Mesostructured CeO2 and Pd/CeO2 nanophases: Templated synthesis, crystalline structure and catalytic properties

Article abstract of DOI:10.1016/j.molcata.2005.04.056

This work reports a ceria solid and Pd/ceria catalyst prepared through a surfactant-templated synthesis route used for simultaneous abasement of NO and CO emissions. The surface features, textural properties and crystalline structure of ceria and Pd/ceria catalyst were studied by means of thermogravimetric analysis (TGA), N₂ physisorption isotherms and in situ Fourier transform infrared (FT-IR) spectroscopy, high resolution electron transmission microscopy (TEM) and X-ray diffraction (XRD) techniques. In the calcination procedure, part of the adsorbed water on the surface of the solid was derived into unidentate and bidentate hydroxyls associated with surface cationic ions of ceria. The surfactant cations were strongly interacted with the solid during the preparation, which induces defects formation in the crystalline structure of the annealed ceria. The retained surfactant in the solid could be combusted to yield CO₂, water and organic molecules with a small amount of coke-like deposits. The resultant ceria showed mesoporous texture and cubic phase containing lattice defects in the crystalline structure. The Pd/CeO₂ catalyst was very active for NO reduction via CO with a high selectivity to N₂. A 100% NO conversion with a selectivity to 100% N₂ was achieved over the Pd/ceria catalyst at a reaction temperature of 300 °C. The catalytic activity and selectivity of this catalyst are much superior to the catalysts of Pt or Rh supported on TiO₂, Al₂O₃, TiO ₂-Al₂O₃ and ZrO₂-Al ₂O₃ prepared by a sol-gel method. A possible reaction mechanism of NO reduction by CO over the Pd/CeO₂ catalyst was discussed.

Full text of DOI:10.1016/j.molcata.2005.04.056

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Journal of Molecular Catalysis A: Chemical 237 (2005) 182–190
Mesostructured CeO2 and Pd/CeO2 nanophases: Templated synthesis,
crystalline structure and catalytic properties
J.H. Zhanga, Y.Q. Yanga, J.M. Shena, J.A. Wangb,∗
a
Chemistry and Chemical Engineering College, Xinyang Normal University, 464000 Xinyang, Henan, PR China
Laboratory of Catalysis and Materials, Superior School of Chemical Engineering and Industrial Extractives,
b
National Polytechnic Institute, Col. Zacatenco, C.P. 07738 Mexico City, Mexico
Received 31 March 2005; accepted 20 April 2005
Available online 6 June 2005
Abstract
This work reports a ceria solid and Pd/ceria catalyst prepared through a surfactant-templated synthesis route used for simultaneous abasement
of NO and CO emissions. The surface features, textural properties and crystalline structure of ceria and Pd/ceria catalyst were studied by means
of thermogravimetric analysis (TGA), N2 physisorption isotherms and in situ Fourier transform infrared (FT-IR) spectroscopy, high resolution
electron transmission microscopy (TEM) and X-ray diffraction (XRD) techniques. In the calcination procedure, part of the adsorbed water
on the surface of the solid was derived into unidentate and bidentate hydroxyls associated with surface cationic ions of ceria. The surfactant
cations were strongly interacted with the solid during the preparation, which induces defects formation in the crystalline structure of the
annealed ceria. The retained surfactant in the solid could be combusted to yield CO2, water and organic molecules with a small amount of
coke-like deposits. The resultant ceria showed mesoporous texture and cubic phase containing lattice defects in the crystalline structure. The
Pd/CeO2 catalyst was very active for NO reduction via CO with a high selectivity to N2. A 100% NO conversion with a selectivity to 100%
N2 was achieved over the Pd/ceria catalyst at a reaction temperature of 300 ◦C. The catalytic activity and selectivity of this catalyst are much
superior to the catalysts of Pt or Rh supported on TiO2, Al2O3, TiO2–Al2O3 and ZrO2–Al2O3 prepared by a sol–gel method. A possible
reaction mechanism of NO reduction by CO over the Pd/CeO2 catalyst was discussed.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Surfactant-templated synthesis; Pd/ceria; Catalyst; NO reduction; CO oxidation
1. Introduction
surface area and controllable porosity, these mesostructured
metal oxides are very attractive for catalysis. Among
these oxides, ceria is particularly interest in a variety of
catalytic processes [9]. For instance, ceria has been used as
catalyst or support in the selective oxidation, fine chemicals
synthesis, methane steam reforming, phenol hydrogenation
at atmospheric pressure and solid oxide fuel cell applications
[10–14].
One of the important applications of ceria is the use in the
area of environmental catalysis, particularly in the design of
the new generation of three-way catalysts for simultaneously
controlling NO, CO and hydrocarbon emissions in oxygen-
rich conditions. This is because ceria has large oxygen
storage capacity and the ability of fast transferring bulk
oxygen to its surface [15,16], the former may uptake oxygen
A novel synthesis approach of mesoporous Si-MCM-41
materials by using organic molecule as synthetic template
reported by Mobil scientists, opens new possibilities in the
design of innovative materials with potential applications in
a variety of areas [1]. By using similar or modified synthesis
route, a number of other metal oxides with mesoporous
structures were successfully obtained, these include titania
[2], zirconia [3], niobia [4], ceria [5], alumina [6], tin oxide
[7] and manganese oxide [8]. Since the enhancement of
∗
Corresponding author. Tel.: +52 55 57206000x55124;
fax: +52 55 55862728.
E-mail address: jwang@ipn.mx (J.A. Wang).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.04.056

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183
under fuel-lean conditions and the latter may release oxygen
under reducing conditions, those allow a ceria-containing
catalyst to efficiently work in a relatively wide window of
oxygen partial pressure. In addition, ceria may also improve
metal dispersion on it and promotes surface and bulk oxygen
reducibility of the support when it was used as catalysts
additive [17,18]. All of these properties are very important
for three-way catalysts for the abatement of automobile
exhausts.
Differing from ideas used in the traditional synthesis
methods, ceria obtained by using a surfactant-templated
synthetic approach shows larger surface area and ordered
mesopore structure [5,14]. The mesoporous support of ceria
would give rise to stable and well dispersed metal particles
on its surface, as a result, enhancement of catalytic perfor-
mance may be achieved. It is reported that CO conversion
on the Pd/ceria prepared by using myristyltrimethylammo-
nium bromide as synthetic template is always higher than
that achieved on the one prepared by using a precipitation
method, this resulted from larger surface area and better metal
dispersion [5].
In the current work, mesoporous ceria nanophase was
obtained in a base condition, through a templated synthesis
route, using cetyltrimethylammonium chloride as the
template. The surface features and texture properties of
the resultant ceria are characterized by thermogravimet-
ric analysis (TGA), Fourier transform infrared (FT-IR),
Brunaur–Emmett–Teller (BET) and transmission electron
microscope (TEM) techniques. Its crystalline structure
was refined by applying Rietveld method on the basis of
X-ray diffraction (XRD) analysis. Evidence of surfactant
association with the pores of the solid is provided and the
catalytic activity and selectivity of NO reduction via CO
over the ceria and Pd/ceria catalyst are reported.
100 ◦C for 5 days. The solid was then filtered and washed
with water and methanol for several times. The resultant ma-
terial was dried at 80 ◦C for 24 h and then was calcined at
200, 400 and 600 ◦C for 4 h for further characterization.
2.2. Preparation of 3 wt.% Pd/CeO2
The 3 wt.% Pd/CeO2 catalyst was prepared by impreg-
nating the ceria support annealed at 600 ◦C with a calculated
amount of an aqueous solution of Pd(NO3)2·2H2O. The
metal supported catalyst was dried at 120 ◦C for 4 h and
then was calcined at 600 ◦C for 4 h. Before the catalytic test,
the catalyst was reduced by using 99.9% H2 at 400 ◦C for
1 h in order to obtain metallic palladium particles on the
catalyst.
2.3. Textural properties
The textural properties of the ceria solids were measured
in a Digisorb 2405 sorptometer by means of N2 physisorption
isotherms at −195 ◦C. Before the N2 adsorption, 0.5 g of
the sample was thermally treated at 200 ◦C under vacuum
condition for 6 h in order to remove water from the sample.
The surface area was determined according to the standard
Brunaur–Emmett–Teller method and the total pore volume
was evaluated from the amount of adsorbed N2 at a relative
pressure (P/P0) of about 0.99. The pore diameter distribu-
tions were calculated based on the desorption isotherms by
the Barrett–Joyner–Halenda (BJH) algorithm.
2.4. Analysis of thermogravimetric–Fourier transform
infrared (TG–FT-IR)
TG analysis was carried out in flow air using a Dupont
Model 950 thermoanalyzer from 25 to 800 ◦C at a heating rate
of 20 ◦C/min to determine weight losses during the thermal
treatment and to verify if the CTACl incorporation with the
solid materials. The gaseous products produced during TG
procedure were simultaneously monitored by means of an on
line coupled FT-IR spectroscopic technique.
2. Experimental
2.1. Ceria preparation
To prepare ceria solid, two solutions were pre-
pared: the first solution was prepared by dissolving
16.0 g of cetyltrimethylammonium chloride (CH3 (CH2)15
N(CH3)3Cl, referred as CTACl) in 500 ml deionized hot wa-
ter (around 50 ◦C) with stirring, followed by adding 120 ml
of aqueous ammonia (28 wt.%) to obtain a clear micellar so-
lution; the second solution was prepared by dissolving 21.7 g
of Ce(NO3)3·6H2O in 500 ml deionized water. The cerium
solution was added, drop-by-drop, into the surfactant solu-
tion with a proper agitation to disperse the droplets before
local concentrations become excessive. The pH value of the
mixture remained at approximately 11 in order to induce the
hydrolysis and the polycondensation of the inorganic precur-
sor around the formed micelles. Afterwards, the slurry was
continuously stirred for 4 h until gel was formed, and then it
was sealed in a Teflon bottle for hydrothermal treatment at
2.5. Surface characterization by in situ FT-IR
The surface dehydroxylation and removal of the residual
surfactants from the dried samples were characterized by
using in situ FT-IR spectroscopic technique on a Nicolet
Magna-IR 550 spectrometer. The sample was ground by
hand with a pestle in a mortar and then pressed at 4 tonnes to
give a self-supporting wafer (10 nm in diameter). The sample
wafer (around 10 mg) was placed inside an IR cell, which
was coupled with a vacuum and heating systems. The sample
could be exposed to various gaseous environments with
different pressures at different temperatures not exceeding
400 ◦C. The in situ FT-IR spectra reported herein were
recorded at 25, 100, 200, 300 and 400 ◦C, respectively.

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J.H. Zhang et al. / Journal of Molecular Catalysis A: Chemical 237 (2005) 182–190
50 to 400 ◦C. Catalyst loading was 50 mg. The effluent
discharged from the reactor after the reaction was analyzed
by on-line gas chromatography–FT-IR analysis system (HP-
5890 Chemical Station and Nicolet-8220 Gas Analyzer).
N2 and CO2 were analyzed by gas chromatography and
CO, N2O, NO, NO2 were analyzed by IR. The light-off
temperature, T50, is defined as the temperature at which the
conversion of CO and NO reaches 50%, respectively.
Table 1
Atomic coordinate of the cubic structure with space group Fm3m (2 2 5)
Atom
Site
x
y
z
Ce
O
4e
4e
0.00
0.25
0.00
0.25
0.00
0.25
2.6. X-ray diffraction analysis and Rietveld refinement
The power X-ray diffraction data were collected at room
temperature in a Siemens D-5000 diffractometer with Cu K␣
radiation and a secondary beam monochromator. Intensities
of the diffraction lines were obtained in the 2θ range between
20◦ and 110◦ with a step of 0.02◦ and a measuring time of
2.5 s at each point. Crystalline structures were refined with
the Rietveld technique by using FULLPROF98 code [19].
The atom atomic coordinates used to refine the ceria with
cubic structure are reported in Table 1. For the crystalline
structure refinement, the solid structure was fitted with
a cubic unit cell with space group Fm3m. The Rietveld
refinement provided not only the concentration and average
crystallite size of each phase present in the sample but also
the lattice parameter and lattice defect information and
microstrain in the crystalline structures. The lattice defects
were determined by comparing the cation occupancy number
with that present in the perfect ideal ceria. The standard
deviations, corresponding to the variation of the last signifi-
cant figures, are given in parentheses. When they correspond
to refined parameters, their values are not estimates of the
probable error in the analysis as a whole, but only of the
minimum possible errors based on their normal distribution.
3. Results and discussion
3.1. Thermal analysis
Fig. 1 shows a TG–DTG profiles of an as-made ceria solid.
The temperature range was set between 25 and 900 ◦C. Two
weight loss stages were observed in the TG profile: the first
stage (T < 150 ◦C) led to a 2.41 wt.% weight loss that is as-
signed to desorption of adsorbed water on the sample. The
second stage located in the temperature range between 150
and 400 ◦C which exhibited a maximal rate of weight loss
around 3 wt.%/min, showing a pronounced weight loss of
17 wt.%. Above 400 ◦C, the base line slightly declined down,
resulting in a weight loss less than 1 wt.%.
For identifying the gaseous products produced at various
weight loss stages in Fig. 1, infrared analysis was simul-
taneously carried out. Fig. 2 shows a set of FT-IR spectra
of the gaseous mixture discharged from the TG system.
In the first stage of TG test (<150 ◦C), the IR spectrum
exhibited several groups of peaks: for example, one between
3900 and 3500 cm−1 attributed to surface hydroxyls, and
another between 1700 and 1300 cm−1 (Fig. 2a), these were
characteristic of molecular water. This confirms that the
first weight loss stage in TG profile is resulted from the
desorption of water adsorbed on the sample. It is noted
that CO2 was present in the exit stream because double
peaks between 2400 and 2300 cm−1 and the ones around
650 cm−1 in the IR spectra were formed. Taking into account
the fact that the surfactant is thermal stable below 150 ◦C,
obviously, the CO2 observed in this temperature range was
2.7. Images of transmission electron microscope
High resolution TEM images of CeO2 and Pd/CeO2 were
carried out in a JEOL 4000 EX electron microscope equipped
with a pole piece with spherical aberration coefficient of
Cs = 1.00 mm. The CeO2 and Pd/CeO2 powder samples were
grounded softly in an agate mortar and dispersed in isopropyl
alcohol in an ultrasonic bath for several minutes. A few drops
were then deposited on 200-mesh copper grids covered with
a holey carbon film. The electron micrographs were recorded
in electron negative films and in a digital PC system attached
to the electron microscope. The mean particle diameter (d)
ꢀ
ꢀ
was calculated by using the d = dini/ ni formula, where,
ni is the number of particles and di the diameter of the particle
i, and the observed particles were arranged by class differing
by 0.5 nm in size.
2.8. Catalytic test
The catalytic evaluation of the Pd/ceria for NO reduction
by CO was performed on a microreactor system. The reac-
tants mixture fed into the microreactor consisted of 1.5 vol.%
CO and 0.5 vol.% NO, the balance was helium. The reactant
flow was 30 cm/min and reaction temperature ranged from
Fig. 1. TG–DTG profiles of the as-prepared ceria dried at 80 ◦C.

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J.H. Zhang et al. / Journal of Molecular Catalysis A: Chemical 237 (2005) 182–190
185
Fig. 2. A set of in situ FT-IR spectra of the exit gases discharged from TG
system. The sample used for TG analysis was ceria solid dried at 80 ◦C. (a)
60 ◦C; (b) 180 ◦C; (c) 200 ◦C; (d) 220 ◦C; (e) 500 ◦C.
Fig. 3. A set of in situ FT-IR spectra of the as-prepared ceria recorded during
the heating procedure in the IR cell. (a) 25 ◦C; (b) 100 ◦C; (c) 200 ◦C; (d)
300 ◦C; (e) 400 ◦C.
not produced by surfactant decomposition but likely by the
desorption of adsorbed CO2 on the sample because it was
exposed to air before the TG measurement.
In the second weight loss stage (150–400 ◦C), very sharp
IR bands corresponding to CO2 and H2O were observed
(Fig. 2b–d). The intensities of the bands related to CO2
greatly increased as the temperature was increased until they
reached a maximum at approximately 200 ◦C. This result
indicates that surfactant combustion to produce CO2 takes
place and its burning-off rate increases with increasing of
the temperature. Two bands at 2950 and 2800 cm−1 were
also observed, these are ascribed to the stretching vibration
modes of C H bonds in the hydrocarbon compounds in the
outlet gases. Appearance of hydrocarbon compounds also
indicates that some residual surfactants were decomposed
into small organic molecules and left from the solid to outlet
in good agreement with the TG-IR characterization shown in
Figs. 1 and 2. There are twin bands at 2950 and 2875 cm−1
,
which are characteristic of the stretching vibrations of C H
bond in hydrocarbons (νCH3as and ν(CH2as + νCH3as)), i.e.,
herein the surfactant. In the C C stretching and C H defor-
mation vibrations regions, two very tense bands located at
approximately 1575 and 1325 cm−1, and the third appeared
at 720 cm−1, these peaks are assigned to the vibrations of
bending mode (scissoring and rocking vibrations) of the C H
bonds in the groups of CH2 and CH3 in the surfactants
[20,21]. It was also observed that a sharp band appeared
around 1080 cm−1, it was due to the (CH3)3N group in
the surfactant. These observations confirm, once again, that
the surfactant cations were indeed strongly interacted with
the solid during the preparation, and they were not removed
in washing step. In addition, the small double peaks around
2300 cm−1 were related to the presence of CO2. Since the
sample was exposed to the air before the IR measurement, it
probably adsorbed some CO2 on the surface, as stated above.
When the temperature in the IR cell was increased to
100 ◦C, the intensity of the wide band between 3700 and
3000 cm−1 was remarkably decreased, and it was replaced
by two bands at 3550 and 3625 cm−1. These two bands
were attributed to the stretching frequencies of the unidentate
(Ce OH) and bidentate (HO Ce OH) species, respectively;
while, the double bands corresponding to C H bonds at 2950
and 2875 cm−1 seem to become sharper in comparison with
these at 25 ◦C; however, when the covering effect resulted
from the broad peak of the adsorbed water at 25 ◦C is taken
into account, they almost remained unchanged. The bands in
the region between 1200 and 1700 cm−1 also remained the
same as that at 25 ◦C. These results show that most of the
adsorbed water is desorbed, however, the surfactants inside
the pores are not yet removed at 100 ◦C.
stream. In addition, it is possible, during the synthesis,
(4−x)
that polyhydrous cerium oxide Ce(OH)x
·yH2O was
formed. The decomposition of the hydrous cerium oxide
usually occurs in the temperature range between 200 and
300 ◦C. Therefore, the largest weight loss in the TG profile
probably resulted from both the combustion/oxidation of the
surfactant and decomposition of hydrous cerium oxide.
In the temperature above 400 ◦C, CO2 together with trace
water was identified (Fig. 2e). This is likely resulted from the
combustion of trace residual coke-like deposits produced by
surfactant decomposition.
3.2. Surface characterization by in situ FT-IR
In order to analyze the changes of the ceria surface features
during the calcination, in situ FT-IR technique was applied.
Fig. 3 shows a set of in situ FT-IR spectra of the as-prepared
ceria sample recorded at various temperatures in the IR cell.
When the IR spectrum was recorded at 25 ◦C, a broad absorp-
tion band covers the region between 3700 and 3000 cm−1
which is due to water adsorbed on the sample surface. This is
,