Effect of barium addition over palladium catalyst for CO-NO-O2 reaction

Article abstract of DOI:10.1016/j.molcata.2011.08.025

CO-NO-O₂ reaction over Pd supported on barium fixed alumina (Pd/Ba/Al) and ceria-zirconia mixed oxide (Pd/Ba/CZ) catalysts was investigated by XPS, TPR and in situ DRIFT during light-off under a stoichiometric condition. The activity of Pd supp

Full text of DOI:10.1016/j.molcata.2011.08.025

Journal of Molecular Catalysis A: Chemical 349 (2011) 94–99
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Effect of barium addition over palladium catalyst for CO–NO–O reaction
2
∗
K. Tanikawa , C. Egawa
School of Energy and Environmental Science, Utsunomiya University, 7-1-2 Yoto, Utsunomiya 321-8585, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 April 2011
Received in revised form 18 August 2011
Accepted 27 August 2011
Available online 3 September 2011
CO–NO–O2 reaction over Pd supported on barium fixed alumina (Pd/Ba/Al) and ceria–zirconia mixed
oxide (Pd/Ba/CZ) catalysts was investigated by XPS, TPR and in situ DRIFT during light-off under a stoi-
chiometric condition. The activity of Pd supported on barium fixed alumina catalysts for CO oxidation as
well as NO reduction was improved by increasing the amount of barium loading. In contrast, Ba addition
decreased the activity of Pd on ceria–zirconia mixed oxide for CO oxidation, while it had little effect on
NO reduction. Enhancement of CO oxidation on Pd/CZ is caused by the activated oxygen supplied from
ceria–zirconia mixed oxide to Pd particles. The decrease of the activity for CO oxidation on Pd/Ba/CZ is
derived from the reduced supply of the activated oxygen. On the other hand, NO reduction is independent
of barium addition for CZ, indicating that the desorption and/or dissociation of NO is not affected by the
activated oxygen supplied from CZ under stoichiometric CO–NO–O2 reaction conditions.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Palladium
Alumina
Ceria–zirconia
CO oxidation
NO reduction
1
. Introduction
Conventional three-way catalysts consist of noble metals, oxide
to the overall efficiency of three-way catalysts. The ceria–zirconia
mixed oxide also plays a crucial role in enhancing the activity in
reduction atmosphere by supplying more oxygen available for the
oxidation processes. Furthermore, the oxygen vacancies associated
with reduced ceria in the proximity of noble metal particles have
been suggested as promoting sites for CO and NO reaction. Hence,
the investigation of additive effect of barium on CZ is required for
improving NOx reduction on Pd catalyst.
In our previous work, it has been confirmed that Ba has the
effect of improving the dispersion of palladium on alumina, while
for palladium on ceria–zirconia mixed oxide it inhibits the supply
of oxygen from CZ to palladium under CO–O2 reaction condition
[8]. In the reaction including NO as a reactant, NO predominantly
adsorbs on Pd compared to CO [9] and further NO partially adsorbs
as nitrite and/or nitrate on support materials such as alumina and
ceria–zirconia [10,11]. Therefore, the desorption and/or dissocia-
tion of NO related species would affect to the catalytic activities.
The purpose of this research is to clarify the effect of barium
addition on the activity of the palladium catalysts supported on
ceria–zirconia mixed oxide (Pd/Ba/CZ) for CO oxidation and NO
reduction in comparison with that on alumina (Pd/Ba/Al) in the
supports such as alumina (Al) and ceria–zirconia mixed oxide (CZ),
and some additives. It is well known that Rh possesses superior
NOx reduction activity, but its precious value and high cost demand
minimum usage. As a replacement, palladium is very beneficial
because of its higher activity for hydrocarbon oxidation and its rel-
atively lower cost than other precious metals. However, palladium
has a lower activity for NO reduction compared to rhodium. Then,
in order to maximize the catalytic activity of palladium, alkaline
earth metals such as barium have been added to some automotive
catalysts in practical use [1]. Barium is generally known to play a
supplemental role as NOx absorber in NOx storage-reduction (NSR)
catalysts under lean operation conditions. Besides NOx absorber in
NSR, the effect of alkaline earth metals such as barium on alumina
or alumina–ceria supported-palladium has been investigated in
several papers; enhancement of thermal durability of alumina [2],
suppression of hydrocarbon chemisorption on palladium [3,4] and
increase of palladium dispersion [5–7] were reported in three-way
catalysts operated under stoichiometric conditions.
On the other hand, understanding of the effect of barium on
palladium supported on ceria–zirconia mixed oxide is not suffi-
ciently established. The oxygen storage and release capacity of
ceria–zirconia mixed oxide is technologically important for con-
tributing an enlargement of the operating air/fuel ratio and leading
reaction of CO–NO–O . NO species during the reaction was espe-
cially characterized using TPR and in situ DRIFRS.
2
2. Experimental
Pd/Ba/Al and Pd/Ba/CZ were prepared by two-step incipient
wetness impregnation. Commercially available ␥-alumina (Al)
2
∗ Corresponding author. Tel.: +81 28 686 5814x602; fax: +81 28 686 5791.
E-mail address: kenji.tanikawa@mattheyasia.com (K. Tanikawa).
(Sasol Ltd., SSA 160 m /g) and ceria–zirconia mixed oxide (CZ) with
2
cubic phase (Rhodia Co., Ltd., SSA 85 m /g) were used. The molar
1
381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.08.025

K. Tanikawa, C. Egawa / Journal of Molecular Catalysis A: Chemical 349 (2011) 94–99
95
Table 1
for pore volume also indicates that the pores are not covered with
barium. Similar tendency was also revealed in Pd/CZ with barium.
SSA of Pd/CZ and Pd/Ba(10)/CZ were 79 and 70 m /g. The reduction
of 11% is consistent with the barium content. It also indicates that
the pores are not covered with barium for CZ.
The list of each component loading for samples employed.
2
2
Pd loading
wt.%)
Ba loading
(wt.%)
SSA (m /g)
Pore volume
(cm /g)
3
(
Pd/Al
1.00
1.00
0.99
0.99
1.00
1.02
1.06
1.03
0
152
163
150
130
79
80
78
70
0.48
Pd/Ba(1)/Al
Pd/Ba(5)/Al
Pd/Ba(15)/Al
Pd/CZ
Pd/Ba(1)/CZ
Pd/Ba(5)/CZ
Pd/Ba(10)/CZ
1.0
5.5
15.3
0
0.94
4.0
9.5
3.2. Catalytic activity
0.40
0.22
Fig. 1a and b shows the QMS intensities of AMU28 (CO and N ),
2
AMU30 (NO), AMU32 (O2), AMU44 (N2O and CO2) during light-
off reaction over Pd/Al and Pd/Ba(15)/Al. On Pd/Al, CO oxidation
and NO reduction simultaneously proceed in the CO–NO–O2 mix-
ture. The temperature (T50) at which half of CO was converted to
0.18
ratio of CeO /ZrO2 in CZ was adjusted to 1. Initially barium was
impregnated on Al or CZ by aqueous solution of barium acetate.
◦
CO2 was 219 C, and similarly T for NO and O2 were observed at
50
2
◦
◦
2
18 C and 220 C, respectively. On Pd/Ba/(15)/Al, CO oxidation and
◦
The Ba loaded oxides were dried at 150 C for 2 h and calcined at
NO reduction occurred in the same manner as Pd/Al, although T
50
◦
5
00 C for 2 h in air. Subsequently, palladium was impregnated on
◦
◦
of CO and NO shifted to 179 C and 185 C, respectively (Fig. 1b).
These results demonstrate the promoting effect of barium addition
in consistent with the previous study showing the improvement of
CO oxidation by barium [4]. It is to be noted that the increase of NO
these oxides by using aqueous solution of palladium nitrate and cal-
cined at the same condition as the first step, resulting in Pd loading
of 1 wt.%. Thus, Pd/Ba/Al catalysts with 0, 1, 5 and 15 wt.% Ba, and
Pd/Ba/CZ catalysts with 0, 1, 5 and 10 wt.% Ba were prepared.
Specific surface areas (SSA) were determined by physisorption
of N2 at the liquid nitrogen temperature, using the BET equation.
The samples were degassed in vacuum for at least 1 h at 200 C prior
to the adsorption measurements. Pore volumes were obtained from
the desorption curve of the isotherm.
◦
intensity was observed above 200 C on Pd/Ba(15)/Al in Fig. 1b. It is
reported that NO seems to be converted to N2 and N O under lean
2
◦
conditions up to 200 C but further increase in temperature would
◦
inhibit N O generation [8]. In the present study, AMU44 signals due
2
to CO2 and N O actually decreased in the same temperature range.
2
Accordingly, the reactant gas is considered to be in a slightly lean
condition although a stoichiometric mixture is introduced.
Catalytic activity using
CO + 0.2% NO + 1.0% O2 (He balance) was examined in a pyrex
glass flow reactor system where a gas flow rate of 50 mL/min
a stoichiometric mixture of 2.2%
The activity of Pd supported on CZ supports for CO–NO–O reac-
2
tion is shown in Fig. 2a and b. Compared to the activity of alumina
−1
◦
◦
(
4
SV = 6000 h ) and a linear heating rate of 10 C/min from 50 C to
00 C were used. Prior to catalytic reaction, samples were calcined
◦
based catalysts, CO oxidation started already at 50 C and CO was
◦
◦
completely converted at 110 C on barium free CZ supported Pd
◦
in situ under 10% O2 (He balance) at 500 C for 30 min and cooled
in 10% O2 gas and finally purged by He at 50 C. Reactant gases
catalyst, which is in good agreement with the previous study [9].
This promoting effect of CZ is caused by the activation of oxygen
supplied from CZ support [10]. After completion of CO–O2 reac-
tion, NO reduction seems to start through the reaction with residual
◦
were regulated with mass flow controllers and analyzed using a
quadrupole mass spectrometer (QMS) on line.
DRIFTS analysis of adsorbed species on the catalysts under reac-
tion conditions was carried out using a JASCO FTIR-610. A DRIFT
cell was fitted with KBr windows and a heating cartridge for sam-
(a)
Pd/Al
◦
ple heating. Samples were pre-treated at 500 C for 30 min in 10%
◦
O (N balance) and cooled to 30 C. Then, each spectrum was mea-
2
2
◦
◦
◦
sured every 20 C with a heating rate of 10 C/min up to 410 C in a
CO/N2
reaction mixture gas (5.0% CO + 0.45% NO + 2.3%O , N2 balance).
2
XPS measurements were carried out by an Omicron EA125 spec-
trometer using the Mg K␣ radiation (1254.6 eV) at 150 W. The XPS
system has a preparation chamber for samples in which in situ
reduction and oxidation were fulfilled by the introduction (200 Pa)
CO /N O
2
2
NO (x 10)
O2
◦
of hydrogen or oxygen to samples kept at 175 C, respectively.
All the data were referenced with C 1s binding energy fixed at
2
85.0 eV. The obtained peaks were deconvoluted with CasaXPS
5
0
100
150
200
250
300
software, using a Shirley type background subtraction and sym-
metric Gaussian–Lorentzian line shapes.
(
b)
Pd/Ba(15)/Al
3
. Results and discussion
CO/N2
3.1. Catalyst characterization
CO /N O
2
2
NO (x 10)
Table 1 summarizes the catalyst properties; palladium and bar-
ium contents, specific surface area and pore volume. ICP analysis
showed that all the samples included Pd close to 1 wt.%. The speci-
fied amount of barium supported on each material was determined
by XRF. It was observed that the deposition of barium decreased the
SSA and pore volume of alumina. The decrease of 14% in SSA from
O2
5
0
100
150
200
250
300
2
2
1
52 m /g for Pd/Al to 130 m /g for Pd/Ba(15)/Al, corresponds to the
Temperature (ºC)
addition of 15 wt.% barium. It indicates that the barium exists on
the alumina surface without covering the pores of alumina because
pore blockages should lead to a sharp drop in SSA. The decline of 17%
Fig. 1. QMS intensity profile of the effluent for the CO–NO–O2 reaction over Pd/Al
(a), Pd/Ba(15)/Al (b) catalysts.

9
6
K. Tanikawa, C. Egawa / Journal of Molecular Catalysis A: Chemical 349 (2011) 94–99
Table 2
◦
50% isoconversion temperatures (T50, C) observed for Pd/Ba/Al and Pd/Ba/CZ cata-
(
a)
lysts in the CO–NO–O2 reaction.
Pd/CZ
T50 (CO conv.)
219
197
180
179
82
87
126
179
T50 (NO conv.)
T50 (O2 conv.)
NO (x 10)
Pd/Al
218
200
186
185
174
197
178
198
220
198
180
179
81
85
126
180
Pd/Ba(1)/Al
Pd/Ba(5)/Al
Pd/Ba(15)/Al
Pd/CZ
Pd/Ba(1)/CZ
Pd/Ba(5)/CZ
Pd/Ba(10)/CZ
CO /N O
CO/N2
2
2
O2
5
0
100
150
200
250
300
confirmed for NO reduction as well. These results demonstrate the
promoting effect of barium addition. Considering the behavior that
CO oxidation and NO reduction start at the same temperature for
each catalyst, it is most likely that a determining step controls com-
monly both reactions regardless of the present of Ba.
(
b)
Pd/Ba(10)/CZ
NO (x 10)
CO/N2
On Pd/Ba/CZ, T for CO oxidation drastically increased from
2 C to 179 C in the order of Pd/CZ, Pd/Ba(1)/CZ, Pd/Ba(5)/CZ and
CO /N O
50
2
2
◦ ◦
8
Pd/Ba(10)/CZ. Finally, the activity of Pd/Ba(10)/CZ resembles that
of Pd/Ba(15)/Al. On the contrary, NO reduction was not affected by
Ba addition, and all the catalysts showed approximately the same
activity as that of Pd/Ba/Al. These results indicate that NO reduction
is less influenced by support materials, in contrast to CO oxidation.
CO conversions based on the QMS intensities of CO during
CO–O2 and CO–NO–O2 reactions on Pd/Ba(0, 15)/Al and Pd/Ba(0,
O2
5
0
100
150
200
250
300
Temperature (ºC)
1
0)/CZ are compared in Fig. 3a and b. On Pd/Al, T of CO con-
50
◦
◦
version varied from 185 C to 218 C by NO addition. Similarly, T
50
Fig. 2. QMS intensity profile of the effluent for the CO–NO–O2 reaction over Pd/CZ
a), Pd/Ba(10)/CZ (b) catalysts.
◦
◦
of CO conversion shifted from 145 C to 179 C on Pd/Ba(15)/Al. It
was, thus, clarified that the activity for CO oxidation is remarkably
deteriorated by the presence of NO in the reaction gases irrespec-
tive of barium addition. It is most likely caused by NO adsorption
on Pd surface, inhibiting CO adsorption and/or the dissociation of
O2 on palladium. Hence, the desorption and/or dissociation of NO
adsorbed on Pd is considered to be the limiting step of CO–NO–O2
reaction [10].
(
CO. Just before the occurrence of NO reduction, NO desorption was
observed at 90 C on Pd/CZ. Similar NO desorption was reported for
Rh deposited on CZ [11] as well as CZ support alone [11,12] where
NO acts as ligand on the cationic Rh species, or adsorbs strongly
at defect sites presented on CZ support surface. Accordingly, it is
supposed that molecular NO species adsorb over cationic Pd , or
◦
x+
x+
nitrite and/or nitrate species are deposited over CZ surface, because
NO desorption is absent from Pd/Ba(0, 15)/Al.
100
90 (a)
The presence of barium caused an apparent decrease of CO oxi-
dation in the CZ supported system (Fig. 2b). T50 of CO conversion on
the CZ catalyst with 10 wt.% Ba drastically shifted to a higher tem-
perature. This indicates that barium prevents the contact between
Pd–CZ and inhibits oxygen supply from CZ to Pd, resulting in the
decrease of the activity of Pd/Ba/CZ for CO oxidation. In contrast, T50
of NO conversion was not noticeably different from that of Pd/CZ,
and as a result NO reduction on Pd/Ba(10)/CZ started simultane-
ously with CO oxidation as observed on Pd/Ba(0, 15)/Al. It suggests
that NO reduction is not influenced by the presence of oxygen
from CZ. The QMS intensity of NO increased in a similar manner as
Pd/CZ but the desorption peak from Pd/Ba(10)/CZ shifted to a higher
80 Pd/Ba(0, 15)/Al
70
60
Ba(0)/CO-O2
Ba(0)/CO-NO-O2
5
4
0
0
Ba(15)/CO-O2
Ba(15)/CO-NO-O2
30
2
1
0
0
0
50
100
150
200
250
300
1
00
(b)
9
0
0
0
0
0
0
0
0
0
◦
temperature of 150 C. The peak shift seems to be related to the
8
7
6
5
4
3
2
1
x+
Pd/Ba(0, 10)/CZ
change of the adsorption strength between NO and cationic Pd
,
or between nitrite and/or nitrate and CZ surface due to Ba addi-
tion. Further investigation is needed for the origin of the different
Ba(0)/CO-O2
Ba(0)/CO-NO-O2
Ba(10)/CO-O2
Ba(10)/CO-NO-O2
◦
adsorption strength. The decrease of NO conversion above 200 C
on Pd/Ba(10)/CZ is also caused by the decrease of N O generation
2
under a slightly lean condition as mentioned before.
The temperatures of T50 observed over Pd/Ba/Al and Pd/Ba/CZ
in the CO–NO–O2 reaction are summarized in Table 2. On Pd/Ba/Al
0
◦
system, T for CO conversion was constantly lowered from 219 C
5
0
50
100
150
200
250
300
◦
to 179 C in the order of Pd/Al, Pd/Ba(1)/Al, Pd/Ba(5)/Al and
Pd/Ba(15)/Al. T50 of Pd/Ba(5)/Al was approximately the same as that
of Pd/Ba(15)/Al, indicating the amount of 5 wt.% barium is enough
to improve the activity under this condition. Similar tendency was
Temperature (ºC)
Fig. 3. CO conversion in CO–O2 and CO–NO–O2 reactions as a function of the amount
of Ba loading for Pd/Ba(0, 15)/Al (a), Pd/Ba(0, 10)/CZ (b) catalysts.

K. Tanikawa, C. Egawa / Journal of Molecular Catalysis A: Chemical 349 (2011) 94–99
97
◦
Fig. 4. DRIFT spectra for Pd/Al (a), Pd/Ba(15)/Al (b) in a temperature range of 30–410 C under CO–NO–O2 reaction conditions.
On the other hand, the activity of Pd/CZ for CO oxidation was
slightly influenced by the addition of NO. T of CO conversion
oxidation and NO reduction started at the similar temperatures.
Therefore, the desorption and/or dissociation of NO adsorbed on
50
◦
◦
◦
shifted from 65 C to 82 C by NO in the reaction gases, indicating
that NO molecules prevent somehow the progress of CO oxidation.
On the other hand, the activity of Pd/Ba(10)/CZ stayed almost con-
stant irrespective of the presence of NO in contrast to the case of
Pd/Ba/Al (Fig. 3a). NO actually gives little influence on CO oxida-
tion over CZ based catalysts. Therefore, it is interpreted that the
metallic Pd is a limiting step in this reaction. Above 250 C, a peak
−
1
−1
was newly observed at 2255 cm with a shoulder at 2232 cm
due to the vibration of Al–NCO of tetrahedral and octahedral Al3+
species, respectively [10]. It means that NCO species is formed via
the reaction of dissociated NO and CO on palladium and spills over
onto Al O3 surface, because NCO species accumulated on alumina
2
activation of oxygen supplied through O dissociation is not signif-
icantly affected by NO, although barium certainly influences Pd–CZ
interaction.
is stably present at higher temperatures as reported in the litera-
ture [13]. The vibrations of carbonate and nitrates on Al O surface
2
2
3
−
1
appeared in the 1200–1650 cm region. An unambiguous assign-
ment of all the frequencies is not possible. However, the band at
−
1
−1
3.3. In situ DRIFTS
1298 cm
and 1585 cm could be assigned to bridging and/or
chelating bidentate nitrate on Al O3 based on the adsorption band
2
In order to elucidate the reaction mechanism, the adsorption
reported for NO on Al2O3 surface [15].
states of each reactant on Pd/Al and Pd/CZ catalysts during the
On Pd/Ba(15)/Al, noticeable absorption bands related to CO on
Pd were absent under the present condition (Fig. 4b), but a weak
reaction were evaluated by using DRIFTS in a temperature range
◦
−1
of 30–410 C under a stoichiometric gas mixture of CO, NO and O .
absorption band at 1764 cm assignable to NO adsorbed on metal-
2
−1
◦
In the CO–O2 reaction, on-top CO (2099 cm ) on Pd metal was
observed even after O2 pre-treatment, indicating that PdO is read-
ily reduced by CO at 30 C under NO free condition and CO adsorbs
lic Pd developed from 90 C, overlapping with that of Ba nitrate [15].
It indicates that CO on Pd desorbs more readily than that of Pd/Al.
In agreement with the observation by XPS that the presence of Ba
suppresses the growth of Pd particles [16], the surface of palladium
particles is most likely covered by carbonates or nitrates detected
◦
on metallic Pd sites [9]. However, in the spectra of DRIFT on Pd/Al
−
1
as shown in Fig. 4a, an absorption band around 2164 cm corre-
sponding to CO adsorbed on PdO overlapped to the absorption of CO
−
1
in the 1200–1650 cm region due to the reaction of CO/NO and O2
with barium. As a result, the reactivity could be improved by the
desorption of CO and NO at relatively lower temperatures, leading
◦
gas at 30 C, although absorption of NO gas was not observed due to
◦
its low concentration. Above 70 C new peaks instead appeared at
1
−1
−1
◦ ◦
to the evolution of CO2 gas absorption at 190 C. Above 210 C a
940 cm and 1995 cm which can be assigned as CO adsorbed
−
1
on bridged sites of metallic Pd [13]. As for the adsorption of NO, an
new band was observed at 1745 cm which could not be assigned
in the present study, but it is related to the interaction of bar-
ium and CO because the same peak was also observed on Pd/Ba/CZ
(Fig. 5b) as well as Ba/Al without Pd under CO–O2 condition (not
−1
◦
absorption band was detected at 1771 cm above 110 C which is
assignable to NO contacting with metallic Pd [13,14]. Upon increas-
ing the temperature the peak intensity of bridged CO on Pd was
lowered while that of NO on Pd was enhanced. Thus, NO predom-
inantly adsorb on active sites of Pd and suppress the adsorption of
◦
shown). In the temperature region higher than 230 C, two bands
−
1
−1
at 2230 cm and 2168 cm were observed assignable to Al–NCO
and Ba–NCO, respectively [17]. It is confirmed that the formation
of isocyanate species on alumina and barium does not affect the
reaction on the Pd/Ba/Al. The bands assignable to bridging and/or
chelating bidentate nitrate on Pd/Ba/Al were detected immediately
◦
CO [13]. When the temperature reached to 210 C, the intensity of
−
1
adsorbed NO gradually weakened and instead a peak at 1974 cm
due to bridged CO appeared with the absorption of CO2 in the gas
phase. It corresponds well to the result of TPR (Fig. 1a) in which CO

9
8
K. Tanikawa, C. Egawa / Journal of Molecular Catalysis A: Chemical 349 (2011) 94–99
◦
Fig. 5. DRIFT spectra for Pd/CZ (a), Pd/Ba(10)/CZ (b) in a temperature range of 30–410 C under CO–NO–O2 reaction conditions.
after the reaction at 1294 cm−1 and around 1600 cm similar to
Pd/Al [10].
−1
at 1787 cm assignable to Pd –NO was gradually enhanced until
−1
0
◦
130 C and weakened with increasing temperature. This behavior is
corresponding to the desorption of NO over 130 C observed in TPR
(Fig. 2a). On the other hand, Pd–NO at 1709 cm maintained its
intensity over 150 C and disappeared at 210 C, namely T
conversion in TPR. It indicates that the desorption of NO and/or the
dissociation of NO on Pd is the limiting step for NO reduction in a
similar manner as Pd/Al. This is consistent with a reaction mecha-
nism proposed in the previous studies. Chuang et al. have proposed
as the NO–CO reaction mechanism on Rh/Al that Rh–NO dissoci-
ates to form adsorbed nitrogen and oxygen, and adsorbed oxygen
reacts with Rh (CO) to produce CO [19]. Furthermore, it has been
shown that Pd –NO and Pd–CO are the active adsorbates involved
in the NO–CO reaction on Pd/Al [13]. In the temperature region
◦
Fig. 5a displays the intensive peak of on-top CO adsorbed on
−1
◦
ı− −1
PdO at 2164 cm at 30 C over Pd/CZ, where oxygen can be sup-
plied from CZ support to Pd particles. The stability of CO on PdO is
in agreement with the previous study [12]; the absorption peak of
CO on Pd/ceria–zirconia/alumina disappeared at 150 C. Concurrent
with the decrease of CO on PdO, two large peaks assigned to Pd –NO
◦
◦
of NO
100
◦
ı−
0
−
◦
1
ı−
−1
(
1787 cm ) and Pd–NO (1709 cm ) [13,14,18] were observed
−
above 90 C. It seems that NO species adsorb on the metallic Pd
sites reproduced by the reduction of PdO in the progress of CO oxi-
dation reaction because there are no other peaks assigned to CO on
Pd. This result corresponds to the result of TPR where O2 is com-
+
2
2
0
◦
pletely consumed at 110 C as shown in Fig. 2a. The broad peak
1
9
8
7
6
5
.E+05
.E+04
.E+04
.E+04
.E+04
.E+04
Ce3+
Ce3+
Ce 3d
U’’’
Pd/CZ(red)
Pd/CZ(oxi)
8
75
880
885
890
895
900
905
910
915
920
Binding energy (eV)
Fig. 6. The effect of reduction and oxidation treatment on Ce 3d lines for Pd/CZ.

K. Tanikawa, C. Egawa / Journal of Molecular Catalysis A: Chemical 349 (2011) 94–99
99
◦
−1
−1
higher than 250 C, two bands at 2184 cm and 2151 cm were
observed which can be assigned to CZ–NCO [20]. The appearance
of two bands suggests the presence of two adsorption sites such as
stoichiometric condition. The improvement due to barium addi-
tion was clearly observed on Pd/Ba/Al, although negative effect was
demonstrated on Pd/Ba/CZ. The activity of Pd/Ba/Al for CO oxidation
as well as NO reduction was improved by increasing the amount
of barium loading. In the DRIFT spectra, CO and NO species on
Pd predominantly appeared with nitrates and carbonates on oxide
supports. On both Pd/Al and Pd/Ba/Al, CO oxidation and NO reduc-
tion occurred simultaneously with the decrease of NO absorption
bands on Pd. Thus, NO desorption and/or dissociation process is the
rate-limiting step for both reactions regardless of the presence of
barium on alumina. In Pd/Ba/CZ system, it was revealed that CO
oxidation was enhanced by the activated oxygen supplied from CZ
to Pd particles, which is inhibited by barium addition, resulting in
the decrease of CO oxidation. In contrast, NO reduction was inde-
pendent of barium addition, indicating that barium has no effect on
the desorption and/or dissociation of NO over palladium supported
on CZ.
4+
3+
3+
Ce and Ce . The presence of Ce is observed for Pd/CZ in Ce 3d
XPS as shown in Fig. 6. The Ce 3d states at 885 eV and 904 eV B.E.
due to Ce increased together with the decrease of so-called U
line at 917 eV B.E. characteristic to Ce by reduction. Inversely, the
Ce 3d states due to Ce3 decreased by oxidation. However, it was
revealed that a certain amount of Ce remained as Ce3 , which is
nicely correlated with two bands assigned to CZ–NCO in the DRIFT
Fig. 5a). On CZ surface, bridged or chelated bidentate nitrates are
observed as intensive absorption bands at 1572 cm , 1254 cm
and 1006 cm from 30 C [21]. These nitrates species remained up
to 210 C, namely T1 of NO conversion in TPR. However, the reduc-
tion of peak intensity at 1298 cm and 1612 cm below 110 C
seems to contribute to NO desorption in TPR, because there was no
peak derived from molecular NO species on cationic Pd [11].
On Pd/Ba(10)/CZ, PdO–CO (2164 cm ) was clearly identified at
0 C as shown in Fig. 5b, which is different from the case of Pd/Ba/Al
3
+
ꢀꢀꢀ
4+
+
+
(
−
1
−1
−1
◦
◦
00
−1
−1
◦
x+
−1
◦
3
References
without any noticeable peaks assigned to PdO–CO. However, the
[
[
1] S. Matsuura, A. Hirai, K. Arimura, H. Shinjoh, SAE Paper 950257, 1995.
2] L.F. Liotta, G. Deganello, D. Sannino, M.C. Gaudino, P. Ciambelli, S. Gialanella,
Appl. Catal. A 229 (2002) 217–227.
peak of CO on PdO dropped its intensity to half of that on Pd/CZ
◦
(
Fig. 5a) but remained until 190 C. It indicates that to some extent
oxygen supply from CZ support materials to Pd particles is inhib-
[3] H. Shinjoh, N. Isomura, H. Sobukawa, M. Sugiura, Stud. Surf. Sci. Catal. 116
(1998) 83–91.
◦
0
ited by Ba addition. Above 150 C, the peaks assigned to Pd –NO
[
4] T. Kolli, K. Rahkamaa-Tolonen, U. Lassi, A. Savimaki, R.L. Keiski, Catal. Today
00 (2005) 303–307.
ı−
−1
−1
−
and Pd–NO were observed at 1766 cm and 1706 cm , respec-
1
1
tively and bridged Pd–CO also appeared around 1972 cm . These
CO and NO bands depleted in a temperature range from 190 C to
30 C, corresponding to the simultaneous occurrence of CO oxi-
[5] T. Kobayashi, T. Yamada, K. Kayano, Appl. Catal. B 30 (2001) 287–292.
[6] T. Kolli, U. Lassi, K. Rahkamaa-Tolonen, T.-J.J. Kinnunen, R.L. Keiski, Appl. Catal.
A 298 (2006) 65–72.
◦
◦
2
[
7] F. Klingstedt, H. Karhu, A. Kalantar Neyestanaki, L.-E. Lindfors, T. Salmi, J. Vayry-
nen, J. Catal. 206 (2002) 248–262.
dation and NO reduction at the same temperature as observed
on Pd/Ba/Al. In consistent with the elimination of Pd–NO band,
[8] T. Kolli, K. Rahkamaa-Tolonen, U. Lassi, A. Savimaki, R.L. Keiski, Catal. Today
100 (2005) 297–302.
−1
Ba–NCO species was detected (2166 cm ), providing evidence of
NO dissociation. These results suggest that Ba has no effect on the
desorption and/or dissociation of NO over palladium supported on
CZ but inhibits oxygen supply from CZ to Pd particles. In the region
[
9] M. Fernandez-Garcia, A. Martinez-Arias, A. Iglesias-Juez, A.B. Hungria, J.A.
Anderson, J.C. Conesa, J. Soria, Appl. Catal. B 31 (2001) 39–50.
[10] Martinez-Arias, M. Fernandez-Garcia, A. Iglesias-Juez, A.B. Hungria, J.A. Ander-
son, J.C. Conesa, J. Soria, Appl. Catal. B 31 (2001) 51–60.
11] G. Djega-Mariadassou, Catal. Today 90 (2004) 27–34.
12] Iglesias-Juez, A. Martinez-Arias, A.B. Hungria, J.A. Anderson, J.C. Conesa, J. Soria,
M. Fernandez-Garcia, Appl. Catal. A 259 (2004) 207–220.
13] K. Almusaiteer, S.S.C. Chuang, J. Catal. 184 (1999) 189–201.
14] K. Almusaiteer, S.C.C. Steven, J. Catal. 180 (1998) 161–170.
15] Y. Chi, S.C. Steven, J. Chuang, Phys. Chem. B 104 (2000) 4673–4683.
[
[
−
1
of 1000–1650 cm , several peaks assigned to nitrates on Pd/Ba/CZ
◦
were detected at 30 C. However, these intensities were weakened
[
[
[
compared with Pd/CZ because the CZ supports were partly cov-
ered with barium, and no noticeable variation is recognized for the
◦
nitrate species related to NO desorption in the range of 100–170 C.
[16] K. Tanikawa, C. Egawa, Appl. Catal. A 403 (2011) 12–17.
[
[
17] H. Adbulhamid, J. Dawody, E. Fridell, M. Skoglungh, J. Catal. 244 (2006) 169–182.
18] R. Di Monte, J. Kaspar, P. Fornasiero, M. Garzinai, C. Paze, G. Gubitosa, Inorganica
Chimica Acta 334 (2002) 318–326.
19] S.C. Steven, C.D. Tan Chuang, J. Catal. 173 (1998) 95–104.
20] T. Bansagi, T.S. Zakar, F. Solymosi, Appl. Catal. B 66 (2006) 147–150.
21] B. Azambre, I. Atribak, A. Bueno-Lopez, A. Garcia-Garcia, J. Phys. Chem. C 114
(2010) 13300–13312.
4
. Conclusions
[
[
[
Effect of barium addition on Pd/Ba/Al and Pd/Ba/CZ systems
was investigated by XPS, TPR and DRIFTS under CO–NO–O2