4796
Langmuir 1999, 15, 4796-4802
Characterization of High Surface Area Zr-Ce (1:1) Mixed
Oxide Prepared by a Microemulsion Method
A. Martı́nez-Arias, M. Fernández-Garcı́a, V. Ballesteros, L. N. Salamanca,
J. C. Conesa,* C. Otero, and J. Soria
Instituto de Catálisis y Petroleoquı́mica, CSIC, Camino de Valdelatas s/n, Campus U.A.M.,
Cantoblanco, 28049 Madrid, Spain
Received October 30, 1998. In Final Form: February 17, 1999
A Zr-Ce mixed oxide with ca. a 1:1 atomic ratio is prepared by a microemulsion method and studied
by X-ray diffraction, transmission electron microscopy, and Raman, X-ray photoelectron (XPS) and electron
paramagnetic resonance (EPR) spectroscopies. The results show the formation of a high surface area
material (SBET ) 96 m2 g-1) constituted by homodispersed particles of a major pseudocubic phase t′′ (as
shown by Raman); the stabilization of the latter phase, instead of the normally more stable tetragonal
phase t′, is probably due to the small crystallite size (ca. 5 nm). XPS indicates a moderate degree of surface
enrichment in cerium. An EPR study is carried out on the superoxide species formed on the material by
O2 adsorption after outgassing at temperatures up to Tv ) 773 K; this shows that the reduced surface
centers thermally formed on this mixed oxide are similar to those found on pure ceria but are generated
more easily than on the latter, thus evidencing a surface redox reactivity higher than that of the CeO2
single oxide.
Introduction
Cerium oxide is a common promoting component of the
three-way catalysts used for the elimination of toxic
exhaust gases in automobiles.1 One of its most important
beneficial roles is related to the particular redox properties
acquired by the catalysts upon establishment of contacts
between this component and the active phases (usually
precious metals) deposited on it.2 In particular, achievement of a high oxygen storage capacity (OSC), related to
the particular ability of CeO2 to undergo rapid reduction/
oxidation cycles, has been shown to be of major relevance
for the enhancement of the catalyst performance.3 From
a practical point of view, this produces more efficient
catalysts able to work in a broader region around the air/
fuel stoichiometric value and leads to a large enhancement
of the overall catalytic performance in real conditions, in
which oscillations around this stoichiometric point are
produced.4
Recent work has shown that the redox behavior of
cerium oxide can be severely modified by incorporation of
zirconium and formation of mixed Zr-Ce oxides.5-8 Thus,
a large enhancement of the bulk ceria reduction process
has been reported to be produced upon subjecting
CexZr1-xO2 systems to repeated reduction-oxidation cycles
at high temperatures (up to 1200 K). These processes have
been shown to be dependent on the structure of the mixed
oxides and on the composition of the samples, both aspects
being mutually related. Thus, the highest OSC is
* E-mail: [email protected].
(1) Heck, R. M.; Farrauto, R. J. Catalytic Air Pollution Control:
Commercial Technology; Van Nostrand Reinhold: New York 1995.
(2) Trovarelli, A. Catal. Rev.-Sci. Eng. 1996, 38, 439.
(3) Yao, H. C.; Yu Yao, Y. F. J. Catal. 1984, 86, 254.
(4) Harrison, B.; Diwell, A. F.; Hallett, C. Plat. Met. Rev. 1988, 32,
73.
(5) Fornasiero, P.; Di Monte, R.; Ranga Rao, G.; Kaspar, J.; Meriani,
S.; Trovarelli, A.; Graziani, M. J. Catal. 1995, 151, 168.
(6) Fornasiero, P.; Balducci, G.; Di Monte, R.; Kaspar, J.; Sergo, V.;
Gubitosa, G.; Ferrero, A.; Graziani, M. J. Catal. 1996, 164, 173.
(7) Balducci, G.; Kaspar, J.; Fornasiero, P.; Graziani, M.; Saiful Islam,
M.; Gale, J. D. J. Phys Chem. B 1997, 101, 1750.
(8) Trovarelli, A.; Zamar, F.; Llorca, J.; De Leitemburg, C.; Dolcetti,
G.; Kiss, J. T. J. Catal. 1997, 169, 490.
observed5-8 for mixed oxides of composition CexZr1-xO2
(x close to 0.5) showing a pseudocubic phase (phase t′′,
with an X-ray diffraction (XRD) pattern corresponding to
the cubic Fm3m space group but with internal tetragonal
symmetry due to oxygen displacement from the ideal
fluorite lattice sites).6,9,10 On the basis of extended X-ray
absorption fine structure experiments, it has been proposed that the distortion of the oxygen sublattice could
make the oxygen ions more mobile, which would lead to
improved redox properties.11 The presence of a high
concentration of ions with redox properties (i.e., cerium
ions) is proposed to also be relevant in this respect, and
the best balance between this factor and the presence of
phase t′′ is obtained for materials with x > 0.5.8 Another
factor proposed to influence the redox properties of these
systems concerns the textural characteristics of the
starting materials, because it has been reported that
subjecting a Ce0.5Zr0.5O2 sample (prepared by precipitation
following a homogeneous gel route and showing an initial
relatively high surface area) to redox cycling at high
temperature leads to a decrease in the temperature of the
reduction of the solid solution from 900 to 700 K.6 It is
noted, however, that results on mixed oxides prepared by
high-energy mechanical milling similarly show a promotion of bulk reduction, suggesting a relatively low relevance
of the textural properties of the materials.8 Other aspects,
such as the dimensions of the crystallites or the presence
of defects and lattice mismatches, leading to nanoscalesegregated ceria phases, are also proposed to affect
beneficially the redox properties of the systems.8
The characteristics of the materials depend strongly on
the preparation method employed. Thus, high-temperature calcination of a mixture of the oxides produces5 mixed
oxides with very low surface areas; although an important
promotion of bulk reduction has been shown to occur upon
(9) Yashima, M.; Arashi, H.; Kakihana, M.; Yoshimura, M. J. Am.
Ceram. Soc. 1994, 77, 1067.
(10) Yashima, M.; Morimoto, K.; Ishizawa, N.; Yoshimura, M. J. Am.
Ceram. Soc. 1993, 76, 1745.
(11) Vlaic, G.; Fornasiero, P.; Geremia, S.; Kaspar, J.; Graziani, M.
J. Catal. 1997, 168, 386.
10.1021/la981537h CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/04/1999
Characterization of Zr-Ce (1:1) Mixed Oxide
deposition of rhodium on these materials,5 favoring NO
decomposition,12 these low surface materials are in
principle not adequate for catalytic purposes because
dispersion of metals on them is strongly limited. The best
results in this respect are obtained by sol-gel methods,6,13
although in some cases nonmonophasic materials are
produced.6 Another method consists of high-energy mechanical milling of mixtures of ceria and zirconia; this
produces materials which have relatively low surface areas
but show promising redox properties.8 Another approach
for the preparation of these materials, which might lead
to the production of ultrafine powders,14 is the coprecipitation within microemulsions containing both Ce and Zr.
Recent results15 have shown the formation of high surface
area ceria-zirconia samples (in the whole series Ce1-xZrxO2, 0 < x < 0.9) by this method. Thus, uncalcined
materials (rather amorphous as revealed by XRD) display
values of SBET up to 285 m2 g-1, which in the case of a
Ce0.2Zr0.8O2 sample (only one examined in detail in that
work) decreases upon calcination at 773 K to ca. 80 m2
g-1, and the material is then well-crystallized. In this work,
the characteristics of a mixed oxide of zirconium and
cerium prepared by the microemulsion method are
examined, focusing mainly on structural characterization
aspects.
Experimental Section
Preparation. The Zr-Ce mixed oxide was prepared using as
precursor salts zirconyl nitrate (Aldrich, purity ) 99.99%) and
cerium (III) nitrate hexahydrate (Aldrich, purity ) 99.9%). An
aqueous solution containing the same amount (0.25 M) of both
Zr and Ce was prepared. An inverse microemulsion (water in
organic) was prepared by mixing while stirring 8 g of this aqueous
solution with 58 g of heptane, 15 g of hexanol, and 19 g of a
surfactant (Triton X-100, supplied by Aldrich). This emulsion
was mixed while stirring with another emulsion having similar
characteristics, except that the aqueous solution now contained
1.5 M tetramethylammonium hydroxide pentahydrate (Aldrich).
Immediately after both emulsions were mixed, a brownish
turbidity appeared, denoting reaction (precipitation in the
dispersed aqueous phase); the fluid was stirred for 24 h, after
which the resulting suspension was centrifuged and decanted;
the remaining solid was washed with methanol. Then, after
centrifuging and decanting again, the solid was first dried for a
short time at room temperature then at 383 K for 24 h, and
finally it was calcined in air at 773 K for 2 h (after a slow increase
of temperature at a rate of 2 K min-1). The resultant specimen
was orange-yellow; according to chemical analysis, performed
by inductively coupled plasma atomic emission spectrometry
(ICP-AES) in a JY model 70+ apparatus following sample
dissolution by acid digestion methods,16 the material has a Ce/Zr
) 0.52:0.48 cation ratio. Its BET surface area, determined from
N2 adsorption isotherms in a Micromeritics 2100 automatic
apparatus, amounts to 96 m2 g-1.
A commercial CeO2 sample (supplied by Rhône-Poulenc)
showing a surface area (SBET ) 109 m2 g-1) comparable to that
of the Zr-Ce mixed oxide sample is used as a reference in some
of the experiments.
Techniques. Powder XRD patterns were recorded on a
Siemens D-500 diffractometer using nickel-filtered Cu KR
radiation operating at 40 kV and 25 mA and with a 0.025° step
size. Peak simulation was achieved with a conventional fitting
(12) Ranga Rao, G.; Kaspar, J.; Meriani, S.; Di Monte, R.; Graziani,
M. Catal. Lett. 1994, 24, 107.
(13) Sun, Y.; Sermon, P. A. J. Mater. Chem. 1996, 6, 1025.
(14) (a) Boutonnet, M.; Kizling, J.; Stenius, P.; Maire, G. Colloid
Surf. 1982, 5, 209. (b) Gao, L.; Qiao, H. C.; Qiu, H. B.; Yan, D. S. J. Eur.
Ceram. Soc. 1996, 16, 437.
(15) Masui, T.; Fujiwara, K.; Peng, Y.; Sakata, T.; Machida, K.-I.;
Mori, H.; Adachi, G.-Y. J. All. Comput. 1998, 269, 116
(16) Larrea, M. T.; Gómez-Pinilla, I.; Fariñas, J. C. J. Anal. Atom.
Spectrosc. 1997, 12, 1323.
Langmuir, Vol. 15, No. 14, 1999 4797
program,17 using Pearson VII functions and allowing for separation of KR1 and KR2 contributions.
Photoelectron spectra were acquired with a VG ESCALAB
200R spectrometer equipped with a hemispherical electron
analyzer and a 120 W Mg KR X-ray source. A PDP 11/04 computer
from Digital Equipment Co. was used for collecting and analyzing
the spectra. The powder sample was pressed into a small
aluminum cylinder and then mounted on a sample rod placed in
an in situ pretreatment chamber where it was outgassed at room
temperature for 1 h prior to being moved into the analysis
chamber. The pressure in the ion-pumped analysis chamber was
maintained below 3 × 10-9 Torr (1 Torr ) 133.33 N/m2) during
data acquisition. Spectra in the relevant energy windows were
collected for 20-90 min, depending on the peak intensities, at
a pass energy of 20 eV, which is typical of high-resolution
conditions. The intensities were estimated by calculating the
integral of each peak after subtraction of the “S-shaped”
background (Shirley-type baseline) and taking into account
experimental factors affecting quantitative data, viz., ionization
cross section, electron escape depth, and instrumental sensitivity.
All binding energies (BE) were referenced to the adventitious C
1s line at 284.9 eV. This reference gave BE values within an
accuracy of (0.2 eV.
Raman spectra were obtained at room temperature with a
Bruker Fourier transform Raman instrument using the 1064
nm exciting line, at a resolution of 4 cm-1, and taking 100 scans
for every spectrum.
Electron paramagnetic resonance (EPR) spectra at the X band
frequency (∼9.5 GHz) were recorded at 77 K with a Bruker ER
200D spectrometer calibrated with 2,2-diphenyl-1-picrylhydrazyl
(g ) 2.0036). Portions of sample were placed inside a quartz
probe cell provided with greaseless stopcocks, where they could
be submitted to outgassing treatments in a conventional highvacuum line (residual pressure ) ca. 6 × 10-3 N m-2) or to O2
adsorption experiments; in these, a sequence was followed
consisting of the admission of doses of ca. 50 µmol of O2 per gram
of sample at 77 K, followed by prolonged outgassing at 77 K, and
further warming in the closed cell at 298 K for 30 min, prior to
final recooling to 77 K and obtention of the spectrum. The amounts
of unpaired spins detected in the spectra were quantified by
comparison of their doubly integrated intensity with that of a
copper sulfate standard.
Samples for transmission electron microscopy (TEM) were
prepared by crushing in an agate mortar, dispersing in isobutanol,
and depositing on perforated carbon films supported on copper
grids. TEM data were obtained on a JEOL 2000 FX II system
(with 3.1 Å point resolution) equipped with a LINK probe for
energy-dispersive spectroscopy (EDS) analysis. Electron diffraction diagrams taken with the same system were analyzed in
detail by digitizing the diffraction images with a scanner and
carrying out radial (angle-integrated) densitometric analysis of
the circular pattern with a homemade computer program.
Results
Data from Structural Techniques. The powder XRD
pattern of the ceria-zirconia sample is shown in Figure 1.
It agrees well with the presence of a cubic fluorite-type
phase; in the following, all Miller indices are referenced
to the unit cell in this structure. The pattern shows
resolved peaks with relatively large line width, ascribable
to the presence of small crystallites formed after calcination at 773 K. The average crystallite size (L) was
estimated from the X-ray line widths of the resolved peaks
using the Scherrer equation β ) Kλ/L cos θ, where K is
a constant taken as 0.94,18 λ is the X-ray wavelength
(λ ) 1.5406 Å), and β is the line width (fwhm in radians).
For this estimation, only the most intense (111) and (220)
lines at ca. 2θ ) 29° and 48.5° are used, taking the fwhm
values obtained for the Cu KR1 component after simulation
of the corresponding peaks. This gives for L a value of 5.3
(17) Krumm, S. Winfit!, version 1.2; http://www.geol.uni-erlangen.de.
(18) Yao, M. H.; Baird, R. J.; Kunz, F. W.; Hoost, T. E. J. Catal. 1997,
166, 67.
4798 Langmuir, Vol. 15, No. 14, 1999
Martı́nez-Arias et al.
Figure 1. X-ray diffraction pattern of the ceria-zirconia sample.
( 0.3 nm. Estimation of the surface area which would
correspond to this figure, considering a single-crystallite
spherical particle model, yields a value somewhat higher
(about 2 times) than that obtained by the BET method.
Thus, a moderate degree of crystallite aggregation is
present in this specimen.
The lattice parameter obtained from such computer
simulations [examining the (111) and (220) peaks] is 5.280
Å, its error being estimated to be ca. 0.02 Å (10% of the
fwhm). The accuracy of the procedure was checked by
determining the cell parameter of well-crystallized, pure
CeO2, in the same way, which yielded a lattice parameter
of 5.412 Å, in good agreement with the literature value
of 5.411 Å.18
Although the XRD data agree in principle with a cubic
structure, it is worth noting that previous literature
reports have shown that a metastable tetragonal phase
(phase t′), with a structure corresponding to space group
P42/nmc, occurs at room temperature for Ce0.5Zr0.5O2 and
nearby compositions; such a phase is found, for example,
in CexZr1-xO2 samples (with x between 0.35 and 0.65)
prepared by high-temperature calcination of the mixed
oxides, followed by quenching to room temperature and
annealing in air.10 According to available literature data
on phase t′,10,19 for compositions near x ) 0.5, the axial
ratio c/a of this phase would be close to 1.01, leading to
splittings of ca. 0.20° and 0.37° for the (220) and (311)
XRD peaks, respectively; these are rather lower than the
line width measured for these reflections on the sample
(fwhm ) ca. 2.0°). Thus, these measurements cannot
distinguish whether some degree of tetragonality, at levels
such as indicated by the literature data, is present here.
In fact, similar residual errors are obtained in simulated
fits of the experimental (311) peak to either one or two
peaks (in the second case, at positions corresponding to
those of phase t′, using the lattice parameters given in
Fierro et al.19), fixing in both cases the position for the
overlapped reflection (222). A similar conclusion (inability
to discern such small tetragonality within the observed
line widths) is reached if a similar analysis is carried out
on the (220) peak. In any case, XRD results do allow the
presence of segregated pure ceria and (tetragonal) zirconia
phases in substantial amounts to be ruled out, as these
would produce new peaks at locations clearly distinct from
those observed in Figure 1.
Transmission electron micrographs taken for the sample
(a representative image is given in Figure 2) show the
presence of a polycrystalline phase with relatively round
particles (some of these showing images with straight line
edges, which imply the presence of flat crystal faces,
however) clustered in agglomerates typically 50-100 nm
(19) Meriani, S.; Spinolo, G. Powder Diffr. 1987, 2, 255.
Figure 2. Bright field TEM image of the ceria-zirconia sample.
in size. These particles appear with narrowly distributed
sizes, around 5.1 ( 0.5 nm, thus corresponding roughly
to the crystallite sizes detected in XRD. Specific preferential crystal orientations were not observed. An analysis
with X-ray energy-dispersive spectroscopy shows an
average composition of Ce0.57Zr0.43O2, thus indicating, in
line with the ICP-AES results, a Ce mole fraction
somewhat above 0.50. The difference between the results
from both techniques is hardly significant, taking into
account the limited accuracy of the X-ray EDS analysis.
More important is the observation that no relevant
variations in the Zr/Ce ratio were detected across the
specimen (within the resolving power of the measurement,
estimated to be around 100 nm). Thus, no chemical
composition heterogeneity is detected in this experiment.
Selected area electron diffraction patterns were also
taken from this material (Figure 3); they show rings
(somewhat grainy, indicating incomplete sampling of all
possible orientations) that can be indexed according to a
cubic fluorite structure with a lattice parameter of 5.3 (
0.5 Å, in relatively good agreement with XRD results. To
better analyze these data, the diffraction images were
scanned and subjected to radial densitometry. The resulting curves, after subtraction of an approximate baseline
(Figure 4), displayed clearly resolved peaks up to scattering
angles corresponding to spacings of 0.7 Å. All of the peaks
agreed with the positions expected for the fluorite
structure, although again their line widths are too large
to allow the presence of some tetragonal character to be
excluded, at least at levels of ca. 1%. Although accurate
values of the lattice parameter cannot be obtained from
these data, the good resolution displayed by the high
deflection angle (low spacing) peaks indicates a high
homogeneity in lattice constants and, therefore, in the
composition of the crystalline material.
Characterization of Zr-Ce (1:1) Mixed Oxide
Langmuir, Vol. 15, No. 14, 1999 4799
Figure 5. Raman spectrum of the ceria-zirconia sample.
Figure 3. Electron diffraction pattern obtained from the ceriazirconia sample.
Figure 4. Radial (angle-integrated) densitometry pattern
obtained from the electron diffraction rings shown in Figure 3.
Spectroscopic Results. The Raman spectrum of the
sample is shown in Figure 5. It exhibits broad bands at
ca. 470 (with shoulders at ca. 550 and 640 cm-1), 305, and
120 cm-1, superimposed on a relatively strong background.
The EPR spectrum of the sample, outgassed at room
temperature, displays only weak, sharp signals in the g
< ge (ge ) 2.0023) range that are not affected by adsorption
of O2 at 77 K; they are similar to those found in impure
or doped ceria and can be ascribed to bulk defects,20 because
of their small intensity (roughly 0.04% of the whole amount
Figure 6. EPR spectra (at 77 K) after O2 adsorption on samples
outgassed at different temperatures Tv: (a) and (b), ceriazirconia sample; (c) and (d), pure CeO2 (from Soria et al.26); (a)
and (c), Tv ) 473 K; (b) and (d), Tv ) 573 K.
of cerium and zirconium in the sample), or to Zr3+ ions;21
they will not be discussed further here.
Oxygen adsorption at 298 K on the sample previously
evacuated at Tv ) 373-773 K leads to the appearance in
the EPR spectra of several signals which can be ascribed
to different superoxide species (Figure 6a,b). Their characteristics and assignment are summarized in Table 1,
and the evolution of the overall integrated intensity of the
spectrum in this field range as Tv is increased is shown
in Figure 7. Apart from a signal OZ, which has one value
(gx) close to ge (the usual situation found for metal cationbonded O2-) and which, being very similar to the one
formed by O2 adsorption on pure ZrO2,22,23 must correspond
to an O2- species bonded to Zr4+ ions, the main components
of the spectrum are signals having all of their g values
clearly above ge. The displacement of gx above ge is a
characteristic of O2- radicals bonded to Ce ions, being
interpreted as due to covalent mixing of O2- π* orbitals
with 4f orbitals of Ce.24,25 The main differences between
(20) Fierro, J. L. G.; Soria, J.; Sanz, J.; Rojo, J. M. J. Solid State
Chem. 1987, 66, 154.
(21) Liu, H.; Feng, L.; Zhang, X.; Xue, Q. J. Phys. Chem. 1995, 99,
332.
(22) Martı́nez-Arias, A.; Fernández-Garcı́a, M.; Belver, C.; Baranda,
J.; Conesa, J. C.; Soria, J., in preparation.
(23) Che, M.; Tench, A. J. Adv. Catal. 1983, 32, 1.
(24) Che, M.; Kibblewhite, J. F. J.; Tench, A. J.; Dufaux, M.; Naccache,
C. J. Chem. Soc., Faraday Trans. 1 1973, 69, 857.
(25) Martı́nez-Arias, A.; Coronado, J. M.; Conesa, J. C.; Soria, J. In
Rare Earths; Sáez Puche, R., Caro, P., Eds.; Editorial Complutense:
Madrid, 1997; p 299.
4800 Langmuir, Vol. 15, No. 14, 1999
Martı́nez-Arias et al.
Table 1. EPR Parameters and Characteristics of the Superoxide Signals Obtained upon Oxygen Adsorption on Samples
Outgassed at Different Temperatures Tv (see text for details)a
formation conditions (K)
signal
parameters
OC1
gz ) 2.037-2.034,
gx ≈ gy ) 2.011
gz ) 2.042-2.037,
gy ) 2.010,
gx ) 2.008
OC2
OZ
a
(Zr, Ce)O2
gz ) 2.037,
gy ) 2.009,
gx ) 2.002
CeO2
Tv g 373
Tv g 373
Tv g 473
Tv g 573
(Zr, Ce)O2
Tv g 473
ZrO2
Tv g 473
assignment
ref
O2--Ce4+
(isolated vacancies)
O2--Ce4+
(associated vacancies)
26
O2--Zr4+
22
26
Axes assignment follows conventions of previous works.23-26
Figure 7. Dependence upon Tv of the overall integrated
intensities measured for the EPR signals obtained upon O2
adsorption on the ceria-zirconia sample.
Table 2. XPS Data Obtained for the Ceria-Zirconia
Samplea
Ce 3d5/2
Zr 3d5/2
O 1s
u′′′ (%)
(Ce/Zr)at
882.8
182.2
530.0
7
2.6
a
Ce 3d5/2, Zr 3d5/2 and O 1s binding energies (eV), intensity of
the Ce4+ satellite (u′′′) at 915.2 eV (expressed as percentage of total
Ce peaks intensity), and Ce/Zr atomic ratio.
the Zr-Ce mixed oxide sample and a pure CeO2 sample
(Figure 6c,d) concern (a) the substantially higher overall
intensity of oxygen radicals shown by the first one in the
whole Tv range examined and (b) formation of signal OC2
at a lower Tv in the Zr-Ce mixed oxide (Tv ) 473 K) than
that in pure CeO2 (Tv ) 573 K). This can be detected, as
shown in Figure 6, from the appearance of a shoulder at
g ) 2.008, representative of the presence of signal OC2,
at the mentioned Tv values.
Signals OC1 and OC2 have been observed previously
upon O2 adsorption on outgassed pure CeO2;26 they are
due to O2- ions that are stabilized at surface sites which
can be described respectively as isolated and associated
surface-anion vacancies.
XPS results are summarized in Table 2. It must be noted
that the complete analysis of the Ce features is made
difficult due to the overlap with the ZrLMM Auger peak,
appearing with relatively large line width (ca. 6.5 eV) and
intensity at Ek in ) 324 eV. This decreases the accuracy
with which the overall intensity of the Ce 3d peaks can
be estimated. Here, to perform the surface concentration
determinations, the Zr Auger line was approximated by
a Lorentzian shape and its integrated intensity was
subtracted from the total intensity to extract the Ce 3d
contribution. The Ce/Zr atomic ratio determined from this
intensity and that of the Zr 3d XPS peak, although affected
by some uncertainty due to the above-mentioned reason,
(26) Soria, J.; Martı́nez-Arias, A.; Conesa, J. C. J. Chem. Soc., Faraday
Trans. 1995, 91, 1669.
indicates that the surface of the specimen is enriched in
Ce. It is worth mentioning that these results have been
reproduced using another spectrometer. Cerium enrichment at the surface has already been seen in Zr-Ce oxide
systems prepared by sol-gel methods.13
The Ce binding energy obtained for the sample is typical
of fully oxidized materials.13,27,28 On the other hand, the
relatively small intensity of the peak appearing in the Ce
3d manifold at BE ) 915.2 eV (usually called u′′′), which
is observed only in pure Ce4+ compounds and accounts for
14% of the total integrated intensity of the Ce lines in the
case of CeO2,29 may suggest that the material studied
contains Ce3+ in some amount; it must be noted, however,
that arguments have been put forward against the use of
the u′′′ peak intensity for the quantification of the degree
of Ce reduction in mixed oxides.30
Discussion
Structure of the Crystalline Phase. Analysis of the
resolved peaks appearing in the X-ray diffractogram
suggests the presence of a single cubic phase, as expected
for the fluorite structure (Fm3m space group). As mentioned above, these data do not indicate a deviation from
the cubic character, as would occur if a t′-type tetragonal
phase were present. However, it must be recalled that the
structure of the “cubic” phase detected for Ce-rich (Ce, Zr)
mixed oxides has been claimed to have tetragonal symmetry at the atomic level, even if displaying equal values
for the a and c axes;6,9,10 i.e., one would have a tetragonal
form without tetragonality (c/a ) 1), also sometimes called
“pseudocubic”. Such a phase, usually designated t′′, has
been reported to be the most stable phase in the CexZr1-xO2
system only for x > 0.65, whereas for lower x values (in
particular, in the range 0.3 e x e 0.65) the abovementioned t′ phase would occur instead. The internal
tetragonal character of that phase t′′, assumed to correspond also to space group P42/nmc, would arise, as
happens in the t′ phase, from displacement of the oxygen
atoms along the c axis, away from the special positions of
the cubic Fm3m group. In such situations, with c ) a, i.e.,
without any peak splitting observable, this departure from
the internal cubic symmetry is rather difficult to distinguish using XRD, because of the small atomic scattering
factor of oxygen in comparison to those of Zr and Ce, which
dominate the diffractograms.10
Better assessment of the phase(s) present can be
achieved by complementing the XRD data with those
(27) Shyu, J. Z.; Otto, K.; Watkins, W. L. H.; Graham, G. W.; Belitz,
R. K.; Gandhi, H. S. J. Catal. 1988, 114, 23.
(28) Fernández-Garcı́a, M.; Gómez-Rebollo, E.; Guerrero Ruiz, A.;
Conesa, J. C.; Soria, J. J. Catal. 1997, 172, 146.
(29) Shyu, J. Z.; Weber, W. H.; Gandhi, H. S. J. Phys. Chem. 1988,
92, 4964.
(30) Schmitz, P. J.; Usman, R. K.; Peters, C. R.; Graham, C. W.;
McCabe, R. W. Appl. Surf. Sci. 1993, 72, 181.
Characterization of Zr-Ce (1:1) Mixed Oxide
obtained by Raman spectroscopy. In the case of a pure
cubic fluorite structure, only one band would be Ramanactive; pure CeO2, for example, gives one band at around
465 cm-1, whereas ZrO2 stabilized in the cubic fluorite
phase by doping gives one band at ca. 490 cm-1.6 On the
other hand, phases showing tetragonal symmetry in space
group P42/nmc would show six Raman-active modes; this
is the case of Zr-Ce mixed oxides showing phase t′ 9,11 or
of Y2O3-stabilized zirconia.31 However, in the case of ZrCe mixed oxides showing phase t′′, experimental work
suggests that some of the modes observable in phase t′
become degenerate so that only four bands are detected.9,11
The fact that only four bands are observed in the Raman
spectrum of the sample (apart from the weak shoulder at
ca. 640 cm-1, which is ascribed to a localized substitutional
defect vibration)9 thus agrees with the presence of mainly
phase t′′. Actually, this Raman spectrum is rather similar
to those reported by Yashima et al.9 for mixed oxides
having Ce mole fractions between 0.7 and 0.8 and
attributed to phase t′′ [a result which was supported in
that work by XRD data that had sufficient resolution in
the (400) peak to distinguish between t′ and t′′ phases] or
to those results obtained by Fornasiero et al.6 for oxides
with compositions similar to that of the material examined
here and also attributed to that phase.
Thus, a combination of Raman and XRD data indicates
that the crystalline material detected here corresponds
mainly to phase t′′. It is noteworthy that here this phase
is found for compositions close to a cation ratio Zr/Ce )
1:1, for which the literature indicates that phase t′ is
preferentially formed. The stabilization of the thermodynamically unfavored phase t′′ may be attributed in the
present case to the formation of extremely small particles,
because it has been observed that below certain critical
size (in the case of ZrO2 a value of 17 nm has been
reported)32 the cubic phase (on the basis of XRD observations, i.e., probably phase t′′) becomes stabilized.6,14,19,33
Composition and Homogeneity of the Material.
Concerning the (cubic) lattice parameter value measured
for our sample, it should be noted that it is somewhat
lower than what would be expected for the sample
composition (as determined by chemical analysis) in light
of either the values extrapolated from JCPDS files for
Zr-Ce mixed oxides showing the cubic phase18 (up to 0.4
Zr mole fraction) or those given by Yashima et al.10 for
other well-oxidized materials of this kind (annealed under
air at 627 °C). Taking into account these two sets of
literature data, the crystalline material in the sample
would have a composition with atomic ratio Zr/Ce ) 52:48
or 55:45, respectively, rather than the value of 48:52
determined by chemical analysis.
This discrepancy should in no case be attributed to the
presence in this sample of (EPR-detected) lattice defects,
because their quantity is too small to justify any significant
crystallographic effect. Thus, the sample should formally
be regarded as a fully oxidized, i.e., “undefective”, type of
oxide,10 as expected when taking into account that the
final step of its preparation is a calcination in air at 773
K.
Correlation of the results obtained by the different
techniques allows one to establish a structural model for
the sample. Thus, crystallographic data and Raman
spectroscopy show that the largest fraction of the ions is
forming a Zr-Ce mixed oxide crystalline phase t′′, which
(31) Kim, D.-J.; Jung, H.-J.; Yang, I.-S. J. Am. Ceram. Soc. 1993, 76,
2106.
(32) Chaterjee, A.; Pradhan, S. K.; Datta, A.; De, M.; Chakravorty,
D. J. Mater. Res. 1994, 9, 263.
(33) Keramidas, V. G.; White, W. B. J. Am. Ceram. Soc. 1974, 57, 22.
Langmuir, Vol. 15, No. 14, 1999 4801
is slightly richer in Zr. On the other hand, XPS data show
a (Ce/Zr)at ratio higher than that determined by chemical
analysis. Certainly that higher composition cannot correspond to a significant fraction of crystallized material;
this would have been detected in the XRD and electron
diffraction profiles and perhaps in the Raman spectrum
as well. Most likely, this reflects the presence of a Ce-rich
layer at the external surface of the particle agglomerates
(evidenced by TEM) and is not representative of particles
located inside the agglomerates, which probably provide
the largest contribution to the sample surface.
Thus, some degree of heterogeneity appears to be
present in the sample, arising probably from the preparation process. The results suggest that a slightly preferential zirconium precipitation occurs during preparation,
leading after calcination15 to a crystalline phase somewhat
enriched in Zr. This might be related to the lower charge/
radius ratio of Ce3+ with respect to Zr4+ cations, which
would favor precipitation of the latter. Then, at the final
stages of the precipitation process, some Zr depletion in
the aqueous phase would appear, leading to certain cerium
enrichment in the last fractions which precipitate and
form the external part of the crystallite agglomerates,
according to XPS. It is worth noting that Ce-enrichment
at the sample surface has been shown to occur for Zr-Ce
mixed oxide samples prepared by sol-gel methods.13
Surface Characteristics. With these observations in
mind, it is important to discuss whether the surface of the
material is actually composed of a thin layer of cerium
dioxide, which would lead to a masking of all influence of
the zirconium component on the surface reactivity properties. The EPR results obtained upon O2 adsorption on
the outgassed sample shed some light on this issue.
Formation of superoxide species by oxygen interaction
with this type of sample can be assumed to follow the
process22-26 (described using formal charges)
O2 + Ce3+(Zr3+) f O2- - Ce4+(Zr4+)
Thus, evaluation of the amount of superoxide species
generated by this interaction (Figure 7) gives an indication
of the degree of surface reduction attained after outgassing
(at least for moderate reduction levels in which further
electron transfer to give O22- or other EPR-silent species
will be negligible),25 and the analysis of the characteristics
of the species formed yields information on the nature of
the adsorption centers present in the samples. At first
sight, the superoxide species formed here appear more or
less identical to those detected in similar experiments
performed on pure CeO2 (for signal OC-type)24 or ZrO2
(for signal OZ).22,23 However, close examination of the
results indicates that the amount of radicals formed for
low or moderate pre-outgassing temperatures is distinctly
higher than that found on those single oxides. Thus, the
signal OC2, indicating the presence of associated anion
vacancies at the surface of the cerium-containing phase,
already appears for Tv ) 473 K, whereas on pure ceria it
first appears for Tv ) 573 K (and with lower intensity).
This indicates that the Zr-Ce mixed oxide sample shows
a distinctly higher surface reducibility.
Also significant is the plateau observed in the intensity
of superoxide radicals for Tv g 573 K (Figure 7), which
contrasts with results on pure CeO2,22,26 in which a
monotonic increase in the intensity of superoxide species
with Tv is observed up to the limit Tv used. One possible
explanation could be that, assuming that a more strongly
reduced surface is present for Tv > 573 K, a more advanced
degree of electron transfer to a part of the adsorbed oxygen
occurs, leading to the formation of O22- or other diamag-
4802 Langmuir, Vol. 15, No. 14, 1999
netic species (as is found for more strongly reduced ceriacontaining catalysts),34 and producing at the equilibrium
a similar amount of superoxide radicals as that for Tv )
573 K. In the latter case, however, such transfer of a second
electron, which would lead to the formation of diamagnetic
species, seems to require a certain activation energy and
completion only at temperatures significantly higher than
77 K, as indicated by the observation that a significant
decrease in the amount of superoxide radicals occurs when,
following O2 adsorption at Ta ) 77 K, the sample is warmed
to room temperature. Such signal decrease upon warming
was not observed here for the mixed oxide, so that this
type of explanation does not seem probable. Taking into
account the known properties of this mixed oxide material,
the apparent saturation in the amount of reduced surface
centers produced for Tv g 573 K (on the basis of evaluation
of the amount of superoxide radicals generated upon
oxygen adsorption; see Figure 7) may be explained rather
as due to the onset of anion vacancy equilibration between
the surface and the bulk of the material, which is made
possible by the higher ion mobility in this system; indeed,
previous TPR experiments on Zr-Ce mixed oxides have
shown that the onset of bulk reduction occurs in the same
range of temperatures.6,34
It is also worth noting that signal OZ, due to O2- species
coordinated to Zr cations, appears in the mixed oxide with
substantially higher intensity than that observed, under
the same experimental conditions, for a microemulsionprepared pure ZrO2 sample.22 This indicates that the Zr
ions present at the surface of the (Zr, Ce)O2 sample are
also under the influence of cerium entities. The precise
mechanism of this influence is however not clear. It could
be that the Zr ions are more easily reduced because of the
proximity of Ce ions in the mixed oxide (e.g., because the
oxide ions bonded to Zr become more labile). Other possible
explanations would be that the superoxide radicals
actually form on cerium (by oxidizing a Ce3+ ion via the
path given above) and then they are transferred to a
neighboring Zr ion, i.e., a spillover-type mechanism, or
that electron transfers from nonexposed, easily reducible
Ce3+ ions toward the surface Zr4+-O2 complexes are
produced; similar effects have been observed in other
samples combining ions with different reducibilities, such
as in the case of a Co/MgO system.35 Further work is
required to clarify this point.
Thus, the surface of (Zr, Ce)O2 does not behave as just
the superposition of the properties of CeO2 and ZrO2
surfaces. Rather, the behavior observed by EPR is likely
to be that of the surface of the mixed (Ce,Zr) oxide itself
(possibly with influence from a certain degree of enrichment in Ce, as indicated by XPS). The material indeed
(34) Martı́nez-Arias, A.; Fernández-Garcı́a, M.; Conesa, J. C.; Soria,
J. J. Catal. 1999, 182, 367.
(35) Giamello, E.; Sojka, Z.; Che, M.; Zecchina, A. J. Phys. Chem.
1986, 90, 6084.
Martı́nez-Arias et al.
appears more active than the simple parent oxides in the
formation of reduced-surface centers, a feature already
known to be a characteristic for the bulk of this mixed
oxide but which is here shown to also specifically affect
the surface. Overall, the best description of the surface of
this material therefore corresponds to a mixed (Ce, Zr)
oxide (with average Ce/Zr ratio higher than that of the
chemical composition, according to XPS), having both
cations exposed and where O2--stabilizing vacancies are
formed more easily than on either of the pure oxides.
Conclusions
The inverse microemulsion method is shown to be wellsuited for preparing (Zr, Ce) mixed oxides with high surface
area (approaching 100 m2 g-1) and cation stoichiometries
close to 1:1; these are formed mainly by regular crystallites
with well-defined composition and narrow size distribution, having the t′′ (pseudocubic) phase structure already
reported in the literature. The stabilization of this latter
phase, rather than of the tetragonally distorted t′ phase
(the thermodynamically preferred one for this composition), is probably due to the small crystallite size. EPR
data show that this material displays higher surface redox
reactivity than either of the parent single oxides prepared
by the same method.
The moderate degree of compositional heterogeneity of
the specimen obtained in this way (surface enrichment in
Ce) is ascribable to the different relative speeds of the
precipitation of the Zr and Ce ingredients inside the
microemulsion droplets. The degree of this heterogeneity,
and of any surface enrichment in one or another cation,
is likely to depend, among other factors, on the nature of
the precursor salts; therefore, one may expect to be able
to influence this aspect and, correspondingly, the surface
chemical properties of the system by changing the
precursor salts. Work is in progress to examine this point.
Acknowledgment. Thanks are given to the CAICYT
(Project No. MAT 97-0696-C02-01) and to the Comunidad
de Madrid (Project No. 06M/085/96) for financial help
which allowed us to carry out the research reported here.
Prof. J. L. G. Fierro and Mr. E. Pardo are greatly
acknowledged for obtaining the XPS data and Dr. R. X.
Valenzuela is acknowledged for obtaining the Raman
spectra. Thanks are also due to Prof. G. Munuera and the
XPS technical staff at the Universidad de Sevilla for XPS
reanalysis of the sample. Dr. C. Prieto is acknowledged
for the use of the X-ray diffractometer. A.M.-A. and M.F.G. wish to thank, respectively, the Comunidad Autónoma
de Madrid and the CSIC for, respectively, a postdoctoral
grant and a postdoctoral contract, under which this work
has been carried out.
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