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Applied Catalysis B: Environmental 69 (2007) 219–225
Effect of ceria–zirconia ratio on the interaction of CO with
PdO/Al2O3–(Cex–Zr1�x)O2 catalysts prepared by sol–gel method
Gabriela Perez-Osorio a, Felipe Castillon b, Andrey Simakov b, Hugo Tiznado a,Francisco Zaera c, Sergio Fuentes b,*
a Posgrado en Fısica de Materiales, Centro de Investigacion Cientıfica y de Educacion Superior de Ensenada, BC (CICESE),
Apdo. Postal 2732, Ensenada, BC, Mexicob Departamento de Catalisis, Centro de Ciencias de la Materia Condensada (CCMC UNAM), Apdo. Postal 2681, Ensenada, BC, Mexico
c Department of Chemistry, University of California at Riverside (UCR), Riverside, CA 92521, USA
Received 15 December 2005; received in revised form 31 May 2006; accepted 6 July 2006
Available online 21 August 2006
Abstract
The current work is devoted to study of CO interaction with PdO/Al2O3–(Cex–Zr1�x)O2 catalysts. Ceria–zirconia–alumina supports with
different Ce/Zr ratio were prepared by sol–gel technique. The FT-IR characterization of CO adsorbed at �120 and 25 8C on oxidized and reduced
samples revealed that Ce/Zr ratio modifies the surface properties of support and oxidation state of palladium. The catalyst with Ce/Zr molar ratio
0.5/0.5 was characterized with the highest ability to stabilize palladium in oxide state and the highest activity to oxidize CO. Redox treatment of
catalysts improves their catalytic activity.
# 2006 Published by Elsevier B.V.
Keywords: Palladium oxide; Al2O3–(Cex–Zr1�x)O2; CO adsorption; In situ FT-IR; Catalytic CO oxidation
1. Introduction
The palladium-only three-way catalysts (Pd-only TWC) are
used for automotive catalytic converters with closed-loop
control systems [1–11], due to their lower cost, their ability to
catalyze the hydrocarbon oxidation and their durability under
high temperature conditions. In fact, the fourth generation of
these catalytic converters has been available in the market since
1995 [12,13]. Pd-only catalysts can be effective as TWC for the
simultaneous removal of NO, CO and hydrocarbons in
automotive exhaust gases [7,14], but they typically show
lower efficiency in NO conversion compared with TWC based
on Rh [15,16]. Nevertheless, the catalytic activity of Pd-only
TWC could be improved by doping with rare earth oxides. For
instance, Trovarelli [17] reported that due to its properties,
structure and capabilities of storing and releasing oxygen, CeO2
is an important component in automotive emission-control
(oxidation) catalysts. The role of oxygen vacancies and their
* Corresponding author. Tel.: +52 646 1744602; fax: +52 646 1744603.
E-mail address: fuentes@ccmc.unam.mx (S. Fuentes).
0926-3373/$ – see front matter # 2006 Published by Elsevier B.V.
doi:10.1016/j.apcatb.2006.07.001
mobility on ceria surfaces, has been recently studied by Esch
et al. [18]. The mechanism of oxygen vacancies appearance and
their interaction upon further ceria reduction have been recently
studied in detail by scanning tunneling microscopy and density
functional calculations [18]. It was shown that on the slightly
reduced surface single vacancies prevail and can be distin-
guished as two types. One type appears as depressions
surrounded by three paired lobes and can be assigned to
surface oxygen vacancies. A second type appears as triple
protrusions centered about of the third layer oxygen sites. Upon
further reduction almost all vacancies interact with each other
with formation so-called linear surface oxygen vacancies with
length of several nm. In addition the authors concluded that
single vacancy formation around single Zr4+ dopants is
facilitated.
The catalytic activity of the palladium catalysts depends as
much on their method of preparation [19,20] as well as the
composition of the support [21–25]. For Pd supported on Al2O3
catalysts modified with ceria and zirconia, the extent of the
effect of this promotion on both the reduction of NO and the
oxidation of CO depends on the interaction between the support
and palladium, which determines the nature of the Pd species
G. Perez-Osorio et al. / Applied Catalysis B: Environmental 69 (2007) 219–225220
and its particle size. Thus, the physical and chemical
characteristics of modifier play an important role in these
reactions [26,27], which requires a strict control of the
composition of the support to obtain the best thermal stability
and optimal catalytic activity [28,29].
Incorporation of Ce–Zr by sol–gel into the alumina matrix
can improve more efficient degree of Ce–Zr dispersion and
thermal stability of alumina. The sol–gel method provides the
capacity to mix several components in a single stage, and
allows controlling the structure and composition of the final
solid mixture at molecular level [30]. In addition, sol–gel
methods give a homogeneous mixture of Al2O3–MOx binary
oxides with enhanced efficiency for NO reduction. In the past,
we have reported that palladium catalysts deposited on Al2O3–
La2O3 supports prepared by the sol–gel method show an
enhancement in the reducibility properties, and that those
changes influence the activity of reduction of NO by hydrogen
[31]. It was suggested in that work that the higher selectivity
towards the production of N2 found on the catalysts of Pd/
Al2O3–La2O3, compared with the Pd/Al2O3 catalysts at
temperatures below 350 8C is due to the interaction of
palladium particles and reduced species of lantana.
The aim of this work was to determine the effect of ceria–
zirconia ratio on the interaction of CO with PdO/Al2O3–(Cex–
Zr1�x)O2 catalysts in order to obtain further information about
the catalyst structure and surface configuration by FT-IR
spectroscopy, that is important in their efficiency in CO
oxidation. The samples of alumina–ceria–zirconia mixed
oxides were prepared by sol–gel method in order to get
homogeneous support for catalysts and to promote the
interaction between PdO and support.
2. Experimental
2.1. Preparation of catalysts
Alumina supports were prepared with different Ce/Zr ratios
(Al2O3–(Cex–Zr1�x)O2 with x = 0, 0.25, 0.33, 0.5, 0.66, 0.75
and 1.0) by the sol–gel method from organic precursors
following a procedure reported previously [32]. A solution of
acetylacetonate of cerium (Aldrich) and/or acetylacetonate of
zirconium (Alfa Aesar) in ethanol with moderate agitation was
added to a mixture of aluminum sec-butoxide (Aesar Alpha) in
Table 1
BET surface area and experimental composition of catalysts
Catalysts Surface area BET (m2 g�1) Exp
Pd
PdO/Al2O3–CeO2 246 0.2
PdO/Al2O3(Ce0.75Zr0.25)O2 253 0.2
PdO/Al2O3(Ce0.67Zr0.33)O2 260 0.2
PdO/Al2O3–(Ce0.5Zr0.5)O2 246 0.1
PdO/Al2O3(Ce0.33Zr0.67)O2 253 0.2
PdO/Al2O3(Ce0.25Zr0.75)O2 257 0.2
PdO/Al2O3–ZrO2 249 0.1
PdO/Al2O3 214 0.2
a Calculation was made by difference based on stoichiometry of Ce, Zr and Al
2-metil-2,4-pentanediol (Aesar Alpha), staying in reflux for
3 h, with moderate agitation at 94 8C. Hydrolysis was made by
adding deionized water. The obtained gel was aged for 10 h.
Samples were dried at vacuum (about 10�2 Torr) at 100 8C for
12 h and then calcined in N2 atmosphere at 450 8C for 12 h,
with a later treatment in air at 650 8C for 4 h.
The support impregnation was carried out using a palladium
chloride solution of 0.12% (w/v), to give a concentration of
0.3 wt.% of palladium. After impregnation, the samples were
calcined in air at 650 8C for 4 h.
Chemical analysis of prepared samples by inductive coupled
plasma (ICP) technique was done using Optical Emission
Spectrometer Optima 4300D. Before analysis samples were
dissolved in sulphuric acid solution (1:1). The experimental
composition of the prepared catalysts determined by ICP as
well as the specific surface area determined by nitrogen
adsorption and BET method are presented in the Table 1.
According to the data presented in a previous paper [33] for
most of the catalysts it could be concluded that (Ce1�xZrx)O2
mixed oxide is present like nano-crystallites which size is
below the XRD detection limits. Only for the PdO/Al2O3–
(Ce0.75Zr0.25)O2 catalyst it was observed formation of a
(Ce1�xZrx)O2 mixed oxide solution. The particle size of PdO
was about 6 nm for PdO/Al2O3–CeO2, it decreased to 3.3 nm
for PdO/Al2O3–(Ce0.25Zr0.75)O2 and finally it increased a little
for PdO/Al2O3–ZrO2 (4.5 nm).
2.2. CO adsorption by FT-IR
The CO adsorption was performed as described in previous
report [34]. Typical experiments were carried out step by step
from low temperatures (LT,�120 8C) to room temperature (RT,
25 8C) and from 5 Torr of CO up to evacuation for 40 min at
10�2 Torr. Infrared spectra of CO adsorbed on the catalysts
were registered with a resolution of 4 cm�1, with a Bruker
Equinox 55 Fourier-transformed infrared (FT-IR) spectrometer
in transmittance mode with DTGS detector [35]. Experiments
were carried out in a quartz laboratory-made cell with windows
of NaCl able to work from �120 to 550 8C and pressures of
10�2 to 760 Torr. The catalyst powders were pressed into self-
supporting discs of 13 mm of diameter and less than 20 mg of
weight. Samples were exposed to 5 Torr of carbon monoxide
(Matheson Research grade) purified using a trap cooled with
erimental composition ICP (wt.%) Calculated valuea (wt.%)
Ce Zr Al2O3
0 4.45 <0.007 95.34
0 3.42 1.45 94.93
4 3.09 2.03 94.64
9 2.27 2.84 94.70
0 1.37 3.68 94.75
0 1.07 4.12 94.61
8 <0.006 5.5. 94.31
9 <0.006 <0.006 99.70
oxides.
G. Perez-Osorio et al. / Applied Catalysis B: Environmental 69 (2007) 219–225 221
Fig. 1. FT-IR spectra of catalysts recorded at �50 8C (after introduction of
5 Torr of CO at �120 8C, 40 min evacuation and heating up to �50 8C). The
samples were pretreated at (O550). (A) PdO/Al2O3, (B) PdO/Al2O3–ZrO2, (C)
PdO/Al2O3–(Ce0.25Zr0.75)O2, (D) PdO/Al2O3–(Ce0.33Zr0.67)O2, (E) PdO/
Al2O3–(Ce0.50Zr0.50)O2, (F) PdO/Al2O3–(Ce0.67Zr0.33)O2, (G) PdO/Al2O3–
(Ce0.75Zr0.25)O2 and (H) PdO/Al2O3–CeO2.
liquid nitrogen and then continuously evacuated. Adsorption
experiments were initiated at �120 8C (LT) in order to avoid
changes of catalysts surface, later, temperature was increased
gradually with ramp of 10 8C/min up to room temperature (RT).
Infrared spectra of CO adsorption were obtained after
subtraction of the background recorded before CO adsorption.
Catalysts were pretreated in situ before FT-IR experiments.
Oxidized catalysts refer to samples calcined in oxygen
(100 Torr) at 550 8C for 1 h, while reduced ones correspond
to samples with oxygen treatment followed with reduction in
hydrogen (100 Torr) at 550 8C for 1 h. These treatments were
labeled as O550 and O550/H550, respectively.
2.3. Catalytic activity in CO oxidation
Catalytic activity test using a 1% CO + 1% O2 (helium
balance) at 40 ml/min was carried out in a fixed bed glass U-
tube reactor (i.d. 4 mm). About 75 mg of sample was packed
between two layers of glass wool. Gases of UHP grade were
controlled with mass flow controllers. Catalysts were preox-
idized in situ in O2 flow at 550 8C for 1 h, cooled at the same
atmosphere and purged by helium at room temperature. During
catalytic activity test the temperature of catalyst bed was
increased from room temperature up to 250 8C with ramp of
2 8C/min. Reagents and products were analyzed by gas
chromatography with a SRI instrument 8610C equipped with
a TCD detector and two columns packed with molecular sieve
and silica-gel, operating at 100 8C.
CO oxidation was carried out on two sets of catalysts, where
the first set refers to fresh samples after oxidation treatment in
O2 flow at 550 8C for 1 h and the Second set corresponds to
redox catalysts. Redox treatment includes two cycles of
reduction in H2/Ar (10/90) flow with temperature increase
programmed from 25 up to 550 8C with ramp of 20 8C/min and
re-oxidation of samples in O2 flow at 550 8C for 1 h.
The catalytic activity data were presented in terms of
temperature required for 50 % CO conversion.
3. Results and discussion
3.1. CO adsorption on oxidized (O550) catalysts
In order to minimize changes on the catalysts surface due to
reaction of CO probe with adsorbed oxygen, the temperature of
adsorption was initially lowered and then gradually increased
stepwise at definite rate.
The effect of Ce/Zr ratio on CO chemisorption over PdO
species was studied comparing FT-IR spectra of catalysts
recorded at �50 and 25 8C.
Experiments of CO adsorption were performed introducing
5 Torr of CO for 20 min at �120 8C, followed by 40 min of
evacuation (10�2 Torr) at this temperature then heating from
�50 to 25 8C. The temperature increase from �120 to �50 8Ccaused gradual diminution of bands attributed to CO
chemisorbed on the support. Therefore, at �50 8C it was
possible to observe the CO adsorption corresponding to
palladium oxide species.
Spectra of CO adsorbed on catalysts after O550 treatment
(Fig. 1) presented at �50 8C five main bands, three of them
already observed on supports [34,36], S3 (2185 cm�1) and S4
(2200–2205 cm�1) on alumina, Ce4 (2162 cm�1) on ceria, and
two more related to palladium oxide, L1 (2120 cm�1) and L2
(2156 cm�1)) as in [34,24]. In agreement with reference [34]
the S3 and S4 sites could be assigned to CO on Al3+ tetrahedral
and Al3+ octahedral, respectively. The band at 2162 cm�1 may
be assigned to CO coordinated on Ce4+ cations. This band is
shifted upward from physisorbed CO (2157 cm�1) due to
strong interaction with Lewis acid sites in accordance with
studies of CO adsorption on polycrystalline ceria [26,37]. The
appearance of band at 2185 cm�1 in the absence of Ce ions as in
the case of PdO/Al2O3–ZrO2 sample [34] indicates the role of
alumina sites in stabilization of such type of CO species.
However, we can not exclude overlapping of this band with
band of CO adsorbed on Ce4+ ions modified by chlorine, which
was described in the report of Binet et al. [37] and Bensalem
and co-workers [38]. Indeed, modification by chlorine could be
due to PdCl2 used as palladium precursor under the catalysts
preparation. Intensity of signal at 2185 cm�1 could be sum of
two bands from different CO adspecies stabilized on alumina
and ceria.
The L1 and L2 bands similar to those of reference [34,24]
were associated to Pd+ and Pd2+ species, respectively [39]. The
O550 treatment caused a different L1/L2 intensity ratio
depending on the Ce–Zr ratio. So, for PdO/Al2O3 catalyst
the L1/L2 ratio was slightly less than one, it later increased
above one for the PdO/ Al2O3–ZrO2 sample and decreased
drastically when Ce concentration was gradually increased.
Finally, for the PdO/ Al2O3–CeO2 catalyst only the L2 band
was detected. Notice that frequency of band at around
G. Perez-Osorio et al. / Applied Catalysis B: Environmental 69 (2007) 219–225222
Fig. 2. FT-IR spectra of catalysts recorded at 25 8C (after exhibition to 5 Torr of
CO and 40 min evacuation at 25 8C). The samples were pretreated at (O550). (A)
PdO/Al2O3, (B) PdO/Al2O3–ZrO2, (C) PdO/Al2O3–(Ce0.25Zr0.75)O2, (D) PdO/
Al2O3–(Ce0.33Zr0.67)O2, (E) PdO/Al2O3–(Ce0.50Zr0.50)O2, (F) PdO/Al2O3–
(Ce0.67Zr0.33)O2, (G) PdO/Al2O3–(Ce0.75Zr0.25)O2 and (H) PdO/Al2O3–CeO2.
Fig. 3. Spectra of catalysts within range 1100–1600 cm�1 (carbonate region)
recorded at 25 8C (after sample exhibition with 5 Torr of CO at �120 8C,
40 min evacuation and heating up to 25 8C, exhibition to 5 Torr of CO and
40 min evacuation at 25 8C). The samples were pretreated at (O550). (A) PdO/
Al2O3, (B) PdO/Al2O3–ZrO2, (C) PdO/Al2O3–(Ce0.25Zr0.75)O2, (D) PdO/
Al2O3–(Ce0.33Zr0.67)O2, (E) PdO/Al2O3–(Ce0.50Zr0.50)O2, (F) PdO/Al2O3–
(Ce0.67Zr0.33)O2, (G) PdO/Al2O3–(Ce0.75Zr0.25)O2 and (H) PdO/Al2O3–CeO2.
The bands assignation is showed in Table 2. Bold curve (E0) corresponds to CO
adsorption over bare Al2O3–(Ce0.50Zr0.50)O2 sample.
Table 2
FT-IR in the 1100–1600 cm�1 region for CO2 adspecies formed at CO
adsorption at 25 8C after O550 treatment of catalyst samples
Band wavenumber
(cm�1)
Assignation Literature
value (cm�1)
Da
(cm�1)
1160 Bridged on alumina 1135b �25
1193 Hydrogen carbonate on ceria 1218b 25
1227 Bridged on ceria 1232c 5
1334 Monodentated on ceria 1351b 17
1437 Polydentated on ceria 1462b 25
1490 Monodentated on ceria 1504b, 1497d 14, 7
a Difference between literature and experimental values.b Ref. [37].c Ref. [40].d Ref. [41].
2120 cm�1 depends on the Ce/Zr ratio reflecting different
nature of Pd–support interface. Changes in position of signal
around 2156 cm�1 manifests different contribution of L2 and
Ce4 bands for samples with different Ce/Zr ratio.
From these results it is concluded that ceria stabilizes
preferentially Pd2+ species. However, some portion of ceria
surface sites is not occupied by palladium species in accordance
with CO adsorption data.
Further increase of temperature up to 25 8C (Fig. 2)
diminished the relative intensity of band assigned to Pd2+
species and affected the relative contribution of Pd+. Note, the
position of L1 band was shifted by about 10 cm�1 for PdO/
Al2O3–ZrO2 and PdO/Al2O3(Ce0.33Zr0.67)O2. This blue shift
could be attributed to formation of some intermediate
palladium species between Pd+ and Pd2+ as was also observed
in Ref. [34].
Adsorption of CO at room temperature obviously should
result in appearance of reduced palladium ions as Pd+ due to
partial reduction of Pd2+, because reduction can proceed even at
�50 8C as was shown by our own experiments presented in
Fig. 1. On the other hand one did not expect any changes in PdO
dispersion at room temperature. So, we rather prefer to assign
observed shift of L1 band to appearance of some Pd species as a
result of an ensemble contribution of Pd+ and Pd2+ ions.
For some oxidized catalysts during heating up carbonate
species were detected (Fig. 3), indicating that oxide palladium
species were reduced by CO. The intensity of bands assigned to
CO adsorption on support sites S3 and Ce4 also changed when
temperature was increased.
The formation of carbonate species in the 1100–1600 cm�1
region was monitored at RT (25 8C) after CO adsorption
(5 Torr, 25 8C). The spectra of all samples shown in Fig. 3
develop carbonate bands at 1160, 1193, 1227, 1334, 1437 and
1490 cm�1. The assignment of these bands is presented in
Table 2. The shift of experimental bands from those reported by
Binet et al. [37] on polycrystalline CeO2 could be attributed to
differences in dispersion of Ce species in our samples. Such an
effect of dispersion is clearly indicated for monodentate CO2
species reported for bulk CeO2 [37,40] and that supported on
alumina [41]. As a result of Zr addition one can expect strong
distortion of Ce oxide structure and it could be the reason for
red shift of experimental bands observed for studied samples in
carbonate region. A similar red shift for bands of methanol
adsorbed over Ce–Zr–O mixed oxide in comparison with pure
ZrO2 was observed in Ref. [42].
The presence of CO2 bands reveals that oxidation of CO
already happened at these temperatures. The formation of
G. Perez-Osorio et al. / Applied Catalysis B: Environmental 69 (2007) 219–225 223
Fig. 5. FT-IR spectra of catalysts recorded at 25 8C (after exhibition to 5 Torr of
CO and 40 min evacuation at 25 8C). The samples were pretreated at (O550/H550).
(A) PdO/Al2O3, (B) PdO/Al2O3–ZrO2, (C) PdO/Al2O3–(Ce0.25Zr0.75)O2, (D)
PdO/Al2O3–(Ce0.33Zr0.67)O2, (E) PdO/Al2O3–(Ce0.50Zr0.50)O2, (F) PdO/Al2O3–
(Ce0.67Zr0.33)O2, (G) PdO/Al2O3–(Ce0.75Zr0.25)O2 and (H) PdO/Al2O3–CeO2.
carbonate bands started during heating from �50 to 25 8C.
These carbonate bands are attributed to adsorption of CO2
formed by reaction between PdO and CO adsorbed on it.
Note, that for samples PdO/Al2O3–(Ce0.5Zr0.5)O2 and PdO/
Al2O3–(Ce0.33Zr0.67)O2 the bands with more pronounced
intensity were detected indicating their high activity in CO
oxidation.
3.2. CO adsorption on reduced (O550/H550) catalysts
The spectra of CO adsorbed on reduced catalysts are
shown in Fig. 4. For the majority of samples the O550/H550
treatment produced an intense band at 2103 cm�1 (L0)
attributed to CO adsorbed on Pd0. Indeed, the presence of
marked shoulders L1 and L2 in many cases indicated the
presence of oxidized palladium species being dependent on
the content of Ce. That was particularly the case for the
Pd/Al–Ce0.50Zr0.50 catalyst, where the L0 band was not
detected, but L1 and L2 were clearly observed. It can be
proposed that Pd2+ and Pd+ species were reduced to Pd0 and
then it was reoxidized by oxygen species provided by
surrounding ceria species. The ability of palladium to be
reoxidized in this catalyst may be determined by the unique
high oxygen mobility characterized for mixed Ce/Zr oxides
and particular for those with Ce/Zr ratio close 1 according to
[41,43].
A shoulder at 2111 cm�1 (Ce3) corresponding to Ce3+–CO
interaction [38] was clearly observed for the reduced catalysts
particular for those with high Ce/Zr ratio.
The spectra in Fig. 5 recorded at 25 8C show the same bands
but with low intensity. The L0 band was predominant for all
Fig. 4. FT-IR spectra of catalysts recorded at �50 8C (after introduction of
5 Torr of CO at �120 8C, 40 min evacuation and heating up to �50 8C). The
samples were treated at (O550/H550). (A) PdO/Al2O3, (B) PdO/Al2O3–ZrO2, (C)
PdO/Al2O3–(Ce0.25Zr0.75)O2, (D) PdO/Al2O3–(Ce0.33Zr0.67)O2, (E) PdO/
Al2O3–(Ce0.50Zr0.50)O2, (F) PdO/Al2O3–(Ce0.67Zr0.33)O2, (G) PdO/Al2O3–
(Ce0.75Zr0.25)O2 and (H) PdO/Al2O3–CeO2.
catalysts except for Pd/Al2O3–(Ce0.50Zr0.50)O2 where L1 and
L2 bands still were detected.
The spectrum of carbonate bands in Fig. 6 at room
temperature shows that a less amount of CO2 was formed over
reduced samples, compared to oxidized catalysts. Also in this
case, catalyst Pd/Al2O3–(Ce0.50Zr0.50)O2 develops carbonate
bands with the highest intensity.
Fig. 6. Spectra of catalysts within range 1100–1600 cm�1 (carbonate region)
recorded at 25 8C (after sample exhibition with 5 Torr of CO at �120 8C,
40 min evacuation and heating up to 25 8C, exhibition to 5 Torr of CO and
40 min evacuation at 25 8C). The samples were pretreated at (O550/H550). (A)
PdO/Al2O3, (B) PdO/Al2O3–ZrO2, (C) PdO/Al2O3–(Ce0.25Zr0.75)O2, (D) PdO/
Al2O3–(Ce0.33Zr0.67)O2, (E) PdO/Al2O3–(Ce0.50Zr0.50)O2, (F) PdO/Al2O3–
(Ce0.67Zr0.33)O2, (G) PdO/Al2O3–(Ce0.75Zr0.25)O2 and (H) PdO/Al2O3–CeO2.
The bands assignation are shown in Table 2.
G. Perez-Osorio et al. / Applied Catalysis B: Environmental 69 (2007) 219–225224
Fig. 8. Catalytic activity in CO oxidation for redox catalysts. The data for
upwarding curve correspond to light-off curve with temperature increase and
those for downwarding curve to that with temperature decrease.
3.3. CO oxidation
All catalysts are characterized by a gradual increase of CO
conversion from room temperature up to 150 8C. Fig. 7 shows
catalytic activity in CO oxidation of fresh catalysts (O550) with
different Ce/Zr ratio and data for fresh supports. Values for
supports are shifted about 200 8C to high temperature
compared with those for catalysts. The PdO/Al2O3–ZrO2 and
PdO/Al2O3 catalysts presented relatively low activity in CO
oxidation. Addition of ceria–zirconia mixed oxide enhanced
activity of catalysts, compared with those based on individual
oxide supports. The best catalysts were those situated around
the middle of the Ce–Zr composition range.
It has been reported that ceria plays a key role in CO
oxidation by supplying oxygen to the Pd–Ce interface [41,43].
The fact that the lowest temperature for 50% CO conversion
was achieved with catalysts PdO/Al2O3–(Ce0.67Zr0.33)O2, PdO/
Al2O3–(Ce0.5Zr0.5)O2 and PdO/Al2O3–(Ce0.33Zr0.67)O2 corre-
lates with the highest oxygen mobility for bulk Ce–Zr system
with similar composition reported in [42,44].
The redox O550/H550/O550 pretreatment slightly modified the
catalytic properties of catalysts in CO oxidation, as shown in
Fig. 8. As a result of such a treatment the catalytic activity of
PdO/Al2O3–CeO2 and PdO/Al2O3 catalysts was diminished; on
the other hand, PdO/Al2O3–(Ce0.5Zr0.5)O2 and PdO/Al2O3–
(Ce0.33Zr0.67)O2 catalysts increased their activity. Light-off
profiles of redox catalysts are very similar to those of fresh
catalysts and manifest the same tendency with respect to Ce/Zr
ratio, except that they are shifted to lower temperatures.
There are two possible explanations for enhancement of the
activity of Ce–Zr catalysts after redox treatment. The
dispersion of palladium oxide species could be increased by
consequent reduction/oxidation procedures [34]. Otherwise,
redox treatment can result in formation of new vacancies in
Fig. 7. Catalytic activity in CO oxidation for fresh catalysts and for bare
supports. The data for upwarding curve correspond to light-off curve with
temperature increase and those for downwarding curve to that with temperature
decrease. Data for bare supports are presented by dash line.
ceria containing species improving the oxygen mobility to the
palladium–support interface [18].
In addition, for all cases the catalytic activity obtained at
temperature increase (upwarding curve) was higher than that
with temperature decrease (downwarding curve) indicating
some changes in the active sites of the catalysts in the course
of the catalytic reaction. Note, that the difference in
upwarding and downwarding curves decreased slightly after
redox treatment of samples. The later permits to assume that
the main reason for catalyst deactivation during activity test is
partial sample reduction then the blocking of active sites
with carbon deposits formed at CO oxidation. Indeed, for
sample with the highest oxygen mobility (PdO/Al2O3–
(Ce0.5Zr0.5)O2) as could be expected according to data in
[42] the difference in up-warding and down-warding curves is
practically absent.
4. Conclusions
Preparation of Al2O3–(CexZr1�x)O2 oxides by sol–gel
allowed the incorporation of ceria and zirconia additives with
different Ce/Zr ratio to alumina producing homogeneous
supports with high surface area for palladium containing
catalysts.
As revealed by FT-IR studies of CO adsorbed the oxidation
state of palladium in PdO/Al2O3–(CexZr1�x)O2 catalysts is a
function of the Ce/Zr ratio due to specificity of oxygen mobility
of Ce–Zr mixed oxides. So, in PdO/Al2O3–(Ce0.5Zr0.5)O2
catalyst palladium presents preferably as Pd2+ even after
reductive sample pretreatment due to reoxidation by bulk
support oxygen.
Ceria–zirconia mixed additives enhance the catalytic
activity of palladium oxide catalysts compared with those
based on alumina or alumina with Ce or Zr individually. The
G. Perez-Osorio et al. / Applied Catalysis B: Environmental 69 (2007) 219–225 225
catalysts showing Ce/Zr ratios about 0.4–0.6 presented the
lowest temperature for 50% conversion of CO.
The redox O550/H550/O550 pretreatment increased the
activity of PdO/Al2O3–(Ce0.5Zr0.5)O2 and PdO/Al2O3–
(Ce0.33Zr0.67)O2 catalysts. The highest activity for CO
oxidation was observed for PdO/Al–(Ce0.5Zr0.5)O2 catalyst.
The sol–gel method used for catalysts preparation promotes
the palladium–support interaction, which in turn is very
appropriate for CO oxidation. The observed red shift of the
bands in FT-IR spectra for carbonate region permits to propose
formation of Ce–Zr–O nanocrystallites (islands) on the surface
of alumina prepared by the sol–gel technique where structure of
cerium oxide is hardly distorted by presence of Zr. Similar
conclusion could be done based on the fact that change of
catalytic activity with Ce–Zr ratio for our samples is
comparable with that published for bulk Ce–Zr oxides [39].
Interaction of these nanocrystallites with palladium precursor
produces highly reactive PdO sites for CO oxidation, probably
by increase of dispersion of palladium species and/or by supply
enough oxygen transport in the course of the reaction.
Acknowledgments
The authors acknowledge E. Flores-Aquino, J. Peralta, J.
Palomares and P. Casillas for their technical assistance. The
financial support was obtained by grants from CONACyT
(41331-Y and CO 1-3), UC-MEXUS-CONACYT, U.S.
Material Corridor Initiative and DGAPA-UNAM, IN119602-
3 projects. F.F. Castillon acknowledges support from Depart-
ment of Chemistry of the University of California at Riverside.
G. Perez-Osorio, acknowledges support from CONACyT for
the scholarship 122975.
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