X-Ray Diffraction, FTIR, and NMR Characterization of Sol-Gel Alumina Doped with Lanthanum and Cerium

Article abstract of DOI:10.1006/jssc.1996.7135

Alumina doped with La and Ce was prepared by using the sol-gel technique. The doping concentrations were alternatively 2 and 5 wt% for each element. Samples were characterized as a function of temperature by using X-ray powder diffraction and FTIR spectroscopy. MAS-NMR studies showed that lanthanum and cerium interacted with alumina and produced a compound with aluminum ions in tetrahedral symmetry. These interactions stabilized alumina structure and shifted its transformations to higher temperatures. When cerium-doped samples were heated at temperatures higher than 1000°C, CeO₂was segregated. For La doping such segregation was not observed. Bronsted acid sites were generated for the low doping concentrations of both elements. At the high doping concentration, however, only Lewis acid sites existed in a higher concentration than in undoped alumina.

Full text of DOI:10.1006/jssc.1996.7135

JOURNAL OF SOLID STATE CHEMISTRY 128, 161—168 (1997)
ARTICLE NO. SC967135
X-Ray Diffraction, FTIR, and NMR Characterization
of Sol—Gel Alumina Doped w ith Lanthanum and Cerium
A. Vazquez,* T. Lopez,*ꢂR R. Gomez,*ꢂR Bokhimi,Sꢂmꢂꢀ A. Morales,S and O. NovaroSꢂꢁ
*Department of Chemistry, Universidad Autonoma Metropolitana-Iztapalapa, A.P. 55-534, 09340 Mexico, D.F., Mexico; RUniversidad de Guanajuato,
Lascurain de Retana s/n, Guanajuato, Gto., Mexico; SInstitute of Physics, National University of Mexico (UNAM) A.P. 20-364, 01000 Mexico, D.F., Mexico;
m
and National Institute of Nuclear Research (ININ),Me xico, D.F., Mexico
Received December 13, 1995; in revised form August 27, 1996; accepted August 28, 1996
generated during dehydration and dehydroxylation. Prep-
aration, aging, and drying procedures determine the final
Alumina doped with La and Ce was prepared by using the
sol–gel technique. The doping concentrations were alternatively xerogel properties (19, 20).
2 and 5 wt% for each element. Samples were characterized as
a function of temperature by using X-ray powder diffraction and
FTIR spectroscopy. MAS-NMR studies showed that lanthanum
and cerium interacted with alumina and produced a compound
with aluminum ions in tetrahedral symmetry. These interactions
stabilized alumina structure and shifted its transformations to
higher temperatures. When cerium-doped samples were heated
at temperatures higher than 1000 °C, CeO2 was segregated. For
La doping such segregation was not observed. Brønsted acid sites
were generated for the low doping concentrations of both ele-
ments. At the high doping concentration, however, only Lewis
When c-alumina is doped with La and Ce it is stabilized
and used as a support in catalysis (21—26). CeO and La O
ꢁ
ꢁ ꢃ
not only stabilize c-alumina but also improve its selectivity
to definite products. Cerium is the most frequent base addi-
tive used in the support of automobile converter catalysts
that transform carbon monoxide, hydrocarbons, and nitro-
gen oxides (27—29). Cerium-doped catalysts can supply
oxygen in reduction and oxidation environments. The sta-
bilization of c-alumina by La is supposed to be caused by
nucleation of LaAlO on the alumina surface (21). LaAlO
ꢃ
ꢃ
acid sites existed in a higher concentration than in undoped interacts with the c-alumina support and inhibits its trans-
alumina. ( 1997 Academic Press
formation toward a-alumina.
In this work, we synthesized alumina xerogels doped with
La and Ce. These xerogels were characterized by X-ray
powder diffraction and FTIR spectroscopy after being
heated at different temperatures. They were also character-
ized by MAS-NMR spectroscopy.
1. INTRODUCTION
In the traditional synthesis of alumina, the initial precur-
sor hydroxide determines its different phases and properties
(1—6).
2. EXPERIMENTAL
Alumina of high thermal stability and high specific sur-
face area is obtained by using the sol—gel technique (7—10).
Usually, Al and O atoms form a nearly amorphous struc-
ture, with Alꢃ> and Oꢁ\ ions forming octahedra (11).
When supercritical drying is used in the sol—gel process,
aerogel alumina is obtained (12, 13). This alumina has a
large specific surface area and macropores that make it
attractive for use as a support in catalysis, because it favors
the interaction with the metal to be supported.
Synthesis of sol—gel Al2O3. Aluminium sec.-butoxide,
Al(OC H s) (0.207 mol) was mixed with 6.5 mol of abso-
ꢄ
ꢅ ꢃ
lute ethanol at 60°C, refluxing and stirring constantly. Dis-
tilled water (2.94mol) was added to this mixture, giving
a water/alkoxide molar ratio of 14.2. The gelling reaction
was completed 1 h after having added this solution. There-
after, the gel was dried in air at 70°C for 24 h.
Synthesis of sol—gel ¸a/Al2O3 with 2 and 5 wt% ¸a. Alu-
The sol—gel process also can produce xerogel alumina minum sec.-butoxide (0.207 mol) was mixed with 6.5 mol of
(14—19). Xerogels are made from a colloidal solution gelling absolute ethanol at 60°C, refluxing and stirring constantly.
at low temperatures at atmospheric pressure. They are For each La concentration, a water solution of lanthanum
solids with collapsed porosity produced by the surface stress nitrate containing 2.94 mol of distilled water was added to
this mixture. The gelling reaction was completed 1 h after
having added this solution. Thereafter, the gels were dried
under the same conditions as alumina gel. After drying at
ꢀ To whom correspondence should be addressed.
ꢁ Member of El Colegio Nacional, Mexico.
161
0022-4596/97 $25.00
Copyright ( 1997 by Academic Press
All rights of reproduction in any form reserved.

162
VAZQUEZ ET AL.
70°C, the samples of all three systems were annealed in air
for 12 h at temperatures between 500 and 1100°C.
Synthesis of sol—gel Ce/Al2O3 with 2 and 5 wt% Ce.
Aluminium sec.-butoxide (0.207 mol) was refluxed with
6.5 mol of absolute ethanol at 60°C, stirring constantly. For
each Ce concentration, a water solution of cerium nitrate
containing 2.94 mol of distilled water was added. The gell-
ing reaction was completed 1 h after having added this
solution. Thereafter, the gels were dried under the same
conditions as alumina gel.
X-ray powder diffraction. The structure (crystalline or
amorphous) of the phases in the samples, annealed at differ-
ent temperatures, was obtained at room temperature by
using X-ray powder diffraction. The reflection diffracto-
meter used CuKa radiation, and had a graphite mono-
chromator in the secondary beam. Each specimen was
prepared by packing sample powder in a glass holder.
Intensity data were measured by step scanning in the 2h
ranges between 2° and 110° and between 2° and 70°, with
a 2h step of 0.02° and a measuring time of 1 s per point.
F¹IR spectroscopy. The dried solids were characterized
in situ with a Nicolet 170-SX Fourier spectrometer, which
uses controlled atmospheres. Before measuring, samples
were pressed until transparency was achieved. Spectra were
measured in a vacuum of 1;10\ꢆ Torr. To identify and
characterize the acid sites, pyridine was adsorbed by fluxing
FIG. 1. X-ray powder diffraction pattern of undoped sol—gel alumina
after annealing the sample at different temperatures. a and c indicate a- and
c-alumina. The indexing of the pattern corresponds to a-alumina.
N on the samples at room temperature. Pyridine excess
ꢁ
was removed by making a vacuum in the camera of the
spectrometer. During the analysis in situ, pyridine was de-
sorbing while the samples were heated to 300°C.
MAS-NMR spectroscopy. Powder of the samples was
analyzed in a Bruker NMR spectrometer Model ASX-300.
The study was done with the ꢁꢇAl isotope at a frequency of
78.21 MHz with a spinning speed of 12 kHz. Chemical shifts
were referred to aluminum in Al(H O )ꢃ> ion.
ꢁ ꢆ
3. RESULTS AND DISCUSSION
X-ray Diffraction Analysis
The X-ray diffraction patterns of the fresh dried samples
in all systems were similar (Figs. 1—5). They corresponded
to an aluminum hydroxide with a crystalline structure hav-
ing some diffraction peaks as in bohemite (5) and others
diffraction peaks as in d-Al(OH) (30). The crystalline struc-
ꢃ
ture, however, did not correspond to any of them.
Undoped sol—gel alumina was transformed into c- and
a-alumina when it was annealed in air (Fig. 1). After the
sample was heated at 1000°C, c-alumina and impurities of
a-alumina were formed. The transformation of amorphous
into c-alumina was associated with the dehydration of the
sample hydrated during the sol—gel process. When the
sample was heated from 1000 to 1100°C, the c-alumina was
FIG. 2. X-ray powder diffraction pattern of sol—gel La/Al O with
2 wt% La after annealing the sample at different temperatures. a and
c indicate a- and c-alumina.
ꢁ
ꢃ

DOPED SOL—GEL ALUMINA
163
FIG. 3. X-ray powder diffraction pattern of sol—gel La/Al O with
5 wt% La after annealing the sample at different temperatures. c indicates 5 wt% Ce after annealing the sample at different temperatures. a, c, and
FIG. 5. X-ray powder diffraction pattern of sol—gel Ce/Al O with
ꢁ
ꢃ
ꢁ ꢃ
c-alumina.
c indicate a- and c-alumina and cerianite, respectively.
totally transformed into a-alumina, which has a MAS-
NMR spectrum with a chemical shift of !2.1 ppm (Fig. 6),
which falls in the range of chemical shifts corresponding to
aluminum ions with octahedral symmetry (31).
Lanthanum doping of sol—gel alumina inhibited the
transformation of alumina phases (Figs. 2 and 3). After these
doped samples were heated at 500°C in air, the structure
was amorphous, but, after heating at 1000°C, it transformed
into c-alumina, which was stable even at 1100°C. The MAS-
NMR spectrum of the sample heated at this temperature
(Fig. 6) shows one peak at !3.6 ppm and a second peak
that was very broad and centered around 35 ppm. The first
peak corresponds to an aluminum ion in octahedral sym-
metry; the second one corresponds to an aluminum ion in
tetrahedral symmetry (31). Since the area of the first peak is
larger (Fig. 6), more aluminum ions are in octahedral sym-
metry. At this temperature, however, the X-ray diffracto-
gram shows that almost all sample is c-alumina. The num-
ber of sites with tetrahedral symmetry increases with La
concentration (Fig. 6). The sample with 2 wt% La had im-
purities of a-alumina (Fig. 2), in contrast to the sample
corresponding to 5 wt% La (Fig. 3). This means that for
totally stabilizing the sample in the c-alumina phase, the
concentration of 2 wt% is insufficient.
FIG. 4. X-ray powder diffraction pattern of sol—gel Ce/Al O with
2 wt% Ce after annealing the sample at different temperatures. a, c, and
c indicate a- and c-alumina and cerianite, respectively.
ꢁ
ꢃ
The X-ray diffraction patterns of the La-doped samples,
however, did not show the presence of the LaAlO phase
ꢃ

164
VAZQUEZ ET AL.
FIG. 6. MAS-NMR spectra of the samples after annealing them at
1100°C: (a) sol—gel Al O , (b) 2 wt% La/Al O , (c) 5 wt% Al O , (d)
FIG. 7. FTIR spectra of the La/Al O samples.
ꢁ
ꢃ
ꢁ
ꢃ
ꢁ
ꢃ
ꢁ ꢃ
2 wt% Ce/Al O , and (e) 5 wt% Ce/Al O . SSB in the figure indicates the
ꢁ
ꢃ
ꢁ ꢃ
spinning side bands.
metry, similar to the way lanthanum did. The transforma-
tion of c-alumina into a-alumina was nearly total at 1100°C
reported to be responsible for the stability of La-doped (Fig. 5). This transformation also destroyed Al—Ce interac-
alumina (21). In this phase aluminum ion has octahedral tion, decreasing with that the number of aluminum ions
symmetry, and according to the MAS-NMR spectra, lan- with tetrahedral symmetry (Fig. 6). At this temperature, Ce,
thanum doping gives rise to aluminum ions with tetrahedral Al, and O did not combine to give a CeAlO crystalline
ꢃ
symmetry. LaAlO did not appear even after the sample was phase (32); instead, they segregated, forming Al O and
ꢃ
ꢁ ꢃ
heated at 1200°C; at this temperature the only crystalline CeO .
ꢁ
phase observed was a-alumina. This suggests that in the
Ce doping also stabilized alumina phases. The stabiliz-
present case La atoms were integrated into the alumina ation was weaker than with La. Since the interaction be-
structure instead of being supported on the surface (21).
tween cerium and aluminum ions gave rise to aluminum
Lanthanum doping stabilized c-alumina. Lanthanum and ions with tetrahedral symmetry (Fig. 6), the CeAlO phase
ꢃ
aluminum ions interacted and produced a local arrange- was eliminated as being the phase responsible for the
ment with aluminum ions in tetrahedral symmetry. This alumina’s of stability. In CeAlO aluminum ion has oc-
ꢃ
eliminated LaAlO as the phase responsibile for the tahedral symmetry.
ꢃ
alumina’s stability, because in this phase the aluminum ion
has octahedral symmetry.
Cerium doping also inhibited the transformation of
FTIR Studies
alumina phases (Figs. 4 and 5). After these samples were
In the sol—gel alumina, the vibrations of OH ions and,
heated at 500°C in air, they had an amorphous structure C—H, Al—OH, and Al—O bonds generated the observed
that was transformed into c-alumina after heating at bands in the infrared region. The stretching vibration of the
1000°C. Here, Ce stabilized this structure, at least to this OH ions of residual water and solvent in the gel produced
temperature. By heating the sample at 1050°C (Fig. 4), a very intense broadband at 3472 cm\ꢀ (Fig. 7), whereas
doped c-alumina was partially transformed into a-alumina their bending vibration generated the band at 1632 cm\ꢀ.
and cerianite (CeO ). MAS-NMR analysis after annealing The two small bands at 2971 and 2921 cm\ꢀ were produced
ꢁ
the sample at this temperature (Fig. 6) showed that cerium by the asymmetric bending vibrations of the C—H bond in
doping gave rise to aluminum ions with tetrahedral sym- the uncondensed butoxy groups. The stretching vibrations

DOPED SOL—GEL ALUMINA
165
of the Al—OH bond gave rise to the band at 1555 cm\ꢀ. The
weak bands observed at 1120 and 1072 cm\ꢀ were prod-
uced by the Al—O bonds. The spectrum at low energies was
undefined.
The high-energy infrared spectrum of La-doped alumina
with 2 wt% La (Fig. 7) was similar to that observed for
undoped alumina. The OH ions produced the same bands
reported for these ions in undoped alumina. The intensity of
the band at 2971 cm\ꢀ produced by the asymmetric vibra-
tions of the C—H bond, however, slightly decreased in inten-
sity, but the band at 2921 cm\ꢀ completely disappeared,
indicating that the aluminum sec.-butoxide reaction was
completed and that the amount of residual butoxy groups
was low. The band at 1555 cm\ꢀ observed in undoped
alumina and produced by the vibrations of the Al—OH bond
shifted to 1588 cm\ꢀ and had a higher intensity. This shift
was produced by the interaction of lanthanum ions with
alumina.
Contrasting with undoped alumina FTIR spectrum, the
low-energy region of the La-doped alumina spectrum was
well defined. It showed bands at 832, 616, and 472 cm\ꢀ,
which probably were produced by vibrations of Al—O bonds
corresponding to aluminum ions with tetrahedral sym-
metry.
FIG. 8. FTIR spectra of the Ce/Al O samples.
ꢁ
ꢃ
Doping alumina with 5 wt% of La produced bigger
changes (Fig. 7). For example, the band at 1555 cm\ꢀ disap-
peared, indicating the absence of Al—OH bonds. The bands a vacuum (33). Pyridine is chemisorbed on oxides in two
in the low-energy region moved to the higher energies, 892, ways: on Lewis acid sites (coordinatively unsaturated or
622, and 490 cm\ꢀ. This shift was produced by a stronger electron-pair acceptor sites) by coordination of nitrogen
Al—O bond corresponding to the higher stability of the lone pair of electrons, and on Brønsted acid sites by transfer
structure than in undoped alumina.
of protons from the site to nitrogen (34). With this method
The FTIR spectrum of Ce/Al O with 2 wt% Ce was very the Brønsted acid sites (protonic acidity) can be clearly
ꢁ ꢃ
similar to that of undoped alumina (Fig. 8). The principal distinguished from the Lewis acid sites (nonprotonic acid-
differences were in the high-energy region. Here, Ce stabil- ity), because Brønsted sites produce the pyridinium ion
ized OH groups, causing the band produced by its stretch- (PyH>) and promote the formation of pyridine species
ing vibrations to move to 3352 cm\ꢀ. For 2 wt% Ce the bonded to hydrogen, which produce the bands in the FTIR
band at 1555 cm\ꢀ, associated with Al—OH bonds, shifted spectrum between 1700 and 1400 cm\ꢀ.
to 1520 cm\ꢀ, but it disappeared for 5 wt% Ce. As with
In undoped alumina only Lewis acid sites were present. In
undoped alumina, the spectrum at low energies was unde- these samples, the adsorption of pyridine produced two
fined. These results also suggest that the interaction between principal bands in the FTIR spectrum (Fig. 9), one at
aluminum and cerium ions is weaker than that between 1590 cm\ꢀ and another at 1442 cm\ꢀ, with a relative inten-
aluminum and lanthanum ions.
sity of 2.1; they were characteristic of Lewis acid sites.
Another band with a relative intensity of 0.4 appeared at
1490 cm\ꢀ. The sample acidity was low and the acid sites
disappeared with temperature; therefore, after the sample
Pyridine Adsorption Studies
When materials are produced by the sol—gel technique, was heated at 300°C the intensity of the bands almost
the OH groups play an important role in the acidity of the vanished.
product. For example, dehydration of alumina produces an
The bending vibration of the OH ions of residual water
acid—base site pair. In general, the most common acid sites and solvent in the gel gave rise to a band at 1632 cm\ꢀ with
are of the Lewis type, but in sol—gel alumina Brønsted sites an intensity decreasing with temperature (Fig. 9) because of
are also generated.
sample dehydratation.
The acidic properties and the nature of the sites generated
Doping alumina with 2 wt% La generated Lewis and
in the samples are determined by studying the adsorption of Brønsted acid sites. The acidity produced by Lewis acid sites
pyridine as a function of temperature and its desorption in gave rise to one band at 1448 cm\ꢀ with a relative intensity

166
VAZQUEZ ET AL.
FIG. 10. FTIR spectra of pyridine absorbed in a sol—gel La/Al O
sample with 2 wt% La.
FIG. 9. FTIR spectra of pyridine absorbed in sol—gel alumina.
ꢁ
ꢃ
of 2.56 and another at 1490 cm\ꢀ with a relative intensity of
2.4 (Fig. 10). These sites were more stable to annealing than
in undoped alumina, because they disappeared only after
heating the sample at 400°C. The Brønsted acid sites gene-
rated the band at 1540 cm\ꢀ with a relative intensity of 0.79.
The bending band at 1632 cm\ꢀ (Fig. 10) was produced
by water OH groups. This band diminished with temper-
ature and disappeared at 500°C.
For the La doping concentration of 5 wt% the number of
Lewis acid sites increased whereas the Brønsted sites disap-
peared (Fig. 11). Therefore, the band at 1540 cm\ꢀ was not
observed anymore. Since the number of Lewis acid sites in-
creased, the band at 1442 cm\ꢀ had a relative intensity of 4.12.
Cerium-doped alumina samples had variable Lewis and
Brønsted acid site concentrations. In the samples with
2 wt% Ce, the number of acid sites increased in the main,
being thermically stable even at 500°C (Fig. 12). At
1448 cm\ꢀ these samples had an adsorption band charac-
teristic of Lewis acid sites; it had a relative intensity of 6.71
when it was fresh and a relative intensity of 0.59 after it was
heated at 500°C. Brønsted acid sites generated the band at
1540 cm\ꢀ, with a relative intensity of 1.06. This intensity
corresponded to 30% more Brønsted sites than in the
samples doped with 2 wt% lanthanum. For Ce doping,
Brønsted sites were thermoresistant even at 300°C, where
the relative intensity of the corresponding band is 0.13
(Fig. 12). The band of Lewis acidity, observed at 1490 cm\ꢀ,
had a relative intensity of 2.07; it was also higher than in the
2 wt% lanthanum sample.
FIG. 11. FTIR spectra of pyridine absorbed in a sol—gel La/Al O
sample with 5 wt% La.
ꢁ
ꢃ

DOPED SOL—GEL ALUMINA
167
At this Ce doping concentration, the bending vibration of
OH ions of residual water and solvent in the gel gave rise to
a band at 1632 cm\ꢀ with an intensity decreasing with
temperature (Fig. 12) because of sample dehydratation.
When cerium concentration was increased to 5 wt%
(Fig. 13) the Brønsted sites disappeared. The number of
Lewis acid sites increased, producing an increase in the
intensity of the bands at 1590 and 1448 cm\ꢀ, reaching
a relative intensity of 7.97. These were the samples with the
highest concentration of Lewis acid sites.
4. CONCLUSIONS
When alumina was doped with La or Ce atoms and was
prepared by the sol—gel technique, the doping atoms were
incorporated into the alumina structure and generated
structures with aluminum ions in tetrahedral symmetry.
This produced an alumina—dopant interaction that stabil-
ized alumina structures, shifting their transformations to
higher temperatures. The interaction was stronger for La
doping.
CeO was segregated when the samples doped with Ce
ꢁ
were heated at high temperatures, instead of forming any
Ce—Al—O compound. For La doping, we observed, even
after heating the sample at 1200°C, neither segregation nor
FIG. 12. FTIR spectra of pyridine absorbed in a sol—gel Ce/Al O
sample with 2 wt% Ce.
ꢁ
ꢃ
the formation of LaAlO phase supposed to be responsible
ꢃ
for alumina stabilization.
Low Ce doping concentrations produced Brønsted acid
sites, but they disappeared at the high doping concentra-
tions. The number of Lewis acid sites significantly increased
when the samples were doped with the high concentrations
of La and Ce. These sites were more stable than in undoped
alumina.
ACKNOWLEDGMENTS
We thank CONACyT (Mexico), CNRS (France), and the National
Science Foundation (United States) for financial support.
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ꢁ
ꢃ

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