Thermal, solution and reductive decomposition of Cu-Al layered double hydroxides into oxide products

Article abstract of DOI:10.1016/j.jssc.2009.02.003

Cu-Al layered double hydroxides (LDHs) with [Cu]/[Al] ratio 2 adopt a structure with monoclinic symmetry while that with the ratio 0.25 adopt a structure with orthorhombic symmetry. The poor thermodynamic stability of the Cu-Al LDHs is due in part to the

Full text of DOI:10.1016/j.jssc.2009.02.003

ARTICLE IN PRESS
Journal of Solid State Chemistry 182 (2009) 1193–1199
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Thermal, solution and reductive decomposition of Cu–Al layered double
hydroxides into oxide products
Ã
Sylvia Britto, P. Vishnu Kamath
Department of Chemistry, Central College, Bangalore University, Bangalore 560 001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Cu–Al layered double hydroxides (LDHs) with [Cu]/[Al] ratio 2 adopt a structure with monoclinic
symmetry while that with the ratio 0.25 adopt a structure with orthorhombic symmetry. The poor
thermodynamic stability of the Cu–Al LDHs is due in part to the low enthalpies of formation of Cu(OH)2
and CuCO3 and in part to the higher solubility of the LDH. Consequently, the Cu–Al LDH can be
decomposed thermally (150 1C), hydrothermally (150 1C) and reductively (ascorbic acid, ambient
temperature) to yield a variety of oxide products. Thermal decomposition at low (400 1C) temperature
yields an X-ray amorphous residue, which reconstructs back to the LDH on soaking in water or standing
in the ambient. Solution decomposition under hydrothermal conditions yields tenorite at 150 1C itself.
Reductive decomposition yields a composite of Cu2O and Al(OH)3, which on alkali-leaching of the latter,
Received 7 November 2008
Received in revised form
16 January 2009
Accepted 1 February 2009
Available online 12 February 2009
Keywords:
Layered double hydroxide
Solution decomposition
leads to the formation of fine particles of Cu2O (o1
mm).
& 2009 Elsevier Inc. All rights reserved.
1. Introduction
Gibbsite is also a layered hydroxide in which the layers have
the composition [Al2/3 1/3(OH)2] (&: octahedral vacancy). In
&
gibbsite the layers are stacked in the sequence AB BA AB BA- - - -.
Among the gibbsite-based LDHs, the most common are the I–III
LDHs with Li+ occupying the cation vacancies of gibbsite [8,9]
leading to the layer composition [Al2/3Li1/3(OH)2]1/3+. These LDHs
have the composition [Al2Li(OH)6]Cl ꢁ 3H2O. Recently, however,
O’Hare and co-workers [10] have synthesized a series of II–III
Layered double hydroxides (LDHs) are derived either from the
structure of mineral brucite [Mg(OH)2], or from that of gibbsite/
bayerite [Al(OH)3] [1], the material obtained in the latter case
being Al-rich. Brucite comprises a close packing of hydroxyl ions,
in which alternative layers of octahedral sites are occupied by
divalent ions, yielding charge-neutral layers having the composi-
tion [M(OH)2] (MII ¼ Mg, Ca, Mn, Fe, Co, Ni). Isomorphous
substitution of a fraction of the divalent ions by trivalent ions
M0 (III) (M0 ¼ Al, Cr, Fe, Ga, In) results in layers having the
composition [M(II)1ꢀxM0(III)x(OH)2]x+ (xp0.33). The positive
charge on the layers is compensated by the incorporation of
anions such as CO23ꢀ, NO3ꢀ, Clꢀ in the interlayer region to yield
LDHs having the formula [M(II)2M0(III)(OH)6]Cl ꢁ mH2O corre-
sponding to x ¼ 0.33. Among the brucite-based LDHs, those
containing Cu2+ as the divalent ion are unique in that the bivalent
hydroxide, Cu(OH)2, has a structure closer to that of lepidocrocite
LDHs derived from gibbsite having the composition [Al2-
II
M
0.5
(OH)6]Cl ꢁ mH2O. Here also the LDH containing Cu was found
to contain impurity phases. The synthesis of II–III LDHs based on
the gibbsite structure creates the possibility of studying a much
wider compositional range ([Cu]/[Al] ¼ 2–0.25) than was possible
within the brucite-based systems.
The synthesis of LDHs has most commonly been carried out by
one of two routes: (i) through coprecipitation at constant pH [11]
or (ii) through an oxide route wherein the oxide of the divalent
metal is soaked in a salt solution of the trivalent metal [12,13]. In
view of the difficulties seen in obtaining phase-pure Cu–Al LDHs,
we attempted their synthesis through both the routes given
above. While coprecipitation yields a Cu-rich Cu–Al LDH, the
oxide route yields an Al-rich Cu–Al LDH.
The interest in Cu–Al LDH stems from the importance of the
oxide products derived from these materials in catalysis [7,14–16].
Cu–Al oxides of the spinel structure are used in the oxidation of
phenol from aqueous solution [14]. They have also been used in
the selective C-alkylation of phenol with methanol [15] and
ternary Cu–Zn–Al mixed oxides derived from LDHs have been
used as catalysts for low-temperature methanol synthesis [16].
(g-FeOOH) [2] rather than brucite. The difficulties seen in
obtaining phase-pure Cu–Al LDHs may be related to the instability
of Cu(OH)2. In recent years, many attempts have been made to
synthesize phase-pure Cu–Al LDHs [3–7] and in only a few cases
has this been achieved [5]. In most studies, malachite [Cu2(OH)2
(CO3)], gibbsite and gehrardtite [Cu2(OH)3(NO3)], were also
obtained as impurity phases.
Ã
Corresponding author.
E-mail address: vishnukamath8@hotmail.com (P. Vishnu Kamath).
0022-4596/$ - see front matter & 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2009.02.003

ARTICLE IN PRESS
1194
S. Britto, P. Vishnu Kamath / Journal of Solid State Chemistry 182 (2009) 1193–1199
The oxides derived from these LDHs exhibit a high surface area,
basicity and greater resistance to sintering [1].
2.4. Reversible thermal behavior
The thermodynamic stability of the LDH is an important factor
guiding its decomposition pathway. There are several approaches
to understanding the thermodynamic stability of a LDH [17].
A 0.5 g batches of the LDHs were calcined at 400 1C for 4 h in a
muffle furnace. Complete decomposition is indicated by the mass
loss. In separate experiments, the amorphous residue was stirred
for two days in a Na2CO3 solution or equilibrated in a water-
saturated environment. The product was collected in a previously
weighed sintered glass crucible and the mass gain monitored.
(1) As the LDHs are generally decomposed by heating, thermal
stability is obtained from calorimetric measurements of the
enthalpy of formation [18].
(2) As the LDHs are obtained by alkali precipitation from a mixed
metal salt solution, Johnson and coworkers [19] have
measured the solubility product of LDHs and related it to
their thermodynamic stability. The amphotericity of Cu–Al
LDHs, seriously impacts the stability of the LDH in solution.
(3) In contrast to other LDHs, the Cu–Al LDHs are unique owing to
the reducibility of the Cu2+ ion. Therefore the stability of the
2.5. Reductive decomposition
Three hundred and seventy milliliter of a solution containing
2.6 g of ascorbic acid was adjusted to pH 10 using 1 M NaOH. To
this solution, 2 g of the CuAl4-LDH was added and the suspension
stirred at room temperature for 3 h. The yellow product obtained
was centrifuged, washed several times with deionized water,
twice with ethanol and dried in a dessicator. The resultant product
was found to be a composite of Cu2O–Al(OH)3. One gram of the
composite was soaked in 50 mL of 6 M NaOH in a screw capped
polypropylene vessel and kept at 65 1C in an air oven for 6 h to
LDH is also dictated by the redox potential of Cu2+
.
(4) An electronic structure approach can be envisaged taking into
consideration such factors as the octahedral CFSEs of the
metal ions and the mode of coordination of the anions. This
approach has particular significance to the Cu–Al LDHs due to
the Jahn-Teller distortion of the Cu2+ ion coordination.
leach out the Al(OH)3. The residue was filtered through
previously weighed sintered glass crucible and the mass loss
monitored.
a
In this paper we report the synthesis and characterization of
brucite and gibbsite-based Cu–Al LDHs. We study their break-
down through the thermal, solution and reductive decomposition
routes and examine the products obtained from each and their
possible applications.
2.6. Characterization
Powder diffraction data were collected on a Bruker D8 Advance
˚
powder X-ray diffractometer (CuK
a radiation, l ¼ 1.5418 A). Data
were collected at a continuous scan rate of 11 2
rebinned into steps of 0.021 2 . TG data were obtained using a
y
minꢀ1 and
y
Mettler Toledo TGA/SDTA Model 851e system (30–800 1C, heating
rate 5 1C minꢀ1, flowing air). IR spectra were obtained using a
Nicolet Model Impact 400D FTIR spectrometer (KBr pellets,
4000–400 cmꢀ1, resolution 4 cmꢀ1). Scanning electron micro-
graphs were obtained using a JEOL Model JSM 840 microscope.
The powder samples were dispersed on a double sided conducting
carbon tape. The samples were sputter coated with gold to
improve conductivity.
2. Experimental
2.1. Synthesis of Cu2Al–CO3 LDH
A mixed metal nitrate solution containing stoichiometric
amounts of Cu and Al (2:1) was added drop-wise to a NaHCO3
solution (100 mL) containing four times the stoichiometric excess
of carbonate ions. The pH was maintained at 10 using a Metrohm
Model 718 STAT Titrino operating in the pH STAT mode. The
reaction was carried out at room temperature under vigorous
stirring. The blue precipitate was aged in the mother liquor (16 h).
It was then washed 3–4 times with deionized Type-II water
(Millipore Elix-3 water purification system, specific resistance
The Cu and Al content in the LDHs was estimated by
gravimetric analysis. Cu was estimated using the benzoin-
a
-oxime (cupron) method [20]. Aluminium was estimated using
8-hydroxyquinoline as the precipitant [20] after the removal of
Cu as CuS.
15 MO cm), once with ethanol and dried in an air oven at 65 1C.
3. Results
2.2. Synthesis of CuAl4–NO3 LDH
3.1. Synthesis and characterization of Cu2Al–CO3 LDH
A 1 g of CuO was made into a slurry in 15 mL of 0.7 M Al(NO3)3
solution and aged in an air oven at 65 1C for four days. The
precipitate was then washed several times with deionized water,
once with ethanol and dried in an air oven at 65 1C.
In Fig. 1(a) is shown the PXRD pattern of the Cu–Al LDH
obtained by coprecipitation. All the observed reflections (up to 651
2y) are indexed on a monoclinic cell using the cell parameters
˚
˚
given by Yamaoka and coworkers [3] (a ¼ 15.21 A, b ¼ 2.9 A,
˚
c ¼ 5.86 A,
b ¼ 100.31) (Table 1). There are no impurities of
2.3. Hydrothermal and thermal decomposition
a crystalline nature. The wet chemical analysis and TGA data
(Table 2) yield an approximate formula [Cu0.7Al0.3(OH)2]
(CO3)0.15 ꢁ 0.8H2O. The basal reflections are sharp while the non-
Both the Cu2Al–CO3 LDH and the CuAl4–NO3 LDH were
hydrothermally treated at temperatures ranging from 110 to
180 1C. In a typical procedure, 0.5 g of LDH was dispersed in 40 mL
of deionized water (50% filling) and hydrothermally treated in a
Teflon-lined autoclave at the required temperature for 24 h. We
refer to the decomposition of the LDH under hydrothermal
treatment as solution decomposition. In separate experiments,
the LDHs were also calcined in a muffle furnace at 400, 600 and
800 1C for 18 h each.
basal reflections (4301 2y) are considerably broadened. Such non-
uniform broadening is due to the presence of stacking faults
[21–23]. On hydrothermal treatment at 110 1C for 24 h, the LDH
structure is retained and the reflections in the mid-2
(30–501 2 ) are distinctly sharper indicating structural ordering
(see Fig. 1(b)).
y region
y
The CO23ꢀ ion intercalated in the D3h symmetry exhibits three
IR active modes corresponding to the out-of-plane bending n2

ARTICLE IN PRESS
S. Britto, P. Vishnu Kamath / Journal of Solid State Chemistry 182 (2009) 1193–1199
1195
(A200: 870 cmꢀ1), antisymmetric stretching n3 (E0: 1360 cmꢀ1) and
in-plane bending n4 (E0: 670 cmꢀ1). The IR spectrum of the
Cu–Al–CO3 LDH in the carbonate absorption region (Fig. 2)
however shows a splitting of the doubly degenerate n3 (1362
and 1380 cmꢀ1) and n4 modes (600 and 640 cmꢀ1) corresponding
to the A1+B2 modes of C2v symmetry. This combined with
activation of the n1 mode (1025 cmꢀ1) indicates lowering of the
carbonate symmetry from D3h to C2v in the Cu–Al–CO3 LDH. The n1
mode is more pronounced in the hydrothermally treated sample
(data not shown).
Other CO23ꢀ-containing LDHs crystallize in a three-layered
rhombohedral structure and the CO23ꢀ ions occupy the trigonal
prismatic interlayer sites [24]. This structure is preferred to other
polytypes as the prismatic site maximizes the H-bonding between
the oxygen of the CO23ꢀ ions and the hydroxyl groups of the metal
hydroxide slab [25]. However there are other polytypes, notably
those of hexagonal symmetry, that also provide prismatic sites for
CO23ꢀ incorporation [26]. Laboratory prepared samples are there-
fore obtained as intergrowths of two or more polytypes. The
intergrowth regions manifest as regions comprising stacking
faults. The stacking faults once incorporated cannot be eliminated
by hydrothermal treatment [27]. The Cu–Al system is therefore an
exception among the LDHs in that ordering is observed on
hydrothermal treatment. Although detailed structure of the Cu–Al
LDH is not known, the facile ordering of the LDH is a manifesta-
tion of the differences in the structure of this LDH when compared
to other LDHs.
(a) In contrast to other II–III LDHs, the Cu–Al LDH crystallizes
with monoclinic symmetry.
(b) On monoclinic distortion, the interlayer sites lose three-fold
symmetry. The incompatibility between the local symmetry
of the interlayer site and symmetry of the CO23ꢀ ion,
destabilizes the stacking disorders whereby partial ordering
is observed on hydrothermal treatment. Complete ordering
however is preceded by decomposition at 150 1C.
3.2. Synthesis and characterization of CuAl4–NO3 LDH
Fig. 1. PXRD patterns of Cu2Al–CO3 LDH (a) as prepared and (b) on hydrothermal
treatment at 110 1C (inset: 301–701 2y region expanded).
In Fig. 3(a) is shown the PXRD pattern of the product obtained
by soaking CuO in Al(NO3)3 solution. The pattern is indexed to an
Table 1
Calculated and observed d-spacings of the PXRD patterns of the Cu2Al–CO3 LDH
and CuAl4–NO3 LDH.
Cu2Al–CO3 LDH (monoclinic cell)
CuAl4–NO3 LDH (orthorhombic cell)
d (calc.)a (A)
h k l
d (obs.) (A)
d (calc.)b (A)
h k l
˚
˚
˚
˚
d (obs.) (A)
7.481
3.749
2.734
2.520
2.414
2.231
2.032
1.891
1.721
1.589
1.569
1.545
1.522
1.466
1.441
7.481
3.741
2.741
2.519
2.389
2.227
2.039
1.891
1.721
1.587
1.568
1.559
1.521
1.465
1.442
200
8.605
4.285
3.168
3.079
2.511
2.291
2.219
2.009
1.481
1.459
8.595
4.298
3.176
3.099
2.513
2.279
2.224
2.004
1.482
1.459
200
400
221
420
112
022
431
241
523
651
400
102
111
211
311
ꢀ212
610
710
711
113
ꢀ413
213
ꢀ204
004
a
˚
˚
˚
b
a ¼ 15.207 A, b ¼ 2.9 A, c ¼ 5.86 A,
¼ 100.31.
b
˚
˚
˚
a ¼ 17.19 A, b ¼ 8.94 A, c ¼ 5.30 A.
Fig. 2. Infrared spectrum of the Cu2Al–CO3 LDH.
Table 2
Elemental analysis and TGA mass loss data of the Cu2Al–CO3 LDH and CuAl4–NO3 LDH.
Sample
Cu/Al ratio
% Mass loss from TGA
Approximate formulaa
Cu2Al–CO3 LDH
CuAl4–NO3 LDH
2.3
35.7
47.7
Cu0.7Al0.3(OH)2(CO3)0.15 ꢁ 0.8H2O
0.24
Cu0.17Al0.67(OH)2(NO3)0.33 ꢁ 0.35H2O
a
Computed on the basis of Cu/Al ratio and water content calculated from TGA mass loss.

ARTICLE IN PRESS
1196
S. Britto, P. Vishnu Kamath / Journal of Solid State Chemistry 182 (2009) 1193–1199
vibrations in the IR spectrum indicates breakdown of the LDH
structure. On soaking the amorphous residue in Na2CO3 solution,
the LDH is reconstructed (Fig. 5(b)). However, only 79% of the
original mass is regained possibly due to partial bulk dissolution.
Reconstruction is also observed on equilibrating the oxide residue
at the ambient temperature under 100% relative humidity
(Fig. 5(c)). In this case 99% recovery of mass was observed.
A
further evidence of reconstruction is the restoration of
CO23ꢀ-related vibrations in the IR spectrum of the reconstructed
samples (data not shown). Decomposition at higher temperatures
leads to the formation of the more stable CuO which does not
reconstruct. At even higher temperatures (800 1C), CuAl2O4-
related peaks appear in the PXRD pattern (Fig. 6(a)).
In Fig. 7(a) is shown the PXRD pattern of the sample obtained
on decomposing CuAl4-LDH at 400 1C. The disappearance of basal
reflections indicates breakdown of the LDH structure. The pattern
obtained on soaking the residue in Na2CO3 solution is shown in
Fig. 7(b). The pattern could be indexed to a mixture of Cu2Al–CO3
Fig. 3. PXRD patterns of CuAl4–NO3 LDH (a) as-prepared and (b) after
hydrothermal treatment (120 1C).
100
˚
˚
˚
orthorhombic cell (a ¼ 17.19 A, b ¼ 8.94A, c ¼ 5.29A) (see Table 1).
Wet chemical analysis yields an approximate formula [Cu0.17
Al0.67(OH)2](NO3)0.33 ꢁ 0.35H2O (see Table 2). This structure is
reported to be closely related to the gibbsite-based Li–Al LDHs
except that only half the octahedral vacancies in gibbsite are
occupied by Cu2+ [10]. Gibbsite-based Li–Al LDH is prepared by an
‘imbibition’ reaction in which the Li+ ions are topotactically
inserted into the [&1/3Al2/3(OH)2] (&: cation vacancy) slab
through the triangular faces of the [&(OH)6] octahedra by a
mechanism called diadochy [28]. Insertion of divalent ions into
gibbsite is however not facile and requires harsh conditions [10].
We are able to synthesize the Cu–Al4 LDH by a simpler route
involving soaking of CuO in Al(NO3)3 solution. Fig. 3(b) shows the
PXRD pattern of the product obtained upon hydrothermal
treatment of CuAl4–NO3 LDH (120 1C, 24 h). The LDH has
decomposed leaving behind Al(OH)3. The absence of any peaks
due to CuO and the blue color of the supernatant indicates the
deintercalation of Cu2+ ions. An estimation of Cu content in the
supernatant confirms leaching out of copper from the LDH (Obs.:
9.4%; Exp.: 11.7%).
0.00
80
-0.25
60
100 200 300 400 500 600 700 800
DTG
100
TGA
0.00
80
-0.25
60
100 200 300 400 500 600 700 800
T (°C)
Fig. 4. TGA and DTG curves of the (a) Cu2Al–CO3 LDH and (b) CuAl4–NO3 LDH.
3.3. Thermal behavior
The TGA curve of the Cu2Al-LDH is shown in Fig. 4(a). The LDH
loses mass in two steps. The lower temperature mass loss is
observed from 30 to 550 1C and a higher temperature mass loss is
seen from 550 to 800 1C. The DTG curve shows that the LDH
decomposes at 150 1C. The total mass loss (35.7%) over the
30–800 1C range is consistent with the following reactions:
½Cu0:7Al0:3ðOHÞ2ꢂðCO3Þ0:15 ꢁ 0:8H2O ! 0:15 CuAl2O4 þ 0:55 CuO
In contrast, the TGA curve of the CuAl4–NO3 LDH (Fig. 4(b))
indicates a single step mass loss with decomposition occurring at
250 1C (the mass loss below 100 1C is attributed to loss of
physisorbed water). The mass loss (47.7%) is consistent with the
following reaction:
½Cu0:17Al0:67ðOHÞ2ꢂðNO3Þ0:33 ꢁ 0:35H2O ! 0:165 CuAl2O4
þ 0:165 Al2O3
In Fig. 5(a) is shown the PXRD pattern of the oxide residue
obtained on thermal decomposition (400 1C, 4 h) of the
Cu2Al–LDH. The material is X-ray amorphous. Disappearance of
basal reflections in the PXRD pattern and of the CO23ꢀ-related
Fig. 5. PXRD pattern of the (a) oxide residue obtained on thermal decomposition
(400 1C, 4 h) of the Cu2Al-LDH, (b) product obtained on soaking (a) in Na2CO3
solution, and (c) on equilibrating (a) in a water-saturated atmosphere.

ARTICLE IN PRESS
S. Britto, P. Vishnu Kamath / Journal of Solid State Chemistry 182 (2009) 1193–1199
1197
LDH and Al(OH)3. On decomposing the LDH at even higher
temperatures (600–800 1C), poorly crystallized CuAl2O4 is ob-
tained (Fig. 6(b)) which does not reconstruct back to the LDH.
Among the layered hydroxides, the reconstruction phenomen-
on has been exhibited only by the LDHs of Mg and Zn with Al
[29,30]. In these LDHs, Al3+ substitutes for some of the Mg2+ in the
MgO to form a defect oxide having the rocksalt structure. Due to
the compositional metastability of this oxide residue, it rapidly
reverts back to the LDH structure. The LDHs of Ni and Co, which
form spinels at a low temperature, do not exhibit reversible
thermal behavior. In the case of the Cu–Al system also, thermal
decomposition leads to an X-ray amorphous metastable oxide
intermediate of unknown structure. Formation of the stable oxide
(tenorite, CuO) and reconstruction to the LDH are competing
pathways accessible to the oxide residue. At lower temperatures
as formation of LDH from the metastable intermediate is
kinetically favored, the intermediate reverts back to the LDH
structure by absorption of CO2 and moisture from the atmosphere.
On heating to higher temperatures however, the activation barrier
to formation of tenorite is overcome and the metastable
intermediate is transformed to the stable tenorite, which being
more stable than the LDH, does not reconstruct. In the case of the
CuAl4-LDH, the presence of broad peaks at 371 and 451 indicate
possible formation of the more stable g-Al2O3. Reconstruction is
therefore not observed in humid atmosphere. On soaking in
Na2CO3 solution however, dissolution of the oxide residue is
facilitated and the more stable phases, Cu2Al–CO3 LDH and
Al(OH)3 are precipitated in preference to CuAl4-LDH.
3.4. Solution decomposition
As discussed earlier, while hydrothermal treatment at 110 1C
leads to structural ordering of the Cu2Al-LDH, treatment at higher
temperatures (150 1C) leads to the formation of crystalline CuO
(major phase) and boehmite, AlOOH (minor phase) (Fig. 8(a)). The
formation of crystalline CuO at much lower temperatures than
during thermal decomposition is related to the fact that under
these conditions decomposition proceeds via dissolution and
reprecipitation. As explained by Cuddenec and co-workers [31],
upon dissolution, the Cu2+ ion adopts a square planar coordination
in the complex anion [Cu(OH)4]2ꢀ. This is a precursor to CuO
which also has Cu2+ in square planar coordination.
In contrast, under solid state decomposition conditions,
formation of tenorite requires rearrangement of the coordination
environment of Cu2+ from that in the LDH to the square planar
environment in tenorite. The kinetic barrier leads to formation of
an intermediate metastable amorphous oxide residue below
600 1C before formation of tenorite at higher temperatures.
The CuAl4-LDH also breaks down at lower temperatures under
hydrothermal conditions than under thermal conditions. As it is
Al3+-rich, it gives rise to boehmite and bayerite (Fig. 8(b)). Cu2+ is
leached out upon hydrothermal treatment.
3.5. Reductive decomposition
Due to the reducibility of the Cu2+ ion, the Cu–Al LDH lattice
can also be broken down by reductive decomposition. In Fig. 9(a)
is shown the PXRD pattern of the product obtained on the
reduction of CuAl4-LDH by ascorbic acid. The pattern indicates a
composite of Cu2O and Al(OH)3. We explore the potential of this
material as a precursor to fine particulate Cu2O. Due to the
insolubility of Cu2O in alkali, Al(OH)3 may be selectively leached
out leaving behind fine particles of Cu2O. This strategy is similar
Fig. 6. PXRD pattern of the oxide residue obtained on thermal decomposition
(800 1C, 4 h) of the (a) Cu2Al-LDH and (b) CuAl4-LDH.
Fig. 7. PXRD pattern of (a) oxide residue obtained on thermal decomposition
(400 1C, 4 h) of the CuAl4-LDH and (b) product obtained on soaking (a) in Na2CO3
solution.
Fig. 8. PXRD pattern of the product obtained on hydrothermal treatment of (a)
Cu2Al–CO3 LDH and (b) CuAl4–NO3 LDH at 150 1C.

ARTICLE IN PRESS
1198
S. Britto, P. Vishnu Kamath / Journal of Solid State Chemistry 182 (2009) 1193–1199
to that used by other authors [32–34] for the synthesis of
macroporous materials. In Fig. 9(b) is shown the PXRD pattern of
the residue obtained on soaking the composite in alkali. Selective
leaching out of the Al(OH)3 leaves behind single phase Cu2O. The
mass loss obtained on leaching of Al(OH)3 is quantitative. The SEM
images of the Cu2O–Al(OH)3 composite is shown in Fig. 10(a). It is
synthesis of Cu–Al LDHs may be related to the lower thermo-
dynamic stability of this LDH. We now look more closely at the
stability of the Cu–Al LDH from the point of view of thermo-
chemistry, solubility and electronic structure as has been
elucidated in the introduction to this paper.
Navrotsky and co-workers [18] have pointed out that for
systems where thermodynamic data are not available, a simple
approach to determine the enthalpy and free energy of formation
would be to consider the LDH to consist of a mechanical mixture
of binary hydroxides and carbonates. Lack of detailed structural
information pertaining to Cu–Al LDHs limits the extent to which
such an approach may yield accurate thermodynamic values for
the Cu–Al LDHs. Nevertheless, a qualitative understanding of the
relative stability of the Cu–Al LDHs with respect to other M2+–Al
LDHs may be made. This ‘mechanical mixture’ model shows that a
large contribution to the stability of a LDH is derived from the
divalent salt of the interlayer anion with a smaller (but not
negligible) contribution from the divalent hydroxide. Unlike
carbonates of other divalent ions which have the calcite structure,
the carbonate of copper is stable as the basic carbonate, malachite.
Cupric carbonate (CuCO3) decomposes in a facile way to CuO
and CO2. Furthermore, the free energy of formation of Cu(OH)2
is lower than that of the other divalent hydroxides such
seen to consist of large monolithic particles in the
mm range. On
leaching of Al(OH)3, the SEM image (Fig. 10(b)) shows smaller sub-
micron sized particles.
Attempts were made to synthesize fine particle Cu2O from the
Cu2Al-LDH precursor. However on reduction of Cu2Al–CO3 LDH in
ascorbic acid, only Cu2O peaks are observed (see Fig. 11). The
possibility that the Al(OH)3 is present as an amorphous phase is
however ruled out by the absence of any mass loss on soaking the
product obtained in alkali. This implies that the Al(OH)3 has
undergone dissolution during the reduction stage itself.
4. Discussion
There has been a considerable degree of confusion in the early
literature regarding the formation of Cu–Al LDHs. It was believed
that Cu2+ can be incorporated into LDHs only in the presence of
another divalent ion [1]. The difficulties associated with the
as Mg(OH)2 and Co(OH)2
[D
G0298[Cu(OH)2] ¼ ꢀ85.3 kcal molꢀ1
,
Fig. 9. PXRD pattern of product obtained on (a) reductive decomposition of CuAl4-
Fig. 11. PXRD pattern of the Cu2O product obtained on reductive decomposition of
LDH and (b) soaking (a) in 6 M NaOH.
Cu2Al-LDH using ascorbic acid.
Fig. 10. SEM image of (a) the Cu2O–Al(OH)3 composite obtained on reductive decomposition of CuAl4-LDH and (b) Cu2O obtained on leaching of Al(OH)3 from (a).

ARTICLE IN PRESS
S. Britto, P. Vishnu Kamath / Journal of Solid State Chemistry 182 (2009) 1193–1199
1199
D
G0298[Mg(OH)2] ¼ ꢀ199.3 kcal molꢀ1
,
D
G0298[Co(OH)2] ¼ ꢀ110 kcal
hydrothermal and reductive routes yielding a bouquet of oxide
products with potential for application in different areas.
molꢀ1] [2]. The lower thermal stability of the Cu2Al-LDH (150 1C) in
comparison with other M2+–Al LDHs such as Mg–Al LDH (2751C) is
therefore understandable.
LDHs are generally prepared by coprecipitation at such a pH
that the solubility products of both bivalent and trivalent
hydroxides are simultaneously exceeded. In general, LDHs may
be precipitated over a range of pH values from 7 to10. Johnson and
co-workers [19] have shown that for LDHs of the type
M2Al(OH)6(CO3)0.5 ꢁ H2O, the LDH phase is more stable than the
divalent hydroxides up to a pH of 10, 9 and 8 for M ¼ Zn, Co and
Ni, respectively, and up to a pH of 12 for M ¼ Mg. Further, above
the pH range 7–10, the LDH phase is more stable than the related
hydroxysalt phases due to the stabilizing influence of the
interlayer CO23ꢀ ion. With the Cu–Al system however, as we have
explained earlier, CO23ꢀ does not confer additional stability to the
LDH as it coordinates in C2v mode. Therefore hydrogen bonding
interactions which are primarily responsible for the high stability
of other M2+–Al–CO3 LDHs, do not play a significant role in the
stability of Cu-Al LDHs. Consequently, the Cu2Al-LDH precipitates
over a much narrower pH range (10–11). At lower pH ranges (7–9)
competing hydroxysalt phases (such as malachite) are more
stable. At high pH ranges (411), the amphotericity of Cu(OH)2
causes facile decomposition to CuO. The lower stability of
Cu–Al–CO3 LDHs in solution is also reflected in its lower
temperature of decomposition under hydrothermal conditions
(150 1C) when compared to other M2+–Al–CO3 LDHs (180 1C for
M ¼ Zn).
Acknowledgments
The authors thank the Department of Science and Technology
(DST), Government of India (GOI) for financial support. PVK is a
recipient of the Ramanna Fellowship of the DST. SB thanks the
University Grants Commission, GOI, for the award of a Senior
Research Fellowship (NET). Authors thank the Department of
Metallurgy, Indian Institute of Science for extending electron
microscopy facilities.
References
[1] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173.
[2] H.R. Oswald, R. Asper, Preparation and Crystal Growth of Materials with
Layered Structures, D. Reidel Publishing Company, Dordrecht, 1977.
[3] T. Yamaoka, M. Abe, M. Tsuji, Mater. Res. Bull. 24 (1989) 1183.
[4] Y. Lwin, M.A. Yarmo, Z. Yaakob, A.B. Mohamad, W.R. Daud, Mater. Res. Bull. 36
(2001) 193.
[5] R. Trujillano, M.J. Holgado, F. Pigazo, V. Rives, Physica B 373 (2006) 267.
[6] I.Y. Park, K. Kuroda, C. Kato, Solid State Ionics 42 (1990) 197.
[7] A. Alejandre, F. Medina, P. Salagre, X. Correig, J.E. Sueiras, Chem. Mater 11
(1999) 939.
[8] A.M. Fogg, A.J. Freij, G.M. Parkinson, Chem. Mater. 14 (2002) 232.
[9] A.V. Besserguenev, A.M. Fogg, R.J. Francis, S.J. Price, D. O’Hare, V.P. Isupov, B.P.
Tolochko, Chem. Mater. 9 (1997) 241.
[10] A.M. Fogg, G.R. Williams, R. Chester, D. O’Hare, J. Mater. Chem. 14 (2004)
2369.
[11] H.C.B. Hansen, C.B. Koch, R.M. Taylor, J. Solid State Chem. 113 (1994) 46.
[12] H. Boehm, J. Steinle, C. Viewager, Angew. Chem. Int. Ed. Eng. 16 (1977) 265.
[13] M. Rajamathi, P.V. Kamath, Bull. Mater. Sci. 23 (2000) 355.
[14] A. Alejandre, F. Medina, X. Rodriguez, P. Salagre, J.E. Sueiras, J. Catal. 311
(1999) 188.
[15] S. Velu, C.S. Swamy, Appl. Catal. A: General 145 (1996) 141.
[16] C. Busetto, G. Del Piero, G. Manara, F. Trifiro, J. Vaccari, J. Catal. 85 (1984) 260.
[17] S. Britto, A.V. Radha, N. Ravishankar, P.V. Kamath, Solid State Sci. 9 (2007) 279.
[18] R.K. Allada, A. Navrotsky, H.T. Berbeco, W.H. Casey, Science 296 (2002) 721.
[19] C.A. Johnson, F.P. Glasser, Clays Clay Miner. 51 (2003) 1.
[20] Vogel’s Textbook of Quantitative Inorganic Analysis, fourth ed., Longman
Group Ltd, 1978.
[21] V.A. Drits, A.S. Bookin, Layered Double Hydroxides: Present and Future, in: V.
Rives (Ed.), Novo Scientist, New York, 2001, pp. 39–92.
[22] A.V. Radha, C. Shivakumara, P.V. Kamath, Clays Clay Miner. 53 (2005) 521.
[23] G.S. Thomas, M. Rajamathi, P.V. Kamath, Clays Clay Miner. 52 (2004) 693.
[24] R. Allmann, Acta Cryst. B 24 (1968) 972.
It is well known that Cu2+ is stabilized by a Jahn-Teller
distorted octahedral coordination environment. If the Cu(OH)6
octahedra are only locally distorted, then an average Cu–O bond
distance is obtained and the symmetry of the structure as a whole
is unaffected. This is the case in the Cu–Cr LDHs where XRD shows
that the structure retains rhombohedral symmetry. EXAFS how-
˚
ever indicates two Cu–O bond distances (1.97 and 2.31 A)
characteristic of Jahn-Teller distortion [35].
In the Cu–Al LDHs reported here however, it appears that the
Jahn-Teller distortion leads to a lowering of the symmetry of the
entire structure to monoclinic. It appears that in the Cu–Al LDHs
the gain in electronic stability derived from Jahn-Teller distortion
around Cu2+ is offset by a decrease in lattice energy due to
lowered hydrogen bonding interactions of the carbonate anion
with the hydroxyl groups of the layer.
All the above factors put together serve to explain the
difficulties associated with synthesis of Cu–Al LDHs as well as
the lower thermal and aqueous stability of these LDHs in
comparison with other M2+–Al–CO3 LDHs.
In conclusion, Cu–Al LDHs have been prepared by coprecipita-
tion and through an oxide route. While coprecipitation yields
a Cu-rich brucite-based LDH, the oxide route yields an Al-rich
gibbsite-based LDH. The Cu–Al LDHs have a low thermodynamic
stability which facilitates their decomposition by thermal,
[25] H.F.W. Taylor, Min. Mag. 39 (1973) 377.
[26] A.S. Bookin, V.A. Drits, Clays Clay Miner. 41 (1993) 551.
[27] M. Bellotto, B. Rebours, O. Clause, J. Lynch, J. Phys. Chem. 100 (1996) 8527.
[28] S. Komarneni, N. Kozai, R. Roy, J. Mater. Chem. 8 (1998) 1329.
[29] T. Sato, K. Kato, T. Endo, M. Shimada, React. Solids 2 (1986) 253.
[30] F. Kooli, C. Depege, A. Ennaqadi, A. De Roy, J.P. Besse, Clays Clay Miner. 45
(1997) 92.
[31] Y. Cuddenec, A. Lecerf, Solid State Sci. 5 (2003) 1471.
[32] M. Rajamathi, S. Thimmaiah, P.E.D. Morgan, R. Seshadri, J. Mater. Chem. 11
(2001) 2489.
[33] L. Zou, F. Li, X. Xiang, D.G. Evans, X. Duan, Chem. Mater. 18 (2006) 5852.
[34] L. Zou, X. Xiang, J. Fan, F. Li, Chem. Mater. 19 (2007) 6518.
[35] H. Roussel, V. Briois, E. Elkaim, A. de Roy, J.P. Besse, J. Phys. Chem. B 104
(2000) 5915.