T
T
T
T
T
T
T
T
O
0
10
20
30
40
50
60
70
2θ (Cu Kα)
Fig. 12. PXRD diagrams of CuO calcined under nitrogen and oxygen at 1000ꢁC for 2 h (T—Tenorite; C—Cuprite). Peaks marked Al are due to the
sample holder and used as reference.

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328
also substantiated the formation of Cu2O, while CuO
remains unchanged without any weight loss even after
calcination at 1000ꢁC when the atmosphere was oxygen.
The PXRD pattern of CuCoAl-HT thermally treated
in oxygen showed peaks corresponding to tenorite at
400ꢁC while at 650ꢁC, peaks corresponding to a spinel
phase appeared (cf. in situ PXRD results). Further
increase in temperature enhanced the crystallinity of
both tenorite and spinel phases. In contrast, cuprite
formation along with tenorite was noticed even at 400ꢁC
when the sample was thermally treated in nitrogen. At
650ꢁC, the crystallinity of both phases was improved
and a spinel phase was formed. Thermal treatment at
1000ꢁC did not yield peaks corresponding to tenorite
but only cuprite and a spinel phase were detected. The
formation of the cuprite phase at lower temperatures
may be caused by a catalytic role played by cobalt in
reducing Cu2+ to Cu+. The formation of Cu2O at lower
temperatures poses a doubt on the quantification of the
fourth stage of transformation for this sample. How-
ever, we believe, the concentration of such species is less
Fig. 14. FT-IR spectra of CuCoAl-HT thermally treated at (a) 400ꢁC,
(b) 650ꢁC, and (c) 1000ꢁC in nitrogen and oxygen.
(although it is well crystalline in nature) at lower
temperatures while it increases with an increase in
thermal treatment temperature (at the expense of the
concentration of CuO). However, one cannot comple-
tely exclude the contribution of the thermodynamically
allowed decomposition of Co3O4 to CoO under nitrogen
at around this temperature (850ꢁC) during the fourth
stage of transformation [32].
In order to elucidate further on these findings, FT-IR
spectra of the calcined samples were recorded at various
temperatures (similar to PXRD) and are reported in
Figs. 13 and 14. The spectra of authentic samples,
namely Cu2O and CuO, are also presented for compar-
ison in Fig. 13.
The spectrum of the CuNiAl sample thermally treated
in nitrogen at 400ꢁC does not exhibit well-defined bands
except for a weak band around 830 cmꢂ1, which is
assigned to the out-of-plane deformation mode (n2) of
carbonate [33]. A broad absorption band around
512 cmꢂ1 was observed, which could not give any
relevant information. The spectrum recorded in oxygen
was identical to that observed in nitrogen consistent
with PXRD results, which suggested that no redox
transformation occurs at this temperature. However,
when the sample was thermally treated at 650ꢁC in
either nitrogen or oxygen, no band was noted at
830 cmꢂ1 which suggests the absence of carbonate or
that the carbonate is present in undetectably low
Fig. 13. FT-IR spectra of CuNiAl-HT thermally treated at (a) 400ꢁC,
(b) 650ꢁC, and (c) 1000ꢁC in nitrogen and oxygen; spectra of Cu2O and
CuO are included for comparison.

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329
OH
OH
OH
OH
Cu
OH
Al
HO
HO
OH
OH
OH
OH
OH
OH
OH
OH
Cu
Cu
M
OH
Cu
OH
OH
OH
Cu
OH
Al
HO
HO
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
H
OH
O
HO
O
OH HO
OH
OH
Cu
M
O
OH
O
H
H
H
OH
OH
O
OH
H
H
OH
O
C
OH
O
O
100°C, vacuum
-H2O
C
O
H
C
O
O
O
H
H
H
OH
OH
Cu
OH
OH
OH
O
O
HO
HO
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
Cu
OH
OH
Cu
HO
HO
HO
OH
OH
OH
OH
Cu
OH
OH
OH
OH
M
Al
Cu
Cu
M
Al
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
M = Ni, Co or Mg
Oxide Lattice
Cu
O
OH
OH
OH
Cu
OH
O
O
OH
OH
OH
OH
Cu
M
Al
C
OH
OH
O
OH
OH
O
O
OH
O
OH
OH
OH
O
C
Cux(Ni)yOz(CO3)m
C
O
O
O
OH
Cu
OH
OH
Al
OH
O
OH
OH
OH
OH
OH
OH
OH
M
Cu
O
OH
OH
OH
OH
OH
CuO
CuO
850°C
850°C
CuO + Spinel
Cu2O + Spinel
Oxide Lattice
1000°C
1000°C
Cu
O
+ CuO
C
Cu2O + Spinel
+ NiO
CuO + Spinel
+ NiO
O
O
Cux(M)yOz(CO3)m
Scheme 2. Thermal evolution processes involved in heating of Cu-rich hydrotalcites.
concentration. The spectral vibrations due to M–O
vibrations were better resolved and absorptions were
in oxygen, again corroborating PXRD results, which
showed poor crystallinity of tenorite formed in oxygen
in contrast to nitrogen. The spectra recorded both in
oxygen and nitrogen showed clear features when the
sample was thermally treated at 1000ꢁC. In the case of
nitrogen, a sharpband characteristic of curpite was
noted around 625 cmꢂ1 (see Fig. 13). No bands were
noted in the region around 500 and 600 cmꢂ1, char-
acteristic of tenorite, as supported by PXRD (Fig. 10).
The bands due to the spinel phase also sharpened with
clear features around 725 cmꢂ1 (n1 vibration of spinel
noted around 494 cmꢂ1 (n1; nCu2O
)
and around
600 cmꢂ1 confirming the presence of CuO [34]. In
addition, very weak bands around 730 and 800 cmꢂ1
(including a broad absorption around 600 cmꢂ1) were
noted, probably assigned to absorptions of spinel,
although PXRD did not reveal a crystalline spinel
phase at this temperature [35], suggesting that they are
either amorphous or poorly crystallized. No well-
defined absorption was noted for the sample calcined

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S. Kannan et al. / Journal of Solid State Chemistry 177 (2004) 319–331
330
phase) and 790 cmꢂ1. Although we do not have an
evidence to prove the band around 800 cmꢂ1 is for the
spinel phase, we do observe such a weak band around
800 cmꢂ1 for pure binary CoAl hydrotalcite calcined at
400ꢁC in air for 5 h and for pure Co3O4 [21]. However,
bands characteristic of tenorite were detected in oxygen
(bands at 593, 530 and 480 cmꢂ1) along with bands of
the spinel phase. The band positions of the spinel phase
were similar irrespective of the atmosphere, suggesting a
similar nature of the spinel phase in both cases.
ratio around 5.0 were synthesized. The thermal analysis
pattern of these samples showed four weight loss stages
in nitrogen with the fourth stage being ascribed to
reduction of CuO to Cu2O (not observed when recorded
in oxygen). Formation of CuO (by in situ PXRD)
through partial dehydroxylation of the layered network
was noted well below the structural decomposition.
Retention of carbonate at temperatures as high as 700ꢁC
(in situ FT-IR) was noted suggesting the presence of
oxycarbonate. Based on the discussions presented
above, thermal evolution processes involved on heating
Cu-rich hydrotalcites in both inert and oxidizing atmo-
spheres are given in Scheme 2.
Subtle differences were noted for sample CuCoAl-HT
when the thermal treatment was carried out in different
atmospheres. FT-IR spectra (Fig. 14) of the sample
treated both in nitrogen and oxygen at 400ꢁC, showed a
broad absorption from which no discrete information
could be inferred. However, similar to CuNiAl-HT, the
n2 absorption of carbonate was also observed for this
system. When the treatment temperaturꢂe1was raised to
Acknowledgments
S.K. thanks the Alexander von Humboldt Founda-
tion for a research fellowship. S.K. also thanks the
Council of Scientific and Industrial Research, New
Delhi, for financial assistance under Young Scientist
Scheme. V.R. thanks MCyT (Spain) for Grant
MAT2000-1148-C02-01.
650ꢁC, a sharpband around 670 cm
was noted,
ascribed to a spinel vibration [35]. A broad band was
noted in the region 650–500 cmꢂ1 with weak bands
around 600 and 560 cmꢂ1, although PXRD showed
well-crystallized Cu2O peaks for this sample. In other
words, although PXRD showed the formation of
cuprite, FT-IR did not clearly reveal its formation
(band around 635 cmꢂ1 might be buried in the broad
band between 650 and 550 cmꢂ1), suggesting that the
concentration of this phase must be very low, however,
highly crystalline. However, when the sample was
thermally treated at 1000ꢁC in nitrogen, a well-defined
band characteristic of cuprite was noted at 634 cmꢂ1 and
no band due to tenorite was detected. Further, a shift in
the band position for the spinel phase occurs, probably
due to compositional variations, due to incipient
crystallization of the cuprite phase. Bands (585, 525
and 480 cmꢂ1) corresponding to tenorite were noted in
oxygen for the sample calcined at 1000ꢁC. However, in
oxygen, no significant shift in the band position was seen
for the spinel phase, whose intensity was much larger
than for the spinel phase observed in nitrogen (similar to
PXRD results; see Fig. 11). Further, the bands due to
spinel are relatively broad, suggesting the presence of
multivalent metal ions (in other words, occurrence of
oxidation) present both in tetrahedral and octahedral
coordination. Weak shoulders were also noted for the
sample calcined in oxygen around 725 and 800 cmꢂ1
substantiating the presence of the spinel phase. In
addition, both samples showed a sharp band around
320 cmꢂ1 when calcined in oxygen at 1000ꢁC, for which
the attribution is not clear.
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