Effect of hydrothermal treatment on properties of Ni-Al layered double hydroxides and related mixed oxides

Article abstract of DOI:10.1016/j.jssc.2008.09.014

The Ni-Al layered double hydroxides (LDHs) with Ni/Al molar ratio of 2, 3, and 4 were prepared by coprecipitation and treated under hydrothermal conditions at 180 °C for times up to 20 h. Thermal decomposition of the prepared samples was studied using the

Full text of DOI:10.1016/j.jssc.2008.09.014

ARTICLE IN PRESS
Journal of Solid State Chemistry 182 (2009) 27–36
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Effect of hydrothermal treatment on properties of Ni–Al layered double
hydroxides and related mixed oxides
a,
Ã
a
b
c
d
ˇ
´ ˇ
ˇ
ˇ
ˇ
´
´
´
Frantisek Kovanda , Tomas Rojka , Petr Bezdicka , Kveta Jiratova , Lucie Obalova ,
d
e
b
ˇ
´
´ ˇ
Katerina Pacultova , Zdenek Bastl , Tomas Grygar
a
´
Department of Solid State Chemistry, Institute of Chemical Technology, Prague, Technicka 5, 166 28 Prague, Czech Republic
b
c
ˇ
ˇ
Institute of Inorganic Chemistry of the ASCR, v. v. i., 250 68 Rez, Czech Republic
´
Institute of Chemical Process Fundamentals of the ASCR, v. v. i., Rozvojova 135, 165 02 Prague, Czech Republic
d Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava, Czech Republic
e
ˇ
J. Heyrovsky´ Institute of Physical Chemistry of the ASCR, v. v. i., Dolejskova 3, 182 23 Prague, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
The Ni–Al layered double hydroxides (LDHs) with Ni/Al molar ratio of 2, 3, and 4 were prepared by
coprecipitation and treated under hydrothermal conditions at 180 1C for times up to 20 h. Thermal
decomposition of the prepared samples was studied using thermal analysis and high-temperature X-ray
diffraction. Hydrothermal treatment increased significantly the crystallite size of coprecipitated
samples. The characteristic LDH diffraction lines disappeared completely at ca. 350 1C and a gradual
crystallization of NiO-like mixed oxide was observed at higher temperatures. Hydrothermal treatment
improved thermal stability of the Ni2Al and Ni3Al LDHs but only a slight effect of hydrothermal
treatment was observed with the Ni4Al sample. The Rietveld refinement of powder XRD patterns of
calcination products obtained at 450 1C showed a formation of Al-containing NiO-like oxide and a
presence of a considerable amount of Al-rich amorphous component. Hydrothermal aging of the LDHs
resulted in decreasing content of the amorphous component and enhanced substitution of Al cations
into NiO-like structure. The hydrothermally treated samples also exhibited a worse reducibility of Ni2+
components. The NiAl2O4 spinel and NiO still containing a marked part of Al in the cationic sublattice
were detected in the samples calcined at 900 1C. The Ni2Al LDHs hydrothermally treated for various
times and related mixed oxides obtained at 450 1C showed an increase in pore size with increasing time
of hydrothermal aging. The hydrothermal treatment of LDH precursor considerably improved the
catalytic activity of Ni2Al mixed oxides in N2O decomposition, which can be explained by suppressing
internal diffusion effect in catalysts grains.
Received 20 June 2008
Received in revised form
12 September 2008
Accepted 20 September 2008
Available online 8 October 2008
Keywords:
Layered double hydroxides
Hydrotalcite-like compounds
Thermal decomposition
Nickel–aluminum mixed oxides
Porous structure
N2O decomposition
& 2008 Elsevier Inc. All rights reserved.
1. Introduction
thermal decomposition at moderate temperatures, LDHs give
finely dispersed mixed oxides of MII and MIII metals with large
surface area and good thermal stability. Calcined Ni–Al LDHs are
used as catalysts of various reactions, for example in the steam
reforming of methanol [4] or selective reduction of aldehydes to
alcohols [5], and exhibit also a high activity in N2O decomposition
[6]. Thermal decomposition of LDHs was studied by many authors,
a great number of studies were performed namely with synthetic
Mg–Al hydrotalcite [7–9]. Two characteristic processes accom-
panied by a considerable weight loss and an endothermic effect
can be observed during hydrotalcite heating: The release of
interlayer water at 150–200 1C accompanied by a collapse of the
LDH basal spacing [9,10] and the complete decomposition of the
layered structure at 350–600 1C. The resulting structurally
disordered oxide mixture crystallizes to form MgO-like phase
above 400 1C; MgO and MgAl2O4 spinel are detected above 900 1C.
The Ni–Al LDHs exhibit very similar thermal behavior with
Layered double hydroxides (LDHs), which are also known as
hydrotalcite-like compounds or anionic clays, represent a class of
layered materials with chemical composition expressed by the
III
general formula [MII
M
x(OH)2]x+[Anꢀx/n ꢁyH2O]xꢀ where MII
1ꢀx
and MIII are divalent and trivalent metal cations, Anꢀ is an
n-valent anion and x has usually values between 0.20 and 0.33.
The MII/MIII isomorphous substitution in octahedral sites of
the hydroxide sheets results in a net positive charge, which is
neutralized by the interlayers composed of anions and water
molecules. LDHs are often used in heterogeneous catalysis as
precursors for preparation of mixed oxide catalysts [1–3]. After
Ã
Corresponding author. Fax: +420 2 24311082.
E-mail address: Frantisek.Kovanda@vscht.cz (F. Kovanda).
0022-4596/$ - see front matter & 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2008.09.014

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F. Kovanda et al. / Journal of Solid State Chemistry 182 (2009) 27–36
thermal decomposition between 300 and 400 1C followed by
crystallization of NiO (bunsenite). The NiAl2O4 spinel together
with NiO is found by XRD in samples calcined at temperatures
900 1C and higher [11]. The formation of oxide phases during
heating of Ni–Al LDHs was examined also using FTIR spectroscopy
[12,13].
10 wt% of solid, which was hydrothermally treated at 180 1C for
20 h. After filtration, the product was washed thoroughly with
distilled water and dried overnight at 60 1C. The remaining portion
of the filtration cake was washed and dried without hydrothermal
aging. The dried samples were heated for 4 h at 450 or 900 1C in
air to obtain mixed oxides.
LDHs are usually prepared by coprecipitation, when a solution
containing MII and MIII metal cations in adequate proportions
reacts with an alkaline solution. The product crystallinity can be
affected by various reaction parameters (pH, temperature, con-
centration and flow rate of added solutions, and hydrodynamic
conditions in the reactor) and/or post-synthesis operations (e.g.,
aging of the precipitate). A substantial improvement of the
product crystallinity can be achieved by hydrothermal treatment
at temperatures, which do not exceed the decomposition
temperature of the LDHs. The hydrothermal crystallization is
usually carried out at temperatures up to ca. 200 1C under
autogenous pressure for a time ranging from hours to days. Some
aspects of the hydrothermal treatment of hydrotalcite-like
compounds at high temperatures have been discussed in the
comprehensive article by Cavani et al. [1]. During hydrothermal
treatment of Mg–Al hydrotalcite, the maximum crystallite size
was achieved after long-time crystallization at 150–200 1C
[14–18]. The hydrothermal treatment was also applied in order
to improve the crystallinity of some LDHs containing nickel: Ni–Al
[11], Ni–Al and Ni–Cr [19], Ni–Al–Cr and Ni–Al–Fe [20], and Ni–Fe
[21]. Hydrothermal treatment of Ni–Al LDHs using microwave
heating was also reported [22,23]. Well-crystallized products with
a narrow distribution of particle size can be obtained during
homogeneous precipitation in the presence of urea [24]. Recently,
the microwave-assisted preparation of Ni–Al LDHs by urea
hydrolysis [25] and reconstruction of calcined products in
aqueous media [26] were reported. An alternative route for
obtaining Ni–Al LDHs in the form of thin films is electrosynthesis
using cathodic reduction of Ni and Al nitrates [27].
The layered crystal structure of hydrotalcite-like compounds
collapses during thermal decomposition and the formed oxides
are poorly crystalline with some amorphous portion gradually
disappearing on further heating [28]. A relation between the
precursor crystallinity and properties of the obtained mixed
oxides can be expected. In the present work, the effect of
hydrothermal treatment on physical chemical properties of both
LDH precursor and related mixed oxides is studied. Thermal
stability of the Ni–Al LDHs, changes in phase composition of the
samples during heating, and structural and textural properties
of the LDHs and related mixed oxides are examined. An effect
of the precursor hydrothermal treatment on catalytic properties of
the related mixed oxides is also explored, using catalytic N2O
decomposition as a testing reaction.
The prepared samples were denoted by acronyms with the
molar ratios of the constituents and time of the hydrothermal
treatment. For example, Ni2Al and Ni2Al-20 h denote the samples
with Ni/Al molar ratio of 2 without hydrothermal aging and
hydrothermally treated for 20 h, respectively. The calcined
samples were denoted by analogous acronyms with the tempera-
ture value; for example, Ni2Al-20 h/450 denotes the Ni2Al-20 h
sample calcined at 450 1C.
The system with Ni/Al molar ratio of 2 was studied in more
details. The coprecipitated precursor was hydrothermally treated
at 180 1C for 4, 8 and 20 h. The related mixed oxides prepared by
LDH heating at 450 1C were crushed and sieved to obtain the
fraction with particle size of 0.160–0.315 mm, which was used for
surface area, porosity, TPR and catalytic measurements.
The reference NiO sample was prepared by calcination of
b-Ni(OH)2 at 450 1C for 4 h in air. The b-Ni(OH)2 was synthesized
by precipitation of an aqueous solution of nickel sulphate
according to Yang [29].
2.2. Experimental techniques
Chemical composition of the prepared LDHs was determined
by atomic absorption spectroscopy (AAS) using Perkin-Elmer 3100
instrument after sample dissolution in hydrochloric acid. Carbo-
nate content was determined by analysis of CO2 evolved during
thermal decomposition of the sample using Eltra CS 500
instrument.
Thermal analyses, including thermogravimetry (TG), differen-
tial thermal analysis (DTA) and evolved gas analysis (EGA), were
carried out using a Netzsch STA 409 instrument equipped by the
quadrupole mass spectrometer QMS 403/4 (Balzers). The heating
rate of 10 1C minꢀ1, air flow rate of 75 ml minꢀ1 and 50 mg of
sample were used. Gaseous products were continually monitored
for chosen mass numbers m/z (18-H2O+ and 44-CO2+).
Powder X-ray diffraction patterns were recorded using a
PANanalytical X’Pert PRO instrument with Co
Ka radiation
(l
¼ 0.179 nm) in 2 range 7–1001, step size 0.021. The primary
y
beam intensity was regularly checked between the measurements
to make sure that the integral intensities of the diffraction lines
are not affected by its variations; the variations were less than
71%. The XRD measurements using silicon (SRM640c, NIST) as
an internal standard were performed to evaluate the content
of crystalline phases in mixed oxides obtained by calcination of
Ni2Al LDH precursors at 450 and 900 1C. For refinement of lattice
parameters and estimation of the mean coherence length
(approximately equal to crystallite size), DiffracPlus Topas, release
2000 (Bruker AXS, Germany) was used. The structural models
were taken from the Inorganic Crystal Structure Database ICSD,
Retrieve 2.01 (FIZ Karlsruhe, Germany).
2. Experimental
2.1. Preparation of samples
The Ni–Al LDHs were prepared by coprecipitation. An aqueous
solution (450 ml) of Ni and Al nitrates with Ni/Al molar ratio equal
to 2, 3 or 4 and total metal ion concentration of 1.0 mol lꢀ1 was
added with flow rate of 80 ml minꢀ1 into batch reactor containing
200 ml of distilled water. The flow rate of simultaneously added
alkaline solution of 0.5 mol lꢀ1 Na2CO3 and 3 mol lꢀ1 NaOH was
controlled to maintain the reaction pH of 10. The coprecipitation
was carried out under vigorous stirring at 25 1C. The product was
filtered off and a part of filtration cake was placed into 100 ml
Teflon lined stainless steel bomb and resuspended in filtrate
(mother liquor) to obtain 75 ml of suspension containing about
High-temperature powder XRD (HT XRD) measurements were
performed in Anton Paar HTK16 high-temperature chamber
with PANalytical X’Pert PRO diffractometer (Co K
a radiation,
X’Celerator multichannel detector) as described earlier [28]. The
acquisition of XRD patterns at each temperature within several
minutes was sufficiently fast to approach the dynamic conditions
of TG/DTA/EGA measurements. To study the precursor decom-
position and oxide phases crystallization, the scan in 2
4–801 (Co K radiation) with an effective heating rate of 0.85 1C
minꢀ1 between 25 and 400 1C (the step of 25 1C) was used. Then
y range
a

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29
the heating rate was raised to 3.3 1C minꢀ1 between 400 and
900 1C (the step of 100 1C). HT XRD measurements were used to
identify LDH decomposition temperatures.
TEM images were obtained using Transmission Electron
Microscope Philips EM 201 at accelerating voltage of 80 kV and
magnification of 130,000.
Surface area and mesoporous structure of the samples was
determined by adsorption/desorption of nitrogen at 77 K using
Micromeritics ASAP 2010 instrument. The total porous structure
was characterized by Micromeritics AutoPore 9600 instrument
working in the range 0.1–400 MPa.
Table 1
Chemical composition of the coprecipitated and hydrothermally treated LDHs
Sample
(wt%)
Ni
Ni/Al molar ratio
Al
Na
CO23ꢀ
Ni2Al
35.4
35.6
35.4
35.8
8.40
8.60
8.40
8.40
0.019
0.029
0.016
0.021
10.65
9.70
9.52
9.63
1.94
1.90
1.94
1.96
Ni2Al-4 h
Ni2Al-8 h
Ni2Al-20 h
Ni3Al
38.0
40.2
5.80
6.40
0.004
0.019
9.27
8.15
3.01
2.89
Ni3Al-20 h
The temperature-programmed reduction (TPR) measurements
were carried out with 50 mg sample weight in a hydrogen–nitrogen
mixture (10 mol% H2) using flow rate of 50 ml minꢀ1
. The
Ni4Al
40.6
41.6
4.56
4.66
0.002
0.012
8.85
7.98
4.09
4.10
Ni4Al-20 h
temperature was linearly raised at the rate of 20 1C minꢀ1 up to
800 1C. After freezing of rising water at –76 1C, a composition
of gases was analyzed by thermal-conductivity detector.
Table 2
The XPS spectra were measured by an ESCA 310 (Gammadata
Scienta, Sweden) electron spectrometer with a base pressure
Results of XRD analysis of the coprecipitated and hydrothermally treated LDHs
1 ꢂ10ꢀ8 Pa. Samples were excited with Al K
a radiation. The total
Sample
(003) integral (003) FWHM Crystallite
Lattice parameter
intensity
sizea
(A)
˚
energy resolution of the instrument was 0.65 eV as measured by
FWHM of the Au 4f7/2 photoelectron peak. The powdered samples
were deposited on the surface of gold plate and spectra were
measured at room temperature and a pressure of 6 ꢂ10ꢀ8 Pa. The
following photoelectrons were recorded: Ni 2p, Ni 3p, Al 2p, Al 2s
and O 1s. From all spectra a Shirley base was subtracted with a
help of Gaussian–Lorentzian curve. In order to obtain information
about the structure of the surface and the dispersion of the active
phases, the surface atomic ratios were calculated as the ratio of
the corresponding peak intensities, corrected with theoretical
sensitivity factors based on Scofield’s photoionization cross-
sections [30].
(a.u.)
(nm)
a
c
(12y)
Ni2Al
17,428
23,666
24,167
24,859
2.29
0.66
0.55
0.37
4.1
14.0
16.9
25.2
3.022
3.026
3.027
3.027
22.73
22.70
22.70
22.71
Ni2Al-4 h
Ni2Al-8 h
Ni2Al-20 h
Ni3Al
16,253
66,260
2.19
0.36
4.2
3.042
3.042
23.44
23.09
Ni3Al-20 h
25.8
Ni4Al
17,473
19,232
2.02
1.23
4.6
7.5
3.050
3.055
23.57
23.54
Ni4Al-20 h
a
Crystallite size calculated as XRD coherent domain in [001] direction using
The N2O decomposition reaction was performed in a fixed-bed
reactor of 5 mm internal diameter in temperature range from 330
to 450 1C with total flow rate of 100 ml minꢀ1 NTP (273 K,
101.325 kPa), 0.1 g catalyst and 1000 ppm N2O balanced by He.
Before each run, the catalysts were pre-treated by heating in the
He flow (50 ml minꢀ1, the heating rate 10 1C minꢀ1) up to 450 1C
and maintaining the temperature for 1 h. Then the catalysts were
cooled to reaction temperature and the steady state of N2O at the
reactor output was measured. The gas chromatograph Agilent
Technologies 6890N equipped by TCD detector was used to
determine N2O concentration in the reactor inlet and outlet.
Scherrer’s formula.
intensity of the hydrotalcite diffraction lines (Fig. 1). A trace
amount of AlOOH (boehmite) was detected in the powder XRD
pattern of the Ni2Al-20 h sample. From XRD data processing, the
(003) diffraction line integral intensity, FWHM value, crystallite
size in [001] direction and lattice parameters of the prepared
LDHs were evaluated (Table 2).
Powder XRD patterns of the Ni2Al samples showed that the
FWHM value of (003) diffraction line decreased with increasing
time of hydrothermal treatment, which indicated an increase of
the LDH crystallite size. A marked increase of the LDH crystallite
size (calculated as a mean coherence length using Scherrer’s
formula) was observed already after a relatively short time of
hydrothermal treatment (4 h) and the prolonged aging time
resulted in a further ordering of the LDH crystal structure. The
increasing time of hydrothermal treatment resulted also in an
increase of the integral intensity of LDH (003) diffraction line,
which can indicate rising content of the crystalline LDH phase in
the samples. In dependence on aging time, a marked increase in
the (003) line integral intensity was observed especially between
the hydrothermally treated Ni2Al samples and the non-treated
one. The maximum difference in the (003) line integral intensity
was observed between the Ni3Al and Ni3Al-20 h samples but the
corresponding FWHM values were comparable to those evaluated
for Ni2Al and Ni2Al-20 h samples, respectively. On the other hand,
only a minimum effect of the hydrothermal aging was observed
for the Ni4Al sample with high Ni/Al molar ratio. The lattice
parameters were not changed during LDH hydrothermal treat-
ment. Only an insignificant increase in lattice parameter a, and
decrease in lattice parameter c between hydrothermally aged
samples and non-treated ones was found (Table 2). Benito et al.
[23] reported on similar changes in lattice parameter c and
3. Results and discussion
3.1. Characterization of the Ni–Al LDH precursors
The results of chemical analysis of the coprecipitated and
hydrothermally treated Ni–Al LDHs are summarized in Table 1
and the structural characteristics of the LDHs evaluated from the
XRD data are presented in Table 2. The Ni/Al molar ratio in
products dried at 60 1C corresponded approximately to those
in the nitrate solution used for coprecipitation. The hydrothermal
treatment did not influence the content of Ni and Al cations in
the LDH. Slightly increased content of carbonate was found in the
coprecipitated samples without hydrothermal aging. A trace
amount of residual sodium cations (o0.03 wt%) remained in the
washed and dried samples. In the powder XRD patterns of the
dried samples without hydrothermal treatment, only a hydro-
talcite-like phase exhibiting relatively low crystallinity was
detected. The crystalline hydrotalcite-like phase was formed in
spite of the application of relatively rapid flow rate of nitrate
solution (80 ml minꢀ1 instead of the conventional dropwise
addition). The hydrothermal treatment resulted in an increasing

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Fig. 1. Powder XRD patterns of the coprecipitated and hydrothermally treated
Ni–Al LDHs.
Fig. 2. HT XRD patterns of the non-treated Ni2Al and hydrothermally treated
Ni2Al-20 h samples. O—Ni–Al mixed oxide, *—Pt support.
assigned them to strengthening interaction between interlayer
anions and hydroxide layers.
300 1C. Further heating of the samples resulted in decreasing
intensity of the LDH diffraction lines, which disappeared com-
pletely at 350 1C. The HT XRD patterns of the Ni2Al and Ni2Al-20 h
samples are shown in Fig. 2.
3.2. Thermal decomposition
Thermal decomposition of the prepared Ni–Al LDHs was
studied using conventional thermal analysis and HT XRD as in
[28]. During HT XRD measurements, powder XRD patterns with
the temperature step of 25 1C were obtained between 25 and
400 1C. In general, two essential stages can be observed during
phase transformation of LDHs upon heating: (i) a shift of LDH
basal spacing associated with release of interlayer water and (ii)
disappearing of the LDH diffraction lines and formation of oxide
phases. Stanimirova et al. [10] discussed the mechanism of LDH
collapse; the collapsed phase retaining the layered structure of
the pristine LDH was denoted as metahydrotalcite B. An analogous
phase observed during heating of cobalt-containing LDHs we
denoted as collapsed LDH; it had basal spacing decreased typically
The results of thermal analysis of the Ni2Al samples hydro-
thermally treated for various times are demonstrated in Fig. 3.
Two major endothermic effects, characteristic for hydrotalcite-like
compounds, were observed in DTA curves. The first one was
associated with H2O evolution and corresponded to a release
of interlayer water. It was observed at 240 1C for the Ni2Al
sample and at 275–280 1C for the hydrothermally treated ones.
The second endothermic effect was accompanied by simultaneous
H2O and CO2 evolution associated with dehydroxylation of
hydroxide layers and decomposition of interlayer carbonate.
Both thermal analysis and HT XRD measurements showed that
hydrothermal aging enhanced thermal stability of the coprecipi-
tated LDHs (Table 3).
˚
from about 7.6 to 6.6 A [28]. Similar shift of the basal spacing was
found also for the examined Ni-Al LDHs.
The Ni3Al and Ni4Al samples exhibited an analogous thermal
behavior like Ni2Al one before and after hydrothermal aging.
˚
The basal spacing d003 of about 7.55 A was evaluated for the
˚
dried Ni2Al sample. During HT XRD measurement, the basal
The basal spacing of about 7.8 A evaluated for the Ni3Al sample
˚
spacing was not changed up to 150 1C and then it was slightly
gradually decreased between 100 and 175 1C to achieve ca. 6.8 A.
˚
˚
shifted to about 7.35 A. Between 250–300 1C, a marked shift to
The collapsed LDH with basal spacing of about 6.7 A was detected
˚
about 6.85 A was observed and the LDH basal diffraction lines
at 200–300 1C and this phase disappeared at 325 1C. The
disappeared at 350 1C. Hydrothermally treated samples (Ni2Al-4 h,
Ni2Al-8 h and Ni2Al-20 h) exhibited very similar changes in
basal spacing in dependence on heating temperature: At lower
hydrothermally treated Ni3Al sample exhibited basal spacing of
˚
˚
7.7 A. A marked shift of the basal spacing to 6.6 A was observed
between 175 and 200 1C. The collapsed phase was stable at
200–300 1C and disappeared completely at 350 1C (a residual
(003) diffraction peak of the collapsed phase was detected up to
325 1C). A dehydration of the interlayer space during heating of
the Ni–Al LDH with Ni/Al molar ratio of 3 was recently discussed
˚
temperatures (25–175 1C) the same basal spacing of 7.55 A was
evaluated. At 200 1C, slightly shifted basal diffraction lines (003)
and (006) with lower intensity were observed and collapsed LDH
˚
phase with basal spacing of 6.6 A was found between 225 and

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F. Kovanda et al. / Journal of Solid State Chemistry 182 (2009) 27–36
31
˚
20 h sample with basal spacing of 7.85 A was gradually converted
´
´
by Perez-Ramırez et al. [31]. A continuous decrease of the
interlayer space was observed in the temperature range of
150–200 1C. Contrarily to the Mg–Al LDH, the rehydration of
dehydrated Ni–Al sample was not fully reversible and no
reconstruction of the Ni–Al mixed oxide prepared by LDH
calcination at 450 1C was observed.
˚
to the collapsed LDH with d003 of about 6.7 A, which disappeared
completely at 325–350 1C. The phase changes detected by HT XRD
were observed at lower temperatures than that corresponding
DTA effects and gases evolution maxima measured during thermal
analysis. This event can be explained by a difference in the used
heating rates. On the other hand, similar disagreement between
HT XRD and thermal analysis results was observed with cobalt-
containing LDHs [28], when the same heating rates were used. It
can be expected that maximum evolution of gases (and corre-
sponding DTA effect) takes place after at least partial destruc-
tion of the ordered LDH crystal structure and that the
gaseous products are evolved from already partly amorphous
intermediate.
˚
A gradual shift of the basal spacing from about 7.85 to 6.6 A in
the temperature range 100–150 1C was observed also with
the Ni4Al sample. The collapsed phase with d003 of 6.55 A was
detected at 200–275 1C and it disappeared at 300–325 1C. The
hydrothermal aging of the Ni4Al sample did not affect signifi-
cantly its thermal behavior. Between 125 and 175 1C the Ni4Al-
˚
TEM images (Fig. 4) showed a marked increase of the LDH
crystal size during hydrothermal treatment. The aggregates
composed of small thin crystals with hexagonal morphology were
observed in the coprecipitated samples. The product aging under
hydrothermal conditions increased the crystal size to achieve
hexagonal platelets with diameter of nearly 100 nm after 20 h
treatment. The size and hexagonal morphology of the primary
LDH crystals was kept even after their thermal decomposition at
450 1C, when Ni–Al mixed oxide was formed. However, the
integrity of initially compact hexagonal platelets was slightly
affected by narrow pores formed likely due to a release of volatile
components (H2O and CO2) during LDH heating.
A partial
destruction of the hexagonal platelets can be seen in the TEM
images of the samples calcined at 900 1C. It can be explained by
recrystallization processes and formation of other oxide phases at
high calcination temperatures.
3.3. Formation of the mixed oxides
HT XRD measurements showed that characteristic LDH
diffraction lines disappeared around 350 1C in all examined
samples and rather amorphous calcination products are formed.
A gradual crystallization of NiO-like mixed oxide with increasing
temperature can be seen in Fig. 2. Ambient XRD measurements
showed starting crystallization of NiO (bunsenite) at ca. 400 1C
[11,32]. Increasing calcination temperature enhanced the crystal-
lization of NiO-like phase and additionally NiAl2O4 spinel was
detected in the samples calcined at 800–900 1C and higher
temperatures.
The powder XRD patterns of the mixed oxides obtained by
heating of Ni2Al samples at 450 and 900 1C (Fig. 5) were evaluated
in more details to examine an influence of the LDH precursor
hydrothermal treatment on phase composition of the calcination
products. The lattice parameter of the NiO-like phase formed
at 450 1C was slightly lower in comparison with reference NiO
Fig. 3. Results of thermal analysis of the Ni2Al samples hydrothermally treated for
various times.
Table 3
Thermal decomposition characteristics of coprecipitated and hydrothermally treated LDHs obtained by thermal analysis (temperatures of DTG minima, DTA minima and
maximum H2O and CO2 evolution) and high-temperature X-ray diffraction
Sample
-H2O (EGA) (1C)
-CO2 (EGA) (1C)
LDH dehydration
LDH destruction
T of
D
m minima
T of DT minima
(HT XRD) (1C)
(HT XRD) (1C)
(DTG) (1C)
(DTA) (1C)
Ni2Al
232; 356
265; 370
270; 370
269; 371
240; 365
275; 378
280; 381
280; 380
265; 389
290; 400
296; 400
297; 400
368
380
379
380
200
225
225
225
350
350
350
350
Ni2Al-4 h
Ni2Al-8 h
Ni2Al-20 h
Ni3Al
(135, 189); 331
240; 349
145; 340
248; 351
207; 340
250; 350
340
357
175
200
325
350
Ni3Al-20 h
Ni4Al
147; 318
182; 326
158; 330
180; 330
162; 330
221; 331
330
333
150
150
300
325
Ni4Al-20 h

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F. Kovanda et al. / Journal of Solid State Chemistry 182 (2009) 27–36
Fig. 4. TEM images of the non-treated Ni2Al and hydrothermally treated Ni2Al-20 h samples and related mixed oxides obtained at 450 and 900 1C. a—Ni2Al, b—Ni2Al/450,
c—Ni2Al/900, d—Ni2Al-20 h, e—Ni2Al-20 h/450, f—Ni2Al-20 h/900.
(Table 4). No separate Al-containing phase was observed by
XRD that could mean either a segregation of Al cations to an
amorphous components or their incorporation into the NiO-like
phase or both. The Al incorporation into NiO lattice seems to be in
line with decreasing the NiO lattice parameter on decreasing Ni/Al
molar ratio in the samples. It is obvious that the lattice shrinkage
is larger in oxides formed from hydrothermally aged LDHs;
the aging also resulted in larger crystallite size of the calcined
products (Table 4) and lower content of amorphous components
(Table 5). The content of the amorphous components is approxi-
mately equal (non-treated sample) or lower (hydrothermally
treated ones) compared to the total content of Al2O3 in the
system (25.44 wt% for mixed oxide with Ni/Al molar ratio of 2).
The amorphous components were not found in the samples
calcined at 900 1C but the content of NiAl2O4 spinel is still lower
than it would correspond to a portion of aluminum available in
the formed oxide mixture: if all aluminum was included in the
NiAl2O4 spinel, its content would be equal to 44.08 wt%. The
results of Rietveld refinement can be explained by formation
of a mixture of Al-rich amorphous component and Al-containing
NiO-like oxide at 450 1C, followed by recrystallization at 900 1C
to NiAl2O4 spinel and NiO still containing of about 20 mol% Al in
the cationic sublattice. Trifiro et al. [33] reported a substantial
dissolution of Al3+ ions during treatment of calcined Ni–Al LDHs in
a NaOH solution; this event was explained by presence of an
alumina-type phase, probably grafted on the spinel-type phase.
However, this is not the case in phases here reported, because XPS
analysis did not indicate a surface enrichment by Al in relation to
the bulk (see below). With respect to stoichiometric NiO both
evaluated contents of components and decrease in the lattice
parameter of NiO-like phase in the calcination products (900 1C)
indicate a rather enhanced incorporation of Al cations into the NiO
lattice in hydrothermally treated samples.
3.4. Porous structure
The physical chemical properties of the Ni2Al LDHs hydro-
thermally treated for various times and related mixed oxides
obtained at 450 1C were studied in more details because these
mixed oxides were also tested in the catalytic decomposition of
N2O. The BET surface area of the dried LDHs increased after short-
time hydrothermal treatment (from 70 to 87 m2 gꢀ1 measured for
Ni2Al and Ni2Al-4 h samples, respectively). Long-time hydrother-
mal treatment decreased significantly the LDH surface area
(52 m2 gꢀ1 measured for Ni2Al-20 h sample), which can be
explained by dissolution and recrystallization of the smallest
crystallites and amorphous particles and subsequent increase of
the LDH crystallite size during hydrothermal aging. The growth of
LDH crystals during hydrothermal aging was confirmed by XRD
and TEM results. The calculated volumes of micropores with
diameter lower than 2 nm (Table 6) showed that all LDH samples
have very low volume of micropores and evaluated surface area
can be ascribed to mesopores. Pore size distribution was evaluated

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33
from nitrogen desorption isotherm measured at –196 1C. The
Ni2Al sample exhibited a sharp maximum corresponding to the
pore radius of 1.9 nm and then a low and broad maximum at rmax
of about 11 nm. Hydrothermal aging of the LDH precursor resulted
in a marked increase of the pore size, which increased with time
of hydrothermal treatment (Table 6).
Total intrusion pore volume increased with time of hydro-
thermal treatment and increased from 0.31 to 0.65 cm3 gꢀ1
measured for Ni2Al and Ni2Al-20 h samples, respectively. Bulk
density is inversely connected with the total pore volume and
decreased with increasing time of hydrothermal treatment.
Helium density of the LDH samples was not affected significantly
by hydrothermal aging and corresponded approximately to
2.62–2.66 g cmꢀ3. The increasing porosity of the LDHs can be
explained by growth of the crystallites during hydrothermal
treatment. For Ni2Al sample, the mercury intrusion showed much
wider pores (rmax of 10 and 24 nm) in comparison with nitrogen
desorption (rmax of 1.9 nm). On the other hand, pores with only
slightly lower size were found in hydrothermally treated LDHs. In
all measured samples, no pores with rmax higher than 100 nm
were detected (Fig. 6).
The BET surface area of the calcination products increased
more than two times in comparison with the LDH precursors
and slightly increased with time of LDH hydrothermal aging.
The volume of micropores was also increased but it remained
negligible in relation to the total pore volume as evaluated surface
area corresponded to mesopores. Larger pore volume resulted in
lower bulk density and higher porosity of the calcined samples.
Helium density of the obtained mixed oxides slightly increased
with the time of LDH hydrothermal treatment from 4.02 to
4.26 evaluated for Ni2Al/450 and Ni2Al-20 h/450 samples,
respectively. Pores with rmax of 2.5 nm were detected in the
Ni2Al/450 sample by nitrogen desorption. Slightly wider pores
in comparison with the parent LDHs were found also for
the hydrothermally treated samples (Table 6) but a minor part
of the pores detected in these samples exhibited rmax of about
1.7 nm. Formation of such narrow pores could be connected with
release of the volatile components during thermal decomposition
of the LDH precursors.
Fig. 5. Powder XRD patterns of the Ni2Al samples hydrothermally treated for
various times and calcined at 450 and 900 1C. B—NiO (bunsenite), S—NiAl2O4
(spinel), *—Si (internal standard).
Table 4
Lattice parameter and crystallite size of NiO-like mixed oxide obtained by
calcination of the LDHs at 450 1C
Crystallite sizea (nm)
˚
Sample
Lattice parameter (A)
3.5. TPR results
Ni2Al/450
4.148
4.138
4.137
4.138
3.3
3.9
4.0
5.0
Ni2Al-4 h/450
Ni2Al-8 h/450
Ni2Al-20 h/450
The TPR patterns of the Ni2Al samples calcined at 450 1C are
demonstrated in Fig. 7. They exhibit broad reduction peak with
maximum in the temperature range 523–614 1C, which corre-
sponds to reduction of Ni2+-Ni0; the reducibility of the samples
worsened with increasing time of hydrothermal treatment. The
reduction maxima of the calcined Ni2Al samples were observed at
much higher temperatures than the reduction maximum of the
reference NiO (330 1C). A formation of small and stable NiO
particles can be expected during calcination of Ni2Al LDH
precursors at 450 1C; however, they are worse reducible in
Ni3Al/450
4.166
4.158
4.6
4.9
Ni3Al-20 h/450
Ni4Al/450
4.173
4.170
5.1
4.8
Ni4Al-20 h/450
NiO
4.177
n.d.
n.d.—Not determined.
a
Crystallite size was obtained by Rietveld refinement (whole pattern fitting).
Table 5
Results of Rietveld refinement of oxide phases formed by heating Ni2Al LDHs hydrothermally treated for various times (content of amorphous components was obtained
using addition of known amount of Si internal standard)
LDH precursor
450 1C
900 1C
˚
˚
NiO (wt%)
Amorphous (wt%)
NiO (wt%)
NiO lattice parameter (A)
Spinel (wt%)
Spinel lattice parameter (A)
Ni2Al
74.6
78.8
82.0
77.7
25.4
21.2
18.0
22.3
62.1
64.8
64.9
64.9
4.177
4.176
4.176
4.174
37.9
35.2
35.1
35.1
8.052
8.065
8.067
8.068
Ni2Al-4 h
Ni2Al-8 h
Ni2Al-20 h
Note: No amorphous components were found in the samples calcined at 900 1C.

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F. Kovanda et al. / Journal of Solid State Chemistry 182 (2009) 27–36
Table 6
Surface area and the porous structure of the hydrothermally treated Ni2Al LDH precursors and related mixed oxides obtained by calcination at 450 1C
Sample
Nitrogen adsorption
Mercury intrusion
Surface area
(SBET) (m2 gꢀ1
Pore volume
Volume of
micropores
Pore size
Total pore
volume
(cm3 gꢀ1
)
Bulk density
Porosity (%)
Pore size
)
(cm3 gꢀ1
)
(rmax) (nm)
(g cmꢀ3
)
(rmax) (nm)
(mm3 gꢀ1
)
Ni2Al
70
87
74
52
0.19
0.34
0.33
0.19
0.9
2.1
1.1
0.7
1.9
9.5
13
0.31
0.44
0.51
0.65
1.51
1.24
1.17
0.98
46.5
54.8
59.9
63.6
10; 24
Ni2Al-4 h
Ni2Al-8 h
Ni2Al-20 h
6
9
17
17; (28)
Ni2Al/450
145
144
154
161
0.40
0.60
0.75
0.75
3.2
3.5
5.5
2.5
2.7
0.48
0.67
0.74
0.92
1.34
1.06
1.08
0.85
64.6
70.9
80.2
77.9
2.8; (11); 18
7.5; (12)
10; (18)
17
Ni2Al-4 h/450
Ni2Al-8 h/450
Ni2Al-20 h/450
(1.7); 12
(1.7); 16
(1.7); (12); 28
Fig. 7. TPR patterns of hydrothermally treated Ni2Al samples calcined at 450 1C.
ordering and increased crystallite size of hydrothermally treated
Ni2Al LDH precursors enhanced the stability of Ni2+ species in
calcination products against reduction as it was formerly reported
[23]. Worsened reducibility of the calcination products prepared
from hydrothermally aged LDH precursors could be also caused by
an enhanced incorporation of the Al cations into the NiO lattice as
it was deduced from the results of Rietveld refinement.
Fig. 6. Pore size distribution of the Ni2Al samples hydrothermally treated for
various times and related mixed oxides obtained at 450 1C (mercury intrusion
porosimetry).
3.6. Surface composition
comparison with stoichiometric NiO. It is known that reduction of
Ni–Al LDH-related mixed oxides is hindered by the presence of
nickel aluminate-type phases formed likely upon heating of the
LDH precursor [11,23,33].
The XPS spectra of Ni2Al samples calcined at 450 1C were
collected and compared with reference NiO. The samples were
deposited on a gold support and examined in the powdered form.
The reference NiO and the Ni2Al-8 h/450 sample were addition-
ally examined after 30 min heating at 400 1C to identify the form
of surface oxygen in the samples. Oxidation state of nickel on the
surface was determined from binding energy and its chemical
shift in the XPS spectra (Table 8). According to the published data
determined by examination of oxides, hydroxides and oxohydr-
oxides [34], the binding energies of Ni 2p3/2 for oxidation state Ni0,
Ni2+, and Ni3+ are equal to 852.8, 854.6 and 856.1 eV, respectively.
Electron structure and theoretical models of pure Ni and NiO are
discussed in [34–36]. Interpretation of the Ni 2p spectra of oxides
and other compounds is based on the transfer of charge of both
the main peak at 854.6 eV and the wide satellite with binding
Data obtained from TPR measurements are summarized in
Table 7. Only a slight difference was observed in amount of
hydrogen consumed for total reduction of all examined samples,
the lowest amount of reducible components being detected in the
sample without hydrothermal treatment. Two individual peaks
were distinguished by fitting the total reduction peak. These two
peaks are associated with two different arrangement of the
primary particles or occurrence of two different phases in the
samples. Positions of both peaks maxima shifted to higher
temperatures and the amount of easily reducible components
(corresponding to the first reduction peak) decreased with
increasing time of hydrothermal treatment. The better structural

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35
Table 7
TPR results of the Ni2Al samples hydrothermally treated for various times and calcined at 450 1C
Peak areaa (%)
H2 consumptionb
(mmol H2 gꢀ1
)
a
Sample
H2 consumption
(20–800 1C) (mmol H2 gꢀ1
Tmax (1C)
)
Ni2Al/450
8.58
8.68
8.80
8.82
523; 585
561; 609
560; 602
570; 614
44; 56
54; 46
36; 64
21; 79
3.77
4.69
3.17
1.85
Ni2Al-4 h/450
Ni2Al-8 h/450
Ni2Al-20 h/450
a
Determined from peak fitting.
b
Hydrogen consumption corresponding to the first reduction peak.
Table 8
Binding energy of internal electrons and FWHM values of the photoemission peaks (in parenthesis)
Sample
Al 2p3/2
Ni 2p3/2
Ni 3p3/2
O 1s
Al3+
Ni2+
Satellite
Ni2+
O2ꢀ
O2ꢀ OHꢀ
H2Oads.
NiO
–
855.5 (4.1)
855.1 (4.7)
855.7 (4.4)
855.5 (4.2)
855.7 (4.0)
855.4 (4.0)
855.5 (3.9)
861.5
861.1
861.7
861.4
861.5
861.5
861.6
67.9 (3.8)
68.2 (4.7)
67.9 (3.9)
68 (4.3)
530.0
531.7 (1.9)
NiO/ha
–
530.0 (3.9)
530.0
Ni2Al/450
73.6 (2.2)
73.6 (2.2)
73.5 (2.2)
73.6 (2.7)
73.6 (2.2)
531.7 (2.3)
531.6 (2.3)
531.6 (2.3)
531.7 (2.4)
531.6 (2.3)
533.0
533.2
533.1
Ni2Al-4 h/450
Ni2Al-8 h/450
Ni2Al-8 h/450/ha
Ni2Al-20 h/450
530.0
68.1 (4.0)
68.2 (4.3)
68.1 (4.0)
530.0
530.0
530.0
533.2
Data are related to the O 1s line (binding energy of 530.070.2 eV).
a
Samples heated for 30 min at 400 1C before XPS measurement.
Table 9
Surface concentration of Ni and Al cations (CS) and ratio of surface to bulk
concentration (CS/CV)
Sample
CS (mol.%)
CS/CV
Al3+
Ni2+
Al3+
Ni2+
NiO
0
1.00
1.00
0.67
0.69
0.66
0.67
0.66
0
0
1
1
1
1
1
1
NiO/ha
0
1
Ni2Al/450
0.33
0.31
0.34
0.33
0.34
1.02
1.11
0.97
1.02
0.97
Ni2Al-4 h/450
Ni2Al-8 h/450
Ni2Al-8 h/450/ha
Ni2Al-20 h/450
a
Samples heated for 30 min at 400 1C before XPS measurement.
energy of 861 eV corresponding to cd9L (c ¼ vacancy in a nucleus,
L ¼ ligand vacancy) and naked cd8 configurations.
Concentrations of the elements present on the samples surface
were calculated from the Al 2p, Ni 2p and O 1s photoelectron lines.
The C 1s photoelectron line was not measured, as carbon was not
either present or only in a very low concentration in the examined
samples. The electron spectra of O 1s consisted of three over-
lapping components, from which the component with the highest
binding energy, i.e., the low intensive one likely belongs to
adsorbed H2O. This component disappeared in the XPS spectrum
of the Ni2Al-8 h/450 sample after its heating at 400 1C, which
confirmed this assumption. The surface concentrations of Ni and
Al close to the bulk ones were found and only a very slight
enrichment of mixed oxide surface with nickel could be expected
(Table 9).
Fig. 8. Temperature dependences of N2O conversion over hydrothermally treated
Ni2Al samples calcined at 450 1C. (Conditions: 1000 ppm N2O balanced by He, total
flow 100 ml minꢀ1, catalyst weight 0.1 g)
correlation between structural and catalytic properties of solids
[6]. The catalytic decomposition offers also a simple method for
abatement of N2O in waste gases. The temperature dependence of
N2O decomposition over Ni2Al samples calcined at 450 1C is
shown in Fig. 8. It can be seen that hydrothermal treatment
enhanced the catalytic activity of related mixed oxide catalysts;
the Ni2Al-20 h/450 sample exhibited the highest N2O conversion
from all examined catalysts. Temperature T50, at which 50%
conversion of N2O was observed, decreased with increasing time
of hydrothermal treatment and the difference of T50 between the
Ni2Al/450 and Ni2Al-20 h/450 samples was about 25 1C.
3.7. Catalytic activity in N2O decomposition
The enhanced catalytic activity of hydrothermally treated
samples could be related to a decreased amount of amorphous
admixture (as it was observed by XRD). The surface area of the
Catalytic decomposition of nitrous oxide is a simple redox
reaction, which was often chosen as a testing reaction for finding a

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F. Kovanda et al. / Journal of Solid State Chemistry 182 (2009) 27–36
calcined products was influenced only slightly by hydrothermal
treatment of the LDH precursor. We have found [6] that the total
surface area is not a crucial factor affecting the catalytic activity of
Ni–Al mixed oxides in N2O decomposition. Actually the pore size
distribution in the examined samples changed in dependence on
time of LDH hydrothermal aging (Fig. 6), and there is obviously a
positive correlation between catalytic activity and porous struc-
ture. In such a case, the influence of the diffusion processes can be
very likely considered.
Hence, we have estimated the Weisz–Prater criterion [37] to
determine whether the internal diffusion in the pores is limiting
the reaction rate. A negligible concentration difference in the
catalyst grains can be expected for Weisz–Prater criterion 51 and
consequently no diffusion limitations occur at such conditions. On
the other hand, the internal diffusion causes a decrease in the
observed reaction rate for Weisz–Prater criterion b1. Following
parameters were chosen for calculation of the criterion values:
Bulk density of the catalyst ranging from 600 to 1800 kg mꢀ3 and
radius of the grains ranging from 0.8 ꢂ 10ꢀ4 to 1.6 ꢂ10ꢀ4 m. The
Weisz–Prater criterion, calculated within these limits, changed
from 10.7 to 0.2 for Ni2Al/450 and Ni2Al-20 h/450 samples,
respectively. Based on the calculated values of the Weisz–Prater
criterion, the differences in catalytic activity of the examined
Ni–Al mixed oxides can be explained by a decelerating effect of
the internal diffusion in the catalysts grains.
and the efficiency of related mixed oxides in N2O decomposition
increased with increasing time of hydrothermal aging.
Acknowledgments
This work was supported by the Czech Science Foundation
(Project No. 104/07/1400) and by the Ministry of Education, Youth
and Sports of the Czech Republic (Projects No. MSM 6046137302
´ ˇ
and 6198910016). TEM was kindly performed by Jaroslav Bohacek
(Institute of Inorganic Chemistry ASCR).
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