Thermal decomposition of crystalline NiII-CrIII layered double hydroxide: A structural study of the segregation process

Article abstract of DOI:10.1021/jp050639v

A structural study of the thermal evolution of Ni_0.69Cr _0.31(OH)₂(CO₃)_0.155?·nH ₂O into NiO and tetragonal NiCr₂O₄ is reported. The characteristic structural parameters of the two coexisting crystalline phases, as well as their relative abundance, were determined by Rietveld refinement of powder x-ray diffraction (PXRD) patterns. The results of the simulations allowed us to elucidate the mechanism of the demixing process of the oxides. It is demonstrated that nucleation of a metastable nickel chromite within the common oxygen framework of the parent Cr^III-doped bunsenite is the initial step of the cationic redistribution. The role that trivalent cations play in the segregation of crystalline spinels is also discussed. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp050639v

J. Phys. Chem. B 2005, 109, 10137-10141
10137
II
III
Thermal Decomposition of Crystalline Ni -Cr Layered Double Hydroxide: A Structural
Study of the Segregation Process
,†
,†
‡
§
Elsa E. Sileo,* Mat ´ı as Jobb a´ gy,* Carlos O. Paiva-Santos, and Alberto E. Regazzoni
INQUIMAE, Facultad de Ciencias Exactas y Naturales, UniVersidad de Buenos Aires, Pabell o´ n II,
Ciudad UniVersitaria, C1428EHA - Buenos Aires, Argentina, Instituto de Qu ´ı mica, UniVersidade Estadual
Paulista, UNESP, Araraquara, 14801-970, S a˜ o Paulo, Brazil, and Unidad de ActiVidad Qu ´ı mica,
Comisi o´ n Nacional de Energ ´ı a At o´ mica, Centro At o´ mico Constituyentes, AVenida General Paz 1499,
B1650KNA - San Mart ´ı n (Pcia. de Buenos Aires), Argentina
ReceiVed: February 4, 2005; In Final Form: March 29, 2005
2 3 2 2 4
A structural study of the thermal evolution of Ni0.69Cr0.31(OH) (CO )0.155‚nH O into NiO and tetragonal NiCr O
is reported. The characteristic structural parameters of the two coexisting crystalline phases, as well as their
relative abundance, were determined by Rietveld refinement of powder x-ray diffraction (PXRD) patterns.
The results of the simulations allowed us to elucidate the mechanism of the demixing process of the oxides.
It is demonstrated that nucleation of a metastable nickel chromite within the common oxygen framework of
III
the parent Cr -doped bunsenite is the initial step of the cationic redistribution. The role that trivalent cations
play in the segregation of crystalline spinels is also discussed.
1. Introduction
Layered double hydroxides (LDHs), of general formula,
II
-x
III
x
M1 M (OH)2Ax/m‚nH2O, are lamellar, brucite-like, mixed
II
hydroxides where M cations are partially substituted by the
III
M
ones; commonly, substitution degrees range between 0.33
1
m-
and 0.20, expressed as cation molar fraction. The anions A
occupy the interlamellar space and counterbalance the excess
of positive charge due to the substitution. If the interlamellar
anions are volatile species such as carbonate or nitrate, thermal
II
III
decomposition of LDHs easily leads to mixed M , M oxides
at significant lower temperatures and shorter times than
2
-6
traditional ceramic procedures.
In addition, the chemical
7
decomposition (CD) route, which involves moderate annealing,
allows one to preserve (or even increase) the surface area of
the starting precursor. This characteristic is highly appreciated
8
Figure 1. Indexed PXRD pattern of the starting LDH.
in the field of heterogeneous catalysis, where the crystalline
and chemical properties, as well as the textural ones, are
tion of metastable nonstoichiometric solid phases. The similari-
ties and differences compared to related systems are discussed.
9-17
crucial.
Many interesting materials have been obtained upon
thermal decomposition of LDHs, ranging from multicationic
1
8,19
20
oxides
to novel single-phase spinels.
2
. Experimental Section
2.1. Synthesis of LDH. The layered Ni -Cr hydroxide was
Despite the large number of reports concerning the thermal
decomposition of LDHs, many structural aspects related to the
crystallization of the mixed oxides during annealing still remain
obscure, mostly because the presence of amorphous phases blurs
traditional PXRD analysis.21 However, certain LDHs, such as
Ni1-xCrx(OH)2(CO3)x/2‚nH2O, decompose into crystalline mixed
oxides at relatively low temperatures,2 providing a suitable
system to explore.
II
III
synthesized following the procedure outlined earlier.2
3,24
This
involves aging a Cr(NO3)3-Ni(NO3)2-urea solution in a
commercial microwave digester at 493 K for 60 min; the
composition of the solution was 0.02 M Cr(NO3)3, 0.044 M
Ni(NO3)2, and 0.5 M urea. The solid was separated by
centrifugation, washed with water, and dried in vacuo at room
temperature. Its composition, assessed by chemical analysis, was
Ni0.69Cr0.31(OH)2(CO3)0.155‚nH2O. Its PXRD pattern (Figure 1)
indicates that the sample is a pure LDH phase.
2
Here, we report, using Rietveld refinement, the structural
features associated with the solid-state transformation during
II
III
the thermal decomposition of highly crystallized Ni -Cr
LDHs in air, to characterize the demixing through the segrega-
2.2. Thermal Treatments. Successive thermal treatments
were performed in a Shimadzu TGA-51 instrument, under air
-
1
II
III
*
Corresponding authors. E-mail: sileo@qi.fcen.uba.ar (E.E.S.),
flow (20 mL min ). In the first thermal cycle, the Ni -Cr
LDH was heated from room temperature to 723 K (Ti) at a rate
jobbag@qi.fcen.uba.ar (M.J.).
†
Universidad de Buenos Aires.
Universidade Estadual Paulista.
Comisi o´ n Nacional de Energ ´ı a At o´ mica.
-
1
‡
§
of 10 K min ; once Ti was attained, the temperature was kept
constant for 30 min, and then, the sample was allowed to cool
1
0.1021/jp050639v CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/26/2005

10138 J. Phys. Chem. B, Vol. 109, No. 20, 2005
Sileo et al.
Figure 2. FTIR spectra of the samples annealed in air at different
temperatures.
back to room temperature. In the subsequent cycles, the sample
was heated to Ti at 20 K min-1 and then at 10 K min from Ti
to the next desired temperature (Ti+1), which was kept constant
for another 30 min. After each cycle, the samples were analyzed
by PXRD and Fourier transform infrared (FTIR).
-1
PXRD patterns were recorded in a Siemens D5000 diffrac-
tometer with a Bragg-Brentano geometry, using Cu KR
radiation; generator settings were 40 kV and 30 mA. Divergence,
scattered, and receiving radiation slits were 1°, 1°, and 0.2 mm,
respectively. A curved graphite monochromator was used. Data
were collected in the 16.000° e 2θ e 140.000° range, with
variable scanning steps and counting times. The step width of
each run assured a minimum of about twelve intensity points
for the narrower peaks. Crystallite size and strain were calculated
taking into account the instrument broadening function that was
determined using NIST SRM 660 lanthanum hexaboride (LaB6)
standard.
Figure 3. (a) PXRD patterns of the samples annealed in air at
increasing temperatures; the asterisks denote the peaks corresponding
to the spinel phase. (b) Zoom showing the cubic to tetragonal transition
of the spinel phase.
Transmission FTIR spectra of the samples were collected in
a Nicolet 510 spectrometer.
VI
35
teristic of Cr -O vibrations. These bands fade with increasing
annealing temperature, becoming negligible at 923 K. The trend
2
.3. Data Analysis. PXRD data were analyzed by the
VI
III
reveals the reduction of the previously formed Cr back to Cr .
In line, the TG trace (cf. Supporting Information) shows a slight
weight loss between 800 and 900 K, which corresponds to the
elimination of O2 according to the general stoichiometry 2CrO3
25
26
Rietveld refinement method using the GSAS routine. The
27
Thompson-Cox-Hasting pseudo-Voigt function, with the
microstrain broadening description of Stephens,28 was used for
fitting the peak profiles. A polynomial function was employed
for fitting the background signal. Cell parameters, sample
displacement, full width at half-maximum, microstrain broaden-
ing, scale factor, and positional parameters for all atoms were
refined. The isotropic thermal parameters were fixed at the
values measured for an NiO single crystal.29 The preferred
orientation in the sample heated at 1773 K was corrected using
3
f Cr2O3 + /2O2.
PXRD patterns of the samples annealed at different temper-
atures are presented in Figure 3A. All of them show the presence
III
of bunsenite. The formation of M -doped NiO at this relatively
low temperature was also reported for the thermal decomposition
of Ni0.70Co0.30(OH)2(CO3)0.15‚0.67H2O. At 923 K, nickel
chromite is already detected. In agreement with a previously
reported work, no peaks corresponding to NiCrO4 are observed
in any of the samples annealed below 923 K, thus indicating
36
3
0
the March model. Only Lorentzian and Gaussian coefficients
of peak profiles were refined to obtain the respective crystallite
size and microstrain parameters.
35
VI
that Cr is well-dispersed within the oxidic matrix; in the case
II
III
of the Mg -Cr LDH, on the other hand, segregation of
3
. Results and Discussion
3
5
MgCrO4 was observed. Figure 3A also indicates changes in
the cell parameter and crystallite size of both bunsenite and
spinel phases, which are denoted by the variations in the position
and width of the diffraction peaks. At 1773 K (Figure 3B), the
splitting of the peaks corresponding to nickel chromite indicates
a clear change in the crystallographic structure of the spinel
phase.
Thermogravimetric analyses (Supporting Information) show
that the thermal decomposition of Ni0.69Cr0.31(OH)2(CO3)0.155‚
nH2O occurs in three main steps, as expected for a carbonate-
containing LDH.3 In the first two regions (298-450 K, and
1,32
4
50-580 K), the mass loss is associated with the release of
superficial and interlamellar water, respectively; from the second
step, n ) 0.64 mol/mol is derived. The third stage (580-780
K) corresponds to the irreversible decomposition of the LDH,
a process in which the evolution of CO2 overlaps the dehy-
The PXRD patterns of the solids annealed within 723-1123
K can, in principle, be described using two different structural
models. In the first one, proposed by Gazzano et al.,37 the
changes in the patterns are ascribed to a single-phased NiO super
cell (Fd3m, where Cr ions are randomly distributed only in
octahedral positions) that modifies its site occupancies with
temperature; the nucleation and growth of the spinel phase are
3
3
34,35
droxylation of the structure. According to previous reports,
III
VI
III
Cr ions are oxidized to Cr within the temperature range.
Figure 2 shows the IR spectrum of the sample annealed at 723
K, which shows bands at 780 and 950 cm that are charac-
-
1

Thermal Decomposition of Crystalline Ni -CrIII
II
J. Phys. Chem. B, Vol. 109, No. 20, 2005 10139
TABLE 1: Agreement Factors and Lattice Cell Parameters for the Refined Samplesa
T/K
a Bn/Å
strain Bn
a Sp/Å
c Sp/Å
GofF
R
wp
R
p
R
B
wt % Bn
7
8
9
023
123
773
23
23
23
4.1539(6)
4.1541(2)
4.1657(3)
4.1692(2)
4.1748(2)
4.1758(1)
1.25
1.24
1.11
1.04
0.85
0.32
1.28
1.51
1.34
1.38
1.38
1.36
3.47
5.04
6.71
7.52
8.16
11.02
2.56
3.93
5.19
5.77
5.84
8.52
2.30
3.00
3.40
3.27
3.75
4.77
100
100
65(1)
60(1)
56(1)
53(1)
8.332(1)
1
1
1
8.3234(7)
8.3174(9)
5.8520(1)
8.4014(3)
a
)2/Σ w 2]0.5. R
Strain for the NiO-like phase is also shown. Bn ) bunsenite. Sp ) spinel. R
p
) 100 Σ|I
o
- I
c
|/Σ I
o
. Rwp ) 100[Σ w
i
i
(I
o
- I
c
i o B
I )
2
1
I
00 Σ|Iko - Ikc|/Σ Iko. GofF ) Σ w
i
(I
o
- I
c
) /(N - P). I
o
and I
c
) observed and calculated intensities. w ) weight assigned to each step intensity.
ko and Ikc ) observed and calculated intensities for Bragg k-reflection. N and P ) number of data points in the pattern and number of parameters
refined. Cell parameters for pure NiO, cubic NiCr , and tetragonal NiCr are 4.1758(1), 8.3155(7), and 5.8369(4), 8.4301(6), respectively.
2
O
4
2 4
O
bunsenite phase increases with temperature, whereas that of the
spinel decreases, both tending toward those reported for the pure
phases.
Bunsenite is the only crystalline phase that is produced by
thermal decomposition of Ni0.69Cr0.31(OH)2(CO3)0.155‚0.64H2O
at 723 and 823 K. Within this temperature range, other LDHs
evolve into doped bunsenites and an amorphous phase that is
3
8-40
richer in the trivalent metal ion.
In the present case, the
possible presence of an amorphous solid precludes establishing
the actual composition of the bunsenite phase. However, the
large contraction of the a parameter, the high strain (Table 1),
VI
and the evident presence of Cr (Figure 2) and absence of
NiCrO4 indicate that the formed bunsenite is highly defective,
III
with a rather high degree of chromium doping, with Cr
VI
occupying octahedral sites and Cr located probably at the
III
particles surface (cf. ref. 41). In iron-doped bunsenites, Fe
IV
42,43
and Fe occupy tetrahedral positions.
According to Del Arco et al.,38 upon annealing Ni0.7Fe0.3-
Figure 4. Observed (b) and calculated (solid line) XRD patterns of
the samples annealed at 1123 K (lower) and 1773 K (upper); the
difference plots are also shown.
(
OH)2(CO3)0.15‚nH2O at 723 K, most of the iron segregates from
III
NiO and forms an amorphous solid in which Fe adopts the
tetrahedral coordination. Likewise, heating of Ni0.72Al0.28(OH)2-
II
viewed as the passing of Ni from octahedral to tetrahedral sites.
III
(
CO3)0.14‚nH2O at 923 K produces a mixture of Al -doped
Despite the close relationship that exists between the structures
of NiO and cubic NiCr2O4, this model is unable to fit the present
data; no unique peak profile function that could simulate all
the diffraction peaks could be found, even though they were
corrected for anisotropic line-broadening. The peaks corre-
sponding to the spinel phase are always broader than those of
bunsenite, indicating the existence of two different crystal-
lographic phases and the formation of NiCr2O4 via a nucleation
and growth mechanism.
II
39
bunsenite and amorphous Ni -doped alumina. These results
seem to contradict our former conclusion. However, the
dissimilar thermal behavior of these LDHs is in line with the
energetic requirements associated with the transfer of a trivalent
metal ion from an octahedral site to a tetrahedral one; the
electronic configuration of the 3d Cr ions, which stabilizes
the octahedral coordination, renders a much higher barrier for
the diffusion jumps, thus hindering the expulsion of Cr from
3
III
44
III
the NiO lattice.
At 923 K, where CrVI is quantitatively reduced (Figure 2),
homogeneous nucleation of cubic nickel chromite takes place
The second structural model considers that the PXRD patterns
of the solids heated at 723 and 823 K correspond to NiO with
Cr ions distributed in octahedral sites, whereas those of the
4
5
III
(Figure 3A). As suggested by Chen et al., the very close
matching of NiO and NiCr2O4 lattices must ease this process.
Indeed, the data in Table 1 show that the value of the cell
parameter of the spinel phase doubles that of the parent
bunsenite, indicating that nucleation of the cubic nickel chromite
involves the redistribution of cations within a common frame-
work of oxo ions. The driving force for nucleation and growth
samples heated at T g 923 K correspond to a biphasic
crystallographic system composed of bunsenite and a spinel
phase. This model describes the experimental patterns excel-
lently. Figure 4 presents the observed, calculated, and difference
PXRD profiles for the samples annealed at 923 and 1123 K.
Table 1 collects the indicators of the goodness of the fits, as
well as structural information for the various samples; the quality
of the refinements is similar for all samples.
III
is the high Cr supersaturation of the bunsenite phase; the
III
reported Cr solubility data span from 1% to 10% in the range
4
6-48
1
100-1800 K;
these values are much lower than the
The PXRD pattern of the sample heated at 1773 K, in which
NiCr2O4 only presents its tetragonal variety, was simulated using
the Fm3m and I41/amd spatial groups (Figure 4). The change
from cubic to tetragonal, taking place within the range 1123-
chromium content of our sample. These two factors should
account for the rather low temperature required to produce the
spinel phase; for example, the analogous Ni0.72Al0.28(OH)2-
49
(CO3)0.14‚nH2O yields NiAl2O4 above 1173 K.
1
773 K, is clearly seen in the blowup presented in Figure 4.
As the temperature is increased, the cell parameters of the
The evolution of the refined cell parameters a (for bunsenite)
bunsenite and the spinel phases shift in opposite ways (Table
1). The contraction of the cell of nickel chromite is compatible
and a/2 (for the spinel) in the range 723-1123 K is presented
in Table 1; reported values for pure NiO and cubic NiCr2O4
are also included for comparison. The cell parameter of the
III
with a relative enrichment in the smallest Cr ions. On the other
hand, the expansion of bunsenite and the strain reduction reflect

10140 J. Phys. Chem. B, Vol. 109, No. 20, 2005
Sileo et al.
place, the cation mole fraction of both phases is almost equal
to that of the starting solid. At 1123 K, the cell parameters reveal
almost stoichiometric NiCr2O4 and NiO phases. Their weight
fractions correspond to those expected from the composition
of the starting LDH (see Table 1), indicating that demixing is
almost completed.
Finally, upon annealing at 1773 K, the spinel phase adopts
the tetragonal structure, which is the stable form at room
temperature. Clearly, the metastable cubic NiCr2O4 phase is
stabilized by surface energy, owing to the nanometric size of
Figure 5. Crystal sizes of the bunsenite (b) and spinel phases (O) as
a function of the annealing temperature.
5
4
the crystallites. The transition from cubic to tetragonal takes
place readily, once the size of the parent crystallites reaches a
5
5
certain critical value.
4
. Conclusions
This work illustrates that Rietveld refinement of XRD data
of mixtures of double oxides gives sound structural information
that enables the characterization of the whole demixing. The
method rules out the possible occurrence of a single-phased NiO
super cell at the very early stages of the process and indicates
that the bunsenite and spinel phases coexist after nucleation of
the latter. Formation of nonstoichiometric cubic spinel takes
2
-
place at 923 K; at this point, the O frameworks of both
crystalline phases are coincident. Then, cation migration leads
to segregation. Once redistribution of cations is completed, the
cell parameters of the final crystalline phases are very close to
those of the stoichiometric phases. At 1773 K, cubic NiCr2O4
transforms into tetragonal NiCr2O4.
Figure 6. Cell parameters of a family of Ni1-xCr
a function of the Ni content in the parent LDHs: (O) this work; (b)
x
O
1+x/2 bunsenites as
II
data taken from ref 24. Cell parameters for NiO and cubic NiCr
are also included.
2
O
4
Comparison with related systems indicates that the demixing
III
of the parent M -doped bunsenite phases is governed by the
III
electronic configuration of M , which determines the path by
which nucleation of the spinel phase takes place. In the case of
III
mobile M ions, nucleation occurs at bunsenite/amorphous solid
III
interfaces. In the case of the sluggish Cr , on the other hand,
III
homogeneous nucleation takes place in the bulk of Cr -
supersaturated bunsenites. The latter process is further favored
by the structural matching of NiO and NiCr2O4.
Supporting Information Available: Thermogravimetric
analyses. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. Support from ANPCyT (PICT 06-06631)
is gratefully acknowledged. A.E.R. and M.J. are members of
CONICET. M.J. is also a Gabbo’s fellow.
Figure 7. Changes in the NiII content in the bunsenite (b) and the
spinel (O) phases in the 923-1123 K temperature range.
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(
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III
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II
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